Ion intensity, background

Figure 3. Raw experimental data of background ion intensity before...

Hastie [131] coupled for the first time a quadrupole mass spectrometer with a Knudsen cell. One of the quadrupole mass spectrometer - Knudsen cell systems used at our laboratory is shown in Fig. 4. The system has been developed to study small alkali metal clusters under equilibrium conditions (see Sect. 3.2). Broad-band photoionization by a 1 kW Hg/Xe lamp is used for the first time in Knudsen effusion mass spectrometry to reduce fragmentation. Other quadrupole mass spectrometer - Knudsen cell systems have for example been developed by Hilpert [132, 133], Fraser and Rammensee [134], Plante [135], Ono et al. [136], Kematick et al. [137], and Edwards et al. [138]. Cryogenic pumping is used in the device by Hilpert to reduce mercury background ion intensities for the study of amalgams [132], The instruments described in Refs. 134,135 use a chopper to modulate the molecular beam from the Knudsen cell. Interfering background ion intensities can, thereby, be subtracted. The apparatus developed by the authors of Refs. 137, 138 renders possible the simultaneous application of Knudsen effusion mass spectrometry and the mass-... 

Another ion profile often encountered from background ions is one in which the intensity increases or falls at a regular rate throughout the analysis. This often occurs in LC-MS during gradient elution when the ion is associated with only one component of the mobile phase. 

Figure 9.29 Ion intensities of transient 64Zn+ signals in one selected protein spot in 2D gel of an S layer compared to the background signal of gel blank.

An optical consideration specific to the use of the TOF-MS with high-intensity sources is the removal of background ions, plasma gas ions, and matrix ions to prevent detector saturation. To date, this has been accomplished with parallel-plate deflection in the flight path, which is depicted in Fig. 12.6. Removal of specific m/z ions is accomplished by the application of a time-dependent potential to one, or both, of the plates at some time delayed with respect to the repeller pulse. In this way, those ions between the plates at the time of the pulse experience a field transverse to the flight axis and are removed from the flight path. The voltage pulses employed here must have fast rise and fall times (<10-20 nsec), and be applied at precise delay intervals to ensure that mass resolution is not compromised, and to allow for the unimpeded passage of the previous and subsequent masses. Also, the... 

Fig 9.29 illustrates Zn determination in a Zn binding protein in a selected protein spot after separation of the proteins of an S layer used as biomolecular template, as described above, by 2D gel measured by LA-ICP-SFMS at medium mass resolution (m/Am = 4000). ° The Zn ion intensity in the protein spot (compared with the background signal in the gel blank) was measured at three different positions in the selected spot. Using LA-ICP-SFMS, the highest Zn intensity was found in the middle of the protein spot investigated. Additional measurements by MALDI-MS on Zn containing protein spot in 2D gel can be useful to identify the structure of the protein. ... 

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