Gas Chromatography

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CHEM-ENG 590E: Microfluidics and Microscale Analysis in Materials and Biology

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Introduction

Chromatography is a technique in the field of analytical chemistry that separates a fluid mixture into its parts for analysis. The field of chromatography is very broad and is used in many different ways. Chromatography can be done on a small strip of paper to large industrial columns for pharmaceutical purification [1]. Types of chromatography include gas chromatography (GC), high performance liquid chromatography (HPLC), size exclusion, ion exchange, hydrophobic interaction, and affinity chromatography. The different types of chromatography, use different properties of molecules to separate and identify mixtures [2].

Figure 1 A high performance gas chromatography machine (center) that is used in conjunction with a mass spectrometer (left) [3].

The analytical tool known as gas chromatography was founded by Archer Martin and Richard Synge in 1941. The basis of this new technique, separating itself from other forms of chromatography at the time, was the concept that chromatography was not limited to just the liquid phase. It took nearly 10 years after the idea of gas chromatography, in 1951, for the concept to proven to be an effective analytical tool [4]. Figure 1 shows a state of the art GC machine that is paired with a mass spectrometer for more streamline analysis.

How It Works

Figure 2 Flow sheet of a gas chromatography system with carrier gas, injector, column, detector and data system [5].

Gas chromatography can be broken down into a flow sheet diagram like in Figure 2. Gas chromatography uses a gas mobile phase and a liquid stationary phase for separation. The gas mobile phase is made up of a carrier gas, usually an inert gas like hydrogen, and the sample. The sample at is injected into a vessel where it is turned into a volatile state using heat to be carried through the machine. Once the sample is ready, it is opened to the carrier gas and sent through the column through the help of pneumatic valves. Once the sample is carried into the column, it is allowed to interact with the stationary phase through different molecular interactions. In the column, the sample is broken down into its components to be analyzed. The analyzer instrument in the system detects the chemical properties of the sample and generates an electronic signal to be read. The electronic signal is then converted into a physical graph called a chromatogram for interpretation [4].

Figure 3 The inside of a GC column detailing the mobile (flow) phase and the stationary phase [6].

Components in the flow are traditionally eluted in order of descending vapor pressure. Therefore, the molecules that are usually seen in the gaseous phase are seen first on the chromatograms, followed by the heavier components of the analyzed mixture. To actively change the elution of gas chromatography, the stationary phase shown in Figure 3 can be modified. Increasing the thickness will allow for more gases to be analyzed but with decreased resolution. Decreasing the thickness will increase resolution but at the expense of retention time. The temperature and pressure of the system can be changed to get different results, where increasing temperature of the column or pressure of the sample can decrease the retention time but greatly increase resolution. By modifying equipment conditions, gas chromatography can be used in a variety of analytic work [6].

Advantages

  • Gas chromatography is fast, efficient, and can analyze many components at once.
  • Resolution of the chromatogram is very high, giving very clear data and results.
  • GC analysis is very accurate and reliable for sample analysis.
  • The volume of sample needed to analyze the solution is small.
  • Gas chromatography has been around for over 50 years and commercially available to most laboratories [4].

Shortcomings

  • Samples need to be able to be stable in the gas phase.
  • The system is run at a high temperature so samples may break down during the analysis.
  • Cannot determine the structure of the sample [4].

Analysis

The analysis of chromatography starts at the molecular scale, looking at the separation of molecules in the mobile and stationary phases. The value that describes this partition between the two phases is called the retention value (Rf) [2].

[math]Rf=\frac{(distance \, traveled \, by \, sample)}{(distance \, traveled \, to \, solvent \, front)}=1-\alpha[/math]

Where the closer Rf gets to one, the faster the separation happens, while the closer it gets to zero, the slower the separation happens. To determine the Rf value, α, the partition coefficient needs to be determined [2].

[math]\alpha=\frac{(Concentration \, in \, stationary \, phase)}{(Concentration \, in \, stationary \, and \, mobile \, phase)}[/math]

Chromatography runs are analyzed through a plot of detector signal versus time they produce called a chromatogram. The different components can be seen as peaks in the graph. The separation between two peaks, or components of a mixture, can be defined as the resolution [6].

