Determination of chromium in nitric acid extracts of soils by flame and electrothermal atomic absorption spectrometry

Abstract

The direct determination of Cr in soil samples following extraction of the metal in a microwave oven was accomplished by using flame (FAAS) and electrothermal (ETAAS) atomic absorption spectrometry. Both techniques are required owing to the broad concentration range over which Cr occurs in soils. The operating conditions for FAAS were optimized; use of a lean air-acetylene flame significantly reduced the severe chemical interferences of Fe³⁺, Ni²⁺, Al³⁺, Mg²⁺, at the expense of slightly decreased sensitivity. The limit of detection achieved was 0.11 µg ml^–1 (7.33 µg g^–1). The temperature programme of the graphite furnace, the use of chemical modifiers, the atomization technique employed (wall or L'vov platform) and the effectiveness of deuterium background correction were investigated. Chromium was reliably determined by using the L'vov platform and no chemical modifier or background correction. The sensitivity thus achieved was 3.82±0.04 pg (_n=10) and 0.73 µg l^–1 (48.6 ng g^–1) as characteristic mass and limit of detection, respectively. The within-batch precision (_n=10) and analytical recovery were both acceptable. Two certified reference materials (BCR CRM 141, Calcareous Loam Soil, and BCR CRM 277, Estuarine Sediment) were used for validation. Both pseudo-total Cr extraction and determination were accomplished in a straightforward, fast and effective way, without the need for special reagents or a prior separation of Cr from the soil matrix. The optimized methods were used to determine Cr in a variety of roadside soils, using direct calibration with aqueous standards.

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