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Perfecting Polymer Measurements: OMNISEC Q&A

25 March 2015 No Comment

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OMNISEC at PittconA couple of weeks ago I had the opportunity to present a live webinar through Chemical & Engineering News about how our latest solution for Gel Permeation Chromatography/Size Exclusion Chromatography (GPC/SEC), OMNISEC, can help our customers with their synthetic polymer applications.

OMNISEC is a complete system for GPC/SEC measurements including a pump/degasser/autosampler module in OMNISEC RESOLVE and an advanced multi-detector module including RI, UV/Vis, Light scattering and IV detectors in OMNISEC REVEAL.

During the webinar we covered measurements from simple polymers like polystyrene to fascinating drug delivery polymers such as PLGA. At the end of the webinar we received a whole host of questions but we only had time to answer a handful.  Now the link to the recording of that webinar is available on our web site (click here to play it back) I have written answers to most of the questions here.  I found them very interesting as they offer a broad insight into the applications and concerns of our customers – I hope you find the answers are also interesting!  I’ve listed some extra resources you may find helpful below the Q&A.

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Q. Can Chloroform be used as the mobile phase?

OMNISEC has great solvent compatibility.  Organic solvents such as THF, DMF, DMSO, HFIP, chloroform, acetone, and methanol will all work with no problem.


Q. What is the molecular weight (MW) range for the RALS/LALS detector?  &  What is the upper limit in MW? We are interested in very high MW polymers – 12 to 14 million Da.

A. The lower limit we’ve specified is 200 Da.  We made some great measurements of a 200 Da PEO sample so that’s where we’ve set the limit.

The specified upper limit for OMNISEC is 107 Daltons.  However, I should say that this isn’t the limit of the detectors.  Really the upper limit for a SEC system is limited by the columns and the separation.  It isn’t really possible to separate anything larger and about 100 nm radius using SEC.  This equates to about 107 Daltons so is why the limit is set there.


Q. What is the communication protocol between computer and instrument? USB?

A. Yes, a single USB connection between the detector module OMNISEC REVEAL and the PC.  OMNISEC  RESOLVE communicates directly to OMNISEC REVEAL.


Q. What is the limit of quantitation (LOQ) and percent uncertainties in this type of analysis?

A. We have measured the limit of quantification to be about 100 ng of a 105 KDa polystyrene sample which we think demonstrates excellent sensitivity.  In those tests, the percent RSD was around 5%, which I think is reasonable at the limit of quantification.


Q. Do you have equipment for GPC analyses at high temperature? Can this system be used to measure polyethylene?

Yes we can, but not these systems.  We have a dedicated HT-GPC system for the measurements of polyolefins at high temperatures up to 160°C.


What’s the preferred form of sample? Solid, liquid?

For polymers, most samples come as solids.  This allows you to accurately weigh the sample out and measure its concentration for measurement and get the most accurate data.  A liquid sample is ok but since most polymers are solid and dissolved in some liquid, it means there is less certainty about the sample’s concentration.


How can you get a Zimm plot with only two light scattering measurement angles?

The simple answer to this is that we don’t.  The RALS/LALS detector in OMNISEC is not a typical MALS detector.  With a MALS detector, you are right, two angles would not be enough.  The RALS/LALS detector works slightly differently.  We use the RALS detector to make a single measurement of scattered light and calculate molecular weight.  This works for small isotropic scatterers but clearly not for larger molecules; its great sensitivity makes it ideal for this.  The LALS detector is used to measure scattered light (and therefore molecular weight) with no extrapolation or data fitting.

For more information on the principles behind the RALS/LALS and MALS detectors, I’d like to point out our whitepaper on the topic.  You can download it by clicking here


What are negative peaks on some chromatograms? And why they not on all of them?

Negative peaks appear on two detectors, the RI and the viscometer.  Negative peaks on the RI detector represent solvent peaks.  This is the solvent the sample was dissolved in eluting at the end of the measurement.  In the viscometer, a negative peak represents part of the sample eluting from the delay column which is part of the way the detector works.

All of my chromatograms would have them.  If some of the images showed no negative peaks, it was because I had rescaled the images to magnify the peaks.


The RI, RALS, LALS, and UV data show peak values that have pretty big variation (example is PS star vs. linear PS). How does one handle those variations when deciding what the elution value is?

