1. Sample preparation

The sample has to be as dry as possible. In addition, the boiling point of the compound should be low enough in order to be suitable for this technique. In other words, it makes very little sense to inject polymers, proteins, sugars or other high molecular compounds, because they are usually not volatile enough unless they are functionalized i.e., using trifluoromethyl (-CF₃) or acetyl groups (CH₃C(=O)-) to make them more volatile (see example above for carbohydrate derivatives).

The sample should be completely dissolved in a low boiling, weakly polar solvent like diethyl ether, dichloromethane, hexane, etc. This way the solvent will elute first and will not interfere with the rest of the spectrum (see in isoborneol spectrum). The process should be reversed if low boiling compounds i.e., gasoline were analyzed. A typical concentration should be ~1-5 mg/mL for FID and GC/MS systems, and higher for TCD systems because they are less sensitive. The sample should never be injected in neat form! The GC samples cannot contain any solids.

2. Sample introduction (manual injection)

The sample solution is introduced through the injection port, using a micro-liter syringe. The injection port is very hot in order to allow for the solvent and the compounds evaporate instantaneously. The syringe has to be cleaned thoroughly before it is used. To load the syringe, the plunger is slowly pulled to draw the sample from the solution. If the plunger is drawn too fast, a lot of air will be drawn into the syringe as well, which makes it necessary to start over. The loaded syringe taken out of the sample solution, and the plunger then pulled a little more to suck the liquid out of the needle. This way, the sample solution is not in the needle anymore. The syringe is brought to the injection port and the needle pushed through the septum. Then, quickly and smoothly the plunger is pushed down to inject the sample. Afterwards, the syringe needle is pulled out of the septum and the injection port. Care has to be taken to avoid the bending of the plunger by applying too much force during the injection. A bent plunger causes leaks in the syringe and makes it difficult to draw the solution afterwards. After the injection, the syringe should be immediately rinsed with a low boiling solvent. This also avoids the build-up of a residue on the inside of the upper part of the syringe, which is very difficult to remove.

Normally, the amount of sample entering the column is rather small because most instruments are operated in the split mode to avoid the overloading of the column (meaning only a small amount usually enters the column, ~1-2 % typically). Only in cases of a very low concentration sample the splitless mode is used.

3. Temperature issues

a. The temperature of the injection port has to be ~10-20 ^oC above the boiling point of the compounds to be investigated to ensure complete evaporation upon injection. The injection port is usually heated to 200-300 ^oC. The column temperature has to be above the dew points of the compounds to prevent condensation in the column.

b. The temperature of the oven can be kept constant (isothermal), or gradually be changed (gradient). The second method often provides a better result due to the fact that the differences in boiling points can additionally be used to separate the compounds. The separation is greatly improved, the peaks are less broad, and the higher boiling components leave the column much earlier (see example discussed above).

c. Most FIDs require a certain minimum temperature in order to ignite (>150 ^oC). One reason for this measure is to avoid condensation of water in the detector.

d. To ensure an acceptable lifetime of the column, the column should not be. The absorbent may decomposes or boil out and deposit in the detector. This would destroy the column and the detector at the same time. Every column has a tag attached that states its composition and operation temperatures. If the experimenter was not sure which column was installed at a given point, the tag on the column should be checked. Despite proper care, most columns have a very limited lifetime since the stationary phase gradually degrades upon heating i.e., column bleeding, thermal degradation of the stationary phase, etc.

4. Gas flow

A high carrier gas flow produces sharp peaks, but a poor separation. A very slow flow rate leads to significant peak broadening and causes a bad separation due to increased overlap of peaks. For a given problem, this parameter has to be optimized as well.

5. Columns

Capillary columns (WCOT=wall coated open tubes) generally have higher total number of theoretical plates (25,000-200,000) compared to packed columns (1000-5000). However, their capacity and the flow rates are much lower than for packed columns. Column material can greatly vary in their polarities and their upper temperature limit.

Non polar: Squalane (branched hydrocarbon), 150 ^oC; 100 % methylpolysiloxane, 300 ^oC; 5 %-phenyl column on diphenyl methylsiloxane (HP-5, DB-5), 320 ^oC

Moderately polar: 14 % cyanopropylphenyl/86 % methylpolysiloxane, 300 ^oC; phenylsiloxane carborane, 370 ^oC

Polar: 50 % cyanopropylphenyl/50 % methylpolysiloxane, 275 ^oC; polyethylene glycol, 225 ^oC

Highly polar: 70 % cyanopropylpolysilphenylenesiloxane, 250 ^oC; poly(diethyleneglycol succinate), 200 ^oC

Chiral: 20 % -cyclodextrin on methylpolysiloxane, 220 ^oC

6. Maintenance

One very important point to make here is the proper maintenance of a GC or GC/MS. Those instruments cost anywhere between $80-120K (more than most luxury cars!) and need to be properly used and maintained. Changing a septum, cleaning the liner and injection port itself, baking the column (to remove the remaining compounds which is highly advisable after installation and prior to its use), providing a high quality gases, column changes, etc. significantly enhances the performance and the lifetime of the instrument. A person with a basic technical understanding like a technician or person with a B.S. in chemistry or biochemistry should be able to perform these tasks.

7. Glossary

8. Applications

• Environmental laboratories i.e., determination of volatile organic compounds in water or solid
  
• Food industry i.e., quality check for additives and flavoring agents (i.e., identifying flavoring agent in wine: linalool, citronellol, nerol, geraniol)
  
• Cosmetics i.e., test of fragrances
  
• Forensics i.e., identification of drugs and trace components like toxins
  
• Pharmaceutical industry i.e., purity test for intermediates and final products
  
• Petroleum industry i.e., detection of sulfur compounds in gasoline and natural gas
  

9. Useful links

GC Troubleshooting