Absolute Laser Spectrometric Amount Fraction Measurements: Impact to Traceable Breath Gas Analysis

Abstract

Laser spectroscopic techniques such as tunable diode laser absorption spectroscopy (TDLAS), quantum cascade laser absorption spectroscopy (QCLAS) and cavity ring down spectroscopy (CDRS) have been shown to be capable of performing absolute amount fraction measurements of gas species such as CO2 and CO. These techniques have been proven to be very sensitive, selective and have real-time responses. The aim of this work was to: perform absolute amount fraction measurements of breath gas species using TDLAS, QCLAS and CRDS, reliably quantify breath gas species, address metrological data quality objectives, i.e. uncertainty and traceability issues, as well as define and reduce the uncertainty of amount fraction results from the typical 10 % to levels suitable to fit breath analysis purposes, 5 % and below. Thus, aiming at traceable amount fraction results, measurements have been performed using TDLAS, QCLAS and CRDS based on the absolute method TILSAM. GUM compliant uncertainty budgets for spectrometric amount fraction results were developed. TDLAS in combination with single-pass and multipass gas cells has been used to perform absolute measurements of the CO2 amount fractions. To check the TDL spectrometer for its feasibility for absolute amount fraction measurements and to be operated on the basis of the TILSAM method, gravimetric gas mixtures of CO2 in the range of 20 to 60 mmol•mol-1 were quantified. At the 50 mmol•mol-1 level (exhaled breath level) the relative standard uncertainties of the spectrometric CO2 amount fraction results are in the ±0.7 % range. The intra-pulse mode QCLAS has been utilized to measure absolute CO amount fractions at the 100 µmol•mol-1 and 1000 µmol•mol-1 levels based on the TILSAM method. Although, not at the exhaled breath level of 1-3 µmol•mol-1, the feasibility of intra-pulse mode QCLAS for CO measurements has been shown. The standard uncertainty of the CO amount fraction results, limited by the uncertainties of the line strengths used which were in the range of 2-5 % relative, are in the range of ±2.3 % relative. A CRDS spectrometer has been used to carry out absolute CO2 amount fraction measurements referring to the TILSAM method. The spectrometric results were in good agreement with the respective gravimetric reference values. The standard uncertainties of the CO2 amount fraction results, also limited by the uncertainty of the used line strength, were in the range of ±2.1 % relative. In a separate measurement, it has been shown in coperation with other partners that CO amount fractions in the nmol•mol-1 levels can be quantified using CRDS. It has been found that the TILSAM method suffers from the unavailability of traceable line data. Thus, line strengths and broadening coefficients of CO2 in the ro-vibrational band around 2 µm have been measured. The derived line data are in agreement to a high degree with published data. Compared to literature, improved GUM compliant standard uncertainties in the ±0.6 % range for the measured line strengths have been reported. The validity of the absolute method, TILSAM, has been further proven in a measurement campaign. The TDLAS-based quantifications were performed on CO2 at the 300 and 500 µmol•mol-1 level. The spectrometric results from the different laboratories were in good agreement, expressed by a degree of equivalence being in the 1 % range, with the respective comparison reference values (CRVs).