r flame, it is unusual for the number of excited atoms to exceed 1% of the total number of atoms. Any variable which affects this percent age, such as matrix, solvent, or flame temperature, will seriously change the intensity of the emission temperature. For example, if the number of excited atoms increases from 1.0% to 1.1 %, the emission in tensity should increase about 10%. However, in absorption we are deal ing with the other 99% of the atoms and the same variable will change the population of ground state atoms from 99% to 98.9%. This effect is minor and in practice can often be neglected. An equally im portant feature is the high degree of sensitivity of the procedure (Table I, page 24 A). Another attraction to this field is the simplicity of the equipment required. In many cases only a simple flame photometer with a modulated light source and detector is required. In some cases—e.g., analysis of alkali metals—light filters provide sufficient spectral resolution. However, it is better to use a simple prism or grating as a monochromator, particularly where transition and noble metals are in volved.
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Disadvantages
It would be unusual indeed if any system, new or otherwise, could be devised which did not suffer from some built-in problems. Atomic ab sorption spectroscopy is no excep tion. Perhaps the most important problem is that when a flame is used to produce the atomic state, several elements are not detectable. These elements include aluminum, titanium, tungsten, molybdenum, silicon, and a few others. At pres ent, the most probable explanation appears to be that these elements form oxides in the flame. This for mation would prevent realization of the atomic state which is essential for detection. As described below, it appears likely that this problem can be overcome for metal samples. But an alternative method of atomization other than the flame seems to be essential for liquid samples. Attempts to break down these oxides with a high temperature
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47 Circle No. 121 on Readers' Service Card VOL 32, NO. 8, JULY 1960
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