Ultisil® series, as Welch’s flagship product, has been widely used by numerous customers. You may have noticed a subseries under it, called “LP.” If you’ve paid attention to the LP series, your initial impression is likely “resistant to strong acids.” But is LP only about acid resistance? Product information generally provides only the most basic characteristics, and as we delve deeper, we discover many hidden “treasures.” Let’s explore these “treasures” together and discuss how we can utilize them to our advantage.
Let’s first take a closer look at a few key terms in the LP special design:
- Introduction of alkyl side chains with larger functional groups
- Highly hydroxylated silica gel surface
Next, we will delve into the discussion with these key terms in mind.
The first key term, “Introduction of alkyl side chains with larger functional groups”:
Since the silica gel itself is not acid-resistant, the main reason why the chromatographic columns with a silica gel matrix are not resistant to strong acids is that their bonded phase (including the main chain and endcapping) can undergo hydrolysis under strong acidic conditions. In the case of the main chain, LP series products introduce alkyl side chains with larger functional groups, and the steric hindrance of these larger functional groups inhibits the degradation of the bonded phase in high-temperature, strong acidic environments. Therefore, this series of products exhibits better stability under high-temperature, strong acidic conditions. Additionally, the “larger functional groups” themselves have stronger hydrophobic properties and a unique spatial structure, leading to significantly stronger interactions with the weakly polar parts in compounds compared to regular bonded phases. This results in a special selectivity for a wide range of compound structures. So, despite having generally lower bonded phase density, the LP series retains weakly polar compounds quite effectively.
The second key term, “Non-endcapped”:
As mentioned earlier, the LP main chain has a special design. When facing smaller, more easily detached endcapping groups, the LP series takes the most straightforward approach by not endcapping.
At this point, many people may have numerous questions about “non-endcapped,” and some may even have doubts about the performance of the LP series directly because of this feature. As a user, I understand everyone’s concerns, as the statement that “the ionizable silanol groups carry negative charges and can adsorb to alkaline compounds carrying positive charges, leading to tailing” is like a mantra ingrained in the DNA of every chromatographer. But does this statement really explain the reasons for tailing?
Clearly, for the situation of “ionizable silanol groups carrying a negative charge adsorbing to alkaline compounds carrying a positive charge, resulting in tailing,” two conditions must be met simultaneously: silanol group ionization and ionization of alkaline compounds.
This requires that our pH value cannot be too low or too high, but clearly, this situation accounts for only a small part of our daily work. However, even outside of these conditions, our chromatographic peaks often still exhibit tailing. The reasons for chromatographic peak tailing, or extending it to the reasons for peak asymmetry, are so complex that we won’t delve into them here. We can summarize it briefly with the statement: “Compounds on the stationary phase experience more than one force, and at least one of these forces is in an overloaded state.
Therefore, if we view the non-endcapping due to “lack of selectivity” as an addition of “a special type of force,” such as significantly strengthened hydrogen bonding under acidic conditions, weak ion-exchange interactions under near-neutral conditions, or stronger hydrophilicity compared to regular endcapped reversed-phase columns…
Indeed, doesn’t this make the development of our analytical methods more interesting? Since we can’t avoid the laws of nature, let’s use them to our advantage and turn “disadvantages” into “advantages.” This is not an overstatement; the LP series’ “special selectivity” has long been widely recognized in the market.
Exploring these unique properties and selectivities can lead to more versatile and effective analytical methods, providing us with valuable tools in our scientific endeavors.
The third key term, “Highly hydroxylated silica gel surface”
as you’ve pointed out, hasn’t been discussed yet. It seems like you’ve intentionally saved it for last. Please feel free to provide more information or questions related to this key term, and I’ll be happy to discuss it further.
While this surface may not appear to have a direct impact, it’s, in my opinion, the most thoughtful design in the LP series. Due to the “non-endcapped” feature, the impact of differences between silica gel batches on the LP series is far greater than for other series. Therefore, Yuexu has introduced a “highly hydroxylated” process in the production of LP series products. This process treats the surface of silica gel spheres, eliminating some of the micropores that can cause a decrease in column efficiency. It creates a very smooth surface with fewer and uniformly distributed acidic silanol groups on the silica gel. This minimizes the impact of batch-to-batch variations as much as possible.
From the very beginning of its design, Welch deeply recognized the “complexity” of LP forces and made every effort to keep these forces as stable as possible, laying a solid foundation for users to develop stable and reproducible methods by reasonably utilizing these forces.