Injection and Pulsed Sources

Following upon a method devised by the writer and Grindley and adopted later by Bradbury, a second method was developed by Powell, Starr and the writer which has been extensively used in the Bristol laboratory.. .. The actual priority of publication must go the van de Graaff who developed an essentially similar method independently in the Oxford Electrical laboratory he did not however apply it to any important cases. 

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At-line LIF methods are either based on intrinsic detection or extrinsic approaches. The former often involves static measurements which are prone to photobleaching and thermal effects. These problems can often be addressed by optimizing the excitation source output (i.e., optical power and pulse rate) or by sample agitation. Flow injection analysis or other autonomous sample prepreparation schemes are possible to facilitate various at-line extrinsic methods such as various selective fluoroimmunoassays. ... 

A stable activable tracer (SAT) is a stable material that is injected into a system under study and whose concentration in the system is measured by a post sampling activation analysis. The advantages of such "artificiar tracers as compared with the naturally occurring trace elements in various systems (which act as natural tracers) are as follows artificial tracers have a controlled emission rate (either pulse or continuous injection) and control of the amoimt injected, both of which are valuable in model validation studies they can be injected in amounts suflBcient to ensure easy detection in the system under study and they lend themselves better to simultaneous tracing of several similar pollutant sources. 

A remarkable adaptation of technology borrowed from another analytical method was demonstrated for an electrospray ionization (ESI) IMS instrument ion injection to a drift tube was achieved through modulation with a mechanical chopper as found in atomic absorption spectrometry. The chopper was a disk with a small hole that would align with the source and drift tube and would operate as an ion injector. The disk had a second window that was used with optical sensors to synchronize ion injection and drift time, and ion injections were made at pulse rates of 5 to 200 Hz with pulse widths of 200 to 500 ps. 

Certain ion sources are not continuous, as with ESI and Ni, and instead are intermittent, requiring electrical pnlses, snch as with CDs or optical pulsing like that with multiphoton ionization to make ions from a sample. Ion injection with such sources can be coincident with ion formation, and ion injection may be arranged without ion shutters. In this instance, an ion shntter may be seen as optional or unnecessary. While a pulsed ion sonrce conld be attractive for reduced costs of drift tube manufacture and simplicity of design, pnlsed sources can introduce time-dependent chemistry or ion intensity into a mobility measurement, and neither of these is easily controlled or desirable. 

Figure 7.19 Rectangular pulse injection and corresponding response (source LeGojfand Midoux 1981, p. 205). 

Figure 1.2 Typical arrangement for a conventional Ion mobility spectrometer. Ions are produced In the upstream region (left-hand side of the figure), in this case via a radioactive source, and are then drawn from left to right by an electric field applied through a series of electrodes (the guard rings ). Ions are injected in pulses using an electrical shutter (a Bradbury-Nielson (BN) gate) and the time taken to reach the detector is then determined. The ion detector in the figure is a simple Faraday plate (see Section 3.6).

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