ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
lower than those found by the proposed method and other published values. The other published values (15, 16), however, were in excellent agreement with the higher Pb and Bi values obtained using the proposed method. Values obtained for P b and Zn on W-1 and BCR-1 were considerably below the accepted literature values when digested with KCIOBand hydrochloric acid while the Ag, Cd, and Cu were near the accepted values. The low recovery of P b and Zn suggests isomorphous substitution of these metals in the silicate lattice. When the optional hydrofluoric acid digestion was used, good agreement with the literature values was obtained. Mean values of replicate analyses using the “total” digestion for the eight samples are included in Table
111. CONCLUSION Although the described technique was developed for geologic materials, it may be applied to other materials where the major element interferences of a variable matrix are a problem. Also, the concentration effect of an organic extraction and the increased atomization efficiency of an organic solution may be advantageous for trace metal determinations in waters or other immiscible liquids. Determination of the six metals using a heated graphite furnace would require a
1101
smaller organic phase and conceivably yield values several orders of magnitude lower than those obtainable using conventional flame atomic absorption spectrometry.
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9)
(10) (11) (12) (13) (14) (15) (16) (17)
F. L. Moore, Anal. Chem., 36, 2158 (1964). F. L. Moore, Anal. Chem., 37, 1235 (1965). T. Groenewald, Anal. Chem., 40, 863 (1968). P. D. Rao, At. Absorp. Newsl., 8, 131 (1970). P. D. Reo, At. Absorp. Newsl., 10, 118 (1971). C. W. McDonald and F. L. Moore, Anal. Chem., 45, 983 (1973). C. W. McDonald and T. Rhoades, Anal. Chem., 46, 300 (1974). F. G. Seeley and D. J. Crouse, J . Chem. Eng. Data, 11, 424 (1966). J. Dolezal, P. Povondra, and 2. Sulcek, “Decomposltlon Technlques in Inorganic Analysls”, American Elsevier, New York, N.Y., 1968. M. Olade and K. Fletcher, J . Geochem. Explor., 3, 337 (1974). F. Felgl, ”Qualitative Analysis by Spot Tests”, Elsevier, New York, N.Y., 1947. C. L. Luke, Anal. Chlm. Acta, 39, 497 (1952). P. W. West and J. K. Carlton, Anal. Chlm. Acta, 6, 406 (1952). Q. H.Allcott and H.W. Lakln, “Qeochemlcal Expioratlon 1974”, Elsevler Scientific, Amsterdam, 1975, p 659. R. F. Sanzoione and T. T. Chao, Anal. Chlm. Acta, 86, 163 (1976). W. H.Flcklln and F. N. Ward, J . Res. U . S . Geol. Surv., 4, 217 (1976). F. J. Fianagan, U . S . Geol. Surv., Prof. Pap., 640 (1976).
RECEIVED for review January 16, 1978. Accepted April 13, 1978. Use of brand names in this report is for descriptive purposes only and in no way constitutes endorsement by the U.S.Geological Survey.
Correction of Spectral Overlap Interference by Zeeman Atomic Absorption Spectrometry Hideakl Kokuml’ Lawrence Berkeley Laboratory, Universlfy of Callfornla, Berkeley, Callfornla 94 720
Interference caused by dlrect spectral overlap can be corrected by Zeeman Atomlc Absorptlon Spectroscopy. Typlcal cases of Interference of thls klnd appearlng In the analysis of Sb were examlned. Sb llnes at 217.023 nm and 231.147 nm overlap the Pb absorptlon llne at 216.999 nm and the NI absorptlon llne at 231.096 nm, respectlvely. I n both cases, spectral Interference can be ellmlnated by utlllzlng the Polarked Zeeman AA Technlque.
