Crime scene shows, especially those that focus on forensic evidence, have become very popular in the last few years. There are many instruments and techniques used in such shows that are also commonly used by chemists in research labs.

These instruments are crucial to the identification of compounds, both by crime scene investigators and chemists working to prepare new prescription drugs. As a scientist, I can become somewhat frustrated watching these types of shows as they give an unrealistic expectation of the science being performed. There is a documented effect of these types of shows, known as the “CSI effect,” which has influenced the judicial process. The general public, which includes potential jurors in criminal prosecutions, has come to expect the type of evidence and definitive identifications that are made week-in and week-out on these shows. An understanding of the limitations of these methods and more realistic expectations of their performance can help to relieve this “CSI effect” and the frustration of some scientists.

A commonly used method of identifying an unknown compound is Mass Spectrometry, usually abbreviated as MS. The principle behind MS is to measure the mass of a compound and then, using the mass, identify the compound. In order to detect the mass of the compound it must first be ionized or made charged, usually positively charged. These now charged molecules are prone to breaking apart into smaller molecules, sometimes in a predictable pattern called a fragmentation pattern. Samples can be prepared for analysis by MS by dissolving in different types of solvents, depending on the method to be used to ionize the compound. There are many different methods that can be used in MS, which can also make it difficult for a compound to be identified.

While watching a crime scene analysis show, you may have seen a scientist inject an unknown substance into a Mass Spectrometer and then, in a flash, the computer flashes the identity of the compound. This idea is based upon the use of extensive libraries of compounds which have already been analyzed using MS. When an unknown sample is analyzed and the mass and the fragmentation pattern are obtained, it can then be compared to the library of the mass and fragmentation patterns of known compounds. In principle, this should be a very straight forward process. However, like so many other things in life, this analysis is not that easy and is most definitely not as easy as it is portrayed on television.

First, for some compounds it can be very difficult to obtain the mass of the in-tact molecules before it beings to fragment into smaller pieces. This is very dependent upon the method used to ionize the compound.

Second, the type of fragmentation of the compound is also greatly dependent upon the method used to ionize.

So, in order for an unknown compound to be compared to a library of compounds, the method used should be the same (i.e. it may not be valid to compare compounds analyzed using different methods). While the libraries that are available to forensics laboratories are very large, it is very likely that there will be no conclusive identity assigned to a compound based upon MS alone. Also, in the vast majority of cases samples that require analysis are mixtures of many different compounds. For example, look at the number of ingredients listed in shampoo. Mixtures of compounds must first be broken down into their constituent parts before they can be analyzed. This can be an incredibly difficult, if not impossible, task.

There are other methods that can be used to identify unknown compounds. An interesting example is the use of Infrared Spectroscopy, usually abbreviated as IR, to identify automotive paints. If a sample of paint from a vehicle is left behind at a crime scene it can be analyzed using infrared light. Infrared light is higher wavelength than visible light and is used to analyze the vibrations of molecules. The IR spectra of an unknown paint sample can be compared to libraries of spectra of known automobile paints. This can allow for the identification of the make, possibly even model, of the unknown car or can establish the presence of a certain type of vehicle in the area of the crime scene.

The use of science in the investigation of crimes has become a vital piece of the judicial puzzle. However, there are limitations to any scientific method which make science difficult and fulfilling at the same time. The nature of an hour-long crime show is to wrap-up a story line in a short amount of time and, as such, the true nature of forensic investigation has become lost. Forensic evidence is often circumstantial when it is present and, in some cases, there is no forensic evidence available to aid the investigators. In my next article I will focus on the testing of DNA in forensic science.

Weedsport native Lauren O'Neil holds a Ph.D. in organic chemistry from the University of Notre Dame. She will tackle current trends in her monthly column by explaining the science behind such matters