Anal. Chem. 1987, 59, 2709-2710
for their continued technical assistance and support. Megabore GC columns were donated by J&W, Inc., through the assistance of R. Freeman. We extend our appreciation to D. Forsyth for helpful discussions regarding the photochemical mechanisms and E. Rogers for advice on the optimization of the capilliary GC methods. We are also grateful for the assistance offered by W. LaCourse and R. Nelson in reading early drafts of the paper. Registry NO.I-, 20461-54-5; IO,, 15454-31-6;IO,, 15056-35-6; 1-iodopentane, 628-17-1; allyl iodide, 556-56-9; 1,2-diiodoethane, 624-73-7; iodobenzene, 591-50-4; 1,4-diiodobenzene, 624-38-4.
LITERATURE CITED (1) Selavka, C. M.; Krull, I. S. And. Chem.. preceding paper In this Issue. (2) Kropp, P. J.; Polndexter. G. S., Plenta, N. J.; Hamliton, D. C. J . Am. Chem. SOC. 1978, 9 8 , 8135. (3) Nagaoka, S.; Takemura, T.; Hlraoakl, 8. Bull. Chem. SOC.Jpn. 1985, 58, 2082. (4) Sharma, R. K.; Kharasch, N. Angew. Chem., Int. Ed. Engl. 1988, 7 , 36. (5) Kropp, P. J.; Jones, T. H.; Polndexter, G. S. J . Am. Chem. SOC. 1973, 9 5 , 5420. (6) Polndexter, G. S.; Kropp, P. J. J . Am. Chem. SOC. 1974, 96, 7142. (7) McNeely, S. A,; Kropp, P. J. J . Am. Chem. SOC. 1978, 9 8 , 4319. (8) Fery-Forgues, S.; Palllous, N. J . Org. Chem. 1888, 5 1 , 627. (9) Crlstol, S. J.; Lee, G. A. J . Am. Chem. SOC. 1989, 91, 7554. (10) Lacourse, W. R.; Selavka, C. M.; Krull, I. S. And. Chem. 1987, 5 9 , 1366. (11) Selavka, C. M. Ph.D. Thesis, Northeastern University, 1987
(16) (17) (18) (19) (20) (21) (22) (23)
2709
Batley, G. E. Anal. Chem. 1984, 5 6 , 2261. Ta-Shma, R.; Rappoport, 2. J . Am. Chem. SOC. 1983, 105, 6082. Johnson, C. D. Tetrahedfm 1980, 36, 3461. Harrls, J. M.; Clark, D. C.; Becker, A,; Fagan, J. F. J . Am. Chem. Soc. 1974, 9 6 , 4478. Karton, Y.; Pross, A. J . Chem. Soc., ferkin Trans. 2 1977, 1860. McClelland, R. A.; Banak, N.; Steenken, S. J . Am. Chem. SOC. 1988, 108, 7023. Rltchie, C. D. J . Am. Chem. Soc. 1975, 9 7 , 1170. Halmovlch, 0.;Treinln, A. Nature (London) 1985, 207, 185. Fery-Forgues, S.; Palllous, N. J . Org. Chem. 1988. 51, 627. Smothers, W. K.; Schanze, K. S.; Sattiel, J. J . Am. Chem. SOC. 1978, 101, 1895. Butler, L. J.; Hlntsa, E. J.; L e e , Y. T. J . Chem. fhys. 1988, 8 4 , 4104. Nelleborg, P.; Lund, H.; Eriksen, J. Tetrahedron Lett. 1985, 2 6 , 1773.
RECEIVED for review December 9, 1986. Accepted June 25, 1987. Funding was provided by a Phase 1 NIH-SBIR (No. IR43ES04057-01)grant, an NIH Biomedical Research Grant (No. RR07143), and a grant from Pfizer, Inc., (Groton, CT). Support for C.M.S. was provided by a National Institute of Justice, U.S.Department of Justice, Graduate Research Fellowship (No. 86-IJ-CX-0058),and a scholarship from the Northeastern Association of Forensic Scientists. Points of view or opinions stated in this paper are those of the authors and do not necessarily represent the official position or policies of the US. Department of Justice. This is contribution number 328 from the Barnett Institute of Chemical Analysis and Materials Science at Northeastern University.
Real-Time Library Search of Vapor-Phase Spectra for Gas Chromatography/Fourier Transform Infrared Spectrometry Eluents Robert E. Fields, 111, and Robert L. White*
Department of Chemistry, University of Oklahoma, Norman, Oklahoma 73019
A gas chromatography/Fourler transform Infrared spectrometry (GCIFT-IR) data cdiectlon/iibrary search system Is described whlch provldes Infrared spectral characterization of gas chromatographic eluents during separatlon. Parallel computlng and multitasking are employed to arbitrate data acquisition and llbrary search functions based on prlorlty assignment. Various methods are described for Increasing the efflciency of real-time ilbrary Searching. The qualitative analysis capabUlty of the system Is established for a synthetic alcohol mixture. Real-time search results for unknown composition hydrocarbon and flavor mixtures are described. I t is determined that the slgnal-to-noise ratlos of GC/FT-IR eluent spectra are an important factor In the reproducibliity and quailty of real-time library search Identiflcatlon.
