Single-channel instruments

Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. 

The design of all single-channel instruments stems from the original Technicon AAI system, which comprises several interconnected units, called modules. The liquid streams flow through the modules, each of which performs a different analytical function (Figure 6.6). The subsequently developed AAII analysers did not consist of separate modules but included an analytical cartridge which combined several functions in one unit. 

As was the case for Fs, minor modification of Fsnr is necessary for the case of a single channel instrument. The time that determines SNR is ts. which would occur instead of tM in Eq. (4.24) for a single-channel instrument. Keeping Esnr as a parameter based on total measurement time requires the substitution t = tm /Nr, and... 

So Esnr is decreased for the single-channel instrument when compared to a multichannel system with Nr resolution elements. 

The spectrometers shown in Figs. 15-1(a), 15-4, and 15-5 are single-channel instruments, in the sense that they have only one counter, which is regarded as a channel through which information is received. The various spectral lines are measured sequentially by moving the counter from one line to another, either manually or by a mechanical drive. 

The kind of crystal and kind of counter in each channel are selected to be best suited to the wavelength to be measured in that channel. No compromise has to be made in order to cover a certain wavelength range, as in a single-channel instrument. 

Multichannel spectrometers may contain, in addition to the fixed channels, a scanning channel in which a moving crystal and counter can sequentially scan the entire spectrum, just as in a single-channel instrument. This feature is useful for qualitative analysis or for quantitative determination of an element for which a fixed channel has not been preset. 

Apart from single-channel instruments, with which only measurements at a single wavelength in one channel can be performed, dual-channel instruments are also used. They contain two independent monochromators which enable measurements to be taken simultaneously at two different wavelengths. They are of use for the simultaneous determination of two elements, where e.g. one element can be the analyte and the second a reference element. Two lines with widely different sensitivities can also be used so as to determine one element over a wide concentration range. 

Among wavelength-dispersive spectrometers, a distinction can be made between single-channel instruments and multi-channel spectrometers. In the former type of instrument, a single dispersive crystal/detector combination is used to sequentially measure the X-ray intensity emitted by a sample at a series of wavelengths when this sample is irradiated with the beam from a high power (2—4 kW) X-ray tube. In a multi-channel spectrometer, many crystal/detector sets are used to measure many X-ray Unes/elements simultaneously. 

Beta Scientific make three analyzers, the 4200, the 4201 and the 4203. These instruments were designed for unattended and fully automatic operation, complete analyses in less than 10 minutes, cost effectiveness and an ASCII, R232 interface for computer or LIMS connection [196]. The model 4200 is a single channel instrument with no R232 interface and manual calibration the model 4201 also includes an R232 interface, and the moel 4203 is a three channel version without sample preparation but with an R232 interface. The model 4210 is a th station sample preparation unit, a required accessory for model 4203. 

Data logging instruments hard-wired to the monitoring devices may be single or multi-channel. Single-channel instruments have the advantage that they may be mounted local to the monitoring point to minimize the length of the cable run. 

The trade-off between the three axes of Fig. 5.14.1 is an important, but not the only consideration in the design of a radiometric instrument. The electrical bandwidth needed to transmit the information may become a significant parameter for scanning radiometers or for spectrometers. So far, the discussion has been limited to single-channel instruments. If one uses an array of detectors or an instrument with a multiplex property, such as the Michelson interferometer, Eq. (5.14.3) still applies to each detector element or to each spectrally resolved interval, but the electrical bandwidth or the bit rate, if the information is converted into digital format, increases according to the number of elements in the array, or the multiplex factor, both expressed by M. The required bit rate is proportional to... 

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