Ferrioxalate solutions for calibration in oxygen and carbon dioxide measurements

Abstract

Air-equilibrated, acidic aqueous solutions containing Fe3+ and oxalate are used to generate levels of 02 and C02 upon irradiation at X = 300-500 nm which are useful for calibration of blood gas measurements. Irradiation times <3 min yield p02 and pC02 levels which are repeatable to within 0.7 and 0.3 mmHg, respectively. Variation of the Fe3+ and oxalate concentrations alters both p02 and p C02 levels and enables rapid and precise generation of two levels of each gas using only two solutions. Thus, an air-equilibrated (25 °C) ferrioxalate solution with a high oxalate level gives nominal p C02 and p02 levels of 74 and 13 mmHg, respectively. A similarly equilibrated ferrioxalate solution with a low oxalate level gives nominal pC02 and p02 levels of 36 and 180 mmHg, respectively. The within-run precision for these irradiated solutions when run as normal samples on a commercial blood gas analyzer Is well within the accepted repeatability specifications for such instruments. Storage of ferrioxalate solutions with high oxalate levels at 85 °C for 28 days yields p02 and pC02 values upon air-equilibration and irradiation which are unchanged from levels in freshly prepared solutions to within ±0.4 and ±1.0 mmHg, respectively. Ferrioxalate solutions with low oxalate levels show a lesser degree of thermal stability for C02 photogeneration, with declines in p C02 values >2 mmHg for storage >20 days at 25 °C.