ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
of exchange site environment in Nafion is responsible for its excellent selectivity properties with regard to univalent metal ions. The upturn in the cesium ion-hydrogen ion isotherm at low fraction of hydrogen ion, which is seen to a smaller extent for other isotherms, may reflect the relatively abrupt decrease in overall water content which would be expected to occur in this region. This point needs to be studied further. Nafion shows promise as a selective ion-exchange material which can be manufactured in various forms and used in corrosive environments. Future work will concentrate on its selectivity properties for divalent cations.
ACKNOWLEDGMENT T h e authors thank Richard Huntrods and Dale Cole for their help with experimental portions of this work, and the staff a t the Chemistry Division, National Research Council of Canada, for their help in preparation of the manu,script. We also thank Daniel Vaughan of Du Pont for supplying samples of Nafion.
LITERATURE CITED J. E. Harrar and R. J. Sherry, Anal. Chem., 47, 601 (1975). R. E. Adams, S. R. Betso, and P. W. Carr, Anal. Chem., 48, 1989 (1976). B. Kratochvil and K. R . Betty, J . Nectrochem. Soc., 121, 851 (1974). F. W. Dampier, J . Appl. Electrochem., 3, 169 (1973). C. H. Lochmulier, J. W. Galbraith, and R. L. Walter, Anal. Chem., 48, 440 (1974). (6) W. J. Blaedel and T. R. Kissel, Anal. Chem., 44, 2109 (1972).
(1) (2) (3) (4) (5)
(7) (8) (9) (10) (11) (12) (13)
(14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) 1281
865
G. L. Lundquist, G. Washinger, and J. A. Cox, Anal. Chem., 47, 319 (1975). J. A. Cox and J. E. DiNunzio, Anal. Chem., 49, 1272 (1977). Chem. Eng. News, 20 (March 20, 1978). R . Schiogl, Ber. Bunsenges Phys. Chem., 82, 225 (1978). S. C. Ye0 and A. Eisenberg. J . Appl. Polym. Sci., 21, 875 (1977). I. M. Hodge and A. Eisenberg, Macromolecules, 11, 289 (1978). T. D. Gierke, "Ionic Clustering in Nafion Perfluorosuifonic Acid Membranes and Its Relationship to Hydroxyl Rejection and Chior-Aikaii Efficiency," presented at the 152nd National Meeting, The Electrochemical Society, Atlanta, Ga., October 10-14, 1977. K. A. Mauritz, C. J. Hora, and A. J. Hopfinger, Polym. Prep., Am. Chem. Soc., Dlv. Polym. Chem.. 19, 324 (1978). S. G. Cutler, P o l y , , Prepr., Am. Chem. Soc., Div. Polym. Chem., 19, 330 (1978). K. A. Mauritz and S. R. Lowry, Polym. Prepr., Am. Chem. Soc., Div. Polym. Chem., 19, 336 (1978). R. A. Komoroski and K. A. Mauritz, J. Am. Chem. Soc., 100, 7487 (1978). M. Lopez, B. Kipling, and H. L. Yeager, Anal. Chem.. 48, 1120 (1976). M. Lopez, B. Kipling, and H. L. Yeager, Anal. Chem., 49, 629 (1977). D. Reichenberg in "Ion Exchange", Voi I, J. A. Marinsky, Ed., Marcel Dekker, New York, 1966. 0. D.Bonner and L. L. Smith, J . Phys. Chem., 81, 326 (1957). Unpublished results. 0.D. Bonner, W . J. Argersinger, and A. W. Davidson, J . Am. Chem. Soc., 74, 1044 (1952). 0. D. Bonner and V . Rhett, J . Phys. Chem., 57, 254 (1953). 0.D. Bonner and W. H. Payne, J . Phys. Chem., 58, 183 (1954). 0. D. Bonner, J . Phys. Chem., 59, 719 (1955). G. Eisenman, Biophys. J . , 2, part 2, 259 (1962). G. Eisenman, in "Ion-Selective Electrodes", R. A. Durst. Ed.. Natl. Bur. Std. Spec. Pub/., 314, Washington, D.C., 1969
RECEIVED for review November 6, 1978. Accepted February 26, 1979. This work was supported by the National Research Council of Canada and the University of Calgary.
