A microfluidic probe head for providing a sequence of separate liquid volumes separated by spacers, the separate liquid volumes including a respective target substance associated with a respective target area, the microfluidic probe head including an inlet and an outlet; a first fluid channel fluidly connected to the inlet, the first fluid channel configured for delivering an injection liquid from the inlet to a respective target area; a second fluid channel fluidly connected to the outlet, the second fluid channel configured for delivering liquid volumes from the respective target area to the outlet; and a spacer insertion unit fluidly connected to the second fluid channel, the spacer insertion unit configured for inserting spacers into the second fluid channel between the liquid volumes to provide the sequence of separate liquid volumes separated by spacers.