Instrumentation Richard F. Browner School of Chemistry Georgia Institute of Technology Atlanta, Ga. 30332
Andrew W. Boorn Sciex 55 Glen Cameron Road, #202 Thornhill, Ontario L3T 1P2, Canada
Sample Introduction Techniques for Atomic Spectroscopy Selection of the best sample introduction procedure for an analysis requires consideration of a number of points. These include: • the type of sample (e.g., solid, liquid, gas), • the levels, and also the range of levels for the elements to be determined, • the accuracy required, • the precision required, • t h e a m o u n t of material available, • t h e number of determinations required per hour, and • special requirements, such as whether speciation information is needed. T h e measurement techniques available, whether flame atomic absorption spectroscopy (FAAS), inductively coupled plasma (ICP) atomic emission spectroscopy, or dc plasma (DCP) atomic emission spectroscopy, will also have a major effect on the choice of t h e procedure selected. Although this paper will concent r a t e specifically on sample introduction techniques, in any real analysis sample introduction is an extension of sample preparation. As a consequence, t h e selection of a suitable sample introduction technique can depend heavily on available and effective sample preparation procedures. Generally, though, sample preparation will not be discussed here, except where sample introduction and sample preparation are intimately linked. 0003-2700/84/0351-875A$01.50/0 © 1984 American Chemical Society
T o u n d e r s t a n d the limitations of practical sample introduction systems it is necessary to reverse the normal train of thought, which tends to flow in t h e direction of sample solutionnebulizer-spray chamber-atomizer, a n d consider the sequence from the opposite direction. Looking a t sample introduction from the viewpoint of the atomizer, t h e choice of procedure will hinge on what t h e atomizer can usefully accept. Bearing in mind t h a t every atomizer has certain reasonably well defined, b u t different, properties of t e m p e r a t u r e , chemical composition etc., an introduction procedure must be selected t h a t will result in rapid breakdown of species in the atomizer, irrespective of the sample matrix. T o ensure efficient free atom production, the following parameters m u s t be known for each analyte-matrix-atomizer combination: • m a x i m u m acceptable drop size, • o p t i m u m solvent loading, both aerosol and vapor, • m a x i m u m acceptable analyte mass loading, • appropriate gas flow p a t t e r n s for effective plasma penetration (for the ICP), and • suitable observation height. T h i s last parameter should be selected in conjunction with the gas flow p a t t e r n of sample introduction such
t h a t a d e q u a t e residence time is provided for t h e introduced material to desolvate, vaporize, and atomize. In certain cases, for instance, when organic solvents are introduced to an ICP, it also may be necessary to adjust t h e atomizer operating characteristics to account for the change in plasma properties induced by t h e solvent. Here, an increase m forward power to t h e plasma (e.g., from 1.25 to 1.75 kW) is necessary to aid the decomposition of organic species. Overall, then, it is the properties of t h e atomizer t h a t dictate the design a n d operation of the sample introduction system. This is particularly true for liquid sample introduction with p n e u m a t i c nebulization.
Present Understanding of Sample Introduction Processes T h e r e is a great deal of intuitive, b u t relatively little experimentally based, knowledge in this field. Clearly, t h e r e is some upper limit to the size of d r o p t h a t can be vaporized in the typically 1-2 ms available in the atomizer. Yet there are no tables available t h a t specify the upper limit of drop size suitable for each matrix and atomizer. Such tables would be of great help to
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984 · 875 A
ACID DIGESTION BOMBS Parr '®
®
Teflon Lined
With the abilil, tn hold strong acids or alkalis at temperatures well above normal boiling points, Parr Acid Digestion Bombs offer rapid and safe procedures for dis solving inorganic materials in HF, HCI, HNO3 or other strong acids, and for digesting organic samples in strong alkalis or oxidizing acids, all with complete sample recovery without losing any trace elements. These bombs are available in 23 and 125 ml sizes for tempera tures to 150°C and pressures to 1200 psig, and in a 23 ml, high strength design for temperatures to 285°C and pressures to 5000 psig. All have removable, thick-walled Teflon cups in stainless steel bodies with convenient closures, positive seals and other exclusive Parr features. For details, write or phone Parr Instrument Company, Moline, Il. 61265. 309/762-7716 CIRCLE 159 ON READER SERVICE CARD
