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need to be carefully monitored. The antibiotics of interest included, oxacillin, cloxacillin and
dicloxicillin Blanchflower et al. [25] developed a procedure for simultaneously monitoring five
penicillins in muscle and kidney tissue and milk. The authors employed an LC/MS tandem instrument
fitted with an electrospray interface, and utilized single ion monitoring to selectively locate and measure
each penicillin. Most biological samples require complex sample preparation and the measurement of
antibiotics in animal tissue and animal products is no exception. Tissue sample were pulverized, spiked
with the standard and homogenized. Acetonitrile was added, sonically mixed and then centrifuged. A
portion of the supernatant liquid was treated with phosphoric acid, mixed with dichloromethane and
again centrifuged. Acetonitrile and n-hexane were added, the mixture shaken and centrifuged. The
lower layer was washed with water. The solvent mixture was then extracted with phosphate buffer,
centrifuged and the lower layer treated with tetrabutylammonium hydrogen sulfate. The solution was
then extracted with dichloromethane, the extract evaporated to dryness and dissolved an acetonitrile
water mixture. The liquid chromatograph was a Merck-Hitachi Model L6000 pump, a Rheodyne 7125
injector and an Intersil ODS-2 reversed phase column 15 cm long and 4.6 mm I.D. The outlet from the
column was coupled to a Megabore probe of a VG Platform ES-MS which was operated in the negative
ion mode. The source was maintained at 120°C and the flow rates of the drying and nebulizing gas was
10 1/hr. The extraction and focus voltages were about 17 and 24 V respectively

It was found that the fragmentation pattern could be significantly changed by adjusting the voltage on
the extraction cone. As the voltage was increased the degree of fragmentation increased. This effect is
shown in Figure 9.29. It is seen that 5 V on the extraction cone produced just two ions above m/z of
160, i.e. m/z = 434 and 436. Increasing the potential to 10 V produced another peak at m/z 293. At a
potential of 20 V the peak at m/z of 293 has markedly increased and has been joined by a significant
peak at m/z=295. At the same time the original major peak at m/z=434 had shrunk to a minor peak and
the peak at m/z=436 was barely visible. It is seen that the operating conditions of the interface can be
critical in