Talk:Mass/Charge Ratio Comments

I think that we may be stuck with the unitless m/z. But you are correct, this is a confusing point.

Sparkman recommends against the Thomson unit.

- Kermit Murray

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Unfortunately I don't have the book of Sparkman available. What is his line of argument? I used to be in favor of the "unitles" m/z. Now I think it is a folly. Maybe we can declare m/z as dimension- and unitles. We could also declare current, voltage, length, time, speed, weight, force and any other property as dimensionles. We could also remove all the labels from every bottle, we could purge the names of all streets, cities and countries. However, the confusion only gets bigger. The concept of dimensions and units is an important scientific and cultural achievements of humanity. It helps naming and measuring things, and enables communication about it. And we, the mass spec community, make fools of ourselves if we ignore this achievement. Or did I miss something?

-Ionwerker

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Here's Sparkman (p 27 of Mass Spec Desk Reference)

????????????????????????Although this term is the official usage as prescribed in the Current IUPAC Recommendations and the ASMS guidelines, unfortunately, m/z is a mass spectrometry neologism. In SI units, the lowercase letter m is the symbol for the meter. The symbol for atomic mass is the lower case u. Therefore, the correct abbreviation for a mass spectral ion or a peak in a mass spectrum should be u/z ?????????????????????? This term has never been used. The single term m/z is a symbol, not an abbreviation.??????????????????????

He goes on to say that ????????????????????????m/z is a symbol, not a mathematical formula?????????????????????? even though it looks like a mass divided by a charge number (and can often be treated in this way. The current convention is to use dimensionless "m/z units" and hope the confusion stays at a minimum. The thomson unit is an attempt to reduce this confusion, but it adds some new confusions due to similar terms in physics.

Ken Busch says in his Mass spectrometry Forum (PDF) that "a more correct unit for use on the x axis of a mass spectrum would therefore be u/z. Mass spectrometrists simply choose not to use it." We could, and I take it that this is your suggestion.

- Kermit Murray

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Kermit, The following definition is taken from here:([1])

Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor n:

Q = n x [Q]

some examples:

• l = 3 m (the physical quantity length is three meters)
• m = 5 kg (the physical quantity mass is 5 kilos)
• m = 5 u ( the physical quantity is 5 amu)
• q = 3.5 C ( three and a half Coulombs)
• q = 2 e (2 elementary charge units)

1) Sparkman seems to mix up Q and [Q], that is the symbol for a physical quantity with its unit. ????????????????????????Therefore, the correct abbreviation for a mass spectral ion or a peak in a mass spectrum should be u/z.?????????????????????? No! u/z are the units, corrsponding to [Q]. We need also a symbol for the physical quantity mass/charge. This symbol can be m/z (even though m/q would be better). Hence a mass spec x axis needs both:

m/z (u/e).

2) m is the symbol of a physical quantity mass, and m is the abbreviation of the unit meter.

3) Let us investigate where the z is coming from: start with Q = n x [Q] and use it for a charge:

q = n x [q]

Now, if [q] is the elementary charge, n will always be a whole number. Whole numbers are often indicated with z. Therefore in this special case we could write:

q = z x e

This means z is not the symbol of charge (as it is used by mass spectrometrists (m/z), it is only the numerical factor belonging to the unit. Hence z is the number that you read from the x-axis and should not appear in the label. Compare this with a length measurement:

l = n [l] or, when measuring in meters: l = n m

Now, if you label a x-axis that represents a distance, do you label it with ????????????????????????n?????????????????????? ? No! It is labeled with l (m). Consequently we should eliminate the z because it is the source of the whole confusion.

Here are the basic facts:

1. mass spectrometers measure the physical property mass/charge
2. the most common used symbol for mass/charge is m/q
3. the symbol m/q is completely independent on the units [m/q] that are being used

Ok, now we have settled the question of the symbol. Let?????????????????????????s turn to the second part, the units to be used. Obviously, everyone in MS wants to measure the mass in amu and the charge in elementary charge units.

m = n [m] if [m] is amu it becomes: m = n u

q = n [q] turns into q = n e or maybe q = z e as argued above.

Hence, the unit mass spectrometrists use is [m/q] = u/e. That is what everyone uses. The only problem with this: e is not an official unit. Here we have a severe problem: everyone uses a unit that does not exist officially. There are four ways out:

1. we make e official (in which case we should not call it e, but for example Millikan (Mi).
2. we create the Th (which de facto creates e through the back door)
3. we mumble about m/z being dimensionless
4. we use kg/C

I will now argue why (3) is nonsense: The argument why m/z supposedly is unitles goes like this:

in q = z e, z is unitless.

That is correct! But as we have seen above, we should use q and not z, and q has unit e. This means the wrong use of z as a symbol of the physical quantity for charge is the root of the confusion.

Let?????????????????????????s make the same mistake with a distance quantity, which makes the error more obvious.

d = n m is a distance quantity measured in meters.

Now we could label an x-axis representing a distance with n and declare it as dimensionless (n really is dimensionless). Nobody would do this!

t = n s is a time quantity in seconds.

Here again we could lable time-axis with n and declare the measurement dimensionless. The same can be done for any other physical quantity. But it is wrong! We did measure the time in seconds, the distance in meters and so on. The same holds for m/z. We do measure the charge in e! That is simply a fact and we cannot deny it. Nor should we. Hence, we will have to either introduce a new unit for charge (because we use it anyway), or we stop using the elementary charge as a unit and turn to kg/C. These are the two only legal ways out, and since nobody wants to use kg/C there is only one way out: declaring the elemental charge unit officially as a unit. Using the Thomson is fine with me, however, it is a unit derived from e, hence it is not really appropriate to define Th without making e legal.

kind regards,