Determination of Parts-per-Billion Concentrations of Aqueous Nitrate by Derivatization Gas Chromatography with Electron Capture Detection Roger L. Tanner," Ruby Fajer, and Jeffrey Gaffney Atmospheric Sciences Division, Brookhaven National Laboratory, Upton. N e w York
An analytical method is described for determining nanogram quantltles of nitrate in microliter aqueous extracts of a variety of environmental samples. The method is based on the reaction of nitrate with an electron capture-sensitive benzene analogue to form a substituted nitrobenzene derivative which may be separated and analyzed by gas chromatography with electron capture detection (GC-ECD). Its application lo aqueous nitrate determination in the 2 X lo-' to 2 X M range (10 ppb-10 ppm) is demonstrated. I n the analysis of aqueous extracts of airborne partlculate samples, low results were frequently obtained. A more detailed study revealed serious negative halide interferences, and the mechanism of bromide interference was elucidated through GC/MS/computer analysis. Addition of silver ion to remove halide interference was evaluated.
Determination of nitrates and nitrites in various environmental media at the trace level in small amounts of samples is of interest for many reasons, in particular, the potential for in vivo production of nitrosamines from the reaction of nitrite ion with secondary amines in the human stomach ( I ) . Also, 0003-2700/79/0351-0865$01 .OO/O
1 1973
nitrates in gaseous and airborne particulate form are important end products of NO, reactions in polluted atmospheres ( 2 ) although present in trace (ppb) quantities. Existing automated techniques for nitrate and nitrite ( 3 ) involving reduction, diazotization, and colorimetry have a limit of detection of about 0.04 pg/mL in aqueous solution and require 2-4 mL of sample. The recently developed technique of ion chromatography ( 4 ) is somewhat more sensitive (limit of detection -0.01 ppm, as low as 0.001 ppm with a concentrator column) b u t still requires about 2 mL of sample, is susceptible to interferences from phosphate and bromide if present in lox quantities, and requires substantial attention from an experienced analyst. Other techniques for nitrate (5-7) have been reported but are too complicated or insufficiently sensitive for comparison here. T h e need for a method for determination of sub-ppm concentrations of nitrate and nitrite in microliter quantities of environmental samples led to the development of a new technique based on derivatization-gas chromatography with electron capture detection (GC-ECD). As reported by several groups (8-11), the method involved reaction of benzene (or a benzene derivative) with traces of nitrate in the presence of 5&98% sulfuric acid. The resultant nitrobenzene derivative C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
was extracted into organic media and analyzed quantitatively by GC-ECD. T h e limit of detection (LOD) reported was only about 0.1 ppm but the method has been applied t o a variety of sample types including ambient nitric acid collected in impregnated filters, and t h e sample size required for blood and saliva samples was only 50 pL. Interference has been reported for chloride, thiocyanide, and some amino acids when present in 500-fold excess. W e report in this work the extension of the derivatization GC-ECD technique using electron capture-sensitive derivatizing agents (u,a,a-trifluorotoluene a n d 2,3,5,6-tetrafluoroanisole) which reduces t h e LOD of t h e method t o less than 2 x M (about 10 ng/mL). Application of t h e alternate acid catalyst, trifluoromethanesulfonic acid, produces t h e nitro derivative with higher and more reproducible yields. Interference studies of common anions are reported and elimination of t h e halide interference by addition of silver(1) salts was investigated in some detail. T h e modified method reported is simple, rapid, and moderat.ely precise. Nearly a n order of magnitude more sensitive t h a n any other comparable nitrate method, it is applicable t o microliter-sized samples and has been applied t o analysis of aqueous extracts of airborne particle samples.
