Automated flash chromatography systems have helped synthetic chemists speed up their synthetic research. One major advancement with these systems over the past 15 or so years has been the addition of photo-diode array ultraviolet (PDA-UV) UV detectors with which chemists can detect and fractionate using one, two, or multiple wavelengths. Enabling detection and fractionation with multiple wavelengths increases the likelihood that target and by-product compounds will be isolated with increased purity.
Flash chromatography is a purification technique used by chemists to isolate their targeted compound from by-products and impurities. Because the reaction mixture (or natural product extract) may have its best solubility in a solvent that is chromatographically “stronger” than the mobile phase, liquid sample loading can be problematic causing early eluting and/or broad peaks with poor purity. In those cases, a technique called dry loading is frequently used.
Flash chromatography is a purification technique used by chemists to isolate their targeted compound from by-products and impurities. Because the reaction mixture (or natural product extract) may have its best solubility in a solvent that is chromatographically “stronger” than the mobile phase, liquid sample loading can be problematic causing early-eluting, broad peaks which can reduce purification efficacy and product purity. In those cases, a technique called dry loading is a better alternative.
With all forms of chromatography there are limitations relating to sample load – both mass and volume. These are independent variables which, for the best results, should be investigated separately. In this post, I will address the impact of increasing solvent volume on flash chromatographic separations.
A good question I get asked frequently to which there is no specific value. When asked this question I answer, “it depends on your sample and how good your separation is”; not a satisfying response, but it is the truth.
Chemists using silica columns for normal-phase flash chromatography typically equilibrate their columns prior to loading their samples. Companies manufacturing automated flash purification systems often have the equilibration volume and flow rate pre-programmed and tied to a column size. Some of these flash system companies allow equilibration volume to be edited while others have the volume fixed. Is one of these options better than the others? In this post I discuss how equilibration volume impacts flash chromatography results.
Have you ever been faced with the challenge of evaporating solvent from a reaction mixture where the target compound and perhaps some by-products are temperature sensitive and degrade or decompose with elevated heat? How do you efficiently accomplish this evaporation challenge?
For chemists isolating their synthesized product in maximum yield and purity is a primary goal. Sometimes the crude reaction mixture stays in solution, sometimes it does not. In these cases, is it better to just redissolve in a strong solvent, say DMSO, or to filter, wash, and then purify? After all, the precipitating material may be unreacted starting material and could potentially complicate the subsequent flash purification step. On the other hand, it may be product crystallizing on its own and worthy of your attempt to isolate it without further work-up other than filtration.
Purifying your synthetic product efficiently in high yield with minimal impurities is every chemist’s goal. At discovery-scale, flash chromatography is the go-to purification technique as it is relatively simple and effective.
For many synthetic chemists the primary purification goal is to isolate as much synthetic product as possible with a minimum of 80% purity. The go-to technique for product isolation is flash purification (flash chromatography), especially for intermediates.
An interesting question for synthesis chemists as the need for reversed-phase flash chromatography for both intermediates and final compounds increases. Historically, normal-phase flash chromatography has been the “go-to” purification tool for synthetic intermediates. Today, however, intermediates and final products are increasingly polar. These polar reaction mixtures cause purification challenges in both separation and recovery with normal-phase (silica) thus requiring use of reversed-phase flash chromatography.
Over the course of my career, I have had terrific interactions with a multitude of chemists discussing chromatography. When it comes to flash chromatography the approaches from these chemists ranged from running generic 0-100% ethyl acetate in hexane gradients to modeled gradients based on tribal knowledge for a type of synthetic molecule to always using TLC for method development to prep HPLC. Each of these techniques are used because they provide some level of success. To me, though, if I spend time and resources synthesizing a highly valuable and unique molecule, then I want to purify the reaction mixture with the best possible method.
The term “Green Chemistry” has become a major part of the science community’s lexicon. When I think about green chemistry and its relationship to flash column chromatography I think of two specific areas where it applies...
Applying green chemistry principals to flash purification is becoming increasingly important. In this post, I discuss ways to make flash column chromatography greener by reducing solvent use through optimization of gradient shape.
This is a follow-on to my earlier post where I presented some greener alternatives to DCM as a solvent in flash column chromatography.
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.
Many chemists I talk to understand that TLC data is useful for flash chromatography method development. Most also know that they should try to get their target compound to elute with an Rf between 0.15 and 0.4 by adjusting TLC solvent strength. Have you ever wondered why this is important and how Rf values impact flash chromatography results?
In this post I will explain the relationship between TLC Rf and flash elution volumes (CV) and why having your target compound elute in the Rf range is needed.