Desalting, biological samples

Solid-phase microextraction (SPME) [63] is normally used for sample collection, pre-concentration and desalting before analysis. It is usually used off-line. However, SPME can be implemented in temporal monitoring of biomolecules with low to medium temporal resolution (minutes, hours) [64]. In one method, SPME fibers were used to sample metabolites from live animals followed by analysis using LC-MS [65]. The method enabled extraction of metabolites directly in the tissue of moving animals. It was not necessary to withdraw a representative biological sample for analysis. In this case, the amount of analyte extracted into the SPME fiber was independent of the sample volume [65]. Recently, SPME was also coupled on-line with a MS ion source operated at atmospheric pressure [66]. 

To avoid the above problems we routinely use an extraction method in which DNA fragments become bound to hyroxyapatite powder (9). This is handled in the form of a centriftiged chromatography column for the purposes of washing with various buffers and final elution of pure DNA in 0.5 M potassium phosphate. This DNA is concentrated and desalted in a centrifugal ultrafiltration device (Centricon-10 from Amicon). Many commercial kits are available for the isolation of pure DNA from biological samples therefore a detailed description of the above method seems inappropriate, however, choice of the method should take into account the above considerations. 

Direct application of samples, particularly fluids, may be possible for the TLC of sugars, that is, without sample cleanup. However, various sample cleanup procedures may be required for some biological samples, particularly whole-body extracts of plant and animal tissues. Such cleanup procedures may include extraction, desalting, deproteinization, and other techniques described in Chapter 4 and in Churms (1981). 

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