It is a pre-coated pad, from the same batch, that is loaded into the sampler and put in its brown airtight vial to be used as a ‘control’. This ‘control’ sampler should accompany the samplers that are to be exposed, so it is subject to the same temperatures and conditions as the others, but the control sampler should never be opened, except to remove the pads and placed into an airtight shipping vial.