lK)3Y-Yl40:XninhnI-o477sn? no/n T,h,tra. Vol 27 pp 477 to 493 GAS CHROMATOGRAPHY AND GAS CHROMATOGRAPHY-MASS SPECTROMETRY OF ORGANOStiLPHUR COMPOUNDS AND OTHER LABILE MOLECULES MICHAEL THOMPSON and MIRIAM STANISAVLJEVIC Department of Chemistry, Lash Miller Chemtcal Laboratories. University of Toronto. Toronto. Ontario. Canada (Recerred 7 Scytcwher 1979. Accrptd 2 I Dmwher 1979) Summary--A survey is made of mvestigations into the adsorption of polar and labile substances on surfaces within gas chromatographic equipment and of the methods which have been developed to minimize it. A description is given of a gas chromatograph-mass spectrometer in which such methods have been applied for the analysis of trace quantities of sulphur compounds. Gas chromatography (GC) combines unexcelled sep- aration capability with facile compound detection. The technique in one form or another has become the premier method used by analytical chemists for the detection and determination of traces of organic and organometallic compounds. The recent analytical literature shows that despite the rise of HPLC. gas chromatography still accounts for approximately 5O?J, of work published in trace organic analytical chemis- try. The choice of column materials for a particular analysis becomes extremely important, not only for the complete separation of all the components in the sample. but also to reduce losses due to adsorption. This has been a source of concern for many years in the chromatographic analysis of highly reactive and polar molecules, such as organosulphur compounds, which are highly sensitive to adsorption and catalysis. A considerable number of papers have stressed the need to use inert materials in the gas chromato- graphic determination of these substances at trace levels, since any interaction with either the column walls or the support material results in serious errors in quantification. It has been suggested that these losses are due to processes more complicated than simple adsorption effects. For instance, catalysis by the metal surface can lead to structural changes in the component. In any event, these effects need to be minimized so that the separation depends entirely upon the partition process. THE PROBLEMS OF ADSORPTION Hodges and Matson’ reported that an inert sup- port was required to reduce tailing in the chromato- graphic analysis of sulphur-containing gases. Adsorp- tion problems were also encountered by other workers in the same field. ‘~4 Losses of sulphur com- pounds seem to occur when the vapours come into contact with metal surfaces. Harrison and Coyne’ believed that the non-reproducibility encountered in the detection of thiols and hydrogen sulphide in beer or water headspace samples at the 0.01-0.1 ppm level, when stainless-steel, nylon or glass columns were used, was due to an interaction with the metal capil- lary tube connecting the column exit to the flame photometric detector (FPD). Freedman6 noted the poor recovery of low molecular-weight alkyl thiols and sulphides at low concentrations from stainless- steel capillary columns. He believed that it could be improved by using an all-glass system. A loss of hydrogen sulphide was reported in the quantitative determination of sulphur compounds in the gas phase of cigarette smoke,7 and this was attributed to the use of stainless-steel tubing in the lines. By replacing them with Teflon tubing an improvement was obtained. Vitenberg et ~1.~ also advocated the replacement of metal surfaces by glass or Teflon in the determination of sulphur compounds in industrial effluents. Adsorp- tion problems are not confined solely to the chroma- tographic system, but can occur with the vessels used to prepare standard samples. Thompson’ used per- meation tubes to eliminate such effects. A copper eva- porator used in the gas-chromatographic determina- tion of heterocyclic compounds” was found to cause transformations to occur within some of the mol- ecules before they entered the column. This was related to the presence of a tertiary butyl group at a sulphide sulphur bond with the thiophene ring, or the sulphide sulphur bond with this ring through a meth- ylene group. Again, in the determination of organo- sulphur species extracted from marine sediment,” at least one sulphur compound was believed to have undergone a photoreaction. These phenomena have been observed with other types of molecules. For instance, the calibration curves for chelates of cerium-group lanthanides” give an intercept of 0.18 pg at zero response. This is due to a loss of sample to the column material. Cadmium, lead and cobalt chelates of monothioacetylacetone’3 showed considerable decomposition during separ-
Transcript
lK)3Y-Yl40:XninhnI-o477sn? no/n T,h,tra. Vol 27 pp 477 to 493
GAS CHROMATOGRAPHY AND GAS CHROMATOGRAPHY-MASS SPECTROMETRY OF
ORGANOStiLPHUR COMPOUNDS AND OTHER LABILE MOLECULES
MICHAEL THOMPSON and MIRIAM STANISAVLJEVIC
Department of Chemistry, Lash Miller Chemtcal Laboratories. University of Toronto. Toronto. Ontario. Canada
(Recerred 7 Scytcwher 1979. Accrptd 2 I Dmwher 1979)
Summary--A survey is made of mvestigations into the adsorption of polar and labile substances on surfaces within gas chromatographic equipment and of the methods which have been developed to
minimize it. A description is given of a gas chromatograph-mass spectrometer in which such methods have been applied for the analysis of trace quantities of sulphur compounds.
Gas chromatography (GC) combines unexcelled sep- aration capability with facile compound detection. The technique in one form or another has become the premier method used by analytical chemists for the detection and determination of traces of organic and organometallic compounds. The recent analytical literature shows that despite the rise of HPLC. gas chromatography still accounts for approximately 5O?J, of work published in trace organic analytical chemis- try. The choice of column materials for a particular analysis becomes extremely important, not only for the complete separation of all the components in the sample. but also to reduce losses due to adsorption. This has been a source of concern for many years in the chromatographic analysis of highly reactive and polar molecules, such as organosulphur compounds,
which are highly sensitive to adsorption and catalysis. A considerable number of papers have stressed the need to use inert materials in the gas chromato-
graphic determination of these substances at trace levels, since any interaction with either the column walls or the support material results in serious errors in quantification. It has been suggested that these losses are due to processes more complicated than simple adsorption effects. For instance, catalysis by the metal surface can lead to structural changes in the component. In any event, these effects need to be minimized so that the separation depends entirely upon the partition process.
THE PROBLEMS OF ADSORPTION
Hodges and Matson’ reported that an inert sup- port was required to reduce tailing in the chromato- graphic analysis of sulphur-containing gases. Adsorp- tion problems were also encountered by other workers in the same field. ‘~4 Losses of sulphur com- pounds seem to occur when the vapours come into contact with metal surfaces. Harrison and Coyne’ believed that the non-reproducibility encountered in
the detection of thiols and hydrogen sulphide in beer or water headspace samples at the 0.01-0.1 ppm level, when stainless-steel, nylon or glass columns were used, was due to an interaction with the metal capil- lary tube connecting the column exit to the flame photometric detector (FPD). Freedman6 noted the poor recovery of low molecular-weight alkyl thiols and sulphides at low concentrations from stainless- steel capillary columns. He believed that it could be improved by using an all-glass system. A loss of hydrogen sulphide was reported in the quantitative determination of sulphur compounds in the gas phase of cigarette smoke,7 and this was attributed to the use of stainless-steel tubing in the lines. By replacing them with Teflon tubing an improvement was obtained. Vitenberg et ~1.~ also advocated the replacement of metal surfaces by glass or Teflon in the determination of sulphur compounds in industrial effluents. Adsorp- tion problems are not confined solely to the chroma- tographic system, but can occur with the vessels used to prepare standard samples. Thompson’ used per- meation tubes to eliminate such effects. A copper eva- porator used in the gas-chromatographic determina- tion of heterocyclic compounds” was found to cause transformations to occur within some of the mol-
ecules before they entered the column. This was related to the presence of a tertiary butyl group at a sulphide sulphur bond with the thiophene ring, or the sulphide sulphur bond with this ring through a meth- ylene group. Again, in the determination of organo- sulphur species extracted from marine sediment,” at least one sulphur compound was believed to have undergone a photoreaction.
These phenomena have been observed with other types of molecules. For instance, the calibration curves for chelates of cerium-group lanthanides” give an intercept of 0.18 pg at zero response. This is due to a loss of sample to the column material. Cadmium, lead and cobalt chelates of monothioacetylacetone’3 showed considerable decomposition during separ-
478 MICHAEL THOMPSON and MIRIAM STANISAVLJEV~
ation on a chromatographic column and none was eluted unchanged. The nickel chelate gave a barely discernible peak when less than 0.3 pg was injected. A similar adsorption pattern was observed for the other chelates, especially those of zinc and cobalt, whilst lead thioacetylacetonate underwent thermal decom- position. Utsunomiya I4 investigated the chromato- graphic behaviour of rare-earth chelates of isobutyryl- pivalylmethane and reported that the peak height for the terbium chelate decreased on repeated injection of sample. The terbium chelate was not eluted when pure solvent (benzene) was injected into the column, but a peak corresponding to terbium appeared when isobutyrylpivalylmethane (IBPM) was used. This peak also decreased in height on repeated injection of IBPM and further injection of terbium samples pro- duced a reasonable peak. It was thought that sample decomposition was the reason for these peculiar results, and that the terbium might be oxidized to Tb(IV). This effect was not observed in the case of the other rare-earth chelates. During a week’s storage of standard solutions of chromium(III) hexafluoroacetyl- acetonate [Cr(hfa)s] in 2-dram vials with polyethyl- ene-lined caps, the concentration of Cr(hfa)s de- creased.i5 The loss of Cr(hfa)s when contained in borosilicate glass flasks for the same period was negli- gible. Stainless-steel columns containing silicone grease and Apiezon M supported on firebrick have been shown to adsorb completely tin(IV) chloride. titanium(IV) chloride and iron(II1) chloride. This is thought to be due to a reaction between the chlorides. the greases and the walls of the column.”
