Addition of internal standards to particulate sample matrices for routine trace analyses of semivolatile organic compounds: A source of systematical and random errors

This article is a criticism of the strategy of adding (isotope labelled) internal standards of semi volatile hydrophobic organic compounds directly on the surface of particulate samples matrix such as sediment, soil and fly ash, etc. in a small aliquot (mL) of solvent, before trace level analysis. The use of the internal standard is intended to compensate for incomplete extractions, clean-up losses, dilution errors and instrument variations. However, direct addition of internal standards to sample matrices creates two possibilities for inaccurate results by processes only affecting the internal standard: First, evaporation losses of standard from the sample matrix during evaporation of the carrier solvent. Second, the native analyte and internal standard sorb to the sample matrix with differing force. Both processes can introduce systematic and random error to the result. A systematic error of 74% due to evaporation losses of tetra chlorinated dibenzo-p-dioxins is observed, while the corresponding error for octa chlorinated dioxin is 0%. The associated random error is 45% for tetra down to 1–4% relative standard deviations for hepta and octa chlorinated dioxins. For laboratory staff the evaporation losses of standard (and native) compounds causes, besides dust, an additional risk of inhalation exposure. The internal standard should instead be added to the extraction solvent after the extraction. Smaller systematical errors (10–20%) and associated random errors due to irreversible sorption are discussed. Received: 4 September 1997 / Revised: 26 January 1998 / Accepted: 31 January 1998