WO2011117572A1

WO2011117572A1 - Analysis of breath

Info

Publication number: WO2011117572A1
Application number: PCT/GB2011/000405
Authority: WO
Inventors: Graham Hancock; Robert Peverall; Grant Andrew Dedman Ritchie
Original assignee: Isis Innovation Limited
Priority date: 2010-03-25
Filing date: 2011-03-22
Publication date: 2011-09-29

Abstract

A method of detecting simultaneously acetone and carbon dioxide, and preferably methane, in breath using cavity-enhanced absorption spectroscopy. The method utilises a broadband light source emitting light in the range 1640 to 1700nm such as a superluminescent light emitting diode. The light is coupled into an optical cavity into which the breath sample is admitted and the characteristic spectral features of carbon dioxide, acetone and methane are identified in the absorption spectrum output. Wavelength selectivity can be provided by use of a tunable fibre filter on the input side of the cavity or by an FTIR on the output side. The apparatus is suitable for widespread use as it is relatively portable and durable and it can thus be used for screening for diseases in which the presence of acetone is a marker, such as diabetes.

Description

ANALYSIS OF BREATH

The present invention relates to an apparatus and method for analysing the breath of a human or animal subject, in particular for simultaneously measuring the amount of acetone and carbon dioxide in the breath.

The presence of acetone in the breath has been suggested as a marker for diabetes as it is related to the level of glucose in the blood. Breath acetone can also be used as indicator of dietary behaviour and of exercise. The significant advantage of breath monitoring is that it is relatively non-invasive and is thus comfortable for the subject and can be suitable as a large- scale screening technique.

Presently breath acetone can be measured by either mass spectrometric or optical techniques. It is essential that the breath sample does not deteriorate (for example with reaction with container walls) before analysis and for this reason direct measurement (i.e. in which the patient breathes directly into the instrument) is essential. Although mass spectrometric techniques are accurate, they require large and expensive instruments which are unsuitable for widespread use. Lower cost techniques of measuring breath acetone have been proposed based on absorption spectroscopy of reaction products of acetone in the breath. However these require a sophisticated calibration procedure in order to derive the absolute

concentration of the acetone from the measurement carried out on the reaction product.

A problem with the spectroscopic measurement of acetone directly is that acetone is a relatively large and complex molecule (in spectroscopic terms) and thus it absorbs over a relatively broad wavelength range. Consequently spectroscopic techniques based on the use of a laser, which typicall looks at only a very small part of the acetone absorption feature lacks specificity (i.e. it cannot indicate without doubt whether the absorption measured is due to acetone or some other species). Another problem is that with acetone the level of absorption is relatively low because of low number density in breath and so it is difficult with absorption spectroscopy to achieve adequate sensitivity to detect acetone in breath.

These difficulties are discussed in the paper "A New Acetone Detection Device Using Cavity Ringdown Spectroscopy at 266nm: Evaluation of the Instrument Performance Using Acetone Sample Solutions" by Wang and bi (Measurement Science and Technology 18(2007)2731 - 2741 ). In that paper the authors propose and evaluate the performance of cavity ringdown spectroscopy (CRDS) on gas samples containing acetone vapour mixed with air and water vapour. The authors show that it is possible to detect such acetone samples by the CRDS method, and suggest the use of CRDS for a portable, fast-response acetone detection device. The same authors have also proposed NIR CRDS (see Applied Spectroscopy 58, (2004) 784) in which they show part of a low resolution spectrum of acetone between 1632.7 and 1672.2 ran. However cavity ringdown spectroscopy is instrumentationally difficult and in practice is not suitable for widespread use, for example in doctors' surgeries. Each ringdown measurement is also carried out at a specific wavelength which again means that to achieve specificity in the case of a sample like breath, which will include other absorbers at each wavelength, the ringdown measurement must be repeated at a number of different wavelengths. It also requires good temporal resolution in order to be able to measure the ringdown time. At 266nm the technique requires UV-grade optics, a relatively expensive laser and is relatively power hungry. Also the technique is subject to interference because of strong UV scattering by aerosols.

