Determination of ultra-trace levels of Mo in plants by inductively coupled plasma tandem mass spectrometry (ICP-MS/MS)

Abstract

In this work a new procedure for the determination of ultra-trace concentrations of Mo in plants is proposed. Microwave-assisted acid digestion of plants were performed without H2O2 in order to minimize contaminations and obtain lower procedure's blank. Solutions with different HNO3 concentrations were evaluated to digest the samples and 7 or 1molL−1 HNO3 solutions led to similar carbon residual contents. Since, analytical blank was a critical issue all samples were microwave-assisted digested using only 5mL of 1molL−1 HNO3. For Mo determinations in plant materials an ICP-MS with tandem configuration (ICP-MS/MS) with an octopole reaction system (ORS3) pressurized with oxygen gas and operating in MS/MS mode was employed to overcome polyatomic interferences caused by K, a major element in plants. Higher sensitivity was achieved for the determination of Mo using the mass-shift dioxide form (MoO2+) obtained with O2 gas flow rate at 0.2mLmin−1. The limit of detection of the analytical procedure was 260pgg−1. Molybdenum was quantified in six certified reference materials: apple leaves (NIST 1515), peach leaves (NIST 1547), trace elements in spinach leaves (NIST 1570), tomato leaves (NIST 1573), wheat flour (NIST 1567a) and rice flour (NIST 1568b) for accuracy assessment. Recoveries for Mo were in the 100.0–109.6% range and analytical results were in agreement with certified values at a 95% confidence levels (t-test). The procedure was applied for determination of Mo in 7 different samples of sugar cane, corn leaves and roots, and rice flour and the concentrations were in the 0.39 to 3.81μgg−1 range.