Automated on‐line comprehensive two‐dimensional LC×GC and LC×GC–ToF MS: Instrument design and application to edible oil and fat analysis

Abstract

After a successful off‐line feasibility study, the automation of comprehensively coupled liquid chromatography and gas chromatography (LC×GC) has been studied. Important aspects to consider when developing automated LG×GC include the relative speeds of the two dimensions, the compatibility of the LC eluent (type and flow rate) with the GC dimension, and the column loadabilities. Because the GC separation is relatively slow, the LC instrument has to be operated in the stop‐flow mode. Two interfaces for transferring large numbers of subsequent LC fractions to the GC were constructed: one based on a six‐port switching valve, and one which uses a dual side‐port syringe. Both interfaces were found to perform fully acceptably. The actual transfer of the LC fraction to the GC was realised using a standard split injector to vaporise the compounds and LC eluent. Gas phase splitting was applied to match LC mass load and GC column loadability. The standard deviations of the peak areas obtained in this way were better than 7% (n = 6). The reliability of the system was demonstrated by the problem‐free analysis of large series of oil and fat samples, with the focus on both intact triglycerides and their fatty acid methyl esters (FAMEs). Finally, the hyphenation of the automated LC×GC system to a sensitive and rapid‐scanning time‐of‐flight mass spectrometer was realised. By using LC×GC–ToF MS, the information density of the chromatograms could be improved even further, which allowed easy identification of individual compounds as well as compound groups.