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Absorption profiles of flames used in atomic-absorption spectroscopy
Spectrochimica Acta,Vol. 25B.pp. 620 to645. PergamonPrens1070. Printedin Northern Ireland
Absorption profles of flames used in atomic-absorption spectroscopy* CRTJNI L. CEAKRABARTI~, MOHAN KATYAL and DOUGLASE. WILLIS Chemistry Department, Carleton University, Ottawa 1, Onfario, Canada (First received 29 Januaq
1970; in retied form 15 August 1970)
Al&r&--A study has been made of the distributions of metallic atoms in their ground electronic at&e when 8n aqueous or an organic solvent containing the met81 present in the solution in various combined forms is sprsyed into various flames used in atomic-absorption spectroscopy. It has been found that with almost all elements studied, rich flames give greater peak absorbance8 than lesn flames, 8nd the fl8me stoichiometry determines the number of free atoms in 8 flame. It has also been observed that the spatial distribution of free 8tOmf3in 8 flame depends not only on the flame stoichiometry but also on the species which the desired met81 forms in the solution and the flame, on the other anions, complexing agents, 8nd cstions present in the solution, on chemicel and physical properties of the solvent, on the type of atomizer-burner and on the flame temperature.
profiles of a limited number of elements have been reported by RANN and and HAMBLY and RANN [2]. From a study of the flame profiles of ten elements in an air-acetylene flame of a pre-mix atomizer-burner fitted with a IO-cm slot, RANN and HAMBLY [l] made some very important observations, all of which with the exception of one have been found to be valid by the present authors-the exception being the following observation [I, p. 8821. “The distributions, shown in Fig. 3, for the air-acetylene flame suggest that the same number of atoms is released into the flame regardless of the flame conditions. The peak absorbances in the distributions show little differences between the rich and lean flames”. The above observation, if true, would be extremely important both from theoretical and practical aspects. However, it should be noted that none of the ten elements reported by RANN and HAMBLY [l] form a monoxide having a high dissociation energy. Preliminary studies made by CHAKRABARTI et al. [3-51 indicated that the sensitivity of many elements is enhanced by fuel-rich flames, and these findings [3-51 made questionable the above observation of RANN and This study was undertaken in order to resolve the above-mentioned HAMBLY [l]. discrepancies, and also, to study the flame profiles of the elements which are not well atomized, and also some metal-complex and solvent systems. When this FLAME
HAMBLY [l],
* Presented in p8rt 8t the 16th Spectroscopy Symposium of Canada, October 21, 1969, and at the Joint CIC/ACS Conference, Toronto, Canada, M8y 27, 1970. t To whom all correspondence should be addressed. [I] C. S. RANN and A. N. HAMBLY, Anal. Chem. 37, 879 (1965). [2] A. N. P&~LY 8nd C. 5. RANN in Flame En&&n and Atomic Absorption Spectmmetry (edited by J. A. DEAN and T. C. RAINS), Vol. 1, pp. 241-265, Theory. Marcel Dekker, New York and London (1969). BARTI and S. P. SINOEAL,Canadian &e&y. 14,168 (1969). [3] c. L. cHAxJ%A [4] V. S. SASTRI, C. L. CEAKRA~ARTIand D. E. WILLIS, Can. J. Chem. 47, 687 (1969). [5] C. L. CHAKRABARTIand S. P. SINOHA~,Spectrochim. Acta MB, 663 (1969). 1
629
630
CHUNI
L. CHAKBABARTI,MOHAN KATYAL and DOUULA~ E. WILLIS
manuscript was in press it was brought to the notice of the authors that, as a result of studies of flame profiles of some elements in nitrous oxide-acetylene flames of various stoichiometries, FASSELet al. [6] also have questioned the validity of the above observation of RANN and HAMBLY[l] in the case of the elements which form stable monoxides. DEFINITIONOF TERMS Rich flame: a fuel-rich flame. Lean flame: a fuel-lean flame. Atomization: the process that converts the elements to be determined, or its compounds, to free atoms in the ground electronic state. Atomized: converted to free atoms, in the ground electronic state. Sensitivity: the concentration, in part per million, that gives 1% absorption under optimized experimental conditions. EXPERIMENTAL Apparatus Spectrophotometer : 8 Techtron Model AA-3 atomic-absorption photometer fitted with a R-136 photomultiplier tube.
spectro-
Atomizer-burner A Techtron (No. AB 51), lo-cm, air-acetylene, pre-mix, laminar-flow slot burner. A Techtron (No. AB 40), s-cm, nitrous oxide-acetylene, pre-mix, laminarflow, slot burner. A Beckman (No. 4030), “total-consumption” turbulent-flow, medium-bore burner. Hollow-cathode lamp “High-intensity” hollow-cathode lamp for copper; lamps for all other elements.
standard hollow-cathode
Readout system A meter readout and a Sargent Recorder, Model SRL. Reagelats Reagents used in the study were analytical-reagent grade salts: calcium chloride, chromic chloride, copper sulphate, hafnium oxy-chloride, potassiumniobium heptafluoride, potassium-tantalum heptafluoride, potassium titanyl oxalate, lead nitrate, and zirconium oxy-chloride; and cyclopentadienyl compounds of hafnium, titanium, and zirconium. Aqueous solutions of the salts were prepared containing 5-1Oo/o hydrochloric acid (in the case of Ca, Cr, Cu, Hf, Pb, Ti and Zr) or 10% hydrofluoric acid (in the case of Nb and Ta). Solutions of the cyclopentadienyl compounds were made in ethanol and methyl isobutyl ketone. Chemically pure acids, organic solvents, and other chemicals, and doubly-distilled water were used throughout. [6] V. A. FASIEL, J. A. RASMUSON,R. N. KNISELEY and T. G. Cowmy, &pe&vchim. Part B (in press).
Acta
Absorption profiles of Bames nsed in Alec-abortion
~~~o~opy
631
E~~~~~~~t~ co~~t~~ The optical arrangement for the measurement of the flame profiles was the usual set-up of a Techtron, Model AA-3, atomic-absorption spectrophotometer with a vertioal slit of 12 mm height and two condensing lenses. The lenses were firmly clamped on the optical bench, one on each side of the atomizer-burner-the lens nearer to the hollow-cathode lamp to be called lens No. 1 and the lens nearer to the mono&romator to be called lens No. 2. A thin metal disc with a pinhole aperture 1 mm in dia. was plaoed between the burner and the condensing lens No. 2. Other experimental set-up and the operating oouditious which were employed to obtain absorption profilea are s~lmmari~edin Tables 1-3. The incident Table I. Experimental conditions with the Techtron &cm N&%--C&H, burner Ele-
SOlVent
Taken as
ment
Llbmp current (mA)
Wavelength (A)
SpectrEI Flow rates(l/ruin) band pess --..--Rich flame* Lean flamet (A) N,O C,H, N,O C,H,
20
3653.6 3643
3.3
%6
8
5.5
4.8
Ti-cupferrate Cp,TiCl,
Aq. soln. containing 6 % (v/v) HCl iMIBK C,H,OH
20 20
3.3 3.3
6.6 6.6
5 5
-
-
Ti
CP*Ti~ln
MIBK
20
3.3
6.5
6
-
-
Zr
ZrOCi,
20
3.3
6G
6
-
-
Zr Zr Hf
CP&% CP&r% HfOCI,
3.3 3.3 3-3
&6 &6 5.6
5 6 6
-
-
3tIf
Cp,HfCl,
Aq. sol& contsining 10% (v/v) HCl C,H,OH MIBK Aq. aoln. containing 10% (v/v) HCI C,H,OH MIBK Aq. soln. containing 10 % (v/v) HF Aq. eoln. containing 10% (v/v) HF
3643 36036 3643 3863.6 3643 3801
Ti
K,TiO (Ox),
Ti Ti
CP,HfCl, K,NbF,
K,TaF,
Ta
20 20 20
20 20 20
3073 3073 4069
3.3 3.3 3.3
5.5 6.6 6-b
5 6 5
-
-
20
2714
3.3
5.5
.6
-
-
* 40-60
mm red “feather”. t 4-B mm red “feather”.
