Metrohm 828 IC Dual Suppressor - Manual (Page 11)

1.2 How it works
828 IC Dual Suppressor/ 8.828.1003 Instructions for Use
3
The counter ions (Na + ) of the eluent are exchanged for the hydrogen
ions (H + ) of the resin in the cell. If NaOH is used as the eluent then
water will be formed; if NaHCO
3
/Na
2
CO
3
is used as the eluent then
carbon dioxide is produced. At the same time the counter ions of the
sample (M + = metal cation) are exchanged for protons from the resin.
Together with the sample anions these form acids which have an
increased conductivity, e.g. hydrochloric acid, nitric acid, etc. This
provides an improved signal-to-noise ratio and an improved detection
sensitivity.
2. During the operation of the 828 IC Dual Suppressor a direct current
is constantly applied to the suppressor cell electrodes.
Water in the cell is electrolyzed and the following electrode
reactions take place:
Anode: 2 H
2
O š 4 H + + O
2
(g) + 4 e -
Cathode: 2 H
2
O + 2 e - š 2 OH - + H
2
(g)
At the anode hydrogen ions and gaseous oxygen are produced; at
the cathode hydroxide ions and gaseous hydrogen. In this way the
hydrogen ions produced at the anode continuously regenerate the
cation exchanger. The sodium cations of the eluent and the cations
of the sample wander toward the cathode in the direct current field.
The eluate that leaves the suppressor cell on the cathode side
contains the hydroxide salts of these cations, gaseous hydrogen
and some sample anions (in the form of their sodium salts). The
eluate at the anode side contains carbonic acid or water together
with gaseous oxygen and some sample ions (protonated form). The
protonated sample anions are eluted toward the detector with
carbonic acid or water.
3. After leaving the suppressor cell all eluates pass through a
degassing unit. Oxygen and hydrogen are removed from the eluate
flows at the anode and cathode sides respectively before they
reach the waste container. The eluate at the OUT connection (see
Figure 1, Figure 10) contains the sample anions and carbonic acid
(if a carbonate/bicarbonate eluent is used) and passes the
degassing unit before reaching the conductivity detector. In the
degassing unit the carbonic acid dissociates to form carbon dioxide
and water. Carbon dioxide is removed and water remains.
This further reduces the background conductivity of the eluate to
provide improved sensitivity and a more stable baseline. This
means that it is possible to use carbonate/bicarbonate gradients.
The injection peak (water dip), which is often overlapped by quickly
eluting anions, and the system peak, which interferes with the
detection of anions which coelute with it, are eliminated.