Determination of Perchlorate Anion in Foods by Ion Chromatography−Tandem Mass Spectrometry

A rapid, sensitive, and specific method was developed for determining perchlorate anion in lettuce, cantaloupe, bottled water, and milk. A test portion of chopped crop homogenate was extracted with diluted nitric acid and filtered. Milk proteins were precipitated with acetonitrile, and the supernatant, after centrifugation, was cleaned up on a graphitized carbon solid-phase extraction column. Water samples were analyzed directly. All test solutions were syringe filtered and mixed with an 18O4-labeled perchlorate internal standard before ion chromatography-tandem mass spectrometry. A strong anion exchange column eluted with 100 mM ammonium acetate in 50:50 (v/v) acetonitrile/water was interfaced via electrospray ionization to a triple stage quadrupole mass spectrometer operated in the negative ion mode. The labeled internal standard corrected for any sample matrix effects on measured signals. Four parent-to-product ion transitions, for loss of oxygen, were monitored for native and 18O4-labeled perchlorate anion, respectively: 35Cl-perchlorate, m/z 99 --> 83 and 107 --> 89; 37Cl-perchlorate, m/z 101 --> 85 and 109 --> 91. The limit of quantitation was 1.0 microg/kg in lettuce, 2.0 microg/kg in cantaloupe, 0.50 microg/L in bottled water, and 3.0 microg/L in milk. Native perchlorate was recovered from fortified test portions in the range 93-107% for lettuce, 107-114% for cantaloupe, 100-115% for bottled water, and 99-101% for milk.