Even without the rise of online pharmacies, there have been multiple food and medicine adulteration cases, some due to carelessness, some due to greed. One unfortunate part of the story is that most cases of adulteration are pretty clumsy, and lives could have been saved if we had simple and widely available tests for contaminants.

That is precisely what a team of engineers has recently tried to achieve. They have taken some pretty old ideas and rejigged them to create a rather innovative testing system that can detect adulteration in liquid medicines and maybe even food.

Sounding off

The challenge with making a generic test for contamination is that all sorts of things can end up in food and medicine. The key to this new idea is that you don’t necessarily need to know what has been added, only that it is different from the standard formulation. In almost all cases, changing the formulation changes the density of a liquid. A sensitive mass sensor, then, should be able to detect medicines that have not been produced properly.

The key to this sensitivity lies in acoustics: the tone of an instrument depends on how the sound wave is resonated by the instrument. The pitch of a guitar string, for instance, depends on its tension, mass, and length. This property has been used to great effect beyond music. A quartz crystal, for instance, will change its resonant frequency if its mass changes. This provides for a very sensitive balance to measure tiny changes in mass. But quartz mass balances, as they are called, are quite expensive.

What these researchers were looking for is a cheap alternative. In the developing world especially, hospitals and individuals cannot shell out thousands of dollars for highly sensitive mass balances. They do, however, have access to musical instruments.

The principle remains the same, though: change the mass of the vibrating material, and the tone of the note it produces will change. To this end, the researchers decided to use a modified version of an ancient African instrument called the mbira. The mbira consists of a set of steel tines suspended over a sound box. The length, width, and density of the tines determines their notes.

The researchers removed the tines and replaced them with a single hollow metallic tube, tuned to a G sharp two octaves above middle C. When the tube is filled with water, the tone drops to F sharp in the same octave. The change is clearly distinguishable by ear.

Tuning in to smaller changes

Smaller changes in density, such as adding small amounts of salt to the water, cannot be distinguished by ear. To help in these cases, the researchers created a script to analyze the sound online. A user records the sound on their smartphone and uploads the sound file to the researchers' website. The script finds the instrument sounds and tries to locate a point where only the fundamental tone—the fundamental tone is the lowest note that the tine can produce—is present. It uses that window of sound to calculate the frequency of the tine for the user.

The researchers showed that a single-note, mbira-like instrument can detect salt concentrations with a resolution of about 12 mg/ml, which is enough to distinguish sea water from fresh water but not much in between. However, that test is almost a worst case, because the mass difference between sea water and fresh water is only 2.5 percent.

The researchers also showed that a common adulteration—switching glycerol for diethylene glycol during the production of cold medicine—was easily detectable.

This only works, however, if medicines are reliably consistent in their production, since they provide the baseline. To test this, the researchers obtained cold medicine produced in different batches and measured their frequency. They found that the medicines were all the same to within 0.025 percent, while the change between glycerol and diethylene glycerol is over one percent.

Now, most people don’t want to destroy a musical instrument to make a drug detector. However, you don’t need to in order to do this. The same result can be obtained from an instrument that consists of a solid plank as a sounding board with something that holds the tube firmly in place above the plank.

I love cheap sensors

This may, in fact, be a record in terms of cheap sensors: scrap timber, a couple of screws, and a bit of metal tubing, plus a cellphone. And as the author points out, smartphones are becoming more common worldwide.

I also think the sensitivity can probably be improved and is probably a necessity. At the moment, the mbira detector can detect really clumsy adulteration, but it would struggle with smaller density differences. Maybe that can be improved, either by using a more sophisticated analysis or through changes on the instrumental side.

ACS Omega, 2018, DOI: 10.1021/acsomega.8b01673 (About DOIs)