ASAH PEMAHAMAN (prepare for examination)

1. Split/splitless injector 

- Split injector adjusted for high concentrate substance. In this mode, the sample solution is pass trough the injector and only a small amount of evaporated sample pass trough into the column. The gas flow of this method is continuously flows in the column. 

- Splitless injector adjusted for lower concentration substances. In this mode, two different modes are release. A stand by mode is before the injection started and the split vent is opened. it's means that the main part of the gas flows is leave trough the split vent. 

Then when injection is started, sample injected and GC system is start- the split vent is closed and all of the evaporated sample are injected into the column.

2. Flash evaporation 

- Flash evaporation is a process which some liquid sample instantly boils-or flashes- after it has been heated at high temperature system. It is a characteristic evaporation on GC system. The solution sample which contains of analyze substances are flash evaporated when it were injected into the column trough the hot injector. 

3. Thermal stress 

- To running of GC system, we need temperature programming. On temperature programming, GC system is set in range between 60 - 300 degree Celcius. Because of that, every substances which would analyze by GC should be thermal stable. 

4. Capillary column 

- Capillary column is one of the example of column type in GC system. Capillary column is built by thin silica glass tube. The length of this column is between 10 - 150 m, but column which has length between 25 - 60 is commonly used. Diameter of this column is between 0.1 - 0.53 mm and has a film thickness in range 0.1 - 5 micrometers. Capillary column has higher separation capacity than packed column but it has lower substance capacity than packed column. Capillary column is mainly used for complex sample separation. 

5. Carrier Gas

- Carrier gas or commonly called mobile phase is contains of pure gases or inert gases. It should be purely to avoid of background noise. Helium is preferable uses as carrier gas on GC system. It cause He is highly inert, non-explosive and deliver reasonable chromatographic efficiency. If we consider of van Deemter curve Helium's uopt is at a higher average linear velocity but less efficiency. Helium's van Deemter curve is much flatter that nitrogen, then changes in the average velocity do not decrease efficiency by a large amount.  Nitrogen provides the best efficiency on Van Deemter curves; however its uopt is at a low average linear velocity. The low average velocity make it the least desirable carrier gas for capillary GC. We also can explain that Nitrogen has lowest HETP then it have high separation capacity than the other.  

Hydrogen's uopt is the highest than three common carrier gases, we also can see that the van Deemter curve of Hydrogen is very flat. The high of uopt Hydrogen means that Hydrogen has shortest analysis times than Nitrogen and Helium. Besides of that, the wide range of average linear velocity of Hydrogen make it as the best carrier gas for analysis sample which has wide temperature range by GC. But, we can't choose it as a carrier gas because Hydrogen would be explosive if it react with Oxygen. 

 
http://www.chem.agilent.com/cag/cabu/carriergas.htm

6. Oven Temperature
GC system used "temperature programming" to runs of the analysis. Temperature program means that we can set the temperature from the lower to higher temperature. Usually, the temperature program which used in GC system is :

60 degree (1 min) - 10 degree (per min) - 280 degree (15 min)

its means that we should set the initial temperature of GC system at 60 degree when liquid sample injected trough the injector and make it increases 10 degree per minute until reach the temperature at 280 degree Celcius.

Temperature programming is one of the efficient ways to separates of closely related compounds. When the temperature of column increases, then vapor pressure of component would be increases and make the equilibrium of dissolve component in the mobile phase and in the stationary phase leads to the gas phase. This condition would decreasing of the retention time of substances. By temperature program, the temperature of the system would be increase gradually and it cause the substances of the sample would be separates based on their retention times.

Beside of temperature program, GC system also has isotherm condition.Temperature of this condition is sets at 175 degre Celcius. 

_wish me luck for exam_ Amin...