The importance of combining complementary structure specific detection methods with gas chromatography for complex mixture analysis is well established. A great deal of effort has been expended to develop routine gas chromatography/infrared/mass spectrometer analysis systems (GC/ IR/MS) (1-7). The effectiveness of GC/IR/MS analysis is heavily dependent on the method selected for interpreting 0003-2700/87/0359-2709$01.50/0
infrared and mass spectrometric information obtained during chromatographic separation. Initial GC/IR/MS identifications were performed by manually comparing library search results obtained from infrared and mass spectrometric data (1,2). This method was time-consuming and often required days for complete data evaluation. Williams et al. subsequently proposed several schemes for automating GC/IR/MS data evaluation ( 8 , 9 ) . Recently, an automated GC/IR/MS data evaluation procedure based on library search comparisons was reported (5).This automated analysis system was tested by using a 17-component synthetic mixture. Gas chromatographic separation of the 17 components required 25 min but the total analysis time (including chromatographicseparation) was 140 min. These results indicate that current limitations of GC/IR/MS analysis systems lie in available data reduction methods. GC/IR or GC/MS data reduction is usually considered an off-line function. As such, gas chromatographic separation must be completed prior to initiating spectral interpretation procedures. However, real-time data evaluation can be achieved by using parallel processing data acquisition hardware (10). In this paper, we describe a GC/FT-IR mixture analysis system incorporating real-time (during the separation) library searching for chromatographic eluents. Analysis 0 1987 Amerlcan Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
procedures presented here enhance GC/FT-IR capabilities and could be incorporated into existing GC/IR/MS automated data evaluation software. The quantity of useful information generated during GC/FT-IR analysis is enormous (11). Acquisition of hundreds or even thousands of infrared spectra during a typical separation is common. The time required for evaluating this information is often much longer than the actual analysis. This situation is continually compounded by development of more sensitive FT-IR spectrometers and greater resolution capillary gas chromatographic systems. Algorithms for generating chromatograms from infrared data (12-15) and for automated data evaluation (16,17) have been developed to minimize the number of repetitive tasks that must be performed by the operator. Previous automated data evaluation was not performed during chromatographic separation because data processing requirements were excessive and incompatible with data acquisition (17). A GC/lV-IR data colledion/library search system featuring simultaneous interferogram acquisition and spectral search is described here. The data system incorporates dual processors with multitasking and parallel computing capabilities. One of the processors is dedicated to signal averaging. The other is used for data file storage, real-time chromatogram generation, and library search. High priority is given to data file storage and real-time chromatogram generation. Library search is assigned low priority and is performed only if interferogram data storage and chromatogram generation functions are not pending. Program priorities are arbitrated by a multitasking UNIX operating system.
EXPERIMENTAL SECTION A Hewlett-Packard Model 5890 gas chromatograph equipped with a split/splitless capillary column injector was used for all mixture separations. The infrared spectrometer used for GC/ FT-IRmeasurements was a Mattson Instruments, Inc., Sirius 100 FT-IR. The FT-IR was equipped with a narrow-band MCT detector and was operated at 8 cm-' resolution (2048 data points) with an interferometer scan velocity of 2.5 cm/s. The alcohol mixture was prepared from reagent grade components without further purification. The flavor additive was obtained from Mattson Instruments, Inc. Flavor additive measurements were made by using a Mattson Instruments, Inc., light pipe accessory. The capillary column used for flavor additive separation was a 10-m X 0.32-mm DB-5 column with a 0.25-gm stationary-phase film thickness. A l-wL aliquot of the flavor additive was injected by using a split ratio of 1O:l. The column flow rate was adjusted to 2 mL/min and the gas chromatographic oven temperature was programmed from 60 to 175 "C at a rate of 2 deg/min. GC/FT-IR interferogram files were collected at a rate of 2.4 files/s. All other mixture analyses were performed by using a light pipe interface constructed in our laboratory and designed to fit within the sample compartment of the FT-IR spectrometer. This interface is described in detail elsewhere (18). The alcohol mixture was separated by using a 25-m X 0.31-mm Hewlett-Packard Ultra-l column (part no. 19091A-112) with a 0.52-wm methyl silicone stationary-phase film thickness. For alcohol analysis, the column flow rate was adjusted to 2 mL/min and a 0.5-gL sample was injected by using a split ratio of 1001. The gas chromatographic oven was temperature programmed from 40 to 150 "C at a rate of 20 deg/min after an initial isothermal period at 40 "C for 1min. Ten interferograms were signal averaged per data file requiring 1.6 s/file. The naphtha mixture was separated by using the same capillary column as the alcohols. Column flow rate was adjusted to 0.8 mL/min and a 0.5-pL aliquot was injected by using a split ratio of 34:l. The gas chromatographic oven was temperature programmed from 30 to 50 "C at 1.5 deg/min and then from 50 to 120 "C at 3 deg/min following an initial isothermal period at 30 "C for 10 min. Twenty-five interferograms were signal averaged for each FT-IR spectrum, requiring 4.0 s/file. Data System. A Mattson Instruments, Inc., Sirius 100 FT-IR spectrometer data system was employed without modification for the measurements described here. A block diagram of the data
PRINTER
MEM
B
I
DISK
Figure 1. Block diagram of the GC/FT-IR parallel computing data
system. iaa
;C/FI-IR
CPU I I O L t T I M k
-
I
I 8 8
5 0
1 0 8
:i0
7
~
a
2 5 0
No. SIGNAL AVERAGED SCANS
Figure 2. Percentage of CPU 1 data acquisition idle time as a function of the number of signal-averaged scans per data file.
