[habbababba]

The amount of deflection of ions inside a mass spectrometer depends on 2 factors:
1) The mass of the ion.
2) The charge of the ion.

The instrument detects the ions based on their mass/charge (m/z) ratio. This means that 2 ions with the same m/z ratio do not necessarily have the same relative mass units. For example, an ion with 18 mass units and a +1 charge will have a m/z = 18. Also another ion with 36 relative mass units and a +2 charge will have a m/z = 18. However, isn't the purpose of a mass spectrometer to find out the relative mass of a chemical species? The 2 ions just mentioned will register the same peak on the mass spectrum despite having different mass units.

Does the mass spectrometer 'filter' the ions in a way that they all end up having the same charge? If yes, how? If no, how is the issue solved?

Thank you.

[Corribus]

No instrument is perfect. Sometimes you just have to deal with interferences.

That said.

In the case of mass spec: first, resolution is better than m/z = 18. The chances of two completely independent chemicals having exactly the same m/z ratio is small. You might be able to resolve them even if they are close. Second, chromatographic separation on the front end (e.g., GC/MS) can provide a lot of additional information and help separate away interferences. Third, in some types of MS (like ICP-MS) collision cells or reaction cells can be used to "react away" interferences that have the same mass number as the analyte of interest. Third, there are fragmentation patterns and other means of getting around interferences. While a parent ion may have an m/z of XX, daughter ions may also result from ionization that have different m/z values. So while one of the ions may have an interference, others may not.

[habbababba]

The chances of two completely independent chemicals having exactly the same m/z ratio is small.

I understand the impact of making assumptions in science can have on formulating plausible models. However, in this particular case, I can't rely on such assumption because it can heavily alter the meaning of the measurement. Your statement could hold a decent degree of truth for large and complex molecules, but for a sample of a monatomic element, I can't see how the chances are small.

So is it imperative that we couple MS with other separation techniques in order to enhance the accuracy of determining the mass of a chemical species?

Thanks.

[Corribus]

I think it's important not to try to overgeneralize. The solution to avoid interferences depends a lot on the technique you are trying to use, what your analyte is, and so forth. Even for very light analytes, exact m/z ratios are not common and resolution may be good enough to separate them. For example, we often analyze for sulfur in our lab, which is roughly isobaric with dioxygen (molecular weight = 32) and a few other potential interferences. But it's not EXACTLY the same. Isotope patterns are different, and the resolution of the ICP-MS may be good enough to resolve in certain instrument modes.  In cases where resolution is not good enough to separate ICP-MS interferences, collision cells are used. There's no one "magic bullet" to get rid of an isobaric interference. It all depends on what your analyte is, what your sample is, and what instrumentation you have available. And this is only for ICP-MS. LC/MS, GC/MS, etc., all have many but complex solutions to potential interferences.

 

[habbababba]

we often analyze for sulfur in our lab, which is roughly isobaric with dioxygen (molecular weight = 32) and a few other potential interferences.But it's not EXACTLY the same.

So if I have a sample of bromine and I would like to find out the relative abundance of the different isotopes of bromine, will ⁷⁹Br⁺ and ⁷⁹Br-⁷⁹Br²⁺ give the same m/z peak? If no, why? If yes, then the second molecular ion is affecting the actual percentage abundance of the⁷⁹Br isotope and so how does resolution solve this issue?

Thanks again.