Page 370 - Tandem Techniques

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The design of a more recent form of thermospray interface, the Vestec Model 201 used by Via and
Taylor [16], is shown in Figure 9.18. This interface is designed to cope with column flow rates of up to
1.5 ml/min, and requires the use of two oil diffusion pumps backed by a single mechanical pump. The
two pumps differentially exhaust the vacuum manifold, the source, and the analyzer regions of a
quadrupole mass spectrometer. In addition, a further two diffusion pumps, backed by a single
mechanical pump, are also coupled directly to the source, opposite the sample inlet. This pump removes
about 99% of the vaporized mobile phase, whereas the heavier molecules pass through an ion aperture
in the sampling cone, and into the mass spectrometer. The ions are formed immediately after the
nebulization, and pass alongside a repeller plate, held at a high potential, that impels the ions through a
hole into the ion source. Once in the ion source, the ion optical system directs them into the analyzer
section of the spectrometer. The reagent gas is methane and its flow is controlled by separate needle
valves.

An example of the use of the thermospray ionization source for the measurement of furazolidone in
some pharmacokinetic studies is afforded by he work of McCracken et al. [17]. Furazolidone (N-(5-
nitro-3furfurylidene-3-amino)-2-oxazolodinone is a 5-nitrofuran antibiotic that is added to animal feeds
to help prevent such infections as Escherichia coli and Salmonella in cattle, pigs and poultry.
Consequently, it is important to determine the amount of furazolidone residues (if any) that might
remain in the meat after slaughter for human consumption. In Europe the maximum level that is
tolerated is 5 µg/kg of animal foodstuff.

Furazolidone is light sensitive and so operations must be carried out under artificial yellow light. Liver
and muscle tissue were examined and 2 g samples were homogenized with 40 ml of a mixture of
McLlvaine buffer and methanol (7+3) and then centrifuged for 15 minutes. The supernatant liquid was
evaporated to 15 ml at 40°C and 25 ml of dichloromethane was then added and the mixture shaken for
about 1 minute. The upper aqueous layer was discarded and the solvent extract evaporated dryness and
taken up in 2 ml of dichloromethane and 6 ml of hexane. The sample was passed through a prepared
Bond-Elut NH extraction cartridge, washed with 5 ml
2