Give Loretta D'Costa a murder weapon. Say, a battered, bloodstained golf club with bits of human tissue attached. If you do, chances are that whoever committed the crime won't go free for long. But D'Costa (BSc'90, MSc'93) won't be able to tell you much about it. Her results only get talked about in court. D'Costa, who's part of a high-tech team of DNA sleuths at the Centre of Forensic Sciences in downtown Toronto, is sworn to secrecy and took an oath to that effect when she got the job.

    Her official title is "forensic technologist" which is just another way of saying D'Costa can help prove ? beyond a shadow of a doubt ? who dunnit. D'Costa, who works under a supervising scientist, specializes in applying something called "polymerase chain reaction" (don't ask) techniques to DNA samples. "It's like generating multiple copies of a sample for analysis ? like Xeroxing it."

    The tools of her trade are a microscope, a stereoscope and a large, lamp-mounted magnifying glass. After a painstaking visual examination of each sample, portions of the DNA obtained from semen, blood or saliva are used to create a profile of the criminal, says D'Costa. "It's almost like a biological fingerprint."

    A smudge on a gun barrel, a streak of blood on a knife, a stain on a tire iron or baseball bat, or material found under a homicide victim's fingernails, can yield evidence valuable to a Crown prosecutor. Once the criminal's profile is established, police may obtain a DNA warrant that allows them to take a DNA sample from a suspect to see if there is a match.

    Some cases take only a week, others may involve months of her time, she says. The supervising scientist writes up the case report,works with Crown prosecutors and testifies about the Centre's findings in court.

    Does her work ever get to her? "Sometimes," she says, although she's learned to professionally distance herself from the cases she deals with. "But when you come across children ? well, then it's hard."

Photo: SUSAN KING