F. W. McLafferty, ed., "Mass Spectrometry of Organic ions," Academic Press, New York, 1963. H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds", Holden-Day, San Francisco, 1967. H. M. Grubb and S. Meyerson. "Mass Spectrometry of Organic Ions", F. W. McLafferty, Ed., Academic Press, New York, 1963. L. L. Thurstone, "Multiple Factor Analysis", University of Chicago Press, Chicago, 1947. H. H. Harman, "Modern Factor Analysis", University of Chicago Press, Chicago, 1967. P. Horst, "Factor Analysis of Data Matrices", Holt, Reinhart and Winston, New York, 1965. R. J. Rummel. "Applied Factor Analysis", Northwestern University Press, Evanston, 1970. E. R. Malinowski, Doctoral Dissertation, Stevens Institute of Technology, Hoboken, NJ, 1961. P. H. Weiner, Doctoral Dissertation, Stevens Institute of Technology, Hoboken, NJ, 1971. J. T. Bulmer and H. F. Shurvell, J. Pbys. Chem., 77, 256 (1973). E. R. Malinowski and P. H. Weiner, J. Am. Cbern. Soc., 92, 4193 (1970). P. H. Weiner, E. R . Malinowski, and A. Levinstone, J. Pbys. Cbem., 74,
ISOMERS Flgure 10. Three experimental intensities for each of the twenty-two isomers of CioH14 in turn (Figure 5). The masses are 119, 105, and 9 1 amu in that order. The intensities at each mass are normalized to 1
4537 (1970). E. R . Malinowski and P. H. Weiner, J. Pbys. Chem., 75, 1207 (1971). P. T. Funke. E. R. Malinowski, E. E. Martire, and L. 2. Pollara, Sep. Sci., 1, 661 (1966). P. H. Weiner and D. G. Howery, Anal. Chem., 44, 1189 (1972). P. H. Weiner and J. F. Parcher, Anal. Chern., 45, 302 (1973). D. Macnaughton. Jr., L. E. Rogers, and G. Wernimont, Anal. Cbern., 44,
tant to realize that normalization conventions differ in different programs. We have consistently used those of BMDX72. The terminology loadings and scores, rather than property factor matrix and molecular factor matrix also corresponds to BMDX72. More details are available concerning terminology, conventions, and computational procedures (27).
1421 (1972). J. J. Kankare, Anal. Chem., 42, 1322 (1970). N. Ohta, Anal. Chem., 45, 553 (1973). J. C. Stover, Doctoral Dissertation, Fordham University, New York,
1974. R. W. Rozen and E. McLaughlin, Twenty-Second Annual Conference on Mass Spectrometry, Philadelphia, PA, May 19-24, 1974, p 441. John Jalickee, private communication. Mass Spectral Data, API Project 44, Nos. 210, 212, 214, 439-441,
ACKNOWLEDGMENT We thank William Lawlor for his help, and recognize the preliminary work of Vincent Waldron (Honors Thesis, Fordham University, 1969).
459-463, 486, 494, 863, 934, 1184, 1429, 1431, 1432, 1570, 1655, 1957. W. J. Dixon, Ed., BMD, Biomedical Computer Programs, X Series Supplement, University of California Press, Berkeley, 1970, p 90. E. McLaughlin Petersen. Doctoral Dissertation, Fordham University, New York, 1975.
LITERATURE CITED (1) K. Biemann, "Mass Spectrometry-Organic Chemical Applications", McGraw-Hill. New York, 1962. (2) J. H. Benyon, "Mass Spectrometry and Its Applications to Organic Chemistry", Elsevier, Amsterdam, 1960.
RECEIVEDfor review November 5, 1974. Accepted March 12, 1975.
