26 Analytical Atomic Spectroscopy 1.3 FLAMES 1.3.1 Fundamental Studies Perhaps the most important c o n t r i b u t i o n f o r some years t o the study o f flames and t h e i r use i n analysis was the appearance i n 1982 o f a book by Alkemade e t a l . (2302). E n t i t l e d "Metal Vapours i n Flames", and containing d e t a i l e d descriptions o f the processes i n v o l v i n g the e x c i t a t i o n , i o n i z a t i o n , and reactions o f metal vapours, the book w i l l be essential reading f o r a l l a n a l y t i c a l chemists i n t e r - ested i n the fundamental basis o f the measurement o f atoms i n flames.ison, developments i n the nature and optimization o f flames i n the past year appear t o be t r i v i a l . I n one communication, the use o f the a i r l x y l e n e flame f o r the determination of Au and Pd by e x t r a c t i o n w i t h o c t y l disulphide i n t o xylene, was compared t o the conventional air/C2H2 flame (633).o f 20 pg 1-1 were reported f o r both elements. f o r the operation o f an 02-sandwiched air/C2H2 flame has been reported t o give a maximum flame temperature o f 2900 K, and 2-30 f o l d improvements i n AAS sensit- i v i t y compared t o a conventional air/C2H2 flame f o r elements such as A l , Bay Cay S i and V (477, 478).a i r / C 2 H 2 mixture and two 1.0 x 100 mm s l o t s f o r 02, positioned on e i t h e r side o f the flame. Interferences o f H3P04 and H2S04 on Ca, and A1 on Mg were suppressed i n the h o t t e r sandwiched flame. N20/C2H2 flame was reported t o give greater s t a b i l i t y t o the flame and improve- ments i n the r e p r o d u c i b i l i t y o f the AAS measurement o f A1 , Ca and S i (1603).-- new burners have been reported i n the Russian l i t e r a t u r e , one o f f e r i n g a longer absorption path length compared t o s l o t burners, and consequently improved AAS s e n s i t i v i t y (1796), and the other w i t h burner openings consisting o f " t h i n I n compar- Detection l i m i t s Some o r i g i n a l ideas on burner design have appeared i n the past year.A burner The burner consisted o f a 0.5 x 100 mm central s l o t f o r the The cooling o f a commercial burner f o r the TwoAtomization and Excitation 27 projections", which apparently reduced the heating of the burner and made i t e a s i e r and s a f e r t o operate (1954).f o r both air/C2H2 and N20/C2H2 flames. been reported f o r the measurement of flame temperatures (763, 1460). Measurement of rotational excitation temperatures i n premixed air/C3H8 flames was achieved using the rotational f i n e s t r u c t u r e of the CH band a t 431.4 nm (763). middle U . V . (0-0) band of OH i n the region of 306 nm was investigated i n labor- atory flame experiments, i n an attempt t o develop a method f o r temperature measurements f o r coal , gas and o i l b o i l e r flames using remote-sensing spectro- scopic techniques based on a medium resolution monochromator (1460).instrument, i n which, tion measurements t o optimize the flame gas flow r a t e s f o r best s e n s i t i v i t y (C99, C103, C310, see a l s o Section 2.5.2 and Table 2.6C).Useful practical information f o r the analyst not fortunate enough t o possess such an instrument has appeared i n the form of a modified standard addition method (366), useful comments on the c r i t e r i a f o r application of the standard addition method i n AAS (570), and a simple numerical method f o r the evaluation of AAS calibration data (870).internal standards f o r use i n AAS w i t h the air/C2H2 flame. internal standards should match the t e s t analyte i n the dependence of their degree of atomization on such variables as flame temperature and composition. Three novel applications of atomic spectra observed i n flames have appeared during 1982. by AAS using an air/C2H2 flame and HCLs manufactured from 6 L i and 7 L i respect- ively (2227).The absorption obtained exhibited i n each case a non-linear dependence on the t o t a l L i concentration and the isotopic composition, but measurements w i t h both lamps allowed the l a t t e r t o be calculated. been measured by AAS i n a N20/C2H2 flame w i t h a detection l i m i t of 9 ug ml-1 (183.0 nm) using an atmospheric pressure discharge tube as the l i g h t source and and Ar-sheathed optical system (545).Bramall and Thompson (1549) proposed the use of the 430.3 nm non-resonance 5 f o r the determination of h i g h concen- t r a t i o n s of Ca i n sewage sludges and liquors, using AAS and a N20/C2H2 flame. The s e n s i t i v i