Where CSI Gets it Wrong

Where CSI Gets it Wrong

I recently had the privilege of teaching my FTIR Analysis of Controlled Substances course at a well known forensics lab; one of the ones they make TV shows about (course outline is here: http://www.spectros1.com/c-forensic.html). The simple act of writing the letters "CSI" on the board elicited a chorus of groans and laughter from the roomful of forensic scientists taking the course. For those of you who don't watch much TV, the letters "CSI" stand for "Crime Scene Investigation", a series of shows about how forensic scientists help solve crimes. The chorus of groans and laughter is based on the fact that the science on these shows is so inaccurate as to be laughable.

The first thing CSI gets wrong is the role of forensic scientists in crime fighting. They portray lab workers donning bullet proof vests, carrying guns, and chasing down and arresting bad guys. Now in some states forensic scientists may go to the occasional crime scene, but there are no forensic lab workers that I am aware of that carry a gun and arrest people. In many forensics labs there is a strict division of labor between the police, who arrest the bad guys and collect evidence at crime scenes, and the civilian scientists who analyze crime scene evidence and testify about it in court.

Another thing CSI gets wrong is the speed of the analyses performed in a forensics lab. In the world of CSI it apparently only takes a few minutes to run a DNA analysis and identify the bad guy. In the real world it usually takes days or weeks to get DNA results back, and although DNA is a powerful forensics tool it may not be definitive because you can not identify someone who is not in your DNA database.

It is true of CSI, and most movies and TV shows, that the actors and actresses are exceedingly good looking. Now, I have great respect and admiration for the professionalism of forensic scientists and the important role they play in promoting public safety. However, I can tell you that they all don't look like supermodels :-).

But what CSI really gets wrong is the way it portrays the use of FTIR in forensic labs. If you watch the show closely you may have noticed there is a Thermo Nicolet FTIR on the set of one of the CSI shows. Several years ago a Nicolet salesman told me the story of how this came to be. The producers of the show approached Nicolet and asked them for a free FTIR in return for the free publicity Nicolet would enjoy by having the instrument appear on TV. The folks at Nicolet were a little leery of handing over an instrument worth tens of thousands of dollars for free. They had the intelligence to ask the producers of CSI, "Will you ever use the FTIR to perform an analysis?". The answer was no. So, instead of giving the show a complete instrument Nicolet proposed giving them the plastic shell that covers the instrument but with nothing inside of it. This was acceptable to the producers, and that is how a Nicolet "FTIR" came to be featured on a TV show.

Several years ago an episode of CSI featured the use of the FTIR in one of their shows, and they got it terribly wrong. On TV they showed a red visible light laser, apparently a He-Ne laser, as the light source of the FTIR As I teach in my Fundamentals of FTIR course all FTIRs contain a visible light laser that is used to measure the optical path difference of the interferometer. However, this laser is NOT the infrared source because you can't measure an infrared spectrum with visible light. Also, because a laser gives off only one wavelength of light it is impossible to use it to measure a spectrum, which requires many wavelengths of light. Another thing I teach in my Fundamentals of FTIR course is that FTIR is a form of molecular spectroscopy. Individual atoms are not chemically bonded to anything, do not possess vibrations, and hence generally don't have a mid-infrared spectrum. This makes FTIR inappropriate for elemental analysis. On the same show mentioned above they were using the FTIR to perform an atomic analysis on a sample. This is more than I could take, and I have not watched an episode of the show since.

These scientific inaccuracies may seem amusing, but they can have a negative effect upon the public safety of our country. A number of forensic scientists and police officers that I have talked to have said there exists a "CSI effect". Potential jurors, defense attorneys, and even some prosecutors have been so swept up into the imaginary world of the TV show that they have totally unrealistic expectations of what a crime lab can do. Prosecutors expect DNA test results back in hours and get cranky when they do not get what they want. But the scariest story I heard involved a juror. This person had watched so much CSI that he fancied himself an expert in the field. When the police did not run the tests that this juror thought they should have run, he assumed the police were hiding something and as a result voted to acquit a person who may very well have been guilty. So, as entertaining as these shows may be, there is always a cost to portraying things that are not true.

ATR II : Depth of Penetration

The depth of penetration (DP) in an ATR experiment is a measure of how far the evanescent wave penetrates into a sample. Understanding the variables that determine DP tell us a lot about how the technique works, why ATR spectra look the way they do, and the sorts of interesting applications that can be pursued via ATR. The equation for depth of penetration in an ATR experiment is (the envelope please):

DP = 1/2πWnc(sin2θ – n2sc)1/2

(please pardon the appearance of the equation, it had a rough night...and blogger does not allow subscripts and superscripts. Red is for superscripts, blue is for subscripts.)

Where
DP = Depth of Penetration (in cm)
W = Wavenumber in cm-1
nc = Refractive index of ATR crystal
θ = Angle of incidence of IR beam with crystal surface
nsc = refractive index of sample divided by refractive index of crystal

Note that all the paramaters in the DP equation are in the denominator, so when any of them goes up, DP goes down. The next several blog posts will cover the different parameters in this equation and what they teach us about the ATR experiment.

FTIR Sample Preparation for the 21st Century: ATR

The holy trinity of FTIR sample analyses is speed, accuracy, and cost. Ideally an analysis will be carried out quickly, accurately, and as fast as possible. Sample preparation has long been the Achilles' heal of FTIR, too frequently involving long, tedious, manual operations. For example, even in the hands of a skilled analyst it can sometimes take over 1 hour to prepare a KBr pellet. This is unacceptable in a 21st century lab where time is money and speed is of the essence. Fortunately, there exists a sample preparation technique that is up to the challenge of giving us accurate, fast, and inexpensive analyses, it is called Attenuated Total Reflectance (ATR). This blog post will be the first in a series exploring how ATR works, what applications it is suited for, and why it is such an advantageous technique.

In the ATR technique the infrared beam is brought to a focus on the face of a prism-shaped crystal made of a material that is infrared transparent and has a high refractive index. Common examples of ATR crystals include diamond, Zinc Selenide (ZnSe), and Germanium. The infrared light is refracted by the crystal and travels towards the top surface of the crystal as illustrated above. The magic ocurrs when the infrared beam reaches the top surface of the crystal. Because of a phenonenon too complicated to explain, here a small portion of the infrared beam sticks up above the surface of the crystal. I call this region of space a "hot spot" but it is more properly called the "evanescent wave". Infrared spectra are obtained by bringing samples into contact with the evanescent wave so they can absorb some of the infrared radiation. More to follow...

For more on the topic of ATR you can consult my book Fundamentals of FTIR, available for purchase here: FTIR Books , or take my Hands-On FTIR Sample Preparation Course as outlined here: FTIR Sample Prep. Course Outline.