Selective Excitation of Probe Ion Luminescence (SEPIL) - ACS

1 Selective Excitation of Probe Ion Luminescence (SEPIL)

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J O H N C. W R I G H T , F R E D E R I C K J. G U S T A F S O N , and L A U R A C. P O R T E R Department of Chemistry, University of Wisconsin, Madison, W I 53706

There have been numerous s t u d i e s t h a t illustrate the e x c e l l e n t d e t e c t i o n l i m i t s obtained when a l a s e r e x c i t a t i o n source generates o p t i c a l emission from a sample (1-7). When working a t such low c o n c e n t r a t i o n l e v e l s , one must b a t t l e the new problems of contamination and i m p u r i t i e s . The l a s e r can provide a potential advantage here as w e l l because o f the very narrow s p e c t r a l bandpasses o f modern l a s e r s . I f the a n a l y t i c a l system has sharp line o p t i c a l t r a n s i t i o n s whose wavelength depends upon the partic u l a r a n a l y t e , the narrow bandwidth can provide a high degree o f selectivity f o r the analyte of i n t e r e s t . A common method o f p r o v i d i n g the narrow t r a n s i t i o n s i s t o convert the a n a l y t i c a l sample t o a gas as is commonly done in atomic a b s o r p t i o n and atomic fluorescence experiments (7). The techniques f o r accomp l i s h i n g t h i s transformation have a long h i s t o r y i n the field and there can be little doubt about the eventual success. We have been studying the feasibility of a very d i f f e r e n t method o f a c h i e v i n g narrow t r a n s i t i o n s t h a t are c h a r a c t e r i s t i c o f an a n a l y t e . The lanthanide ions have narrow l i n e t r a n s i t i o n s , even i n condensed phases, as a r e s u l t o f s h i e l d i n g o f the o p t i c a l l y a c t i v e 4f e l e c t r o n s h e l l by the outer 5s 5p o r b i t a l s (8,9). A small c r y s t a l f i e l d s p l i t t i n g i s produced by the immediate surroundings. T h i s s p l i t t i n g can be used as a s h o r t range s p e c t r o s c o p i c probe o f the condensed phase. In p a r t i c u l a r i f analyte ions are a l s o present i n the condensed phase near a l a n thanide probe i o n , a c r y s t a l f i e l d s p l i t t i n g w i l l be produced that i s c h a r a c t e r i s t i c o f the presence of that a n a l y t e . n

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0-8412-0459-4/78/47-085-001$05.00/0 © 1978 American Chemical Society

Hieftje; New Applications of Lasers to Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

