Two-dimensional chromatography
Different combinations of one-dimensional GC and LC produced the analytical chromatographic technique that is known as two-dimensional chromatography.
This methodology first appeared in the literature with a 1944 publication by A. J. P. Martin and coworkers detailing an efficient method for separating amino acids – "...but the two-dimensional chromatogram is especially convenient, in that it shows at a glance information that can be gained otherwise only as the result of numerous experiments" (Biochem J., 1944, 38, 224).
[2] The chief advantage of two-dimensional techniques is that they offer a large increase in peak capacity, without requiring extremely efficient separations in either column.
GC-MS is the single most important analytical tool for the analysis of volatile and semi-volatile organic compounds in complex mixtures.
[7] It works by first injecting the sample into the GC inlet where it is vaporized and pushed through a column by a carrier gas, typically helium.
[8] The compounds eluted from the column are converted into ions via electron impact (EI) or chemical ionization (CI) before traveling through the mass analyzer.
Sometimes GC-MS utilizes two gas chromatographers in particularly complex samples to obtain considerable separation power and be able to unambiguously assign the specific species to the appropriate peaks in a technique known as GCxGC-(MS).
[13] Nonetheless, the problem can be resolved by 2D LC-MS, as well as other various issues including analyte coelution and UV detection responses.
It was shown that 2D-LC could offer quite a bit more resolving power compared to the conventional techniques of one-dimensional liquid chromatography.
In the 1990s, the technique of 2D-LC played an important role in the separation of extremely complex substances and materials found in the proteomics and polymer fields of study.
Early work with 2D-LC was limited to small portion of liquid phase separations due to the long analysis time of the machinery.
Two-dimensional liquid chromatography is better suited to analyzing complex mixtures samples such as urine, environmental substances and forensic evidence such as blood.
Closely related compounds have similar chemical properties that may prove difficult to separate based on polarity, charge, etc.
Using a subsequent LC technique, the similar basicity between the peptides can be further separated by employing differences in apolar character.
Compared to comprehensive 2D-LC, heart-cutting 2D-LC provides an effective technique with much less system setup and a much lower operating cost.
Based on linear solvent strength theory (LSST) of gradient elution for reversed phase chromatography, the relationship between retention time, instrumental variables and solute parameters is shown below.
[18] While a lot of pioneering work has been completed in the years since 2D-LC became a major analytical chromatographic technique, there are still many modern problems to be considered.
GCxGC provides a high range of sensitivity and produces a greater separation power due to the increased peak capacity.
