Guide to solvent evaporation

insights from industryDr Induka AbeysenaApplication SpecialistGenevac

An interview with Dr Induka Abeysena, Application Specialist, Genevac, conducted by April Cashin-Garbutt, MA (Cantab)

What range of applications is solvent removal used for and how important is evaporation for sample preparation?

Solvent removal covers a broad range of Chemical, Biological and Analytical applications where it is necessary to either remove an existing solvent from a solution completely, or to concentrate the solvent to facilitate analysis of a molecule of interest. This could be a single sample or many samples in parallel.

The application focus of the many labs worldwide using evaporators and concentrators is diverse. While preparative purification is the most common application, use in medicinal chemistry, natural products research including food flavours and fragrances, oligonucleotide production and forensic research and crystallisation are also widespread.

In some applications, there is a need to evaporate strong acids like hydrochloric acid (HCl) and higher boiling point solvents such as dimethyl sulfoxide (DMSO). Increasingly evaporation is being acknowledged as one of the most crucial steps during sample preparation. You could be loading an expensive set of samples and/or purified samples into an evaporator and it is important that the solvents are evaporated safely to retain sample integrity and prevent sample loss.

What are the most common evaporation problems and why do many facilities use systems that are slow and cumbersome?

After spending weeks and sometimes months trying to synthesize a target molecule followed by purification before analysis, the worst thing is to find out is that the sample has become contaminated!  

Many scientists are familiar with solvent bumping (uncontrolled, vigorous boiling that causes some of the sample to eject from its container, think of heating soup in a microwave!)  during solvent removal.   An uncontrolled evaporation process can cause sample contamination, sample loss and therefore waste a lot of time and money.

Some evaporators do not have the correct technology to detect how much energy is put in during evaporation. This could over heat the samples and cause damage.

In most environmental analysis applications scientists deal with very volatile analytes and if the heat input and vacuum cannot be precisely controlled during evaporation it is common to lose these components leading to inaccurate quantitative results.

There are still many solvent removal technologies in the market where the waste solvent is released to the environment which creates an unsafe workplace for the scientists and also leads to environmental damage.

While many scientists are used to conventional techniques like rotary evaporators and blow down systems, these popular lab tools suffer from the above mentioned evaporation issues and involve a lot of time consuming manual operation. By comparison Genevac centrifugal evaporators have advanced technologies to overcome these problems and include many productivity enhancing features.

Can you please give an overview of the different types of evaporation system available and explain how the technologies have advanced over recent years?

The most common types of evaporation systems found in the laboratory are rotary evaporators, freeze dryers, centrifugal evaporators, blow down systems and vortex evaporators.

Freeze dryers freeze the samples and then at low pressure sublime the solvent directly from a solid phase to a gas phase to give a ‘fluffly’ sample. This is a slow process and ideal for sensitive samples such as proteins, and can achieve high level of dryness.

There are basic manifold bench top freeze dryers and shelf freeze dryers, where samples can be frozen within the unit in a controlled manner. It is common to use an oil pump in a freeze dryer to achieve low vacuum and also a refrigerated cold trap to trap the solvents.  

The technology is limited to samples that can easily be frozen using vacuum alone, therefore the solvents used are mainly water, with some organics that freeze and sublime easily, such as tertiary butanol or 1,4-dioxane.

Centrifugal evaporators utilize a vacuum pump and a cold trap like a freeze dryer but the low vacuum employed enables removal of a wide range of liquid solvents at a low temperature. Since evaporation occurs from a liquid to a gas phase it is much faster than freeze drying. Also due to the centrifugal force, samples evaporate from the surface thereby avoid bumping.

Heat energy is used to accelerate the rate of evaporation, but note this, not to increase the temperature of the sample.  Advanced systems such as those developed by Genevac can easily process high concentrations of aggressive acids such as HCl or more complex mixtures.

A blow down evaporator uses an inert gas such as Nitrogen or Argon is blown down onto the samples via a needle to create a flow over the surface of the liquid. This changes the equilibrium between the vapour phase and the liquid phase. The source of energy is usually hot air.  The speed of evaporation using blowdown systems is good for volatile solvents but poor for high boiling point solvents. This method is ideal for sample concentration and some units can automatically detect the end point, some are simply gas tube and a pipette tip rigged up on a retort.  They are open access, but typically manual.

Vortex evaporators rely on swirling the sample tubes to create a vortex.  This action increases the surface area to aid the evaporation rate but the samples will dry all over the tube. This Swirling action  could help prevent bumping but due to insufficient  g-force,  it is not  guaranteed. Rotary evaporator is a vortex evaporator but for a single sample.

Most modern evaporators, like the Genevac HT Series 3 high throughput evaporator, come with a touch screen user interphase, similar to an iPhone. Built-in pre-set methods enable users to simply press a start button to remove solvent from their samples. However, it is also really easy to programme a new method for more complex solvent removal applications.

Innovative design features in the HT Series 3 help users to maximise productive solvent removal time, freeze dry HPLC fractions and make polymorphs using their centrifugal evaporator. As with SP freeze dryers, advanced technologies like, control nucleation, TDLAS (Tunable Diode Laser Absorption Spectroscopy) and SMART software, helps scientists to optimize freeze drying cycles and save valuable time.  The latter are mainly used for method development for pharmaceutical formulations requiring freeze drying.

Are technologies available for specific applications such as for heat sensitive samples?

This depends on the kind of samples you may have. For labs preparing thermally sensitive biological samples - I would most often recommend using a freeze dryer.  If your sample is more robust, it is possible to dry these sensitive samples using the ‘no heat’ option on a Genevac centrifugal evaporator such that just ambient heat is used for the evaporation.