Isoprene. acetaldehyde and acrolein have been identified as constituents in the gas phase of cigarette smoke.” All of the isoprene and acrolein. but only 80?,, of the acetaldehyde was recovered after adsorp- tion and desorption on Tenax, followed by gas chromatography. Srinivasan rr al.” reported an assay for pentadecylcatechols in poison ivy extracts by gas chromatography. When pure pentadecylcatechol was injected into the columns (glass, with 1% SE-30 or I% NGS on Chromosorb W) a broad peak was observed. A second injection produced a sharp peak superim- posed on the first peak. Subsequent injections showed that the sharp peak increased in height at the expense of the broad peak, which finally disappeared. If the compound was injected after a period of 1 hr, the broad peak was once more observed, but disappeared again with further injections. It is believed that penta- decylcatechol reacted with the support or liquid phase. This problem was overcome by preparation of a less reactive derivative. When the poison ivy extract was injected into the column, no peak corresponding to pentadecylcatechol was seen until the column was first saturated by injection of that compound.
These problems are also encountered when a gas chromatograph is linked with a mass spectrometer
(GC-MS). Bruner et al. used a Watson-Biemann separator and compared the results with those obtained in a direct coupling method, with cholesterol
as the test material. I9 The signal on the total ion- current monitor was lost when the separator was installed. A reversal of the ion intensities showed that adsorption and thermal decomposition had occurred. It was reasoned therefore that direct coupling was needed when polar, high-boiling and thermally un- stable compounds were being handled. A palladium separator used by Lovelock et al.” produced similar results. The structure of certain compounds could be
changed by catalysis at the metal surface. Honour et al.” found that the steroids present in the urine of patients with hypertension were degraded in the GC-MS system used for analysis. Modifications of the all-glass interface were necessary to overcome the problem.
EVALUATION OF COLUMN MATERIALS
Large numbers of reports have been published in recent years concerning the adsorptive properties of different column materials and methods for their deactivation. It appears that the adsorption phenom- enon can be related to either the solid support or the column material used in a particular analysis. Grob22 studied the effects of different metals (gold. gold/ platinum, platinum and platinum/iridium) used for the tubing connecting a capillary column with a flame-ionization detector (FID). A glass capillary column was used with OV-I, SE-52, OS-124, Ucon HB 5100, Emulphor ON 870 (E). PEG 20,000 or Silar 1OC as stationary phase. No permanent inactivation of the surface was obtained, in fact he observed intense adsorption and only transient inactivation could be produced. as in the case of the PEG column. To achieve low activity on the surface. the inactivat- ing agent needed to be applied constantly. The most effective agent was found to be the carrier leaving the column containing Ucon HB 5100. When the plati- num tubing was replaced by inactivated glass. practi- cally permenent inactivity was obtained. which seemed to be independent of the column coating. For these reasons Grob recommended the use of glass instead of metal in GC-MS systems. 2-Mercaptoben- zothiazole (2-MBT) and benzotriazole (BTA) were assessed as reagents for the deactivation of a stainless- steel transfer line between a SCOT capillary column and a flame photometric detector.23 The line was first conditioned with hydrogen sulphide, 2-MBT or BTA and any adsorption effects were detected by passing butanethiol through the column. The detector gave no response when an untreated line was used. Hydro- gen sulphide was found to be partially effective as a deactivating agent, but the effect decreased with time. 2-MBT gave a better performance but was found to bleed off eventually and was not suitable for tempera- ture programming. The detector response was highest when the line was coated with BTA but again the effect decreased, although over a longer period of time. Welsch et a1.24 tested a variety of silaning agents for the deactivation of glass capillary columns,
GC-MS of labile molecules 479
viz. dimethyldichlorosilane (DMCS), trimethylchloro- silane (TMCS) and hexamethyldisilazane (HMDS). Three different column coatings were used, squalane, OV-1 and Ucon 550 LB. Silaning was accomplished by pushing a plug of the reagent through the column, sealing the ends and applying heat. It was found that deactivation by heating with HMDS at 300” for about 20 hr produced the best results. Silanation of glass capillary columns has received further attention from Novotnj, and Bartle. 25 They saturated glass capillary columns with the vapour of DMCS, TMCS, HMDS, allyltrichlorosilane (ATS) or phenyltrichlorosilane (PTS) at 150” for 48 hr. The columns were then coated with loo/, SF-96 and dinonyl phthalate. They found that silanation with HMDS and TMCS was only effective for non-polar stationary phases and a negative effect was observed for polar phases. Results with the dinonyl phthalate stationary phase were best if ATS-treated columns were exposed to oxygen at high temperatures.
Adsorption on aluminium tubing26 manifests itself by causing very long tailing peaks, double peaks due to dehydration of alcohols, or loss of sensitivity due to irreversible adsorption. Tailing was found to be more common with old tubing, probably because of oxide layer formation on the inner wall. In order to reduce this tailing effect, columns of aluminium and glass were packed with 80/100 mesh Porapak Q. or loO/, SE-30 on 80/100 mesh Celite 560 AW, treated with DMCS and tested by measuring the peak shapes obtained for n-butanol, that obtained with n-butanol on Porapak Q in glass columns being used as refer- ence (‘zero” adsorption). Initially the inner walls were coated with polar tailing-reducers, such as Gas-Quat L. Antarox CO 880, sodium laurate and FFAP. These gave a small improvement if the original tailing was not serious, but were ineffective if serious adsorption was encountered. Removal of the oxide layer with methanolic hydrogen chloride made tailing much worse. Good peak shape and detection limits were obtained by treating a packed column with trifluoro- acetylacetone (TFAA). This treatment appeared per- manent (up to a period of 6 months). Similar results were given by hexafluoroacetylacetone (HFAA).
The materials used for columns and connections in the gas chromatography of sensitive, high molecular- weight compounds were compared by Arnold and Fales.27 They compared copper, aluminium and stainless steel with silaned glass as standard. Test solutions of codeine, adrenosterone, phenazocine, cholestane, cholesterol-3-methyl ether, cholesterol and narcotine, were separated on 1% SE-39 on siliconised Gas Chrom P, 100/120 mesh. Glass was found to be only slightly superior to aluminium or stainless steel. Basic substances (codeine, phenazocine) were almost completely adsorbed and the other compounds showed a diminished response when a copper system was used. The peaks reappeared to some extent upon
subsequent injections. It was noted that the Teflon connections used on the glass column caused peaks to tail and some compounds were lost at elevated tem- peratures. Teflon was later compared with stainless steel for the determination of sulphur-containing pol- lutants such as hydrogen sulphide.” Teflon was found to be adsorbent, in contradiction of the com- mon assumption. Different types of Teflon, e.g., FEP and TFE. were also investigated. In studies carried out on the chromatographic analysis of Kraft Mill sulphides, 29 three types of glass container were exam- ined for adsorptive properties and compared with “Scotchpak” bags. It was found that the latter were unsuitable as containers and therefore glass was used (which showed little adsorption). Greased glass stop- cocks caused losses of methanethiol when compared with Teflon-clad stopcocks. whilst neoprene and silicone-rubber stoppers showed no loss over 48 hr. Farwell et ~1.~~ noted the problems encountered when trying to determine sulphur-containing gases with an all-glass cryogenic enrichment and capillary gas- chromatographic system. The sample was collected in a glass U-tube (containing glass beads) cooled in liquid oxygen. The sample was then transferred (by heating) to a glass capillary trap before entering the gas-chromatographic column. Four types of glass were evaluated for use as the U-tube viz., soda-lime, borosilicate, conventional quartz and clear fused quartz. It was found that untreated glass, Pyrex and conventional quartz showed a minimum of adsorp- tion. Pyrex was chosen since the U-tube made with it compared well with a Teflon tube packed with Teflon (FEP)* (40/60 mesh), but this unfortunately produced non-quantitative recoveries and possessed memory effects. It was reported that a surface-deactivated WCOT column was required to minimize peak tailing and to achieve good separation of polar compounds. For this reason all glass parts were deactivated, with a variety of chemicals. Glass capillary columns were not recommended for the determination of sulphur diox- ide or dimethyl sulphoxide, because of bad tailing and peak broadening. Some conditioning of the column was required for hydrogen sulphide, carbon oxysul- phide (COS), carbon disulphide, methanethiol and dimethyl disulphide. The most efficient deactivating agent proved to be a combination of polysiloxane and methyl silicone (SE-30 or SP-2100).
Uden and Jenkins31 investigated the adsorption and displacement effects of aluminium(III), chro- mium(II1) and iron(II1) /?-diketonates, using different types of column materials, liquid phases and supports. Peak broadening and tailing were more evident for iron than for chromium, especially before the column had been conditioned by successive injections. Aluminium showed less tailing than iron, but more than chromium. Peak shapes were improved by re- peatedly silaning the column. Firebrick and Phase Sep P showed more tailing and adsorption than Chromosorb W or Phase Sep N. Adsorption of all the chelates was much reduced by using a Teflon support.
