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become more important in environmental analysis, pharmaceutical analysis and in the many facets of
biotechnology, the need to understand and clearly define the measurement becomes extremely urgent.
In general, the units that are most important to the majority of scientists and, in particular, analysts, are
the units of mass and concentration (mass/unit volume). If the mass and concentration sensitivity are
known, the values can be converted to any other units that may be of interest. If the sensitivities of all
sensing and measuring devices are defined in terms of mass and concentration, then they can all be
compared on a rational basis.

Concentration Enhancement Techniques

One of the problems that arise when associating an electrophoretic separation technique with the mass
spectrometer, or for that matter, any other spectroscopic instrument, is the low masses and
concentrations of solute that are involved. Consequently, a number of concentrating techniques has
been developed to circumvent this difficulty. One of the early methods, developed by Cai and Rassi [6],
was to use a capillary coated with an octadecyl bonded phase, to concentrate the sample by dispersive
interactions (hydrophobic) onto the surface of the tube. The device was, in fact, the micro-form of a
solid state extraction cartridge. Two fused silica tubes were used, joined by a Teflon sleeve. The first
contained the dispersive coating of octadecyl chains, and the second was used for the electrophoretic
separation. In one example, the concentration tube was 20 cm long and 50 µm I.D., and the
electrophoresis tube was 60 cm long (30 cm to the point of detection) and 50 µm I.D. The inner surface
of the pre-concentration tube was roughened by etching with a 5% solution of ammonium hydrogen
bifluoride, at 250-300°C, under sealed conditions. The roughened tube was then filled with a 20% w/v
solution of dimethyloctadecyl chlorsilane and heated at 110°C for an hour. The silanizing process was
repeated twice. The sample was accumulated on the walls of the tube from an aqueous electrolyte
solution, and was subsequently displaced into the electrophoresis tube, with an electrolyte solution
containing acetonitrile. This process is similar to the recovery of sample from a solid state extraction
cartridge. The recovery was claimed to improve the lower detection limits by a factor of 10 to 35.