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Some unexpected interferences in flames
Spectrochlmsa
Acta. Vol. 448. No
12, pp. 1273-1284,
058GG47/89 $3 cm + Alo Pergamon Press plc
1989
Printed in Great Britan
Some unexpected interferences in flames* E. E. PICKEXT,M. ALRESHAIDAT, S. BROADWAY and S. R. KOIRTYOHANN? Departments of Biochemistry and Chemistry, University of Missouri-Columbia, Columbia, MO 65211, U.S.A. (Receioed 12 April 1989) Abstract-A series of interferences has been observed in emission and absorption for rare earth elements in the nitrous oxide-acetylene flame and for some first row transition elements with air and acetylene. Signal changes, mostly suppressions, are observed at low (ppm) concomitant levels with the degree of interference consistently showing an abrupt slope change at mole ratios of concomitant to analyte element of between l-l and 2-l. The effects are observed for high boiling, oxygen containing mineral acids and for many organic compounds which contain oxygen and which decompose instead of vaporizing when heated. Some non-volatile, nitrogen containing compounds also give suppressions but here the behavior is less consistent. The effects are very sensitive to flame richness and often can be overcome by proper fuel to oxidant ratios. Also, the effects are much more severe when only analyte and concomitant are present in contrast to solutions containing other salts, i.e. a sample matrix. Though we cannot yet offer a complete explanation for the suppressions, definite stoichiometry is indicated in reactions involving the solid phase after desolvation and prior to vaporization.
1. I~TR00ucT10N INTER-ELEMENT effects in flames were studied extensively until about two decades ago when most researchers in the field (including two of the present authors) felt that they were reasonably well understood, predictable, and readily controlled [l, 21. It first became apparent that this was not the case when the emission intensities from rare earth elements in the nitrous oxide-acetylene flame were found to vary significantly when different mineral acids were added in low (ppm) concentrations. Further investigation revealed that a few ppm of many different concomitants caused variations in emission and absorption responses for about half of the rare earth elements and several members of the first row transition elements. Relatively few reports of interferences of this type have appeared. Roos and PRICE [3] described the effect of citric acid in the concentration range of hundreds of ppm on iron absorption in the air acetylene flame, finding significant suppressions and indications of definite stoichiometry. They found that phosphoric acid addition overcame the interference. Our data suggest that interferences of this type are much more common than previously believed. This paper presents a partial catalog of the observations without making any serious attempt to explain them. The investigations are continuing and more detailed explanations should be forthcoming.
2. EXPERIMENTAL 2.1. Apparatus Three different instruments were used in these studies. Most emission observations were made on an instrument assembled in our laboratory from commercial and locally constructed components. It consisted of a Perkin-Elmer model 0040-146 premixed tlame burner with a 50 mm slot head. Gas flows were controlled by Brooks model 13558800 flow controllers which were plumbed to allow the oxidant to be readily changed between nitrous oxide and air. The flame was imaged on the 25 pm wide (0.04 nm band pass) by 2 mm high entrance slit of a Jarrell-Ash Model 82-000 monochromator. Signals from the RCA 4840 photomultiplier were processed through a locally made d.c. amplifier and fed to a 1OmV Linear Instruments strip chart recorder. For absorption measurements on this instrument, a hollow cathode lamp, powered by a Kepco regulated d.c. supply, was mounted on the optical axis, the light modulated bv a chopper and the signal processed by a locally made a.c. amplifier.
* Dedicated to Walter Slavin in recognition of his receipt of the 1988 Anachem Award and presented as an invited lecture at the Anachem Award Symposium during the FACSS Meeting, Boston, 1 November 1988. t Author to whom correspondence should be addressed. 1213
