Transient recorders

Typical mass resolution values measured on the LIMA 2A range from 250 to 750 at a mass-to-charge ratio M/ Z= 100. The parameter that appears to have the most influence on the measured mass resolving power is the duration of the ionization event, which may be longer than the duration of the laser pulse (5—10 ns), along with probable time broadening effects associated with the l6-ns time resolution of the transient recorder. ... 

Limitations in the digitizer s dynamic range can be overcome by using multiple transient recorders operating at diflerent sensitivities, or by adding logarithmic preamplifiers in the detection system. From the preceding discussion it appears, however, that quantitative analysis is not the primary area of application of LIMS. Semiquantitative and qualitative applications of LIMS have been developed and are discussed in the remainder of this article. 

Time-resolved microwave conductivity measurements with electrodes in electrochemical cells can conveniently be made with pulsed lasers (e.g., an Nd-YAG laser) using either normal or frequency-doubled radiation. Instead of a lock-in amplifier, a transient recorder is used to detect the pulse-induced microwave reflection. While transient microwave experiments with semiconducting crystals or powders have been performed... 

The fastest reliable PMC transients recorded at electrodes (ZnO single crystals24) were limited by the lifetime of a 10-ns laser flash. It was apparent from the nondeconvoluted signal at shorter time scales that much faster decay processes took place and would be accessible with faster laser pulses. 

In order to distinguish more clearly between effects induced by the varying potential and kinetic contributions, the continuous oxidation of the three Cj molecules was followed at a constant potential after the potential step. The corresponding faradaic and mass spectrometric (m/z = 44) current transients recorded after 3 minutes adsorption at 0.16 V and a subsequent potential step to 0.6 V (see Section 13.2) are reproduced in Figs. 13.5-13.7. In all cases, the faradaic current exhibits a small initial spike, which is associated with double-layer charging when stepping the electrode potential to 0.6 V. 

The basic components include a Nd YAG pulsed laser system which is coaxial with a He Ne pilot laser and visible light optical system. The latter system enables the analytical area of interest to be located. The TOF-MS has a flight path of 2m in length, with an ion detection system that includes an electron multiplier detector, a multichannel transient recorder, together with a computer acquisition and data processing system. 

The analytical area of interest is positioned in the focal spot of the He Ne laser beam, the transient recorder is armed to record, and the Nd YAG laser is fired. This laser pulse of between 5 and 15 ns duration produces a packet of ions that is accelerated from the sample surface and injected into the TOF-MS. All the... 

The turn-on and turnoff measurements were performed at room temperature using a power supply voltage of 300V and a load resistance of approximately 20Q in common emitter mode. Typical turnoff and turn-on transients recorded at 25°C are shown in Figures 6.21 and 6.22. The device was turned off by simply removing the base current. A turn-on rise time of 160 ns and a turnoff fall time of 120 ns were observed at 25°C. In addition, a storage time of approximately 40 ns was observed. 

Figure 17.34 Photocurrent transients recorded on (a) Co(DTB)32 +-mediated DSSC. Electrolyte composition Co(II) — 0.15 M, Li+ -0.5M, and Tbpy = 0.1M. (h) Lil/I2 0.3/0.03 M 0.1 M Tbpy in acetonitrile. From Bignozzi et al., unpublished results.

Fig. 6.52. (a) Three-dimensional representation of current transients recorded for a polycrystalline gold electrode in a solution that contains 5 x 10 4 M pyridine over the potential region -0.75 V to +0.6 V (vs. saturated calomel electrode), (b) Three-dimensional representation of charge transients obtained by integration of the current transients shown in (a). In this drawing AcrM = AgM. (Reprinted from J. Stolverg, J. Richer, and J. Lipkowski and D. E. Irish, J. Electroanal. Chem. 207 213, copyright 1986, Fig. 4, with permission of Elsevier Science.)... 

The temporal resolution of both methods is limited by the risetime of the IR detectors and preamplifiers, rather than the delay generators (for CS work) or transient recorders (SS) used to acquire the data, and is typically a few hundred nanoseconds. For experiments at low total pressure the time between gas-kinetic collisions is considerably longer, for example, approximately 8 /is for self-collisions of HF at lOmTorr. Nascent rotational and vibrational distributions of excited fragments following photodissociation can thus be obtained from spectra taken at several microseconds delay, subject to adequate SNR at the low pressures used. For products of chemical reactions, the risetime of the IR emission will depend upon the rate constant, and even for a reaction that proceeds at the gas-kinetic rate the intensity may not reach its maximum for tens of microseconds. Although the products may only have suffered one or two collisions, and the vibrational distribution is still the initial one, rotational distributions may be partially relaxed. 

Huxley, H. E., Simmons, R. M., Faruqi, A. R., Kress, M., Bordas, J., and Koch, M. H. (1981). Millisecond time-resolved changes in X-ray reflections from contracting muscle during rapid mechanical transients, recorded using synchrotron radiation. Proc. Natl. Acad. Sci. USA 78, 2297-2301. 

Time resolved A state fluorescence is measured with a transient recorder-signal averager, as described previously.(6J A typical fluorescence decay curve is shown in Figure 3. It is composed of at least three exponentials, independent of excitation wavelength. The short component lifetime is extracted by fitting approximately the first third of the decay to the expression. 

---