Terminology and Nomenclature for Gas Chromatography, Part 2: Reading Chromatograms and Presenting the Data

In Part 1 of this series, we discussed the fundamental nomenclature and terminology, based on the International Union of Pure and Applied Chemistry (IUPAC) recommendations, that should be used when discussing gas chromatography (GC). We now turn to the data and fundamental data analysis that should be part of analyzing, understanding, and discussing GC results. We will start by examining a chromatogram and explore the fundamental meaning and reporting of terms we use every day, including retention time, hold-up time, peak width, retention factor, and more. As chromatographic literature has evolved, we see GC data described using many formats and terms; it is important to return to the IUPAC definitions to ensure that all reported data are easily comparable across the literature. This will lead into a discussion of the fundamental chemistry underlying retention and separation in GC, which we will explore more deeply in Part 3 of this series.

The correct nomenclature and terminology for chromatography have been defined by the IUPAC and should be used whenever analyzing, discussing, and publishing chromatographic data.^1,2 We obtain a lot of data and can perform many calculations from any chromatogram. Using the correct terminology and definitions ensures that other scientists can understand, review, and repeat our work. New users especially should become familiar with the correct terms, and experienced users should be reminded of them.

The data plot generated in a chromatographic experiment is called a chromatogram, not a chromatograph, spectrum, or other terminology. Figure 1 shows a typical chromatogram, with several important parameters labeled. This chromatogram was generated using Pro-EZGC, an online chromatogram simulator, and shows the separation of hydrocarbons on a nonpolar stationary phase under isothermal conditions.³ First, note the axis labels, which are measures of time (x-axis) and detector signal (y-axis). Whenever presenting a chromatogram, be sure to clearly label the axes with appropriate units. I will often import a chromatogram from my data system into a graphics program and relabel the axes to make them more readable. Axes copied directly from screens and data systems are often difficult to read. Way back in the day, chromatograms were printed on paper, and axes were labeled by hand or using rub-on lettering.

The chromatogram, which is a plot of signal versus time, has several characteristics. The areas where the plot rises, reaches a maximum, and falls are called peaks. The flat areas before, after, and between the peaks are called baseline. If the baseline slowly rises or falls but a peak is not generated, this is called drift. Drift due to decomposition of the stationary phase, called column bleed, is very common in temperature-programmed separations as the temperature approaches the maximum allowable temperature for the stationary phase.

The first and most common term shown in Figure 1 is the retention time (t_R), which is defined as the length of time from the instant of injection to the maximum signal of a peak. Note that for most calculations and determinations that use retention times to identify analytes, the retention time assumes a symmetrical, Gaussian peak. Retention time will shift if the peak is not symmetrical. If a peak can be viewed as representing the mass distribution of molecules that pass through the column over time, the retention time of a symmetrical peak represents the average or center of mass of the population of molecules represented by the peak.

As a population of analyte molecules passes through the column, each molecule can be in one of two phases: adsorbed on (in gas-solid chromatography) or dissolved in (in gas-liquid chromatography) the stationary phase or vaporized in the mobile phase. When in or on the stationary phase, the molecules are not moving down the column, although they are diffusing. When in the mobile phase, they are moving along the column, driven by the flow of the mobile phase (in GC, the carrier gas).

This leads to two additional time definitions, the hold-up time, t_M and the adjusted retention time, t’_R. Hold-up time is the amount of time required for an analyte that is not retained at all by the stationary phase to traverse the column, from injection to the detector. This is often measured by injecting methane or another small molecule, an easily detected gas, and measuring its retention time. Classically, a butane lighter could be used to provide a sample of butane for manual determination of hold-up time. Modern data systems automatically calculate hold-up time using the column dimensions and experimental conditions. Hold-up time is discussed in detail in a previous installment of GC Connections.⁴

The difference between the retention time and the hold-up time yields the adjusted retention time, as shown in Equation 1. Subtracting time spent in the mobile phase from time in the column provides the adjusted retention time, the time an analyte spends in or on the stationary phase. In the isothermal (constant temperature) separation seen in Figure 1, we see that there is one hold-up time, and each peak has a different adjusted retention time and a different retention time. Note that as the hydrocarbon chain gets longer, the adjusted retention time and the retention time increase due to the increased strength of intermolecular forces between the analyte and the stationary phase with the longer chains, while the hold-up time remains the same.

There are several additional calculations that are often performed using retention time data. Table I provides a summary of the IUPAC definitions of several commonly calculated terms used in gas chromatographic data analysis.

We will now define these terms practically, and we will discuss their theoretical background in part 3 of this series. The retention factor, k, formerly known as the capacity factor, k’, is calculated from the adjusted retention time and the hold-up time as shown in Equation 2.

