Mutual Cation Interference Effects in Flame Photometry WALTER H. FOSTER, Jr.,and DAVID N. HUME Department o f Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge
b Mutual cation enhancement effects with the alkali and alkaline-earth metals were investigated in various flames and atomizer-burner systems. These effects were definitely established to b e true spectroscopic interferences, causing enhancement of the monochromatic radiation measured above continuous background. The trends observed when flame temperature and metal ionization potential were varied indicate that repression of the ionization of the test element in the flame i s the major cause of cation enhancement effects. This displacement of ionization equilibria became strikingly apparent upon simultaneous examination of atomic and ionic lines in the flame spectra of the alkalineearth metals, calcium, strontium, and barium. O n the basis of the results obtained, several recommendations are made regarding optimum conditions for flame spectrochemical analyses.
in emission intensities were considered which could be attributed to factors affecting the rate of addition of sample to the flame and the flame temperature ( 7 ) . I n this investigation, cation interference effects are considered which are not due to any instrumental errors or t o the factors studied previously. These effects are true enhancements of the e iission intensity, nieasurrd above background. Cation and :inion interference effects in flame photometry have been reported by many invebtigators and the results have been characterized by a striking lack of agreunent. Positive effects (enhanrcment) and negative effects (depression) have frequently been at-' tributed to the same test elementinterferer system. Meloche (14) has recently reviewed the subject in addition t o other general background material that can be found (5,8,12, I S , 1 7 ) . Because of the large variety of instruments and flames used by different investigators, variations in emission intensity caused by instrumental factors have usually been confounded with those due to spectroscopic interferences in the flame. This study of cation interference effects was carried out while instrumental factors, such as those studied in the preceding paper, were under control. ARIATIONS
There are several schools of thought regarding cation interference effects. Smit, Alkemade, and cor~orkers(2, 3, 16) explain mutual cation enhancement on the basis of shifting ionization equilibria. Because most of the lines used in flame photometry are due to transitions within the neutral atom, any substance which will suppress the ionization of the test element m i l l enhance its atomic emission. The use of low temperature flames is recommended for determining elements appreciably ionized in the flame source, such as the alkali and alkaline-earth metals. Dippel (6) attributes these enhancements to energy exchange between atoms in excited states. On the other hand, hfargoshes and Vallee (10) attribute most reported enhancements to poor background correction. Monochromatic radiation, measured above continuous background, is reported unaffected by extraneous components of the samplc solution. The importance of using monochromators with high resolution is emphasized along with the use of higher temperature flames t o excite more elements and to decrease anion effects due to compound formation. The purpose of the present investigation was to test the ionization theory of cation enhancement by direct measurcment under unambiguously controlled conditions. As the degree of ionization of a metallic vapor in a high temperature flame increascs with increasing flame temperature, with decreasing ionization potential of the element, and with decreasing concentration of the element (2, S), the per cent enhancement for a given system of test element and interfering element should change in the same way if ionization is the major cause of cation enhancement effects. Thrse relationships have been examincd using the alkali and alkaline-earth mctals. INSTRUMENT AND MATERIALS
The instrument and materials used have been described ( 7 ) . I n addition, a Baird Associates flame photometer with discharge type atomizer and separate Meker-type natural gas burner (19) was used for some of the studies, RESULTS AND DISCUSSION
The degree of ionization of a metallic
39, Mass.
