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There are a lot of questions here looking for simple methods to measure caffeine content (or extract it), including one of mine here and on the Coffee site:

But here I'd like to ask something different:

Question: Is there a standard industrial method for measuring caffeine content in food and drinks?

For example, if there were government requirements to post the amount of caffeine in a product there may be some standard way that government advised that it be done (i.e., "this is the way we're going to do it to verify what you report."). While I don't know of any such requirements, that doesn't mean there aren't any.

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The answer is fortunately rather simple. Use a C18 reversed phase column and a UV detector in an HPLC system. One would make a calibration curve by injecting various concentrations into the column. Now, if an unknown sample of a drink is injected, and one sees a peak at the same retention time in the sample chromatogram, you know it is caffeine. By interpolating the area, we know its concentration. When an HPLC method is developed, it is ensured that no other component elutes at the same time. This is achieved by using gradients of mobile phase composition.

If there is a legal issue and one has to provide a 100% foolproof test, then one has to use a mass spectrometer along with a UV detector to confirm the identity of the peak at various locations, i.e., one ensures that the peak is pure.

See the examples in Agilent's note "Determination of Caffeine in Coffee Products According to DIN 20481 by Edgar Naegele from where this figure is taken:

enter image description here

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    $\begingroup$ I could see series caffeine aficionados laying out $50k for an HPLC. How else do you tune in the espresso machine for that perfect cup? $\endgroup$
    – Jon Custer
    Nov 18, 2022 at 3:16
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    $\begingroup$ Determination of Caffeine in Coffee Products According to DIN 20481 (also here) compare figures 4 & 5. Nice! $\endgroup$
    – uhoh
    Nov 18, 2022 at 11:14
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    $\begingroup$ "DIN"="Deutsches Institut für Normung" - the German ISO member. DIN norms are pretty much the requested "standard industrial method". $\endgroup$
    – MSalters
    Nov 18, 2022 at 14:07
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    $\begingroup$ @uhoh, Mini portable and complete HPLCs of the size of a brief case are available now. I see no problem in running caffeine stds and samples. $\endgroup$
    – AChem
    Nov 18, 2022 at 16:48
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    $\begingroup$ @uhoh We used to keep a 5 pound bottle of caffeine citrate next to the always on lab break room coffee pot. The sign on it, due to me, was “Two scoops per pot, to combat the Santa menace.” So you could just start with decaf and add caf, to desired level of wiredness. (We took the big bottle of caffeine citrate away after a few weeks, because someone might have not understood and actually added the two scoops!) $\endgroup$
    – Ed V
    Nov 18, 2022 at 23:40

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