Question

1. Explain the various factors responsible for separation of components of a mixture in chromatography.
2. Explain about the classification of chromatography with few examples.
3. Draw a block diagram of gas chromatograph and explain the function of each component
4. Write a note on the micro syringe use to inject the liquid sample into gas chromatograph.
5. Write a note on principle of working of Gas chromatograph. (How to prepare GC for analysis)
6. Write a note on analysis of chromatogram (Types the method of analysis)
7. Write a note on applications of GC analysis in petroleum and petrochemicals industries.
8. Why do you think GC is very popular analytical technique?

Answer 1

1. Various factors responsible for separation of components of a mixture in chromatography are-

a) Mobile phase-mixture interaction is one of the most important two factors. Based on interaction with the mobile phase, components tend to get eluted. Greater interaction leads to faster elution & reverse for lower interaction. Polarity is the property that is harnessed in chromatography. If polarity oof mobile phase matches with any component, that travels after than another component whose polarity is different from that of eluent.

b) Stationary phase-mixture interaction is another most important factor. Variety of stationary phase-component interaction causes variation in component elution, that plays major role in separation.

c) Time of elution is also responsible. Generally, due to interaction with stationary phase, components may get struck if long time is spent in elution.'

d) Packing of columns affects elution & separation. Uniformly packed columns tend to separate mixtures with greater efficiency.

2. Classifications are-

• 1. Techniques by chromatographic bed shape:
  • 1.1 Column chromatography
  • 1.2 Planar chromatography
    • 1.2.1 Paper chromatography
    • 1.2.2Thin-layer chromatography (TLC)
• 2. Techniques by physical state of mobile phase:
  • 2.1 Gas chromatography
  • 2.2 Liquid chromatography
• 3. Affinity chromatography:
  • 3.1 Supercritical fluid chromatography
• 4. Techniques by separation mechanism:
  • 4.1 Ion exchange chromatography
  • 4.2 Size-exclusion chromatography
  • 4.3 Expanded bed adsorption chromatographic separation
• 5. Special techniques:
  • 5.1 Reversed-phase chromatography
  • 5.2 Hydrophobic interaction chromatography
  • 5.3 Two-dimensional chromatography
  • 5.4 Simulated moving-bed chromatography
  • 5.5 Pyrolysis gas chromatography
  • 5.6 Fast protein liquid chromatography
  • 5.7 Counter current chromatography
  • 5.8 Periodic counter-current chromatography
  • 5.9 Chiral chromatography
  • 5.10 Aqueous normal-phase chromatography

3. Block diagram of gas chromatograph looks like this-

Sample injector Flow controller Waste Column Detector Carrier gas Column oven

a) The eluant (carrier gas) is introduced from a gas cylinder outside the machine. It's called the carrier because that's exactly what it does—carry the sample being studied through the machine. In gas chromatography, the carrier gas is the mobile phase.

b) Flow controller is the moderator that operates flow rate of the carrier gas for optimum separation.

c) Sample injector is used to inject liquid sample with mixture through the injection port & the sample instantly vaporizes at high temperature.

d) Column is used to separate the mixture into its components. The column is a very thin (capillary) tube, sometimes as much as 30–60m long, coiled and entirely contained inside an oven that keeps it at a high enough temperature to ensure that the sample remains in gas form. The temperature of the oven can be carefully controlled.

e) As the sample separates out and its constituent gases travel along the column at different speeds, a detector senses and records them. Various different detectors can be used, including flame ionization detectors, thermal conductivity detectors, and mass spectrometers (usually separate machines).

f) The data analyzer attached to the machine draws a chromatogram with peaks corresponding to the relative amounts of the different chemicals in the sample.

