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The role of internal standards and their interaction with soils impact accuracy of volatile organics determinations

Both US Environmental Protection Agency (EPA) SW-846 Methods 8260C/5035 and 8261A include mixing soil with water and addition of internal standards prior to analyses but the equilibration of internal standards with the soil is not required. With increasing organic carbon content and no effort to equilibrate internal standards with the matrix, results are less likely to be accurate. Adding internal standards to soils prior to diluting the sample with water gives more accurate determinations but less reliable quality control (QC). Extending times for equilibration of internal standards improves accuracy but is conducive to analyte degradation not normally observed during analyses. Soil-matrix effects on a given analyte can be greatly understated using a single internal standard as described in Method 8260C while the use of multiple internal standards as described in Method 8261A is more accurate. Method 8261A's reporting error when spiking soils before adding water provides confidence intervals with accuracy near the experimental rule (75.2, 95.7 and 99.5%) with the exception of two analytes that require overnight equilibration.