Detecting a substrate

A system comprising:

• a picker comprising a picker tip;
• a substrate comprising
• a first side comprising a first target analyte, and
• a second side;
• a substrate holder comprising the substrate,
• a position detector comprising
• a sensor fixed at a set distance along a first axis relative to the substrate holder,
• a target plate fixed at a set distance along the first axis relative to the picker tip;
• and
• at least one processor programmed to execute a method comprising
• driving the picker along the first axis towards the first side of the substrate,
• wherein, during the driving step, the sensor or the target plate moves toward or away from the target plate or the sensor, respectively,
• contacting the substrate with the picker tip,
• overtraveling the substrate with the picker tip,
• withdrawing the picker tip from the substrate to at least the point along the first axis at which the contacting step occurs, wherein the withdrawing step is performed after the overtraveling step and before the isolating step, and
• isolating the first target analyte from the substrate with the picker.