t y was reduced by a f a c t o r of 600 compared t o the normal Ca resonance l i n e .This procedure was proposed as a simple a l t e r n a t i v e t o other methods of s e n s i t i v i t y reduction such as burner rotation. The study of the e f f e c t of wavelength modulation frequency, modulation amplitude and modulation waveform on SNR i n continuum source flame atomic absorption which was reported l a s t year (see ARAAS, 1981, 11, 25) has now been published (972).In the l a t t e r case, burners were reported Two methods, based on the measurement of molecular emission i n t e n s i t i e s , have The A manufacturer has produced a new microprocessor control led atomic absorption amongst other features, the computer uses atomic absorp- Takada and Nakano (423) established c r i t e r i a f o r the selection of I t was suggested t h a t The L i isotopic composition of geological materials was determined Iodine has A 3-step square waveform, a bi-gaussian waveform and a sine28 Analytical Atomic Spectroscopy wave were compared.quencies, although the advantage diminished a t frequencies greater than 80 Hz. A d i f f e r e n t type of waveform, a s t a i r c a s e modulation waveform, which uses f i v e measurement positions across the wavelength modulation interval , has been reported t o give improvement i n SNR f o r measurements a t the l e s s s e n s i t i v e position and other s i g n i f i c a n t advantages including decreased computation time (C2158).The use of continuum source AAS t o carry out simultaneous multi- element analysis necessitates the adoption of compromise flame conditions , and the i d e n t i f i c a t i o n of s u i t a b l e air/C2H2 flame conditions f o r eight elements was examined by the analysis of a wide range of SRMs (1572).I t has a l s o been shown t h a t the optimum s l i t parameters (width and height) which g i v e the best SNR i n continuum source AAS vary from element t o element (1145). system uses a common entrance s l i t , the use of compromise s l i t parameters will have t o be adopted, and the data presented will be invaluable i n selecting them.Miller-Ihli e t a l . (888) have a l s o shown t h a t the continuum source AAS instru- ment can be used t o observe AAS spectral interferences d i r e c t l y . In continuum source AAS these will include any element which has an absorption l i n e w i t h i n the wavelength modulation interval. Four examples were i d e n t i f i e d , three o f which have not previously been characterized in AAS ( s e e also Section 2.2.2).The f i r s t two yielded an improvement i n SNR a t a l l f r e - Since the Additional reference on the preceding topic - 1770 1.3.2 Atomization and Interference Studies Apart from a more extensive evaluation of background correction using Zeeman e f f e c t AAS w i t h flame atomization, very l i t t l e new information on atomization phenomena has appeared i n the past year.calculation of the distribution of Al, C and Si between gaseous and condensed phases i n an air/C2H2 flame (1357). Calculated values of f r e e atom f r a c t i o n s were in agreement w i t h published experimental data.Some new measurements of the degree of atomization of Cr, Mg, Mo and Na i n an air/C2H2 flame were obtained using a r e l a t i v e peak absorption method (624). contribution, concerned w i t h the e f f e c t of flame stoicheiometry on the measurement of Fey involved the introduction of the analyte in gaseous form as iron carbonyl (C446).I t was shown t h a t removal of the nebulization stage reduced the absorption noise. Mass spectrometry a l s o revealed the presence of Fe' and Fe(0H); i n an air/CO diffusion flame. Low concentrations of NO2 have been measured i n an air/CH4 flame on a f l a t burner u s i n g the photo-acoustic e f f e c t (1403). A chromatix CMX-4 flashlamp-pumped dye l a s e r a t 490.0 nm was used t o excite the NO2 molecule.A detailed examination of molecular halide emission bands i n H2/N2 diffusion flames revealed a number of s u i t a b l e band systems which would be a l t e r n a t i v e s t o the well known A and B systems of the Ga and In halides (440). Useful r e s u l t s An equilibrium model was used f o r the Another i n t e r e s t i n gAtomization and Excitation 29 were obtained f o r InF (A, B and C systems), GaF (A, B and C systems), A1F (A system), TlC1, T l B r , T l I , and C systems o f the GaC1, GaBr, InC1, and InBr mole- cules.The I n bands gave useful detection l i m i t s i n the mg 1-1 range f o r B r , C1, F and I. Determination