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In an a r b i t r a r y condensed phase, lanthanide ions w i l l encounter a number o f d i f f e r e n t surroundings e i t h e r because o f d i f f e r e n t ways the ions o f the phase can be ordered around the lanthanide i o n or because other ions (analytes) have a l s o entered the phase. The a b s o r p t i o n and fluorescence spectra can t h e r e f o r e become q u i t e complex because o f the d i f f e r e n t s p l i t t i n g s o f the lanthanide ions i n d i f f e r e n t surroundings. This complexity may be s i m p l i f i e d i f a tuneable l a s e r i s used to e x c i t e a t a wavelength t h a t matches an absorption l i n e o f a lanthanide i o n with a p a r t i c u l a r surroundings (10). Since the lanthanide ions with d i f f e r e n t surroundings have d i f f e r e n t c r y s t a l f i e l d s p l i t t i n g s , these ions w i l l not be e x c i t e d . The fluorescence spectrum then c o n t a i n s only l i n e s from a lanthanide with one type o f surroundings. T h i s process i s c a l l e d s e l e c t i v e l a s e r e x c i t a t i o n o r s i t e s e l e c t i v e spectroscopy. S i m i l a r l y , an e x c i t a t i o n spectrum o f one s i t e may be obtained by monitoring a s p e c i f i c fluorescence l i n e while scanning the dye l a s e r e x c i t a t i o n wavelength. Thus a high s e l e c t i v i t y f o r a p a r t i c u l a r analyte may be obtained by e i t h e r e x c i t i n g o r monitoring s p e c t r a l t r a n s i t i o n s o f the l a n t h a nide i o n t h a t has the analyte o f i n t e r e s t i n the immediate surroundings . In order t o make t h i s idea p r a c t i c a l , one must solve the chemical problem o f b r i n g i n g about an a s s o c i a t i o n between an analyte i o n and a lanthanide i o n . There are a number o f methods that can be used t o accomplish t h i s . The simplest method i s t o make the condensed phase i t s e l f from lanthanide ions, i . e . a pure lanthanide compound. Any f o r e i g n i o n t h a t enters the c r y s t a l l i n e l a t t i c e o f such a compound w i l l have to perturb a lanthanide i o n . In a second method, the presence o f analyte ions could l e a d to the establishment o f a second c r y s t a l l i n e phase which w i l l p a r t i c i p a t e i n the p a r t i t i o n i n g o f t r a c e amounts o f lanthanide i o n s . I f two phases a r e present, one c o n t a i n i n g the analyte and the other an innocuous d i l u e n t , the lanthanide spectra w i l l r e f l e c t the presence o f both phases. A t h i r d method introduces both the lanthanide and a n a l y t e ions a t t r a c e concentrations i n a host compound and r e l i e s upon i n t e r a c t i o n s between them to form assoc i a t e s o r complexes. The i n t e r a c t i o n s can be the r e s u l t o f chemical bonding o r o f d i f f e r e n c e s i n i o n i c r a d i i or charge s t a t e between analyte and lanthanide. F o r example, s u b s t i t u t i o n o f a t r i v a l e n t lanthanide f o r a d i v a l e n t cadmium i n cadmium molybdate produces a charge imbalance which c o u l d be compensated by r e p l a c i n g t h e hexavalent molybdenum by a quadravalent niobium. The e f f e c t i v e p o s i t i v e charge o f the lanthanide i o n ( r e l a t i v e t o the l a t t i c e ) and the e f f e c t i v e negative charge o f the niobium


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Probe Ion Luminescence

WRIGHT E T AL.

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w i l l cause Coulombic a t t r a c t i o n s which w i l l favor formation o f a lanthanide-niobium complex i n the l a t t i c e . The p e r t u r b a t i o n s o f the l o c a l c r y s t a l f i e l d s by the niobium w i l l produce c r y s t a l f i e l d s p l i t t i n g s on the lanthanide i o n t h a t are unique and permit the use o f s e l e c t i v e l a s e r techniques to e x c i t e only lanthanide ions that have nearby niobium i o n s . The i n t e n s i t y i s p r o p o r t i o n a l to the number o f niobium i o n s . Any o f the lanthanide ions which are f l u o r e s c e n t can be used f o r these procedures. In a p r a c t i c a l a n a l y s i s , i t would be b e s t to choose the lanthanide i o n and the s p e c i f i c t r a n s i t i o n s t h a t gave the best s e l e c t i v i t y and s e n s i t i v i t y . In order to l i m i t the number o f v a r i a b l e s t h a t need to be optimized, we have chosen to use europium as the lanthanide i o n i n a l l o f our s t u d i e s because of s e v e r a l unique p r o p e r t i e s t h a t make Eu p a r t i c u l a r l y s u i t a b l e f o r p r e l i m i n a r y work. The Eu energy l e v e l s t r u c t u r e i s shown i n F i g u r e 1 (8). ^The ground s t a t e ^ F Q and the most f l u o r e s c e n t e x c i t e d s t a t e DQ are both s i n g l e t l e v e l s (a J=0 s t a t e can have^ only an Mj =0) and t h e r e f o r e f l u o r e s c e n t t r a n s i t i o n s from D Q " * F Q o r a b s o r p t i o n t r a n s i t i o n s from ^ F - ^ D Q have only one t r a n s i t i o n . Thus, when one monitors the "^p^ f l u o r e s c e n c e t r a n s i t i o n s with an instrument of s u f f i c i e n t l y l a r g e bandpass to i n c l u d e t r a n s i t i o n s from the Eu ions i n a l l p o s s i b l e c r y s t a l l o g r a p h i c s i t e s as one scans a tuneable l a s e r over the r e g i o n of " ^ F Q - ^ D Q t r a n s i t i o n s , the r e s u l t i n g spectrum w i l l c o n t a i n o n l y one l i n e f o r each type o f c r y s t a l l o g r a p h i c environment t h a t Eu encounters i n the sample. T h i s procedure permits a r a p i d c h a r a c t e r i z a t i o n o f the s i t e s present i n a sample by t a k i n g a s i n g l e scan. The advantage i s p a r t i a l l y o f f s e t because the p o s i t i o n o f the ^ F Q - ^ D Q t r a n s i t i o n r e s u l t s only from a second order p e r t u r b a t i o n and i t s i n t e n s i t y i s lower because i t i s forbidden i n f i r s t order. There i s a l s o a p o s s i b i l i t y o f having the l i n e from a Eu with a nearby analyte a c c i d e n t a l l y overlapping l i n e s from Eu ions i n other s i t e s . These problems can be e l i m i n a t e d by using the other Eu t r a n s i t i o n s such as ^ Q ~ ^ 2 ky using other lanthanide i o n s . Q

