Glossary of GCxGC terms

Jan Beens, Vrije Universiteit Amsterdam, the Netherlands

Abstract Nomenclature and conventions for further use in GCxGC technique

KeywordsComprehensive Two-Dimensional Gas Chromatography, nomenclature, conventions

LevelBasic

Nomenclature and conventions

In the First Symposium on Comprehensive Two-Dimensional Gas Chromatography, held in March 2003 in Volendam, the Netherlands, the participants, after an extensive discussion, unanimously agreed upon the following nomenclature and conventions for further use in this technique [P. Schoenmakers, P. Marriott. J. Beens, Nomenclature and Conventions in Comprehensive Multidimensional Chromatography, LC-GC Europe, 16 (2003) 335-339

General nomenclature

Examples of abbreviations involving the multiplex (×) sign.

Abbreviation	Full term
GC×GC GC×GC-FID GC×GC-MS	Comprehensive two-dimensional gas chromatography Comprehensive two-dimensional GC with flame-ionization detection Comprehensive two-dimensional GC with mass spectrometric detection
LC×LC LC×SEC	Comprehensive two-dimensional liquid chromatography Comprehensive two-dimensional (liquid×size-exclusion) chromatography
LC×GC	Comprehensive two-dimensional (liquid×gas) chromatography
SFC×GC	Comprehensive two-dimensional (supercritical-fluid×gas) chromatography
GC×GC×GC	Comprehensive three-dimensional gas chromatography
LC–GC×GC	On-line liquid chromatography-Comprehensive two-dimensional gas chromatography
SFC–GC×GC	On-line supercritical-fluid chromatography-Comprehensive two-dimensional gas chromatography

Definitions of “orthogonality” in various fields of science.

Field	Definition of orthogonality
Mathematics*	Of two vectors or functions: perpendicular; having an inner product equal to zero. Of a set of vectors or functions: such that the inner product of any two is zero if and only if the two are distinct.
Statistics*	Of a set of variates: statistically independent. Of an experimental design: such that the variates under investigation can be treated as statistically independent.
Analytical Chemistry	Of two separation dimensions: such that the elution times in both dimensions can be treated as statistically independent.

* From the Oxford English dictionary (on-line edition).

Other Definitions

In the table a number of other relevant definitions are collected. A number of these are specifically derived from the field of GC×GC, but their use is not exclusive for this technique. For example, the word modulator may equally well be used to describe the intermediate stage in a number of other comprehensive two-dimensional separation methods. One result from the discussion was that a modulator does not necessarily need to incorporate a focusing effect. The latter is easily achieved by thermal modulation in GC×GC, but it is not usually encountered in the comprehensive two-dimensional combination of two liquid-phase separation methods.

Nomenclature suggested for comprehensive two-dimensional (gas) chromatography.

Term	Definition
Modulator	Interface device between the two columns in a comprehensive two-dimensional separation system that accumulates or samples narrow bands from the eluate of the first column for fast re-injection into the second column.
Modulation time or Modulation period (P_M )	The duration of a complete cycle of modulation in a comprehensive two-dimensional separation system (equals the length of a second dimension chromatogram, i.e., the time between two successive injections into the second column).
Modulation frequency (f_M )	Number of modulations per unit of time.
Modulation number (n_M )	The number of modulations for a given first-dimension peak.
Single-stage modulation	Accumulation and focusing during one series of processes at one location in the modulator.
Dual-stage modulation	Accumulation and focusing during two successive series of processes at two locations in the modulator.
Focusing effect	Reduction of the band width (in time, distance and/or volume units) (= band width without modulation/band width with modulation).
Sensitivity enhancement	Ratio between peak height with and without modulation (peak-amplitude enhancement) (note: sensitivity refers to the signal, not to the noise!).*
Zone compression	The effect of reducing a chromatographic peak (width) in space or time to give a higher concentration within a chromatography column.
Separation space	The region within the two-dimensional GC×GC plot in which compounds are, or may be, distributed.
Wrap-around	The occurrence of second dimension peaks in subsequent elution sequences, caused by second-dimension retention times that exceed the modulation time of a comprehensive two-dimensional system [46].
Modulation phase (F_M)	The pattern of modulated peaks caused by the time relationship between peak distribution and the pulsing process of the modulator in a comprehensive two-dimensional separation system [98].
In-phase modulation	The modulation phase that produces a symmetrical sequence of peaks with a single maximum pulse peak [4].
Out-of-phase modulation	Any modulation phase that produces a non-symmetrical peak-pulse distribution [98].
Column set	The combination of columns used for a given GC×GC experiment.
Column set cross sectional area ratio	The relative change in cross sectional area for the ¹D to ²D columns of the column set (¹D/²D).
Chromatographic structure	The observed ordering of chemically related compounds in the plane of a comprehensive two-dimensional separation.
Colour plot	Two-dimensional plot representing a comprehensive two-dimensional separation, in which the colour represents the signal intensity of the separation system.**
Contour plot	Two-dimensional plot representing a comprehensive two-dimensional separation, in which similar signal intensities are connected by means of a line.**
Apex plot	Two-dimensional plot representing a comprehensive two-dimensional separation, in which peak apices of second-dimension peaks are displayed by a symbol on the co-ordinates of the maximum of the second dimension signal intensity.**

* A reduction in the detection limit may also be achieved. This reduction is proportional to the product of the sensitivity enhancement and the noise reduction.
** The x-axis represents the first-dimension retention time, the y-axis the second-dimension retention time of the separation system.

Symbols

This table lists a number of symbols that are recommended for use in comprehensive two-dimensional separations in general and GC×GC in particular. The use of the superscript prefix 1 or 2 is suggested to distinguish between the first- and second-dimension columns.

Non-exhaustive list of symbols suggested for use in GC×GC (and other comprehensive two-dimensional separation methods):

Symbols	Definition
¹d_c , ²d_c	Internal diameters of the first- and second-dimension columns (respectively) used in a comprehensive two-dimensional system.
¹D, ²D	First dimension and second dimension of a GC×GC system.
¹t_R , ²t_R	Retention times of a peak in the first and second dimension of a comprehensive two-dimensional system respectively.
¹t_M , ²t_M	Hold-up times (or “dead” times) of the first and second columns of a comprehensive two-dimensional system respectively.
¹k, ²k	Retention factors of a peak eluting from the first-and second-dimension columns of a comprehensive two-dimensional system respectively.
¹I, ²I	Retention indices of a peak eluting from the first- and second-dimension columns of a comprehensive two-dimensional system respectively.
¹N, ²N	The numbers of theoretical plates of the first and second columns of a comprehensive two-dimensional system respectively.
¹σ, ²σ	Standard deviations of a peak eluting from the first-and second-dimension columns of a comprehensive two-dimensional system respectively.
¹w_b, ²w_b	Peak widths at base of a peak eluting from the first-and second-dimension columns of a comprehensive two-dimensional system respectively.
¹R_s, ²R_s	Resolution values of a peak pair eluting from the first and second column of a comprehensive two-dimensional system respectively.
¹n, ²n	Peak capacities of the first and second columns of a comprehensive two-dimensional system respectively.
¹d_f, ²d_f	Film thicknesses of the first and second columns of a comprehensive two-dimensional system respectively.
¹û, ²û	Average linear velocities in the first and second columns of a comprehensive two-dimensional system respectively.
¹T_e, ²T_e	Elution temperatures for a peak eluting from the first dimension and second dimension of a comprehensive two-dimensional system respectively.