Terminology and Nomenclature for Gas Chromatography, Part 1: Fundamental Definitions

In analytical chemistry we depend not only on the precision of our results but on the precision of our language.Too often in scientific and promotional publications, we see a variety of names and symbols used to describe chromatographic data and methods.In this column, we will review key terminology and symbols for gas chromatography using the correct definitions, as published by the International Union of Pure and Applied Chemistry, the organization that sets definitions across all of chemistry, in their 1993 and 2017 recommendations.As we proceed through a multi-part series, we will discuss the differences between the accepted and colloquial or classical definitions and when to be aware of them.Authors and readers of the scientific literature should take care to use the correct terminology, nomenclature and symbols always when presenting and discussing scientific results.We will also discuss the fundamental underpinnings of the definitions as we review the related theory.

Chromatography, or “color writing,” was first described around the turn of the 20th century by Ramsey, who separated gas mixtures using adsorbents such as charcoal, and, more famously, by Tswett, who separated plant pigments by passing mixtures down a packed column using liquids.¹ Of course, in today’s gas chromatography, the sample mixtures may be colored, but the gaseous analytes generally are not.

Over the ensuing century and a quarter, many terms and symbols have been used to describe the techniques and data collected in chromatographic experiments. The International Union of Pure and Applied Chemistry (IUPAC) is an umbrella organization comprising most national chemical societies, such as the American Chemical Society. One major responsibility of IUPAC is to set important international terms and definitions throughout the field, such as the names and symbols of the chemical elements. This ensures that all chemists “speak the same language” when discussing and publishing chemical data.

The IUPAC had defined all the terms and symbols for chromatography in two documents, published in 1993 and 2017.^2,3 We can begin with the definitions of chromatography and elution chromatography, which describe modern gas chromatography and go well beyond Tswett’s idea of “color writing.”

“Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one of which is stationary (stationary phase), while the other (the mobile phase) moves in a definite direction. Elution chromatography is a procedure in which the mobile phase is continuously passed through or along the chromatographic bed and the sample is fed into the system as a finite slug.”^2,3

It is useful to parse the key words in this definition, as it is an excellent thought and learning exercise to examine the variables in a new equation or the exact words in any definition. First, chromatography is a physical method. It involves physical, not chemical changes. Within the chromatographic system, we generally do not perform a chemical reaction; chromatography is based on physical phase transitions.

The next key phrase is distributed between two phases. This phrase defines the presence of two of the classical phases of matter: solid, liquid, gas, supercritical fluid, or plasma. Although chromatography generally does not employ a plasma phase, the other four are all used in chromatography. The other half of the phrase, distributed between, demonstrates the basic separation and transport mechanism that underlies both chromatography and extraction, which separates based on differences in the distribution constants (equilibrium constants) and the underlying thermodynamics of the phase transition between the two phases.

The next clause defines the two phases. One phase (the mobile phase) moves, while the other phase (the stationary phase) does not. With the phases of matter being solid, liquid, gas, or supercritical fluid, we can next define several forms of chromatography based on these phases. The three phases that easily move, gas, liquid, and supercritical fluid, define the three fundamental modes of chromatography with which we are familiar: gas chromatography (GC), liquid chromatography (LC), and supercritical fluid chromatography (SFC). The nature of the mobile phase determines the mode of chromatography in use.

Two of the phases, liquid and solid, are easily kept from moving, so these make up nearly all stationary phases. This leads to subcategories within each mode of chromatography: gas-solid chromatography GSC, gas-liquid chromatography (GLC), liquid-solid chromatography (there are several modes), and SFC, which usually uses solids as the stationary phase. Figure 1 shows a summary chart of the various modes of gas and liquid chromatography and their acronyms. We will come back to more details on the definitions of the acronyms for GC.

The requirement that the mobile phase moves in a definite direction limits the physical configuration of chromatographic systems. In gas chromatography, the only way to ensure that the gaseous mobile phase moves in a definite direction is to drive or pull it through a tube or column. As we will discuss later, this column can either be packed with particles of the stationary phase or be an open tube, with the stationary phase coated or bound to the wall of the tubing.

By contrast, if the mobile phase is a liquid, the movement in a definite direction can be achieved by pushing the liquid through a packed (most common) or open tube, or the mobile phase can move by capillary action on a planar surface, as in thin-layer chromatography or paper chromatography. For SFC, the stationary phase is almost always a solid.

