Analytical Limbo: How Low Can You Go?

Aug 31, 2006

In analytical chemistry, the continual quest for enhanced sensitivity and specificity — in gas chromatography (GC), this can be equated to separation power — remain the common goal in the development of new analytical methodologies. Today, GC is still the most widely used method for the analysis of volatile and semivolatile organic compounds. When coupled with the right choice of detector for the specific application, a wide linearity range and low limit of detection (LOD) can be met. For GC analyses, many approaches can be used to achieve greater sensitivity and lower LOD. They can be classified broadly into four categories: improved sampling (sample preparation) strategies; sample introduction methods; improved chromatographic performance; and alternative (selective–sensitive) detection transducers. This article provides an up-to-date review of existing and emerging chromatographic innovations, based upon these four strategies, that will improve sensitivity and detection limits of trace analysis in GC.

The demand for enhanced sensitivity and detectability in analytical technologies has never been greater. The need to measure trace levels of numerous environmental pollutants is regulated by strict laws in many countries, as most of them pose serious health risks and threats to the ecosystem. These include polychlorinated biphenyls (PCBs), polyaromatic hydrocarbons (PAHs), and a vast array of modern pesticides and toxicants. Thus, when complex sample matrices are involved, almost invariably, a tedious sample workup and a highly sensitive analytical solution are required for their separation and detection.


Figure 1
The more frequent use of hair and other alternative matrices (for example, saliva and nails) in clinical and forensic applications also demand greater sensitivity and lower detection limits for qualitative and quantitative measurements of a variety of possible analytes (for example, drugs), as they generally are present in low concentrations (1,2). For postmortem analyses in particular, the limited quantity of some biological matrices (3) (for example, due to severe putrefaction), also intensifies the quest for sensitive analytical methods. By IUPAC definition, sensitivity of an analytical method is defined by the slope of the calibration curve (4). Hence, when comparing two analytical methods, the method that produces the steeper linear calibration curve for the same analyte is considered to be more sensitive, provided that their "noise" is approximately equivalent. While limits of detection (LOD) provide good indication of the instrument performance, LOD is not considered a direct measure of sensitivity (5,6). Despite this, LOD is generally an unofficial yardstick for estimating analytical sensitivity or suitability of an instrumental technique for trace analysis. For the purpose of this article, a broad definition of sensitivity will be adopted with the term sensitivity referring to both the previous official and unofficial meanings.

Gas chromatography (GC) is the method of choice for analyzing thermally stable volatile and semivolatile substances due to its acknowledged "speciation" or molecular isolation capability and the generally accepted sensitivity of the method. The use of appropriate derivatization methods for nonvolatile and thermally unstable components extend GC applications to a much wider suite of compounds (7). When coupled with the right choice of selective detector, the requirement for wide linearity range and low detection limits can be met. Figure 1 lists the working range of some of the most common GC detectors (8).


Figure 2
For GC analyses, many approaches can be used to achieve greater sensitivity and reduce the analytical LOD. They can be classified broadly into four categories: improved sampling (sample preparation) strategies; sample introduction methods; improved chromatographic performance; and alternative (selective–sensitive) detection transducers, as illustrated in Figure 2. Each of these components is integral to the overall analytical GC process, and their individual contribution fuels the primary aim of this article, which is to review approaches that will permit the analyst to "go low" to meet the sensitivity needs of ultratrace analysis in GC. The following discussion provides an overview, rather than an exhaustive treatise, on these approaches.

Sample Preparation (Sample Workup)

In reality, analytes rarely are present in their pure forms in a sample and, thus, sample preparation to isolate or release the target analytes from matrix interferences is almost always necessary; a signal increase through coelution is clearly an artificial contributor to sensitivity enhancements. Most sample preparation processes incorporate a concentration step before the reconstitution of the extract in an appropriate solvent before GC analysis. This concentration step is not only time consuming and labor-intensive but also risks the loss of volatile analytes. The right choice of derivatization method also can confer sensitivity to the analysis by the simultaneous improvement in an analyte's peak height (and symmetry) and chromatographic performance.