ANALYTICAL CHEMISTRY, VOL. 50, NO. 11, SEPTEMBER 1978
paper for other materials, including petroleum products and petroleum process catalysts. LITERATURE CITED
1585
The British Petroleum Company Limited BP Research Centre Chertsey Road Sunbury-on-Thames, Middlesex TW16 7LN
(1) J. N. Wilson and C . Z. Marczewski, Anal. Chem., 4 5 , 2409 (1973). (2) Ernst Hohmann et al., €rdolKb;hle, 26, 647 (1973).
J. N. Wilson* C. Z. M a r c z e w s k i
RECEIVED for review April 26,1978. Accepted May 30,1978.
Exchange of Comments: Analysis of Complex Volatile Mixtures by a Combined Gas Chromatography-Mass Spectrometry-Computer System Sir: We are writing to correct several errors in the paper of Gates et al. (I)concerning previously published results from our laboratory ( 2 , 3 ) . In a detailed discussion of our work in the above paper, the authors seek to compare our methods with their own. During the course of this discussion, and in other places in their otherwise excellent paper, erroneous statements are made which in our judgment reflect badly on both laboratories and will result in considerable confusion in the minds of readers of the paper. First, a very important comparison is never explicitly stated in their paper. Their system ( I ) is designed specifically to quantitate previously observed compounds and is applicable to mixtures where the only interest is quantitation of these constituents. Our efforts (2, 3) have been directed toward the more general problem of detecting and quantitating both previously observed and new compounds. Both methods have their advantages and disadvantages. These are confused, however, by the discussions of Gates et al. (I). Second, we wish to correct the following specific points: (1)None of our methods utilize reverse library search ( I , p 439 and p 440). The HISLIB (3) program utilizes “forward” library search and comparison in all phases of operation. (2) T h e CLEANUP (2) program applies its doublet resolver recursively to deconvolute multiplets (2, p 1372). Although one might criticize this approximation, the program is not now, nor has it ever been, limited to simple doublets ( I , p 437 and
p 440) and routinely handles complex multiplets with limitations similar to those discussed by Gates et al. for their own procedures. (3) Our work makes explicit use of internal standards (3, p 1623 (abstract) and throughout the remainder of paper), not external standards as alleged ( I , p 440). (4) Although we have not published data on the accuracy, sensitivity, and linear range of our methods, we have published data on its precision (3).
Sir: In a recent paper ( I ) we made erroneous statements about the techniques developed at Stanford University (2,3) for the analysis of complex volatile mixtures by a combined gas chromatography-mass spectrometry-computer system. As Rindfleisch, Smith, and Yeager have indicated above, the HISLIB program (3)uses “forward” library search algorithms rather than “reverse” library search and quantitation is based on internal standards ( 3 ) ,not external standards. Furthermore, Smith et al. (3) have reported percentage standard deviation of their procedure in the analysis of relative concentrations of reference hydrocarbons and of urinary organic acids obtained either by organic solvent extraction or by anion-exchange chromatography. We regret that these errors have appeared in our discussion of the Stanford system, which is an alternative approach to our MSSMET system for quantitative metabolic profiling. I t does have several important advantages that are not yet imp!emented in MSSMET, including detection and quantitation of previously unobserved (novel) compounds. We certainly did not intend to denigrate the excellent work (2, 3) by this group.
The objection by Rindfleisch, Smith, and Yeager about the capability of their CLEANUP program (2) to deconvolute multiplets needs to be clarified. Our opinion, after having read their paper (2),was that several points were not clearly presented and documentation was minimal. In retrospect, we should not have said (1) that “their system can resolve doublets but not multiple overlapped mixtures of substances” because it was explicitly stated (2, p 1372) that sequential use of the doublet resolver can be used for the multiplet case. They go on to point out, however, that the full procedure has not been implemented beyond the doublet case (2, p 1372), and that the procedure may not perform very well with complex multiplets, in terms of producing accurate amplitude information (2, p 1374). We interpreted these statements to mean t h a t they were not running the system routinely on complex multiplets and had not yet documented a capability to deal with such mixtures effectively. The analysis of multiplets by MSSMET depends 011 the choice of a single, unique designate ion, rather than a cleaned-up spectrum, for each substance in the multiplet and has, indeed, been shown
0003-2700/78/0350-1585$01 .OO/O
LITERATURE CITED (1) S. C. Gates, M. J. Smisko, C. L. Ashendei, N. D. Young, J. F. Holland, and C. C. Sweeley, Anal. Chem., 5 0 , 433 (1978). (2) R. G. Drorney, M. J. Stefik, T. C. Rindfleisch, and A. M. Duffield, Anal. Chem.. 48, 1368 (1976). (3) D. H. Smith, M. Achenbach, W. J. Yeager, P. J. Anderson, W. L. Fitch, and T. C. Rindfieisch, Anal. Chem.. 49, 1623 (1977).