o f B by molecular emission spectrometry, by measur- i n g the green boron oxide band emission a t 548 nm, has also been proposed (1752, C2451).Boron was introduced i n t o an air/C2H2 flame f o l l o w i n g vapour generation as the trimethoxyboron ester. An a l t e r n a t i v e t o the normal AAS o r AFS procedures, based on molecular emission, has also been proposed f o r the determination o f As a f t e r hydride generation (1457).The arsenic oxide emission was observed i n an air/H2 flame on a quartz burner using a f i l t e r photometer w i t h a 10 nm h a l f width a t 495-505 nm. achieved w i t h the a i d o f a l i q u i d N2 t r a p c o l l e c t i o n system. o f the e f f e c t s o f organic solvents on AAS s e n s i t i v i t y (568, 886, 895, 1083), including a t i m e l y review by Cresser (1083).Kumina and Karyakin (895) d i s - cussed the e f f e c t s o f d i f f e r e n t solvents on the basis o f t h e i r chemical nature, and physical properties such as spraying e f f i c i e n c y , surface tension, volat- i l i t y , and v i s c o s i t y . Desolvation rates i n flames contribute t o the r e l a t i v e l y high enhancements given by ethers and ketones compared t o alcohols and carb- o x y l i c acids.value, and lowest density, viscosity, surface tension, b o i l i n g point, and s o l u b i l i t y i n H20 tended t o give the greatest enhancements. - by phosphine i n the C2H2 has been studied (C97, 863). As a r e s u l t o f the r a t e o f atom appearance w i t h respect t o flame height, i t was suggested t h a t the interference was caused by reaction o f PX molecules ( X = 0 o r H) w i t h Ca atoms diffusing away from the solute p a r t i c l e s .t o 600 p l 1-1 o f PH3 and p u r i f i e d grade less than 50 u l 1-1. The problem can be removed by use of La3' as a releasing agent combined w i t h a scrubber t o remove most o f the PH3 from the C2H2 before combustion (see also Anal. Chem., 1981, 53, 2363).A mechanism explaining the enhancement o f atomic absorption s e n s i t i v i t y by surfactants has been proposed (1746). Anionic surfactants were shown t o enhance the air/C2H2 flame s e n s i t i v i t i e s f o r Cay C r , Cu, Mg, Mn and N i by up t o 150%, whereas non-ionic and c a t i o n i c surfactants had e i t h e r a small negative o r n e g l i g i b l e e f f e c t .ment of oppositely charged analyte species a t the a i r j w a t e r i n t e r f a c e a t the surface o f large droplets of solution. analyte i n the small droplets and a depletion o f analyte i n l a r g e r droplets which run t o waste. The greatest enhancement occurs a t the c r i t i c a l micelle concentration. Enhancement o f the signals f o r T i , U and Z r i n a N20/C2H2 flame A low detection l i m i t o f 20 pg m l - 1 (10 m l sample) was A number o f publications have reported t h e o r e t i c a l and p r a c t i c a l investigations L i u (568) also reported t h a t solvents w i t h the highest combustion The depression o f AES and AAS signals f o r Bay Ca and S r i n air/C2H2 flames Commercial grade C2H2 contains 200 It was proposed t h a t anionic surfactants promote the enrich- This r e s u l t s i n an accumulation o f30 Analytical Atomic Spectroscopy has been reported on a d d i t i o n o f A1 (1157).Both the T i atomic absorption signal and the T i 0 emission signal were enhanced under the same conditions i n d i c a t i n g t h a t competition f o r oxygen would n o t s a t i s f a c t o r i l y explain the e f f e c t o f A l .A d r o p l e t generator was used t o show t h a t mixture o f A1 w i t h the analyte species produces solute p a r t i c l e s t h a t undergo explosive fragmentation i n the flame, g r e a t l y increasing the solute p a r t i c l e surface area and increasing the r a t e of vaporization. flame AAS (C59, C60, C62, 137, C294, 411, 1149).The spectral features o f atomic magneto-optical r o t a t i o n spectroscopy o r atomic Faraday e f f e c t were described f o r a number o f elements using an air/H2 flame (137). a l l y calculated Zeeman s p l i t t i n g patterns were i n agreement w i t h the experi- mental observations. Koizumi e t a l . (411) have shown t h a t application o f a stable magnetic f i e l d o f 10 kG t o air/C2H2 o r N20/C2H2 flames on premixed s i n g l e s l o t burners, combined w i t h polarized r a d i a t i o n from a HCL, provided measurements t h a t were unaffected by flame conditions, flame f l u c t u a t i o n s , background absorption and baseline s h i f t s .obtained f o r Cd, Pb and Zn