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Anal. Chem. 1905, 57, 1472-1474
presence of new species and provide limited structural information. Registry No. I (R, = R3 = OAc, R2 = Re = H, R4 = R5 = OH), 95673-99-7;I (R1= R3 = OAC,& = R4 = Rb = & = H), 38818-51-8; 50722-38-8; I [R, = OAC,Rz = H, R3 = R4 = OH, R5 = & = (*)I, 11, 18374-83-9; 111, 90044-33-0; IV, 90044-34-1; culmorone, 95648-62-7.
(13) Brumley, W. C.; Andrezejewski, D.; Trucksess, E. W.; Dreifuss, P. A,; Roach, J. A. G.; Eppley, R. M.; Thomas, F. s.; Thrope, C. W.; Sphon, J. A. Blomed. Mass SDectrom. 1982. 9 . 451-458. (14) Smith, R. D.; Usdeth, H. R. Anal. Chem. 1983, 55, 2266. (15) Greenhalgh, R.; Meier, R.-M.; Blackwell, B. A,; Miller, J. D.; Taylor, A,; ApSimon, J. W. J. Agrlc. Food Chem. 1984, 32, 1261-1264. (16) Scott, P. M.; Lau, P.-Y.; Kanhere, S. R. J. Assoc. Off. Anal. Chem. 1981, 64,1364.
J. R. Jocelyn Pare* Roy Greenhalgh Pierre Lafontaine
LITERATURE CITED Surman, D. J.; Vickerman, J. C. J. Chem. SOC.,Chem. Commun. 1981, 324-325. Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N. J. Chem. SOC., Chem. Commun. 1981, 325-326. Rinehart, K. L., Jr. Science 1982, 218, 254-260. Barber, M.; Bordoll, R. S.; Elliot, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982, 54, 645A-657A. Joffe, A. Z.I n "Microbial Toxins, Vol. VII"; Kadls, S., Ciegler, A,, Ajl, S. J., Eds.; Academic Press: New York, 1971: Chapter 5. Bamburg, J. R.; Strong, F. M. I n "Microbial Toxins, Vol. VII"; Kadis, S., Clegler, A., Ajl, S. J., Eds.; Academic Press: New York, 1971; Chanter 7. COG-R. J.; Cox, R. H. "Handbook of Toxic Fungal Metabolites"; Academic Press: New York. 1981. Mirocha, C. J.; Pathre, S:V.; Christensen, C. M. I n "Mycotoxic Fungi, Mycotoxicoses: An Encyclopedic Handbook"; Willie, T. D., Morehouse, L. G., Eds.; Marcel Dekker: New York, 1978: Vol. I, pp 365-420. Blackwell, B. A.; Greenhalgh, R.; Bain, A. D. J. Agrlc. Food Chem. 1984, 32, 1078-1083. Par& J. R. J. Ph.D. Thesis, Carleton University, Ottawa, ON, Canada, 1984. Busch, K. L.; Cooks, R. G. Anal. Chem. 1983, 55,38A. Plattner, R. D.; Bennett, G. A. J. Assoc. Off. Anal. Chem. 1983, 66, 1470.
Chemistry and Biology Research Institute Agriculture Canada Ottawa, Ontario, Canada K1A OC6
John W. ApSimon The Ottawa-Carleton Institute for Research and Graduate Studies in Chemistry Carleton University Ottawa, Ontario, Canada K1S 5B6
RECEIVED for review October 18, 1984. Resubmitted January 28,1985. Accepted February 19,1985. We thank the Natural Sciences and Engineering Research Council-Canada for the award of a Visiting Fellowship in Biotechnology to J.R.J.P. This paper is C.B.R.I. contribution No. 1495 and was taken in part from the Ph.D. Thesis of J. R. J. Par& Carleton University, July 1984.
