Demonstration of Techniques and a Suitable Atomizer for Practical Multielement Atomic Absorption Analysis

S T E P H E N R. L A W S O N , * J O H N A. N I C H O L S , P U L I G A N D L A R A Y W O O D R I F F Department of Chemistry, Montana State University, Bozeman, Montana 59717

V I S W A N A D H A M , a n d

Simultaneous multielement atomic absorption spectroscopy has not become a common laboratory workhorse for elemental anal- yses despite recent advances in instrumentation. Two major obstacles preventing its implementation are the sometimes se- vere matrix interferences which occur in some pulsed atomizers and the Hmited working range of atomic absorption spectros- copy compared to the wide linear dynamic range of inductively coupled plasma-atomic emission spectroscopy. Use of a constant temperature furnace in conjuction with techniques such as peak width at fixed height, random dilutions with element rationing, and monitoring two or more wavelengths of different sensitiv- ities of an element are effective methods for eliminating or reducing these obstacles. Determination of trace zinc in un- weighed samples of reagent grade CdC12o 2.5H20 and the deter- ruination of lead in unknown volumes of blood using hemoglobin iron as the internal standard are examples of analyses per- formed on the dual channel monochromator used in this work. Problems in selecting appropriate compromise conditions of atomization are exemplified in work done on solid samples. Although full recoveries of Zn and Cd were obtained at 1800 K in biological samples, low recoveries for Zn were obtained in NBS coal fly ash at this temperature. Atomization at 2100 K was necessary to restore full recovery. Index Headings: Analysis, multielement; Atomic absorption spec- troscopy; Internal standards; Unweighed solid sampling.

I N T R O D U C T I O N

During the last 25 years, atomic absorption spectros- copy (AAS) has become a widespread and highly prac- tical analytical tool. Combined with electrothermal atom- ization, AAS makes ul tratrace element analyses and di- rect solid sampling accessible to even budget-limited laboratories. Thus, the chance to even double the number of elements tha t can be determined at one t ime with minimum additional cost led many workers in the field to a t t empt development of a practical mult ie lement atomic absorption spec t rometer ) -5 This interest led to the development of a commercial dual-channel AAS system (Instrumentat ion Laboratories). Workers who de- sire more elements usually employ modified direct read- ers, which is a practical route for laboratories tha t ac- quired them as par t of an emission setup.

Hollow cathode lamps are available which contain up to six or seven elements, al though line intensities tend to decrease as the number of elements increases. Alder et al. ~ used an array of nine hollow cathode lamps to analyze up to nine elements and still retain graphite furnace atomic absorption spectroscopy (GFAA) detec- tion limits comparable to commercially available atomic absorption instruments using single e lement hollow cath- ode lamps and electrothermal atomization.

Received 15 September 1981. * Present address: Chemistry Department, Carleton University, Ottawa

K1S 5B6, Canada.

Volume 36, Number 4, 1982

Harnley and co-workers 6 have recently provided the best demonstra t ion of a practical s imultaneousmult ie le- ment atomic absorption system which they dubbed SI- MAAC. A xenon arc cont inuum lamp was the pr imary source and wavelength modulat ion provided background correction at each analytical line for up to 16 elements. Instrumentally, it appears tha t simultaneous multiele- ment analyses are equally feasible using AAS or atomic emission spectroscopy (AES). However, the use and growth of mult ie lement AAS has, in fact, been quite slow. Two main factors have contr ibuted to this slow growth: (1) some electrothermal atomizers have been prone to chemical interferences and /o r preatomizat ion losses; (2} AAS typically has a limited working range in contrast to the wide linear dynamic range for AES, particularly inductively coupled plasma-AES. These problems tend to force the analyst to provide custom-tailored atomiza- tion conditions and dilution factors for each sample-an- alyte combination. ~'s When several elements must be considered simultaneously, the search for a chemical p re t rea tment to resolve all possible interferences be- comes insurmountable. A matrix interference-free at- omizer is, therefore, as essential to a mult ielement AAS system as the light source and polychromator . This be- comes especially evident when we go beyond the syn- thetic solutions which have thus far been the pr imary focus of mult ie lement AAS inst rument development studies.

I. E X P E R I M E N T A L

A. A p p a r a t u s . The monochromator is a variable- wavelength, dual-channel, single grating model built in this laboratory. Construct ion details have been described previously) I t is exceptionally versatile since two pho- tomultiplier tubes pivot around the Rowland circle by mechanical drive while still allowing room for 5 to 10 fixed channels.