system is shown in Figure 1. Data acquisition and real-time search software was written in our laboratory by using C programming language. Two 68000 microprocessors (CPU 1 and CPU 2) operated independently and in parallel. The processors were linked via dual ported, double buffered memory and a communication register. CPU 2 was used to acquire interferogram data and signal average in one of the two buffers (e.g. MEM A). CPU 1 was signaled by CPU 2 when averaging was completed via the communication register link. After relinquishing control of the buffer containing averaged interferograms, CPU 2 continued to acquire interferogram data by using the other buffer (e.g. MEM B) for averaging. While CPU 2 was acquiring the next data file, CPU 1 stored the first file on disk storage media, computed a chromatogram intensity,and plotted this value on the terminal display screen. These functions were completed well before the next signal-averaged interferogram was available. Prior to adding real-time search capability, it was determined that CPU 1 was idle during 50-80% of the chromatographic analysis period (Figure 2). This previously wasted idle period was used to perform library searches for GC/FT-IR eluent absorbance spectra. Software. To optimize CPU 1data acquisition efficiency and maximize time available for library searching, procedures for rapid chromatogram generation and gas chromatographic eluent detection were investigated and the most efficient methods were selected. Chromatograms were generated by using the GramSchmidt vector orthogonalization technique (13),which does not require Fourier transformation of interferograms prior to chromatogram intensity calculation. Chromatogram peaks were detected in real-time by using a point by point slope comparison method. This method was adopted because it required minimal calculation and was insensitive to chromatogram base-line fluctuations. The slope comparison method identified chromatographic peaks for most components. However, small chromatographic peaks and some unresolved components were not detected by this method. A flow chart for the chromatographic peak detection algorithm used in this study is depicted in Figure 3. Each chromatogram intensity (CURRENT) was compared with the previous value (LAST) to calculate the slope of the chromatogram. Previous slope tendencies were saved for comparison by setting software flags (POS-SLOPE, NEG-SLOPE) to true or false as appropriate. A positive slope indicated the beginning of a possible chromatographic peak elution. When a positive slope was first encountered, the chromatogram intensity at this point was saved (START) and subsequent intensity values were compared until a negative slope was detected. When this occurred, the maximum chromatogram intensity of the elution was saved (PEAK) and the next occurrence of a positive slope was sought. The next positive slope marked the end of the potential chromatographic elution. Discrimination between chromatogram
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
(-)
2711
R t R L - T I M € SCRQCH t F F I C l t N C y
.-.
?0 8 -
8 0
2 0 4 0 6 R 6 0 SIMULTANEOUSLY SEARCHED SPECTRA
1 0 0
Figure 4. Time required for library search when multiple spectra are searched at once: (top) searches employing the 3276 spectra of the EPA library of vapor-phase spectra; (bottom) searches employing a 232 spectra hydrocarbon subset library.
0 IEIUP
Flow chart of the algorkhm used to detect GWFT-IR chromatogram peaks during data acquisition.
Flgure 3.
base-line noise and a valid chromatographic peak was based on comparisons of peak height with a preset threshold (TRESH). If both the leading (P-THRESH) and trailing (N-THRESH) peak heights exceeded the threshold, the chromatogram peak was assumed to be real. In order to take advantage of operating system multitasking capabilities to arbitrate data acquisition and library search functions, a separate program was written for real-time library search. Data collection software detected chromatographicpeaks by using the method described previously and tabulated disk locations for interferograms containing eluent information in a file. This file was accessed by the search program during GC/ FT-IR data acquisition. The search program retrieved interferograms, performed necessary Fourier transformations, computed absorbance spectra, and searched spectra against the EPA library of vapor-phase spectra. Because the search program was assigned low priority, it was active only when CPU 1would have otherwise been idle. Library search comparisons were made at 16 cm-l resolution by using a square difference metric (19). The match factor reported in library search results was computed as match factor = 1.0 -
C[lib(i) - unk(i)12 C[unk(i)I*
Where lib(i) represented library spectra intensities and unk(i) denoted GC/FT-IR spectrum intensities. Equation 1produced match factors near unity for close spectral comparisons. Off-line searches of the 3276 spectra in the EPA library of vapor-phase spectra (including FFT and absorbance computation) required approximately 30 s per spectrum for comparisons over a 4000-700 cm-' spectral range. Real-time searches required approximately 1min/spectrum. Search software could be configured to restrict spectral comparisons to selected wavelength regions to reduce search time. Alternatively,search times were reduced by searching several GC/FT-IR spectra simultaneously. The time required for reading the EPA vapor-phase spectra library from disk storage into computer memory constituted a significant portion of library search time. With a search of several spectra simultaneously, search times per spectrum could be reduced because the library was read from disk only once. Effects of searching multiple spectra at one time are shown graphically in Figure 4. When 10 spectra were searched simultaneously,the time per spectrum for real-time
;
19.2
4
IIII ///I I1
1.1
2.3
3.4
4.5
RCTLNTION T l M C
5.6
6.6
7.9
9.0
Inlnl
Flgure 5. GC/FT-IR Gram-Schmidt chromatogram for a synthetic
10-component alcohol mixture. searching was about 36 s. Real-time search software employed in this study could search up to 10 spectra simultaneously. The actual number of spectra searched at once was determined by the number of peaks which had eluted prior to library search execution. If fewer than 10 peaks had eluted, all spectra were searched together. If more than 10 peaks had eluted, only 10 were searched simultaneously. In effect, gas chromatographic eluent spectra were queued. Library search times were further reduced by restricting the size of the library searched. This required a priori knowledge of the sample and was implemented only for the naphtha sample. This sample was found to contain primarily hydrocarbon components. A subset library containing only hydrocarbon infrared spectra was extracted from the EPA library of vapor-phasespectra. The subset library contained 232 spectra and accounted for 7% of the entire library. Real-time searches with the subset library required only 7 s/spectrum (Figure 4). GC/FT-IR search identifications for the naphtha sample with the subset library were often identical with those obtained by using the entire 3276 spectra EPA library of vapor-phase spectra. Subset library searching was not employed for the flavor mixture analysis. Alcohols, aldehydes, acids, and esters were expected to be major constituents of the flavor mixture. Spectra representing these functionalitiescomprise approximately half of all the entries in the EPA library of vapor-phase spectra.