E. E. PICKETTet al.
1274
Most absorption measurements for the transition elements were made on a Perkin-Elmer model 2380 AA unit. In order to eliminate the remote possibility that the unexpected results were caused by the Perkin-Elmer burner system, part of the observations were verified on a Varian SpectrAAAA unit. 2.2. Reagents Stock solutions of the analyte elements were purchased as Aldrich Chemical Co. atomic absorption standards (for the rare earths) or prepared by dissolving reagent grade metals, oxides, or carbonates in a minimum amount of redistilled HCI and diluting to a concentration of 1000~gml-l. Working standards were prepared by dilution of the stock solutions using conventional volumetric techniques. Except in cases where comparisons of the effect of different acids were being made, all solutions to be measured contained 0.06 M HCI. Most stock solutions of concomitants were prepared by dissolving reagent grade chemicals in deionized water. They were added to the analyte solutions at appropriate concentrations and a blank was always prepared to avoid errors due to contamination from the concomitant. 2.3. Procedures
Emission intensities or absorbances were measured at the wavelength normally recommended for the analyte element in the presence of varying concentrations of the concomitant. For measurements on the locally assembled instrument, the flame zone sampled was 2 mm high starting 2 mm above the primary reaction zone. (Preliminary experiments in which sampling height was varied are not reported here.) The exact zone sampled is less readily determined on the commercial instruments: these were operated under standard conditions (except for fuel flow) for each analyte in atomic absorption. In most cases, metal concentrations were selected to give signals which were easily measured with good precision. The analyte concentration was varied for some experiments. Effects of concomitants on Yb ion and oxide emission were studied at the appropriate wavelengths. The species for which data are reported and the wavelengths used are given in Table 1. In one series of experiments, the oxidant flow was split and two similar nebulizers were fitted into a slightly modified spray chamber. Analyte and analyte plus concomitant were first sprayed through one nebulizer with distilled water being aspirated by the other. The analyte solution was then put into one nebulizer and concomitant into the other. Since the two nebulizers did not give identical performance, the experiment was repeated, reversing the solutions going into each nebulizer and the results were averaged. The observed effects are critically dependent on flame richness. Richness was varied by keeping the oxidant flow constant and changing the fuel flow. Nitrous oxide flow was 11.91m-’ and acetylene Table 1. Chemical Species measured
Cr Mn Fe co Ni CU Zn Y MO Nd Sm
Species measured
Wavelength, Absorption 3578.7 2794.8 2483.3 2407.2 2320.0 3247.5 2138.6 3623 3132.6
Eu DY Ho Er Tm Yb Lu Yb+ YbO
and wavelengths
used
A Emission
3453.5 3414.8
3903.0 4924.5 4760.3 4594.0 4046.0 4053.9 4008.0
3988.0
3717.9 3988.0 4518.6 3994.2 4975.0
Some unexpected interferences in flames
1275
varied from 4.0 to 5.9 1m- ‘. The term “lean flame”, is used for an acetylene flow of 4.5 1m- r, giving a flame with a 2mm high red zone. These conditions gave the highest signal/background for Yb emission. The rich nitrous oxide-acetylene flame used 5.91m- ’ of acetylene, had a red zone about 15 mm high and was bordering on incandescence. Air flow was 11.0 1m- ’ and fuel ranged from 1.85 to 2.18 1m- r. The lean flame was operated at a fuel flow just below the point where a light blue interconal zone became visible in the flame. The rich flame bordered on incandescence.
3. RESULTS One of the first puzzling observations is shown in Fig. 1. Yb (2 ppm) in 0.06 M HCl gave a well defined, stable emission signal in the lean nitrous oxide flame. The same amount of Yb in 0.18 M (1% v/v) H,SO, gave about a 15% enhancement relative to the HCl solution. However, if the HCl solution was measured again with minimum time delay, there was an initial suppression which gradually disappeared over about a 2 min period. Data to be used later to help explain these results are given in Fig. 2 which shows the behavior of the emission signal from several rare earth elements in the presence of low concentrations of H,SO,. Nd and Eu show no significant effect at these or higher concentrations. Sm and Yb show an initial suppression followed by increasing signals at higher H,SO, concentrations, eventually resulting in a net enhancement relative to the HCl solution (not shown in the figure due to limited concentration range). For Lu, the emission quickly approaches zero as the H,SO, is increased and never recovers. All of the elements that show any effect have a minimum or a knee or break in the response curve at approximately equimolar amounts of analyte and concomitant. Figure 3 shows the effect of H,S04 on both emission and atomic absorption signals for Yb. There are no significant differences. Figure 4 compares Yb atomic, ionic and monoxide emission. While the plots are not superimposable, all show nearly the same behavior with a break at the same H,SO, concentration. The effect of H,SO, on Yb is much less pronounced in the rich N,O flame as shown in Fig. 5. The intensity of the atomic line for Yb in the absence of concomitants is about 20% less in the rich than in the lean flame; for Y Gd, Tb, Dy, Ho, Er, and Lu, it is 8-60 times greater.
C
C
-
_
/
L
Time Fig.
Effect of acids on Yb emission. (A) Yb in 0.06 M HCI; (B) Yb in 0.18 M H,SO, (1% v/v) and (C) Yb in 0.06 M HCI.
1276
E. E. PICKEI-I et al.
100
30
60 : 5 P P e s
40
d
20
0 0
100
200
ppm Fig. 2. The effect of sulfuric acid concentration elements in a lean nitrous oxide-acetylene
300
sulfuric
400
500
600
acid
on emission intensities from several rare earth flame. A Nd, Eu; A Sm; 0 Yb and n Lu.
