Laser Enhances Spectroscopy Technique's Sensitivity

Brent D. Johnson

A forensic chemist in the trace evidence department at the South Carolina Law Enforcement Div., Lt. Joe Powell catches criminals with a 213-nm Nd:YAG laser attached to an inductively coupled mass spectrometer. This detection technique can identify the elemental signature of bullets and other solid samples.

Firearm identification is made by comparing the striated marks on ammunition components that come in contact with a weapon. However, in cases where the bullets have been severely damaged (left), analysis with inductively coupled plasma mass spectroscopy can be used to match a family of bullets that have a characteristic elemental signature.

Powell explained that, when someone is shot, the bullet can be mutilated, making it impossible to link it with the gun it was fired from using traditional methods. He is developing a technique whereby a bullet can be matched to a family of bullets that were used in a certain gun by the ratio of the trace elements it contains. Preliminary data indicate a high probability of having a match between such known and otherwise unidentifiable samples.

The basis for his method is inductively coupled mass spectrometry, where lasers ablate materials and the resulting products are analyzed to obtain chemical and elemental profiles of the sample constituents. However, the lasers' ability to produce particles small enough to be aerosolized limits the effectiveness of the technique.

Previously, Nd:YAG lasers emitting at 266 nm were employed, but the longer-wavelength lasers required more energy to ablate the material and produced particle sizes that were 0.8 µm, on average. The shorter wavelengths that Powell is using ablate at lower energies, because they are more easily absorbed by sample material, and produce particles smaller than 0.5 µm.

Powell uses a UP213 Nd:YAG laser from New Wave Research to ablate the bullets and sweeps the chemical aerosol into the inductively coupled mass spectrometer through a mixing chamber, which increases the sensitivity of the sample by as much as two or three times. He looks for homogeneity of the sample as well as chemistry changes in the bullet before and after firing.

In the past, laboratories performed elemental analysis with x-ray fluorescence, atomic absorption and neutron activation analysis, but these techniques did not provide enough discrimination and sensitivity to examine for trace elements. With inductively coupled mass spectroscopy, trace elements can identify materials in the same way that fingerprints identify humans.

The UP213 laser also has a resonator flat beam that controls the depth of the ablation so that certain layers of the sample containing different elements can be detected. A Gaussian beam is not able to provide this kind of precision.

A major benefit of the technique is that it requires no sample preparation, which not only saves time, but also eliminates the obscuration of important elements that can occur with sample preparation. The entire process takes only five or six minutes. "The data is overwhelming us," said Powell.

He said that the technique was helpful in a recent case where a man made the mistake of telling his wife that he didn't care what she did. She promptly struck him on the head with a teakettle, putting an end to him. Powell and his team were able to link pieces of ceramic embedded in the man's head with the teakettle, which sealed the case.