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Spectral interferences and background overcompensation in inverse zeeman-corrected atomic absorption spectrometry Part 3. Study of eighteen cases of spectral interference
Analytica Chimica Acta, 186 (1986) 155-162 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
SPECTRAL INTERFERENCES AND BACKGROUND OVERCOMPENSATION IN INVERSE ZEEMAN-CORRECTED ABSORPTION SPECTROMETRY Part 3. A Study of Eighteen Cases of Spectral Interference
ATOMIC
G. WIBETOE and F. J. LANGMYHR*
Department (Norway)
of Chemistry,
University of Oslo, P.O. Box 1033, Blindern, 0315 Oslo 3
(Received 18th February 1986)
SUMMARY Atomic line tables were surveyed in order to select pairs of elements which have contiguous lines and so are likely to suffer from spectral interference. Spectrometric measurements of these element pairs revealed eighteen cases of spectral interference. Four of these cases occurred at the recommended wavelengths for the analyte: the 459.4-nm europium line (interference from vanadium and cesium), the 247.6-nm palladium line (interference from lead) and the 265.9-nm platinum line (interference from europium).
In conventional atomic absorption spectrometry (a.a.s.), spectral interferences caused by direct overlap of foreign atomic lines does not normally present any serious problems. However, the Zeeman splitting of the absorption lines into II- and u-components increases the probability of encountering spectral interference. In the two previous papers [l, 21 in this series, cases were reported of spectral interference which caused overcompensation in inverse Zeeman-corrected a.a.s. In these studies of thirty analyte elements and four matrix elements (cobalt, iron, manganese and nickel), five cases of overcompensation were revealed, all resulting from the presence of matrix element lines 0.0040.007 nm apart from the analyte lines. In the present investigation, tables of atomic lines were surveyed first and then analyte/matrix element pairs which had lines about 0.01 nm apart or closer were examined in detail. EXPERIMENTAL
Instruments, apparatus, reagents and solutions The a.a.s. measurements were made with a Perkin-Elmer (P-E) model 5000/inverse Zeeman instrument and a P-E HGA-400 graphite furnace. Timeresolved absorbance data were collected and displayed with the P-E Data System 10 and the P-E 660 Printer/Plotter. The graphite tubes were of the standard or pyrolytically-coated type; the 0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
166
atomization cells were purged with argon (>99.9% by volume). Solutions were transferred to the furnace from plastic micropipettes. The hollow-cathode lamps and electrodeless-discharge lamps were all delivered by Perkin-Elmer. Stock solutions (10 or 1 mg ml-‘) of antimony, chromium(III), gold, mercury, platinum, titanium, tungsten and zirconium were of Spectrosol quality (BDH Chemicals); those of beryllium and cobalt were of Titrisol quality (Merck). Standard solutions of cesium, potassium, magnesium, rhodium, rubidium, scandium and strontium were prepared by dissolving the reagent-grade chlorides in water or a minimum amount of the appropriate acid. Barium, calcium, lithium and sodium stock solutions were prepared from reagent-grade carbonates with addition of acid, as required. The arsenic, europium and lanthanum standard solutions were prepared from the pure oxides; the silicon solution was prepared by decomposing the pure oxide with sodium carbonate. Stock solutions of the other elements were prepared from high-purity metals dissolved in minimum volumes of the appropriate acids (Suprapur, Merck). Measurements
For the present investigation a selection of 33 electrodeless-discharge and hollow-cathode lamps was available. For the corresponding analytes, the Perkin-Elmer instrument manual was used to compile a list of all recommended wavelengths which provided a relative sensitivity of about 10 or less. Atomic line tables [3, 41 were then surveyed to find other atomic lines situated about 0.01 nm or closer to any of the above-mentioned recommended lines. In some instances, gas-fill lines and other lines, which were assumed to be contained in the bandpass of the monochromator, were also checked for adjacent lines. Table 1 lists the analytes studied, the wavelengths and slit widths used during the measurements, and the matrix elements having adjacent lines TABLE 1 Analytelmatrix element combinations tested for spectral interference [The elements underlined did not show spectral interference. Matrix elements causing background overcompensation are marked with (-). All the other elements gave a positive absorbance signal, and a spectral interference cannot be excluded. Matrix elements given in brackets have lines close to the analyte lines, but were not tested. Except where mentioned, hollow-cathode lamps were used. The general furnace parameters are given in Table 2. The superscripts on the upper left of the separate elements have the following meanings: a question mark indicates that the line may be atomic or ionic (status not given in the wavelength tables); asterisks show that a spectral interference is already known [ 6, 71; ne means that the matrix line is close to a neon line.] Anelvte
Wavelength Olm)
Slit width (nm)
Mat&X elementa tested
Analyte
Wavelength Inm)
Slit width (nm)
Matrix element tested
&
328.1 309.3 396.2
0.7 0.7 0.7
?Eu 77 ?Mo
Pb Pb Pd
213.6 261.4 247.6
0.2 0.7 0.2
zr co (-_). %n, ya Co. Fe. ?Pb (--_).I
Al
157 TABLE 1 (continued) Analyte
Ash AU B Bi cab CO
Cr
cu
EU
Fe
Ga
Geb
I-U In Mn
MO
Ni
aMatrix discharge
AnaIyte
Wavelength (nm)
Slit width (nm)
Matrix element tested
308.2 231.3 257.5 193.7 242.8 267.6 249.1 223.1 227.7 220.8 242.6 241.2 252.1 243.6 357.9 359.4 360.5 426.4 427.6 324.0 327.4 216.6 459.4 462.1 466.2 248.8 302.1 252.7 312.0 287.4 294.4 417.2 266.1 259.2 269.1 263.6 303.9 451.1 279.8 280.1 403.1 313.3 379.0 319.4 390.3 232.0 231.1 362.5 341.6 306.1 346.2 361.6
0.7 0.2 0.2 0.7 0.7 0.7 0.7 0.2 0.2 0.7 0.2 0.2 0.2 0.2 0.7 0.7 0.7 0.1 0.7 0.7 0.7 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.7 0.7 0.7 0.2 0.2 0.2 0.1 0.7 0.7 0.2 0.2 0.2 0.1 0.7 0.7 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2
?Mn, *V (-_) Ba. co Zn Pt Pt
element lamp
Pt
v.w Co (-_). v In. XL 7Co (-_). v sbb
Ge. Or), pt (3 7V
Seb Si
El!
