+
4-DODECANOL
LITERATURE CITED
?
x
mol
B. M. Fabuss, J. 0 . Smith, and C.N. Satterfield, Adv. Pet. Chem. Refin., 9, 156-195 (1964). D. Kunzru. Y . T. Shah, and E. 8.Stuart, ind. Eng. Chem., Process Des. Dev., 11, 605 (1972). D. L. Fanter, J. Q . Walker, and C. J. Wolf, Anal. Chem., 40, 2188 (1968). E. J. Levy and D. G. Paul, J. Gas Chromatogr., 5, 136 (1967). D. L. Fanter, R. L. Levy, and C. J. Wolf, Anal. Chem.,'44, 43 (1972). H . Groenendyk, E. J. levy, and S. F. Sarner, J. Chromatogr. Sci., 8, 599 (1970). C. Merritt, Jr., and D. H. Robertson, Anal. Chem., 44, 60 (1972). C. Merritt, Jr., and C. DePietro, Anal. Chem., 44, 57 (1972). J. H. Dhont, Analyst(London), 89, 71 (1964). H. L. C. Meuzelaar and R. A . in't Veld, J. Chromatogr. Scb, I O , 213 11972) -, H, R. Schuiten, H. D. Beckey, A . J. H. Boerboom, and H. L. C. Meuzelaar. Anal. Chem.. 45. 2358 11973). H. L. C. MeuzelaarandP. G. Kistemaker, Anal. Chem.. 45, 587 (1973). P. G.Simmonds. G. P. Shulmann, and C. H. Stembridge, J. Chromatogr. Sci., 7, 36 (1969). P. G. Simmonds. Appl. Microbiol., 20, 567 (1970). B. D. Boss and R. N. Hazlett, Can. J. Chem., 47, 4175 (1969). B. D. Boss and R. N. Hazlett. lnd. Eng. Chem., Prod. Res. Dev. 14, 135 (1975). J. E. Taylor, D. M. Kulich, D. A. Hutching, and K . J. Frech, Am. Chem. Soc., Div. Pet. Chem. Prepr , 17 (2). 847 (1972). F. W. McLafferty, "Interpretation of Mass Spectra", 2nd ed., W. A. Benjamin, Inc.. Reading, Mass., 1973, p 58. J. Guillot, H. Bottazzi, A. Guyot, and Y. Trambouze. J. Gas Chromatogr., 6 , 605 (1968).
1-45 6 7 8 9 IO I1 I2 12 12 12
5-DODECANOL; "7
6
3
s
:IE
6-OODECANOL
2 m
3
0 m
1-45 6 7 6 9 10 11 12 I2 12 12
CARBON
NUMBER
Figure 4. Pyrograms of the dodecanoi pyrolysis products under conditions discussed in Figure 2 The thin lines represent a-olefins as well as isomeric C I 2 olefins. The repeated carbon number 12 is used to separate the products and show increasing retention time. Other products are identified as follows: (1) acetone, (2) hendecanai, (3) propanal 2-butanone, (4) decanal, (5) 2-hendecanone, (6) butanal. (7) 2-pentanone, (8) nonanal, (9) 2-decanone, (10) pentanal. (111 2hexanone, (12) octanal, (13) 2-nonanone, (14) hexanal, (15) 2-heptanone, (16) heptanal. (17) 2-octanone
+
ACKNOWLEDGMENT The authors thank F. U'.Williams for considerable cooperation in obtaining the mass spectra.
RECEIVEDfor review April 23, 1975. Accepted October 22, 1975. This work was supported by the Naval Air Systems Command, Washington, D.C. 20360, and was presented t o the Petroleum Division, 167th National Meeting, American Chemical Society, Los Angeles, Calif., March 31-April 5, 1974.