RESULTS AND DISCUSSION Alcohol Analysis. The automated qualitative analysis capability of the GC/FT-IR data system was evaluated by analyzing an equal volume of 10 alcohols. Approximately 375 ng of each alcohol entered the GC/FT-IR light pipe. All of the selected alcohols were represented in the EPA library vapor phase spectra. Each of the 10 chromatographic peaks
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
a
Table I. Alcohol Mixture Search Results
eluent
alcohol
top search match
1 2 3
methanol ethanol 2-methyl-2-propanol 1-propanol 2-methylpropanol 1-butanol 1-pentanol 1-hexanol 2-octanol 1-octanol
methanol ethanol 2-methyl-2-propanol 1-propanol 2-methylpropanol 1-butanol 1-butanol 1-hexanol 2-heptanol 1-nonanol
4 5 6 7
8 9
10
alcohol match position 1 1 1 1 1 1
10 1
6 2
shown in Figure 5 were detected in real time and spectra were extracted for library search. Library search results for alcohol mixture components are compiled in Table I. Seven of 10 alcohols were correctly identified by the top library search match. Homologs differing by one carbon atom were best matches for the other three alcohols. Search results for 1pentanol were particularly poor. 1-Pentanol was listed as the 10th library match. Comparison of the measured 1-pentanol spectrum with the 1-pentanol library spectrum revealed significant band shape differences in the Tierprint region, which accounted for the poor match. Clearly, success of automatic qualitative analysis is dependent on the quality of the library used for searching. Naphtha Analysis. To assure that GC/FT-IR chromatographic resolution was not degraded by adding peak detection and data reduction capabilities, chromatograms generated with and without peak detection were compared. Figure 6 contains GC/FT-IR and flame ionization detector (FID) chromatograms for the same naphtha sample separated under identical conditions. The chromatogram in Figure 6a was generated by software incorporating real-time peak detection and library search. Figure 6b is a chromatogram generated without chromatographic peak detection or data reduction. The temporal resolution for digitized infrared spectra in chromatogram 6b was 4.0 s/file while the resolution in chromatogram 6a was 4.25 s/file. The difference in temporal resolution (250 ms) was attributed to operating system overhead during data acquisition and library search process swapping. The chromatogram in Figure 6c is a flame ionization chromatogram for the naphtha mixture. Relative chromatographic peak intensities in parts a and b of Figure 6 compare favorably with relative intensities in part c. FID detector response was proportional to the number of carbon atoms in eluting components. Gram-Schmidt chromatographic peak intensity was proportional to the number of C-H stretching groups contained in eluting hydrocarbon components. A total of 47 gas chromatographic elutions were detected for the naphtha mixture by GC/FT-IR. The top three infrared search matches for these eluents are compiled in Table 11. Signal-to-noise ratio (S/N) was defined as the ratio of the absorbance of the largest infrared band in the spectrum to the root-mean-square (rms) noise computed in the 2200-2000 cm-l region of the spectrum. Match factors for the top three search matches were usually very similar. Unequivocal component identification could not be achieved from vapor-phase search results alone. In fact, several chromatographic eluent searches resulted in the same top search match. For example, butylcyclopentane was listed as the top search match eight times and was included in the top three matches for 15 of the 47 eluents. Heptane and 3,3-dimethylheptane were listed as top search matches for five different eluents and 2,4dimethylpentane was repeated as the top search match once. GC/F=I“-IRchromatographic resolution was lower than
b
C
8 8
4.5
9 8
11.5
1 6 8
R L l L Y l i O l i TIME
2 2 5
2 1 0
315
1 6 0
Ii4nl
Flgwe 6. Gas chromatogram for a naphtha hydrocarbon mixture: (a) GC/FT-IR Gram-Schmidt chromatogram obtained with chromatographic eluent detection and real-time library search; (b) GC/FT-IR
Gram-Schmidt chromatogram obtained without eluent detection; (c) flame ionization chromatogram. that obtained by using the FID detector (Figure 6) so several GC/FT-IR eluent spectra undoubtedly represented component mixtures. GC/FT-IR eluent absorbance spectra were computed by using a reference single-beam spectrum obtained prior to sample injection. Due to long-term FT-IR instability, this reference may not have been appropriate for spectra obtained near the end of the separation. To investigate this potential problem, GC/FT-IR eluent spectra were extracted manually with extra care taken to select reference single-beam interferograms from the chromatographic base line near peak elutions. Search results for manually selected GC/FT-IR spectra are designated as “off-line” in Table 11. For all 47 eluents, match factors for off-line spectral searches were higher than corresponding real-time searches. This suggested that the spectral quality of manually selected spectra may have been higher than real-time generated spectra. For 5 of the 47 eluents (eluents 9,21,33, 36, and 46), there were no common matches between the top three real-time search matches and the top three off-line matches. S/Nfor these five eluents were low, ranging from 27 to 64. Search results for high S / N
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987 12
G C R E T E N T I O N T I M E = 0.994
2713
rnin
8.0164
0
R
R T H
b
a
#.a148
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o
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0.8064
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8.0047
4
I
i , 1 5
1 7 2
229
286
3
4
~ 4 8 1
i 458
Flgure 7. GC/FT-IR GramSchmidt duomatogram for a flavor mixture.
spectra were not significantly altered by manually selecting reference single-beam spectra. Search results contained in Table I1 indicate that the naphtha mixture contained exclusively hydrocarbon components. The fact that relative GC/FT-IR chromatographic peak intensities compare favorably with FID relative intensities adds additional support to this claim. The most suitable library for this type of mixture would be a library containing only hydrocarbon spectra. Unfortunately, the EPA library of vapor-phase spectra contains less than 10% hydrocarbon spectra. Thus, 90% of the vapor-phase spectra in the library used for real-time searching was not applicable to the naphtha sample. To increase real-time search speed, a 232 spectra hydrocarbon subset was created from the 3276 spectra in the EPA vapor phase library and this library was used for searching. Real-time searches using the subset library required approximately 7 s/spectrum. The top three search matches obtained by using the subset library were identical with those obtained by using the entire library for 42 of 47 chromatographic eluents. Search results for eluents 6, 26, 27, 35, and 46 did not correlate because full library searches contained nonhydrocarbon matches among the top three listed. Flavor Analysis. A GC/FT-IR Gram-Schmidt chromatogram for a flavor mixture is shown in Figure 7. More than 50 components were identified in the chromatogram. Of these, 48 were detected and library search results were obtained. In most instances, search results were consistent with components typically found in flavor additives. Often, search matches identified esters which are known to be components of flavor mixtures (20). GC/FT-IR library search results alone could not provide unequivocal component identification. Significant discrepancies between the top and subsequent search matches for the first three eluents indicated that top matches were significantly closer than any other library spectra. Library search top match and mixture component spectra are shown for eluent 2 in Figure 8. These two spectra overlap almost completely and it is probable that eluent 2 was ethyl alcohol. Like the naphtha mixture, some gas chromatographic eluent spectral searches resulted in the same top search match. Cyclohexyl butyrate (eluents 13 and 40), 1,1,3,3-tetrachloro2-propanone (eluents 23 and 24), the y-lactone of 4hydroxynonanoic acid (eluents 33 and 46), 4-hydroxy-3methoxybenzaldehyde (eluents 36 and 39), and trans-1,2dichloroethylene (eluents 43 and 47) were duplicated in eluent search results. Compared with naphtha search results, flavor search matches were repeated fewer times. This was probably due to the fact that a larger portion of the EPA library (about 50%) was applicable to the types of components found in the flavor mixture (e.g. alcohols, aldehydes, esters, acids). For 16 of 48 eluents, the top three real-time search matches contained no matches in common with the top three from
4000
3573
h II
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A
a. 8030
5 7
I I
I \
3147
2720
li Bi 2293
!RET
la40
1013
Wavenumber
Flgure 8. Flavor mixture eluent spectrum (solid line) and library search top match infrared spectrum (broken line) for a flavor mixture component.