respectively, and dynamic ranges o f 10 were recorded. It was claimed t h a t the Zeeman background correction system a1 lowed measurements t o be made i n very f u e l r i c h flames, which successfully eliminated both i o n i z a t i o n There appears t o be increasing i n t e r e s t i n the use o f the Zeeman-effect i n The t h e o r e t i c - Detection l i m i t s o f 0.5, 5 and 0.6 pg 1-1 were 4 on interferences and chemical interferences due t o metal oxide molecule format (C59, C60).A simple s o l u t i o n has been suggested f o r the problem o f double valued c a l i b r a t i o n graphs i n Zeeman AAS using flames (1149). A t concentrat beyond the r o l l - o v e r p o i n t on the c a l i b r a t i o n graph, f a s t t r a n s i e n t absorpt peaks are observed a t the beginning and end o f sample aspiration, and these give a c l e a r i n d i c a t i o n t o the analyst t h a t correction i s necessary. Other references o f i n t e r e s t - Cerium(II1) as a releasing agent f o r Ca and S r : 1889.Interferences i n the AAS determination o f B i : 985. ons on 1.3.3 Devices f o r Sample Introduction 1.3.3.1 Nebulizers A1 though research i n t o nebulization processes and the design o f nebulizers i s very active, most o f the e f f o r t i s aimed a t improving them f o r use w i t h the I C P and i s consequently reviewed i n Section 1.2.1.2.The only improvement i n nebulizer performance aimed s p e c i f i c a l l y a t flame application was a new method o f modulating the sample i n t r o d u c t i o n process (C98). As noted previously (ARAAS, 1981, 11, 28), the major noise component i n flame AAS i s analyte f l i c k e r noise, which o r i g i n a t e s from the nebuliration/transport processes.To reduce t h i s source o f noise, the i n t r o d u c t i o n o f sample may be modulated a t the nebulizer, and the r e s u l t i n g signal demodulated. Pearce and Boss (C98) described a novelAtomization and Excitation 31 modulation system using coulometric forces on e l e c t r o s t a t i c a l l y charged aerosols t o modulate analyte i n t r o d u c t i o n i n t o air/C2H2 flames.I n t h e i r system, a r i n g - shaped charging electrode was suspended i n s i d e the mixing chamber 1-2 cm above the brass nebulizer. When a p o s i t i v e voltage was applied between the r i n g and the nebulizer, the r e s u l t i n g e l e c t r i c f i e l d caused the aerosol produced t o have a negative charge.r i n g and d i d not migrate towards the burner. e f f e c t i v e l y modulated the sample i n t r o d u c t i o n process. The publication o f r e s u l t s from t h i s study would appear t o be o f genuine and general i n t e r e s t . o f KBH4 solutions, and samples containing hydride forming elements (535, 627).The design appears t o be of some i n t e r e s t . surrounded by a tapered tube which ended i n a nozzle w i t h an i n l e t f o r a i r . a i r flow o f 5 t o 7 1 min-’ caused both the solutions, KBH4 i n the inner tube, and the a c i d i c sample s o l u t i o n i n the outer tube, t o be drawn i n t o the nozzle a t approximately 3 m l min-l.AAS measurements i n an air/C2H2 flame gave detection l i m i t s such as 0.0037 ug 1- f o r As, and 0.0043 ug 1-1 for Se. a s p i r a t i o n system (987) and a branched c a p i l l a r y nebulizer was again described as a means o f introducing i o n i z a t i o n b u f f e r s (1499, see also ARAAS, 1978, 8, 20). Although i n t e r e s t continues t o be shown i n the d i r e c t a s p i r a t i o n of samples i n the form o f s l u r r j e s and emulsions, e f f o r t s i n 1982 were centred on p r a c t i c a l applications and these are described i n Sections 3.1.4.2 and 3.1.4.3.been investigated (929, 2257). I n t h i s procedure, i n which samples are introduced v i a the nebulizer i n a conventional manner, analyte atoms are trapped on a water cooled tube held i n an air/C2H2 flame, and subsequently released as a pulse when the cooling water f l o w i s stopped.The superior cooling o f metal tubes compared t o s i l i c a (see ARAAS, 1981, 11, 30) gave greater e f f i c i e n c y o f trapping o f ele- ments such as Cd and Se, b u t the performance o f d i f f e r e n t tube materials varied from element t o element (929).S i l i c a tubes were s t i l l preferred f o r Se, despite severe interferences caused by deposition o f and/or attack o f the tube by, i n t e r - f e r i n g elements such as a l k a l i metals (2257). helped t o overcome t h i s l a t t e r problem (see also Section 3.1.2.5). The charged droplets were then a t t r a c t e d t o the charging Modulating the applied voltage A double c a p i l l a r y nebulizer was described f o r the simultaneous i n t r o d u c t i o n Two concentric c a p i l l a r y tubes were An Hydride generation took place i n the nozzle, and 1 Other authors described a double c a p i l l a r y I n the atom trapping method, tubes made o f Cu, N i , stainless s t e e l and T i have Coating o f the tube w i t h A1203 1.3.3.2 Micro SamDlina Devices Apart from a useful review o f d i s c r e t e sample nebulization (361), the most i n t e r e s t i n g paper on t h i s subject was a t h e o r e t i c a l study presented by Bezur et - a1 .