Multidimensional Gas Chromatography The use of two or more columns to resolve a sample (or using a single column in more than one direction. Current column technology is very near the theoretical limit. However, its still not possible to resolve all components in a complex mixture.Examples of complex samplesTobacco smokeOver 1000 peaks identified - each actually can contain two or more components.PCBs207 species but only 180 resolvedCoffeeOver 600 components identified.Multidimensional GCElemental specific and mass detectors are helpful but cant always solve the problem.ExampleMS cant tell o-, m-, -p substitution or cis from trans.Multidimensional GC can assist in some cases and relies on relatively inexpensive technology.Multidimensional GCMethod relies on passing a portion of a columns effluent to a second column using flow switching.injectionportcolumn 1column 2switchingvalvedetector 1detector 2Multidimensional GCheart-cutregioncolumn 1column 2Why bother?Basic assumption is that no single column can resolve all components of interest.Possible choicesBest - if single column/analysis can do the job.Second best - use two separate assays with different conditions or columns.Last resort - multidimensional GC.Why bother?Sample is limitedYou need all of your data from a single runTime is limitedWhile a multidimensional run can be longer than a single assay, its still shorter than two separate runsEquipment is limitedOne GC setup can do the entire assayApproachesEnrichmentUsed to increase amounts of trace componentsHeart-cuttingGrabs an unresolved portion of a sample for improved separationBackflushingReverse column flow to drive off highly retained componentsEnrichmentPre-concentration of trace components initially on a packed column.More sample can be placed on a packed column than a capillary.Only the trace components of interest are passed to the capillary column.Results in more sample being introduced.Enrichment exampleenriched region assayed on column 2column 1HeartcuttingAlso called “cut and transfer.”No single column can resolve all components of interest or a very large peak masks other components.Passing the unresolved area to a second column can be used to fix the problem.The second column can also be a different polarity.Heart-cutting examplecolumn 1column 2Latex Balloon VolatilesLatex Balloon VolatilesBackflushingUsed when you have a sample that contains both volatile and relatively non-volatile species.Total analysis in one direction would take forever.Only a single column is needed.Backflushing - normal modeinjectionportdetectorIn normal operation, flowoccurs as one would expect.After all faster eluting species have evolved, thevalve is switch, reversing the flow.Backflushing - reverse modeinjectionportdetectorNow the higher MW species will evolve.In effect, we are only using the first portions of the column to do our separation.Backflushing ExampleMethod uses single GC ad columnForward analysis using a TCD detector to assay nonflammable gases.Heartcut part of the sample to a 2nd TCD.Backflushing to an FID to assay flammable gases.Backflushing ExampleBackflushing ExampleDual column exampleRestek describes the use to two capillary columns used in parallel to separate gases and volatile components.5A molecular sieve column for gases(Rt-Msieve 5A PLOT column, 30m 0.32 mm ID)A bonded porous layer polymer for volatiles.(Rt-QPLOT, 30m, 0.53 mm ID)Dual column exampleFrom injectorTo detectorColumn 1Column 2Special fitting tosplit/recombineflows.Dual column example1 2 1 - He2 - Ar3 - O24 - N25 - CH46 - CO27 - COBackflushing example!Determination of dissolved gases in transformer fluid - ASTM methods D3613, D2945, D3612!Carbon oxides are catalytically converted to methane for detection as methane using a FID.!Elemental gases are detected using a TCD.!Backflushing is used to remove transformer fluid from one column while results are being obtained from the other.The plumbingExample