Anal. Chem. 1084, 56, 2973-2974 (3) Smith, R. D.; FJeldsted.J. C.; Lee, M. L. J . Chromatogr. 1982, 2 4 7 , 23 1-243. (4) Smith, R. D.; Udseth, H. R. Anal. Chem. 1983, 55, 2266-2272. (5) Fjeidsted, J. C.; Kang, R. C.; Richter, B. E.; Fields, S. M.; Jackson, W. P.; Lee, M. L., paper presented at the Pittsburgh Conference on Ana-’ lytlcal Chemistry and Applied Spectroscopy, Atlantlc City, NJ, March 5-9, 1984, No. 596. (6) Smith, R. D.; Kalinoski, H. T.; Udseth, H. R.; Wright, 6 . W. Anal. Chem. 1984, 56, 2476-2480. (7) Wright, B. W.; Peaden, P. A.; Lee, M. L.; Stark, T. J. J . Chromatogr. 1982, 2 4 8 , 77-64. (6) Benson, W. R.; Damico, J. N. J . Assoc. Off. Anal. Chem. 1988, 57, 347-365. (9) Slivon, L. E.; DeRoos, F. L., Development of a General Purpose LC/ MS Method for Compounds of Environmental Interest, 1963 Final Report U.S. Environmental Protection Agency, Contract 68-03-2960, March 1963.
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B. W.; Smith, R. D. J . Chromatogr., submitted. Richard D. Smith* Harold R. Udseth Henry T. Kalinoski Chemical Methods and Kinetics Section Pacific Northwest Laboratory (Operated by Battelle Memorial Institute) Richland, Washington 99352 RECEIVED for review June 8, 1984. Accepted July 27, 1984. This work has been supported by the U.S. Department of Energy, Office Of Basic Energy Sciences, U d ? r Contract DE-AC06-76-RLO 1830. (10) Wright,
Simple, Compact Visible Absorption Spectrophotometer Sir: Ultraviolet/visible (UV/Vis) spectrophotometry continues to offer a versatile analytical method for a variety of samples. While mechanically scanned double-beam instruments comprise the state of the art for making highly precise and sensitive spectroscopic measurements, alternative systems utilizing optoelectronic imaging devices (OIDs) have recently been introduced (1,2). With an OID, mechanical scanning is eliminated and a rapid readout rate is possible. Since measurements are taken over all of the wavelengths of interest simultaneously, drift problems are reduced and the multichannel advantage in signal-to-noise ratio or time is achieved. Rapid-scan systems incorporating linear photodiode arrays (PDAs) or charge-coupled devices (CCDs) have been described ( 3 , 4 ) ,and the former is now commercially available. In all cases, grating-based polychromators are used as dispersive elements to select the distribution of wavelengths reaching the channels of the OID. The purpose of this paper is to describe how an even further reduction in size, simplicity, and mechanical stability in an OID-based spectrophotometer is possible using a novel concept as follows: (a) Light from a lamp is collected and collimated by a lens and passed through a cuvette containing the sample. (b) The light transmitted by the sample is then analyzed by a “wedge” interference filter. The wavelength of the maximum transmission of this device varies linearly and continuously over its length. It is mounted so that the wavelength variation occurs along the vertical (long) axis of the cuvette. (c) Immediately behind the interference filter is placed a linear PDA. The long axis of this device is parallel to the wavelength variable axis of the wedge filter so that each photoelement observes a different portion of the spectrum. Thus, the PDA acts as an “electronic photographic plate” to capture the entire spectrum at once. This design retains all of the aforementioned advantages of the use of an OID as a detector. Additionally, it provides the benefit of compactness, due to elimination of the dispersive device, and the benefits of ruggedness and ease of servicing, due to elimination of critical optical alignment. EXPERIMENTAL SECTION Apparatus. A diagram of the spectrophotometer is given in Figure 1. The instrument consisted of a quartz halogen bulb, type S4A from Pelican Products, Inc., as a light source, powered by a Lambda Model LL-902 DC power supply operated at approximately 3.5 V; a 1-in.focal length collimating lens; an infrared blocking filter, type KG-1, manufactured by Schott Optical Glass Inc.; a 10-mm path length fused silica cuvetk, a 25 X 60 mm wedge filter obtained from Oriel Corp., no. 5748; a 25-mm focal length Canon TV lens, Model 6367, f no. 0.78; and a Reticon Corp. series
“S” 512 element diode array operated from an RC 1024SA evaluation board as detector. The infrared blocking filter served to eliminate the infrared sidebands which are passed by the wedge filter at twice the peak transmission wavelengths. The wedge filter incorporated blockage of the short wavelength sidebands that occur at two-thirds the peak transmission wavelength. The band-pass of the wedge filter increases linearly from 8 nm at 400 nm to 14 nm at 700 nm over a 42.9-mm active length. Since the diodes of the PDA extend over 12.8 mm, detection of the spectrum transmitted directly through the filter to the PDA would have afforded a wavelength range of only 89.5 nm. Accordingly, the video lens was used to image a slightly greater portion of the wedge onto the array to give a 130-nm range. The readout rate of the PDA was set to 50 kHz. The entire array was read out at this rate once every 50 ms. The diode signals were digitized by a 12-bit A/D converter and processed by a Digital Equipment Corp. PDP 11/04 computer. Procedure. Chromatographically pure tetraphenylporphine (H,TPP) was the kind gift of Professor Martin Gouterman. Absorbance spectra of HzTPP in dichloromethane at concentrations from 0.41 to 35 ppm were measured. Dark signal and dichloromethane blank data were taken and then serial additions of HzTPP to the blank were made without disturbing the cuvette. Data were stored after each addition. The light source power was adjusted so that the signal from each of the array elements did not exceed nine-tenths of the saturation level in any part of the array during a blank reading. All data acquisition times were 10 s, representing 200 readout cycles summed. Absorbance at each array element was calculated as A = -log [(S- D ) / ( B - D)] (1) where S, D, and B are the sample, dark, and blank sums respectively. In order to improve the signal-to-noiseratio and in keeping with the resolution of the wedge filter, the signals from seven adjacent diodes were averaged together before displaying. For comparison, spectra over the range of 0.41-35 ppm HzTPP were obtained by using a Hewlett-Packard (HP) 8450A UV/Vis spectrophotometer, using a 10-sacquisition time for sample and blank measurements. RESULTS AND DISCUSSION Figure 2A gives a spectrum of HzTPP dissolved in dichloromethane a t a concentration of 2.5 ppm obtained with the prototype spectrophotometer described in the Experimental Section. The positions of the three absorbance maxima in this region are in good agreement with literature values (5). For comparison, in Figure 2B we give the spectrum of the same sample as recorded with the H P 8450A; the two are qualitatively quite similar. However, the lower resolution of the prototype when compared to the H P instrument is apparent. This arises from the wider band-pass of the present wedge filter (12 nm) when compared to the polychromator (2 nm).
