The application of infrared spectroscopy to breath CO2 isotope ratio measurements and the risk of spurious results

Hollow waveguide integrated laser spectrometer for 13CO2/12CO2 analysis

Using hollow waveguide hybrid optical integration, a miniaturized mid-infrared laser absorption spectrometer for ¹³CO₂/¹²CO₂ isotopologue ratio analysis is presented. The laser analyzer described focuses on applications where samples contain a few percent of CO₂, such as breath analysis and characterization of geo-carbon fluxes, where miniaturization facilitates deployment. As part of the spectrometer design, hollow waveguide mode coupling and propagation is analyzed to inform the arrangement of the integrated optical system. The encapsulated optical system of the spectrometer occupies a volume of 158 × 60 × 30 mm³ and requires a low sample volume (56 µL) for analysis, while integrating a quantum cascade laser, coupling lens, hollow waveguide cell and optical detector into a single copper alloy substrate. The isotopic analyzer performance is characterized through robust error propagation analysis, from spectral inversion to calibration errors. The analyzer achieves a precision of 0.2‰ in 500 s integration. A stability time greater than 500 s was established to allow two-point calibration. The accuracy achieved is 1.5‰, including a contribution of 0.7‰ from calibrant gases that can be addressed with improved calibration mixtures.

Major influence of oxygen supply on 13CO2:12CO2 ratio measurement by nondispersive isotope-selective infrared spectroscopy

BACKGROUND

The nondispersive isotope-selective infrared spectroscopy (NDIRS) is a valid method for the measurement of the 13CO2:12CO2 ratio in breath samples. Methodical influences have to be considered to obtain valid results.

AIM

To evaluate the effect of oxygen supply to patients on the measurement of 13C:12C ratio in breath samples by NDIRS.

METHODS

Breath samples of 26 healthy volunteers were taken before, immediately after, and 5 minutes after inhalation of 100% oxygen via a continuous positive air pressure (CPAP) mask. Analysis of breath samples was performed by NDIRS.

RESULTS

Delta per thousand before oxygen inhalation was -25.8 +/- 0.2. Immediately after 5 minutes of 100% oxygen inhalation, delta per thousand increased to -14.8 +/- 0.5 (delta over baseline [DOB] 11.0 +/- 0.4) and after additional 5 minutes of room air inhalation, delta per thousand normalized to -25.6 +/- 0.2 (DOB 0.2 +/- 0.1).

CONCLUSIONS

Oxygen supply to patients and, therefore, changes in gas composition in breath samples clearly influence 13CO2 measurement by NDIRS. This has to be taken into account in the clinical setting. Thus, oxygen supply during measurement of exhaled 13CO2 by NDIRS has to be avoided or maintained at a strictly constant level.

Mass spectrometric studies on brain metabolism, using stable isotopes

In fields related to biomedicine, mass spectrometry has been applied to metabolism research and chemical structural analysis. The introduction of stable isotopes has advanced research related to in vivo metabolism. Stable-isotope labeling combined with mass spectrometry appears to be a superior method for the metabolism studies, because it compensates for the shortcomings of conventional techniques that use radioisotopes. Biomolecules labeled with stable isotopes have provided solid evidence of their metabolic pathways. Labeled large molecules, however, cannot homogeneously mix in vivo with the corresponding endogenous pools. To overcome that problem, small tracers labeled with stable isotopes have been applied to in vivo studies because they can diffuse and attain a homogeneous distribution throughout the inter- and intracellular spaces. In particular, D(2)O-labeling methods have been used for studies of the metabolism in different organs, including the brain, which is isolated from other extraneural organs by the blood-brain barrier (BBB). Cellular components, such as lipids, carbohydrates, proteins, and DNA, can be endogenously and concurrently labeled with deuterium, and their metabolic fluxes examined by mass spectrometry. Application of the D(2)O-labeling method to the measurements of lipid metabolism and membrane turnover in the brain is described, and the potential advantages of this method are discussed in this review. This methodology also appears to have the potential to be applied to dynamic and functional metabolomics.

A quantitative evaluation of spurious results in the infrared spectroscopic measurement of CO2 isotope ratios

The possible generation of spurious results, arising from the application of infrared spectroscopic techniques to the measurement of carbon isotope ratios in breath, due to coincident absorption bands has been re-examined. An earlier investigation, which approached the problem qualitatively, fulfilled its aspirations in providing an unambiguous assurance that 13C16O2/12C16O2 ratios can be confidently measured for isotopic breath tests using instruments based on infrared absorption. Although this conclusion still stands, subsequent quantitative investigation has revealed an important exception that necessitates a strict adherence to sample collection protocol. The results show that concentrations and decay rates of the coincident breath trace compounds acetonitrile and carbon monoxide, found in the breath sample of a heavy smoker, can produce spurious results. Hence, findings from this investigation justify the concern that breath trace compounds present a risk to the accurate measurement of carbon isotope ratios in breath when using broadband, non-dispersive, ground state absorption infrared spectroscopy. It provides recommendations on the length of smoking abstention required to avoid generation of spurious results and also reaffirms, through quantitative argument, the validity of using infrared absorption spectroscopy to measure CO2 isotope ratios in breath.

Evaluation of spurious results in the infrared measurement of CO2 isotope ratios due to spectral effects: a computer simulation study

The application of infrared spectroscopy to the measurement of carbon isotope ratio breath tests is a promising alternative to conventional techniques, offering relative simplicity and lower costs. However, when designing such an instrument one should be conscious of several spectral effects that may be misinterpreted as changes in the isotope concentration and which therefore lead to spurious results. Through a series of computer simulations which model the behaviour of the CO2 absorption spectrum, the risk these effects pose to reliable measurement of 13CO2/12CO2 ratios and the measures required to eliminate them are evaluated. The computer model provides a flexible high-resolution spectrum of the four main isotopomer fundamental transitions and fifteen of their most significant hotband transitions. It is demonstrated that the infrared source, infrared windows and breath sample itself all exhibit strong temperature induced errors but pressure effects do not produce significant errors. We conclude that for reliable measurement of 13CO2/12CO2 ratios using infrared spectroscopy no pressure controls are required. window effects are eliminated using windows wedged at a minimum angle of 0.8-2.2 mrad, depending on the material, and the temperature sensitivity of source and gas cells necessitates stabilization to an accuracy of at least 0.2 K.