(C2364). These workers investigated the e f f e c t s o f the volume o f sample used, uptake rate, gas f l o w rate, and the s i z e o f the spray chamber. matical model was based on the mass-balance o f the nebulizer/burner system, and A mathe-32 Analytical Atomic Spectroscopy an a x i a l dispersion process t o describe the dispersion o f the sample on the way t o the flame.The e f f e c t of each experimental v a r i a b l e on the peak shape, peak height, and peak area has been interpreted by means o f two model parameters, the dispersion c o e f f i c i e n t and the mean residence time.It was demonstrated both t h e o r e t i c a l l y using the model, and experimentally, t h a t there i s an increase i n e f f i c i e n c y o f the nebulizer/burner system when the volume o f sample i s reduced, and the average e f f i c i e n c y a t 25 ~1 can be four times higher than continuous nebulization. Kojima and I i d a (1546, 1729) reported f u r t h e r progress w i t h t h e i r automatic- a l l y triggered d i g i t a l i n t e g r a t o r f o r measurement o f peak areas i n the d i s c r e t e nebulization o f serum samples (see ARAAS, 1981, lJ, Ref. 395). The i n t e g r a t o r i s triggered by t h e e l e c t r i c a l conductance o f the sample when i t reaches t h e nebulizer. Methods f o r the determination o f L i and S r i n blood serum have also been reported using the N20/C2H2 flame (1184, 2040).on t h e i r loop microsampling technique and reported a number o f improvements i n the methodology. a l l o y , and Ir they reported t h a t Ir gave the most consistent r e s u l t s and allowed elements w i t h high vaporization temperatures such as Ag, Ca, Co, Cu, In, Mg, Mn and N i t o be determined (1586).The incorporation o f an alumina c o l l e c t o r tube (4.8 cm x 5 mn) i n the flame above the loop gave a 1 0 - f o l d improvement i n detection l i m i t s compared t o the loop alone (537, 1586). The solvent was evaporated from the loop by e l e c t r i c a l heating, and the residue was introduced i n t o the flame by a precise magnetic o r pneumatic mechanism.w i t h t h i s device was reviewed w i t h p a r t i c u l a r emphasis on the determination o f elements i n body f l u i d s (1118). i n j e c t i o n sample i n t r o d u c t i o n has again been described (C261, 405, see ARAAS, 1981, 11, 29). i n serum by FAAS, and the use o f sample dispersion i n the flowing stream f o r c o n t r o l l e d sample d i l u t i o n was described (942).See also Section 3.1.4.2. Berndt e t a l . (429, 431, 537, 571, 1118, 1586) have now published extensively I n a comparative i n v e s t i g a t i o n o f loops made o f Pt, P t / I r Recent progress A t h e o r e t i c a l model which describes the microsampling method based on flow The F I method has now been used f o r the determination of L i 1.3.3.3 Sample Introduction by V o l a t i l i z a t i o n Very l i t t l e novel work has appeared on t h i s t o p i c i n 1982, more e f f o r t being devoted t o sample v o l a t i l i z a t i o n f o r i n t r o d u c t i o n t o the I C P (see Section 1.2.1.2).d i r e c t analysis o f s t e e l s by FAAS using an air/C2H2 flame (974). Phosphorus and S have been determined by chemiluminescence i n a cool Ar/H2/ entrained a i r flame a f t e r sample i n t r o d u c t i o n by evaporation from a graphite tube atomizer and aerosol desolvation (2219).Organo-P compounds have also been determined by MECA, a f t e r separation by HPLC using both normal phase and reversed An aerosol generator based on an a r c discharge was described f o r theAtomization and Excitation 33 phase colums (1523).A special detector was designed t o cater f o r the eluent from the HPLC. d r i l l e d i n t o i t s circumference. Eluent was collected i n t h e c a v i t i e s and the d i s c was rotated stepwise i n t o the air/H2/N2 flame where the HPO emission a t 528 nm was monitored. It consisted o f a water-cooled Duralumin disc w i t h 40 c a v i t i e s Ebdon e t a l .