Determination of Chlorophenoxy Herbicides in Air by Gas Chromatography/Mass Spectrometry: Selective Ion Monitoring S. 0. Farwell,* F. W. Bowes, and D. F. Adams Air Pollution Research, Washington State University, Pullman, Wash. 99 763
The electron-impact mass spectra for the chlorophenoxy family of herbicides were obtained and subsequently examined for their characteristic fragment Ions. These mass ions were used to establish selective ion monitoring techniques for the specific analysis of airborne 2,4-D compounds. A separate diffusion pump for the ion source of the quadrupole mass spectrometer allowed a direct Interface between the gas chromatograph and the mass spectrometer; thus, the total gas chromatographic effluent flows into the ion source. This total-effluent GC/MS system is extremely advantageous for maximum sensitivity and reproducibility in environmental analysis. For instance, if the base peak in the mass spectra of the chlorinated herbicides is monitored continuously, then the detection limit is in the 1-0.1 pg range. In practice, routine air samples were analyzed for 2,4-D butyl ester down to the 1-pg level. Data are also presented for a comparison study between GC/ECD-3H, GC/ ECD-63Ni, and GC/MS-SID quantitative results. 420
Both the agricultural application of 2,4-D herbicides and the concomitant atmospheric contamination from their use have recently increased in frequency and intensity over large areas of the United States and Canada ( I , 2). T h e most apparent environmental hazard in the use of the 2,4-D herbicides is due to the airborne movement of the chemical to nontarget vegetation during or after application. This family of herbicides are potent broad-leaved weed killers, but are unfortunately also extremely toxic t o grapes, cotton, tomatoes, and other susceptible crops (3-5). Therefore, severe injury to crops outside the treated areas can occur from the spray drift and/or the movement of the herbicide vapors. Because of these facts, it has become increasingly important to analyze qualitatively and quantitatively for trace concentrations of 2,4-D compounds in the air. The most popular technique for 2,4-D analysis is a gas chromatograph with an electron-capture detector (GC/
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
ECD). Although the GC/ECD has a detection limit of about 50 pg for the esters of 2,4-D and provides a convenient means of analysis, it is now widely recognized t h a t the qualitative information in an ECD chromatogram is inadequate for positive identification. In addition, the numerous variety of different chemical compounds that could be present in an air sample further limits the chromatographic information to a tentative identification. Even a supplemental clean-up procedure prior to the analysis of atmospheric samples cannot conclusively eliminate this analytical ambiguity. An electron-capture detector will respond to any compound in the sample that has an appreciable electron affinity, and the actual detector response can vary widely depending upon the number and types of halogens on the parent halogenated molecule. Thus, the ECD is capable of detecting any halogenated hydrocarbon and is not selective to one specific group of chlorinated pesticides, such as the 2,4-D herbicides. During the past three years, our laboratory has conducted an extensive atmospheric monitoring program for 2,4-D (6, 7). Because of our need to determine confidently which 2,4-D compounds are present in the collected air sample, and hence are responsible for the subsequent off-target crop damage a t ambient concentrations, the application of a quadrupole mass spectrometer as a chromatographic detector (GC,’MS) for this analysis was examined. The existence of such a specific and sensitive GCIMS analyzer for 2,4-D herbicides would also be useful in the careful appraisal of the GC/ECD technique for its effectiveness in the selective analysis of the airborne 2,4-D herbicides. When the mass spectrometer is operated in its normal mass scanning mode, a GC/MS system can generate the analytical information required for positive identification, but the corresponding sensitivity is a t the low nanogram level. This nanogram sensitivity is frequently inadequate for the trace determinations of various environmental agents such as 2,4-D herbicides in ambient air. Consequently, the application of selective ion monitoring (SIM), or mass fragmentography (MF), is extremely powerful in environmental analyses because of its inherent increased sensitivity. Instead of scanning a complete mass range, the mass spectrometer in the selective ion mode monitors exclusively one or more ions of a specified mass ( d e ) . Thus, the fraction of the corresponding specific ion(s) reaching the ion detector is increased greatly since the detector is concentrated on one or a small number of ions instead of all the ions in the entire mass range. Selective ion monitoring may be separated into the closely related techniques of single ion detection (SID) and multiple ion detection (MID). In the latter method, several distinct ions are monitored sequentially, whereas only one mass ion is monitored during the chromatographic analysis in single ion detection. Consequently, single