Chromatography Forum

OK, I'm really embarrassed about this one.

For a long time, I used to use "amu" to distinguish between a measured m/z and a molecular weight (Da). It was helpful to be able to write that the observed ion of 223amu was probably the doubly-charged sodium adduct M+2Na of the expected product, which had a neutral mass of 400Da.

A little while back I read wikipedia and realised how wrong I was, saw the error of my ways, and stopped using amu. I couldn't start using Da, because the peak is m/z, not just m. Meanwhile I couldn't start using the Thomson because no one has a clue what it is (and I don't think IUPAC embraced it either).

I can't really use "u" as no one really knows what that is either, and it's a unit of mass, ignoring charge. So now I'm reduced to saying things like "the peak of 223 is probably....", to which my clients/their reviewers tend to ask "223 what? give us a unit!". I suppose I could write "the peak of 223 u/e" or "223 Da/e", but they're just weird.

I can write "the peak of m/z 223 is....", which is what I'm mostly doing, but when you describe the kinetic energy of a car, you don't write the car with 1/2mv-squared of 452 is going this speed... You write it has an energy of 452Joules at this speed.

The theorists seem to feel that because mass is relative and because the charge of an electron is also relative, m/z is two unitless numbers divided by one another and therefore also unitless. This may be true, but it's resoundingly unhelpful; in human speech we need a unit.

If the proteomics people can talk about a 30kDa protein, not just the protein of 30,000, I want a unit too.

What do other people do? I know this is a bit esoteric, but I'm getting all mixed up about this!

m/z is the method the instrument uses to sort the ions.

A certain mass with a certain charge will give a signal at a certain m/z.

If you know the charge (+1, +2, -1, -2, etc) you can determine the mass of the "molecule".
You can use Dalton or AMU or grams if you want for your description of the mass.

Eg, the signal at m/z 50 can be atributed to the doubly charged compound (xyz) with a mass of 100 amu.

Yes, absolutely, but I'm frustrated at having to write "The ion at m/z 550 is probably..." because readers, and even reviewers of non-specialist journals, ask "550 WHAT?, what are the units?". And I have no useful answer.

Have a look at:
http://en.wikipedia.org/wiki/Atomic_mass_unit
This explains why "amu" is impossible.

Also have a look at:
http://goldbook.iupac.org/M03752.html and follow links.

I appreciate that m/z is unitless, but given a journal editor who won't publish unless I give it a unit, what am I supposed to do?

Imh, how many 100 000 people have published MS data?
m/z has the dimensions of mass unit/charge. Since all the MS apparati I ever used were adjusted to give m = 1 being 1/12 of a mole of C-12, and since all molecules analyzed were small and gave a charge of 1+ the m/z were taken as g/mol.

Entirely agreed, especially for singly charged ions. I've just had an unlucky run on multiply-charged things lately, and found myself stuck for words, and then ran into a particularly pedantic client who really, really wanted a unit. Best suggestion kicking around our lab so far is "make one up".

I shall remain silent and unitless.

Tell the client that you are calibrated such that g/mol come out.

will do!
Thanks

The thomson (symbol: Th) is a unit that has appeared infrequently in scientific literature relating to the field of mass spectrometry as a unit of mass-to-charge ratio. The unit was proposed by Cooks and Rockwood[1] naming it in honour of J. J. Thomson who measured the mass-to-charge ratio of electrons and ions.

So, give your client m/z 223 Th.