What do I do if a 2-solvent gradient will not separate my sample?

Usually, a 2-solvent or binary gradient will separate your desired compound from the by-products and impurities. Sometimes though, you can encounter a mixture in which some compounds co-elute and are not separable with any binary gradient you try.

I encountered this situation recently while trying to purify a lavender essential oil and have dedicated this post to how I solved the problem.  Continue reading What do I do if a 2-solvent gradient will not separate my sample?

Does a longer flash column really provide better purification?

This is an interesting question that I am asked from time to time. There does seem to be two camps in which chemists reside – one believing longer and thinner columns provide better separations and the other preferring shorter and fatter columns to do the same chromatography.

Which is right? That is a question I will try to answer based on my own data. Continue reading Does a longer flash column really provide better purification?

Very polar compound purification using aqueous normal-phase flash column chromatography

Purifying polar organic compounds can be very challenging. In a previous post I have discussed using reversed-phase flash chromatography to retain and purify ionizable and ionic compounds.  My colleague, Dr. Elizabeth Denton, has also posted a blog on purifying very polar peptides as well.  Sometimes, however, despite all your efforts with reversed-phase, success is elusive. When this happens, what do you do?

In this post I will discuss using normal-phase flash chromatography with aqueous solvents, a form of HILIC (hydrophilic interaction liquid chromatography), to purify those compounds that just will not stick well enough on reversed-phase media.

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How do I decide between normal- or reversed-phase flash column chromatography?

Choosing a good purification strategy is an important for successful crude compound purification. A major factor in your strategy is choosing between normal-phase or reversed-phase chromatography.  How do you choose?

In this post, I will provide some simple guidance on helping determine which route to take.

Continue reading How do I decide between normal- or reversed-phase flash column chromatography?

When should I use a “high-performance” flash chromatography column?

The evolution of flash column chromatography has brought chemists many new and exciting options for crude mixture purification. Among them are so-called high-performance flash columns or cartridges.  These high-performance purification tools are typically filled with silica or other media 15 to 30 microns in particle diameter (versus 40-63 micron for standard flash media) and provide the expectation of better separations and higher purity fractions.  That’s really enticing but how often do you need these types of columns especially since they are, of course, more expensive?  Well, that question is what I address below. Continue reading When should I use a “high-performance” flash chromatography column?

How does solvent choice impact flash column chromatography performance?

Selectivity and solvent strength are the most important factors that determine success or failure of a chromatographic separation. These two independent dynamics apply to both normal- and reversed-phase chromatography and should be optimized, especially when high fraction purity is needed.

In this post I will discuss the impact that elution solvent choice has on both normal- and reversed-phase purification. Continue reading How does solvent choice impact flash column chromatography performance?

How do I determine loading capacity in reverse phase flash column chromatography?

As the popularity of prep-scale, reversed-phase flash chromatography increases, so does the frequency that I get asked this question, “How do I determine loading capacity in reversed-phase flash chromatography?”

In the world of HPLC, loading capacity isn’t normally a concern as it is primarily an analytical technique.  In the synthetic organic chemistry world, most purification is performed with silica gel where flash column purification methods are developed and loading capacity estimated from TLC data.  However, when normal-phase flash does not work and reversed-phase flash is needed, the question of how to determine reversed-phase loading capacity comes up.

In this post I will attempt to provide some guidelines to help you understand and determine reversed-phase flash chromatography loading capacity. Continue reading How do I determine loading capacity in reverse phase flash column chromatography?

Flash column equilibration – is it a waste of time or necessary step?

Flash column chromatography has been practiced by chemists since the 1970s. That practice required the silica in the column be properly wetted to remove trapped gasses to ensure uniform flow (remember those days of not letting air into the column?). Today, with automated flash chromatography systems and pre-packed cartridges as the norm, chemists ask me – do I really need to pre-equilibrate my cartridge?

In this post I explore the impact on chromatography that equilibration, and lack thereof, has on separation performance.

Continue reading Flash column equilibration – is it a waste of time or necessary step?

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.

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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.

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