Trace and residue analysis

FDA monitors foods for half of the approximately 300 pesticides having official EPA tolerances as weU as a number of other pesticides that have no official tolerances. Multiresidue methods, most of which are based on chromatography protocols, are employed (7). Not aU pesticides are monitored on aU foods and sampling (qv) is purposely biased to catch possible problems. The overaU iacidence of iUegal pesticide residue is, however, quite smaU 1% for domestic surveiUance samples and 3% for imported foods. The methods employed can usuaUy quantify residues present at 0.01 ppm. Quantitation limits range from 0.005 to 1 ppm. 

Extremely low level detection work is being performed ia analytical chemistry laboratories. Detection of rhodamine 6G at 50 yoctomole (50 x lO mol) has been reported usiag a sheath flow cuvette for fluorescence detection foUowiag capiUary electrophoresis (9). This represeats 30 molecules of rhodamine, a highly fluoresceat molecule (see Electhoseparations, electrophoresis Spectroscopy, optical). 

Analysis of single mammalian ceUs by capiUary electrophoresis has been reported usiag on-column derivatizatioa and laser-iaduced fluoresceace detectioa (11). Dopamine and five amino acids were determined ia iadividual rat pheochromocytoma ceUs after on-column derivatizatioa. 

Sampling. A sample used for trace or ultratrace analysis should always be representative of the bulk material. The principal considerations are determination of population or the whole from which the sample is to be drawn, procurement of a vaUd gross sample, and reduction of the gross sample to a suitable sample for analysis (15) (see Sampling). 

The analytical uncertainty should be reduced to one-third or less of sampling uncertainty (16). Poor results obtained because of reagent contamination, operator errors ia procedure or data handling, biased methods, and so on, can be controlled by proper use of blanks, standards, and reference samples. 

---