Introduction
Preparing the MME Pro for operation is an essential step in ensuring reliable electrode performance and accurate voltammetric measurements on the 746 VA Trace Analyzer. This article guides you through installing the needle and capillary, filling the electrode with mercury, adjusting the needle valve, and verifying proper electrode operation before use.
Before beginning, ensure you have all required replacement parts, tools, and accessories available, including a new needle, capillary, mercury, the MME key, grey mounting ring, collection tray, electrode stand, and Mercury Catcher. Follow the video in sequence to correctly prepare the electrode and confirm proper DME, SMDE, or HMDE functionality before starting measurements.
Video
Topics Covered
- To put the MME pro into operation, the following are required: Multi-Mode Electrode pro (MME pro), collection tray, electrode stand, a new needle, a new capillary, scissors, mercury, syringe with needle, the key to the MME, and the mercury catcher "Mercury Catcher" for collecting scattered mercury droplets.
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Inserting the needle (0:06)
- The following are required for inserting the needle: Multi-mode electrode pro (MME pro), a new needle, the key for the MME and scissors.
- Turn the needle holder counterclockwise until it can no longer be turned out, and then carefully pull it out using the grey mounting ring.
- Cut open the plastic packaging of the new needle with scissors. Insert the needle into the hole in the membrane and push it in until the white mark is reached.
- Insert the needle holder with the new needle into the MME pro and turn it clockwise until the black sealing ring is just visible. This prevents mercury from leaking out.
- Do not turn the needle holder in too far. The needle valve will be adjusted later on the measuring stand.
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Installing the capillary (01:05)
- The following items are needed for inserting the capillary: Multi-mode electrode pro (MME pro), electrode stand, a new capillary, the key for the MME and scissors.
- To prepare, unscrew the retaining screw for the capillary, loosely reinsert it into the thread but do not tighten it.
- Do not touch the needle with your bare fingers during this process.
- Insert the capillary into the MME pro and, using gentle twisting motions, push it down through the seal until it stops. Then, loosely tighten the retaining screw by hand. To secure the capillary, place the key on top and use one finger to turn the retaining screw clockwise until the key falls off. Do not re-engage the key.
- The application areas of the standard capillary (6.1226.030) and the silanized capillary (6.1226.050) can be found in the corresponding data sheet for the capillaries.
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Filling the electrode with mercury (02:30)
- The following tasks must be carried out over the plastic tray!
- In addition to the MME pro, the following are required for filling the mercury: collection tray, electrode stand, mercury, syringe with needle, the key for the MME, and the mercury catcher for collecting any spilled mercury droplets.
- Slowly pour mercury into the electrode. It should be filled to the "max. fill" mark (max. 6 mL) at most.
- Using the key, tighten the screw cap until it stops. The opening of the mercury reservoir will then be securely closed.
- Small amounts of spilled mercury can be collected using the mercury catcher (part number 6.2406.000).
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Adjusting the needle valve (04:29)
- The needle valve is adjusted in the measuring station.
- Before inserting an electrode with a new capillary, be sure to empty the measuring vessel (any solution entering the vessel can render the capillary unusable)!
- Step 1: Pressurize the electrode (required pressure 1.0 to 1.2 bar). On the "STAND" page, activate the pressure ("Pressure") using the function key "F1". Once a large mercury droplet has formed, add approximately 20 mL of water and a few drops of KCl solution to the measuring vessel.
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Step 2 (coarse adjustment): Turn the needle holder clockwise until no more mercury flows out.
It has proven useful to check after each half turn, by observing the formation of new drops, whether the capillary is already sealed by the needle. -
Step 3 (fine adjustment): Slowly rotate the needle holder counterclockwise until a new droplet forms.
Close the capillary again with a small clockwise turn (approx. 10°). - Step 4: Rotate the needle holder another 90–100° clockwise. The electrode is now ready for use. Droplet formation can be controlled by the software.
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Verifying electrode function in DME, SMDE, and HMDE operating modes (08:08)
- DME – Dropping Mercury Electrode. The classic dropping mercury electrode.
- When the DME electrode type is switched on, the needle valve opens, mercury flows through the capillary and forms a droplet, the actual electrode. Once the droplet reaches a certain size, it falls off. A new droplet then forms. This process repeats itself throughout the entire measurement cycle. In normal operation, the drop-off mechanism ensures that the droplet falls off after a defined time.
- To check the DME function, activate the DME electrode type on the "STAND" page using the function key "F4". Drop formation is correct if the drop falls from the capillary at regular intervals.
- SMDE – Static Mercury Drop Electrode. A dripping mercury electrode, like the DME, but with a static surface, like the HMDE.
- With the SMDE electrode type, the needle valve opens for only a few milliseconds. During this time, mercury flows through the capillary, forming a droplet with a defined surface area. After a short time, this droplet is removed from the capillary by the knock-off mechanism. A new droplet then forms and is knocked off again after a short time. This process is repeated throughout the entire measurement cycle.
- To check the SMDE function, select the SMDE electrode type on the "STAND" page using the "F6" function key. Drop formation is correct if each stroke of the ejection mechanism removes an equally sized droplet from the capillary.
- HMDE – Hanging Mercury Drop Electrode. The hanging mercury drop electrode; a single hanging drop of mercury serves as the electrode.
- When the HMDE electrode type is switched on, any existing mercury droplet is first removed by the knock-off mechanism. The needle valve then opens for a few milliseconds. During this time, mercury flows through the capillary, forming a droplet with a defined surface area, which serves as the electrode for the entire measurement cycle. Only when the electrode is switched on again does a new droplet form.
- To check the function of the HMDE, activate the HMDE electrode type on the "STAND" page using the function key "F5". Drop formation is correct when the old drop is knocked off and a new drop forms. The drop should not increase in size over time. Reactivating the electrode will produce a new drop.
Additional Support
If you encounter any issues or need additional help, please submit a support ticket through Metrohm Technical Support.
For more detailed training on any of these topics, explore our Metrohm Custom Training options.