Metrohm 746 VA Trace Analyzer - Manual (Page 41)

3 Installation
746 VA Trace Analyzer / 747 VA Stand
3-14
3.4 Multi-mode electrode (MME)
The multi-mode electrode combines the most important polarographic and volt-
ammetric mercury electrodes in a single construction:
• HMDE Hanging mercury drop electrode
Mercury is forced through a glass capillary until a drop
forms at the capillary tip and the entire voltage sweep
performed on this single stationary drop; in general
with preceding enrichment (stripping voltammetry).
• DME Dropping mercury electrode
The classical electrode, the mercury drops fall from the
glass capillary at a controlled rate.
• SMDE Static mercury drop electrode
The latest electrode, it combines the features of the
DME and the HMDE: during the measurement, the
drop surface is constant and stationary (as with the
HMDE); however, for the complete voltage sweep sev-
eral drops are needed (renewal as with the DME).
3.4.1 Construction and operating characteristics of the MME
The construction of the 6.1246.020 Multi-mode electrode is shown in Fig. 12. The
mercury in the reservoir 103 103 flows through the glass capillary 109 109 forming a drop at
its end. The mercury flow is controlled by the sealing needle 97 97, which can be
raised or lowered pneumatically. The different types of electrodes (HMDE, DME,
SMDE) are implemented by timed opening or closing of the mercury flow using this
sealing needle.
The operating characteristics of the MME are illustrated by Figs. 11 and 12. After
valve V V
11
(inert gas supply) is opened, the mercury in the reservoir 103 103 is pressur-
ized. In the standby mode, a back pressure is built up in the interior of the slotted
screw 96 96 which causes the built-in spring to press the sealing needle 97 97 onto the
capillary opening of the glass capillary 109 109 thus preventing the outflow of mercury.
Switching the valve V V
33
allows the inert gas to escape thus releasing the back pres-
sure. The inert gas pressure in the mercury reservoir 103 103 presses the sealing nee-
dle 97 97 fixed to the PTFE membrane of the slotted screw 96 96 upwards and the mer-
cury can now flow out. The tapping mechanism of the DME and SMDE is triggered
by brief opening and closing of valve V V
44
.
The mercury drops formed at the end of the capillary are very small and stable and
thus afford a very good signal/noise ratio. The mercury hermetically sealed in the
reservoir comes into contact only with inert gas and other inert materials and suf-
fices for around 200'000 drops.