Accordingly the present invention provides apparatus for simultaneously measuring the amount of acetone and carbon dioxide in the breath of a human or animal subject, comprising: a breathing tube for provision in an airway for the subject,

an absorption spectrometer for measuring the concentration of acetone and carbon dioxide in gas from the breathing tube;

wherein the absorption spectrometer comprises:

an optical cavity disposed to receive gas from the breathing tube; a broadband light source positioned to supply light to the optical cavity and emitting light over a wavelength range encompassing: a) at least one of the carbon dioxide characteristic spectral features at 1642 run and 1649 run; and also b) at least one of the acetone characteristic spectral features at 1672 nm and 1689 nm;

a light detector for detecting light emerging from the optical cavity, thereby to provide a signal representative of the absorption of the light by acetone and carbon dioxide and methane in the gas in the optical cavity; and

the optical cavity being arranged such that the light in the optical cavity retraces some or all of its path to provide cavity-enhancement of the absorption signal, the light detector being connected to a data processor for receiving the absorption signal and adapted to identify therein characteristic spectral features of acetone and carbon dioxide and to calculate from the identified characteristic spectral features the amount of acetone and carbon dioxide in the breath.

The term "cavity-enhancement" is used in this document to refer to techniques in which the signal available due to spectroscopic absorption by a target substance present in an optical cavity is enhanced through repeated reflection of the radiation back along the same path within the cavity (sometimes referred to as re-entrant paths) increasing the path length and giving the possibility to excite one or more cavity modes. This allows trace components in the gas phase to be much more easily detected and their presence quantified. An optical cavity is usually provided by two optically opposed high reflectivity mirrors (reflectivity typically greater than 99%), and the repeated retracing of some or all of the optical path leads to interference effects and the possibility of observable energy density build-up at a specific wavelength.

"Cavity-enhanced" techniques in this application are those often referred to in the scientific literature as Cavity-enhanced Absorption Spectroscopy (CEAS), Integrated Cavity Output Spectroscopy (ICOS), and variations thereof. These measure the (time-integrated) transmitted intensity as a function of wavelength, in contrast to CRDS in which the temporal evolution of a pulse of light is measured (i.e. time resolved not time integrated), using apparatus which needs, therefore, a high temporal resolution.

It should be noted that optical cavities are fundamentally different in nature and construction to optical multipass cells, such as Herriott cells. Multipass cells are not based on interference or resonance effects in which energy builds up in the cavity but instead use careful alignment of the mirrors to permit a light beam to follow a zig-zag extended single path between the entry and exit windows of the cell. They achieve a much lower path length enhancement than an optical cavity. For example a multipass cell of dimension 50cm to 100cm might achieve a path length of order 100m whereas a smaller optical cavity, e.g. of dimension 25cm, can achieve a path length of order 5km.

Thus the use of an optical cavity within a cavity-enhanced absorption assembly enables a high sensitivity to be achieved (e.g. enhancement factors of 5000 to 10000) within a far more compact and lightweight apparatus than could be achieved using an equivalent optical multipass cell. The apparatus is also easier to set-up and align because with a multipass cell any misalignment of a mirror will result in the single light beam not exiting through the exit window. For this reason multipass cells are also not particularly durable for use outside the laboratory environment.

With the present invention a broadband light source is used such as a light emitting diode preferably in superluminescent mode, e.g. a superluminescent light emitting diode (SLD) or Amplified Spontaneous Emission source (ASE). These emit light over a broad wavelength range compared to a laser diode and in this invention a light source emitting over the region 1660 nm to 1690 run, more preferably 1630 to 1700nm, allows detection simultaneously of carbon dioxide, acetone and, optionally, methane. In the more preferred case acetone and methane are measured on their first overtone transitions near 1670nm and a carbon dioxide transition at 1640-1650nm. Embodiments of the invention can detect acetone at l OOppb to 1 OOppm levels in breath. Another alternative for the broadband light source is a

supercontinuum laser source such as the Fianium SC450.

The use of a single broadband light source covering the whole of the wavelength range encompassing the spectral features of the target species allows the detection of the target species (carbon dioxide and acetone, and optionally methane) in a single wavelength scan.

The light detector provides a signal representative of the transmission (or conversely absorption) from the optical cavity as a function of wavelength. The data processor can use a known fitting algorithm to seek the characteristic spectral features of carbon dioxide and acetone in the output from the detector and thus measure the amount of these target substances. Optionally the characteristic spectral features of methane can also be detected. Preferably the methane and water spectra (which are known) are fitted to the absorption signal allowing their removal (and quantification) as they overlap the acetone features. In order to provide wavelength selection it is possible to use as a wavelength selective device either a narrow band tunable filter such as a tunable fibre filter on the input side of the cavity such that only a small wavelength range of the output from the broadband light source is transmitted through the cavity at any time, with the filter being tuned in a scan over the desired whole wavelength range, or a Fourier Transform Infrared (FTIR) spectrometer on the output side to analyse the whole wavelength range at once. Alternatively a grating spectrometer with a CCD detector could be used on the output side to analyse the output as a function of wavelength. Optionally the ratio of carbon dioxide to acetone can be calculated directly from the output of the light detector. This ratio can, itself, be indicative of abnormality as the amount of carbon dioxide in breath, particularly end-tidal breath, is relatively consistent and thus a high amount of acetone compared to that level is potentially indicative of non-standard breath

concentrations.

