J. Sep. Sci. 2008, 31, 1761 – 1768 K.-H. Kim 1761 Ki-Hyun Kim Atmospheric Environment Laboratory, Department of Earth and Environmental Sciences, Sejong University, Seoul, Korea Original Paper A method to test the detectability of GC/PFPD for an extended concentration range with respect to reduced sulfur compounds In this study, a possible means to extensively expand the quantifiable range of reduced sulfur compounds (RSC) in air has been investigated by a combined applica- tion of GC with pulsed flame photometric detector (PFPD) and a multifunction ther- mal desorber (TD) system. To comply with the purpose of this study, gaseous RSC standards containing the equimolar concentrations of H 2 S, CH 3 SH, DMS, DMDS, and CS 2 were prepared at 11 concentration levels (i.e., 10 ppb – 10 ppm (over 10 3 range)). These standards were then used to derive three-point calibrations based on the modified injection through a TD (MITD). If the mean calibration slopes of each con- centration level are normalized to that of CH 3 SH, the relative ordering is found as 0.65 (H 2 S): 1 (CH 3 SH): 1.34 (DMS): 2.24 (DMDS). The reproducibility of MITD method, when assessed in terms of relative standard error (RSE) for all calibration slopes, had the most stable pattern for DMDS (5.77%) and the least stable one for H 2 S (12.8%). The sensitivity of the MITD-based calibration generally improved with an increase in concentration levels of standard gas. Based on our study, the MITD technique is useful to extend quantification of GC/PFPD by allowing RSC detection over a 10 3 range. Keywords: Hydrogen sulfide / PFPD / Reduced sulfur compounds (RSC) / Thermal desorber / Received: December 23, 2007; revised: January 29, 2008; accepted: January 30, 2008 DOI 10.1002/jssc.200700682 1 Introduction It is commonly acknowledged that airborne pollutants are divided into gaseous or particulate phases by their physical nature. If the former phase type is divided fur- ther with respect to the potential of sensory recognition, they can be distinguished as either odorous or odorless components. Malodor issues can hence be accounted for by the instantaneous or continuous build-up of offensive components in air. The odorous compounds produced by and released from many types of man-made activities include diverse chemical groups such as aldehydes or reduced sulfur compounds (RSC) [1]. Because these chem- icals can pose a nuisance and/or health threats above cer- tain concentration levels, accurate quantification of their concentration levels is often recognized as the key component to their regulation. Although the analysis of each individual component from the mixture of odorous chemicals involves the employment of multiple analyti- cal set-ups, the operation of such system does not neces- sarily guarantee the detection of certain components with the fairly low threshold values. Due to several complexities involved in the RSC ana- lysis, there has been a pressing need for the development of methodologies allowing their quantification at vari- ous pollution levels [2]. As GC determination of RSC heav- ily relied on the flame photometric detection (FPD) method in the early stage, a modification of the instru- mental set-up has often been implemented to atone for its limited detectability [3, 4]. Technical limitations encountered in the early stage were however resolved to a certain degree by the introduction of more sensitive instrumental systems (e.g., pulsed FPD (PFPD), sulfur chemiluminesence (SCD), etc.). Moreover, improvements in the focusing technique with the aid of thermal desorber (TD) or solid phase microextration (SPME) allowed us to expand the efficiency of the cryofocusing stage with the substitution of an electronic cooling sys- Correspondence: Dr. Ki-Hyun Kim, Atmospheric Environment Laboratory, Department of Earth and Environmental Sciences, Sejong University, Seoul 143-747, Korea E-mail: [email protected]Fax: +82-2-499-2354 Abbreviations: AS, air server; FPD, flame photometric detection; MFC, mass flow controller; MITD, modified injection through a thermal desorber; MS, multistream selector; PFPD, pulsed flame photometric detector; RSC, reduced sulfur compounds; RSE, rel- ative standard error; SR, square root; TD, thermal desorber i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
Transcript
J. Sep. Sci. 2008, 31, 1761 –1768 K.-H. Kim 1761
Ki-Hyun Kim
Atmospheric EnvironmentLaboratory, Department of Earthand Environmental Sciences,Sejong University, Seoul, Korea
Original Paper
A method to test the detectability of GC/PFPD foran extended concentration range with respect toreduced sulfur compounds