Co (-_),
Fe, n co. nam, Ni. v =ri, zr ?A& *Eu. ?Mn (Ir). n
pt nets c-j. v (4 wr nei&, ne-, ~2 Pt. TI Cr ?Mn Ba
(OS). Tat 3. We).
5
GT Ga, $Ta
&,
Sr Te Ti
?v
*eve,nev 7Ta -
f
Snb
TIb neV
V
(I~). nazr
Zn
ii&y
Wavelength Inm)
Slit width (nm)
Matrix element tested
276.3 340.6 266.9 306.6 203.0 293.0 273.4 270.2 248.1 271.9 206.8 231.2 204.0 261.6 260.7 262.8 252.4 221.7 224.6 270.6 303.4 254.6 300.9 460.7 214.3 365.4 364.3 320.0 363.6 335.6 376.3 334.2 399.9 399.0 276.8 377.6 238.0 318.4 306.0 306.6 306.6 390.2 213.9
0.2 0.2 0.7 0.1 0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.7 0.2 0.7 0.2 0.2 0.2 0.2 0.7 0.2 0.7 0.7 0.7 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.7 0.2 0.2 0.2 0.2 0.2
zr
3” &,
?Q, 7Eu (-_), & $Ir). Ni (-1 *w
Sb 7Ba. 7In. Ni ?Ba, 7Mo 7Mo &
Cr (-_). ?Fe. (Ru)
Ca (-_) Au. naNi Ka &y7Ca_. ?Mo 7Ta -1 -_( ?W Ca. V Co. %%, II a. Fe. Or), (Ru). 1 (Ir), Rh E. 0). Ra 0). CQ. 9. &, (Ir). La. 7Ta.v Co, Zr
?m,
m. CA, ?Cr, (Ru), Co Rb 7% -1-v Fe 7%0 _. _. Ca Ge Ni Zr _. Fe -1 _*_._ naFe -
B!
?~,?~,~
Cr. Fe ?cr. v Co. v ?Mn -v_ Zr Co (-_) co. V (-_)
co a. nezr -
with atomic
line close to an analyte
or neon
emission
line. bElectrodeless
used.
likely to cause spectral interference. These analytelmatrix element pairs were then examined experimentally to reveal cases of overcompensation. For this purpose, 5 ~1 of a 0.1% (w/v) solution of the matrix element was
158 TABLE 2 General furnace parameters for studying the interfering effect of the elements listed in Table 1. The Zeeman-effect background corrector, pyrolytically-coated graphite tubes and 5-~1 samples were used throughout Step Temp. (“C) Ramp (s) Hold (s) Read (s) Int. flow
1 120 10 20 -
2 400 10 20 29 -
3 2400 0 5
4s 2600 1 4
-b
-
gas stop
-
*Clean step. bRead step.
pipetted into the atomization cell and the element was atomized; the general furnace parameters are given in Table 2. In those instances where background overcompensation was observed (as was evident from a negative absorbance signal), the measurements were repeated with the use of the furnace parameters recommended in the instrument manual; in some cases the element concentration was also changed. In order to avoid memory effects, a new graphite tube was used for each matrix element studied. The appropriate acids were used as blanks. Table 3 lists revised charring and atomization temperatures for those elements which had been found to be subject to overcompensation; the other furnace parameters were the same as those given in Table 2. Measurements were also made without Zeeman background correction (i.e., in the single-beam mode) to decide whether a positive spectral interference, i.e., caused by direct overlap of the analyte emission line with the unsplit matrix element absorption line (no magnetic field applied), could be excluded or not. TABLE 3 Charring/atomization temperatures for elements subject to background overcompensation (the other furnace parameters were the same as those given in Table 2). Except where mentioned, pyrolyticallycoated graphite tubes were used Analyte
Al B Bi co Cr Eua
Temperatures (“C) Charring
Atomization
1500 1000 500 1000 1200 1300
2400 2700 1000 2200 2300 2600
Analyte
Ni Pb Pd Pt Si Sn
Temperatures (“C) Charring
Atomization
1000 500 1000 1400 1400 800
2600 1100 2300 2500 2700 2700
*Standard graphite tubes were used for atomization of cesium at the europium line.
