Signal acquisition mode

Data-dependent acquisition ability has been developed and incorporated into most software packages [MetaboLynx, Xcalibur, and Analyst Information Dependent Acquisition (IDA)]. In data-dependent acquisition mode, a mass spectrometer decides on the fly whether to collect MS/MS or MSn data, remain in full scan MS mode, or conduct other survey scans based upon user-defined criteria. Product ion spectra of potential metabolites can be automatically acquired in a single LC/ MS run. However, false positives may be generated due to highly intense matrix ion signals that may inadvertently trigger MS/MS or MSn scan functions. 

Fig. 12. Sequences for volume selective single voxel spectroscopy. Both techniques work with three slice-selective RF-pulses. (a) The Point RESolved Spectroscopy (PRESS) sequence generates a volume selective double spin-echo. The entire time delay between the initial 90° excitation and the echo is sensitive to transverse relaxation, (b) The Stimulated Echo Acquisition Mode (STEAM) sequence generates a stimulated echo. Maximal signal intensity (without relaxation effects) is only half the signal intensity of PRESS under comparable conditions, but slice profiles are often better (only 90° pulses instead of 180° pulses) and the TM interval is not susceptible to transverse relaxation, (c) The recorded echo signal is only generated in a volume corresponding to the intersection of all three slices.

Fluorescence from pharmaceutical capsule shells and tablet coatings has hindered measurement of their composition by Raman spectroscopy. By switching from the conventional backscattering mode to a transmission mode, Matousek et al. demonstrated that fluorescence could be eliminated in many instances [8]. Backscattering- and transmission-mode Raman spectra of several samples are shown in Figure 7.5. Each spectrum was acquired in 10s with 80mW 830-mn laser power. Matousek et al. also speculate that signal acquisition times could be relatively easily shortened to well below 0.1 s when the transmission mode is combined with optimized optics [8]. 

The popular spin-echo imaging scheme (Fig. 6.2.1(e)) requires execution of a 180° pulse for formation of the Hahn echo. This sequence provides the maximum signal without phase distortions for image construction. However, a 180° pulse also requires considerable rf power, in particular, when it is applied to large diameter coils. As an alternative to Hahn echoes, stimulated echoes can be used for imaging [Burl, Finl, Fral]. They are excited by three instead of two rf pulses (cf. Section 2.2.1). Imaging schemes based on stimulated echoes are also referred to as STEAM (stimulated-echo acquisition mode) images [Fral]. 

Snively et al. used a PA-IR spectrograph equipped with a 64 x 64 MCT rollingmode FPA to push the time resolution of PA-IR spectroscopy to less than 100 ts per spectrum [27]. The small size and the acquisition mode of this FPA, both contributed to enable such time resolution. First, a smaller FPA allows faster frame rates than a larger one (3.2 ms per image in this specific case), at the expense of bandwidth and/or resolution. Second, the 100% duty cycle of a rolling-mode FPA was put to profit. In a snapshot-mode FPA, all pixels measure the signal intensity simultaneously for -100 ts, but the electronic read-out time can be as... 

A solution to this problem is to subtract a background signal immediately after each fluorescence signal acquisition and repeat until all signals have been acquired. This mode of scanning is not available on the... 

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