In this study, a possible means to extensively expand the quantifiable range ofreduced sulfur compounds (RSC) in air has been investigated by a combined applica-tion of GC with pulsed flame photometric detector (PFPD) and a multifunction ther-mal desorber (TD) system. To comply with the purpose of this study, gaseous RSCstandards containing the equimolar concentrations of H2S, CH3SH, DMS, DMDS, andCS2 were prepared at 11 concentration levels (i. e., 10 ppb–10 ppm (over 103 range)).These standards were then used to derive three-point calibrations based on themodified injection through a TD (MITD). If the mean calibration slopes of each con-centration level are normalized to that of CH3SH, the relative ordering is found as0.65 (H2S): 1 (CH3SH): 1.34 (DMS): 2.24 (DMDS). The reproducibility of MITD method,when assessed in terms of relative standard error (RSE) for all calibration slopes, hadthe most stable pattern for DMDS (5.77%) and the least stable one for H2S (12.8%).The sensitivity of the MITD-based calibration generally improved with an increasein concentration levels of standard gas. Based on our study, the MITD technique isuseful to extend quantification of GC/PFPD by allowing RSC detection over a 103
Received: December 23, 2007; revised: January 29, 2008; accepted: January 30, 2008
DOI 10.1002/jssc.200700682
1 Introduction
It is commonly acknowledged that airborne pollutantsare divided into gaseous or particulate phases by theirphysical nature. If the former phase type is divided fur-ther with respect to the potential of sensory recognition,they can be distinguished as either odorous or odorlesscomponents. Malodor issues can hence be accounted forby the instantaneous or continuous build-up of offensivecomponents in air. The odorous compounds produced byand released from many types of man-made activitiesinclude diverse chemical groups such as aldehydes orreduced sulfur compounds (RSC) [1]. Because these chem-icals can pose a nuisance and/or health threats above cer-
tain concentration levels, accurate quantification oftheir concentration levels is often recognized as the keycomponent to their regulation. Although the analysis ofeach individual component from the mixture of odorouschemicals involves the employment of multiple analyti-cal set-ups, the operation of such system does not neces-sarily guarantee the detection of certain componentswith the fairly low threshold values.
Due to several complexities involved in the RSC ana-lysis, there has been a pressing need for the developmentof methodologies allowing their quantification at vari-ous pollution levels [2]. As GC determination of RSC heav-ily relied on the flame photometric detection (FPD)method in the early stage, a modification of the instru-mental set-up has often been implemented to atone forits limited detectability [3, 4]. Technical limitationsencountered in the early stage were however resolved toa certain degree by the introduction of more sensitiveinstrumental systems (e. g., pulsed FPD (PFPD), sulfurchemiluminesence (SCD), etc.). Moreover, improvementsin the focusing technique with the aid of thermaldesorber (TD) or solid phase microextration (SPME)allowed us to expand the efficiency of the cryofocusingstage with the substitution of an electronic cooling sys-
Correspondence: Dr. Ki-Hyun Kim, Atmospheric EnvironmentLaboratory, Department of Earth and Environmental Sciences,Sejong University, Seoul 143-747, KoreaE-mail: [email protected]: +82-2-499-2354
Abbreviations: AS, air server; FPD, flame photometric detection;MFC, mass flow controller; MITD, modified injection through athermal desorber; MS, multistream selector; PFPD, pulsed flamephotometric detector; RSC, reduced sulfur compounds; RSE, rel-ative standard error; SR, square root; TD, thermal desorber
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1762 K.-H. Kim J. Sep. Sci. 2008, 31, 1761 – 1768
tem for liquid nitrogen [5–8]. As such, acquisition of RSCat sub-ppb concentration levels has been facilitated notonly for grab sampling method [9] but also for an onlineanalysis [10].
Although advances accomplished in the GC-based anal-ysis of RSC are substantial, there are still several restric-tions to overcome in the present technologies. Because ofthe uniqueness of sulfur specific detection systems (e. g.,FPD and PFPD), the detectability of such systems gener-ally falls on a narrow quantifiable range [9]. In an effortto investigate the detectability of RSC contained in vari-ous environmental samples, we investigated the per-formance characteristics of the GC/PFPD system for bothlow concentration samples (e. g., ppb range with the aidof TD [2]) and high concentration samples (e. g., ppmrange without a complicated supporting system [11]). Asa result, we were able to describe the fundamental prop-erties of the PFPD application for each analytical mode(e. g., the reproducibility, equimolarity of S response, andLODs). In the present study, we report an overview of ournew experimental approach, namely the modified injec-tion through a TD (MITD) technique to explore the quan-tification of RSC over a wide concentration range (i. e.,103) under the unified operation settings of the TD sys-tem.