159 RESULTS
AND DISCUSSION
The results of the preliminary investigations are shown in Table 1; the effects of the matrix elements can be classified as producing (a) no signal, (b) a negative signal, or (c) a positive signal when atomized at the wavelength of the analyte tested. As is apparent from Table 1, most of the matrix elements belong to group (a), and for these elements spectral interference need not be considered. The lines of these matrix elements are probably only emission lines or very weak absorption lines. Among the matrix elements, eighteen were found to belong to group (b), which comprises those giving a negative absorbance signal caused by a ucomponent of the matrix element overlapping with the analyte emission line. In the absence of the magnetic field, the matrix absorption line may still overlap with the wing of the analyte emission line, and this causes a positive spectral interference in conventional a.a.s. Several cases of overlap of absorption with emission lines have been reported [5], even when peak-to-peak distances are as large as 0.05 nm. As the lines of the matrix elements belonging to group (b) are all absorbing lines and, in addition, are situated less than 0.01 nm away from the analyte line, a positive spectral interference is probable in conventional a.a.s. Experiments conducted with the instrument in the single-beam mode could only establish, from the absence of a signal, that a positive spectral interference was not present. Only in two of the present cases, under the conditions where the overcompensation effect with Zeeman a.a.s. existed, could it be ascertained that a positive spectral interference did not occur; 1000 pg ml-’ vanadium solution measured at the 305.1-nm nickel line and 100 pg ml-’ lead solution measured at the palladium line gave no signal in the single-beam mode. However, at higher matrix concentrations or under other conditions, spectral interference can still be experienced. For all elements of group (b), factors which broaden, shift and/or change the profile of the peaks can also influence the interference, and the interfering effect should therefore always be established experimentally. The total number of spectral interferences detected in the present series of investigations now adds up to 22. Only four of these seem to have been observed or described previously as possible overlapping lines in conventional a.a.s.; the eighteen new cases should therefore be added to the list of known spectral interferences in a.a.s. [6]. Table 4 gives a more detailed description of the eighteen cases of overcompensation encountered in the present study; the wavelengths of the lines believed to overlap, and the negative peak areas obtained with the recommended furnace program, are also listed. For those cases in which the matrix element solution is contaminated by the analyte, the actual negative spectral interferences will be larger than the values listed in Table 4. Some matrix elements have an absorption line adjacent to a gas-fill line of the emission source lamp, and both lines are contained in the bandpass of
160 TABLE 4 Analyte/matrix element Zeeman-corrected 8.8.8. Analyte
Line (nm)
RS*
combinations
subject
to
background
Inter- Overlapping lines (nm) Slit widtbb ferent Emission Interfering (nm) A, A,
overcompensation
h, -h,
Cont.= (Mgml-‘)
in
Peak aread
Al
308.2
1.6
0.7
Ve
308.2155
308.2111
-0.0044
10 100 1000
-0.15 -0.78 -1.07
B
249.7
1.0
0.7
co
249.6778
249.671
-0.007
1000
-0.014
Bi
227.6
14.0
0.2
co
227.6578
227.6532
-0.0046
1000 1000
no si nal -1.15 f
co
243.6
2.9
0.2
Pte
243.6657
243.6689
+ 0.0032
100 1000
-0.14 -0.54
Cr
360.5
2.2
0.7
toe
360.5333
360.5356
+ 0.0023
100 1000
-0.07 -0.28
EU
459.4
1.0
0.2s
V
459.402
459.4108
+ 0.009
10 100 1000
-0.23 -0.66 1.62h
Eu
459.4
1.0
0.2s
cs
459.32431
459.3177
-0.0066
10 000
Ni
341.5
3.5
0.2
co
341.4765
341.4736
-0.0029
1000
-0.067
Ni
305.1
4.5
0.2
V
305.0819
306.0890
+ 0.0071
100 1000
-0.23 -0.97
Pb
261.4
10.0
0.7
co
261.4178
261.4128
-0.0050
1000 1000
no si nal -0.20 P
Pd
247.6
1.0
0.2
Pb
247.6418
247.6379
-0.0039
1000 100 1000
no signal -0.04f -0.28f
Pt
265.9
1.0
0.7
Eu
265.9454
265.942
-0.003
1000
-0.21
-0.0089
1000
-0.54
-0.10
Pt
306.5
2.1
0.7
Nie
306.4712
306.4623
Pt
273.4
4.1
0.2
Fee
273.3961
273.4004
+ 0.0043
1000
-0.04
Si
250.7
2.8
0.2
co
250.6899
250.6877
-0.0022
100 1000
-0.12 -0.67
Si
250.7
2.8
0.2
Ve
250.6899
250.6905
t 0.0006
10 100 1000
-0.06 -0.37 -h
Sn
303.4
2.3
0.2
Cr
303.4121
303.4190
to.0069
1000
-0.24
Sn
300.9
4.1
0.7
Ca
300.9147
300.9205
tO.0058 10000
-0.24
sRelative sensitivities taken from the Perkin-Elmer literature for flame a.a.s. (standard conditions). bExcept where mentioned the recommended low slit is used. CConcentration of interferent; 5 ~1 added. dExcept where mentioned the furnace parameters given in Table 3 are used. eThe matrix metal solution is contaminated by the analyte element. ‘The general furnace parameters given in Table 2 are used. sThe high slit had to be used. hThere is a positive signal superimposed on the negative absorbance. ‘Neon emission line.