Table 1. Ionization of Alkali and Alkaline Earth Metals in Flame
70Ionization Hydrogen- AcetyleneIonization oxygen oxygen Potential, flame, flame, EleE.V. 2450" K. 2800" K. ment Li 5 390 0 9 16 1 5.0 26.4 Ka 5 138 K 4 330 31.9 82.1 Rb
4 176
44 4
89 6
vapor can be calculated using the Saha equation (15) :
where z is thc dcgrce of ionization, P is the partial pressure in atmospheres of metal in all forms in the burnt flame gas, V , is the metal ionization potential in e. v., T is the absolute temperature in OK., and B is a constant: 6.49 for alkali metals and 5.89 for alkaline earths. Using this equation and assuming the partial pressure of metal in the flame to be 10-6 atm. (4, 13), the per cent ionization values shonn in Table I have been calculated for thc alkali and alkaline-earth metals for the assumed wet flame temperatures given. Alkali Metals. T h e effect of flame temperature on a cation enhancement effect is illustrated by thc, d a t a shomn in Table 11. T h e effect of 0.01M cesium chloride (1330 p.p.m. of cesium) on t h e emission intensity of 100 p.p.m. of potassium as potassium chloride u a s examincd in flames of three different t t m p r a t u r c s . Blanks werc run to check for impurities, and background was carefully subtracted. Slit widths less than 0.05 mni. nere used on the Beckman DC spectrophotometer so that line interfcrrnce could be ruled out. The spectral slit nidth for 0.05mm. instrument slit \I iclth corresponds to 1 mp at 405 mp and 6 nip a t 769 mp. The increase in per cent enhancement with increasing flame tcnqmxture is apparent from these results. I n the Beckman instrument, the 769-mp line (excitation potential 1.61 e.v.) and the 405-mp line (excitation potential 3.06 e.v.) were tested in both flames, and the percentage enhancement in a given flame was found t o be the same for both VOL. 31, NO. 12, DECEMBER 1959
2033
ties. These values are given in Table Iv for calcium, strontium, and barium in each of the flames used in the BeckEstimated % man atomizer-burners. The ionic lines Temp., K. Instrumept Atomizer-Burner Enhancement Flame were measured a t higher instrument Katural gas-air 2100 Baird Discharge +4.i sensitivities as they were \$eak. The Hydrogen-oxygen 2475 Beckman Total-consumption $44.0 emission intensity values used to calcuAcetylene-oxygen 2800 Beckman Total-consumption +66.0 late the ratios given in Table I V were corrected for instrument sensitivity, slit idth, and phototube response. An Table 111. Effect of Sodium on Potasincrease in the ion-to-atom ratio indiTable IV. Relative Ionization of Calsium in Two Atomizer-Burner Systems cates a n increase in degree of ionization. cium, Strontium, and Barium The values obtained illustrate how the Beckman Atomizer-Burner, HydrogenIon Line Intensity Oxygen Flame ionization of a metal vapor in the flame Atom Line Intensity increases with increasing flame teniperP.P.M. Ka Added to Emission Intensity, Hydrogen- &4cetvlene- ature and decreasing element ionization 100 P.P.M. Ka Cm. of Chart Paper oxygen ouygen potential. Kone 11.12 flame flame 500 11.i2 Element The ionization theory of cation en1000 11.88 ( + 6 . 8%) hancement effects would predict that 20 p.p.m. Ca as 2000 12.25 the per cent enhancement for i? constant CaC12 0 029 0 058 20 p.p.m. Sr as Reflux Atomizer, Hydrogen-Oxygen quantity of added electrons should inSrC12 0 060 0 142 Flame6 crease with increasing ion-to-atom in50 p.p.m. Ba as Emission Intensity, tensity ratio in Table IT'. BaC12 0 14G 0 544 YGT Readings on The choice of vehicle by which the DU electrons are added to the flame is None 40 complicated by mutual line interference. Table V. Atomic and Ionic Lines 600 53 As can be seen from Table 5',each of the 1000 $0 ( 505 i 1 -4tomic Lines, Ionic Lines, blue atomic lines of potassium, rubid5000 i4 Element 3Ip N p 10000 90 ium, and cesium, the three alkali K 404.4 ... metals capable of supplying an appreciAll salts present as nitrates. 404.7 ... able quantity of electrons to the flanir, Data taken from Gardiner (8). Rb 420.2 ... fall in close proximity to either a n atom 421.6 ... CS 455.5 ... or ion line of one of the three alkaline459.3 earth metals. lines. Results presented in the preCa 422, 7 a 393'.iQ Rubidium was chosen as the electron ceding paper showed that a given change 396.9 donor because of its low ionization poSr 460. 7a 407.80 in flame temperature would affect high 421.6 tential and because the rubidium line at energy lines much more than low energy Ba 553, 6a 455,4a 421.6 mp could be resolved from the lines. As the two potassium lines were 493 4 calcium atomic line at 422.7 mp. The enhanced to the same extent in both the a Lines chosen for analysis. 