A Study on Losses of PAHs During Sample Concentration for Chromatographic Analysis: Evaporation with a Stream of Nitrogen

Abstract

Losses of polynuclear aromatic hydrocarbons (PAHs) in methylene chloride during evaporation with a stream of nitrogen was systematically studied. The starting sample concentration levels were 0.167 μg/mL and 0.00333 μg/mL for each PAH. These two sets of test solutions were both evaporated from 300 mL to final volumes of 50, 30, 5, 3, and 1 mL, with a constant stream of nitrogen and in a water bath kept at 40°C. Each sample was analyzed by GC-FID.

Factors affecting the percentage of analyte recovery include boiling points of analytes, the final sample volume and the starting sample concentration. When the diluted solutions were reduced from 300 mL to 1 mL, the recoveries for PAHs were all higher than 90%. However, when the concentrated solutions were evaporated to 1 mL, recoveries of all analytes dropped below 85%, and naphthalene, the most volatile PAH, dropped to 77.5%. if evaporation was halted to a final volume of 3∼5 mL, the recovery for PAHs in both concentrated and diluted solutions were still almost all higher than 90%. This implies that during evaporation of methylene chloride with a nitrogen stream, no significant losses of semivolatile analytes, regardless of their boiling points or concentration levels, were found until or unless the final volume reaches below 3∼5 mL. Apparently, the solute losses are mainly due to coevaporation instead of vapor partition.

Key Words:

• PAH
• concentration
• evaporation
• nitrogen
• Kuderna-Danish