In order to obtain an absolute measurement of the amounts of carbon dioxide and acetone it is possible to include a further light source and further detector which are arranged to perform single pass infrared absorption spectroscopy on the breath sample. This measurement can be made using a diode laser operating close to 2um. This gives an absolute measurement of the amount of carbon dioxide in the sample, which in turn allows calibration of the cavity- enhanced signals. Further, the detection of the carbon dioxide level allows the detection of the end-tidal part of the breath (by looking for the maximum carbon dioxide). The breath collection system for the cavity-enhanced measurement can be triggered to collect this end- tidal breath which, because it has had maximum exchange with alveolar blood, will give the most appropriate signal.

Alternatively the apparatus can be calibrated using air as a calibrant by relying on the fact that the normal methane concentration in atmosphere is 1.8ppm. Preferably the characteristic spectral features of acetone at around 1670nm (e.g. within the range 1660-1685nm), carbon dioxide at around 1645nm (e.g. within the range 1635-1 55nm) and methane at around 1665nm (e.g. within the range 1660-1670nm) are identified.

The invention also provides a corresponding method of simultaneously measuring the amount of acetone, carbon dioxide and methane in the breath of a human or animal subject.

Because of the relative ease of set-up and the durability of the equipment, together with its high sensitivity, the invention allows the detection of breath acetone by a compact, portable apparatus. It is therefore suitable for use in doctors' surgeries, clinics and other locations and thus as a screen for conditions marked by breath acetone, e.g. diabetes.

The invention will be further described by way of example with reference to the

accompanying drawings in which:-

Figure 1 is a schematic illustration of a first embodiment of the invention utilising a tunable fibre filter;

Figure 1 A schematically illustrates in more detail the breath delivery system used in the Figure 1 or 2 embodiment;

Figure 2 is a schematic illustration of a second embodiment of the invention using a Fourier transform infrared spectrometer; and

Figure 3 shows the results of analysis of various mixtures of acetone, air and breath obtained using the second embodiment of the invention. Figures 1 , 1 A and 2 illustrate schematically embodiments of the invention. Both

embodiments use the same light source, optical cavity and delivery system and both give the same output (namely absorption or transmission as a function of wavelength) but the Figure 1 embodiment utilises as a narrow band filter a tunable fibre filter to couple into the optical cavity only a small range of the wavelength from the light source at any one time thereby providing the required wavelength selectivity required to output absorption as a function of wavelength. The Figure 2 embodiment couples the whole output of the broadband light source into the optical cavity but uses a Fourier transform infrared spectrometer (FTIR) to analyse the output and provide as an output absorption as a function of wavelength. In more detail in Figures 1 and 2 the apparatus 1 comprises a broadband light source 3 such as a superluminesent light emitting diode, e.g. DenseLight Semiconductors l OmW SLD using a Thorlabs LDC205C current driver, mounted on a heat sink 5 and whose output is supplied to an optical fibre 7. Alternatively a supercontinuum laser source can replace the SLD. In the case of the Figure 1 embodiment the optical fibre couples the light into a tunable fibre filter 9 selected to be tunable over the desired wavelength range and which supplies a selectable portion of the wavelength to the optical cavity 1 1 . The optical cavity 1 1 consists of two high reflectivity mirrors 13, 15 separated by about 25cm enclosed within a vacuum vessel. The apparatus 1 also includes a delivery system 20, shown in more detail in Figure 1 A, which takes a sample of breath from a subject 24 via a ventilation tube 22 and supplies to the interior of the optical cavity 1 1. It includes a valve 21 under control of a valve controller 23 to supply breath via a branch 25 to an exit, or via a branch 27 to the apparatus. This allows selection of breath from the desired part of the breathing cycle to be sampled (e.g. end-tidal).

