I need a method for quantification of trimethoprim residues in animal tissues. My problem is sample preparation. I need LOQ < 50 ppb, but it is very difficult to obtain with TMP's maximum of absorbance - below 210 nm. I can't get chromatogram that is clean enough and still have a good recovery. Could anyone give me a hint please?

There is a publication by my colleague Y.-F. Cheng in J. Chromatogr. A 828 (1999) p 273 that discusses some of the basic principles. The idea is to use both pH and solvent composition to get a clean extract. If you give me your e-mail, I can get you more details directly.

Suggestions, in order of complexity;

(1) Try monitoring at the lambda max of
trimethoprim- it has 2 phenyl groups so 210 is not
the uv max for this compound. You ought to get a
better S/N ratio at the uv max i.e.greater
sensitivity.
(2) You can also increase the sensitivity be going
to narrower column diameters
(3) Failing this you may need to go to more
sensitive detectors – A RP column with 0.25%
TEA/0.1% formic acid (pH 5.8) mobile phase will
allow you to use LC/MS
(4) Kunkel (Electrophoresis, {12},2379, Sept
20,1999) describes a capillary chromatography
system applied to human plasma which has an LOD of
20 ppb.There may be a problem w LOQ but its
worth looking at