The effect of H,SO, on Yb was completely removed in the lean flame when the analyte and concomitant were aspirated through separate nebulizers. Figure 6 shows the effect of oxalic acid on Ni atomic absorption in the rich air-acetylene flame on the Perkin-Elmer instrument. Again, a break in the response curve is seen at low concomitant concentrations where the molar ratios are about unity. The experiment was repeated on the Varian instrument described above with nearly identical results. The effect of fuel flow on the behavior of Ni in the presence of tartaric acid is shown in Fig. 7. It is
1277
Some unexpected interferences in flames
80
80
40
20
0 0.0
0.2
0.4
Percent
0.8
0.8
sulfuric
acid
1.0
1.2
Fig. 3. Relative Yb emission and absorption response as a function of sulfuric acid concentration in the lean nitrous oxide-acetylene flame. A atomic absorption and A emission.
apparent that the response change is a function of fuel flow as well as concomitant concentration up to about 25 ppm. There was little further change with tartaric acid concentration up to 100 ppm, the highest level used in these tests. Only small effects are seen for the lean flame. This is consistent with the fact that a lean flame is normally recommended for determination of Ni [23. This is not always the case, however, because glutamic acid caused a suppression in Fe absorption in both rich and lean flames (Fig. 8). Roes and PRICE C33 reported suppressions on iron absorption due to citric acid in both lean and rich flames.
1278
E. E.
PICKET-Tet al.
60
0
200
400
ppm
600
sulturlc
600
1000
1200
acid
Fig. 4. Relative emission intensity of Yb, Yb+ and YbO, as a function of sulfuric acid concentration in the lean nitrous oxide-acetylene flame. A Yb atomic emission; A Yb ionic emission and Cl YbO emission.
A complete description of all such effects we have observed is beyond the scope of this paper. Table 2 summarizes observations with respect to analytes, and Table 3 lists the concomitants for which effects have been noted to date. In general, the effects are much less pronounced in real sample matrices than in dilute solutions of only the analyte and concomitant. This is illustrated in Table 4 by the effect of glutamic acid on spike recoveries for several elements in the presence and absence of alfalfa ash. The ash from l.Og of air dried plant tissue was dissolved in 1OOml of solution giving about 3 g l- ’ of dissolved salts, chiefly chlorides of K, Ca, and Mg. In no case is the recovery
Some unexpected interferences in flames
1279
80
80
40
20
0 0
200
400
800
ppm
sulfuric
800
1000
1200
acid
Fig. 5. The effect of flame richness on the degree of H,SO, interference for Yb atomic emission. A rich flame (15mm red zone) and A lean flame (2 mm red zone). from alfalfa ash plus glutamic acid significantly different from that for alfalfa ash alone, even though the values deviate significantly from 100% for the ash solution in several cases. That is, alfalfa ash produces matrix effects on these elements but also overcomes the effect of the added glutamic acid. 4. DISCUSSION We are far from being able to offer detailed explanations for all of the above observations. The data do support a few general conclusions and some possible explanations can be ruled out.
E. E. PICKER et al.
1280
80
-
20 l-
aI--
i 0
20
I
ppm Fig. 6. The effect of oxalic
I
I
80
60
40
oxallc
acid on Ni atomic absorption (1OppmNi).
100
acid in the fuel rich air-acetylene
flame
The delay in getting the proper reading on a HCl solution following HISO, aspiration shown in Fig. 1 is undoubtedly due to a slight carryover in the spray chamber. Note that the H$O, concentration in the previous sohrtion was several orders of magnitude higher than that needed for maximum suppression (Fig. 2). The carryover causes an effect similar to a low concentration of H,S04. As aspiration of HCl solution is continued and the H,SO, from the previous sample is rinsed from the chamber, the signal returns to normal. Solution nebulization and transport are ruled out as probable causes because numerous elements are well behaved. Many other examples have been examined, but the behavior of
1281
Some unexpected interferences in flame.9
100
80
20
0 1.8
1.9
2.0
Flow
2.1
2.2
Rate
Fig. 7. Suppression of Ni atomic absorption by tartaric acid as a function of fuel flow. (10 ppm Ni) (absorption for 0 tartaric acid normalized at each fuel flow). A 0 ppm tartaric acid; A 5 ppm tartaric acid, 0 10 ppm tartaric acid W 25 ppm tartic acid 50 and 100 ppm tartaric acid gave a curve nearly identical with 25 ppm.