Atmospheric Pressure Ionization (API) Mass Spectrometry: Formation of Phenoxide Ions from Chlorinated Aromatic Compounds lsmet Dzidic, D. 1. Carroll, I?. N. Stillwell, and E. C. Horning Institute for Lipid Research, Baylor College of Medicine, Houston, TX 77025
Phenoxide ions, (M - CI -I- 0)-, are formed by ion-moleO2 (M - CI 0 ) - OCI, and 02cule reactions: M-I- M --+ (M CI 0 ) - -I-OCI, when certain chlorinated aromatic compounds are ionized in an API source in the presence of nitrogen containing approximately 0.5 ppm of oxygen and also in air. While u- and pthloronitrobenzenes form mainly chloride and nitrophenoxide ions, mthloronitrobenzene ionizes under the same conditions to form a negative molecular ion. Chlorobenzene and odichlorobenzene yield only chloride ions, while more highly substituted polychlorobenzenes form phenoxide ions. Subpicogram detection of 2,3,4,5,6-pentachlorobiphenyl is demonstrated by selective monitoring of the corresponding phenoxide ion.
- +
+
-.
+
+
The usual method of analysis for residues of many insecticides, herbicides, fungicides, and polychlorobiphenyls is 1308
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
gas chromatography with electron capture (EC) detection. A wide variation in detector response with structural changes is a feature of EC detection of organic compounds. The direct experimental observation of negative ion formation, under conditions approximating those of EC detection (atmospheric pressure, carrier gas environment), is not possible with conventional mass spectrometers. The development of the "Plasma Chromatograph" ( I ) provided an opportunity for the measurement of mobilities of negative ions formed in a carrier gas under conditions of temperature and pressure similar to those used in EC detection. When mobilities of negative ions were measured for several polychlorobiphenyls (2),and for isomeric chloronitrobenzenes ( 3 ) ,it was found that some compounds formed several negative ions. These ions were assumed to correspond to M-, C1-, and (M - C1)- ( 2 , 3 ) .The nature of these ions was not, in fact, firmly established, since mobilities cannot be used for ion identification ( 4 ) .
1
ULTRA HIGH PURITY NITROGEN
- 0 5 p p r r of 02 200°C
Figure 1. Panel A. Ions from l802 (added)and l602 in High Purity nitrogen carrier gas. Temperature, 100 OC. Panel 6.Reaction product from rnchloronitrobenzene under t h e same conditions as for Panel A. Panel C. Reaction products from pchloronitrobenzene under the same conditions as for Panels A and B
Atmospheric pressure ionization (API) mass spectrometry is a novel technique with subpicogram sensitivity of detection. Ionization reactions based upon charge transfer, protdn addition, and proton removal (leading to M+, MH' or (M - H)-, respectively) were defined in initial studies (5, 6). An application of obvious importance is the use of API mass spectrometry for the detection and quantification of toxic or potentially toxic agents in the environment; the chief examples of organic compounds in these categories include pesticides, herbicides, and polychlorinated compounds which are now detected by EC methods. The results described here were obtained as a prerequisite for the development of methods for detection and quantification of polychlorobiphenyls by API mass spectrometry. Unlike EC procedures, it is not possible to use API high sensitivity detection methods without knowing the nature of the ions formed in the source. Accordingly, a study of negative ion formation was undertaken. These studies show that ion-molecule reactions can be used as a basis for detection of certain types of chlorinated aromatic compounds by API techniques.
EXPERIMENTAL Instrumentation. Details of the design of the API mass spectrometer, and studies of ion-molecule reactions occurring in a n API source, have been published (5, 6 ) . T h e primary source of electrons is a 0.75 mC 'j3Ni foil. Continuous sampling of the ions (and neutral molecules) in the source chamber is achieved by employing a 25-p aperture in a disk separating the chamber from the low pressure region of the mass analyzer. Data acquisition and display are accomplished with the aid of a PDP-S/E computer and pulse counting techniques. Nitrogen (Linde High Purity Grade, and Linde Ultra High P u rity Grade) and air (Linde, Dry Grade) were used as carrier gases. T h e gases were cleaned prior to use by passage over Linde Molecular Sieve (type 13X). Separate sieves were used in the air and nitrogen lines, rather than a common unit. T h e system was purged and baked at 250 O C for 12 hr between gas changes. Carrier gas flow rates were 5-10 ml/min. T h e source temperature was 100 or 200 OC, as indicated. Samples were introduced by vaporization from a platinum wire placed in the heated carrier gas stream. Reagents. Chlorobenzenes, nitrochlorobenzenes, nitrophenols, and polychlorobiphenyls were from Eastman Kodak Company. T h e chlorobenzenes, chloronitrobenzenes, and dichloronitrobenzenes were purified by preparative gas chromatography. T h e other compounds were used as received. The H2180 and l 8 0 z were 90% isotopic purity and were obtained from Miles Laboratories.
