ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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Direct Current Atmospheric Pressure Argon Plasma Emission Echelle Spectrometer as a Specific Metal Gas Chromatographic Detector R.
J. Lloyd, R. M.
Barnes, P. C. Uden,” and W. G. Elliott
Department of Chemistry, GRC Tower
I, University of
Massachusetts, Amherst. Massachusetts 0 1003
An atmospheric pressure dc argon plasma emission source for an echelle spectrometer has been interfaced for specific element gas chromatographic detection. Details of an interface design are given which enables quantitative transfer of relatively high boiling metal complexes and organometallic compounds without band broadening or sample degradation. Sensitivities in the range 10-10-10-‘2 g s-’of metal are reported and spectral selectivity is discussed for a number of metals and carbon. Gas chromatographic applications are cited to illustrate the applicability of specific element detection and the potential of multichannel operation.
T h e development of gas chromatographic detectors has always followed divergent paths. The need for “universal” detectors arising from t h e great predominance of GC applications to C-H compounds has been largely met by thermal conductivity and flame ionization methods, although each falls short of true universality in particular respects of sensitivity or versatility. When combinations of classical chromatographic parameters of resolution, speed, and capacity have prevented successful application of universal detectors, or when trace levels of specific element species must be determined, “specific” or “selective“ detectors responding solely to single compound classes or elemental compositions are preferred. T h e electron capture, alkali flame, and electrochemical detectors typify this approach. In inorganic gas chromatography, especially with metalcontaining compounds, the characterization of metal content of eluent peaks is very attractive, particularly if high sensitivity is also possible. Spectroscopic detectors have been variously exploited utilizing emission, absorption, and fluorescence for some nonmetals as well as for metals. Sternberg and Paulson ( I ) described a nonselective spectroscopic detector which monitored the intense C2 band emission as organic eluates entered a capillary tube in which was maintained a discharge produced by a Tesla coil leak detector. No filter or monochromator were employed. Juvet and Durbin ( 2 , 3 )used a flame photometric detection system to determine volatile metal chelates of chromium(III), iron(III), and rhodium(II1). Application to GC determination of organometallics has been carried out both in the normal air-rich mode and under fuel-rich conditions ( 4 ) . T h e sensitivity of this detector is strongly dependent on flame conditions and metal emission characteristics, and some metals give sensitivities comparable to flame ionization detection levels while some have much poorer responses. Specific chemiluminescent emission for sulfur and phosphorus compounds was reported by Brody and Chaney ( 5 ) . Problems of reduction of detector response through chemiluminescence quenching by simultaneously eluting interfering species were evident, however, and linearity was lost by self-absorption at high sample concentration levels. A GC interface to an atomic absorption spectrophotometer allowed determination of a range of chromium complexes (6) to na0003-2700/78/0350-2025$01.OO/O
nogram levels; various other applications of GC-AA interfaces have been reviewed (7). Nonflame technology has also been employed for element selective detection. McCormack et al. ( 8 ) used a 2.45-GHz electrodeless, atmospheric pressure argon discharge. Bache and Lisk ($22) improved the design, and developed a helium plasma microwave discharge operating a t 5-10 m m pressure. T h e microwave plasma discharge (MPD) offers similar advantages and disadvantages to the flame photometric detector but eliminates the need to optimize a flame system. Problems occur due t o depositing carbon from solvents on the quartz emission tube, and eluting solvent may extinguish the plasma (13-17). Oxygen scavenger gas may be added to reduce carbon deposits; the effect of this on emission characteristics has been studied. Detection limits to electron capture detection levels have been reported (13,18). However, carrier gas flow rates must be accurately controlled for a stable discharge to be maintained. Furthermore, use of a reduced-pressure discharge adds the complexity of maintaining a constant vacuum at the exit to the gas chromatograph. Such experimental considerations for these conventional microwave emission detectors led to the development of a metal sensitive detector using a commercially available, dc atmospheric-pressure argon plasma as the excitation source, together with a single-channel echelle spectrometer. If general application is envisaged, the ability to employ readily available spectroscopic instrumentation for GC detection is important, and appropriate interface design for quantitative sample transfer is a necessity. We have recently reported on the application of this system to the specific determination of methylmanganese tricarbonyl ( M M T ) in gasoline employing manganese detection (29) and now report on details of the construction of the detection system and its application to a range of metal specific gas chromatographic separations. EXPERIMENTAL Equipment and Interface Design. An atmospheric pressure, dc argon plasma was used as the emission source for the echelle spectrometer (prototype model Spectraspan 111. Spectrametrics, Inc., Andover, Mass.). A Varian 1200 gas chromatograph equipped with flame ionization detection was interfaced with it. Argon, helium, and nitrogen carrier gases were employed. The interface constructed is depicted schematically in Figure 1. The transfer line from the chromatograph to the nozzle below the dc discharge was constructed of 1/16-in.o.d. by 0.016-in. i.d. grade 304 stainless steel tubing. This was placed inside ‘/8-in. o.d. copper tubing to facilitate even heat transfer throughout the transfer line. The copper tubing was in turn wrapped with glass electrical tape (Scotch No. 69. 