Nd and Eu in Fig. 2 establishes the point. Mechanisms involving variations in excitation efficiency are not viable because of the similarity in emission and absorption behavior. Major shifts in the atom-ion-monoxide equilibria do not appear to occur for the few cases tested, though a general conclusion to that effect would be premature. Flame stoichiometry obviously has a profound effect on behavior but not always in simple ways. Interferences are consistently more severe for transition elements in the rich than the lean air-acetylene flame. However, Yb and Ho emission is suppressed to a greater extent by tartaric acid, oxalic acid,
E. E. PICKET et al.
1282
120
80
f 3 4 d
40
20
ppm Fig. 8. The effect of glutamic
glutamlc
acid
acid or iron absorption in the air-acetylene flame and A lean flame.
flame (4 ppm Fe) A rich
and lysine in the lean than the rich N,O flame, while richness has the opposite effect when citric acid, glucose, aspartic acid, or histidine are the concomitants. The differences in behavior with fuel flow probably are not due entirely to temperature changes because interferences are observed in the nitrous oxide flame for transition elements (Table 4). The fact that the effect disappeared when analyte and concomitant were nebulized separately argues against gas phase reactions playing an important role, at least for the case tested. Additional dual nebulizer experiments are planned. It has long been the practice to add La or other salts at high levels to overcome various interferences, especially that of phosphate on calcium. Preliminary work suggests that
1283
Some unexpected interferences in flames Table 2. Analyte elements for which effects due to low concentrations (10 to 50 ppm) of concomitants have been observed MO (severe)* Sm DY Ho Er Tm Yb Lll
Cr (small) Mn Fe co Ni (large) Cu (small) Zn (none)
*not yet studied in detail.
Table 3. Concomitants for which effects on emission or absorption have been noted for one or more analyte elements at low (ppm) concomitant concentrations glucose lactose mannose maltose galactose fructose HW, HP%
glutamic acid oxalic acid citric acid phthalic acid tartaric acid malic acid aspartic acid succinic acid EDTA serine lysine histidine alanine arginine
Table 4. The etfect of 50 ppm glutamic acid on spike recoveries for elements added to an alfalfa ash solution
Analyte and concomitants
% spike recovery air-acetylene
N,O-acetylene
Co + glutamic acid Co + alf. ash Co + alf. ash + glutamic acid
lean 97 91 97
rich 55 108 108
lean 96 99 99
rich 82 108 109
Ni + glutamic acid Ni + alf. ash Ni + alf. ash + glutamic acid
100 100 100
59 110 110
99 104 103
82 108 105
Ho + glutamic acid Ho + alf. ash Ho + alf. ash + glutamic acid
68 25 25
61 122 124
Er + glutamic acid Er + alf. ash Er + alf. ash + glutamic acid
61 24 22
64 112 113
selection and use of releasing agents for the rare earths may be more complex but feasible. Thus 200ppm La overcomes the 33% suppression of Yb and 30% suppression of Ho by 20ppm citric acid in the rich N,O-C,H2 flame but not entirely in the lean flame. Interferences generally are observed from oxygen-containing compounds which are not readily vaporized but which decompose when heated. No exceptions to this have been seen.
1284
E. E. P1CKEl-T et al.
Acetic and nitric acids show little effect, for example, while sulfuric and oxalic acids interfere. Many nonvolatile nitrogen-containing compounds show effects but here the behavior is less consistent. Reasonably definite stoichiometry is indicated by break points on the curve at molar ratios of concomitant to analyte between l-1 and Z-l. The exact ratios depend on flame conditions as well as analyte concentration. The variations should be studied for clues to the chemistry involved. The data suggest formation of reasonably definite compounds between the anlyte and concomitants or their decomposition products after the solvent has vaporized and before the analyte does so. New or different compounds of a currently unknown nature are formed which affect the analyte vaporization or atomization steps. Matrix components present in large amounts serve to dilute solid phase concentrations of analyte and concomitant, preventing formation of the specific compounds and overcoming the observed effects. A similar result often can be observed when the concomitant is present in large excess (1OOOppm).This may help to explain why the effects reported here apparently have been overlooked for so many years. Interferences generally are tested at high concomitant levels on the assumption that signal changes will be enhanced and therefore be more readily apparent. This obviously is not always the case. Also, the fact that Roos and PRICE [3] were able to overcome citric acid suppression of iron absorption by addition of excess H,PO, may be due to dilution of concomitant and anlyte rather than specific chemical reactions as they suggest. Given the variety of effects observed, it is unlikely that a single type of chemical reaction can explain all of them. The investigations are continuing. 5. REFERENCES [l] W. Slavin, Atomic Absorption Spectroscopy. Interscience, New York (1968). [2] B. Welz, Atomic Absorption Spectrometry, 2nd ed. Verlag Chemie, Weinheim FRG (1985) [3] J. T. H. Roos and W. J. Rice, Spectrochim. Acta 26B, 279 (1971).
