GC (Gas Chromatography)

Gas Chromatography (GC) is one of common separation method which used to separates of volatile compound or it must be vaporisable without degradation and preferable use to non-polar substance. This method runs by interaction of mobile phase (gas) and stationary phase (solid particles or liquid film).

GC is running by temperature program or temperature dependent which modified from the lower temperature to higher temperature. At the end of analysis, chromatogram will appear as a result of analysis. From the chromatogram we can see the retention time of every substances in the compound. Every substances has different signal and different retention time. 

Gas Chromatography is consist of the following assembling such as: 

1. Carrier gas supply

2. Pressure Pump 

3. Injector Tube 

4. Oven which include with column 

5. Detector 

6. Signal registration and data processing

* Carrier Gas Supply 

As a carrier gas, He; N2 or H2 are commonly uses. Carrier gas must have a high purity then it would minimize of background noise and avoid reaction of the samples with stationary phase. 

The mobile phase is responsible to transport the substances into the GC system. Only if the mobile phase (gas flow) optimized the separation capacity of a capillary column will reach its maximum. Separation capacity of sample is depend on the speed of carrier gas and HETP calculation. Lowest HETP number has higher separation capacity. The flows of mobile phase is adjusted in a range of 0.2 to 50 mL/min depending on the column type. 

* Injector Tube 

The sample analyze should be injected into the column trough the injector tube manually (by microliters syringe) or by auto sampler. The volume of sample which is injected into the column is 1-2 microliters. Trough the injector tube, sample solution is "flash evaporation" at the temperature about 220 - 300 degree Celcius. After that, sample solution will transferred into the carrier gas flow.  

The common uses injection method are: 

1. Split injection : in this method, the gas flow is continuously split and only a small part of the evaporated sample which injected into the column. For example, a split ratio of 1:10 means that 1 mL/min gas flows through the column and 10 mL/min trough the split vent. If 1 microliters sample solution is injected, then only 0.1 mikroliters are transferred into the column. This injection technique is used to the sample which has high concentration. 

2. Splitless injection : this method is opposite to split injection. In case of a splitless injection two different modes are passed trough. At stand by mode before the injection is started the split vent is opened, means that the main part of the gas flow leaves the GC through the spli vent. When the sample injected and GC-analysis is starts the split vent will be close (splitless mode) and the evaporated sample solution is transferred into the column. Therefore this injection mode is mainly used for the residue analysis of medium - to low - volatile analytes. 

* GC Oven 

After sample solution injected, it would be pass into the column. In the GC oven, the column and carrier gas are heated and the analytes are transported as a gases. At the isothermal GC, the oven temperature is constant during the whole chromatographic process. For more complex substances, gradient temperature are needed. The gradient temperature is adjusted by "Temperature Programming". On this condition, a start and end temperature is set normally in a range 60 and 300 degree Celcius. 

* GC Column 

   GC system has 2 different column types, Packed column and Capillary column.

- Packed Column is filled by carrier/porous material and covered with a liquid layer. This column is mainly used for separation of low complex mixtures at relatively high concentration. This column has low separation capacity if compared to capillary column. 

- Capillary column are built of very thin silica glass tubes. The length of this column is between 10 -150 m, but the column which has length range of 25 -60 m commonly used. Capillary column is coated with the stationary phase or a thin layer of solid particles. This column has higher separation capacity but lower substance capacity if compared to packed column. This column is mainly used for the separation of complex sample for the analyses of substance traces. 

There are 3 steps which release in Oven GC : 
a. Sample vaporized
b. Sample transferred into the column 
c. GC oven program initiated 

Analytes are pass trough the column and separates by 2 factors : 

- Sample volatility : 

   Volatile analytes will move faster through the column  

- Sample polarity 

   Polar analytes interacts more strongly with polar column. 

* Detector 

a. Thermal Conductivity Detector (TCD) 

TCD is consist of two cells, each containing a heating wire in different gas flows and in stand by mode both cells are passed by pure carrier gas. The difference of thermal conductivity between stand by mode and analysis mode should be measured as a signal detection.

b. Flame Ionization Detector (FID) 

The flame is burning in an electrical field between the flame nozzle (cathode) and the collector anode. Ions that are formed in the flame are detected as current flow and detected as signal. FID is non-selective method so potential to other compounds in a sample. It also response to most hydrocarbon but insensitive for another hydrocarbon likes Hydrogen gas, Helium, Nitrogen gas, Carbon Monoxide, Carbon Dioxide, Water, Oxygen, Ammonia and Nitro Oxide.Very sensitive for wide range component between 0 - 10000 ppm and detection limit : 0.1 ppm and commonly used in GC.

c. Thermal Ionization Detector (TID)

TID or NPD (Nitrogen-Phosphor Detector) is modified FID and has similar works. Used for selective detection of nitrogen and phosphor compounds which contains an alkaline salt pearl (as active element). 

d. Electron Capture Detector (ECD)

ECD is applied in trace analyse and mainly used for detection of halogenated and nitrated compounds as these compounds are very sensitive detected. This detector is contains of metal foil which coated by radioactive nickel isotope.  

   

Note: summaries from Practical Course Analytical Chemistry of Technische Universitat Braunschweig, Institute of Environmental and Sustainable Chemistry and also from the class lecture :)