RESULTS AND DISCUSSION Formation of Phenoxide Ions from 0 - and p-Chloronitrobenzene. The introduction of 0- or p-chloronitrobenzene into the API reaction chamber, by volatilization from a platinum wire inserted into the stream of preheated
Figure 2. Production of negative molecular ions, nitrophenoxide ions, and chloride ions from pchloronitrobenzene in Ultra High Purity nitrogen at 200 O C
carrier gas (nitrogen), gave two ions as reaction products. One was C1-; this was identified by amu value and the typical isotope distribution. The organic reaction product corresponded in amu value to a nitrophenoxide ion. Linde High Purity nitrogen carrier gas contains approximately 100 ppm 0 2 and 10 ppm water, according to the manufacturer (7). Halogenated benzenes will react with water a t elevated temperatures to form phenols, and it was considered possible that some or all of the phenolic products were produced in this way. When a small amount of water enriched in lS0 was added to the carrier gas stream, the solvated protons showed the expected change. Both (H2160),H+ and (Hzl*O) ,H+ ions were present. The phenoxide ions, however, were not labeled with lS0,indicating that water was not the source of the oxygen required for phenoxide ion formation. An analogous experiment was carried out with l 8 0 2 , a t a source temperature of 100 "C. A small amount of oxygen enriched in l8Oz was added to the carrier gas stream. The superoxide ions ( 0 2 - ) showed the appropriate isotope distribution, and the phenoxide ions were labeled with lS0.Figure 1 shows the chart records for an experiment in which approximately equal amounts of added l 8 0 2 and l 6 0 2 were present in the carrier gas. The background ions are shown in panel A. In the absence of water, l 6 0 2 and l 8 0 2 - would be the dominant ions. Water desorbed from the l8O2 inlet system walls, however, clustered with the oxygen ions to give the spectrum shown. Panel B shows the effect of introducing rn-chloronitrobenzene into the source; the oxygen ions disappear, and the product ion corresponds to M-. Panel C shows the ionic products from p-chloronitrobenzene. The aromatic ion did not contain chlorine, and it was labeled with l80 in the same proportion as the label in the superoxide ions (and molecular oxygen). The predicted reaction in nitrogen for all three chloronitrobenzenes is electron attachment to form M-. I t was considered possible that the substitution reaction was one between M- and molecular oxygen; the experiments illustrated in Figure 1 do not, however, give any indication of the molecular ion. At attempt was made to detect the M- ion from p-chloronitrobenzene through use of Linde Ultra High Purity nitrogen. In this case, the oxygen concentration was less than 1 ppm, and 0 2 - ions were barely detectable. The results for p-chloronitrobenzene are shown in Figure 2. Even under these conditions, the predominate ion was (M - C1 + 0)-, but, as shown, a small peak corresponding to M- was present. ANALYTICALCHEMISTRY,
VOL. 47, NO. 8, JULY 1975
1309
Similar results were observed for 2,3-dichloronitrobenzene when Ultra High Purity nitrogen was used. This compound gave C1- ions, (M - C1+ 0)-ions and M- ions. The M- ions were not observed when Linde High Purity grade nitrogen was used. When Linde Dry Grade air was used as a carrier gas, the ratio of oxygen ions to electrons was greater than lo6;thermal electrons do not contribute to the ionization of trace levels of samples under these conditions. When m-chloronitrobenzene was introduced in air, C1- and (M - C1+ 0)ions were observed in addition to the M- ion observed in nitrogen; the C1- and (M - C1 0)- ion intensities were about 10% of the M- intensity. Samples of 0- and p - chloronitrobenzene yielded only phenoxide ions; the result was the same as that observed in High Purity nitrogen. These ion molecule reactions may be written schematically in the following way:
+
02
M