480 MICHAI L THOMPSON and MIRIAM STANISAVLJFV~
Severe adsorption occurred with Carbowax 20M and DEGA as liquid phases. Apiezon L and M were found to be better than SE-30. The nature of the column material. copper, glass or stamless steel, had little effect. One chelate which had been adsorbed could later be displaced by another, e+ chelates of aluminium and Iron. and thts effect was not changed by silanation of the column or the use of a Teflon support. However. the stationary phase had consider- able effect. the displacement being more apparent with polar liquids. A strange phenomenon was observed in the chromatography of iron compounds in that a portion of the chelate was gradually eluted before the rest of the sample. Substantial column in-
teractions have been observed when glass or Teflon columns and deactivated supports are used for the chromatography of /&diketonates, except for those of chromium(III). aluminium(lll). beryllium(I1) and a few others.“’ These interactions manifest themselves in the form of asymmetrical or spurious peaks and sample losses. Quantitattve analysis below the /lg level becomes difficult. Ntckel heptafluorobutanoylpi- valylmethanate showed considerable tailing on a Tef- lon column containing 30”;, silicone gum on 60/85 mesh Universal B. Chelate samples less than 0.2 /lg were not eluted. Substitutton of stainless steel or cop- per for Teflon or glass resulted in almost complete loss of the chelate. Other more polar stationary phases appeared to adsorb and/or decompose these complexes. A stainless-steel column containing 3”,, QF-I on Varaport 30 (SO/l00 mesh) showed no re- sponse for Ni(ATFP), when the sample level was less than 5 ng [H(ATFP) = 4-amino-l.l,l-trifluoropent-3-
en-2-one]. The detection limit rose to 20 ng when the column was inserted into a different gas chromato- graph containing a steel injection port. Samples of less than 0.5 jig of Cu(ATFP), were not detected, owing to interactton with the column. Koppe and Adams3’ evaluated a large number of support mater- ials and stationary phases on stainless-steel columns
for the determination of gaseous sulphur compounds below the ppm level. With empty columns losses occurred. Recoveries were quoted for hydrogen sul- phide, sulphur dioxide and methanethiol from glass, stainless-steel and Teflon columns. No sulphur com- pounds were recovered when aluminium tubing was used, and stainless steel was chosen as the column material. Hydrogen sulphide and sulphur dioxide were determined in an inert gas at the 10-1000 ppm level, with a variety of column materials.34 Reproduc- ible results could be obtained only by reducing con- tact between the effluent and the metal surfaces and by a general use. of Teflon. Adsorption problems were encountered and it was found necessary to condition columns by repeated injections of the sample to obtain a uniform response. Figures 1 and 2 illustrate this point, for two different colums. Some adsorption was still present, but it was much less pronounced when the stainless-steel column was replaced by Teflon (with which the response was 8 times as great).
(A)
FIN. 1. (A) Successive injections of H,S (1000 ppm) at I20 .34 Column: stamless steel I m x l/8 in. with 20’fj0 Carbowax 400 on Diatoport S. (B) Successive injecttons of SO> (1000 ppm) at I20 .34 (Reproduced by kind pcrmis-
sion of the author.)
When the Diatoport S support was replaced by Teflon 6, a great improvement was observed in the case of sulphur dioxide. but little change was noticed for hydrogen sulphide. The adsorption of the latter practically disappeared when dinonyl phthalate was used instead of Cdrbowax 400 as the stationary phase, but the reverse took place with sulphur dioxide, there being a reductton in peak height after the first injec- tion.
SH, IOOOppm SO, 1000 ppm
Fig. 2. Successive injections of H2S and SO1 (loo0 ppm) at 120”.“4 Column: Teflon 1 m x l/8 in. with 207; Carbowax 400 on Teflon 6. (Reproduced by kind permission of the
author.)
GC-MS of labile molecules 481
Sokolov et ~1.~’ also found it necessary to con-
dition the column with the compound under study,
e.g., copper trifluoroacetylacetonate [Cu(tfa), J. Glass or Teflon columns were used with a variety of liquid phases and supports. The first sample of Cu(tfa), in- jected on Chromaton NAW and Polychrom-1 coated with SE-54 was completely adsorbed. Subsequent injections resulted in gradual distribution of the adsorbed compound along the length of the column. Significant adsorption was caused by the glass-wool plugs (2530”/, of the total amount adsorbed) com- pared with 10-12x on PTFE wool. Adsorption was also found to increase with increase in the amount of liquid phase used. Little difference was found between silaned and non-silaned Chromaton but a large increase in adsorption was found when Polychrom-I was used. An increase in temperature caused a de- crease in adsorption, however.
Non-ideal column behaviour has been reported for metal fl-diketonates. The peak shape is usually asym- metric and the column HETP is lower than that for organic compounds of comparable volatility. A mix- ture of liquid phases and supports was suggested for the determination of mixed-ligand complexes of lan- thanides. Dexsil required the least loading by suc- cessive injections of the terbium complex. With QF-I and SE-30, but not Dexsil 300, there was displace- ment of europium by terbium. Glass columns were then evaluated after silanation to various degrees. All the silaned columns showed unsatisfactory chromato- graphic behaviour. Spurious peaks and shoulders occurred when QF-1 and SE-30 were used, possibly owing to chelate decomposition and exchange of the ligand.
A chromatographic study of several volatile metal halides on different stationary phases (n-octadecane, squalane, Apiezon T, silicone oil and paraffin) showed that tin(IV) chloride and titanium(IV) chloride were completely adsorbed on Apiezon L. Branched
alkanes, such as Apiezon grease and silicone oil, led to reactions on the column, therefore normal alkanes were prescribed. Kusy’* reported the separation of polar and non-polar compounds on columns contain- ing various stationary phases. He noted that adsorp- tion took place, especially with polar compounds. Substances able to form hydrogen bonds were believed to undergo adsorption, with the strength of the bond dependent on the structure of the molecule. He found evidence of irreversible adsorption on the support (Chromosorb P), but this decreased with increase in column loading.
The contribution of the support materials to adsorption phenomena has received a great deal of attention. Ettre3’ investigated firebrick, Chromosorb, Chromosorb W, Celite and Teflon for the separation of polar and non-polar substances. In accordance with popular opinion he reported that Teflon should only be used for highly polar samples. Another study compared Chromosorb W, Firebrick P, Carborun- dum, Fluoropak, glass beads, nichrome beads and
stainless-steel beads.40 for a mixture of nine ketones.
Adsorption decreased, especially for polar com- pounds. when the supports were silaned. except for
Fluoropak. which showed no adsorption properties
anyway. Ottenstein 41 has discussed diatomite and non-diatomite supports and methods for their deacti- vation. Sze rt a1.42 realized the need to eliminate adsorption on the solid support in their separation of lower aliphatic amines. Potassium hydroxide, tetra- hydroxyethylethylenediamine (THEED) and tetraeth- ylenepentamine (TEP) were used as deactivating agents for Chromosorb W 60/80 mesh. Potassium hydroxide (2”!,), when used with 15% Carbowax 400 or 1540. eliminated tailing. THEED showed no deac- tivation. whereas TEP improved tailing. A combina- tion of these two reagents was eventually decided upon for the best separation. The trifluoroacetylaceto- nates of chromium(II1) and ruthenium(II1) have been separated on a glass column containing squalane or Apiezon L on 80/100 mesh Chromosorb W and DMCS.43 The chelates reacted with the uncoated unsilaned support and no peaks were observed unless the material was silaned. Figure 3 shows the chroma- tograms for Cr(tfa), at various stages of silanation. No further improvement was obtained after 500 ~1 of dimethyldichlorosilane (DMCS) had been added to the support. At low stationary-phase loadings, some adsorption was seen even with silanation. It was
I E
I I I I I I I I
3
500 p I. DCDMS
u z ii
100 u I. DCDMS
Y _---_---__---_-----_-----
30 pl. DCDMS
0 p,i., DCDMS
I I I I I I I I I I 0 3 6 9 I2 15 16 21 24
MINUTES -
Fig. 3. Chromatograms for Cr(tfa), at various s’iages of silanation4’ Column: 3.4 g of Chromosorb W (no liquid loading), at 120”; Sample: 3O.pg of Cr(tfa), in 3 ~1 of solu- tion. Carrier-gas velocity 4.0 cm/set. (Reproduced from Journal of Gas Chromamgraphy by permission of Preston
Publications Inc.)
482 MICHAEL THOMPSON and MIRIAM STANISAVLJEVU?
stated that highly loaded columns should be used to minimize adsorption with these support materials, Acid-washed Chromosorb W, alone or silaned with DMCS, and containing 6% OV-101 or 6% DC-200 was evaluated for column performance with five organophosphorus compounds.44 The conditioned columns were treated with Carbowax 20M vapour and ng amounts of material were injected into the column. Only ronnel exhibited no adsorption on silaned Chromosorb with OV-101. Improvements were achieved after Carbowax treatment, as shown by the azinphosmethyl oxygen derivative which could not be detected on either column (AW or DMCS) before deactivation. It was noted that very gradual bleeding of the Carbowax occurred with time. Chro- mosorb 102 gives tailing of the peaks observed with amines, reported by Hertl and Neumann,45 who ascribed the problem to unreacted vinyl groups behaving as active sites on the support material. They treated the support with hydrofluoric acid, followed by a coating of 2% Carbowax 20M and found that less interaction was observed with the Carbowax- treated support, i.e., the peaks were more symmetri- cal. It was conjectured that the Carbowax blocked the active sites by covering the support surface, while hydrofluoric acid was supposed to react with the vinyl groups. Suprynowicz et ~1.~~ reported that silaning agents react with active sites to produce trimethylsilyl groups. They illustrated this point by using pure and silaned Diatomite D covered with 1%. 2% and 5% dinonyl phthalate. Silanation decreased the adsorp tive nature of the support. In many of the papers mentioned in this article, Carbowax 20M has been used as a deactivating agent. Its performance in this role was evaluated with Celite 545 as support and lx, 3% and 10% Apiezon L and OV-210 as stationary phases.47 Bare Celite and coated Celite columns were run side by side, a valve being used to switch flows to an electron-capture detector (ECD). The difference became less pronounced as the load or polarity of the liquid phase increased. With bare Celite, no peaks were observed for the first injections. On subsequent injections the peaks obtained were still smaller than those obtained with the modified Celite.