2 Materials and methods
2.1 Theoretic basis of the MITD method
The MITD technique is a modified approach developed toanalyze gaseous samples over an extended concentrationrange with the aid of the multifunction TD systemequipped with air server (AS) unit (UNITY model, MarkesInternational, UK) (Fig. 1). (Here, the term multifunctionTD is used, as it is built to facilitate the analysis of volatilecompounds collected either by tube sampler or by bagsampler with the aid of a programmable gas flow controlsystem (AS)). Considering that the AS system is built totransfer a sample with large volume (e. g., above a fewtens to several hundreds of mililiters) at a given flowrate, its usage is basically limited to the samples of signif-icantly low concentrations (samples with ppb or ppt lev-els). This is inevitable, unless extra dilution is applied tothe concentrated samples. Because of uncertainties inlow sample transfers by the mass flow controller (MFC)unit (at significantly low flow rates) or the system con-tamination during the treatment of concentrated sam-ples, the AS/TD system is not feasible to directly analyzehighly concentrated samples (e. g., ppm level) with asmall sample volume size. Accordingly, if the analysis ofhighly concentrated RSC samples is desirable, one needsto employ the system that fits such purpose (e. g., directinjection into the GC injector or into a loop injection sys-tem). Although a number of dual instrumental system
had been built with FPD or PFPD to treat RSC samples ofdifferent concentration ranges, relatively little is knownabout a single system to cover a wide concentrationrange at once [9].
For the purpose of extending the applicability of thePFPD system to cover samples over a wide concentrationrange, we investigated an MITD technique in which bothambient and polluted samples can be analyzed by the
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Figure 1. An instrumental set-up for the RSC analysis in thepresent study: a schematic diagram of a combined set-up forthe preconcentration system using an AS and a TD unit.
Figure 2. Pictures of the MITD technique: (A) the upper twopictures show the connection between Tedlar bag and anoutlet of the MS; and (B) the lower two pictures show that thesilicone tubing line used for connecting MS and Tedlar bag isused as the temporary injector to receive standard gasesloaded in a gas-tight syringe.
J. Sep. Sci. 2008, 31, 1761 –1768 Gas Chromatography 1763
single analytical set-up consisting of AS/TD system. Toapply this approach, a back-up (ultrapure) N2 gas filled ina Tedlar bag (instead of sample bag) is initially connectedto the multistream selector (MS) unit (Fig. 2). The transferof the N2 gas into the TD unit proceeds in the same man-ner as the normal operation conditions of the TD setting,as discussed below. Standard gases, prepared at varyingconcentration range (Table 1), are then drawn into a gas-tight syringe and injected into the AS unit via a tempo-rary injection port (refer to the lower part of Fig. 2). Thisinjection port is built to connect the silcosteel tubingline of the AS outlet (MS) and the N2 gas container (Tedlarbag). Because a short piece of silicone tubing is used as atemporary injection port, the syringe needle can pene-trate into the inner tubing line of the MS outlet to allowdirect delivery of standard gas into the TD unit. By this
modification of the TD loading condition, a variablequantity of samples (e. g., small volume of concentratedsamples and large volume of low concentration samples)can be introduced into the TD without extra sampletreatment. As such, the AS/TD system can be used tocover samples over a wide concentration range throughthe application of the MITD method.
2.2 Analysis of RSC standards by MITD methodover 103 concentration range
In order to perform a series of calibration experimentsbased on the proposed MITD approach, the AS/TD systemwas operated in the following sequence. As a preparationof calibration experiments, the working standard sam-ples were prepared at a total of 11 concentration levels
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Table 1. A design of experiment for RSC calibration over 103 concentration range; results of computation are given for the abso-lute quantity of RSC supplied to the TD system by means of MITD technique
a) The original concentrations expressed in ppb (v/v) ratio are converted into mol/volume ratio assuming T= 208C andP= 1atm.
b) While RSC standard gases are supplied into TD system, the back-up N2 gas is delivered from a 10 L Tedlar bag via AS systemat constant flow rate of 40 mL/min for a duration of 5min.
