*2.3.2 Temperature control*

In general, homogeneous temperature control of the reaction solution has to be achieved. It is possible to submerge the whole microfluidic device and tubing in a water / oil bath. However, for *in situ* investigation, there needs to be unimpeded access to the channel, and this approach is thus not viable. In this case, custom designed heating elements, e.g. heated enclosures, are employed to regulate temperature. Temperature control is only limited by the geometric constraints of the measurement, and the heat transmission of the device material. For example, we have implemented copper heating tubes for surrounding the glass capillary of hybrid microfluidic chips, incorporating a window for the X-ray beam that provided excellent thermal control of the measurement [26, 27]. A key to good thermal stability is to also incorporate heating elements for the fluidics systems, to keep the reaction solutions at appropriate temperatures and ease the thermal load on heating elements in the device.
