As synthetic chemistry has evolved, so has flash chromatography. Target molecule synthesis is becoming more complicated and the synthetic products more polar. This shift in compound polarity has changed purification strategy from almost entirely normal-phase flash chromatography using silica to a significant percentage of flash chromatography now being reversed-phase during the past 9 or so years.

Scaling up reversed-phase flash chromatography methods is often necessary as reaction scale increases. This is especially true when other non-chromatographic purification techniques do not work or meet purity and/or yield needs.

The bane of organic synthesis for most chemists is purification rather than synthesis. Synthetic reaction mixtures are rarely devoid of impurities so some type of purification is necessary.  Most often flash chromatography is used but for many chemists, it is less well understood than their chemical reaction and provides some level of anxiety.

In this post, I will summarize the five most important steps to creating a successful flash chromatography method and thus the anxiety associated with it.

For medicinal chemists, maximizing the synthetic yield of their newly created intermediate compound is their priority. More times than not, flash chromatography is used to purify these intermediate compounds to at least 80% purity. Final compounds, however, not only require high yield but maximum attainable purity, typically in excess of 95%. For this purity level, chemists will either send the reaction mixture to an in-house prep HPLC lab or perform their own preparative HPLC compound purification, if it is available in the lab.

In my previous post, I talked about the "Chemistry Behind Normal-phase Flash Chromatography", the most common form of liquid-solid chromatography. In this post, I focus on reversed-phase flash chromatography and how it differs from normal-phase.

The challenges organic, medicinal, and natural product chemists face are many: from designing reactions, to optimizing synthesis, work-up / extraction, and purification / isolation of the desired compound or compounds. Among those issues related to purification / isolation is the common problem of separating compounds with similar chemistry that either co-elute or separate poorly.

In this post I will discuss some tips on how to "resolve" this issue (yes, pun intended).

With reversed-phase flash column chromatography becoming increasingly popular for routine purification, understanding how to make the cartridges last (since they cost more) is important to know. 

In this post I will mention a few tips to prolong reversed-phase cartridge life.

I am often asked why reversed-phase TLC data does not translate well to reversed-phase flash column chromatography.  There are several reasons for this and in this post I will attempt to explain the challenges associated with reverse-phase TLC as a method development tool for reversed-phase flash chromatography.

In my role as senior technical specialist at Biotage I am often asked about compound detection options. For most flash chromatography methods, UV is the default detection tool since many compounds do absorb some UV light.

Diode array UV detectors provide chemists choices in wavelength selection providing the ability to widen or narrow the wavelength range needed to detect specific compounds and enhance their sensitivity.

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.

When it comes to the purification of polar, water-soluble compounds reversed-phase chromatography is the most commonly used approach. However, because of strong stationary phase – mobile phase repulsion forces, the use of highly aqueous (90-100% water) solvent systems has been shown to provide less retention than needed.  This issue has led to the development of “aqueous compatible” reversed-phase media.

Organic and medicinal chemists frequently utilize flash chromatography to purify their reaction mixtures. Normal-phase flash chromatography is most often used but may not the best methodology, especially when the compounds are quite polar and/or ionizable.

For these molecules, reversed-phase flash chromatography is preferred but often is not used due to an uncertainty regarding the best solvent choices and the reversed-phase mechanism.  In this post, I will discuss how organic solvent choice in reversed-phase chromatography can influence the chromatographic separation.

Compounds precipitating during flash chromatography is at best an inconvenience when working up your crude reaction mixture.  Precipitation during purification typically happens in the column or in the tubing exiting the column.

In this post, I will propose a strategy that can minimize and perhaps prevent this issue from occurring.

The newly released Biotage® Selekt flash chromatography instrument can be run at a maximum flow-rate of 300 mL/min or a maximum pressure of 30 bar. These high flowrates and pressures enable a user to perform chromatography using not only dry-packed, single-use plastic flash columns containing small (≥20 μm) spherical silica particles, but also semi-preparative, slurry-packed
HPLC columns for multiple use with smaller (≤20 μm) spherical silica particles.

One of the more challenging purifications is that of water-soluble, ionizable compounds. Typically, normal-phase with silica is not used because of the probable non-reversible interactions, especially between the ionized amines interacting and the ionizable silanols.  With normal-phase out of the purification solution that leaves ion exchange and reversed-phase as chromatographic options.

In this post I will discuss the use of reversed-phase and the influence pH and buffers have on the chromatography of some ionic, water soluble compounds.

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.