161
oL,-
3 2.5
0 Time (5)
Fig. 1. Absorbance profiles of 5-~1 portions of a 1% (w/v) cesium solution measured at the 459.4-nm em-opium line with Zeeman background correction and different slit widths. Slit widths: (1) 0.7 nm; (2) 0.2 nm; (3) 0.07 nm. For experimental conditions, see Table 3.
the monochromator/detector system. A u-line of a matrix element may then overlap with the gas-fill emission line, this again causing overcompensation. In the present study, this type of interference probably occurred in one analytelmatrix element system. Figure 1 shows the absorbance profiles for 5~1 portions of a 1% cesium solution measured at the 459.4~nm europium line. Profiles 1 and 2, recorded at slit widths of 0.7 and 0.2 nm, respectively, show overcompensation probably caused by a u-line of the 459.3177-nm cesium resonance line overlapping with the 459.3243~nm neon line. When the slit width was decreased to 0.07 nm, profile 3 was obtained; the two interfering lines were thus excluded from the bandpass and overcompensation was no longer observed. This type of spectral interference is not likely to cause any serious problems in practical work, but it remains a type of interference that is not normally taken into account. Most of the present eighteen cases of overcompensation do not appear at the main lines recommended for measurement of the analyte, but some are affected, such as the 459.4~nm europium line (interference from vanadium and cesium), the 247.6~nm palladium line (interference from lead) and the 265.9~nm platinum line (interference from europium). The other cases of spectral interference encountered here occur at the less sensitive wavelengths, but these lines can also be of importance in a.a.s. Selection of absorption lines of similar or different sensitivity makes it possible to avoid interferences and to provide greater flexibility in covering different concentration ranges. In electrothermal a.a.s., certain interferences can be avoided by taking advantage of the different volatilities of the analyte and the matrix element. In some cases, the difference in volatility even permits an element to be atomized before the other element starts to evaporate. It is also possible to utilize spectral interferences for analytical purposes, such as has been previously
162
described [l, 21 for the systems Hg/Co and Zn/Fe; this method may also be applicable to the element pairs Bi/Co, Pb/Co and Pd/Pb in the present investigation. The remaining matrix elements are those belonging to group (c), i.e., the elements that gave a positive absorbance in the preliminary measurements (see Table 1). The interpretation of these results is complicated by the fact that some of the metal solutions were contaminated, and extensive work would be needed to decide whether or not a positive absorbance for a matrix element is caused by a contamination of the analyte, a positive spectral interference or both. A background overcompensation effect may be overlooked because of a contamination of the analyte. Some of the matrix elements belonging to group (c) are known to give spectral interference in conventional a.a.s. (see Table 1). For the other elements of this group, nothing definite could be concluded from the present experiments. The present series of papers describes 22 cases of spectral interferences in inverse Zeeman-corrected a.a.s. and demonstrates that disturbances by absorbing atomic lines are more common than previously believed. Further work is required for the elucidation of other spectral interferences. Work in this field would be facilitated if highly resolved spectra of the emission sources, a data bank of current wavelengths and wavelength data, and appropriate software were available. REFERENCES 1 G. Wibetoe and F. J. Langmyhr, Anal. Chim. Acta, 165 (1984) 87. 2 G. Wibetoe and F. J. Langmyhr, Anal. Chim. Acta, 176 (1985) 33. 3 G. R. Harrison, M.I.T. Wavelength Tables, The Technology Press, Wiley, New York, 1939. 4 P. W. J. M. Boumans, Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectroscopy, Pergamon Press, Oxford, 1980. 5 J. D. Norris and T. S. West, Anal. Chem., 46 (1974) 1423. 6 See, e.g., B. Welz, Atomabsorptionsspektrometrie, 3rd edn., Verlag Chemie, Weinheim, 1983. 7 T. Kantor, P. Fodor and E. Pungor, Anal. Chim. Acta, 102 (1978) 15.