corresponding off-line searches. These 16 eluent spectra had S / N ranging from 2 to 42. For 8 of the 48 eluents, match factors did not increase for spectra selected manually. These eight eluent spectra had S/N ranging from 2 to 24. In general, inconsistent search results were associated with low S/N eluent spectra. Poor search results for low S / N spectra have been reported previously (21).
CONCLUSIONS We have shown that library search can be combined with GC/FT-IR data acquisition without using an expensive array processor. In fact, no modifications of the commercially available FT-IR data acquisition hardware were required. However, parallel computing and multitasking capabilities were required. The most efficient priority-based processing method would incorporate interrupt driven software. Unfortunately, this was not possible for the system described here, which employed a UNIX operating system. A 250-ms data acquisition delay resulted from using operating system priority arbitration. This delay was found to be insignificant on the time scale of GC/FT-IR data acquisitions employed (1.6-4.0 s/file) and was outweighed by the advantages of obtaining library search information during chromatographic separation. The GC/FT-IR data acquisition/real-time search analysis system described here permits preliminary evaluation of eluent spectra during the separation. For separations requiring 30-40 min, search results for as many as 50 components can be obtained prior to completion of chromatographic separation. If a gas chromatograph autoinjector were added to the described GC/FT-IR system, an enormous quantity of component information could be generated in a short period of time without operator intervention. GC/FT-IR time savings resulting from real-time searching depends on the number of chromatographic peaks and the length of the separation. For the naphtha mixture, 39 searches were completed during the separation and the total analysis time was 40.6 min (including chromatographic separation). For the flavor additive, 45 searches were completed during the separation and the total analysis time was 47.5 min. Without real-time search capability, analysis times would have been 59.5 min (assuming 30 s/search) for the naphtha mixture and 68.5 min for the flavor additive. Library search efficiency was enhanced by creating a library containing only spectra which were representative of expected mixture components. Spectral search inconsistencies were sometimes associated with low S/N spectra for mixtures analyzed in this study. Thus,spectral S/N is an important factor and should be considered when interpreting library search results. Unfortunately, infrared analysis alone could not
2714
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
Table 11. Naphtha Mixture Search Results ret. time, min S / N 1
5.53
42
2
6.60
201
3
7.20
660
4
7.67
538
5
8.14
108
6
9.21
565
7
10.42
683
8
10.69
103
9
11.29
64
10
11.69
86
11
12.03
75
12
12.70
90
13
14.11
61
14
14.98
327
15 16
15.72 16.12
470 96
17
16.86
76
18
17.40
366
19
18.13
278
20
18.73
488
21
19.87
27
22
20.48
255
23
21.28
456
index 0.8864 0.8863 0.8858 0.9682 0.9676 0.9672 0.9826 0.9790 0.9786 0.9806 0.9767 0.9764 0.9765 0.9732 0.9732 0.9815 0.9773 0.9758 0.9858 0.9849 0.9839 0.9299 0.9292 0.9291 0.8860 0.8849 0.8846 0.9586 0.9580 0.9575 0.9194 0.9190 0.9181 0.9229 0.9222 0.9208 0.9141 0.9140 0.9139 0.9623 0.9594 0.9579 0.9688 0.9672 0.9642 0.9334 0.9322 0.9311 0.9165 0.9164 0.9160 0.9770 0.9766 0.9741 0.9680 0.9674 0.9617 0.9809 0.9798 0.9789 0.7960 0.7956 0.7966 0.9650 0.9631 0.9623 0.9746 0.9744 0.9738
real-time 2,4-dimethylpentane 2,5-dimethylhexane 2,2,4-trimethylhexane ethylcyclohexane (5,6,7,8-tetrahydro-2-naphthyl)hexadecane butylcyclohexane 3,3-dimethylheptane butylcyclopentane 2,2-dimethylheptane 3,3-dimethylheptane 3-ethylpentane 3-methylpentane 3,3-dimethylhepane 2-methylbutane butylcyclopentane heptane amyl disulfide octane (5,6,7,8-tetrahydro-2-naphthyl) hexadecane cyclododecane butylcyclohexane 2,2-dimethylheptane methylcyclopentane butylcyclopentane butylcyclopentane pentane 3,3-dimethylheptane 2,4-dimethylpentane 2,5-dimethylhexane 2,2,4-trimethylhexane butylcyclopentane 2,2-dimethylheptane methylcyclopentane methylcyclopentane 2,2-dimethylheptane 2,2,5-trimethylhexane 2-methylbutane 2,5-dimethylhexane 2,4-dimethylpentane butylcyclopentane 2,6,10,14-tetramethylpentadecane
3,3-dimethylheptane cis-1,3-dimethylcyclohexane
1,4-dimethylcyclohexane 6,9,12-tripropylheptadecane
1,l-dimethylcyclohexane 6-penty1(5,6,7,8-tetrahydro-2-naphthyl)-
undecane 6,ll-dipentylhexadecane butylcyclopentane 2,2-dimethylheptane methylcyclopentane trans-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane
6,ll-dipentylhexadecane cis-1,3-dimethylcyclohexane
1,4-dimethylcyclohexane 6-penty1(5,6,7,8-tetrahydro-2-naphthyl).
