DS-1 Deuterated Water Analyser

Overview

The isotope dilution technique is used to determine a patients total body water. A small, tracer quantity of deuterated water (sometimes called "heavy water" is drunk. Sometime later, the deuterium concentration in body water pools will become a mixture of the pre-existing water with the labelled tracer. Once complete mixing is achieved, the deuterium concentration will reflect the extent to which the tracer has been diluted by the body water.

This technique is recognised as a "gold standard" for determining total body water, and hence body composition. Historically, the deuterium concentration was measured on an isotope ratio mass spectrometer (IRMS) after off-line extraction of the hydrogen/deuterium from a urine sample. More recently, on-line systems have been developed to reduce the water prior to admission to the mass spectrometer. The DS-1 directly measures the deuterium concentration in the water vapour in breath, which in turn reflects the concentration in the blood at the lung interface. This not only makes sampling a whole lot easier but also, because of its real-time nature, allows body pool dynamics to be investigated.

Sampling simplicity

Aqueous samples are analysed without the need for preparative chemistry. The system measures the deuterium concentration in water vapour, for example, directly in breath with a low flow impedance inlet, or in any other physiological fluid which can be contained in a capped sample vial. Headspace samples from multiple vials can be sequentially admitted with an autosampler.

Breath samples contain an instantaneous indication of the deuterium in the blood. This allows the dynamics of transfer from the tracer to be observed. The same applies to other sources which can be sampled repeatedly. In one experiment, the transfer across a peritoneal membrane was measured.

The analytical system is based on a new technique called flowing afterglow mass spectrometry (FA-MS). In this, the plume of a microwave induced plasma is used to ionise the sample, which is then analysed by a quadrupole mass spectrometer. This means that chemistry takes place in the gas phase, eliminating the need for time consuming off-line equilibration or high-temperature reduction or pyrolysis.

Unlike IRMS instruments, the FA-MS powers up quickly. Samples can be analysed in real time, within fifteen minutes of plugging the instrument into a standard power socket. For breath, the deuterium profile in individual exhalations can be monitored, allowing the non-alveolar portion to be rejected.

Low level instrument functions are managed by an on-board microcomputer. The machine is operated via a desk or lap-top computer which "talks" to the on-board computer via a standard ethernet link using the TCP/IP (internet) protocol. Powerful application software handles the instrument control and mathematics, so that the user is presented with meaningful results.

Robust technology

The FA-MS and its parent product the SIFT-MS are redefining the state of the art in trace gas analysis. These systems have been designed to be transportable, have moderate power requirements, require minimal consumables and be cost effective. The hardware has been developed and refined by the team at Trans Spectra Limited (led by Prof. David Smith F.R.S.) to handle real world applications resulting from the clinical research program at the University Hospital of North Staffordshire (U.K.). Instrument Science Limited has a sole maufacturing and distribution license.

Specifications The D to H ratio of a sample containing 5% water vapour at natural abundance can be determined with a relative precision of 1%. Dimensions: 600 x 600 x 600 mm. Consumables: Argon carrier gas. Power: Standard laboratory or hospital mains supply. Purpose designed analyser and electronics, communicating with a host computer via Ethernet and TCP/IP.

Further reading

• Detailed technical note
• FA-MS website
• On-line determination of the deuterium abundance in breath water vapour by flowing afterglow mass spectrometry, FA-MS, with applications to measurements of total body water. D. Smith and P. Spanel, Rapid Communications in Mass Spectrometry, 15 (2001), 25-32
• Accuracy and precision of flowing afterglow mass spectrometry for the determination of the deuterium abundance in the headspace of aqueous liquids and exhaled breath water. P. Spanel and D. Smith, Rapid Communications in Mass Spectrometry, 15 (2001) 867-872.
• Rapid measurement of deuterium content of breath following oral ingestion to determine body water. S. Davies, P. Spanel, D. Smith. Physiol. Meas. 22 (2001) 651-659.
• Comparative measurements of total body water in healthy volunteers by online breath deuterium measurement and other near-subject methods. David Smith, Barbara Engel, Ann M. Diskin, Patrik Spanel, Simon J. Davies.American Journal of Clinical Nutrition 76, (2002), 1295-1301.
• Measuring transport of water across the peritoneal membrane. Ramzana B. Asghar, Ann M. Diskin, Patrik Spanel, David Smith, Simon J. Davies.Kidney International 64 (2003) 1911-1915.
• Selected ion flow tube mass spectrometry (SIFT-MS) and flowing afterglow mass spectrometry (FA-MS) for the determination of the deuterium abundance in water vapour" Patrik Spanel, David Smith, in Pier A. de Groot (Ed.) Handbook of Stable Isotope Analytical Techniques (2004), Elsevier.