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LANTHANIDE ANALYSIS Our research i n t o the f e a s i b i l i t y o f implementing these ideas as an a n a l y t i c a l methodology has been d i v i d e d i n t o two s e c t i o n s development of trace methods f o r lanthanide i o n a n a l y s i s where the a s s o c i a t i o n with an analyte i s not a f a c t o r (the lanthanide i o n i t s e l f i s the analyte) and development of methods f o r other ions where the analyte ion must be a s s o c i a t e d with the l a n t h a n i d e . The o v e r - a l l problem can then be approached i n smaller s e c t i o n s while simultaneously p r o v i d i n g new a n a l y t i c a l methods. The procedure f o r performing a lanthanide a n a l y s i s c o n s i s t s of adding Ca(N0 )2 to the s o l u t i o n c o n t a i n i n g unknown amounts o f lanthanide ions and p r e c i p i t a t i n g with the a d d i t i o n of NH4F (11). The lanthanide ions c o - p r e c i p i t a t e with a very f a v o r a b l e 3


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Figure 1. The Eu electronic energy levels of the free ion are shown on the left. The splittings and shifts that occur when the Eu ion is placed within two different crystalfieldsis shown on the right for the D and Ό manifolds. Note the scale expan sion required to see the crystal field splittings. The MA:Eu represents Eu doped in a httice MA and the MA:Eu,X repre sents Eu and an analyte X in association in the MA Ixittice. 3+

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d i s t r i b u t i o n c o e f f i c i e n t i n the CaF2. T h i s step serves both as an e x t r a c t i o n , s e p a r a t i o n , and p r e - c o n c e n t r a t i o n . The p r e c i p i t a t e i s f i l t e r e d , washed, and i g n i t e d . The i g n i t i o n step converts the f l u o r i d e i n t e r s t i t i a l charge compensation o f the c o - p r e c i p i t a t e d lanthanide ions to an oxygen charge compensation. T h i s conversion a l s o reduces the number of d i f f e r e n t s i t e s present and i n c r e a s e s the o s c i l l a t o r strengths o f the o p t i c a l t r a n s i t i o n s . The procedure works q u i t e w e l l . No s e p a r a t i o n steps are r e q u i r e d f o r i s o l a t i n g i n d i v i d u a l lanthanide i o n s , l i n e a r c a l i b r a t i o n curves are obtained up t o c o n c e n t r a t i o n s o f 1 ppm, and d e t e c t i o n l i m i t s o f 200 p a r t s i n 1 0 are p o s s i b l e . The l a t t e r l i m i t i s expected to be lowered by a t l e a s t an order of magnitude because o f s e v e r a l improvements i n our d e t e c t i o n e l e c t r o n i c s . 1 5