undecane octane 6,9,12-tripropylheptadecane
nonane butylcyclopentane 2,2-dimethylheptane 3,3-dimethylheptane 6,9,12-tripropylheptadecane 6,12-diethyl-9-pentylheptadecane
octane 1,l-dimethylcyclohexane 1,4-dimethylcyclohexane 6-penty1(5,6,7,8-tetrahydro-2-naphthyl)-
index 0.9734 0.9729 0.9726 0.9832 0.9820 0.9813 0.9840 0.9809 0.9799 0.9821 0.9775 0.9773 0.9822 0.9785 0.9782 0.9862 0.9814 0.9790 0.9871 0.9866 0.9856 0.9766 0.9742 0.9722 0.9720 0.9714 0.9674 0.9696 0.9688 0.9687 0.9740 0.9710 0.9709 0.9707 0.9683 0.9683 0.9706 0.9702 0.9702 0.9783 0.9736 0.9719 0.9723 0.9706 0.9673 0.9818 0.9746 0.9735 0.9714 0.9708 0.9672 0.9862 0.9857 0.9814 0.9765 0.9760 0.9680 0.9872 0.9854 0.9845 0.9714 0.9695 0.9688 0.9853 0.9827 0.9796 0.9814 0.9813 0.9806
off-line isooctane 2,4-dimethylpentane 2,5-dimethylhexane cyclohexanebutyronitrile
ethylcyclohexane cyclohexane 3,3-dimethylheptane 2,2-dimethylheptane butylcyclopentane 3,3-dimethylheptane 3-ethylpentane 2-methylbutane 3,3-dimethylheptane 2,2-dimethylheptane 2-methylbutane heptane amyl disulfide octane (5,6,7,8-tetrahydro-2-naphthyl)hexadecane cyclododecane propylcyclohexane 2,2-dimethylheptane methylcyclopentane 2,2,5-trimethylhexane methylcyclopeptane 2,2-dimethylheptane 2,5-dimethylhexane 2,4-dimethylpentane 2,5-dimethylhexane 2,2,4-trimethylhexane 2,2-dimethylheptane methylcyclopentane butylcyclopentane methylcyclopentane 2,2,5-trimethylhexane 2,2-dimethylheptane 2-methylbutane 2,Fj-dimethylhexane 2,4-dimethylpentane butylcyclopentane 2,6,10,14-tetramethylpentadecane
3,3-dimethylheptane cis-1,3-dimethylcyclohexane
1,4-dimethylcyclohexane 6,9,12-tripropylheptadecane
1,l-dimethylcyclohexane isobutylcyclohexane 6,ll-dipentylhexadecane 2,2-dimethylheptane methylcyclopentane butylcyclopentane trans-1,2-dimethylcyclohexane trans-1,2-dimethylcyclohexane
6,ll-dipentylhexadecane cis-1,3-dimethylcyclohexane
1,4-dimethylcyclohexane 6-penty1(5,6,7,8-tetrahydro-2-naphthyl)-
undecane octane 6,9,12-tripropylheptadecane
nonane 2,5-dimethylhexane 2,4-dimethylpentane 2,2,5-trimethylhexane 6,9,12-tripropylheptadecane 6,12-diethyl-9-pentylheptadecane
octane 1,l-dimethylcyclohexane isobutylcyclohexane 1,4-dimethylcyclohexane
undecane 24
21.82
216
0.9740 0.9691 0.9650
2,6,10,14-tetramethylpentadecane
butylcyclopentane 2,2-dimethylheptane
0.9803 0.9732 0.9705
2,6,10,14-tetramethylpentadecane
butylcyclopentane 2-tert-5-methylcyclohexanol
ANALYTICAL CHEMISTRY, VOL. 59, NO. 22, NOVEMBER 15, 1987
2715
Table I1 (Continued) ret. time, min
S/N
25
22.48
301
26
23.42
27
23.82
28
24.56
29
25.10
30
25.77
31
26.44
32
27.17
33
28.18
34
29.18
35
29.58
36
29.85
37
30.52
38
30.79
39
31.06
40
31.39
41
31.79
42
32.26
43
33.20
44
33.80
45
34.07
46
34.47
47
36.48
index
0.9689 0.9674 0.9663 144 0.9567 0.9532 0.9485 95 0.9211 0.9194 0.9175 178 0.9455 0.9437 0.9437 49 0.8839 0.8829 0.8819 295 0.9682 0.9652 0.9633 277 0.9614 0.9594 0.9565 250 0.9586 0.9586 0.9585 51 0.8330 0.8328 0.8324 578 0.9814 0.9813 0.9796 103 0.9046 0.9044 0.9038 57 0.8861 0.8860 0.8860 86 0.8922 0.8922 0.8916 225 0.9540 0.9532 0.9530 25 0.9196 0.9194 0.9193 28 0.8529 0.8522 0.8520 93 0.9106 0.9102 0.9092 68 0.8641 0.8641 0.8637 38 0.7780 0.7777 0.7776 33 0.7963 0.7954 0.7952 53 0.7947 0.7943 0.7941 27 0.7563 0.7563 0.7560 42 0.7664 0.7664 0.7662
real-time 3,3-dimethylheptane 2,4-dimethylhexane 2,5-dimethylhexane carvomenthene
index
