GPC detector – OMNISEC – Your Questions Answered

OMINISEC Malvern Instruments

Thanks so much to those of you who joined us for the launch of  OMNISEC, our new GPC/SEC system.  If you missed the event you can play it back by clicking here, to see how the new instrument can really enhance the characterization of polymers and proteins.  

Amy and I really enjoyed the launch and were thrilled by the interest it generated.  Many of you sent through questions during the event, and though I answered as many as possible inevitably there were too many to cover live.  As promised, we’ve gone through the questions and provided answers below.  I hope they continue to add to your first impression of the system. Below the Q&A you will find a list of relevant resources which show more about the capabilites of OMNISEC and how they could benefit your research. If you have any more questions or would like a demo of the system, please visit our website, or get in touch with your local representative. 

If you are working with polymers, you may also want to sign up for this C&EN webinar that I am presenting: Addressing polymer characterization challenges & maximising product potential using the latest in SEC technology.

When will the recording link be made available to those registered?

The recording of the launch event is already available on the web site to view. Please tell your colleagues who couldn’t attend to play it back. Click here to access the recording.

Which organic solvents can the system handle? Can Chloroform be used as a 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.

What columns can be used on the system?

OMNISEC has standard HPLC fittings throughout so any standard analytical GPC/SEC column can be used with it. This includes our own column ranges and you can find more information about them by clicking here.

How is the cooling system for samples in this instrument? Is it improved from the previous version?

The autosampler cooling uses air recirculation around the samples to hold them to a particular temperature. The temperature range for the autosampler is 4-60°C (I think I said 65°C on the day, so I apologise for the mistake). Condensation is handled much better in OMNISEC RESOLVE with a well-defined run-off pathway to a waste bottle beneath the instrument.

How many angles does the light scattering detector have?

The light scattering detector is a RALS/LALS detector, which measures at two angles, 90° on the RALS (right-angle light scattering) and 7° on the LALS (low-angle light scattering).  If the system was a MALS detector, then it would be important to have more angles to best fit the MALS extrapolation.  However, the RALS/LALS detector uses a slightly different principle.  The RALS detector is used to measure the molecular weight of small isotropic scatterers with maximum sensitivity while the LALS detector is used to accurately measure the molecular weight of larger anisotropic scatterers.  Using the RALS/LALS principle, OMNISEC’s light scattering detector is as accurate as a MALS detectors, such as the Viscotek SEC-MALS 20.  For more information on the differences between the different types of light scattering detectors, please click here to download our white paper on the topic.

What’s the difference between Viscotek TDAmax vs. OMNISEC?

While the systems are similar in terms of their major components, it is in their performance that OMNISEC overshadows our older system, the TDAmax.  Both contain degasser, pump, autosampler, column oven, and the four detectors (RI, UV, light scattering and intrinsic viscosity).  However, OMNISEC’s performance is superior in every way.  The degasser is lower volume making solvent changes quicker and easier.  The pump is new including a seal backwashing design to protect them from salt-related wear.  The autosampler can cool samples and make injections as small as 1 μL with no sample waste and can control sample temperature.  The RI detector and light scattering detectors are 8 and 10 times more sensitive respectively.  Finally, the viscometer has been redesigned with a self-balancing mechanism and user exchangeable capillaries.  As you can see, improvements have been made in all areas to make OMNISEC a truly superior system.

Is injection volume the same as the existing system and what about manual injection for protein samples?

The injection volumes in OMNISEC RESOLVE range from 1 – 300 μL.  This is a wider range than the GPCmax.  Remember also, this is available with no injection overhead (up to 100 μL) and sample temperature control.

It is possible to make a manual injection if desired.  OMNISEC REVEAL can accept a trigger signal from a manual loop which you could connect in the flow path.  However, most people would do that to inject low volumes and for minimum waste.  When OMNISEC RESOLVE can inject so little and with no waste, I don’t know why you’d want to do a manual injection.

Can we transfer data from OmniSEC 5.0 to new software and evaluate?

If you wanted to compare results from OmniSEC 5.0 and OMNISEC 10, then yes, you could transfer data into the newer software version.  However, I should make clear that OMNISEC 10 cannot be used with the TDAmax or other products from the Viscotek range.  It is designed to work with the new OMNISEC system.

Do you still need AD converter if you use OMNISEC software with an alternate HPLC system?

All the A-to-D capabilities are in OMNISEC REVEAL, so that is effectively operating as your board.  This new version of OMNISEC software is not available as a stand-alone product, I think the situation you are describing wouldn’t really apply.  However, if you want to use OMNISEC REVEAL with a 3rd-party GPC/SEC system, then that is absolutely possible.  You just connect a trigger signal from your autosampler to OMNISEC REVEAL and start collecting data.

Is the light scattering wavelength the same as in the TDA? What did you do to make the light scattering more sensitive?

The light scattering wavelength is similar to the TDA but not quite the same.  It is 640 nm as opposed to 670 nm in the TDA.  It is matched to its RI detector though for the highest accuracy in the molecular weight measurement.
The sensitivity comes from a number of areas.  A higher laser power and better focussing in the cell means higher collection efficiency.  Quieter electronics and better signal processing all mean reduced baseline noise.  The net result is a signal-to-noise increase in OMNISEC REVEAL of about 10 times over the TDA.

Can I measure the molecular weight of polymers (such as MW around 5K to 20K), is that similar to GPC?

Yes, absolutely, OMNISEC is using GPC as the separation process so all of the measurements are done after the separation.  The light scattering sensitivity improvements means making measurements of samples around 5 – 20 kDa should be no problem at all.

The column and detectors are in separate modules in the new system, compared to the old GPCmax-TDA. How does the small temperature variation may play a role in the signals?

We’ve slightly changed the format in OMNISEC from the TDAmax and first I want to explain why.  The real benefit is that it allows a better separation of the chromatography and the detection.  This makes OMNISEC REVEAL far more ‘plug-and-play’, meaning it can be placed next to any GPC/SEC system.  It also means you don’t have to open the entire detector module just to change the columns.

You are right though, there’s a small distance now between the column oven and detectors.  Truthfully though I don’t think it will make much difference, if any, to the result.  The time the sample will spend in that region is very small and the total tubing length is also short so band broadening won’t be a problem.  I really don’t think you would see any difference at all.

What is the minimum dn/dC where you would expect to see reasonable signals from the RI and LS detectors assuming you have a Mw ~ 10k or less and any concentration you want?

This is a difficult question.  The relative light scattering sensitivity to a particular sample is based on the sample’s molecular weight, concentration, and dn/dc as you correctly say.  I don’t think I could give you an exact figure for the lowest dn/dc but some of the measurements we showed were of PLGA which has a dn/dc around 0.05 and in our measurements had molecular weights around 10 – 15 kDa.  We had good signal at 1 mg/mL and 100 μL injection volumes.

I hope that gives you an idea of what we could do.

Does the software work with other GPC equipment?

OMNISEC REVEAL is available as a detector on its own, so we can integrate this with any 3rd party SEC system to make these measurements. Integration of the trigger signal would be no problem in order to synchronise the measurements.  However, you’d have to type in the sequences in both pieces of software to match them up.  They wouldn’t be able to communicate with each other.

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Relevant Resources:

Tech Note: An introduction to OMNISEC RESOLVE

Tech Note: An introduction to OMNISEC REVEAL

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

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

App Note: Structure-molecular weight relationships of synthetic polymers