Interference of Molecular Spectra Due to Alkali Halides in NonFlame Atomic Absorption Spectrometry Bruce R. Culver and Terry Surles Varian Instrument Division, 6 1 1 Hansen Way, Palo Alto, CA 94303
When using nonflame atomizers for atomic absorption analyses involving complex matrices, the primary interference encountered is nonatomic background absorption. Nonatomic absorption can be attributed to molecular absorption and/or light scattering. The latter occurs when solid particulate species are generated within the light path and is related in a complex fashion to the reciprocal of the wavelength of the incident radiation. Molecular absorption, on the other hand, occurs when matrix species are vaporized along with the analyte atomic species and absorb a portion of the analyte atomic resonant line emitted from the hollow cathode. Both effects result in false absorbance signals which must be corrected if accurate results are to be realized, i.e., the nonatomic portion of the signal must be measured and subtracted from the total signal. Two commonly available techniques are used to measure nonatomic absorption. One method measures the absorbance a t a nonabsorbing line which is adjacent to the analyte resonant line. If the nonabsorbing line does not lie within one or two nm of the resonant line and/or if the nonatomic background is due to molecular absorption, the measurement a t best only approximates the absorbance a t the analyte wavelength. The other method involves measuring the nonatomic absorption a t the same wavelength using a continuum source. This method was first described by Koirtyohann and Pickett ( I ) . They measured nonatomic absorption for NaCl, NaN03, KC1, and H2SO4 a t various wavelengths using an oxy-hydrogen flame and a simplified Fuwa-Vallee burner arrangement. The origin of the band spectra observed was not investigated, but it was shown that the continuum source background correction technique was quite adequate for the elements investigated. Above 350 nm, the continuum source intensity is weak however, and the adjacent line correction technique should be employed. However, for most atomic absorption determinations when using flame atomization, nonatomic background absorption does not present a problem except for samples containing high salt concentrations. When nonflame atomizers are used, on the other hand, special care must be taken to ensure that the absorbance signal is entirely atomic in nature and the origin of the nonatomic signal becomes very important if the adjacent line correction technique is employed. The purpose of this work was to establish the nature of the nonatomic background commonly encountered from different matrices when using nonflame atomization.
EXPERIMENTAL Apparatus. The measurements were carried out on a VarianTechtron Model AA-6DA Atomic Absorption Spectrophotometer. A hydrogen hollow cathode lamp was used as the continuum source to obtain the molecular spectra. The slit on the instrument was left at the 0.5-nm SBW position for all of the determinations. A Model BC-6 background corrector in conjunction with the AA6DA was used to obtain the P b and Cd calibration curves. A Varian-Techtron Model 63 Carbon Rod Atomizer was used to vaporize the samples after they had been dried on the atomizer. The dry conditions were adjusted so that the sample dried evenly and uni920
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, MAY 1975
formly on the atomizer. The atomize temperature was adjusted so that each salt studied completely vaporized within 1.5 seconds into the atomize cycle. Atomize temperatures reached in this time period were sufficient to completely vaporize the salt. The CRA is capable of reaching 3000 “C in 2 seconds (2, 3 ) . Procedure. Alkali halides were investigated because they represent the most commonly encountered problem matrices in nonflame atomic absorption determinations. All the solutions studied were aqueous solutions containing 0.1% (w/v) of the indicated salt (see Figure captions). The salts studied were taken from reagent grade stock. Five 1 1 of solution was pipetted onto the atomizer using an Autopette fixed volume dispenser. After the sample was dried, volatilized, and the absorbance recorded a t one wavelength, the wavelength was changed and the process repeated until the entire spectra covering a wavelength range from 200 to 380 nm was recorded.
RESULTS AND DISCUSSION Spectra were obtained for aqueous solutions of NaF, NaCl, KCl, KBr, Nal, CaC12, Na2S04, NaN03 and Na2HP04. For all oxy-anion salt solutions, little absorbance was observed in the wavelength region of interest although with the nitrate and sulfate salts, a weak band was observed near 220 nm. This is probably due to NaO, which would be expected as a decomposition by-product of these salts in the carbon rod a t high temperatures. Significant absorbance maxima were observed, however, for the solutions containing halide anions. Representative spectra for NaF, NaCl, and NaI are shown in Figure 1. The spectra for KCl, KBr, and CaC12 are shown in Figure 2. In all cases, the absorbance values are proportional to salt concentration. The absorption maxima are related to the anion and cation of the salt. The observed absorbance maxima can be compared to the calculated values listed by Herzberg ( 4 ) .From Table I, it can be seen that excellent correlations are obtained, considering the analytical technique which was used. The nonatomic background spectra obtained for these salts is thus primarily due to the molecular absorption of the continuum source radiation by the gaseous diatomic alkali halide species. Salt scattering may have some contributory effect, but this effect is indeed small compared to the effect of the molecular absorption. In most cases, the electronic transition which accounts for the molecular bands is IZ+ l2+.This refers to a transition from a ground state which is primarily ionic in character to the first excited state which has predominantly molecular character. These transitions were not observed for the oxy-anion salts. I t appears that these salts decompose a t the high temperatures of the Carbon Rod Atomizer. Any ionic species remaining, such as NaO, NaN, or NaS, do not appear to have strong band maxima in the wavelength region studied under the conditions implemented for this study. Other salts which can be vaporized without decomposing probably would exhibit a similar spectra. Calibration curves for P b and Cd were determined using the BC-6 continuous background corrector (Figure 3). The nonatomic background, which was determined by running several of the standards with the device in the “Background Only” mode, was 0.23 absorbance and 0.57 absorbance a t the P b 217.0-nm and the Cd 228.8-nm lines, respec+
'0
h
9
+'\
Table I. Molecular Spectra of Alkali Halide Solutions
t
8
A max nm
NaCl c NaF Nal
i.
