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and also the flux of the reagent into the eluent flow stream. Band spreading due to unequal fiber lengths or diameters in multifiber units can be minimized by adjusting the size of the hollow fiber unit so that i t has a total volume inside the fibers which is smaller than the volume of the peaks eluted. In this case only moderate end effects on the sample peaks will be observed in most systems. Reagent Addition. Several parameters should be considered for reagents in postcolumn reactions. T h e flux rate through a membrane is dependent upon the concentration gradient across the membrane. Thus as the reagent is depleted from the reservoir, the flux into the eluent will decrease. The effect this has on a particular analysis will depend on the system being studied. For example, if a large reservoir is used or a high reagent concentration is used to permeate excess reagent into the eluent, little effect will be seen on the analysis. Clearly the point should be determined for a given system where reagent depletion begins to affect the analysis and regeneration of the reagent performed prior to that time. Alternatively, a slow flow of fresh reagent may be used to replenish the reservoir by flowing around the fiber bundle. Attention should also be given to the choice of solvent used for the reagent as it may have either a benificial or detrimental effect in the flux of reagent through the membrane. Other Potential Uses for Hollow Fiber Reactors. This new technique of passive reagent addition or p H change is also applicable to electrochemical, turbidimetric, and conductometric detection as will as to vibrational and electronic
wavelength detection. These devices may be used both following chromatography and in flow injection analysis applications. The use of two or more reactors and multiple detectors may be desirable with particularly difficult sample matrices. Registry No. 2,4-DNP, 88-85-7; 4-OH, 93037-41-3; 2-OH, 94042-50-9;benzylamine, 100-46-9;glycine, 56-40-6.
LITERATURE CITED Frei, R. W.; Scholten, A. H. M. T. J. Chromatogr. Sci. 1979, 17, 152-160. Takata, Y.; Muto, G. Anal. Chem. 1973, 4 5 , 1864-1868. Stevens, T. S.;Davis, J. C.; Small, H. Anal. Chem. 1981, 53, 1488- 1492. Stevens, T. S.; Jewett, G. L.; Bredeweg, R. A. Anal. Chem. 1982, 54, 1206-1208. Davis, J. C. United States Patent 4448691, 1984. Peterson, D. P.; Davls, J. C. United States Patent 4451 374, 1984. Schulman, S. G. Rev. Anal. Chem. 1971, 1 , 85-111. Udenfriend, S.;Stein, S.; Bohlen, S.; Dairman, P.; Leimgurber, W.; Weigele, M. Science 1972, 178, 871-872.
’
Present address: Surface Sclence Laboratories, Standard Oil (Ohio), Cleveland, OH 44128. ‘Present address: PPG Research Center, Barberton, OH 44203.
James C. Davis*’ Dennis P. Peterson2 Dow Chemical USA Midland, Michigan 48640 RECEIVED for review January 27, 1983. Resubmitted October 15, 1984. Accepted November 15, 1984.
Adding Forward Searching Capabilities to a Reverse Search Algorithm‘ for Unknown Mass Spectra Sir: Mass spectrometry, especially with direct coupling to the gas chromatograph (GC/MS), is the most widely used tool for the identification of unknown organic molecules. The modern GC/MS can produce dozens or hundreds of unknown electron-ionization mass spectra per hour, for which many computer matching systems have been proposed (1-13). Of these, it is well-known that forward searching algorithms are substantially superior for unknown mass spectra of pure compounds (1-3) and that reverse searching (4-6) is advantageous for spectra of mixtures. We describe here a combination of forward and reverse searching that shows substantially improved performance for spectra of both pure compounds and mixtures without prior knowledge of purity. Reverse searching, which is considered necessary even for capillary column GC/MS ( I , 2,14), gives matching credit only for peaks of the reference spectrum that are in the unknown, not vice versa. Because of this, matching an unknown spectrum of pure dioctyl phthalate retrieves those of octenes (Table I), a logical impurity, as the fragmentation of C8HI6 in the phthalate is the source of many peaks in its spectrum similar to those from octenes (Figure 1). This causes the reliability value (the probability that the degree of match found indicates the correct answer) predicted by the probability based matching (PBM) system (4, 6,8,12, 13,15-17) to be misleadingly high (32%) for 3-octene; 14 of the top 20 matches are C,H2,, Compounds. However, these would be rated poor matches by forward searching, which would also require the presence of phthalate peaks. The scheme described here for combining reverse and forward searching capabilities is based on the automatic subtraction of matching
reference spectra (8).
EXPERIMENTAL SECTION Calculations were carried out with a Hewlett-PackardHP-1000 computer, matching against a library of 76 663 different mass spectra of 67 128 different compounds from the Wiley/NBS data base (13, 18) (spectra of isotopically labeled compounds were omitted). A total of 392 different compounds, each of which was represented in the file by more than one different spectrum, were selected at random from the file. The spectrum of highest “quality index” (19) of each compound was used to make the “pure spectrum” file of unknowns. Groups of three of these spectra, ordered by increasing molecular weight, were combined in 60:3010 (totalions) proportion for the fde of 130 unknown mixture spectra. Reference spectra used as unknowns were omitted from the PBM results. The PBM program used (“system a”) includes several modifications of the original ( 4 , 6 ) ,such as ranking of retrieved spectra by reliability values (15),abundance based flagging of spurious peaks (15-1 7), and quadratic scaling to minimize experimental abundance variations (16, 17). Class I matches are the same compound or a stereoisomer; class IV matches represent compounds that are sufficiently similar structurally to make difficult their differentiation by mass spectrometry (6,15).For retrieved spectra the maximum “% contamination” (% C) and minimum “% component” values allowed are 80% (75% for Table I) and l o % , respectively, restricting the retrieval of minor mixture components. In forward searching modification “b” the predicted reliability value is adjusted t o reflect the magnitude of the %C value; the adjustment is based on PBM results using the 392 unknowns (17). For the final system “c”,each reference spectrum which matches with a predicted reliability value >lo% is subtracted from the 0 1985 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table I. PBM Matching (Class IV) of Mass Spectra of Pure Bis(2-ethylhexyl) Phthalate and Its 3:l Mixture with 3-Octene pure serial no. 56290
compound
b
a
bis(2-ethylhexyl) 47e phthalated diisooctyl phthalate 42 2-methyl-1-butene 34 (Z)-3-octene 32 3,4-dimethyl-l-pen- 31 tene bis(2-ethylhexyl) 29 phthalate all octenesg 25 (no. of octenes (2) retrieved) all C,Hz, 24 compounds (no. of C,Hz, (14) compounds)
78723 67538 2942 1545 78728
mixture c
50 61
a
b
c
56
22
45
37 50 35
