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Scan Frequency for Data Acquisition

The chromatographic trace resulting from a succession of mass spectra, as seen previously, brings up a question. During the programming of an acquisition mode, at what frequency is it wise to record mass spectra One scan every 5 seconds Ten scans per second The answer is less simple than it seems at first glance. To answer this question, we must first define the scan and microscan parameters in GC-MS programs. [Pg.80]

The scan is the time interval between two consecutive mass spectra in a chromatogram. The acquisition frequency (or sampling rate) is generally expressed as scans per second. The microscan is the effective duration of the scan of an analyzer. Its duration depends on the range of m/z ratios to be scanned set by the user for the acquisition method. If, for example, a quadrupole scans at a speed of 6000 Th/ second and the acquisition method scans m/z ratios in a range from 50 to 249 Th (200 Th), the duration of a microscan will be 200/6000 = 1/30 of a second. [Pg.80]

The microscan is the shortest duration necessary for recording a mass spectrum. The fastest spectrum acquisition frequency corresponds to 1 scan = 1 microscan. By definition, the duration of a scan cannot be inferior to that of the microscan. Therefore, if one programs, for example, a scan of 0.2 second (or a spectrum every 0.2 second) with an ion trap presenting a microscan duration of 0.3 second on the specified scanning range (50 to 500 Th, for example), the software will send an error message because the demanded frequency is superior to that authorized by the performance of the device. [Pg.80]

FIGURE 5.3 Chromatogram extract plotted with frequencies of four scans/second (left) and one scan/second (right). [Pg.81]

One must also take into consideration that the TIC associated with the mass spectrum under these conditions will be approximately four times superior to that from 1 scan = 1 microscan and that the resulting chromatographic response will be more intense. This last point is obviously very important for trace analysis. [Pg.81]


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