Figure 4 An example of what a chromatograph would look like plotted by time [4].

[math] Resolution = \frac{0.589\Delta t_{r}}{1/2w_{av}} [/math]

Where Δtr is the difference in time between the two peaks and wav is the width of the peak in units of time. Area under the peak can generally tell how much of a compound or molecule is in a sample solution. Figure 4 shows what a typical GC chromatograph would look like. To determine which peak corresponds to which compound, a known sample can be run through the system as reference or a standard from literature or industry can be used to best analyze the results [6].

Applications

Medicine Gas chromatography is a valuable resource to the medical field due to its quick and reliable analysis. By developing sample preparation and validation techniques, biological samples can be tested for drugs and proteins for medical testing. It was found in a 2004 study, that gas chromatography, in conjunction with mass spectroscopy, can determine concentrations of lidocaine, prilocaine, and other drugs. This sampling of blood can be done in just one minute, providing quick results to patients and doctors [7].

Pharmaceuticals Uses for gas chromatography in pharmaceuticals happens in not only development, but also in post market screening. Due to regulations, drugs need to be extremely pure with consistent properties. Using gas chromatography to analyze drugs product solutions when developing industrial purification process is important due to its quick and accurate analysis. The faster the properties of a drug product can be determined, the quicker a company can bring a drug to market [4].

Pharmaceutical companies also have to do research into how effective their products are doing. Testing for efficacy can include urine drug screening of patients on different drugs to see how its working. Gas chromatography can be used to find drug fragments or different biological indicators to best determine the drugs true effect on on patients [4].

Food Characterization of food molecules is critical for food companies as so many tight regulations. Analyzing food mixtures for quality and consistent is important for company image. Unilever research and development, has looked into oil compositions using gas chromatography. Oil and fats are usually not made up of a single molecule but a collection of many different variations of similar molecules. It is vital for companies to know how much of different types of oils and fats so the contents can be labeled on the products mandated by the regulatory agencies. Gas chromatography is used as a quick and effective analysis of different types of food [8].

Petroleum Due to the large variations in chemicals in petroleum in diesel, it is commonly seen for product validation and characterization. Octane numbers for gasoline are determined through the composition of gas. Providing as close to real time data on the composition of affluent gas streams of a refinery are key to providing reliable products to the consumers. Due to the quick characterization of gas chromatography, it is one of the most popular analysis techniques used in the petroleum industry [4].

References

[1] "Principles of Chromatography." Khan Academy. N.p., 2016. Web.

[2] University of Kentucky. Chromatography. Lecture 6 presented in Chemistry 554 at University of Kentucky. 2013.

[3] High Performance Gas Chromatography Mass Spectroscopy. 2016. Technology University of Malaysia, Makmal.

[4] "Theory and Instrumentation of GC." GC's CHROM Academy (n.d.): 1-24. Crawford Scientific. Web.

[5] Schoenmakers, Peter. "Introduction to Capillary GC Injection Techniques." Introduction to Capillary GC Injection Techniques. - Introduction to Capillary GC Injection Techniques. - Chromedia. Chromedia, n.d. Web.

[6] University of California Irvine. "Background - Gas Chromatography." UCI.edu. Robert M Corn, 2016. Web

[7] Altun, Z., M. Abdelrehim, and L. Blomberg. "New Trends in Sample Preparation: On-line Microextraction in Packed Syringe (MEPS) for LC and GC ApplicationsPart III: Determination and Validation of Local Anaesthetics in Human Plasma Samples Using a Cation-exchange Sorbent." Journal of Chromatography B 813.1-2 (2004): 129-35. ELSEVIER.

[8] Janssen, Hans-Gerd, Herrald Steenbergen, and Sjaak De Koning. "The Role of Comprehensive Chromatography in the Characterization of Edible Oils and Fats." European Journal of Lipid Science and Technology 111.12 (2009): 1171-184. Wiley.