ANALYTICAL AIDS Chemistry for Toxicity Testing C.W. Jameson and Douglas B. Walters A practical approach to the chemistry requirements for the toxicological evaluation of environmental chemicals. 1984 248 pp ill. 250-40547-4 $29.95
Analytical Pyrolysis Kent J. Voorhees An up-to-date review of the theory, technique and applications of analytical pyrolysis. 1984 496 pp ill. 408-01417-2 $69.95
High Resolution Spectroscopy J. Michael Hollas A comprehensive guide to the principles and tech niques of modern high resolution spectroscopy. 1982 638 pp ill. 408-10605-0 $115.00
Handbook of Anion Determination W. John Williams Offers single-volume coverage of the most important methods for the determination of over 70 anions. 1979 630 pp ill. 408-71306-2 $139.95 To order, or for more information, write:
Butterworth Publishers 80 Montvale Avenue Stoneham, MA 02180
CIRCLE 31 ON READER SERVICE CARD 876 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
practicing analytical chemists. Of course, the production of these tables is not a trivial matter, and would re quire rather involved experimental procedures at the state of the art in particle generation and characteriza tion. Nevertheless, when these data do become available, which undoubtedly they will in due time, this should allow researchers to steer around the major ity of interference problems other than those of spectral overlaps, for which tabular data are already avail able (i, 2). ICP Systems. The data available on solvent-loading limitations for or ganic solvents with the ICP have been characterized recently (3). Some typi cal limiting aspiration rates are shown in Table I, together with evaporation factors, E, of the solvents. These data are useful as a guide for organic sol vent introduction to the ICP. The evaporation factor is a measure of the rate of mass loss from an evaporating drop, and is given by: Ε = 48 DvaPsM2(5RT)-2
(1)
where Dv is the diffusion coefficient of the solvent vapor, σ is the surface ten sion, Ps is the saturated vapor pres sure, M is the molecular weight of the solvent, δ is the density, R is the gas constant, and Τ is the absolute tem perature. In general, the ICP has de creasing tolerance to solvents as their evaporation factors increase, and there is an inverse correlation between evaporation factor and limiting aspi ration rate. However, the alcohols have a much greater quenching effect on the plasma than their evaporation factors would indicate, and they may readily extinguish the plasma under normal operating conditions. It is always possible to remove at least part of the solvent vapor by con densation. Two groups of workers have attempted this and shown that the tolerance of the ICP to organic sol vents is greatly improved when a large fraction of the solvent vapor is re moved from the gas stream passing to the plasma (3, 4). This is an indication of how the sample introduction pro cess can be modified to produce analyte closer to the optimum for the at omizer. No published data are avail able on limitations of aqueous sample introduction to the ICP, although clearly water loading in the plasma has a direct influence on plasma prop erties. In fact, it has been shown for certain ionic lines that doubling the water loading entering the plasma can cause a 100-fold reduction in analyti cal signal (5). From a practical standpoint, three important conclusions can be reached. First, it is necessary to introduce sam ple to the atomizer with drops no larg er than a certain maximum size
Chemical Research Faculties: An International Directory fhernica' j?esearcn Faculties
^W-m-»"-"*·*^fldglfgf 198*
H&n\ For the first time — a new in ternational directory from the ACS. Contains all the informa tion you need on chemical re search and researchers at uni versities around the world. Offers data on who's doing what in chemistry, chemical engineering, biochemistry, and pharmaceutical/medicinal chemistry. Gives you informa tion on over 735 academic de partments worldwide and lists more than 8,900 faculty mem bers and their areas of spe cialization. Also provides in valuable data on graduate programs at universities in more than 60 countries as well as names, addresses, and im portant information on 63 chemical societies around the world. Cross-indexed three ways for easy use. An indis pensable reference for indus try and academia alike. 640 pages (1984) Clothbound LC 83-22323 ISBN 0-8412-0817-4 US & Canada $129.95 Export $155.95 Order f r o m : A m e r i c a n Chemical Society Distribution Office Dept. 60 1155 S i x t e e n t h St., N.W. W a s h i n g t o n , DC 20036 or CALL T O L L FREE 800-424-6747 and use y o u r MasterCard, VISA, or A m e r i c a n Express credit c a r d .
