2818
Anal. Chem. 1988, 60, 2818-2821
control applications, by providing a valuable piece of information that may be monitored; effective molecular size of polymers in mixtures. The flow method was linear with injected polymer mass for over 2 orders of magnitude, making qualitative analysis attractive. As a tool for theoretical studies, the flow method offers a viable alternative to either light scattering or ultracentrifugation for providing molecular size information,as correlated to translational diffusion properties of macromolecules. The capability of the device to study very dilute solutions, a 5 X to about 1 X lo4 g mL-' injected solution concentration range, allows one to assume "infinite" dilution conditions for many applications. Recent improvements in the RIG detector design, employing a position sensitive detector, have reduced the required injected concentration by about a factor of 5 (37). The RIG detector may also prove useful if interfaced with field-flow fractionation (FFF) (13-15). Process analysis application of the RIG flow method includes the ability to accurately measure changing polymer polydispersity (2-4). The RIG flow method appears to be an exciting analytical technique that should find wide interest and applicability.
ACKNOWLEDGMENT Valuable discussions with B. E. Eichinger were appreciated.
LITERATURE CITED (1) Smlth, C. G.; Nyquist, R. A.; Mahle, N. H.; Smith, P. B.; Martin, S. J.; Pasztor, A. J. Jr. Anal. Chem. 1987, 5 9 , Il9R-141R. (2) Batzer, H.; Lohse, F. Introduction to Macromolecular Chemistry; WIley: New York, 1979; Sectlon C. pp 147-200. (3) Koenig, J. L. Anal. Chem. 1987. 59. 1141A-1155A. (4) Hlrschfleid, T.; Callls, J. B.; Kowalskl, B. R. Sclence 1984, 226, 312-31 8. (5) Callls, J. 8.; Illman, D. L.; Kowalskl. B. R. Anal. Chem. 1987, 5 9 , 625A-637A. (6) Ouano, A. C.; Kaye, W. J . Polym. Sci. 1974, 12, 1151-1162. (7) Kaye, W. Anal. Chem. 1973, 45, 221A-225A. (8) Ouano, A. C. J . Chromatogr. 1978, 118, 303-312. (9) Qruska, E.; Maynard, V. R. J . h y s . Chem. 1973, 7 7 , 1437-1442. (10) Qrwka, E.; Klkta, E. J., Jr. J . h y s . Chem. 1974, 7 8 , 2297-2301. (11) Gruska, E.; Klkta, E. J., Jr. J . Am. Chem. Soc. 1978, 9 8 , 643-648. (12) Katz, E. D.; Scott, R. P. W. J . Chromatogr. 1983, 270, 29-50.
(13) Giddlngs, J. C. J . Chem. phvs. 1988, 49, 81-85. (14) Klrkland, J. J.; Yau, W. W. Sclence 1982. 218. 121-127. (15) Caldwell, K. D. In Chemical Analysk: Modem Methods of fartlcle Slze Analysis; Barth, H. G., Ed.; Wlley: New York, 1984; Vol. 73, Chapter 7. (16) Roovers, J. E. L.; Bywater, S. Macromolecules 1972, 5 . 384-388. (17) Roovers, J. E. L.; Bywater, S. M a c m k u l e s 1974, 7 , 443-449. (18) Bywater, S. In Advances In Pdymer Sclence; Cantow, H. J., et al., Eds.; Springer-Verlag: New York, 1979; Vol. 30, pp 89-116. (19) Bauer, 8. J.; Hadjlchristidls, N.; Fetters, L. J.; Roovers, J. E. L. J . Am. Chem. SOC. 1980, 102, 2410-2413. (20) Roovers, J.; Hadjlchrlstidls, N.; Fetters, L. J. Macromolecules 1983, 16. 