[jandrewrogers]

I am not up-to-date on the bleeding edge but that explanation doesn’t seem correct? The use of x-rays in analytical chemistry is for elemental analysis, not molecular analysis. (There are uses for x-rays in crystallography that but that is unrelated to this application.)

At an elemental level, the materials of a suitcase are more or less identical to an explosive. You won’t easily be able to tell them apart with an x-ray. This is analogous to why x-ray assays of mining ores can’t tell you what the mineral is, only the elements that are in the minerals.

FWIW, I once went through an airport in my travels that took an infrared spectra of everyone’s water! They never said that, I recognized the equipment. I forget where, I was just impressed that the process was scientifically rigorous. That would immediately identify anything weird that was passed off as water.

[wyldfire]

Here's an article that talks about Dual-energy CT [1]. And another one talking about material discrimination using DECT [2].

[1] https://en.wikipedia.org/wiki/Spectral_imaging_(radiography)

[2] https://pmc.ncbi.nlm.nih.gov/articles/PMC2719491/

[jandrewrogers]

Neither of those articles seem to support the idea that you can do molecular analysis with x-rays. They are all about elemental analysis, which is not useful for the purpose of detecting explosives.

[littlecranky67]

Not sure if they use dual-energy x-ray as in [0], but you don't need to if you take x-ray shot from different angles. Modern 3D reconstruction algorithms you can detect shape and volume of an object and estimate the material density through its absorption rate. A 100ml liquid explosive in a container will be distinguishable from water (or pepsi) by material density, which can be estimate from volume and absorption rate.

https://en.wikipedia.org/wiki/Dual-energy_X-ray_absorptiomet...

[codethief]

See also beepblap's comments further below where they elaborate on this a bit (it's not just simple dual-energy xray apparently).

[don_esteban]

Hm, isn't it enough to just detect water and flag everything else as suspicious?

If your liquid is 80%+ water (that covers all juices and soft drinks), it is not going to be an explosive, too much thermal ballast.

[palata]

> FWIW, I once went through an airport in my travels that took an infrared spectra of everyone’s water! They never said that, I recognized the equipment. I forget where, I was just impressed that the process was scientifically rigorous. That would immediately identify anything weird that was passed off as water.

Something like 10 years ago, I had my water checked in a specialised "bottle of water checker" equipment in Japan. I had to put my bottle there, it took a second and that was it. I have been wondering why this isn't more common ever since :-).

No idea if it was an "infrared spectra machine" of course.

[regularfry]

Cynically, it's so they can sell you another bottle on the secure side. If they spend money to give themselves a working mechanism to distinguish water from not-water, they lose the ability to create retail demand.

[palata]

I understand the idea, but it's not completely true: I empty my bottle before the security check, and fill it after in a fountain.

[regularfry]

Then you have successfully circumnavigated a problem that more forgetful people will run into head-first. It doesn't have to catch everyone for the shops who are tenants on the secure side to complain about lost sales.

[marcosdumay]

There's still no evidence that peroxide-based explosives are stable enough to be practical. And nobody every explained why the few liquid ones are so dangerous, but the solid ones get a pass when they are more stable.

It's a good thing that airport brought some machinery to apply the rule in a sane way. But it's still an insane rule, and if it wasn't the US insisting on it, the entire world would just laugh it off.