(dmax)- Second, the solvent introduc tion rate must fall within a certain permissible band of values. Third, to maintain good system reproducibility, it is essential that all these parameters be controlled carefully over the long term. Any significant change in drop size or solvent loading reaching the at omizer could have an adverse effect, both on system accuracy and system reproducibility. From this standpoint, the need to maintain a constant tem perature in the plasma box in an ICP system becomes clear, as a means to reduce the baseline drift caused by variable solvent vapor loading (5). Atomic Absorption Systems. Flames are generally far less suscepti ble to variations in solvent loading than ICPs are, although the introduc tion of organic solvents to an air-acet ylene flame can lead to a significant temperature drop. This in turn could cause the onset of interferences due to sample matrix problems. For flame AAS systems, the design of nebulizers and spray chambers appears to have been empirically optimized to provide the best aerosol drop size in the flame for interference-free analyte vaporiza tion. Solvent loading appears to be a secondary factor. In the past 20 years AA nebulizers and spray chambers have undergone a steady progression. They have changed from devices producing very coarse aerosols, with a corresponding high incidence of vaporization inter ferences, to devices producing much finer aerosols, which are largely free from this type of interference. In fact, recent data indicate that it is possible to virtually eliminate all matrix-in duced vaporization interferences in AAS (e.g., calcium-phosphate, siliconaluminum, silicon-manganese, etc.) (6, 7). This is accomplished by shift-
Table I. Limiting Organic Aspiration Rates for ICP a
Solvent
Limiting Aspiration Rate *
Evaporation
(mL/min)
Factor
Methanol 0.1 47.2 Ethanol 2.5 45.6 18.5 Xylenes 1.0 Acetone 0.1 264 MIBK 3.0 77.3 Diethyl ether CO c
g
100 /XL-
50
'co
Έ
50 μ ι
LU
10 uL
0 5
0
10 Time (min)
Figure 10. Flow injection peaks ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984 · 887 A
i n t r o d u c t i o n . It is therefore possible, with t h e I C P , t o inject s a m p l e s a t t h e r a t e of a p p r o x i m a t e l y 4/min, as op posed t o 1.5/min w i t h conventional s a m p l e i n t r o d u c t i o n . Additionally, in A AS, where t h e a d d i t i o n of ionization buffers, l a n t h a n u m releasing agents, etc., m a y be desirable, it is possible to a d d t h e a n a l y t e as a spike i n t o a flow ing s t r e a m of t h e desired buffer, m a k ing for a relatively simple e x p e r i m e n tal system (26). Other Techniques for Sample Introduction T h e t e c h n i q u e s considered so far have achieved s u b s t a n t i a l practical use; t h e r e are o t h e r s which h a v e m o r e specialized applications. Laser abla tion, in which t h e power from a fo cused r u b y laser is used t o vaporize a spot of m a t e r i a l directly from a solid surface, h a s considerable promise (27, 28). A n o t h e r a p p r o a c h of g r e a t inter est in metallurgy is t h e use of s p a r k or arc vaporization (29, 30). S o m e inter esting studies have b e e n m a d e in which s a m p l e is i n t r o d u c e d i n t o t h e I C P with a carbon rod a n d placed into t h e torch in t h e region of t h e p l a s m a coils b u t below t h e p l a s m a itself (31, 32). Direct inductive h e a t i n g of t h e carbon occurs, a n d t h e s a m p l e v a p o rizes directly into t h e p l a s m a . W i t h t h i s system, very efficient t r a n s p o r t of
The Chemist's Catalog f;ld;;gl ldfkldfs f;ld;;gl ldfkldfs
OdfsdB«sse OdfsdB«ss OdfsdB«ss
s a m p l e t o t h e p l a s m a is readily accom plished. However, vaporization is n o t always very rapid, a n d t h e b r o a d emis sion p e a k s t h a t result can s o m e t i m e s lead to poor detection limits. O t h e r devices a i m e d a t obtaining ef ficient s a m p l e transfer of solid a n d liquid s a m p l e s t o t h e I C P h a v e been described recently, including a s y s t e m where t h e rf p l a s m a is led i n t o a c h a m ber below t h e t o r c h for s a m p l e vapor ization (33). M a n y of t h e devices p r e s e n t l y pro posed as a l t e r n a t i v e s t o liquid s a m p l e i n t r o d u c t i o n offer g r e a t promise for specific applications; however, t o achieve w i d e s p r e a d use, t h e y will have t o d e m o n s t r a t e t h e reliability, free d o m from interference, a n d t h e ease of use t h a t liquid s a m p l e i n t r o d u c t i o n c u r r e n t l y enjoys. Finally, t h e r e is al ways t h e possibility t h a t some t r u l y new s a m p l e i n t r o d u c t i o n t e c h n i q u e , with general applicability, will be de veloped. T h e need is certainly t h e r e . Acknowledgment T h i s m a t e r i a l is based on work s u p p o r t e d b y t h e N a t i o n a l Science F o u n dation under Grant No. CHE8019947. References (1) Parsons, M. L.; Forster, Α.; Anderson, D. "An Atlas of Spectral Interferences in
FREE! The Chemist's Catalog SPORTSWEAR, GIFTS AND ACCESSORIES from the American Chemical Society The latest t-shirt—"Keep your ion a chemist!" PLUS brand new duffle bags, sports caps, and coffee mugs. And just for fun—playing cards, 'California Flyers" and (for the kids, of course) yo yo's! Even a handsome new tie with woven silk insignia. And, the same popular items that were sell-outs last year—barbecue/lab aprons, tote bags, baseball shirts and a wide range of t-shirts in children's and adult sizes. Quantity discounts, too! Call (202) 8 7 2 - 4 5 8 8 , write, or mall coupon for free catalog. Education Division, American Chemical Society 1155 Sixteenth Street, N.W., Washington, DC 2 0 0 3 6 Please send me free catalog describing ACS sportswear, gifts and accessories.
