Spectral acquisition mode

Figure 1.9 Optical schematic of the Bruker Optics ATR objective used in (a) alignment mode and (b) spectral acquisition mode. Illustration courtesy of Bruker Optics.

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. 

The last equation tells us what value of the dwell time we have to use to establish a particular spectral width. In practice, the user enters a value for SW and the computer calculates DW and sets up the ADC to digitize at that rate. It is important to understand that with the simultaneous (Varian-type) acquisition mode, there is a wait of 2 x DW between acquisition of successive pairs of data points. The average time to acquire a data point (DW) is the total time to acquire a data set divided by the number of data points acquired whether they are acquired simultaneously or alternately. The spectral window is fixed once the sampling rate and the reference frequency have been set up. The spectral window must not be confused with the display window, which is simply an expansion of the acquired spectrum displayed on the computer screen or printed on a paper spectrum (Fig. 3.15, bottom). The display window can be changed at will but the spectral window is fixed once the acquisition is started. 

Time-related variables are critical in GD work. One of the key features of any analytical method used in a production facility is the sample analysis time. Because GD is a kinetic-based system, the evolution of the plasma processes to, hopefully, an equilibrium situation, is of special concern. It is sufficient to say that any analysis (spectral acquisition) performed in a sequential mode requires that the plasma reach a steady state before analyses can be started — which is obviously arguable for simultaneous detection. Thus, for most analyses, the plasma stabilization time can be a substantial fraction of the overall sample analysis time. 

The photodiode-array detector is a powerful analytical instrument that has provided enhanced detection capabilities with the addition of detailed spectral information via its multisignal detection technology. Its applications are HPLC based and can be found in basic research, automated analysis, pharmaceutical product development, and the clinical laboratory environment. Through spectral acquisition and analysis, a wealth of information can be obtained about the identity and purity of a compound. Combined with high selectivity and sensitivity, this mode of de-... 

Water resistant, front-surface aluminum mirrors can be used as deposition substrates, followed by spectral acquisition in the R-A mode. The smooth and hard surface properties of mirrors complicate efficient analyte deposition (spreading of the analytes) when the eluent is not completely evaporated. The spectral data recorded from these substrates should be in close resemblance to the spectra obtained from transmission measurements, because the band intensities are controlled by a double-pass transmittance mechanism. However, still spectral differences between R-A and KBr disk spectra can be observed, including absorption-band shifts and asymmetries. Furthermore, the effect of light scattering (Christiansen effect) may become apparent when the spot thickness exceeds a certain level and anomalous relative band intensities may be observed in R-A spectra of certain analytes deposited on flat substrates when compared to transmission spectra acquired from KBr disks. In order to minimize these... 

Triple quadrupole instruments (Section 6.4.3) have not previously been considered suitable for use with MALDl sources in view of their nature as scanning analyzers (serial recording of a mass spectrum resulting in very low duty cycle in full spectral acquisitions) with a limited miz range. However, for the present purpose of small molecule quantitation by MALDl, neither of these limitations applies. In particular, if the so-called multiple reaction monitoring (MRM) mode of tandem mass spectrometry is... 

While this level of mass measurement accuracy is rarely necessary in quantitative analyses, the fast spectral acquisition rates of TOFs have been used to advantage in methods employing extremely fast GC separations. Standalone TOF analyzers cannot readily mass select ions, thus precluding the use of SIM mode. As such, quantitative assays performed on TOFs must currently make use of full scan MS mode. Moreover, TOFs have a low duty cycle when operated with continuous ionization sources since the slowest (highest m/z) ions in one ion packet accelerated into the flight tube must be allowed to reach the detector before the next packet is selected. At the other extreme, TOF analyzers have an essentially 100% duty cycle when directly interfaced to pulsed ionization sources like MALDI if the ionization pulse is synchronized with the TOF acceleration. 

Table 5.29 shows the main characteristics of EPMA. The EDS mode provides several inherent advantages relative to WDS (i) simultaneous spectral acquisition (minimising beam damage) (ii) allowance for quantitative microanalysis of rough surfaces or particles and (Hi) spectrum imaging [250]. It is waste of time to proceed with quantitative microanalysis from a XEDS spectrum without first carrying out qualitative analysis. This requires that every peak in a spectrum be identified unambiguously and with statistical certainty. 

The instrument response depends on several factors, such as optical imperfections, image motion, the nraizero spatial area of each detector, as well as the filtering produced by electronic elements. Except for the image motion, all the elements influencing the instrument response can be attributed to the acquisition mode. In particular the type of scanning system and the spectral selection mode can have a strong impact on the precision of the measurements. 

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