When the carrier gas is air, Reaction 2 is suppressed because of the overwhelming concentration of oxygen. A charge transfer sequence (Reactions 1, 3, and 5) leading to M- would be expected to occur under these conditions when the electron affinity of M is greater than that of oxygen. Ions corresponding to M-, however, are not observed in air for either 0- or p-chloronitrobenzene. This does not signify that both compounds have a lower electron affinity than oxygen, but rather that Reaction 6 is more exothermic and thus has a larger rate constant than Reaction 5. When the oxygen concentration is very low, Reactions 2, 4,and 6 become the main route of reaction. As the oxygen concentration nears zero, Reaction 4 is gradually suppressed and, as a result, the ratio of M-/phenoxide- increases. The phenoxide ions formed in the presence of varying intermediate concentrations of oxygen will be derived both from 0 2 - reacting with M, and M- reacting with 0 2 . The proportion of product derived from each pathway will depend upon the relative rate constants of the reactions in each pathway and on the concentration of the reagents. The experimental record in Figure 2 shows a small M- ion yield. The major product is phenoxide ion, however, suggesting that the energy relationships are such that Reactions 2, 4, and 6 are highly favored. The concentration of M- is small even when the oxygen concentration is very low, and use of air, High Purity, or Ultra High Purity nitrogen, all result in the formation of phenoxide ions from the ortho and para isomers. In fact, the experimental results suggest that the best way to detect and measure very low concentrations of oxygen in nitrogen would be to use 0 - or p-chloronitrobenzene as a reagent, and to monitor formation of the phenoxide ion. These observations for 0 - , m-, and p-chloronitrobenzene raise questions about the nature of the presumed collision complex, the transition intermediate, and the origin of the C1- ions. The halflife of the ion collision complex [MO2]- is presumably very short, but its existence as a transient entity does not seem an unreasonable assumption. The transition state or intermediate for p - chloronitrobenzene is presumably: 1310
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
The meta isomer can not form this type of structure; this suggests that reaction of the meta isomer either will not occur a t all, or that it will occur by a different reaction path. The ortho isomer can form a structure analogous to the proposed para structure. The origin of the C1- ion is not known, but the results in air with the meta isomer suggest that it is associated with phenoxide ion formation. I t is possible that the neutral radical C10, which is presumed to be the other product when phenoxide ions are formed, reacts further to form C1-. Formation of Phenoxide Ions from 0-, m-,and p-Nitrophenols. A remaining question is the reason for the exothermic nature of the reaction of phenoxide ion formation in the case of the para and ortho isomers, in contrast to the situation observed for the nieta isomer. If it is assumed that the C-C1 bond dissociation energies are comparable for all three chloronitrobenzenes, a pronounced difference in rate of phenoxide formation can be due only to 0 - and p-nitrophenoxy radicals having a higher electron affinity than the rn-nitrophenoxy radical, or to a difference in activation energy requirements. The electron affinity of these radicals is also related to acidity of the phenol. For a gas phase acid, the acid strength increases with the difference between the electron affinity of the corresponding radical and the dissociation energy (8).These relationships suggest that 0 - and p-nitrophenol should be stronger acids than rn-nitrophenol in the gas phase as well as in solution. Additional experiments were carried out to examine these questions. m- and p-Nitrophenol were introduced into the source (vaporization from a platinum wire) in the presence of C1- (from chloroform). p - Nitrophenol was ionized to phenoxide ions by Reaction I; m- nitrophenol was not ionized. This experiment confirmed the expected relationships. A separate study of gas phase acidities indicated that 0 2 - was a stronger base than C1-, and that all three nitrophenols are ionized in the gas phase by proton transfer to 0 2 - ions (8).