Effect of Concentration Gradients on Spectra in Gas Chromatography/Fourier Transform Infrared Spectrometry Sir: One problem generally disregarded in GCIFT-IR is the question of whether changing analyte concentration during the collection of a single interferogram will detrimentally affect the spectral results. Lephardt and Vilcins addressed this problem theoretically and came to the conclusion that changing concentration has the same effect as an increased apodization (2). With the relatively broad peak widths of packed column GC, modern FT-IR scan rates arre sufficiently rapid to ensure negligible concentration gradients during single interferometric scans. However, the growing popularity of smaller, slower scanning FT-IR spectrometers and the development of capillary GC/FT-IR have made it possible for significant concentration changes to occur during the collectior of a single interferogram. With capillary peak elution times of 2-5 s and FT-IR spectrometer scan cycles of 2 s, only one to two interferograms will be collected over a single GC peak. Under these conditions, a noticeable concentration gradient will occur across each sample interferogram. The effects of this gradient upon spectral results are examined in this correspondence. EXPERIMENTAL SECTION Instrumentation. Reference and sample interferogramswere collected using an IBM Instruments IR-85 FT-IR spectrometer with a DTGS detector. The sample was a thin polystyrene film. The interferometric data were transferred to a Vax 11/780 minicomputer for spectral calculations. Procedure. A Gaussian chromatographic profile was used to model the changing analyte concentration. By variation of the width of this Gaussian function, it was possible to study the effects of different interferometric scan rates. Representation of the concentration gradient was established by assuming a valid Beer's
law relation. From this it follows that
where Ti and Ai represent the transmittance and absorbance of the sample at time i. The ratio of absorbances at a given frequency can be determined from the Gaussian profile &/Ap = G(6)
(2)
where G(6) is the Gaussian value at mirror displacement 6, A , is the maximum sample absorbance, and A6 is the sample absorbance at displacement 6. Substitution of eq 2 into eq 1results in T&u)= T , ( v ) ~ ( * )
(3)
Equation 3 simulates the change in transmittance as a function of mirror displacement 6 and the peak transmittance T,(v). It is the sample concentration gradient through the light pipe that causes transmittance to change with mirror displacement. To represent this change in the interferogram domain requires the calculation of a new transmittance spectrum for each mirror displacement. Sample interferograms containing concentration gradients can then be calculated Z(6) = ~ T , ( U ) ~ ' * )cos R(U (27rvS) )
(4)
where R(u)represents the reference power spectrum. In this work, these calculated sample interferograms were transformed and ratioed with the original reference power spectrum, R(u), to produce the sample gradient spectra which result from a changing concentration profile. To evaluate the spectral distortions caused by this concentration gradient, the sample gradient spectra were fit to a sample spectrum (containing no gradient). This was accomplished by utilizing the least-squares method described by
0 1985 American Chemical Society 0003-2700/85/0357-1472$01.50/0
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985 MEAN=10@0
0.98 SCANS/FWHM 0.110
T 0.020
0.000
1473
J
109.6
0.0
819.2
1220.8
1636.1
I
4
L .
2048.0
8.088 0. 08
MEAN
1.08
2.00
Flgure 1. Dependence of spectral distortion upon the interferogram
3.88
5.88
4.00
SCANWFWHH
Figure 3. Spectral distortion dependence on concentration gradient severity. The gradient is centered at interferogram point 1000.
location of the Gaussian chromatographic profile center. MEAN=17B (BURST)
0.25
h
\
0.m
0.20 8.38
r
0.15
1
0
2 0.10
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3.00
4.08
1.50 4E00
5.00
3330
2060
1990
1320
15%
1320
I
658
SCANS/FVHM
Figure 2. Dependence of spectral distortion on the width of a Gaussian concentration gradient. The peak is centered at the light burst.
B 0.08
Haaland and Easterling ( 2 ) . The following parameters were calculated:
var(k) =
11 li
U2
CAri2
1.20
..
