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Improving the Analysis and Collection of Trace Evidence Samples

Notes From the Field
Date Published
November 2, 2020

Trace evidence recovered from crime scenes can provide powerful leads for law enforcement. It can assist not only in identifying a suspect but in revealing strong associations between the suspect and the crime.

In 2003, detectives asked our laboratory to analyze glass fragments from an active double homicide investigation in Galt, California. It was one of the first cases in which we analyzed glass using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), a capability we had just invested in at the time.[1]

In this investigation, law enforcement officers found a wooden baseball bat in a suspect’s vehicle and the bat had glass fragments embedded within it. At the lab, we compared those glass fragments to a broken window at the victims’ home. Our analysis determined that the pieces of glass from the bat were indistinguishable from the window pane glass, indicating that the bat could have been used to break the window. This corroborated the prosecution’s theory in the case, and at trial the suspect in question was convicted on murder charges.

The 2003 Galt case demonstrates the beauty of trace evidence: Criminals don’t know that they’re carrying it or that they’ve transferred it to a crime scene.

Unlike DNA analysis, trace evidence analysis can’t provide a statistical probability that a piece of evidence, such as fiber, tape, or broken glass, came from a victim or suspect’s person or property. But every piece of trace evidence analyzed has a compound effect. Think of trace evidence associations as spokes in a wheel. Every time we can make an association using a piece of evidence from a scene, we add another spoke to the wheel, thereby strengthening the case.

For trace evidence units across the country, maintaining the most up-to-date forensic technology, planning for staff departures, and coordinating training with crime scene investigators are critical steps needed to provide law enforcement with essential information just as we did in the 2003 Galt case.

Maintaining State-of-the-Art Equipment

In 2003, the Sacramento County District Attorney’s Laboratory of Forensic Services was awarded a state grant to create a trace evidence resource center for providing advanced trace evidence capabilities to public safety agencies across California. The grant, which was partially funded by NIJ’s Paul Coverdell Forensic Science Improvement Grants Program, allowed us to invest in laser ablation analysis technologies, a significant step forward for us at the time.

Although the grant has long since ended, we continue to serve as an unofficial trace evidence resource center for law enforcement agencies throughout California and maintain the capabilities of the technology we were first able to acquire with the support of the grant.

In many ways, trace evidence analysis is highly dependent upon instrumentation that is often very expensive. Unfortunately, in some laboratories across the country, leadership doesn’t see “the bang for your buck” to support a trace evidence unit.

We’re grateful to have the strong support of District Attorney Anne Marie Schubert, which is a key reason why we’re able to maintain an active trace evidence unit with some of the most advanced technologies available to laboratories. In fact, we are the only laboratory in California with analytical capabilities for LA-ICP-MS on trace evidence materials, including glass evidence.

In a normal cycle, we try to replace our equipment about every 10 years, but that’s certainly a luxury compared to many labs. Some trace evidence units may still be using equipment that is 20-25 years old; equipment whose age may hinder the quality of analysis it is able to provide.

For example, a modern gas chromatograph mass spectrometer is many times more sensitive than one that is 20 years old. Having the ability to provide analysis using state-of-the-art technology is critical in a courtroom.

Judges and juries expect labs to perform testing using the most advanced technologies available. If I’m asked whether any further elemental analysis on a glass piece of evidence can be performed, I can assure the jury that we have already conducted the fullest extent of elemental analysis that can be done to differentiate between glass samples. I can testify to that because of the equipment we maintain in our lab.

Approximately 80%-85% of glass can be discriminated by refractive index alone. For the remaining 15%-20% of glass that is indistinguishable by refractive index, LA-ICP-MS becomes valuable. What we’re able to do with LA-ICP-MS is to look at major, minor, and trace elements of the glass that provide outstanding discrimination capabilities. Prior to having an LA-ICP-MS instrument, we lacked the capability of detecting trace elements in glass samples.

Some laboratories without the proper technology or financial support for trace evidence analysis have simply stopped doing the casework.

We frequently get requests from agencies outside of our region to analyze evidence using our advanced trace evidence capabilities. We decide whether to test these samples on a case-by-case basis. We have to gauge our case backlog and ultimately determine if we can get them a response in a reasonable time frame, while also maintaining an efficient turnaround time for the cases from Sacramento County.