3M Company) to provide electrical insulation. A type K thermocouple was placed within the transfer line and electrically insulated with glass tape. The transfer line was heated by 26-gauge, glass-insulated. nichrome heating wire wrapped tightly around the copper tube with minimal separation between the wires. The entire transfer line was then wrapped with glass electrical tape for further insulation, and flexible silicone rubber tubing (Ja-Bar Silicone Corp., Andover, N.J.) was placed around the transfer assembly to provide overall thermal insulation. The sheath gas preheater was also constructed. A 5-ft length of 1/4-in.o.d. copper tubing was filled with 20/60-mesh copper 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 Nozzle Ceramic Insulator Electrode
,Electrode
Holder
Epoxy Glass Base Positioning Block
Preheater
Sb5A-H
From G C r i ac
Flgure 1. Diagram of nozzle, sheath gas preheater, and transfer line
from gas chromatograph to dc argon plasma filings which were retained with glass wool plugs. This tubing was then wrapped between the individual heat dissipation fins of a 110-V, 250-W cartridge heater. The entire assembly was insulated with glass fiber, wrapped with aluminum foil and glass tape for rigidity. Since flowing argon removes heat from the preheater, the voltage needed to maintain a specific equilibrium temperature is a function of flow rate. To minimize this dependence and to reduce the thermal lag time of the preheater, a thermostat was placed inside the assembly and set at 250 "C, which corresponded to the temperature found most suitable in this study. This system maintained a sheath-gas temperature of 230 "C at the nozzle outlet. Some heat loss occurred as the gas travels to and out of the nozzle. The outlet side of the argon sheath gas preheater was connected to the sidearm of a 1/4-in. Swagelok tee fitting, and the transfer line was attached to the base of the tee. The nozzle was fitted concentrically within the sheath gas tube which was connected to the top of the tee fitting. The temperatures of transfer line and sheath gas preheater were individually regulated by variable voltage transformers. The epoxy glass base of the dc plasma source was modified to allow the accurate positioning of the nozzle and effluent gas stream relative to the dc plasma discharge. The nozzle was held in place by three set screws mounted in the epoxy glass board. The gas chromatographic columns used were 6 f t X in. 0.d. stainless steel. Dexsil300 GC and SE 30 methylsilicone stationary phases were employed on Chromosorb 750, 100-120 mesh (Johns-Manville Corp., Denver, Colo.) or other diatomite substrates. Argon or helium were used as carrier gases, and flame ionization was employed for nonselective detection. The effluent from the column was split 1:1 between flame ionization and dc plasma. A delay line to the former was constructed of a length of in. 0.d. X 0.016 in. i.d. stainless steel tubing equal to that employed in the plasma transfer line. Typical operating conditions for the dc plasma were argon sheath gas flow rate, 3.0-3.5 standard cubic feet per hour (SCFH); argon cathode gas flow rate, 2.1 SCFH; argon anode gas flow rate, 2.1 SCFH, current, 7.4; voltage, 40-60 V. The voltage depended on the condition of the electrodes; since the source is a constant current device, the plasma resistance determined the actual voltage. Spectrometer entrance and exit slits were 100 Fm x 200 pm. A chopper in front of the entrance slit was employed with a chopping frequency of 99 Hz. An amplifier time constant between 1 and 3 s was used. Photomultiplier tubes R446 and 1P28 run at 450-750 V were employed. Chemicals. The metal compounds chosen for investigation represented typical volatile metal chelates and organometallics previously separated by gas chromatography. Chromium(II1) trifluoroacetylacetonate Cr(TFA), was prepared by standard methods (20). Copper(I1) N,N-ethylenebis(trifluoracety1acetoneimine) (Cu(enTFAz)), its nickel(I1) and palladium(I1) analogues, (Ni(enTFA2) and Pd(enTFA,)) and the analogous propylene derivative compounds (Cu(pnTFA,), Ni(pnTFA,)) were prepared and purified as noted previously (21). Benzenechromium tricarbonyl was purchased (Strem Chemical Inc., Danvers, Mass.),
C A S F,S\',
5Zc-,
Figure 2. Plot of the relative unit response of the dc argon plasma detector to chromium in Cr(TFA), vs. the flow rate of the heated sheath
gas. Chromium emission measured at 267.7 nm and plasma zone measured reoptimized at every flow rate and cyclopentadienylchromium dicarbonylnitrosyl was obtained from E. Mintz and M. D. Rausch (22). Zinc bis(diethy1dithiocarbamate) (Zn(DTCz))was prepared by an established procedure (23).