SPECTRAL INTERFERENCES AND BACKGROUND OVERCOMPENSATION IN INVERSE ZEEMAN-CORRECTED ABSORPTION SPECTROMETRY Part 3. A Study of Eighteen Cases of Spectral Interference
ATOMIC
G. WIBETOE and F. J. LANGMYHR*
Department (Norway)
of Chemistry,
University of Oslo, P.O. Box 1033, Blindern, 0315 Oslo 3
(Received 18th February 1986)
SUMMARY Atomic line tables were surveyed in order to select pairs of elements which have contiguous lines and so are likely to suffer from spectral interference. Spectrometric measurements of these element pairs revealed eighteen cases of spectral interference. Four of these cases occurred at the recommended wavelengths for the analyte: the 459.4-nm europium line (interference from vanadium and cesium), the 247.6-nm palladium line (interference from lead) and the 265.9-nm platinum line (interference from europium).
In conventional atomic absorption spectrometry (a.a.s.), spectral interferences caused by direct overlap of foreign atomic lines does not normally present any serious problems. However, the Zeeman splitting of the absorption lines into II- and u-components increases the probability of encountering spectral interference. In the two previous papers [l, 21 in this series, cases were reported of spectral interference which caused overcompensation in inverse Zeeman-corrected a.a.s. In these studies of thirty analyte elements and four matrix elements (cobalt, iron, manganese and nickel), five cases of overcompensation were revealed, all resulting from the presence of matrix element lines 0.0040.007 nm apart from the analyte lines. In the present investigation, tables of atomic lines were surveyed first and then analyte/matrix element pairs which had lines about 0.01 nm apart or closer were examined in detail. EXPERIMENTAL
Instruments, apparatus, reagents and solutions The a.a.s. measurements were made with a Perkin-Elmer (P-E) model 5000/inverse Zeeman instrument and a P-E HGA-400 graphite furnace. Timeresolved absorbance data were collected and displayed with the P-E Data System 10 and the P-E 660 Printer/Plotter. The graphite tubes were of the standard or pyrolytically-coated type; the 0003-2670/86/$03.50
o 1986 Elsevier Science Publishers B.V.
166
atomization cells were purged with argon (>99.9% by volume). Solutions were transferred to the furnace from plastic micropipettes. The hollow-cathode lamps and electrodeless-discharge lamps were all delivered by Perkin-Elmer. Stock solutions (10 or 1 mg ml-‘) of antimony, chromium(III), gold, mercury, platinum, titanium, tungsten and zirconium were of Spectrosol quality (BDH Chemicals); those of beryllium and cobalt were of Titrisol quality (Merck). Standard solutions of cesium, potassium, magnesium, rhodium, rubidium, scandium and strontium were prepared by dissolving the reagent-grade chlorides in water or a minimum amount of the appropriate acid. Barium, calcium, lithium and sodium stock solutions were prepared from reagent-grade carbonates with addition of acid, as required. The arsenic, europium and lanthanum standard solutions were prepared from the pure oxides; the silicon solution was prepared by decomposing the pure oxide with sodium carbonate. Stock solutions of the other elements were prepared from high-purity metals dissolved in minimum volumes of the appropriate acids (Suprapur, Merck). Measurements
For the present investigation a selection of 33 electrodeless-discharge and hollow-cathode lamps was available. For the corresponding analytes, the Perkin-Elmer instrument manual was used to compile a list of all recommended wavelengths which provided a relative sensitivity of about 10 or less. Atomic line tables [3, 41 were then surveyed to find other atomic lines situated about 0.01 nm or closer to any of the above-mentioned recommended lines. In some instances, gas-fill lines and other lines, which were assumed to be contained in the bandpass of the monochromator, were also checked for adjacent lines. Table 1 lists the analytes studied, the wavelengths and slit widths used during the measurements, and the matrix elements having adjacent lines TABLE 1 Analytelmatrix element combinations tested for spectral interference [The elements underlined did not show spectral interference. Matrix elements causing background overcompensation are marked with (-). All the other elements gave a positive absorbance signal, and a spectral interference cannot be excluded. Matrix elements given in brackets have lines close to the analyte lines, but were not tested. Except where mentioned, hollow-cathode lamps were used. The general furnace parameters are given in Table 2. The superscripts on the upper left of the separate elements have the following meanings: a question mark indicates that the line may be atomic or ionic (status not given in the wavelength tables); asterisks show that a spectral interference is already known [ 6, 71; ne means that the matrix line is close to a neon line.] Anelvte
Wavelength Olm)
Slit width (nm)
Mat&X elementa tested
Analyte
Wavelength Inm)
Slit width (nm)
Matrix element tested
&
328.1 309.3 396.2
0.7 0.7 0.7
?Eu 77 ?Mo
Pb Pb Pd
213.6 261.4 247.6
0.2 0.7 0.2
zr co (-_). %n, ya Co. Fe. ?Pb (--_).I
Al
157 TABLE 1 (continued) Analyte
Ash AU B Bi cab CO
Cr
cu
EU
Fe
Ga
Geb
I-U In Mn
MO
Ni
aMatrix discharge
AnaIyte
Wavelength (nm)
Slit width (nm)
Matrix element tested
308.2 231.3 257.5 193.7 242.8 267.6 249.1 223.1 227.7 220.8 242.6 241.2 252.1 243.6 357.9 359.4 360.5 426.4 427.6 324.0 327.4 216.6 459.4 462.1 466.2 248.8 302.1 252.7 312.0 287.4 294.4 417.2 266.1 259.2 269.1 263.6 303.9 451.1 279.8 280.1 403.1 313.3 379.0 319.4 390.3 232.0 231.1 362.5 341.6 306.1 346.2 361.6
0.7 0.2 0.2 0.7 0.7 0.7 0.7 0.2 0.2 0.7 0.2 0.2 0.2 0.2 0.7 0.7 0.7 0.1 0.7 0.7 0.7 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.7 0.7 0.7 0.2 0.2 0.2 0.1 0.7 0.7 0.2 0.2 0.2 0.1 0.7 0.7 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2
?Mn, *V (-_) Ba. co Zn Pt Pt
element lamp
Pt
v.w Co (-_). v In. XL 7Co (-_). v sbb
Ge. Or), pt (3 7V
Seb Si
El!