(C277, 1511, C2067, C2300) described investigations o f f o u r methods by which as chromatography eluates can be detected using FAAS. most s e n s i t i v e method, which bettered the s e n s i t i v i t y o f ETA-AAS, employed an air/H2 d i f f u s i o n flame f o r i n i t i a l combustion o f the GC e f f l u e n t . products from t h i s were swept i n t o a ceramic tube suspended above a conventional air/C2H2 flame i n the AAS instrument.speciation o f organometallic compounds o f As, Hg, Pb, Se and Sn (see also ARAAS, 1981, 2, 31). The The The apparatus was used t o study the 1 .3.4 F1 ame Atomi c F1 uorescence Spectrometry Although the use o f the I C P and DCP as e x c i t a t i o n sources f o r flame AFS has again generated some i n t e r e s t (C1689, C2117), these experiments appear t o be overshadowed by the commercial development o f the I C P as an atom c e l l f o r AFS, which i s reviewed i n Sections 1.2.1.5 and 2.5.3.The use o f AFS f o r m u l t i - element analysis has again been discussed (484, C1689), the review by Ullman (484) being p a r t i c u l a r l y useful.Applications o f flame AFS t o r e a l sample analysis are s t i l l l i m i t e d . A non-dispersive atomic fluorescence system was described and used f o r the deter- mination o f Au i n geological materials (C449). To avoid the necessity f o r s c a t t e r correction using an a u x i l i a r y lamp, the author preferred t o separate the Au from the matrix by solvent extraction.The a l t e r n a t i v e approach was demonstrated by the development o f an accurate method f o r the determination o f Pb i n blood, using a dispersive AFS system w i t h a continuum source f o r s c a t t e r background correction and a N2-separated air/C2H2 flame (C306). the samples were simply d i l u t e d 1 + 5 w i t h Triton-X o r n i t r i c a c i d before aspiration.I n continuum source AFS, background correction can be achieved by wavelength modulation but the r e s u l t i n g signal includes both AE and AF components. double modulation system was developed i n order t o measure n e t AF signals without the AE component (C2120). This used two l o c k - i n - a m p l i f i e r s operating a t d i f - f e r e n t frequencies, one l i n k e d t o a source i n t e n s i t y modulation system, and the other t o a wavelength modulation system.i n o v e r a l l s e n s i t i v i t y f o r elements w i t h l i n e s i n the v i s i b l e region. I n t h i s case A -- As expected, t h i s resulted i n a loss 1.3.5 Applications o f Lasers Lasers continue t o generate much i n t e r e s t i n research and fundamental studies,34 Analytical Atomic Spectroscopy but there i s s t i l l l i t t l e sign of any p o t e n t i a l a n a l y t i c a l i n t e r e s t .Useful reviews again appeared during 1982 (1046, 1642), t h a t by Gelbwachs (1046) being mainly concerned w i t h l a s e r fluorescence , including saturated o p t i c a l non-reson- ant emission spectroscopy (SONRES) and harmonic saturated spectroscopy. 1.3.5.1 Atomic Fluorescence Spectrometry Further r e s u l t s have been published on the measurement o f l a s e r excited AF from electrothermal atomizers. Extremely low detection l i m i t s f o r Pb have been recorded using a graphite cup atomizer, and w i t h the AF measurement being performed i n the vapour plume above the atomizer (885, see ARAAS, 1981 , 11, 33).Detection l i m i t s f o r a wide range o f elements, including Ag, Coy Cu, Eu, Fey Ir, Mn, Nay Pb, and P t were reported t o be f a c t o r s o f 2 t o 1000 lower than those obtained by ETA-AAS (594). o f Ir i n process solutions, Co i n vegetable matter, and Coy Cu and Fe i n s o i l . Very low detection l i m i t s were also reported by Falk e t a l . (C2113, C2393, C2439) using a s i m i l a r atomizer w i t h a frequency-doubled dye-laser pumped N2 l a s e r as the l i g h t source.only estimated by extrapolation o f the signals obtained w i t h s u b s t a n t i a l l y more concentrated aqueous solutions. paring stable solutions w i t h Pb concentrations near the detection l i m i t s . escence i n the i n d u c t i v e l y coupled plasma (C108, C2115, C2116) and microwave plasma (C200).marginal a n a l y t i c a l use, b u t w i l l be p r i m a r i l y useful f o r the measurement of plasma diagnostics. He and h i s co-workers (C139, C2116) have pointed out, how- ever, t h a t the AF spectra