Report for Analytical Chemists
Advantages and Disadvantages of Electron Spectroscopy as Analytical Technique
A t this point after extolling the virtues of electron spectroscopy for several pages, it might be well to consider w h a t the pros and cons of the technique are for the analytical chemist. T h a t it has much potential in both qualitative and q u a n t i t a t i v e analysis is evident and t h a t the potential applications to chemistry have not yet, to use a pun, "scratched the surface." Before anyone undertakes a program in electron spectroscopy, however, there are a number of factors which should be known, some of which have been mentioned before b u t which will be summarized here. Electron spectrometers require residual magnetic fields of ca. 10~4 G and a vacuum of better t h a n 10~5 Torr. T h e technique is good for solid, liquid, and gaseous samples. Solid samples, both amorphous and crystalline, can be placed on t a p e or p u t into pellets, similar to the use of K B R in t h e infrared. Gas samples can be studied by using differential pumping techniques, the sample c o m p a r t m e n t serving as the containers. Liquid samples, assuming they h a v e reasonable vapor pressure a t room t e m p e r a t u r e , must be frozen on a cold finger to reduce vapor pressure. Electron spectroscopy to d a t e has not been effective in studying solutions because of the high v a c u u m necessary—even using differential pumping techniques to keep the solvent out of t h e m a i n v a c u u m c h a m b e r ; a n y studies done on solutions under these conditions would be of questionable validity, with the solvent boiling from the surface of the solution. If v a c u u m p u m p oil containing carbon is used, unless precautions arc taken t o minimize diffusion of the p u m p oil into the main v a c u u m system, one obtains hydrocarbon coatings on all samples. This can be both an a d v a n t a g e and a disadv a n t a g e . T h e c a r b o n - I s photoelectron line can be used as a s t a n d a r d ; however, it can also be annoying when studying surfaces or doing q u a n t i t a t i v e analysis. T h e technique is both qualitative and q u a n t i t a t i v e . Usually photoelectron lines of adjacent elements of t h e periodic t a b l e are far a p a r t
so that overlap between lines from similar electrons of nearest neighbors does not occur. For example, the nitrogen Is line is at ca. 400 eV, oarbon at 290 eV, and oxygen at 530 eV. A similar situation is found for silicon, phosphorus, and sulfur. Electron spectroscopy is useful for almost every element in the periodic table. It is a particularly valuable technique for determining one element in the presence of many others. For example, Siegbahn has observed a cobalt signal from vitamin B J 2 where cobalt is one in 180 atoms. However, electron spectroscopy under current conditions of resolution is not good for distinguishing one atom only slightly different from say 20 others. In this case the photoelectron peak from the one atom in question would appear only as a slight shoulder on a much higher peak and would probably be lost in the noise. The size of the sample required can be quite small. Photoelcctron spectra are obtainable from micrograms of material, and Siegbahn has reported spectra from samples as small as 10~8 g. I t is not inconceivable that electron spectroscopy could do a chemical analysis on a fingerprint. In addition to its sensitivity, the technique should be, with reasonable advances in calibration, capable of 1 % precision. Electron spectroscopy is generally a nondestructive technique. However, some organic compounds do decompose under bombardment from X-rays. For example, pnitroaniline has been shown to decompose as have some chlorinated anilines. When an electron gun source is used for excitation, generally sample decomposition will be greater. For the determination of trace quantities of one element in the presence of others, electron spectroscopy should be sensitive to the tenths of a per cent range. ACKNOWLEDGMENTS
The author wishes to acknowledge the valuable assistance of John Jack, William Swartz, and Anthony Waraksa in setting up our electron spectroscopy program. In
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ANALYTICAL CHEMISTRY, VOL.. 4 2 , NO. 1 , J A N U A R Y 1 9 7 0
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