EXPERIMENTAL Apparatus. All analyses were conducted using a HewlettPackard Model 5710A gas chromatograph equipped with a 63Ni electron capture detector (ECD). The ECD employs variable pulse frequency and thus has a linear dynamic range exceeding lo4. Argon ( 5 7 ~methane) was the carrier gas. Peak areas were integrated on a Varian CDS 111 chromatographic data system. The optimum packed column for separation of the derivatizing agent and the nitro derivative was found to be a 6-ft by '/,-inch 0.d. glass column, 4 % poly-rn-phenyl ether (six-ring) on 80/100 mesh chromsorb G, acid washed, DMCS treated (Applied Science Laboratories, Inc., State College, Pa.). GC/MS/computer analysis was accomplished using a computer-controlled Hewlett-Packard 5985A GC /MS spectrometer with Hewlett-Packard 2113 data system. Reagents. Three derivatization agents were used: pentafluorobenzene (PCR, Inc.), a,n,a-trifluorotoluene (benzotrifluoride, BTF) (Aldrich Chemical Co.), and 2,3,5,6-tetrafluoroanisole(TFA) (Fairfield Chemical Co., Blythewood, S.C.). 2,3,4,5,6-Pentafluoronitrobenzene and m-nitrobenzotrifluoride (both from PCR, Inc.) were used to confirm the retention volume of the nitrobenzene derivative peak and also to measure the efficiency of the was not nitration reactions. 4-Nitro-2,3,5,6-tetrafluoroanisole commercially available, but its retention volume was initially established by analogy and later confirmed by gas chromatography-mass spectrometry (GC-MS) (see Results and Discussion). The acid catalysts were reagent grade sulfuric acid (Mallinckrodt) and trifluoromethanesulfonic acid (Aldrich Chemical Co.). The extraction solvent was "Baker analyzed", reagent grade cyclohexane (Baker Chemical Co.). All reagents were used without further purification. Procedure. Benzotrifluoride and Sulfuric Acid. To a 0.1-mL portion of the nitrate-containing analyte solution was added 0.5 mL H,SO, in a 2-mL microreaction vessel with a Teflon-lined cap. After the solution was cooled to 0 "C, 1 p L BTF was introduced and the reaction mixture was vortex-stirred using a Thermolyne test tube mixer for 10 min (other stirring or shaking procedures were ineffective). The solution was then extracted with 1 mL cyclohexane and a 0.5-mL aliquot of the organic layer was removed for GC analysis. One-microliter injections of the cyclohexane solution were analyzed to determine the amount of nitro derivative present. 2,3,5,6-Tetrafluoroanisole and Sulfuric Acid. In this procedure, 1 pL of TFA was introduced directly into the 0.1 mL of nitrate solution with subsequent addition of 0.5 mL H2S04without a cooling step. The remainder of the procedure was identical to that for BTF and H2S04. 2,3,5,6-Tetrafluoroanisole and Trifluoromethanesulfonic Acid. One to 50 pL of nitrate solution and 1 6 1 2 small glass beads were placed in a 2-mL microreaction flask and evaporated to dryness
in a vacuum oven a t 60 "C. The sample was cooled to room temperature, then 1 ILLof TFA and 25 pL of trifluoromethanesulfonic acid were added. After the flask was capped, the reaction mixture therein was stirred for 4 min and neutralized with 0.5 mL of 1 M KOH. Extraction into cyclohexane and subsequent GC analyses were performed as per the sulfuric acid procedure. GC-ECD Analysis. The GC conditions varied with derivatization procedure as follows: for both nitro derivatives, the injection port temperaturere was 200 "C, and the detector temperature was 300 "C with a carrier flow rate of 30 mL/min; for the m-nitrobenzotrifluoride, the column temperature was 150 "C isothermal, and for the 4-nitro-2,3,5,6-tetrafluoroanisole, it was 175 "C. Calibration curves for the above reactions were prepared as described below. For the H2SO4method, 1- to 50-pL portions of a standard NaNO, solution (24 pg/mL) were diluted to 0.1 mL and the reaction was carried out as described above. For the trifluoromethanesulfonic acid method, 1 to 50 pL of the same standard NaN03 solution (24 pg/mL) were evaporated to dryness and reacted as previously described. In all cases, peak area was plotted vs. amount or concentration of NaNO,. Gas Chromatography/Mass Spectroscopy Analysis. Using the H P 2113 data system and software package, 70-eV electron impact mass spectra were obtained with a glass capillary jet separator as the GC/MS interface. The GC/MS interface, ion source, and analyzer were maintained a t 300, 150, and 200 "C, respectively, during the analysis. GC/MS data were corrected for mass spectral background contributions. GC conditions were the same as in the GC/ECD analysis except that helium was used as the carrier gas. Perfluorotributylamine (PFTBA) was used routinely to calibrate the hyperbolic quadrupole mass spectrometer a t m / e 69, 219, and 502 using H P software (AUTOTUNE). Since the fluorinated compound and its derivatives under study were more sensitive to electron capture detection, the nitration procedure was scaled up to facilitate the GC/MS characterization.
