# Talk:Nominal mass

--Ionworker 12:22, 6 Jan 2005 (CST)

Atomic weight should not be used because weight is a force and not a mass.
Minimum nominal mass is a better name for the nominal mass of the lightest isotope.
Isotope base mass would be more appropriate for the mass of the most abundant isotope.

## Proposed Entry

The mass of an ion in atomic mass units rounded to the nearest integer.

M = n Da = round(m)

The concept of nominal mass is useful because the mass of (small) molecules is always very close to their nominal mass. Many types of mass spectrometers do not have sufficient resolving power to distinguish molecules with the same nominal mass. For example CO and N2 both have the same nominal mass M = 28 and cannot be resolved by many mass spectrometers even though the accurate mass m is slightly different for the two molecules.

Moved from front page.

-- K. Murray 16:27, 18 Jan 2005 (CST)

## Nominal Mass of Larger Molecules

Sometimes the nominal mass of larger molecules can be confusing. A molecular formula of C45H74O has a "molecular mass" of 630.574 Da using the most abundant isotopes of C, H, and O. Calculated as:

```    (12.00000x45) + (1.00782506x74) + (15.99491475x1) = 630.574
```

It has a "molecular weight" of 631.087 Da from the averaged masses of all the C, H, and O isotopes. Based on the proposed entry, would the nominal mass of this molecule be 630 or 631 Da?

I propose that nominal masses be calculated using nominal masses of each atom involved. For example, C45H74O in the above example would be calculated as:

```    (12x45) + (1x74) + (16x1) = 630
```

thus making the nominal mass 630 Da. This eliminates the cumulative effect of mass defects for larger molecules.