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It's a spectrometer. By measuring how different individual wavelengths of light interact with a chemical, you can make a profile of its atomic makeup. How accurate the device is depends on several factors, including which kind of spectroscopy the device is using, how pure the sample is, and what kind of substance it is. The device is almost certainly better at detecting drugs and explosives, for example, but would likely struggle to identify something more complex like mashed potatoes.

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Likewise, it wouldn't probably be great at, for example, telling you which alloy of aluminum something you found in a scrapyard is, unlike some other ways of zapping things to know what they are.

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Ah, cool. So probably one of these FTIR or Raman spectrometers?

It would be able to say with fair certainty that it is not a known explosive or drug due to wavelengths that are missing though in the case of unknown items.

So if I wanna get my explosives through, mix them into some mashed potatoes, got it.

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I'm going to go out on a limb and suggest it is not raman. raman scattering is very weak, and would work better with prepared samples.

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Some law enforcement has access to handheld/portable versions of chemical identification tools that have been historically restricted to laboratory use. Customs agents frequently use FTIR and Raman spectrometers but some have even portable mass spectrometers that they can use (relatively newer tools).

FTIR/Raman works by using light (infrared and visible respectively) to identify chemicals in a sample. Briefly, this works by looking at the 'color intensity' of the light that went in and looking at how much is missing in the light that goes through the sample. The color of light absorbed by a chemical depends on that chemical's structure. Most chemicals (when pure) can be uniquely identified by their FTIR or Raman spectrum (the spectrum is a graphical/numerical description of the colors that were absorbed and by how much from the sample).

How accurate it is depends on the sample. If the sample is pure, the technique is highly accurate. If its a mixture, it's going to depend on what the components are, how much of each there is, and what you're trying to look for. FTIR/Raman often struggle to specifically identify inorganics like minerals, but are very good at identifying organic compounds like cocaine and methamphetamine. FTIR/Raman also struggles with low concentration formulations. For example, if cocaine is present in a sample at less than 5% with other material such as lactose composing the majority of the sample, it may be difficult to identify by FTIR/Raman.

Often, law enforcement will use a variety of techniques to identify material to overcome the shortcomings of any one technique.

For the most part, the hand-held versions of these devices are designed to operate automatically - interpreting the data for the user and identifying it as accurately as it can. The device usually gives some indication of confidence or whether or not it's a mixture. It's important to note that the identifications from these devices are almost exclusively used as a presumptive analysis, meaning that confirmatory identification from a lab is necessary for any type of criminal charge. This means they can use it as presumptive cause to take some action like seizure or arrest.

Thanks for taking the time to answer, that's really cool.