Evaluating the impact of GC operating settings on GC–FID performance for total petroleum hydrocarbon (TPH) determination

Interlaboratory comparisons for the analysis of mineral oil have indicated that many laboratories have problems in producing data of acceptable quality, mainly because of variations in the gas chromatographic settings used in the determination. A D-optimal design was therefore utilized to study the effects of six different GC operating settings on the GC performance criterion proposed by ISO and CEN draft standards ISO/FDIS 16703:2004 and CEN prEN 14039:2004:E for total petroleum hydrocarbon (TPH) determination. Both qualitative and quantitative factors were investigated. The results indicate that the performance criterion can only be achieved if the splitless injection settings are carefully optimized. Otherwise mass discrimination readily affects the validity of the results. The most critical factors affecting GC performance were the inlet liner design, inlet temperature and injection volume. The methods, however, were robust with respect to small changes in split vent time, GC column flow and FID temperature. The results show that non-discriminating splitless injection can only be obtained by optimizing the injector settings with respect to the significant factors. The main conclusion that can be drawn is that, if no further standardization is to be given for TPH determination by GC–FID, then a proper estimate of the expanded uncertainty should be appended to the TPH results. Only then can the reliability of the TPH results be guaranteed and further justification thus gained to support the end-use of the data.