Evaluation of a Vidicon Scanning Spectrometer for Ultraviolet Molecular Absorption Spectrometry Michael J. Milano’ and Harry L. Pardue2 Department of Chemistry, Purdue University, West Lafayette, Ind. 47907
A silicon target vidicon tube with a fused silica faceplate has been evaluated for molecular absorption spectrometry in the visible and near ultraviolet region of the spectrum. The computer interfaced spectrometer is shown to be useful down to about 200 nm for scan rates up to 250 Hz and repetitlon rates between 250 and 0.2 Hz. Error data are discussed in terms of the emission spectrum of a deuterium lamp as modified by the dispersion optics and the spectral response of the vidicon and in terms of error coefficients which are used to identify the sources of the error. The spectrometer Is evaluated for the quantitative determination of phenolic compounds based upon difference spectra and is applied to the kinetic study of a four-step reaction which involves three different intermediates with lifetimes between ten milliseconds and several minutes. Rate constants determined for intermediate steps in the reaction agree well with constants determined by conventional instrumentation after appropriate wavelengths had been identified on the rapid scan vidicon spectrometer.
A previous report from this laboratory summarized early applications of array detectors for rapid scanning spectrometry and presented qualitative and quantitative data from an operating vidicon spectrometer which demonstrated the potential of these devices for analytical spectrometry ( I ). Subsequent to that report, review articles appeared which included experimental data to further demonstrate the potential of these devices (2, 3 ) . More recently, a series of papers describing applications to stoppedflow kinetic studies of fast reactions involving transient intermediates ( 4 , 5 ) ,and simultaneous determination of multiple elements by atomic absorption and emission spectrometry (5-8) have confirmed the projections made in that early report ( I ). Some of these studies (6-8) have been based upon a commercial instrument while others have utilized a custom designed vidicon spectrometer interfaced to a small general purpose digital computer ( 4 , 5). This latter system incorporates versatile drive and detector circuitry which permits scan times t o be varied between 4 milliseconds and 10 seconds, repetition rates between 250 and 0.1 Hz, real time dynamic range of 2000, and manually adjustable dynamic range of a t least 10,000. This instrument system was used for an extensive study of the characteristics and applications of the silicon target vidicon for molecular and atomic spectrometry in the visible region of the spectrum. This report utilizes a silicon target vidicon with a fused silica faceplate to extend the study to the near ultraviolet region. The tube is useful for spectral studies between 200 and 1100 nm with scan times down to 4 msec. This paper includes quantitative data similar to those reported earlier for the visible region (5) and identifies the magnitudes of l Present address, Department of Chemistry, State University of New York at Buffalo, Buffalo, N.Y. Correspondence should be addressed to this author.
different types of error coefficients (9, I O ) which are used to identify the probable sources of the errors and suggest methods to reduce these. Applications reported in this paper include the quantitative determination of phenols based upon difference spectra between acidic and basic forms and an elucidation of the several s1;eps involved in the hydrolysis of a Pd(I1) complex of triglycine. Results obtained in this and an earlier study ( 5 ) demonstrate unequivocally that the silicon target vidicon is a useful detector for molecular absorption spectrometry in the region between 200 and 800 nm.
EXPERIMENTAL Apparatus. Details of the design of the vidicon spectrometer have been presented (5). Although the system is capable of performing reliably for scan times down to 4 msec , most of the data reported here are for a scan time of 10 msec. The only changes made for operation in the ultraviolet were the substitution of a vidicon tube with a fused silica faceplate (No. C232.16, Radio Corp. of America, Harrison, N.J. 07029) and a deuterium source and a power supply to drive the deuterium source (source No. 9780 and supply No. 71701, Beckman Instruments Inc., Fullerton, Calif. 92634). The 3-mm pathlength stopped-flow mixing system, described previously, was used in the study of the hydrolysis of palladium triglycine. A 1.00-cm quartz cell thermostated to within fO.l “C and with manual mixing was used for the phenol measurements. Data Acquisition A P D P 12-30 computer with 8 K of core memory is used to record and process the spectral data. The assembly language data acquisition program records 59 spectra per experiment. The conversion and storage of each 128-point spectrum is initiated by a O.2-msec, %volt pulse produced by the scanning circuits. There is maximum time uncertainty of f 3 psec associated with the detection of the start pulse by the computer. Although this uncertainty results in each recorded spectrum being slightly displaced in the wavelength axis, it has a negligible effect (
