Gas chromatography

GAS CHROMATOGRAPHY

Rodrigo Carretero Villaseñor Código: 216304824 Instrumentación química analítica II. 2019B Departamento de química Universidad de Guadalajara

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Origins of Gas Chromatography

The development of GC as an analytical technique was pioneered by Martin and Synge in 1941; they predicted that the mobile phase need not be a liquid, but could be a vapor. Benefits of using gas-liquid partition chromatograms: Columns much more efficient Separation times much shorter Martin and his co-worker, A. T. James demonstrated the technique by separating and quantitatively determining the twelve components of a C1-C5 fatty acid mixture.

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Caption: : Anthony T. James, Archer J. P. Martin & Richard L. M. Synge

Gas chromatograph

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Components of the GC system Gas Inlets Pneumatic controls Injector Column Column Oven Detector Data System Hyphenated gas chromatography techniques Pre-gas chromatograph Automated On-line sample preparation Static headspace (HS)  Dynamic headspace (PT) Large volume injection (LVI) Solid-phase microextraction (SPME) Post-gas chromatograph sample analysis 

Caption: : Generalized diagram of a gas chromatograph

Hyphenated gas chromatography refers to the coupling of a GC to information-rich detectors and coupling of gas chromatograph to automated sample preparation systems. Static headspace analysis is based on the theory that equilibrium between a condensed phase and a gaseous phase can be reproducibly maintained for the analytes of interest and that the gaseous phase containing the analytes can be sampled reproducibly. Working steps: Thermostatting the sample at a given temperature and for a given time until it has reached a state of equilibrium.  Pressurization of injection is carried out. Take an aliquot from the headspace and inject it into the GC. Limitations:  Analytes of interest must posses very low vapor pressures.

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Static headspace (HS) 

Caption: : Static head space working steps overview

Dynamic headspace (PT)

Dynamic headspace analysis based on the principle is that the change in mass of a volatile or semivolatile analyte with time can be expressed in terms of the volumetric flow rate of stripping inert gas. Working steps: Place the sample into a chamber, at a pre-selected temperature, that is sparged with carrier gas at a specified rate and time. Carrier gas removes the analytes from the matrix and transports them to a trap. Dry purge. Heat and backflush the trap to lead to the desorbing the analytes from the sorbent material and transferring into GC. Materials which are used for the trapping of analyte include polymers, carbon, silica, and alumina.

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Caption: : Dynamic headspace working steps overview

Large volume injection (LVI)

Large volume injection technique allows larger aliquot of the sample analyte to be deposited on to the column while simultaneously eliminating the solvent, whereas the three most widely used common injection techniques (split, splitless and cold on-column injection system) the sample volume is limited to less than 2µL. Limitations: Proper quantitative analysis requires the vapor pressures of analytes are significantly higher than that of the solvent.  

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Caption: : Diagram of a PTV inlet system

Solid-phase microextraction (SPME)

Solid-phase microextraction is fundamentally a solvent-free sample preparation technique in which relatively thin film extracting phase of very small volume, less than 1µL, is firmly coated and bound to a fused silica fiber which in turn can be exposed to a sample matrix. Room air, aqueous solution or organic solvents acts as a sample matrix. The extracting phase bound to the fiber is very similar to the phases used in capillary GC. Applications: Determination of enantiomeric distributions of volatile components in essential oils, seprating optical isomers and  characterization of cheese aroma compounds using MS an as detector.

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Caption: : Solid-phase microextraction working steps overview

MULTIDIMENSIONAL GC

Multidimensional gas chromatography is defined as a GC system of two or more columns of different selectivity and a device that enables the selective transfer of a portion of a chromatographic run from one column to the second column. Applications:  This technique is used in various fields such as food and fragrance, forensic and health science and in petroleum industries.   

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Caption: : Diagram of a multidimensional GC system

Components of the GC system

Gas Inlets: Carrier (H2, He, N2), Make-up gas (H2, He, N2), Detector Fuel Gas (depending on the detector type). Pneumatic controls: These controls regulate the correct gas pressure and then fed to the required part of the instrument. Injector: Here the sample is volatilized. Many inlet types exist including split/splitless, programmed thermal vaporizing (PTV) and Cool-on-column (COC). Column: The sample is separated into its constituent components in the column. Columns vary in length and internal diameter depending on the application type and can be either packed or capillary.  Column Oven: Temperature is controlled via a heated oven. The oven heats rapidly to give excellent thermal control. The injector and detector connections are also contained in the GC oven. Detector: The detector responds to a physicochemical property of the analyte, amplifies this response and generates an electronic signal for the data system to produce a chromatogram. Main detectors are Flame Ionization (FID), Electron Capture (ECD), Flame Photometric (FPD), Nitrogen Phosphorous (NPD), Thermal Conductivity (TCD), and Mass Spectrometer (MS).  

Post-gas chromatograph sample analysis 

GAS CHRMATOGRAPHY/ MASS SPECTROSCOPY The mass spectrometer is a universal detector for gas chromatographs, since any compounds that can pass through a GC is converted into ions in the mass spectrometer. Applications: GC/MS currently finds a very wide array of applications including analyses of biological tissue, petroleum residues, pesticides, essential oils and pharmaceutical. GAS CHRMATOGRAPHY / FOURIER TRANSFORM INFRARED SPECTROSCOPY This is a coupled technique in which the gas chromatograph does the separating and the infrared spectrometer does the identifying. GAS CHRMATOGRAPHY/ FOURIER TRANSFORM MATRIX ISOLATED INFRARED SPECTROSCOPY  It consists on complete isolation of the effluent of a GC column at temperatures approaching 10K in an inert noble gas (usually argon) matrix. Such low temperature virtually eliminates all intermolecular interactions and reduces molecular rotation.  Applications: This technique is useful for the study of reactive molecules such as free radicals and reaction intermediates and detecting difference between cis and trans isomers in the selected compounds.  Data System: The data system receives the analogue signal from the detector and digitizes it to chromatogram.