ANALYTES ASSOCIATED WITH LANTHANIDES The simplest method of e s t a b l i s h i n g an a s s o c i a t i o n between the lanthanide i o n and an analyte was by forming an ordered s t r u c ture o f a lanthanide compound i n the presence o f an a n a l y t e . This method i s i l l u s t r a t e d i n F i g u r e 2 f o r E u ( S 0 ) 3 produced by r a p i d evaporation o f s o l u t i o n c o n t a i n i n g 0.5 mole % Na^PO^ r e l a t i v e to SO^. The spectrum i n F i g u r e 2 was obtained by scanning a dye l a s e r over the e x c i t a t i o n r e g i o n o f the ^ F Q ^ ^ D Q t r a n s i t i o n while monitoring the ^ D Q ~ ' ^ 2 fluorescence with a very wide bandpass. Each peak represents a s i n g l e Eu s i t e . The two small peaks to the l e f t o f the main i n t r i n s i c peak are only obtained with PO^~ i n the o r i g i n a l s o l u t i o n and they have an i n t e n s i t y which i s p r o p o r t i o n a l to the P0^~ c o n c e n t r a t i o n . The same_behavior i s obtained i f A s O ^ i s p r e s e n t although the two AsO^ peaks occur a t d i f f e r e n t wavelengths from those of PO^~. Although the analyte peaks do not look very s i g n i f i c a n t i n compari s o n with the main i n t r i n s i c peak and would seem s u s c e p t i b l e to being l o s t i n the main l i n e a t lower c o n c e n t r a t i o n s , t h e i r absol u t e i n t e n s i t y i s c o n s i d e r a b l e . In f a c t , the i n t e r f e r e n c e from the main peak can be e l i m i n a t e d e i t h e r by tuning the l a s e r to e i t h e r of the e x c i t a t i o n wavelengths o f the PO4- peaks and s e l e c t i v e l y e x c i t i n g f l u o r e s c e n c e from only the Eu s i t e s with P04~ nearby or by s e t t i n g a monochromator with a narrower bandpass_to monitor a f l u o r e s c e n c e t r a n s i t i o n o f Eu s i t e s with nearby PO^ and o b t a i n i n g a s i n g l e s i t e e x c i t a t i o n spectrum. E i t h e r method produces s p e c t r a completely f r e e o f the main i n t r i n s i c l i n e s and permits one to f o l l o w the P0^~ peaks to much lower c o n c e n t r a t i o n s . w

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A s s o c i a t i o n between the lanthanide and analyte ions can a l s o be achieved i f the two ions provide a mutual charge compensation f o r each other i n a l a t t i c e o f ions with d i f f e r e n t valences (12). T h i s method i s more complex than the previous example because s e v e r a l a d d i t i o n a l f a c t o r s become important. The Coulombic i n t e r a c t i o n s w i t h i n the charge compensating p a i r t h a t promote t h e i r a s s o c i a t i o n are opposed by the l a t t i c e d i s t o r t i o n s t h a t might


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r e s u l t i f the p a i r becomes a s s o c i a t e d and by entropy considerat i o n s which favor randomization of the i o n s . I f these a s s o c i a t e s have favorable f r e e energies, they w i l l cause an increase i n the d i s t r i b u t i o n c o e f f i c i e n t s f o r the p a r t i t i o n i n g o f the ions i n t o the c r y s t a l l a t t i c e . A d d i t i o n a l l y , both ions can be compensated by native d e f e c t s that a r e present i n the l a t t i c e g i v i n g r i s e to i n t r i n s i c s i t e s that comprise the s p e c t r a i n the absence o f a n a l y t e s . One can again have a s s o c i a t e s o f e i t h e r lanthanide or analyte ions with the native defects i n a number o f d i f f e r e n t arrangements. A lanthanide i o n w i l l be i n v o l v e d i n a competition between a s s o c i a t i o n with an analyte, a s s o c i a t i o n with a n a t i v e d e f e c t , and d i s s o c i a t i o n . Each o f the p o s s i b l e e q u i l i b r i a w i l l a l s o a f f e c t the native d e f e c t e q u i l i b r i a o f the host l a t t i c e . T h i s s i t u a t i o n can be compared d i r e c t l y with the f a m i l i a r e q u i l i b r i a that a chemist considers f o r complexation o f EDTA with a t r a n s i t i o n metal i n an ammonia b u f f e r s o l u t i o n . A c o n d i t i o n a l formation constant can be assigned f o r p a r t i c u l a r values o f pH and ammonia l i g a n d c o n c e n t r a t i o n . In the same way, success o f t h i s method r e q u i r e s favorable " c o n d i t i o n a l formation constants" f o r the lanthanide-analyte a s s o c i a t e s . There are d i f f e r e n t l i m i t i n g cases that can be encountered f o r d i f f e r e n t values o f the e q u i l i b r i a constants. 1.