RESULTS AND DISCUSSION Parameter Optimization. Volatile chromium chelates have been among the metal compounds most extensively studied by gas chromatography (24). Since chromium is also a suitable test element for emission spectroscopic analysis, Cr(TFA)3was employed in initial evaluation of the dc plasma system as a metal specific detector. In early experiments, the interface was operated without a sheath gas flow. Under these conditions the emission below the intersection (the inverted V) of the plasma was very diffuse, and thus t h e sensitivity was poor. Sheath gas introduced to flow concentrically around the nozzle prevented excessive diffusion as the sample travelled from the nozzle to the plasma. Presence of the sheath gas appeared to increase the residence time of sample within the discharge region by preventing its rapid escape around the edges of the plasma, and t o minimize the quantity of entrained air introduced into the emission volume, thereby reducing background emission. T h e variation of the sheath gas flow rate on detection response is shown in Figure 2. At high sheath gas flow rates, the plasma was disturbed by turbulence and easily extinguished. Heating t h e sheath gas to approximately 225 "C aided in preventing condensation of gas chromatographic eluents on the tip of the nozzle. Sheath gas was employed in all subsequent investigations. T h e position of the sample entry nozzle relative to the discharge was found to affect sensitivity. Directing the sample toward the cathode electrode side enhanced response whereas pointing it toward the anode electrode side diminished the response. The discharge was positioned to image the selected discharge region on the spectrometer entrance slit. Furthermore, the volume of primary analyte emission was below the main arc column, and the best signal-to-noise ratio was always achieved by monitoring this region. Since the nozzle was aimed at this area, spectrometer alignment with the discharge was optimized whenever electrodes were repositioned or changed. The discharge source used zirconiated tungsten electrodes placed concentrically inside ceramic tubes. Electrode erosion requires slight periodic adjustment of the position of either t h e electrodes or viewing location. Operation of the detection system was satisfactory over gas flow rates between 10 and 100 mL min-' typically encountered in gas chromatography. Various carrier gases were found t o be equally suitable; argon, helium, and nitrogen were studied although the latter could not be used if cyanogen bands created spectral interference or if nitrogen selectivity was required.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 3
a!
2
4
3
6
cc P
2
4
6
E
2027
C
a : DC P
i
0
2
4
6
TIVQ (Mln)
Figure 3. Comparative detection of Cr(TFA,) (fac and mer unresolved) by flame ionization and dc argon plasma detection. Column: 3% Dexsil 300GC on Chromosorb 750. Column TemDerature: 170 OC. Helium flow: 60 mL/min
Figure 4. Calibration curve for chromium metal (injected as Cr(TFA),) using dc argon plasma detection
Sensitivity and Spectral Selectivity. The sensitivity and spectral selectivity were investigated for chromium emission from Cr(TFA)3. Gas chromatographic conditions are noted in Figure 3. T h e transfer line to the nozzle was maintained at 265 "C to minimize band spreading in the interface. T h e dual detector chromatogram is given in Figure 3. With a 1:l effluent split, the responses are offset for clarity. T h e flame ionization response for chloroform solvent and the imperfectly resolved Cr(TFA)3 isomers is given at the bottom. The plasma response monitoring t h e chromium emission a t 267.7 n m is recorded at the top. No response in Cr emission was observed for 1 pL of solvent passing into the plasma, while 54 ng of chromium gave a full scale detector signal (1 mV). The selectivity for chromium was at least lo7, the form of the definition used being t h a t at least 10 mg of a n-alkane hydrocarbon having similar elution characteristics would be required t o generate a peak area response equal to t h a t of a metal compound containing 1 ng of chromium. The identical peak profiles recorded for both detectors confirms the integrity of the plasma interface and t h e absence of hot or cold spots in i t which might cause degradation or condensation. T h e quantitative response of the dc plasma detector is represented in Figure 4. A linear range from 2 to 150 ng of chromium is illustrated. T h e nonlinearity above 150 ng was attributed to saturation of the photon counting detection electronics, and the linear range was subsequently extended to 500 ng by using current amplification and conversion to a n analog voltage. T h e detection limit (at S/N = 2) was 3.4 x lo-" g Cr s-l for a peak width of 60 s.
