Flash chromatography is a chemical separation technique used to purify chemical mixtures. Because it is a purification technology, the process is also referred to as flash purification.
Using a “dry” loading technique with flash chromatography typically improves compound purity and overall separation quality compared to liquid loading. The reasons for this I have prophesized previously and include:
How does flow rate impact my flash column chromatography separation? This is the kind of question I frequently get. After all, we all know that flow rates that are too high or too low can result in bad prep HPLC chromatography. Well, this is not necessarily true in flash chromatography.
In chromatography there are three inter-related variables which impact your separation and are represented on the chromatographer’s triangle.
Getting the most benefit from your crude sample purification with column chromatography or flash chromatography involves optimizing many variables. In previous posts I have talked about selecting the best solvents, their ratios, and maximizing load based on TLC Rf data. These are all important chromatography-generated variables but now I would like to share some tips on actual technique differences and their impact on purification performance.
In particular in this post I will focus on the benefits and drawbacks of liquid loading and dry loading. Both have their place in liquid chromatography but when should one technique be used over another?
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.
Method transfer from reversed-phase TLC (thin layer chromatography) to reversed-phase flash column chromatography can be very challenging. Because of this, I often recommend using HPLC for reversed-phase flash chromatography method development. This really is a straight-forward process if you start with the right HPLC column and know a little about your HPLC system's detector.
In this post, I will share some tips on how to develop a reversed-phase flash purification method using HPLC.
In order to perform flash chromatography consistently, the equipment you use must be properly maintained by following some “best practices”. These best practices include using clean solvents (I typically use ACS grade), volatile organic modifiers, and quality columns.
In a previous post I shared results of experiments where I evaluated selected organic solvents for sample dissolution and injection for reversed-phase flash purification. I demonstrated that DMF and DMSO both are excellent solvents for this purpose and actually provide better chromatography than methanol, acetonitrile, and acetone.
In this post I report some surprising results from follow-on work evaluating the impact of increased injection volume using DMF and DMSO as the sample diluent/injection solvent.
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.
When it comes to the purification of polar organic compounds many chemists turn to normal-phase flash chromatography with dichloromethane and methanol as the mobile phase. This solvent system often can be challenging to optimize due to methanol’s high polarity and protic chemistry.
I have found that acetonitrile can often replace methanol as the polar modifier in DCM-based solvent systems. In this post I will show an example where this is true.
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.
Challenging separations, we all have faced this vexing problem. You synthesized your compound, analyzed it, and know your molecule is in there, based on LC-MS or TLC. Then, you do some method development using a silica TLC plate and see a major spot with some minor, early-eluting impurities. You think that the purification will be easy only to find that your “purified” compound has some co-eluting impurities. Now what? Should you change solvents or change stationary phase?
In this post, I will show how changing the column media but keeping the same solvents removed a co-eluting impurity in one of my reaction mixes.
Various flash chromatography sample loading options are available including liquid and dry loading. Choosing the right technique is important because your sample loading choices (sample solvent and dry load sorbent), can have a major impact on the results.
In this post, I compare the two techniques and show the benefits dry loading with a form of diatomaceous earth can bring to your purification.
For chemists, flash chromatography is part of their everyday synthesis workflow. For most syntheses, crude reaction mixtures are purified by normal-phase (aka adsorption) chromatography. There are times, however, where the crude mixture’s complexity and polarity make normal-phase chromatography very challenging. For these situations, reversed-phase (aka partition) chromatography may be a preferred option.
But, if you have only one flash system available, can you, should you, and how do you efficiently switch from non-polar, normal-phase solvents to polar, reversed-phase solvents – and back again without issues? In this post I'll attempt to shed some light on the topic.
Previously, I have posted on a normal-phase flash chromatography method to separate and isolate CBG from a CBD-rich hemp distillate. CBG is just one of many naturally occurring minor cannabinoids of interest in this fast-growing market.
The answer to this question is yes, reversed-phase can sometimes provide a better separation and thus better purification than normal-phase. When is reversed-phase likely to be the better choice is a different, and likely better, question.
In this post I will try to demonstrate when reversed-phase is likely the better purification mode.