Antec Scientific VT-03, Руководство пользователя

Страница: 29 / 36

CHAPTER 4 Maintenance 29
2. Rinse the polishing disc with demi water before applying the diamond
slurry!
3. Apply a few drops of slurry on the wetted polishing disc, and polish the
electro
de with a ‘figure 8’ motion for about one minute. Apply only gentle
pressure.
4. Clean the electrode with a wetted tissue and check the surface visually,
repeat the procedure if necessary until the shining metal REF surface
appears.
5. Clean the polishing disc with demi water.
6. Store the polishing disc dust free in its plastic bag.
Ag/AgCl salt bridge
Three aspects determine the proper function of an Ag/AgCl reference
electrode.
1. The chloride concentration must be kept at a strictly fixed level. This is
best guaranteed by using a saturated chloride salt solution at a constant
temperature.
2. The salt bridge must allow proper electrical contact with the mobile
phase. The higher the leakage through the frit the better the conduction.
This conflicts with the previous point.
3. Air bubbles inside or close to the salt bridge will lead to instability of the
three-electrode configuration. Because of their extreme compressibility,
changes in conductivity and the ionic equilibrium of the REF occur. This
increases the noise considerably.
The REF is factory filled with KCl unless specified otherwise. Other chloride
salts should be used when the mobile phase contains perchlorate (use NaCl)
or a high percentage of organic modifier (use LiCl).
Saturation and air bubbles
After prolonged use the salt bridge in the REF will not be saturated any more,
which usually leads to a poor reproducibility in electrochemical detection. The
potential of the REF is determined by the chloride concentration (see page
13). If the salt bridge is not saturated and the KCl concentration changes:
1. the noise in the system will slowly but continuously increase,
2. the background current will increase,
3. sensitivity for movements and pump noise will increase.
If an air bubble is trapped in the salt bridge or in the cotton plug that
separates the salt bridge and the mobile phase the flow cell becomes
extremely sensitive towards flow fluctuations and vibrations. This is caused
by the high compressibility of the trapped air.