Table 2. Experimental conditions with the IO-cm air-acetylene burner. All elements taken as shown below in the Table were dissolved in aqueous solvent containing 5 % (v/v) hydroohIoric acid Element
Taken as
cu
cuso,
Ca Sn Pb Cr
C&C&$ SnCl, PbfNOs), (‘rC1,
Lsmp current
Wave-
tmA)
(4
4 10 IO 10 10
3247 4227 2863 2170 3579
length
Spectral Flow rates (I/min) band pass ____-~ Rich flame Lean flame W Air C$l$ Air C,H, 1.7 1.7 3.3 6.6 3.3
8.6 8.6 8-5 8.6 8.5
1.5 l-5 l-5 1.5 I.5
8.5 8-5 6*8 S-6 8.5
0.9 0.9 0.9 O-9 o-9
-
632
CWNI L. c EA~RABARTI, MOIIAN KATYAL
and DOUULASE.
WILLIS
Table 3. Experimental conditions with the Beckman oxy-acetylene burner. All elements taken as shown below in the table were dissolved in aqueous solvent containing 6 o/0 (v/v) hydrochloric acid Element
cu Ca Sn Pb Cr
Taken as
cusoa CaCl, SnCl, Pb(NO,), CrCl,
Lamp current (mA) 4 10 10 10 10
Wavelength (A) 3247 4227 2863 2170 3579
Spectral band pass (A) 1.7 1.7 3.3 66 3.3
Plow rates (l/mm) Rich flame
Lean flame
0s
CsBs
0,
C&l
1.3 1.3 1.3 1.3 I.3
3.1 3-l 3.1 3.1 3.1
1.3 1.3 I.3 1.3
1.3 1.3 1.3 1.3
radiation from the hollow-cathode lamp was focussed in the flame by lens No. 1, and this image refocussed by lens No. 2 on the monochromator entrance slit. The burner slot of the Techtron burners was aligned on the optical axis; the Beckman burner was mounted in the usual way. The distribution of the ground-state atoms in the flame was measured by the atomic-absorption spectroscopic technique. The refraction pattern (1) of the incident radiation caused by the flame was scanned past the pinhole aperture (1 mm in dia.) by movement of the flame, performed by horizontal and vertical traversing screws attached to the burner support. An absorbance reading was taken at each point of 1 mm of the horizontal traverse, and 2 mm of the vertical traverse in the case of Techtron burners, and 2 mm of the horizontal traverse and 2 mm of the vertical traverse in the case of the Beckman oxy-acetylene burner. At each point, distilled water and then the test solution were nebulized. The blank absorbance value obtained by nebulizing the solvent (either distilled water or organic solvents) in the flame, was subtracted from the gross absorbance obtained by nebulizing the test solution; the result was the net absorbance, which was shown on the absorption profiles. RESULTSAND DISCUSSION Figures l-5 show absorption profiles in flames produced by the air-acetylene pre-mix slot atomizer-burner. Figures 6-16 show absorption profiles in flames produced by the nitrous oxide-acetylene pre-mix slot atomizer-burner (laminar flames). Figures 17-21 show absorption profiles in flames produced by the oxyacetylene “total-consumption” burner (turbulent flames). Although the Figs. l-16 relate to laminar flames whereas Figs. 17-21 relate to turbulent flames, wherein eddy-currents make flame profiles more difficult to interpret, it may still be interesting to compare the similarities and dissimilarities of the flame profiles in general between the rich and lean flames, and between the flame profiles of the same element in the laminar and turbulent flames. One feature which is common to all figures irrespective of the flames and the systems investigated is that, with the exception of Cu in Fig. 1, rich flames give greater peak absorbance than lean flames, and that the flame stoichiometry determines the number of free atoms found in the flame. These observations are contrary to the observations
Absorption profiles of flames used in atomic-absorption spectroscopy RICH FLAME 14
LEAN
633
FLAME
/ ~''35~'~
12 E E I0-
0 In,,. i,i
8" n,, m
6
-
4
-
0 m
I,-
S2 w -r.
2 _ -~
8mm
•
(a)
4
8 m m -----~
(b)
Fig. l. Absorption profile in the 10-ore air-acetylene flame. 1 pg/ml t a k e n as CuSO a in aqueous solution.
Cu 3247A.
Cu
j!o 14
E 12 E
n,.' W 2: n,,"
-'8 m
W
<
b-lW
2 t
6mm
(a)
~
~
6 mm
(b)
Fig. 2. Absorption profile in the 10-cm air-acetylene flame. P b 2170A. P b 100 ~g[rnl taken as Pb(NOs) s in aqueous solution; Fig. (a) rich flame; Fig. (b) lean flame.
634
CEUNI L. CHAKRABARTI, MOHAN KATYAL and DOUGLASE. WILLIS RICH
-
LEAN
FLAME
FLAME
-4mm-
4mmi--,
(0)
(b)
Fig. 3. Absorption profile in the lo-cm air-acetylene flame. 20 pg/ml taken 6s C&l, in aqueous solution. RICH
FLAME;
-4mm-
250RPM.
LEAN
FLAME;
Ca 422lA.
Ca
500RRM.
-4mm(0)
(b)
Fig. 4. Absorption profile in the lo-cm air-acetylene flame. Cr 3679A. Cr taken as CrCI, in aqueous solution; Fig. (a) Cr 250 pg/ml; Fig. (b) Cr 500 pg/ml.
Abeorp[ion profi]e~ o f
fl~mes used in e,tom~e.sbso~p~-~:on s p e o ~ r o ~ p y
R|CN F L A M E ; 5 0 0 P , P.WL
L E A N FLAME;5OOOPP, M~
14
o}~O z
nbJ >. 0 m
< 6 i3: :: 4
Fig. 5. /kb~¢pkion profile in t ~ e tO.¢rn a&r-ce~ylene fia,me. Sn 2 8 6 3 A . S~ tskon ~s SnC]~ in aqueous solution; Fig, (v~) Sa 500 pg/ml; Fig. (b) Sn 5000 Fg/ml. RICH FLAME 2O E E ]8
t~J z
LEAN FLAME
m .05 JO
m ~ = 6 1
a
635
CEU-NI
L.
c ~KRAB~BTI, RICH
-
MOEAN
KATYAL
end DOUULA~E. WILLIE
FLAME
3mm
-
+
3mm
-
Fig. 7. Absorption profile in the S-cm nitrous oxide-acetylene flame. Rich flame. Ti 36438. Ti 2600 pg/ml taken as titanium potassium oxalate in aqueous solution.
