I J. W. Hosking, N. B. Sneil, and B. T. Slurman Western Australian lnstitute of Technology South Bentley, W.A., 6102, Australia
An Atomic Absorption Interferences Experiment An advanced exercise for the undergraduate analytical laboratory
Chemical interferences can be a problem in analyses by atomic absorption spectrometry particularly with air-acetylene flames. Although the various types of interference are described in hooks which deal with atomic absorption spectroscopy ( 1 4 ) there is a need to demonstrate these interferences to students in instrumental analvtical courses. Most experimentsdescribedin the literature (for example (5, Ware only intended to introduce students to the technique of atomic absorption spectrophotometry although one experiment in a recently published book (7) does briefly consider the interference problem. The experiment described here aims not only to demonstrate the maenitude of chemical interferences but also to show some of tce ways to overcoming these interferences. As part of their undergraduate program chemistry major students at the Western Aurtralian Institute of Technolorn study several instrumental analytical techniques. This pap& describes one of the four 3-hr atomic absorption spectrometry laboratory sessions which involves a study of the interferences observed in the analysis of calcium. Medical Technology and Physics major students also do this exercise as part of their atomic absorption program. Interferences occur when matrix components cause a variation in the observed absorbance compared with that obtained for oure standards. Chemical interferences are often greatest at the flame position of maximum sensitivity but are cenerallv reduced or eliminated hv usine". w i t i o n s hieh in the game, or by using the hotter nitious oxide acetylene flame. The magnitude and variety of interferences observed in the analysis of calcium have been the subject of numerous reports, for examole references (8)-(12). I t has been claimed that many of these interferenc&ck be eliminated,by the addition of releasing aeents such as strontium. lanthanum and the disodium s a i of ethylenediaminetetraketic acid (8,12). Procedure For students who are not familiar with the imnortance of flame conditions i t is advisable that they first deiermine the name profiles of calcium under lean (oxidizing),stoichiometric and fuel rich (reducing) air-acetylene flames (Fig. 1). The magnitude and number of interferences in calcium analysis were investigated by aspirating solutions of constant calcium concentration and a varvine matrix under different flame conditions (see the table). ~hhescaleex~ansion facility was used to eive an aovarent constant absorbance for a reference solu$on under-the different flame conditions. This enables the results to be more readilv interpreted. The efficiency of strontium, lanthanum, add the disodium salt of ethylenediaminetetraaoetic acid as releasing agents for the anilysis of calcium in the presence of and aluminum was also investigated under the same conditions. Finally students were given an unknown calcium solution (containing phosphate and chloride) and pure calcium standards and asked to analvze the unknown solution under the most favorable conditiotk. Solutions for a 'standard addition' analysis were also provided. Experimental Solutions of &(NO&, CaC12, and Ca(ClO& were prepared by adding minimum quantities of appropriate acid to CaC03. 128 1 Jownal of ChemicalEducation
ABSORBANCE Figure 1. Flame profile for calcium under lean (L), stoichiometric @)andfuel rich (R) mndiiions.
Mg, Al, Sr and La were added as chlorides, and C1- and PO& as the acids. The Sr solution contained some Ca which was determined by the standard addition technique. Analytical reagents ware used in all cases, except for the LaCI3solution which was prepared by dissolving calcium free Laz03in hydrochloric acid. The solution containing lanthanum and phosphate was stable for 1 week; precipitation occurred after approximately 2 weeks, when some calcium was lost by coorecioitation. The calcium concentration in all solutions was 5 rng/l.' The results quoted in this report were obtained with a Varian AA4 atomic absorption spectrophotometer with an AA5 burner although the same trends were obtained using a Perkin-Elmer model 403 atomic absorption spectrophotometer. The flame profiles obtained with the Perkin-Elmer model 403 show less variation than those in Figure 1because of the different burner design and the broader light beam. The unknown solution contained 2.5 mgA of calcium together with 500 mgll C1- as HCl and 500 mgA P O P as H3POa. The standard solutions contained 2 , 4 and 6 mgh of calcium. The standard addition solutions were the same as the unknown with the addition of 2 , 4 and 6 mgA of calcium. All these solutions contained 2% HC104 as a preservative. Results The flame profiles (Fig. 1)for calcium are readily explained bv atomization and atom oxidation vrocesses. Flame orofiles f i r calcium in the presence of the other species can be calculated for the fuel rich flame using the results of Figure 1and the table.
