Unless samples are introduced into the mass spectrometer by either gas orliquid chromatography, samples with multiple components generally give complicated mass spectra that are difficult to interpret reliably. The purity of all samples should be assessed by NMR and/or chromatography prior to submission for mass spec analysis. Impurities are especially important for ESI and FAB analyses because one component can suppress the signal for another component. Signal suppression may vary with the amount of sample used for analysis, with less sample often leading to less suppression. Finding different peaks in a spectrum when the same sample is analyzed multiple times may indicate that the sample contains multiple components.
Good sample preparation procedures are essential for good mass spec analyses. Common sources of problems include contamination by materials extracted from containers, vials, tubing etc. used to prepare or store the sample. Organic solvents can extract plasticizer from many plastic containers. EI mass spectra of common phthalate plasticizers have peaks at (ask Yaqub John for masses). Acidic solutions can extract large quantities of Na+, K+, etc. from soft glass containers. These salts can be highly detrimental for ESI and FAB analyses. Concentration of large volumes of solvent makes this problem much worse. High concentrations of salt can often be detected by a high abundance of sodium or potassium adducts in ESI and FAB spectra. In the case of FAB, these cations can replace the proton in matrix cluster ions. For example, the sodium adduct of the glycerol matrix peak at m/z 185 has an m/z of 207. In general, peaks differing in mass by 22 or 38 mu usually indicate sodium or potassium adducts, respectively. High concentrations of salt can be removed by reversed phase HPLC, or by similar solid phase extraction procedures using Sep-Paks or Zip-Tips.
Although picogram quantities of many compounds can be detected by mass spectrometry, it is useful to have at least 0.1 mg of material submitted. The quality of ESI and FAB spectra depends very much on the amount of sample introduced into the mass spectrometer. Too much or too little sample may lead to unsuccessful analyses. In the case of ESI, we request that you submit exactly 0.1 mg of material. Since sample introduction into the mass spectrometer usually requires dissolving the sample in a suitable solvent, it is essential that you indicate which common solvents will dissolve your sample.