14-
2-4mm-
Fig. 8. Absorption profile in the 5-cm nitrous oxide-acetylene flame. Rich flame. Ti 3643A. Ti 160 rg/ml taken as titanium cupferrate in 100% methyl isobutyi ketone.
Absorption pro&s
of flsmes used in atomic-absorption
spectroscopy
637
28’
42_
C-
3mm(a)
-
3mm(b)
Fig. 9. Absorption profile in the S-cm nitrous oxide-acetylene flame. Rich flame. Ti 364311. Ti 60 lug/ml taken as cyclopentadienyltitanium chloride; Fig. (a) solvent 100 oAethyl alcohol; Fig. (b) solvent 100 % methyl isobutyl ketone.
made by RANN and HAMBLY[l] for the elements which are common to both papers, viz., Cu, Ca, and Cr. The differences in the absorbances given by the rich and lean flames are greatest for those elements which are not well atomized in flame, e.g., Ti, Zr, Hf, Nb, Ta, etc., and are least for those elements which are well atomized in flames, e.g., Pb. In the case of Cu in Fig. 1 whioh is very well atomized in flame, the above difference disappears. The reason the lean flame shows greater peak absorbance than the rich flame in Fig. 1 is probably that the copper atoms in the latter suffer greater dilution because of the greater volume of a rich flame compared to that of a lean flame. It is interesting to compare Fig. 1 with Fig. 19 and note that with the same copper solution, and with the turbulent-flow oxy-acetylene burner as in Fig. 19, the rich flame gives greater peak absorbance than the lean flame. The other features which are also common to all elements which are poorly atomized in lean flames is that, compared to the situation in lean flames, in rich flames the free atom population is not only greater but extends to a much greater height over the burner-top. This enhancement of the sensitivity by rich flames may be explained [4-S] as due to the greatly diminished concentration of atomic oxygen in rich flame, and the highly exothermic nature of the reaction, C + 0 + CO (which releases 257 kcal mole-l at 25’0, which makes the reduction of metal oxides thermodynamically more favourable in fuel-rich flame. The other feature which
Cnurrr L. CHA~RABARTI,MOEAN KATYAL and DOUGLAS E. WILLIS
26V 26. 24E E .
22 20-
!
18’
w” 5
16-
m
14-
Y 0 4
12. IO-
5 (3
6
’
642. c-
3mm (al
-
C-
3mm
--+
(b)
Fig. 10. Absorption profile in the 6-cm nitrous oxide-acetylene flame. Rich flame. Ti 3663GA. Ti 60 pg/ml taken as cyclopentadienyltitanium chloride; Fig. (a) solvent 100 oA ethyl alcohol; Fig. (b) solvent 100 0/0methyl iaobutyl ketone.
-4mm-
-4mm(a)
(b)
Fig. 11. Absorption profile in the 6-cm nitrous oxide-acetylene flame. Rich flame, Zr 3601A. Aqueous solution; Fig. (a) Zr 2600 rg/ml taken aa ZrOCl,; Fig. (b) Zr 600 pg/ml taken as cyclopentadienylzirconium chloride.
Absorption profiles of flames used in atomic-absorption
-4
-
mm-
spectroscopy
639
5 Mom-
(b)
(a)
Fig. 12. Absorption profile in the &cm nitrous oxide-acetylene flame. Rich flame. Zr 360~4. Zr 500pg/ml taken as cyclopentadienylzirconium chloride; Fig. (a) solvent 100’~ ethyl alcohol; Fig. (b) solvent 100°A methyl isobutyl ketone.
-4
mm-
Fig. 13. Absorption protie in the 5-w nitrous oxide-acetylene flame. Rich game. Hf 3073A; Fig. (a) Hf 6,000 pg/ml taken as HfOC1, in aqueous solution; Fig. (b) Hf 600 pg/ml taken as cyclopentadienylhafnium chloride in 100 % ethyl alcohol. to alI figures are that the peak absorbance is limited to such a small area in the flame that the small-area absorbance technique as suggested by RANN and HAMBLY [l] would give greater sensitivity than the large-area absorbance technique normal@ used; also, the distribution of absorbance values in the flame show a steep gradient on the horizontal direction but a low gradient in the vertical direction. Therefore, with the large-area absorbance technique which is normally used, greater sensitivity will be obtainable [I] ifthe incident radiationis concentrated are common
640
t&UN1
L. c HAKRABAR!l!I,
MOEANI(aTYAL and DOUULAC( E.
WILLIS
-4mm----*
Fig. 14. Absorption profilein the &cm nitrous oxide-acetylene flame. Rich 5ame. Hf 3073A. Hf 600 rg/ml taken as cyclopsntadienylhafnium chloride in 100% methyl isobutyl ketone.
Fig. 16. Absorption profilein the 6-cm nitrous oxide-acetylene flame. Rich 5ame. Nb 4068A. Nb 2,600 rg/ml taken as I&NbF, in aqueous solution containing 10 % (v/v) HF.
in a vertical direction, as can be accomplished by the use of a hollow-cathode of height five times greater than the width (instead of the circular hollow-cathode normally used). These observations agree with those made by RANN and HAMBLY [l] for the few simple systems studied by them in the flame of an air-acetylene pre-mix slot burner. The striking dissimilarity between the profiles given by the laminar and turbulent flames is that in the rich flames of the former, the peak absorbance occurs at a much lower height in the flame, and the absorbance values fall off with the increasing height from the burner-top (because of increasing dilution with the increasing height in the flame), whereas in the rich flames of the latter, the peak absorbance occurs at a much greater height over the burner-top, and in the case
Absorption
proSleaof flames used in atomic-absorption
spectroscopy
641
Fig. 16. Absorption profile in the s-cm nitrous oxide-acetylene flame. Rich flame. Ta 2714A. Ta 2500 @g/ml taken as &TaF, in equeous solution containing 10 % (v/v) HF.
RICH
-I2
FLAME
mm-
-12
(0) Fig. 17. Absorption
mm(b)
profilein the oxy-acetylene
flame. Ca 422711. Ca 10 ,ug/ml taken as CaCla in aqueous solution.
of Figs. 19 (Cu) and 20 (Pb), the peak absorbance occurs at the maximum height (measured) above the burner-top, and the absorbance values fall off with the decreasing height over the burner-top. This dissimilarity can be rationalized in terms of the difference in the diameter of the droplets produced by the above burners, the burning velocity and the temperature of the two flames and the cooling effect of the nebulized solution on the flame temperature. The diameter of droplets which enter into the flame of a Iaminar-flow indirect (chamber-type) burner (the Techtron air-acetylene burner) is, in general, smaller than what enters into the flame of a turbulent-flow “total-consumption” direct burner (the Beckman
642
CHUM L. CHAKRABAETI,MOHAN &TYAL RICH
OFig.