METHODS FOR DEACTIVATION OF
CHROMATOGRAPHIC MATERIALS
Schieke and Pretorius4* recently described several different methods of deactivating whisker-walled open-tubular glass columns. The whiskers increase the surface area but are highly active, causing exces- sive tailing, and therefore need to be removed. Silana- tion was attempted with a solution of DMCS in toluene, which was passed through the column with a stream of dry nitrogen. A second column was filled with HMDS and TMCS vapours and sealed at the ends before being heated at 200” for 48 hr. The vapour was also heated at 200” for 24 hr. Adsorption of surface-active agents, such as benzyhriphenylphos-
phonium chloride, was tried. Several materials were chosen as reagents for surface carbonization. Di- chloromethane vapour was sealed in a column and heated at 550” for 30-45 min; acetylene was passed through the column, the ends were sealed and the acetylene pyrolysed at 550” for the same length of time; n-hexane was injected until 10% of the column was filled and the heating process was repeated. The application of non-extractable polymer layers was investigated by saturating a column for 3-6 hr with a 2% solution of Carbowax 20M in dichloromethane. After flushing with dry nitrogen, the ends were sealed and the column was heated at 280” for 24 hr. A mix- ture of polar and non-polar compounds was injected into each prepared column and the amount of tailing was calculated. The most effective deactivation involved the passage of HMDS and TMCS vapours through the column at 200” for 24 hr.
Column conditioning
There is some controversy regarding the “carrier effect” observed in GC-MS systems. This phenom- enon was recently discussed by Blazer and Chait.49 They reported that it was advisable to inject a large quantity of the carrier substance (a compound which the adsorbing system is unable to distinguish from the compound of interest,50 usually the same compound, but containing a stable heavy isotope) either simulta- neously with the sample, or before the analysis. This covers any active sites in the column and connections between the GC and MS. This effect is only useful for selected ion monitoring analysis of small quantities. The need to coat gas chromatographic columns before analysis has received special attention in many cases where highly reactive or polar compounds are used. For instance, Gumbmann and Burr” reported that the initial response to sulphur compounds extracted from potatoes was low and erratic on three different columns, but improved with repeated use of the columns. Two reports*2*53 concerned with the separation of a mixture of gases on two columns con- nected to a thermal conductivity detector (TCD), both stated that the systems needed to be conditioned in the case of sulphur dioxide.
The determination of elemental sulphur with an ECD and an FPD54 required repeated sulphur injec- tions on glass columns containing three different liquid phases before reproducible results could be obtained. Black et al. ” found it necessary to con- dition a Teflon column packed with Supelpak S by using high concentrations (five 60-ng/ml injections) of hydrogen sulphide and sulphur dioxide. Devonald et
a1.56 minimized the adsorption of sulphur-containing species on flasks by pretreating the flasks with sul- phur vapour. They found that sulphur dioxide and dimethyl disulphide were adsorbed onto the syringes used. This effect has been observed for other types of molecules, such as imidan and imidoxon, which are phosphorus-containing pesticides.57 The glass column used for this particular determination contained loO/,
GC-MS of labile molecules 483
DC-200 on Gas Chrom Q SO/l00 mesh and needed to be conditioned by repeated injections. The same authors’s encountered similar problems in the analy- sis of 138 pesticides and their metabolites with glass columns containing a variety of stationary phases on Gas Chrom Q 80/100 mesh.
An all-glass system was utilized by Juvet and Dur- bin59 to reduce reactions with metal chelates to a minimum. They found that the columns used needed to be conditioned, especially for iron(II1) hexatluoro- acetylacetonate. Chelates of chromium and alumin- ium (acetylacetonates, trifluoroacetylacetonates and hexafluoroacetylacetonates) were separated on a stainless-steel column containing 20% Dow Coming Silicone Fluid 710R on Gas Chrom Z.60 At least 6 yl-injections containing about 1 mg/ml were necess- ary to condition the column. Gallium, aluminium, indium and beryllium trifluoroacetylacetonates have also been investigated. 61 It was found necessary to condition a silaned glass column containing silaned glass microbeads as support, by successive injections of the gallium and indium complexes. Brunnee et ~1.~’
tested different types of material as connection lines between a GC and an MS. They found that the sur- face was deactivated by the sample itself, which in this case was cholesterol.
Silanation
Glass columns have been advocated for the chro- matographic analysis of drugs, pesticides and other compounds that undergo thermal decomposition in metal columns. However, glass itself must be pre- treated. A solution of DMCS (5%) in toluene was suggested by Bach63 as a useful silaning agent which should react with and block any active sites. A solu- tion of HMDS was used by Dewar and Maier64 to deactivate glass beads used as a solid support for squalane. Novotny and TesaIik6’ used a combination of HMDS and TMCS to silane glass capillary columns after they had been internally etched with a gaseous mixture of hydrogen chloride and hydrogen fluoride. Several stationary phases were then applied and the separation efficiency of silaned and unsilaned columns was determined. The silaned surface showed a favourable effect only for non-polar stationary phases, whereas a strongly negative effect was observed for polar phases. It is now common practice to treat columns and column packings with silaning agents. An example is the determination of trace mer- captans and sulphides in natural gas66 with stainless- steel columns treated with Siliciad and packed with 5% polyphenyl ether (PPE) on acid-washed Chromo- sorb G, 80/100 mesh, treated with DMCS. Stainless steel was preferred to Teflon for the columns and a glass sample loop gave a greater response than either Teflon or stainless steel. A Siliclad solution was used by the same author in a later determination of sul- phur gases in hydrocarbon streams.67 There was still some adsorption on one column containing PPE and phosphoric acid on a siianed support. Goode6s recog-
nized the need to use silaned columns for the determination of sulphur compounds in North Sea natural gas. in preference to conditioning unsilaned columns by repetitive sample injection. A solution of Silyl-8 was placed in the appropriate column and heated at 250” for 12-16 hr. Adsorption losses on treated and untreated aluminium cylinders used for collecting samples were also studied. In any given cylinder, adsorption losses were found to increase with increasing molecular weight of the compound under investigation and to be proportional to the ini- tial concentration, this loss occurring entirely within the first few hours. It is of interest that this author used an aluminium tube to connect the column to the GC detector. Heating tape maintained the tempera- ture of this tube at 30&400”, which was believed to reduce adsorption of sulphur compounds. Nickel columns and connections have been used in a heart- cutting technique in high-resolution gas chroma- tography applied to the analysis of sulphur com- pounds in cigarette smoke.69 Although nickel is now regarded as being as inert as glass, these authors realized the need to silane all metal parts coming into contact with the sample.
A film of SE-30 containing fine particles of silaned silicic acid was deposited on silaned glass capillary columns for the determination of human urinary steroids.” The silaning agent, DMCS in toluene, was also used in the precolumn tubing and splitter. This precaution was taken since metal columns destroy many biological samples. No change in the column properties was apparent after 6 months. Glass separa- tors used in a combined GC-MS system can be the cause of sample losses if precautions are not taken. MacLeod and Nagy” found it necessary to treat their fritted glass molecular separator in situ by injection of bistrimethyisilylacetamide (BSA) through a septum into the transfer lines. The compound was drawn by vacuum into the separator for reaction with surface OH groups. The sensitivity for selected terpenoids increased after treatment with BSA.
Carbowax 2OM
Carbowax 20M has proved particularly useful in gas chromatography. not only as a stationary phase, but more recently as a deactivating agent. Schomburg et al.” found that compounds were adsorbed if glass capillary columns were not treated with Carbowax. This was not true, however, if polar stationary phases were used. Free silanol groups on the glass surface have been suggested to be the cause of active sites.73 The remedy proposed was to coat the column with Carbowax and heat it under nitrogen. The coating was removed and then reapplied. The coated capillary column showed excellent separation power and long- term stability when used for 2-undecanpne and low- boiling aliphatic alcohols. Before treatment the column showed tailing of the alcohol peaks, but after treatment resolution was improved and tailing totally absent. Blomberg74 also deactivated Pyrex capillary
484 MICHAEL THOMPSON and MIRIAM STANISAVLJEVII?
columns, using a thin layer of non-extractable Car- bowax 20M coated with SF-96. for the separation of sulphur compounds.
Misceliuneous liqurds
Several other liquid phases have been assessed as deactivating agents. A glass tube packed with 5% PEG 20M on Chromosorb W, AW was inserted into the injection port of a gas chromatograph and con- nected to a glass capillary column with PTFE tub- ing.” PEG 2OM was allowed to bleed through the capillary column overnight. The deactivated column was coated with SE-30 and its performance compared with that of one that had not been deactivated. Diel- drin and endrin (a substance very sensitive to adsorp- tion) were used as the test materials. It was found that endrin was not eluted from the non-deactivated column containing a thin film of SE-30, but decom- posed to produce two products. Decomposition was thought to be due to the column wall activity, since this effect was not observed for the deactivated column containing a thin film, or the non-deactivated column with a thick film. Withycombe rt LI/.‘~ noted the need to replace metal surfaces with glass tubing. However, the sample splitter alone was deactivated by injecting SE-30 and heating to 400”. Heckman et al.” used 6M hydrochloric acid as the deactivating agent for glass capillary columns. The ends were sealed and the column was heated overnight at 10&150”. The presence of benzene on molecular sieve 13X was found to reduce the adsorption capacity of the sieve for thiophene ‘s in the determination of low concen- trations of sulphur compounds. Averill showed that if the stainless-steel tubing used on a GC-MS system was treated with 2,4-pentanedione. greatly improved peak shapes and lowered detection limits for steroids were obtained.
Gases
The mechanical properties of glass make it prefer- able to Teflon for chromatographic supports, accord- ing to Diez rt al. *’ The surface activity of glass is due
to the presence of silanol (Si-OH) and siloxane (Si- GSi) groups, which behave as electron donors and acceptors respectively. Glass columns were therefore deactivated by high temperature treatment with a mixture of nitrogen and hydrazoic acid (I :3). Com- pounds such as ethanol, benzene, methyl ethyl ketone, nitromethane and pyridine were eluted from the treated columns. It was observed that adsorption de- creased for those products which did not contain nitrogen atoms, probably because an Si-N-B bond had been formed. Bruner et al.*’ conditioned their glass columns by heating in a nitrogen atmosphere and found that no adsorption occurred with sulphur compounds and therefore concluded that there was no need to use PTFE. Bruner et al.*’ also found that a glass system gave similar results to PTFE, provided
that it was conditioned by passage of dry helium or hydrogen at 130” for 6 hr.
