Sample Volume by Pressure Injection in CE

The volume of the sample that gets injected in the capillary is given by:

Figure 1



in which A is the cross-sectional surface of the capillary, u the average velocity during injection and tinj. the injection duration. The internal diameter of the capillary is know and the injection time is set in the method. The average velocity is calculated from:

Figure 2




Here ΔP is the injection pressure as set in the injection method, dc is the internal diameter of the capillary η is the sample solvent viscosity and L the length of the capillary (here total length and not the length to the point of detection). In practice, the instrument will apply a time-dependent pressure profile during the injection time such that the integral of ΔP.tinj.  has the correct value.

Substitution of equation 2 in equation 1 results in:

Equation 3

For example, a capillary with 50 µm i.d. and 500 mm length, injection pressure 50 mbar and sample solvent water with viscosity 1 cP results in an injected volume of 16.3 nL in case of injection time of 10 s.