Scan modes

The scan mode display is divided into a number of windows, that display the data recorded from the active inspections. In addition, the A-scan data from the ultrasonic probes can be displayed in probe monitor windows, for monitoring the signal quality. Figure 7 shows the scan mode display for simultaneous recording of two P-scan inspections (displayed in the same presentation window) and a T-scan inspection together with 3 probe monitor windows. 

The analyze mode display is similar to the scan mode display used online. Analyze mode includes functions for evaluation of data, e. g. markers, measure functions, zoom function and selection of cross-section views. In addition, A-scan data can be reconstructed into images and displayed. 

In a (B/E)(l - E) -scanning mode, a mass difference is seiected. For exampie, in this case a precursor ion m, is chosen (it is shown as being made up of two parts of mass mj, n,). After fragmentation, the product ion is mj accompanied by a neutral particle of mass n,. The mass difference (n, = m, - mj) can be specified so only pairs of ions connected by this difference are found. 

The mass scan mode is used to survey the entire mass spectrum within a certain volume of the specimen. 

In scanning mode the sequential detection of single pixels (picture elements) and voxels (volume elements) results in long measurement times in practice, therefore, only small volumes (10 x 10 x 1 p,m ) can be measured [3.56]. 

Before considering these fonr scan modes in detail, it is worthwhile considering the types of instrnment that have MS-MS capability because, as two stages of mass spectrometry are involved, not all systems will provide this facility. 

Figure 3.29 Structure of atrazine. Reprinted from J. Chromatogr., A, 915, Steen, R. J. C. A., Bobeldijk, I. and Brinkman, U. A. Th., Screening for transformation products of pesticides using tandem mass spectrometric scan modes , 129-137, Copyright (2001), with permission from Elsevier Science.

Data acquisition Scan mode Selected-ion-monitoring mode Erratic component quantitation Reproducible component quantitation... 

Figure 5.27 Selective detection of lactolated peptides from a tryptic digest of / -lacto-globulins by LC-electrospray-MS-MS, showing (a) the total-ion-cnrrent trace in full-scan mode, and (b) the total-ion-current trace in neutral-loss-scanning mode. Figure from Selective detection of lactolated peptides in hydrolysates by liquid chromatography/ electrospray tandem mass spectrometry , by Molle, D., Morgan, F., BouhaUab, S. and Leonil, J., in Analytical Biochemistry, Volume 259, 152-161, Copyright 1998, Elsevier Science (USA), reproduced with permission from the publisher.

As in the case of STEM, the main benefit arising from the use of the scanning mode is that the incident electron probe can be stopped or controlled in its motion and a variety of detector types and configurations can be used to obtain particular signals, giving information beyond that obtained in the normal Imaging modes. 

X-ray Diffraction. Eliffiactograms were obtained with a ShiimdzuXD-Dl instrament with monochromator using CuKa, radiation. It was operated in continuous scan mode at 0.5° (20) min. ... 

Specificity is unsurpassed. Traditionally, MS was performed on very large and expensive high-resolution sector instruments operated by experienced specialists. The introduction of low-resolution (1 amu), low-cost, bench-top mass spectrometers in the early 1980s provided analysts with a robust analytical tool with a more universal range of application. Two types of bench-top mass spectrometers have predominated the quadrupole or mass-selective detector (MSD) and the ion-trap detector (ITD). These instruments do not have to be operated by specialists and can be utilized routinely by residue analysts after limited training. The MSD is normally operated in the SIM mode to increase detection sensitivity, whereas the ITD is more suited to operate in the full-scan mode, as little or no increase in sensitivity is gained by using SIM. Both MSDs and ITDs are widely used in many laboratories for pesticide residue analyses, and the preferred choice of instrument can only be made after assessment of the performance for a particular application. 

The full-scan mode is needed to achieve completely the full potential of fast GC/MS. Software programs, such as the automated mass deconvolution and identification system (AMDIS), have been developed to utilize the orthogonal nature of GC and MS separations to provide automatically chromatographic peaks with background-subtracted mass spectra despite an incomplete separation of a complex mixture. Such programs in combination with fast MS data acquisition rates have led to very fast GC/MS analyses. 

The instrument scan mode called selected reaction monitoring (SRM) is generally used for quantitative applications. SRM is similar to selected ion monitoring (SIM) in single quadrupole MS. The difference is that a product ion from the decomposition reaction in the collision cell is measured instead of a single ion formed in the... 

The MS detector is a particularly suitable tool for confirming the identity of an analyte which has been detected with ECD or NPD. Confirmation of identity should be performed particularly in those cases in which the MRL appears to have been exceeded or in which a compound seems to be present which is not expected in the sample being analyzed. In this case, the scan mode is used in order to identify the compound by means of its mass spectrum. 

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