C
2
4
6
3
1
3
Tlmz ( ' + I - )
Figure 5. Chromatographic separation of Cu(enTFA,) and n-tetracosane in. column of 3 % Dexsil 300 GC on 100/120 mesh on a 6 ft X Chromosorb 750 showing comparative responses with flame ionization and dc argon plasma detection. Column temperature, 220 OC
An important practical feature enhancing the utility of the detector is that solvent volumes as great as 100 pL do not extinguish the discharge. This enables the useful range of sample component concentration t o be extended. Also, the absence of a confining quartz capillary tube found in microwave discharges prevents carbon deposition formation from organic solvents. Similar selectivity was observed for copper in t h e gas chromatography of Cu(enTFA,). This complex is more chromatographically difficult than C'r(TFA)3 since if t h e temperature falls below about 215 "C in the interface, the complex will partially condense and result in peak broadening or reduced response. Since above 260 "C some thermal decomposition may occur, a nozzle temperature of about 225 "C and a transfer line temperature of 250 "C must be maintained. A dual detection chromatogram of 41 ng of copper as the complex is presented in Figure 5. n-Tetradecane was included in this sample because it is sometimes an internal reference for quantitative flame ionization detection of similar chelates (25). The column conditions chosen give incomplete separation illustrated in the flame ionization trace. The D C P selectivity of a t least lo7 provides no interference from either solvent (benzene) or n-tetradecane. The detection limit for copper was calculated as 5.6 X g s-l for a peak width of 120 s. Although simultaneous multielement detection was not investigated, since additional electronics and readout systems were not available for the echelle spectrometer, this mode of operation was simulated through successive identical injections of a mixture of Cu(pnTFA,), Ni(pnTFA,), and Pd(pnTFA,). Wavelengths of 324.7 nm, 341.4 nm, and 340.5 nm, respectively, were monitored and the chromatograms are shown in Figure 6. The copper and nickel chelates were not resolved by the Dexsil 300GC column used under these conditions. However, both are readily quantitated using the DCP detector a t the appropriate wavelength. The sensitivity and selectivity toward carbon is of practical importance since the possibility exists of using DCP carbon detection as an alternative to flame ionization detection in a dedicated plasma emission detection system. If multichannel detection including carbon were available, effluent splitting would not be required. The carbon emission response a t 247.8 n m was compared with flame ionization response for a series
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
7
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2C P
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7
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8
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Figure 8. Chromatographic separation of Cu(pnTFA,), Ni(pnTFA,), and Pd(pnTFA,) on a 6 ft X 1 / 8 in. column of 2.5% Dexsil 300 GC on 100/120 mesh Chromosorb 750 with dc argon plasma detection for carbon (247.8 nm) 2
4
8
'
2
c i 4
r
2 4 .
,
Time (Mir,)
Figure 6. Chromatographic separation of Cu(pnTFA,), Ni(pnTFA,), and Pd(pnTFA,) on a 6 ft X in. column of 2.5% Dexsil 300 GC on 100/120 mesh Chromosorb 750 showing comparative detection with flame ionization and dc argon plasma detection for Cu, Ni, and Pd. Column temperature, 230 OC 2
2 4
E.
c
4
2
a
T ' r 2 i W n !
