On-line heart-cut LC–GC and, more recently, comprehensive LC–GC (LC×GC) are very powerful analytical techniques because of the combination of the selectivity features of LC with the high efficiency of GC. Such a combination can overcome the drawbacks related to sample manipulation in off-line approaches because of the automation of the transfer procedure. Dirty samples, or non-volatile components, can be pre-purified in the LC step, allowing the transfer to the GC system of the volatile fraction, or of selected fractions, exploiting the high selectivity of the LC process. This article presents an overview of the most recently used interfacing systems, as well as applications in the food analysis.
Food samples are often formed of compounds belonging to numerous chemical groups, present in a wide range of concentrations; thus, one-dimensional gas and liquid chromatographic techniques often cannot provide the required efficiency and selectivity for safety or quality assessment.
Nowadays, great attention is being devoted to the application and development of multidimensional chromatographic techniques because of their higher separation power and selectivity. Among the chromatographic combinations, the hyphenation of liquid and gas chromatography in a heartcut system, or, in the last decade, in a comprehensive one, has proven to be of considerable interest. The reason for this attractiveness is related to the capability of such methods to elucidate samples, or sample fractions, characterized by high complexity; furthermore, in many situations the LC process acts as a sample preparation step, avoiding the presence of nonvolatile components in the GC system.The main strength of multidimensional LC–GC lies in the high selectivity of LC and the enhanced separation power of GC. On-line LC–GC methods are particularly suited for the separation of compounds with similar physico-chemical properties, or of target components from a complex matrix. The coupling of the second dimension to a mass spectrometer generates a very powerful three-dimensional analytical method, heart-cut LC–GC or comprehensive LC×GC coupled to MS (LC–GC–MS or LC×GC–MS), characterized by the potential to generate highly pure spectra, which enables a much easier interpretation compared to those generated from GC–MS analysis.
The main problem, from an instrumental point of view, is to introduce a large volume of liquid effluent into a GC injector. As a consequence, the coupling of these two techniques is a more difficult compared to multidimensional GC or LC approaches. However, technological developments have enabled the development of efficient interfaces for on-line applications. Fully automated interfacing is now possible by using commercially available hardware, enabling either heart-cut or comprehensive analysis.
On-line LC–GC and LC×GC Transfer Devices
In modern-day multidimensional LC–GC analysis, loop-type and syringe-based interfaces are mainly used for efficient solvent removal and chromatography band transfer, coupled to on-column or vaporizing injectors. For a detailed description of LC–GC interfaces and instrumental features, detailed contributions are present in the literature (1,2).
The selection of the most suitable interface and injection system is mainly based on the analytical problem at hand, and on the selected LC conditions. The amount of solvent to be eliminated depends on the column dimensions and can vary between a few microlitres and 1 mL. On the other hand, the nature of the solvent is of the utmost importance and very polar solvents, such as methanol or water, are difficult to remove effectively. For this reason, often relatively highly volatile apolar to medium-polar solvents are used, even though different approaches have been developed to allow the use of reversed-mode LC.
The programmable temperature vaporizing (PTV) injector is, from a practical point of view, the easiest to use. In the PTV injector, the solvents are removed via the split vent, while non-volatile components remain in the liner, a part that can be easily cleaned. The PTV injector is the most commonly used solvent-removal device in LC×GC, where a high number of transfers are performed. On-column injection is particularly suitable in the presence of low boiling components because of the capability of such an injector to reduce discrimination during the solvent evaporation step. A brief description of current-day interfaces for LC–GC systems now follows.