I need to develop an assay that needs sub picograms/mL quantitation limit. What I want to do is to use pentafluorobenzoylchloride (I)(acylation), pentafluorobenzylbromide (II)(alkylation), or di-methylpentafuorobenzylsilylchloride (III)(silylation) of my compound to form a electron rich derivative so that the negative electron captive ionization may enhance the signal. Part structure of my compound is like this:

Ph-NH-(C=S)-O-C(CH3)-Ph

Ph: benzene ring

I want to form a derivative either on sulfur or on nitrogen. I talked to a couple of organic chemists. Both of them told me that it is more possible to add an agent to S atom than to N. however, it is difficult. I would like to hear a suggestion from chromatographers who are experienced with derivatization on GC. If you can give me a little procedure, that will be great. I did some of the work using (I) and (II). But I did not get a signal. Either the derivative was not produced or the derivative was not stable - decompose in the derivatizing, GC injector port, or in MS source. My procedure is: my compound in toluene plus (I) or (II) plus triethylamine, shake at 60 C for 60 min. wash with water 3 times. toluene injected. No signal. Thank you very much. -- Philip Jawangate