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The electrospray ion-producing inlet system is probably the most commonly used interface handling liquid sample solutions. An early report describing this interface was that by Whitehouse et al. , The explanation given for ion formation is as follows. Electrospray ionization occurs as the result of strong electric field acting on the surface of a sample solution as it is sprayed into a dry gas such as nitrogen . This process produces a cloud of charged droplets that rapidly evaporate and, as a consequence. shrink in size. The accompanying increase in charge density that results from the decrease in volume, surface area, and radius of curvature of the droplets causes very strong electric fields to be formed. As each drop continues to shrink, the electric fields become sufficiently strong to cause the droplets to explode, producing ions. Due to the strength of the electric field, and the large number of ions that are produced, many of the ions that are formed will contain multiple charges. It will be seen later that as the mass spectrometer discriminates on the basis of the m/z value of the ion, multiple ion production will, in effect, extend the mass range of the spectrometer.
A diagram of the Hewlett-Packard Electrospray Ionization LC/MS interface is shown in figure 23.
Courtesy of the Hewlett-Packard Company.
The sample solution is mixed with a nebulizing gas and the spray jet directed onto a disk target. The target is set at a high potential relative to that of the spray nozzle. In the center of the target is a pinhole entry into the source and the jet is directed slightly to the side of this aperture. The fine droplets at the edge of the spray are drawn into a chamber (held at a reduced pressure) through the pinhole aperture. Inside the chamber, the droplets are entrained in a stream of hot nitrogen gas. The hot gas rapidly evaporates the solvent, producing ions in the process described above. The core of the jet is skimmed by a set of conical screens to selectively remove the drying gas so that the ions pass directly into an ion-optical arrangement. The ion optical system directs the ions, so formed, into the mass analyzer. In this system, ions with multiple charges are produced as a result of a molecule becoming associated with more than one proton. As a result, an ion of molecular weight 1000, carrying three charges, will appear at an m/z value of 333.3 on the Spectrum where an ion of mass 333.3 and unit charge would normally appear. It is clear that the production of multiple charged ions, in effect, increases the mass range of the spectrometer. As an example of the use of the ion spray used as an LC/MS interface, the total ion current chromatogram of a sample from the tryptic digest of lysozyme is shown in figure 24.
Courtesy of the Hewlett Packard Company.
Courtesy of the Hewlett-Packard Company.
Figure 25 displays three peaks at m/z values of 554.1, 876.0 and 1106.8. It is seen from figure 25 that the peak was complex and all the components were not resolved from one and other. The question arises did all three peaks originated from the same solute, or did any pair of the peaks result from multiple charges on the same molecule. A program is included in the Hewlett-Packard data handling software that tests the inter-relationship between the individual peak masses in the mass Spectrum. In practice the software determines whether the molecular weights of any pair or group of peaks are linearly related to each other. If the respective peaks increase or decrease proportionally with one another, then they must originate from the same parent molecule, and probably represent multiple charges on the same molecule. The peaks in figure 25 were tested in this manner, and the results from the correlation program are shown in figure 26.
Courtesy of the Hewlett-Packard Company.
The data demonstrated that the peak at 876.0 m/z was not related to the other two and, furthermore, the peaks at 554.1 and 1106.9 were doubly and singly charged species of the same ion.
e.g. (1106.0-1(H+))/2 + 2((H+) = 554.95 cf 554.1 (from Spectrum)
Unfortunately, the system, in some forms, does have some certain disadvantages. The electrospray system may not work well with solvents having high water content, but certain interface designs work better than others with aqueous mixtures. Some electrospray nebulizers are designed specifically to cope with solvents of high water content. It follows that the electrospray system must be chosen to suit the particular analyses for which it is to be used. Its area of application has increased rapidly over the past years, and some particular applications that illustrate its versatile capabilities will be described.
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.