1764 K.-H. Kim J. Sep. Sci. 2008, 31, 1761 – 1768
(10 ppb to 10 ppm: Table 1) by conducting a one-step dilu-tion of the primary standard with an ultrapure N2 gas(99.999% purity with a moisture content of less than 5%).For the purpose of this study, the primary standard con-taining all four S compounds at an equimolar concentra-tion of 10 ppm (ppm, 10 – 6) was purchased (Ri Gas, Korea);this original standard gas was prepared gravimetricallyusing a concentration of 5% (by ISO 6142 method).
To initiate MITD-based calibration experiment, a Ted-lar bag of a 10 L capacity was filled with ultrapure N2 gasand connected to the inlet of an MS system in the AS unitusing 2 cm long silicone tubing (Cole-Parmer, VernonHills, US) at 3.2 (id) and 6.4 mm (od) (Fig. 1B). The quantityof RSC required to derive the three-point calibrations(16.6, 33.3, and 66.6 pmol) was then drawn individuallyby gas-tight syringes with respective capacity. To reducethe possible contamination of the system due to the useof high concentration samples with the glass syringe, sev-eral syringes of the identical capacity were prepared andused. Because memory effects were seen occasionallyafter the injection of standards with the highest concen-tration (e. g., near 10 ppm standard), syringes werereplaced and treated immediately to eliminate the sour-ces of contamination.
These standard gases were then injected into the innerwall of the MS tubing line across the silicone connector,while the back-up N2 gas was supplied to the TD unit at afixed condition (e. g., flow rate of 40 mL/min for a dura-tion of 5 min) (Fig. 1). Because RSC standard gas is trans-ferred directly into the MS tubing line, the sum of gassupply into the TD (e. g., RSC standard plus back-up N2
gas) is controlled at the total volume of 200 mL. Throughan application of the MITD method, quantification ofRSC was made with the single TD system to cover bothsmall (highly concentrated) and large (diluted) samplevolumes, without any kind of transition between differ-ent analytical system (or modes). Hence, the performanceof the MITD method is clearly distinguished from that ofthe complicated dual mode set-up for RSC analysis intro-duced in the previous work (e. g., [9]).
The analysis of the RSC samples transferred to the GC/PFPD system via the TD unit was made under the follow-ing conditions. The temperature (T) conditions for the GCsystem were set as follows: (i) T (initial): 808C for 2 min;(ii) T (ramping): 68C/min rate; and (iii) T (final): 2108C at2 min. To acquire a good resolution in the GC systembetween different RSCs, a BP-1 column (60 m60.32 mmid, 5 lm film thickness, SGE) was used at a column flowrate of 1.2 mL/min (at the end of the column for the N2
carrier gas) with each running cycle ending at 20 minintervals. To provide pulsed flames into the combustor(15 mm length and 2 mm id), gases were provided:H2 = 11.5, Air1 (wall gas) = 10, and Air2 (combus-tor) = 10 mL/min. In addition, to allow a simple compari-son of the PFPD responses between different S com-
pounds, integration of their peak areas was made in thelinear mode with the square root (SR) function on. Theuse of this SR function can efficiently help mask the SRresponse (i. e., due to the conversion of S atoms to an S2
complex) of the detector. Hence, the calibration proce-dure of the PFPD system can be facilitated, as if one werehandling a simple, first-order equation.
The fundamental analytical parameters covering theanalytical performance of the GC/PFPD system have alsobeen examined. As an indirect means to evaluate the reli-ability of RSC standard used in this study, we routinelycompared its calibration results with those made by apermeation tube (DYNACAL permeation device: VICI Met-ronics, Poulsbo, USA). A fairly good agreement (e. g., a5%)was commonly achieved between the two standard types.General information about the QA for our RSC analysiswith and without the TD has been described elsewhere[2, 11]. The datasets produced by the GC/PFPD systemwith the aid of the AS/TD unit showed a reasonably goodreliability in terms of relative standard error (RSE); theRSE values quantified from triplicate analyses of identi-cal samples were generally seen between 1 and 5% [2].The absolute LOD values of the four RSCs, when esti-mated under the AS/TD operation conditions, generallyfell into the range of l10 (CH3SH, DMS, and DMDS) to30 pg (H2S) [2].