0.9802 0.9800 0.9786 0.9642 0.9611 2,6,10,14-tetramethylpentadecane 0.9572 2-ethylhexylamine 0.9565 tri-2-nobornylmethanol 0.9525 1,3-dimethyladamantane 0.9503 2,2,5-trimethylhexane 0.9780 3,3-dimethylheptane 0.9744 2-methylbutane 0.9741 3-ethylpentane 0.9703 heptane 0.9690 hexane butylcyclopentane 0.9689 0.9794 heptane 0.9750 2,6,10,14-tetramethylpentadecane 0.9732 hexane 0.9742 hexane 0.9740 heptane 0.9716 2,6,10,14-tetramethylpentadecane 0.9748 cis-1,3-dimethylcyclohexane 0.9743 1,4-dimethylcyclohexane 0.9739 6,9,12-tripropylheptadecane hexane 0.9665 0.9664 heptane 0.9660 carvomenthene nonane 0.9868 decane 0.9863 0.9846 6,9,12-tripropylheptadecane 0.9793 1-carbonitrile-1-cyclohexene 0.9766 chlorocyclohexane 2,2-dimethylheptane 0.9698 0.9755 6,9,12-tripropylheptadecane 0.9752 nonane 0.9745 octane 0.9773 heptane 0.9762 hexane 0.9726 2,6,10,14-tetramethylpentadecane 0.9903 (5,6,7,8-tetrahydro-2-naphthyl)hexadecane 0.9891 isobutylcyclohexane 0.9891 butylcyclohexane 0.9728 2,5-dimethylhexane 0.9726 2,2-dimethylheptane 0.9717 butylcyclopentane 0.9671 butylcyclopentane 0.9636 2,6,10,14-tetramethylpentadecane 0.9633 heptane 0.9757 butylcyclopentane 0.9751 2,6,10,14-tetramethylpentadecane 2,2-dimethylheptane 0.9747 butylcyclopentane 0.9770 0.9720 hexane 0.9716 pentane hexane 0.9377 0.9373 pentane 0.9369 butylcyclopentane hexane 0.9586 0.9571 heptane 0.9569 pentane 0.9482 heptane 6,9,12-tripropylheptadecane 0.9469 0.9462 hexane 0.9561 2-dodecyloxy-3-tetradecyloxy-1-propanol 0.9554 1,2,3,4,5,6,7,8,9,10,11,12-dodecahydrotriphenylene 1,2-difluoro-1,1,2,2-tetrachloroethane 0.9537 nonane 0.9734 0.9722 6,9,12-tripropylheptadecane octane 0.9721
provide unequivocal component identification. However, functional group information and isomer differentiation were obtained by using GC/FT-IR. Commonly, mass spectrometry is combined with GC/IR (GC/IR/MS) t o provide unambig-
off-line 3,3-dimethylheptane 2,4-dimethylhexane 2,5-dimethylhexane carvomenthene 2,6,10,14-tetramethylpentadecane
2-ethylhexylamine tri-2-norbornylmethanol 1,3-dimethyladamantane 1-adamantaneethanol 3,3-dimethylheptane 2-methylbutane 2,4-dimethylhexane heptane 6,9,12-tripropylheptadecane
amyl disulfide heptane 2,6,10,14-tetramethylpentadecane
amyl disulfide hexane heptane 2-ethylhexylamine 1,4-dimethylcyclohexane cis-1,3-dimethylcyclohexane 6,9,12-tripropylheptadecane
amyl disulfide 6,9,12-tripropylheptadecane 6J2-dimethy1-9-pentylheptadecane
decane nonane hendecane 1-carbonitrile-1-cyclohexene
chlorocyclohexane bromocyclohexane 6-(5,6,7,8-tetrahydro-2-naphthyl) hexadecane myristonitrile 1-decanethiol heptane 2,6,10,14-tetramethylpentadecane amyl disulfide (5,6,7,8-tetrahydro-2-naphthyl) hexadecane butylcyclohexane ethylcyclohexane 2,5-dimethylhexane 2,4-dimethylhexane 3,3-dimethylheptane 2,6,10,14-tetramethylpentadecane
butylcyclopentane 2,4-dimethylhexane butylcyclopentane 2,6,10,14-tetramethylpentadecane
3,3-dimethylheptane butylcyclopentane heptane hexane pentane hexane butylcyclopentane hexane 2,6,10,14-tetramethylpentadecane
heptane 6,9,12-tripropylheptadecane
heptane octane amyl disulfide heptane octane decane nonane 6.11-di~entvlhexadecane
uous component identification. In addition, gas chromatographic retention index libraries are currently available and may be used in conjunction with spectral comparisons for unequivocal compound identification (22, 23).
2716
Anal. Chem. 1987, 59, 2716-2718
ACKNOWLEDGMENT The authors thank Donald L. Beduhn of Mattson Instruments, Inc., for providing the library of vapor-phase spectra and for helpful discussions concerning software development.
LITERATURE CITED Wilkins, C. L.; Giss, G. N.; Brissey, 0. M.; Seiner, S. Anal. Chem. 1081, 53. 113. Crawford, R. W.; Hirshfeid, T.; Sanborn, R. H.; Wong. C. M. Anal. Chem. 1982, 5 4 , 817. WHkins, C. L.; ass, 0. N.; White, R. L.; Brissey, G. M.; Onyiriuka, E. C. Anal. Chem. 1082, 5 4 , 2260. Wilklns, C. L. Science 1083, 222, 291. Laude, D. A., Jr.; Brissey, G. M.; Ijames, C. F.; Brown, R. S.; Wilkins, C. L. Anal. Chem. 1984, 56, 1163. Laude, D. A., Jr.; Johiman, C.; Wilkins, C. L. Opt. Eng. 1985, 2 4 , 1011.
Cooper, J. R.; Bowater, 1. C.; Wilkins, C. L. Anal. Chem. 1088, 58, 279 1. Williams, S.S.; Lam, R. B.; Sparks, D. T.; Isenhour, T. L.; Hass, J. R. Anal. Chlm. Acta 1982, 138, 1. Williams, S. S.; Lam, R. B.; Isenhour, T. L. Anal. Chem. 1983, 55, 1117.
