The use of breath analysis to determine the state of a person’s health is a concept that dates back millennia (Gouma 26). Cardiovascular disease is an ailment for which current analysis methods involve invasive sampling. It has been suggested that the amount of cholesterol in blood can be measured by the amount of isoprene (Karl et al. 762) or carbon dioxide (Amann 34) in the breath.
One of the early methods to detect isoprene in breath is based on proton transfer reaction mass spectrometry (PTR-MS) (Karl et al. 762-763). In this technique, the breath sample is made to flow through a source of hydronium (H3O+) ions. A proton is transferred from the hydronium ion to the analyte to produce protonated species. The hydronium does not react with the major components of air, yet is able to react with the volatile organic compounds (VOCs) present in human breath. Hence, this technique shows analytical specificity towards VOCs. When isoprene adds a proton, it is then measured at a mass-to-charge ratio (m/z) of 69. As this technique can be employed online, samples can be measured every few seconds. The sensitivity of this technique is determined by two rate constants: the rate constant for proton transfer and the rate constant for deprotonation.
A related approach is the preparation of mixed breath gas (Kushch et al. 1012). In this technique, the breath samples were obtained by a straw and the sample was stored in a Tedlar (polyvinylfluoride, PVF) bag. The sample thus collected was analyzed for isoprene by PTR-MS. The collection of samples in this manner allows one to make replicate readings to determine the variability in the measurements. However, the samples must be processed within 12 h as the isoprene diffuses through the bag to a measurable limit over this time frame.
In order to eliminate the errors that arise due to the diffusion of gases through Tedlar, the use of stainless steel canisters has been recommended for breath sampling (Buszewski 558-559). The VOCs do not permeate through the walls; moreover, they do not adsorb to the surface of the stainless steel. Hence, the sample integrity can be maintained for much longer than with Tedlar bags. The samples thus collected need to be pre-concentrated before they can be separated by gas chromatography and analysed by mass spectrometry (GC-MS). GC-MS is a technique whereby all the gaseous components of the sample can be measured; however the separation process is not instantaneous. The analytical time depends on the chromatographic conditions. The separation can be improved by applying pre-concentration techniques. Pre-concentration is primarily required to remove moisture and to increase the concentration of analytes. Adsorption and cryotrapping are two methods employed.
Nanotechnology techniques have been employed to aid in the sensing of VOCs in breath. These include the use of binary oxide nanowires (Gouma 28). These materials have a very high aspect ratio and thus are very sensitive probes. This method has been used to detect a number of VOCs; however a suitable probe is yet to be developed for isoprene. As with other nanoprobes, this technique provides a non-invasive and portable means for VOC determination in human breath.
The methods outlined so far are indirect measures of blood cholesterol. A fast direct method to measure blood cholesterol has been recently reported (Wisitsoraat 8661). As this is a direct measurement, it is also an invasive technique: blood needs to be drawn. However, the sensitivity of this technique – based on cyclic voltammetry of cholesterol – allows for accurate measurements with smaller sample volumes than traditional titration-based techniques.
References
Amann, Anton, ed. Volatile Biomarkers: Non-invasive Diagnosis in Physiology and Medicine. n.d.
Buszewski, Boguslaw, et al. "Human exhaled air analytics: biomarkers of diseases." Biomedical Chromatography 21 (2007): 553-566.
Gouma, Perena. "Nanoceramic sensors for medical applications." American Ceramic Society Bulletin 91.7 (2012): 26-32.
Karl, Thomas, et al. "Human breath isoprene and its relation to blood cholesterol levels: new measurements and modeling." Journal of Applied Physiology 91 (2001): 762-770.
Kushch, Ievgeniia, et al. "Breath isoprene – aspects of normal physiology related to age, gender and cholesterol profile as determined in a proton transfer reaction mass spectrometry study." Clin Chem Lab Med 46.7 (2008): 1011-1018.
Wisitsoraat, Anurat, et al. "High Sensitivity Electrochemical Cholesterol Sensor Utilizing a Vertically Aligned Carbon Nanotube Electrode with Electropolymerized Enzyme Immobilization." Sensors 9 (2009): 8658-8668.