t
7
Solution
Obiewed
Calculated
0.1% KCI
246
249 243 234
235 210n 255 278 0.1%NaI 220 257 325 0.1% NaF 215 0.1% CaC12 214 a No value reported by G . Herzberg ( 4 ) .
0.1% NaCl 0.1% K B r
"--
i
0-7200
~
-
\
P, I
300 WAVELENGTH lnm)
400
Flgure 1. Molecular spectra of NaCI, NaF, and Nal using 5 MI of 0.1 % ( w h ) solutions 1.0 -
P KCI 1 KBr CaCIZ
Figure 2. Molecular spectra of KCI, KBr, and CaCI2 using 5 MIof 0.1 YO (w/v) solutions
r A T Pb
-
0.23abs.
BKGD A T Cd
-
0.57abr.
BKGD
0 Cd ng/ml
0
2.5
5.0
7.5
10.0
Figure 3. Calibration curve for determining Pb and Cd in 0.1 % NaCl
tively. The RSD for a 2 ng/ml Cd standard was f3.2%, while the RSD for a 10 ng/ml P b standard was f3.7%. Similar RSD's were obtained using the adjacent line correction technique for both elements; however, the corrected answer proved to be inaccurate. The Bi 227.8-nm line was used to measure nonatomic absorption near the Cd line. Subtracting the background from the total absorbance signal resulted in a 12.5% undercorrection. The nonabsorbing P b line a t 220.3 nm was used to measure the background near the P b 217.0-nm line resulting in a 12% overcorrection.
2 54 277 213 258 325
The use of the nonabsorbing adjacent line method to correct for background absorption should not be used for nonflame AA determinations involving samples which contain excessive amounts of thermally stable, nonvolatile salts unless it is unavoidable (i.e., above 350 nm). The steep slope of the molecular absorption bands make it very difficult to find a nonabsorbing line which is close enough to the resonant line to achieve an accurate correction. On the other hand, when using a continuum source, whether simultaneous or sequential, it is necessary that the continuum lamp emission be optically coincident with the hollow cathode lamp emission and that the two beams have the same aperture through the absorption zone to achieve accurate compensation for the nonatomic background absorption. With either correction technique, if alkali halide solutions or solutions containing alkali halide salts (i.e., seawater, brines, serum, etc.) are being analyzed by nonflame atomic absorption, it is necessary to always minimize the nonatomic contribution of the signal prior to atomization to obtain accurate results. If this cannot be accomplished instrumentally, a chemical separation of the analyte elements and matrix or the use of an alternate resonant line in a region of the spectrum where the molecular bands are not predominant is necessary. Work is being continued on a number of other salt matrices. After completion of this work, a paper presented a t the 1973 Florence Spectroscopy Conference by Guger and Massmann ( 5 ) was brought to our attention. Their paper concentrated on absorption data of salt solutions as a function of time and temperature using a carbon tube furnace. Where similar studies were carried out, data are in general agreement.
LITERATURE CITED (1)S.R. Koirtyohann and E. C. Pickeft, Anal. Chem., 37,601 (1965). (2)D. G. Brodie and J. P. Matousek, Anal. Chem., 43, 1557 (1971). (3) M. D. Amos, "Nonflame Atomization in AAS (a current review)," Amer. Lab, Aug. (1972). (4) C. Herzberg, "Molecular Spectra and Molecular Structure I. Spectra of
Diatomic Molecules." 2nd ed., Van Nostrand and Co., Inc., Princeton, NJ. 1950,pp 501-581. (5) S. Guger and H. Massmann, Uber physikalische and chemische Prozesse bei der Atomabsorptionanalyze mit Graphitrohrofen, XVll Coloquim Spectroscopium Internationale. Florence, Italy, Sept. 1973.
RECEIVEDfor review October 7, 1974. Accepted January 13, 1975.
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