ion detection is more sensitive because the detector signal from one fragment ion is recorded continuously but, for the same reason, also results in the loss of considerable qualitative information. Therefore, it is advantageous to establish an area ratio using several different masses which represent the most intense and diagnostic ions for the compound. When unequivocal identification is required, the observed responses of the sample ions can be compared to the relative abundances obtained for the identical ions of the standard. Furthermore, any changes in the relative area ratios for a component can be used to indicate a coeluting chromatographic impurity. A recent dodecapole mass spectrometer system has the ability to monitor up to five groups of four characteristic ion fragments over the entire mass range of the instrument, one group a t a time, during a chromatographic
analysis. T h e length of time each group is measured is a controllable variable. For these and other reasons, quadrupole mass spectrometers are advantageous compared to conventional magnetic-sector types for multiple ion monitoring (8, 9). T h e selective ion monitoring techniques have been the subject of a number of recent reviews and research articles (9-14). Furthermore, Grimsrud et al. (15)of our research section have showed that mass spectrometric analysis with gas chromatography can be applied to the direct analysis of chlorofluorocarbons in background air samples. Until now, there has been no evaluation of the selective ion monitoring technique for the analysis of 2,4-D herbicides and their related compounds. This paper describes a gas chromatograph/mass spectrometer system which was designed specifically for the direct introduction of the total gas chromatographic effluent into the ion source of the mass spectrometer. The total-effluent capability has been combined with the technique of selective ion monitoring to provide a specific and sensitive analytical method for the analysis of 2,4-D herbicides. T h e experimental basis and application of the combined GC/MS technique to actual atmospheric samples are reported along with the results of a correlation study between data obtained by corresponding GC/ECD and GC/MS-SID analyses. In addition, the mass spectra and generalized fragmentation for the chlorophenoxy herbicides and closely related compounds are reported.
EXPERIMENTAL Reagents. The analytical standards of the various chlorophe-
noxy herbicides were prepared from the following pure compounds: 2,4-D acid (or 2,4-dichlorophenoxyaceticacid), 98%;2,3-D acid, 98%; 3,4-D acid, 96%; 2,4-D dimethylamine, 97.7%; 2,4-D methyl ester, 99+%; 2,3-D methyl ester, 98%; 3,4-D methyl ester, 96%; 2,4-D isopropyl ester, 98.7%; 2,4-D n-butyl ester, 98%; 2,3-D n-butyl ester, 98%; 3,4-D n-butyl ester, 96%; 2,4-D isobutyl ester, 98.4%; 2,4-D n-pentyl ester, 99+%; 2,4-D PGBE (or propylene glycol butyl ether) ester, 96%; 2,4-D butoxyethanol ester, 99.8%; 2,4-D 2-ethyl hexyl ester, 97.7%; and 2,4-D isooctyl ester, 99.6%. Except for the methyl and butyl esters of 2,3-D and 3,4-D,all of the chlorophenoxy chemicals were obtained commercially from the Dow Chemical Co., Chemical Manufacture Laboratory, and Aldrich Chemical Co. The methyl and butyl esters of 2,3-D and 2,4-D were synthesized from their respective parent acids by esterification with BFs/methanol or BF3ibutanol reagents acquired from Analabs, Inc. The esterifications were performed according to the procedure described by Horner et al. (16). The 2,4-D standards for GC/MS and GC/ECD analysis were prepared in Nanograde benzene from Mallinckrodt or in twice distilled n-decane from the Humphrey Chemical Co. The 2,7-dichlorodibenzo-p-dioxin of 99% purity was obtained from Analabs, Inc. The purity and identity of all the chlorophenoxy compounds and the polychlorinated dibenzo-p-dioxin were confirmed by gas chromatography and mass spectrometry. Sample Collection. The atmospheric 2,4-D samples employed in this study were collected with the differential air-sampling system developed by Adams et al. ( 2 7 ) .However, the impactor sections of the original samplers were modified by replacing the pressfit glass solvent cup with a new threaded-seal glass cup for a more leak-proof assembly ( 7 ) .The impactor cup contained a two-phase collection solution of 10 ml n-decane and 10 ml of 5% aqueous NaHC0:I. The midget impinger contained 10 ml of n-decane, which was purified by fractional distillation. The middle fraction of the n-decane distillation was collected for use in the air samplers. The field monitoring sites were located in the grape-growing areas of Eastern Washington and this sampling network has been fully described elsewhere (7). Instrumentation. The gas chromatograph/mass spectrometer system was a Hewlett-Packard Model 5700A GC interfaced to a Hewlett-Packard Model 5930A MS with a dual chemical/electron ionization (CI/EI) source. The mass spectrometer’s highspeed vacuum system consists of two 4-inch diffusion pumps, one attached directly to the ion source housing and one connected to the mass analyzer. Thus, the ion source and mass analyzer volumes are pumped separately. T w o rough pumps are also used, one for the ANALYTICAL CHEMISTRY, VOL. 48, NO.
