Rapid Scanning Detectors

Quantitative HPLC analysis was carried out on a Spectraphysics 8720 chromatography system, a rapid scan detector by Barspec on a Zorbax ODS column with acetonitrile water 75/25 as the eluent. 

A more popular approach for detemining peak purity has been the use of diode-array detectors which were first introduced in 1982 [50]. Peak homogeneity of similiar benzodiazepines [51] and theophylline have been determined by this technique [52]. Rapid-scan detectors are also useful in confirming the presence of known components such as colorants, which are commonly used in drug products [53,54] and identification of related substances [55,56]. 

The scope of ultraviolet and visible spectrophotometry can be further extended when combined with a chromatographic separation step such as HPLC. The development of rapid-scanning detectors based on the linear photodiode array permits spectra to be acquired during the elution of peaks. Computer-aided manipulation of these spectra has led to new strategies for the examination of chromatographic peak homogeneity, based on classical techniques in spectroscopy. The use of microcomputers enables the development of archive retrieval methods for spectral characterisation (A. F. Fell etal, J. Chromat., 1984, 316, 423-440). 

A spectrum in a specified ranalogue signals from eadi photodiode are digitised and transferred to a computer, where they e corrected for dark current response and transformed to absorbance. A number of digital techniques are available to increase sensitivity and to extend the use of rapid-scanning detectors to multicomponent analysis, reaction kinetics, tablet dissolution tests, process control, and detection in HPLC (A. F. Fell et al, Chrom-atographia, 1982, 16, 69-78). 

The limited availability of affordable commercial RSSF instruments has been an important factor that has prevented the widespread application of RSSF spectroscopy to the study of biological systems. However, in the past year, a significant change in the availability of commercial instrumentation hats come about. There currently are at least five manufacturers of computerized rapid-scanning detector systems. The choices in commercial instrumentation range from a mechanically scanned system with a single photomultiplier detector to photodiode array detector systems. This review includes descriptions of the currently available commercial systems. Because the authors experience in the field of RSSF spectroscopy is limited to the use of diode array detector systems and because most of the commercial instruments have appeared on the market just within the past 12 months, it has not been possible to make detailed performance evaluations and comparisons of the new commercial systems. 

Closed-open systems can also be used for individual and differential kinetic determinations [56,57], a relatively unexplored area In which rapid scan detectors have great potential. 

Experimental limitations initially limited the types of molecular systems that could be studied by TRIR spectroscopy. The main obstacles were the lack of readily tunable intense IR sources and sensitive fast IR detectors. Early TRIR work focused on gas phase studies because long pathlengths and/or multipass cells could be used without interference from solvent IR bands. Pimentel and co-workers first developed a rapid scan dispersive IR spectrometer (using a carbon arc broadband IR source) with time and spectral resolution on the order of 10 ps and 1 cm , respectively, and reported the gas phase IR spectra of a number of fundamental organic intermediates (e.g., CH3, CD3, and Cp2). Subsequent gas phase approaches with improved time and spectral resolution took advantage of pulsed IR sources. 

Since modern FTIR spectrometers can operate in a rapid scan mode with approximately 50 ms time resolution, TRIR experiments in the millisecond time regime are readily available. Recent advances in ultra-rapid scanning FTIR spectroscopy have improved the obtainable time resolution to 5 ms. Alternatively, experiments can be performed at time resolutions on the order of 1-10 ms with the planar array IR technique, which utilizes a spectrograph for wavelength dispersion and an IR focal plane detector for simultaneous detection of multiple wavelengths. ... 

Rapid scanning mass spectrometers providing unit resolution are routinely used as chroaatographic detectors. Ion separation is accomplished using either a magnetic sector, quadrupole filter or ion trap device. Ions can also be separated by time-of-flight or ion cyclotron resonance mass analyzers but these devices are not widely used with chromatograidiic inlets and will not be discussed here [20]. 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

In the past decade, as systems have become simpler to operate, mass spectrometry (MS) has become increasingly popular as a detector for GC. Of all detectors for GC, mass spectrometry, often termed mass selective detector (MSD) in bench-top systems, offers the most versatile combination of sensitivity and selectivity. The fundamentals of MS are discussed elsewhere in this text. Quadrupole (and ion trap, which is a variant of quadrupole) mass analyzers, with electron impact ionization are by far (over 95%) the most commonly used with GC. They offer the benefits of simplicity, small size, rapid scanning of the entire mass range and sensitivity that make an ideal detector for GC. 

Do not have to continually replace sample with blank when obtaining a molecular absorption spectrum 2) errors due to fight source and detector fluctuations are minimized and 3) accurate rapid scanning of wavelengths is possible. 

The introduction of rapid-scanning UV-VIS detectors based on diode-array technology has been of great assistance in confirming the identity of peptides that contain aromatic residues (Phe, Tyr, and Trp). The advantage of these detectors lies in their ability to store spectra for subsequent use. 

Fig. 5.6. A block diagram of an optical coherence tomography/Raman spectroscopy system C, circulator RSOD, rapid scanning optical delay BP, 785 bandpass BSO, beam shaping optics DM1, dichroic mirror at 990 nm DM2, dichroic mirror at 800-950 nm LP, long pass at 808 nm GP, galvanometer pair BD, balanced detector BPF, electronic band-pass filter AI-AO DAQ, analog input-output data acquisition (reprinted with permission from [34]. Copyright 2008 Optical Society of America)...

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