Measurement of total CO2 in microliter samples of urine and other biological fluids using infrared detection of CO2

The purpose of this study is to describe a low-cost and simply made instrument capable of measuring the total CO2 content of microliter volumes of biological fluids utilizing a commercially available CO2 sensor based on a NDIR detector. The described instrument is based on transformation of dissolved HCO3 (-) to CO2 by acidification and subsequent measurement of the produced CO2. The instrument has a linear response in the range 0.025-10 mu mol HCO3 (-), which enables measurements in fresh urine and plasma samples down to 5 mu l. The values from plasma were compared to measurements made on 65 mu l whole blood in an automatic blood gas analyzer and found not to differ significantly. Compared to currently commercially available instruments applying the same principles to measure total CO2, this study provides a simple and robust alternative which even can be used on smaller sample volumes.