Using a traditional comparison microscope, a firearms examiner compares the striations on a test-fired bullet against the marks on a bullet recovered from a crime scene.
Illuminating potential evidence
One of the bigger challenges for forensic science, according to Houck, is that, unlike in a standard laboratory, where sampling is carefully planned in advance, forensic technicians have to play the hand they’ve been dealt. Finding potential evidence is the critical first step.
The alternate light source (ALS) is a handheld device that contains a xenon, tungsten or halogen lamp and tunable filters; investigators adjust the emitted wavelength to highlight different things. These commercially available instruments help investigators find bits of trace evidence, such as tiny clothing fibers that the perpetrator may have left at the scene. Many ALS systems include goggles with tinted lenses that are paired with the filter passbands.
Semen stains, illuminated at 450 nm, “pop like a pale blue fluorescence,” Catalani said. Urine stains will look yellow-green. Blood will absorb the blue light and thus look black. Teeth and bone fragments will glow with an eerie pale white/blue.
As lasers have shed weight and cost over the years, crime scene investigators have taken them into the field too. The first generation of such devices consisted of water-cooled argon lasers that required three-phase power and were mostly confined to the forensic lab, said Andrew Masters, marketing director for Coherent Inc. of Santa Clara, Calif.
About five years ago, Coherent introduced a second-generation diode-pumped solid-state laser called the Incriminator, Masters said. It used recirculating cooling water and standard electrical power, but still cost approximately $100,000.
Last year, the company debuted the TracER, a self-contained 6-W, 532-nm laser in a portable unit with a rechargeable battery. The light source has a bandwidth of 1 nm instead of 50 nm for a typical ALS, making more green light available for better depth of field when photographing fingerprints on a curved piece of evidence, Masters said.
According to Catalani, a device called the NarTest made its CSI debut in the episode aired on January 25. The NarTest NTX 2000 is a tabletop-sized infrared spectrometer that is preset to look for the spectral fluorescence signatures of the four most commonly abused illicit drugs: methamphetamine, cocaine, heroin and marijuana.
The manufacturer, NarTest Technologies of Morrisville, N.C., claims that the instrument can detect the presence of any of the four drugs in 5-10 minutes. Together, the machine and personnel training will cost a law enforcement agency about $40,000.
To distinguish inks on different parts of a document, researchers can use commercial or homemade instruments that vary the incident and reflected wavelengths of light. Multispectral imaging, combined with statistical techniques, can highlight slight compositional differences.
Back at the crime lab
Manufacturers are willing to lend CSI Productions new “toys” for inside the lab, too, Catalani said. For example, laser ablation—in which an infrared laser boils off a small amount of a solid surface for analysis in a mass spectrometer—provides another opportunity for one of those “cool CSI shots.” The special-effects people show a red laser beam vaporizing the item in question inside a clear box, whereas in real life the infrared laser—producing as much as 109 W/cm3 of energy—usually operates inside an opaque chamber.
Baldwin’s group at Ames Laboratory, in collaboration with researchers at Lawrence Berkeley National Laboratory, has been studying forensic applications, such as identification of solid materials, for laser ablation.
One traditional method of identifying an unknown solid involves taking several milligrams of material, dissolving it in acid and spraying it into a stream of argon in a plasma system. In that old technology, it’s difficult to analyze tiny samples, and the process introduces other chemicals into the mix.
“With laser ablation, if you can see it, you can shoot it and get data out of it,” said Glenn A. Fox of Lawrence Livermore National Laboratory.
Laser ablation offers less likelihood of contaminating the sample, and of course it saves a lot of time, Baldwin said. One limitation is that sometimes technicians don’t know whether they are removing a sample that truly represents the bulk of the material. Also, laser ablation is more suited to metals and elemental analysis because the high energies involved would destroy molecular bonds.
Future laser-ablation systems will make use of far-ultraviolet and femtosecond lasers, although the latter are still very expensive, Baldwin said. Ablated particles that are too big won’t be fully digested for the mass spectrometer, and shorter-pulse and UV lasers make smaller particles come out of the ablation process.
Fourier-transform infrared (FTIR) spectroscopy has melded with microscopy to analyze the chemical composition of small fragments of evidence, down to sub-millimeter field sizes. In his previous lab career, Catalani put suspected drug samples inside potassium bromide pellets in order to perform transmission FTIR spectroscopy. Today, FTIR microscopy permits reflectance FTIR spectroscopy with virtually no need for sample preparation.
FTIR microscopy can determine the chemical composition of each layer in a paint chip—which gives investigators higher confidence that two paint chips might have come from the same source, Baldwin said.
Multispectral and hyperspectral imaging techniques use multiple emission spectra under different lighting conditions in order to distinguish subtle differences, such as whether all the ink on a check came from the same pen. Baldwin said that these techniques also could be used on fibers, paint samples and other chemicals. These techniques are fairly common in remote sensing, but are new to forensics.
Laser microdissection systems, which consist of a laser mounted to a microscope, allow laboratory workers to separate different kinds of cells in a tissue sample. Such techniques can help distinguish the sperm cells from multiple rapists and analyze old samples from cold cases, Baldwin said.
The “rape kit” that is mentioned on TV crime shows collects a mix of epithelial cells from the female as well as sperm cells from the male. The mix of DNA from the two genders makes it difficult to get a sure match from the rape kit to a suspect’s DNA. Chemical treatments can wash out the female cells, but this limits the sensitivity of the DNA test.