4. Microsyringe:

The sample is introduced into a heated small chamber via a syringe through a septum – the heat facilitates volatilization of the sample matrix. The carrier gas then either swiftly sweeps the entirety (splitless injection mode) or a portion (split injection mode) of the sample into the column. In split mode, a part of the sample/carrier gas mixture in the injection chamber is exhausted through the split ventilator. Split injection is preferred when working with samples with high analyte concentrations whereas splitless injection is appropriate for trace analysis with low amounts of analytes. In splitless mode, the split valve opens after a pre-set amount of time to purge heavier elements that would otherwise contaminate the system. This pre-set (splitless) time should be optimized, the shorter time ensures less tailing but a loss in response, the longer time increases tailing but also signal.

5. Gas chromatograph working principle-

Tiny sample of the mixture of substances being studied is placed in a syringe and injected into the machine. The components of the mixture are heated and instantly vaporize. Next, the carrier gas is added which is simply a neutral gas such as hydrogen or helium or nitrogen, designed to help the gases in sample move through the column. In this case, the column is a thin glass usually filled with a liquid that has a high boiling point (either a gel or an adsorbent solid). As the mixture travels through the column, it's adsorbed and separates out into its components. Each component emerges in turn from the end of the column and moves past an electronic detector that identifies it and prints a peak on a chromatogram. The final chromatogram has a series of peaks that correspond to all the substances in the mixture. Gas chromatography is sometimes called vapor-phase chromatography (VPC).

6. GC process involved two types of analysis of its chromatogram-

a) Chromatographic data is presented as a graph of detector response (Ordinate) against retention time (Abscissa), which is called a chromatogram. This provides a spectrum of peaks for a sample representing the analytes present in a sample eluting from the column at different times. Retention time can be used to identify analytes if the method conditions are identical. Also, the pattern of peaks will be constant for a sample under identical conditions and can identify complex mixtures of analytes. This is the qualitative analysis mode.

b) The area under a peak is directly proportional to the amount of analyte present in the chromatogram. By calculating the area of the peak using the mathematical function of integration, the concentration of an analyte in the original sample can be determined. This is the quantitative analysis mode.

7. Application of GC in Petroleum & Petrochemical industry is as follows-

Gas chromatography has proved to be one of the most important analytical tools in the area of petroleum hydrocarbons. With the invention of the multidimensional column switching system and hyphenated techniques GC has further proved to be an indispensable technique for the industry in process control, process development, monitoring the product quality and formulating specifications as well as waste and pollutant analysis. In petroleum industry GC finds wide application to estimate the potential of crude oil for lighter hydrocarbons, naphthas, higher fractions, and specific products.

a) Lighter Hydrocarbon Analysis- Analysis of gaseous hydrocarbon is required in various refinery streams, petrochemical industry, gas and oil condensate in fields, exhaust and, mine gases etc. Natural gas analysis is one of the important analyses in Petroleum industry. Estimation of C1-C5 hydrocarbon in gasoline was one of the first gas-liquid chromatographic applications in petroleum industry for monitoring gasoline volatility.

b) Analysis of individual HC Upto C8- One of the most difficult and time-consuming analyses confronting petroleum and petrochemical laboratories is the determination of individual HC in gasoline fraction up to 180°C. More than 200 compounds of wide chemical nature, consisting of normal and branched paraffin, mono and bicyclo paraffin and mononuclear aromatics exist normally in straight run naphtha.

c) Separation of Mono-Aromatics- Petroleum streams are seemingly an endless source of aromatic hydrocarbons for a number of industries, hence their qualitative and quantitative estimations in a straight run, reformed and cracked naphtha, gasoline are desirable. Monoaromatics in gasoline can be analyzed using a polyethylene glycol capillary column.

d) Analysis of n-Paraffins in Kerosine- The determination of normal alkanes distribution in hydrocarbonm~ture is of considerable importance in the petroleum industry. Normal alkanes from petroleum sources are important feedstocks for petrochemical industries which can be swiftly purified using GC techniques. Long-chain alkanes can be converted to lubricant and fuel additives, plasticizers, industrial surfactants, etc.

8. GC is popular because of its efficiency in environmental monitoring and industrial applications with reliability, accuracy & low expense. Coupled with Mass Spectrometer, GCMS becomes a much more powerful technique for complex compound analysis.