An Ithaco model 353 DL lock-in amplifier system consisting of two independent linear lock-in amplifiers sharing a common power supply was connected to a dual pen omniscribe strip char t recorder for peak height mea- surements (Houston Instruments). A logarithmic ampli- fier, V-F converter, and counter were used for peak area measurements . This setup provided simultaneous read- out from both channels on the same chart paper and thereby produced a pe rmanent t ime history of the rela- tive appearance t ime of each analyte species.

One power supply was used for bo th photomult ipl ier tubes to assure tha t any slight voltage variations would cause similar changes in their responses. Both photo- multiplier tubes have approximately the same response characteristics.

A Varian CRA-63 graphite cup electrothermal atom-

APPLIED SPECTROSCOPY 375

izer was used for pulse-type atomization. Dry and ash temperatures were measured with a Chromel-Alumel thermocouple. A 30 ms time constant was used for all graphite cup work.

A constant temperature Woodriff furnace was used for all measurements except the blood analysis. It is an ideal furnace for multielement AAS due to its freedom from matrix interferences 9' lo and the ability to run solid sam- ples without pretreatment. ~1

Hollow cathode lamps were the primary source for all measurements. Either multielement or single element lamps combined with a half-silvered mirror were utilized.

B. P r o c e d u r e . NBS orchard leaves SRM 1571, bovine liver SRM 1577, coal SRM 1632, and coal fly ash SRM 1633 were analyzed in the Woodrift furnace. The coal and coal fly ash were run without pretreatment in closed graphite cups. 9 Large samples (>5 rag) of orchard leaves and bovine liver were ashed prior to introduction into the furnace as described by Nichols et al. ~1 to reduce particulate scattering and molecular background.

Blood samples were obtained from the Montana State University Health Center and Bozeman Deaconess Hos- pital. The samples were 1 to 5 days old at the time of analysis. Whole blood (2.3 #1) was pipetted into a Varian CRA-63 cup along with 2.3 pl of 6 N nitric acid. The mixture was dried at 110°C for 30 s, and then the tem- perature was increased to 150°C for an additional 30 s. This allowed complete and even drying of the viscous sample without loss due to splattering. Ashing was per- formed in air at 600°C for 55 s. Finally, the sample was atomized to a final atomization temperature of 2600°C for6 s.

II. RESULTS AND D I S C U S S I O N

A . D u a l E l e m e n t A n a l y s i s i n S o l i d s . Many workers have stated that compromise conditions of atomization in multielement AAS are not a problem because one can simply use the atomization temperature of the least volatile analyte, s This laboratory has found that complex samples can make it more difficult to find compromise conditions. This is especially true with solid sampling which eliminates sample digestion and pretreatment. In general, we have found that solid biological tissue sam- ples can be run successfully under a wider variety of conditions than siliceous materials. One reason for diffi- culties with glasses and rock samples is the high back- ground (~0.25 absorbance unit at 2670 K in a constant temperature furnace for less than I mg of NBS SRM 616 glass) which appears due to vapor of silica if the atomi- zation temperature must exceed 2470 K for any of the analytes sought. 1° Thus, sample weight may be exces- sively restricted if the maximum temperature desired for one or more elements produces uncorrectably high back- ground. Here the sequential volatilization available with pulsed atomization would be advantageous provided complete recoveries are obtained.

For more volatile elements, samples such as coal f ly ash may show poor recovery under atomization condi- tions which work well for standard solutions and some biological samples (Table I). Cadmium and zinc give good recoveries (Cd 95% ± 5%; Zn 105% ± 10%) in orchard leaves and bovine liver using closed sample crucibles 1~ at 1800 K. However, in NBS coal fly ash, good recoveries

376 Volume 36, Number 4, 1982

were obtained only for Cd at this temperature (Table II). It was necessary to work at 2100 K in order to obtain recoveries of Zn without compromising cadmium.

B. Ana lys i s o f Major and Minor Cons t i t uen t s in U n w e i g h e d S a m p l e s . A multielement spectrometer provides AAS with a unique opportunity for use of an internal standard by which analysis of unweighed sam- ples or solutions diluted to an unmeasured volume (a desirable practice when performing ultratrace analysis, where each transfer to volumetric glassware brings fur- ther possible contamination) can become additional standard methods in AAS analysis.

1. Determinat ion o f Trace Zinc in C a d m i u m Samples . The following study of zinc contamination levels in re- agent grade cadmium compounds illustrates some typical problems and possible solutions in the use of the second channel of the spectrometer to weigh the cadmium con- tent of a solid sample of reagent. In this case the internal standard is not a separate, deliberate addition; knowledge of the stoichiometry of the compound and the assump- tion that it is at least 95 to 99% pure compound allow us to calculate the gross sample weight from the measured amount of cadmium.