The cavity mirrors 13 and 15 have a reflectivity of about 99.97% and light exiting the cavities is either coupled into a detector 31 such as an InGaAs detector (e.g. Thorlabs DET410) in the Figure 1 embodiment, or is supplied to such a detector 32 via an FTIR 30 in the Figure 2 embodiment. The FTIR 30 is schematically illustrated in Figure 2, a suitable example is a Perkin Elmer Spectrum 100. The output from the detector 31 , 32 is analysed by a data processor 35.

Figures 1 and 2 schematically illustrate the spectrum being displayed on a display 37, though in practice the data processor uses a fitting algorithm to detect the known characteristic spectral features of carbon dioxide, methane and acetone in the wavelength regions concerned and directly outputs the measured levels of the target substances and, as mentioned above, optionally the ratio of carbon dioxide to acetone.

As illustrated in Figure 1 A the apparatus preferably further includes in the delivery system 20 a light source 40, for example a VCSEL operating at 2 microns, and light detector 41 , e.g. an InGaAs photodiode, positioned to make a single pass IR absorption measurement to detect the absolute C0₂ level in the breath sample. This measurement can be made using the strong C0₂ absorption at a wavelength near 2um. The detection of the absolute C0₂ level together with the measurement of the relative levels of C0₂, methane and acetone from the cavity- enhanced signals allow absolute levels of methane and acetone to be calculated.

Further, the detection of the carbon dioxide level allows the detection of the end-tidal part of the breath (by looking for the maximum carbon dioxide). The delivery system 20 for the cavity-enhanced measurement is triggered to collect only this end-tidal breath which, because it has had maximum exchange with alveolar blood, will give the most appropriate signal.

It should be appreciated that the relatively weak absorption signals of acetone can be detected because of the use of the optical cavity in which the light at least partially retraces some of the same path repeatedly (unlike a multipass cell) between the mirrors 13 and 15 which can cause some degree of interference and results in increased signal sensitivity. Also the mirrors 13 and 15 are smaller and lighter than those typically used in multipass optical cells and the light enters and exits the cavity through the mirrors themselves rather than through separate entry and exit pupils.

To illustrate the sensitivity and effectiveness of the invention Figure 3 illustrates the absorption spectrum obtained with a Figure 2 embodiment of the invention for three different mixtures of acetone, air and breath. In each of the cases the carbon dioxide (double peak) feature can be identified between 1640 and 1655nm, becoming stronger as the proportion of breath in the sample increases. The methane feature (a small peak) at about 1665nm can be seen in the 30% and 50% breath samples (and the broad acetone feature can be seen between 1665 and 1680nm in all three samples. Spectral features of water can also be detected at 1 90nm and above though these are relatively weak and the output of the broadband light source 3 is falling away at these wavelengths.

Claims

1 . Apparatus for simultaneously measuring the amount of acetone and carbon dioxide in the breath of a human or animal subject, comprising:

a breathing tube for provision in an airway for the subject,

wherein the absorption spectrometer comprises:

an optical cavity disposed to receive gas from the breathing tube; a broadband light source positioned to supply light to the optical cavity and emitting light over a wavelength range encompassing: a) at least one of the carbon dioxide characteristic spectral features at 1642 run and 1649 nm; and also b) at least one of the acetone characteristic spectral features at 1672 run and 1689 nm;

2. Apparatus according to claim 1 wherein data processor is adapted to identify in the absorption signal the characteristic spectral features of acetone, carbon dioxide and methane and to calculate from the identified characteristic spectral features the amount of acetone, carbon dioxide and methane in the breath.

3. Apparatus according to claim 1 or 2 wherein the broadband light source emits light over a wavelength range 1630 to 1700nm.

4. Apparatus according to claim 1 , 2 or 3 further comprising a wavelength selective device and controller therefore, the controller being adapted to control the wavelength selective device to scan over the wavelength range of the characteristic spectral features such that the detector detects the transmission through the optical cavity as a function of wavelength.

5. Apparatus according to claim 1 , 2, 3 or 4 wherein the light detector, wavelength selective device and controller are constituted by a Fourier transform infrared spectrometer.

6. Apparatus according to claim 1 wherein the broadband light source is coupled to the optical cavity by a tunable optical fiber filter constituting said wavelength selective device and controller.

7. Apparatus according to any one of the preceding claims further comprising a further light source and further detector arranged to pass light through the gas from the breathing tube to measure by single pass infrared absorption the concentration of carbon dioxide in the gas from the breathing tube; and wherein the data processor receives the signal from the further detector and is adapted to calculate therefrom the carbon dioxide concentration thereby to calibrate the cavity-enhanced absorption signals.

