This question is one that is increasing in frequency. Over the past 10 or so years reversed-phase flash chromatography use has increased dramatically. Likewise, reversed-phase preparative HPLC (RP pHPLC) use has also increased. Chemists need to know when to choose between the speed and low solvent use of flash column chromatography and the ultimate purification of RP pHPLC. With this as the backdrop, let me give you my thoughts on hot to choose between flash chromatography and when it is best to use RP pHPLC.
I have recently posted on how solvent choice influences the separation of hard to resolve compounds using normal-phase flash chromatography. As a chemist with an inquiring mind, I thought I would expand my research beyond normal-phase and see what happens in reversed-phase.
In this post, I share my results.
How to choose between normal- and reversed-phase flash column chromatography is an excellent question and one that my readers often ask. Those who use column chromatography know that as long as the reaction products or compounds are fairly non-polar and near neutral pH they will have successful purifications. However, when your mixture's chemical characteristics are more challenging (polar, non-polar, basic, acidic) there are other options that are available to successfully separate pure compounds.
In this post, I will discuss the criteria you can use to guide your choice between normal- or reversed-phase flash chromatography.
Acetone, as you know, is a terrific solvent. It dissolves many organic molecules, evaporates easily, is both water and organic soluble, and is cheap (relatively). These attributes tell me it should be a good polar modifier for normal-phase flash chromatography.
Many microwave assisted organic synthesis (MAOS) reactions use polar solvents such as alcohols, DMF, DMSO, because they absorb and transfer microwave energy very efficiently. However, the downside of using polar, microwave absorbing solvents is that they can interfere with normal-phase flash chromatography.
In this post, I discuss why dry loading can be advantageous when purifying polar-solvated reaction mixtures.
Sometimes it feels as if organic chemistry and chromatography are a mixture of art and science. Maybe its because of the necessary creativity needed to address the variety of challenges that we face almost daily. Frankly, its what I find most interesting about this world.
One of the bigger challenges facing chemists is the ability to detect and collect compounds with little or no UV absorption during flash purification. In this post I will talk about a technique that I have found to be quite useful when trying to purify mixtures containing one or more poor absorbers.
Gradients, used in chromatographic methods, assist with chemical separation and elution. They begin with “weak” elution conditions and end with “strong” elution conditions.
In previous posts I have touched upon various sample loading options and how they impact flash chromatographic performance, primarily in normal-phase flash purification. As the use of reversed-phase flash chromatography has steadily increased over the past few years I thought it would be a good idea to discuss one of the most important factors impacting its success.
In this post I discuss the results of some of my original research studying the impact of injection solvent choice on reversed-phase flash separations.
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.
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.
Tetrahydrocannabinol, aka THC, is a hallucinogen found in cannabis and, to a lesser degree, in hemp. Though THC is legal in some locations in the US and Canada, there is a growing market for its non-hallucinogenic cousin, cannabidiol (CBD), which has purported medical benefits.
The problem with isolating CBD from cannabis and hemp is contamination from THC, which is typically present at a moderate to high percentage. In this post, I will provide some insight into rapidly purifying CBD to remove THC.
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.
Your choice of sample loading technique can, and likely will, affect the separation and purity of your targeted compound. While liquid loading is easy and often fit for purpose, it can provide some issues especially if large sample volumes are required relative to column size (> 1% of a column volume) or the dissolution solvent is too strong for the chosen purification method (e.g. injecting a methanol-solvated sample into a hexane/ethyl acetate mobile phase).
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.