Interference Effects in Furnace Atomic Absorption Spectrometry Edward J. Czobik and Jaroslav P. Matousek" Department of Analytical Chemistry, University of New South Wales, 2033 New South Wales, Australia
Interferences caused by various metal chlorides have been studied in a Varian Techtron CRA 63 furnace using both slow conventional and fast oscilloscopic detection. Signal averaging has also been used to improve the signal-lo-noise ratio of fast measurements. Comparison of analyte atomization temperatures with time-resolved interferent population confirms that atom population is depleted by chloride formation in the vapor phase. Time-resolved measurements for the Pb-NaCI system show a low temperature shift of the lead atomic absorption peak and also reveal potential problems for background correction in this system. Treatment with acids has been employed to eliminate both chemical and spectral interferences in the systems studied.
In contrast to widespread application of furnace atomization t o practical analytical problems, relatively few fundamental studies have appeared dealing with the mechanisms of chemical interferences affecting this technique. Based on measurements with the King graphite furnace ( I ) , interferences were classified as: interferences due to physical effects, formation of molecular compounds in the gas phase resulting in molecular absorption, formation of refractory compounds (usually carbides), and interaction between elements and gaseous compounds. A t the same time, formation of relatively volatile compounds of the analyte element was suggested as the cause of chemical interferences in furnace atomizers ( 2 ) , a mechanism entirely different from t h a t experienced in flames. Following a study correlating volatilities of some interfering chlorides with the extent of their interference effect. it was later proposed t h a t the vapor phase process is responsible (3). A different type of interaction, namely occlusion of the analyte in the interfering matrix, was also suggested to contribute t o furnace interferences ( 4 , 5 ) . High-temperature equilibrium calculations were performed for a multicomponent system in order to study the interference from chlorine in furnace systems (6). Based on these calculations, hydrogen generated in high-temperature reactions was suggested t o be effective in eliminating the chlorine interference (7). There are some aspects of several interference studies which restrict their general applicability and suitability as a basis for conclusions about the nature of interference effects: (1)Contrary to the established approach of investigating flame interferences as primarily the effect of an isolated cation or anion, the effect of the interferent as a compound has to be considered in furnace work (2). Frequently, this fact is not recognized (8, 9). (2) Most of the work on interferences in furnace atomization is devoted t o the determination of a particular element in a matrix and results are usually reported as an enhancement or depression when a single quantity of interferent is added to the analyte. T h e simplified approach using only a single or a few concentration points may lead to erroneous conclusions about the extent of interferences and it is therefore necessary to study a widest possible concentration range ( 5 ) . 0003-2700/78/0350-0002$01 OO/O
(3) The effect of the temporal response of the detection system on the transient signals in furnace systems has been widely documented (2, 10-13). The use of the standard atomic absorption instrumentation in the majority of interierence studies results necessarily in some distortion of the observed transient signals. The extent of the distortion depends both on the time constant of the measurement system and on the residence time of the atomic vapor in the furnace and hence on the type of furnace used. The residence time of the atomic vapor may in turn be affected by the added interferent. Consequently, the results obtained with slow responding detection systems will be influenced by several instrumental parameters and as such cannot be considered to reflect the true nature of processes taking place,in the furnace. A t the same time, these results may be perfectly valid for the particular application. In this paper we report the results of a detailed study on the interference effect of metal chlorides in furnace atomic absorption. Both a slow conventional and a fast response detection system are used over-a wide concentration range of the interferents. The interference mechanism is discussed and some chemical methods for the elimination of the interference are demonstrated.