422.7-mp calcium line is very intense and hydrogen-oxygen and acetylene-oxygen the use of a slit width of 0.015 nini. proflames, the increase in emission intensity vided a spectral slit nidth of 3.75 A. is due t o a n increase in concentration of which was sufficient to resolvc thc t n o atomic vapor and not to an increase in Although Huldt discussed the repression lines which differ by 11 A. The atom flame temperature. of strontium ionization due to added and ion lines chosen for analysis arc inThe data in Table 111 show that enelectrons, this effect apparently was indicated in Table V. hancement effects are lower for a given sufficient to explain the enhancement of The results obtained u hcn rubidium flame with a total-consumption type strontium atomic emission observed. Bas added to solutions of calcium, atomizer-burner than with a reflux type. Disturbance of dissociation equilibria strontium, and barium arc shown This can be attributed to the cooling and transfer of energy between atoms graphically in Figures 1 and 2 . The effect of the relatively large amount of were discussed as possible explanations abscissa represents the amount of rubidwater taken u p into the flame mith the for the observed effect. ium added in the form of rubidium total-consumption type system. The I n the present investigation simulchloride. The ordinate represents the magnitude of this cooling effect was detaneous changes in the concentration of relative change in emission intensity of termined and is discussed in the preneutral atoms and ions-Le., degree of the atom and ion lines as electrons were ceding paper ('7). ionization-were observed for the alkaadded to the flame. I n the absence of Alkaline-Earth Metals. Several line earths. The absolute magnitudes interference, the atom lines (the ones workers have noted t h a t lines due t o of the enhancements observed during usually used for analytical work) would neutral atoms, arc lines, a n d ionized these studies are not theoretically follow the dotted line a t unity, indiatoms, spark lines, are observed in the significant because of the dependence of cating no change. The curve in the flame spectra of the alkaline-earth metthese values upon element concentralower half of the plots represents the als. Ahrens (1) describes how the relation. the form of the calibration curve, change in ionic emission and is a general tive intensities of atomic and ionic lines and instrumental characteristics. S o one shoning the behavior of the ion lines have been used to study the physical attempt has been made previously to for all three elements. features of the direct current arc source. obtain absolute values; rather, the K h e n the same experiments were perThe only use of atom and ion lines in the relative values have been used to indiformed in t n o different fla'mes, hystudy of flame spectra appears to be the cate how the degree of enhancement drogen-oxygen and acetylene-oxygen. work reported by Huldt (9), who inchanged as the physical and chemical the effects increased with increasing vestigated the enhancement of stronproperties of the system were varied. flame temperature. At a fixed flame tium atomic emission due to added The relative extent of ionization of the temperature, the per cent enhancement calcium by measuring the relative inalkaline earth metal vapor in different tensities of the strontium atom line at for a fixed quantity of added rubidium flames can be estimated by measuring 461 mp and the ion line a t 408 mp. the ratio of ionic t o atomic line intensiincreased in the relative order, calcium, Table II.
Effect of Cesium on Potassium Emission at Three Flame Temperatures
-
+
0
2034
ANALYTICAL CHEMISTRY
I
14
0 0.4 6 k
02t
L 1
OO
I
0
85
2 M x IO” 170
3 4 R b C l Added 255 340
5
6
425
510
I I
OA
85
0
p.p.m. R b Added
I
I
2
M x IO” 170
!
3 4 R b C l Added 255 340
5 425
510
pp.m. R b Added
Figure 1. Variation of atomic and ionic lines of calcium, strontium, and barium with added rubidium in hydrogen-oxygen flame
Figure 2. Variation of atomic and ionic lines of calcium, strontium, and barium with added rubidium in acetylene-oxygen flame
Barium atomic line, 554 m p ; 50 p.p.m. barium as barium chloride A Strontium atomic line, 461 m p ; 20 p.p.m. strontium as strontium chloride W Calcium atomic line, 4 2 3 mp; 20 p.p.m. calcium as calcium chloride 0 Ionic lines: Barium, 455 mu; strontium, 408 mp; calcium, 394 mp
Barium atomic line, 5 5 4 m p ; 2 0 p.p.m. barium as barium chloride A Strontium atomic line, 4 6 1 mp; 20 p.p.m. strontium as strontium chloride W Calcium atomic line, 4 2 3 m p ; 2 0 p . p m calcium as calcium chloride 0 Ionic lines: Barium, 455 mp; stroitium, 4 0 8 m p ; calcium, 394 m u
.;trontiuni, barium, as predictable from the ionization potentials of the three elements. I n each case, the atomic emission intcnsitj increased sharply and then leveled off until the enhancement n as constant with added rubidium. This leveling off was due to essentially complete repression of the ionization of thc test element, as can be seen from obswvation of the simultaneous decrease in thc ionic line intensity. These results indicate that the radiation liuffcr tecahiiique (18) can provide a useful means of compensating for cation enhxncement effwts. The only case 11here enhancement was not observed wac for calcium in the hydrogen-oxygen flame. I n this case, the ionization was veri slight and a small decrease in emission intensity n as observed in the presence of the added rubidium chloride, Thii depression was felt to be due to the qalt effects discussed in the preceding paper. These results are in agreement M ith those published by Margoshes and Vallee (11),who obswved no enhancement of calcium emission in the hydrogeno\ygen flame n hen extraneous cations were added to the solution. Honever, the present ~ o r kshows calcium in the hydrogrn-oxygen flame to be a case of very small ionization a here enhancement nould not be expected. I n the acetylene-oxygen flame, calcium ionization is much higher and enhancement of atomic emission is observed.
