Spectrophotometer method, soil sample

Results of research and actual casework were presented by Croft and Pye (2004a) and show the spectrophotometer method to provide a precise and rapid method for soil sample comparison. In a study assessing the... 

Acetone or acetonitrile are commonly used as solvents to extract TNT from soils, although a mixture of DMSO/Ethanol was used as a solvent to extract TNT in a report on toxicity studies [8], These colorimetric methods can be used to detect a class of explosives, such as nitroaromatics, nitramines, and nitrate esters. However, explosives within a class cannot be distinguished very well. The most preferred on-site method relies on colorimetric detection as explained in the EPA Method 8515 for TNT. These methods require manual sample extraction for soil samples and preconcentration for water samples. Further, calibration with a control solution needs to be performed manually, the sample and reagents have to be mixed manually, and the absorbance from a spectrophotometer is noted manually. 

A recent modification to EPA Method 7196 has been published and will be implemented in this lab exercise (4). The method uses a hot alkaline solution (pH 12) to solubilize chromates that are to be found in soils obtained from hazardous waste sites. One portion of the aqueous sample would then be aspirated into the FIAA for a determination of total Cr, whereas diphenylcarbazide dissolved in acetone will be added to another portion, and the absorbance of the red violet complex will be measured at 540 nm using a visible spectrophotometer. In this manner, both total Cr and Cr(VI) can be determined on the same sample. Thus, the ratio of the concentration of Cr(III) to the concentration of Cr(VI) in a soil sample can be calculated from the data generated in this experiment. 

PCBs in soils and wastewaters can be rapidly screened on site or in the laboratory by immunoassay technique (Chapter 1.13). Immunoassay test kits are now commercially available from many suppliers. The samples can be tested at the calibration levels of 1 to 50 ppm. The kit primarily contains antibody-coated test tubes or magnetic particles, assay diluent, PCB-enzyme conjugate, a color-forming substance, and a solution to quench the reaction. The method does not distinguish accurately one Aroclor from another. PCBs can be measured semiquantitatively by comparing the optical density of the color formed in the sample against a set of calibration standards using a spectrophotometer. 

For chemical analysis of each sample, approximately 10 g of soil was transferred to a Gooch crucible. Surface debris such as twigs were removed, and the crucibles were heated in a muffle furnace at 500 °C for 24 h to remove organic components. The ashed soil was then pulverized with a pestle in a porcelain mortar. For each soil position, four 0.5-g portions of fine powder were analyzed separately, and the results were averaged. Soil analyses were performed in-house by atomic absorption spectrometry on a Varian Model 1250 spectrophotometer according to our previously reported method, which involves total dissolution of the sample in acid (11,12). The elements assayed were strontium, zinc, magnesium, calcium, sodium, lead, iron, aluminum, manganese, and potassium. 

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