Dual-channel instruments

In the dual-channel system two radiation sources are used. The beams from these sources are crossing the same atomizer and then they are handled independently by two monochromators, two detectors, and two electronic units. 

These systems are very versatile. Absorption and emission measurements can be run by both channels. A dual-channel system permits the simultaneous determination of two elements. The sample amount and time needed are then only half of that required by a conventional AA system. This is of great use in analyses of small samples. The background correction can be performed by the other channel. Then the difference of the absorbance readings of the two channels are measured. It is also possible to obtain the absorbance ratio of the two channels, which can be used in working with the internal standards. 

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Dual-channel instruments are very similar to their single-channel counterparts and differ only in the fact that they analyse each sample for two constituents simultaneously. The sample stream is divided into two fractions, each one flowing independently through its own system of tubing with the appropriate reagents and detectors. 

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. 

Molecular absorption methods described in the literature are based on the use of continuum light sources (deuterium lamp with a thermal cathode or hollow cathode) or line-like radiation sources (hollow cathode lamps). Measurements using a continuum light source are carried out with a dual-channel instrument. The other channel is needed for the background correction. With a line-like radiation source, a conventional AA spectrometer can be used. In this technique the non-specific absorption is measured with a continuum radiation source. 

At this point, it is also appropriate to mention the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft, that flew past Jupiter in December 2000 and will arrive at Saturn in July 2004, and the Planetary Fourier Spectrometer (PFS) on the Mars Express spacecraft, to arrive at Mars in December 2003. Both spectrometers are dual channel instruments consisting of short- and long-wave interferometers. First, we discuss CIRS. 

From the practical point of view, dual-channel phase detectors operated in quadrature appear to be the best hardware-detection choice. Unlike other techniques, phase detection is sensitive to RF frequency offset from resonance and to the RF phase which, on the one hand, makes the signals more complex to use (as well as and more sensitive to instrument instabilities) but, on the other hand, leads to a number of important advantages, such as ... 

Figure 2.8 shows the general arrangement of the instrument with 100 optical cuvettes radially dispersed around the vertical axis of the reaction rotor. The optical components of the dual-channel monochromator are rigidly mounted within a machined aluminium housing (which is not shown in the diagram), and the housing itself is sohdly fixed to the top of the rotor. 

The most recent generation of NDIR analyzers have evolved to satisfy the frequently harsh industrial environments encountered. These analyzers utilize solid-state sensors for the detection of infrared radialion. Most frequently used sensors are lead selenide (PhSc). thermopiles, or pyroelectric detectors. The gas analyzers generally are configured as single-path instruments, dual-beam with a reference palh. or dual-channel with a reference filter. 

A fast FID consists of a main control unit and two remote sampling heads (which house the FIDs). The dual channel nature of the instrument enables simultaneous real-time measurement in two locations allowing, for example, evaluation of catalyst performance. 

The method development process will be aided if we are able to use sophisticated instrumentation (see also section 1.7.2). Automated injection and data handling will allow a number of experiments to be performed without the requirement of an analyst being present. Moreover, we have seen in chapter 5 (section 5.6) that the use of sophisticated detection techniques (dual-channel or multi-channel detectors) may be of help in the optimization process. 

Figure 13 represents an instrument having dual-channel optics. This means that there are facilities for two line sources and one continuum source, each line source having its own monochromator and detector. Many workers have advocated the usefulness of this type of instrument. Certainly the recently introduced microcomputerised dual channel models are easier to use than their earlier counterparts, so it is worthwhile to summarise the possible attractions of simultaneous dual element analysis. One obvious possibility is the analysis of two elements in a sample at the same time, thus halving analysis time. A less obvious attraction is the possibility of analysing via an internal standard. This is where a second element, either already present or added to the sample, is measured and ratioed to the analytical element. The... 

UV-visible spectra were obtained using a Cary 118C spectrophotometer. Insertion probe mass spectra were obtained with a Dupont 21-49IB mass spectrometer, the FAB spectra with a VG MM-ZAB instrument, and the FD spectra with a JEOL DX-300 mass spectrometer. High pressure liquid chromatography measurements were made with a Waters instrument fitted with dual channel (405 and 546 nm) optical detection and using C18 analytical and semipreparative columns. 

Dionex also offers visible, fluorescence, and pulsed amperometric detectors for use with the series 4000i. Dionex also supply a wide range of alternative instruments, e.g., single channel (20101) and dual channel (2020i). The latter can be upgraded to an automated system by adding Autoion 100 or Autoion 300 controllers to control two independent ion chromatograph systems. Dionex also supply 20001 series equipped with conductivity pulsed amperometric, UV/visible, visible, and fluorescence detectors. 

The ultimate limit in digitizers are the ultrafast digitizers by Intertechnique of France and sold by Tektronix. These expensive devices can take data at rates up to 1000 Ghz and with a response time of less than 60 picoseconds. These instruments are not easy to use, can be easily damaged and do not offer the many options that traditional digitizers offer such as dual channel or dual time bases. However they do provide speed. 

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