Chemistry Reference
In-Depth Information
CE can easily be made portable because the electrophoresis process in a capillary consumes little power,
so small-sized high voltage power supplies can be used. Optical detection is not currently well suited to
portable instruments; however, the emergence of small-sized light-emitting diodes will soon change that
situation. Da Silva [29] and Zemann [30] have developed a useful detection device: a contactless
conductivity detector (CCD) that measures the conductance of a small gap between tubular electrodes laid
on the separation capillary. The device is inherently small, and most analytes can be detected with a CCD.
Several groups have developed designs for new portable CE instruments. Hauser's group [31] has developed
(and optimized) a portable CE instrument with CCD for the sensitive field measurement of ionic compounds
in environmental samples. It is battery-powered, and the high voltage modules are capable of delivering up
to 15 kV at either polarity for more than one day. Inorganic cations and anions, including ions of heavy
metals and arsenates, can be determined with detection limits of approximately 0.2 to 1 mM. The instrument
was field-tested in a remote region of Tasmania. Nitrite and ammonium were determined on-site at
concentrations as low as 10 ppb, in the presence of other common inorganic ions at concentrations two to
three orders of magnitude higher. In another publication, Haddad's group demonstrated the use of CE for
the detection of explosives in the environment [32]. Instead of CCD, Haddad's team used indirect
photometric detection. They proved that it is possible to analyse blast residues at a crime scene, where they
can be sampled simply by wiping hard surfaces with a wet cloth, rather than by transporting the residues
back to the laboratory. They found that they could separate and detect the 12 cations at concentrations as
low as 0.11 mg l
-1
and separate and detect the 15 anions at concentrations as low as 0.24 mg l
-1
. In both
cases, the analyses took less than 10 minutes. However, they found that CCD performed better than indirect
photometric detection [33].
Seiman
et al.
[34] attempted to develop a robust sampling procedure for on-site analysis. In this project,
the CE analyser consisted of two pieces of capillary that were separated by a narrow gap (30
m). To
introduce the sample, a plastic syringe was inserted into a socket connected to the gap, and the background
electrolyte in the cross-section of the sampler was flushed out by the sample stream injected by the syringe.
The sample between the capillaries was then carried into the separation channel by electroosmosis flow
(EOF), and the background electrolyte filled the junction between the two capillaries when high voltage
was applied. This technique reduces the manipulation of buffer vials. The method developed for this
instrument has an LOD of 4-8
μ
μ
M for phosphonic acids and 0.3-0.5
μ
M for cations, and an RSD (internal
standard) of 8
.
Sampling complexity can be reduced even further by means of a simple manual buffer/sample vial
exchange at the capillary inlet. A possible design for what is conceivably the simplest portable instrument
is shown in Figure 9.2. An analysis compartment with two buffer vials and a CCD sensor is located at
the front of the instrument and a touch-screen computer on top. A typical electropherogram of the
phosphonic acids recorded with this instrument is shown in Figure 9.3. This figure is discussed further
in Section 9.3.
The design of a sampling instrument can vary according to the analyst's requirements. For example, the
robustness of a portable sampling instrument meant for field use must take into account that the operator
might have to work in a protective suit and gloves. In this case, split sampling or manipulating the sample
with a syringe rather than operating with vials would be more convenient (see Figure 9.4).
Electrophoresis is a key technology for further micronizing analytical separation methods by making use
of an advanced concept based on lab-on-a-chip platforms. It is believed that this technology will open the way
to many inexpensive point-of-care medical diagnostic devices. Many reports on portable CE analysers based
on microfluidics or lab-on-a-chip (LOC) platforms have been published in recent years (see the following
publications for examples [35,36]). CE-based microfluidic devices will be described fully in Chapter 17 on
Miniaturization
. We will now discuss one overlooked feature that impedes the potential greenness of LOC
devices.
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