Sample preparation in mass spectrometry

The mass spectrometry sample preparation is used to optimize a sample for analysis in a mass spectrometer. These have, depending on the ionization method, different requirements of volume, concentration, and composition of the analyte solution.

MALDI

Conventionally, the sample is mixed with a matrix solution and applied to the target for MALDI mass spectrometry. Here, the matrix with the sample crystallized and composed of the analyte molecules are converted by the pulsed laser radiation in the gas phase. The salt content of the sample is not as critical as in the case of the electrospray ionization ( ESI = ), however, it may be noise due to side reactions with the matrix, for example, Alkali metal ions are affecting the readability of the spectra. A separate desalting step is still often not necessary, since by appropriate selection of buffer salts in the preceding steps of sample preparation, for example, can be in-gel digestion of proteins for protein identification by peptide mass fingerprinting, the use of alkali metal salts is avoided. Furthermore, the washing of the matrix / analyte mixture crystallized on the target or the addition of ammonium phosphate to the matrix / analyte solution to improve the signal quality.

ESI

The electrospray ionization places higher demands on the properties of the sample due to the Ionisationsprinzips. Thus, the volume of the needle which in the off-line measurements prevents ( that is, the sample is not via the coupling to a liquid chromatography mass spectrometer to be introduced ) is used, larger sample volumes. In comparison to the higher limit of detection of MALDI therefore normally requires a concentration of the sample. Evaporation of the solvent during the spray makes high demands on the salt freedom of the sample solution, otherwise the salts with the continuous reduction of the volume in spray or needle begin to crystallize and thus make a successful measurement impossible. An important application of the ESI -MS device is the area of ​​proteomics. Here, the mass spectrometer for the identification and sizing of proteins is used. The identification of the protein samples can be done at ESI measurements de novo peptide sequencing and peptide mass fingerprinting. Both methods require a previous digestion of the protein into peptides, which mostly takes place enzymatically using proteases. Both the digestion in solution and in the in-gel digestion buffered solutions are needed, whose salt content and volume of the analyte is too high and too low for the ESI measurement. Therefore, a combined desalting and concentration step is carried out before the measurement. Usually this is a reverse phase liquid chromatography (RP- LC ) are used, wherein the peptides bound to the chromatography matrix, the salts, however, may be removed by washing. The elution of the bound peptides may be carried out with a small volume of organic solvent. In the LC / MS coupling desalination / concentration is usually implemented with a guard column, for off-line measurements RP- micro columns are used, which can be plugged directly onto microliter pipettes. Elution is carried out in this case with the spray solution contains a sufficiently high proportion of organic solvents. The spray solution provided with the enriched analyte can then directly into the spray capillary and thus can be transferred into the mass spectrometer.