RESULTS AND DISCUSSION T h e principal innovation reported in this paper is t h e demonstrated determination of nitrate traces in amounts one t o two orders of magnitude lower t h a n existing methods through nitration of electron-capture (EC) sensitive aromatics t o form nitrobenzene derivatives which may be analyzed easily and quantitatively a t t h e picogram (pg) level. T h e obvious choice for t h e most EC-sensitive starting material is pentafluorobenzene (PFB) based on t h e literature d a t a for t h e EC-sensitivity of the pentafluorophenyl and pentafluorobenzyl groups (12, 13). However, initial experiments showed that pentafluorobenzene did not react with ppm levels of aqueous nitrate even in 88% HzS04,nor did nitration of PFB with solid nitrate traces proceed measurably in the presence of the other strong acid catalyst used in this study-trifluoromethanesulfonic acid (TMSA). Alternate starting materials were investigated and, on the basis of availability, reactivity, E C D sensitivity, and chromatographic suitability, two were selected for further study-benzotrifluoride (BTF) and 2,3,5,6-tetrafluoroanisole (TFA). BTF. Benzotrifluoride will react with nitrate in t h e presence of 5&90% aqueous H,SO, t o form nitro derivatives. T h e optimum concentration of H2S04was found t o be 83 % ; lower concentrations resulted in reduced nitration efficiency whereas concentrations 1 88% decomposed t h e B T F , presumably t o benzyl alcohol as reported by Le Fave (14). Two peaks are observed in the cyclohexane extract of this nitration: A, eluting a t 3.0 min from the polyphenylether column under representative conditions and B, (about 10 area % of a A) eluting at 3.9 min under t h e same conditions. Authentic samples of m-nitrobenzotrifluoride were used to identify t h e first, larger peak as the meta derivative and t h e second peak was identified as a n isomer of m-nitro-BTF by GC-MS. Based on the electron-withdrawing nature of the CFs
ANALYTICAL CHEMISTRY, VOL. 5'1, NO. 7 , JlJNE 1979
18-
6l
X
:
0 ;
EEUZOTRIFLUORIDE ANISOLE
+
0.6 1 /
0.4
-
./
H2S04
H2S0,
'
/x
/
+
867
/
/
1
/
0 '
1
0
/ /
p q / m hONO,
Flgure 1. Comparison of calibration curves as a function of nitration starting material; (x) benzotrifluoride, (0)2,3,5,6-tetrafluoroanisole; acid catalyst, 83 YO H2S04
group, weak meta direction of the nitration reaction would indeed be indicated. A small amount of the ortho or para product is also formed (eluting after the meta product). All response data reported for nitration of B T F were obtained by summing the peak areas of the two nitro derivatives using t h e grouping function of the chromatography data system. By use of the procedure described above, the response curve for the nitration products of B T F as a function of concentration of standard sodium nitrate was generated and is shown in Figure 1. Since the slope in the linear portion of the curve is about 105/pg/mL N a N 0 3 and area changes of lo4 are easily detected, sensitivity to concentration changes of go% peak area reduction at 4-fold excess bromide) and is signalled by a new peak on the chromatogram eluting after the T F A reagent but before the nitro derivative. T h e additional peak has been identified as the bromo derivative (4-bromo-2,3,5,6-tetrafluoroanisole) by GC-MS. T h e mass spectra of the TFA reagent, its nitro and bromo derivatives are shown in Figure 6. The TFA reagent (mol wt 180),typical of an aromatic ether, yields ions of m / e 165 and 137, corresponding to loss of methyl followed by loss of carbon monoxide leading to the stable C5F4H+ion. By analogy the 4-nitro- or 4-bromo-TFA derivatives should yield a C5F4+ radical ion ( m / e 136) during fragmentation which, followed by loss of fluorine radical, would yield a C5F3+ion ( m / e 117). Both peaks are observed to be abundant in the spectra