EXPERIMENTAL Instrumentation. A Yarian Techtron model 63 CRA furnace mounted in a Varian Techtron AA-5 atomic absorption spectrometer and a Mace FBQ-100 chart recorder were used. The tubular furnaces coated with pyrolytic graphite were modified by removing the coating from the contact area on the furnace exterior in order t o ensure a reproducible electrical contact. For measurements with a fast response detection system. the signal from the photomultiplier was led directly to the input of a 549 Tektronix storage oscilloscope. Later in the study, a Hewlett-Packard model 5480B multichannel signal averager was used, This arrangement provided either a single shot operation or signal averaging of successive atomizations with a time constant of 1 ms and storage in four quarters of a 1024 channel memory. Both the oscilloscope and the signal averager were triggered by the furnace power supply when it commenced the atomization stage. Furnace temperatures of up to 1100 "C were measured using a chromel-alumel thermocouple with a fine hot junction while an Ircon series 6000 radiation thermometer (Ircon Inc., Skokie, Ill. 60076) was employed between 1000 and 3000 "C. Operation. Solutions were introduced intci the furnace with a microliter pipet incorporating a Chaney adaptor and disposable Teflon tips (Diagnostics Division, Pfizer Inc., New York?N.Y.I. They were dried for 20 s at a selected voltage so that the furnace temperature at the end of drying reached 110 " C and then atomized for 3 s. The atomization voltage was selected so as to achieve complete atomization of the analyte element and complete removal of the interferent. No ashing was applied, unless otherwise stated. Background correction was applied by subtracting the peak absorption signal measured for a pure interferent from the peak absorption signal for the solution containing both the analyte element and the interferent using the same hollow-cathode line source, This procedure avoided errors due to imperfect matching of beams from line and continuum sources (12). Ultrapure reagents were used and a final check with a hydrogen continuum 1977 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, J A W A R Y 1978
10-
loo
10 *
10'
C U , ng
3
io3
a s CuCI,
Figure 1. Effect of CuClp on peak absorbance by 0.5 ng lead measured at Pb 217.0 nrn line. ( 0 )Pb atomic absorption (0)background absorption due to CuCI,
lamp was made to ensure that only a molecular absorption signal was measured for the pure interferent. RESULTS AND DISCUSSION Metal chloride interferences may be conveniently classified as effects due to the presence of: (a) transition metal chlorides which decompose readily a t elevated temperatures; (b) alkali metal chlorides which are relatively stable. These two groups show distinctly different interference patterns. I n t e r f e r e n c e i n T r a n s i t i o n M e t a l Chloride Systems. Selection o f Detection S y s t e m . Although fast oscilloscopic measurements proved useful to establish the relative time (and temperature) position of individual atomic and molecular absorption peaks, the high noise level associated with these measurements makes them less suitable for quantitative evaluation of interference effects. For this reason, the slow conventional amplifier-recorder system (time constant of 0.3 s) was used. The selection of the slow system may seem to be in contradiction to the comments made in the introduction about the effect of the temporal response of the detection system on the transient signals. In this instance, however, the slow detection system is adequate since the risetime of the atomic absorption peak is virtually unaffected by the presence of the interferent, as confirmed by the oscilloscopic measurements. The slow system also provides an adequate time resolution of the atomic and molecular absorption peaks. Interference Measurements. Figures 1 and 2 show the effect of CuClz on the lead and nickel peak atomic absorption signals. They are typical of the chemical interference caused by transition metal chlorides. The depression of the atomic
absorption signal is usually observed when 0.01 to 0.1 pg of the interferent is added. Further increase in the amount of the interferent up to approximately 1 pg results in a complete elimination of the atomic population. The depression corresponds with the appearance of a molecular absorption signal due to the interferent. For a given instrumental system, the separation of atomic and molecular absorption signals on the time scale depends on the difference between the at,omization temperature of the analyte element ;and the vcllatility of the interferent. Interference Mechanism Using Boi1i;zg Point Data. The depression is apparently due to the depletion of the atom population by chloride formation in the vapor phase. In order to explain the interference mechanism, the boiling point and the decomposition temperature of the interferent may be used as a first approximation. For example, in the Ni--CuC12system (Figure 2), CuClz decomposes to CuCl at 990 "C (14). Since CuCl has a boiling point of 1490 "C, it is retained in the furnace and in turn decomposes to copper. Chlorine is released in these reactions and due to the very fast heating rate applied in the atomization step, it can be expected to persist in the furnace up to relatively high temperatures. Since nickel atomizes a t 1710 "C (the temperatwe corresponding to the maximum of the nickel signal), a reaction between gaseous nickel atoms and chlorine is possible and the chemical interference takes place. In contrast, in a Ni-PbC12 system, no depression of nickel signal is observed. The fact that the atomization of lead occurs a t 990 "C, Le., 720 "C lower than the atomization temperature for nickel, account!; for different behavior. Also, PbC12 has a boiling point of 950 "C and therefore it is likely that by the time the atoniization tem-
1
'
-
-
-
-
' . ,/.L
-
L
-
s:H-riMISTFY, VOL 50, NO 1, JANUARY 1978
L
1
I
Io-'
loa
10'
C u , ng as ' D i
t 1
-
t
T
CdCI,
riicfiel is reached, all of PbC1, and its decomb ~ ~ ~ dhill \ ~have ~ t diffused > out of the furnace. For
r t . c t w f i . 210 interference IS observed for a Ki-CdCI2 A r > i x agdiIi, CdC12has a boiling point of 960 "C, which ' , e l e ~ the a atomization temperature for nickel. Coni:i\ [lit- iriterferent and its decomposition products are rr\ .:ii tile furnace before they can react with the nickel >iiiatioli. It ic interesting to note that in this system. 1)) i n t of CdClz :f 960 " C is very close to the w ~ t i of t NiCl? of 970 "C. For this reason, selective '11 < I t these chlorides cannot occur. The fact that TIC e i b observed indicates that NiC12 is not formed ( 5 >tare 011 drying. This reaction would have to result o t i i i c ke! atomic population since the volatilization \~ocddoccur well below the atomization temperature
+ t i
1
\
atutilliation temperature for lead of 990 "C means *ith the nickel determination. Thus, CdC12
~ ~ 1 ~ er(. i . t
consequently removed from the furnace edrly ztage cf the atomization.