How important is it to choose the correct vacuum pump and cold trap?

Modern centrifugal evaporators incorporate a high performance vacuum pump and a cold trap, but also manage them with their controls platform to deliver optimum performance.

Users ultimately want the simplicity of ‘walk away’ automation when they operate their evaporator system, in the confidence that the equipment is taking care of things. Achieving this requires a significant consistent investment in R&D on the part of the equipment supplier, and will affect the price of a system. You may be able to save money by configuring your own system, however, it will probably not perform as well as a fully integrated evaporation system.

What other tips do you have for choosing the best evaporator for a specific application?

This is a great question! And this would be my process,

  • Are the samples heat sensitive? – If yes use a Freeze Dryer, if not select an evaporator.
  • What is the throughput - Is it one or many samples? If one sample, select a single sample evaporator if not select a parallel evaporator.

Then before selecting which brand of evaporator to purchase you should consider the following:

  • What kind of solvents do you want to evaporate, volatiles, aqueous or high boiling point solvents?
  • Do you use chlorinated acids like HCl? Remember not all systems are HCl resistant, by comparison Genevac evaporators are built for this purpose.
  • Will you need to handle potentially explosive solvents like diethyl ether?  Or are your samples moisture sensitive? Only Genevac offer an Inert Gas Purge option with their evaporators.
  • Do you often have to remove mixed solvents from your samples? If YES, then only choose an evaporator that comes with validated anti-bumping technology.
  • Can the evaporator accommodate the sample formats (flasks, vials, tubes, microplates etc) you are commonly tasked with removing solvent from?
  • Would you like to also freeze dry samples using your evaporator?
  • How easy is it to use and maintain the evaporator?

Can you please give an overview of the different evaporators available from Genevac?

The Genevac range of evaporators cover all applications from a simple benchtop unit to a high throughput automated system.

Mivac is a basic modular concentrator ideal for biological applications. It is compatible with removing aqueous and many organic solvents from a variety of sample formats including tubes, vials and microplates.

Mivac can also be configured to freeze dry a few samples. Two types of Mivac concentrators are available for low  and higher throughput samples  with a standard condenser and 3 options for pumps for different applications. The miVac pressure controller helps to further optimise the methods.

EZ-2 is a compact personal evaporator with 4 models, Envi, Standard, Plus and Elite. These models with its unique features are suitable for a wide range of applications. It is renowned for its user friendliness and has options for HCl removal and inert gas purge applications.

The EZ-2 also accommodates the standard sample formats as well as round bottom flasks and the SampleGenie system that allows the user to concentrate or dry a larger volume sample directly into a vial. Infrared lamps provide heat energy for evaporation and it is much faster than miVac concentrators.

Genevac patented Dri-Pure® sample protection technology prevents cross contamination and comes as a standard. Other features like, pre programmed methods, the ability to do timed runs and automatic runs and low maintenance make this model popular in a multi user lab.

The HT Series 3 high throughput evaporator was launched just 2 years ago. Since then we have sold over 60 units. This product line has all the great features in a EZ and more. Two models are available, the HT-6 and the HT-12 for maximum throughput.

HT Series 3 evaporators are the benchmark systems for high throughput applications like library synthesis and purification. HT series 3 evaporators enable solvent removal from the widest variety of sample formats. The systems touch screen user interface sets a new standard for ease of use and built is condenser save the valuable space in a busy lab. Thermally sensitive samples can benefit from precise temperature control using sample guard technology.

For scientists faced with removing large volumes of  solvent such as those in natural product labs, we offer Rocket Synergy evaporators. The Rocket 4D Synergy evaporator is fitted with an autofeed system to evaporate large volumes, over than 5L, with no user intervention. It can also accommodate a 6-position rotor to evaporate smaller volumes.

The Rocket Synergy evaporator comes as standard with a flask rotor though it also has the option to use a 5L bowl to evaporate batch samples. Both systems come with pre-programmed methods and automatic drain as standard. Steam is used as the source of heat energy to give excellent  evaporation rates.

All Genevac evaporators have oil free dry pumps and applications support is free of charge  during the life time of the evaporator.

Does SP Scientific also offer other evaporation related technologies such as freeze drying and rotary evaporators?

Yes, SP Scientific is a leading manufacturer of  Virtis, FTS and Hull  brand freeze dryers. The product range  covers from bench top to production scale and utilises in research and manufacturing applications. SP also offer affordable rotary evaporator systems. For further information please follow this link https://www.spscientific.com/ProductCategory/98/  

What do you think the future holds for solvent evaporation? What innovations are in the pipeline?

Efficient solvent evaporation will always be a critical part of sample preparation regardless the number of samples or the volume. I would think in the future there may be novel purification and synthetic techniques to minimize the amount of solvents used during the sample preparation but this may be many decades away.

Also in some countries, there are regulations on how much vapor can be release into the environment and scientists are obliged to use solvent removal systems which are much greener. I think this will be compulsory in many countries in the near future.

With a product launch around the corner,  SP Scientific aims to produce quality, reliable products with short delivery times

Watch this space….

Where can readers find more information?

The following links offer you access to an extensive bibliography of white papers, application notes and an ever-expanding range of informative webinars for freeze dryers and evaporators

About Dr Induka Abeysena

As Genevac Applications Specialist I am responsible for supporting customers and distributors with their application queries.

After completing my PhD in Synthetic Organic Chemistry from University of East Anglia, Norwich, UK, I joined Genevac in 2005. After various positions within the company, I have been the Applications Specialist for over 5 years.

Genevac is a global leader in centrifugal evaporation systems and is part of SP Scientific, a leading manufacturer of specialty equipment for pharmaceutical, biotechnology and industry.

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