2, FEBRUARY 1976
421
MINUTES
MINUTES 4
(d )
2
6 IO MINUTES
14 78 6 -
Figure 1. GC/ECD chromatograms of a 2,443 mixture
Sample components: (1) methyl ester, (2) isopropyl ester, (3) isobutyl ester, (4) n-butyl ester, (5) PGBE ester, (6) isooctyl and ethyl hexyl esters, (7) butoxyethanol ester. Chromatographic conditions: injector temperature of 200 O C , detector temperature of 300 O C , column temperature of 180 ‘C, carrier gas flow rate of 45 mllrnin analyzer foreline and a smaller one for the inlet system. The addition of a separate diffusion pump for the ion source permits a “direct-coupled” interface between the gas chromatograph and the mass spectrometer. Consequently, no molecular separator or enricher is required with our differentially-pumped mass spectrometer, and the entire GC column effluent is transferred into the MS ion source. This direct interface system allows a maximum GC effluent of 20 ml/min directly into the ion source; however, flow rates of approximately 10-15 ml/min are routinely used in our laboratory. Flow rates of up to 20 ml/min may be admitted directly to the ion source while maintaining source and analyzer Torr, respectively. The dipressures in the ranges of and rect GC/MS interface has a number of advantages in efficiency, simplicity, reproducibility, and sensitivity (9, 18, 19). Naturally, these characteristics are extremely valuable in the trace analysis of environmental pollutants. In addition, the GC/MS system has the standard features such as an independently heated direct insertion probe, a heated expansion volume, a Bendix continuous dynode electron multiplier (CDEM), oscilloscope, recording oscillograph, and strip chart recorder. Although a variety of GC columns, ranging from support-coated open tubular (SCOT) capillary columns to %-in. 0.d. packed columns have been used in our GC/MS system, the mass spectral data reported in this article were obtained with 3-ft X %C-in.stainless steel columns. Four different column packings were evaluated for their suitability in the GC/MS analysis of the chlorophenoxy compounds: a) 1.5% SP-2250/1.95% SP-2401 on 100/120 mesh Supelcon AW-DMCS; b) Durapak (Low K’) Carbowax 400/Porasil F, l O O / l Z O mesh; c) Bondapak Clx/Porasil C, 120/150 mesh; and d) 5% Dexsil 300 on 60/80 mesh Chromosorb W-AW-HMDS. A 10-ft X Vs-in. glass column packed with 1.5%SP-2250/1.95% SP-2401 on 100/120 mesh Supelcon AW-DMCS was used in the GC/ECD analyses. The GC/MS carrier gas was ultra-pure helium, and 95% argon5% methane was employed in the Perkin-Elmer Model 3920 gas chromatograph since it is equipped with a “”Ni electron-capture detector. An Aerograph Hy-Fi Model 600 gas chromatograph with a 3H electron-capture detector was also employed in the correlation study, and nitrogen was used as the carrier gas in this instrument. A Hewlett-Pnckard Model 3380A recording intergrator was used with the Perkin-Elmer and Aerograph ECD chromatographs. Integrated peak areas of the GC/MS single ion chromatograms were computed with the aid of a manual planimeter.