M
+
C1-
4
(M
-
H)-
+
HC1
(7)
As a consequence of these energy relationships, it is to be expected that replacement of chlorine by oxygen in an aromatic chloro compound will occur through these reactions only if the corresponding phenol is a stronger gas phase acid than HC1. If the corresponding phenol is a relatively weak gas phase acid (for example, phenol or p-chlorophenol), the reaction will not occur. This was confirmed for chlorobenzene and p - dichlorobenzene which did not form the corresponding phenoxide ions. A more precise definition of the thermodynamic relationships must await determination of bond dissociation energies for halogenated benzenes and phenols, of electron affinity estimates for the corresponding neutral radicals, and estimates of activation energies. Formation of Phenoxide Ions from Polychlorinated Benzenes and Biphenyls. The ion intensity ratios (M C1 O)-/C1- of several polychlorobenzenes were measured in air a t 200 "C. The results are in Table I. The chlorophenoxide ion was not observed for 1,2-dichlorobenzene. The intensities of the chlorophenoxide ions increased sharply from trichloro- to hexachlorobenzene. Similar results were obtained for a group of chlorinated biphenyls. Assuming that the bond energies (C-Cl) do not change significantly with increasing chloro substitution, the rate constants for the production of C1- from polychlorinated benzenes
+
-
Table I. Ratios of Phenoxide to Chloride Ions Obtained from Polychlorinated Benzenes at 200 "C and Sample Concentration of 1 ppm, in Air Carrier Gas
-
1
1,2 -Dichlorobenzene 1,2,4-Trichlorobenzene 1,2,3,5-Tetrachlorobenzene Pentachlorobenzene Hexachlorobenzene
l r I W " +O)- c1-
0 0.08 0 .go
GNO Compound
6H5N02
o-ClCGHdN02 p-NOZCgH40H 117 -ClCGH,N02 ClzCsH3N02
Karasek (6, 12)
(15)
1.88 1.86 1.86 1.74 1.73
1.79 1.77 1.77
Ion species
CBHSNO; N02GHdON02CGHdOCICGH4NO2ClNO,CgH,O-
3.20 58.0
should not change significantly. Thus, the approximately 700-fold increase in the ratio of the polychlorophenoxide to chloride ions in going from a trichloro- to hexachlorobenzene indicates a sharp increase in the rate constants and exothermicities of phenoxide anion production. The increase in phenoxide production is almost certainly linked with an increase in gas phase acidity of the phenols. In separate experiments, it was found that gas phase acidities increase sharply in going from dichloro to pentachlorophenol. This should be paralleled by increasing electron affinities of the corresponding polychlorophenoxy radicals. Comparison of A P I Results w i t h Observations Made t h r o u g h Use of O t h e r Techniques. Phenoxide ions of the form (M - C1 0 ) - were observed by Tannenbaum and Dougherty in studies of the negative ion chemical ionization mass spectra of polycyclic chlorinated pesticides (9). Similar phenoxide ions were also observed in spectra of halogen-substituted nitrobenzenes and polychlorinated biphenyls ( I O , 11). Since M- ions were also observed, it was assumed that phenoxide ion formation occurred through reaction of M- and oxygen. These reaction conditions were not identical with API conditions; they represent a combination of a low concentration of oxygen and a very much lower ion source pressure. Under these conditions, the halflife of M- was increased. Taken together, these results and the API results unequivocally demonstrate that phenoxide ion formation occurs for certain chlorobenzenes and chlorobiphenyls a t all concentrations of oxygen from trace amounts to the concentration in air. It is possible to observe M- ions, but only when the concentration of oxygen is extremely low, and preferably under relatively low pressure conditions to decrease the frequency of collision of Mwith 0 2 . During early work with a Plasma Chromatograph-Mass Spectrometer (PC/MS) combination, and with nitrogen carrier gas, pentachlorophenoxide and C1- ions were observed as the only ions in the negative ion spectrum of hexachlorobenzene ( 1 2 ) . Since the PC/MS was being used to investigate mass-mobility relationships ( 4 ) , these results were not pursued a t that time. They indicate, however, that phenoxide ion production was observed under Plasma Chromatograph conditions, and that M- was not a detectable product from hexachlorobenzene. When these observations are added, it is evident that all three techniques (API, PC, chemical ionization negative ion mass spectrometry) lead to the same conclusion. The replacement of chlorine by oxygen occurs for many but not all chlorinated benzenes and biphenyls under ionizing conditions which have hitherto been expected to lead only to M- or to (M - C1)-. The extent to which this reaction occurs for a specific compound is related to the gas phase acidity of the corresponding phenol. These experimental results and conclusions contradict the interpretation of relevant PC data by Karasek and his colleagues. For example, it was reported in a study of the ionization of 0 - , m-, and p-chloronitrobenzenes that the
