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THE SPECTRO-PAEDIA

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The Particle Beam Interface

The particle beam interface involves nebulizing the sample solution, and the solvent free particles of solute are then passed into the ionization chamber of the mass spectrometer. Electron impact or Chemical Ionization spectra can be produced, and the system has been given name the of the somewhat vainglorious title of the monodisperse aerosol generation sampling interface. In addition, it has been endowed with the very pretentious acronym (MAGIC). Willoughby and Browner [37] and Winkler et al. [38] were two of the early groups working on this interface, which consists of two parts, the aerosol generator and the momentum separator. A diagram of the aerosol generator developed by Winkler et al. is shown in figure 59.

Figure 59. Cross-Sectional View of the Aerosol Generator.

Nebulization takes place at the end of a fused silica tube, 25 μm I.D., similar tubing employed in capillary gas chromatography. The short length of fused silica tubing is connected to the conduit tube from the LC column (220 μm I.D.) by a low dead volume fused silica tubing connector. The liquid jet forms at the end of the small-diameter tubing, and was found to be very efficient and seldom clogged. Nevertheless, precautions must be taken to ensure that no solid particles are carried into the conduit from the column. A diagram of the second part of the interface, the momentum separator, is shown in figure 60.

Figure 60. A Cross-sectional View of the Momentum Separator

In previous work [39[ [40] the most important sources of sample loss from this type of interface were found to be particle sedimentation, poor nozzle/skimmer alignment and loss due to turbulence. Consequently, the skimmers were designed to provide an undisturbed path from their point of generation, until they were in the ion source of the mass spectrometer. The body of the momentum separator was made of stainless steel and the nozzles and skimmers were machined from 6010 grade aluminum. The separation between the nozzle and the first skimmer was about 10 mm and was adjustable by the use of shims. The first skimmer had a 100˚ exterior angle and a 95˚ interior angle with a 0.5 mm orifice. The second skimmer had a 45˚ exterior angle and a 30˚ interior angle and a 1 mm orifice. The distance between the two skimmers was also about 10 mm and was also adjustable. Both interlock chambers were pumped with mechanical vane (hot oil) pumps, having a capacity of 21.6 m3/h.

The interface was found to be easy to operate, the skimmer provided little turbulence and it had high transport efficiency. The device was tested by using the normal phase separation of the cis and trans isomers of retinol acetate. The separation obtained is shown in figure 61.

The total mass of sample injected was 50 ng and the chromatogram has not been smoothed in any way. It is seen that the separation does not appear to be impaired by the interface and the sensitivity that is achieved, compares well with other types of interface. Another example of the use of the interface in the separation of some aliphatic fatty acids shown in figure 61.

Figure 61. The Total Ion Chromatogram of the Separation of a retinol Mixture


Figure 62. The Total Ion Current Chromatogram of Some Mixed Fatty Acids

In this example a reversed phase column was employed with gradient development from 80% methanol and 20% water (containing 3% acetic acid) and 100% methanol . It is seen that there is some indication of tailing, although whether this arises from the column or the interface is not clear. It is also seen that the nebulizer functions well with aqueous solution containing 20% of water. It is not reported whether or not higher water contents work equally as well. The electron impact Spectrum obtained for lauric acid is shown in figure 63.


Figure 63. Electron Impact Spectra of Lauric Acid.

The EI Spectrum exhibits a significant molecular ion and agrees completely with the reference spectra of that compound. It should be noted that the Spectrum is presented without background subtraction. There is no evidence of thermal degradation in the interface, despite the relatively high temperature of the ion source (240˚C).


Cappiello and Famiglini [41] extended the work and designed a micro-flow particle beam interface. They started with the Particle Beam Interface manufactured by Hewlett-Packard, (similar in form to the device described previously) and modified the nebulizer to operate at lower flow rates. The basic design is shown in figure 64. The end portion of the coaxial tubing, carrying the helium, was widened to keep any slow-growing liquid droplets away from the internal gas conduit wall. This enlargement is sharply reduced at the capillary tip, thus keeping the contact surface between the capillary and the wall of the gas conduit to a minimum.

 

Figure 64.The Micro-flow Nebulizer

The unique character of this device is its capacity to nebulize very small liquid flows. In the absence of a helium flow, the liquid is not forced out by the following liquid but remains as an expanding droplet. When the helium flow is commenced, as soon as the droplet size exceeds the diameter of the tip, the energy of the gas breaks the surface tension forces and droplets are formed. An example of the performance of the interface is shown in figure 65.

Figure 65. Ion Chromatograms of caffeine and Testosterone

The peaks in Figure 65 represent the injection of three samples of 40 pg of caffeine (m/z of monitoring ion of 194) and three samples of 600 pg of Testosterone (m/z of monitoring ion 124). The flow rate was 2 μl/min. In general, the authors claimed very little nebulizer contamination over long periods of time, better overall performance over a wide range of mobile phase composition, efficient nebulization over a wide range of flow rates and more simple operational procedures.

 

 

About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.

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