0003-2700/84/0356-2973$01.50/00 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 14, DECEMBER 1984 COLLIMATING LENS
SAMPLE CUVET
IR
SOURCE
IMAGING LENS
0. I.D
WEDGE FILTER
FILTER
Flgure 1. Optical schematic of the compact spectrophotometer. 0.08-
A
w
0
3 0.04-
m
(r
0 v,
::0.02525
575
625
WAVELENGTH (nm)
H I
0.06
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475
525
575
525
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WAVELENGTH (nm)
Figure 3. Base-line noise level of the compact spectrophotometer. Absorption spectrum of solvent referenced to solvent.
0.06-
0.004 475
OOOJ 475
625
WAVELENGTH (nm)
Figure 2. Absorption spectrum of H,TPP, concentration 2.5 ppm, in dichloromethane,(A) as obtained with the compact spectrophotometer, and (9)as obtained with a commerclal diode array spectrophotometer. We next assessed the linearity and dynamic range of the prototype instrument. A log-log plot of measured absorbance at 514 nm vs. actual concentration of HzTPP showed good linear correlation (0.997). We expected a slope of unity but actually obtained a value of 0.87. This undoubtedly arises from stray light (6),produced by imperfect functioning of the wedge interference filter. For comparison, we obtained a similar calibration curve using the H P 8450A and observed a slope of unity within experimental error. We must also point out that the aforementioned data was obtained without moving the cuvette. We found that below an absorbance value of 0.1, spectra measured when the cuvette was removed and replaced after blank data acquisition were not reproducible. This is due to the fact that the paths of light reaching the PDA trace back to the light source along paths that are spatially distinct. Altering the cuvette position places a different optical density into the paths due to inhomogeneity of the fused silica down the length of the cuvette, irrespective of the sample. Clearly, this system will work best when used in conjunction with a flow cell.
Figure 3 is a blank vs. blank spectrum, which can be used for estimation of the detection limit of the instrument. As can be seen, the base-line noise of approximately 4 x 10-4 absorbance units consists of two components, a high frequency, randomly varying part which originates from the preamplifier, and a low frequency component which is thought to arise from 60-Hz line interference. This noise corresponds to a detection limit of 8 ppb for HzTPP at a wavelength of 514 nm. Here, the detection limit is defined as 3 times the rms base-line noise. In comparison, the calculated detection limit for the commercial instrument was a factor of approximately 4 lower, reflecting its lower base-line noise level, which is probably due to more careful shielding and better grounding. The above studies show that it is quite feasible to develop a visible spectrophotometer with the virtues of a good signal-to-noise ratio, compactness, and the potential to perform well in adverse environments. Moreover, the instrument is inexpensive to construct and servicing will not involve critical optical or mechanical alignment. Of course, further improvements are highly desirable especially in the areas of spectral resolution, wavelength coverage, and stray light rejection. We plan to address these problems by custom fabrication of a wedge filter with the desired properties. Our eventual goal is to directly bond the filter to the photodiode array. While the system presented here will probably never equal the performance of a conventional double-beam instrument, there are numerous applications where it could be successfully used.
ACKNOWLEDGMENT We are indebted to J.D. S. Danielson for useful discussion and for assistance with the electronics of this instrument.
LITERATURE CITED (1) Talmi, Y., Ed. “Multichannel Image Detectors”; American Chemical Soclety: Washington, DC, 1979; ACS Symp. Ser. No. 102. (2) Talml, Y., Ed. “Multichannel Image Detectors”; American Chemical Soclety: Washington, DC, 1983; ACS Symp. Ser. No. 236. (3) Mllano, M. J.; Lam, S.; Grushka, E. J . Chromafogr. 1976, 725, 315-326. (4) Ratzlaff, K. L.; Paul, S. L. Appl. Specfrosc. 1978, 33, 240-245. (5) Rosseau, K.;Dolphin, D. Tetrahedron Lett. 1974, 48, 4251-4254. (6) Sharpe, M. R. Anal. Chem. 1984, 56, 339A-356A.
John C. Pfeffer D. Bruce Skoropinski James B. Callis* Center for Process Analytical Chemistry Department of Chemistry, BG-10 University of Washington Seattle, Washington 98195
RECEIVED for review May 10, 1984. Accepted July 11, 1984. This work was supported in part by Grant GM-22311 from the National Institutes of Health.
