Positive rays

In its simplest form, a mass spectrometer is an instmment that measures the mass-to-charge ratios ml of ions formed when a sample is ionized by one of a number of different ionization methods (1). If some of the sample molecules are singly ionized and reach the ion detector without fragmenting, then the ml ratio of these ions gives a direct measurement of the molecular weight. The first instmment for positive ray analysis was built by Thompson (2) in 1913 to show the existence of isotopic forms of the stable elements. Later, mass spectrometers were used for precision measurements of ionic mass and abundances (3,4). 

Historical That positive rays could be deflected in electric and magnetic fields was shown as early as 1898 by Wien, but it was not until 1912 that what was to become the forerunner of the modem mass spectrometers was built by JJ. Thompson, who became known as the father of mass spectrometry. The existence of two isotopes of neon (m/e 20 and 22) was demonstrated by Thompson with this instrument. The discovery of stable isotopes of elements has been generally considered the... 

W. Bleakney. A New Method of Positive Ray Analysis and Its Application to the Measurement of Ionization Potentials in Mercury Vapor. Phys. Rev., 34(1929) 157-160. 

Two fundamental discoveries about the structure of the atom and electromagnetic radiation also occurred during this period and provided a foundation for instrumentation that would be fundamental in furthering our understanding of soil chemistry. One was the discovery of X-rays, also sometimes called Rontgen rays, discovered in 1895, by W. Rontgen [24], The second was made by J. J. Thomson in 1912. He observed positive rays and described how these could be used to identify compounds and elements. Subsequently, he presented a clear description of the process in 1913. This led to the development of mass spectrometry [25],... 

Mass spectrometry is one of the oldest instrumental analytical methods. Positive rays were discovered by Goldstein in 1886 (after Barrie Prosser, 2000). The first mass spectrometer for routine measurements of stable isotope abundances was reported in 1940 and improved upon over the following ten years Nier, 1940, Nier, 1947, Murphey, 1947, McKinney et al, 1950, after Prosser, 1993. It is remarkable that the vast majority of active gas spectrometers in use today are little changed from those described around 50 years ago. For most people, mass spectrometry now means organic molecular structure determination. However, within the last 15... 

M.A. Grayson, Measuring Mass From Positive Rays to Proteins, Chemical Heritage Press, Philadelphia, 2002, pp. 149. 

The positive rays are produced by the ionisation of the vapour of the element. 

Experiments on positive rays in the early 1900s showed the positive electricity never existed independently from particles of matter similar to ordinary atoms. Also see William Ramsay, "The Electron as an Element,"... 

Measuring Mass - From Positive Rays to Proteins Grayson, M.A., editor ASMS and CHF Santa Fe and Philadelphia, 2002. 

THE IONIZATION POTENTIALS OF HYDROGEN AS INTERPRETED BY POSITIVE RAY ANALYSIS... 

General Description of Method.— The gas is ionized by impact electrons emitted by a hot tungsten filament, and, by means of an electric field the positive ions formed are drawn through a narrow slit into a magnetic field, where they are resolved into constituents of different ratios of charge to mass by a method very similar to that employed by Dempster in his positive ray analysis. The ionization potential necessary to produce each ion is determined by gradually reducing the potential applied to the impact electrons until no trace of the particular ion can be detected. 

In their studies with cathode rays, researchers observed different rays traveling in the opposite direction of cathode rays. In 1907, Thomson confirmed the rays carried a positive charge and had variable mass depending on the gas present in the cathode-ray tube. Thomson and others found the positive rays were as heavy or heavier than hydrogen atoms. In 1914, Ernest Rutherford (1871-1937) proposed that the positive rays were composed of a particle of positive charge as massive as the hydrogen atom. Subsequent studies on the interaction of alpha particles with matter demonstrated that the fundamental positive particle was the proton. By 1919, Rutherford was credited with identifying the proton as the second fundamental particle. 

Aston, with experience in chemistry as well as physics, began working at the Cavendish Laboratory in 1909 on separating isotopes using mass spectrography. He started to assist Thomson in his study on positive rays. Together they improved on Thomson s original apparatus. 

F. W. Aston 60 has shown that the positive ray spectrum of lithium shows the existence of two isotopes of at. wt. 6 and 7 sodium shows no isotope potassium shows two isotopes with at. wt. 39 and 41 rubidium shows two isotopes of at. wt. 85 and 87 and caesium has possibly two isotopes. 

As the speed of the impacting electrons is still further increased no new processes are observed by the positive ray or clean-up methods. Some experimenters have observed additional ionization setting in at about thirty volts. Of more importance, however, are the results of Franck and Blackett, who showed that the emission of Balmer lines was a primary result of impact of thirty volt or higher speed electrons. In other words, they showed dissociation may occur with excitation of one (or perhaps both) atoms. 

Thomson, J. J. (1912) Further experiments on positive rays. Philosophical Magazine, 24, 209-53. 

Such positive rays can be obtained from any kind of gas and they have been investigated by J. J. Thomson and F. W. Aston with very interesting and important results. They can... 

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