Fast amplifier

Figure 8.7.5 Schematic diagram of apparatus employed for the temperature-jump method. The laser pulse is passed through a neutral density filter (ND) and irradiates the thin film electrode at the bottom of the cell. The dark rectangles are an auxiliary electrode and a QRE for measurement of the potential change. The potentiostat (Pot.), which adjusts the electrode potential before irradiation, is disconnected immediately before the laser pulse. The change in potential is measured with a fast amplifier (Amp.). [Reprinted from J. F. Smalley, L. Geng, S. W. Feldberg, L. C. Rogers, and J. Leddy, J. Electroanal. Chem., 356, 181 (1993), with permission from Elsevier Science.]...

FIG. 6. Normalized response functions [see Eqs. (74) and (75) and associated discussion] obtained using the fast amplifier (O) with the titanium-sapphire laser k = 798 nm fwhm = 100 fs) and using the old amplifier ( ) with the NdYAG laser (k = 1060 nm fwhm = 8 ns). Solid lines are optimized fits generated using Eq. (76) with parameter values given in text. 

Files containing circuit diagrams, part numbers, etc., for the potentiostat and fast amplifier are accessed using URLs http //www.inst.bnl.gov/cgi-bin/view io. pl table=ionumbers id=45 and http //www.inst.bnl.gov/cgi-bin/view io.pl table=ionumbers id=553 respectively. Click on the numeral in table entry Archive CD and follow instructions to download the desired files. 

In this instrument ions produced in the source are accelerated to a given velocity. The unresolved beam is then injected into a field-free region and the ions drift towards the collector. The velocities will be inversely proportional to the square roots of the masses. This means that a pulse of ions will split up according to the ionic masses. The unresolved beam thus becomes resolved in time. Provided that the response time of the electronics is sufficiently fast a spectrum can be recorded. Obviously an average over many such pulses is necessary to provide a reliable signal. Once again the electronics lie at heart of this problem, which demands very fast amplifiers. Initially the time-of-flight mass spectrometer (TOF) was the province of physicists and later of chemists but, with the tremendous advance in electronics, instruments are now produced that are capable of routine operation by relatively untrained operators. 

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