How much is a kilogram? Sorry, that is incorrect…

I like to brag about how precise and sensitive Agilent instruments are.  Here’s a story that highlights the importance of accuracy in science.

The world is going to change how much a kilogram weighs.

Here’s the history.  There is an actual kilogram.  It’s called “Le Grand K,” and it was forged in 1879.  It sits in a guarded, vacuum-sealed vault in the Bureau International des Poids et Mesures outside of Paris.  It is the standard by which all other kilograms are calibrated.

Here’s the problem.  Several years ago, scientists discovered that one of the mirrored “witness” kilos (the one at the National Institute of Standards and Technology in the U.S.) had a different weight than Le Grand K.  The discrepancy is currently 45 micrograms and slowly increasing.  This is about the weight of an eyelash or a grain of sugar, but it’s the kind of thing that drives scientists nuts.

Think about it.  A “joule” defines energy in kilograms per meter.  A “candela” defines luminosity in joules per second.  If one measurement becomes unreliable, the problem cascades to numerous other measurements.

Here’s the solution.  Scientists are going to change the definition of a kilogram.  Rather than basing it on the mass of an object – which may change over time – they want to use an absolute, unchanging physical quality.  There is a precedent for this.  The “meter” was originally defined using a brass bar; it is now based on the speed of light.

Candidates for the new kilogram include Planck’s Constant (from quantum mechanics) and the Avogadro Constant (from atomic mass).  The winner, to be decided in 2018, will be the one that can be measured most precisely.

Speaking of which, how precise and sensitive are Agilent instruments?

  • Our mass spectrometers can measure the mass of a molecule to six significant digits
  • Our LC/MS systems can analyze blood samples of less than 100 µL (millionths of a liter)
  • Our DNA Analysis Kits can analyze samples of 100 picograms in a microliter (the amount of DNA in a single human cell)
  • Our ion pumps can create a near vacuum so perfect, it enables scientists to measure an instrument variance of one part in a thousand billion billion (that’s 1 in 1,000,000,000,000,000,000,000)

How’s that for precision?


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