Topics - Quantitative Analysis

Quantitative Analysis In chromatography there are two types of analysis ‘qualitative analysis’ which identifies the solute or solutes present in a mixture and ‘quantitative analysis’ which determines how much of a substance or substances is present in a mixture. Qualitative data is obtained from retention measurements; quantitative data is obtained from peak height or peak area measurements. Quantitative analysis makes certain demands on the chromatographic apparatus, particularly the injection system and the detector. In the first instance, a truly representative sample must be placed on the column by the injection system and secondly, the detector must have a linear response that is known and defined by its response index. For accurate work the response index of the detector should lie between 0.97 and 1.03 over the concentration range used. Providing the separation is highly reproducible, peak heights can be used as a relative measurement of the quantity of material present. To obtain a value proportional to the mass of solute present, a ‘response factor’ must be used for each substance determined and these response factors are obtained by prior calibration. Alternatively peak areas can be used for quantitative assessment and peak area measurements are claimed to be more precise. Peak area measurement may also be necessary if peak profiles are not close to Gaussian in shape and significantly distorted. Almost all quantitative measurements are obtained using an internal or external standard chosen so that it is eluted discretely and well separated from other components of the mixture. Quantitative analysis is probably the major application of chromatography techniques.