150 4see
3330
zw8
6.3
IAVENWERS
where k is the concentration ratio parameter, var(k) is the variance, 2 is the error variance, and SE(C,) is the standard error in the concentration. Cr is the concentration of the reference and is assumed to be unity. The standard error was used as an indication of the amount of spectral distortion which had occurred as a result of the concentration gradient.
RESULTS AND DISCUSSION When a GC peak passes through the light pipe, there is no way to determine where the peak will occur in relation to the interferogram light burst. It was predicted that the amount of distortion would depend upon the position of the Gaussian peak. T o determine what effect the peak position does have, a series of calculations was done in which the peak width was kept constant a t 0.98 scans/fwhm and the peak position was varied. Figure 1 shows the results of these calculations. Distortion is plotted vs. the mean of the Gaussian. A 2048-
Figure 4. Absorbance spectrum of polystyrene (A) with no concentration graident and (B) with a concentration gradient corresponding to 0.488 scan/fwhm.
point interferogram with the burst a t 170 was used. The plot shows that the distortion does depend on where the peak is with respect to the light burst. The highest distortion was observed a t low mean values (around the burst). The distortion decreases until the mean is a t 1000 and then begins to increase again. The distortion depends not only on where the peak maximum is but also on the severity of the concentration gradient. In a GC/FT-IR run, the gradient depends upon the scan rate and the GC peak width. For the remainder of this discussion, the peak width will be related to the number of scans taken across the peak by using the number of interferograms per full width a t half maximum (scans fwhm). Figure 2 illustrates
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Anal. Chem. 1905, 57. 1474-1476
the amount of spectral distortion introduced by varying the number of scans fwhm. The peak position is constant a t 170, the light burst. Above 2-2.5 scans/fwhm, the distortion is small and relatively constant. Below 2 scans/fwhm, a rise occurs in distortion, indicating that the gradient has become severe enough to effect changes in the spectrum. The same calculations were repeated with the mean at 1000, the interferogram point corresponding to the minimum in the distortion vs. mean curve (Figure 1). Figure 3 shows the results of these calculations. The curve is very similar to that in Figure 2. Above 2 scans/fwhm the distortion is small and constant. Below 2 scans/fwhm, a rise is again observed. These results indicate that 2 scans/fwhm are necessary t o avoid the region of increased spectral distortion, regardless of peak position. The type of distortion that occurs is also of interest. Figure 4A is the original polystyrene spectrum. Figure 4B is a spectrum containing the concentration gradient. The latter was calculated with a Gaussian centered a t the burst with a width of 0.488 scans/fwhm. Note first that the intensities of the major peaks remain relatively unchanged. However, the second spectrum exhibits some loss of resolution. Side shoulders are no longer apparent and smaller split peaks have collapsed into single peaks. This loss of spectral detail could lead to misidentification, particularly when done by computerized search. Changing analyte concentration is not one of the major problems encountered in GC/FT-IR. However, it is a problem
that can detrimentally affect the spectral results. I t is important that researchers be aware that the problem exists in order to take the appropriate steps to prevent the occurrence of a significant concentration gradient during a single scan. ACKNOWLEDGMENT The authors wish to thank Daniel T. Sparks for his advice and assistance on this project. LITERATURE C I T E D (1) Lephardt, J. 0.; Vilcins, G. Appl. Spectrosc. 1975, 2 9 , 221. (2) Haadland, D. M.; Easterling, R. G. Appl. Spectrosc. 1980, 3 4 , 539.
’
Present address: Department of Chemistry, USMA, West Point, NY 10996. Present address: Department of Chemistry and Biochemistry. Utah State University, Logan, UT 84322.
J a c k i e Getz White P a t r i c k M. Owens’ Thomas L. Isenhour*2 University of North Carolina a t Chapel Hill Chapel Hill, North Carolina 27514
RECEIVED for review May 31, 1984. Resubmitted January 10, 1985. Accepted January 23, 1985. This work was supported by the National Science Foundation Grant No. CHE 8026747 and by the Department of the Army. This paper was presented in part a t the 1984 Pittsburgh Conference and Exposition, Abstract No. 265.