produced are much less complex than the I C P spectrum i t s e l f and w i l l allow the use o f monochromators of more moderate resolution.The resonance l i n e s f o r halogens l i e i n the V.U.V. and d i r e c t observation o f these elements i n the I C P has only been found possible near the power c o i l (see ARAAS, 1981, fly Ref. 1659). Fredeen and Bastians (C108) have shown t h a t C1 can be detected using a non-resonance t r a n s i t i o n from the f i r s t excited s t a t e using l a s e r AF i n the ICP.The most e f f i c i e n t detection was achieved when the e x c i t - a t i o n and fluorescence were a t d i f f e r e n t wavelengths, eliminating the s c a t t e r problem. background emission. The processes t h a t i n h i b i t saturation o f t r a n s i t i o n s i n the I C P have also been reviewed by Bastians (C2115). induced transfer from the o r i g i n a l excited s t a t e t o d i f f e r e n t excited states , and t o i o n i z a t i o n l e v e l s were considered, and used t o determine the degrees o f saturation f o r a number o f atomic t r a n s i t i o n s (see also Section 1.2.1.5).Omenetto e t a l . (1505) have now published t h e i r study on saturation spectral This system was a c t u a l l y used f o r the determination I n both cases, detection l i m i t s a t the f g l e v e l were This was because o f the d i f f i c u l t y o f pre- There appears t o be increasing i n t e r e s t i n the study o f l a s e r excited f l u o r - --- Winefordner (C139) suggested t h a t such systems w i l l only be o f Modulation o f the source also allowed discrimination against d.c.The rates o f c o l l i s i o n a l l y irradiance (see ARAAS, 1981, lJ, 34).S i m i l a r conclusions were reached by deAtomization and Excitation 35 Olivares and Heiftje (135) i n a practical study of atomic t r a n s i t i o n s i n a i r / C2H2 and air/H2 flames. spectral power density, was found t o be independent of the analyte atom concen- t r a t i o n and flame conditions, but was markedly affected by the l a s e r wavelength used t o excite the atoms, the rates of the d i f f e r e n t deactivation paths, and the degeneracy of the two energy levels involved i n the t r a n s i t i o n excited.optoelectronic cross correlation method has been developed f o r the determination of the lifetime of atomic and molecular excited s t a t e s , and has been applied t o the study of Na i n an air/CZH2 flame (2328), and in a MIP (C200).Such measurements are claimed t o provide an indication of the r a t e s of quenching processes which deactivate excited atoms i n the d i f f e r e n t sources , and hence define the observable emission i n t e n s i t y . of the excited s t a t e is d i r e c t l y proportional t o the quantum efficiency of the fluorescence process.found t o be 0.72 agreement w i t h 1 i t e r a t u r e values (2328). The use of l a s e r saturated fluorescence measurements f o r s p a t i a l diagnostic purposes i s receiving increased i n t e r e s t and more published work i s now avail- able. Measurements of the s p a t i a l d i s t r i b u t i o n of OH radicals in flames have been reported by a number of workers, and provide a useful i n s i g h t i n t o the chemistry of the combustion processes i n premixed and turbulent air/CH4 flames ( 1 459 , 1480) , and ai r/C2H2 flames (2326) , and a hi ghly sooty a i r/C3H8 flame ( 1 480).Temperature measurements i n flames u s i n g l a s e r excited AF have again been reported. The advantages and disadvantages of a method based on thermally assisted AF have been discussed (903, C2119), and the two-line AF method has been proposed as a means t o study non-steady s t a t e conditions i n turbulent combusti on gas f 1 ows (2259).Other references of i n t e r e s t - In t h i s case, the parameter defined as the saturation --- A new In addition, the measured lifetime In an air/C2H2 flame, the Na fluorescence lifetime was 0.07 ns and the quantum efficiency of 0.044 was i n good Shuttle LIDAR resonance fluorescence investigations of K , Na and Mg' number densities i n the upper atmosphere : 1404, 1405. 