I$
t
I
i t - - + rof ~
a n exception since the amount of the order of 10 p,g is required to completely I J I I I I C absorption signal for copper. This can
em
irit
8
11
1nterfere:it exceeding 1 fig resulted in a IS
io3
CuCt2
on peak absorbame by nickel measured at Ni 232 0 nm line Ni atomic absorption ( 0 )0 4 ) r h.$o,iiid absorption d u e to CuCI,
ft+
1
10
ng, (E) 0 7 ng, (A)1 0
be explained by a relatively large difference between the boiling point of PbC12 (950 "C) and the atomization temperature for copper (1430 "C). Only a large amount of PbClz introduced into the furnace will ensure the presence of its decomposition products at the time when the atomization temperature for copper is reached.
Interference Mechanism Using Direct Time-Resolued Measurements. The spproach using the boiling point of the interferent to predict the behavior of interfering species within the furnace is not strictly correct. It does not take into account the kinetic parameters determining the transient population of these species in the furnace. Became of the difficulty of specifying kinetic parameters such as the rate of volatilization, the rate of atomization, and the rate of diffusion for the variable furnace temperature, a direct measurement was made to predict the behavior of the interfering species in the furnace. T h e concentration of chlorine, however, cannot be measured directly. Since chlorine is generated by decomposition of metal chloride molecules, it can be assumed that the first appearance of the metal atoms marks also the first appearance of chlorine, provided that the metal chloride decomposes directly to metal atoms. This assumption is valid for the metal chlorides studied with the exception of CuC1, which initially yields stable CuCl and chlorine. For the remaining chlorides, the vapor phase population of the interferent can be monitored with high sensitivity by measuring the corresponding metal atom concentration, provided the diffusion rate of the metal atom does not differ appreciably from that of the interfering species.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
5
Table I. Summary of Transition Metal Chloride Systems Analy te atomization temper atu re, 'C
System Pb - CUC1 Ni-Cu C1, Ni-PbCl, Ni-Cd C1, Pd-CdCl? Pb-HgC1, CU-P b C1, Cu-Pb C1,
Amount Pg
990 1710 1710 1710 990 990
0.7
0.7 0.7
0.07 0.07
...
Interferent Temperature range for which T = 0, "C
-bp, C
1280-1770 1280-1770 54 0-1 160 180-1090 180-1090
1490' 1490' 950 960 960
...
300
1430
20
5 10-1460
1430
0.7
540-1160
950 950
Interference observed Yes
Yes N CI N CI
Yes NCI Yes Ncl
' Data for CuC1. A
1 -
T
0 -
*'
I
0 -
Figure 4. Oscilloscope traces showing tirne overlap oetween copper atomic population and interfering species. ( 1 ) 0.4 ny Cu recorded at Cu 324.8 nm; (2) 6 ng and (3)20 pg PbCI, recorded at Pb 217.0 nm. Duration of trace, 5 s
E 1 -
"
'M
J
T
// Figure 3. Oscilloscopic traces showing time overlap between nickel atomic population and interfering species. A and B: (1) 1.0 ng Ni recorded at Ni 232.0 nm. A: (2) 0.7 kg PbCI, recorded at Pb 217.0 nm. B: (2) 0.7 kg CuCI, recorded at Cu 324.8 nm. Duration of trace,
5s
Calculations using the expression for the diffusion loss from a furnace (15) show t h a t atoms dissipate very rapidly from the short length furnace used in this study. For example, zinc atom population in nitrogen atmosphere at 2060 "C is reduced to 0.1 % of the original level within 0.1 s compared with 1% for both lead and molecular chlorine under identical conditions. Consequently, the resulting transmittance-time or transmittance-temperature profiles are determined predominantly by the atom production process and, as such, they can be used to follow the behavior of gaseous chlorine in the furnace. Figure 3 shows time-resolved oscilloscopic measurements for atomizations of PbClz and CuClz in relation to the nickel atomic absorption peak. T h e relative position of the traces confirms the earlier conclusions based on the boiling points
