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Determination of Polychlorinated Dibenzo-p -dioxins Using Capillary Gas Chromatography with Microwave-Induced Plasma Detection Ahmad H. Mohamad, Mantay Zerezghi, a n d Joseph A. Caruso* Department of Chemistry] University of Cincinnati, Cincinnati, Ohio 45221
An lnltlal lnvestlgatlon uslng a low-power (20 min) unless conditions were changed. By use of the listed conditions, an excellent chromatogram of the six isomers was
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with the GC flow and importantly increase analyte residence time in the plasma providing more excitation and subsequently leading to a greater signal to noise ratio, resulting in better limits of detection, even at the low mass flows hssociated with capillary GC.
ACKNOWLEDGMENT The authors are grateful to D. Firestone of the USFDA who kindly provided several of the isomers. I TIME (SEC)
Figure 4. Chromatogram for six dioxin isomers. Order of elution: 2,7di-, 1,2,4-trii, 1,2,3,4-tetra-, 1,2,4,6,7,8-hexa-, 1,2,4,6,7,8,9-hepta-, and octasubstituted isomers. The breaks in the chromatogram are for the purpose of presentation.
obtained (Figure 4). All the dioxins were eluted in less than 18 min. The varying emission intensities in the chromatogram were due to varying concentration of the dioxins in the mixture. Limited supply of some of these and difficulty of dissolving certain others resulted in such variations. However, the emission intensities of the compounds of known concentration did not decrease significantly when compared to the intensities obtained during single dioxin determination. Further work is in progress in two areas: first, improving the sensitivity and, second, using a polychromator to do simultaneous multielement detection for elemental ratioing. With the recent development of a laminar flow torch in these laboratories (12), flow rates of less than 30 mL/min are possible through the plasma torch. This would be compatible
LITERATURE CITED (1) McCormack, A. J.; Tong, S.C.; Cooke, W. D. Anal. Chem. 1985, 37, 1470. (2) Bache, C. A.; Llsk, D. J. J . Assoc. Off. Anal. Chem. 1987, 50. 1246. (3) Mulligan, K. J.; Caruso, J. A.; Fricke, F. L. Analyst (London) 1980, 705,1060. (4) Estes, S. A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981, 53, 1829. (5) Carnahan, J. W. Am. Lab. (Fairfied, Conn.) 1983 (Aug). (6) Nestrick, T. J.; Lamparski, L. L.; Stehl, R. H. Anal. Chem. 1979, 57, 2273. (7) Hariess, R. L.; Oswald, E. 0.; Wllkinson, M. K.; Dupuy, A. E., Jr.; McDaniel, D. D.; Tai, Han Anal. Chem. 1980, 52, 1239. (8) Karasek, F. W.; Onuska, F. I. Anal. Chem. 1982, 5 4 , 309A. (9) Mitchum, R. K.; Moler, G. F.; Korfmacher, W. A. Anal. Chem. 1980, 52,2278. (IO) Haas, D. L.; Caruso, J. A. Anal. Chem. 1985, 57,846. (1 1) Bruce, M. L.; Caruso, J. A. Appl. Spectrosc. 1985, 39,942. (12) Bruce, M. L.; Workman, J. M.; Caruso, J. A. Appl. Spectrosc. 1985, 39,935.
RECEIVED for review July 1, 1985. Accepted October 3,1985. The authors are grateful to the National Institute of Environmental Health Sciences for support of this work through Grant No. ES-03221.
Thermospray Mass Spectrometry. Use of Gas-Phase Ion/Molecule Reactions To Explain Features of Thermospray Mass Spectra Anthony J. Alexander and Paul Kebarle* Chemistry Department, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Gas-phase lon/molecule reactions have a strong Influence on the relative Ion Intensities In thermospray mass spectra. The relative lntensltles of the posltlve Ions In the thermospray (TSP) spectra of 0.1 M ammonium acetate solution follow a pattern that can be predlcted on the basis of gas-phase lon/molecule equlllbrla. Analyte Ions form clusters wlth gasphase solvent, NH, and CH,COOH molecules. The cluster ratlos follow gasphase lon-cluster equlllbria predlctlons. With volatlle and moderately volatile analytes B In ammonlum acetate solutlon, the BH' Ions are formed by gas-phase protonatlon of gaseous molecules B by the ammonium reagent Ions. At [B],, < lo-' M, the gas phase protonatlon Is slow (kinetic control) and the BH' signals are llnear with concentration of B. At [BIap > lo5 M the protonation becomes progresslvely faster, proton transfer equlllbrla conditions are approached, and the BH+ response is nonilnear. The observed Ion lntenslty changes wlth [B],, can be predlcted by gas-phase lon/molecule klnetlcs expresslons.
The thermospray interface and ionization method for liquid chromatography/mass spectrometry (TSPMS), discovered and developed by Vestal and co-workers (1-3), is becoming an important analytical tool of liquid chromatography/mass 0003-2700/86/0358-047 1$01.50/0
spectrometry. This is attested by the rapid increase of applications reported from a number of analytical laboratories (4-7). Further evidence for the widening interest in TSPMS is the large number of reports on this method at the 1985 American Society for Mass Spectrometry conference in San Diego. Two distinct stages are involved in the formation of TSP mass spectra ultimately observed with the mass spectrometer. The first stage is the formation of the gaseous ions out of the microdroplets produced by the vaporizer. The ions produced in the first stage will be called the primary TSP ions. The second stage involves the primary TSP ions in gas-phase ion/molecule reactions. The mechanism for the formation of the primary TSP ions has received considerable attention (1-3,8). It is believed that the charge on the microdroplets results from statistical fluctuations in the distribution of the positive and negative ions in the droplet, a charging mechanism known from earlier work (9,10).Once the microdroplets are charged,field-induced ion evaporation occurs after solvent evaporation has shrunk the droplet to sufficiently small size; see Thomson and Iribarne (11, 12). The second stage, in which gas-phase ion/molecule reactions may remove the primary TSP ions and produce new product ions, has been recognized as of possible importance (1-3) but has received much less attention. The present work is a 0 1986 American Chemical Society