+
Table 11. Calculated Mobility (cm2/V sec)
meta isomer formed only M- ions, and that the ortho and para isomers formed (M - C1)- and C1- ions by dissociative electron capture (3, 13). It is a virtual certainty that the reported (M - C1)- ions were, in fact, phenoxide ions; this is also true for the ion product from 2,3-dichloronitrobenzene. The conditions of operation were such that a low concentration of oxygen would have been present. The identification of the ion products as (M - C1)- rested upon mobility measurements, and upon an assumption that (M - C1)- should be the product. It is not likely, however, that these unwarranted assumptions would have been characterized as experimentally proved facts if the mobility data and mobility comparisons had not seemed to support this interpretation. The reason for the error becomes apparent when the mobilities of the ions from nitrobenzene, o-chloronitrobenzene, p-nitrophenol, m- chloronitrobenzene, and 2,3-dichloronitrobenzene are compared. These values are listed in Table 11. They were determined by Karasek (3, 14) and by Franklin GNO Corp. (15). Included in this table are the ionic species as determined by API techniques. As shown, the mobilities found by two investigators, for the ion products from 0- chloronitrobenzene and p - nitrophenol, are identical and different by only about 1%from that of nitrobenzene. The difference in values between the two laboratories is most likely due to errors in the measurement of absolute temperature (15).From these data, the difference in mobility between [C&N02]and the phenoxide ion [N02C&I50]- is estimated to be on the order of the experimental error of f0.02 cm2/V sec listed by Karasek ( 1 3 ) for PC work. In this case, mobility data cannot differentiate between [C&jN02]- and [NO&sH40]- ions, or between [ClC&4N02]- and [ClN02C&@]- ions. These results are of considerable interest because they show that the addition of oxygen to nitrobenzene does not greatly affect the mobility of the resulting ion, although it represents a significant change in mass. This implies that the diffusion cross-sections for the related ions are much the same. We believe that these two examples of mistaken ion identification clearly illustrate the dangers involved in making structural identifications based upon mobility data. The recently introduced practice of illustrating PC ion yields in the bar graph format used in mass spectrometry, which suggests that ion identification is assured by mobility measurements, implies a certainty of identification which the technique does not in fact possess. Sensitivity of Detection of Negative Ions. Subpicogram sensitivity of detection was demonstrated earlier for a compound ionized by proton addition. In this study, 2,3,4,5,6-pentachlorobiphenylwas used as a model compound. Air was used as a carrier gas in these experiments at a source temperature of 200 "C. Selective ion monitoring techniques were used to detect formation of the phenoxide ion. The distribution of these ions is shown in Figure 3. The amu values indicated that these were tetrachlorophenoxide ions in the biphenyl series, and the isotope distribution was that expected for a tetrachloro compound. The structure of the phenoxide ion is not known, but it is probably the ion ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1311
2.3,4.5.6-PC B
5 00
SIM ( M - C l + O ) SUBPICOGRAM SAMPLES 2 00
I 305
31
Figure 3. Tetrachlorophenoxide ions formed from 2,3,4,5,6-pentachlorobiphenyl by gas phase nucleophilic substitution with Oz-.Carrier gas, air: temperature, 200 O C
Flgure 4. Response observed for subpicogram samples of 2,3,4,5,6-pentachlorobiphenyl in air carrier gas. The tetrachlorophenoxide ion at 307 a m u was monitored. Temperature, 200 O C
