Murray V. Johnston

Post source pulse focusing: a simple method to achieve improved resolution in a time-of-flight mass spectrometer

Abstract The theoretical basis for a simple method to improve resolution in a linear time-of-flight mass spectrometer is presented. This method, post-source pulse focusing, involves the application of a focusing voltage pulse to a short field-free region located after the source and acceleration regions. The pulse is timed so that it occurs after the ions of interest have entered this region. Instrumental requirements are identical to those for time-lag focusing, but the disadvantages of time-lag focusing are eliminated. For a typical linear time-of-flight mass analyzer design, resolution improvements of a factor of three or more should be easily attainable. Theoretical considerations also suggest this technique will be particularly valuable for surface desorption/ionization and heated sample introduction where much greater resolution enhancements are predicted. In addition, various instrumental alterations are discussed which suggest possibilities for achieving resolutions in excess of 10 000 for a linear time-of-flight mass spectrometer.

Resolution enhancement in linear time-of-flight mass spectrometry by post-source pulse focusing

Abstract Resolution enhancement in linear time-of-flight mass spectrometry is demonstrated for a new technique, post-source pulse focusing. The relatively simple procedure followed to perform pulse focusing is described including a discussion of the need to choose proper experimental conditions. A resolving power of ca. 1500 for m/z 228 is demonstrated. Finally, equations are presented which allow the electric field conditions necessary for optimum pulse focusing to be estimated for instruments with a wide range of instrumental configurations.

Sheath Flow Jet Expansions of Liquids and Supercritical Fluids

A method is described for performing supersonic jet spectroscopy of compounds dissolved in a liquid or supercritical fluid. The fluid is expanded through a capillary restrictor into a concentric sheath gas flow of argon. The resulting mixture undergoes further expansion through a 200-μm tapered nozzle. Laser-induced fluorescence of naphthalene is used to probe the cooling and focusing properties of the nozzle. A rotational temperature of 12 K has been obtained for a variety of fluid carriers. No fluorescence enhancement is observed along the centerline of the expansion relative to the results with a conventional free jet expansion.

Multiphoton ionization of transition-metal tetraphenylporphines. Metal complexes which display molecular ionization

Abstract The tetraphenylporphine (TPP) complexes of cobalt, nickel, and copper were studied by multiphoton ionization mass spectrometry. These complexes were found to exhibit molecular ionization without ligand predissociation using radiation at 266 nm. The laser power dependences for Cu(TPP) + and Cu + ion formation show that Cu(TPP) + is not formed by an ion/molecule reaction with Cu + . The ionization behavior of these complexes is discussed in terms of a gas-phase macrocyclic ligand effect which inhibits efficient molecular dissociation prior to ionization.

Fourier Transform Fluorescence Excitation Spectroscopy in a Supersonic Jet

Laser analytical methods are usually preferred for supersonic jet spectroscopy since a high photon flux across a narrow spectral region is required for sensitive detection. Lamp sources have been used for fluorescence excitation, but their performance has been poor due to low photon fluxes. The major problems associated with lamp excitation lie not in the radiant output of the lamp but in the ability to transfer the radiation to the experiment. For example, commercial 1000-W xenon arc lamp systems can deliver up to 40 mW/nm in a

Sheath Flow Focusing in Supersonic Jet Spectroscopy

The mechanism of cooling in sheath-flow-focused supersonic jet expansions is examined. Cooling is found to be strongly influenced by the sheath gas flow properties but independent of the carrier gas in the sample stream. These results indicate that considerable turbulence and mixing between the sheath and sample gases occur downstream from the orifice. However, mixing cannot be complete, since, relative to results with a conventional jet expansion, a substantial enhancement of analyte is obtained along the centerline of a sheath-flow-focused jet expansion. Spectral broadening at “high” analyte mass flow rates within the sample stream is found to arise from inefficient cooling. There are limits to both how large and how small the nozzle orifice can be. Small orifices result in spectral broadening, even at very low analyte mass flow rates. Large orifices may have Reynolds numbers sufficient to cause turbulent flow, which degrades the focusing effect. The optimum nozzle geometry and gas flow conditions for sheath-flow-focused jet expansions are discussed.

Chromatographic interfaces for supersonic jet spectroscopy

Abstract Until recently, applications of supersonic jet spectroscopy to chemical analysis have been plagued by relatively poor detection limits and the lack of suitable interfaces to standard chromatographic techniques. Supersonic jet nozzles based upon new technologies have been developed which permit low picogram detection limits to be obtained and provide convenient interfaces for capillary gas chromatography. Extension of these methods to liquid and supercritical fluid chromatography may also be possible.