Acta. Vol. 448. No
12, pp. 1273-1284,
058GG47/89 $3 cm + Alo Pergamon Press plc
1989
Printed in Great Britan
Some unexpected interferences in flames* E. E. PICKEXT,M. ALRESHAIDAT, S. BROADWAY and S. R. KOIRTYOHANN? Departments of Biochemistry and Chemistry, University of Missouri-Columbia, Columbia, MO 65211, U.S.A. (Receioed 12 April 1989) Abstract-A series of interferences has been observed in emission and absorption for rare earth elements in the nitrous oxide-acetylene flame and for some first row transition elements with air and acetylene. Signal changes, mostly suppressions, are observed at low (ppm) concomitant levels with the degree of interference consistently showing an abrupt slope change at mole ratios of concomitant to analyte element of between l-l and 2-l. The effects are observed for high boiling, oxygen containing mineral acids and for many organic compounds which contain oxygen and which decompose instead of vaporizing when heated. Some non-volatile, nitrogen containing compounds also give suppressions but here the behavior is less consistent. The effects are very sensitive to flame richness and often can be overcome by proper fuel to oxidant ratios. Also, the effects are much more severe when only analyte and concomitant are present in contrast to solutions containing other salts, i.e. a sample matrix. Though we cannot yet offer a complete explanation for the suppressions, definite stoichiometry is indicated in reactions involving the solid phase after desolvation and prior to vaporization.
1. I~TR00ucT10N INTER-ELEMENT effects in flames were studied extensively until about two decades ago when most researchers in the field (including two of the present authors) felt that they were reasonably well understood, predictable, and readily controlled [l, 21. It first became apparent that this was not the case when the emission intensities from rare earth elements in the nitrous oxide-acetylene flame were found to vary significantly when different mineral acids were added in low (ppm) concentrations. Further investigation revealed that a few ppm of many different concomitants caused variations in emission and absorption responses for about half of the rare earth elements and several members of the first row transition elements. Relatively few reports of interferences of this type have appeared. Roos and PRICE [3] described the effect of citric acid in the concentration range of hundreds of ppm on iron absorption in the air acetylene flame, finding significant suppressions and indications of definite stoichiometry. They found that phosphoric acid addition overcame the interference. Our data suggest that interferences of this type are much more common than previously believed. This paper presents a partial catalog of the observations without making any serious attempt to explain them. The investigations are continuing and more detailed explanations should be forthcoming.
2. EXPERIMENTAL 2.1. Apparatus Three different instruments were used in these studies. Most emission observations were made on an instrument assembled in our laboratory from commercial and locally constructed components. It consisted of a Perkin-Elmer model 0040-146 premixed tlame burner with a 50 mm slot head. Gas flows were controlled by Brooks model 13558800 flow controllers which were plumbed to allow the oxidant to be readily changed between nitrous oxide and air. The flame was imaged on the 25 pm wide (0.04 nm band pass) by 2 mm high entrance slit of a Jarrell-Ash Model 82-000 monochromator. Signals from the RCA 4840 photomultiplier were processed through a locally made d.c. amplifier and fed to a 1OmV Linear Instruments strip chart recorder. For absorption measurements on this instrument, a hollow cathode lamp, powered by a Kepco regulated d.c. supply, was mounted on the optical axis, the light modulated bv a chopper and the signal processed by a locally made a.c. amplifier.
* Dedicated to Walter Slavin in recognition of his receipt of the 1988 Anachem Award and presented as an invited lecture at the Anachem Award Symposium during the FACSS Meeting, Boston, 1 November 1988. t Author to whom correspondence should be addressed. 1213
