Glossary of Gas Chromatography Terms

Baseline

The baseline is the line drawn by the data system when the only signal from the detector is from the mobile phase.

Capillary Gas Chromatography (Capillary GC)

Gas chromatographic separation in a capillary column

Capillary Columns

Glass capillaries filled with stationary phase diameter: 0.2 – 1.0mm, length: 5-50m

Carrier gas

inert gas that constantly flows through the chromatography column and detector. Carrier gas mobilizes the sample by pushing it through the column. Inert refers to non-reactivity of the carrier gas with the sample and a low response of the detector to the carrier gas.

Chromatography

Separation method

Chromatography column
length of metal or fused silica tubing filled or coated with one or more packing materials that separates the compounds of interest as the sample mixture passes through.

Detector
sensor that produces a measurable signal when certain molecules pass through it.

Detectors in GC

• ECD -   electron capture detector
• ELCD – electrolytic-conductivity detector
• FID -    flame ionization detector
• FPD -   flame photometric detector
• MSD -  mass selective detector
• NPD -  nitrogen phosphorous detector
• TCD -   thermal conductivity detector

Direct injection

Occurs when sample enters an inlet and is swept into a column by carrier-gas flow. No sample splitting or venting occurs during or after the injection.

Electron-capture detector (ECD)
detector that is especially sensitive to halogen-containing molecules. Thermal electrons generated by the radioactive decay of 63Ni create a small, steady current within the detector. This current is measurably reduced when certain molecules capture electrons as they pass through the detector.

Electrolytic-conductivity detector

In ELCD, the detector catalytically reacts halogen-containing solutes with hydrogen (reductive mode) to produce strong acid by-products that are dissolved in a working fluid. The acids dissociate and the detector measures increased electrolytic conductivity. Other operating modes modify the chemistry for response to nitrogen- or sulphur-containing substances.

Flame ionization detector (FID)

Most common GC detector – detects all solutes containing at least 1 CHn unit.

Flame photometric detector (FPD)

Highly selective for solutes containing S- or Sn-atoms

Gas chromatograph (GC)

type of instrument based on the injection of component mixtures onto a chromatography column, separation of the components as the mixture passes through the column, and sensing of the components of interest as they elute into a detector.

Gas sampling valve (GSV)

A multiple-port, two-position valve that changes the direction of gas flows through chromatography columns and other parts of an instrument. Used to fill a sample loop with sample then inject the loop contents onto the column.

Headspace sampling

Gas-phase sampling technique in which solute is removed from an enclosed space above a solid or liquid sample.

Ion-trap detector

A mass spectrometric (MS) detector that uses an ion-trap device to generate mass spectra.

Inert gases

N2, He, H2

Nitrogen phosphorous detector (NPD)

Nitrogen/phosphorus selective detector detects only solutes with N and/or P-atoms.

On-column injection (OCI)

In on-column injection, sample enters the column directly from the syringe and does not contact other surfaces. On-column injection usually signifies cold injection for capillary columns.

Packing material
granular, solid-phase powder, sometimes also coated with a liquid phase, that is used to fill a chromatography column. The packing material of a column is chosen for its ability to separate the molecules of interest from one another. Packing materials typically separate molecules by their size (molecular sieves), boiling points, or polarities.

Peak

When the detector registers the presence of a compound, the normal baseline signal it sends to the data system changes, resulting in a deflection from the baseline called a peak. Well resolved peaks are symmetrical, touch the baseline, and do not interfere with other peaks.

Peak capacity

The amount of solute that can be injected without a significant loss of column efficiency.

Purge-and-trap sampling

A concentration technique for volatile solutes. Sample is purged with an inert gas that entrains volatile components onto an adsorptive trap. The trap is then heated to desorb trapped components into a GC column.

Sample loop
length of hollow metal tubing whose fixed volume is filled with sample to a repeatable pressure and its contents injected onto a chromatography column. Consistent sample loop volume, pressure and temperature ensure that the same amount of sample is injected for every analysis.

Selectivity

The fundamental ability of a stationary phase toretain substances selectively based upon their chemical characteristics, including vapour pressure and polarity.

Sensitivity

The degree of detector response to a specified solute amount per unit time or per unit volume.

Split injection

The sample size is adjusted to suit capillary column requirements by splitting off a major fraction of sample vapours in the inlet so that as little as 0.1% enters the column. The rest is vented.

Splitless injection

A derivative of split injection. During the first 0.5–4 min of sampling, the sample is not split and enters only the column. Splitting is restored afterward to purge the sample remaining in the inlet. As much as 99% of the sample enters the column.

Thermal Conductivity detector (TCD)

Unselective, detects all types of solutes