Calibration of the electron-capture detector for the determination of polychlorinated biphenyls
The response of the constant current electron capture detector (CC-ECD) to 21 polychlorinated biphenyls is discussed. Responses were non-linear from the detection limit upwards, showing that the CC-ECD is a fundamentally non-linear detector. The extra sum of squares principle was used to assess systematically the validity of several non-linear calibration functions with their respective calibrated ranges. The power function could be applied in the small amount range. The maximum amount to which the power function applied was of the order of several tens to several hundreds of picograms, depending on the compound. Using a slightly modified power function, the calibrated range could be extended by a compound specific factor of 2–45 (maximum amount of the order of several hundred to >1000 pg). Logarithmic and linear interpolation could be used to increase the calibrated range substantially without significant loss of accuracy, from the detection limit to >1000 pg injected. Model calculations showed that the use of appropriate internal standards can limit the effect of volume errors on the quantification to less than 1%.