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Maybe it wasn’t all those butter tarts

Feeling like you’ve packed on a few micrograms over the holidays? Perhaps the kilogram is to blame.

Researchers from Newcastle University in Britain have shown the kilogram itself has put on weight.

The original kilogram — known as the International Prototype Kilogram or the IPK — is the standard against which all other measurements of mass are set. Stored in the International Bureau of Weights and Measures in Paris, 40 official replicas of the IPK were made in 1884 and distributed around the world in order to standardize mass.

Using a state-of-the-art Theta-probe XPS machine — the only one of its kind in the world — the team has found the original kg is likely to be tens of micrograms heavier than it was when the first standard was set in 1875. It seems that over time, contaminants have built up on the surface of the platinum-based originals.

“It doesn’t really matter what it weighs as long as we are all working to the same exact standard. The problem is there are slight differences. Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original,” said lead researcher Peter Cumpson.

“But mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge. There are cases of international trade in high-value materials — or waste — where every last microgram must be accounted for.

The researchers used cutting-edge X-ray Photoelectron Spectroscopy (XPS) to analyze surfaces similar to the standard kilogram to assess the buildup of hydrocarbons — and how to remove them.

Publishing their findings this month in the journal of Metrologia, they reveal how giving the kg a suntan could be the answer. “By exposing the surface to a mixture of UV and ozone we can remove the carbonaceous contamination and potentially bring prototype kilograms back to their ideal weight.”

Sounds like a good excuse for a winter holiday.

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