Ask someone what gases they would expect to find in our breath and oxygen, carbon dioxide and water may well be high on the list. But did you realize that acetone could be in there too? Acetone, apart from being a common solvent and lab glassware cleaner, is also a key indicator of a potentially fatal condition called ketoacidosis, a complication of diabetes. In ketoacidosis, the body transforms fatty acids into three compounds called ‘ketone bodies’ (shown in Figure 1) in response to a lack of insulin.)
Figure 1: Ketone bodies produced in ketoacidosis.
This is intended to provide more food for the body, as ketone bodies can be used as fuel if there is a shortage of other options such as glucose. However, the acidity of the carboxylic acid groups can lead to dangerously low pH in the blood. Similar effects to diabetic ketoacidosis can happen after intense exercise or starvation as the body tries to make fuel available for respiration. The acetone that is produced is easily able to escape the blood into the lungs, because it is highly volatile, where it is breathed out. Monitoring acetone levels in breath is therefore a useful way to help diagnose ketoacidosis, as the authors point out. Such measurements are also useful for research purposes in linking breath acetone levels to medical conditions.
Previously, ketoacidosis has been detected by smelling acetone on the breath – it has a sweet smell similar to nail varnish remover (these products often contain acetone). As easy as that may be, it does not quantify the amount of acetone, so it is hard to compare across different samples. In previous research work, the paper states, mass spectrometry has often been the method of choice. The response of the mass spectrometer to different molecules can be calibrated by running standards of known concentration, allowing the amount of acetone, and other molecules to be measured. In this work, the authors report a different way of measuring acetone using a portable device they have made.
The device works by coupling two key components together. The first part is a preconcentrator, which collects the acetone by adsorbing it onto a polymer. The second part contains a cavity in which the amount of acetone is measured by cavity enhanced spectroscopy.
To outline how cavity enhanced spectroscopy works it is worth thinking about how light is absorbed by the sample. The absorbance of light of a certain wavelength depends on the sample, its concentration and its thickness, which is related by the Beer-Lambert law.
In cavity enhanced spectroscopy, a laser beam is bounced between two mirrors inside a cavity containing the sample, so that the beam of light keeps passing back and forth through the sample. The mirrors reflect over 99.99 % of the light each time, but a small amount passes through the mirror instead of being reflected. A detector measures the amount of light passing through the mirror, which tells us how much sample is present.
This technique is a way of getting high sensitivity in a small device. The reflections force the light to travel a greater distance through the sample so that even extremely small concentrations give measurable absorbance changes (in this paper they can detect down to around 159 parts per billion). The wavelength of light had to be carefully chosen to avoid absorbance by other common molecules in our breath.
The sensor was put to the test by analysing breath samples from volunteers, who were asked to fast and perform exercise – the device captured the rising acetone levels in their breath. The same samples were also tested by mass spectrometry to test whether the readings given by the device were accurate (Figure 2).
Figure 2: Comparison of the concentrations of acetone measured with the new device against mass spectrometry results. Reprinted with permission from Anal. Chem. 2016, 88, 11016−11021. Copyright (2016) American Chemical Society.
Perhaps after future development the cost of the device and its tuneability to detect other trace gases will further improve. Although this device is still at an early stage, it is an example of many showing how analytical chemistry can be beneficial for human health in clinical or research settings. It may be worth checking out the patent literature as well – you can find interesting chemistry that is not published elsewhere!
 The Beer-Lambert Law states that Absorbance, A, is given by A=αcl, where α is the extinction coefficient (depends on wavelength and sample), c is concentration and l is the distance the light travels. The further light travels into the sample, the greater the amount absorbed. This helps us understand why cavity enhanced spectroscopy is much more sensitive than simply passing light through a cuvette…because the light is bounced through the sample many times, the value of l is effectively increased.
*Featured image reprinted with permission from Anal. Chem. 2016, 88, 11016−11021. Copyright (2016) American Chemical Society