ADDITION OF INTERNAL STANDARDS TO PARTICULATE SAMPLE MATRICES FOR ROUTINE TRACE ANALYSES OF SEMIVOLATILE ORGANIC-COMPOUNDS - A SOURCE OF SYSTEMATIC AND RANDOM ERRORS

This article is a criticism of the strategy of adding (isotope labelle d) internal standards of semi volatile hydrophobic organic compounds d irectly on the surface of particulate samples matrix such as sediment, soil and fly ash, etc. in a small aliquot (mL) of solvent, before tra ce level analysis. The use of the internal standard is intended to com pensate for incomplete extractions, cleanup losses, dilution errors an d instrument variations. However, direct addition of internal standard s 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 ca rrier solvent. Second, the native analyte and internal standard sorb t o 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 ob served, 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 labora tory staff the evaporation losses of standard (and native) compounds c auses, besides dust, an additional risk of inhalation exposure. The in ternal standard should instead be added to the extraction solvent afte r the extraction. Smaller systematical errors (10-20%) and associated random errors due to irreversible sorption are discussed.