PEG normally flies as the Na+ adduct in MS. The COOH groups will be protonated at the pH of the MALDI matrix, so you should see the PEG-COOH as a Na+ adduct in positive ion mode. If you run in negative ion mode you might see the PEG-COO-, depending on the number of COOH groups per PEG.

Luke V Schneider Thank you for your inputs sir. So, if I have COOH-PEG-COOH and if i run in negative mode, what kind of peak will be observed?

In addition I would like to know in order to get the peak for COOH-PEG-COOH, which will be a better option whether MALDI or ESI. Does ESI Have an impact on Na+ adduct formation?

ESI will give you multiple charge states to deal with, not impossible unless the PEG is very large. MALDI will give you singly charged species (generally). Unless you have a mono-disperse PEG (very unlikely), you should see a series of isotopic peaks spaced 44.05 Da apart. Depending on how the PEG polymerization was initiated, you may be able to match the exact structure to the masses. The HOOC-PEG-COO- should be the structure.

Ok sir thank you for your inputs.

In negative mode ESI, with addition of alkali metal salts, carboxylic acids show also (RCOO-)M+(RCOO-) type clusters.

Jean-Francois is correct. PEG acts a lot like crown ethers in that it chelates metal ions, which is why it typically flies as a Na+ adduct in positive ion mode.

Luke V Schneider Sir, I've got the following graph when subjecting the COOH-PEG-COOH to MADLDI -TOF. I just want to confirm whether 2212.47 is the molecular weight of the compound or no. Because I got a series of peak spaced with 44 Da from 393 to 1700m/z (which may be because of the monomers in PEG). Can you please have a look at the graph at help me out with this?

Thank you.

I doubt that your compound HCOO(CH2CH2O)nCH2CH2COOH (?) is monodisperse , i.e., n having a single value. m/z 2212.47 may be an impurity cationized (by Na+?) or protonated (M+H+).

Jean-François Gal sir, the following attachment is the structure of my compound. I have also got the monomers peaks with the spacing of 44Da. I have attached the image of that as well. Please help me to find whether, my compound has shown the molecular peak at 2212.47 or not. Whether i can consider this 2212.47 as molecular weight of the compound or no.

I suppose that the MALDI MS was in positive mode. Have you a synthesis step using a potassium salt ? One solution is a singly charged K+ adduct : for example, n = 10, using nominal masses, 134(end groups)+(44*10)+39 = 613. Just a guess !

Jean-François Gal Sir, is that mean the molecular peak at 2212.47 also has K+ adduct? Can i consider that 2212.47-39.0983= 2173.371 as the molecular weight of my compound?

So in that case no of monomers in my PEG would be 2173.371-134 (end groups) =2039.37 /44.04 (monomer mol.weight)= 46.03.

Is the above calculation correct? Kindly help me with this sir.

Thank you,

S.K.Janani.

Your calculations seem correct, but it is strange that this polymer with n = 46 is out (not part) of the enveloppe of the distribution culminating at n = 10 (m/z = 613.35) and ending at about n = 28-30.

Jean-François Gal Sir, actually the no: of monomers does'nt end with 28-30, it is having equal distribution of peaks with spacing 44.04 Da till 1700 m/z and beyond that. So, by considering that I hope that the no of monomers would be 46. Correct me if I'm wrong. I have also attached zoomed data of the same for the reference.

So in that case my no of monomers would be 46 with molecular weight of 2173.371 Da.

Further, if I consider the peak 481.23, the formula would be COOH-CH2-O-(CH2CH2O)7-CH2-COO-.

Kindly let me know whether i can consider this calculation.

Thank you,

S.K.Janani

OK, n seems extends above 40, but I don't see why n = 46 has a special meaning (some larger intensities than peaks for n > 40?).

So far we spoke about positive ions (K+ adducts, still I don't know if the presence of K+ is justified) and COOH-CH2-O-(CH2CH2O)7-CH2-COO- is a negative ion.

My question was "where is the ion m/z 2212.47?". Is it the last significant peak on the spectrum?

The mass spectrum "44da" shows a mixture of polymer, probably mostly constituted of compounds corresponding to n about 6 up to n about (or larger than) 40, considering the zoom "zoomdata" on the end of the spectrum.

For me the peak m/z 481.23 is not different from the others, because 133.8517 is in fact 134, no need to remove a proton. If you add K+ to the negative ion COOH-CH2-O-(CH2CH2O)7-CH2-COO-, you obtain a neutral salt not detected by the mass spectrometer.

The peak 437.19 is strangely about twice the expected intensity considering the distribution of peaks for n = 5, n= 7, 8, etc.

There are a lot of other peaks which seems also to be spaced by 44.

1-I lost the spectrum but I agree that it pertains to the series of polymers.

2-You speak bout "my compound" in one answer. I mean that your compound is a mixture of numerous polymers with n = about 5 to up to 46.

3-Yes it is the K+ adduct with n = 7.

4-You are right, the matrix peak interfere.

5-See answer 2.

Jean-François Gal thank you sir,

So as a conclusion we can say that the molecular weight of the compound is 2212.47-39.0983=2173.3717.

With n=46 and the peak can be described as COOH-CH2-O-(CH2CH2O)n-CH2-COOH (where only n varies based on the no of monomers in the peak) with K+ adduct right sir?

I fear that we were wrong !

The isotopic distribution in a mass spectrum can be made using

ChemCalc for example : http://www.chemcalc.org/

The results for the K+ adducts for n = 46 and 47 are attached. The differences with rounded (nominal) masses shows up when there is a large number of atoms of H and O, because the exact masses of the isotopes 1H and 16O are slightly different from 1 and 16.

The isotopic "cluster" of peaks may be seen on you spectra with a zoom depending on the resolution.

Note also that the calibration of the m/z scale is crucial for comparisons with simulations.

Sorry for misleading you with my oversimplified estimations.😫

Ok sir i'll see and work accordingly. Thank you.

The simulation is a good way to validate the ion formula by comparing the isotopic profile (if the resolution is sufficient.)

I am sorry for the waste of time, ions at high masses are more difficult to deal with.