Chemistry Reference
In-Depth Information
(a)
Sampler
Column
(b)
Detector
Capillary
High pressure
pump
Detector
High voltage
power supply
Eluent
Waste
Buffer/
sample
Buffer
Figure 9.1
(a) Schematic diagram of an HPLC instrument. (b) Schematic diagram of CE equipment.
For sampling, the buffer reservoir is replaced by a sample reservoir.
typically utilizes high electric fields for separation of an analyte, which might be based on its charge, size, or
hydrophobicity. The same detector could be used for both HPLC and CE, but the column, which is the
essence of the separation method, differs significantly. CE uses capillaries 50 cm in length with an internal
diameter of 10-100
m. In contrast, HPLC uses standard columns of 15 cm length with a 4.7 mm diameter.
Whereas most HPLC separations are performed using an H
2
O/ACN gradient, which elutes samples in a
reverse phase column, the choice of CE buffer systems is much more flexible. In CE, the sample is introduced
by immersing the end of the capillary into a sample vial and applying pressure - vacuum or voltage. HPLC
uses mechanical six-port two-position rotary valves for sample introduction. The basic schematics of typical
CE and HPLC instruments are shown in Figure 9.1. Note the functional similarity of both separation methods.
However, there are differences: the driving force of the mobile phase in CE is fundamentally different from
that of HPLC and detection in CE must be performed on the column in order to not compromise the efficiency
of the separation. HPLC needs a special device for sample introduction, whereas CE sampling, as mentioned
above, can be accomplished by exchanging sample/buffer vials.
μ
9.2.2
CE versus HPLC from the point of view of Green Analytical Chemistry
CE is a powerful separation technique that uses minimal amounts of solvent. As already mentioned, a typical
CE column is a narrow capillary made of fused silica with an inner diameter (i.d.) of 50
μ
m and a length of
approximately half a meter. It has a volume of about 5
μ
l. Because electroosmosis - the force that propels the
eluent in CE - has a flow rate of sub-
l min
−1
, given a typical runtime of 10 min, it follows that eluent
consumption during a CE run is almost nonexistent. ACN and other harmful solvents are rarely used as eluents
in CE because most separations are achieved in non-toxic buffer systems such as phosphate and borate.
Despite the striking functional similarity between the two techniques (as demonstrated in Figure 9.1), the
capillary electropherograph has a much simpler construction because of the lack of moving mechanical parts.
Instead of high precision, elevated pressure pumps and rotary valves, only a high voltage power supply is
required for CE.
In Green Analytical Chemistry, reduction of the usage of harmful eluents (like ACN) seems to be the main
criterion for a method to be considered green. How does CE compare with different modes of HPLC in this
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