A scavenger is a chemical substance that is added to a mixture in order to remove or deactivate impurities and marring by-products. ln many reactions, an excess of starting materials is used to drive a solution phase reaction to completion. The final mixture is composed of the desired product and one or more unwanted side products. When the reaction is completed, the unwanted side product must be removed. A common technique to remove unwanted substances from a reaction mixture is scavenging. Scavengers are often added in excessive amounts to accelerate the removal of impurities. Mixing the scavengers with power-ultrasound into the reaction mixture provides a fine-size dispersion so that the scavenger is broken into very small particles or droplets. The smaller the particle or droplet size, the higher the reactive surface area. This leads to a more economical use of scavenging chemicals and faster removal of impurities or unwanted by-products.

Scavengers are frequently used in

• chemistry: In solution phase combinatorial chemistry, scavengers are frequently used to remove un-consumed excess reagents and/or spent reagents.
• pharmaceutical development and production: active compounds of high purity are required in order that the compounds can excert their effects fully.
• oil & gas industry: e.g. during the desulphurization. A common example is the removal of hydrogen sulfide (H₂S) to avoid any unfavorable reactions (especially for corrosion control).
• biology and bio-tech: e.g. act nanoparticles as radical scavenger to prevent tissue against damage.

The Power of Ultrasonic Mixing

Powerful ultrasound waves introduced into a liquid medium result in cavitation. Cavitational shear forces are very effective to disperse and emulsify. The fine size distribution, which is achieved by powerful ultrasonic mixing, can be precisely controlled so that micron- and nano-sized particles or droplets are easily created. The diminute size of the scavenger particles and their even distribution lead to a high particles surface, which means a highly active surface area to bond the impurities in order to remove them from the reaction mixture.

The Benefits of Ultrasonic Scavenging

By the efficient ultrasonic dispersion, the scavenger can be used sparingly. The ultrasound forces disperse and distribute the particles or droplets at very fine size, so that the full potential of the scavengers is used. Thereby, an excessive use of the scavenger reagents is avoided. This saves scavenger and facilitates the removal of the scavenged by-products.
Ultrasonic assisted scavenging allows high throughput parallel synthesis and purification. This makes your process more efficient and improves the product quality. Additional benefits result from the fact that scavenging is an important alternative to time-consuming liquid chromatography and liquid-liquid extraction.
Ultrasonic mixing contributes to all kind of scavenger types, such as solid scavengers (e.g. powders, gels, scavenger polymers, scavenger resins), and liquid scavengers.

Batch Sonication or Inline Sonication

Ultrasonic processes can be run as batch process as well as in a continuous flow-through mode. Especially for smaller volumes, e.g. for sonication of samples and smaller production lots, the ultrasonification process can be easily done in an beaker or batch. However, for higher volume streams is the inline sonication recommended to ensure even and consistent process results. In accordance to the volume stream, Hielscher offers a broad range of ultrasonic flow cell reactors.

Process Steps of Ultrasonic Assisted Scavenging

• 1. Prepare the reaction slurry and let react the substances completely. (Note that ultrasound promotes chemical reactions! It may be beneficial to sonicate the reaction mixture also during the first step.) After the reaction, the solution contains the final product and the unwanted by-products. Add the scavenger of your choice
• 2. Add the scavenger and prepare a crude pre-mixed slurry or emulsion. After the slurry/ emulsion is formed, sonicate the mixture.
• 3. When the scavenging reaction is completed, the by-products are bonded and can either be removed or can remain in the reaction mixture as they are due to their bonding with the scavenger. Purification can be performed by collecting the pure product by filtration, centrifugation or by removing the supernatant with an eyedropper/ pipette.