Sample Volume by Pressure Injection in CE

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

in which A is the cross-sectional surface of the capillary, u the average velocity during injection and t_inj. 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:

Here ΔP is the injection pressure as set in the injection method, d_c 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.t_inj.  has the correct value.

Substitution of equation 2 in equation 1 results in:

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.