of the derivatives. T h e presence of peaks a t m / e 30, 46, 179, and 195, characteristic of NO and NO2 fragmentation, along with the abundant molecular ion intensity ( m / e 225) confirm the as the identification of the 4-nitro-2,3,5,6-tetrafluoroanisole nitration product. T h e characteristic isotope pattern of the molecular ion ( m / e 258, 260) as well as the observed fragmentation ion due to loss of methyl ( m / e 243, 245) followed by loss of CO ( m / e 215, 217) confirm that the interference is a mono-bromo product and is bound to the aromatic ring system. The mass spectral data presented clearly indicate that this compound is 4-bromo-2,3,5,6-tetrafluoroanisole. I t is hypothesized that the bromide interferes by reducing the nitrate in strongly acid solution, forming bromine which then brominates the anisole (reactions 3 and 4). HBr
+ "OB
CF,S03H
Bra + H 2 0 + NO
F
F
F
F
F
F
F
F
(3)
(4)
Chlorine does not interfere (no chloroanisole derivative ap-
ANALYTICAL CHEMISTRY, VOL. 51, NO. 7, JUNE 1979
1
I
W 0
1
I
z
869
a
n
z 2
m
a W
? I-
a 1 W
K
0
30
60
90
120
150
180
210
240
270
300
M/E
Figure 6. Mass spectra of 2,3,5,6-tetrafluoroanisole and fis nitro and bromo derivatives; molecular formulas, (a)C7H,F40, (b) C7H3F4N03,(c) C7H3F40Br
pears on the chromatogram) at equivalent concentrations because nitrate cannot oxidize chloride to chlorine and probably also because C12 is less effective in the nucleophillic addition to the benzene ring. Unfortunately the bromo derivative is not formed quantitatively or reproducibly when the nitrate concentration is constant, thus the method cannot be used for indirect analysis of nitrate or directly adapted for bromide determination. However, it is clear that bromide must be absent for quantitative nitrate analysis. Halide interference may be removed by addition of silver ion as suggested by Tesch et al. (9). However, it was found in this study that such interference removal introduced other difficulties. Since there are few soluble silver salts, tests could be conducted only with silver acetate and silver perchlorate. Our source of the latter was contaminated with nitrate and was thus unsuitable for testing halide interference removal. Silver acetate (AgAc) solutions were added to various amounts of NaC1- or KBr-nitrate mixtures and the GC-ECD response of the nitro-TFA peak relative to a control experiment with pure N a N 0 3 was measured and is shown in Table I. Addition of equimolar amounts of Ag' and halide removes the interfering effect of the halide. Excess AgAc appears to repress t h e formation of the nitro derivative when bromide was originally present, albeit the results are too scattered to make conclusions in the case of chloride. Additional experiments were conducted with very large excess of chloride present (-0.05 M) in which solid silver acetate was added and the mixture shaken to metathesize the AgAc to AgCl in proportion to the amount of chloride present. In this case, excess AgAc addition clearly reduced the response of the nitro derivative peak probably because of interfering reactions of the acetate with the acid catalyst. Comparison was then made of the effect of silver acetate addition on the area of the nitro derivative peak for nitrate in aqueous extracts of airborne particulate samples collected on Long Island, N.Y., during late January 1978. Since the
Table 1. ~~~~~d of Halide Interference with Derivatization of Standard Nitrate Samples by Means of Silver Acetate Addition
sample composi tiona A (control) A + 4D A+B+4D A + 5B + 20D A + 10B + 40D A + 8.7C + 35D A + 7.1C + 29D A + 6.5C + 26D A
A+71C A + 7.1C A + 7.1C
+ 7.1D + 29D
nitro derivative peak area, relative units 682 i 6 3 8 7 4 i 130 856i 13 669 i 2 756 2 1 1 5 248 i 26 480 + 8 8 7 0 i 30 744 t 25 40.8 i 0.7 739+ 80 4751 7
% of control
_-_ 128. 126 98. 111.