Co (-_),
Fe, n co. nam, Ni. v =ri, zr ?A& *Eu. ?Mn (Ir). n
pt nets c-j. v (4 wr nei&, ne-, ~2 Pt. TI Cr ?Mn Ba
(OS). Tat 3. We).
5
GT Ga, $Ta
&,
Sr Te Ti
?v
*eve,nev 7Ta -
f
Snb
TIb neV
V
(I~). nazr
Zn
ii&y
Wavelength Inm)
Slit width (nm)
Matrix element tested
276.3 340.6 266.9 306.6 203.0 293.0 273.4 270.2 248.1 271.9 206.8 231.2 204.0 261.6 260.7 262.8 252.4 221.7 224.6 270.6 303.4 254.6 300.9 460.7 214.3 365.4 364.3 320.0 363.6 335.6 376.3 334.2 399.9 399.0 276.8 377.6 238.0 318.4 306.0 306.6 306.6 390.2 213.9
0.2 0.2 0.7 0.1 0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.7 0.2 0.7 0.2 0.2 0.2 0.2 0.7 0.2 0.7 0.7 0.7 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.7 0.2 0.2 0.2 0.2 0.2
zr
3” &,
?Q, 7Eu (-_), & $Ir). Ni (-1 *w
Sb 7Ba. 7In. Ni ?Ba, 7Mo 7Mo &
Cr (-_). ?Fe. (Ru)
Ca (-_) Au. naNi Ka &y7Ca_. ?Mo 7Ta -1 -_( ?W Ca. V Co. %%, II a. Fe. Or), (Ru). 1 (Ir), Rh E. 0). Ra 0). CQ. 9. &, (Ir). La. 7Ta.v Co, Zr
?m,
m. CA, ?Cr, (Ru), Co Rb 7% -1-v Fe 7%0 _. _. Ca Ge Ni Zr _. Fe -1 _*_._ naFe -
B!
?~,?~,~
Cr. Fe ?cr. v Co. v ?Mn -v_ Zr Co (-_) co. V (-_)
co a. nezr -
with atomic
line close to an analyte
or neon
emission
line. bElectrodeless
used.
likely to cause spectral interference. These analytelmatrix element pairs were then examined experimentally to reveal cases of overcompensation. For this purpose, 5 ~1 of a 0.1% (w/v) solution of the matrix element was
158 TABLE 2 General furnace parameters for studying the interfering effect of the elements listed in Table 1. The Zeeman-effect background corrector, pyrolytically-coated graphite tubes and 5-~1 samples were used throughout Step Temp. (“C) Ramp (s) Hold (s) Read (s) Int. flow
1 120 10 20 -
2 400 10 20 29 -
3 2400 0 5
4s 2600 1 4
-b
-
gas stop
-
*Clean step. bRead step.
pipetted into the atomization cell and the element was atomized; the general furnace parameters are given in Table 2. In those instances where background overcompensation was observed (as was evident from a negative absorbance signal), the measurements were repeated with the use of the furnace parameters recommended in the instrument manual; in some cases the element concentration was also changed. In order to avoid memory effects, a new graphite tube was used for each matrix element studied. The appropriate acids were used as blanks. Table 3 lists revised charring and atomization temperatures for those elements which had been found to be subject to overcompensation; the other furnace parameters were the same as those given in Table 2. Measurements were also made without Zeeman background correction (i.e., in the single-beam mode) to decide whether a positive spectral interference, i.e., caused by direct overlap of the analyte emission line with the unsplit matrix element absorption line (no magnetic field applied), could be excluded or not. TABLE 3 Charring/atomization temperatures for elements subject to background overcompensation (the other furnace parameters were the same as those given in Table 2). Except where mentioned, pyrolyticallycoated graphite tubes were used Analyte
Al B Bi co Cr Eua
Temperatures (“C) Charring
Atomization
1500 1000 500 1000 1200 1300
2400 2700 1000 2200 2300 2600
Analyte
Ni Pb Pd Pt Si Sn
Temperatures (“C) Charring
Atomization
1000 500 1000 1400 1400 800
2600 1100 2300 2500 2700 2700
*Standard graphite tubes were used for atomization of cesium at the europium line.
