Two-Dimensional Techniques - Gas Liquid Chromatography & Mass Spectrometry
A truly multidimensional system is one where a separation dimension must be coupled with a spectroscopic dimension and the following conditions are thus applicable:  The components of a mixture are subject to two or more separation steps, in which their displacements depend on different factors and  when two or more components are separated in any single step, they always stay separated until the total separative operation is finished.
High Performance Liquid Chromatography-Gas Chromatography (HPLC-GC)
Multidimensional liquid chromatography-gas chromatography (LC-GC) combines the selectivity of LC with the high efficiency and sensitivity of GC, thus giving a relatively high peak capacity. The advantages of on-line LC-GC are:  less sample required,  no sample workup,  fully automated sample pre- or post-treatments are possible as well as  no evaporation or dilution is necessary. Coupling of LC-GC is not a trivial issue, as both the LC and GC operate in phases that are in two different physical states. Normal-phase liquid chromatography (NPLC) is more easily coupled with GC than reversed-phase liquid chromatography (RPLC), and thus the eluent is usually a non-polar volatile solvent. In 1989 the first fully automated LC-GC instrument was pioneered for commercial use. Since then, LC-GC has been shown to be applicable to many areas of analysis, including industrial samples and petrochemicals, environmental samples and biological and pharmaceutical samples.(1,2)
Comprehensive Two-Dimensional Gas Chromatography (GCxGC)
Comprehensive Two-dimensional gas chromatography (GCxGC) has been shown through different types of research to be applicable to many areas of analysis. This type of two-dimensional gas chromatography was first described 12 years ago, and since then the number of publications in the field have grown rapidly. GCxGC is now considered the main technique used to provide a very high separation power, as well as providing an enhancement of sensitivity and structured chromatograms. It is a multidimensional technique in which the resolving power of two or more different columns is applied to some or all of the components in a sample. Many advantages are associated with this technique including:  complex samples can be separated into many distinct peaks (providing an alternative for group-type analyses),  superior resolution when compared to conventional GC,  boiling-point distributions for different classes of compounds concurrently, and  acts as a means for rigorous validation of existing and recently developed techniques.(3)
Gas Chromatography-Mass Spectrometry (GC/MS)
Gas Chromatography-Mass Spectrometry permits the analysis of two or more compounds eluted simultaneously. This can be achieved in two ways: either direct coupling or via an open-split coupling.
Liquid Chromatography-Mass Spectrometry (LC/MS)
Liquid Chromatography-Mass Spectrometry is far more difficult to achieve than GC/MS. Limitations include the the limited detection procedures. It does though have the advantage that it is connected with isolation, pre-purification and analysis in a single chromatographic step. Therefore the most applicable LC/MS interface for a specific application must be selected, based on analyte polarity.
Tandem Mass Spectrometry (MS/MS)
Tandem Mass Spectrometry is where two mass analyzers are used: one for selecting the precursor ion from the ions created in the ion source and the other for analyzing the product ions after the collisions. It is fairly useful in quantitative environmental and bioanalysisReferences:
(1) Grob, K (1991). On-line coupled LC-GC. Huthing, Heidelberg, Germany.
(2) Mondello, L., G. Dugo, et al. (2003). "Recent applications in multidimensional chromatography." 1: 2.
(3)Beens, J., J.Blomberg (2000). Comprehending Comprehensive Two-Dimensional Gas Chromatography. 23: 2.