Standard gases used to calibrate anesthetic vapor analyzers: are they stable?

OBJECTIVE

Gas cylinders containing volatile anesthetic vapors often are used to calibrate anesthetic agent analyzers. Differences seen between manufacturers in device calibration may be the result of error in the preparation of these calibration standards or drift in their concentration.

METHODS

We measured the stability of 29 calibration gases in 2-L aluminum cylinders over a 15-month period. We also measured the stability of 18 gases in 1-L aluminum mini bottles.

RESULTS

We found an average change of less than 0.02 vol% in the concentration of volatile anesthetic vapors for the 2-L aluminum cylinders containing 1.0 vol%. The maximum change was 0.04 vol%. For the 2-L cylinders, the manufacturer's certified concentration was 0.03 to 0.09 vol% higher than the concentration we measured on receipt. For the mini bottles, we found an average change of less than 0.04 vol% during a 6-month period; the maximum change was 0.06 vol%. The maximum change in 12 months was 0.14 vol%.

CONCLUSIONS

Our results indicate that calibration gases containing volatile anesthetic vapors appear to be stable when stored in suitable cylinders. Aluminum cylinders sealed with a stop-cock seem to be suitable. In contrast, mini bottles seem to be less stable, probably due to the sealing construction. The difference of up to 0.09 vol% between our measurement of vapor concentration and the manufacturer's certified concentration may result from adsorption on surfaces in the cylinders after preparation or may reflect differences in calibration technique.