D e n n i s H. S m i t h * William J. Yeager T h o m a s C. Rindfleisch Department of Genetics Stanford University School of Medicine Stanford, California 94305
RECEIVED for review April 3, 1978. Accepted May 26, 1978.
1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO, 11, SEPTEMBER 1978
to be capable of quantitation of mixtures with a large number of unresolved components. We felt that the differences in the approach of these two methods in handling multiplets were important to the reader and regret that our concept of the status of their method was not accurate. The problem of peak deconvolution is of key importance, obviously, because it is central to the choice of combined gas chromatography-mass spectrometry systems rather than gas chromatography alone for quantitative metabolic profiling.
(2) R. G.Dromev, M. J. Stefik, T. C. Rindfleisch, and A. M. Duffield, Anal. Chem., 48, i 3 6 8 (1976). (3) D. H. Smlth, M. Achenbach, W. J. Yeager, P. J. Anderson, W. L. Fitch, and R. C.Rindfleisch, Anal. Chem., 49, 1623 (1977).
C h a r l e s C. Sweeley* S t e p h e n C. Gates John F. Holland Department of Biochemistry Michigan State University East Lansing, Michigan 48824
LITERATURE CITED (1)
S.C. Gates, M. J. Smisko, C. L. Ashendel, N. D. Young, J. F. Holland, and C. C.Sweeley, Anal. Chem , 50, 433 (1978).
RECEIVED for review May 8, 1978. Accepted May 26, 1978.
Chlorinated Benzyl Phenyl Ethers: A Possible Interference in the Determination of Chlorinated Dibenzo-p -Dioxins in 2,4,5-Tric hlorophenol and Its Derivatives Sir: The routine determination of the 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) content of 2,4,5-trichlorophenol, 2,4,5-trichlorophenoxyaceticacid esters and 2,4,5-trichlorophenoxypropionicacid esters is by a procedure involving silica gel column chromatography sample preparation followed by GC-MS detection ( 2 , 2 ) . The method is quite specific requiring proper elution through the silica gel column, proper retention time in the gas chromatograph, proper molecular weight as monitored at m / e 320, 322, and 324, and proper isotope ratios as measured from the peak heights of each ion monitored. However, a t the high sensitivities required (less than 0.1 ppm) for this determination, other peaks are observed in the mass spectrometer output, some of which are in the TCDD retention time region with the correct isotope ratios for tetrachloro compounds. By preparing a larger than normal 2,4,5-trichlorophenol sample, we have been able to identify several "caustic insoluble" components some of which have a potential to be mistaken for chlorinated dibenzo-p-dioxins by GC-MS. EXPERIMENTAL Apparatus. Mass spectra were obtained on an LKB-9000 with a System Industries S-150 computer system. The gas chromatography column was 9 f t X 2 mm i.d. glass packed with 370OV-1 silicone on 80/100 mesh Chromosorb W-HP. The column temperature was programmed from 100 to 240 "C at lO"/min. Accurate mass measurements were performed on an AEI MS-30 GC-MS. The gas chromatography column used was 6 f t X 2 mm i.d. OV-210 silicone on 80/100 mesh Chromosorb W-HP temperature programmed from 125 to 220 "C a t 6'/min. Mass measurements were performed by the DS-50 data system and the results reported are the average of a t least five determinations. The mass chromatograms in Figures 2 and 3 were obtained using a Finnigan 3000 GC-MS with the PROMIM attachment. The gas chromatograpy column used was 6 ft X 2 mm i.d. glass packed with 3% OV-3 silicone on 80/100 mesh Gas Chrom Z. Column temperatures are listed in the Figures. The chromatogram in Figure 1 was obtained by gas chro0003-2700/78/0350-1586$01.00/0
Table I. Mass Spectrum of Chlorobenzyl Trichlorophenyl Ether possible identity mle rel. abund." 320 1.0 321 322 323 3 24 325
0.1 1.8 0.1
196 197 198 199 200
0.8 0.1
167 169 171
2.1 2.1
125 126 127 128
100.0
99 101
11.5 2.5
89 90
31.8 9.9
G3 62 61
14.4
molecular ion, M' (measd 319.9231, theor. 319.9329)
0.8
0.1
C,H,OCl,', trichlorophenol ion
0.8
0.1 0.3
C,H,Cl,+
0.8
7.9 39.3 2.5
C,H6Cl+,chlorobenzyl (tropyllium) ion
C,H,Cl' C,H,+ C7H6'
6.8 3.1
a The relative abundances, especially for the weaker ions, may be in error by + 50% because of the presence of background which was subtracted by the data system.
matography with flame ionization detection using a 10 ft X 2 mm i.d. glass column packed with 1070 OV-101 silicone on 100/120 mesh Gas Chrom Q. The column temperature was programmed from 120 to 250 "C at 8"/min. Reagents. 2,6-Dichlorobenzyl-2,4,5-trichlorophenyl ether was synthesized by refluxing a solution of 2,6-dichlorobenzyl bromide 'C 1978 American Chemical Society
