The stability of ethanol in unstoppered tubes

The exact time frame within which ethanol can be reliably measured in unstoppered tubes is not known. The aim of this study was to investigate the stability of alcohol concentration in unstoppered tubes.

44 samples with alcohol concentration >2.7 mmol/L were included in the study. Measurements were done on Vitros 250 analyzer with original Vitros reagents. After the initial alcohol measurement, each sample was aliquoted into two separate clean tubes (1 mL). One of the aliquoted tubes was stoppered immediately after aliquoting and remained stoppered during the experiment; while the other two tubes (original sample tube and the second aliquoted tube) remained open. During the experiment all three tubes were kept at room temperature. Alcohol concentration was measured at 30 minutes, 1, 2 and 3 hours after the initial measurement in all 3 tubes. The differences between the time intervals for each test tubes were examined using repeated measures Anova or Friedman test. The deviation from the initial concentration was calculated for all three test tubes for each time interval. The calculated deviations were compared with desirable imprecision specifications (DI) according to the RiliBÄK (DI<9%).

We found a statistically significant difference between the initial concentration and the concentration in unstoppered tubes for all the investigated time intervals; however, the DI was exceeded only in the original tube and in the tube B, 3 hours after the initial measurement (-9.2% and -12.6%, respectively).

Alcohol concentration can be accurately measured in the unstoppered samples within two hours upon decapping the tube, when stored at room temperature. Longer storage time (>2 hours) in the unstoppered samples introduces significant bias in alcohol concentration.