Exhaled breath VOCs are often collected and stored in sorbent tubes before thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) analysis. Information about the stability of VOCs during storage is needed to account for potential artifacts and monitor for losses. Additionally, information about the stability of VOC standards in solution is required to assess their performance as quality control and internal standards. We evaluated the stability of a standard mixture of 42 VOCs in dual-sorbent tubes containing Tenax® TA and Carbotrap 1TD over 60 days at commonly used storage conditions: room temperature (~21 °C), 4 °C, and -80 °C. The same 42 VOCs were also evaluated for their stability in methanol over 60 days while stored at -20 °C. All samples were analyzed using TD-GC-MS. During storage, most VOCs were stable on sorbent after 60 days: 36/42 (86%), 39/42 (93%), and 41/42 (98%) had not statistically changed for room temperature, 4 °C and -80 °C, respectively, based on Spearman rank correlation coefficients and linear regression analysis. The isotopically labelled VOCs tested here are good candidates for use as internal standards for pre-analysis or storage. Degradation of VOCs in solution was apparent after 60 days: 27/42 (64%) of VOCs had statistically decreased. The total VOC mixture had dropped to 90% of its original intensity after ~22 days and a subset of VOCs typically used as internal standards dropped to 90% in ~16 days. Analysts using similar mixtures should make a fresh solution at least every two weeks to ensure analytical accuracy. This study provides important insights into storage practices for both sorbent tubes and standard solutions, guiding analysts toward improved reliability and accuracy in exhaled breath analysis. 
.
Read full abstract