Multichannel instruments

Another approach to multielemental analysis is to use a multichannel instrument that allows for the simultaneous monitoring of many analytes. A simple design for a multichannel spectrometer consists of a standard diffraction grating and 48-60 separate exit slits and detectors positioned in a semicircular array around the diffraction grating at positions corresponding to the desired wavelengths (Figure 10.50). 

Multichannel instruments are capable of measuring the intensities of the emission lines of up to 60 elements simultaneously. To overcome the effects of possible non-specific background radiation, one or more additional wavelengths may be measured and background correction (see Section 21.12) can be achieved. 

The 1950s saw the introduction of a completely new approach to automation, in the form of continuous flow analysis. This made a significant contribution to the advance of automated analysis and subsequent development has been in the form of flow injection analysis. The original instruments were single channel and capable of measuring only one constituent in each sample. Multichannel instruments were then developed which could simultaneously carry out several different measurements on each sample. These were useful in laboratories where many samples required the same range of tests. 

Multichannel instruments These are equipped with a photodiode array detection system. The radiation from a tungsten or deuterium lamp is focused on the sample or solvent cell, and then passes to a diffracting grating. The scattered radiation arrives at the diode array, which simultaneously detects and analyzes various wavelengths. 

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. 

Two major problems with both types of system are standardization and sample identification. With multichannel instruments a serum standard or reference is almost obligatory. This must be standardized... 

An ingenious multichannel instrument for tissue imaging was developed at the University of Illinois [162], The thrust of the research was to develop a frequency-domain instrument for noninvasive, real-time NIR optical tomography of tissue in... 

A similar false rejection problem arises when multichannel instrument systems are controlled using the I2J rule on each of several channels. For one control material being analyzed by 4, 8, 12, and 20 channels, the chances that the control value on at least one channel exceeds its 2s limits are 18%, 33%, 46%, and 64%, respectively. Such a high rate of values exceeding the control limits may cause the same percentage of work to be routinely repeated, obviously compromising the efficiency of the laboratory and increasing its costs. 

CCDs and CIDs are appearing in ever-increasing numbers in modern spectroscopic instruments. In spectroscopic applications, charge transfer devices are used in conjunction with multichannel instruments, as discussed in Section 26B-3. In addition to spectroscopic applications, charge transfer devices find widespread applications in solid-state television cameras and microscopy. 

Because all channels operate simultaneously, a multichannel instrument is fast. 

Continuous-flow instruments may also be single-channel (batch) instruments that analyze a continuous series of samples sequentially for a single analyte (Figure 23.4). Or they may be multichannel instruments in which the samples are split at one or more points downstream into separate streams for different analyte analyses, or separate ahquots of samples may be taken with separate streams in parallel. 

Array-Based Multichannel Instruments. Spectrometers that offer the versaliliiy of spectrographs willl photographic recording and the speed and precision of spectrometers equipped with multiple photomultipliers arc widely ivailable. These instruments are the... 

Fach transducer in a multichannel instrument has its own amplifier, pulse-height selector, scaler, and counter or integrator. These instruments are equipped with a computer for instrument control, data processing, and display of analytical results. A determination of twenty or more elements can be completed in a few seconds to a few minutes. 

Multichannel instruments are widely used for the determination of several components in industrial materials such as steel, other alloys, cement, ores, and petroleum products. Both multichannel and singlechannel insirument.s are equipped to handle samples in the form of mct ils, powdered solids, evaporated films, pure liquids, or solutions. When necessary, the materials are placed in a cell with a Mylar or cellophane window. 

With single-beam designs, the array dark current is measured and stored in conipulcr memory. Next, the specirum of the source is obtained and stored in memory after dark-current subtraction. Finally, the raw spectrum of the sample is obtained and. after dark-current subtraction, the sample values are divided by the source values at each wavelength to give absorbances. Multichannel instruments can also be configured as double-beam-in-time spectrophotometers. 

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

Flame OES can be used to determine the concentrations of elements in samples. The sample usually must be in solution form. Generally, one element is determined at a time if using an AAS system in emission mode. Multichannel instruments are available for the simultaneous determination of two or more elements. Detection limits can be very low as seen in Appendix 7.1, Table Al. Detection limits for the alkali metals are in the ppt concentration range when ionization suppression is used. One part per trillion in an aqueous solution is 1 pg of analyte per mL of solution or 1 x g/mL. Most elements have detection limits in the high ppb to low ppm range. 

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