For QC and R&D labs working with polymers, knowing the molecular weight, repeating units and end groups of the polymer sample is vital, while monitoring incoming goods for impurities is important for sample quality.
Compared to other analytical methods, MALDI-TOF mass spectrometry can retrieve more structural information of polymers in a single, straightforward and rapid direct measurement – avoiding lengthy gradient and method development associated with traditional LC methods.
In a SelectScience webinar, now available on demand, MALDI-TOF expert Dr. Volker Sauerland explains the benefits of the technique and how it can enhance your QC or R&D lab’s polymer analysis. He also explores fast QC methods and direct measurement of various polymers; how complex mixtures can be readily analyzed with TLC-MALDI; and how spatial distributions of polymers can be probed with MALDI Imaging.
Read on for highlights from the webinar Q&A session or register to watch the webinar on demand.
Q: Is it possible to analyze copolymers with MALDI-TOF?
VS: Yes. It's possible to analyze copolymers, it depends a bit on the mass range, of course. At higher masses, the complexity of the copolymers is too high - there are just too many possible combinations of monomers. The typical mass range that you can analyze is up to approximately 5 kDa. In this case, you would want to use Polymerix software, rather than PolyTools. The Polymerix software can generate nice 2D plots of the distributions of the different monomers.
Q: Is end-group analysis easy for homopolymers and copolymers?
VS: For homopolymers it’s absolutely straightforward. Both Polymerix, and PolyTools can be used here. PolyTools will give you the answer right away as long as monomer and end-groups are defined in the program. Polymerix will make suggestions for the chemical composition of the combined end-groups depending on the pre-settings you choose. In the end, you always get the combination of the end groups. You don't get single end-group analysis when using MALDI-TOF alone. However, single end-group analysis is possible with a MALDI-TOF when you use the fragmentation possibility of a TOF/TOF instrument.
For copolymers again it’s only Polymerix which can do the analysis of the combined end-groups in the lower molecular weight range. For a lower molecular weight EO/PO, it's definitely possible to do an end-group analysis.
Q: Is it possible to run very high-mass polymers?
VS: It is possible to analyze higher mass polymers. The largest polymer I have run was 600 kDa. We also have a customer who recently ran a 1 MDa polymer. The thing to remember is, the higher the mass, the less information you get out of your sample.
You will still be able to generate average masses, which are often a very good thing to have. But, if you go for very high masses, in the end, you won’t be able to do end-group analysis because you won’t have the oligomeric resolution anymore. Extrapolation down to zero just won’t be possible.
Q: Is it possible to analyze ionic polymers?
VS: Yes, it’s possible to analyze ionic polymers. Polyanions can be analyzed after ion exchange. This will be your first step because you need to be sure that it's always the same counterion. Typically, polyanions are polyacids, and therefore you have to transfer them completely into the hydrogen form. This can be done by an ion exchange step. Afterwards, they fly quite nicely, but the preparation has to be done in advance.
If you have a mixture of counterions, for example, a mixture of sodium and potassium and hydrogen and your polymer contains 10 acid groups, these groups could be bound to 10 hydrogen, 10 sodium, 10 potassium, or any mixture in between. There are so many possible combinations. With every step, with every possible combination, you get a drop in the signal intensity of your ion. This makes the overall ion signal intensity too weak. Therefore, you need to have an ion exchange step before you really analyze this type of sample.
The second part of the ionic polymers is the polycation. Only very low molecular weight samples can be ionized. So far, I have been unable to get any good spectra for high-mass samples.
Q: Can MALDI-TOF be used for block polymers, like EO/PO, for example?
VS: Yes. This depends again on the molecular weight range. EO/PO is one of our standard examples that we use in the lab and for training. This works nicely up to a limited mass range of around 5 kDa.
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Q: Are there any guidelines which determine if a polymer flies well or not?
VS: There are no set guidelines. However, I always recommend just googling NIST polymer and MALDI. At the National Institute of Standards and Technology, they have a nice database of sample preparations listed there. If you have a polymer and you're not sure how to prepare it, then take a look at this database.
Otherwise, join meetings in your country. This is where you find other people working on synthetic polymers and mass spectrometry. We all have the same problems, independently of which spectrometer we have. The community and maybe the NIST database are your best resources here. Of course you’re also welcome to contact us via the Bruker MALDI application support.
Q: How do you determine branching in polymers if they have intrinsic microporosity (PIMs)?
VS: Independent if the polymer has intrinsic microporosity or not, branching is only detectable by mass spectrometry if there is a mass change related to the branching. If this is the case, the number of observed oligomeric distributions will increase with every branch increasing the complexity of the spectrum. As long as the spectrum complexity is not getting too high, you can also calculate the degree of branching. However, if there is no mass change between the linear and the branched form, the only way is to analyze by MS/MS, then we can see more.
Q: Is solvent-free sample preparation a standard method, and how is the sample fixed to the sample carrier? Are there alternative methods?
VS: I would not call a solvent-free sample preparation the standard method. Solvent-free sample preparation has one big advantage over all the other preparations: the solvent-free preparation avoids any separation of different molecular masses, for example, or different end groups.
If you have different solubilities, it might happen that these molecules precipitate later in the preparation, and therefore they will be found more in the center of the sample, giving you an inhomogeneous preparation. That's something that you avoid using solvent-free preparation.
The solvent-free method we use involves mixing your sample, matrix and salt in a mortar or in an Eppendorf vial together with two steel balls and vortex it thoroughly. You should then have a fine powder. Then, put just a very low amount of the powder on the sample carrier, press it down and scratch it off again, so only minor amounts are really remaining on the sample carrier plate. That's how it's fixed on the carrier plate. There are also other methods available.
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