Gas Chromatography (GC) is an analytical technique for separating, identifying, and measuring individual compounds within a mixture.
In contrast to liquid chromatography, a stream of carrier gas serves as the mobile phase to carry volatilized samples through a solid or liquid stationary phase.
Based on polarity, individual analytes adsorb to the stationary phase and flow through the column at different rates until they reach detectors, of which there are various types.
The retention time, or the times at which compounds separately elute is the analytical basis of GC.
GC and GC-MS are widely used in the life sciences in areas such as environmental monitoring, forensics, food and beverage analysis, drug detection and medical screening, and metabolomics.
Gas Chromatography Considerations:
Sample Injection:
Modern sample injection ports for introducing sample at the head of the column typically include a heating element for subsequent vaporization.
In addition to manual injection, automated sample loading is a common feature or accessory for many GC instruments.
These are offered in forms of auto-injectors (small to medium throughput), autosamplers (higher throughput), or robotic handling.
Automation can provide both higher processivity and less variability from manual handling.
Detection:
- Thermal conductivity detector (TCD)
- TCD measures the change in thermal conductivity of eluting analytes in reference to the flow of a carrier gas.
It is frequently referred to as the universal detector, is nonspecific, and can respond to all compounds.
- Flame ionization detector (FID) -
FID uses hydrogen to pyrolyze compounds eluting from the column, producing ions that generate measurable current.
It is a common detection method that is very sensitive, but limited, to organic and hydrocarbon-containing compounds.
- Electron capture detector (ECD) -
ECD emits electrons that, when captured by eluting analytes, produces a measurable change in current.
It is extremely sensitive to halogenated compounds.
- Flame thermionic detector (FTD) and nitrogen phosphorus detector (NPD) -
With thermionic detectors such as FTD and NPD, analytes are ionized by heat, resulting in a measureable change in current.
They are especially sensitive for organic compounds containing nitrogen and phosphorus.
- Flame photometric detector (FPD) -
In FPD, a flame burns eluting analytes to produce spectral emissions whose wavelengths are detected by optical filters.
It is highly selective for sulfur- and phosphorus-containing compounds.
- Pulsed discharge detector (PDD) -
PDD is based on pulsed high voltage discharge between electrodes resulting in ionization of eluting analytes.
Different configurations allow for measurement of: electrons produced from ionization (helium photoionization), or current change from electron capture (electron capture).
PDD can be used for detection of gases, inorganic, organic, and halogenated compounds.
Carrier Gas:
The choice of mobile phase carrier gas will depend on the type of detector used, the properties of the sample, as well as safety and availability of the gas.
In certain applications, gases with a purity as high as 99.9999% and filtered for hydrocarbons, moisture, and oxygen are ideal for maximum sensitivity.
Common carrier gases include helium, hydrogen, nitrogen, argon, and air.
Check if the instrument can safely accommodate certain gases, such as hydrogen.
With many modern GC instruments, gas flow rates and pressure can be monitored and controlled electronically.
Column and Temperature:
In choosing a GC column, consider the properties of stationary phase, column length and diameter, and film thickness.
Temperature also directly affects the rate at which the sample travels through the column; higher temperatures moves samples faster.
Ensure that the GC oven and column can accommodate the temperature ranges of the desired applications.
GC-MS:
Gas chromatography-mass spectrometry (GC-MS) integrates a mass spectrometer in-line or downstream of the GC instrument, allowing for even finer identification and quantification of trace amounts of compounds.
Mass spectrometers break down analytes into smaller ionized fragments.
The resulting mass and charge signatures is used to identify chemical structures of compounds.
GCxGC:
GCxGC, or two-dimensional gas chromatgoraphy, uses two columns connected in series separated by a modulator.
The modulator in essence traps small analyte fractions from the first column for a fixed period and then transfers them onto the second column as a narrow pulse.
The refocusing results in a two-dimensinal chromatogram that allows for increased sensitivity.
Gas chromatograph selection will depend on the intended research applications and desired throughput.
Keep an eye out for potentially useful features such as modular or interchangeable components, compatibility for GC-MS, number of accomodated sample injections, and types of detectors.
For the most up-to-date information, be sure to consult the manufacturer specifications.
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