Analytical detector tubes

Figure 8.36 Schematic diagram of a gas-fiiied X-ray detector tube. He fiiier gas is ionized by X-ray photons to produce He+ ions and eiectrons, e-. The eiectrons move to the positiveiy charged center wire and are detected. (Modified from Parsons, M.L., X-ray methods, in Ewing, G.W. (ed.). Analytical Instrumentation Handbook, 2nd edn., Marcei Dekker, ino., New York, 1997. Used with permission.)...

In recent years passive samplers have become popular. They come with various kinds of analyte-specific sorbents, and the rate of diffusion for that particular device has been determined by the manufacturer. Some of these passive devices are color detector tubes and can be read directly after the sampling period. Other passive samples must be sent back to the supplier s laboratory for analyses. Figure 6.3.1.3 shows a passive sampling badge. 

The ion signal is measured as a function of time and should therefore consist of a series of peaks corresponding to ions with different mobilities arising from different chemical compounds in the analyte. Drift tube transit times depend on the length of the tube but are typically on the order of several tens of milliseconds. By comparison, the injection time for ions is < 1 ms, and until all of the ions in the injected bunch reach the end of the drift tube, a second bunch of ions cannot be injected. Consequently, the duty cycle, which is a measure of the fraction of ions that reach the detector out of the total number of ions that could reach the detector if the experiment was not pulsed, is rather low and is typically 1%. This is an important factor in limiting the sensitivity of IMS. Nevertheless, detection of compounds... 

The dimensions of the exit tube from the detector are not critical for analytical separations but they can be for preparative chromatography if fractions are to be collected for subsequent tests or examination. The dispersion that occurs in the detector exit tube is more difficult to measure. Another sample valve can be connected to the detector exit and the mobile phase passed backwards through the detecting system. The same experiment is performed, the same measurements made and the same calculations carried out. The dispersion that occurs in the exit tube is normally considerably greater than that between the column and the detector. However, providing the dispersion is known, the preparative separation can be adjusted to accommodate the exit tube dispersion and allow an accurate collection of each solute band. 

The resolution of the ToF analyser is dependent upon the ability to measure the very small differences in time required for ions of a similar m/z to reach the detector. Increasing the distance that the ions travel between source and detector, i.e. increasing the length of the flight tube, would accentuate any such small time-differences. The implication of such an increase is that the instrument would be physically larger and this goes against the current trend towards the miniaturization of all analytical equipment. 

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