Gas Chromatography

In Gas Chromatography (GC), a gaseous mobile phase transports a gaseous solute through a long, thin column containing solid or liquid stationary phase.

Classification: GC is divided into 2 major categories: 1. Gas solid chromatography(GSC) GSC employs a suitable adsorbent such as silica gel, alumina etc. as the solid stationary phase. Here separation occurs by distribution of solutes between the gaseous mobile phase and the surface of the adsorbent. 2. Gas liquid chromatography(GLC) GLC employs liquid stationary phase which is bound to inert supporting material. Here separation occurs by partitioning a sample between a mobile gas phase and liquid stationary phase.

Instrumentation: A Gas chromatograph consists of 3 basic units: 1. The chromatographic unit 2. The temperature control & Signal Amplification Unit 3. The recorder unit. 1. The chromatographic unit consists of the following parts: i. The carrier gas source ii. A gas pressure regulator and flow control system iii. A sample injection port iv. A chromatographic column v. A thermostatic column oven vi. A detector vii. A gas exit port

i. Carrier Gas Source: H 2 and He give better resolution than N 2 at high flow rate. But N 2 is inexpensive & gives reduced sensitivity. The solutes diffuse more rapidly through H 2 and He, therefore equilibrate between the mobile and stationary phase more rapidly. However, H 2 is explosive when mixed with air and there is also a chance of reacting with unsaturated compounds (Hydrogenation). He is commonly used but it is highly expensive. Air is used only when the O 2 in the air is useful for the detector or separations.

ii. A gas pressure regulator and flow control system: This unit control and monitor the carrier gas flow. iii. A sample injection port: The sample injection port may be a side arm at the beginning of the column closed by a self- sealing rubber septum. A microliter syringe is used to inject the sample directly into the gas flow at the top of the column. For liquid samples or solids dissolved in liquid. The injection port is ordinarily heated to a temperature above that of the column to ensure vaporization of the sample. The gaseous sample may be injected directly.

iv. GC Column: The column may be straight, U shaped or coiled, 1-4m in length and 2-4 mm in diameter. It may be made of glass or stainless steel or nickel. Length can vary from a few centimeters to over a hundred meters. 2 types of columns are used in GC. 1. Packed columns & 2. Capillary columns

Packed columns are usually made of stainless steel or glass. In GSC, columns are packed with size graded adsorbent or porous polymers while in GLC, the packing is prepared by coating the liquid phase over a size graded inert solid support.

Capillary columns have a very small internal diameter on the order of a few tenths of millimeters and lengths between meters are common. These columns are very flexible so that a very large column can be wound into small coil. They are also known as open tubular columns that are usually made of fused silica (SiO 2 ). They consist of a liquid or solid stationary phase coated on the inside wall, e.g. a) WCOT – Wall Coated Open Tubular column b) SCOT – Support Coated Open Tubular column c) PLOT – Porous Layer Open Tubular column

v. Thermostated column oven: The chromatographic column is placed within a thermostated column oven. There is a heater within the oven and the oven temperature is thermostatically controlled from the temperature control and signal amplification unit. Vi. Detectors: a) Thermal conductivity detector (TCD), b) Flame ionization detectors (FID), c) Electron capture detector (ECD), d) Discharge ionization detectors (DID), e) Helium ionization detectors (HID), f) Flame Photometric detectors (FPD), g) Hall Electrolytic Conductivity detectors (HECD), h) Nitrogen Phosphorous detectors (NPD), i) Mass selective detectors (MSD) etc.

Among them the most common are the TCD & FID. Both work over a wide range of concentrations. While TCDs are essentially universal and can be used to detect any component other than the carrier gas. FIDs are primarily sensitive to hydrocarbons. TCD is non- destructive and can be can be operated in series before FID (destructive).

Thermal Conductivity Detector: It measures the changes in thermal conductivity due to the sample. A TCD detector consists of a heated wire or thermistor with an applied current. The heated filament is cooled by the flow of carrier gas. When organic molecules (sample) displace some of the carrier gas, thermal conductivity of the mixture of carrier gas and compound gas reduces. The filament becomes hotter than the control column. The temperature difference of the control and sample filament is measured and a signal is recorded. The TCD is not as sensitive as other detectors but it is non-specific and non-destructive.

Flame Ionization Detector: The effluent from the column is mixed with hydrogen and air and then ignited electrically. A potential of a few hundred volts is applied An FID consists of a hydrogen/air flame and a collector plate. Most organic compounds, when introduced into a high temperature of a hydrogen/air flame, produce ions and electrons that can conduct electricity through the flame. The resulting current (~ A) is then measured. The flame ionization detector exhibits a high sensitivity, large linear response range and low noise. Its only disadvantage is that it destroys the sample.

Electron Capture Detector: The basis of this type of detectors is the electron affinity of different substances. The ECD uses a radioactive (63Ni, 65Ni, 3H) beta emitter (electrons) to ionize some of the carrier gas and produce a current between a biased pair of electrodes. When organic molecules that contain electronegative functional groups, such as halogens, phosphorous, and nitro groups pass by the detector, they capture some of the electrons and reduce the current measured between the electrodes. ECD detects positive ions (N2+) of carrier gas by the anode electrode. Ionization: N 2 + β (e) → N e, the N 2 + establishes a “BASE LINE”. X - (F, Cl or I) + N 2 + → N 2 + X

So, the “BASE LINE” due to N2+ will decrease and this decrease constitutes the signal. The more the halogen containing X compounds in the sample, the less the N2+ in the detector. The ECD is as sensitive as the FID but has a limited dynamic range and finds its greatest application in analysis of halogenated compounds.

2. Temperature Control & Signal Amplification Unit: Temperature conditions of the chromatographic unit need to be accurately regulated by heaters. Oven temperature, injector port temperature, detector temperature are controlled separately. Function of the signal amplification unit is to amplify the signal produced by the detector and to transmit the amplified signal to the recorder unit.

3. Recorder Unit: The amplified detector signal is sent to this unit which produce a graph of detector signal response against time.

Applications: Compound must exist as a ____ at a temperature that can be produced by the GC and withstood by the column (up to 450°C) Alcohols in blood Aromatics (benzene, toluene, ethylbenzene, xylene) Flavors and Fragrances Permanent gases (H2, N2, O2, Ar, CO2, CO, CH4) Hydrocarbons Pesticides, Herbicides, PCBs, and Dioxins Solvents

Advantages: Requires only very small samples with little preparation Good at separating complex mixtures into components Results are rapidly obtained (1 to 100 minutes) Very high precision Only instrument with the sensitivity to detect volatile organic mixtures of low concentrations Equipment is not very complex (sophisticated oven).

Question: Make a difference between GC and HPLC.