214-220. (21) Klein, J.; Fletcher, D.; Fetters, L. J. Faraday Symp. Chem. Soc. 1983, 18, 159-171. (22) Huber, K.; Burchard, W.; Fetters, L. J. Macromolecules 1984, 17, 541-548. (23) Edwards, C. J. C.; Kaye, A.; Stepto, R. F. T. Macmmolecules 1984, 17, 773-782. (24) Zlmm, 8. H. Mecromo/ecules 1984, 17, 795-798. (25) Renn, C. N.; Synovec, R. E. Anal. Chem. 1988, 60, 200-204. (26) Tljssen, R.; Bos, J.; VanKreveld, M. E. Anel. Chem. 1988. 58, 3036-3044. (27) Hancock, D. 0.;Synovec, R. E. Anal. Chem. 1988, 6 0 , 1915-1920. (28) Pawllszyn, J. Anal. Chem. 1986, 5 6 , 243-246. (29) Pawllszyn, J. Anal. Chem. 1988, 58, 3207-3215. (30) Pawllszyn, J. Anal. Chem. 1988, 80, 766-773. (31) Golay, M. J. E.; AtWood. J. G. J . ChrOmtOgr. 1979, 186, 353-370. (32) Atwood, J. G.; Golay, M. J. E. J . Chromatogr. 1981, 218, 97-122. (33) Hupe, K.-P.; Jonker, R. J.; Rozing, G. J . Chromatogr. 1984, 285, 253-265. (34) Garell, P.; Rosset. R. J . Chromatogr. Sci. 1985. 2 0 , 367-371. (35) Rocca, J. L.; Hlgglns, J. W.; Brownlee. R. G. J . Chromatogr. Sci. 1985, 23, 106-113. (36) Flow Cytometry and Sorting, Hydrodynamlc Roperties of Flow Cytometric Instruments; Kachel, V., et ai., Eds.; Wlley: New York, 1979; Chapter 3. (37) Hancock, D. 0.; Synovec, R. E., unpubllshed results. (38) &oh, R.; Hallsz, I. Anal. Chem. 1981, 53, 1325-1335. (39) Cantor, C. R.; Schimmel, P. R. Biophysical CheMtry: Technkpes for the Study of Blobgical Structure and Function; W. H. Freeman: New York, 1980; Part 11. Chapter 10. (40) Merck Index, loth ed.; Merck and Co.: Rahway, NJ, 1983; no. 8732. (41) Barth, H. G.; Carlln, F. J., Jr. J . Liq. Chromatogr. 1984, 7 , 1717-1738.
RECEIVED for review May 9, 1988. Accepted July 18, 1988.
D.O.H.and R.E.S. thank the NSF Center for Process Analytical Chemistry for support of this work (Project Number 86-2).
CORRESPONDENCE Microwave Induced Plasma Atomic Absorption Spectrometry with Solution Nebulization Sir: In the earlier inductively coupled plasma (ICP) dev loping research, the ICP had been investigated for atomic a sorption spectrometry (AAS) by Wendt and Fassel (11,by Greenfield et al. (2),by Veillon and Margoshes (3), by Morrison and Talma ( 4 ) , by Mermet and Trassy (5), and by Bordonali and Biancifiori (6). A multiple pass system (I), a T-shaped plasma cell (2),and a long-path torch (5)have been used to increase the absorption path length. Bordonali and Biancifiori (7) also received a patent in 1972 covering the analysis of trace elements by ICP-AAS. However, researchers of the ICP field supported and verified that the energetic plasma was more suitable for atomic emission spectrometry (AES). In an attempt to reduce spectrum complexity, researchers such as Winefordner et al. (8) have performed atomic fluorescence spectromery (AFS) on the ICP. In light of the spectral interference in ICP-AES, Greenfield (9) has recently
!