Recently, the author developed the Polarized Zeeman Atomic Absorption Spectroscopy (PZAA) technique and described the principle ( I ) , the instrumentation (2) and the applications ( 3 , 4 )of this technique in previous papers. With this method, a steady magnetic field of about 11kG is applied to the sample vapor in a direction perpendicular to the light beam. A difference of absorption is observed for the polarized constituents of radiation, PL and PII,which are polarized perpendicular and parallel to the magnetic field, respectively. The difference of absorption is proportional to the true density of the analyte atoms, but is not affected by molecular absorption and light scattering, i.e., background absorption, caused by the thermal decomposition of the sample matrix. This technique has many advantages over various conventional methods of background correction. (5-9). Namely, the Permanent address, N a k a Works, H i t a c h i Ltd., Katsuta, Ibaraki,
312, Japan.
background is precisely corrected at exactly the same wavelength as that of the atomic absorption line; the baseline does not change with time even when the radiation intensity does; the double beam optics are optimized because both the sample light and reference light follow exactly the same path through the sample vapor. In addition to these advantages we have recently found that interference caused by direct spectral overlap of absorption lines can be corrected by this technique. Interference by the direct spectral overlap occurs if another element is capable of absorbing radiation meant for the analyte (10). This kind of spectral interference brings about serious errors in atomic absorption spectrometry. An apparent signal is obtained even when the analyte is not present in the sample. This type of interference cannot be eliminated by using a smaller spectral band pass or the standard addition method. A number of researchers, including Fassel, Winefordner, and L’vov, et al., have studied this type of interference (10-19). On more than 10 lines, interference due to direct spectral overlap was reported and about 50 cases of spectral overlap within 0.03 nm were found in an Atlas of Spectral Lines. Some researchers have investigated the possibility of correcting for this kind of interference by applying the wavelength modulation technique to continuum source atomic absorption (10, 11). However, correction is not possible unless the band pass is so narrow that it is comparable to the separation between the interfering and the analytical line. In this paper we report a correction method that utilizes the Polarized Zeeman AA technique. With this technique,
0 1978 American Chemical Society 0003-2700/78/0350-1101$01.00/0
1102
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978 EMISSION L I S E OF THE I N T E R E S T A T O M
Table I. Character of Atomic Lines of Interest (25-27)
N I K G O F THE I N T E R F E R I K G ABSORPTION L I N E
Element
Wavelength, nm
Energy levels, cm-l
Sb
206.833
0-48332
4S3,, - 4P,,, 0.41
217.581
0-45945
4S3,, - 4P,,, 0.18
231.147
0-43249
4S3,2-
217.023 216.999 283.306 231.096 232.003
Note la
Pb I
h &VELENGTH
Flgure 1. Correction of direct spectral overlap by the Polarized Zeeman AA technique the interference caused by direct spectral overlap can be corrected without depending upon the monochromator to provide an extremely narrow band pass. Figure 1 shows the principle of this method, which depends on the fact that the absorption intensities of the H and ut u- components are almost the same in the wings of the interfering absorption line. An absorption line profile is generally expressed by the convolution of the Gaussian and Lorentzian functions (Voigt function) (20). If pure adiabatic collisions are assumed, the following equation can be derived for the two wings of an absorption line (21).
+
6
=I1:
5
- (blue wing) (red wing)
Here, K(V) = absorption coefficient at a frequency v, v = frequency of the light, vo = frequency of the line center. When the sample vapor is placed in a magnetic field, the interfering absorption line splits into H and u Zeeman components, which absorb the polarized radiation P, and PI,,respectively. Then, when the emission line lies a t a frequency in the wing of this absorption line, the ratio of the absorption intensity of PII(sample light) and P , (reference light) is expressed by the following equation in the case of a simple Zeeman triplet pattern.
Here, A v , = Zeeman shift of the absorption line, K,, K , = absorption coefficients of T and u components a t frequency v, respectively. If v - v o >> AuZ,k,/k, becomes 1. In this case absorption of the component equals the absorption of the u components and no interference occurs because the difference of absorption between PIIand P, is observed in the Polarized Zeeman AA technique. We look at two typical cases of direct spectral overlap of the analytical lines of Sb with the absorption lines of P b and Ni which were reported by Slavin et al. (16,22,23) and L'vov e t al. (15, 24), respectively. These interferences frequently cause problems in the analysis of Sb.
