Low Vacuum
Conventional SEM analysis is undertaken with the sample chamber at high vacuum, and insulating samples require prior coating with carbon or gold in order to prevent an electrical charge from accumulating on the sample that will degrade the images.
Operating a SEM in low vacuum mode allows imaging and analysis of uncoated samples. In this mode, the sample chamber contains water vapour or nitrogen gas that can prevent electrical charge accumulation. The secondary electron signal is also enhanced by cascade amplification by the vapour/gas. Results of X-ray microanalyses, however, must be interpreted with care owing to scattering of primary electrons.