There are many ways to diagnose health conditions. One of the most common methods is blood testing. This sort of test can look for hundreds of different kinds of molecules in the body to determine if an individual has any diseases or underlying conditions. Not everyone is a fan of needles, however, which makes blood tests a big deal for some people. Another method of diagnosis is breath analysis. In this process, an individual's breath is measured for different molecules as indicators of certain health conditions. Breath analysis has been fast progressing in recent years and is continuing to gain more and more research interest. It is, however, experimentally challenging due to the extremely low concentrations of molecules present in each breath, limited number of detectable molecular species, and the long data-analysis time required. Now, a JILA-based collaboration between the labs of NIST Fellows Jun Ye and David Nesbitt has resulted in a more robust and precise breath-testing apparatus. In combining a special type of laser with a mirrored cavity, the team of researchers was able to precisely measure four molecules in human breath at unprecedented sensitivity levels, with the promise of measuring many more types of molecules. The team published their findings in the Proceedings of the National Academy of Sciences (PNAS).
Mirrors and Lasers
In order to make an effective breath-testing apparatus, the team of researchers needed a way to "code" the different molecules found in breath into usable data. They did this through a "fingerprinting" process. Using a laser known as a frequency comb, the team could shine over 10,000 different colors of infrared light at the breath sample. According to first author Qizhong Liang, the variation in color was important: "Molecules absorb infrared light in a selective manner. They give different absorption strengths to light at different optical frequencies. How the absorption pattern looks is governed by the molecular rotational and vibrational properties." Since each molecule in the breath absorbed light at a different frequency, this "fingerprinted" each molecule, associating it with a unique absorption pattern, making it easier for the researchers to measure and analyze the data. Liang added that "measuring the optical absorption signals over a broad spectral range, one can simultaneously determine what molecular species are present." As many other devices take tens of minutes, or could only test one molecule species at a time, this new apparatus increased the number of analyzed molecules in breath-testing significantly by analyzing breath in real-time–a reduction in analysis time and presumably, cost.
The implementation of the frequency comb was essential for the apparatus to work. The colors within this special type of laser are evenly spaced in frequency, making them easier to fine-tune than other lasers. In order for the frequency comb to work properly, it has to be coupled to the mirrored cavity by matching the cavity's resonance–a specific frequency that corresponds to the longitudinal mode of that cavity. Depending on the size and shape of the cavity, the resonance may vary. Matching the cavity resonance frequency to the laser frequency helped the team to better measure molecules. "By controlling and matching the light frequency to a specific cavity resonance frequency, one can measure ultrasensitive molecular absorption signals over a broad frequency range in a simultaneous manner," Liang explained. "In our experiment, we can measure absorption signals at 15,000 isolated optical frequencies in just three minutes. This allows us to detect multiple molecular species in a highly time-efficient manner." The increased efficiency made the apparatus capable of measuring and analyzing data in almost real-time.
In building their effective apparatus, the researchers realized that some molecules in breath had very weak light absorption. To boost this absorption, the team built a cavity with a pair of high-reflectivity mirrors. The mirrors enhanced the interaction length between the laser light and breath molecules by a factor of several thousand in order to make the absorption stronger in just one breath. The mirrored cavity increased the sensitivity of the apparatus, furthering its precision.
Testing the Breath: Bananas...and Booze?
After the apparatus was constructed, the researchers needed to test its effectiveness. They decided to look at methanol as a target molecule. In order to see possible changes in methanol levels, they had a test subject consume foods and drinks in an effort to change the methanol levels in their breath. "We actually started with alcohol, because there are some literature reviews in the past that suggest some change in the methanol levels of breath," Liang grinned. "This sounds like a fun experiment because your test subject gets the opportunity to drink alcohol. We tried brandy, whisky and soju, a South Korean wine. It turns out none of these alcohols actually gave some obvious change in molecular concentrations." Though drinking alcohol in the name of science would have been a rather whimsical endeavor, the team ultimately had to abandon the idea.
Instead, they turned to fruit, and found that collecting data in 15-minute intervals, while their test subject ate ripe bananas, resulted in a gradual increase of methanol concentration in the breath. Liang found the entire process to be: "…very impressive. We could monitor several other molecules simultaneously, like methane and partially-deuterated water. We could confirm their concentrations did not change over the time after the banana consumption."
After seeing success in their apparatus, the team of researchers is shifting their focus towards diagnosing COVID-19 in people. According to postdoctoral researcher Jutta Toscano: "We are currently conducting a campus-wide study to understand how much the molecules present in people's breath can tell us about the state of their health, including the presence of various conditions that could be affecting them, such as COVID-19, diabetes, and asthma, among others." Having a less invasive method to diagnose COVI9-19 will not only make it easier to contain the virus, but can also be a cheaper and faster option for the government in the long run. Toscano found that: "Collaborating and learning from people in other fields of research (from engineering to physiology) has been a very exciting part of this project. Building bridges across disciplines and sharing expertise to reach a common scientific goal is both fulfilling and formative." Such collaborations as this can result in timely and beneficial real-world applications, like the breath-analyzer apparatus, which may change the way COVID-19 infections are analyzed and treated.
Written by Kenna Castleberry, JILA Science Communicator