EXPERIMENTAL A Polarized Zeeman Atomic Absorption spectrophotometer, which was previously described (2)was used with a magnetic field strength of 11.0 kgauss. The difference of absorption between the polarized light P , and PI,was observed, utilizing rotation of
Ni
0-46069 0-35287 0-43259 0-43090
Terms
gfvalue
4P,,, 0.12
3PP,-3D, 3P0- ,P, 3F,-3F, ,F4- ' G ,
0.048 0.22 0.60 0.86
a Note 1. The Sb line at 217.023 nm is clearly observed in the hollow cathode discharge. However, a term assignment for this transition has not been made as far as we know. This line is described in the MIT table (27) and coincidence table ( 2 8 ) as a weak emission line, but it is not found in the NBS tables (25, 26). We determined that this line was not from the Ne gas or impurities contained in the Sb hollow cathode which means that this line at 217.023 nm is surely due to Sb. Observations by Manning and Slavin on this line in a hollow cathode lamp in their laboratory showed that it was even 20% brighter than the nearby Sb line at 217.581 nm (29, 30).
a linear polarizer at 77.7 Hz. A cup type graphite cuvette was employed for sample atomization. For comparison, a conventional type AA spectrophotometer (Hitachi 170-50 AA spectrophotometer) with a graphite furnace atomizer (Hitachi GA-2 graphite atomizer) and a D2lamp background correction system was also used. The monochromator used for the Polarized Zeeman AA spectrophotometer was exactly the same as the monochromator in the conventional AA spectrophotometer. The same hollow cathode lamps (Hitachi hollow cathode lamp HLA3 and HLA4) were used in both the Polarized Zeeman AA and the conventional AA instruments. The temperature of the graphite furnace was measured by an optical pyrometer which was calibrated at the melting point of Mo.
RESULTS AND DISCUSSION Table I shows the related lines of Sb, Pb, and Ni; their wavelengths, relevant energy levels, terms, and gf values. Figure 2 shows the emission spectrum from the hollow cathode lamp of Sb operated at 10 mA. This spectrum was obtained using a monochromator with resolving power of 0.18 nm. The strong emission lines of A, B, and C correspond to the resonance lines of SbI a t 206.833, 217.581, and 231.147 nm, respectively. Spectral interference is caused by a Pb resonance line a t 216.999 nm when the atomic absorption of S b is measured at the Sb emission line at 217.581 nm (16,22,23). Spectral interference is also caused by the Ni resonance line at 231.096 nm when the atomic absorption of Sb is measured a t the Sb emission line a t 231.147 nm ( 1 5 , 2 4 ) . These cases of spectral interference were both eliminated by using the Polarized Zeeman AA technique. Interference with Pb. The atomic line a t 217.581 nm emitted from the Sb hollow cathode lamp cannot be absorbed by the P b resonance absorption line a t 216.999 nm, because the separation between these lines is 0.582 nm. In this case the spectral interference is caused by the S b line at 217.023 nm in Figure 2 (16). Since the separation between the Sb line and the absorption line of P b at 216.999 is only 0.024 nm, P b atoms absorb this Sb line. The intensity of this line at 217.023 is more than half of the Sb resonance lines a t 217.581 nm. Both the lines pass through the exit slit of the monochromator unless the slit width is sufficiently small, because the separation between these Sb lines is only 0.582 nm. Figure 3 shows the observed spectral interference of Sb with P b in conventional atomic absorption. The Sb hollow cathode
ANALYTICAL CHEMISTRY, VOL. 50,
NO. 8,
-&...
JULY 1978
1103
,
IGHT SOURCE : Sb AVELENGTH : 217.6
'7%1
nm I
Figure 4. Elimination of spectral interference caused by Pb in the determination of Sb by the Polarized Zeeman AA technique (wavelength, 217 nm; light source, Sb)
I
--
r---_-
Emission spectrum from a S b hollow cathode lamp
__
-1I
WAVELENGTH ( m )
Figure 2.
-
.._
-
.--A
WAVELENGTH BAhD PASS
__
Figure 3.
