Chemical ionization interfaces

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

Xu AS, Peng LL, Havel JA, Petersen ME, Fiene JA, et al. 1996. Determination of nicotine and cotinine in human plasma by liquid chromatography-tandem mass spectrometry with atmospheric-pressure chemical ionization interface. J Chromatogr B Biomed Appl 682 249. 

Tanaka, Y., Otsuka, K., and Terabe, S. (2003). Evaluation of an atmospheric pressure chemical Ionization Interface for capillary electrophoresis-mass spectrometry. /. Pharm. Biomed. Anal. 30, 1889-1895. 

Takada, Y, Sakairi, M., and Koizumi, H. (1995). Atmospheric pressure chemical ionization interface for capillary electrophoresis mass spectrometry. Anal. Chem. 67, 1474—1476. 

Following separation on conventional gas chromatographic columns, electron-capture detector (402) has been used for the determination of the hydroxy metabolite of dimetridazole in swine muscle with good sensitivity and specificity. To confirm the presence of lasalocid residues in bovine liver, gas chromatography coupled with mass spectrometry via a chemical ionization interface (387) has been successfully applied. 

Figure 22-19 (a) Atmospheric pressure chemical ionization interface between a liquid chromatography column and a mass spectrometer. A fine aerosol Is produced by the nebulizing gas flow and the heater. The electric discharge from the corona needle creates gaseous ions from the analyte. [Adapted from E. C. Huang, T. Wachs, J. J. Conboy, and J. D. Henion, Atmospheric Pressure Ionization Mass Spectrometry," Anal. Chem. 1990,62,713A ] (b) Atmospheric pressure chemical ionization probe. [Courtesy Shimadzu Scientific Instruments, Columbia, MD.J... 

However, only a portion of the eluent actually provides ions that enter the mass spectrometer. A diagram of the atmospheric pressure chemical ionization interface is shown in figure 28. 

The introduction of the thermospray interface in the mid to late 1980s provided the first efficient LC-MS connecting technique. With the relatively new interface techniques of electrospray interface and the complementary atmospheric pressure chemical ionization interface (APCI), the full potential of the LC-MS system can now be achieved. 

One of the earliest reports of SFC interfaced with APCI was by Huang et al. [121]. The authors used a pin-hole restrictor to maintain supercritical fluid conditions in a packed-column (pcSFC) system. Results for a mixture of five corticosteoids were described with an injection of 25 ng of each of the components. The system was also amenable for capillary SFC/MS applications with minimum modification. Sadoun and Virelizier [122] reported an SFC interface with ESI in which a two-pump SFC and a packed column were used with the outlet directly interfaced to an ESI source of a quadrupole mass spectrometer. Also, 1-30% (v/v) of polar organic modifier (Me0H-H20 95 5) was added to CO2 mobile phase to help elute polar organic compounds. The setup was shown to allow analysis of polar organic compounds that were difficult to analyze with earlier implementations of SFC-MS with a chemical ionization interface. A recent review article is available on pcSFC-MS [123]. 

The criteria for optimum performance of atmospheric pressure chemical ionization and electrospray are different. For electrospray, it is important to control the flow rate and composition of the sheath-flow liquid, the mobile phase flow rate, and the dimensions of the electrospray and sheath-flow capillaries, as well as their relative position with respect to each other. Sample ionization with carbon dioxide alone does not occur in the absence of sheath-flow liquid. For atmospheric pressure chemical ionization, apart from those factors that affect the performance of the nebulizer, the flow dynamics throughout the interface and temperature in the ionization region, are the most important parameters. Moistened air is sometimes used as a makeup gas to maintain stable operation of the atmospheric pressure chemical ionization interface with HaO ions supplementing solvent ions in the reaction gas plasma. The ionization efficiency of both methods is similar to liquid chromatography. 

Applications using atmospheric pressure chemical ionization interface (APCI)... 

Mass spectrometric detection allows analysis of most nonionic surfactants without deriva-tization. Thermospray, electrospray, or atmospheric pressure chemical ionization interfaces permit direct introduction of the effluent of the LC into the MS and make the MS a very selective detector for nonionics. Quasimolecular ions are produced for each discrete compound, so that the HPLC system is not required to separate both by degree of ethoxylation and by alkyl character. A relatively simple HPLC separation, coupled with MS anal-... 

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