Instruments are also used to determine the % of a constituent in a sample. This is quantitative analysis.

For quantitative instrumental analyses, the size of a response is measured and compared to the response of a standard under the same conditions. For example, in chromatography the area of peaks are measured and compared to the area of peaks of a standard. When absorption spectroscopy is used, the intensity of the absorption of the sample is compared to the intensity of absorption of the desired constituent in a known standard.