High-voltage supply

The high-voltage supply provide potentials up to about 30 kV, and the typical field strengths used are in the range of 200-400 Vcm . Separations can be performed with constant current or constant potential, the latter is preferred in most cases. Typical current is about 100 pA at 30 kV, but the current will vary with the conductivity of the electrolyte solution. The heat generated, because of the current, may lead to temperature variations in the system. 

a cooling system is most often used to give a constant temperature in the separation system. Both air cooling and liquid cooling systems are used in commercial instruments. 

Because high voltages are used, up to 30 kV, the separation system needs to be insulated for safety reasons. 

The EOF can transport the analytes through the capillary depending on their 

EOF can be minimized by using capillaries with a deactivated surface. Deactivation is obtained by chemically binding a ligand to the silica surface or by adding compounds (additives) to the electrolyte solution. Such deactivation will also aid in preventing unwanted adsorption of analytes to the capillary wall (which may be a problem with basic analytes.) 

In comparison with the other sources of uncertainty discussed above, electronic noise in scintillation detector systems is a minor problem. More important, as we shall see, is gain drift caused by instability in the high-voltage supply. The priorities when selecting electronic modules for scintillation counting are somewhat different from those which determine a system for high-resolution (semiconductor) spectrometry. 

The high-voltage units described in Chapter 4, Section 4.2 would not be satisfactory for scintillation detector 

In Section 10.9, it was explained that the dynode electron multiplication factor depended upon the potential difference between successive dynodes. This implies that the overall multiplication, G, of the complete photomultiplier chain should be related to the overall anode to cathode voltage, V, in the following manner  

For example, the voltage regulation might be specified as within 0.001 % and the temperature stability as + 0.005% per °C compared to 0.1% and 0.08%, respectively, for units intended for semiconductor detector systems. The long-term stability must also be good and figures of 0.01 % over a 1 h period and 0.03 % over 24 h would be typical. 

As with all detectors, the pulse of current at the output, in this case the PMT anode, must be integrated to provide the signal. Because electronic noise is usually not a problem, preamplifiers for scintillation systems need not have a particularly low noise specification. AU three types of preamplifier - voltage, current and charge-sensitive - are in common use. Charge-sensitive preamplifiers are often offered for routine use but low cost voltage-sensitive types are also common. For normal gamma spectrometry 

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Schematic drawing of a temperature-jump apparatus (adapted from Ref. 13). Shown are the analyzing lamp, observation cell, monochromator, photomultiplier, oscilloscope, spark gap, and high-voltage supply.

Obviously, increasing the dynode voltage, and hence the amplification, tends to increase these unwanted signals. Thus, the high voltage supply (commonly up to 1000 V) must be highly stabilized and is only increased as necessary. 

Figure 1. Block diagram of single-photon time-correlation apparatus from Barker and Weston 11 HV, high-voltage supplies L, lamp PI, photomultiplier M, monochromator FURN, furnace C, sample cell LP, light pipe F, interference filter P2, photomultiplier AMP, amplifier DISCI, discriminator D1SC2, discriminator T-S, timer scaler DL, delay line TAC, time-to-amplitude converter BA, biased amplifier MCPHA, multichannel pulse-height analyzer TTY, teletype printer and paper-tape punch REC, strip-chart recorder.

Fig. 5. The schematic diagram of the pulsing system and the ion beam pulse radiolysis system with an optical emission spectroscopy. PMT denotes photomultiplier tube HV, high voltage supply CFD, constant fraction discriminator TAC, time to amplitude converter and PH A, pulse height analyzer. From Ref. 36...

The independence from high-voltage supply and the small amounts of technical equipment required allow construction of mobile units for multielement analysis of mineralogical or geological samples in the field. 

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