Long Optical Path Length Thin-Layer Spectroelectrochemistry. Quantitation of Adsorbed Aromatic Molecules at Platinum Sir: The design and demonstration of a long optical path length, thin-layer electrochemical cell (LOPTLC) for high sensitivity optical monitoring of solution species were previously described (1)from our laboratory. I t was also proposed that this cell, with its long optical path length and its large electrode surface area to solution volume ratio, could provide the requisite optical sensitivity to quantitate adsorption of species from the solution onto the electrode surface a t monolayer levels. Such quantitation has now been achieved for the molecules of 1,4-hydroquinone (1,4-HQ) and 1,2,4-trihydroxybenzene (triol) adsorbed onto Pt, utilizing the method of successive solution injections previously described by Soriaga and Hubbard for electrochemical thin-layer cells (2-4). EXPERIMENTAL SECTION Electrode Preparation. Electrodes were cut from a sheet of thick polycrystalline Pt foil (Alfa Products, Danvers, MA) to fit precisely into the LOPTLC. The P t surface was polished successively with graded wet emery paper and with 0.2-pm diamond particles which were dispersed on a nylon polishing pad (Buehler, Ltd., Evanston, IL). Between polishing steps, the electrode was cleaned in an ultrasonic bath containing triply distilled water. Finally, the electrode was “cleaned”by immersion into a solution of hot, concentrated nitric acid. The surface area of Pt was determined by the charge consumed in the hydrogen underpotential deposition region (5,6). The cyclic voltammetric response for an electrode with adsorbed iodine provided the base line for the charge integration (7). A value of 210 pC/cm2 for the monolayer coverage of hydrogen on Pt (8) was used to calculate a Pt surface area of 1.07 cm2. Solution Injection Procedure. Prior to the first solution injection, the Pt electrode in 1 M H2S04was scanned repetively
between the potential limits of $1.2 to -0.15 V vs. Ag/AgCl (saturated KC1) reference electrode until a steady-state voltammetric response was obtained. The scan rate was 2 mV/s. Immediately after injection of working solution, which contained either 1,4-HQor triol, a potential of +0.2 V vs. Ag/AgCl (saturated KC1) was applied to the Pt electrode. An absorbance spectrum ( A vs. A) was obtained 3 min after injection. Repetitive spectra taken thereafter indicated that loss of species from solution due to adsorption was completed within the first 3 min. Thus, in most of the experiments, the change in absorbance at a fixed wavelength (A-E) during linear sweep voltammetric experiments was measured immediately after acquisition of the absorbance ( Avs. A) spectrum. The cell was then emptied, rinsed with electrolyte, and refilled with working solution (second injection) for the second set of optical and electrochemical data. This process could be repeated for a third solution injection. These injections followed a vacuum degassing/filling procedure previously described ( I ) . Instrumentation and Reagents. The cell body of the LOPTLC was fabricated from Kel-F (3M Co., St. Paul, MN) with gaskets made from Kalrez (Du Pont Chemical Co., Wilmington, DE). Chemically inert materials for the cell were used to minimize sources of impurities. The LOPTLC specifications were as follows: volume, 11.0 p L ; optical path length, 1.26 cm; solution layer thickness, approximately 103 pm. Slight differences in alignment of the cell in the spectrophotometer light path between experiments caused small but significant differences in the optical path length; these differences were eliminated by the method of calculating the solution absorbance changes between the first and second injections. Spectral measurements were made with a DMS 90 UV-VIS spectrophotometer (Varian Instrument Co., Palo Alto, CAI. A conventional three-electrode potentiostat with data acquisition on a Houston Model 2000 x-y recorded (Houston Instrument Co.,
0003-2700/85/0357-1474$01.50/00 1985 American Chemical Society
