Detailed hydrocarbon analysis (DHA) using ASTM method D6729 and D6729 appendix X2.

Introduction

Detailed hydrocarbon analysis (DHA) is a separation technique used by a variety of laboratories involved in the petrochemical industry for analysis and identification of individual components as well as for bulk hydrocarbon characterisation of a particular sample. Bulk analysis looks at gasoline composition in terms of PONA components (Paraffins, Olefins, Naphthalenes and Aromatics) and other fuels in the C1-C13 range since this gives an indication of overall quality of the sample.

The analysis of gasoline for spark ignition components is essential for quality control. Owing to the complex nature of gasoline samples, good resolution between eluents is required and therefore a long column is used (typically 100m). Several methods are routinely used for DHA which differ in their oven temperature ramp rates or in the length of column used. Each method has its advantages and disadvantages since some improve peak resolution of low boiling compounds whereas others provide better resolution of heavier compounds at the end of the chromatogram. The complex nature of the methodology coupled with the use of such a long column means that run times can easily exceed 120 minutes when using helium carrier gas. However, the use of hydrogen can vastly increase run rates because of its efficiency at higher linear velocities. This is a particularly attractive prospect for oil analysis laboratories since faster throughput of sample means increased profitability. The benefits of using hydrogen in terms of improved chromatography combined with the increasing cost of helium along with supply issues means that laboratories switching from helium to hydrogen can become much more profitable whilst maintaining standards of analysis that conform to industry standards.