How long may a breath sample be stored for at −80 °C? A study of the stability of volatile organic compounds trapped onto a mixed Tenax:Carbograph trap adsorbent bed from exhaled breath

Abstract

Thermal desorption is used extensively in exhaled breath volatile organic compound (VOC) analysis, and it is often necessary to store the adsorbent tube samples before analysis. The possible introduction of storage artefacts is an important potential confounding factor in the development of standard methodologies for breath sampling and analysis. The stability of VOCs trapped from breath samples onto a dual bed Tenax^® TA:Carbograph adsorbent tube and stored −80°C was studied over 12.5 month. 25 samples were collected from a single male participant over 3 h and then stored at −80 °C. Randomly selected adsorbent tubes were subsequent analysed by thermal desorption-gas chromatography-mass spectrometry at 5 times points throughout the 12.5 month of the study. Toluene-d₈, decane-d₂₂ and hexadecane-d₃₄ internal standards were used to manage the instrument variability throughout the duration of the study. A breath-matrix consisting of 161 endogenous and 423 exogenous VOC was created. Iterative orthogonal partial least squared discriminant analysis (OPLS-DA) and principal components analysis (PCA) indicated that it was not possible to detect storage artefacts at 1.5 month storage. By 6 month storage artefacts were discernible with significant changes observed for 27% of the recovered VOC. Endogenous VOC were observed to be more susceptible to storage. A paired two-tailed t-test on the endogenous compounds indicated that the maximum storage duration under these conditions was 1.5 month with 94% of the VOCs stable. This study indicates that a prudent approach is best adopted for the storage of adsorbent samples; storage times should be minimised, and storage time examined as a possible discriminatory factor in multivariate analysis.