The zinc analyses were performed using the 307.6 nm line instead of the excessively sensitive 213.9 nm zinc line. This permitted use of convenient sample sizes of the reagents being analyzed (on the order of 1 mg). However, such sample sizes also produced extremely large re- sponses for cadmium line at 326.1 nm. The response was pegged in excess of 0.8 absorbance unit for at least 40 s due to 0.5 mg of cadmium.

Fortunately, the diffusion-controlled loss of sample vapor from a constant temperature furnace can be ex- pressed as one or a sum of exponential terms containing time constants related to residence time and the time required to attain a particular absorbance value, la The length of time that the response is above a fixed peak height (in this case, 0.8) can thereby be related to the logarithm of the analyte quantity. Normally, a linear relationship between these two parameters can be ob- tained over a concentration range for cadmium 1000

TABLE I. SampleresultsforsimultaneousdeterminationofCu and Zn in mineral feed and NBS orchard leaves, a

Cu (~g/ml) Zn (~g/ml) Sample b

Found Ac tua l F o u n d Actual Orchard leaves 0.103 0.10 0.25 0.21 Mineral feed 13.0 12.8 12.7 11.5

a Results read vs simple aqueous standards. b Microwave digested with HNO3.

TABLE H. Simultaneous determination of zinc and cadmium in NBS coal fly ash (SRM 1633). =

Zinc/ng Cadmiura/ng Sample

size (rag) Ob- Ex- % Re- Ob- Ex- % Re- served pected covery served pected covery

0.9 100 189 52.9 4.3 1.3 340 b 3.8 285 800 35.6 6.4 5.5 116 8.6 590 1800 32.8 10.8 12.5 86 9.7 710 2035 34.9 11.6 14.0 83

18.5 1345 3880 34.7 24.0 26.8 90 a Atomization temperature: 1770 K; rz. ffi 307.6 nm; ~'cd ffi 326.1 nm. b High Cd recovery due to inhomogeneity effects encountered at small

sample sizes.

times higher than the concentration at which the re- sponse first exceeds the chosen height (0.8 absorbance unit).

Some of our responses were in excess of this linear range and so it was necessary to establish empirically the curvature of the relationship at higher concentrations. Onset of curvature appeared to vary from compound to compound as shown in Fig. 1.

Once the ability to weigh these samples by use of the 326.1 nm cadmium line was established, it was quite straightforward to measure simultaneously the zinc re- sponses with the other channel of the spectrometer set at 307.6 nm. An example of the data obtained is shown in Table III.

An alternative to solid sampling, in the above case, would be to dissolve the reagent and pipet small enough aliquots to avoid part of the difficulties of calibration for cadmium. However, certain samples may be insoluble or difficult to dissolve completely (for example, analysis of arsenic in some samples of antimony oxides), and solid sampling may be appreciably faster and simpler.

2. Use of Iron in Hemoglobin as the Internal S tandard in Blood. The same technique was applied to the deter- ruination of lead in human whole blood analyses using the iron in hemoglobin as the internal standard. The lead levels of nine different blood samples were determined by standard additions. A plot of lead concentration against the ratio of iron to lead absorbance (peak areas) produced a linear graph from 25 to 90 ppb Pb (Fig. 2). The range of lead concentration in the samples studied was between 25 and 45 ppb. For Pb 30 ppb is considered average and 80 ppb or above is toxic. 14 Data at higher

35-

25-

~Y / I

15-

10 ~ 140 1000

Cd u.lgl FIG. 1. Plot of the peak width in millimeters measured at a fixed absorbance level (0.8 absorbance unit) vs the weight of cadmium a s :

0, solution of nitrate salt; D, solid chloride salt; ×, solid surface salt.

TABLE HI. Ca lcu la t ion of zinc concentration in a n u n w e i g h e d sample.

