Ultrasonic Bath Sample Prep in Pharmaceutical Research

Ultrasonic bath, sonicator and ultrasonic cleaner are terms describing equipment employed for sample preparation steps in pharmaceutical research projects aimed at drug design and development. Because the term “ultrasonic cleaner” is associated with parts cleaning we’ll use ultrasonic bath in this post.

Sample preparation is defined by the International Union of Pure and Applied Chemistry (IUPAC) as the process used to transform analytes into a measurable form. 

For example, in Sample Preparation of Pharmaceutical Dosage Forms, “sonicators, also referred to as ultrasonic extraction, can be considered both particle size reduction techniques and an agitation technique.  Because of its ability to disperse, mix and dissolve samples, it is widely used in the sample preparation of dosage forms.”

In this post we describe

• The purpose of sonicators
• How the process works
• Ultrasonic bath equipment
• A typical sample prep cycle

Ultrasonic Baths as Sonicators

As noted, sonication is one of the processes used in sample preparation to extract an active pharmaceutical ingredient (API) from its carrier, or excipient, prior to conducting content uniformity and potency assay tests. 

Other methods of achieving desired results may be employed.  These include shaking, stirring, vortexing, grinding, milling, blending and homogenizing.

Certain of these options, because of how they operate, can heat samples, which can degrade the API.

An ultrasonic bath employed as a sonicator can achieve rapid sample preparation while avoiding chemical degradation of the API due to excessive heat. 

How Ultrasonic Baths Perform Pharmaceutical Sample Prep

Ultrasonic is defined as sound above the range of human hearing (nominally 20,000 cycles per second or 20 kHz).   In sample prep applications it is used to create high-frequency energy in liquids. 

One of the methods to accomplish this is employing an ultrasonic cleaner (employed as a sonicator bath), its tank containing water and a surfactant. 

When activated, the unit’s generators excite transducers bonded to the bottom of the tank that vibrate at ultrasonic frequencies.  The tank bottom serves as a membrane; its vibration creates millions of microscopic vacuum bubbles in the sonicator bath. 

When bubbles contact anything immersed in the bath they implode with violent force. This is termed ultrasonic cavitation.  In ultrasonic cleaning applications these implosions loosen and carry away contaminants. 

In ultrasonic sample prep applications cavitation energy passes through the walls of glass beakers and flasks positioned in the tank.  These contain solvents and samples to be analyzed.  Now serving as a “bath” the solvent acts on the samples, quickly performing dispersing, mixing and dissolving.

Ultrasonic Bath Equipment

The Elmasonic S150  available from Tovatech is ideally suited for USP methods that specify an ultrasonic bath for sample preparation. 

It consistently gives reproducible sample prep results across a wide range of applications including solubilizing, dissolving, digesting, dispersing, emulsifying, homogenizing and mixing. 

The efficiency of the S150 as a sonicator for sample preparation is due to a combination of

• A 37 kHz ultrasonic frequency
• 300 watts average ultrasonic power
• a relatively large surface area (20 inches x 12 inches)
• but a depth of only 4 inches.  

The result is a significantly higher power per unit volume compared to conventional ultrasonic cleaner models.  This means the sonication process is completed before heat buildup, which as noted earlier is a natural result of ultrasonic energy that can cause API degradation.

A caution:  

Simply reducing the liquid volume in a deeper model ultrasonic cleaner will not achieve the same result.    Operating equipment with liquid substantially below the fill line can cause damage to the ultrasonic generator and tank.

Instead, the Elma S150 is specifically tuned for optimum performance with a liquid depth of under 4 inches. 

A Typical Sample Prep Cycle

• Prepare samples in accordance with established operating procedures and place them in flasks along with the recommended solvent. 
• Volumetric flasks are placed in the stainless steel basket of the S150 and Erlenmeyer flasks are fitted into clamps affixed to the mesh bottom of the basket.  The basket in turn is placed into the sonication bath solution consisting of water and a sonication amplifier –  such as a few drops of dishwashing liquid.  The flasks need to be only partially immersed.
• Set the timer for the sonication cycle. 
• The set time and remaining time are exhibited on an LED display. 
• Turn the unit on.  Ultrasonic transducers powered by the ultrasonic generator will start the cavitation action producing the millions of minute bubbles in the sonication bath solution.
• The ultrasonic waves will penetrate the walls of the flasks causing cavitation to occur in the flask solution as well.  These cavitation bubbles implode when contacting the samples, causing them to disperse and dissolve. 
• At the end of the timed cycle the unit will shut off and sample analysis proceeds.    Flasks should be thoroughly cleaned and inspected before reuse. 

A Suggested Sample Pre-Prep Step

Fresh sonication solutions should be degassed before sample preparation steps begin.  This process removes entrapped air that reduces the efficiency of cavitation.

The Elmasonic S150 has an auto-degas function that accomplishes the process in approximately 10 minutes.  Note that ultrasonic units without a degas function will degas during normal operation, but this process can take 10-15 minutes or more depending on the amount of solution involved. 

Another Option: Sonicator Horns as Mini Ultrasonic Baths

Sonicator horns, also called ultrasonic cell disruptors, operate on the same principal as using the Elma S150 for larger batch processing. 

In this case, the bath is replaced by an assembly consisting of an acrylic sleeve that is fitted over a cup horn available in 2 or 3 inch diameters.  

The horn is attached to a converter that delivers 20 kHz frequency ultrasonic vibration.  Coolant such as cold water can be continually circulated through the acrylic sleeve via a lower inlet and upper outlet to avoid heat damage to the samples.

Check our post on small scale nanoparticle dispersion for more information on sonicator horns as mini ultrasonic baths.

In Summary

Sample preparation is an exacting science to achieve accurate quantitation when conducting assays for drug products. Sonication employing an ultrasonic bath or sonicator horn is a common means of extracting APIs from solid matrices.  Contact the scientists at Tovatech for help in specifying and using the correct ultrasonic bath equipment for your assays.