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Gas Chromatographic Separation Methods

A gas chromatograph typically has a carrier gas (it is common to refer to the mobile phase in gas chromatography as the carrier gas since the mobile phase has no influence at all over the separation – its only purpose is to carry the solutes through the column), heated injection port, column in a heated oven, and detector. 

The carrier gas must be inert and helium or nitrogen is used for most gas chromatographic applications.  Sample injection is most commonly accomplished through the use of a small-volume syringe (1 μL is a typical injection volume).  The liquid sample is injected into a hot zone that is sealed with a polymer septum.  The syringe pierces the septum, and on injection the liquid sample is flash-volatilized in the heated injection port and flushed into the column by the carrier gas.  With fused silica capillary columns, 1 μL of liquid sample will overload the column so the injector is designed with a splitter.  Adjusting the flow of the splitter gas allows you to calibrate the split ratio.  Ratios of 50:1 to 100:1 (only the one part is injected) are common.  Most injection ports of this type have glass liners.  Different glass liners are inserted under the septum depending on whether you are doing split or splitless injections.

Another gas chromatographic injection technique that is used in certain applications involves thermal desorption from a polymer trap.  The technique is illustrated in Figure 70 for the analysis of organic chemicals in air. Assume that we have a small piece of tubing filled with a polymer. The polymer is a material that organic chemicals will adsorb to. Water in the air will pass through unretained. A polymer that is used quite frequently for this application is Tenax. Tenax is a p-polyphenylene oxide polymer that is stable at very high temperatures.  In the adsorption step, we use a pump to draw some large volume of air (this might be as high as 15 liters of air) through the Tenax trap, which is held at room temperature. The organic chemicals from the air will adsorb onto the polymer.

Figure 70. Use of a sorbent trap to remove organic compounds from an air sample.

The organic chemicals can be desorbed from the Tenax trap by heating the gas under a flow of helium.  The illustration in Figure 71 shows how the Tenax trap is desorbed in a backflush mode (the end that had the higher concentrations of organic chemicals is put closer to the gas chromatograph).  Switching valves comparable to liquid chromatographic injection valves make it easy to redirect the flow of gases through these Tenax traps between the adsorption and backflushing orientation. 

Figure 71. Desorption of a sorbent trap in a backflush mode.

Desorption might typically be done at a temperature of 225oC over a period of five minutes.  One thing to realize is that we would never be able to tolerate a five-minute injection of a sample, since this would mean that all of the peaks might be as long as five minutes.  It is essential to maintain a “plug” injection, which is achieved by thermal focussing.  Thermal focussing is accomplished by cooling a region in the injector (or in older gas chromatographs, the entire oven) to a temperature of –50oC.  This cooling is accomplished using either liquid carbon dioxide or liquid nitrogen.  At –50oC, the organic compounds that desorb from the Tenax trap freeze in a small band at the head of the column.  Rapid heating of this frozen band injects the sample at a comparable rate to a normal syringe injection.

Sorbent traps are frequently used for the analysis of trace levels of volatile organic chemicals in water as well.  In this case, the method is known as a purge and trap technique and is shown in Figure 72.  A sample of water is taken (usually 5 mL – too large to ever inject into a gas chromatograph) and purged with helium gas.  After exiting the water, the helium gas flows through a Tenax trap.  The helium gas bubbling through the water displaces the dissolved volatile organic chemicals, which adsorb onto the Tenax trap.  The Tenax trap is desorbed as described above for the analysis of volatile organic chemicals in air.

Figure 72. Purge and trap device used to analyze volatile organic compounds in water.

Information on a variety of commercially available gas chromatographic stationary phases is provided in Table 3.  The distinction between different stationary phases is based on a comparison of their polarity.  Five different compounds are typically used to represent different types of functional groups.  Individual indices for these five compounds are measured, and a composite value (P) is determined as well.  These polarity indices are referred to as Rohrschneider constants.  The higher the number the more polar the phase.  In reality, a sampling of four or five stationary phases would be good enough to span the range of polarities that is needed for gas chromatographic separations.

