Determination of Pb in the ash fraction of plants and peats using the Energy-dispersive Miniprobe Multielement Analyser (EMMA)

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Abstract
Dry ashing at 550 °C was used to pre-concentrate Pb in peat and plant material prior to analysis using the Energy-dispersive Miniprobe Multielement Analyser (EMMA). Using approximately 1 g of starting material, precise determination of the ash content using a conventional laboratory balance (±1 mg precision) was possible provided the mineral matter concentration exceeded about 3.5%, yielding approximately 35 mg of ash for analysis. To assess possible loss of Pb upon ashing, certified plant reference materials and independent analytical methods (ICP-MS of acid digests and EMMA analysis of whole peats) were used for comparison. No significant systematic loss of Pb occurred during ashing. The accuracy of Pb determinations using dry ashing was within 10% in plant materials containing as little as 0.5 µg g–1 (estimated from certified plant material). The EMMA determinations of Pb in peat by dry ashing generally agreed with the ICP-MS analyses of acid digests within 12% down to 0.2 µg g–1. The average limit of detection for a sample containing 2% of mineral matter was 100 ng g–1, more than five times better than the detection limit obtained using EMMA analyses of whole peats. Overall accuracy and precision were better when Pb was measured by dry ashing compared to direct analysis of solid peats or plants.

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