Observed interference by Pb in the determination of conspectrometry (wavelength, 217 nm; light source, Sb)
AA
lamp operating at a current of 12 mA, was set in the conventional AA spectrophotometer, and the wavelength was adjusted so as to peak light intensity around 217.5 nm. Widths were 0.8 and 1.0 nm for the entrance and the exit slits, respectively, and the resolving power was 2.25 nm. Standard solutions of P b of 10 p L were atomized by utilizing the graphite atomizer at 2000 "C. D2 lamp correction was employed to eliminate the interference of background absorption. Although Sb was absent in the P b standard solutions, the radiation from the Sb hollow cathode lamp was clearly absorbed as shown in Figure 3. Figure 3 shows that only a few ppm of P b in a sample brings about a serious error on the analysis for Sb. Figure 4 shows that this kind of spectral interference can be eliminated by the Polarized Zeeman's AA technique, even
: 217.6 nm : 2.2
.-
nm
SAMPLE VOLL'ME : 10 p 1 -
FLZL S C A L E : A b 3 . 0 . j (100 div.) -
-_ -
Figure 5. Signals caused by Pb at 217 AA with a Pb hollow cathode lamp
ventional
__.-
L I G H T SOURCE : Pb
-
-
~
I
nm using Polarized Zeeman
the high concentration Pb standard solution of 1000 ppm did not give rise to any absorption signal. (Sb solution of 0.05 ppm shows absorbance of 0.05 under these conditions.) The monochromator and the slit width in the Polarized Zeeman AA spectrophotometer were exactly the same as those used in the conventional AA spectrophotometer and also a similar type of graphite furnace atomizer was used in both instruments. Therefore, we can say that the difference between Figures 3 and 4 is totally caused by the difference between Polarized Zeeman AA and conventional AA spectroscopy. Using this Polarized Zeeman AA spectrophotometer and a P b hollow cathode lamp but keeping the same conditions for the wavelength, the atomization temperature, etc., P b absorption was observed, as shown in Figure 5 . The sensitivity of the P b line at 216.999 nm is 1.8 times larger than that of the P b line at 283.306 nm when Polarized Zeeman AA is used. (In conventional AA, the sensitivity ratio of these lines is 2.5. This difference in sensitivity ratio is due to the difference between the Zeeman patterns of these lines.) The precision of the correction in this technique is estimated from Equation 2. The Pb line at 216.999 nm is due to the transition between 3P0and 3D,. The Zeeman shift and the intensity of the Zeeman components are calculated from equations in a previous paper ( I ) . The Zeeman pattern of this line shows a simple triplet and Avz, the shift of 'u and ucomponents, is 12.1 x nm at the magnetic field strength of 11.0 kG. In this case, the wavelength difference between
-s ,d
loo
-
8 0 -
,
Table 11. Sensitivities for the Resonance Lines of Sb, Pb, and Ni in the Polarized Zeeman AA at the Atomization Temperature of 2000 C
: Eleabsorp- Detection O
1%
c
ment
Wavelength
Sb
206.833 217.581' 231.147 216.999 283.306' 231.096 232.003'
Pb Ni I
0 0
0.01
0.005
0.015
0.02
0.025
003
0.035
0.04
HAVELEUGTR D I F F E R E h C E ( m )
Figure 6. Precision of the correction vs. the wavelength difference between the analytical line and the interfering line (interfering line, Pb at 216.999 nm; magnetic field, 11.0 kG)
,
-0
-8
'
~
I
-
-
; -
0.1
-
LIGHT S O U R C E : Sb
tion,a pg
limit,b pg
9.9 8.9
6.8 4.1 2.6 3.4 1.7
7.2
8.5 15. 530. (49)d 440. (40)d
17.d
14.d
a 1%abs. in Polarized Zeeman AA is defined by the quantity of the element which gives rise to a difference of absorption between Pi1 and PI equivalent to 0.004365 absorbance unit. Detection limit is defined by the quantity of the element which gives rise to a signal twice larger than the noise level. ' The line most frequently used in AA spectrometry. Atomization temperature: 2500 "C.
__
WAVELENGTH : 231.1 m
-(IO0
div.)