This is exactly right.  For many samples the peaks from different detectors will be at slightly different retention volumes.  The reason for this is that they all respond to different properties.  For example, since the light scattering responds to molecular weight multiplied by concentration, it is skewed to higher molecular weights.

We always use the concentration detector such as the RI or UV to determine the real peak retention volume as this is the most representative and comparable between samples.


Do you need to know dn/dc separately or can it be calculated during the run?

Either.  You can use a dn/dc you know and then use the RI detector to measure the sample concentration and then calculate molecular weight.  This will give you the best repeatability between multiple injections and it will also allow you to calculate your sample’s recovery.

The alternative is to inject a known concentration and then use the RI peak area to calculate a dn/dc.  This is best for samples with unknown dn/dc but it does assume 100% recovery from the column so depends on good chromatography.


How well does the system respond to frequent changes of the solvent system? E.g. from THF to water and back.

OMNISEC is really robust and designed to work with all sorts of aqueous and organic solvents.  It is therefore possible to switch back and forth between them.  Parts like our new viscometer pressure transducers are robust to significant pressure changes and the system includes overpressure protection to stop them being damaged during solvent changeover.

Nevertheless, this procedure must always be carefully performed.  When switching between solvents, you should always monitor the back pressure and the viscometer pressures to make sure they are not too high.  You also have to be careful to avoid any salt precipitation in the lines as this can block the capillaries.  No instrument can completely protect against such considerations so always treat this procedure with significant care.


Are there situations under which it would be better to have a multi-angle light scattering detector instead of LALS/RALS?

This is an interesting question.  The two technologies, MALS and RALS/LALS, use the same principles to make measurements of the same properties but they do so using different technologies.  When comparing molecular weights from the two, you’d find that they are extremely comparable and they are difficult to differentiate in this respect.

On the other hand Rg comes from the extrapolation to 0 degrees that is performed by the MALS.  With many angles to make this extrapolation, MALS will give you a better Rg result but remember that it is highly dependent on the model used so always be careful to quote this when quoting measured Rg values.


Can OMNISEC system be used with aqueous SEC to characterize water soluble polyelectrolytes?

Yes, absolutely water-soluble polymers are not a problem for OMNISEC.  The only consideration here might be the columns and buffers.  In order to make good measurements on any system, you need a good separation so column selection and buffer composition are important considerations when trying to measure something.


What is the chromatography condition – solvent and column used for running PLGA?

For the PLGA measurements we just used a typical set up of styrene-divinyl benzene columns (our T-columns) using THF as a solvent.


What GPC/SEC columns can be used with OMNISEC?

Any standard analytical columns can be used with OMNISEC.  We offer our own, or you could use your preferred columns from another supplier.


How do you compare the OMNISEC and SEC-MALS?

The two instruments are by their nature, slightly different.  OMNISEC is a complete integrated multi-detection system whereas the SEC-MALS 20 is a modular MALS detector primarily designed to be added on to existing SEC systems to provide just measurements of molecular weight (and Rg).  So their intended purposes are slightly different but their measurement of molecular weight is of course, very similar.

In terms of performance, OMNISEC have superior sensitivity to the SEC-MALS 20.  Its integration will also reduce band broadening (dispersion) resulting in slightly better chromatography.  On the other hand, the SEC-MALS 20 is quicker to setup and integrate with an existing system and being MALS, (as I mentioned above) it will give a superior Rg result.


Is it possible for sample mw to change based on sample prep? That is, if sample is a small cube of polystyrene and is dissolved in entirety vs. if sample is cut with razor to make solubility faster?

I think this is difficult to say for sure.  If in your example, the range of molecular weights within your cube is evenly distributed then a small cutoff should be representative.  My advice here would be that repeatability breeds confidence.  If you are not sure how representative a sample is, make a couple of repeat measurements using fresh sample preps.  If these are consistent, then you can trust the results obtained.


Further Resources

Application Note: GPC/SEC analysis of polylactic acid (PLA) and poly(lactic-co-glycolic acid) (PLGA) using OMNISEC

Application Note: Measurements of absolute molecular weight of synthetic polymers using OMNISEC

Application Note: Structure-molecular weight relationships of synthetic polymers

An introduction to OMNISEC RESOLVE

An introduction to OMNISEC REVEAL