18.
+8mm-
FLAME
end Doua~as
LEAN
E. WILLIS
FLAME
-6mmm---r (b)
(a)
Absorptionprofile in the oxy-acetylene
flame. Cr 36798. taken as CM& in aqueous solution.
RICH
c--
LEAN
FLAME
18-
(al
c--
Cr 609 pg/d
FLAME
12 mm(b)
Fig. 19. Absorption profile in the oxy-acetylene flame. Cu 3247A. taken a8 CuSO, in aqueous solution.
Cu 10 pg/ml
oxy-acetylene burner) [7, p. 281. Also, the burning velocity of the latter flame is much greater than that of the former [7, p. 771; hence, in a stable flame produced by these two burners, the vertical rise velocity of the latter flame is usually much greater than that of the former flame. Thus, in the former flame the processes of desolvation, vaporization, and dissociation which lead to the formation of free atoms aze allowed more time to proceed fhan in the latter flame. In other words, in the former flame at a lower height above the burner-top, the number of free at,oms formed is greater than that in the latter flame. In the above comparison of the absorption profiles shown in Figs. 1-13, 15-16 (Fig. 14 is an exception) (treating all these figures as one group) with those of Figs. 19-20 (treating these two figures as another group), the difference in the actual flame temperature profiles and in the distribution of temperature in the two flames, have not been [7]
R. HERMANN and C. T. J. ALICEMADE, Chemical Alaalysis by Flame Photmetry by P. T. GILBERT, JR.), 2nd Edn. Intorscience (1903).
(Translated
Absorption pro&s of fiamw used in ache-abortion
E
RtCH
-12
FLAME
mm--w
LEAN
c---l2
spectrowupy
613
FLAME
mm--4 (b)
(a)
Fig. 20. Absorption profile in the oxy-acetylene flame. Pb 217OA. Pb 25 pg/ml taken as PbfNO,), in aqueous solution.
-6mmPig. 21. Absorption profile in the oxy-acetylene f&me. Sn 2363A. Sn 200 pg/mI t&m aa &&I, in aqueous solution.
considered although such differeuoes are important and must be considered in any rigorous comparison. However, certain generalizations can be drawn, Although the theoretical temperature of a ‘dry” stoicbiometric oxy-acetylene flame is about 800°C [7, p. 771 greater than that of a Wry” stoichiometrio airacetylene flame, in the former case the oooling effect of the much larger volume of nebulized solution may wipe away most of this difference of 800°C between these two flames [8, p. 106]. ln any case, it is seen from Figs. 19 and 20 that the droplets of larger diameter entering into the oxy-aoetylene flame take longer time for @] C. TE, J. ALKEMLU)I,in PEarneI&&w& J. A.
DBAX
and T. C. ms),
und Atom& Abrpt&m Spm ry (edited by Vol. I, Theory. &mxl Dekker, New York and London (1969).
644
CIIUNIL. c ~BARTI,
MOHAN KATYALand DOIJULA~ E. WILLIS
desolvation than the droplets of smaller diameter which enter into the airacetylene flame in spite of a somewhat higher temperature and a greater rate of desolvation due to the turbulent motion of the gases in the turbulent oxy-acetylene flame [S, p. 281. It should be noted that the actual temperature (not measured) of “dry” flames of the stoichiometry used in this study is lower than the theoretical temperature of stoichiometric “dry” flames. However, the above discussion concerning the difference in temperature of the two flames may still have some validity. As mentioned above, Fig. 14 is an exception to the general trend shown by the group of Figs. 1-13 and 15-16 since in Fig. 14 the absorbance values increase with the increasing height. However, besides this apparent similarity of Fig. 14 with those of the group of Figs. 19-20, the spatial distribution pattern of free atoms in Fig. 14 is quite different from that in Figs. 19-20. Comparison of Fig. 13(b) with Fig. 14 leads one to suspect that with the same experimental conditions, the same compound in both cases, viz., cyclopentadienyl titanium chloride but different organic solvents, the droplets formed by methyl isobutyl ketone (Fig. 14) take a longer time in desolvation than those formed by ethyl alcohol. Figures 17, 18 and 21 show flame profiles which are quite different from the group of Figs. 19-20 and resemble the group of Figs. l-13 and 15-16 in the way Figs. 17, 18 and 21 show peak absorbance lower in the flame, and the absorbance values then decrease with the increasing height above the burner-top. This can be rationalized in terms of the processes mentioned earlier. However, the discussion given in this paper is enough to show that the flame profile of an element is a complex function of many variables. The interpretation of flame profiles is further complicated by a large number of simultaneous and consecutive reactions which occur in flame. Comparison of Fig. 8 and Fig. 9(b), shows that, with the same solvent (100% methyl isobutyl ketone), titanium as CpzTiCI, (titanium cyclopentadienyl chloride) shows 22.5 fold enhancement in sensitivity over titanium as titanium cupferrate. Since the solvent was same in both cases, and the flame was strongly reducing in both cases, it is not unreasonable to conclude that the enhancement is not due to the solvent but is due to the particular species formed by titanium in these two cases. Comparison of Fig. 11(a) with Fig. 11(b) shows that with the same solvent (aqueous solution containing 5% (v/v) of hydrochloric acid), zirconium as Cp,ZrCl, (zirconium cyclopentadienyl chloride) gives a five fold enhancement in sensitivity over zirconium as zirconium oxychloride. Figure 12(a) and (b) show that the substitution of organic solvents viz. ethanol or methyl isobutyl ketone for the above-mentioned aqueous solution of zirconium brings about only a slight increase in sensitivity over that shown by Fig. 11(b). Similar enhancement in sensitivity has been observed in the case of hafnium as can be seen from Fig. 13 and 14, and also in the case of niobium and tantalum (see Figs. 15 and 16 and Ref. 4, p. 690), and indeed for any element which forms oxides of high dissociation energy [4-51. The reason for such enhancement in sensitivity has been explained by CHAKRABARTIet al. [4-51. Comparisons of Fig. 9(a) with Fig. 9(b), Fig. 10(a) with Fig. 10(b), Fig. 12(a) with Fig. 12(b), and Fig. 13(b) with Fig. 14, show the effect of solvents on Ti, Zr,
Absorption prof3lm of flames used in a~mic-abso~tion
spectroscopy
645
and Hf as cyclopentadienyl compounds in ethyl alcohol and methyl isobutyf ketone. To summarize, the spatial distribution of free atoms in a flame depends not only on the flame stoichiometry but also on the species which the desired element has formed in the solution and in the flame [4], on the other anions, complexing agents, and cations present in the solution, chemical and physical properties of the solvent, type of the atomizer-burner, the flame temperature, etc. It is evident from the above observations that any change (deliberate or unintentional) in any of the above variables (and also any change in other experimental conditions) is likely to change the absorption profile. Any such change should be followed by re-determination of the abso~tion profile under the changed conditions, and resetting the apparatus to take advantage of the changed pro&. Act7mowledgeme&s-!&e authors are indebted to the Ontario Department and the National Research Council of Canada, for research grants.
