The transdermal assay apparatus includes first, second, and third members. The first member has one or more sample surfaces, each of which is configured to receive a sample thereon. The second member defines one or more reservoirs, each of which has an opening on a surface of the second member. Each sample surface is substantially the same size as each opening. The transdermal assay apparatus also includes a magnetic clamp configured to clamp a tissue specimen between the sample surface and the opening. The magnetic clamp preferably includes a magnet having a strength that is selected based on the clamping force required between the first member and the second member. The invention also provides a method for using a transdermal assay apparatus.