Analyte-lanthanide

a s s o c i a t i o n i s strong.

2.

Lanthanide-native d e f e c t a s s o c i a t i o n i s strong while a n a l y t e native d e f e c t a s s o c i a t i o n and analyte-lanthanide a s s o c i a t i o n are weak.

3.

Both lanthanide-native d e f e c t a s s o c i a t i o n and a n a l y t e - n a t i v e d e f e c t a s s o c i a t i o n a r e strong.

4.

Lanthanide a s s o c i a t i o n with e i t h e r analyte o r native defects i s weak.

Each o f the d i f f e r e n t cases has a d i s t i n c t i v e i n f l u e n c e on the observed s p e c t r a . The f i r s t case i s the a n a l y t i c a l l y u s e f u l one i n which new l i n e s appear i n the s p e c t r a t h a t are d i r e c t l y r e l a t e d to the presence o f an a n a l y t e . T h i s case i s shown i n F i g u r e 3 f o r CdMo0 with Eu and Nb present i n t r a c e q u a n t i t i e s . These s p e c t r a were obtained by monitoring the D Q - * F transition with a very broad bandpass while scanning the wavelength o f a dye l a s e r over the p o s s i b l e F Q - * D Q t r a n s i t i o n s . The l i n e s charact e r i s t i c o f Nb can be s e l e c t i v e l y e x c i t e d or monitored to o b t a i n s p e c t r a o f only those Eu ions with nearby Nb. 4

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For case 2, the a d d i t i o n o f analyte to a m a t e r i a l can cause marked changes i n t h e spectrum of a Eu doped system because o f the disturbance i n native d e f e c t concentrations. The a d d i t i o n o f analyte i s accompanied by an increase i n the n a t i v e d e f e c t concent r a t i o n r e q u i r e d to charge compensate t h a t a n a l y t e . I f there i s l i t t l e a s s o c i a t i o n between the analyte and i t s native d e f e c t , the concentration o f the unassociated n a t i v e d e f e c t w i l l become l a r g e


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Figure 2. The excitation spectrum of the F -» Ό transition in Èu (SO^) with PO ~ added obtained by monitoring the fluorescence from D -» F with a broad bandpass instrument. The two small peaks on the left are characteristic of the presence of PO ~. 7

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Figure 3. The excitation spectra of the F -> F> transition in CdMoO with europium alone and with europium and niobium added in trace quantities. This spectra was obtained by monitoring the D -» F fluorescence with a broad bandpass instrument. The additional line associated with the niobium is readily observed. The niobium concentration is 1 mol%. 0