When you don’t have the capability to analyze a particular kind of trace evidence within your own lab, it’s important not to simply push evidence aside. Look for outside opportunities to analyze this critical evidence. Oftentimes, the mantra “you don’t know what you don’t know” is true in that you’re able to find avenues to analyze evidence that you didn’t think were possible.

Succession Planning

From an administrative standpoint, forensic laboratories often overlook succession planning — bringing in new analysts to replace those leaving — when it comes to trace evidence analysis.

It can take anywhere from five to nine years to fully train someone as a generalist trace evidence examiner. I remember my own training years ago: It took about five to six years before I was capable of doing most of the trace evidence casework that we encounter. It’s a large commitment for a laboratory to make that type of investment in an analyst.

So when someone with 20, 30, or even 40 years of experience in the field decides to retire, it is a struggle to replace that knowledge base.

In our laboratory, we have three full-time examiners assigned to our trace evidence unit. Even as the director, I handle some casework, and we have a retired colleague who works for us part time. We recognize that this is a real benefit since other trace evidence units across the country may be staffed by only one or two examiners.

For our succession plan, we’ve transferred laboratory staff into the trace evidence unit and are currently training those people for the day when one of our more experienced staff members retires. It’s important to prepare for departures now so you don’t lose your capabilities as you lose staff.

Maintaining Regional Databases

Here in Sacramento, we maintain a regional glass database built from casework and additional local samples. Our samples include windshields, architectural glass, containers, and many other objects. As with everything we analyze, regional databases help to provide an additional layer of context in trace evidence analysis.

We maintain this database to evaluate the frequency of occurrence. Should a jury ask “How common is this glass sample,” we can run the sample through our database and give them an answer. Maintaining a database won’t tell you anything about the crime, but the entries are valuable for determining the range of possible sources and prevalence of the material.

There is no national database for glass samples, but if there were, it could present some drawbacks. For instance, there may be no value for me in searching a database of glass in Florida when the glass will likely be different than glass in my region of California. Additionally, if you combined all of the occurrence rates from all of the regional databases, the resulting frequency rate could be biased and skew the overall conclusion.

Improving Trace Evidence Collection

Some of the local crime scene investigation teams have routine workshops, and we’ve partnered with them to coordinate training. We ask these agencies what evidence collection techniques they need training on, and then we provide hands-on, practical exercises so they can experience the techniques in action.

Following a workshop session, I’ve often had investigators approach me to discuss cases with similarities to what we covered in the training. After learning more about our capabilities, they’re interested in submitting evidence that they didn’t realize we could analyze.

This is just one of many benefits that come from these training sessions. We ask investigators to consider the trace evidence component in every case they handle. Trace evidence may not be used in every case, but if it’s collected, at least there’s the opportunity to use it in the future, should it be needed.

By educating our investigators on proper evidence collection techniques, we also improve the quality of samples submitted to the lab. If we receive quality trace evidence, we can develop quality case results. If we get suboptimal samples that weren’t collected properly, we may be unable to analyze them.

Although it’s easy for a trace evidence unit to just sit back in the laboratory and wait for cases to come in, it’s critical to maintain communication with law enforcement agencies and ensure they are aware of all of our capabilities. Otherwise, trace evidence disciplines will die on the vine, with no evidence submitted for analysis. One small piece of evidence may not seem critically important as law enforcement clears a crime scene, but add up the many forms of trace evidence and your case becomes stronger, just like the many spokes in a wheel.

About Notes From the Field

The National Institute of Justice (NIJ) is the research, development, and evaluation arm of the U.S. Department of Justice. NIJ aims to address the critical questions of the criminal justice field, particularly at the state and local levels.

NIJ Director David Muhlhausen developed the Notes From the Field series to allow leading voices in the field to share their strategies for responding to the most pressing issues on America’s streets today.

Notes From the Field is not a research-based publication. Instead, it presents lessons learned by on-the-ground criminal justice leaders, from years of experience and thinking deeply about criminal justice issues.

Date Published: November 2, 2020