recalled attention to ICP-AFS and ICP-AAS. The ICP-AAS technique has been performed by Magyar and Aeschbach (lo), by Downey and Nogar ( I I ) , and by Gillson and Horlick (12). Like flame AAS, ICP-AAS exhibits high selectivity, and because of the energetic argon plasma, sample atomization is complete and proceeds without chemical interferences (10). The ICP-AAS system is suggested for use with complex samples, in which high selectivity is desired without the concern for sensitivity (10). These researchers have found the ICPAAS to give high detection limits and low sensitivity (10-12), and they have attributed these disadvantages to the very short absorption path length, which is several times shorter in the plasma than in an AA flame. Mermet and Trassy (5) have addressed this difficulty in their design and construction of an ICP torch for AAS. The ICP-AAS also is useful in some physical measurements for the plasma. These ICP-AAS and ICP-AFS investigations have indicated that there is an
0003-2700/88/0360-2818$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 24, DECEMBER 15, 1988
2810
Table I. Experimental Components component
model/size
manufacturer
microwave cavity microwave generator coaxial cale tuning stubs discharge tubes spectrometer glass-frit nebulizer peristaltic pump pneumatic nebulizer spray chamber support gas chart recorder hollow cathode lamps
Modified Beenakker TM(010), aluminum, 20 mm deep, 89 mm dia Model MPG 4 M (0-120 W) RG 214/U (50 Q) Model 1878B 99.8% alumina, -55 mm long, 5 mm i.d., 7 mm 0.d. Model 82-500 20 mm disk diameter, fine porosity Masterflex variable speed drive, motor, pump head, and tubing (0.8 mm i.d.) concentric glass (low uptake rate) ARL style (163 mm) 99.5% Ar Model 355 multielement (Ca, Mg, Al, Li) multielement (Cr, Fe, Mn, Ni, Co, Cu) single element (Cu)
laboratory built Opthos Instrument, Inc. consolidated Maury Microwave, Inc. Coors Porcelain Co. Jarrell-Ash Corning, lab bulit Cole-Parmer Spex Industries, Inc. Spex Industries, Inc. Liquid Air Corp. Linear Instruments, Inc. Westinghouse Beckman Westinghouse
abundance of ground-state species in the plasma. Recently, the technique of ICP-IAS (ion absorption spectrometry) has been examined by Umemoto and Kubota (131,who found that the detection limits are comparable to those of the ICP-AES. Recently, Liang and Blades (14) have developed and reported a capacitively coupled plasma for AAS and obtained comparable detection limits to those of the graphite furnace AA. The system consists of an conventional ICP generator and an electrothermal vaporization device for sample introduction. Microwave induced plasmas (MIPS)can be generated at the 2450-MHz range with argon (15), helium (16),nitrogen air (I@, or combinations of gases (19). The conventional MIP is operated with less than 120-W power levels and a gas flow rate of less than 1L/min. In contrast, the ICP operates typically a t 1kW and 15 L/min argon flow. Therefore, the MIP is less expensive both for the instrumentation and for the operation costs. In the past, the low-poweredMIP has been used successfully for vapor and gaseous samples. Its use on liquid samples was difficult until the development of the Beenakker cavity (20). This system allowed liquid aerosols to be nebulized directly into the atmospheric discharge. Recent studies on AES with the solution nebulization into the MIP (105 W) by Ng and Shen (15)have shown detection limits comparable to those of the ICP. Long and Perkins (21)have succeeded in using an even lower power (36 W) argon MIP-AES for solution nebulization although the system gives low sensitivity. The aforementioned MIP works were performed on AES. The low-powered MIP might be suitable also for AAS. This paper describes a solution nebulization MIP-AAS system for trace element analysis, using conventional hollow cathode lamps and an AA spectrometer.
(In,
EXPERIMENTAL SECTION Atomic Absorption Plasma Torch. The torch was designed to possess the following characteristics. First, it must allow the hollow cathode lamp radiation to pass through. Second, it must withstand the high temperature of a plasma. Third, the path of the nebulized liquid from nebulizer to plasma must be minimal to prevent condensation. Fourth, the torch must be demountable, easily constructed, and inexpensive. The torch design is shown in Figure 1. The discharge tube is alumina, which is mounted to the torch base with a Swagelok nut (point 8) equipped with a Velcore plastic ferrule (point 7), to a drilled out Swagelok union (point 6). The Velcore ferrule was selected for a superior fit to the alumina tube as compared to a typical brass ferrule. The Velcore has a melting point of 400 "C. The union was drilled to allow the placement of the sample delivery tube (point 5), which was soldered in place. The union is then connected to a reducing adaptor (point 4) which has been drilled to accept the shielding gas delivery tube (point 5) which is soldered in place. The shielding gas protects the quartz optical
1
2 3
4
6
7
0
9
5
Figure 1. Construction (not drawn to scale) of the microwave induced plasma torch for atomic absorption spectrometry: (1) window crush nut (10 mm hole); (2) neoprene O-rings (14.3 mm 0.d.; 9.5 mm i.d.); (3) quartz window (14.5 mm diameter, 2.3 mm thick): (4) in. pipe to 3/e in. pipe adaptor (8.5 mm i.d.); (5) in. i.d. copper tubes sddered in place; (6) Swageiok in. pipe to '/, in. tube union (7 mm 1.d.); (7) Veicore plastic ferrule (mp 400 OC); (8) cover nut; (9) aluminum oxide discharge tube (4-5 mm i.d., 6.5-7 mm o.d., 55-60 mm long).
