Basic Instrumental Requirements and Considerations

The main concern in implementing a multidimensional separation solution will be how to design the instrumental set-up. The greater the difference between the two dimensions, the greater the potential difficulty in their coupling, since there will be greater dissimilarity in the mechanisms of separation. The wide choice of column chromatography separation methods explains why the coupling or interfacing may present a challenge. If the two dimensions are of the same chromatographic type - GC-GC, HPLC-HPLC, supercritical fluid chromatography (SFC)-SFC etc. - the task is not so problematic. Where different carrier phases are required for the two dimensions, chromatographic integrity must be maintained. Transferring a solution phase from HPLC to capillary GC or SFC requires an interface that can effectively introduce analyte into the narrow-bore column without compromising band dispersion, and whether analytical scale or large volume solvent injection is to be used determines the interface complexity.

Table 1 presents a summary of the potential successful multidimensional column chromatography methods. Some options will be generally incompatible, such as IC coupled with GC, since exclusion of the electrolyte carrier fluid from the GC system will be difficult, and ionic analytes for which IC is usually used will not be suited to GC analysis. The suitability of HPLC-GC and GPC-GC for volatile organics is the reason why these liquid-phase first-dimension separations are useful for sample differentiation prior to the GC step. Table 2 further outlines various procedural aspects of selected multidimensional methods.

For multidimensional GC (MDGC) analysis, it is not a difficult task when dealing with relatively low boiling mixtures to couple two columns together, and to have gas-sampling valves or a flow-switching device to allow transfer of effluent. The classical Deans switch relies on pressure differences to pass carrier flow in different directions. Figure 1 presents a schematic diagram of a typical commercial MDGC system, comprising one oven, two columns, two detectors, a midpoint restrictor at which point the diversion of column flow to either the first detector or the second column occurs. A cold trap focuses heart-cut

Table 1 Possible multidimensional coupling of separation dimensions in column chromatography

Second dimension

GC NPHPLC RPHPLC IC GPC SFC CE

First dimension

NPHPLC RPHPLC IC

GPC SFC CE

y y y y y y y y y y y y y y y y y y y y y y y y y y y y y

GC, Gas chromatography; NPHPLC, normal-phase high performance liquid chromatography; RPHPLC, reversed-phase high performance liquid chromatography; IC, ion chromatography; GPC, gel permeation chromatography; SFC, supercritical fluid chromatography; CE, capillary electrophoresis.

Table 2 Selected multidimensional (MD) chromatography modes and application areas

Dimension 1 Dimension 2 Interface Method

Packed GC Capillary GC

Packed/capillary GC Packed/capillary GC

Capillary GC Capillary GC

Capillary GC Capillary GC

HPLC Capillary GC

HPLC-GPC Capillary GC

Heart-cut valve

Direct coupling; pressure tuning Heart-cut valve, with options (see Table 1) Continuous transfer; peak compression Large volume injection Large volume injection

Trace enrichment

Multi-chromatography

Conventional high resolution MDGC

Comprehensive 2D gas chromatography

Multidimensional HPLC-GC

Prior class separation before GC step fractions, and a solenoid-controlled shut-off valve closes the flow through to the monitor detector and effects the transfer of the flow of column 1 to column 2.

Direct coupling of two or more columns, with all the effluent from one column passing wholly into the second column without hindrance (see later for variations on this theme), is normally not considered a MDGC analysis because MDGC methods should lead to greater peak capacity for the total system. Capacity may be thought of as the total available or achievable peak separation on a column. In simple terms, this is the total retention space divided by an average peak width parameter defining acceptable neighbouring peak resolution, i.e. the maximum number of peaks, resolved to a given extent, which can be produced by the system. Consider temperature-programming analysis. Assuming that the total chromatographic adjusted retention time is 90 min, and each peak basewidth is 10 s (peak widths may be approximately constant across the whole analysis, depending on the temperature ramp rate chosen), then a maximum of 540 baseline separated peaks could be recorded on this column. In practice, the actual number would be much smaller since the peaks are not eluted uniformly over the total time of the analysis.

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