corresponding to 2,3,5,6-tetrachloro-4-hydroxybiphenyl. than 1 ppm oxygen. Chlorobenzene and o-dichlorobenzene Samples of 150 femtograms to 5 picograms were used, with yield only chloride ions. m- Chloronitrobenzene shows a small yield of phenoxide ions in air but only M- in nitroprecautions as described earlier (6) to avoid syringe contamination. Figure 4 shows the result of sequential injecgen. “Dissociative electron capture” to yield phenyl negations of samples of increasing size. The limit of detection is tive ions (M - C1)- has not been observed for any of the approximately 100 femtograms. compounds studied. These conclusions are based on ion mass measurement; ion mobility measurements can not Figure 4 is intended to illustrate the sensitivity of detecdistinguish between the related aromatic negative ions diftion found under API conditions. I t is not suggested that subpicogram response curves be used for absolute measurefering in mass by an oxygen atom. The sensitivity of API ments. Internal standards, preferably stable isotope labeled mass spectrometry is Comparable to that of electron capcompounds, are required for quantitative work. The peak ture detection; this sensitivity coupled with its structural width from the 150 femtogram injection is about 15 secspecificity recommends API as the method of choice for deonds. This represents a sensitivity of 1 x g/sec with a termination of many chlorinated aromatic compounds as well as of trace concentrations of oxygen. signal-to-noise ratio of 4, which is comparable to that of the electron capture detector for similar compounds (16). A g/sec with a unity signal-to-noise sensitivity of 4 X LITERATURE CITED ratio has been shown for 2,4,6-trinitrotoluene by Plasma (1) F. W. Karasek, Anal. Chem.. 46, 710A(1974). Chromatograph techniques (17). (2) F. W. Karasek. Anal. Chem., 43, 1982 (1971). (3) F. W. Karasek. 0. S. Tatone, and David M. Kane, Anal. Chem., 45, Gas chromatography with an EC detector has been, and 1210 (1973). is, widely used in environmental studies. The identification (4) G. W. Griffin, I. Dzidic. D. I. Carroll, R. N. Stillwell, and E. C. Horning, Anal. Chem., 45, 1204 (1973). of a specific compound, and its quantification, is based (5) E. C. Horning, M. G. Horning. D. I. Carroll, I. Dzidic, and R. N. Stillwell. upon retention time data and an observed EC response. Anal. Chem., 45, 936 (1973). Since many chlorinated compounds give an EC response, (6) D. I. Carroll, I. Dzidic, R. N. Stillwell, M. G. Horning, and E. C. Horning, Anal. Chem., 46, 706 (1974). quantitative data for complex mixtures of chlorinated pes(7) Matheson Gas Data Book, Fifth Edition, Matheson Gas Products, East ticides, polychlorobiphenyls and the like are likely to be Rutherford, NJ. (8) I. Dzidic, D. I. Carroll. R. N. Stillwell, and E. C. Horning, J. Am. Chem. subject to frequent misinterpretation, particularly when Soc., 96, 5258 (1974). packed columns are used. A combination of APT mass spec(9) H. P. Tannenbaum and R. C. Dougherty, Abstracts from the 21st Annual trometry with liquid (18, 19) or gas chromatography should Conference on Mass Spectrometry, San Francisco, CA, May 1973, p 243. be regarded as the method of choice when very high sensi(10) R . C. Dougherty, J. Dalton, and F. J. Biros, Org. Mass Spectrom., 6, tivity in detection is to be used in identification and quan1161 (1972). (11) R. C. Dougherty, J. D. Roberts, and F. J. Biros, Anal. Chem., 47, 53 tification studies for highly complex mixtures of these com(1975). pounds. When determinations are made in the subpico(12) S. N Lin. G. W. Griffin, E. C. Horning, and W. E. Wentworth, Chem. gram range, however, the use of stable isotope labeled inPhys., 60, 4994 (1974). (13) F. W. Karasek and David M. Kane. Anal. Chem., 46, 780 (1974). ternal standards is recommended.
CONCLUSION Atmospheric pressure ionization mass spectrometry permits the experimental investigation of negative ion formation under the temperature and pressure conditions characteristic of electron capture gas chromatographic detection. The principal ions formed from polychloroaromatic compounds and from chloroaromatics activated by an ortho or para nitro group are chloride ion and the phenoxide ion formed by replacement of a chlorine by an oxygen atom. The source of the oxygen is 0 2 present in the carrier gas; these ions are formed even in nitrogen containing less
1312
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
F. W. Karasek and D. M. Kane. J. Chromatogr., 93, 129 (1974). R. F. Wernlund, Franklin GNO Corp., personal communication, 1975. M. Krejci and M. Dressler, Chromatogr. Rev., 13, 20 (1970). M. J. Cohen, Technical Report 5305, Franklin GNO Corp., West Palm Beach, FL. (18) E. C. Horning, D. I. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R. N. Stiilwell. J. Chromatogr. Sci., 12, 725 (1974). (19) E. C. Horning, D. I. Carroll, I. Dzidic, K. D. Haegele, M. G. Horning, and R. N. Stillwell, J. Chromatogr., 99, 13 (1974). (14) (15) (16) (17)
RECEIVEDfor review September 18, 1974. Accepted February 10, 1975. This work was supported by Grants GM13901 of the National Institute of General Medical Sciences, HL-05435 of the National Heart and Lung Institute, and ($125 of the Robert A. Welch Foundation.
