Advances in Volatile Organic Compounds Detection

Volatile organic compounds (VOCs) are a collection of compounds characterized by their high vapor pressure and low water solubility – a combination of properties that allows them to vaporize into the air with ease.

Many VOCs are man-made chemicals and solvents that have been developed for industrial use, such as in the manufacturing of paints, pharmaceuticals and refrigerants. VOCs may also be formed as combustion byproducts in vehicle emissions and industrial processes.

Plants, soils and volcanoes are natural sources of VOCs. To some extent, the human body is also a natural emitter of VOCs – biogenic volatile compounds are often present in the breath as a result of secondary metabolic processes.

Having effective methods for VOC detection is a crucial safeguard in protecting human health and the environment. Many VOCs are recognized as air pollutants that can have a negative impact on the environment and may pose a health risk with long-term exposure. Since the body also produces VOCs, there is also an interest in developing VOC detectors that can be used in diagnostic medicine to detect the presence of certain biomarkers in the breath, such as those linked to lung cancer or diabetes.

Next-generation VOC detection tools

The analysis of VOCs is largely performed using gas chromatography coupled to mass spectrometry (GC-MS) as the gold standard. The United States Environmental Protection Agency’s (US EPA’s) standardized and validated test method for determining VOCs in solid waste samples, water samples and wipe samples (Method 8260D) makes use of GC-MS for the analysis of more than 100 key analytes, for example.

While GC-MS does offer comprehensive high-quality analysis, ideal for the detection and quantification of a vast range of different volatile compounds, the technique is less well-suited for on-site or real-time VOC monitoring. MS instruments are generally bulky, must be housed in a laboratory and operated by a skilled technician, with high up-front costs and power demands.

To overcome these limitations and enable the detection of VOCs in a wider range of environments and scenarios, scientists are currently investigating numerous alternative analysis methods.

Miniaturized GC

There has been significant interest in miniaturizing GC instruments for many years – not just for VOC detection, but for numerous different analytical applications. Current iterations of miniaturized GC technology intended for temporary stationary laboratories can deliver analytical performances on par with their full-size equivalents, while consuming less power, materials and space. In these instruments, the core components of the GC system – the injector, column and detector – are shrunk down to fit in a smaller overall footprint.

Handheld, mini-GC or micro-GC (µGC) units are the most extreme form of this miniaturization. In these devices, the column may be replaced by etched channels on a semiconductor chip to provide a different, miniaturized mode of chromatographic separation. Mini/µGC devices are more limited in performance compared to regular GC or stationary miniaturized GC apparatus, but they offer greater functionality than a simple gas sensor on account of their ability to be selective towards specific target analytes.

Innovation in miniaturized GC systems for VOC detection is ongoing. In recent studies, researchers have successfully used portable GC systems for the real-time monitoring of VOCs in indoor and outdoor air by combining the system with a miniature carbon nanotube sponge preconcentrator to boost sensitivity. One group of researchers has even developed a belt-mounted prototype µGC system that is intended to measure personal exposures to VOCs in industrial workplace environments.

Electronic noses

The electronic nose (e-nose) is a system first introduced in the 1980s, comprising gas sensors coupled to an array of automated recognition systems designed to mimic the human olfactory system. Since their introduction, the popularity of research involving e-noses and sensor arrays has surged. There has been  a five-fold increase in related publications since the turn of the millennium from less than 200 scholarly publications in 2000 compared to over 1000 in 2023.

When the multi-sensor array of an e-nose is exposed to a gas, the analytes will interact with the sensitive sensor material surfaces, generating physical changes in the sensors that can be converted into a readable digital signature that is used to identify the “odor”.

Compared to traditional GC-MS techniques, e-noses are relatively inexpensive, simple to use and provide rapid analysis results. The integration of different nanomaterials, alongside the use of alternative pattern recognition algorithms, can also help to boost the sensitivity, selectivity and accuracy of these devices.

E-nose devices based on metal oxide semiconductor sensors have found regular use in environmental engineering applications for the detection of VOCs and other air pollutants at parts-per-million (ppm) and sub-ppm levels.

E-noses have also been utilized for clinical diagnosis. Similarly to how analytes in the air can be detected by their unique signatures, specific “breathprints” can be useful in diagnosing, monitoring or phenotyping various diseases.

While e-noses have certainly increased in popularity, there are several limitations facing this type of technology. Most notably, these devices work by detecting patterns – not individual molecules – and thus they cannot be used to identify and quantify specific compounds present in a given sample. Still, positive results in early diagnostics studies do indicate that this technology may find effective clinical use as a complementary diagnostic tool that can be used by non-specialists in analytical science.  

Wearable VOC detectors

At the extreme end of portability, wearable VOC sensors enable real-time in situ measurements of local VOC levels.

These patch-like devices typically consist of a lightweight wearable substrate material (i.e., textiles or polymers), a conductive electrode material and a VOC sensing material (i.e., a metal oxide semiconductor, carbon-based nanomaterials or small molecules). These are combined using a variety of advanced industrial coating and printing techniques.

Wearable VOC sensors are of particular interest in agriculture. Here, the VOCs naturally emitted by plants can be assessed to determine if the plants are suffering from high stress, or if they are emitting any pathogen-specific biomarkers that would indicate an infection. There are wearable VOC sensors reported in the scientific literature that can be applied to a plant's leaf and indicate potential pathogenic infections several days before visual symptoms appear.

Non-agricultural applications for similar devices have also been explored. For example, there are publications detailing the use of wearable microfluidic sensors that can detect the presence of certain diabetes-related VOCs in human sweat. Wearable fluorescent badges and patches for industrial workplaces have also been prototyped, with the sensor devices producing a highly visible color change when exposed to specific VOCs.

Like the e-noses, much of this wearable technology is not capable of quantifying or identifying a broad spectrum of VOCs in the local environment. Wearable sensors based on electrochemical detection methods may also suffer from certain specificity issues, as the presence of multiple structurally similar VOC molecules could result in overlapping electrochemical responses and false results.

However, these devices are generally not being built with a view to replacing the superior sensitivity and specificity of GC-MS analysis – they are intended to be rapid, first-line warning technologies that can indicate an immediate environmental danger or an undiagnosed infection in time-sensitive real-world environments. With further research into improved sensor materials, better signal processing and cost-effective mass production technologies, wearable VOC sensors could help to enhance agricultural and environmental monitoring.

The future of air monitoring

The ideal VOC sensor would be able to balance high sensitivity and specificity, with the need for portability, stability, rapid response times, low costs and the ease of mass fabrication for widespread use. While such a sensor is perhaps unrealistic, significant efforts have been invested in producing new technologies that can act as a complement to gold standard GC-MS techniques – expanding the possibilities for real-time in situ air monitoring.

With the development of miniaturized GC-MS devices, e-noses and wearable sensor technologies, scientists are continually moving closer to a future where air quality monitoring and VOC detection are more accessible than ever – generating new insights into the world around us, as well as into human health and agriculture.