159 RESULTS
AND DISCUSSION
The results of the preliminary investigations are shown in Table 1; the effects of the matrix elements can be classified as producing (a) no signal, (b) a negative signal, or (c) a positive signal when atomized at the wavelength of the analyte tested. As is apparent from Table 1, most of the matrix elements belong to group (a), and for these elements spectral interference need not be considered. The lines of these matrix elements are probably only emission lines or very weak absorption lines. Among the matrix elements, eighteen were found to belong to group (b), which comprises those giving a negative absorbance signal caused by a ucomponent of the matrix element overlapping with the analyte emission line. In the absence of the magnetic field, the matrix absorption line may still overlap with the wing of the analyte emission line, and this causes a positive spectral interference in conventional a.a.s. Several cases of overlap of absorption with emission lines have been reported [5], even when peak-to-peak distances are as large as 0.05 nm. As the lines of the matrix elements belonging to group (b) are all absorbing lines and, in addition, are situated less than 0.01 nm away from the analyte line, a positive spectral interference is probable in conventional a.a.s. Experiments conducted with the instrument in the single-beam mode could only establish, from the absence of a signal, that a positive spectral interference was not present. Only in two of the present cases, under the conditions where the overcompensation effect with Zeeman a.a.s. existed, could it be ascertained that a positive spectral interference did not occur; 1000 pg ml-’ vanadium solution measured at the 305.1-nm nickel line and 100 pg ml-’ lead solution measured at the palladium line gave no signal in the single-beam mode. However, at higher matrix concentrations or under other conditions, spectral interference can still be experienced. For all elements of group (b), factors which broaden, shift and/or change the profile of the peaks can also influence the interference, and the interfering effect should therefore always be established experimentally. The total number of spectral interferences detected in the present series of investigations now adds up to 22. Only four of these seem to have been observed or described previously as possible overlapping lines in conventional a.a.s.; the eighteen new cases should therefore be added to the list of known spectral interferences in a.a.s. [6]. Table 4 gives a more detailed description of the eighteen cases of overcompensation encountered in the present study; the wavelengths of the lines believed to overlap, and the negative peak areas obtained with the recommended furnace program, are also listed. For those cases in which the matrix element solution is contaminated by the analyte, the actual negative spectral interferences will be larger than the values listed in Table 4. Some matrix elements have an absorption line adjacent to a gas-fill line of the emission source lamp, and both lines are contained in the bandpass of
160 TABLE 4 Analyte/matrix element Zeeman-corrected 8.8.8. Analyte
Line (nm)
RS*
combinations
subject
to
background
Inter- Overlapping lines (nm) Slit widtbb ferent Emission Interfering (nm) A, A,
overcompensation
h, -h,
Cont.= (Mgml-‘)
in
Peak aread
Al
308.2
1.6
0.7
Ve
308.2155
308.2111
-0.0044
10 100 1000
-0.15 -0.78 -1.07
B
249.7
1.0
0.7
co
249.6778
249.671
-0.007
1000
-0.014
Bi
227.6
14.0
0.2
co
227.6578
227.6532
-0.0046
1000 1000
no si nal -1.15 f
co
243.6
2.9
0.2
Pte
243.6657
243.6689
+ 0.0032
100 1000
-0.14 -0.54
Cr
360.5
2.2
0.7
toe
360.5333
360.5356
+ 0.0023
100 1000
-0.07 -0.28
EU
459.4
1.0
0.2s
V
459.402
459.4108
+ 0.009
10 100 1000
-0.23 -0.66 1.62h
Eu
459.4
1.0
0.2s
cs
459.32431
459.3177
-0.0066
10 000
Ni
341.5
3.5
0.2
co
341.4765
341.4736
-0.0029
1000
-0.067
Ni
305.1
4.5
0.2
V
305.0819
306.0890
+ 0.0071
100 1000
-0.23 -0.97
Pb
261.4
10.0
0.7
co
261.4178
261.4128
-0.0050
1000 1000
no si nal -0.20 P
Pd
247.6
1.0
0.2
Pb
247.6418
247.6379
-0.0039
1000 100 1000
no signal -0.04f -0.28f
Pt
265.9
1.0
0.7
Eu
265.9454
265.942
-0.003
1000
-0.21
-0.0089
1000
-0.54
-0.10
Pt
306.5
2.1
0.7
Nie
306.4712
306.4623
Pt
273.4
4.1
0.2
Fee
273.3961
273.4004
+ 0.0043
1000
-0.04
Si
250.7
2.8
0.2
co
250.6899
250.6877
-0.0022
100 1000
-0.12 -0.67
Si
250.7
2.8
0.2
Ve
250.6899
250.6905
t 0.0006
10 100 1000
-0.06 -0.37 -h
Sn
303.4
2.3
0.2
Cr
303.4121
303.4190
to.0069
1000
-0.24
Sn
300.9
4.1
0.7
Ca
300.9147
300.9205
tO.0058 10000
-0.24
sRelative sensitivities taken from the Perkin-Elmer literature for flame a.a.s. (standard conditions). bExcept where mentioned the recommended low slit is used. CConcentration of interferent; 5 ~1 added. dExcept where mentioned the furnace parameters given in Table 3 are used. eThe matrix metal solution is contaminated by the analyte element. ‘The general furnace parameters given in Table 2 are used. sThe high slit had to be used. hThere is a positive signal superimposed on the negative absorbance. ‘Neon emission line.
