Tag Archives: metrology

Chemistry: Not quite so elementary

Urey
Harold Urey, father of isotopic chemistry. Image from Wikipedia.

Chemistry, like many of the sciences, is laden with a horribly confusing vocabulary that persists for merely historical reasons. For example, atoms are not indivisible even though atom comes from an ancient Greek word atomos meaning “indivisible”. “Free” energy is not free, at least not as most speakers of modern English would understand the word.

And so it is with the chemical “elements”: they aren’t elementary. Take carbon for example. This supposedly elementary substance can be chemically separated into (at least) two types, each with a different reactivity. I’m talking, of course, about isotopes. The heavier type of carbon is carbon-13 or 13C for short, and is made of atoms that have six protons and seven neutrons in its nucleus. The lighter, more prevalent type is carbon-12 (12C), with the same number of protons, but only six neutrons instead of seven. Admittedly, the differences in chemical reactivity of 13C and 12C is very slight — entirely negligible for most practical purposes. So in a sense, calling “carbon” an “element” is only a slightly misleading, but there are certainly times when the difference can be very important.

Don’t just take my word for it. Poul Anderson explained the same thing in, well, slightly different terms back in 1989:

Unclefts with the same tale of firstbits but unlike tales of neitherbits are called samesteads. Thus, everyday sourstuff has eight neitherbits with its eight firstbits, but there are also kinds with five, six, seven, nine, ten, and eleven neitherbits. A samestead is known by the tale of both kernel motes, so that we have sourstuff-13, sourstuff-14, and so on, with sourstuff-16 being by far the most found. Having the same number of bernstonebits, the samesteads of a firststuff behave almost alike minglingly. They do show some unlikenesses, outstandingly among the heavier ones, and these can be worked to sunder samesteads from each other.

One other crazy thing about isotopes (or samesteads if you prefer). Not all neutrons have the same mass. The 13C atom’s mass 13.0034 daltons (Da), and the 12C atom’s is 12.0000 Da (by definition). Since those atoms differ only by a neutron, it would seem that a neutron in a carbon atom has a mass of 13.0034 – 12.0000 = 1.0034 Da. But hydrogen also has two isotopes: 2H with one neutron and one proton, and 1H with no neutrons at all, only a single proton. But a 2H atom’s mass is 2.0141 Da, and a 1H atom’s is 1.0078 Da. If you do the same subtraction, you’ll find that the mass of neutron in a 2H atom is 1.0063 Da. Those numbers are close but they are NOT the same. A neutron in a hydrogen atom is about 0.3% heavier than a carbon atom’s neutron.

The reason for the difference has to do with Einstein’s famous mass-energy relationship and the strong nuclear force and…well, let’s make it a subject for another time. For now, let’s end by saying that these seemingly slight differences in neutron mass from atoms of one element to another — called the mass defect — can be detected in modern mass spectrometers. And what of it, you say? Well, I’m glad you asked: that will be the subject of some upcoming posts. (Here’s a taste.)

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The mess of enzyme activity units

Trying to quantify enzyme activity can be a messy business. In this post, I’ll discuss some of the complexities that can haunt both students and practitioners of enzymology. Since this post gets a bit discursive, all the meat is below the fold.
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