2.3 An instrumental set-up for the RSC analysis
In order to investigate the reliability of the MITDmethod, the GC system for RSC separation (Model DS6200, Donam Instruments, Korea) was interfaced with apulsed flame photometric detector (PFPD model 5380,O.I. Analytical, College Station, Texas, USA). In order toexamine the extendibility of RSC analysis with the GC/PFPD system, a multifunction TD system was operated toselectively cryofocus S compounds contained in workingstandard gases prepared over a 103 concentration range(refer to Table 1 for the experimental configuration).Basic information concerning the combined applicationof the GC/PFPD method with the AS/TD system has beendescribed elsewhere [6].
Equipped with an internal vacuum pump and MFC,the AS unit can pull up any type of gas mixtures (stand-ard gas or air samples) stored in various container types(e. g., Tedlar bag (232 Series, SKC, US)) into the TD unitthrough any channel in the MS. Hence, the supply rate ofsample gases into the TD system can be controlled over adesirable flow rate (i. e., between 5 and 100 mL/min) for avariable duration (i. e., between 0.1 and 999.9 min). Abuilt-in cold trap unit of TD (a mixture of silica gel andcarbotrap B) then allows for cryofocusing the S gases viaelectronic cooling by Peltier cooler (PC). As the precon-centration was performed by the PC at –158C, S com-pounds were desorbed thermally by rapidly heating the
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J. Sep. Sci. 2008, 31, 1761 –1768 Gas Chromatography 1765
cold trap at 2508C (5 min duration for heating). Finally,the analytical components were detected by the GC/PFPDsystem for the quantification of the individual S com-pounds [6].
As shown in Fig. 2A, samples stored in any containertype (e. g., Tedlar bag in this study) are pulled by the vac-uum pump via MFC unit in the AS and delivered into the(focusing) cold trap unit under the normal TD operationconditions. For the application of the MITD method, sam-ple transfer into TD proceeds only via gas tight syringe,while the delivery of back-up gas is induced simultane-ously by the MFC unit (Fig. 2B). It should be noted thatthe operation of the MFC system can be subject to a largebias, if the flow control for samples departs from its max-imum capacity [12]. Thus, there is a potential risk thatoperation of the AS/TD unit far below its maximum sup-ply rate can yield erroneous information for both sampleloading volume and the resulting concentration level ofunknown samples.
3 Results
In this study, the analysis of RSC standard samples wasmade over a concentration range of 103 (e. g., samples ofas low as 10 ppb to the maximum of 10 ppm) by the GC/PFPD with an application of the MITD technique. Theresults of RSC calibration made using standard samplesof 11 concentration levels are provided in Table 1 withtheir respective peak area values. Despite significant dif-ferences in the concentration levels between differentstandard types (i. e., 103), all calibrations were designed tofit identical quantities for each RSC at all three calibra-tion points (16.6, 33.3, and 66.5 pmol). For instance, tomake three-point calibrations of a 10 ppb standard, 40,80, and 160 mL of standard samples were drawn by thesyringe and introduced into the TD unit for analysis. Onthe other hand, for the calibration of the standard at thehighest concentration level (i. e., 10 ppm), a three-pointcalibration was made at sample volumes of 40, 80, and160 lL. Each calibration was then made by directlyinjecting standard gas drawn by the gas tight syringeinto the inner tubing line of the MS unit, while the backup N2 gas contained in a 10 L Tedlar bag was supplied tothe TD unit at a constant flow rate of 40 mL/min. Figure 3shows the calibration results of each RSC made usingstandard samples of 11 different concentration levels.Here, all calibration results of five RSCs are simultane-ously shown for each respective concentration level. Abrief inspection of the calibration results indicates thatH2S is the most variable, while those of DMDS are themost stable. The results indicate that relative ordering ofRSC calibration slopes is maintained consistently acrossthe entire concentration range.
4 Discussion
In the analysis of gaseous pollutants collected from vari-ous environmental settings, one should consider thetechnical problems arising from unique linearity rangesof different GC detector systems in the case of S (eitherwith a conventional FPD or PFPD). For readers' reference,the linearity of PFPD system for the RSC analysis falls in afairly narrow range depending on species such as l101
for DMDS, although H2S exhibits a more extended rangeover l103 [11]. The limited linearity ranges of the S-spe-cific detectors contrasts sharply with those of otherdetector types such as FID, well known for its extensivelinearity coverage (e. g., over 107). Hence, where the anal-ysis of significantly low RSC concentrations is concerned,the use of the TD technique is the most reliable choice bycryofocusing samples of a large volume (e. g., a few tens ofmilliliters to a few liters). On the other hand, if one hasto analyze samples of significantly high concentrations(a few ppm level), direct injection of samples into the GCinjector (or loop system) is a primary choice to consider[9].
