Mass spectrum, ‘quadrupole like

Molecular ion mass interferences are not as prevalent for the simpler matrices, as is clear from the mass spectrum obtained for the Pechiney 11630 A1 standard sample by electron-gas SNMSd (Figure 4). For metals like high-purity Al, the use of the quadrupole mass spectrometer can be quite satisfiictory. The dopant elements are present in this standard at the level of several tens of ppm and are quite evident in the mass spectrum. While the detection limit on the order of one ppm is comparable to that obtained from optical techniques, the elemental coverage by SNMS is much more comprehensive. 

We discussed the fundamentals of mass spectrometry in Chapter 10 and infrared spectrometry in Chapter 8. The quadrupole mass spectrometer and the Fourier transform infrared spectrometer have been adapted to and used with GC equipment as detectors with great success. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-infrared spectrometry (GC-IR) are very powerful tools for qualitative analysis in GC because not only do they give retention time information, but, due to their inherent speed, they are also able to measure and record the mass spectrum or infrared (IR) spectrum of the individual sample components as they elute from the GC column. It is like taking a photograph of each component as it elutes. See Figure 12.14. Coupled with the computer banks of mass and IR spectra, a component s identity is an easy chore for such a detector. It seems the only real... 

TOP analyzers, like quadrupoles, scan the mass spectrum rapidly. Resolutions of 500 can be obtained. These analyzers are popular for high mass ion detection since they have no real upper mass Limit. 

Product-ion scan The first quadrupole is set to transmit a selected precursor ion falling within a mass window of usually 1 Da. These ions enter the second quadrupole and are fragmented by collision induced dissociation. The fragment ions are mass resolved by the third quadrupole operated in the full-scan mode. The result is an electron impact-like mass spectrum obtained indirectly from a molecular-adduct ion or other characteristic ion formed in the ion source by a soft ionization method. Also known as a daughter ion scan... 

Analysis was performed by desorbing the organics from the traps with a Curie-point pyrolyzer unit in series with a quadrupole mass spectrometer. The data produced were similar to Py-MS data, although, quite likely, thermal desorption was taking place rather than pyrolysis. The typical mass spectrum obtained from the contaminated areas was dominated by the major ions of PCE. Table 7.5 shows the various compounds that were identified in spectra obtained from the 25 samples spaced around the contaminated area. Table 7.6 shows the compounds identified by static trapping from a particular location and by purge-and-trap GC/MS analysis of water from an adjacent well. 

An alternative to the 3D quadrupole ion trap (Paul trap) is the linear quadrupole ion trap. The linear ion trap is akin to a hybrid of the quadrupole mass filter and the 3D ion trap in that it consists of a four-rod assembly, like the quadrupole filter, but also it has entrance and end electrodes like the 3D ion trap. Confinement of ions along the axial direction is provided by DC potentials applied to the end electrodes. The quadrupole rods produce radial motion of the ions through application of an RF electric field, in a similar manner to that already described for the quadrupole mass filter. To record a mass spectrum axial ion ejection, initiated by RF excitation, can be used in a procedure similar to that used for the 3D ion trap. 

There are many types of mass spectrometer, from high-resolution double-focusing instruments, which can distinguish molecular and fragment masses to six decimal places, to bench-top machines with a quadrupole mass detector which can resolve masses up to about m/z = 500, but only in whole-number differences. Routinely you are most likely to encounter data from bench-top instruments and therefore only this typie of spectrum will be considered. 

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