RESULTS AND DISCUSSION Because of our extensive 2,4-D monitoring study during t h e past three years (1973-1975), we have been measuring herbicide levels in the atmosphere by GC/ECD. However, this chromatographic analysis cannot absolutely establish 422
1 -
8
6 4 MINUTES
u q 2
6 4 2 MINUTES
Figure 2. Single ion chromatograms of a 2,4-D mixture
Mass monitored: ( a ) m/e = 175, ( b ) m/e = 162, ( c ) m/e = 111, (4 m/e = 185, and (e) m/e = 199. Sample components: (1) methyl ester, (2) isopropyl ester, (3) isobutyl ester, (4) n-butyl ester, (5) PGBE ester, (6) isooctyl and ethyl hexyl esters, (7) butoxyethanol ester. GUMS conditions: column temperature-programmed from 178 to 210 OC at 4’/min, detector gain of 2, bandwidth of 73, helium flow rate of 10 ml/min the identity or the corresponding concentrations of these airborne herbicides. Even gas’ chromatographic analysis based upon retention time data on different stationary phases does not conclusively prove the identity of a chromatographic peak. Futhermore, the optional confirmation technique of transesterification is often excessively time consuming for a large number of routine field samples and is also frequently inadequate since the esterification procedure accounts for the total 2,4-D in the sample, and therefore can result in an inconsistent mass balance when compared to the total for the individual 2,4-D compounds previously identified in the sample. For example, t h e retention time identification of unknown chromatographic peaks can only be as exhaustive as the set of standards available to the analyst, and the actual sample chromatograms repeatedly contain peaks not identified by our present set of standards. Larose e t al. (20) have reported a linear relationship t h a t can be used to predict the retention times for a series of esters of a particular chlorophenoxy acid, but the use of these retention indices for qualitative information is restricted to provisional identification. In addition to our own concern regarding the sole analysis of atmospheric samples for 2,4-D herbicides by GC/ ECD, Sutherland and co-workers (21) have recently expressed similar apprehensions. For instance, they were able to confirm only 63% of their gas chromatographic multicolumn identifications by chemical derivitization via butylation. In this same article, they report that 57% of the individual 2,4-D compounds were unaccounted for by their two-column confirmation procedure. Because of these analytical ambiguities, we examined the potential of gas chromatography/mass spectrometry, and especially the technique of selective ion monitoring for the absolute analysis of airborne herbicides and pesticides. T h e initial GC/MS experiments involved the selection of a chromatographic column packing with the desired characteristics of low column bleed and adequate separation for the 2,4-D esters. T h e compromise selection for the column material used in this evalution of the methodology was the
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
2 , 4 - 0 " ~ B " t f 1 Ester
2 . 4 - 0 Methyl Ester
-'
60.
1
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,
8
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23
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M e t h y l Ester
Figure 3. El mass spectra for the methyl esters of ( a )2,4-D, ( b ) 2 - 3 and ( c )3,4-D
D,
1.5% SP-2250/1.95% SP-2401 on 100/120 mesh Supelcon AM;-DMCS since this column packing had also been used in t h e routine GC/ECD analyses. Figures 1 and 2a illustrate t h e GC/ECD and t h e GC/MS single ion chromatograms for a mixture of 2,4-D esters with this column material. However, the chemically-bonded Bondapak Cih/Porasil C column provides adequate resolution and a lower bleed rate than the SP-2250/SP-2401 column, and might be perferable for the routine analysis of numerous samples on a GC/MS system. T o investigate thoroughly t h e potential for selective ion monitoring by GC/MS, it was imperative to acquire the electron-impact mass spectra for each of t h e chlorophenoxy compounds. Because space limitations preclude presentation of all these individual mass spectra. Figures 3-5 show several representative spectra for the esters of 2,4-D, 2,3-D, a n d 3,4-D. T h e mass spectrum and generalized fragmentation scheme of 2,4-D acid have been described by Safe et al. (22). For those who are interested, t h e complete set of the mass spectra can be obtained from the authors. The composite group of mass spectra for the 2,4-D herbicides permitted correlation of specific mass ions for t h e ensuing single m/o chromatograms. For example. t h e mass fragments of m / e 175, m/e 162, and m/e 111 are large ions in the mass spectra of all t h e dichlorophenoxy esters, and therefore can be used to monitor selectively the chromatographic effluent for the presence of these compounds. On t h e other hand, the mass fragment a t m/e 199 is specific for t h e methyl esters of 2,4-D and 2,3-D. while t h e mass ion a t m/e 185 is selective for the ethyl, pentyl. and butyl esters. Table I summarizes the relative intensities for the major fragment ions of t h e chlorinated phenoxy compounds and illustrates their respective potential for use in selective ion monitoring. An interesting observation was noted when the mass spectra for the methyl and n-butyl esters of 2,3-D, 2,4-D and 3,4-D were compared. As shown in Figures 3 and 4, only the 2,3-D and 2,4-D methyl esters displayed peaks a t m / e 199 (P - C1)+ and likewise, only the 2,3-D and 2,4-D n-butyl esters showed a peak a t rn/e 185 [P - (C1 C?H,)]+. In addition. n-butyl 2,3-D is the sole ester in this
+
Figure 4. El mass spectra for the n-butyl esters of ( a ) 2.4-D. ( b ) 2,3-D, and
( c )3,4-D
.'_
-.
