Mass, atomic spectrograph

HW noted that past atomic number 27 the whole number rule did not work for the atomic weights. They attributed this fact either to the presence of isotopes for all of these elements or to some phenomenon which they did not understand. The first explanation (isotopes) is correct. The mass spectrograph, already mentioned as an invention of Aston, shows that all isotopic masses on the 160 scale (or the very similar 12C scale used today) tend to differ from integer values by less than 0.1%. 

Analysis. Atomic absorption, emission, and mass spectrographic separation are the most sensitive methods for the analysis of Si. Electrothermal atomization-atomic absorption spectroscopy ETAAS has a sensitivity of 10 ppb, ICPAES 1 ppb, and ICPMS 10 ppb. Colorimetric agents permit spectrometric analysis down to about 10 ppb. 

Within two decades of inventing the mass spectrograph, Aston succeeded in identifying 212 of the 281 naturally occurring isotopes of all the elements. He realized that atomic weights measured from... 

A typical analysis using the spark-source mass spectrograph shows the following impurities N = 400, C = 300, Ta = 200, Fe = 90, Mo = 40, V = 10 atomic p.p.m. All other impurities are below the 10-p.p.m. limit it is interesting that only 1 p.p.m. each of Cu and W have been detected, even though these are the major construction materials of the furnace used. 

Analysis. There are five important methods for measuring the ratios of isotopes, making use respectively of—the spectrograph, the mass spectrograph, density measurements, atomic weight measurements and refractive index measurements. Any of these methods must be pushed to the present limits of accuracy or beyond in order to meet the requirements for tracer experiments. If, for example, an increase in concentration of C13 is effected from 1 per cent to 1.1 per cent and an accuracy of five per cent in the increase is required the determination must be accurate to one part in 20,000. 

If a mass spectrograph is available, how would you use D2 to show whether hydrogen gas that is absorbed reversibly on a metal is absorbed as atoms or as molecules ... 

Aston mastered this new approach to an extremely delicate analysis of the chemical elements and developed it with surprising accuracy. A narrow beam of positive rays was passed into an electro-magnetic field which bent the stream of ions. This deflected beam of rays was then photographed on a sensitized plate. If the stream of ions was composed of atoms of equal mass only one band of light appeared on the plate. Positive rays consisting of atoms of different masses, however, were split into an electric spectrum, the number of bands depending upon the number of isotopes. Even the relative proportion of the isotopes could be determined from the size and darkness of the bands on Aston s mass spectrograph. ... 

The chemical method of accurately determining atomic weights involves the determination of stoichiometric ratios. The procedure is illustrated by the examples given below and the problems at the end of the chapter. In addition to this method, extensive use is made of the method of gas densities, and, in recent years, of the mass-spectro-graphic method. The gas-density method is discussed in Chapter 8, and the mass-spectrographic method in the section following this one. 

In 1907 Sir j. J. Thomson developed a method of determining the ratio of charge to mass of an ionized atom (or ionized gas molecule) by measuring the deflection of a beam of the ionized atoms in electric and magnetic fields. The apparatus is called a mass spectrograph. 

The great accuracy of modern mass spectrographs makes the mass-spectrographic method of determining atomic weights about as useful and important at present as the chemical method. 

The value given in the International Atomic Weight Tables for gold at present is 197.2 it is not unlikely that this will soon be revised to the mass-spectrographic value by the International Committee. 

For an element consisting of several isotopes the chemical atomic weight is found by determining the masses of the different isotopes and also their relative amounts. The way that this is done is indicated in Figure 7-4, which shows the results of a mass-spectrographic investigation of nickel. The isotopic composition of this element is found in this way to be Ni , 67.4% 26.7% ... 

Determination of atomic weights by chemical methods by yse of the mass spectrograph by the X-ray method. 

Calculate (to five. places) the atomic weight of iron on the chemists scale from the following mass-spectrographic data ... 

The Existence of Stable Ions. In the discussion of the mass spectrograph in the preceding chapter it was mentioned that atoms in a gas have the power to add an extra electron, forming a negative ion such as I, or to lose an electron, forming a positive ion such as I+. This affinity for electrons or ability to lose electrons is so great for many "elements as to cause their anions or cations to be very stable, arid to be present in most of the compounds of these elements. 

At temperatures of activation of 356° and below, the methyl hydrogen atoms exchange more rapidly than those on the ring. Runs at 215, 244, and 327° gave distribution patterns similar to those shown for 275, 300, and 356°. The amounts of exchange into the ring and into the methyl were determined by NMR (12) since mass spectrographic analysis on... 

The more common negative observations are understandable for two reasons. First, very careful measurements are required to detect the amounts of desorbed gas corresponding to the mass spectrographic observations or to the ESR measurements (ca. H atoms per gram),... 

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