However....Temperature can be parameter targeted by method optimization to speed up liquid chromatography. Generally, it is known that temperature increase leads to an increase of brownian motion and causes faster separation (lower retention times (k) but also peak broadening and a change in selectivity due to differences in mass transfer.
Other positive side effects are lower backpressure which enables the usage of longer columns or smaller particle to increase efficiency and lower solvent consumption.
This dependency can be explained by the Horvath Rule, named after the hungarian Csaba Horvath who is known for a lot inventions in the field of LC, for instance pellicular particles. The rule points out that an temperature increase of 4-5 degree celisus has similar effects as increasing the organic modifier by 1%.
When it comes to temperature gradients in conventional LC and peak broading one has to differentiate between an radial and axial gradient. The radial gradient (in to out) causes intense peak broadening, whereas the axial gradient (front to back) leads to peak shapening and decrease retention times. In tradional LC pre-column heater and post-column cooler can prevent these effects related to the so called thermal mismatching.
If you are performing LC on chip device the interface with the heater is much larger compared to an conventional LC column and therefore one is going to have a much faster thermal response time. Solvent pre-heating is obsolate. Heat dissipation is reduced since there is no air contact.
Due to dependency Horvath has described one does not need a solvent gradient to increase peak capacity for on chip LC temperature gradients one can be used instead. This saves ressources and reduces the ecological impact of analyical labs.
https://pubs.acs.org/doi/abs/10.1021/acs.analchem.7b00142
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