Ion Chromatography in High Pressure: Analytical Scale FAQ (I)
This past year, we ran a very popular four-part series of webinars on our recently released High-Pressure Ion Chromatography (HPIC) technique. Attending chemists had so many interesting and pertinent questions that I have been writing posts on the Q&A sessions at the end of each webinar and providing useful links for our readers.
This post covers Part 1 of the Q&A session from the analytical scale applications webinar; Part II to follow soon.
To view the webinar on-demand, click this link: High-Pressure Ion Chromatography—Analytical Scale (4 mm and 2 mm) Applications (you will need to fill out a brief registration form before viewing) and learn more about fast separations and highly resolved peak separation in 4 mm and 2 mm formats for water testing, food and beverages, and petrochemical applications.
The webinar was moderated by Celia Henry Arnaud, Senior Editor, C&EN and the speaker at the webinar was Chris Pohl, Vice President Chromatography Chemistry Chromatography and Mass Spectrometry, Thermo Fisher Scientific, Inc.
The transcript has been edited for clarity and readability.
I already have a Thermo Scientific Dionex ICS-5000 system: can I run the new 4 µm IC columns?
Yes, you can run the 4 µm Ion Chromatography (IC) columns but you won’t be able to run them above 3000 psi if you have an eluent generator because the new eluent generator is only works with the new Thermo Scientific Dionex ICS-5000+ system.
I thought PEEKTM started to deform at 3000 psi. How do you run at the higher pressures? Do you use different tubing materials?
The pressure rating of PEEKTM is dependent on a number of factors but with the PEEKTM tubing materials we are using the tubing can withstand up to about 7000 psi.
How difficult is it to flip a system between anion and cation modes? Or, do you suggest having two dedicated systems?
It is not difficult to switch back and forth; in our lab we have done this in less than two hours. However, you have to be fairly fastidious about your protocol to not have any problems with blanks when you switch back and forth. Our recommendation is to have a dedicated system for each class of ions unless your budget is extremely limited.
Which fast column do you recommend for inorganic anions in food matrices, such as starch, or matrices containing fats and proteins? Can you provide some examples of applications on sugar analysis?
We recommend using our anion-exchange column (Thermo Scientific Dionex IonPac AS11-HC Capillary Column) as the best option although the focus of this particular column is not speed mainly because food matrices invariably contain a lot of organic acids that would otherwise compromise the quantitation of the inorganic ions.
As regards to sugar analysis, we have not focused on sugar analysis as yet because we haven’t started making 4 um sugar analysis columns. We are actively developing a smaller particle size for our existing carbohydrate column (Thermo Scientific Dionex CarboPac SA10) using this technology and hope to release it soon.
What are the benefits of the macroporous structure bead versus the pellicular microporous bead other than an increase in the number of theoretical plates per unit column length? Also, are the anionic macroporous beads more prone to sample matrix contamination, (i.e. metals and metal oxides), since there is penetration into the bead itself?
To clarify, in our labs we see similar efficiencies regardless of whether or not the particle is porous or non-porous. The reason for using the wide-pore macroporous structures is that we get significantly higher capacity when we have the stationary phase coating not just the exterior surface of a non-porous particle but also the interior. We started with the macroporous resins as our first product launch is because they tend to be the most popular with customers. But in the coming months, we will be adding some non-porous materials as well. In general, macroporous structures provide higher capacity and generally better peak shape than non-porous materials do.
Answer to part 2 of the question: No, in general, anionic macroporous beads are not any more prone to sample matrix contamination and usually the contamination tends to be on the exterior surface which is most accessible to the sample and the higher molecular weight is, the more pronounced that effect is. So in fact the macroporous structures are somewhat less prone to contamination than non-porous materials.
Is it possible to use HPIC in big polymeric molecules? For example molecular weights of about 10,000 Da? Is there a size limit for using HPIC?
Yes and no. The macropore structure we are using has a size exclusion limit of over a million. So from a permeation point of view, very large molecules can access the entire surface, but what limits the molecular weight at which you can resolve any analyte from--say if it’s a polymer, an N+1 oligomer of the same polymer--is the exclusion of the stationary phase, especially if it is cross-linked as with some of our anion-exchange columns, such as the Thermo Scientific Dionex IonPac AS18 Hydroxide-Selective Anion-Exchange column or the Thermo Scientific Dionex IonPac AS11-HC Capillary column, is the ultimate determinant of the upper limit of the molecular weight that you can separate.
Therefore, if you do a gradient separation, have an anionic polymer, and can resolve individual oligomer chain lengths up to a certain link, typically the maximum you can resolve is somewhere between a 60-mer and 120-mer depending on the polymer. It tends to be in that range. You can, indeed, resolve 10,000 molecular weight polymers in some cases, but it depends a bit on the charge density of the polymer.
By the way, do check out our online resource on this high-pressure ion chromatography solution featuring product videos, on-demand webinars, and downloadable technical and application notes.
Watch for the next blog post in a couple of days. Also, do you have questions not answered by this FAQ? Do let us know using the comments box below: we look forward to hearing from you!