Elution refers to the process of passing the mobile phase through the system and collecting or detecting the separated compounds at the outlet of the column or developing the separation as spots on a paper or plane. When compounds are detected at the column outlet following separation, they are said to have eluted. Finally, chromatography requires that the sample mixture be separated by being introduced to the system (column or plane) as a narrow, finite plug or band. The term band, derived from the colored stripes originally seen in classical column liquid chromatography, is still used to describe chromatographic sample compounds that are still on the column.

Referring to Figure 1, for gas chromatography, we can see that there are two types of columns: packed columns, in which the stationary phase consists of solid particles; and liquid-coated solid particles that are packed into a tube. In this case, there are two modes of GC: GSC, in which the stationary-phase particles are only solid, and GLC, in which the stationary-phase particles are coated with a liquid. This liquid is usually a viscous substance, such as a silicone polymer. Packed-column gas chromatography is not widely used today, except for training and the analysis of highly volatile analytes. At its height in the 1960s–1980s, there were hundreds of stationary phases available for packed-column GC. Packed-column GC remains the simplest and least expensive entry point for introduction to GC.

If the column is an open tube, also termed a capillary column, most used in today’s instruments, there are three stationary-phase modes available. The simplest is GLC, or wall-coated open-tubular (WCOT), in which a liquid stationary phase is thinly coated or chemically bound to the inside surface of the column. For GSC in capillary columns, the terminology is PLOT (porous layer open tubular). In this configuration, a porous solid adsorbent is bound to the inside wall of the tubing. Finally, in a few applications, a SCOT, or support-coated open-tubular configuration, combines packed and open-tubular columns by coating the inside of the tube with liquid-coated solid particles. Most modern gas chromatography is done using GLC with WCOT columns.

Table 1 provides a summary of the most important terms and symbols in chromatography, as recommended by IUPAC, and that follow from the definition of chromatography described above. We will be discussing the terms and definitions in Table I over the next couple of columns. As you review them, be reminded that chromatography, along with all separation methods such as extraction, is based on the drive to equilibrium of a simple phase transition.

Note that in all forms of column chromatography, if the analyte is injected into the system as a finite plug, it must enter the column and first contact the stationary phase while it is dissolved or distributed in a mobile phase, so the mobile phase appears as the reactant in equation 1.

In gas-liquid chromatography, by far the most common form of capillary gas chromatography, the first critical term is the distribution constant for equation 1, shown as an equilibrium constant expression in equation 2.

In a later column, we will discuss details of the underlying thermodynamics; think about the basics that you learned back in first-year college chemistry and how this connects to the retention times that we measure.

Both the numerator and denominator of equation 2 are concentration terms that are usually expressed as mass divided by volume. This leads to the need for definitions for volume terms, V_M, V_S, and V_R. V_M is the volume of the mobile phase, V_S is the volume of the stationary phase, and V_R is the retention volume, the volume of mobile phase needed to be added to the column to elute an analyte from the moment of injection to its emergence from the column outlet.

The analogous time-based definitions are the more familiar terms, retention time, t_R, the total time required between sample injection and the emergence of the peak maximum from the column outlet, and holdup time, t_M, the time required to elute a compound that is not retained by the stationary phase. These are related to the retention volume and mobile phase volume through the volumetric flow rate, F_C, which is usually expressed in mL/min. Volume (mL) divided by flow rate (mL/min) gives time.

Please note that when reporting chromatographic or any scientific data, the correct use of symbols and definitions is extremely important. For example, in data tables and discussions, it is better to use the correct symbol, t_R, to abbreviate retention time, not “R.T.,” which seems to have come into common usage. There are also several older common usages that we will discuss in the next column as we dive more deeply into the definitions in Table I.

In this first installment of a series on terminology and nomenclature, we have defined and reviewed some theory behind several of the most fundamental terms in chromatography. We saw how the definition of chromatography defines the many chromatographic methods used in modern analytical science. In the next column, we will dive more deeply into the various terms and figures of merit that we calculate when interpreting and evaluating chromatographic data. Finally, in part 3, we will examine the fundamental chemistry and thermodynamics that underlie retention in gas chromatography. Understanding these terms, definitions, and theory will help us all to communicate, discuss, and think about chromatographic data more clearly and effectively.

References

1. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography; John Wiley & Sons: Hoboken, NJ, 2019, Chapter 1 and references.
2. Ettre, L. S. Nomenclature for Chromatography. Pure Appl. Chem. 1993, 65 (4), 819–872.
3. Maryutina, T. A.; Savonina, E. Y.; Fedotov, P. S. et al. Terminology of Separation Methods (IUPAC Recommendations 2017). Pure Appl. Chem. 2018, 90 (1), 181–231. DOI: 10.1515/pac-2017-0111