Analysis of Caffeine in Dietary Products by Multiple Injection Capillary Electrophoresis - Caffeine: Chemistry, Analysis, Function and Effects - page 168

Table 9.5 Accuracy of the caffeine determination at different concentrations

(n 5 10). Operation conditions as those given in Figure 9.3.

Caffeine Concentration (mgmL -1 ) Accuracy % (¡RSD)

25 98 ¡ 6.0%

d n 0 t 2 n g | 4

50 99 ¡ 5.8%

75 104 ¡ 2.1%

100 101 ¡ 3.4%

125 100 ¡ 2.9%

200 100 ¡ 2.4%

250 99 ¡ 2.8%

300 100 ¡ 2.5%

350 100 ¡ 1.9%

400 101 ¡ 1.6%

450 102 ¡ 2.3%

500 100% ¡ 2.6%

600 101% ¡ 1.5%

650 102% ¡ 2.5%

700 100% ¡1.3%

800 99.4% ¡ 1.2%

900 100.2% ¡ 1.3%

times were shorter at higher temperature, i.e.,25 o C, as a result of increased
EOF and decreased viscosity of the BGE. The migration times were longer at
pH 5.3 because of a decrease in the electroosmotic mobility.
The observed variations in the separation parameters did not complicate the
conversion of these modified methods to the multiple-injection mode. The
relatively small changes in the migration times and Dt mig could be compensated
for by the adjustment of the t PE in order to maintain the original number of
injections (i.e., n inj 5 5), Table 9.6.
The results illustrate the necessity of performing an initial separation in
single injection mode upon any changes in the separation conditions, e.g.,
replacement of the capillary column (Lod´n et al 2008). The primary analysis
in single injection mode can be considered as a system suitability test.
The robustness of the developed method was further demonstrated by
constructing a standard graph consisting of twenty one different caffeine
concentrations analyzed in different capillaries and days, Figure 9.5.
The results demonstrated that the separation system was robust towards the
induced changes. The slight changes in the separation parameters could be
compensated by adjusting the t PE .

9.6 System Suitability and Acceptance Criteria
The migration time of caffeine in the first injected sample should be between 2
and 4 minutes. A significant drift in the migration times is a result of a change
in the BGE composition, e.g., evaporation of acetonitrile, or in the inner
surface of the capillary causing variations in EOF. The later issue can be