Optimisation of the selectivity of a pulsed flame photometric detector for unknown compound screening.

Identification of traces of chemical warfare agents (CWA) is generally performed with spectral methods such as mass spectrometry (MS) or NMR, but the use of element-specific detectors is most often required to extract interesting signals from gas chromatographic (GC) data heavily masked by natural interferents. The pulsed flame phosphorus detector (P-FPD) is able to detect phosphorus, sulphur and arsenic, and thus should be very well-suited to CWA detection. However, first results using standard operating conditions recommended by the manufacturer of P-FPD led to false positive detection of phosphorus-containing compounds on the sulphur line. Therefore, an optimisation process of the selectivity of P-FPD for phosphorus versus sulphur or arsenic was undertaken, and allowed to identify gate delay and gate width as crucial parameters for the performance of P-FPD. While selectivity could be significantly improved, unexpectedly, this resulted in a concurrent important loss of sensitivity (ca. 45%) for arsenic, which suggests that this detector should be carefully optimised with respect, and prior to its purported use.