Instrumentation
Rapid Scanning Spectroscopy: Prelude to a New Era in Analytical Spectroscopy Robert E. Santini, Michael J . Milano, and Harry L. Pardue Department of Chemistry Purdue University Lafayette, Ind. 47907
Future developments in rapid scanning spectroscopy will depend heavily upon recent advances in analog and digital electronics, computer technology, and opto-electronic systems. Among these opto-electronic systems are included the vidicon tube, solid state array detector, acousto-optic filter, and electrically controlled refracting elements
Rapid scanning spectroscopy (RSS) is a technique in which a selected re gion of the electromagnetic spectrum is scanned on a time scale ranging from several seconds to a few microseconds. The technique has been developed primarily for the study of chemical reactions or processes which involve short-lived intermediates having spectral properties which differ significantly from reactants and products. Repetitive scans of the appropriate spectral region can provide informât ion on t he number and types of species present as well as the kinetics of the formation and decay of intermediates. Such data can provide explicit mechanistic information which could only be inferred from measurements made at a single wavelength. Typical examples which lend themselves to this ty pe of study include
enzyme -substrate complexes ( / ), mixed complexes in ligand-exchange reactions (2), and products of electrochemical [3) or flash photolysis (il experiments. An entire issue of Applied Optics (5) was devoted to the subject of RSS instrumentation, including a comprehensive review of instrumental developments and applications reported prior to 1968 (6). One of the most striking features of the literature on this topic is that it involves a series of specialized instrument systems applied to equally specialized problems. There have been virtually no examples in which RSS instrumentation has been used for such routine applications as multielement analyses or for developmental work on analytical procedures or even for fundamental studies of slow reactions or systems at equilibrium. It is probable that most of the common methods involving analytical spectroscopy can profit from time resolved spectra generated with a minimum of effort. There is significant overlap between the performance requirements of the conventional re-
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cording spectrophotometers and many of the specialized rapid scanning spectrophotometers. It appears logical to consider designs which would permit a single spectrophotometer to fulfill many of the functions now performed by these two different types of instruments. In our opinion, the technology necessary for a major step forward in the design of scanning spectrophotometers which would accomplish this goal is now available. For many types of problems, the manual and /or conventional mechanical designs can be replaced by versatile electronically controlled systems. The time and effort required to obtain one or several scans of a spec trum covering several hundred nanometers should be little more than that now required to obtain measurements at one wavelength byuse of conventional instrumentation. Decisions involving trade-offs among speech spectral resolution, and photometric accuracy would be implemented by simple external control switches. These developments will depend heavily upon recent advances in ana-
Dispersed Source
Dispersing element
ι I λ
Sample
Detector Read out
Ce||
Spectrum Slit a
Sample Source „ „ Cell
Dispersing element
Dispersed { ι Spectrum
Array „ Detector
Read out
Figure 1. Two basic approaches to dispersion scanning spectrometers A. Scanned s p e c t r u m
B, array detector
ANALYTICAL CHEMISTRY, VOL. 45
NO
11. SEPTEMBER
1973 · 915 A
REQUIRED READING for better Chromatographic Separations
log and digital electronics, c o m p u t e r technology, a n d o p t o e l e c t r o n i c syst e m s . Among these opto-eleetronic s y s t e m s are included t h e vidicon t u b e , solid s t a t e array detector, acousto-optic filter ( 7). and electrically controlled refracting e l e m e n t s . However, t h e reader should not lose sight of t h e fact t h a t most of t h e basic concepts will have h a d their b i r t h in t h e R S S studies s p a n n i n g t h e last two decades. We have two major goals in preparing t h i s report. O n e of these is to develop a perspective of t h e different general a p p r o a c h e s which have been developed for R S S and to present r e p r e s e n t a t i v e e x a m p l e s of each of these a p p r o a c h e s . T h e other is to project d e v e l o p m e n t s which can be expected in t h e near future as well as long-range d e v e l o p m e n t s which will be necessary to achieve near o p t i m a l performance. Space r e q u i r e m e n t s have forced us to pay m a x i m u m attention to those e x a m p l e s which are judged to have been most useful in the recent past or which show maxim u m promise for the future. Nevertheless, some designs which are viewed to be i m p r a c t i c a l in their present forms are given brief coverage with t h e prospect t h a t they m a y whet t h e a p p e t i t e of one or more readers who m a y a p p l y new or different technologies which could result in the development of viable s y s t e m s .
General Considerations Most s c a n n i n g s p e c t r o m e t e r s can be grouped u n d e r one of two major
classifications which we have chosen to label dispersion and multiplex s p e c t r o m e t e r s . Most c o m m o n instrum e n t s employ t h e dispersion mode in which a prism or grating s e p a r a t e s t h e s p e c t r u m into narrow b a n d s of energy (resolution e l e m e n t s ) which can be monitored i n d e p e n d e n t l y . S p e c t r o m e t e r s o p e r a t i n g in t h e multiplex mode [Fourier (