8. Apparatus according to claim 7 wherein the further light source and further detector measure the concentration of carbon dioxide by direct absorption.

9. Apparatus according to claim 8 further comprising a delivery system to deliver breath form the breathing tube to the cavity, the delivery system being triggered to deliver only the end-tidal breath upon detection of a maximum carbon dioxide level by the further detector.

10. Apparatus according to any one of the preceding claims wherein the data processor is adapted to identify the characteristic spectral features of acetone at around 1670nm, carbon dioxide at around 1645nm and methane at around 1665nm.

1 1 . Apparatus according to any one of claims 1 to 9 wherein the data processor is adapted to identify the characteristic spectral features of acetone within the range 1 60 to 1685nm, carbon dioxide within the range 1635 to 1655nm and methane within the range 1660 to 1670nm.

12. Apparatus according to any one of the preceding claims wherein the broadband light source is a light emitting diode used in superluminescent mode or a supercontinuum laser.

13. Apparatus according to any one of the preceding claims wherein the data processor is adapted to calculate the ratio of acetone to carbon dioxide from the respective identified characteristic spectral features.

14. Apparatus according to any one of the preceding claims wherein the optical cavity is formed by two mirrors arranged to face each other along a common optical axis which is transverse to the gas flow through the breathing tube.

15. Apparatus according to claim 1 1 wherein the optical cavity is formed by two mirrors arranged to face each other along a common optical axis and being about 99.9% to 99.99% reflective to light at the wavelength used to detect the acetone concentration.

16. A method of simultaneously measuring the amount of acetone and carbon dioxide in the breath of a human or animal subject, comprising:

passing breath from the subject to an optical cavity;

supplying to the optical cavity light emitted by a broadband light source over a wavelength range encompassing: a) at least one of the carbon dioxide characteristic spectral features at 1642 nm and 1649 nm; and also b) at least one of the acetone characteristic spectral features at 1672 nm and 1689 nm;

detecting light emerging from the optical cavity, thereby to provide a signal representative of the absorption of the light by acetone and carbon dioxide in the gas in the optical cavity;

wherein the optical cavity is arranged such that the light in the optical cavity retraces some or all of its path to provide cavity-enhancement of the absorption signal, processing the absorption signal to identify therein characteristic spectral features of acetone and carbon dioxide, and calculating from the identified characteristic spectral features the amount of acetone and carbon dioxide in the breath.

17. A method according to claim 16 further comprising the step of processing the absorption signal to identify the characteristic spectral features of methane, and of calculating from the identified characteristic spectral feature the amount of methane in the breath.

18. A method according to claim 16 or 17 wherein the broadband light source emits light over a wavelength range 1630 to 1700nm.

19. A method according to claiml 6, 17 or 18 further comprising controlling a wavelength selective device to scan over the wavelength range of the characteristic spectral features to detect the transmission through the optical cavity as a function of wavelength.

20. A method according to claim 19 wherein the wavelength selective device comprises a Fourier transform infrared spectrometer.

21. A method according to claim 19 wherein the wavelength selective device comprises a tunable optical fiber filter coupling the light from the light source to the optical cavity.

22. A method according to any one of claims 16 to 21 further comprising passing light from a further light source through the gas from the breathing tube and detecting it with a further detector to measure by single pass infrared absorption the concentration of carbon dioxide in the gas from the breathing tube; receiving the signal from the further detector and calculating therefrom the carbon dioxide concentration thereby to calibrate the cavity- enhanced absorption signals.

23. A method according to claim 22 wherein the further light source and further detector measure the concentration of carbon dioxide by direct absorption.

24. A method according to claim 23 wherein only the end-tidal breath is delivered to the optical cavity triggered by detection of a maximum carbon dioxide level by the further detector.

25. A method according to any one of claims 16 to 24 comprising identifying the characteristic spectral features of acetone at around 1670nm, carbon dioxide at around 1645nm and, optionally, methane at around 1665nm.

26. A method according to any one of claims 13 to 24 comprising identifying the characteristic spectral features of acetone within the range 1660 to 1685nm, carbon dioxide within the range 1635 to 1655nm and, optionally, methane within the range 1660 to 1670nm.

27. A method according to any one of claims 16 to 26 wherein the broadband light source is a light emitting diode used in superluminescent mode or a supercontinuum laser.

28. A method according to any one of claims 16 to 27 further comprising calculating the ratio of acetone to carbon dioxide from the respective identified characteristic spectral features.