(10) Whke, R. L. J. Autom. Chem. 1087, 9 ,66. (11) White, R. L. Appl. Spectrosc. Rev., in press. (12) Coffey, P.; Mattson, D. R.; Wright, J. C. Am. Lab. (FairfieM, Conn.) 1078, 10, 126. (13) DeHaseth, J. A.; Isenhour, T. L. Anal. Chem. 1977, 49, 1977. (14) Hanna, D. A.; Hangac, G.; Hohne, B. A,; Small, G. W.; Wieboidt, R. C.; Isenhour, T. L. J . C h r m t o g r . Sc;. 1070, 17, 423. (15) Hohne, B. A.; Hangac, G.; Small, G. W.; Isenhour, T. L. J. Chromatogr. Sci. 1081. 19, 283. (16) Gurka, D. F.; Betowski, L. D. Anal. Chem. 1982, 5 4 , 1819. (17) Hanna, A.; Marshall, J. C.; Isenhour, T. L. J. Chromatogr. Sci. 1970, 17, 434. (18) Fields, R. E., 111; White, R. L. Appl. Spectrosc. 1987, 41, 705. (19) Lowry, S. R.; Huppler, D. A. Anal. Chem. 1981, 53, 889. (20) Morris, W. W. J. Assoc. Off. Anal. Chem. 1073, 56, 1037. (21) Cooper. J. R.; Taylor, L. T. Anal. Chem. 1984, 56, 1989. (22) Lee, M. L.; Vassibros, D. L.; White, C. M.; Novotny, M. Anal. Chem. 1079, 51, 768. (23) Sprouse, J. F.: Varano, A. Am. Lab. (Fairfield, Conn.) 1984, 16,54.
RECEIVED for review March 23, 1987. Accepted July 7, 1987. This work was supported by funds provided by the oMahoma University Associates Foundation.
Spectrophotometric Determination of Poly(viny1 alcohol) in Cadmium Hydroxide Pastes Charles E. Baumgartner
GE Company, Corporate Research and Development, Schenectady, New York 12301
A colorknetrlc method has been developed to spectrophote metrically determtne the poly(vlnyl alcohol) (PVA) content of aqueous Cd(OH), pastes. The method, which Is based upon the formailon of a blue PVA-loQne-borlc acid complex, can quantitatively determine PVA in aqueous soiutlon concentrations of 2-40 mg/L corresponding to PVA levels within the pasle as low as 0.02 % Solution absorbance d the complex, however, depend8 upon the concentration of Cd2+in solution necessitating cognizance of this level for routine quantitative analysis.
.
Poly(viny1 alcohol) (PVA) has been added recently as a binder to many oxide systems as a means of improving the integrity of the formed product. One such area has been its increasing use as a binder in pasted Cd(OH)2 battery electrodes (1-4), where it is used to improve the handling characteristics of formed electrodes. This use in sealed battery systems, which require careful monitoring of all electrode constituents, has required the establishment of a sensitive analytical method for control of the electrode's PVA level. Previous analytical techniques, based upon the formation of a blue PVA-iodineboric acid complex, are well documented in the literature (5-9), however, these methods were typically developed for nonoxide systems such as paper and textile coatings. It has been found that these methods required modification to provide routine quantitative PVA analysis for this system due to sensitivity of the absorptivity to dissolved Cd2+concentrations. This modified technique allows quantitative detection of PVA in Cd2+-containingsolution from concentrations of 2 to 40 mg/L (ppm).
EXPERIMENTAL SECTION Reagents. Boric acid solution is prepared at 0.65 M by the dissolution of 40.0 g of, H3B03into 1L of HzO. An Iz/KI solution is prepared by dissolving 25.0 g of KI into 100 mL of HzO, followed
by the addition of 12.7 g of sublimed 12. Following dissolution, this is diluted to a 1-L volume to yield concentrations of 0.05 M Iz and 0.15 M KI. HC1 solution for dissolution of the Cd(OH), paste is obtained via 1 O : l dilution of concentrated HCl yielding 1.2 M HC1. Procedure. Optimum analytical results are obtained for a sample possessing between 0.5 and 2.5 mg of PVA/100 mL of solution. For a Cd(OH)zpaste containing between 0.1 arid 0.5% PVA by weight, this corresponds to a sample size of approximately 0.5 g. The samples of interest here are always predominately (>90%) Cd(OH)> For samples of lower or variable Cd(OH)z content, the Cd(OH)z level should be known to within a few percent. The electrode or Cd(OH)2paste should be dried briefly at 105 "C prior to accurately weighing out a sample between 0.25 and 0.75 g. Samples prepared by using high molecular weight PVA of low residual acetate content can readily be dried at this temperature overnight, while low molecular weight PVA pastes should be heated only to dryness to avoid PVA loss due to evaporation. Following drying, the sample is placed into a 100-mL Erlenmeyer flask along with 50 mL of 1.2 M HC1 and is stirred until all Cd(OH)zhas dissolved (e.g., 30-60 min at room temperature). The solution is filtered to remove undissolved constituents within the electrode, i.e., insoluble oxides or hydroxides, metallic Cd, organic fibers, etc., added to the electrode to alter either physical or electrochemicalproperties, and is then quantitatively transferred to a 100-mL volumetric flask and placed into a 25 "C constant temperature HzO bath. To this solution is added 25 mL of 0.65 M H3B03. The solution is well mixed and allowed to equilibrate thermally in the water bath. A 3.0-mL aliquot of the Iz/KI solution is pipetted into the sample flask and the volume brought to 100 mL by using 25 "C HzO. (Final solution concentrations are 0.16 M H3B03,1.5 X M Iz, 4.5 X M KI, and 0.6 M HCl.) After 15 min, the absorbance of the yellow-green solution is measured at 660 nm, the PVA complex's absorption maximum, vs a PVA-free reference solution prepared by using 50 mL of HCl solution, 25 mL of boric acid solution,and 3.0 mL of Iz/KI solution diluted to a 100-mL total volume. The concentration of PVA in the unknown solution is obtained from a previously determined calibration plot relating the linear
0003-2700/87/0359-2716$01.50/00 1987 American Chemical Society
