Recently, I posted an article explaining why high performance TLC plates are not needed for method development for high-performance flash chromatography. Based on some excellent feedback, I see a need to discuss silica chemistry and its impact on chromatography.
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In a nutshell, no two silica brands are the same. Sure, many have the same physical properties, such as surface area and porosity, but the surface chemistry between silica brands is almost always different. While the surface of silica particles is comprised primarily of hydroxylated silicon (Si-OH), not all of that chemistry is the same. According to Ewe Neue in his text &#;HPLC Columns &#; Theory, Technology, and Practice (Neue, )&#;, there are several different silanol types including single silanols (Si-OH), silane diols or geminal silanols (HO-Si-OH), and bridged silanols or silyl ethers (Si-O-Si), Figure 1.
Figure 1. Structures of the three silanols types types. Top - geminal. Middle - single. Bottom - silyl ether.
Single and geminal silanols are quite polar and can be acidic. They are good at attracting polar compounds, organic bases, and other nucleophiles. Silyl ethers, on the other hand, are relatively hydrophobic and not very reactive.
So, how do the differences noted above impact what you do with flash chromatography? Well, consider the interactions taking place between your compound and the silica chemistry. Let's say you have a compound in your sample that is well retained on a silica TLC plate that happens to contain a high silyl ether percentage and you transfer the method to a flash column packed with a high single or geminal silanol content. With that scenario, it is very likely that your compound will elute later than you would expect from the flash cartridge. In fact, you may need a very different, more polar solvent system to perform the purification.
The ratio of these three silanols types impacts selectivity as well. Using unmatched TLC and flash silica has even caused compounds to change their elution pattern and/or selectivity.
I did not believe this early in my career but just one sample convinced me it is true. The sample was a mixture of organic dyes and the eluent was 100% toluene for both the TLC and the flash purification. Though I do not have a picture of the resulting flash chromatography, I do have the TLC data and the actual flash elution volumes to share, Table 1.
Biotage
Competitor
Flash Elution
TLC
TLC
volume
Rf
CV
ΔCV
Rf
CV
ΔCV
CV
0.84
1.19
0.82
1.22
1.25
0.34
2.94
1.75
0.33
3.03
1.81
3
0.18
5.56
2.61
0.25
4
0.97
5.5
0.1
10
4.44
0.09
11.11
7.11
9.25
What this data shows is that the more polar the compound (the lower the Rf), the more likely that a difference in retention and selectivity will be encountered when silica chemistry differs. And remember, low Rf values tend to provide better separations so small Rf differences can cause big column volume (CV) and ΔCV shifts.
In the example above, the ΔCV for the last two compounds varies significantly between the Biotage plate and another supplier's plate. Purifying the sample in a flash column packed with Biotage silica (Biotage® KP-Sil) provided elution column volumes with better correlation to the Biotage TLC plate than with the competition's plate.
Yes, I know you probably have a common TLC plate supplier and perhaps you have not experienced this problem so why change brands? Well that is certainly your choice, but for me, I would rather not chance a problem occurring because it is just a matter of time before you have issues which may lead to confusion during purification when transferring your TLC-based method to flash.
Have you ever experienced an issue like this? Share your experiences.
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In the molecule of the silane coupling agent, there are both organic groups that are affinity for organic materials and hydrolyzable groups that are affinity for inorganic materials. Among them, the organic groups have a great influence on the performance of rubber products. Only when the organic group can react with the corresponding organic material can the performance of the rubber material be improved. When the organic group in the silane coupling agent is a non-reactive alkyl or aryl group, it has no effect on polar organic materials, but can be used in non-polar materials (such as silicone rubber, polystyrene, etc.).
Sulfur-containing silane coupling agents are frequently used in the rubber industry, such as TESPT, bis-[(triethoxysilyl)-propyl] disulfide (TESPD or Si75), and γ-mercaptopropyl trimethoxy Base silane (A-189), etc., and the silane coupling agent TESPT is mostly used in tires.
Generally, the principle of choosing silane coupling agent is:
Vinyl silane is mostly used for polyolefin rubber;
Sulfur-containing silane coupling agents (such as Si69 and Si75, etc.) are mostly used for sulfur vulcanizates;
Epoxy resins generally use silanes whose end groups are epoxy or amino groups;
Unsaturated polyesters mostly use vinyl and epoxy silanes.
When choosing a silane coupling agent as an auxiliary agent for rubber materials, in addition to considering the reactivity of the organic groups in the silane coupling agent, the compatibility of the silane coupling agent with organic materials and the storage of rubber materials should also be considered. The impact of stability. Sometimes, it is better to use a composite silane coupling agent or a reaction product of a silane coupling agent and a variety of compounds.
For more silica silaneinformation, please contact us. We will provide professional answers.
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