The use of columns at higher temperatures, e.g., 55”. was advocated by Adams et ds3 to minimize surface adsorption of certain compounds, oi;., hydro- gen sulphide, sulphur-dioxide and methanethiol.
Teflon columns und connectors
Stevens et u/.*4-86 have found that Teflon columns and supports give superior separation and detection of reactive sulphur compounds. They report the loss of a peak corresponding to sulphur dioxide when Tef- lon (FEP) lines are replaced by stainless steel. Soft glass and borosilicate glass also show retention of sul- phur dioxide at levels below 10 ppm. Teflon (FEP) was found not to give this effect and was therefore adopted as the column material. Further sample- metal interaction was eliminated by fitting the column exit directly into the base of the flame-photometric
GC detector. Several packing materials were evalu- ated and all were found to be unsatisfactory, even after silanation. Powdered Teflon was the only one sufficiently inert to be of use. The same was true for the variety of liquid phases evaluated. A mixture of polyphenyl ether (j-ring) and phosphoric acid was finally chosen. For further experiments on the determination of low concentrations of sulphur com- pounds, a Teflon (FEP) column containing poly- phenyl ether and phosphoric acid on @O/60 mesh) Teflon was used.
Many workers in this field have since adopted the use of Teflon for chromatographic materials. Bruner
ef al.” used a Teflon column for the GC determi- nation of sulphur compounds in air. All gas lines, sampling loops and exponential dilution flasks were constructed of Teflon. In this way all adsorption effects were minimized. Baumgardner et al.** reported the use of a Teflon column, connectors and 3-way valve in the measurement of sulphur compounds with an FPD. A similar column was prepared by Blan- chette and Cooper 89 for the determination of hydro- gen sulphide and methanethiol in mouth air at ng/ml levels. Teflon was also used as the material for sam- pling probes and connections. No memory effects were noted with this system. Teflon is now being used whenever adsorption losses need to be minimized. This is shown by its use in the construction of per- meation tubes for calibration purposes,” and for parts in which the surfaces are contacted by the sample, such as sampling valves, columns and even the wool used to plug the ends of columns.”
There seems to be general agreement that there is a need to reduce or minimize effects arising from the interaction of reactive or polar molecules with the column material, stationary phase, support, sample lines and valves. The exact method used seems to vary from one author to another. In fact, some conflicting results are given, which would suggest that a great deal of care must be taken in making such evalu- ations.
GC-MS of labile molecules 485
AN APPROACH TO THE STUDY ORCANOSULPHUR SPECIES BY
INERT CC-MS TECHNIQUE
OF AN
The amount of sulphur present in the atmosphere results largely from natural causes, such as volcanic activity, sea spray and decomposition of organic materials rather than from industrial or man-made emissions from paper mills, fossil combustion and petroleum refining. 92 The latter sources, however, are increasing in importance and are becoming of
major concern to environmentalists. Consequently there is a requirement to monitor trace quantities of low molecular-weight air pollutants such as hy-
drogen sulphide and sulphur dioxide. Attempts to identify the origin of petroleum spills by using the sulphur “fingerprint” is a very recent application.93-95 Other materials which have been investigated for sulphur content include pesticide residues,96-9s fla- vour volatiles,5’.76 beers and wines, 5.99-‘“’ combus_
tion products of tobacco,4*7~69.‘02~103 sulphona- mides ‘o4.1o5 town and natural gas,66.106-109 marine
sedimknts,’ ’ coal’ lo and petroleum samples.’ “-’ l6
This is not intended as a complete list, but merely serves to indicate the various industries relying on sulphur chemistry.
The development of the flame photometric detector
by Brody and Chaney ’ ’ ’ made possible the determin-
ation of sulphur compounds at trace levels by obser- vation of the blue emission from the S2 species pro- duced in a hydrogen-rich flame, through a filter hav- ing maximum transmittance in the 394 nm region. The selectivity and sensitivity of this technique allowed sulphur compounds to be determined at nanogram and picogram levels with relative ease, compared with previous methods.
A new detector introduced by HNU Systems Inc.“s is based on the photoionization of the species being eluted and is claimed to be ten times more sen- sitive to low molecular-weight sulphur compounds than the FPD. It has a wide linear range and since it does not use a flame, a supply of hydrogen and air is not required. A recent development by Photovac Inc.“’ would appear to be capable of detecting com- pounds at a level at least one order of magnitude lower than that detectable with the HNU system.
Gas chromatography-mass spectrometry couples the most powerful separation technique available to the analytical chemist with what can be referred to as the ultimate detector. Determination of sulphur com- pounds by this combined technique should attain detection limits at least comparable with GC-FPD, as well as providing structural identification. Two major problems with this system, however, are removal of the carrier gas, and the pressure differences between the CC and the ion-source. For these reasons, enrich- ment devices or molecular separators are preferred when using packed columns. Three basic types of sep- arator are available,“’ the one of choice here being the semi-permeable membrane separator, developed
by Llewellyn and Littlejohn.“’ This incorporates a
silicone-rubber membrane selectively permeable to organic compounds and largely impermeable to the
carrier gas. Earlier. mention was made of some of the problems
encountered in the analysis of highly reactive and polar compounds and methods were suggested for alleviating the situation. Although Teflon is not com- pletely inert, it does show the least adsorption of sul- phur species and for this reason we chose it for use in an inert GC-MS system instead of glass, which would require deactivation. and constant conditioning by repeated sample injection. By replacing with Teflon all surfaces that are contacted by the sample, any adsorption is kept to a minimum. In a recent article, Fujiwara and Ogata’** showed that hydrogen sul- phide reacts with si)icon( I I I) surfaces. The material was exposed to hydrogen sulphide at room tempera- ture and and the amount adsorbed was measured by Auger electron spectroscopy and low-energy electron- loss spectroscopy. It was found that hydrogen sul- phide molecules were adsorbed non-dissociatively on silicon surfaces at room temperature. After annealing at 550” dissociation produced desorption of hydrogen and formation of silicon-sulphur covalent bonds. Above 650” the sulphur atoms were desorbed to leave a clean silicon surface. This evidence would seem to indicate that labile sulphur molecules react not only
with metal surfaces, but also with those made of glass, and extreme caution should be used in the choice of column materials for any such determination at low concentration levels.
Here we describe an inert system in which the sample contacts only Teflon surfaces from the point of injection to its entry into the ion-source. By use of another detector (FPD) in parallel with the MS, checks can be made on both losses in resolution and
decreases in the signal, caused by adsorption.
EXPERIMENTAL
The compounds studied were of reagent grade and were used without further purification.
Gas chromatoyraph)z
Chromatograms were obtained wtth a dual-column dual-electrometer Varian Aerograph GC (Model 2740) coupled to an FPD (Tracer Inc., Austin, Texas) mounted at the side of the GC. complete with a 750-V power suo~lv. The response was monitored with a dual-pen L&ear Instruments (Model 385) recorder. In this way both FPD (sulphur) and FID (solvent) signals were obtained. The column consisted of a 40 ft x l/8 in. (outer diameter) Tef- lon (FEP) tube packed with 40/60 mesh Chromosorb T coated with 12% polyphenyl ether and 0.5% orthophos- phoric acid (Chromatographic Specialties, Brockville. Ontario). The column exit was fitted directly into the base. of the detector to minimize dead volume and reduce effluent-metal interactions. All gases used (nitrogen, hydro- gen, air) were dried with “Gas-dry” filter traps (Chroma- tographic Specialties). In addition, an oxygen trap (“Oxi- sorb”) was placed on the nitrogen cylinder and a hydro- carbon trap was present on the hydrogen cylinder.
486 MICHAEL THOMPWN and MIRIAM STANISAVLJEV~
To vant From W
UIU Ton’
u Fig. 4. Teflon membrane molecular separator.
Mass spectrometry
The instrument used was an AEI Model 902, high- resolution (variable between 1000 and 30000) double- focusing mass spectrometer using an electron impact source and coupled to an AEI DS-50 data system. Source temperature 150”; electron energy 70 eV; accelerating volt- age 8 kV. Diffusion pumps capable of pumping air through the lines at 2400 I./W were installed in preparation for chemical ionization.
Separafor. The membrane separator (Fig. 4) was con- structed completely from Teflon and had an outside dia- meter of 5 cm. The inside surfaces were tapered at 45” to prevent tearing or creasing of the membrane when the unit was assembled. The inside path was I cm long, 0.24 cm wide and 0.25 cm deep. The silicone-rubber membrane (J) (0.1 mil thick, General Electric, Schenectady, New York) was placed over a Teflon-coated stainless-steel support I .5 cm in diameter (Millipore Ltd., Mississauga, Ontario) covered by a Teflon filter (H) (Millipore Ltd.) and secured with a rubber O-ring (K). Aluminium plates (A) and brass bolts (D) were used to hold the separator halves (C) together after sealing with a high vacuum sealant (Space Environment Labs., Boulder, Colorado). Connections were made to l/g-in. outside diameter Teflon (FEP) tubing (Chromatographic Specialties) at the GC end and l/4-in. outside diameter Teflon tubing (Alltech Associates, Arling- ton Heights, Illinois) at the MS end, with special Teflon unions (Fluoroware, Chaska, MN).
