Understanding the response factors of a GC detector

Gas Liquid Chromatography (GLC) or simply Gas chromatography (GC)is a versatile technique that helps identify and quantify constituent compounds of complex organic mixtures. Primarily such compounds should be volatile in nature and should not decompose at high temperatures inside the injector and the column oven.

 The technique offers a wide choice of detectors. For any application it is desirable that the detector should fulfill the following basic requirements:

Gas Chromatograph cut out view showing detector

Desirable characteristics of GC detectors

• Response to eluting compounds in carrier gas should be reproducible under same operating conditions.
• High sensitivity towards eluting compounds.
• The linearity of response over a wide range of concentrations.
• Small detector volume to avoid peak broadening with resultant loss of resolution.
• The detector should be preferably non-destructive.

The desirable attributes of a detector can be expressed in quantitative terms through detector response parameters. However, before proceeding further a clear understanding of detector response parameters is essential.

Detector response parameters

Dynamic range expresses the range of concentrations of the analyte over which the response of the detector is in direct proportion to change in concentration of the analyte. The maximum of the range is the concentration at which the detector no longer responds to increase in concentration.

Minimum detector sensitivity is defined as the minimum concentration of analyte that can be discriminated from noise. It is the concentration at which the concentration is twice the noise level.

The linear Dynamic range is the concentration range over which the detector response is in a linear relation with the change in concentration of the analyte.

Detector response is the output in volts for a unit change in analyte concentration flowing across the detector sensor. In case of the mass sensitive detector, it is the voltage response for a unit flow of mass across the detector sensor.

Noise results in abrupt fluctuations seen on the baseline which cannot be attributed to changes in the concentration of analyte reaching the detector in the carrier gas stream. Noise can be termed as short term, long term or drift.

Short term noise results because of baseline perturbations that have frequencies significantly higher than those of the eluting compounds. It does not pose serious problems and can be filtered out electronically.

Long-term noise arises from baseline perturbations that have frequencies comparable to those of eluted compound peaks. Such noise is more disturbing as it cannot be easily differentiated from peaks of eluted compounds. It cannot be removed by electronic filtering without affecting the profile of eluted compounds. Long-term noise affects the limits of detection or sensitivity of detectors.

Drift results from perturbations that have frequencies significantly larger than those of eluted peaks. This results in a gradual drift in the slope of the baseline. Common reasons for drifts are changes in operational parameters such as temperature, flow rate, column bleed, etc