E. E. PICKETTet al.
1274
Most absorption measurements for the transition elements were made on a Perkin-Elmer model 2380 AA unit. In order to eliminate the remote possibility that the unexpected results were caused by the Perkin-Elmer burner system, part of the observations were verified on a Varian SpectrAAAA unit. 2.2. Reagents Stock solutions of the analyte elements were purchased as Aldrich Chemical Co. atomic absorption standards (for the rare earths) or prepared by dissolving reagent grade metals, oxides, or carbonates in a minimum amount of redistilled HCI and diluting to a concentration of 1000~gml-l. Working standards were prepared by dilution of the stock solutions using conventional volumetric techniques. Except in cases where comparisons of the effect of different acids were being made, all solutions to be measured contained 0.06 M HCI. Most stock solutions of concomitants were prepared by dissolving reagent grade chemicals in deionized water. They were added to the analyte solutions at appropriate concentrations and a blank was always prepared to avoid errors due to contamination from the concomitant. 2.3. Procedures
Emission intensities or absorbances were measured at the wavelength normally recommended for the analyte element in the presence of varying concentrations of the concomitant. For measurements on the locally assembled instrument, the flame zone sampled was 2 mm high starting 2 mm above the primary reaction zone. (Preliminary experiments in which sampling height was varied are not reported here.) The exact zone sampled is less readily determined on the commercial instruments: these were operated under standard conditions (except for fuel flow) for each analyte in atomic absorption. In most cases, metal concentrations were selected to give signals which were easily measured with good precision. The analyte concentration was varied for some experiments. Effects of concomitants on Yb ion and oxide emission were studied at the appropriate wavelengths. The species for which data are reported and the wavelengths used are given in Table 1. In one series of experiments, the oxidant flow was split and two similar nebulizers were fitted into a slightly modified spray chamber. Analyte and analyte plus concomitant were first sprayed through one nebulizer with distilled water being aspirated by the other. The analyte solution was then put into one nebulizer and concomitant into the other. Since the two nebulizers did not give identical performance, the experiment was repeated, reversing the solutions going into each nebulizer and the results were averaged. The observed effects are critically dependent on flame richness. Richness was varied by keeping the oxidant flow constant and changing the fuel flow. Nitrous oxide flow was 11.91m-’ and acetylene Table 1. Chemical Species measured
Cr Mn Fe co Ni CU Zn Y MO Nd Sm
Species measured
Wavelength, Absorption 3578.7 2794.8 2483.3 2407.2 2320.0 3247.5 2138.6 3623 3132.6
Eu DY Ho Er Tm Yb Lu Yb+ YbO
and wavelengths
used
A Emission
3453.5 3414.8
3903.0 4924.5 4760.3 4594.0 4046.0 4053.9 4008.0
3988.0
3717.9 3988.0 4518.6 3994.2 4975.0
Some unexpected interferences in flames
1275
varied from 4.0 to 5.9 1m- ‘. The term “lean flame”, is used for an acetylene flow of 4.5 1m- r, giving a flame with a 2mm high red zone. These conditions gave the highest signal/background for Yb emission. The rich nitrous oxide-acetylene flame used 5.91m- ’ of acetylene, had a red zone about 15 mm high and was bordering on incandescence. Air flow was 11.0 1m- ’ and fuel ranged from 1.85 to 2.18 1m- r. The lean flame was operated at a fuel flow just below the point where a light blue interconal zone became visible in the flame. The rich flame bordered on incandescence.
3. RESULTS One of the first puzzling observations is shown in Fig. 1. Yb (2 ppm) in 0.06 M HCl gave a well defined, stable emission signal in the lean nitrous oxide flame. The same amount of Yb in 0.18 M (1% v/v) H,SO, gave about a 15% enhancement relative to the HCl solution. However, if the HCl solution was measured again with minimum time delay, there was an initial suppression which gradually disappeared over about a 2 min period. Data to be used later to help explain these results are given in Fig. 2 which shows the behavior of the emission signal from several rare earth elements in the presence of low concentrations of H,SO,. Nd and Eu show no significant effect at these or higher concentrations. Sm and Yb show an initial suppression followed by increasing signals at higher H,SO, concentrations, eventually resulting in a net enhancement relative to the HCl solution (not shown in the figure due to limited concentration range). For Lu, the emission quickly approaches zero as the H,SO, is increased and never recovers. All of the elements that show any effect have a minimum or a knee or break in the response curve at approximately equimolar amounts of analyte and concomitant. Figure 3 shows the effect of H,S04 on both emission and atomic absorption signals for Yb. There are no significant differences. Figure 4 compares Yb atomic, ionic and monoxide emission. While the plots are not superimposable, all show nearly the same behavior with a break at the same H,SO, concentration. The effect of H,SO, on Yb is much less pronounced in the rich N,O flame as shown in Fig. 5. The intensity of the atomic line for Yb in the absence of concomitants is about 20% less in the rich than in the lean flame; for Y Gd, Tb, Dy, Ho, Er, and Lu, it is 8-60 times greater.
C
C
-
_
/
L
Time Fig.
Effect of acids on Yb emission. (A) Yb in 0.06 M HCI; (B) Yb in 0.18 M H,SO, (1% v/v) and (C) Yb in 0.06 M HCI.
1276
E. E. PICKEI-I et al.
100
30
60 : 5 P P e s
40
d
20
0 0
100
200
ppm Fig. 2. The effect of sulfuric acid concentration elements in a lean nitrous oxide-acetylene
300
sulfuric
400
500
600
acid
on emission intensities from several rare earth flame. A Nd, Eu; A Sm; 0 Yb and n Lu.