Re-entrant tube. The tube is shown in Fig. 5. A length of l/4-in. outside diameter Teflon tubing (A) (Alltech Associ- ates) was covered with a double layer of aluminium foil (J) [holding a chrome]-alumel thermocouple (K) in place] and glass tape (L) soaked in leak-sealant. Heater wire (M) (nichrome, 28 gauge) was then wrapped round this and covered by another layer of soaked glass tape. The frame- work consisted of standard 12-mm (C) and l/4-in. (H) Swagelok fittings (Avon Valve and Fitting Ltd, Scar- borough, Ontario) welded to a stainless-steel metal flange (F). Four feedthroughs (G) (Quality Hermetics, Toronto, Ontario) with enamel wire (N) attached were soldered into the flange. Ceramic tubing (P) was used as an insulator on all wires. Pins 2 and 4 were connected to the heater wires and pins 1 and 3 were connected to the thermocouple. A short length of 12-mm outside diameter Pyrex glass tubing (B) which had been internally etched with hydrofluoric acid to produce a wall thickness of 0.05 cm was placed over the end so that the Teflon tubing protruded about l/4 in. Teflon front and back ferrules (Avon Valve and Fitting Ltd) were used as a seal between the tubing and the stainless-steel framework. Once complete, the re-entrant tube was bolted to the source housing by means of the metal flange, with the Teflon tip touching the ion-source.
Procedure
The system is shown schematically in Fig. 6. Liqutd or
Fig. 5. Teflon re-entrant tube.
GC-MS of labile molecules
Column 4-port Membmne
Sample _____________v!v~_-,__Se~o!r?r_______
Moss
Spectrometer -__ --_------_ -- ____ -
F. P D.
Fig. 6. Schematic diagram of GC-MS arrangement showing the alignment of the 4-port valve.
headspace samples are injected into the GC by syringe. On emerging from the column, the effluent passes into a Cport Teflon valve (Hamilton 2x valve with 1/8-m. NPT CTFE fittings) situated in the GC detector oven and heated at 120”, whence it can pass either directly into the detector base of the FPD or to the membrane separator. Since no stream splitter is used, samples need to be injected twice to obtain both an FPD recording and a mass spectrum. From the separator, the sample passes through a silicone-rubber membrane and into the ion-source. The carrier gas (nitro- gen) and any effluent not entering the MS pass back through the Cport valve and are detected by the FPD. The separator and lines to the MS are heated in a specially built aluminium oven. All lines and connections that make contact with the sample from the GC inlet to the ion- source are made of Teflon. The connections between the GC and MS are made as short as possible to minimize dead volume. A triple temperature-control unit is built to provide and control heat to the separator, line and re- entrant tube. The temperatures are adjusted manually to those specified, and measured with chromel-alumel ther- mocouples. Heat is transferred by wrapping all the parts in aluminium foil. Heating tapes (Briscoe Mfg. Co., Col- umbus, Ohio) are wrapped round the separator and re- entrant line and connected to the triple control unit. The third connection is made to the nichrome heating wire through two of the four feedthroughs.
RESULTS AND DISCUSSION
The detector response and all chromatographic
conditions were optimized with a solution containing methanethiol, ethanethiol, n-butanethiol, dimethyl sulphide, diethyl sulphide and dimethyl disulphide in absolute ethanol (100 pg of S per ml). Sample sizes
were usually 1 or 10 ~1 injected into the column. The following conditions were then used for GC and GC-MS experiments: column temperature 120”; injector and FPD temperature 140”; nitrogen carrier gas flow-rate 30 ml/mm; hydrogen flow-rate 60 ml/min; air flow-rate 100 ml/min.
All solutions were prepared in Nalgene polyprop ylene flasks (Canlab, Toronto, Ontario) which had been thoroughly cleaned. The sulphur response was recorded by one recorder pen linked through the electrometer to the photomultiplier tube and the sol- vent response by a second pen (by measuring the re- sponse at a collector-ring housed in the FPD body). The collector ring is sensitive to carbon ions pro- duced during ionization of the effluent in the flame; this system is less sensitive (by a factor of 5 10’) than
487
a normal FID used in gas chromatography but it is adequate for the measurement of the ethanol concen- tration. The FPD response was measured as peak area (peak height x width at half-height) and the efficiency of the column was calculated at different carrier-gas flow-rates. There seemed to be an increase in the number of theoretical plates with a decrease in flow-rate, as predicted by the van Deemter equation. The optimum nitrogen flow-rate would correspond, however, to a long analysis time and so a compromise was made between analysis time and efficiency by using a flow-rate of 30 ml/min ( -4000 theoretical plates). Under these conditions a good separation of the six sulphur compounds was achieved, as shown in
TIME (min)
Fig. 7. Chromatogram of organosulphur compounds in ethanol (S 100 &ml). Sample size 1 ~1. A, Methanethiol; B, ethanethiol; C. dimethyl sulphide; D, diethyl sulphide;
E, butanethiol; F, dimethyl disulphide; G, ethanol.
488 MICHAEL THOMPWN and MIRIAM STANISAVLJEVI~
Slope 2.03
, , 0.1 I IO 100 IOCO
ng S
Fig. 8. Log-log plot of FPD response US. concentration of n-butanethiol (as ng of S).
Fig. 7. Sharp, narrow peaks were obtained in a reasonable time with a minimum of tailing.
The response of the detector was examined by injecting sample sizes in the range 1000-O. I ng of S. A log-log plot of response us. concentration (Fig. 8) was linear in the range 250-O. 1 ng for all six species exam- ined and the slope varied from 1.69 for methanethiol to 2.03 for n-butanethiol. These values were obtained by a least-squares method. This is in agreement with previous reports that the FPD response varies as the square of concentration90*‘L3~‘24 and that a log-log plot has a slope of approximately 2.
The lowest limit of detection, defined as a signal equal to twice the standard deviation of the noise, was found to correspond to about 10 pg of S. Peaks corre- sponding to 100 pg of S were easily seen for all six compounds (Fig. 9). These values seem to be better by a factor of 10-1000 than results previously reported. Table 1 shows a list of GC-FPD or GC-MS determi- nations for inorganic and organic sulphur species in a variety of matrices, together with the columns used and detection limits. In most cases a Tracer FPD was utilized, except in the case of reference 135, where a Varian dual flame detector was used. In less than a quarter of the reports listed is the detection limit in the picogram range. Most authors in these cases used some method to deactivate the column so that losses would not be experienced when handling small quan- tities of material. When this step was omitted, the detection limit rose considerably. This paper confirms that by careful choice of the chromatographic mater- ials and elimination of dead volumes, labile species
can be determined at extremely low concentration levels.
Some preliminary work was carried out with the separator connected to a vacuum system to simulate an MS. All six sulphur species were injected into the GC and the fraction passing through the membrane was determined. These experiments served to empha- size two aspects associated with this system. First, the detector flame was not extinguished on rotation of the Cway valve to divert the effluent flow either to the separator or to the FPD. Secondly, no loss in resolu- tion was produced by the l/&in. Teflon tubing used in the connections between the valve and FPD. In fact, the peaks remained sharp, with no perceptible broadening.
The separator and reentrant tube were attached to the MS-902. A pressure of about 1 x 10e6 mmHg was maintained in the source. This occasionally rose to about 4 x 10m6 mmHg when the separator and reentrant were heated to above 100”. The yield for methanethiol, butanethiol, diethyi sulphide and dimethyl disulphide was determined at different separator temperatures and different carrier-gas flow- rates. Only four sulphur species were used for these experiments, because the solvent peak (ethanol) was not permitted to overlap with any peaks due to
TIME (min)
Fig. 9. Chromatogram of organosulphur compounds in ethanol (S 0.1 pg/ml). Sample size I pl. Peaks as shown in
Fig. 7.
N-MS of labile molecules
Table I. Detection limits for sulphur determination by GC-FPD and GC-MS
HPPOI, on Chromosorb T 40/60 (2) 4.6’ x 0.085” I.D. Teflon (FEP), Carbopack B-HT-100 (3) 6’ x 0.085” I.D. Teflon (FEPL Chromosil310 i4j I’ x 0.085” I.D. Teflon (FEPi Deacttgel 12&14c1 (I) 6 m x 2 mm I.D. Teflon (FEP), IO% Triton X-305 f
H3PO4
(2) I2 m x 2 mm I.D. Teflon (FEP), 0.5% Triton X-305 on Chromosorb G (AW, DMCS) 70/80
80 m x 0.28 mm I.D. glass capillary coated with FFAP
20 m x 0.25 mm I.D. glass capillary coated with SF-96
18’ x l/8” O.D. Teflon, Porapak QS 8O/lOO acetone- washed
1.83 m x 6 mm O.D. glass, 9% OV-101, 1% HIEFF 8 AP on Gas Chrom Q 8O/lOO
3 m x 3 mm glass c&&ining (1) 25% TCEP on Shimalite AW, DMCS 60/80 (2) 25% TCP on Shimalite AW, DMCS 60/80 (3) IO% PPE on Shimalite TPA 60/80 (4) Porapak Q 50/80 (5) Silica gel 60/80 300’ x 0.02” I.D. stainless-steel capillary, Polyethylene
glycol 400 (I) 6’ x l/8” stainless steel, 5% Silicone Oil QF-I on Pora-
pak QS 8O/lOO (2) 10’ x l/8” stainless steel 5% PPE on Chromosorb G,
AW, DMCS 8O/lOO 30 m x 0.5 mm I.D. glass capillary (SCOT), SE-30 6’ x 0.02” O.D. glass with (I) Porapak Q SO/l00 (2) Chromosorb 104 8O/lOO 5.5 m x 3 mm I.D. glass, 25% l,2,3-tris-2-cyanoethoxypro-
pane on Chromosorb W, AW 60/80 (I) 6’ x l/8” O.D. Teflon (FEP), Tracer Special Silica (2) 18’ x l/4” O.D. glass, 20% FFAP on Chromosorb W,
H,S, SQz, CHGH, (CH&S, 0.3 ng (CH3)2S2 I-BU I x lo-‘Og (CH,)zS. CHJSCIH,, <20 pg s (C&)28 (C21U2S2, thio- phene, THT, methyl n-butyl sulphide, other organosul- phur species
WW2S < 10 ppMt 137 Thiophenes <I ng 138 6-Mercaptopurine 20 ng 139
sulphur-containing compounds. It was important to prevent ethanol from entering the separator, where it would flood the membrane. Therefore the valve was switched before elution of the ethanol peak, so that the solvent was diverted to the FPD. Any sulphur species eluted at the same time would also be diverted to the detector and be vented.
