Time step-scan

A further extension of the DFG S19 method was achieved when polar analytes and those unsuitable for GC were determined by LC/MS or more preferably by liquid chromatography/tandem mass spectrometry (LC/MS/MS). Triple-quadrupole MS/MS and ion trap MS" have become more affordable and acceptable in the recent past. These techniques provide multiple analyte methods by employing modes such as time segments, scan events or multiple injections. By improving the selectivity and sensitivity of detection after HPLC separation, the DFG S19 extraction and cleanup scheme can be applied to polar or high molecular weight analytes, and cleanup steps such as Si02 fractionation or even GPC become unnecessary. 

In conventional nanosecond pump-probe dispersive TRIR experiments, also described previously, kinetic data are collected at one frequency at a time. These data can then be used to construct a series of time-resolved IR spectra. Thus, in the dispersive experiment kinetic data are used to construct spectra, and in the step-scan experiment spectral data are used to derive kinetics. 

X-ray diffraction was done using a Siemens D-500 diffractometer utilizing CuKa radiation (1.406 A). The data were collected as step scans, with a step size of 0.05° 29 and a count time of 2 s/step between 10° and 80° 29. 

Vasenkov, S. and Frei, H. (2000). Time-resolved study of acetyl radical in zeolite NaY by step-scan FT-IR spectroscopy. J. Phys. Chem. A 104, 4327 4332... 

In situ infrared spectra were obtained using the set-ups described in the experimental section. The step potential sequence was applied to obtain clean surface. The electrode was pushed onto the window after the potential was held at 1050 mV. The potential was stepped to 100 mV and the spectra were recorded repeatedly until no change was seen. The potential was increased with steps of 20 mV each and held 3 min. An infrared spectrum was recorded at each potential in the final 2 min. of the holding time. 400 scans were accumulated each time, leading to a 4 cm l resolution. 

For chemically stable systems for which time resolution is not needed, Clausen etal. (52) obtained excellent spectral quality by combining normal EXAFS with XRD simply by recording the two data sets sequentially for the catalyst sample in the same setup. In this approach (Fig. 9) a diffractometer was mounted between the first and second ionization chambers in a standard EXAFS spectrometer setup, and the EXAFS was then recorded by step-scanning the monochromator through the energy region of interest (33). The XRD pattern can be acquired at the most convenient wavelength,... 

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

While an intensity profile at the detector as a function of retardation may be acquired in a step-scan mode, two major drawbacks affect this method of interferogram acquisition. First, the mirror(s) requires stabilization times with mirror inertia and time constants of the control loop determining this parameter in achieving a given optical retardation. Second, additional hardware and control mechanisms need to be incorporated into the spectrometer, thus increasing instrument cost and complexity. In certain cases, however, the utility of a step-scan instrument justifies this additional expense. Historically, the step-scan approach was favored with slow detectors. With the advent of fast detectors and electronics, step-scan interferometry became... 

The sensitivity and detection limits of an analytical technique are determined by the SNR of the measurement, an important metric for assessing both the instrumental performance and analytic limits of the spectral measurement. Following typical analytical practices, 3 and 10 times the noise have been suggested as limits of detection and of quantification for IR spectroscopy, respectively. The performance of interferometers in the continuous-scan mode, which is simpler compared with that of the step-scan mode, has been analyzed well. The SNR of a spectrum measured using a Michelson interferometer is given by12... 

P 26] Time-resolved FTIR spectroscopy was performed by operation of an infrared spectrometer in the rapid scan acquisition mode (see Figure 1.59) [110]. The effective time span between subsequent spectra was 65 ms. Further gains in time resolution can be achieved when setting the spectral resolution lower (here 8 cm4) or by using the step-scan instead of rapid-scan mode. 

In the substrate standard method the absorption effect is determined using the transmittance ratio (T =1. /I ), The determination of the transmittance involves additional measurements, i.e., step scanning over the silver profile, to obtain I. and I , IA is measured for each filter sample and if can be obtained several ways as discussed below, gThese additional measurements can be performed very quickly compared to the more lengthy measurements of the internal standard because the silver peak is quite intense, A step scan of the silver diffraction profile plus background counting time can be accomplished in about two minutes with better than 1% precision,... 

---