1.3.5.2 Laser Enhanced Ionization (LEI) Two timely reviews of LEI appeared i n 1982 (1139, 2222), and anyone requiring an introduction t o the fundamental principles of t h i s method, and a state-of- the-art report on i t s analytical potential will find them extremely useful.Flame detection l i m i t s of < 1 pg 1-1 were reported f o r twenty-four elements, and the l a t e s t work on the suppression of ionization interferences, and the analysis of real samples was covered i n depth i n one a r t i c l e (2222). authors were optimistic about the future potential of LEI, and concluded t h a t improved performance may be obtained by separating the high temperature atom- ization stage from a cooler LEI environment, giving low background currents f o r The36 Analytical Atomic Spectroscopy the exci t a t i o n / i o n i zation process.S i g n i f i c a n t improvements i n LEI detection l i m i t s have been reported f o r a number o f elements using two l a s e r beams t o populate high energy e l e c t r o n i c l e v e l s i n an atom (880, C1272, C2111).laser-enhanced i o n i z a t i o n , involved t h e use o f two dye-laser beams, one o f which was frequency doubled. Detection l i m i t s f o r Sn, 0.3 ug 1-1 i n an air/H2 flame; Au, 1 1-19 1 - l ; Cd, 0.1 ug 1 - l ; Coy 0.08 1-1g 1 - l ; Cu, 0.07 ug 1 - I ; N i , 0.08 1-1g 1 - l ; Pb, 0.09 ug 1-1 i n an air/C2H2 flame were s i g n i f i c a n t l y lower than s i n g l e - l a s e r LEI.Spectral s e l e c t i v i t y was also improved i n some cases by use o f the second e x c i t a t i o n wavelength, and accuracy was demonstrated by analysis o f metallurgical RMs. Another group o f workers proposed an extension of t h e LEI approach, c a l l e d The method, c a l l e d stepwise-excitation -- dual l a s e r i o n i z a t i o n (C86, C87, C2112, see also ARAAS, 1981 , 2, Ref. 2033). I n t h i s case, the second l a s e r beam was used t o i o n i z e the atoms excited by the f i r s t l a s e r , i n an analogous approach t o t h a t used i n resonance i o n i z a t i o n spectroscopy. more s e n s i t i v e than s i n g l e - l a s e r LEI and l e s s susceptible t o i o n i z a t i o n i n t e r - ferences. Recent work suggested t h a t DLI i s most advantageous when the energy required from the second l a s e r t o reach the i o n i z a t i o n l i m i t i s close t o the energy o f the second l a s e r photon (C2112).involved two lasers and w i l l be consequently more c o s t l y , the p o t e n t i a l f o r development t o a wider range o f elements and f o r improved s e n s i t i v i t y and freedom from interferences appears considerable.l i m i t a t i o n s o f t u r b u l e n t flames on t o t a l consumption burners are n o t relevant i n the same way as i n spectroscopic measurements. burners i n LEI has been reported i n two conference papers using air/H2, a i r / It was claimed t h a t dual-laser i o n i z a t i o n (DLI) was 2-3 orders Although both these approaches Since LEI measurements are made w i t h electrodes, the l i g h t s c a t t e r i n g The use o f t o t a l consumption C2H2, 02/H2 and 02/C2H2 flames (C2085, C2304).F u l l publication o f the r e s u l t s o f these studies w i l l be o f considerable i n t e r e s t . Other reference o f i n t e r e s t - Laser-enhanced i o n i z a t i o n flame velocimeter : 2325. 1.3.5.3 Other Studies Further studies of l a s e r induced changes i n hollow cathode lamps f i r s t reported i n ARAAS l a s t year (1981, 11, 35) have appeared, b u t these are mostly o f a fundamental nature w i t h l i t t l e a n a l y t i c a l i n t e r e s t (927, 1481). I n a r e l a t e d study (1479), investigations o f several non-laser atomic emission sources f o r t h e i r p o t e n t i a l t o induce optogalvanic signals i n HCLs were reported. The sources included an A r ICP, an 02/H2 flame, a high temperature furnace, EDLs, and HCLs. Successful r e s u l t s were obtained w i t h the A r I C P and w i t h HCLs, and demonstrated s p e c i f i c element optogalvanic detection without the use o f e i t h e r a l a s e r o r an expensive monochromator. Only very low optogalvanic signals wereAtomization and Excitation 37 obtained from the other sources, and d i f f e r e n t i a t i o n o f these from a background signal generated by photoelectric emission and r a d i a t i v e heating o f the cathode was reported t o be d i f f i c u l t . i m p u r i t i e s i n s i n g l e c r y s t a l s o f S i has been reported (779, 783), and represents the f i r s t a n a l y t i c a l application o f t h i s technique. were obtained and the minimum detectable Na l e v e l i n the s o l i d was estimated t o be lo1’ atoms ~ m - ~ . The determination o f Eu by i n t r a c a v i t y atomic absorption using an air/C3H8 flame and the 601.8 nm l i n e has also been described (1106). The use o f resonance i o n i z a t i o n spectroscopy f o r the determination o f Na Semi-quantitative r e s u l t s