The second generation EC-EPR cell

For the second generation of EC-EPR cells the design was drastically simplified, and the main parts were machined from PEEK. All of the cell parts were designed such that only drill and laithe should be needed for the fabrication process. Figure 5.2a shows the schematic representation of the cell where parts 1, 3, 4 and 5 were machined from PEEK, and Figure 5.2b is a photograph of an assembled cell including electrode contacts to thicker wires and a Teflon tube for a sample flow.

Parts 1 and 3 support EPR test tubes, 2a: Q-band EPR tube 1.1 mm ID & 1.6 mm OD, 2b: X- band EPR tube 3.0 mm ID & 4.0 mm OD. 2a is the sample tube holding the solvent and

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therefore the ID determines the sample volume inside the resonator, but also contains the WE and RE. In an assembled set up (Figure 5.3) the inner tube holds the solvent away from the fringing electric fields of the LGR gaps. Thus, if necessary, the ID of 2a can be adjusted by choosing a suitable capillary and the performance of the setup optimised for solvents with different dielectric constants. To date, cells with ID’s (2a) between 0.8 and 1.1 mm have been fabricated. The X band tube (2b) acts as a structural support, making the assembled cell robust and easy to handle, while enabling symmetrical placement of the cell into the resonator.

Screw threads (c) on part 3 allow the attachment of the cell into the resonator (Figure 5.3) and also the adjustment of the WE inside the resonator in the Z-direction for optimal performance. Part 4 fits to part 3, resulting in a small chamber between them where the CE is located. The four channels in part 4 allow the attachment of Teflon tubing for solvent flow, but also permit the connection of the WE, RE and CE to thicker wires outside the cell

Figure 5.2. (a) The EC-EPR cell designed for a LGR. 1 & 3: capillary supports, 3c: threads to attach the cell to the resonator, 2a: Q-band EPR tube for the sample, 2b: standard X-band EPR tube for structural support, 4: adaptor for electrodes and sample flow, 5: pegs for electrode attachment, 6: capillary for inserting the working electrode (WE) to the sensitive part of the resonator, RE: reference electrode, CE: counter electrode. (b) A photograph of an assembled cell.

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for electrical connection through parts 5. This arrangement makes changing the RE and CE easy if necessary, whilst the additional channel enables mixing experiments to be conducted, or insertion of a thermocouple for monitoring the sample temperature.

Part 6 is a fine capillary with dimensions of ca. 0.15 mm ID, 0.4 mm OD through which the WE is guided to the sensitive part of the LGR inside 2a. The capillary can be removed and inserted back through part 4, allowing the WE to be changed when necessary without dissembling the entire cell. The bracketed part, from where the enlarged diagram is taken, represents the sensitive region of the 5-loop 4-gap LGR used in this work and has a length of 10 mm in the Z-direction. The RE, inserted into the cell through one of the pegs (5) is placed as close as possible to the WE to minimise the uncompensated resistance. The CE in the chamber of parts 3 and 4 is far enough from the active region of the resonator so that no interference from CE products is expected.

5.2.2.2.The cell assembly

When assembled, a silicone rubber compound (RS 555-588) was used to attach various parts together whenever possible. Silicon rubber acts as a sealant, making the joints leakage proof but does not acts as an adhesive binding the parts tightly together, and thus the cell can be dissembled much easier when necessary. Ideally the only place where adhesive such as araldite is needed is to connect the thick electric wires to the parts 4 and 5 (Figure 5.2b and Figure 5.3a) to protect the connection to the micro electrode wires inside the EC-EPR cell. To achieve a low resistivity connection between the electrode wires and the thicker electric wires, silver conductive paint (RS 186-3593) was used. Silver paint was preferred over a conductive epoxy, as the electric wires come close to each other at the top of part 4, and thus it is easier to prevent the wires from short circuiting.

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In the second generation design the WE is not attached from both ends anymore, as this would necessitate opening of the entire cell when the WE is changed. Instead, exposing only the tip of a metal wire and inserting it to the cell through part 6 allows the WE to be removed from the cell when fouling occurs and a new one to be inserted, making the design extremely practical. The drawback of this method is that depending on the length of the WE the tip of the wire can be leaning towards the sample tube wall, and thus the diffusion field restricted compared to a situation where the wire is placed symmetrically to the sample tube, as shown in the enlarged part of Figure 5.2a. None the less, the practicality of the design outweighs the issues related to diffusion field, and as shown in Chapter 6 the EC behaviour of the design with WE lengths of 7 mm is still acceptable.

Figure 5.3. (a) Image of an assembled cell attached to the resonator. For experiments the resonator is lowered between the modulation coils. (b) A schematic representation of the cell attached to the resonator, showing the geometry of the 5 loop 4 gap resonator. The WE is located to the centre of the sensitive part of the resonator, 10 mm in height.

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 Electrode wires

WEs can be fabricated for example from polyester coated Pt, Au or Ag wires. The polyester coating can be removed by soaking the wire in saturated KOH, after which the exposed electrode is wiped with acetone and rinsed thoroughly with Milli-Q water. Cells have been fabricated successfully with WE conductor diameters of 25 and 50 μm, and for larger diameters the dimensions of part 6 (Figure 5.2a) should be adjusted accordingly. It should be noted that also a Ag mesh has been successfully used as a WE by wrapping it snugly around part 6, which allows up to 20 times larger charge generation in static solutions when compared to 7 mm long, 50 μm diameter micro cylinder. Therefore, especially if combined with signal averaging or flowing solutions to avoid depletion of the parent molecule for short lived radicals, the sensitivity of the setup could be greatly enhanced. An ideal RE is a chloridized Ag wire with 50 or 125 μm conductor diameter in >0.1 M KCl background electrolyte, although bare Ag and Pt wires can also be used. As a CE, a bare 250 μm diameter Pt wire coiled in the chamber between parts 3 and 4 has proven to be the most reliable option.