161
oL,-
3 2.5
0 Time (5)
Fig. 1. Absorbance profiles of 5-~1 portions of a 1% (w/v) cesium solution measured at the 459.4-nm em-opium line with Zeeman background correction and different slit widths. Slit widths: (1) 0.7 nm; (2) 0.2 nm; (3) 0.07 nm. For experimental conditions, see Table 3.
the monochromator/detector system. A u-line of a matrix element may then overlap with the gas-fill emission line, this again causing overcompensation. In the present study, this type of interference probably occurred in one analytelmatrix element system. Figure 1 shows the absorbance profiles for 5~1 portions of a 1% cesium solution measured at the 459.4~nm europium line. Profiles 1 and 2, recorded at slit widths of 0.7 and 0.2 nm, respectively, show overcompensation probably caused by a u-line of the 459.3177-nm cesium resonance line overlapping with the 459.3243~nm neon line. When the slit width was decreased to 0.07 nm, profile 3 was obtained; the two interfering lines were thus excluded from the bandpass and overcompensation was no longer observed. This type of spectral interference is not likely to cause any serious problems in practical work, but it remains a type of interference that is not normally taken into account. Most of the present eighteen cases of overcompensation do not appear at the main lines recommended for measurement of the analyte, but some are affected, such as the 459.4~nm europium line (interference from vanadium and cesium), the 247.6~nm palladium line (interference from lead) and the 265.9~nm platinum line (interference from europium). The other cases of spectral interference encountered here occur at the less sensitive wavelengths, but these lines can also be of importance in a.a.s. Selection of absorption lines of similar or different sensitivity makes it possible to avoid interferences and to provide greater flexibility in covering different concentration ranges. In electrothermal a.a.s., certain interferences can be avoided by taking advantage of the different volatilities of the analyte and the matrix element. In some cases, the difference in volatility even permits an element to be atomized before the other element starts to evaporate. It is also possible to utilize spectral interferences for analytical purposes, such as has been previously
162
described [l, 21 for the systems Hg/Co and Zn/Fe; this method may also be applicable to the element pairs Bi/Co, Pb/Co and Pd/Pb in the present investigation. The remaining matrix elements are those belonging to group (c), i.e., the elements that gave a positive absorbance in the preliminary measurements (see Table 1). The interpretation of these results is complicated by the fact that some of the metal solutions were contaminated, and extensive work would be needed to decide whether or not a positive absorbance for a matrix element is caused by a contamination of the analyte, a positive spectral interference or both. A background overcompensation effect may be overlooked because of a contamination of the analyte. Some of the matrix elements belonging to group (c) are known to give spectral interference in conventional a.a.s. (see Table 1). For the other elements of this group, nothing definite could be concluded from the present experiments. The present series of papers describes 22 cases of spectral interferences in inverse Zeeman-corrected a.a.s. and demonstrates that disturbances by absorbing atomic lines are more common than previously believed. Further work is required for the elucidation of other spectral interferences. Work in this field would be facilitated if highly resolved spectra of the emission sources, a data bank of current wavelengths and wavelength data, and appropriate software were available. REFERENCES 1 G. Wibetoe and F. J. Langmyhr, Anal. Chim. Acta, 165 (1984) 87. 2 G. Wibetoe and F. J. Langmyhr, Anal. Chim. Acta, 176 (1985) 33. 3 G. R. Harrison, M.I.T. Wavelength Tables, The Technology Press, Wiley, New York, 1939. 4 P. W. J. M. Boumans, Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectroscopy, Pergamon Press, Oxford, 1980. 5 J. D. Norris and T. S. West, Anal. Chem., 46 (1974) 1423. 6 See, e.g., B. Welz, Atomabsorptionsspektrometrie, 3rd edn., Verlag Chemie, Weinheim, 1983. 7 T. Kantor, P. Fodor and E. Pungor, Anal. Chim. Acta, 102 (1978) 15.