Multi-wavelength detection

Thormann W, Meier P, Marcolli C, Binder F (1991) Analysis of barbiturates in human serum and urine by high-performance capillary electrophoresis-micellar electroki-netic capillary chromatography with on-column multi-wavelength detection. J Chromatogr 545 445-460. 

Since multi-wavelength detection has now been mentioned and will feature quite prominently in other apphcations, it is worthwhile discussing the reasons for this and why forensic laboratories have been keen to use and develop new multiwavelength detection techniques. Traditionally, solute characterisation in HPLC is based upon the measurement of retention time. However, for the purpose of sample identification or discrimination retention time is a very poor parameter because of the possibility of this being similar or identical to other compounds. 

Werner, G. Schneider, V. Emmert, J. Simultaneous determination of creatine, uric acid and creatinine by high-performance liquid chromatography with direct serum injection and multi-wavelength detection. J. Chromatogr. 1990, 525, 265-275. 

The word multiplex has been used indiscriminately by many authors to describe both simultaneous and rapid sequential (e.g. rotating filters, fast-scan or slow-scan systems) multi-wavelength detection methods [37]. Other specialists use this designation to refer to spectroscopic methods based on the truly simultaneous measurement of several optical signals at different wavelengths. Finally, others reserve this term for those methods which use a single detector for multi-wavelength detection [17]. 

F. Ldzaro, A. Rios, M. D. Luque de Castro, and M. Valcarcel, Simultaneous Multi wavelength Detection in Flow Injection Analysis. Anal. Chim. Acta, 179 (1986) 279. 

Recently, LC techniques were introduced for the evaluation of dyestuffs. They have much higher sensitivity than TLC, the choice of conditions, mobile phase, and columns again depends on the chemical properties of the dyes, and an extraction procedure is also required. The results obtained by LC can be used for quantitative data processing and, by comparison of retention times, peak height or area ratios, and trace impurity detection by its higher sensitivity than TLC, more detailed discrimination of fiber samples is possible. Multi-wavelength detection using a PDA detector is also useful, as described above. 

Multi-wavelength detection diode array rapid scan methods ... 

Historically, photographic films and plates have been used at the exit plane of the monochromators to detect the spectral patterns from analytes. The main advantage of this approach is its inherent multi-wavelength detection ability. Because photographic films and plates are area detectors with fantastic spatial resolution, the entire spectrum from a sample may be recorded on a 30-cm plate, which ultimately results in simultaneous multi-elemental analysis. Unfortunately, the dynamic range of photographic emulsion is limited and the response function is typically nonlinear. Furthermore, the overall process is extremely time consuming and chemically intensive (need for a darkroom. 

B.G.M. Vandeginste, R. Essers, T, Bosman, J. Reijnen and G. Kateman, Three-component curve resolution in HPLC with multi wavelength diode array detection. Anal. Chem., 57 (1985) 971-985. 

Most compounds can be detected directly as they are able to produce a direct analytical signal. Photometric detection, especially UV (including diode array and multi-wavelength UV detection) is by far the most frequently applied detection technique. The application of mass spectrometry (MS) detection in CE is attractive as it can provide structural information [44]. Hologram-based refractive index detection [45] and electrochemical detection [46,47] were also reported. Conductivity [41,48-50] and amperometric [51,52] detection has shown to have advantages for the analysis of both organic and inorganic compounds. 

Among optical sensors, those based on fluorescence are of major interest because of their ability to use spectral and temporal information multi-wavelength measurements allow simultaneous detection of two or more analytes, and discrimination between analytes is possible by time-resolved measurements. Multiplex capabilities represent the main advantage of such sensors compared to electrochemical devices. 

J Vindevogel, R Szucs, P Sandra, L Verhagen. Analysis of beer iso-ar-acids by micellar electrokinetic chromatography and multi-wavelength UV detection. J High Res Chromatogr 14 584-586, 1991. 

Recently a number of advanced detection methods have been introduced such as multi-wavelength evaluation or measuring the UV spectra by diode-array scanning [86,87]. fluorodensitometry. voltammetry, and fast atom-bombardment mass spectrometry. etc. 

Demorest DM. Distinguishing cyclobenzaprine from amitriptyline and imipramine by liquid chromatography with UV multi-wavelength or full-spectrum detection. Blood levels of cyclobenzaprine in emergency screens. J Anal Toxicol 1987 11 133 (Letter). 

A corollary deserves a mention in this era of multi-wavelength astrophysics presence of 7Be may be detectable by the /3-decay of this unstable nucleus - 7Be(e i/e)7Li with 10% of the decays providing a y-ray at 478 keV. Several searches for the 478 keV y-ray line have been reported. The most recent is by Harris et al. (2001) who report upper limits on the 478 keV flux at least an order of magnitude higher than predictions for the several well observed novae - only weak limits can be placed on the key parameters in the nucleosynthesis and ejection of 7Be . Prospects for detection of nearby novae using INTEGRAL appear quite good. 

The value chosen for resolution R Is dependent upon the requirements of th separation. If co onent detection Is the sole objective, R depends upon the methods available to discriminate between adjacent components. As an example, the sophisticated multi-wavelength spectroscopic detection of overlapping chromatographic components allows the magnitude of Rj to approach very small values (3). Subtle differences In the ss spectra of eluting components can also be used to reduce R. ... 

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