The use of atmospheric pressure ionization at -78 °C for the analysis of a thermally unstable compound, 2,5dimethylene-2,5-dihydrothiophene, dissolved in methylene chloride is demonstrated with a home-built lowtemperature electrospray source connected to a quadruple mass analyzer. To prevent the analyte molecule from polymerizing at elevated temperatures, the sample solution was maintained at -78 °C in the dry-ice-cooled sample injection and sample loop units. The solution was delivered to the spray needle by compressing the nitrogen gas in a syringe which was connected to the sample loop by a Teflon tube. At such low temperature, a strong ion signal was obtained from the analyte molecule as a result of corona discharge ionization.Electrospray ionization (ESI) mass spectrometry has been proven to be very useful for analyzing not only large biomolecules 1,2 but also small organic and inorganic compounds. [3][4][5][6][7] Since most organic and inorganic compounds do not dissolve well in water, organic solvents such as n-hexane, benzene, chloroform, methylene chloride, acetonitrile, and methanol are used alone or mixed together with water for the analysis of such compounds by ESI. [8][9][10][11][12][13][14][15] Most organic solvents freeze at temperatures far below 0 °C. For example, the melting point of methylene chloride (CH 2 -

This study detected the extremely reactive carbenoid, 1,2,3-hexatriene-5-one (HTO), by low-temperature atmospheric pressure ionization (API) mass spectrometry. During the analysis the sample solution was maintained at À78°C in the API source to prevent the analyte molecule from polymerizing. At this low temperature a strong protonated HTO signal was detected. At room temperature the compound polymerized rapidly and the mass spectrum was dominated by dimer ions. Signals due to protonated HTO-(H 2 O) 2 adducts were also detected, and can possibly be attributed to trace water in the organic solvent, or to ionization of the moisture as in atmospheric pressure chemical ionization. # 1998 John Wiley & Sons, Ltd. Received 8 May 1998; Revised 27 May 1998; Accepted 28 May 1998 Detecting thermally unstable compounds by mass spectrometry is relatively difficult owing to the fact that conventional ionization techniques used in mass spectrometry cannot efficiently desorb or generate ions at the subambient temperatures at which the unstable compounds (commonly dissolved in dry organic solvents) are usually stored. In atmospheric pressure ionization (API), ions can be formed at atmospheric pressure without heating.1,2 Because the freezing points of most organic solvents are markedly below 0°C, using such solvents would allow us to obtain the mass spectra at low temperature.3 Our previous work has successfully used low-temperature API mass spectrometry to detect 2,5-dimethylene-2,5-dihydrothiophene, a volatile and reactive compound.3 Oxaphosphin, an extremely airsensitive and thermally unstable intermediate in the Wittig reaction, can also be detected in a similar manner.4 A similar result has also been reported by another research group. 5Carbenoids play an increasingly important role in a variety of synthetic methods, particularly the intramolecular versions.6,7 Many methods are available for preparing carbenes. Conventionally, carbenes are synthesized by photolysis, thermolysis, or metal-ion catalysis in the solution.6-8 However, upon formation the product is readily prone to attack by a reagent or organic solvent molecule. The development of flash vacuum pyrolysis (FVP) has eliminated such an attack. [9][10][11] In FVP, the carbenoid is synthesized in the gas phase by flash pyrolysis under reduced pressure. The products from FVP are initially coldtrapped by liquid nitrogen and then transferred into an inert organic solvent and stored at À78°C (in a dry-ice bath) to decrease the reactivity of the compound.In this study, 1,2,3-hexatriene-5-one (HTO) was synthesized and stored in CH 2 Cl 2 at À78°C.12 Owing to the presence of three consecutive double bonds in the molecule, HTO molecules rapidly polymerize at an elevated temperature. In particular, HTO molecules polymerized and condensed in electron impact ionization and chemical ionization sources, and therefore could not be detected in this way. HTO is also too volatile to be analysed by FAB or LSIMS. Currently, HTO monomer can only be structurally characterized by low-temp...