Zinc absorbance Amount of zinc from calibration curve Peak width at fixed height of Cd signal Amount of Cd from calibration curve Amount of CdC12.2.5H20 corresponding to Cd found Concentration of Zn in the unweighed sample Concentration of Zn according to Baker's analysis Concentration of Zn on the basis of a weighed sample

0.12 0.47 #g

21.2 division 560 ~g

1.09 mg 0.43 #g/mg 0.3 ftg/mg 0.31 #g/mg

90

80-

70-

60. ¢3

so- M,I m l

40-

30-

20-

10-

i ' ' i 4.0 d.o do 7;o 8'.0 d.o lO'.O 11'.o 12.o Fe/Pb ABSORBANCE RATIO

Fro. 2. Plot of the weight of lead against the ratio of absorbances measured simultaneously of iron and lead in each of nine blood samples.

lead concentrations were obtained by addition of stand- ard lead nitrate solutions to several blood samples. This produced a graph which covered the range of lead con- centrations usually found in clinical laboratories with a standard deviation representative of the natural fluctu- ations in iron levels for a small number of people.

Hemoglobin levels can vary from 12 (anemic) to 16 (high altitudes) g of hemoglobin per 100 ml of whole blood (both male and female ranges included). 14 This corresponds to a range of 408 to 544 ppm iron. The average iron concentration among the nine samples used in this study was 460 ± 13 ppm. Using a lead absorbance which corresponds to 44 ppb Pb and the Pb vs Fe/Pb plot, values from 35 to 50 ppb lead are possible due to the iron-related fluctuations in the Fe/Pb absorbance ratio. Thus, a 14 to 18% error is possible at these ex- tremes. However, general screening tests are usually designed to distinguish between a 44 ppb and an 80 ppb lead level. An anemic person might be misinterpreted to have a high lead level, but this would trigger further tests to determine whether the problem was the iron or lead level. A high altitude area would be calibrated for higher iron levels to avoid erroneously low lead readings. Thus, the effectiveness of ratioing Fe to Pb (or other trace metals), which avoids the need for accurate blood volume readings, should be sufficient for mass screening pro- grams or any situation where speed is important.

C. Gene ra l Analys is . It is always desirable in a high volume analytical laboratory to obtain two or more de- terminations with a negligible increase over the labor required for one. In this laboratory, we have made exten- sive use of multielement AAS in analyzing trace elements in metals with both continuous temperature furnace and CRA and generally group elements by volatility to min- imize degradation of the graphite. For the refractory elements in particular, the life time of the HGA furnace tube is an important limiting factor on productivity of automated sequential analyses, as well as an expensive replacement factor. Combinations such as Pb-Cu, Mn- Cu, Mn-Co, Cr-Ni, A1-Si, and Cr-Si are typical examples of dual-element determinations performed in this labo- ratory.

APPLIED S P E C T R O S C O P Y 377

Element ratioing has proven useful in saving time and labor when seeking to establish that two metal items are of the same composition. This obviates the need for standards and simplifies range finding through use of random dilutions. Another approach to range finding is to set each monochromator channel on a different wave- length of the same element but with different sensitivi- ties. High- and low-concentration samples can be run together and one or both wavelengths will give a useful response. If a computer is available and a suitable algo- rithm to fit the curve can be found, the nonlinear portion may be usable.

1. R. Woodriff and D. Shrader, Appl. Spectrosc. 27, 181 (1973).

2. A. Strasheim and H. G. C. Human, Spectrochim Acta 23B, 265 (1967). 3. R. Mavordineanu and R. C. Hughes, Appl. Opt. 7, 1281 (1968). 4. K. W. Jackson, K. M. Aldous, and D. G. Mitchell, Spectrosc. Lett. 6, 315

(1970). 5. J. F. Alder, D. Alger, A. J. Samuel, and T. S. West, Anal. Chim. Acts 87, 301

(1976). 6. J. M. Harnley, T. C. O'Haver, B. Golden, and W. R. Wolfe, Anal. Chem. 51,

2007 (1979). 7. D. J. Hydes, Anal. Chem. 52, 959 (1980). 8. K. Oh]s, J. J. Sotera, and H. L. Kahn, Presented at the 1980 Pittsburgh

Conference, Atlantic City, NJ, Paper 429. 9. L. Hageman, A. Mubarak, and R. Woodriff, Appl. Spectrosc. 33, 3 (1979).

10. L.R. Hageman, J. A. Nichols, P. Viswanadham, and R. Woodriff, Anal. Chem. 51, 1406 (1979).

11. J. A. Nichols, R. D. Jones, and R. Woodriff, Anal. Chem. 50, 2071 (1978). 12. D. A. Bath, PhD thesis. Montana State University, Bozeman (1975). 13. R. Woodriff, M. Marinkovic, and R. A. Howald, Anal. Chem. 49, 2008 (1977). 14. N. W. Tietz, Ed. Fundamentals of Clinical Chemistry (W. B. Saunders Co.,

Philadelphia 1976) p. 1123, 1214.

