Chemistry Reference
In-Depth Information
increased to a much less absorbing region (308nm) where the background response could
be backed off sufficiently for the detector to be used at maximum sensitivity. A
comparison of the conductivity and ultraviolet detectors showed that:
(1) The response is greater for ultraviolet detection by a factor of 5-30. A relatively
stronger, negative ultraviolet response occurs for weakly acidic anions because there is
no dependence on their ionisation.
(2) Linearity ranges using ultraviolet detection are greater than those for conductance.
(3) Conductance detection may give positive or negative peaks depending on eluant
concentration and pH. Ultraviolet detection of non-ultraviolet absorbing ions give a
response in one direction only.
An ultraviolet detector used in series with a conductivity detector is the basis for a
method [9] for the determination of anions in non saline waters. This combination of
detectors greatly increases the amount of information that can be collected on a given
sample. The application of ultraviolet detection has the following advantages:
(1) aid in the identification of unknown peaks;
(2) use in resolving overlapping peaks;
(3) help in eliminating problems associated with the carbonate dip;
(4) reduction of problems associated with ion exclusion in the suppressor column;
(5) ability to detect anions not normally detected by the conductivity detector, eg
sulphide and arsenite.
For maximum sensitivity, the preferred position of the ultraviolet detector is after the
suppressor column. The suppressor column also decreases the background absorbance of
the other common ion chromatographic eluents such as 0.0002M Na
2
CO
3
/0.002M
NaOH, 0.006M Na
2
CO
3
and 0.005M Na
2
B
4
O
7
10H
2
O.
Fig. 12.1 shows a separation of seven common anions monitored using the
conductivity detector. Fig. 12.1 (b) was obtained with the ultraviolet detector after the
suppressor column. As is illustrated in Fig. 12.1, nitrite, bromide, and nitrate absorb
strongly in the ultraviolet, while fluoride,
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