Standartnye Obrazcy (May 2017)
Elemental analysis of geological materials by a glow discharge mass spectrometry
Abstract
Advances of elemental analysis in geology are defined by the development and improvement ofphysical, instrumental methods of chemical analysis. New instrumental methods - emission spectrometry and mass spectrometry with inductively coupled plasma (ICP) sources are the most popular in the area of elemental analysis in recent years. These methods have a very high sensitivity and wide range of elements analyzed (up to 70 items at a time). Both of these methods have been developed and demonstrate their high analytical performance only when analyzing liquids. Consequently, when using these methods in geology, solid samples must be completely transferred into liquid. This complicates the analysis, especially in the analysis of sparingly soluble objects. In some cases, analyzed geological objects are generally insoluble in acids and other solvents (e.g. such as black shales are not dissolved till the end in any acids). Naturally, there is a need to develop an analytical method, wherein the step of dissolution of the sample can be avoided. Such instrumental methods exist but are not used because of low sensitivity. Method of glow discharge mass spectrometry has the greatest potential today, but its use is hampered by lack of a recognized certified ion source capable of converting analyzed (in most cases - not conductive) geological sample into low-temperature plasma. In this paper, the glow discharge source on the basis of the hollow cathode is used as an external source. The hollow cathode has been long and successfully used in analytical practice in emission spectral analysis. However, for use in mass spectrometry, its design has been considerably modified. The analyzed sample, which was formerly in the form of powder or fine particles located at the bottom of the hollow cathode, in a new cathode is installed as a rod along the axis of the inner cathode cavity - it is the emitter of sputtered neutral particles. Hollow cathode source is set on the high resolution mass - spectrometer of ELEMENT 2 type instead of the ICP source. In sample preparation a geological sample is transformed into ultrafine state (particle size of about one micron), this thin layer of powder is applied to the emitter. Particles are held on the emitter (cathode) by van der Waals and electrostatic forces, are bombarded with argon ions, they can be also effected by electrostatic forces - between the emitter and the anode voltage of several hundred volts is applied. As a result, the charged ultra-disperse particles break away from the - emitter and diffuse into the glow discharge. Getting into the negative glow particles are heated by the electron current, then evaporate and ionize. The low-temperature plasma is formed. Mass spectral line of each element consists of many individual spikes lasting 10-30 msec. Each spike is the result of evaporation and ionization of individual particles. Mass spectral lines can be smoothed in the summation series of mass spectra. The proposed method allows direct analysis of geological samples with sensitivity ppb or less without dissolving the sample, without concentration of impurities.
