By measuring a sufficient number of different elements, the probability of accidentally matching two samples can be as rare as the duplication of fingerprints in two individuals. Matching of trace impurities is often called a “fingerprint” method.
With neutron activation analysis, you can obtain the “fingerprints” of the two samples to see if they match. Although this kind of evidence may be difficult to use as proof in court, a positive match will let the detective know that he is on the right track. Also, the suspect might confess if he is confronted with the evidence and realizes that he is “caught”. On the other hand, a mismatch will clear the suspect completely and the detective will know to look elsewhere for the criminal.
You seal each sample in a tiny polyethylene bag about ½ inch square. One sample is taken from the victim’s clothing and the second, about the same size as the first, taken from the damaged area of the automobile. In preparing these samples, you handle all the materials with clean forceps because you realize that the most minute dirt from your fingers will be detected in the analysis.
The two bags are irradiated together for 1 hour in a nearby reactor and 2 hours later you begin counting the samples with a high-resolution, lithium-drifted-germanium, gamma-ray spectrometer. This will give you a match (or mismatch) for elements that yield radioisotopes of fairly short half-life such as manganese (2.56 hours), copper (12.8 hours), sodium (15 hours), arsenic (27.7 hours), etc. You plan on “counting” the samples again later on, if the first counts match, so that you can check on radioisotopes with longer half-lives such as iron (45 days), chromium (27 days), silver (270 days), cobalt (5 years), etc.
The two gamma-ray spectra you obtain look like those in the [figure on the opposite page]. The gamma rays from the irradiated paint taken from the victim’s clothing indicate the presence of the common elements sodium, potassium, and copper, but gold, lanthanum, and europium are also conspicuously present. The gamma rays from the other sample also reveal sodium, potassium, and gold but in rather different proportions. More striking is the absence of copper and the two rare earths, and the presence of manganese and arsenic, which were not indicated in the first sample.
The paint samples definitely do not match. Therefore, you inform the detective that his suspect is innocent after all. You’ve solved your problem, but he still has his. Perhaps the same technique will provide positive proof when he finds the real culprit.
Gamma-ray spectra of two samples of paint. These two spectra are obviously different and, therefore, could not have come from the same source.