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thus depressing the concentration o f the native defects compensat i n g the Eu i o n . T h i s lowered c o n c e n t r a t i o n can be p a r t i a l l y r e s t o r e d by d i s s o c i a t i o n of Eu-native d e f e c t a s s o c i a t e s which i s r e f l e c t e d by the l o s s of l i n e i n t e n s i t y from these a s s o c i a t e d Eu s i t e s . An example o f t h i s i s shown i n F i g u r e 4 f o r PbMoO^ with Eu and As present i n t r a c e amounts. No new l i n e s appear that can be r e l a t e d to As but there i s a strong i n f l u e n c e o f As on the spectrum. I t should be emphasized at t h i s p o i n t that the explanation given above can only be c l a s s i f i e d as h i g h l y s p e c u l a t i v e a t t h i s p o i n t because of the lack o f research i n t h i s area i n our l a b o r a t o r y and others. The s p e c t r a t h a t are observed i n the t h i r d case are not w e l l d e f i n e d . The a d d i t i o n o f analyte can have no a f f e c t on the Eu spectra i f the a s s o c i a t i o n of analyte and i t s n a t i v e d e f e c t i s strong enough that the p r e v i o u s l y e x i s t i n g e q u i l i b r i a of n a t i v e defects i s not a f f e c t e d . In general however, the a s s o c i a t i o n cannot be t h a t strong and changes are expected although not as marked as case 2. No new l i n e s appear that are c h a r a c t e r i s t i c of the analyte. The f i n a l case corresponds to no changes i n the Eu spectra because o f the a d d i t i o n of an analyte s p e c i e s . The Eu i s i s o l a t e d and not i n t e r a c t i n g with e i t h e r analyte nor n a t i v e d e f e c t compens'ations. The model presented above i s an i d e a l i z e d one i n many respects that i s meant only to focus a t t e n t i o n on the important aspects o f t h i s method o f a n a l y s i s . I t neglects many v a r i a b l e s such as s e v e r a l competing e q u i l i b r i a with other n a t i v e d e f e c t s or i m p u r i t i e s , e q u i l i b r i a with the surrounding atmosphere, and the changes that occur i n d i s t r i b u t i o n c o e f f i c i e n t s because o f the a d d i t i o n of an analyte s p e c i e s . I t does p o i n t out the p o s s i b i l i t y of c o n t r o l l i n g the important d e f e c t e q u i l i b r i a f o r o p t i m i z i n g an a n a l y s i s procedure by analogy to the EDTA e q u i l i b r i a where the pH c o n t r o l s the a n a l y t i c a l l y important e q u i l i b r i a . For c r y s t a l l i n e l a t t i c e s , i t may be p o s s i b l e to c o n t r o l the native d e f e c t concent r a t i o n s to increase " c o n d i t i o n a l formation constants" f o r the lanthanide-analyte a s s o c i a t e s making a p a r t i c u l a r procedure f e a s i b l e or o p t i m a l . CONCLUSIONS These methods represent a departure from conventional f l u o r escence approaches t h a t become f e a s i b l e with the a d d i t i o n o f the l a s e r to the l i n e o f a n a l y t i c a l instruments. The approach i s very promising f o r s e v e r a l reasons. I t i s i n h e r e n t l y a method with very low d e t e c t i o n l i m i t s and can be used over a wide dynamic range of concentrations as demonstrated i n the experiments on t r a c e lanthanide a n a l y s i s . I f the p r e p a r a t i o n step involves a


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WRIGHT E T A L .

580

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581

581

Wavelength (nm.)

Figure 4. The excitation spectra of the F -» Ό transition in FhMoO with europium alone and with europium and arsenic added in trace quantities. This spectrum was obtained by monitoring the D —» F fluorescence with a broad bandpass instrument. It can be seen that the addition of the arsenic analyte does not produce any useful lines but it does have a marked effect on the intrinsic europium sites. The arsenic concentra tion is 1 mol%. 7