2
of University Affairs,
Absorption profles of flames used in atomic-absorption spectroscopy* CRTJNI L. CEAKRABARTI~, MOHAN KATYAL and DOUGLASE. WILLIS Chemistry Department, Carleton University, Ottawa 1, Onfario, Canada (First received 29 Januaq
1970; in retied form 15 August 1970)
Al&r&--A study has been made of the distributions of metallic atoms in their ground electronic at&e when 8n aqueous or an organic solvent containing the met81 present in the solution in various combined forms is sprsyed into various flames used in atomic-absorption spectroscopy. It has been found that with almost all elements studied, rich flames give greater peak absorbance8 than lesn flames, 8nd the fl8me stoichiometry determines the number of free atoms in 8 flame. It has also been observed that the spatial distribution of free 8tOmf3in 8 flame depends not only on the flame stoichiometry but also on the species which the desired met81 forms in the solution and the flame, on the other anions, complexing agents, 8nd cstions present in the solution, on chemicel and physical properties of the solvent, on the type of atomizer-burner and on the flame temperature.
profiles of a limited number of elements have been reported by RANN and and HAMBLY and RANN [2]. From a study of the flame profiles of ten elements in an air-acetylene flame of a pre-mix atomizer-burner fitted with a IO-cm slot, RANN and HAMBLY [l] made some very important observations, all of which with the exception of one have been found to be valid by the present authors-the exception being the following observation [I, p. 8821. “The distributions, shown in Fig. 3, for the air-acetylene flame suggest that the same number of atoms is released into the flame regardless of the flame conditions. The peak absorbances in the distributions show little differences between the rich and lean flames”. The above observation, if true, would be extremely important both from theoretical and practical aspects. However, it should be noted that none of the ten elements reported by RANN and HAMBLY [l] form a monoxide having a high dissociation energy. Preliminary studies made by CHAKRABARTI et al. [3-51 indicated that the sensitivity of many elements is enhanced by fuel-rich flames, and these findings [3-51 made questionable the above observation of RANN and This study was undertaken in order to resolve the above-mentioned HAMBLY [l]. discrepancies, and also, to study the flame profiles of the elements which are not well atomized, and also some metal-complex and solvent systems. When this FLAME
HAMBLY [l],
* Presented in p8rt 8t the 16th Spectroscopy Symposium of Canada, October 21, 1969, and at the Joint CIC/ACS Conference, Toronto, Canada, M8y 27, 1970. t To whom all correspondence should be addressed. [I] C. S. RANN and A. N. HAMBLY, Anal. Chem. 37, 879 (1965). [2] A. N. P&~LY 8nd C. 5. RANN in Flame En&&n and Atomic Absorption Spectmmetry (edited by J. A. DEAN and T. C. RAINS), Vol. 1, pp. 241-265, Theory. Marcel Dekker, New York and London (1969). BARTI and S. P. SINOEAL,Canadian &e&y. 14,168 (1969). [3] c. L. cHAxJ%A [4] V. S. SASTRI, C. L. CEAKRA~ARTIand D. E. WILLIS, Can. J. Chem. 47, 687 (1969). [5] C. L. CHAKRABARTIand S. P. SINOHA~,Spectrochim. Acta MB, 663 (1969). 1
629
630
CHUNI
L. CHAKBABARTI,MOHAN KATYAL and DOUULA~ E. WILLIS
manuscript was in press it was brought to the notice of the authors that, as a result of studies of flame profiles of some elements in nitrous oxide-acetylene flames of various stoichiometries, FASSELet al. [6] also have questioned the validity of the above observation of RANN and HAMBLY[l] in the case of the elements which form stable monoxides. DEFINITIONOF TERMS Rich flame: a fuel-rich flame. Lean flame: a fuel-lean flame. Atomization: the process that converts the elements to be determined, or its compounds, to free atoms in the ground electronic state. Atomized: converted to free atoms, in the ground electronic state. Sensitivity: the concentration, in part per million, that gives 1% absorption under optimized experimental conditions. EXPERIMENTAL Apparatus Spectrophotometer : 8 Techtron Model AA-3 atomic-absorption photometer fitted with a R-136 photomultiplier tube.
spectro-
Atomizer-burner A Techtron (No. AB 51), lo-cm, air-acetylene, pre-mix, laminar-flow slot burner. A Techtron (No. AB 40), s-cm, nitrous oxide-acetylene, pre-mix, laminarflow, slot burner. A Beckman (No. 4030), “total-consumption” turbulent-flow, medium-bore burner. Hollow-cathode lamp “High-intensity” hollow-cathode lamp for copper; lamps for all other elements.
standard hollow-cathode
Readout system A meter readout and a Sargent Recorder, Model SRL. Reagelats Reagents used in the study were analytical-reagent grade salts: calcium chloride, chromic chloride, copper sulphate, hafnium oxy-chloride, potassiumniobium heptafluoride, potassium-tantalum heptafluoride, potassium titanyl oxalate, lead nitrate, and zirconium oxy-chloride; and cyclopentadienyl compounds of hafnium, titanium, and zirconium. Aqueous solutions of the salts were prepared containing 5-1Oo/o hydrochloric acid (in the case of Ca, Cr, Cu, Hf, Pb, Ti and Zr) or 10% hydrofluoric acid (in the case of Nb and Ta). Solutions of the cyclopentadienyl compounds were made in ethanol and methyl isobutyl ketone. Chemically pure acids, organic solvents, and other chemicals, and doubly-distilled water were used throughout. [6] V. A. FASIEL, J. A. RASMUSON,R. N. KNISELEY and T. G. Cowmy, &pe&vchim. Part B (in press).
Acta
Absorption profiles of Bames nsed in Alec-abortion
~~~o~opy
631
E~~~~~~~t~ co~~t~~ The optical arrangement for the measurement of the flame profiles was the usual set-up of a Techtron, Model AA-3, atomic-absorption spectrophotometer with a vertioal slit of 12 mm height and two condensing lenses. The lenses were firmly clamped on the optical bench, one on each side of the atomizer-burner-the lens nearer to the hollow-cathode lamp to be called lens No. 1 and the lens nearer to the mono&romator to be called lens No. 2. A thin metal disc with a pinhole aperture 1 mm in dia. was plaoed between the burner and the condensing lens No. 2. Other experimental set-up and the operating oouditious which were employed to obtain absorption profilea are s~lmmari~edin Tables 1-3. The incident Table I. Experimental conditions with the Techtron &cm N&%--C&H, burner Ele-
SOlVent
Taken as
ment
Llbmp current (mA)
Wavelength (A)
SpectrEI Flow rates(l/ruin) band pess --..--Rich flame* Lean flamet (A) N,O C,H, N,O C,H,
20
3653.6 3643
3.3
%6
8
5.5
4.8
Ti-cupferrate Cp,TiCl,
Aq. soln. containing 6 % (v/v) HCl iMIBK C,H,OH
20 20
3.3 3.3
6.6 6.6
5 5
-
-
Ti
CP*Ti~ln
MIBK
20
3.3
6.5
6
-
-
Zr
ZrOCi,
20
3.3
6G
6
-
-
Zr Zr Hf
CP&% CP&r% HfOCI,
3.3 3.3 3-3
&6 &6 5.6
5 6 6
-
-
3tIf
Cp,HfCl,
Aq. sol& contsining 10% (v/v) HCl C,H,OH MIBK Aq. aoln. containing 10% (v/v) HCI C,H,OH MIBK Aq. soln. containing 10 % (v/v) HF Aq. eoln. containing 10% (v/v) HF
3643 36036 3643 3863.6 3643 3801
Ti
K,TiO (Ox),
Ti Ti
CP,HfCl, K,NbF,
K,TaF,
Ta
20 20 20
20 20 20
3073 3073 4069
3.3 3.3 3.3
5.5 6.6 6-b
5 6 5
-
-
20
2714
3.3
5.5
.6
-
-
* 40-60
mm red “feather”. t 4-B mm red “feather”.
