Zeta cell still good to use?
The popular Zetasizer can measure zeta potential in a disposable and reusable cuvette, DTS1070. This capillary cell is more affordable than the traditional dip cell, ZEN1002. But which one is better? So how do you know if your zeta cell is still good? Can you re-use it endlessly? Or throw it out after single use? What about the electrode blackening we may observe?
What the disposable zeta cell is
This widely popular disposable cell offers many advantages to users interested in zeta potential measurements. It is inexpensive and therefore does not need to be shared between different research projects. Scientists can use a cell for a specific range of projects and choose another cell when a research project introduces different chemistry that may have poor interactions with the first set of studies .
The DTS1070 zeta cell is a closed capillary electrophoresis cell. It consists of a closed (or better closable) sample container with electrodes. As a result of that design, the integrated electrodes are part of this compact cell. The cell material itself is poly-carbonate. Whereas gold-plated copper electrodes conduct the outside field into the sample. The gold is there for inertness. We want to be able to measure a good range of samples. Yet we don’t want to destroy the electrodes in the process, as that could contaminate the sample.
How to treat the disposable zeta cell well
Polycarbonate has excellent optical properties but is not particularly solvent resistant. The poly-carbonate is fine for most aqueous buffers. You can rinse samples out with water, methanol, ethanol, or isopropanol but not acetone or other organic solvents. So watch out for some organic solvents though, i.e. no toluene, no acetone. The electrodes, on the other hand, are a different story.
The electrodes of the DTS1070 cell have a thin layer of gold, about 6 atomic layers of gold. They will degrade with use, especially under high ionic strength buffers. Some components are particularly hard on electrode materials, see list below. However, in the case of many aqueous studies a DTS1070 cell can be good for 100’s of measurements. Thin gold films can interact with these compounds
- citric acid / citrates
- sulfates
- phosphates
and lead to blackening. For example, ‘strong’ electric loads for ‘extended’ time can cause redox reactions, leading to electrode blackening. That of course is not good. In extreme cases you may even see pink. (That’s colloidal gold.)
In some situations, the diffusion barrier method can help. It keeps the sample away from the electrodes. And thus minimizes direct sample-electrode interaction. The electrode spacing is 54mm, so it’s relatively easy to put a little distance between the sample and the electrodes.
And when to get new zeta cells
Ideally, when the old cells no longer give good phase plots. Or when you suspect that the electrodes are so dirty that they interfere. Or if it is visibly contaminated. You can measure our transfer standard ZTS1240. But most often, you won’t need to. Completely charred electrodes (i.e. extreme blackening) with rugged surface look different from smooth shiny gold. Take a magnifying glass to check for a closer look. Fortunately, it’s budget-friendly, so everyone in the lab can have their own.
Typically, when the gold electrodes degrade and need to be discarded, they turn black. Or if the sample turns red this is a sign that the gold has stripped away and you should discard the cell. The phase plot diagram in the results may also be consistently of poor quality.
Or go for a dip cell
Another option to consider when making electrophoretic mobility or zeta potential measurements is the Dip Cell.. This cell was first introduced commercially in the early 1990’s and is made of PEEK with permanent palladium electrodes. The dip cell is basically a “dip stick”. With two integrated Palladium electrodes, solid Palladium. These electrodes can withstand harsher conditions. Yet it’s still a good idea to treat them well. In other words, similar to the disposable zeta cells, avoid harsh and extended treatment. Dip cells require cleaning between samples but are not meant to be thrown away. Most dip cells are probably in contact with non-aqueous, lower dielectric constant samples. The electrode spacing is only 2.15mm – perfect for media with low dielectric constants.
Previous posts:
- Can size or zeta confirm binding?
- How to clean the dip cell
- What exactly is field strength in zeta potential measurements?