One contraband detection system proposed by SNL is a hand-portable system that has both swipe and vapor collection capabilities. When combined with a commercial chemical detector, the system can detect substances at parts per trillion. The other proposed detection system is a highly sensitive stationary device with swipe capability only. Both machines have proven effective in finding all types of substances in mail, including methamphetamine, LSD, cocaine, and marijuana. Although both systems have performed well in detention center evaluations, the staff prefers the hand-portable system with both vapor and swipe capabilities. Its portability also allows it to be used in other areas of the facility besides the mailroom. In addition to the equipment purchase, the evaluation has found other ways to improve contraband detection procedures; for example, the detention center now has a policy of refusing to process mail with postage stamps (drugs have been found under postage stamps). Even personal letters must be mailed in prepaid postage envelopes or be taken to the post office and metered. Also, detention staff now destroys the original envelopes that supposedly contain correspondence from attorneys and replace them with clean blank manila envelopes. In addition, all letters written in ink gel are returned.