Table 2. Experimental conditions with the IO-cm air-acetylene burner. All elements taken as shown below in the Table were dissolved in aqueous solvent containing 5 % (v/v) hydroohIoric acid Element
Taken as
cu
cuso,
Ca Sn Pb Cr
C&C&$ SnCl, PbfNOs), (‘rC1,
Lsmp current
Wave-
tmA)
(4
4 10 IO 10 10
3247 4227 2863 2170 3579
length
Spectral Flow rates (I/min) band pass ____-~ Rich flame Lean flame W Air C$l$ Air C,H, 1.7 1.7 3.3 6.6 3.3
8.6 8.6 8-5 8.6 8.5
1.5 l-5 l-5 1.5 I.5
8.5 8-5 6*8 S-6 8.5
0.9 0.9 0.9 O-9 o-9
-
632
CWNI L. c EA~RABARTI, MOIIAN KATYAL
and DOUULASE.
WILLIS
Table 3. Experimental conditions with the Beckman oxy-acetylene burner. All elements taken as shown below in the table were dissolved in aqueous solvent containing 6 o/0 (v/v) hydrochloric acid Element
cu Ca Sn Pb Cr
Taken as
cusoa CaCl, SnCl, Pb(NO,), CrCl,
Lamp current (mA) 4 10 10 10 10
Wavelength (A) 3247 4227 2863 2170 3579
Spectral band pass (A) 1.7 1.7 3.3 66 3.3
Plow rates (l/mm) Rich flame
Lean flame
0s
CsBs
0,
C&l
1.3 1.3 1.3 1.3 I.3
3.1 3-l 3.1 3.1 3.1
1.3 1.3 I.3 1.3
1.3 1.3 1.3 1.3
radiation from the hollow-cathode lamp was focussed in the flame by lens No. 1, and this image refocussed by lens No. 2 on the monochromator entrance slit. The burner slot of the Techtron burners was aligned on the optical axis; the Beckman burner was mounted in the usual way. The distribution of the ground-state atoms in the flame was measured by the atomic-absorption spectroscopic technique. The refraction pattern (1) of the incident radiation caused by the flame was scanned past the pinhole aperture (1 mm in dia.) by movement of the flame, performed by horizontal and vertical traversing screws attached to the burner support. An absorbance reading was taken at each point of 1 mm of the horizontal traverse, and 2 mm of the vertical traverse in the case of Techtron burners, and 2 mm of the horizontal traverse and 2 mm of the vertical traverse in the case of the Beckman oxy-acetylene burner. At each point, distilled water and then the test solution were nebulized. The blank absorbance value obtained by nebulizing the solvent (either distilled water or organic solvents) in the flame, was subtracted from the gross absorbance obtained by nebulizing the test solution; the result was the net absorbance, which was shown on the absorption profiles. RESULTSAND DISCUSSION Figures l-5 show absorption profiles in flames produced by the air-acetylene pre-mix slot atomizer-burner. Figures 6-16 show absorption profiles in flames produced by the nitrous oxide-acetylene pre-mix slot atomizer-burner (laminar flames). Figures 17-21 show absorption profiles in flames produced by the oxyacetylene “total-consumption” burner (turbulent flames). Although the Figs. l-16 relate to laminar flames whereas Figs. 17-21 relate to turbulent flames, wherein eddy-currents make flame profiles more difficult to interpret, it may still be interesting to compare the similarities and dissimilarities of the flame profiles in general between the rich and lean flames, and between the flame profiles of the same element in the laminar and turbulent flames. One feature which is common to all figures irrespective of the flames and the systems investigated is that, with the exception of Cu in Fig. 1, rich flames give greater peak absorbance than lean flames, and that the flame stoichiometry determines the number of free atoms found in the flame. These observations are contrary to the observations
Absorption profiles of flames used in atomic-absorption spectroscopy RICH FLAME 14
LEAN
633
FLAME
/ ~''35~'~
12 E E I0-
0 In,,. i,i
8" n,, m
6
-
4
-
0 m
I,-
S2 w -r.
2 _ -~
8mm
•
(a)
4
8 m m -----~
(b)
Fig. l. Absorption profile in the 10-ore air-acetylene flame. 1 pg/ml t a k e n as CuSO a in aqueous solution.
Cu 3247A.
Cu
j!o 14
E 12 E
n,.' W 2: n,,"
-'8 m
W
<
b-lW
2 t
6mm
(a)
~
~
6 mm
(b)
Fig. 2. Absorption profile in the 10-cm air-acetylene flame. P b 2170A. P b 100 ~g[rnl taken as Pb(NOs) s in aqueous solution; Fig. (a) rich flame; Fig. (b) lean flame.
634
CEUNI L. CHAKRABARTI, MOHAN KATYAL and DOUGLASE. WILLIS RICH
-
LEAN
FLAME
FLAME
-4mm-
4mmi--,
(0)
(b)
Fig. 3. Absorption profile in the lo-cm air-acetylene flame. 20 pg/ml taken 6s C&l, in aqueous solution. RICH
FLAME;
-4mm-
250RPM.
LEAN
FLAME;
Ca 422lA.
Ca
500RRM.
-4mm(0)
(b)
Fig. 4. Absorption profile in the lo-cm air-acetylene flame. Cr 3679A. Cr taken as CrCI, in aqueous solution; Fig. (a) Cr 250 pg/ml; Fig. (b) Cr 500 pg/ml.
Abeorp[ion profi]e~ o f
fl~mes used in e,tom~e.sbso~p~-~:on s p e o ~ r o ~ p y
R|CN F L A M E ; 5 0 0 P , P.WL
L E A N FLAME;5OOOPP, M~
14
o}~O z
nbJ >. 0 m
< 6 i3: :: 4
Fig. 5. /kb~¢pkion profile in t ~ e tO.¢rn a&r-ce~ylene fia,me. Sn 2 8 6 3 A . S~ tskon ~s SnC]~ in aqueous solution; Fig, (v~) Sa 500 pg/ml; Fig. (b) Sn 5000 Fg/ml. RICH FLAME 2O E E ]8
t~J z
LEAN FLAME
m .05 JO
m ~ = 6 1
a
635
CEU-NI
L.
c ~KRAB~BTI, RICH
-
MOEAN
KATYAL
end DOUULA~E. WILLIE
FLAME
3mm
-
+
3mm
-
Fig. 7. Absorption profile in the S-cm nitrous oxide-acetylene flame. Rich flame. Ti 36438. Ti 2600 pg/ml taken as titanium potassium oxalate in aqueous solution.