r-
Figure 7. Elimination of spectral interference caused by Ni in the determination of Sb using the Polarized Zeeman AA technique (wavelength, 231 nm; light source, Sb)
the interfering line (Pb at 216.999 nm) and the analytical line (Sb at 217.023) is 240 X nm. Then, K , / K , in Equation 2 becomes 99.5%. Therefore, if any interference should occur in Polarized Zeeman AA, it would be less than 0.5% of the interference which would occur with conventional AA. The precision of the correction depends upon the wavelength difference between an interfering line and the analytical line. Figure 6 shows the relationship between the precision of the correction and the wavelength difference between the analytical line and the P b line a t 216.999 nm. Interference with Ni. Nickel also brings about this kind of interference in the determination of Sb, because the Sb line a t 231.147 nm overlaps the Ni line a t 231.096 nm (13, 15). Figure 7 shows that the Ni interference can be corrected using Polarized Zeeman AA; 1000 ppm of Ni solution did not give a signal even when the Sb line at 231.147 nm was employed. Table I1 shows that at the atomization temperature for Sb (2000 "C) considerable Ni absorption is observed. The Ni line at 231.096 nm is almost as sensitive as the line at 232.003 nm which is most frequently used for Ni analysis. Using conventional AA with D2lamp correction, another kind of spectral interference caused by the absorption of Ni was observed with the Sb lamp. In this case, the radiation from the Dz lamp was more strongly absorbed by Ni atoms than the radiation from the Sb hollow cathode lamp. This is because many absorption lines of Ni around 231.096 nm fall within the bandpass of the monochromator and absorb the continuum from the Dz lamp, while the Sb radiation at 231.147 nm is absorbed by only the wing of the Ni line a t 231.096 nm. Figure 8 shows the effect of this kind of interference when a conventional AA unit with Dz lamp correction is used.
I --.
,I i
_ .
--
- _
--
-
__
-
x -g:-I
1
Figure 8. Interference caused by Ni when conventional AA with D2 lamp background correction is used (wavelength, 231 nm; the light source, Sb and D, lamps)
Because of interference of this kind, conventional AA instruments are difficult to use for the analysis of trace elements in steel (24). In the analysis of the gun shot residues, Sb in the sample must be determined (31). In this case, the contamination of Pb causes serious errors unless the Polarized Zeeman AA technique is employed. The application of the Polarized Zeeman AA technique to gun shot residue analysis will be reported elsewhere. Interference caused by spectral overlap would also be corrected by the Zeeman AA technique utilizing the line shifted emission source. ACKNOWLEDGMENT The author is grateful to R. D. McLaughlin and T. Hadeishi of the Lawrence Berkeley Laboratory of the University of California, and T. C. O'Haver of the University of Maryland for their helpful discussions and suggestions. LITERATURE CITED (1) H Koizurni and K Yasuda, Spectrochh Acta, Part B , 31, 523 (1976). (2) H Koizurni, K. Yasuda, and M Katayama, Anal Chem , 49, 1106 (1977) (3) H Koizumi and K Yasuda, 28th Pittsburgh Conference on Analytical
Chemistry and Applied Spectroscopy, Cleveland, Ohio, Feb 28-Mar. 4, 1977.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 8, JULY 1978
(4) H. Koizumi, T. Hadeishi, and R. D. McLaughlin, Submitted to Anal. Chem., 1977.
1105
(19) W. P. Kelly and C. 6. Moore, Anal. Chem., 45, 1274 (1973). (20) A. C. G. Mitchell and M. W. Zemansky, "Resonance Radiationand Excited Atoms", 2nd ed., Cambridge University Press, New York, N.Y., 1961. (21) H. Kuhn, Proc. R . SOC. London, Ser. A , , 158, 212 (1937). (22) R. A. Mostyn and A. F. Cunningham. Anal. Chem., 39, 433 (1967). (23) S. Yasuda and H. Kakiyama, Jpn. Anal., 23, 620 (1974). (24) Y. Endo and Y. Nakahara, J . Iron SteelInst. Jpn., 59 (6), 800 (1973). (25) "Experimental Transition Probabilities for Spectral Lines of Seventy Elements", Natl Bur. Stand. (U.S.), Monogr., 53 (1962). Clrc., 467 (1958). (26) "Atomic Energy Levels", Natl. Bur. Stand. (U.S.), (27) "MIT Wavelength Table," John Wiley and Sons, New York, N.Y., 1939. (28) "Coincidence Tables for Atomic Spectroscopy", Elsevier Publishing Co. Amsterdam, 1965. (29) D. C. Manning, Private Communication (1977). (30) S. Slavin, Private Communication (1977). (31) J. A. Galeb and C. R. Midkiff, Jr., Appl. Spectrosc., 29, 44 (1975).
