Determination of liquid chromatography/flame ionization detection response factors for alcohols, ketones, and sugars

In the past, the main focus of flame ionization detector (FID) response studies was set on investigations of gas chromatography (GC) relevant analytes such as aliphatic hydrocarbons and selected functional groups. Only a few data are available for liquid chromatography (LC)/FID responses. Within this research paper, we present the FID response factors for a LC/FID system with an aqueous eluent as mobile phase. The study focus on the most common analytes of LC/FID studies in the past as well as several compounds that are not directly GC compatible because of their polarity. Furthermore, the range of substances was extended to isomers, poly-alcohols, and sugars to obtain more detailed information of the influence of hydroxyl groups on the recorded response. The data show a group-specific correlation of response factors with a correlation coefficient (R²) for, e.g., alcohols and ketones of 0.99. Constant contribution factors of functional groups as mentioned in several GC/FID response studies and prediction models were observed to a limited extent. Interactions of sugar analytes with water showed that transfer of GC/FID to LC/FID data cannot be done in general. The underlying mechanisms revealed several new aspects, which have to be taken into account for future response prediction models, especially of small molecules. Interactions between eluent and analytes show that LC/FID response prediction is more complex and requires more than simple addition of functional group contributions.