A washing agent dispenser including a washing agent case sized and shaped to hold a solid or solidified washing agent, and means for attachment of the case to a support or a surface, the case enabling collection of a surface sample of the washing agent by means of a washing instrument. The track comprises a plurality of interconnected levels so that the items may move along the track between the different levels and the items are readily accessible from all locations on the track. In embodiments of the invention an array of DNA molecules on a substrate comprises a plurality of DNA polymers attached to a trifunctional linker such that at least two DNA polymers are attached to one trifunctional linker that is attached to the surface of the substrate.