BRAND NEW AND EXPANDED!
Name Address _ City/State/Zip _
888 A · ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
ICP Spectroscopy"; Plenum: New York, 1980. (2) Boumans, P.W.J.M. "Line Coincidence Tables for Inductively Coupled Plasma Emission Spectrometry"; Pergamon: New York, 1981; Vols. I and II. (3) Boorn, A. W.; Browner, R. F. Anal. Chem. 1982,54,1402. (4) Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1223. (5) Kull, R., Jr.; Browner, R. F. Spectrochim. Acta Β 1983,38, 51. (6) Smith, D. D. PhD Thesis, Georgia In stitute of Technology, Atlanta, Ga., 1983. (7) Smith, D. D.; Browner, R. F., submit ted for publication in Anal. Chem. (8) Stupar, J.; Dawson, J. B. Appl. Opt. 1968, 7,1351. (9) Novak, J. W.; Lillie, D. E.; Boorn, A. W.; Browner, R. F. Anal. Chem. 1980, 52, 579. (10) Anderson, H.; Kaiser, H.; Meddings, B. In "Developments in Atomic Plasma Spectrochemical Analysis"; Barnes, R. M., Ed.; Heyden and Son: London, U.K., 1981, p. 251. (11) Apel, C. T.; Duchane, D. V. Abstracts of Papers, Pittsburgh Conference on An alytical Chemistry and Applied Spec troscopy, Cleveland, Ohio, 1979. (12) Layman, L. R.; Lichte, F. E. Anal. Chem. 1982,54,638. (13) Olson, K. W.; Haas, W. J., Jr.; Fassel, V. A. Anal. Chem. 1977,49, 632. (14) Boumans, P.W.J.M.; de Boer, F. J. Spectrochim. Acta Β 1975,30, 309. (15) Taylor, C. E.; Floyd, T. L. Appl. Spectrosc. 1981, 35, 408. (16) Berman, S. S.; McLaren, J. W.; Willie, S. N. Anal. Chem. 1980,52, 488. (17) Mermet, J. M.; Trassy, C. In "Devel opments in Atomic Plasma Spectro chemical Analysis"; Barnes, R. M., Ed.; Heyden and Son: London, U.K., 1981, p. 245. (18) Boumans, P.W.J.M.; de Boer, F. J. Spectrochim. Acta Β 1976, 31, 355. (19) Garbarino, J. R.; Taylor, Η. Ε. Appl. Spectrosc. 1980,34, 584. (20) Suddendorf, R. F.; Boyer, K. W. Anal. Chem. 1978, 50, 1769. (21) Mohamed, Ν.; Brown, R. M., Jr.; Fry, R. C. Appl. Spectrosc. 1981,35,153. (22) Kirkbright, G. F.; Gunn, A. M.; Mil lard, D. L. Analyst (London) 1978,103, 1066. (23) Barnes, R. M.; Fodor, P. Spectrochim. Acta Β 1983, 38,1191. (24) Godden, R. G.; Thomerson, D. R. An alyst (London) 1980,105,1137. (25) Greenfield, S. Spectrochim. Acta Β 1983, 38, 93. (26) Tyson, J.; Idris, A. B. Analyst (Lon don) 1981,106, 1125. (27) Carr, J. W.; Horlick, G. Spectrochim. Acta Β 1982, 37,1. (28) Thompson, M.; Goulter, J. E.; Sieper, F. Analyst (London) 1981,106, 32. (29) Human, H.G.C.; Scott, R. H.; Oakes, A. R.; West, C. D. Analyst (London) 1976,102,265. (30) Marks, J. Y.; Fornwalt, D. E.; Yungk, R. E. Spectrochim. Acta Β 1983,38,107. (31) Salin, E. D.; Horlick, G. Anal. Chem. 1979,51,2284. (32) Kirkbright, G. F.; Walton, S. J. Ana lyst (London) 1982,107, 241. (33) Farnsworth, P. B.; Hieftje, G. M. Anal. Chem. 1984,55,1414. (34) Long, S. E.; Snook, R. D.; Browner, R. F., submitted for publication in Spec trochim. Acta B. (35) Fassel, V. Α.; Kniseley, R. N. Anal. Chem. 1974,45,1110 A. (36) Knoller, B. N.; Bloom, H.; Arnold, A. P. Prog. Anal. At. Spectrosc. 1981,4, 81. (37) Nakahara, T. Prog. Anal. At. Spec trosc. 1983, 6,163.
