Let OMNISEC determine if you’ve made a polymer! (According to REACH)
How do you know if you’ve synthesized a polymer? I know, the answer is obviously to analyze it using OMNISEC! But even once you’ve obtained the results, if you’re working with low molecular weight materials, how do you really know your product is polymer? Maybe it’s simply a discrete species with a low concentration of similar impurities of differing molecular weight?
REACH criteria for polymers
According to the European Chemicals Agency (ECHA) and the Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH), a polymer is defined by two parameters:
- Over 50 percent of the weight consists of molecules containing at least three monomer units which are covalently bound to at least one other monomer unit or other reactant; and,
- The amount of molecules presenting the same molecular weight must be less than 50 percent of the weight.
Also mentioned in the thorough document Guidance for the implementation of REACH is that GPC/SEC is the preferred analytical technique to determine if a substance is truly a polymer (section 4.1). Fortunately, OMNISEC has just the features you need to help you through this process!
Over the next few sections I’ll describe how to use OMNISEC to ensure your polymer meets both criteria, as well as some other potential requirements.
REACH criteria point 1
To meet the first requirement of a polymer you’ll need a way to determine the fraction of a sample that is above a particular molecular weight. Fortunately, OMNISEC has a % above / % below feature to do just that!
You’ll need to know the molecular weight of your sample’s monomer. To determine if at least 50 percent of the sample contains at least three monomers, then you’ll set the % above function to a mass greater than a dimer. It should be noted here that the mass of the initiator should also be included.
For instance, if you are working with polystyrene, in which a monomer has a molecular mass of 104 Da, then two monomers would have a molecular weight of 208 Da. If the polymer reaction was initiated with benzoyl peroxide then an additional 121 Da should be added. Therefore, you can set the % above field to 330 Da (> 208 + 121 Da). This will calculate the fraction of sample greater than 330 Da, or rather the fraction of sample that consists of at least three monomers.
How to use % above / % below in the OMNISEC software
You might be wondering how to access these calculations and add them to your customizable report. As described in the link, you first start with the Report Editor by clicking the New report tab (1). Second, you add a results table (2), then click the pointing hand icon (3) to access the item options. Select a Results option, either columns or rows (4).
Next, click on the three dot menu icon at the top right of the Results item window (5). A list of options will appear; click on the capital sigma to configure the % above / % below calculations for your report (6).
Once the % Above and Below Molecular Weight (g/mol) window appears, click the + icon to add a line (7). You will need to specify if you want to calculate the fraction of sample above or below a given molecular weight, or between two molecular weights (8). Once you make a selection, you will need to specify the appropriate molecular weight value (9) and then click OK (10).
After you click OK, you will be taken back to the Report Editor view and may be wondering if you set it up correctly, since nothing will seem to have changed. That’s ok! Simply click the three dot menu icon again (11) and then this time select the gear icon to choose the parameters to be displayed in your report (12).
A new window will appear containing the options available to include in your report. If you expand the % above and below molecular weight section in the left column (13), you will see the recently created calculation. Select it (14), then click the arrow pointing to the right to move the parameter to the Selected portion of the window (15) and then click OK (16).
This time, you will see the parameter added to your report. And once you save it, and open it when analyzing a sample, you will see the calculated result. In this instance I used the value 9,000 Da for demonstration purposes because the polymer I was working with did not contain any fractions with fewer than three monomers.
REACH criteria point 2
To satisfy the second requirement of polymer classification according to REACH, you can use the % above / % below function to determine the fraction of the sample at the peak molecular weight. Since the peak molecular weight indicates the fraction of the sample that is present in the greatest concentration, if that sample fraction doesn’t exceed 50 percent, then the others will not either.
Knowledge of the sample’s monomer molecular weight is also helpful here. In the example below, in which I’m working with the same polystyrene as above, I’ll take the Mp and calculate the percent of the sample represented in the sample band that is 104 Da wide and centered around the Mp.
Using the % between calculation to test REACH criteria point 2
The process for setting up a % between calculation is the same as above, with the only difference being step 8. At this point, you’ll select “between” from the drop down menu and then enter the lower and upper molecular weight limits between which you wish to calculate, as shown below.
At this point, the remaining steps are the same as above. Once you’ve added the calculation to your report, the results will look similar to what I’ve calculated in the image below. As you can see, the data slice centered around the Mp value that is as wide as a single monomer represents 9.46 percent of the sample. This is less than 50 percent, and thus should satisfy the second REACH criteria for polymers.
It is important to note that these % above / % below calculations will work regardless of your detector configuration and molecular weight calculation method. In this example, I’ve used molecular weight data calculation with a light scattering detector, but the calculations will work equally well if you are using a column calibration method.
Additionally, if you can also use multiple sets of limits and the Fraction of sample (%) calculation to determine the relative concentration of each species. The downside here would be the difficulty in maintaining consistency over multiple samples.
Using % above / % below for other regulations
K-REACH, which is the Korean REACH, has additional specifications for polymers, in this case for polymers that can be exempted. As you can see from the details below, calculating the percent of a sample below indicated molecular weight values is a critical part of these analyses.
- Mn >= 10,000 & the contents of molecules weighted < 1,000 is less than 5% & the contents of molecules weighted < 500 is less than 2%
- 1,000 =< Mn molecular weight < 10,000 & the contents of molecules weighted < 1,000 is less than 25% & the contents of molecules weighted < 500 is less than 10%
Final thoughts
In conclusion, I hope the details described here will allow you to use OMNISEC to make it easier for you to characterize and confirm that you’ve synthesized a polymer. The % above / % below calculation function in the OMNISEC software is perfect for meeting the requirements of REACH and other regulatory bodies. If you have any questions, please don’t hesitate to contact us or email me directly at kyle.williams@malvernpanalytical.com.
Related content