of the metal chlorides. No interference is ohserved in the Ni-PbC12 system since the decomposition products of PbClz dissipate from the furnace before the analyte is atomized, as can be seen in Figure 3A. .4t the same time, the overlap between the CuC12 trace and the anrilyte peak confirms that interference takes place due to the presence of high concentration of chlorine at the time when nickel atoms are generated (Figure 3B). The interferent traces can be characterized by the temperature interval within which total absorption is attained. This interval then corresponds to the presence of high concentration of chlorine in the furnace (with the exception of CuC1,) and can be related to the atomization temperature of a given analyte element as is summarized in Table I. For the Pb-CuC12 system, the predicted behavior does not agree with the experimental observation. However. the fact that with CuC12, chlorine is formed a t temperatures lower than those determined from the first appearance of copper atoms, accounts for the discrepancy. It is also interesting to note that a large amount of PbClZ is required to achieve an overlap between the atomization temperature for copper and the temperature range for which T = 0. The fact is demonstrated more clearly in Figure 4 which shows how the increasing amount of PbCl, affects the degree of overlap. These observations confirm the fact established earlier that only a considerable excess of PbC1, causes a marked depletion of copper atomic population. Removal of the T r a n s i t i o n Met a1 Chloride I n t e r f e r ence. Most commonly, fractional volatilization is used to remove interferents from the sample prior to atomization. The success of this approach varies with the matrix composition and a serious loss of the analyte element may occur if there is an insufficient difference between the volatility of the
6
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
n
,L
c--
/
/i
"
't
--+
---e
+-----------.-..L.--L.+-
C - - Lj o_ .-. -'
,
-----__I_
l0
10"
?O'
10'
HNOs
3s
Figure 7. Removal of CuCI, interference on lead by nitric acid as measured at Pb 217.0 nm line. Pb atomic absorption: (A) 0.5 ng Pb; ( A ) 0.5 ng Pb + 0.7 pg CuCI,. Background absorption: (0)0.7 pg
CuCI, Figure 5. Removal of CuClp interference on lead by phosphoric acid as measured at Pb 217.0 nm line. Pb atomic absorption: (A) 0.5 ng Pb; (A) 0.5 ng Pb + 0.7 pg CuCIp. Background absorption: (0) 0.7
I.Lg CUCI,
0
L
(0'
10' "Q
I*
N.CI
Figure 8. Effect of NaCl on peak absorbance by 0 3 ng lead measured at Pb 217.0 nm line. ( 0 )Pb atomic absorption: (0),background absorption . . . L .due to NaCl 10'
-.-I-~.-----L------L-----
10.'
".
10'
'(I'
*E83
."(I
Figure 6. Removal of CuCI, interference on nickel by acids as measured at Ni 232.0 nm line. Ni atomic absorption: (0)1 ng Ni + H3P04; (I) 1 ng Ni + 0.7 fig CuCI, -b H3P04; (A) 1 ng Ni f H2S04;( A ) 1 ng Ni 0.7 pg CuCI, + H2S0,. Background absorption: (0)0.7 pg CuCi,
+
analyte element and that of the interferent. Acid treatment was employed to eliminate interferences due to the presence of transition metal chlorides. The general trend observed was that only the high boiling point acids were able to fully restore the atomic absorption peak to the value prior to the: addition of the interferent. This is demonstrated for the Pb-CuClz and Ni--CuCl* systems in Figures 5 and 6. The interfering chlorides are converted to the corresponding phosphates (pyrophosphates) and sulfates which exhibit very low molecular absorption and therefore a gradual decline of the molecular absorption signal with the increasing acid concentration is observed. Hydrochloric acid which is formed in these reactions is removed during drying or ashing and, consequently, chlorine containing species are removed from the furnace well before the time when the atomic population is generated. The atomic absorption signal is thus restored virtually to its original value. Apart from the two examples, sulfuric and phosphoric acids were used successfully to remove interferences in the following systems: Pb-CdC12, Sn-CuC12, Sn-CdCl,, Zn--PbCI2,Zn-CuC1, and Zn-CdC12. Nitric acid was found to be less effective than phosphoric and sulfuric acids as is shown in Figure 7 . Also, the molecular absorption signal does not seem to be effectively suppressed in the presence of nitric acid. Acetic and hydrochloric acids cannot remove the interference in the Pb-CuC12 system. Acetic acid which has a low boiling point and is a weak acid will not remove chloride during the drying step while hydrochloric acid will be
I.---'*-'
d
1
10'
(0' Na.np
I.