39.b 60. 83.6 _._ 5.5
99. 64.
a A = amount of NaNO,, B = amount of NaC1, C = amount of KBr, D = amount of silver acetate; all units are in relative nanomoles. Based on standard curve response for the amount of nitrate reacted.
amount of halide interferent was unknown but was thought to be less than the molar amount of nitrate, the molar amount of silver acetate added was made equal to the amount of nitrate as determined on the same extract by reduction-colorimetry. T h e results in Table I1 demonstrate that the lower levels of nitrate found by the nitration-GC-ECD method are a t least partially the effect of halide interference since addition of silver increases the calculated nitrate levels to 90% of that found by the standard colorimetric technique. Further work is in progress to develop a procedure for determination of nitrate in atmospheric samples in which the amount of halide interferent is not always known. Application of this method
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JUNE 1979
Table 11. Removal of Halide Interference with Derivatization of Ambient Nitrate Samples b y Means of Silver Acetate Addition PdmL un- by colorknown imetry 03-5 03-6
1.88
03-7
3.76
03-8 04-1 04-2
2.29 0.44 0.45
04-3
0.60
1.89
% b y GC-ECD
-
n o silver
acetate
nitration method silver acetate addeda 82
43 74 63 ND 79 81 ND
89 93 93
-___ x = 68 i 16%
111 ND 70
y =90 1
14%
a An amount of AgAc was added equivalent to the amount of nitrate found by the colorimetric method.
to ambient nitric acid determination is also in progress.
LITERATURE CITED (1) D. H. Fine, D. P. Rounbehier, and P. E. Oettinger, Anal. Chim. Acta. 78. 383 (1975).
(2) C. W. Spicer. Atmos. Environ., 11, 1089 (1977). (3) J. B. Muilin and J. P. Riley, Anal. Chim. Acta, 12, 464 (1955). (4) J. Mulik, R. Puckett, D. Williams, and E. Sawicki, Anal. Left.. 9 , 653 (1976). (5) E. Sawicki, H. Johnson, and T. W. Stanley, Anal. Chem., 35, 1934 (1963). (6) A. Oien and A. R. Selmer-Olsen, Analyst (London), 94, 888 (1969). ( 7 ) G . Norwitz and H. Gordon, Anal. Chim. Acta, 89, 177 (1977). (8) W. D. Ross, G. W. Butler, T. G. Duffy, W. R. Reyh, M. T. Wininger, and R. E. Sievers, J . Chromatogr., 112, 719 (1975). (9) J. W. Tesch, W. R. Rehg, and R. E. Sievers, J . Chromatogr.. 126, 743 (1976). (10) S. A . Hubbard, E. Wlttgenstein, and E. Sawicki, Paper ENVR-9, 172nd National Meeting, American Chemical Society, San Francisco, Calif., Aug. 29-Sept. 3, 1976. ( 11 ) R. J. Hare, M. T. Winninger, W. D. Ross, J. Tesch, and R. E. Sievers, Paper ENVR-99, 176th National Meeting, American Chemical Society, Miami Beach, Fla., Sept. 10-15, 1978. (12) C. A. Clemons and A. P. Altshuller, Anal. Chem., 38, 133 (1966). (13) N. K . McCallum and R. J. Armstrong, J . Chromatogr., 78, 303 (1973). (14) G. M. Le Fave, J . A m . Chem. Soc., 71, 4148 (1949). (15) C. L. Cwn, W. G. Biucher. and M. E. Hill, J . Org. Chem., 38, 4243 (1973).