Analysis of Barium and Strontium in Sediments by dc Plasma Emission Spectrometry

P. C. BOWKER and F. T. MANHEIM United States Geological Survey, Woods Hole, Massachusetts 02543

T h e dc p l a s m a arc is su i ted to a n a l y s i s o f b a r i u m a n d s tront ium in a w i d e r a n g e o f s e d i m e n t a r y r o c k m a t r i c e s , f r o m s a n d s , shales , and carbonates , to f e r r o m a n g a n e s e nodules . S a mp l e s c o n t a i n i n g 10 p p m to m o r e t h a n 3000 p p m b a r i u m a n d s tront ium w e r e s t u d i e d . B o t h a l k a l i (3500 ppm Hthium borate , f r o m a p r e l i m i n a r y f u s i o n ) a n d l a n t h a n u m s a l t s (1%) in the f inal solu- t ion a r e n e e d e d to a c h i e v e f r e e d o m f r o m s y s t e m a t i c e f f e c t s d u e to e x t r e m e v a r i a t i o n i n m a t r i x . I n the absence o f La , n e i t h e r Li, Na , K, n o r Cs t o t a l l y e l i m i n a t e d e f f e c t s o f A1 a n d other const i t - uent s on emiss ion . Si l ica a d d i t i o n to t h e fus ion he lps ach ieve proper f lux v i s c o s i t y to a i d r e m o v a l o f f u s e d b e a d s f r o m g r a p h - i te crucibles . The ef fect o f r e f r a c t o r y - s u b s t a n c e formers such a s a l u m i n u m w i t h ca lc ium can be reduced or r e m o v e d b y se lec t ion o f a p o r t i o n o f the arc for e m i s s i o n m e a s u r e m e n t . H o w e v e r , i t w a s d e c i d e d not to pursue this approach because of loss i n a n a l y t i c a l s e n s i t i v i t y a n d n e e d fo r g r e a t e r prec i s ion in opt ica l a d j u s t m e n t . A n a l y s i s o f s t a n d a r d r o c k sample s s h o w e d g e n e r - a l l y s a t i s f a c t o r y a g r e e m e n t w i t h prec i s ion m e t h o d s o f analys i s , a n d some n e w s tandard rock d a t a a r e r e p o r t e d .

Index Headings: dc P l a s m a s p e c t r o m e t r y ; B a a n d S r a n a l y s i s ; Sed iment a n a l y s i s ; M a t r i x c o r r e l a t i o n s .

INTRODUCTION

Analysis of barium in marine sediments and earth materials has become an important way to trace drilling mud components in offshore areas. 1 From a more general geochemical point of view both barium and strontium are important trace-to-minor constituents of sediments, in part because of their participation in biological proc-

Received 28 Augus t 1981. * Use of t r adenames in this publicat ion is for identification only and

does not const i tu te endor semen t by the Uni ted Sta tes Geological Survey.

3'78 Volume 36, Number 4, 1982

esses, and in part because of their varying solubility in low-temperature geochemical processes.

The use of the dc (direct current) argon plasma as an excitation source in spectrochemical analysis has permit- ted high analytical sensitivity for barium and strontium while achieving reproducibility close to levels typical for flame (emission or absorption) analysis. Detection limits for Ba in the dc plasma have been decreased to approx- imately 1 ppm 2 in solids, whereas the detection limit for Ba determined by x-ray fluorescence is 45 ppm, ~ that for Ba determined by atomic absorption is 10 ppm, 4 and that for Ba determined by neutron-activation analysis is 10 ppmJ However, variations in emission intensity have been attributed to matrix effects including effects caused by ionization, the amount of solids introduced, back- ground emission levels, the portion of plasma selected for imaging on the photodetector, spectral interferences, and co-volatilization effects in the arc. 6-8 Geological matrices may range from nearly pure Si02 and CaC03 to alumi- nosilicates, and all combinations of these end members exist. Significant concentrations of other constituents may also occur in mineral lattices. The principal objec- tive of this work has been to devise an analytical proce- dure that could be insensitive to large differences in geological matrix, as well as to varying forms of Ba and Sr, e.g., silicate, carbonate, or sulfate binding.

I. EXPERIMENTAL PROCEDURE

A. I n s t r u m e n t s Used. A Spectraspan IIB echeile- grating emission spectrometer with a three-electrode dc plasma excitation source (Spectrametrics, Inc.) was used to carry out all analyses. General characteristics of dc plasma sources and the specifics of the echelle-grating

APPLIED SPECTROSCOPY