The effect of H,SO, on Yb was completely removed in the lean flame when the analyte and concomitant were aspirated through separate nebulizers. Figure 6 shows the effect of oxalic acid on Ni atomic absorption in the rich air-acetylene flame on the Perkin-Elmer instrument. Again, a break in the response curve is seen at low concomitant concentrations where the molar ratios are about unity. The experiment was repeated on the Varian instrument described above with nearly identical results. The effect of fuel flow on the behavior of Ni in the presence of tartaric acid is shown in Fig. 7. It is
1277
Some unexpected interferences in flames
80
80
40
20
0 0.0
0.2
0.4
Percent
0.8
0.8
sulfuric
acid
1.0
1.2
Fig. 3. Relative Yb emission and absorption response as a function of sulfuric acid concentration in the lean nitrous oxide-acetylene flame. A atomic absorption and A emission.
apparent that the response change is a function of fuel flow as well as concomitant concentration up to about 25 ppm. There was little further change with tartaric acid concentration up to 100 ppm, the highest level used in these tests. Only small effects are seen for the lean flame. This is consistent with the fact that a lean flame is normally recommended for determination of Ni [23. This is not always the case, however, because glutamic acid caused a suppression in Fe absorption in both rich and lean flames (Fig. 8). Roes and PRICE C33 reported suppressions on iron absorption due to citric acid in both lean and rich flames.
1278
E. E.
PICKET-Tet al.
60
0
200
400
ppm
600
sulturlc
600
1000
1200
acid
Fig. 4. Relative emission intensity of Yb, Yb+ and YbO, as a function of sulfuric acid concentration in the lean nitrous oxide-acetylene flame. A Yb atomic emission; A Yb ionic emission and Cl YbO emission.
A complete description of all such effects we have observed is beyond the scope of this paper. Table 2 summarizes observations with respect to analytes, and Table 3 lists the concomitants for which effects have been noted to date. In general, the effects are much less pronounced in real sample matrices than in dilute solutions of only the analyte and concomitant. This is illustrated in Table 4 by the effect of glutamic acid on spike recoveries for several elements in the presence and absence of alfalfa ash. The ash from l.Og of air dried plant tissue was dissolved in 1OOml of solution giving about 3 g l- ’ of dissolved salts, chiefly chlorides of K, Ca, and Mg. In no case is the recovery
Some unexpected interferences in flames
1279
80
80
40
20
0 0
200
400
800
ppm
sulfuric
800
1000
1200
acid
Fig. 5. The effect of flame richness on the degree of H,SO, interference for Yb atomic emission. A rich flame (15mm red zone) and A lean flame (2 mm red zone). from alfalfa ash plus glutamic acid significantly different from that for alfalfa ash alone, even though the values deviate significantly from 100% for the ash solution in several cases. That is, alfalfa ash produces matrix effects on these elements but also overcomes the effect of the added glutamic acid. 4. DISCUSSION We are far from being able to offer detailed explanations for all of the above observations. The data do support a few general conclusions and some possible explanations can be ruled out.
E. E. PICKER et al.
1280
80
-
20 l-
aI--
i 0
20
I
ppm Fig. 6. The effect of oxalic
I
I
80
60
40
oxallc
acid on Ni atomic absorption (1OppmNi).
100
acid in the fuel rich air-acetylene
flame
The delay in getting the proper reading on a HCl solution following HISO, aspiration shown in Fig. 1 is undoubtedly due to a slight carryover in the spray chamber. Note that the H$O, concentration in the previous sohrtion was several orders of magnitude higher than that needed for maximum suppression (Fig. 2). The carryover causes an effect similar to a low concentration of H,S04. As aspiration of HCl solution is continued and the H,SO, from the previous sample is rinsed from the chamber, the signal returns to normal. Solution nebulization and transport are ruled out as probable causes because numerous elements are well behaved. Many other examples have been examined, but the behavior of
1281
Some unexpected interferences in flame.9
100
80
20
0 1.8
1.9
2.0
Flow
2.1
2.2
Rate
Fig. 7. Suppression of Ni atomic absorption by tartaric acid as a function of fuel flow. (10 ppm Ni) (absorption for 0 tartaric acid normalized at each fuel flow). A 0 ppm tartaric acid; A 5 ppm tartaric acid, 0 10 ppm tartaric acid W 25 ppm tartic acid 50 and 100 ppm tartaric acid gave a curve nearly identical with 25 ppm.
