Chemistry Reference
In-Depth Information
emerged only recently [7]. However, the development of CE-MS technology does not contribute significantly
to the discussion of CE as a green analytical method and thus remains outside of this review.
Nevertheless, CE is popular in academia (approximately 2000 research papers are published yearly)
because it provides interesting material for studies (there are endless possibilities to alter the composition of
the separation buffer, and the technique demonstrates superior efficiency compared to other separation
methods). Still, the development of CE has stagnated in recent years. The disadvantage of CE with regard to
its inferior reproducibility as compared to HPLC is such an obstacle that the industry is still clinging to the
convenience of HPLC. Relatively few new publications on CE development have been published in the last
year as compared to the number of publications related to its application. This indicates that CE is maturing
rapidly and that its field of application could be quite narrow.
Green chemistry might affect such dismissive and pessimistic attitudes towards the future of CE. If the demand
for green analytical methods continues, it could create new interest in CE. The further advancement of CE as a
green analytical technique could open new research directions in this field - especially those aimed at improving
its reproducibility. CE is one of a number of techniques that meet the definition of a green analytical method. It
is perhaps the only liquid phase separation method that is sophisticated, selective and sensitive enough to solve
a large number of analytical problems and can be made green. Surprisingly, the green chemistry community has
only recently recognized this fact [8-11]. CE consumes little power and it can be miniaturized and made portable,
due to the fact that it requires very little solvent (
l). The latter attribute is a major advantage of CE in light of the
well-known acetonitrile (ACN) crisis in the year 2009 [12]. ACN is a popular HPLC solvent that the typical
analytical laboratory consumes by the litre every day. Up until the recent downturn of the global economy, which
led to decreased automobile parts production, of which process ACN is a byproduct [12], no one thought seriously
about replacing ACN as an HPLC eluent. Now, faced with the ACN crisis, which resulted in a shortage and
enormous price increases, companies are looking at green chromatographic solutions in earnest. However, green
chromatography that uses solvents other than ACN and/or elevated temperatures for separations might not
provide the required solutions and this might lead to CE becoming a viable alternative [13].
The commercially available CE instruments (e.g. from Agilent and Beckman) with their robotic auto-
samplers are bulky and hardly fit the definition of a green instrument. The green features of CE have been
incorporated and demonstrated in several types of custom-made portable CE devices. In this chapter, we will
discuss the characteristics of CE that make it a promising candidate as a genuinely green analytical technique
compared to other separation methods. Special attention is given to comparing CE with different modes of
HPLC and microfluidics-based lab-on-a-chip (LOC) devices. Microscale HPLC and LOC are new, cutting
edge research directions in analytical separations. We will attempt to prove that, contrary to widespread
belief, the greening of these methods faces more serious obstacles than classical CE. The possible ways of
overcoming the disadvantages of CE will also be discussed.
μ
9.2
Capillary electrophoresis among other liquid phase separation methods
9.2.1
Basic instrumentation for liquid phase separations
A typical analytical separation instrument has the following functional elements: a device for creating eluent
flow, separation column, detector and data recording system. In order to actuate the eluent in an HPLC
instrument, pressure is applied. Highly technical mechanical pumps generate the pressure. The electrolyte
flow in CE is established by electroosmosis, which is generated by a high voltage power supply. The separation
mechanism in HPLC is the partition of a sample between mobile and stationary phases. In contrast, CE
separates sample compounds by differences in their migration in an electric field; however, the complete
separation process could be rather elaborate and include participation as well. Capillary electrophoresis (CE)