Table VI. Effect of Rubidium on Thallium Emission
+
Emission Intensity, Cm.a Hydrogenhcetyleneoxygen oxygen flame flame
Element Interferer 100 p.p.m. T1 as TlKOj 100 .p.m. T1 as TPNOa 0.006M RbCl
10.54
8.01
+
I
50
loo
I
I50
I 200
I 250
p.Pm Bo
Figure 3. Enhancement of 554-mp atomic line of barium from addition of rubidium chloride; plotted as a function of barium concentration using acetylene-oxygen flame
The increase in per cent enhancement with decreasing concentration of the element to be determined is illustrated in Figure 3. This plot shows the extent of enhancement, expressed as a ratio of barium atomic emission a t 553.6 mp in the presence of a fixed amount of added rubidium (O.OOSM, 510 p.p.m.) t o that of the barium alone. When this ratio is plotted as a function of the barium concentration, the per cent enhancement is seen to increase from 25 to 57y0 as the barium concentration is decreased from 250 t o 50 p.p.m.
(510 p.p.m. Rb) 10.57 8.37 0 Emission intensity read a t 378 mp.
Other Metals. Enhancement effects would be expected t o decrease a s t h e ionization potential of t h e test element increases. When thallium (ionization potential 6.07 e.v.) was examined in two different flames, t h e results shown in Table V1 were obtained. S o effect was observed in the hydrogen-oxygen flame, while in the acetylene-oxygen flame, the thallium emission was enhanced by 4.5y0. This value is considerably less than those observed for the alkaline-carth metals in the presence of the same amount of added rubidium. Significance for Internal Standard Methods. Lithium has been widely used as a n internal standard in t h e flame photometric determination of alkalies and even alkaline earths. The VOL. 31, NO. 1 2 , DECEMBER 1 9 5 9
2035
\V. A,, Ph.L). tlirsis, Princeton Univ., 1951. ( 7 ) Foster, IT. H., Jr., Hunie, D. S . , B s ~ L CHEX. . 31, 2028 (1959). (8) Gardiner, II., Vallee, B. L., Chapter on F,lame Photometry and Spectrometry in “Xethods of Bio, I ’ D. Glick, ed., 1-01, 111, Interscience, Xew York, 1956. (13) 3\lavrodineanii, R., Boiteux, H., “hnalyse Spectrale Quantitative par la Flanime,” 3Iasson et Cie, Paris, 1951. (11) Mrloche, v. K.,~ A L Cwsir. . 28, 1814 (195G). (15) Saha, 31. S . , Saha, S . IC., “-1Trentise on llodern Physics,” Vol. I, p. 630, Indian Prrss, Calcutta, 1931. (16) Smit, J. A , , Vendrik, *\. J. H., Physica 14, 505 (1918). ( I 7 ) Valiec, B. L., Reports of Sixth Internat,ional Spectroscopic Colloquium, Amst,erdam,1956. (18) West,, P. K,, Folse, P., Montgomery, D., .\N.iL. CHEM.2 2 , 667 (1950). (I!)) White, J. V.,Ibid., 24, 391 (1952). ( 6 ) Dippel,
Table VII.
Effect of Ionization Interference on‘lnternal Standard Results
A , Potassiiini Emission Cesimn Added Intensity, Cm: h-one 7.“ O . O I A I C$(,*I(1330p.p.ni, Cs) 11.1T I?acli result average of duplicates.
