In this lesson, we focus on three fundamental aspects of gas chromatography (GC) instrumentation that directly determine the quality and reliability of an analysis. First, we introduce retention indices, which provide a standardized way to describe retention behavior and enable comparison of chromatographic data between instruments and laboratories. Next, we examine sample-introduction techniques, highlighting both the widespread use of split/splitless injectors and their limitations due to discrimination effects, as well as advanced approaches such as cold on-column and programmed-temperature vaporization (PTV) injection. Finally, we discuss the wide variety of GC detectors, ranging from universal and selective to element-specific, and consider their key performance characteristics. By the end of this lesson, you will understand how retention indices, injection methods, and detectors together form the foundation of robust and reproducible GC separations.
Learning Goals #
After this first class of the course you will be able to
- Explain the principle of retention indices and apply them to compare retention data across different GC systems.
- Describe the operation, advantages, and limitations of the main injection techniques (split/splitless, cold on-column, PTV, headspace, and thermal desorption).
- Recognize and explain discrimination effects during injection and their impact on quantitative accuracy.
- Distinguish between universal, selective, and element-specific detectors, and summarize their key performance parameters (sensitivity, detection limit, dynamic range).
READ SECTIONS 2.1.1 & 2.1.2
Basic Instrumentation and Operation of GC
1. Introduction #
Gas chromatography (GC) is a well-established technique, widely used in laboratories and, to a lesser extent, in process and portable instruments. Most GC systems are designed for high resolution and precise retention-time measurement, which requires accurate oven temperature control. High-thermal-mass ovens offer precise programming but are slow and energy-intensive, while low-thermal-mass systems enable faster operation but struggle with temperature homogeneity.
A second major challenge is the unbiased introduction of volatile samples, which will be a central focus of this lesson. Detectors in GC are highly diverse, but since these are extensively covered in the textbook, we will limit ourselves to general aspects.
In this lesson, we will address three core aspects of GC instrumentation: retention indices, injection techniques, and detection principles.
2. Retention indices #
We have seen in Lesson 10 that gas chromatography (GC) is a true high-resolution separation method with in excess of 100,000 plates being routinely available. This also means that small variations in the stationary phase, the temperature, and sometimes even the mobile phase may affect the resolution of peaks. Small variations in the flow rate will also affect retention times, making these hard to reproduce between different columns and instruments.
Retention indices (I) are based on the Martin rule, which applies to homologues series, such as n-alkanes. It states that in non-programmed (isothermal) GC the logarithm of the net retention times of the members of the series increases linearly with the number of carbon atoms. This is illustrated in the figure below.
Table 1. SWOT (Strengths, Weaknesses, Opportunities & Threats) analysis of gas chromatography.
Retention indices have been developed as a means to record analyte retention data that can be tabulated or stored in libraries, for example in combination with mass spectra.
Figure 1. Schematic depiction of a gas chromatography instrument.
Here x indicates the analyte and the last n-alkane eluting before and the first n-alkane eluting after are denoted by “before” and “after”, respectively.
Retention indices are – in principle – independent of the mobile-phase velocity and the column dimensions (length, internal diameter, stationary-phase film thickness) but they do depend on the temperature and on the exact composition of the stationary phase.
Figure 2. Different stationary-phase formats in GC and properties of fused silica.
1.2. Stationary phases #
Cross-linked polymers are typical stationary phases. Polysiloxanes are by far the most popular, because they show the highest analyte diffusion coefficients. Poly(dimethyl siloxane) (PDMS) is one of the most widely used stationary phases, particularly for non-polar and moderately polar analytes. It is chemically inert, thermally stable, and offers high analyte diffusion coefficients, which contributes to efficient mass transfer and sharp peaks. The polymer is typically crosslinked to ensure it remains immobilized on the column wall and to minimize column bleed, especially at elevated temperatures.
