Simple method of electron capture detector calibration for quantitative measurements of F-11 and F-12 freons in the atmosphere

A simple calibration method of the electron capture detector for a quantitative analysis of freon F-11 and F-12 concentrations in the air is presented. The detector is supplied with pulses of a constant repetition time and operated at two sensitivity levels (high with 3200 mu s and low with 80 mu s) which depend on the air sample volume and analytical method. The detector works first at low sensitivity and is calibrated by a credible liquid standard of freons at the nanogram concentration level. The concentrations of freons in the air container are determined by a sample enrichment method in which a sample volume of ca. 100 cm(3) is trapped on a trap with glass pellets using liquid nitrogen to lower the temperature to 77 K and then thermally desorbed. At the high sensitivity, the calibration is performed using the air from the container as a standard. Different volumes of air samples from the container (from 0.5 to 5 cm(3)) are introduced to the chromatotographic column. This allows the calibration curve of the detector, working at a high sensitivity, to be determined. Such a calibration curve may be used for direct analysis of the freon concentrations in the atmosphere. The accuracy of the method (estimated at ca. 6-7%) depends mainly on the quality of the original freon standards and on the precision of the analytical procedures.