Vapor is provided locally at a sample surface to allow fluorescence of the fluorescent markers in a vacuum chamber. For example, a nanocapillary can dispense a liquid near a region of interest, the liquid evaporating to increase the vapor pressure near the fluorescent markers. The increase in vapor pressure at the fluorescent marker is preferably sufficiently great to prevent deactivation or to reactivate the fluorescent marker, while the overall pressure in the vacuum chamber is preferably sufficiently low to permit charged particle beam operation with little or no additional evacuation pumping.