Background ions

Positive ions are obtained from a sample by placing it in contact with the filament, which can be done by directing a gas or vapor over the hot filament but usually the sample is placed directly onto a cold filament, which is then inserted into the instrument and heated. The positive ions are accelerated from the filament by a negative electrode and then passed into a mass analyzer, where their m/z values are measured (Figure 7.1). The use of a suppressor grid in the ion source assembly reduces background ion effects to a very low level. Many types of mass analyzer could be used, but since very high resolutions are normally not needed and the masses involved are quite low, the mass analyzer can be a simple quadrupole. 

Organics produce no useful positive ions, but the ions produced by inorganic samples are remarkably free from background interference, and the resulting mass spectra are relatively simple. The ion currents derived from the positive sample ions at each m/z value, being free from background ions, represent an accurate measure of the amount of each element. 

Direct analysis with the fluoride lon-selective electrode requires addition of total ionic strength adjustor buffer solution (TISAB) to the standard and to unknown samples Some advantages of this addition are that it provides a constant background ion strength, ties up interfenng cations such as aluminum or iron, which form a complex with fluoride ions, and maintains the pH between 5 0 and 5 5 According to the manufacturer s claim, reproducibility of direct electrode measurement IS 2 0%, and the accuracy for fluonde ion measurement is 0 2% [27]... 

An additional difficulty, especially noticeable when the product ion, NO+ in this case, has a stable neutral counterpart is the presence of background ions. The vacuum pump used in this experiment, a titanium... 

Figure 2.3 TIC traces, having been brought about by using ions in the m/z ranges (a) 35 to 400, and (b) 200 to 400, showing the improvement in signal-to-noise ratio obtained by excluding background ions.

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. 

For liquid metals, one has to set up density functionals for the electrons and for the particles making up the positive background (ion cores). Since the electrons are to be treated quantum mechanically, their density functional will not be the same as that used for the ions. The simplest quantum statistical theories of electrons, such as the Thomas-Fermi and Thomas-Fermi-Dirac theories, write the electronic energy as the integral of an energy density e(n), a function of the local density n. Then, the actual density is found by minimizing e(n) + vn, where v is the potential energy. Such... 

TFC was used recently with an analytical column to obtain a desired chromatographic separation. The second column also reduced background ion suppression. 

Due to the limited dynamic range the QIT is not a particularly good analyzer for quantification. For pure samples the problem is less pronounced, but if a background is present the background ions will constitute a substantial portion of the total number of ions entering the QIT and, hence, affect the quality of the quantification. Typical scan speeds arc 5 kTh/s (several commercial systems offer the possibility of different scan speeds). Higher resolution is attained by reducing the scan speed. 

Figure 5.18. The peak shape of the background ions of m/z 28 versus resolving power R of a mass spectrometer.

Example In the partial TIC obtained by GC-EI-MS of an unknown mixture on a HP-5 column the chromatographic peak at 32.6 min is rather weak (Fig. 12.7). Scan 2045 extracted from its maximum yields a spectrum that is mainly due to background ions from column bleed as demonstrated by comparison to the average of scans 2068 2082. Finally, manual background subtraction ((2035 2052)-(2013 2020)) delivers a spectmm of reasonable quality, although some background signals are not completely erased. 

Non-volatile buffers such as phosphates, borates, perchlorates and phosphoric acid should be avoided at all costs because of high background ion current, source contamination and blockages, and in the case of perchlorates, explosions. Figure 6.4 shows the mass spectrum of typical background when using phosphoric acid in the eluent. If the solvent system for a particular analysis does not assist the electrospray process, it is possible to enhance ionisation by postcolumn addition of a suitable volatile buffer. 

---