Instrumentation
Figure lamp
6.
Electrodeless
discharge
spectroscopy as a major technique has occurred mainly because early workers selected a more suitable source a n d p r o ceeded to develop practical units such as hollow cathode lamps (9). T h e basic design of hollow cathode lamps is now well known. Essentially, they include a cathode containing t h e particular element t o be tested a n d a noble gas a t low pressure. I n opera tion, the gas discharge sputters off cathode material to form a n atomic cloud. This becomes excited and radi ates the atomic line spectrum of the ele ment in question. T h e lines are nor mally narrower than the corresponding absorption lines, so both linearity and sensitivity a r e , achieved. A monochromator m a y be utilized to reject undesired lines, although it has a negli gible effect upon t h e analytical line it self. T h e simple one-element lamps first used have been followed b y multiele ment lamps. These are constructed b y including several elements within the single cathode. Generally, elements are selected on the basis of construction compatibility and freedom from over lapping lines. Use of intermetallic compounds helps provide uniform sputtering rates as desired to avoid preferential loss of one element over others. Also, consideration is given to group combinations commonly related to each other in analytical work. Another trend in the development of hollow cathode lamps has been to in crease brightness. This is particularly desirable in situations where signal-tonoise ratios are critical. Improvement in this regard has resulted from t h e use of auxiliary electrodes mounted in front of t h e cathode. These serve to improve atomic excitation without increasing the sputtering rate. Still another modification has been the development of selective modula tion lamps which in some instances eliminate the need for a monochro-
m a t o r (10). Actually, t w o lamps are used. T h e first generates a line spec t r u m in the conventional manner. This radiation enters a second hollow cathode l a m p of t h e same element. T h e second lamp is periodically turned on and off. As a result, its atomic vapor acts as a selective and modulating absorber for the radiation from t h e first lamp. If the photometric system is tuned t o t h e frequency of modulation, it responds only to t h e analytical line. T h e dc component m a y increase t h e noise level but this can often be reduced b y using solar blind detectors. The electrodeless discharge lamps (Figure 6) discussed in connection with atomic fluorescence are also useful in aa spectroscopy work. These a r e p a r ticularly attractive for elements where conventional hollow cathode lamps have proved ineffective—i.e., volatile elements such as arsenic and selenium. However, great care must be taken with electrodeless discharge lamps to avoid line broadening with the resulting loss of sensitivity. Summary
This discussion on sources has p r o vided an opportunity to describe dif ferent types of sources and to point out various aspects in selecting sources for a particular analytical function. Success or failure often depends upon availability of suitable sources; in fact, m a n y techniques became practical only after significant breakthroughs were made in this area. Atomic absorption spectroscopy is one example; R a m a n spectroscopy is likely to be another. Even in relatively successful fields such as infrared, new sources could still bring significant improvements. F o r this reason, source technology remains a vital and exciting field which will continue to add new dimensions t o analytical spectroscopy. References
(1) Ε . Ε . Pickett and S. R. Koirtyohann, Spectrochim. Acta, 24B, 325 (1969). (2) G. F . Kirkbright and T. S. West, Appl. Opt., 7, 1305 (1968). (3) A. A. Javanovic, Spectrochim. Acta, 25B, 405 (1970). (4) C. D. West and D. N . Hume, ANAL. CHEM., 36, 412 (1964).
(5) R. H. Wend and V. A. Fassel, ibid., 38,337 (1966). (6) J. M. Mansfield, Jr., M. P . Bratzel, Jr., H. O. Norgordon, D. O. Knapp, K. E. Zacha, and J. D. Winefordner, Spec trochim. Acta, 23B, 389 (1968). (7) R. M. Dagnall, K. C. Thompson, and T. S. West, Talanta, 14, 551 (1967). (8) R. G. Smith, ANAL. C H E M . , 41 (10),
75A (1969). (9) W. G. Jones and A. Walsh, Spectro chim. Acta, 19, 249 (1960). (10) J. V. Sullivan and A. Walsh, Appl. Opt., 7, 1271 (1968).
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