Evaluation of the source of bias caused by losses of solvent vapour during sample preparation

Evaporation losses of organic solvents used to dissolve reference solutions and samples lead directly to errors in the measurement of amount of substance. This is due to changes in the mass fraction composition and the mass:volume concentration of solutions. We have found that measuring the amount of solute added to a solution by weighing by difference is vulnerable to negative bias in the measured amount of solute added, because solvent vapour is lost from opened vials and from syringe needles. Relative negative bias became larger with higher solvent volatility and with smaller volumes, reaching −28% for 10 μL of dichloromethane. Straightforward precautions that reduce the impact of evaporation on gravimetric operations with volatile solvents are presented.