Last Month's Most Accessed Feature: Spokes beat spirals: A novel flow cell design for chemiluminescence

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  • Published: Mar 1, 2018
  • Categories: Detectors
thumbnail image: Last Month's Most Accessed Feature: Spokes beat spirals: A novel flow cell design for chemiluminescence

Spokes beat spirals: A novel flow cell design for chemiluminescence

Transparent flow cell

A team of Australian chemists has come up with a new flow cell design for chemiluminescent detection. Not only is this new design quicker and easier to produce via 3D printing than conventional designs, but it is also able to produce a larger chemiluminescent signal, making it more sensitive.

In chemiluminescence, analytes are detected by the light generated when they react with another compound. One common example is the reaction between luminol and hydrogen peroxide, which generates a bright blue light. Not only does this offer an effective way to detect hydrogen peroxide, but, because the reaction is catalyzed by various metals, it can also be used to detect a wide range of metal-containing molecules. And because organic molecules such as phenols can interfere with this reaction, reducing the emission of light, it can be used to detect these molecules as well.

To ensure the various analytes, reactants and catalysts interact efficiently, and also to ensure the emitted light can be detected, chemiluminescence is often conducted in a flow cell made from some kind of transparent plastic. The conventional design for such a flow cell is a spiral, which can easily be produced by coiling a plastic tube and provides sufficient time and space for the reactants and catalysts to interact as they travel through.

Radial alternative

Producing flow cells that are small enough to work with microchip-based analytical systems, however, requires other fabrication techniques, such as 3D printing, and producing a spiral with such techniques turns out to be quite difficult. Support structures are often required to prevent the printed spiral from collapsing and these structures need to be removed after fabrication, adding time and complexity to the whole process.

This prompted Brett Paull and his colleagues at the University of Tasmania to come up with a new flow cell design, which they hoped would be just as effective but easier to produce by 3D printing. This is a radial design: the flow cell is shaped like a bicycle wheel, with a central hub and multiple spoke-like channels leading from this hub to a circular outer galley. Each channel is 700μm wide and 3.6mm long, while the outer galley is 1800μm wide.

A solution containing the analytes, reactants and catalysts, with the analytes probably eluted from a chromatography column, are inserted into the hub. They then travel from the hub through the columns into the galley, giving them time and space to interact with each other and generate light, before exiting through an outlet in the galley.

“Given the design was to allow capture of as much light as possible across a detector window arising from a single inlet, this design seems to make sense,” Paull told separarionsNOW.

Fast printing

Paull and his team were able to fabricate this radial flow cell design in transparent plastic materials using two different 3D printing techniques. One was fused deposition modeling (FDM), in which molten plastic is deposited as layers that harden as they cool, and the other was PolyJet, in which a photopolymer is deposited as drops that harden on exposure to UV light. FDM could print the radial flow cell directly, whereas PolyJet required supports that subsequently needed to be removed. Paull and his team used water-soluble supports that could be removed by soaking the flow cell in sodium hydroxide for 10 hours.

In contrast, they were unable to fabricate a spiral design with FDM at all, because the spiral channel kept on collapsing. They could fabricate it with PolyJet, but it took around 360 hours to remove all the supports by soaking. This is because the sodium hydroxide can only pass in one direction through the spiral, whereas in the radial design it can simultaneously pass between the hub and galley through all of the channels.

When they tested their fabricated radial flow cell on the reaction between luminol and hydrogen peroxide, catalyzed by cobalt, they found that it produced a stronger signal than spiral flow cells, whether created by coiling a tube or PolyJet printing. Using different concentrations of hydrogen peroxide, they showed that the radial flow cell generated peaks for hydrogen peroxide that were higher and larger than the peaks generated by the spiral flow cell.

Finally, they connected the radial flow cell to an ion-exchange chromatography column, mixing luminol and cobalt with the eluent from the column, and showed that it could detect hydrogen peroxide in both urine and coffee extracts.

Analytica Chimica Acta (Article in Press): "A new 3D printed radial flow-cell for chemiluminescence detection: Application in ion chromatographic determination of hydrogen peroxide in urine and coffee extracts"

Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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