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__
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Figure 5. El mass spectra of ( a )2,4-D butoxyethanol ester, ( b ) 2,4-D isooctyl ester, and (c) 2,4-D PGBE ester
group t h a t has a mass spectral peak a t m/e 241 (P- C1)+. On the basis of these preliminary data. it appears t h a t the ease of chlorine loss from t h e radical cation of t h e parent chlorophenoxy esters is 2,3 > 2,4 > 3,4. Apparently, the steric crowding of the ortho chlorine affects t h e mass fragmentation scheme. In our laboratory analysis of air samples for 2,4-D, the acid and amine formulations of 2,4-D are isolated in t h e cleanup procedure and are subsequently methylated for GC analysis ( 6 , 7 ) .Thus, the chromatographic quantitation for the methyl ester of 2,4-D represents the total acid and amine types of 2,4-D in t h e sample. Fortunately, the 2,4-D methyl ester is the only 2,4-D formulation with a fragment a t m/e 199 (P - Cl)+ and consequently, has this specific ion for t h e G C i M S determination of t h e 2,4-D methyl
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
423
Table 11. Major Mass Fragments of Chlorophenoxy Compounds m/e
A-
T:
E N
Em
k-
m
24 1 220 213 199 185 175 162 145 133 111
:. .: n E m m m E E %or a
109
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0
m
m
42
-
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3
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3
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424
. .
.
ester. In fact, the ion a t m / e 199 is the base peak for the methyl ester which naturally augments the overall sensitivity. Similarly, it is important to analyze specifically for the n-butyl ester of 2,4-D because it is an extensively used formulation of high-volatile 2,4-D. T h e base peak in the mass spectrum of n-butyl 2,4-D is m / e 185, and this unique ion provides a sensitive and selective GC/MS analysis for this ester. The combination of gas chromatographic retention times and specific m / e ions permits a selective method for 2,4-D herbicide analysis. Figure 2 displays several single-ion chromatograms for a standard mixture of the methyl, isopropyl, isobutyl, n-butyl, PGBE, isooctyl, and butoxyethano1 esters of 2,4-D. These mass chromatograms demonstrate the changes in the peak intensities t h a t occur when different mle values are monitored by the mass spectrometer. The probable compositions of the fragment ions responsible for the major mass spectral peaks of the chlorophenoxy compounds and their corresponding m / e values are listed in Table 11. After these preceding experiments designed to provide a basis for the selection of specific ions and chromatographic columns for G U M S - S I M analysis, the sensitivities of the single-ion technique were explored. Figures 6 and 7 illustrate typical SIM results for the determination of n-butyl respectively. 2,4-D ester and 2,7-dichlorodibenzo-p-dioxin, T h e 2,7-dichlorodibenzo-p-dioxin is included in the development of the analytical methodology for 2,4-D herbicides since this particular polychlorinated dioxin would seem t o be the most probable dioxin contaminant in commercial 2,4-D formulations. T h e electron-impact fragmentation and ion abundance data have previously been described for the mono-, di-, and tetrachlorodibenzo-p-dioxins(23, 24). T h e base peak in the E1 mass spectra of all these chlorinated dibenzo-p-dioxins was the parent (P) molecular ion. In addition to their intense parent peaks, the only other fragment ions of significant analytical use are the [P (ClCO)]+ and the [P - (ClCO)2]+ species. T h e extremely toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been found as a contaminant of 2,4,5-T where it arises during the hydrolysis of tetrachlorobenzene to form 2,4,5-trichlorophenol, the industrial precursor of 2,4,5-T (25, 26). However, the production of 2,4-dichlorophenol, the precursor of 2,4-D compounds, does not involve any of the conditions necessary for 2,4,5-trichlorophenol production and, consequently, 2,3,7,8-TCDD has not been detected in 2,4-D (27). T h e exceptional sensitivity of the GC/MS single ion monitoring technique for the determination of chlorinated herbicides and related compounds is evident from Figures 6 and 7 . For instance, the detection limit for both 2,4-D butyl ester and 2,7-dichlorodibenzo-p-dioxin is about 0.1 picogram, or 100 femtograms. Since these two compounds
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
I (c)
!
