Adjusted Retention Time (tR')
An analyte's retention time (tR) minus the elution time of an unretained peak (tm).
tR'= tR-tm
Adjusted retention time is also equivalent to the time the analyte spends in the stationary phase.
Capacity Factor (k)
Expression that measures the degree of retention of an analyte relative to an unretained peak, where tR is the retention time for the sample peak and tm is the retention time for an unretained peak. A measurement of capacity will help determine whether retention shifts are due to the column (capacity factor is changing with retention time changes) or the system (capacity factor remains constant with retention time changes).
Thus the higher the capacity factor, the longer the retention time.
Column Efficiency (N)
See Theoretical Plate Number.
Column Evaluation
An application on Thermo GC instruments that measure a column's resistance to flow, accurately controlling gas linear flow rates to ensure consistent retention times.
Detectors
See ECD, FID, FPD, NPD, PID and TCD.
Distribution Constant
A ratio of concentration of solute in the stationary phase versus the mobile phase. Also known as partition co-efficient.
Effective Theoretical Plates (Neff)
A measure of a column performance that accounts for the effects of unretained elution time, where t'R is the adjusted retention time and s is the standard deviation of the peak.
This value also remains constant as retention gaps and guards are used. Depending on the method of peak width calculation, different efficiencies can be reported. This leads to two popular measures:
ECD
Electron Capture Detector. Uses electron emitting source to ionize the carrier gas. Any electron-deficient analyte will reduce this level of ionization. The ECD detector is sensitive to any analyte with electronegative functionality (e.g. Cl-).
FID
Flame Ionization Detector. A "universal" detector for all carbon-containing compounds. A high temperature hydrogen flame ionizes the sample, causing an increase in current through an electrode, proportional to the amount of carbon passing through the flame.
FPD
Flame Photometric Detector. Analytes are passed through a hydrogen rich flame which is monitored by photocells. FPDs are usually used as a specific detector for sulphur- or phosphorous-containing compounds.
Flow Rate
The volumetric flow in mL/min of the carrier gas; this is different from the linear velocity.
Fronting
Distorted peak where the asymmetry of the peak is towards the front; the peak tailing factor is < 1.
Heartcutting (2D GC)
A method of using two columns of different selectivity to gain more information on a sample. In heartcutting, only a selected portion of eluant from the first column is passed onto the second.
HEEP (Heff)
Height Equivalent to an Effective Plate.
Where L is the column length. The smaller the Neff, the more efficient the column's performance.
HETP (H)
Height Equivalent to a Theoretical Plate is a measure of column efficiency where L is the column length and N is the number of theoretical plates:
HETP is based on actual (tR) rather than adjusted retention times (t'R).
Hold Up Time (tm)
The time for an injected substance, which is not retained on the stationary phase, to pass through the column and reach the detector. Hold Up Time is usually measured by injection of a compound such as methane.
Kovat's Index
A value that expresses the retention of a sample compared to two standards eluting before and after it. The Kovat's Index uses simple alkanes. The value is derived by allotting the alkanes a value of 100 times their carbon number and giving the sample a value equivalent to a hypothetical alkane eluting at the same time.
Leak Test
Process to establish the gas tight nature of all connections. This is an automated test on Thermo GC instruments.
Linear Velocity (u)
Mobile phase flow rate expressed in cm/s and is expressed as
Where L is the column length and to is the breakthrough time of an unretained peak
Mass Distribution Ratio [k(Dm)]
Alternative to Capacity Factor. Described as the ratio between the fraction of an analyte in the stationary phase and mobile phase.
NPD
Nitrogen Phosphorus Detector. Selective to the presence of nitrogen or phosphorus in the sample. NPD is often used in environmental studies.
On-Column Injection
Method of injection where the syringe needle enters and delivers the sample onto the top of the column.
Partition Co-efficient
See Distribution Constant.
Peak Width (W)
There are a number of ways to measure peak width. Most common are:
1. Tangential width: measures the baseline between two tangents taken from the peak inflection points and the baseline.
2. Width at half-height: measures width between the peak measured at 50% peak height
Peak width can be expressed in time or volume.
Phase Ratio (beta)
The ratio of a column's volume of stationary phase to mobile phase. An important value when changing the column dimensions of a method.
PID
Photo Ionization Detector. Photons from the detector UV source ionize solutes passing through the detector. The presence of an analyte increases the signal.
PLOT
Porous Layer Open Tubular column. Capilliary column that has a fine adsorbent bonded to it. PLOT is most often used for very volatile liquids or permanent gases.
Purged Splitless Injection
This is the most common splitless method where the injector is set in split mode for a set time but then the split vent is opened to flush any remaining sample.
Resolution
A measure of the separation of two peaks taking into account both the difference in elution time and the peak widths.
Where t2 and t1 are the two retention times, and W1 and W2 are baseline peak widths.
Retention Index
A value that expresses the retention of a sample compared to two standards that elute before and after it. See Kovat's Index.
Retention Time (tR)
Total time from injection to elution. This takes no account of dead volume or tm.
Retention Volume
Total volume from injection to elution.
SCOT
Support Coated Open Tubular column. A capilliary column where the liquid stationary phase is supported on a solid substrate coating the capillary column walls.
Selectivity (alpha)
The relative retention of two adjacent peaks. Selectivity can be calculated using capacity factor or retention volumes.
Separation Factor
See Selectivity.
Sensitivity
An estimation of the smallest analyte signal that can be detected by the method. Sensitivity is usually specified as a signal-to-noise ratio of 2.
Septum
A small disc usually made of rubber or silicone material that is used to seal the injector from the atmosphere. The syringe needle passes through the septum.
Septum Bleed
Compounds generated form the septum by the effect of temperature on the septum material. The compounds released lead to contamination of the injector and noisy baselines.
Split Injection
Method of introducing a sample onto the column. In split injection most of the sample, once vaporized, passes out of the injector to waste with only a small portion entering the column head.
Splitless Injection
Method of introducing a sample onto the column where the purge valve is closed for a period of time allowing all sample to enter the column. The purge valve is then opened to flush the injector.
Theoretical Plate Number
A measure of efficiency of the whole system. There are a number of ways to calculate efficiency because the calculation includes a term for peak width. Depending on the method of peak width calculation used, different efficiencies can be reported. Also known as Column Efficiency.
TCD
Thermal Conductivity Detector. Heated elements form the arms of a wheatstone bridge. Analytes passing through one chamber change the temperature and therefore the resistance which is monitored. It is a universal, non-destructive detector.
Trennzahl Number
A value to describe a separation. The Trenzahl number is calculated from the resolution between two consecutive homologous hydrocarbons. The Trennzahl number represents the number of peaks that can be included between the two hydrocarbon peaks.
van Deemter Equation
This is a relationship that considers the effect of linear velocity on the HETP or h, where A accounts for eddy currents, B describes the diffusion in the mobile phase term, C refers to the resistance to transfer from the stationary to mobile phase and u is the velocity of the mobile phase.
WCOT
Wall Coated Open Tubular column. Column where the stationary phase is bonded to the inside wall of the capillary column. WCOT is the most commonly used column.