Figure 7. Chromatogram of n-decane, n-dodecane, n-tetradecane, and n-hexadecane on a 6 ft X in. column of 10% SE-30 on 60180 mesh Gas Chrom S showing comparative detection with flame ionization and dc argon plasma detection. Column temperature, 170 OC
of n-alkane hydrocarbons (Figure 7). The relative peak heights and areas of the chromatographic peaks from both detectors corresponded exactly. This indicates the validity of quantitation with the DCP carbon mode. Under the conditions employed, however, the detection limit for ntetradecane corresponded to an injection of 0.85 pg of carbon, thus making it more equivalent in sensitivity to a thermal conductivity detector. The DCP carbon monitor is valuable for many analytical applications. The separation of the copper, nickel, and palladium chelate mixture depicted in Figure 6 is shown with the carbon monitor, for example, in Figure 8. The relative peak shapes in Figures 6 and 8 correspond exactly. T h e necessity for periodic adjustment of the position of the electrodes or viewing location because of electrode erosion gives limited absolute long term precision. However, the use of an internal reference eluate containing the measured element has allowed relative standard deviations between 0.8 and 3.470,depending on the state of the electrodes, efficiency of gas flow control, and other operational factors. Sample Integrity. An important feature in the gas chromatography of any metallic compound lies in the proof of the integrity of the eluted species. Metallic compounds are
r-e
/bdlP\
Figure 9. Chromatograms of benzenechromium tricarbonyl and cyin. column clopentadienyl chromium dicarbonyi nitrosyl on a 6 ft X of 3.2 % Dexsil 300 GC on 100/ 120 mesh Chromosorb 750 showing comparative detection with flame ionization and dc argon plasma detection. Column temperature, 190 OC
frequently prone to anomalous column behavior. Spurious peaks may appear if rapid decomposition occurs in the injector or early in the column, while pronounced leading and tailing may occur if degradation takes place more slowly as the band moves through the column. Additionally, adsorption effects are often more pronounced with metallic compounds than with most organic compounds. In many cases, decomposition products which are detectable by flame ionization or thermal conductivity detectors do not contain the metal present in the initial compound. These nonselective detectors do not provide this sometimes valuable distinction. In contrast, information given by specific metal detection, both in terms of the presence of the metal and the shape of the peak may be most valuable in studying anomalous column behavior. Comparison chromatograms for two organometallic chromium compounds are shown in Figure 9. Benzenechromium tricarbonyl is known to be stable to gas chromatography (26),but would show degradation in the transfer line should hot spots be present or catalytic degradation evident. Cyclopentadienylchromium dicarbonylnitrosyl is a new compound the gas chromatographic integrity of which had not been substantiated. DCP detection for chromium proved both compounds to be eluted with no decomposition, and the rapidly eluting peak of the latter compound was shown not
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
3
2
4
6
8
T i m e (Min)
Figure 10. Chromatogram of zinc bis(diethy1dithiocarbamate) on a 6 in. column of 2.5% Dexsil 300 GC o n 100/120 m e s h Chromosorb 750 showing comparative detection with flame ionization ft X
a n d d c argon plasma detection. Column temperature, 2 8 0
O C
Table I. Detection Limits and Wavelengths Measured for Gas Chromatographic dc Argon Plasma Detection wavelength, detection limit, element nm g s-' chromium copper
nickel palladium carbon
267.7 324.7 341.4 340.4 247.8
3.4 x 10." 5.6 x l o - ' * 3.2 x l o - ' ' 1.2x 2.8 X
to result from injection port degradation as had been suspected. Sensitivity for these chromium organometallics also proved identical to t h a t for Cr(TFA),. T h e analogous cyclopentadienyl molybdenum compound was also successfully eluted and detected in the molybdenum specific mode (27). A number of metal dialkyldithiocarbamates have been separated gas chromatographically under suitable conditions (23,28)although high temperatures are required for elution. Quantitation a t low levels has yet to be substantiated. Peak tailing is also often found as illustrated for Zn(DTC), in Figure 10. Flame ionization detection cannot indicate whether the peak tailing of the zinc diethyldithiocarbamate results from on-column degradation or from adsorption and related nonideal behavior of the complex on the column. Further, for a compound like this which is separated with difficulty, the peak tailing might result from a low temperature region between column and flame ionization detector. Identical profiles obtained for flame ionization and zinc specific plasma detection