A comparison of reliability of soil cadmium determination by standard spectrometric methods

Inductively coupled plasma emission spectrometry (ICP-OES) is the most common method for determination of soil Cd, yet spectral and matrix interferences affect measurements at the available analytical wavelengths for this metal. This study evaluated the severity of the interference over a range of total soil Cd by comparing ICP-OES and inductively coupled plasma mass spectrometry (ICP-MS) measurements of Cd in acid digests. Using the emission at 226.5 nm, ICP-OES was generally unable to quantify soil Cd at low (near-background) levels and gave unreliable values compared with ICP-MS. Using the line at 228.8 nm, a marked positive bias in Cd measurement (relative to the 226.5 nm measurement) was attributable to arsenic (As) interference even at soil As concentrations below 10 mg kg. This spectral interference in ICP-OES was severe in As-contaminated orchard soils, giving a false value for soil total Cd near 2 mg kg when soil As was 100 to 150 mg kg. In attempting to avoid these ICP emission-specific interferences, this study evaluated a method to estimate total soil Cd using 1 M HNO extraction followed by determination of Cd by flame atomic absorption (FAA), either with or without preconcentration of Cd using an Aliquat-heptanone extractant. The 1 M HNO extracted an average of 82% of total soil Cd. The FAA method had no significant interferences and estimated the total Cd concentrations in all soils tested with acceptable accuracy. For Cd-contaminated soils, the Aliquat-heptanone preconcentration step was not necessary, as FAA sensitivity was adequate for quantification of extractable soil Cd and reliable estimation of total soil Cd.