Chemical Ionization Mass Spectrometry (CI-MS) for Food and Flavor Analysis

Volatile organic compounds (VOCs) are found in many food and flavor products, and their presence can affect the flavor, aroma, and quality of the product. VOCs are used to analyze food and flavor products to determine their quality, as well as to identify potential contaminants. Common VOCs found in food and flavor products include ethanol, acetaldehyde, methanol, and acetone. VOCs can also be used to detect spoilage in food products, as certain VOCs are released when food begins to spoil. Additionally, VOCs can be used to detect the presence of certain additives, such as preservatives and flavorings. VOCs when present below their threshold limits contribute to a pleasant aroma and flavor of the food, however above threshold limits they are responsible for strange odor and generally degrade the texture, palatability, and appearance of the food products. Knowledge of VOC concentration and the transformations that occur in food is of great importance in the food industry. Identification and quantification of food volatile is vital in maintaining the quality of the food and fermented beverages.

The proton transfer reaction mass spectrometry (PTR-MS) technique based on Chemical ionization mass spectrometry (CI-MS) is highly sensitive and can be used to measure the concentrations of VOCs at very low levels (pptv level). PTR-MS is also used to monitor food safety and to identify adulterants in food and beverages. It is a soft ionization technique that uses a charged gas (usually H₃O⁺) to ionize the sample molecules and then measure their mass-to-charge ratio. The resulting ions are then analyzed with a mass spectrometer to identify the components present in the sample. This technique can provide a detailed analysis of the sample, including identifying the molecular weights and chemical formulas of the molecules present. It is widely used in the fields of pharmaceuticals, environmental analysis, and food science. Ionization of a sample molecule is done, usually through H₃O⁺ ions in an exothermic proton transfer reaction.

$${\rm H}_3{\rm O}^{+}+ \rm M \longrightarrow \rm MH^{+}+{\rm H}_2{\rm O}\,, \qquad \text{Proton transfer}$$ 2^nd order rate equation: $$[{\rm M}] = \frac{1}{\rm k \tau} \frac{[{\rm MH}^{+}]}{[{\rm H}_3{\rm O}^{+}]_o}\,.$$

The concentration of compound [M] can be obtained If the rate coefficient k and reaction time t of the reaction is already known. This can be achieved by obviating instrument calibration and employing the reaction kinetics of ion-molecule reactions given above. The rate coefficient of the reaction k can be calculated using dipole collision theory, in which the dipole moment and polarizability of the neutral molecule are input ingredients.

An effective proton transfer occurs for a fast and exothermic reaction, with the assumption that the reaction occurs on every collision if the proton affinity of the molecule exceeds that of the H₂O molecule by at least 35.0 kj/mol.

The technique is useful for identifying and quantifying both known and unknown components, as well as providing information on their relative abundance. This technique is especially useful in food and flavor analysis, as it can help identify and quantify VOCs present in food and beverages. H₃O⁺ is not the only ionizing agent used in PTR-MS, however many other Commonly used reagent ions in PTR-MS instruments include H₃O⁺, NH₄⁺, NO⁺, and O₂⁺. Ionization via other reagent ions is generally called NH4+-CI-MS, NO+-CI-MS, and so on. Modern PTR-MS instrument capable of switching between reagent ions is called selected reagent ion (SRI) mass spectrometry. CI-MS is soft ionization, means it does not cause fragmentation of the analyte molecules. This makes CI-MS an ideal technique for the analysis of complex mixtures, such as those found in food and flavor samples. Another advantage of CI-MS is that it is a highly sensitive technique, and provide rapid analysis of compounds in complex mixtures.

Despite its advantages, CI-MS has several limitations for food and flavor analysis. CI-MS may cause certain compounds to evaporate, and it is not suitable for the analysis of thermally labile compounds, as the ionization process can cause the compounds to decompose. CI-MS is not suitable for the analysis of compounds with low ionization potentials, as the ionization process may not be efficient. CI-MS is not suitable for the analysis of compounds with high molecular weights. These limitations can be overcome by designing and modifying the PTR-MS instrument as per the applications and the requirements.

CI-MS has been increasingly used in food and flavor analysis. CI-MS in association with TOF has outlooks to provide a comprehensive understanding of the chemical composition of food and flavor samples, including the identification of including organic and inorganic compounds, as well as the quantification of these compounds. CI-MS can find applications in the development of new food and flavor products and maintaining product quality to the highest level.