14-
2-4mm-
Fig. 8. Absorption profile in the 5-cm nitrous oxide-acetylene flame. Rich flame. Ti 3643A. Ti 160 rg/ml taken as titanium cupferrate in 100% methyl isobutyi ketone.
Absorption pro&s
of flsmes used in atomic-absorption
spectroscopy
637
28’
42_
C-
3mm(a)
-
3mm(b)
Fig. 9. Absorption profile in the S-cm nitrous oxide-acetylene flame. Rich flame. Ti 364311. Ti 60 lug/ml taken as cyclopentadienyltitanium chloride; Fig. (a) solvent 100 oAethyl alcohol; Fig. (b) solvent 100 % methyl isobutyl ketone.
made by RANN and HAMBLY[l] for the elements which are common to both papers, viz., Cu, Ca, and Cr. The differences in the absorbances given by the rich and lean flames are greatest for those elements which are not well atomized in flame, e.g., Ti, Zr, Hf, Nb, Ta, etc., and are least for those elements which are well atomized in flames, e.g., Pb. In the case of Cu in Fig. 1 whioh is very well atomized in flame, the above difference disappears. The reason the lean flame shows greater peak absorbance than the rich flame in Fig. 1 is probably that the copper atoms in the latter suffer greater dilution because of the greater volume of a rich flame compared to that of a lean flame. It is interesting to compare Fig. 1 with Fig. 19 and note that with the same copper solution, and with the turbulent-flow oxy-acetylene burner as in Fig. 19, the rich flame gives greater peak absorbance than the lean flame. The other features which are also common to all elements which are poorly atomized in lean flames is that, compared to the situation in lean flames, in rich flames the free atom population is not only greater but extends to a much greater height over the burner-top. This enhancement of the sensitivity by rich flames may be explained [4-S] as due to the greatly diminished concentration of atomic oxygen in rich flame, and the highly exothermic nature of the reaction, C + 0 + CO (which releases 257 kcal mole-l at 25’0, which makes the reduction of metal oxides thermodynamically more favourable in fuel-rich flame. The other feature which
Cnurrr L. CHA~RABARTI,MOEAN KATYAL and DOUGLAS E. WILLIS
26V 26. 24E E .
22 20-
!
18’
w” 5
16-
m
14-
Y 0 4
12. IO-
5 (3
6
’
642. c-
3mm (al
-
C-
3mm
--+
(b)
Fig. 10. Absorption profile in the 6-cm nitrous oxide-acetylene flame. Rich flame. Ti 3663GA. Ti 60 pg/ml taken as cyclopentadienyltitanium chloride; Fig. (a) solvent 100 oA ethyl alcohol; Fig. (b) solvent 100 0/0methyl iaobutyl ketone.
-4mm-
-4mm(a)
(b)
Fig. 11. Absorption profile in the 6-cm nitrous oxide-acetylene flame. Rich flame, Zr 3601A. Aqueous solution; Fig. (a) Zr 2600 rg/ml taken aa ZrOCl,; Fig. (b) Zr 600 pg/ml taken as cyclopentadienylzirconium chloride.
Absorption profiles of flames used in atomic-absorption
-4
-
mm-
spectroscopy
639
5 Mom-
(b)
(a)
Fig. 12. Absorption profile in the &cm nitrous oxide-acetylene flame. Rich flame. Zr 360~4. Zr 500pg/ml taken as cyclopentadienylzirconium chloride; Fig. (a) solvent 100’~ ethyl alcohol; Fig. (b) solvent 100°A methyl isobutyl ketone.
-4
mm-
Fig. 13. Absorption protie in the 5-w nitrous oxide-acetylene flame. Rich game. Hf 3073A; Fig. (a) Hf 6,000 pg/ml taken as HfOC1, in aqueous solution; Fig. (b) Hf 600 pg/ml taken as cyclopentadienylhafnium chloride in 100 % ethyl alcohol. to alI figures are that the peak absorbance is limited to such a small area in the flame that the small-area absorbance technique as suggested by RANN and HAMBLY [l] would give greater sensitivity than the large-area absorbance technique normal@ used; also, the distribution of absorbance values in the flame show a steep gradient on the horizontal direction but a low gradient in the vertical direction. Therefore, with the large-area absorbance technique which is normally used, greater sensitivity will be obtainable [I] ifthe incident radiationis concentrated are common
640
t&UN1
L. c HAKRABAR!l!I,
MOEANI(aTYAL and DOUULAC( E.
WILLIS
-4mm----*
Fig. 14. Absorption profilein the &cm nitrous oxide-acetylene flame. Rich 5ame. Hf 3073A. Hf 600 rg/ml taken as cyclopsntadienylhafnium chloride in 100% methyl isobutyl ketone.
Fig. 16. Absorption profilein the 6-cm nitrous oxide-acetylene flame. Rich 5ame. Nb 4068A. Nb 2,600 rg/ml taken as I&NbF, in aqueous solution containing 10 % (v/v) HF.
in a vertical direction, as can be accomplished by the use of a hollow-cathode of height five times greater than the width (instead of the circular hollow-cathode normally used). These observations agree with those made by RANN and HAMBLY [l] for the few simple systems studied by them in the flame of an air-acetylene pre-mix slot burner. The striking dissimilarity between the profiles given by the laminar and turbulent flames is that in the rich flames of the former, the peak absorbance occurs at a much lower height in the flame, and the absorbance values fall off with the increasing height from the burner-top (because of increasing dilution with the increasing height in the flame), whereas in the rich flames of the latter, the peak absorbance occurs at a much greater height over the burner-top, and in the case
Absorption
proSleaof flames used in atomic-absorption
spectroscopy
641
Fig. 16. Absorption profile in the s-cm nitrous oxide-acetylene flame. Rich flame. Ta 2714A. Ta 2500 @g/ml taken as &TaF, in equeous solution containing 10 % (v/v) HF.
RICH
-I2
FLAME
mm-
-12
(0) Fig. 17. Absorption
mm(b)
profilein the oxy-acetylene
flame. Ca 422711. Ca 10 ,ug/ml taken as CaCla in aqueous solution.
of Figs. 19 (Cu) and 20 (Pb), the peak absorbance occurs at the maximum height (measured) above the burner-top, and the absorbance values fall off with the decreasing height over the burner-top. This dissimilarity can be rationalized in terms of the difference in the diameter of the droplets produced by the above burners, the burning velocity and the temperature of the two flames and the cooling effect of the nebulized solution on the flame temperature. The diameter of droplets which enter into the flame of a Iaminar-flow indirect (chamber-type) burner (the Techtron air-acetylene burner) is, in general, smaller than what enters into the flame of a turbulent-flow “total-consumption” direct burner (the Beckman
642
CHUM L. CHAKRABAETI,MOHAN &TYAL RICH
OFig.
