Sequential acquisition

Multidimensional or hyphenated instmments employ two or more analytical instmmental techniques, either sequentially, or in parallel. Hence, one can have multidimensional separations, eg, hplc/gc, identifications, ms/ms, or separations/identifications, such as gc/ms (see CHROMATOGRAPHY Mass spectrometry). The purpose of interfacing two or more analytical instmments is to increase the analytical information while reducing data acquisition time. For example, in tandem-mass spectrometry (ms/ms) (17,18), the first mass spectrometer appHes soft ionization to separate the mixture of choice into molecular ions the second mass spectrometer obtains the mass spectmm of each ion. 

The most popular, and also a very accurate, experimental method for measuring nonselective spin-lattice relaxation-rates is the inversion recovery (180°-r-90°-AT-PD)NT pulse sequence. Here, t is the variable parameter, the little t between pulses, AT is the acquisition time, PD is the pulse delay, set such that AT-I- PD s 5 x T, and NT is the total number of transients required for an acceptable signal-to-noise ratio. Sequential application of a series of two-pulse sequences, each using a different pulsespacing, t, gives a series of partially relaxed spectra. Values of Rj can... 

Fig. 5.4.6 Distribution of the liquid phase in particles (C) and the lower part comprising the bed comprised of 2-3-mm catalyst beads in inert beads (I) are labeled on the right-hand the course of AMS hydrogenation. Acquisition side of the figure. H2 temperature was 85 °C. of each image took 34 s sequential numbers Flow rates of H2 and AMS are given in Table of images shown are indicated in the figure. 5.4.1.

Continuous Wave (CW) Technology used initially in the acquisition of NMR data. The radiofrequency or the magnetic field was swept and nuclei of different chemical shift were brought to resonance sequentially. 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

Fig. 21. Schematic illustration of MP-HNCA-TROSY antiphase (a) and in-phase (b) spectra with long acquisition time in q. The corresponding subspectra are shown after addition of the antiphase and in-phase data sets (c) and after subtraction of the antiphase and in-phase data sets (d). Due to very small Vcc > the intraresidual cross peaks are almost entirely cancelled out from the antiphase spectrum (a). In the subspectra, the intraresidual cross peaks are shown as doublets, separated by 53 Hz splitting in Fi-dimension, whereas sequential cross peaks are shown as singlets, and they exhibit 53 Hz offset for the upheld and downfield components between the subspectra.

Acquisition times commonly vary from seconds to minutes, often with negligible time between acquisitions, even when measuring multiple locations simultaneously (multiplexing). The dedication of different areas on the charge coupled device (CCD) detector to each measurement point makes this possible. The detectors used for MIR and NIR instruments cannot be multiplexed in the same fashion and must measure multiple samples sequentially. 

Aliasing is a term for choosing slower digitization than required by the Nyquist-theorem to cover the full spectral window, while sign discrimination is accomplished by simultaneous acquisition of real and imaginary phase components (called also complex acquisition) [10, 12]. Sequential acquisition for sign discrimination (either TPPI, or the Redfield-Kunz... 

It is also possible to use an internal standard to correct for sample transport effects, instrumental drift and short-term noise, if a simultaneous multi-element detector is used. Simultaneous detection is necessary because the analyte and internal standard signals must be in-phase for effective correction. If a sequential instrument is used there will be a time lag between acquisition of the analyte signal and the internal standard signal, during which time short-term fluctuations in the signals will render the correction inaccurate, and could even lead to a degradation in precision. The element used as the internal standard should have similar chemical behaviour as the analyte of interest and the emission line should have similar excitation energy and should be the same species, i.e. ion or atom line, as the analyte emission line. 

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