Environmental Engineering Reference
In-Depth Information
Refractive Index Detectors
The concept of the refractive index has been introduced in Section 10.2.2. The
refractive indexes of various HPLC solvents are also given in Table 10.3. In the
presence of solutes, the refractive index of the solution will be changed accordingly. It
is, therefore, important to note that the refractive index detector, unlike UV and
fluorescence detectors, is a universal detector, which responds to nearly all solutes. To
some extent, the refractive index detector of HPLC is equivalent to the TCD in GC.
Like TCD, the measurement of refractive index is not as sensitive as other properties.
Additionally, temperature must be maintained accurately to a few thousandths of a
degree centigrade since temperature has a significant impact on refractive index.
The major design components in refractive index detectors are presented in
Figure 10.11. As shown in this schematic, a light passes through cell windows,
through the sample cell then the window and back to a detector. The same light also
passes through a reference cell (behind the page), then reaches to a separate detector.
The different light-bending properties from the sample and the reference cells are
registered in two photodiodes. The amplified difference in light falling on two
photocells provides the detector responses.
1
2
4
3
5
Figure 10.11 Diagram of a differential refractive index detector. 1¼Light emitting diode;
2¼Thermostatted compartment; 3¼Sample and reference cells; 4¼Mirror; 5¼Photodiode
(Rubinson, Kenneth A.; Rubinson, Judith F., Contemporary Instrumental Analysis, # 2000, p. 658.
Reprinted by permission of Pearson Education, Inc., Upper Saddle River, NJ.)
10.3.3 Detectors for Ion Chromatography
Since IC is essentially a particular application of liquid chromatography for the
analysis of ionic species, detectors used in HPLC can also be used in IC if they can
respond to ionic species. These may include UV-VIS detectors and refractive index
detectors. However, the conductivity detector to date is probably the most commonly
employed in environmental analysis and is described below.
Conductivity is the ability of a solution containing a salt to conduct electricity
across two electrodes. Three related and sometimes confused terms are used
to quantify the ''conductivity'', namely conductance (G), specific conductance or
conductivity (k), and equivalent conductance (
, pronounced as lambda). The con-
ductance (G) measured between two electrodes of area A (cm 2 ) and spacing L (cm)
inserted into a conducting solution is the reciprocal of resistance R (unit in
, ohms):
G ¼ 1
=
R
ð10
:
14Þ
Search WWH ::




Custom Search