ABSORBANCE~
Rich Flame
Solution Containing 5 m g l l Ca as
Lean Flame
Position Polifion of B, Position Position A, bare maximum C. high maximum o f flame f e n l i t i v i f ~ flame sensitivity (1 m m l (5 m m l (15mm) (1 m m l
Absorbance set at 0.50 urlng scale expanslon facility. For relative abrorbance (see Fig. I).
i n 2% HCiO, (Reference Solution1
ca(clo,),
Comments
Even ilolChlOmetrlC quantltler o f the different anoonr can r s w l t n stgnofv cant mfferences n some Darts o f tne flame.
loo mgll A1 + 2% HCIO, loo mg/l AI
+ +
Serious interference due t o formation o f itable calcium aluminater.pH dependent.
+ loo
m g l l Mg + 2% HCIO. Mg + 500 mg/l C l - + 2% HCIO, Ca(Cl0, C m o L + 500 m g l l cI+ 500 mg/i PD,:I+ 2% HciO, + 500 mg/i PO, +
,
~ l t t l cation e interference-perhaps competition for oxygen is rlgnlficant. NOIntwference i n HCIO,.
loo mg/l
g1g12k
~ n clarrlcal s example o f interference. m e r e a r e porltlonr hlgn i n f ame Where no mterference observed.
CaCI, + 2000 mgll Sr ar SrCI, cacl, + 2000 mgll 5 r ar srcl, + loo m s l l A1 c a c l , ~ ~ 2 0 0mgll 0 s r ar srcl, + 500 m s l l
SrCl acts a relearing agent at 6 . I f S ~ ~ N) Ois used the renritivity is reaucdd6ut this still actr ar a relearing agent a t positions near 6 .
PQ.
I n the presence o f HCIO. sr reducer the interference b u t doer n o t comPietelY eliminate the interferences.
CaCl, + 2000 m g l l Sr as SrCI, + 2% HCIO, c a c l + 2000 mgll Sr as SrCI, + 2% HCIO,
".",, ,,.?,.
*. ,%" A""
0 , n.
CaCI, + 2000 mgll Sr as SrCI, + 2% HCiO, + 500 m g l i PO,'Na, E D T A Increaser renritivlty. Na, EDTA acts as a raiea.ing agent to; po,'-at only one pos toon oetween 6 and C. I IS not suitable for 61.
catclo,), + 1%Na, E D T A Ca(CI0, + 1%Na, E D T A + 100 m g l l A1 C a ( c l o ~ ,+ 1%Na, E D T A + 500 mg/l
,
PO,>-
CaCI. + 1%~s ar LaCI, c a c l + 1%~a as Lac!, + 100 mgll AI cacl: + 1% ar LaCI, + 500 mg/l PO,'-
L a increases sensitivity and actr sr a releasing agent for A l and PO.'far 1 1 1 DOlitiOnl.
o r h a r e are mean rerultr for five retr of n u d e n t rerultr. The variation is i 10% due t o different flamt Greater variations rhould be axDected for different burners and instruments.