TIME
(e)
(d) W
z
x w
a.
TIME
Figure 6.
Single ion chromatograms (m/e = 185) of 2,4-D +butyl
ester Sample concentrations: ( a ) 100 ng, ( b ) 10 ng, (c) 1.0 ng, (4 0.1 ng, (e) 0.01 ng, ( 0 1.O pg, (9)0.1 pg. G U M S conditions: column temperature of 205 ‘C, detector gains of 1-1 1, bandwidth at minimum, helium flow rate of 10 ml/rnin
L
n
a. 0 0
rr
TIME
Figure 7. Single ion chromatograms (m/e = 252) of 2,7-dichlorodibenzo-p-dioxin Sample concentrations: ( a ) 1.0 ng, (6)0.1 ng, (c)0.01 ng, (d~ 1.0 pg, (e) 0.1 pg. GCIMS conditions: column temperature of 210 O C . detector gains of 4-1 1 , bandwidth at minimum, helium flow rate of 10 ml/min
have only two chlorine atoms per molecule, their equivalent GClECD detection limit is approximately 50 pg. Thus, the groups of up to four specific mass peaks, one group a t a GC/MS-SIM technique offers three distinct advantages in time, are particularly advantageous in the trace analysis of comparison to GC/ECD for the analysis of the dichlorophemulticomponent air samples. With this multiple ion funcnoxy family of herbicides: a) specific mle qualitative infortion, up to 20 different mass ions can be analyzed during mation in addition to chromatographic retention time data, one GC/MS run while maintaining the picogram sensitivity b) a 5OOX increase in sensitivity, and c) the column oven inherent in measuring four or less ion peaks a t one time. In can be temperature programmed in the GC/MS-SIM systhe analysis of air samples for chlorophenoxy herbicides, tem whereas similar temperature programming of the GC/ this procedure allows the determination of five specific ECD results in excessive base-line drift. Of course, the ab2,4-D esters (e.g., methyl, n-butyl, PGBE, isooctyl, and busolute detection limit of the quadrupole mass spectrometer toxyethanol) since the analyst assigns the length of time for these chlorinated herbicides will vary according to the each 4-ion group is monitored during the chromatographic relative intensity of the particular ion selected for monitorrun. These time assignments for the 4-ion groups are based ing, but in all those cases where the monitored ion was also upon previously established gas chromatographic retention the base peak in the mass spectrum, the detection limit was time windows for each fo the five herbicides. always between 1.0 to 0.1 pg. In practice, we are able to anAfter the conclusion of the analytical development alyze routine air samples for 2,4-D esters in the presence of phase, the GC/MS-SID technique was used in a compariinterfering substances a t the picogram level with single-ion son study with GC/ECD for the analysis of 2,4-D butyl monitoring. T h e selective ion monitoring technique has reester in real air samples collected during our 1974 monitorcently been shown to be a useful method for the sensitive analysis of p,p’-DDT, and 2,3,7,8-tetrachiorodibenzo-p-di- ing program. Representative data from over 100 parallel oxin a t the femtogram and picogram concentrations, reanalyses are presented in Table 111. The samples utilized in spectively (28, 29). this investigation were randomly selected from approxiIf the analyst is willing to sacrifice a small amount of mately 1500 field samples. As noted in Table 111, these atsensitivity for more positive qualitative identification, then mospheric samples were analyzed on two separate electroncapture (“H and “Ni) gas chromatographs. The “constant the technique of multiple ion detection is available. As an current” ( 3 0 )63Ni ECD was linear over an extended range approximation, the overall GC/MS sensitivity for selective ion monitoring decreases as a function of l/\s,where n is of lo4 with a detection limit of approximately 50 pg for the 2,4-D butyl ester. However, the linear range for the “conthe number of different m / e masses monitored during the chromatographic analysis. Thus, the operator can selectivestant potential” (31) 3H ECD was limited to about 10% of its standing current, which corresponds to a linear range of ly determine the butyl esters of 2,4-D by choosing a specific -100. Therefore, it was necessary to dilute the more con4-ion group (e.g., m / e 276, mle 185, mle 175, and m / e 162) centrated 2,4-D samples so that the sample concentrations t h a t in turn can provide additional qualitative identificawere within the linear response region of the constant potion to the corresponding chromatographic peak. Furthermore, the different retention times of the n-butyl and isotential 3H ECD. The GC/MS-SID data in Table I11 were butyl 2,4-D allow specific identification of these two isocollected using a SID mass of 185 and the GC/MS response mers. was linear from 100 ng down to about 10 pg. Obviously, considerable care must be taken to ensure linearity a t the Although there are several different commercially-availlow picrogram levels. In addition to the linear response paable G U M S systems with programmable multiple ion monitoring capability; those with the ability to monitor five rameters of the detectors, nonlinearities a t these trace conANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
*
425
CONCLUSIONS Table 111. ECD and GC/MS-SID Comparison Study for 2,4-D Butyl Estera Sample No.
