Determination of Cr, Zn, Cd and Pb in milk powder by slurry sampling electrothermal vaporization inductively coupled plasma mass spectrometry

Ultrasonic slurry sampling electrothermal vaporization reaction cell inductively coupled plasma mass spectrometry (USS-ETV-RC-ICP-MS) has been applied to the determination of Cr, Zn, Cd and Pb in several milk powder samples. The influences of instrument operating conditions and slurry preparation on the ion signals were reported. Ascorbic acid was used as the modifier to enhance the ion signals. The background ions at the chromium and zinc masses were reduced in intensity significantly by using 0.4 ml min(-1) NH3 as reaction cell gas in the dynamic reaction cell (DRC) while a q value of 0.6 was used. Since the sensitivities of Cr, Zn, Cd and Pb in milk slurry and aqueous solution were quite different, the isotope dilution method was used for the determination of Cr, Zn, Cd and Pb in these milk samples. This method has been applied to the determination of Cr, Zn, Cd and Pb in NIST SRM 1549 and NIST RM 8345 milk powder reference material and a milk powder sample purchased from the market. The analysis results were agreed with the certified values. The method detection limits estimated from standard addition curves were about 3, 37, 0.2 and 2 ng g(-1) for Cr, Zn, Cd and Pb, respectively, in the original milk powder.