As a means to overcome the present limitations in thepractically recoverable linearity range with the GC/PFPD,the MITD approach was explored against RSC standardsamples prepared over a wide concentration range withthe aid of a multifunction TD unit. Our efforts weredirected, especially toward an assessment of analyticalreliability in the measurements of the RSC samples withvariable concentration ranges without transition (ormodification) between different analytical set-ups. Tothis end, we designed and treated a series of experimentsbased on the MITD technique of which applicability wasinitially tested in the previous study of the RSC recoveryrate for the TD system [13]. According to such prelimi-nary study, RSC concentrations determined by directinjection (via GC injector) over a certain concentrationrange were highly comparable to those tested by theMITD method [13]. In this study, however, the MITDmethod was tested extensively to cover a wide standardconcentration range (over 103) for all the RSCs. Theresults derived by the MITD method were then used toevaluate its performance according to the experimentaldesign presented in Table 1. Like the case of the presentstudy, there have been many attempts to develop a tech-nique to accurately control sample transfer quantity forGC analysis such as loop injection method [11] or pres-sure assisted electrokinetic injection (PAEKI: [14, 15]). Asthe maximum sample volume for the direct loop injec-tion method is determined by the loop size, its applica-tion is generally confined to the samples of significantlypolluted samples (e. g., near a ppm level [11]). On theother hand, in the case of PAEKI-type approach, it isadvantageous in that the control of injectable volume isfairly systematic and automatizable. However, its reli-
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1766 K.-H. Kim J. Sep. Sci. 2008, 31, 1761 – 1768
ability can also be limited by the problems associatedwith pressure control, if one needs to transfer signifi-cantly small sample volume (e. g, below a few tens or hun-dreds of microliters). The use of the MITD method, whileits operation is supported partially by the manualmethod, can hence be advantageous from many respects.Most importantly, it allows to control sample volume forthe identical TD set-up to range from as little as microli-ter (for direct GC injection) to several hundred millilitersvolume (with a syringe of a large capacity).
Figure 3 depicts the RSC calibration results obtainedusing standards of each individual concentration level.When calibration slopes of the different RSCs are com-pared at a given concentration level, extremely strong
consistency is seen with the relative ordering of sensitiv-ity: H2S a CH3SH a DMS a DMDS a CS2. To make a mean-ingful comparison of these calibration data, the absoluteslope values of each compound are plotted as a functionof standard concentration level (Fig. 4). Hence, when cali-bration results of each compound derived from theentire concentration range are put together, some com-mon features are found consistently from all calibrationexperiments across a wide concentration range. Mostimportantly, the relative ordering of calibration slopes ismaintained, as discussed above. It is also interesting tonote that absolute calibration slope values do not varysignificantly with increasing standard concentrations,although the results obtained at lower concentration
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Figure 3. Comparison of calibration slope values across different RSCs at a given concentration level. Results are shown for thelower three and upper three concentration ranges of eleven standard prepared for this study. A brief summary of these compara-tive calibration experiments is provided in Table 2.
J. Sep. Sci. 2008, 31, 1761 –1768 Gas Chromatography 1767
ranges are moderately variable. The results of this com-parison thus suggest that the MITD method can be usedto expand the range of sample volume from a few micro-liters to several hundreds of milliliters for TD analysiswithout a significant matrix effect which might haveotherwise suffered from direct GC injection due to thechanging sample volume [2].
In Table 2, the basic statistical parameters were com-puted using all calibration slope values derived usingeach of all standards analyzed in this study. If the meancalibration slopes are normalized to that of CH3SH, therelative ordering of the former four RSCs is computed as0.65 (H2S): 1 (CH3SH): 1.34 (DMS): 2.24 (DMDS). Theobserved relative patterns appear to comply well withthose derived through an application of a loop injectionsystem such as 0.63:1:1.65:3 [11]; in our previous study,the photomultiplier tube (PMT) voltage values werereduced arbitrarily with an increase in standard concen-trations to overcome the inflection problems of PFPD
(i. e., due to its limited linearity range). It should howeverbe noted that the differences in relative calibration slopevalues between the light (H2S) and heavy RSC (DMDS) aremuch narrower in the present study than those observedin our previous one. Because our previous experimentsbased on loop-injection method were significantly modi-fied to arbitrarily control the RSC response (PMT voltage),calibration yielded less stable patterns over a lessextended concentration range of 102. As the calibrationexperiments in the present study were conducted overan extensive range of concentrations, reproducibility ofthe MITD method was examined separately at both lowerand upper bound concentration ranges. The results ofthe ancillary experiments confirmed that the calibrationpatterns derived by the MITD method is highly reprodu-cible with RSE values of less than a few percent. Althoughan involvement of various chromatographic processes(e. g., cryofocusing) is suggested to introduce an inevita-ble bias in the GC response (e. g., [16]), the calibration data
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Figure 4. Comparison of calibra-tion slope values as a function ofthe RSC concentration levels pre-pared as working standard.
