Transient signals

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

The transient signals from and isotopes were used as the indicators of the steel substrate and Ti and were used as the indicators of the... 

The determination of the LOD in connection with transient signals is discussed in Ref. 85. 

Williams, T. W., and Salin, E. D., Hazards of a Naive Approach to Detection Limits with Transient Signals, Anal. Chem. 60, 1988,125-121. 

Transient signals are not observed when a pure Ti02 sol is illuminated with a flash from a frequency doubled ruby laser (A, = 347,1 nm). The charge carriers seem to... 

Detection of transient signals (narrow chromatographic peaks)... 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Clearly RR is attractive as a technique for detecting intermediates. It has, for example, been very successful with intermediates in organic and bioinorganic reactions (55). Unfortunately the high laser intensities that are needed in RR may be incompatible with the photosensitivity of most organometallic intermediates. For example, in an attempt to detect Cr(CO)5 in solution by RR, the only detectable transient signal was emission from excited Cr atoms (55). 

After the data are acquired, transient signals taken at individual wavelengths can be analyzed for kinetic information or an entire time-resolved spectrum can be synthesized. This is achieved by instructing the computer to assemble a point-by-point spectrum, corresponding to a particular delay time after the photolysis pulse. The spectrum is constructed out of points that are 4 cm-1 apart and is similar to a spectrum produced by a normal IR spectrometer but with 30 nsecond time resolution ... 

The typical profile of a CL transient signal (a plot of CL intensity vs. time) is a kinetic response curve that corresponds to a first-order sequence of two consecutive steps, namely (1) generation of the light-emitting product by mixing of the chemical ingredients (the substrate and oxidant), and (2) formation of the end product (Fig. 2). The rate at which each step takes place depends on the formation... 

For FT-ICR-MS, the ICR cell itself must not necessarily differ from one used in scanning ICR-MS (Fig. 4.51). Two of the four side walls (jc-axis) of the ICR cell are connected to the RF power supply during the period of excitation. Then, the image current induced in the detector plates (y-axis) is recorded as transient signal for some period of time (0.5-30 s). The excitation of the ions within the ICR cell has to stop at a level low enough to avoid wall collisions of the lightest ions to be measured. [191,200,201]... 

An interesting correlation exists between the transient signal and the pressure in the ICR cell. [197,201] In a perfect vacuum, the orbiting motion would solely be... 

In kinetic spectroscopy a continuous liglit source is used. The transient signal at a fixed wavelength is detected with a photomultiplier and displayed on a storage oscilloscope. Repeating the kinetic experiments at several wavelength positions again allows us to determine transient absorption spectra. 

N2-saturated saline (0.56 M) solutions of 20 mM Fe " at pH 4.8-6.0 were flash photolyzed at 265 nm. A broad transient absorption was observed with a peak between 700-800 nm and an intensity which was roughly proportional to the laser intensity. The transient signal decayed with tj/2 = 400 ns. It was scavenged by Fe + and Fe ions and did not appear when NjO-saturated solutions were used. Explain. 

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