The heat is on
It's pretty obvious that passing an electric current through a liquid will generate heat. Known as joule heating, this effect is caused by charge-carrying particles, usually electrons, smacking into atoms and molecules as they move through the liquid.
It's also pretty obvious that such heating will occur during capillary electrophoresis (CE), where it is highly undesirable because it can interfere with the analysis by damaging sensitive analytes and causing peaks to spread out. Because of this, analytical scientists often have to limit the size of the current they use in CE to ensure that the temperature inside the capillary doesn't rise too high, at the expense of a longer analysis time.
Up to now, analytical scientists haven't known exactly what size electric current produces what level of heating, because of the difficulty of reliably measuring the temperature inside a capillary. This difficulty has forced scientists to employ temperature-dependent fluorescent dyes, which aren't particularly accurate, or calculate the temperature indirectly from other, more easily measurable parameters, such as conductivity or electroosmotic velocity.
To get a better understanding of the precise relationship between electric current and temperature, two pharmaceutical chemists from the Technical University Braunschweig in Germany, Claudia Cianciulli and Hermann Wätzig, have for the first time directly measured the temperature inside a capillary using an infrared (IR) thermometer. Well, actually, they directly measured the temperature of the outer wall of the capillary, by placing an IR thermometer around 8mm away. Then by knowing the emission ratio of the capillary, meaning how effective it is at emitting IR radiation, they were able to use the temperature measurement of the outer wall to calculate the temperature inside the capillary.
Using this set-up, they quickly showed that, as expected, temperature increases linearly with the size of the electric current along the capillary, measured as power per unit length (W/m). This linear relationship also shows up when the temperature inside the capillary is calculated indirectly from the conductance or electroosmotic velocity. However, Cianciulli and Wätzig found that the temperature rose at a faster rate for the indirect measurements than for the direct measurement.
When the temperature measurements produced by the different methods are plotted on a graph against power per unit length, they all produce straight lines, reflecting the linear relationship between electric current and temperature. But the lines all have different slopes. The temperature measurements made by the IR thermometer produce a line with a slope of 6.3°C m/W, meaning that for every unit increase in power per unit length the temperature in the capillary increases by 6.3°C. But the indirect measurement methods produce lines with slopes ranging from 7°C m/W to 10°C m/W.
Choose your current
There are a number of possible reasons for the different slopes produced by the various temperature measurement methods, but it's likely that the slope produced by the IR thermometer measurements are the most accurate. Furthermore, when Cianciulli and Wätzig applied the same measurement technique to a variety of capillaries with different coatings and containing different buffers, they found that they all generated lines with slopes of around 6.3°C m/W.
This implies that the relationship between temperature and electric current is universal for CE, or at least for all CE cooled by the same method (a fan in this case). Subsequent research has revealed that cooling CE by forced air causes the temperature to rise at a slower rate, producing a line with a slope of 4.0°C m/W.
With these universal lines, analytical chemists can now choose exactly the right size of electric current for their CE analyses, maximising both analytical accuracy and speed. For instance, many CE analyses can tolerate temperatures up to around 50°C, which means that the current needs to be kept below 4W/m (when fan cooled). But some heat-sensitive analytes can't tolerate any temperature above 30°C, in which case the current needs to be kept below 1W/m.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.