Nd and Eu in Fig. 2 establishes the point. Mechanisms involving variations in excitation efficiency are not viable because of the similarity in emission and absorption behavior. Major shifts in the atom-ion-monoxide equilibria do not appear to occur for the few cases tested, though a general conclusion to that effect would be premature. Flame stoichiometry obviously has a profound effect on behavior but not always in simple ways. Interferences are consistently more severe for transition elements in the rich than the lean air-acetylene flame. However, Yb and Ho emission is suppressed to a greater extent by tartaric acid, oxalic acid,
E. E. PICKET et al.
1282
120
80
f 3 4 d
40
20
ppm Fig. 8. The effect of glutamic
glutamlc
acid
acid or iron absorption in the air-acetylene flame and A lean flame.
flame (4 ppm Fe) A rich
and lysine in the lean than the rich N,O flame, while richness has the opposite effect when citric acid, glucose, aspartic acid, or histidine are the concomitants. The differences in behavior with fuel flow probably are not due entirely to temperature changes because interferences are observed in the nitrous oxide flame for transition elements (Table 4). The fact that the effect disappeared when analyte and concomitant were nebulized separately argues against gas phase reactions playing an important role, at least for the case tested. Additional dual nebulizer experiments are planned. It has long been the practice to add La or other salts at high levels to overcome various interferences, especially that of phosphate on calcium. Preliminary work suggests that
1283
Some unexpected interferences in flames Table 2. Analyte elements for which effects due to low concentrations (10 to 50 ppm) of concomitants have been observed MO (severe)* Sm DY Ho Er Tm Yb Lll
Cr (small) Mn Fe co Ni (large) Cu (small) Zn (none)
*not yet studied in detail.
Table 3. Concomitants for which effects on emission or absorption have been noted for one or more analyte elements at low (ppm) concomitant concentrations glucose lactose mannose maltose galactose fructose HW, HP%
glutamic acid oxalic acid citric acid phthalic acid tartaric acid malic acid aspartic acid succinic acid EDTA serine lysine histidine alanine arginine
Table 4. The etfect of 50 ppm glutamic acid on spike recoveries for elements added to an alfalfa ash solution
Analyte and concomitants
% spike recovery air-acetylene
N,O-acetylene
Co + glutamic acid Co + alf. ash Co + alf. ash + glutamic acid
lean 97 91 97
rich 55 108 108
lean 96 99 99
rich 82 108 109
Ni + glutamic acid Ni + alf. ash Ni + alf. ash + glutamic acid
100 100 100
59 110 110
99 104 103
82 108 105
Ho + glutamic acid Ho + alf. ash Ho + alf. ash + glutamic acid
68 25 25
61 122 124
Er + glutamic acid Er + alf. ash Er + alf. ash + glutamic acid
61 24 22
64 112 113
selection and use of releasing agents for the rare earths may be more complex but feasible. Thus 200ppm La overcomes the 33% suppression of Yb and 30% suppression of Ho by 20ppm citric acid in the rich N,O-C,H2 flame but not entirely in the lean flame. Interferences generally are observed from oxygen-containing compounds which are not readily vaporized but which decompose when heated. No exceptions to this have been seen.
1284
E. E. P1CKEl-T et al.
Acetic and nitric acids show little effect, for example, while sulfuric and oxalic acids interfere. Many nonvolatile nitrogen-containing compounds show effects but here the behavior is less consistent. Reasonably definite stoichiometry is indicated by break points on the curve at molar ratios of concomitant to analyte between l-1 and Z-l. The exact ratios depend on flame conditions as well as analyte concentration. The variations should be studied for clues to the chemistry involved. The data suggest formation of reasonably definite compounds between the anlyte and concomitants or their decomposition products after the solvent has vaporized and before the analyte does so. New or different compounds of a currently unknown nature are formed which affect the analyte vaporization or atomization steps. Matrix components present in large amounts serve to dilute solid phase concentrations of analyte and concomitant, preventing formation of the specific compounds and overcoming the observed effects. A similar result often can be observed when the concomitant is present in large excess (1OOOppm).This may help to explain why the effects reported here apparently have been overlooked for so many years. Interferences generally are tested at high concomitant levels on the assumption that signal changes will be enhanced and therefore be more readily apparent. This obviously is not always the case. Also, the fact that Roos and PRICE [3] were able to overcome citric acid suppression of iron absorption by addition of excess H,PO, may be due to dilution of concomitant and anlyte rather than specific chemical reactions as they suggest. Given the variety of effects observed, it is unlikely that a single type of chemical reaction can explain all of them. The investigations are continuing. 5. REFERENCES [l] W. Slavin, Atomic Absorption Spectroscopy. Interscience, New York (1968). [2] B. Welz, Atomic Absorption Spectrometry, 2nd ed. Verlag Chemie, Weinheim FRG (1985) [3] J. T. H. Roos and W. J. Rice, Spectrochim. Acta 26B, 279 (1971).