GC columns can be divided into two types which are packed column and capillary column. Capillary column is also known as open tubular column. Packed column normally has the length of 1m to 4m long while capillary column has a great range of length which is 10m to 100m. The internal diameter (I.D.) of packed column is 2mm to 4mm whereas the I.D. of capillary column is relatively smaller which is 100Â Î¼m to 500Â Î¼m.
Figure : Types of capillary columns
Packed GC column
This type of column is easy to be made and used. It has limited resolution (N<8000). The outside of GC column is usually made up of either stainless steel or Pyrex glass. Glass column is better because it is inert and hence will not interact with compound but it is fragile and can only withstand limited pressure. For elevated pressure and long packed column, stainless steel is favoured. If the column is short, it can be installed vertically in the chromatograph. On the other hand, for longer columns used, they can be U-shaped and if longer than 1m, normally they are coiled. Sometimes, an appropriate silanizing reagent is used to treat the glass columns so that can eliminate hydroxyl groups on surface because they can contribute to asymmetric peaks or can be catalytically active. Meanwhile, for stainless steel column, they are usually washed with dilute hydrochloric acid. After that, washing is followed by extensive water and lastly washed with acetone, methanol, methylene dichloride and n-hexane in order to eliminate any corrosion products or lubricating agents used in the tube drawing process.
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The inside of GC column is tightly packed with inert solid support. The solid supports have high surface area. There are different solid support materials of different Mesh or particle sizes. Typical solid support materials are usually silica material such as Chromosorb W, Celite and Diatoport. Stationary liquid phase is then coated on the solid support. The amount of stationary liquid phase coated is 3 to 10% by weight of the solid support.
As mentioned earlier, packed GC columns are easily made. This can be illustrated on the synthesizing of 10% SE-30 on Chromosorb W of 60-80 Mesh. By using dichloromethane as solvent, 1g SE-30 is dissolved in the solvent and then followed by 9g of Chromosorb W. The mixture is stirred homogeneously and then the solvent is evaporated by using rotary evaporator. By this, the stationary phase is then coated on the surface of the solid support material. It is then following by further drying in an oven and packing the material into column. Conditioning the column in the GC oven at temperature near to the upper temperature limit of the stationary phase used can be done by allowing the carrier gas to flow through the column for one day. The column must be "baked" to make sure the column to be clean.
Capillary column can be further divided into two types: wall coated open tubular column (WCOT) and porous layer open tubular (PLOT). This type of column is mostly used in GC. In the analysis of gas and the separation of hydrocarbons which have low molecular weight, the PLOT columns are popular to be used. The outside of the capillary column is made up of fused silica material. This material is inert and strong. Besides that, capillary columns are very flexible and therefore long column can be wound into very small coils. During processing the tubes of the column, those tubes are coated with a polyimide resin that can obtain mechanical strength as well as prevent the atmospheric erosion.
In the inner of the capillary column, it is open tubular. It has very low resistance to flow. This is an important characteristic that has advantage over packed column because it allows a very long column to be used and thus will increase the efficiency or alternatively, only very short column used but can function at very high mobile phase velocities and thus separation can be done very fast. The stationary phase is a thin and uniform liquid that bonded directly to the inner walls of the column but not solid support material like in packed GC column. It is coated on the wall as a thin film. The thickness of film varies from 0.25Î¼m to 1Î¼m.
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Capillary column is better compared to packed GC column because it has higher resolution. Meanwhile, only small amount of sample is needed for analysis and it produced more peaks in shorter time.
In GC, there must be 2 phases which as mobile phase and stationary phase. Mobile phase is the inert gas while the stationary phase can be a liquid, solid or gel. The most common used stationary phase is in liquid form. To run a GC, the choice of stationary phases is very important. Different samples need to be conducted under different types of stationary phases. This is critical for packed columns but less so for open tubular columns because of high efficiency. Stationary phases must have the ability to separate every compound of mixture from other compound. The criterion to be a good stationary phase is that it should be non-volatile at high temperature of operation of the column. Therefore, most of the stationary phases are polymers since polymers are quite inert. Besides, it must also be inert to any kinds of chemical reaction with solutes. Otherwise, chemical reactions happened will cause separation cannot be done.
Generally, stationary phases can be characterized into 4 types: paraffinic stationary phase, dilute stationary phase, concentrated stationary phase and specific solvents stationary phases. Each type of stationary phase will be discussed more detailed at the following paragraphs below.
Paraffinic stationary phase
From the name itself, we can know that this type of stationary phase is mainly made up of non-polar and non-polarizable groups. It includes all paraffin or mixtures of paraffin. Some carbon black and alkyl silicones like polysiloxane are also grouped under this type of stationary phase. The most common reference non-polar stationary phase is squalane which has the molecular formula of C30H62. It is easy available and totally non-polar. Other examples are Apiezon grease and silicone gum rubber ( R2SiO)2. SE-30, OV-1 and OV-101 are organosilicone polymers that also been used as paraffinic stationary phase. Apiezon is quite common used in the chromatographic analysis of hydrocarbon and other organic substances. There is a new standard of non-polar stationary phase that produced by catalytic hydrogenation of Apiezon grease is better than squalane because it can withstand operating temperature of 200oC and therefore allow column polarities at higher temperature can be evaluated.
Figure : Squalane File:Squalan.svg
In this type of stationary phase, the polarity of solute is insignificant because it will not affect the retention and the retention is actually influenced by the size of solutes. Retention of solute occurred due to the dispersion forces. Dispersion force is solely dependent on size of molecules.
