INSTRUMENTATION Resonance Lamp Detector BY RALPH H. MÜLLER in Atomic Absorption Spectrophotometry ERIODICALLY, b u t all t o o frequently,
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is reminded t h a t m a n y of the best resources of geometric a n d physical optics are largely unexplored a n d t h a t new a n d unexpected applications c a n be m a d e if enough of u s have t h e curiosity t o seek a new approach. T o a degree, it is t h e excellence, versatility and enormous resources of present d a y instrumentation which lure u s t o take the obvious p a t h a n d to ignore alternative a n d possibly simpler solutions. An excellent example of a n original a p p r o a c h h a s been b r o u g h t t o our a t tention b y R i c h a r d E . Reiss of Aztec I n s t r u m e n t s , I n c . This refers t o t h e recent work of D r . Alan Walsh of t h e Division of Chemical Physics, Chemical Research Laboratories, Commonwealth Scientific a n d I n d u s t r i a l Research Organization, Melbourne, Australia. I t was t h e pioneering work of D r . Walsh which led t o t h e widely used methods of atomic absorption spectroscopy. More recently his fertile mind has come u p with t h e application of resonance lamps a s monochromators i n atomic absorption spectroscopy. Practical ins t r u m e n t s a r e already commercially available using resonance detectors a n d intensive research is underway t o d e vise new a n d special detectors of this nature. The physical basis of resonance d e tectors w a s established i n t h e first t w o decades of this century in t h e celeb r a t e d investigations of R . W . W o o d who showed t h a t resonance radiation from atomic vapors provides a method of isolating those lines in a n atomic s p e c t r u m which are absorbed b y such v a p o r s . A n ideal case is provided b y mercury, t h e v a p o r of which, if illuminated b y t h e radiation from a quartz m e r c u r y arc emits primarily t h e 2537 A resonance line. A full understanding of t h e phenomenon w a s obtained in t h e classic experiments of F r a n c k a n d H e r t z who excited resonance radiation in mercury v a p o r b y controlled electron b o m b a r d m e n t . T h e y showed t h a t 4.9 volt electrons will excite t h e first resonance line 2537 A a n d a t 6.7 volts, t h e second resonance line a t 1849 A . A t 10.39 volts, ionization is produced a n d a t this a n d higher potentials, t h e entire arc s p e c t r u m of m e r c u r y is excited. These experiments provided t h e first direct proof of t h e validity of Bohr's
quantized energy states in a n a t o m . T h e quantitative principle extends down to the innermost electronic levels —i.e., to t h e excitation of characteristic x-rays in t h e kilovolt range. I n this discussion, we are concerned, primarily, with optical excitation of metal vapors. Walsh has stated t h a t " t h e possibility of extending a n d developing this t y p e of experiment^ resonance lamps) for more general application t o other elements does n o t a p p e a r t o have been considered, presumably because the experimental difficulties were thought t o b e insurmountable a n d also possibly b e cause t h e potentialities a n d implications of t h e technique were n o t a p p r e ciated." Since 1959, he a n d his associates have been investigating t h e possibility of using atomic resonance lamps as "monochromators in modern t y p e atomic absorption spectrophotometers used for chemical analysis. F o r lowmelting point metals such as calcium, magnesium, sodium, potassium, thallium, a n d lead, resonance lamps can be made in which t h e atomic v a p o r is p r o duced b y indirect electrical heating of the a p p r o p r i a t e metal in a rare gas a t reduced pressure. T h e y have also shown t h a t t h e necessary atomic v a p o r can b e obtained b y cathodic sputtering so t h a t , in principle, t h e method is a p plicable t o all metallic elements, irrespective of melting point, since t h e atomic v a p o r is produced without t h e necessity of heating t h e metal. One disadvantage of t h e sputtering technique is a serious decrease in t h e signal/ noise ratio of t h e o u t p u t signal. This can b e overcome b y t h e use of highintensity hollow cathode lamps a s t h e p r i m a r y source of light. Such lamps have been described b y Sullivan a n d Walsh. A typical arrangement of a n atomic absorption spectrophotometer employing a resonance l a m p a s a monochromator uses a sealed-off hollow cathode lamp, t h e radiation from which passes t h r o u g h t h e flame into which is sprayed the solution for analysis. T h e a t t e n uated beam excites some of t h e atoms in the resonance l a m p detector. T h e resonance radiation is emitted in all directions a n d some falls on t o a p h o t o multiplier. T h e power supply t o t h e light source is modulated, whereas t h a t to t h e resonance l a m p is u n m o d u l a t e d
and thus, b y t h e use of a n ac detection system, a n y radiation from t h e flame or t h e resonance l a m p does n o t produce a n o u t p u t signal. A suitable ac detection system h a s been described b y Box and Walsh. Extensive tests have been made using the resonance l a m p detector system, with careful comparisons with conventional spectral isolation with a spectrometer. Essentially identical a n d equally precise results were obtained. A few minor differences are t o be noted. I n t h e case of sodium, t h e conventional instrument yields a n absorbance concentration relationship which is strictly linear when t h e 5890 A line is isolated and measured. I n using t h e resonance l a m p detector, t h e curve relating a b sorbance t o concentration is slightly concave to t h e concentration axis. This arises from the fact t h a t t h e resonance monochromator passes both components of t h e yellow doublet a t 5890 a n d 5896 A to t h e detector. Since t h e intensities of these two components a n d their a b sorption sensitivities a r e in t h e ratio of 2 t o 1, t h e c u r v a t u r e is unavoidable. Similarly, with potassium, t h e resonance monochromator passes both potassium resonance lines a t 7665 a n d 7699 A a n d thus t h e slope is less t h a n with a conventional monochromator which isolates the more sensitive line a t 7665. T h e new AR-200 atomic absorption spectrophotometer employing t h e resonance l a m p monochromator is m a n u factured b y Tcchtron P t y . , L t d . , under license t o C S I R O , Melbourne, A u s t r a lia, a n d is available from Aztec I n s t r u ments, Inc., 2 Railroad Place, W e s t p o r t , Connecticut 06880. According t o M r . Reiss, instruments a r e available f o r either single or dual element analysis. At present, detectors (thermal or s p u t tering) a n d hollow cathode source lamps ( s t a n d a r d or high intensity) a r e available in these instruments for C a , M g , C a / M g , Cu, Ag, Al, C d , C r , Co, Au, F e , P b , N i , a n d Zn. M a n y other elements will b e added t o this list as life tests are completed on t h e resonance detectors. M a n y exciting things have come o u t of Australia. W e note t h a t D r . Walsh received t h e 1966 Britannica Australia award for science for his work on atomic absorption methods. W e p r e dict this will n o t be the last honor t o come t o this gifted investigator. VOL 39, NO. 8, JULY 1967 ·
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