18.
+8mm-
FLAME
end Doua~as
LEAN
E. WILLIS
FLAME
-6mmm---r (b)
(a)
Absorptionprofile in the oxy-acetylene
flame. Cr 36798. taken as CM& in aqueous solution.
RICH
c--
LEAN
FLAME
18-
(al
c--
Cr 609 pg/d
FLAME
12 mm(b)
Fig. 19. Absorption profile in the oxy-acetylene flame. Cu 3247A. taken a8 CuSO, in aqueous solution.
Cu 10 pg/ml
oxy-acetylene burner) [7, p. 281. Also, the burning velocity of the latter flame is much greater than that of the former [7, p. 771; hence, in a stable flame produced by these two burners, the vertical rise velocity of the latter flame is usually much greater than that of the former flame. Thus, in the former flame the processes of desolvation, vaporization, and dissociation which lead to the formation of free atoms aze allowed more time to proceed fhan in the latter flame. In other words, in the former flame at a lower height above the burner-top, the number of free at,oms formed is greater than that in the latter flame. In the above comparison of the absorption profiles shown in Figs. 1-13, 15-16 (Fig. 14 is an exception) (treating all these figures as one group) with those of Figs. 19-20 (treating these two figures as another group), the difference in the actual flame temperature profiles and in the distribution of temperature in the two flames, have not been [7]
R. HERMANN and C. T. J. ALICEMADE, Chemical Alaalysis by Flame Photmetry by P. T. GILBERT, JR.), 2nd Edn. Intorscience (1903).
(Translated
Absorption pro&s of fiamw used in ache-abortion
E
RtCH
-12
FLAME
mm--w
LEAN
c---l2
spectrowupy
613
FLAME
mm--4 (b)
(a)
Fig. 20. Absorption profile in the oxy-acetylene flame. Pb 217OA. Pb 25 pg/ml taken as PbfNO,), in aqueous solution.
-6mmPig. 21. Absorption profile in the oxy-acetylene f&me. Sn 2363A. Sn 200 pg/mI t&m aa &&I, in aqueous solution.
considered although such differeuoes are important and must be considered in any rigorous comparison. However, certain generalizations can be drawn, Although the theoretical temperature of a ‘dry” stoicbiometric oxy-acetylene flame is about 800°C [7, p. 771 greater than that of a Wry” stoichiometrio airacetylene flame, in the former case the oooling effect of the much larger volume of nebulized solution may wipe away most of this difference of 800°C between these two flames [8, p. 106]. ln any case, it is seen from Figs. 19 and 20 that the droplets of larger diameter entering into the oxy-aoetylene flame take longer time for @] C. TE, J. ALKEMLU)I,in PEarneI&&w& J. A.
DBAX
and T. C. ms),
und Atom& Abrpt&m Spm ry (edited by Vol. I, Theory. &mxl Dekker, New York and London (1969).
644
CIIUNIL. c ~BARTI,
MOHAN KATYALand DOIJULA~ E. WILLIS
desolvation than the droplets of smaller diameter which enter into the airacetylene flame in spite of a somewhat higher temperature and a greater rate of desolvation due to the turbulent motion of the gases in the turbulent oxy-acetylene flame [S, p. 281. It should be noted that the actual temperature (not measured) of “dry” flames of the stoichiometry used in this study is lower than the theoretical temperature of stoichiometric “dry” flames. However, the above discussion concerning the difference in temperature of the two flames may still have some validity. As mentioned above, Fig. 14 is an exception to the general trend shown by the group of Figs. 1-13 and 15-16 since in Fig. 14 the absorbance values increase with the increasing height. However, besides this apparent similarity of Fig. 14 with those of the group of Figs. 19-20, the spatial distribution pattern of free atoms in Fig. 14 is quite different from that in Figs. 19-20. Comparison of Fig. 13(b) with Fig. 14 leads one to suspect that with the same experimental conditions, the same compound in both cases, viz., cyclopentadienyl titanium chloride but different organic solvents, the droplets formed by methyl isobutyl ketone (Fig. 14) take a longer time in desolvation than those formed by ethyl alcohol. Figures 17, 18 and 21 show flame profiles which are quite different from the group of Figs. 19-20 and resemble the group of Figs. l-13 and 15-16 in the way Figs. 17, 18 and 21 show peak absorbance lower in the flame, and the absorbance values then decrease with the increasing height above the burner-top. This can be rationalized in terms of the processes mentioned earlier. However, the discussion given in this paper is enough to show that the flame profile of an element is a complex function of many variables. The interpretation of flame profiles is further complicated by a large number of simultaneous and consecutive reactions which occur in flame. Comparison of Fig. 8 and Fig. 9(b), shows that, with the same solvent (100% methyl isobutyl ketone), titanium as CpzTiCI, (titanium cyclopentadienyl chloride) shows 22.5 fold enhancement in sensitivity over titanium as titanium cupferrate. Since the solvent was same in both cases, and the flame was strongly reducing in both cases, it is not unreasonable to conclude that the enhancement is not due to the solvent but is due to the particular species formed by titanium in these two cases. Comparison of Fig. 11(a) with Fig. 11(b) shows that with the same solvent (aqueous solution containing 5% (v/v) of hydrochloric acid), zirconium as Cp,ZrCl, (zirconium cyclopentadienyl chloride) gives a five fold enhancement in sensitivity over zirconium as zirconium oxychloride. Figure 12(a) and (b) show that the substitution of organic solvents viz. ethanol or methyl isobutyl ketone for the above-mentioned aqueous solution of zirconium brings about only a slight increase in sensitivity over that shown by Fig. 11(b). Similar enhancement in sensitivity has been observed in the case of hafnium as can be seen from Fig. 13 and 14, and also in the case of niobium and tantalum (see Figs. 15 and 16 and Ref. 4, p. 690), and indeed for any element which forms oxides of high dissociation energy [4-51. The reason for such enhancement in sensitivity has been explained by CHAKRABARTIet al. [4-51. Comparisons of Fig. 9(a) with Fig. 9(b), Fig. 10(a) with Fig. 10(b), Fig. 12(a) with Fig. 12(b), and Fig. 13(b) with Fig. 14, show the effect of solvents on Ti, Zr,
Absorption prof3lm of flames used in a~mic-abso~tion
spectroscopy
645
and Hf as cyclopentadienyl compounds in ethyl alcohol and methyl isobutyf ketone. To summarize, the spatial distribution of free atoms in a flame depends not only on the flame stoichiometry but also on the species which the desired element has formed in the solution and in the flame [4], on the other anions, complexing agents, and cations present in the solution, chemical and physical properties of the solvent, type of the atomizer-burner, the flame temperature, etc. It is evident from the above observations that any change (deliberate or unintentional) in any of the above variables (and also any change in other experimental conditions) is likely to change the absorption profile. Any such change should be followed by re-determination of the abso~tion profile under the changed conditions, and resetting the apparatus to take advantage of the changed pro&. Act7mowledgeme&s-!&e authors are indebted to the Ontario Department and the National Research Council of Canada, for research grants.
2
of University Affairs,