The Different Vacuum Modes
High vacuum mode is used to obtain images with the highest possible resolution. In this mode morphological or elemental information can be obtained with secondary electrons for detailed surface analysis, elemental mapping, X-ray analysis, and back-scattered electron detection for elemental analysis. Only samples which are thoroughly dry and conductive can be imaged in high vacuum mode.
Low vacuum mode is used to image non-conductive or poorly conductive samples. Low vacuum imaging eliminates the need for conductive coating. Backscatter imaging can be used in conjunction with low vacuum. Low vacuum allows imaging samples that are not totally dry and not totally conductive, but the images obtained are "softer" (i.e. less contrasted and of lower resolution) than those obtained in high-vacuum mode.
Environmental SEM or "wet mode" permits imaging sample at high humidity. With wet mode samples that are not fully dried ("wet"), uncoated, or both can be imaged under optimum humidity conditions. The purpose of this mode is to image surface detail of biological and non-biological samples. But the images obtained are even softer than in low vacuum mode.