3H E C f , F g P
63Ni E C P , irg/M
GC/MS-SID, ~ g l M ~
1 0.11 0.00 0.00 2 0.91 0.16 0.17 3 0.70 0.29 0.32 4 0.69 0.08 0.09 5 0.11 0.10 0.10 6 2.30 0.18 0.18 7 0.12 0.04 0.06 8 2.60 1.01 1.04 9 0.10 0.08 0.09 10 2.10 0.30 0.31 11 2.60 0.29 0.30 12 6.10 0.71 0.74 13 4.90 0.57 0.55 14 10.0 1.32 1.36 15 3.40 1.45 1.48 16 2.20 1.14 1.00 17 16.0 1.82 1.72 18 0.71 0.66 0.69 19 0.48 0.27 0.33 20 2.50 0.28 0.20 21 5.90 1.29 1.27 22 0.00 0.00 0.00 23 0.14 0.13 0.14 24 26.0 2.55 2.52 25 34.0 2.48 2.20 26 10.0 1.36 1.32 27 0.00 0.00 0.00 28 1.40 0.44 0.43 29 5.90 1.66 1.48 30 0.26 0.15 0.17 31 0.61 0.59 0.63 32 3.90 1.53 1.40 33 0.12 0.10 0.11 34 0.97 0.17 0.15 35 0.31 0.20 0.15 a The quantitative data are the averages of two injections per sample where the two separate values agreed within 25%. b SID m / e = 185.0.
centrations can occur because of losses due to adsorption on the sample container, syringe, GC column, column packing, GC/MS interface, etc. The quantitative results obtained with the 3H ECD for the butyl ester were generally quite higher than the analogous results with the "Ni ECD and MS-SID detectors. The reason for the notable discrepancy in the data acquired with the tritium electron-capture detector is not known. Although considerable effort was expended daily in checking voltage profiles, standing currents, and dc potentials, small changes in the operating conditions of constant voltage ECD's can still produce erroneous results (32, 33). The only apparent explanations include the possibility of intermittent detector contamination due to the impurities in the field samples or some unascertained problem in our Varian Hy-Fi gas chromatograph. The operating temperature of the 3H ECD was 210 "C while the temperature of the 63Ni detector was 300 "C. Consequently, we have found that the 63Ni ECD does not become as readily contaminated by field sample analyses as the 3H detector and, therefore, does not require cleaning as frequently as the tritium detector in order to maintain linearity. Without additional proof, the preceding explanations are quite hypothetical. However, this insinuative analytical incongruity is worthy of further investigation by those atmospheric scientists who are using gas chromatographic results obtained with a tritium electron-capture detector as their only method of analysis. 426
The gas chromatograph/quadrupole mass spectrometersingle ion monitoring technique offers three significant features for the analysis of atmospheric herbicides and similar pollutants: a) low picogram sensitivity, b) qualitative identification based upon characteristic mass spectral ions and their relative responses in combination with the gas chromatographic retention time, and c) the capability of temperature-programming the gas chromatograph. We do not imply that GC/MS-SIM can be realistically employed for the routine analysis of large numbers of field samples, but instead emphasize the importance of this powerful technique to substantiate the qualitative and quantitative data obtained with GC/ECD. The significance of accurate analytical data cannot be overemphasized if we are to provide the scientific information required for future environmental decisions.
~
ACKNOWLEDGMENT The assistance of T. E. Bard, W. J. Powell, and D. L. Kissinger is gratefully acknowledged.
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ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
RECEIVEDfor review June 13, 1975. Accepted October 21, 1975. This project was financed in part by the Washington State Grape Society and Washington Wheat Commission.
