Does Size Really Matter in Flash Chromatography? Part 2

In a previous post I talked about column size, specifically long-thin versus short-fat and the impact of the cartridge’s dimensions on purification performance. With that comparison I showed that in preparative chromatography, purification efficiency is more about the amount of silica than column dimensions. Cartridges of different dimensions containing the same amount of the same media will provide the same separation efficiency.

But there are other chromatographic aspects where size can impact performance. In this post I will focus on some areas where size does matter – media particle size and media surface area.

Continue reading Does Size Really Matter in Flash Chromatography? Part 2

Does size really matter in flash chromatography? Part 1

Yes, the title is a bit salacious but it got your attention, didn’t it? I believe this is a topic worthy of discussion as it relates to flash chromatography for purification because many chemists believe longer but thinner columns perform better than short, wide columns.  The facts of the matter may surprise you.

In this post I discuss the impact that cartridge dimensions have on purification performed using flash purification.

Continue reading Does size really matter in flash chromatography? Part 1

How to choose between linear gradients and step gradients for flash chromatography

Varying the concentrations of mobile phase solvents during  flash purification chromatography enhances the ability of the technique to effectively isolate the desired compound from reaction byproducts and unconsumed reagents.  Choosing how these concentrations will be varied over time has a significant effect on the purity and recovery of desired compounds.

What is the best starting strong solvent %?   What is the ending strong solvent %?  Should the mobile-phase concentrations vary gradually in a linear manner or should they vary step-wise or something else altogether?  Most separations are performed once, occasionally a handful of times.  Because of this, spending effort optimizing a gradient is just not very productive unless there are aids in choosing the gradient profile that provides an effective purification with minimum effort.

Software in flash chromatography instruments, makes it simple to create a gradient.  Now, what should that gradient look like?

In this post I compare isocratic, step, and linear gradients and provide some sage advice on choosing among them.

Continue reading How to choose between linear gradients and step gradients for flash chromatography

How do I create an efficient gradient flash chromatography method?

For most organic and medicinal chemists flash chromatography is just another step in the synthesis work flow – react, analyze, purify, react, analyze, purify… until the final product is made. The desired product of each reaction, and the mixture of other species present are, of course, different with each cycle.  Separating the desired compound efficiently without a lot of hassle is something I have written about in this post as well as in others in this series.

In this post, I’ve written about how that TLC (thin layer chromatography) plate you use for monitoring your reaction can be used to create reliable, efficient, effective gradients.

Continue reading How do I create an efficient gradient flash chromatography method?

Is there an easy way to purify organic amines?

If you synthesize organic amine compounds, especially heterocyclic, secondary, or tertiary amines, you likely have encountered problems with their chromatography using silica columns. With the amine groups being basic and silica being acidic, there is a natural attraction between the two. This sometimes strong attraction often requires the use of a competing amine in the solvent system. Modification of the mobile phase with the addition of a solvent like triethyl amine can provide a successful purification. Often times the use of an amine-modified stationary phase can provide the needed conditions to avoid the acid-base interaction that can interfere with a successful flash chromatography purification.

In this post I will discuss how amine-functionalized silica can simplify organic amine purification and your life (at least in the lab). Continue reading Is there an easy way to purify organic amines?

How do I Choose the Right Column Size for Purification by Flash Chromatography?

In all my years of working with medicinal and organic chemists, I have found that choosing how many grams of silica to use for purification by flash chromatography is something frequently guessed at. Getting the size of the column right is awfully important because using too few grams of silica will doom your purification to failure and using more an optimal mass of the stationary phase means the purification  consumes excess silica, solvents, and a chemist’s time. To determine the optimal amount of silica for a purification, I rely on a factor called ΔCV (delta Column Volume) to identify the best loading capacity on any cartridge. I have also found that ΔCV this is a better loading capacity predictor for flash purification than ΔRf.

Continue reading How do I Choose the Right Column Size for Purification by Flash Chromatography?

How to Optimize TLC to Enhance Purification by Flash Chromatography

In this article I discuss the optimization of solvent ratios to generate ideal Rf (retention factor) values on TLC plates.  Then I show how maximizing efficiency of flash chromatography achieves higher loading with rapid and reliable isolation of compounds, reduced solvent use and improved separation. Continue reading How to Optimize TLC to Enhance Purification by Flash Chromatography

Using TLC to Scout Flash Chromatography Solvents

TLC is the tool most used for normal-phase flash chromatography method development. For many chemists, a solvent system of hexane (or heptane) + ethyl acetate is the first, and sometimes only, solvent system evaluated. Though often useful, ethyl acetate may not always provide the optimal purification conditions. Continue reading Using TLC to Scout Flash Chromatography Solvents

Why Reusing Flash Cartridges is Bad Science

For many chemists lab budgets, especially for consumable items, are limited.  One way of trying to stretch their lab budget is to reuse disposable flash chromatography cartridges.

In this post I will show how regardless of the cartridge brand used, repeated use of silica flash cartridges results in loss of compound resolution and fraction purity.

Continue reading Why Reusing Flash Cartridges is Bad Science