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p r e c i p i t a t i o n , t h i s step w i l l serve as a method of preconcentrat i o n o f the s o l u t i o n i n t o the l a t t i c e . There are two steps i n the method t h a t provide a high s e l e c t i v i t y f o r p a r t i c u l a r analytes thus p o t e n t i a l l y e l i m i n a t i n g the need f o r p r i o r s e p a r a t i o n steps. The p r e p a r a t i o n w i l l exclude o t h e r ions with i o n i c r a d i i and charges incompatible with the c r y s t a l l a t t i c e . The narrow l i n e - w i d t h s o f s p e c t r a l t r a n s i t i o n s permit the s e l e c t i v e e x c i t a t i o n o f only the Eu ions with the analyte o f i n t e r e s t nearby. We have examined a number o f d i f f e r e n t chemical systems to d e t e r mine the range o f a p p l i c a b i l i t y o f such methods. Thus f a r , we have shown a n a l y t i c a l l y u s e f u l l i n e s can be found i n 15 of the elements as w e l l as the lanthanide ions and we b e l i e v e the method can be extended to i n c l u d e a m a j o r i t y i f not a l l o f the ions i n the p e r i o d i c t a b l e . We thus b e l i e v e the method i s widely a p p l i c a b l e and has some powerful advantages but c o n s i d e r a b l y more work i s r e q u i r e d before the method can be c o n s i d e r e d an a d d i t i o n to the a n a l y t i c a l chemist's bag of t r i c k s .

ACKNOWLEDGMENTS We would l i k e to thank the N a t i o n a l Science Foundation f o r the support of t h i s r e s e a r c h under grant number MPS74-24394.

LITERATURE CITED 1.

S t e i n f e l d , J . I . , Tunable Lasers and T h e i r A p p l i c a t i o n in A n a l y t i c a l Chemistry, C.R.C. Crit. Rev. A n a l . Chem. (1975) 5, 225-241.

2.

Fairbank, W.M., Hänsen, R.W. and Schawlow, A.L., Absolute Measurement o f Very Low Sodium-Vapor D e n s i t i e s Using Laser Resonance Fluorescence, J . Opt. Soc. Am. (1975) 65, 199-204.

3.

Hurst, G.S., Nayfeh, M.H. and Young, J.P., A Demonstration o f One-Atom D e t e c t i o n , Appl. Phys. L e t t . (1977) 30, 229-231.

4.

L y t l e , F.E. and Kelsey, M.S., C a v i t y Dumped Argon-Ion Laser as an E x c i t a t i o n Source i n Time-Resolved F l u o r i m e t r y , A n a l . Chem. (1974) 46, 855-860.

5.

Richardson, J.H., W a l l i n , B.W., Johnson, D.C. and Hrubesh, L.W., S u b - P a r t - P e r - T r i l l i o n D e t e c t i o n o f R i b o f l a v i n by Laser Induced Fluorescence, A n a l . Chem. (1976) 98, 620-621.


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AL.


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6.

Bradley, A.B. and Zare, R.N., Laser F l u o r i m e t r y . Sub-PartPer Trillion D e t e c t i o n o f S o l u t e s , J. Am. Chem. Soc. (1976) 98, 620-621.

7.

F r a s e r , L.M. and Windfordner, J.D., L a s e r - E x c i t e d Atomic Fluorescence Flame Spectrometry, A n a l . Chem. (1971) 43, 1693-1696.

8.

Dieke, G.H., "Spectra and Energy L e v e l s o f Rare Earth Ions i n C r y s t a l s " , I n t e r s c i e n c e P u b l i s h e r s , New York (1968).

9.

Wybourne, B.G., " S p e c t r o s c o p i c P r o p e r t i e s o f Rare E a r t h s " , I n t e r s c i e n c e P u b l i s h e r s , New York (1965).

10.

T a l l a n t , D.R. and Wright, J.C., S e l e c t i v e Laser E x c i t a t i o n of Charge Compensated S i t e s in CaF :Er , J. Chem. Phys. (1975) 63, 2074-2085. 3+

2

11.

Gustafson, F . J . and Wright, J.C., Ultrat r a c e Method f o r Lanthanide Ion Determination by S e l e c t i v e Laser E x c i t a t i o n , A n a l . Chem. (1977) 49, 1680-1689.

12.

Wright, J.C., Trace A n a l y s i s o f Nonfluorescent Ions by S e l e c t i v e Laser E x c i t a t i o n o f Lanthanide Ions, Anal. Chem. (1977) 49, 1690-1701.

RECEIVED

August 24, 1978.


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