RECEIVEDfor review October 13, 1978. Accepted February 9, 1979. T h e authors gratefully acknowledge the support of this work by the Division of Chemical Sciences, United States Department of Energy under contract No. EY 76-C-02-0016.
Characterization of Isomeric Compounds by Gas and Plasma Chromatography D. F. Hagen 3 M Central Research Laboratories, St. Paul, Minnesota
55 10 1
The combination gas and plasma chromatographic system has demonstrated the capability of dlfferentiatlng between isomeric compounds. Reduced ion mobility data are inversely proportional to average molecular collisional cross sections. Meta substiiuted positional isomers are generally larger than the para and ortho isomers. 4,4’-Dipyridyl has a cross sectional value approximately 8 A* larger than the 2,2’-dipyridyl species. Geometric cis-trans isomers also give large differences in cross sections as calculated from ion mobility data. Although errors may exist in the absolute value of a cross section, differences as small as 0.2 A* in a given isomeric series are measurable with the system and assumptions described.
T h e plasma chromatograph (Franklin GNO Corp., West Palm Beach, Fla.) has been described as an “atmospheric pressure chemical ionization drift time spectrometer” (1-3) and a number of investigators have discussed the nature of the reactant gases and ion-molecule reactions involved (4-6). A good deal of uncertainty exists as to the exact identity of the product ion mass [M,H+, M(H2O)?H+,M,02-, etc.], and is compounded if solvents or impurities enter into the ion formation process. Spangler and Collins ( 4 ,Carroll et al. ( 5 ) , and Bird and Keller ( I ) discuss the nature of the ion-molecule reactions involved and suggest that the peaks are a composite of interacting species. Bird and Keller ( I ) discussed the dependence of the plasmagrams on sample vapor composition in their studies of “pulse” a n d “steady-state” sampling techniques. They discuss the overload pulse and “steadystate” sampling techniques. They describe the overload pulse sampling situation where an excess quantity of sample causes 0003-2700/79/0351-0870$01.OO/O
the reactant ion species to first disappear, then reappear as the sample concentration decreases with time. They approve of the pulse sampling overload situation on the condition that the resultant ion-molecule peaks are “time stable”. They contend that the peak is “time stable” if the peak maxima do not change by more than 0.2 ms during the lifetime of the peak. They propose that steady-state sampling is desirable to produce reproducible nontransient plasmagrams. This would of course be impossible for dynamic monitoring of peaks as they elute from a gas chromatograph. It has been our experience that reproducible plasmagrams are readily obtainable from GC peak sampling for a reasonable period of time where drift cell temperature and pressure are constant. The multiplicity and deformation of peaks often obtained can in some cases be explained by the presence of isomeric species and impurities. We have been able t o demonstrate that positional as well as stereometric isomers are resolvable with the GC-PC combination and are able to characterize, in one case, five separate positional isomers of a relatively high molecular weight compound. This does not mean that the plasma chromatograph in itself has high resolving power but it does point out the power the technique has in defining a chemical species with regard to the net effect of molecular ion mass and size. T h e size relationship is approximated using the average ion-neutral collision cross section for momentum transfer (0,) discussed and calculated experimentally by Lin et al. ( 7 ) . They utilize both static and rotational models of the ion to calculate a n average collision cross-section (Q,) value. This value is then used to calculate the reduced ion ) on a theoretical basis. They found that mobility ( K O term the mean cross-section values for the static and rotational models gave a satisfactory fit between experimental and 1979 American Chemical Society