Fundamentally, this represents a ratio of the mass or number of molecules of analyte in the stationary phase (m_S) to the mass or number of molecules in the mobile phase (m_M). A higher retention factor indicates stronger interaction between the analyte and the stationary phase. In classical column liquid chromatography, Equations 1 and 2 were calculated using the corresponding volumes of solvents used. Stronger analyte interaction with the stationary phase meant more solvent was needed to elute it.

In gas-liquid chromatography, the phase ratio is the volume of the mobile phase divided by the volume of the stationary phase, seen in Equation 3.

If the retention factor, expressed as mass of molecules in each phase, and the phase ratio are combined and the common variables collected, the partition coefficient for the phase transfer between the mobile phase and the stationary phase is obtained, as seen in Equation 4 and in Equation 1 from Part 1 of this series last month.⁵ This resulting equation is the equilibrium constant expression for a phase transfer process with the analyte (A) starting in the mobile phase and moving to the stationary phase. In GC, the analytes are generally vaporized in the inlet and enter the column in the vapor phase, moving in the mobile phase.

Selectivity (a) is a measure of the separating power of a stationary phase. A higher value for selectivity indicates more separating power. Selectivity is most often determined for closely eluting or similarly structured analytes to demonstrate the separating power of a stationary phase for those analytes. It is calculated as a ratio of the adjusted retention times of two usually adjacent peaks and is shown in Equation 5.

Note that the selectivity measured as the ratio of adjusted retention times is also equal to the ratios of the retention factors and the partition coefficients. We will use this idea in part 3 of this series to discuss the thermodynamic background of differences in retention time that lead to separation in GC.

Besides the retention time, the peak width is a second critical parameter of every peak. We have all noted that peaks get wider as retention times increase. Peaks getting wider as they traverse the column is not a thermodynamic but a kinetic phenomenon, governed by the dimensions and gas flow in the column and diffusion of analyte molecules in both the mobile and stationary phases. We discussed this in more detail in a previous column.⁶

Peak width is measured at one of two points on the peak, the baseline (W_b) and one-half of the peak height (W_1/2). The half-height measurement is useful in situations where the baseline drifts underneath the peak (the baseline is higher or lower at the end of the peak than at the beginning) or when peaks overlap.

The rate of peak broadening is governed by the plate number, N (previously called number of theoretical plates), and the plate height, H (previously called the height equivalent to a theoretical plate, HETP). A plate can be pictured as one phase transfer process between the mobile and stationary phases, and the column can be pictured as a series of these transfers, one after the other, as the analyte molecules move along the column. The number of plates is calculated from the retention time and the peak width as shown in Equation 6.

More plates in a column generally mean more efficient separations, with some limitations that will be discussed in the upcoming Part 3. Most chromatographers use H, the plate height, as a measure of the rate of band broadening; a lower value for H generally means slower band broadening, sharper peaks, giving better separations. H is calculated from the number of theoretical plates and the column length, seen in Equation 7.

In this column, we have now defined the fundamental terms and terminology for chromatographic data analysis. In our next column, we will bring these terms together and connect them to the fundamental thermodynamic and kinetic principles that drive separations and produce resolution, or complete separation of analytes. We will see that the simple, fundamental thermodynamics and kinetics that we learned in introductory chemistry courses can explain much of what drives all separations and extractions, especially chromatography.

References

1. Ettre, L. S. Nomenclature for Chromatography. Pure Appl. Chem. 1993, 65 (4), 819–872. DOI: 10.1351/pac199365040819

2. Maryutina, T. A.; Savonina, E. Y.; Fedotov, P. S. et al. Terminology of Separation Methods (IUPAC Recommendations 2017). Pure Appl. Chem. 2017, 90 (1), 181–231. DOI: 10.1515/pac-2017-0111

3. Pro EZGC Chromatogram Modeler. Restek website.https://ez.restek.com/proezgc (accessed August 21, 2026)

4. McCann, S. P.; Rana, H.; Handzo, B. A. et al. Go With the Flow: Thinking About Carrier Gas Flow in GC. LCGC North Am. 2020, 38 (3), 152–158.

5. Snow, N. H. Terminology and Nomenclature for Gas Chromatography, Part 1: Fundamental Definitions. Chromatography Online website. https://www.chromatographyonline.com/view/terminology-and-nomenclature-gas-chromatography-part-1-fundamental-definitions (accessed August 21, 2026)

6. Snow, N. H. Is Golay’s Famous Equation for HETP Still Relevant in Capillary Gas Chromatography? Part 2: Assumptions and Consequences. LCGC North Am. 2022 40 (2), 69–71. DOI: 10.56530/lcgc.na.mm7085y7