Scanning multichannel instrument

There are different types of spectrometers with detector arrays. Due to the limited number (256, 512 or 1024) of available detector elements compared to the number of resolvable spectral elements, the spectrometers permit either low resolution over the entire spectral range, or high re.solution within a limited range. Alternatively, the technique described by Knoll et al. (1990) may be used They employed scanning a multichannel instrument, which combines the merits of the scanning technique with those of the multichannel technique. 

Currently many thousands of x-ray spectrometers are being utilized for routine qualitative and quantitative analysis. Sensitivities available for most elements reach the low part-per-million range, and the method is equally as applicable at high or low concentration levels. Accuracies of the order of a few tenths of a percent are achievable with analysis times on the order of minutes for scanning spectrometers and even less for multichannel instruments. 

The spectrograph entrance slit of multichannel instruments is usually variable from about the width of one of the array elements to many times wider. Some spectrometers have no entrance slit but instead use a fiber optic as the entrance aperture. Multichannel spectrometers using both photodiode arrays and CCD arrays are capable of obtaining an entire spectrum in a few milliseconds. With array detectors the light can be integrated on a chip or multiple scans can be averaged in computer memory to enhance the signal-to-noise ratio. 

A multichannel instrument is a powerful tool for studies of transient intermediates in moderately fast reactions, for kinetic studies, and for the qualitative and quantitative determination of the components exiting from a liquid chromatographic column or a capillary electrophoresis column. They are also useful for general-purpose scanning experiments. Some have the software necessary to analyze the time dependence at four or more wavelengths for kinetic studies. 

The tail of the plasma formed at the tip of the torch is the spectroscopic source, where the analyte atoms and their ions are thermally ionized and produce emission spectra. The spectra of various elements are detected either sequentially or simultaneously. The optical system of a sequential instrument consists of a single grating spectrometer with a scanning monochromator that provides the sequential detection of the emission spectra lines. Simultaneous optical systems use multichannel detectors and diode arrays that allow the monitoring of multiple emission lines. Sequential instruments have a greater wavelength selection, while simultaneous ones have a better sample throughput. The intensities of each element s characteristic spectral lines, which are proportional to the number of element s atoms, are recorded, and the concentrations are calculated with reference to a calibration standard. 

It is clear that CD measurements can provide unique information which can be used to design and optimize the separation of structurally complex, biologically active materials. However, the fact that HPLC is a time dependent experiment, and the time dependence of the analyte distribution is not compatible with the scanning rate of conventional CD instruments currently limits the CD-HPLC measurement to a single wavelength. This problem will be solved if multichannel detection can be effectively coupled to the CD experiment. 

Recent advances in solid state detectors have led to more efficient laser Raman spectrometers. These spectrometers are based on multichannel detectors (MCD) and are at least an order of magnitude faster than the systems based on PMT detector. However, at present these systems do not match the resolution capabilities of the scanning systems. The Spex Triplemate 1877 A is a new generation instrument which can be fitted with an intensified diode array consisting of 1024 elements. Figure 4.6.3 shows spectra obtained with these two types of detectors. 

Colloid concentrations in the eluent were determined by placing 200 ml of the sample into a Bio-Tek multichannel (optical densitometer with fiber-optics technology Bio-Tek Instruments, Inc., Winooski, VT) microplate reader and scanning at 540 nm. Total Pb concentration in the eluents was allocated to solution phase and colloidal phase. The eluent samples were centrifuged for 30 min at 3500 rpm to separate the soluble contaminant fraction from the colloid-bound contaminant fraction. The colloid-bound Pb was extracted with 1 N HC1-HN03 solution, and along with the soluble Pb fraction, was analyzed by ICP spectrometry. 

Options for multichannel LC-CD detection do exist.f " Stopped-flow accessories for commercial instruments are available that allow part of an eluted fraction to be taken off-line into a microcell placed in the regular sample compartment where data are measured in the normal way. The method still requires rapid scanning capabilities. Repeated injections and multiple scans can be averaged to improve the quality of the signal. A major deterrent to the progress in the early development of HPLC-CD detection was the lack of a dedicated instrument at a reasonable cost, the only option being a fully equipped CD instrument. 

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