Analytical chemistry of carbonyl compounds in indoor air

Carbonyl compounds are ubiquitous in outdoor and indoor air. Due to the high electronegativity of the oxygen atom, they are polar in nature and the CO group opens possibilities for many types of chemical reactions. Their physical and chemical properties are additionally influenced by substituents and conjugated double bonds. The concentration ranges are also highly variable. Formaldehyde can reach 100 ppb or more in indoor air, but reaction products such as 4-oxopentanal (4-OPA) are in the lower ppb range or even below 1 ppb. Another point concerns the dynamics of carbonyls. When examining the emission of formaldehyde in test chambers, an equilibrium concentration is usually established, so that changes over time can be neglected during the measurement. On the other hand, many substances and scenarios are subject to strong fluctuations in concentration over short time periods. The analysis is also made more difficult by the fact that different methods are often required for saturated carbonyls, unsaturated carbonyls and dicarbonyls. This work focuses on aprotic carbonyl compounds such as aldehydes, ketones, lactams and pyrones with relevance for the indoor environment that do not contain any other reactive groups. The range of interesting compounds has grown significantly in recent years, notably through the derivation of health-based guide values, as well as through investigations into new products, human activities and human emissions from the skin and respiratory gas. Classical and modern analysis methods are discussed, which can be considered for the respective research question. Many small molecules require derivatization as a first step, followed by separation via gas chromatography or HPLC. Substance-specific detection without chromatographic separation is routinely used for formaldehyde. With some limitations, the identification of carbonyls in multicomponent mixtures is possible using online mass spectrometry. In particular, proton-transfer-reaction mass spectrometry (PTR-MS) has established as a method with high sensitivity and high time resolution.