Comprehensive Liquid Chromatography and Other Liquid-Based Comprehensive Techniques Coupled to Mass Spectrometry in Food Analysis

The analytical benefits of comprehensive two-dimensional chromatography methods (LC × LC) have been constantly exploited over the last 20 years. The power of LC × LC methods, along with recent advances in mass spectrometry (MS), enabled a much deeper insight into the true qualitative and quantitative composition of real-world food samples. LC × LC experiments are usually carried out on two analytical columns with complementary (orthogonal) selectivity. A transfer device (in most cases one or two-switching valves), positioned between the two dimensions, enables the isolation and reinjection of the chromatographic eluate from the first dimension (1D) to the second dimension (2D) column, throughout the whole analysis. Separations in the 2D are usually carried out in a fast way and ideally must end (not to incur in the so-called “wrap-around effects”) before the following reinjection step. The most striking advantage of LC × LC methods, over the one-dimensional (1D) counterparts, is the enhanced resolving power: in theory, the peak capacity (nc) is multiplicative of the nc values of both 1D and 2D. Such a value is practically never reached for a series of reasons, e.g., lack of complete orthogonality, partial loss of 1D resolution, non-ideal chromatography conditions, etc. To this regard, some tricks have been exploited in the recent years to mitigate such an issue especially for reversed-phase × reversed-phase LC separations (RP-LC × RP-LC). Since its first application in 1978, over 70 original papers have been published for the analysis of real-world food samples and specifically 14 in the last two years. In most cases, the outstanding selectivity and sensitivity of LC × LC methodologies combined with MS detection made trace (ppb level) and ultratrace (ppt level and lower) analysis feasible, thus reducing the need for tedious sample preparation processes. Critical descriptions of significant applications/evolutions are herein reported concerning the last two years on LC × LC and other liquid-based comprehensive two-dimensional chromatography techniques.