Solutions of the four sulphurcontaining species in absolute ethanol (S 100 &ml) were used with a sample size of 1 ~1, which corresponded to 100 ng of sulphur injected into the column. The ion-current at m/e 41 was monitored for each of the compounds studied. This is the most intense peak in the spectrum of methanethiol, the second most intense in that of diethyl sulphide and the fifth most intense in that of both butanethiol and dimethyl disulphide.12s All MS parameters were optimized to give good peak shape with a flat top and maximum sensitivity. Figure 10 gives examples of the traces obtained.
The temperature of the separator, line and re- entrant tube was varied and the response of the FPD noted. In the first run all the sample was diverted to the detector. In the second run the effluent was
diverted to the membrane separator. Any sulphur species not dissolving in the membrane was moni- tored by the FPD. From the difference in the areas of these two peaks, the amount of material entering the MS could be calculated (yield), assuming no loss due to condensation or leakage. The amount of material passing through the silicone membrane depends on the diffusion rate, the solubility of the gas and the membrane thickness. The yields (Y) were calculated according to the equation prescribed by McFadden :’ ’ ’
Y=QMSx looo/, Q GC
where QGc is the quantity of sample leaving the chro- matograph and QWS is the quantity of sample entering the mass spectrometer.
Table 2 shows the results obtained for a number of sulphur species examined at various separator tem- peratures. Values as high as 47% were obtained at the lower temperatures, but this figure decreased with a decrease in the molecular weight of the material under investigation. It is believed that by optimiza-
GC-MS of labile molecules 491
FID RESPONSE
SIM TRACE
FPD RESPONSE ,J_JL L
inj6ct TIME (min)
Fig. 10. FPD/FID response and SIM trace for 4 organo- sulphur compounds (equivalent of 100 ng of sulphur injected). A.A’. Methanethiol: B.B’, diethyl sulphide; C,C’.
butanethiol: D.D’. dimethyl disulphide; E. ethanol.
tion of the separator temperature and flow-rate of the carrier gas, yields greater than 50% could be achieved. It can be seen that with increasing temperature the yield decreases. For diethyl sulphide, a reduction of the nitrogen flow-rate to 15 ml/min gave an increased yield, presumably because more time was available for the sample to dissolve in the membrane before being carried away by the carrier gas. Values for the separation factor N were also calculated by using the equation :
N=&$ MS
where Y is the yield, V& is the carrier-gas volume
Table 2. Yields of different compounds for membrane separator at various temperatures (flow-rate 30 ml/min)
measured at the chromatograph and V,, is the car- rier-gas volume measured at the mass spectrometer. Because the value for V,, was extremely low and could not be accurately measured, the resulting enrichment figures were approximate but certainly very high.
An unusual phenomenon was observed on repeated exposure of the separator to the GC effluent. The value of the yield for a particular compound de- creased with time, i.e., the amount dissolving through the membrane was reduced when the separator was in constant use. The membrane therefore requires a re- covery period between samples although the reason for this has not yet been established. This phenom- enon has been observed at different temperatures. The yield for diethyl sulphide at 80” changed from 25.9% to 18.2% in 150 min. At 60” the change was from 39.3% to 23.1% in only 90 min. This effect is being examined further.
Peaks arising from 40 ng of S in material passing through the separator were of a comparable size to those obtained from the same amount inserted from a glass reservoir fitted with a silicon carbide leak. The large signals obtained with 40 ng of S suggest that much lower quantities could be detected by using single-ion monitoring possibly extending into the picogram range if suitable care and adequate pre- cautions were taken.
.Itcknowledgements-We are indebted to the Imperial Oil of Canada Ltd. and the Natural Sciences and Engineering Research Council of Canada for support for this work. Helpful discussion with A. G. Harrison and D. W. Priddle of the University of Toronto is gratefully acknowledged.
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6. 7.
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REFERENCES
C. T. Hodges and R. F. Matson, Anal. Chem., 1965, 37. 1065. D. F. Adams, R. K. Koppe and W. N. Tuttle, J. Air Pollut. Con& Assoc., 1965, 15, 31. W. E. Rupprecht and T. R. Phillips, Anal. Chim. Acta, 1969, 47, 439. M. R. Guerin. Anal. Letr., 1971, 4, 751. G. A. F. Harrison and C. M. Coyne, J. Chromatog., 1969. 41, 453. R. W. Freedman, J. Gas Chromarog., 1968, 6, 495. A. D. Horton and M. R. Guerin, J. Chromarog, 1974. 90.63. A. G. Vitenberg, L. M. Kuznetsova, 1. L. Butaeva and M. D. Inshakov, Anal. Chem.. 1977.49, 128. B. Thompson, Varian Intrum. Appl., 1977. II, 6. V. I. Yakerson, L. I. Lafer, S. Z. Taits, F. M. Stoyano- vitch, V. P. Litvinov, Y. L. Danyuschevsky and Y. L. Goldfarb, J. Chromurog., 1966, 23, 67. T. S. Bates and R. Carpenter, Anal. Chem., 1979. 51, 551. C. A. Burgett and J. S. Fritz, Tulanta, 1973, 20, 363. R. Belcher, W. I. Stephen, I. J. Thomson and P. C. Uden, J. Inora. Nucl. Chem.. 1971. 33. 1851. K. Utsunomiia, Anal. Chim.’ Acta, 19?2, 59, 147. W. D. ROSS and G. Wheeler, Jr., Anal. Chem., 1964. 36,266. F. M. Wachi, Thesis, University of Illinois, Urbana, Illinois.
492 MICHALL THOMPSON and MIRIAM STANISAVLIEVIC
17 S. G. Zeldes and A. D. Horton, A&. Chern.. 1978. 50, 779.
18. S. Srrnivasan. H. Baer. R. Bowser and A. Karmen. Antrl. &x&m.. 1963. 6. 234.
19. F. Bruner, P. Ciccioh. E. Brancaleoni and A. Longo. Aftrl. Chim.. 1976. 66. 91.
20. J. E. Lovelock, P. G. Simmonds. G. R. Shoemaker and S. Rich. J. Chrornuro<g. Sci.. 1970. 8. 452.
21. J. W. Honour. C. J. W. Brooks and C. H. L. Shackle- ton. 27th Antl~rul Cor~jiwnce Muss Sprclronlrrry and Allied Topics. 1979.
22. K. Grob. Chrotrmtogruphia. 1976. 9. 509. 23. J W. Gramshaw and A. Hussain. J. Chromutoy., 1978,
157. 267. 24. T. Welsch. W. Engewald and C. Klaucke. Chrorlluro-
yruphfu. 1977. IO. 22. 25. M. Novotnj and K. D. Bartle. rhid.. 1970. 3. 272 26. M. K. Wlthers. J. Chrornurog.. 1973. 80. 124. 27. J. E. Arnold and H. M. Fales. J. Gus Chromaron,
28.
29.
30.
31.
32.
1965. 3, 31. R. E. Pecsar and C. H. Hartmann. J. Chromtrfog. Ser.. 1973. Il. 492. I. H. Williams and F. E. Murray. Pulp Pup. Mug. Can.. 1966. 67. T-347. S. 0. Farwell. S. J. Gluck. W. L. Bamesberger, T. M. Schutte and D. F. Adams, Anal. Chem.. 1979. 51, 609. P. C. Uden and C. R. Jenkms. Talanra. 1969. 16, 893. R. Belcher. R. J Martm. W. I. Stephen. D. E. Hender- son. A. Kamahzad and P. C Uden. Anal. Churn., 1973. 45. I 197.
33. R. K. Koppe and D. F. Adams. Enriron. Sci Technol.. 1967. I. 479.
34. D. A. Tourres. Chrorllurc?yrupkirr. 1972, 5. 441. 35. D. N. Sokolov. A. V. Davydbv. S. Yu Prokofyev. S. S.
Travnikov. E. V. Fedoseev and B. F. Mvasoedov. J. Chroruatoy.. 1978. 155. 241.
36 C. A. Burgett and J. S. Fritz. ibid.. 1973. 77. 265. R. A. Keller. ihid.. 1961. 5. 225. 37.
38. 39. 40.
V. Kusy. A~Iu/. Chrm.. 1965. 37. 1748. L. S. Ettre. J. C’hroruaroq.. 1960. 4. 166. D. T. Sawyer and J. K. Barr, Anal. Chem., 1962, 34, 1518.
41. 42.
43.
44.
45.
46.
47. 48. 49.
50.
51.
52.
53. 54. 55.
56.
57.
