Apparatus for dispensing droplets of reagent onto samples includes a probe tip to which droplets of reagent can adhere. The apparatus advances the probe tip toward a sample until a droplet of reagent touches the sample and is pulled off from the probe tip. A sensor detects that the droplet has been pulled off from the probe tip and halts the advance of the probe tip before the probe tip touches the sample. Such apparatus may be used to automatically dispense small volumes of reagent onto fragile samples.