5 essential tips to filter your OMNISEC samples successfully
In a recent post I described my typical process for preparing samples prior to analyzing them with OMNISEC. In the first section of that blog, when discussing the choice of preparation solvent and sample solubility, I recommended that you filter your samples. In this post, I’ll dive deeper into the reasons why, and the things to remember when you filter your OMNISEC samples.
Why you should filter your samples
Since GPC/SEC is a solution-based technique, your sample must be completely dissolved in order for it to pass through the column set and detectors. A completely dissolved sample should be transparent (not necessarily colorless), like the vial on the left in the image below. The vial on the right, containing the cloudy suspension, possesses an unknown concentration of sample that is insoluble. These insoluble particulates can potentially get stuck in a column, diminishing its separating ability, or worse, clog a piece of tubing or detector. And if they do make it through to the detectors, they can make your light scattering and viscometer signals very noisy. Bottom line: you should filter your samples to protect your system!
On occasion, you might find yourself in a situation where you only have a small amount of sample solution and can’t afford to risk losing any during the filtration process. In that case, you can either choose to inject your sample without filtration or use an alternative method (such as centrifugation or spin filters) to remove any potential insoluble material. This scenario occurs more frequently when working with precious protein and biological samples. And don’t forget, your sample will still pass through the post-column filter prior to reaching the detectors. While this can work for a one-off sample, it’s not the best long-term strategy to only rely on the post-column filter.
How you should filter your samples
So you’re convinced that you should filter your samples…but how? When preparing an OMNISEC sample you generally are not working with large volumes. The most convenient way I’ve found to filter samples is to use a syringe filter when I’m transferring the sample solution to an autosampler vial.
I like to use a 5 mL disposable polypropylene syringe to draw up about 2 mL of sample solution, then affix a syringe filter with 0.2 µm pores, and push the solution through the membrane directly into the autosampler vial. I prefer syringes with the Luer Lock fitting in case I’m working with a partially insoluble or viscous sample and need to apply pressure to the syringe plunger. If you’re working with organic solvents, I do not recommend using syringes with a rubber-tipped plunger. The rubber can partially dissolve and contaminate your sample solution.
For the syringe filters, I suggest using ones that are 25-30 mm in diameter (still with 0.2 µm pores). Smaller filters will work, but if your sample contains insoluble material there is a chance the filter does not have the capacity to handle your sample.
Different types of filter membranes
You will want to choose your specific filter type based on your application. The recommendations below are not the only types of filters, but are ones that I have found to work well with the sample types indicated. All of these filter membrane types are available with 0.2 µm pores. And the Nylon filters (part number FIL0072) are available from Malvern Panalytical!
- Nylon
- Use these for all water-soluble polymers (PEO, polysaccharides, etc.)
- Can be used for all organic solvents except HFIP and formic acid
- Second choice when running proteins
- PTFE
- Works well with common organic solvents (THF, DCM, DMF, etc.)
- Use these when running HFIP or formic acid
- Cellulose acetate / regenerated cellulose
- First choice when running proteins
What to do if your sample is not completely soluble
If your sample is completely soluble, then there should be little resistance during filtration. If the filter blocks quickly and you have to really squeeze to push the sample through then there is a good chance particulates are present in the sample.
However, even if your sample is not completely soluble, the filtrate that you collect in the autosampler vial is safe to inject. This can let you know if your sample exhibits partial solubility and allow you to characterize that portion of your sample.
If you do this, it is important to remember that you have changed the concentration of your sample and that the actual concentration of sample in solution is unknown. Therefore, in situations like this, it is critical to use an established dn/dc value for your analysis.
Demonstration of sample filtration
My process for preparing a sample is demonstrated in the video below. This link directs you to the timestamp at which sample filtration is discussed (4:24).
Final thoughts
In conclusion, I hope these details about sample filtration help you obtain better data with your OMNISEC. If you have any questions, please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.
Related content