Chromatography Forum

While doing some API + electrospray ms scans I have come across a few compounds that give an M+23+ signal. The immediate thought was "Sodium Adduct". But the sample was quite diluted from original sample, and diluted in methanol/0.1% formic acid (75/25).
My thought was where is the sodium coming from?
I decided to try fragmenting the ion M+23.
I expected to see a similar pattern of fragments, some possibly 23 amu higher, some not as seen with the M+H+ ion.
I did not see ANY similar peaks, in fact the ion seemed rather hard to break apart and fragments were very low intensity, almost noise. I even scanned for a fragment-peak at 23 amu.

In the past I ALWAYS used the M+H+ peak when in positive mode , using other adducts for quantitative analysis was frowned upon by my employers. As such I have little experience in this sort of thing.

Do sodium adducts give the same sort of fragmentation (same collision energy, same sort of cleavages) as the M+H ion. One would think so. Is this the case?

Alp

You experienced a normal event: sodium adducts tend to produce very low fragmentation and they are not recommended for MRM.

Better stick to the M+H ion as you always did, or possibly to the NH4+ ion if present (that one usually gives reasonable fragments).

Is this always correct?
Why would a sodium adduct form less fragments/lower intensity than a hydrogen (M+H+) adduct?

Would it be because the sodium adduct declusters once it hits the collision gas giving M and na+ (I scanned for but did not see 23+). Or is it some other reason?
Maybe H+ adducts are more prone to re-arrangement/fragmentation when the adduct forms a bond (and Na can't do this)?

Reasons welcome. Educate me!

Alp

Actually I don't really know why, but it obviously has something to do with the high stability of alkali metal adducts which require much higher collision energies than M+H ions.

It is still sometimes possible to fragment Na+ adducts, as can be seen in that paper: Collision-induced fragmentation of underivatized N-linked carbohydrates ionized by electrospray.
David J. Harvey, 2000.

I would be pleased to be educated as well on that topic!

It is ok to use sodium adduct for MRM, because it may have much higher sensitivity than M+1 or negative ESI for some compounds. From my experience, you should see a fragement ions with 23 higher than corresponding M+1 fragmentation. It may depend on how strong the bonding is.

Yangz00g

I would have expected this sort of thing. Some ions with added mass of adduct, some without, depending on where the fragmentation occurs and the charge ends up.

Alp

Sodium adducts don't always fragment the same. I vaguely remember that in carbohydrates, alkali metals can bind to two adjacent groups in a way that hydrogen cannot, which means that the fragments are different. There's something in the Encyclopedia of Mass Spectrometry about it, but I can't face chasing up the reference at the moment.

Whether you succeed depends on both instrument and chemical. Some chemicals (carbohydrate-rich compounds are the classic example) work just fine as sodium adducts in almost any instrument. My experience is that ion traps handle sodium adducts much better than triples, but I'm not a world expert...

Sodium can come from anywhere, particularly glass, and a chemical with a high affinity for sodium will mop it up and form sodium adducts. If you're unlucky, you can start a set of runs with hydrogen adducts, and as the sodium leaches from a glass bottle into a freshly-prepared buffer, a day later the same compound is running mostly as a sodium adduct.

Another thing that can happen, and which causes intense confusion, is surprisingly often the sodium can be lost in a fragment. If you have a part of a molecule that can easily hold a positive charge (big aromatic bit over which the charge can be delocalised) and a leaving-group with a good negative part (sulphate, carboxylate etc.) that can form a salt with the sodium, then you can get a sodium salt as leaving group, and a positive fragment that no longer contains sodium. This means that when you compare fragmentation of sodium adducts with fragmentation of hydrogen adducts, you may find fragments in common in addition to losses in common (as well as fragmentations that are completely different).

Are there effective ways of washing the sodium from the glassware to prevent this type of thing? Or should I "dialyze" my glassware in milliQ water?

Are there effective ways of washing the sodium from the glassware to prevent this type of thing? Or should I "dialyze" my glassware in milliQ water?

The best way to prevent "this type of thing" is to prevent glass ware. You can use polypropylene containers instead.

Thanks

Are there effective ways of washing the sodium from the glassware to prevent this type of thing? Or should I "dialyze" my glassware in milliQ water?

Sonicate mmobile phase bottle with 10% Formic acid or Nitric acid. Ringe with Acetonitrile and then LC/MS grade water.

Note : Do not wash/clean mobile phase reservoir with any detergents.