Analytical Chemistry

Table 4.21 Detectors for capillary electrophoresis

Method Mass detection limit

(moles)

Concentration detection

limit (molar)a

Advantages/disadvantages

UV-vis absorption 10 –^13 – 10 –^1610 –^5 – 10 –^8 • universal

• diode array offers spectral information
  fluorescence 10 –^15 – 10 –^1710 –^7 – 10 –^9 • sensitive


• usually requires sample derivatization
  laser-induced
  fluorescence

10 –^18 – 10 –^2010 –^14 – 10 –^16 • extremely sensitive^

• usually requires sample derivatization


• expensive

amperometry 10 – (^18) – 10 – 19 10 – (^10) – 10 – 11 • sensitive

• selective but useful only for electroactive
  analytes


• requires special electronics and capillary
  modification
  conductivity 10 –^15 – 10 –^1610 –^7 – 10 –^8 • universal


• requires special electronics and capillary
  modification
  mass spectrometry 10 –^16 – 10 –^1710 –^8 – 10 –^9 • sensitive and offers structural
  information


• interface between CE and MS
  complicated
  indirect UV,
  fluorescence,
  amperometry

10 – 100 times less than

direct method

• • universal
  
  
  • lower sensitivity than direct methods
  
  
  
  

others:

Radioactivity, thermal lens, refractive index, circular dichroism, Raman

a Assume 10 nl injection volume.

sample solution and a very small volume (1–50 nl) introduced into the capillary either using gravity,
positive pressure or a vacuum (hydrodynamic method) or by applying a voltage across the capillary
whilst the other end is immersed in the buffer solution causing electrophoretic migration into the tube
(electrokinetic method). Reproducible injections can be difficult to achieve, especially with
electrokinetic injection, because of the number of parameters involved, e.g. time, pressure drop, solution
viscosity and capillary dimensions. Typically, the relative precision of injection is 2–3%, although some
systems now available are capable of better than 1%. With the electrokinetic method, differences in
electrophoretic mobility between sample components can result in discrimination effects whereby the
composition of the injected sample may not be identical to that of the bulk sample.