Table 3. Rohrschneider constants for gas chromatographic liquid phases

Liquid phase Max T
oC
X' Y' Z' U' S' P
Squalane 100 0 0 0 0 0 0
Apiezon L 250 32 22 15 32 42 29
SE-30 300 15 53 44 64 41 43
OV-1, methyl gum 350 16 55 44 65 42 44
OV-3, 10% phenyl 350 44 86 81 124 88 85
OV-7, 20% phenyl 350 69 113 111 171 128 118
Dioctyl sebacate 125 72 168 108 180 123 130
Dilauryl phthalate - 79 158 120 192 158 141
Dinonyl phthalate 150 83 183 147 231 159 161
OV-17, 50% phenyl 375 119 158 162 243 202 177
Versamid 930 150 109 313 144 211 209 197
Trimer acid 150 94 271 163 182 328 218
OV-25, 75% phenyl 350 178 204 208 305 280 235
Polyphenylether 225 182 233 228 313 293 250
Triton X-305 200 262 467 314 488 430 392
Carbowax 20M 225 322 536 368 572 510 462
Carbowax 4000 200 317 545 378 578 521 468
Reoplex 400 200 364 619 449 647 671 550
Carbowax 1540 175 371 639 453 666 641 554
Diglycerol 100 371 826 560 676 854 657
EGSS-X 200 484 710 585 831 778 678
Ethylene glycol phthalate 200 453 697 602 816 872 688
Diethylene glycol succinate 200 496 746 590 837 835 701
Tetrahydroxyethylenediamine 150 463 942 626 - 893 731
Hexakis(2-cyanoethoxycyclohexane) 150 567 825 713 978 901 797
N,N-bis(2-cyanoethyl)formamide 125 690 991 853 1110 1000 929

X' = benzene; Y' = butanol; Z' = 2-pentanone; U' = nitropropane; S' = pyridine
P = (X' + Y' + Z' + U' + S')/5

If we consider what governs retention order in gas chromatography, there are two important parameters.  One is the volatility of the compound, with the observation that more volatile compounds (those with a higher vapor pressure or lower boiling point) elute first.  The other is the attractive forces between the compound and the stationary phase.  If we consider the homologous series of alcohols listed in Table 4, it’s interesting to note that the boiling point goes up by approximately 20oC for each additional methylene group (CH2) in the chain.  The increase in boiling point with each additional methylene group is a little higher for the series of alkanes.  The alcohols have higher boiling points than the corresponding alkanes because they can hydrogen bond with each other.

Table 4. Boiling points for homologous series of alkanes and primary alcohols.

Hydrocarbon Boiling Point (oC) Alcohol Boiling Point (oC)
Pentane 36 1-Pentanol 137
Hexane 69 1-Hexanol 157
Heptane 98 1-Heptanol 176
Octane 126 1-Octanol 196
Nonane 151 1-Nonanol 215
Decane 174 1-Decanol 231

If we consider a homologous series, it turns out that the boiling points are actually determined by the molar volumes of the molecules.  Just as we observed with the size of molecules in steric exclusion chromatography, the molar volume of a molecule is the volume swept out by the molecule as it tumbles.  Molecules with larger molar volumes have higher boiling points, provided the molecules being compared have identical intermolecular forces (you cannot compare hydrocarbons to alcohols).  An interesting comparison is observed by looking at the boiling point of n-octane and iso-octane (2,2,4-trimethylpentane).

                 

          n-octane (bp = 126oC)                  iso-octane (bp = 98oC)

The branched iso-octane would have a smaller molar volume than the linear n-octane, and this is clearly reflected in the boiling points of the two compounds.

When predicting retention order in gas chromatography, the overriding factor is a comparison of the boiling points.  The compound with the lowest boiling point elutes first.  Only when two compounds have very close boiling points (within 5oC or less) does it become important to consider the polarity of the compounds and the polarity of the stationary phase.  Remember that like dissolves like, so a polar stationary phase will show more retention of a polar compound.  The Rohrschneider values are used to determine the polarity of the stationary phase.