D. M. Ottenstein, J. Gas Chromatog.. 1963. I. I I. Y. L. Sze. M. L. Borke and D. M. Ottenstein. Anul. Chrm.. 1963. 35. 240. H. Veenine and J. F. K. Huber, J. Gas C’hrornatou.. 1968, 6. 3%. N. F. Ives and L. GiulTrida. J. Assoc. Off: Anal. Chem.. 1970. 53. 973. W. Hertl and M. G. Neumann, J. Chromatog, 1971. 60. 319. Z. Suprynowicz, A. Waksmundzki and W. Rudzinski. ibid.. 1972. 72. 5. C. R. Hastings and W. A. Aue, ibid.. 1974, 89. 369. 1. D. Schieke and V. Pretorius. ibid., 1977, 132. 217. T. A. Blazer and E. M. Chait, 27th Annual Conference Muss Spectrometry and Allied Topics, 1979. B. J. Millard. Quanrirurioe Mass Specrromerr~. Hey- den, London, 1978. M. R. Gumbmann and H. K. Burr, J. Agrrc. Food Chem.. 1964. 12, 404. E. L. Obermiller and G. 0. Charlier, J. Gas. Chroma- tog., 1968, 6, 446. Idem. ibid., 1969. 7. 580. D. L. Struble, J. Chromutog. Sci.. 1972, 10, 57. M. S. Black, R. P. Herbst and D. R. Hitchcock. Anal. Chrm., 1978,50, 848. B. H. Devonald, R. S. Serenius and A. D. McIntyre, Pulp Pap. Mug. Can., 1972, 73. T-68. M. C. Bowman and M. Beroza. J. Assoc. Ofl Anal. Chem., 1966.49. I 154.
58. Idem. ibid.. 1970. 53, 499. 59. R. S. Juvet and R. P. Durbin. J. Gus Chromaroq..
1963. I. 14. 60. W. D. Ross, Anal. Chem.. 1963. 35. 1596. 61. J. E. Schwarberg. R. W. Moshier and J. H. Walsh.
Tulurllu. 1964, II. 1213. 62. C. Brunnee. L. Delgmann. K. Habfast and S. Meier.
18th Annuu/ Conference Muss Sprctrornrlry und Allied
Topics. 1970. 63. P. H. Bach. Lab. Pracr.. 1975. 24. 817. 64. R. A. Dewar and V. E. Maier. J. Chrornurog.. 1963. Il.
295. 65. M. Novotnji and K. Tesafik. Chromatogruphra. 1968.
1. 332. 66. C. D. Pearson, J. Chromatog. Sci.. 1976. 14. 154. 67. C. D. Pearson and W. J. Hines. Anal. Chrm.. 1977. 49.
68. 69.
70.
71.
72.
73. 74. 75.
76.
77.
78. 79. 80.
81.
82.
83.
84.
85.
86.
87.
88.
89.
90.
91. 92. 93.
94.
95. 96.
9.7.
98.
99.
123. K. A. Goode, J. Inst. Pet., 1970, 56, 33. W. Bertsch. F. Hsu and A. Zlatkis. Anal. Chem.. 1976. 48. 928. A. L. German and E. C. Horning, J. Chromuroy. Sci..
1973. I I. 76. W. D. MacLeod and B. Nagy. Anal. Chem.. 1968. 40. 841. G. Schomburg. H. Husmann and F. Weeke. Chrornu- togruphiu. 1977. 10. 580. D. A. Cronin. J. Chronlutoy.. 1974, 97. 263 L. Blomberg. ibid.. 1976. 125. 389. J. J. Franken, R. C. M. de Nijs and F. L. Schulting. Ibid.. 1977, 144. 253. D. A. Withycombe. J. P. Walradt and A. Hruza. Am.
Chem. Sot. S.wp. Ser.. 1976. 26. 85. R. A. Heckman. C. R. Green and F. W. Best, Anal. Chum.. 1978. 50. 2157. R. Staszewski. J. Chromutog.. 1972. 65. 217. W. Averill. Perkin-Elmer Insrrum. News. 1970. 20, 12. J. C. Diez. M. V. Dabrio and J. L. Oteo, J. Chromu-
roy. Sci.. 1974. 12. 641. F. Bruner. P. Ciccioli and G. Bertoni. J. Chromutog., 1976. 120, 200. F. Bruner. P. Ciccioli and F. di Nardo. i/w/.. 1974, 99. 661. D. F. Adams, R. K. Koppe and D. M. Jungroth.
_ Tappi. 1960,43T. 602. R. K. Stevens. A. E. O’Keeffe and G. C. Ortman. Environ. SCI. Technol.. 1969. 3. 652. R. K. Stevens and A. E. O’KeetTe. Anal. Chem.. 1970. 42, No. 2. 143A. R. K. Stevens, J. D. Mulik. A. E. O’KeefTe and K. J. Krost, ibid.. 1971. 43. 827. F. Bruner. P. Ciccioli and F. di Nardo. ibid.. 1975. 47. 141. R. E. Baumgardner. T. A. Clark and R. K. Stevens, ibid., 1975. 47, 563. A. R. Blanchette and A. D. Cooper. ibid.. 1976. 48. 729. C. H. Burnett, D. F. Adams and S. 0. Farwell. J. Chromatoy. Sri.. 1977. 15. 230. D. S. Walker, Anahwt. 1978, 103. 397. C. F. Cullis. Chem.*Brir.. 1978. 14, 384. E. R. Adlard. L. F. Creaser and P. H. D. Matthews. Anal. Chem.. 1972. 44. 64. M. E. Garza. Jr. and J. Muth. Enriro!l. Ser. Technol..
1974. 8, 249. A. P. Bentz, Anul. Chem., 1976. 48. 454A. J. C. Maitler. L. M. McDonough and M. Beroza. J. Agric. Food Chem.. 1968, 16. 549.
M. C. Bowman, M. Beroza and K. R. Hill, J. Assoc. Ofl Anal. Chem., 1971, 54, 346. R. Greenhalgh and M. A. Wilson, J. Chromatog., 1976, 128, 157. A. Sinclair, R. D. Hall, D. T. Bums and W. P. Hayes, J. Sci. Food Agric., 1970, 21, 468.
GC-MS of labile molecules 493
100. P. Schreier, F. Drawert and A. Junker, Erauwissen- Chromatography-Mass Spectrometry, Wiley-Inter- schaftt, 1974, 27, 205. science, New York, 1973.
101. H. E. Jansen. J. Strating and W. M. Westra. J. Inst. 121. P. M. Llewellyn and D. P. Littlejohn, Pittsburgh Con&
Brew., 1971,77, 154. Anal. Chem. Appl. Spectry., 1966. 102. P. J. Groenen and L. J. Van Gemert, J. Chromatog.. 122. K. Fujiwara and H. Ogata, Surj: Sci., 1978, 72, 157.
1971, 57. 239. 103. M. R. Guerin. G. Olerich and A. D. Horton, J. Chro- 123. J. G. Eckhardt, M. B. Denton and J. L. Moyers, J.
104.
105. 106. 107.
108. 109.
110.
I I I.
112.
113. 114.
115.
116. 117.
118.
119.
120.
matog. Sci.. 1974, 12, 385. Chromatog. Sci., 1975, 13, 133. W. J. Irwin and J. A. Slack, Anal. Pyrolysis Proc. 124. C. H. Burnett. D. F. Adams and S. 0. Farwell. ibid.. _ _ Intern. Symp., 1976, 107. Idem. J. Chromatoa., 1977. 139. 364. Y. Hbshika and Y: iida, ibid., 1977, 134,423. C. F. Spencer, F. Baumann and J. F. Johnson, Anal.
Chem., 1958, 30, 1473. J. A. Schols, ibid., 1961, 33. 359.
E. M. Fredericks and G. A. Harlow, Ibid., 1964, 36, 263. L. J. Darlage. S. S. Black and J. P. Weidner, J. Chro- marog. Sri.. 1973, 11, 272.
D. A. Clay. C. H. Rogers and R. H. Jungers, Anal. Chrm., 1977,49, 126. H. J. Coleman. C. J. Thompson. C. C. Ward and H. T. Rail, ibid.. 1958, 30, 1592. _ R. L. Martin and J. A. Grant. ibid.. 1965.37. 644.
B. B. Agrawal. K. Tesarik and J. Janak, J.‘Chroma- rog.. 1972. 65. 207. H. J. Coleman, C. J. Thompson, R. L. Hopkins and H. T. Rail. ibid., 1965, 20, 240.
Idem, ibid., 1966, 25. 34. S. S. Brody and J. E. Chaney, J. Gas Chromatog..
1966. 24, 42. Phoroionization Detector for Gas Chromatography,
Technical Form P15/677, HNU Systems Inc., Newton, Mass. Photovac Inc., Thornhill, Ontario, Private communi- cation. W. H. McFadden, Techniques in Combined Gas
1978, 16, 68. 125. Index of Mass Spectral Data, ASTM, 1969. 126. J. M. Bremner and W. L. Banwart, Sulphur Inst. J.,
1974, 10.
127. P. Ronkainen, J. Denslow and 0. Lepplnen, J. Chro-
matog. Sci., 1973, 11, 384. 128. G. Goretti and M. Possanzini, J. Chromutog., 1973,
77, 317. 129. T. L. C. de Sousa, D. C. Lane and S. P. Bhatia, Anal.
Chem., 1975,47, 543. 130. J. M. Zehner and R. A. Simonaitis, J. Chromatog. Sci.,
1976, 14, 348. 131. F. Bruner, A. Liberti, M. Possanzini and I. Allegrini,
Anal. Chem., 1972. 44.2070. 132. H. W. Grice, M. L. Yates and D. J. David, J. Chroma-
tog. Sci., 1970, 8, 90. 133. M. C. Bowman and M. Beroza, Anal. Chem., 1968,4&
1448.
134. C. A. Burgett and L. E. Green, J. Chromatog. Sci., 1974, 12, 356.
135. P. L. Patterson, R. L. Howe and A. Abu-Shumays, Anal. Chem., 1978, SO, 339.
136. B. Versino and G. Rossi, Chromatographia, 1971, 4, 331.
137. D. Ellgehausen. 2. Anal. Chem., 1974, 272, 284. 138. E. J. Gallegos, Anal. Chem., 1975, 47, 1150.
139. J. M. Rosenfeld, V. Y. Jaguchi, B. L. Hillcoat and M. Kawai, ibid., 1977, 49, 725.
