Mass Spec Imaging Goes To Court | June 6, 2011 Issue - Vol. 89 Issue 23 | Chemical & Engineering News
Volume 89 Issue 23 | p. 42
Issue Date: June 6, 2011

Mass Spec Imaging Goes To Court

Forensic document analysis with SIMS clearly shows which ink is on top
Department: Science & Technology
News Channels: Analytical SCENE
Keywords: SIMS, imaging, forensics, ink
In this test sample, TOF-SIMS imaging shows that one signature (pseudocolored red) was written on top of another (green).
Credit: Courtesy of Albert Schnieders
In this test sample, TOF-SIMS imaging shows that one signature (pseudocolored red) was written on top of another (green).
Credit: Courtesy of Albert Schnieders

Imagine you signed a contract, but someone later altered the document without your knowledge or approval. Suddenly, you’re in the middle of a legal dispute about what you actually signed. How would you prove your case?

You might hire someone like Albert H. Lyter III, a forensic document examiner at Federal Forensic Associates, in Raleigh, N.C., to find evidence that changes actually had been made. For example, has a “3” been doctored to turn it into an “8”? One way to tell is to look at where different lines cross and see which line is on top—that line is obviously a later addition.

Such analyses have traditionally been performed with conventional optical microscopy. But these methods have been plagued by “questions of reliability,” Lyter says. Take, for instance, the case of black and blue inks.

“If you look at a blue ink stroke and a black ink stroke, the black stroke is always going to look like it’s on top, whether it is or not,” Lyter says. “That’s an illusion that everybody who does this kind of work knows about.”

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) imaging can help answer such questions in a way that provides both chemical information and an easily understood picture to show lawyers, judges, and juries. Lyter collaborates with Albert Schnieders of Tascon USA, an analytical services lab, and Nathan Havercroft of ION-TOF, an instrument firm, both located in Chestnut Ridge, N.Y., to make the measurements. Havercroft described the work at the 23rd Annual Workshop on SIMS & Related Techniques, held in Baltimore last month.

In TOF-SIMS, a “primary” ion beam bombards a surface, knocking off and ionizing molecules—so-called secondary ions—in the process. Those secondary ions are analyzed by a time-of-flight mass spectrometer. By rastering the primary ion beam across the sample, an image of the spatial location of the secondary ions can be built.

“TOF-SIMS is surface sensitive,” Schnieders says. “You analyze only the uppermost one or two monolayers of a surface.”

That surface sensitivity allows Lyter and Schnieders to obtain striking images that even nonexperts can understand. At the point where two ink lines cross, the bottom line simply disappears from the image, whereas the top line is in full view.

“The sampling depth of the technique is within the ink layer of the topmost ink,” Havercroft says. “You don’t even see the bottom layer. If you were to use another technique with a greater sampling depth, you’d still see the second ink and not get such a conclusive result.”

In some circumstances, TOF-SIMS’s surface sensitivity can be a shortcoming.

“These documents are handled a lot,” Schnieders says. “If fingerprints or something else covers those signatures, we wouldn’t see them anymore.”

Also, if the second ink contains a solvent that dissolves components of the first ink, the surface analysis could reveal a mixture of ions from both inks. However, the second ink would smear the first ink in the direction of the second stroke, perhaps providing extra information, Schnieders says.

To test the technique, Schnieders and a colleague signed a printed letter with pens containing different black inks. One signature was on top of the other. Schnieders acquired TOF-SIMS spectra of all of the components individually—the paper, the two inks, and the typed print. After identifying characteristic masses that distinguish the components, he collected a TOF-SIMS image of the intersection of two lines. Despite the similar color of the two inks, they were easily differentiated with TOF-SIMS.

The team is using the method in a legal case that is currently in arbitration, Lyter says. He can’t go into detail except to say that it involves a dispute about what information was on a form when an individual signed it.

“TOF-SIMS imaging of inks is much easier to present and understand than having a lengthy discussion on ink chemistry,” Schnieders says.

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