It's All About Selectivity

Feb 01, 2012

In gas chromatography selectivity is commonly thought to be about exclusivity of the column stationary phases and sometimes extended to the detector. This instalment of "Column Watch" explains how to incorporate selectivity throughout the sample analysis cycle, from the sampling analysis cycle through data analysis and report generation.

Many samples to be analysed using gas chromatography (GC) are complex. There may be many analytes of interest to be separated, possibly at very low concentrations. Some of the analytes may be difficult compounds, meaning they may be thermally labile, active or of too high molecular weight for analysis by GC. There also may be a high loading of matrix and the presence of matrix interferents. From sample collection to data analysis, there are many steps in the process. Of course, the final step is the reporting of the required information about a sample. To get reproducible and accurate results, it is important to think about and understand the sample first, and then to identify the steps to select the best techniques for the analysis.

Figure 1: Steps in sample collection, preparation and analysis.
In chromatography, when selectivity or being selective is mentioned, first thoughts are always directed toward the analytical column's stationary phase, especially in GC where this parameter has the biggest effect on the separation of analytes. In GC, the mobile phase, a gas, has little interaction with analytes, the matrix or the stationary phase, and mainly serves to transport compounds down the column. However, each of the steps identified in the collection, analysis, and reporting of a sample can have selectivity applied to it. Figure 1 identifies some of these key steps. Although some of the steps involved in selectivity enhancement are often the same for gas and liquid chromatography, I will focus this discussion on considerations in GC.

Step 1: Sampling

The first and most important step in any separation is to determine how to collect a representative sample, enough for the analysis. One must consider the nature of the sample (gas, liquid, solid or something in-between); the analytes (volatility, stability); and the location of the sample. It may be relatively easy to collect and return some samples to the laboratory for analysis, while in other cases, sample changes (such as oxidation, chemical breakdown or chemisorption) can occur during the transport or storage process. It may be the case that the analysis needs to be performed in situ.

Selectivity can be considered in this first step — to selectively collect only the portion of the sample of interest with as little matrix as possible. Selectivity can be based on volatility, where only analytes within a certain volatility range are sampled, or on the nature of the analyte, where only target analytes with certain characteristics such as polarity are collected. In a case where there is a high level of matrix interferents, a technique may be selected to avoid these; for example, a reversed-phase solid-phase extraction (SPE) sorbent may be used to concentrate organics and to avoid trapping water.

One technique for selectively sampling on site is thermal desorption; the thermal desorption tube packing material is selected to retain the target analytes and not the matrix. Solid-phase microextraction (SPME) and stir-bar sorptive extraction (SBSE) are other techniques where the phase can be selected for the analytes and not the matrix, and can be used on site.

Step 2: Sample Preparation

The aims of sample preparation can be to make the analytes of interest amenable to GC; to concentrate the analytes; and to remove matrix (if possible). Selectivity can also be considered at this step, to extract only the analytes and not the matrix. Again, selectivity can be based on volatility or the nature of the analytes.

For example, when using SPE, analysts can choose a phase that is selective for either the analytes or the matrix, by thinking about the chemistry of the analytes and selecting a sorbent stationary phase that provides the best interaction to enable extraction of the analytes from the sample. Likewise, in SPME, SBSE and liquid–liquid extraction (LLE), the solvent can be selective. Techniques like headspace and purge-and-trap sampling use volatility to selectively extract the analytes from the sample. Whereas parameters like temperature are important to ensure the best extraction efficiency of target analytes, their effect on the coextraction of matrix components must also be considered. With dynamic headspace and purge-and-trap sampling, the selectivity of the trap can also be chosen to reduce the trapping of matrix coextractives.

Unfortunately, during sample preparation there is often the potential for the loss of analytes and the concentration of unwanted matrix. Given that there are many steps in the analysis of the sample where we can be selective for one over the other, it is important not to lose analytes at this step for the purpose of removing matrix; the latter can be examined again later on in the process. One important factor to consider in sample preparation is that the more individual steps used to isolate or concentrate the analytes of interest, the bigger the chance for analyte loss.