window (point 3) from liquid condensation that might otherwise settle there. These tubes are oriented at a right angle to the torch, creating laminar flow in the torch. The end of the adaptor (point 4) has been polished smooth so that a good seal to the "0" ring (point 2) is facilitated. The quartz optical window (point 3) is sealed by the same "0"ring and cushioned from the crush nut (point 1)by another "0"ring. The quartz window was cut from a spent hollow cathode lamp. In the crush nut a 1cm diameter hole was machined at ita axis to allow the hollow lamp radiation to exit. Components. The experimental components used are listed in Table I. The MIP cavity was similar to the one published earlier (15,19) but was 2 cm deep. The glass frit nebulizer was constructed according to Nisamaneepong et al. (22). The single-element copper lamp was used for convenience at the time. No performance comparison was made between multielement and single-element lamps for copper. Torch and Cavity Mountings. To mount the torch to the cavity, the Swagelok nut (point 8 of Figure 1)was pressed into the axial hole in the back of the MIP cavity with a 0.1 in. under size. This mounting method both centered the torch and dissipated heat accumulated by the torch to the water-cooled cavity base. This method proved to be effective;in fact, the torch body was cool to the touch even after several hours of operation. The MIP cavity and connected torch were mounted into the spectrometer (Figure 2). This was accomplished with the aid of three small brackets that were bolted to the existing burner adjustment arm. By the interaction of the four adjustments it was possible to optimize the hollow cathode beam that passed through the MIP-AAS equipment. System Setup and Experimental Conditions. The setup is also shown in Figure 2 and the solution nebulizer is the glass frit. Optimization for the Glass-Frit Nebulizer System. Power. The MIP operating power was varied from 65 to 105 W (highest obtainable power). Although the strongest signals were obtained with a forward power of 80-90 W, full power operation (105 W) gave the best signal-to-noise ratio. The particular MIP cavity used in the experiment gave higher reflected powers (with or without introducing liquid aerosols) at lower power operations,
2820
ANALYTICAL CHEMISTRY. VOL. 60. NO. 24. DECEMBER 15, 1988 WATER
1 .W.?+O
,
A
8c m
-9
3
4.00e-1
.a d 2.ooe-l 2.71e-20
0
EmLl
60
SO
100
120
Concentration (PPM)
..
Flgure 2. Solution nebullzalion, mlcrowave induced plasma atomlc absorpaon spactromeby system: (1) Mow cathode lamp housing; (2) locuslng optic; (3) burner position adjuster; (4) MIP cavity adaptor/ adjuster; (5) MIP cavity; (6) AA torch; (7) focusing optic; (8) moncchromatu; (9)optical rail: (10) thrwstub tuner: (11) glass frit nebulizer: (12) S hieCJ gas Rowmeter: (13) drain (cW atnwsptmre):(14) nebuliiw gas flowmeter;(15) pump speed wntroller; (16) perlstaiiic pump: (17) sample: (18) high-pressure argon tank. Fw the pneumatic nebulizer 15. and
16.
Table 11. Experimental Conditions for the Solution Nebulization, MIP-AAS Systems GFND
40
Calibration culves A, B, and Care fw calclum (422.7 nm). manganese (279.5 nm). and copper (324.7 nm), respectively. wlth the glass-frit nebulizer, MIP-AAS system. Calibration curve D is fw manganese (279.5 nm) with the concentric glass nebulizer, MlP-AAS system. Flgure 3.
17
system, the wncentric glass nebulizer will replace 11, 13.
20
CGN'
microwave frequency, MKz 2450 2450 forward power, W 105 105 reflected power, W
