Methods and apparatus are employed for determining interactions between different components, of the same or different type of composition. The apparatus provides for arrays of samples in tracks, where light emitting labels are excited and emitted light detected. Headers are provided for defining sites, so that particular interactions can be rapidly detected. Particularly, disks having circular tracks with headers defining sites on the tracks, so that positive signals can be interpreted in relation to the information provided by the header. Various modifications can be made, such as preprepared segments which may then be attached to the disk for assaying.