Miscellaneous Operating Techniques

It is often forgotten that sequential injections onto a column can be overlapped in such a way as to utilise the window before the fastest running peak starts to elute from the column to collect fractions from the previous injection. In this way the column is used at its maximum capacity and solvent usage is minimised. 

The use of the so-called flip-flop technique [13] in which the direction of flow in the column is reversed and the sample is injected alternately at either end of the column has been demonstrated to provide higher throughputs with reduced solvent consumption when used with elution chromatography. This approach requires the use of a column such as the Dynamic Axial Compression system manufactured by Prochrom, Champigneulles, France, where the stability of the packed bed is unaffected by the direction of flow. 

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Paper spray methodology requires no modification of the mass spectrometer. The technique is carried out most effectively on MS/MS instruments (a triple quadrupole was used in these papers) as MS spectra tend to include many miscellaneous co-extracted compounds. CID with detection of the product ion spectra or operation in SRM mode enables both identification and quantification of the analytes. Paper spray is most effective in positive ion mode, although it is possible to use negative mode if the ESI voltage and solvent composition are altered to prevent a discharge. 

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