As stated that retention of solutes depends on the size, thus the larger the size, interaction approaches to stationary phase and solute increases and therefore the retention is greater. For example, two alkanes have the same number of carbons but differ in arrangements where one of them is linear and another one is branched. The linear alkane will experience greater retention compared to the branched alkane due to the larger size. The more branching an alkane has, the smaller the retention will be because the alkane will become more and more compact when the number of branching increases.
Since polarity will not have influence on the retention time, therefore polar solutes are also separated according to the size. The size of solute is directly proportional to its boiling point. However, polar solutes will have higher boiling point because it will occur hydrogen bonding.
Polysiloxane is the most common stationary phase. In fact, it is also the most stable stationary phase. The most basic polysiloxane is 100% methyl substituted. If it is not fully methyl substituted, the amount of substitution of other groups will be indicated as the percent of the total number of groups. For example, it has10% phenyl group and 90% methyl groups if it is a 10% diphenyl-90% dimethyl poplysiloxane. If the percentage of methyl is not stated, it is understood that the necessary amount required in order to make 100%. For instance, 55% phenyl-methyl polysiloxane contains 25% methyl groups. However, the percentage of cyanopropylphenyl can be misleading due to situation of cyanopropyl and phenyl located at the same silicone atom. Therefore, their amount are summed and indicated as summed percentage. Hence, a 16% cyanopropylphenyl-dimethyl polysiloxane means that there are 8% cyanopropyl group and 8% phenyl group whereas the remaining 84% is the methyl group. Basically, siloxane has good thermal stability. It is also resistant to oxidation and the most important is that it can be synthesized with many different types of organic groups to get a big range of selectivity.
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Dilute stationary phase
This type of stationary is made up from combination of non-polar and polar groups. However, the proportion of non-polar groups is larger. There are three types of solvent polar groups. First type is purely polar group such as nitro (-NO2) and nitriles (-CN). Purely polar group will provide very strong retention. Second type is -OH group. This group provides weaker retention compared to purely polar groups but if the solutes are hydrogen donors, extra retention will be obtained due to hydrogen bonding. The third type is ether or ester group. It provides moderate retention on polar solutes but can form hydrogen bonds very well. Examples of dilute stationary phase are organic esters, alkanoids and ethers. OV-17, OV-22 and OV-25 also belong to this group of stationary phase. They are different in the percentage of phenyl methyl siloxane.
http://www.chemicalbook.com/CAS/GIF/26761-40-0.gifDinonyl phthalate Structure
Figure 4: Di-isodecyl phthalate
Figure 3: Di-nonyl phthalate
Figure 6: Dibenzyl ether
Figure 5: Alkyl aryl sulfonate
Figure 7: Tricresyl phosphate
In the paraffinic stationary phase mentioned before, the polarity of solutes does not play role in retention. This situation is changed when using dilute stationary phase since there are some polar groups inside. Due to "like dissolves like", the retention of polar solutes is not only depending on size but also specific interaction between solute and the polar group in stationary phase. Yet, this specific interaction is not that great because there are not many polar groups in the stationary phase. However, alkanes are separated as before which are according to size.
Concentrated stationary phase
Same as dilute stationary phase, this type of stationary phase is a combination of non-polar and polar groups. Now, the proportion of polar groups has increased and occupied the majority of molar volume. Due to this reason, there exists very strong retention for polar compounds. For example, polyethylene glycol and polyesters. For concentrated stationary phase which contain cyanopropyl group such as SILAR 10C and SP-2340, they are useful in separation of fatty acid methyl esters of palm oil.
Alkanes and other hydrocarbons are non-polar, they have very small retentions in these polar solvents and thus this type of stationary phase is not suitable for separation of hydrocarbon. It is still possible to separate hydrocarbon according to size due to dispersion interaction. On the other hand, it is very useful to separate polar solutes because there will be large retention in these solvents.
Since polyethylene glycol is a frequently used concentrated stationary phase, it will be discussed more detailed. For stationary phases which have "WAX" or "FAPP" in their name are some types of polyethylene glycol. Polyethylene glycol is not substituted by other materials, therefore it is 100% polymer of the stated material. Polyethylene glycol is a moderately polar material. Polyethylene glycol has lower temperature limits compared to polysiloxane and it is less stable. Other than that, its lifetime is not long. If there is over heating or over exposure to oxygen, it tends to damage. Nowadays, these deficiencies have been overcome by the cross-linked polyethylene glycol available. It has good ability to separate polar compounds such as amino alcohol, alcohol, amines nitriles and etc. This stationary phase must be in liquid phase under GC temperature conditions. Polyethylene glycol has the oxygen backbone which provides a strong polar dispersive interaction in the phase. This also causes strong dipole interaction as the phase itself is capable to undergo hydrogen bonding. If the analysis of acid solutes is conducted especially for free fatty acids, an acid-treated version of polyethylene glycol is used.
Figure : Structure of polyethylene glycol
Specific solvents stationary phase
This is a type of stationary phase in which chemical can react specifically with a particular class of solute. For example, silver nitrate, AgNO3 dissolves in a stationary phase will cause alkenes and dienes to have large partition coefficient because there occurs formation of complexes between silver ion, Ag+ and C=C double bond.
To separate between m, p-isomer like xylene, we need specific solvents and liquid crystals stationary phase can be used here. Nematic liquid crystal-rod-shaped molecules are arranged parallel in layers. P-isomer is more rod-like shaped and thus will reside more longer in nematic liquid crystal compared to m-isomer. Therefore, p-isomer has longer retention time if nematic liquid crystal is applied.