Table 2. A summary of calibration results of RSC in terms of their slope values
obtained by our MITD method appear to be reasonablyreliable over a wide concentration range without the sys-tem modification.
To account for the effect of concentration differenceson the reproducibility of the RSC calibration, the calibra-tion slope values derived using all 11 concentration lev-els were combined together for the derivation of RSE foreach RSC (Table 2). The results of this computation showthat RSE values (%) change in an ascending order: 5.77(DMDS), 6.68 (DMS), 7.17 (CS2), 8.71 (CH3SH), and 12.8(H2S). This, thus, indicates that the calibration of H2S isthe least reproducible, while that of DMDS exhibits themost reproducible and stable pattern. The observed pat-tern of the relative reproducibility test is in fact compara-ble to those derived on the basis of the thermal desorbingtechnique [2] or loop injection method [11]. For instance,according to the results of the former study [2], the sensi-tivity of H2S detection was found to be highly variable sothat its calibration slopes increase systematically in rela-tion to the initial standard concentration level (the over-all RSE value of H2S calibration slope l20%). However,those of the other RSCs exhibited fairly constant trendswith the RSE values ranging from 2.4 to 5.5%. Althoughthe calibration slope value of H2S measured in thepresent study exhibits the largest variability of all theRSCs, it does not vary systematically with standard con-centration levels. In light of all those aspects with theRSC calibration, the results of present study share somesimilarities and dissimilarities to the previous study. Itshould however be stated that the calibration resultsobtained using the MITD method are reasonably stableenough to maintain good reproducibility in a concentra-tion range over 103.
5 Conclusions
In the present study, we explored a new experimentalapproach named the MITD to evaluate the performanceof GC/PFPD for RSC standards over an extensive concen-tration range by the normal operation setting of the ther-mal desorbing system. By transferring the RSC mesuranddirectly into the tubing line of MS unit in the TD system,we were able to analyze the RSC samples of all concentra-tion levels with a single analytical set-up (i. e., withouttransition between different analytical modes). Becausesample transfers along the TD track can be controlledwith an internal MFC unit, the use of the authentic TDsystem can be optimized by low concentrated sampleswith sufficient volume (e. g., above a few 100 mL). Hence,the application of the MITD method can offer the versa-tility of the TD system to cover samples of the varyingRSC concentration ranges, especially highly concen-
trated samples with sufficiently low sample volume (e. g.,a few lL). As such, we were able to acquire relatively con-stant calibration datasets from samples of an extendedconcentration range (e. g., 103) in a consistent mannerthrough an application of the MITD method.
The results of MITD-based calibration experiments con-firm that the reliability of RSC analysis, when assessed interms of RSE from all calibration slope values of 11 con-centration levels, can be reflected at least in part by theirmolecular weights. It is found that RSE of H2S slopeexhibits the least reproducible value of 12.8%, while thatof DMDS is at 5.77%. The results of the present MITD anal-ysis also show good agreement in relative calibrationproperties of RSC with the results of the previous studymade by loop-based method in which their calibrationtrend was estimated over an extended concentrationrange through the arbitrary adjustment of PMT sensitiv-ity. The results of the present study hence suggest thatthe use of the MITD method is recommendable forextending the detectable range of the GC/PFPD system aswell as the validity of its calibration results by theemployment of the TD technique, if gaseous samples of avarying concentration range must be handled.
This work was supported by a grant from the Korea ResearchFoundation (KRF-2005-201-C00045) funded by the Korean govern-ment (MOEHRD).
The authors declared no conflict of interest.
6 References
[1] Dincer, F., Odabasi, M., Muezzinoglu, A., J. Chromatogr. A 2006,1122, 222 – 229.
