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...

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 :)  

Here I am :)

For some people, going study to another city or going study abroad is one of the most challenges which should be grabs. Of course, if they are going study to another city, they will get more experience, gain their mind set, and get a lot of friends. Others fell that going study abroad gives them a kind of prestige in their home country. In the same way, it may lead to a better job after they finish their study.

But living and studying in another city can also have its problems. Some people have assumes that going study to another city will be make them feels homesick and they also need a long time to adapt. The other crucial aspect is the living cost in the new city. For some people, it will be a problem if they are not prepares well for example if they have to pay their living cost by them self not by scholarship. And a few people go home earlier than expected because their ideas about living or studying in another city were different from the reality.

Thus for me, I just want to grab my future by studying in another city or even another country. I expect that by studying in another city I will gain more my skill and get more experience and of course getting well on learning foreign languages. For this reason, here I am, in Braunschweig, Germany.