The power of SIFT-MS for breath analysis
Wednesday 22 May 2002
Reliable analysis of the trace metabolites in exhaled breath offers the clinician a potentially important non-invasive method for clinical diagnosis and therapeutic monitoring. Researchers at Keele University and the Academy of Sciences of the Czech Republic have developed an analytical method by which metabolites in only single breath exhalations can be accurately measured at the parts per billion (ppb) level. This novel method is called selected ion flow tube mass spectrometry, SIFT-MS. In their latest research work, reported in
Physiological Measurement, Smith
et al demonstrate the extraordinary power of SIFT-MS for breath analysis. They report on the use of SIFT-MS to measure simultaneously the concentrations in exhaled breath of ethanol, acetaldehyde, ammonia, acetone and, routinely, water vapour, following the ingestion of various amounts of ethanol in 500 ml of water into an empty stomach (in the fasting state).
Their analyses were achieved in only single breath exhalations sampled directly into the instrument, the results being available immediately in real time. The breath ethanol reaches concentrations that are approximately consistent with its dilution in the blood and body water, reaching maximum values of about 40 parts per million (ppm) (the legal limit for drivers is 180 ppm) for an ethanol dose of about 20 ml. The breath ethanol decays quickly for small doses, exhibiting first-order kinetics (a single exponential decay) whereas for relatively large doses (20 ml), the initial decay of ethanol from the breath is slower, indicating saturation degradation kinetics in the liver. For smaller doses, and following a meal, the breath ethanol increases only slightly indicating that it is largely metabolised in the stomach.
The study was undertaken partly to investigate the use of ethanol dilution for the determination of total body water, the principle being that, following its ingestion, ethanol is quickly dispersed into the body water and its concentration in breath is then a measure of the total body water. This assumes that the ethanol is not significantly metabolised in the liver or the stomach, but our investigations indicate this to be quite fallacious. Acetaldehyde is detected in the breath following ethanol ingestion, albeit at much lower levels (typically 10 ppb), and its concentration correlates well with the breath ethanol, indicating that it is mostly formed from the metabolism of the ethanol.
The researchers have concluded that total body water is best measured using the deuterium dilution/breath analysis method (
Physiological Measurement,
22 (2001) 651). These simultaneous measurements of breath ethanol and acetaldehyde alone demonstrate the dynamic range of SIFT-MS for on-line analyses of single exhalations, this being at least 10,000. But several other breath metabolites can be quantified simultaneously. In these studies, breath ammonia was also measured and is seen to 'dip' following the water/alcohol drink and this they attribute to the stimulation of blood flow between the stomach/upper intestine and the liver that removes ammonia from the blood. Breath acetone concentrations increased somewhat with time as is expected during the fasting state. Simultaneous measurements of breath water vapour levels are routinely carried out during SIFT-MS breath analyses, these being sufficient to show the differences between the breath (body) temperatures of individuals of less than 1°C. Since water vapour represents about 6% of exhaled breath, i.e. about 6 x 10^7 ppb, this (together with the acetaldehyde measurements) indicates an even wider dynamic range of more than 10^6 for this remarkable analytical method.
This study demonstrates the enormous potential of SIFT-MS for non-invasive physiological measurements. This novel technique is also being used to great effect for medical research to detect the presence of tumours in the body and
helicobacter pylori in the stomach and to investigate other conditions such as renal failure, diabetes, schizophrenia and in health and safety and environmental science. Commercial instruments based on SIFT-MS will soon be available from Trans Spectra Ltd, UK.
The article '
On-line, simultaneous quantification of ethanol, some metabolites and water vapour in breath following the ingestion of alcohol' has been made a Featured Article [at the Institute of Physics website
www.iop.org - BT] and is therefore freely available to all.
This is certainly a dramatically sensitive analysis tool!
BigTrancer