(5) W. Slavin, At. Absorp. Newsl., 24, 15 (1964). (6) S. R. Koirtyohann and E E. Pickett, Anal. Chem., 37, 601 (1965). (7) J. Kuhl, G. Marowsky, and R. Torge, Anal. Chem., 44, 375 (1972). (8) T. Hadeishi, D. A. Church, R . D. McLaughlin, 6. D. Zak, M. Nakamura, and 6. Chang, Science, 187, 348 (1975). (9) H. Koizumi and K. Yasuda, Spectrochim. Acta, Part.6, 31, 237 (1976). (IO) R. J. Lovett, D. L. Welch, and M. L. Parsons, Appl. Spectrosc., 29, 470
(1975). (11) A. T. Zander and T. C. O'Haver, Anal. Chem., 49, 838 (1977). (12) "Contemporary Topics in Analytical and Clinical Chemistry", Volume 2, D. Hercules, Ed., Plenum Press, New York, N.Y., 1978. (13) V. A. Fassel, J. 0. Rasmuson, and T. 0. Cowley, Spectrochlm Acta, Part 6,23, 579 (1968). (14) J. D. Winefordner, W. W. McGee, J. M. Mansfield, M. L. Parsons, and K. E. Zucha, Anal. Chlm. Acta; 36, 25 (1966). (15) 6. V. L'vov, Zh. Anal. Chem., 26, 510 (1971). (16) S. Slavin and T. W. Sattur, At. Absorp. Newsl., 7, 99 (1968). (17) J. E. Allan, Spectrochlm. Acta, Part B , 24, 13 (1969). (18) D. C. Manning and F. Fernandez, At. Absorp. Newsl., 7, 24 (1968).
RECEIVED for review February 13, 1978. Accepted April 7, 1978.
Shape and Position of the Analytical Response in Flameless Atomic Absorption Spectrometry Jdnos Zsak6' Department of Chemistry, University of Constantine, Constantine, Algeria
An equation is proposed for the description of slgnals in flameless atomic absorption spectrometry, by presuming a simplified model and the validity of the Arrhenius equation. Theoretical signals have been constructed and the influence of heating rate and of kinetic parameters Is discussed. The possibility of deriving kinetic parameters from the shape and position of the signals is shown. The physical significance of kinetic parameters is dlscussed.
T = To
+ art (with
QI
d T / d t = const.)
=
(3)
as well as the validity of the following Arrhenius type law:
K = Z exp(-EIRT)
(4)
From Equations 2, 3, and 4, Torsi and Tessari (1) obtain
9 nt = Z q -U exp(-EIRT)
(5)
and Flameless atomic absorption spectrometry (FAAS) is based upon a heterogeneous process occurring in dynamic temperature conditions, viz. the evaporation of atoms during thermal flash. A theoretical model of the process has been proposed by Torsi and Tessari (1). According to these authors, by presuming a monoatomic layer distribution, the rate of evaporation of the sample to be analyzed can be written as
d6
82
dt
a
- - - --
exp(-EIR T )
THEORETICAL SHAPE OF THE SIGNALS The solution of Equation 6 is not discussed by Torsi and Tessari. The variables are separable, but the integration is not possible in finite form. By introducing the variable u = E / R T and the notation
e-" -Jz7 du
p ( ~= ) where q is the surface concentration at d = 1 , d is the fraction of surface coverage, t is the time, and K is the apparent rate constant for the evaporation. The concentration nt of atoms in the gas phase is obtained by dividing Equation 1 by the linear velocity v of the evaporating atoms,
U
one has ( 2 )
E R
J(: exp(-E/RT)dT = - p ( x )
(7)
and the solution of Equation 6 can be given as This magnitude is presumed to be prpportional to the instantaneous absorbance. Thus, the equation of nt gives a theoretical description of the signal obtained in FAAS. Further, a linear temperature variation program is presumed Permanent address, Faculty of Chemistry, Babes-Bolyai University, Cluj-Napoca, Rumania.
0003-2700/78/0350-1105$01 .OO/O
since for T = 0, one has d = 1. Equations 5 and 8 give
1
nt = - exp --x zq u
+p
( x )
Ra zE
0 1978 American Chemical Society
I
(9)
