Instant Notes: Analytical Chemistry

percentage or fraction of the total. Internal normalizationis of particular value in

quantitative chromatography where several components of a sample can be

determined simultaneously, and absolute levels are not of interest. The relative

composition is calculated from the instrument response, peak area in the case of

a chromatographic analysis, for each component in the mixture using the formula

%xi= × 100

where xiis one of ncomponents and Ais the measured area or response.

Example

Figure 5is a chromatographic record (chromatogram) of the separation of a 5-

component mixture. The measured peak areas (using electronic integration with

a computing-integrator, computer and chromatography data processing software

or geometric construction such as triangulation,^1 ⁄ 2 ¥base ¥height) and percent-

ages by internal normalization, which must total 100 percent, are given in Table 1

(e.g., for component 1, relative percent =(167.8/466.94) ¥ 100 =35.9 percent).

Table 1. Peak areas and percentage composition by internal normalization for a

5-component mixture

Component Measured peak area Relative percent

(arbitrary units)

1 167.8 35.9

2 31.63 6.8

3 108.5 23.2

4 80.63 17.3

5 78.38 16.8

Totals 466.94 100.0

Ax






i=n

i= 1

Ai

48 Section B – Assessment of data

Inject

0369121518

Time (min)

1

2

3

4

5

Fig. 5. Chromatogram of a 5-component mixture.