A Few Ions to Avoid or at least recognize

Despite best efforts, ions are encountered that do not appear to have any relationship to the analyte. Such ions are usually due to contaminants. While extensive lists of common contaminants and their respective interfering ions are available, e.g., on instrument manufacturers websites, some merit discussion. 

The phthalates impart flexibility to plastics, particularly to Tygon tubing. A common member of this family is dioctylphthalate, usually di-(ethyl-hexyl)phthalate, the 

El spectrum of which is dominated by m/z 149 (base peak) and has a molecular ion at miz 390 of low intensity ( 1%). In ESI, [M + H]+ at miz 391 and [M + Na] at miz 413 are observed. Various dioctylphthalate spectra, obtained using different ionization methods, are shown in Eigure 2.8. A benefit of the ubiquitous presence of dioctylphthalate is that the miz 391 or miz 413 ion can be used as a lock mass for accurate mass measurements in ESI-TOE instruments in flow injection analysis. 

In ESI spectra the lack of a C ion indicates the presence of inorganic salts. The spectra of these compounds often show peaks that repeat np to several hundred m z, resulting from the formation of clusters where the ions are of the form [salt + charge]+ -. Examples include sodium trifluoroacetate with a repeat unit of 136 (CFjCOONa) that gives ions corresponding to [(CFjCOONa) + Na]+ in the positive ion mode and [(CFjCOONa) + CF3COO] in the negative ion mode. In the case of sodium chloride the repeat is 58 (NaCl), and in this case there are the obvious isotope patterns attributable to the different numbers of chlorine atoms. 

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