Quantitative analysis of oligostyrenes using a charged aerosol detection: linearisation of quasi-linear calibration curves using power functions

The Charged Aerosol Detector (CAD) is a mass-based detector that is useful for solutes that cannot be detected by the UV/VIS detector due to no or weak chromophores. The CAD is being touted as a ‘Universal detector’, however, for the solute to be detected it must be less volatile than the mobile phase. The response of this detector is generally non-linear to concentration compared to the linear response of the UV detector making it difficult to undertake quantitative analysis. The aim of this study was to demonstrate the limitations of the CAD, using as an example, the analysis of a low molecular weight polystyrene that can be detected by both the UV and the CAD. Multiple power functions were applied to the response obtained from the CAD and it was determined for this study that a power function of 2.0 applied linearised the response in accordance to the UV detector derived response. This work shows the importance of finding the correct power function to be applied to the CAD to linearise the data and is dependent on the solute being detected. It is noted that this may be difficult when the solute being analysed cannot be detected by a detector, whose response is linear, such as, the UV detector.