assunil)tion is madcl that c h a n g e in flanic conditions \vi11 affrct the eniissivity of lit’liium and the other elements equhlly. It is clrar from the above rpsults that this assumption should not be valitl for high tcmqxxtture flame systciins in whirli npprt.c.iable ionization of thv u1k:rli and alkaline-Parth rlcnients t:iktss ~ ) l a c r . .4t the temperature of the os!-Rl,ii-:ic.c’t\-lt~ii(~ flame (2800” K.) lithiinn is only about 16% ionized, vhile Imtnssi\Iin, by virtu(, of its l o w r ionizntion 1:otcwtial, is ahout 82Oj, ioniacd ;Tal)Its I). Xtltiitioii of an c,asily ioniztvl cloriicmt surh as ruliidium or cesium ~ I ~ :Imorints of sotlium should or I ~ I Y I:ir.gc o n l i x i ~ , ~thc. t ~ t,missivit>. of 1 otassiuni n i w l i mort’ tli:tn that of lithiuni, thercbj. iii\xIiilxtiiig its us(’ as a n intrrnal staiic1:ml. To vrrif!. t l i i q ronclusion esi)criiiic,iit:LII>,, thc‘ effwt of O . O l M c(’siuni c*liloricic> on the‘ ratio of potassium cmissivitj. ;it i n 9 111,~to t h t of lithium emissivity :tt (iiInip in tlir arc.tylent5o y . g ~ nfl:imtn of the. Rrckni:in total-
B , Lithium
Emission Intensity, Cm: 535
5.53
Ratio A B -1.38 2.02
consumption at oniizc,r-burner was nirasurrd. Each solution contained 100 p.p.ni. of potassium as potassiuni chloride ant1 20 1i.p.m. of lithium as lithium chloridv. Thc r i w l t s (Tabk VII) shonthat thc c)fi‘r,ctsare as prcdictrd and that the use of miall amounts of lithium as ,211 internal standard in high temperature flonic sj.strms is to bc avoided unlcss significant ionization d c c t s :iw known to Iic a i w n t . LITERATURE CITED
(1) Ahrens,
L. H., “Spectrochemical .-\nalyeis,” p. l i , Addison-Wesley Press, Cambridge, .\ln?;s., 1950. ( 2 ) Alkeninde, C. T. J., dissertation, TJtrwht, 1951. (3) Alkrmatie, C. T. *J., Rrports of Sisth International Spectroscopic Colloquiun~, Amsterdam, 1956. (4) Bulewicz, E. AI., Sugden, T. AI., Z’rans. Faraday SOC.54, 1855 (1958). ( 5 ) Uean, J . 4.,i n Willard, H. H., Merrit, L L., Jr., Dean, J. X., “Instrumental Methods of Analysis,” 3rd ed., I7:tnXostrand, Yew Tork, 1058.
RECEIVED for review July 2, 1959. Accepted September 3, 1959. Work supported i n part by the United St,ates At’omic Energy Commission under Contract AT(30-1)905. Taken in part from the Ph.D. thesis of Walter H. Fost’er.Jr., hlassachusetts Iristitiite of Technology, >lay 1959.
A Theory of Spectral Excitation in Flames as a Function of Sample Flow MILTON R. BAKER’ and BERT L. VALLEE Biophysics Research laboratory, Department of Medicine, Harvard Medical School, and the Peter Bent Brigham Hospital,
72 7 Huntington Ave., Boston, 7 5 Mass.
b The intensity of spectral lines emitted b y atoms in flames is a function of the flow rate of the aqueous solution containing them. For each spectral line an optimal flow rate is calculated at which the line intensity is maximal, decreasing a t both higher and lower flow rates. These maximum intensities are determined by the excitation potential of the line, the flow rate of the sample solution, and the interaction of a particular solvent with a particular flame. The curves of intensity vs. sample flow rate for the cyanogenoxygen and hydrogen-oxygen flames with aqueous samples have been derived on the basis of a model which assumes complete mixing, thermal equilibrium, and purely thermal excitation. Knowledge of the position of these maxima is crucial to high temperature flame spectrometry. 2036
ANALYTICAL CHEMISTRY
F
are cooled and ukiniately extinguished by water. Severtheless, it has been customary in flame spectroscopy to increase the sensitivity of spectrochemical detection by increasing the rate of sample aspirated into a flame. This procedure appears t o neglect the cardinal role which flame temperature plays in the thermal excitation process. I t has been assumed that the cooling effect of the sample solution on conventional flames is negligible, possibly because even the maximum volume of solution aspirated per unit time has seemed small. The present paper questions these assumptions and presents a theory which predicts the interrelationships between flow rate of the aqueous sample, flame temperature, and the intensity of spectral lines emitted by flames. The cyanogen-oxygen ( 2 , 3, 9) and the LAMES
hydrogen-oxygen flames are employed as examplrs of high and conventional temperature flames, respectively. Optimal flow rates required to achieve maximum sensitivity of detection for certain spectral lines have been calculated. THEORY
Combustion Products and Flame Temperature. T h e tcmperature of a n y flame is n function not only of t h e heat available from the conibustion reaction, but also of t h e degree t o which this reaction goes t o completion a t t h e temperatures of t h e flaniei.e,, the thermal stability of t h e combustion products ( 6 ) . 1 Present address, Department of Physics, Hnrvard University, Cambridge,
hrass.
