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Second, The scent can be obtained by "swabbing", or wiping down, the surface the criminal had contact with. For example a steering wheel can be wiped with a gauze pad to transfer the scent. Here again, there is the problem of disturbing or ruining other possible evidence.
Third, the scent can be obtained by "passive transfer" where a gauze pad is laid on the surface the suspect had contact with and left there for some time...usually between 20 minutes and an hour. This method will work on some items such as clothing worn by the suspect, or a car seat the suspect sat on, but this method is very time consuming, and the amount of scent transferred is unreliable.
Fourth, and in my opinion by far the best method of scent collection, is to use a vacuum device such as the "Scent Transfer Unit", or "S.T.U." for those of us on a first name basis. This device utilizes a low-draft vacuum to draw the scent onto a sterile gauze pad. These gauze pads, or "scent pads" are then bagged in zip-lock or heat-seal plastic bags and labeled as to content and are now themselves evidence items. It is these scent pads that are presented to the dog to smell when it is time to actually work the case. The use of the STU does not disturb fingerprints, trace hair and fiber evidence, DNA specimens, etc. The STU makes it possible to obtain scent off virtually any object the suspect touched, brushed up against, or had any contact with. Some examples of items I have obtained scent from and successfully used with my bloodhounds on actual crimes are: expended bullet/shell casings, weapons, window sills, door knobs, victim's clothing, victim's body (both victim alive and vicitim dead), fast food wrapper, fried chicken bones, suspect's blood, suspect's semen, suspect's feces, cigarette butt, match stick, Molotov cocktail fragments, pipe bomb fragments, and so on. The ability of the dog to work with the scent from these items is dependant solely on the training of the dog and handler. This will be discussed more later. To obtain more information on the STU, see the "Scent Collection" section or visit the "LINKS" page where you can visit Bill Tolhurst's web page, or email Larry Harris.
Most sophisticated law enforcement agencies, in conjunction with their local prosecuting attorney's office will find the S.T.U., or some similar device, to be the preferred method of scent collection. It is simple to use, produces reliable results, and does not disturb, destroy, or alter other physical evidence. With this method scent can be collected from virtually anything the suspect has had any contact with. The scent pads made can be used right away, or stored for use later. These scent pads that are then stored frozen can be thawed and used reliably years after they were collected.
I know I sound like an advertisement for the S.T.U., and I guess I am, but it is because I believe in it so much. I have no business relationship with either Bill Tolhurst or Larry Harris, and get nothing in return for my statements regarding the S.T.U.

BLOODHOUNDS? WHY BLOODHOUNDS? I am frequently asked, "Why do you use bloodhounds?" My favorite answer goes something like this; if you decide to go duck hunting and you want a dog to retrieve the ducks you shoot (assuming you are a better shot than I am) you would probably get a Labrador Retriever, or some other retriever type dog. If you want to go upland game hunting and you want a dog to help you find and retrieve quail, you would probably get a German Shorthair Pointer or a similar dog. You would make these choices because this is what these dogs have been bred to do. Likewise, if you need to hunt a man, the only breed that has been bred for hundreds of years to do just that one thing is the bloodhound. As the handler of any working dog will tell you, the best dog for the job is the dog that has been bred for the job. Does this mean other dogs can't trail human scent? No. It just means the bloodhound will probably be easier to train, and will probably do the job better than some other breed.
That said, the bloodhound does have its drawbacks. It tends to be a very stubborn dog, which is good when you want a dog to trail for miles and miles and not get bored or distracted, but it is bad when you want a dog that will learn to "sit", or "down", or "off"...bloodhounds tend to not do these things very well. They are odoriferous, unless you bathe them frequently, but if you do that they will lose the oils that make their coat water repellant, thus shouldn't be worked in inclement weather. They tend to slobber...A LOT.
This last point is my opinion, and there are certainly those that disagree. I treat my working dogs like WORKING DOGS, not pets. They live outdoors. Of course they have shelter from the sun and weather. I find that a pampered dog that gets to stay indoors where it is warm and dry will have a difficult time working at "0 dark-thirty" in a cold pouring rain. My dogs' main interaction with me is when we work or train. Working is their function, they know it is their function, they love working (don't you wish you had employees like that?), and work needs to be the source of their greatest rewards. I have yet to have a mean or vicious dog, and my dogs have all worked with great enthusiasm, so don't tell me I mistreat 'em.

TRAINING. This is in no way intended to be a training guide (after all, if I put it all here, what would you need me for?), only a few basic points that must be part of the training program. To say that training is everything in the case of a working Scent Evidence dog and handler team is almost an understatement. The dog and handler team will only be as good as their training. A dog that is normally given a person's T-shirt as a scent article cannot be expected to work reliably off a scent pad made from a single expended .22 casing or a small fragment from an exploded pipe bomb, however, if the dog is used to being given this type of scent article in training, it will have no problem when it comes to the "real thing". Training must mimic the surroundings the dog will be working in...if you will frequently work in ghetto or skid-row type environments, you must train there...if you will frequently work in mountains, or deserts, or swamps, you must train there...if you will frequently work in areas with many loose dogs, or 51-50's (crazy people), or kids playing, you must train there. Get the picture? Don't expect your dog to work well in a situation that is totally foreign to it...that doesn't mean you turn down a case just because it occurred in an area you and your dog haven't prepared for, but then again don't be surprised if the dog's performance is something other than you are accustomed to.