indicated that the complex was not degraded on the column nor was peak broadening in the interface a factor in peak shape. Adsorption factors on the column are almost certainly the cause of the tailing peak. Detection Limits. For the four metals and carbon for which detection limit studies have been carried out in some detail, data are presented in Table I. Comparison with data reported by Skogerboe (29) for the DCP with aqueous solution samples indicates lower observed detection limits in terms of sample element per unit time for the gas chromatographic analysis. Lower background noise level with vapor rather than the liquid introduction may explain this observation. CONCLUSIONS T h e advantages inherent in complexation of analyte elements to form a volatile species free of matrix effects are
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important in considering analytical comparisons between standard solutions and unknown samples. Extraction steps remove other chromatographic interferences and can also serve to concentrate the sample. The use of element-specific spectroscopic detection eliminates the need for complete chromatographic resolution, and the potential of simultaneous multichannel analysis may serve to further enhance t h e practical utility of the gas chromatographic emission spectroscopic combination. T h e atmospheric pressure dc argon plasma gas chromatographic detection system has several major advantages over other specific element detection systems. In contrast to microwave emission detectors, the DCP is not restricted to the use of carrier gas for the discharge, nor is confinement necessary in a capillary tube wherein pyrolytic carbon, or metal, or metal oxide deposits may build up to impair performance. T h e present DCP is limited by air entrainment which would give interference if oxygen or nitrogen were determined selectively. T o excite these elements, more power in the plasma also is needed to enable helium to be used as the plasma gas instead of argon. T o accomplish this, air entrainment could be overcome by building a small chamber around the plasma and passing through it a rapid flow of dry helium. The DCP can accommodate high concentrations of organic material such as solvents containing the species of analytical interest, and high carrier gas flow rates are practical. ACKNOWLEDGMENT Helpful discussions with Carol A. Poirier are greatly appreciated. LITERATURE CITED (1) J. C. Sternberg and R. E. Paulson, J . Cbromatogr., 3, 406 (1960). (2) R. S. Juvet and R. P. Durbin, J . Gas Cbromatogr., 1, 14 (1963). (3) R. S. Juvet and R. P. Durbin, Anal. Cbem., 38, 569 (1966). (4) W. A. Aue and H. H. Hill, Jr., Anal. Cbem., 45, 729 (1973). (5) S. S. Brody and J. E. Chaney, J . Gas Cbromatogr., 4, 42 (1966). (6) W. R. Wolf, Anal. Cbem., 48, 1717 (1976). (7) F. J. Fernandez, Cbromafogr. News/., 5 (2), 17 (1977). (8) A. J. McCorrnack, S. C. Tong, and W. D. Cooke, Anal. Cbem.,37, 1470 (1965). (9) C. A . Bache and D. J. Lisk, Anal. Cbem , 37, 1477 (1965). (10) C. A. Bache and D. J. Lisk, Anal. Cbem., 38, 783 (1966). (11) C. A. Bache and D. J. Lisk, Anal. Cbem., 38, 1757 (1966). (12) C. A. Bache and D. J. Lisk, Anal. Cbem , 39, 786 (1967). (13) R. M. Dagnall, T. S. West, and P. Whitehead, Analyst(London),98, 647 (1973). (14) K. Kawaguchi, T. Sakamoto, and A. Mizuike, Talanfa, 20, 321 (1973). (15) F. A. Serravalio and T. H. Risby, J . Chromatog. Sci., 12, 585 (1974). (16) M. S. Black and R . E. Severs, Anal. Cbem., 48, 1872 (1976). (17) T. Sakamoto, H. Kawaguchi, and A. Mizuike, J . Cbromatogr., 121, 383 (1 976). (18) F. A. Serravalio and T. H. Risby, Anal. Cbem., 47, 2141 (1975). (19) P. C. Uden, R. M. Barnes and F. P. DiSanzo, Anal. Cbem., 50, 852 (1978). (20) R. C. Fay and T. S. Piper, J . Am. Cbem. Soc , 84, 2303 (1962). (21) R. Belcher, K. Biessel, T. J. Cardwell, M. Pravica, W. I . Stephen, and P. C. Uden, J . Inorg. Nucl. Cbem., 35, 1127 (1973). (22) E. Mintz, Ph.D. Dissertation,University of Massachusetts, Amherst, Mass., 1978. (23) T. J. Cardwell, D. J. Desarro, and P. C. Uden, Anal. Cbim. Acta, 85, 415 (1976). (24) P. C. Uden and D. E. Henderson, Analyst(London). 102, 1221 (1977). (25) R. Belcher, R. J. Martin, W. I. Stephen, D. E. Henderson, A. Kamalizad, and P. C. Uden, Anal. Cbem., 45, 1197 (1973). (26) H. Veening, N. J. Graver, D. B. Clark, and B. R. Willeford, Anal. Cbem., 41, 1655 (1969). (27) P. C. Uden, R. M. Barnes, R. J. Lloyd, C. A . Poirier, and B. D. Quimby. "Canadian Chromatography Conference Proceedings (1978)",Chapter 17, Marcel Dekker, New York, in press. (28) J. KruDcik. J. Garai. S. Holotik. D. Oktavek. and M. Kosik. J . Chromatour.. 112, i s 9 (1975) (29) R K Skogerboe I T Urasa, and G N Coleman Appl Specfrosc , 30, 500 (1976)
RECEIVED for review July 31, 1978. Accepted September 27, 1978. Work supported in part by t.he National Science Foundation Grant CHE73 05201.
