Development of a measurement system for certifying ethanol mass fraction in aqueous solutions

In response to the sovereign requirement for national standards the National Measurement Institute, Australia (NMIA) has developed a measuring system using isotope dilution mass spectrometry (IDMS) to certify forensic aqueous ethanol solutions. NMIA participated in an international study, CCQM-K27, organized under the auspices of the International Committee for Weights and Measures to compare our measuring system with the techniques being used for certifying aqueous ethanol solutions in other metrology institutes. This comparison provided objective evidence that the measuring system developed was fit for the purpose of certifying aqueous ethanol solutions that ranged in concentration from 0.8 mg/g to 120 mg/g. A complete measurement uncertainty budget is presented and shows that the largest contribution to measurement uncertainty was from method precision followed by the contribution from the calibration solution. The fundamental technology of the measuring system was gas chromatography of the aqueous ethanol solutions using porous layer open tubular columns, and this effectively produced peak area measurements with both GC/MS and GC-FID. It was found that deactivation of the chromatographic system was critical for obtaining reproducible peak shapes and peak area measurements. A range of measuring systems, all using this gas chromatographic technology, was investigated. When conditions were carefully controlled there was no difference in measurement results from GC-IDMS, GC/MS or GC-FID. There was also no difference in results from on-column or split injection systems. A significant issue with the IDMS system was the fragmentation of 13C2-ethanol to produce an ion with the same mass as the molecular ion of ethanol which lead to isobaric interference; careful measurement of this fragmentation ratio was necessary to calculate accurate mass fraction values. NMIA has adopted the GC-IDMS split measuring system to certify aqueous ethanol solutions for Australian legal requirements since this measuring system provided higher analytical specificity than GC-FID, accuracy that was fit for purpose and was operationally less stringent than on-column techniques.