Determination of formetanate hydrochloride in selected fruits by coupled-column cation exchange liquid chromatography

A strong cation exchange (SCX) liquid chromatographic (LC) method is described for determination of formetanate hydrochloride residue in pome, citrus, and stone fruits. A test portion of fruit, homogenized with the peel left on, was blended with acidified acetonitrile and filtered. A portion of extract was finely filtered, and a 500 microL aliquot (ca 0.2 g test sample equivalent) was loaded onto an SCX solid-phase extraction (SPE) LC column, which replaced the injection loop of the LC injection valve. Cations were selectively enriched; noncations were eluted by acetonitrile in a pre-separation cleanup. Turning the valve to the inject position coupled the SPE column to an SCX analytical column for separation and detection at 250 nm. The mobile phase was 0.4M pH 3.0 ammonium phosphate buffer-water-acetonitrile (50 + 25 + 25). Formetanate cation was quantitated by peak area and regression coefficients from a 5-point linear calibration covering a 100-fold range. Recovery of duplicate fortifications of apple, pear, orange, and peach averaged 89-99% at the respective U.S. tolerances of 3, 3, 4, or 5 ppm and averaged 93-99% at one-tenth of the respective tolerance level. Peel pigments or variable peel bulk of crop varieties tested, as well as other endogenous fruit material, contributed interference that was below the 0.02 ppm limit of detection. In a 1991 limited survey comprising 15 samples, none were found violative. Residues were found in 2 samples, but only 1 measurement was quantifiable, near the 0.06 ppm limit of quantitation.