Gas Chromatography-Microchip Atmospheric Pressure Chemical Ionization-Mass Spectrometry Pekka O 2 stman, † Laura Luosuja 1 rvi, ‡ Markus Haapala, † Kestas Grigoras, § Raimo A. Ketola, † Tapio Kotiaho, ‡ Sami Franssila, § and Risto Kostiainen* ,† Department of Pharmaceutical Chemistry, Faculty of Pharmacy, P.O. Box 56, FIN-00014 University of Helsinki, Finland, Laboratory of Analytical Chemistry, Department of Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland, and Microelectronics Centre, Helsinki University of Technology, P.O. Box 3500, FIN-02015 HUT, Finland An atmospheric pressure chemical ionization (APCI) microchip is presented for combining a gas chromato- graph (GC) to a mass spectrometer (MS). The chip includes capillary insertion channel, stopper, vaporizer channel, nozzle and nebulizer gas inlet fabricated on the silicon wafer, and a platinum heater sputtered on a glass wafer. These two wafers are joined by anodic bonding creating a two-dimensional version of an APCI microchip. The sample from GC is directed via heated transfer line capillary to the vaporizer channel of the APCI chip. The etched nozzle forms narrow sample plume, which is ionized by an external corona discharge needle, and the ions are analyzed by a mass spectrometer. The GC- microchip APCI-MS combination provides an efficient method for qualitative and quantitative analysis. The spectra produced by microchip APCI show intensive protonated molecule and some fragmentation products as in classical chemical ionization for structure elucidation. In quantitative analysis the GC-microchip APCI-MS showed good linearity (r 2 ) 0.9989) and repeatability (relative standard deviation 4.4%). The limits of detection with signal-to-noise ratio of three were between 0.5 and 2 μmol/L with MS mode using selected ion monitoring and 0.05 μmol/L with MS/MS using multiple reaction moni- toring. Gas chromatography combined with mass spectrometry (GC/ MS) is a powerful analytical technique for volatile and thermally stable analytes, as demonstrated by the great number of applica- tions in, for example, environmental, 1 drug, 2 and food analysis. 3 In GC/MS, the separated analytes are traditionally ionized in a vacuum prior to detection by MS. The most common ionization method is electron ionization (EI), which provides high ionization efficiency with reproducible and characteristic mass spectra. Positive and negative ion chemical ionization (CI) are also commonly used in GC/MS, 4 particularly in determination of molecular weights, since fragmentation is considerably less with CI than with EI. Modern liquid chromatography-mass spectrometry (LC/MS) is based on atmospheric pressure ionization (API) techniques: electrospray ionization (ESI), 5,6 atmospheric pressure chemical ionization (APCI), 7,8 and recently introduced atmospheric pressure photoionization (APPI). 9,10 These techniques provide efficient ionization for a wide variety of molecules. ESI is an excellent method for ionic and polar compounds and can be applied for small and large molecules such as peptides and proteins, whereas APCI and especially APPI are better ionization techniques for less polar and neutral small molecules, but they are not suitable for large biomolecules. These methods have become very popular since they are able to ionize molecules that are not amenable to GC/MS. GC/MS and LC/MS have been separate instruments, since in GC/MS compounds are ionized at vacuum (EI and CI) and in LC/MS at atmospheric pressure (ESI, APCI, APPI). 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