Although it is not necessary for the students to aspirate all the solutions in the table, it does serve to demonstrate the capability of the technique to analyze large numbers of samples. These results were ohtained in one and a half hours. The results in the table show the serious depression of the calcium absorbance by phosphate and by aluminum. This may he - - exnlained -~~ r - - ~ ~ in - terms of the formation of refractorv calcium compounds which are less readily reduced to ground state metal atoms. At the less sensitive positions high in the flame the phosphate interference is not significant, hut aluminum causes serious interference at all flame positions. In general interferences are more severe low in the flame even for magnesium and for stoichiometric amounts of ~erchlorate,chloride, or nitrate. The addition of releasing wents was effective in a limited number of flame positions. it should he obvious that an initial investigation would be necessary t o determine the flame conditions and positions at which particular releasing agent is effective. The only flame position where phosphate ion does not interfere sipnificantly in the determination of calcium is the less rensitivehieh oosition. When this nosition was used to analvze -~~~~ the unknown solution (containing 500 mgll P O P and 500 meA C1-) it was found that the instahilitv of the flame Drod&ed ve& noisy absorbance signals for thk standard and the unknown. Because of this the results ohtained varied considerahlg (f25%)from the correct value. Figure 2 shows a set of r e s u h ohtained when thestandard additions techniaue was used to analyze the unknown at the flame position giving the maximum sensitivitv for calcium in the absence of interfering species. Because theealihration curve is not linear it is difficult to extra~olateaccuratelv. The correct result (as shown in Fig. 2) was dbtained fortuitbusly from one of five sets of results. ~
~~
~
~
~
~
~ n d i t i o n rand flame positions,
~
a
~
~
e
Ca ADDED, mgII
.
Figwe 2.
Determination of calcium by standard addition teohnique.
In response to appropriate questions, the student should suggest that the use of lanthanum or the disodium salt of ethvlenediaminetetraacetic acid as a releasine agent should give more reliable results as the analysis can &en he carried out usine more sensitive and less noisv reeions of the flame. The useof the hotter nitrous oxide-acetylene flame can also he investigated. Volume 54, Number 2, February 1977 1 129
LUerature Cued (11 Wiuls, J. 8..'‘*nalytical Flame Spema~py.SeleefodTopi*" (Editor: M a d i n - u . R.),Macmillan. London. 1970.p. 561. (2) Rsmirez-Munoz, J., '"Atomic-Absorption S p t m a c o p y and Analysis by Atomic Ahsorption Flame Photometry? Elsevier Publi8hingCo., Amsterdam. 1968.p. 2M1. 13) L'Vov, a v "Atomic Absorption Spectmehemieal Analysis? Adam Nilger LLtd.. London. 1970. p. 181. (41 ? l i e , W. P.. "~nalytied~ t o m i ~ c h m p t i o nspectrometry," ~ e y d e nand son ~ t d . . London. 1972, p. 86. ( 5 ) Willard. H. H., Merritf L. L, andD~an.LA. "lnstnuncntalMethodnof Analysia,"Sth Ed.. D. van Nostrand Co., NewYork, 1974, p. 381.
130 / Journal of Chemical Education
.I.
(6) Stemporeevaki, S. E., B U ~ I ~R. L A,. and -Barry E. F.. CHEM. EDUC., TI, 332 (1970. o nLtd. (71 Am- M. D., ct al.. "Basic Atomic Abnomtion S p e m a ~ p y : Varian T ~ h ~ Pty Springvale, Australia. 1975, p. 28snd 106. (8) Slsuin, W., "Atomic Absorption Spedrosmpy? Interscience Publi%hera.New Yark. 1968, p. 87. (9) Willis, J.B.,Pmr.RoyolAwf. Chem. Inat., 29.245(19621. (10) Pungor. E.:'AtomieAhaomtionSpetroampy," Internstions1 Unionof P u n a n d Applied chemistry, (~ditora:~agnall,R and irkb bright, 6.F.) ~ u t t e r w o r t v s , ~ o ~ o d ~ n . 1970, p. 51. (11) Harrison, A , and Otfawsy, J. M..Proe. Soc. A n o l Chem.. 9,205 (1972). (121 Mwill, W.A.,and Svchle,G.,Z A m 1 Chem.. 266.177 (1974).
