Background mass spectrum

As previously mentioned, the passage of a continuous flow of mobile phase into the mass spectrometer may give rise to a significant amount of background, as can be seen from the TIC trace shown in Figure 3.13(b). A background mass spectrum, obtained after ca. 0.8 min, is shown in Figure 3.14. 

Figure 3.14 Background mass spectrum obtained from the LC-MS analysis of a pesticide mixture. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Penning ionization is a dominant reaction when nitrogen or neon is used in the DART source. Nitrogen or neon ions are effectively removed by electrostatic lenses and are not observed in the background mass spectrum. When helium is used, the dominant positive-ion formation mechanism involves the formation of ionized water clusters followed by proton transfer reactions. Negative-ion formation occurs by production of electrons by Penning ionization or by surface Penning ionization ... 

A better way of obtaining a background mass spectrum is to employ a catalytic converter, which can scrub the analyte gas clean of VOCs while leaving the water level unchanged [1]. This can be fitted to the inlet system as a bypass unit. Since some background signals... 

However, the two levels may become obvious if the instrument operator tries, for example, to conduct a library search while the computer is trying to acquire input from another mass spectrum the library search has to wait. Acquiring the data is a foreground task. Other functions such as library searching are background tasks. 

Mass spectrum obtained from the NIST Hasteloy Ni-basad standard alloy, using electron-gas SNMSd (Laybold INA-3). The sputtering energy was 1250 V, increasing the sputtered atom flux at the expense of depth resolution. Matrix ion currents ware about 10 cps, yielding background limHed detection at about 2 ppm. 

Figure 4 Mass spectrum obtained from the Aiuminium Peehiney standard Ai 11630, using eiectron-gas SNMSd with a sputtering energy of 1250 V. The AI matrix ion currerrt was significantly greater than 10 cps. yielding a background courrt rate limit less than 1 ppm.

The optimal analytical GDMS instrument for bulk trace element analysis is the one providing the largest analytical signal with the lowest background signal, the fewest problems with isobaric interferences in the mass spectrum (e.g., the interference of with Fe ), and the least contamination from instrument com-... 

Fig. 8. Mass spectrum, with background subtracted, of pbo-toionized (Cfto) Rbv clusters containing both singly and doubly ionized species tbe solid line connects peaks belonging to groups of singly ionized clusters with a fixed value of n. Note tbe dominant peaks corresponding to (C (,Rb, ) Rb and (QoRb,.,) Rb2 (marked...

Because the vacuum in the mass spectrometer and the cleanliness of the ion source, transfer line, GC column, and so forth are not perfect, a mass spectrum will typically have several peaks that are due to background. All GC/MS spectra, if scanned to low enough mass values, will have peaks associated with air, water, and the carrier gas. Other ions that are observed in GC/MS are associated with column bleed and column contamination. 

What effect will the background have on the analysis While the background, in isolation, may look to be significant, its real importance can only be determined by examination of the mass spectrum of a compound as it elutes from the HPLC column. The mass spectrum recorded at the TIC maximum after 4.65 min is shown in Figure 3.16. 

Background may also occur when two components are not completely resolved and a significant contribution from the mass spectrum of the early eluting analyte is found in the mass spectrum of that eluting subsequently. For interpretation purposes, it is equally important that this type of background is removed. The spectrum recorded at the TIC maximum after 5.05 min is shown in Figure 3.18. 

While m/z 229 and 251 were observed in the spectrum of the previously eluting component (see Figure 3.16), it is necessary to confirm that their presence in this spectrum is solely from that source and not from another component whose mass spectrum also contains these ions. If this can be done, the spectrum of the second component can be obtained by background subtraction. 

Further experimental work involving cone voltage studies may provide further confirmatory evidence but the most likely explanation is that the mass spectrum of the component with retention time 4.65 min is that shown in Figure 3.17, while the mass spectrum of the second component is that obtained by background subtraction, and is shown in Figure 3.22. 

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