10'
10'
UCl
Effect of NaCl on peak absorbance by 0.4 ng copper measured at Cu 324.8 nm line
Figure 9.
evaporated and will not alter the composition of the system. Interference in Alkali Metal Chloride Systems. Sodium chloride was selected as a representative of this group of chlorides in view of its presence in a wide variety of matrices. Selection of Detection System. When measured with the slow conventional detection system, a depression of the atomic absorption signal commences generally at a lower interferent level than for transition metal chlorides. Figures 8 and 9 show the effect of NaCl on the lead and copper peak atomic absorption signals; nickel is not affected by the comparable excess of NaCl. As distinct from the corresponding systems containing transition metal chlorides in which a large excess of interferent causes a complete elimination of the atomic population, the initial decrease in the atomic absorption peak here is followed by virtually a plateau region.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1 , JANUARY 1978 E
A
T
7
T
0
0
Figure 10. Time-resolved measdrements for Pb-NaCI system at Pb 217.0 nm line using one (A) and average of eighlt atomizations (D), (1) 0.4 ng Pb; (2) 0.4 ng Pb f 2.0 pg NaCI; (3) 2.0 bg NaCI. Duration of trace, 2 s
Fast oscilloscopic measurements confirmed that the risetime of the copper atomic absorption peak is not affected by the presence of NaCl and thus the slow detection system is adequate for these measurements. For the Pb-KaC1 system. however, both the risetime and the shape of the lead atomic absorption peak depend on t h e amount of interferent. Moreover, the atomic and molecular absorption signals are not completely resolved on the time scale and their relative position is affected by the interferent level. It is therefore necessary to make fast time-resolved measurements to interpret correctly the behavior of this system. In order to overcome the high noise level of fast measurements, a signal averaging technique was employed. An example of signal-to-noise ratio improvement by averaging eight atomizations is given in Figure 10. As a matter of normal practice, signals should be measured at least in triplicate in furnace work because of relatively poor precision of this technique. Signal averaging is obviously ideally suited here, since it provides an averaged signal with improved signal-to-noise ratio. Interference Measurements. The sequence of time-resolved traces in Figure 11 depicts changes in the lead atomic absorption peak when the amount of NaCl present increases from 0.2 ng to 10 pg. T h e corresponding peak readings are also plotted in Figure 12. Several important observations can be made: (1) In the presence of the interferent, the lead peak shifts to the lower temperature range. ( 2 ) The initial portion of the lead peak rises rapidly in the presence of between 2 and 20 ng of NaC1. A double peak is apparent. (3) T h e molecular absorption peak due to NaCl shifts to higher temperatures when the amount of NaCl is increased. As a consequence, the relative position of the atomic absorption and the molecular absorption peaks for the Pb-NaC1 system changes with the amount of the interferent. Currier E f f e c t . Observations described under (1)and (2) can be explained by the "carrier effect" or "carrier distillation" as known in emission spectroscopy (16). I t is reasonable to expect that evaporation of NaCl would cause simultaneous introduction of lead into the vapor phase. In order to support this hypothesis, i t is necessary to compare the temperature a t which the sharp increase of the lead atomic absorption signal occurs with temperature marking the first appearance of the NaCl molecules in the vapor phase. Since molecular absorption measurements do not provide the high sensitivity needed to detect the first appearance of NaCl molecules, the first appearance of sodium atoms in the vapor phase was monitored instead. There is a difference of 110 "C between the first appearance of lead atoms a t 470 "C and sodium atoms at 580 "C. This
result supports the suggested mechanism since the relatively small temperature difference can be explained b?, the fact that a time (and temperature) delay can be expected between the first appearance of NaCl molecules and the first appearance of sodium atoms. There are two factors uhich have to be considered, namely the relative stability of NaCl molecules and the discrepancy between the wall and the vapor (excitation) temperature. The latter factor probably plays a major role since it was established that the excitation temperature in the small furnace used in this study lags behind the wall temperature by between 50 to 200 "C, depending on the heating rate ( 3 ) . The observed temperature difference occurs since the carrier distillation and the corresponding appearance of lead atoms depends on the furnace wall temperature while the dissociation of NaCl and the appemance of sodium atoms is determined by the excitation temperature. It is interesting to note that very pronounced carrier distillation seems to occur only between 2 and 40 ng of NaCl present. The carrier distillation still takes place a t higher quantities of interferent. as can be seen from the decreased atomization temperature for lead, but increased dilution of the analyte element probably slows down the initial fast increase. Some additional insight into furnace processes may be gained by considering why effects resembling the carrier distillation are apparent for neither the P b C u C 1 2 and Pb-CdC12 systems, nor for the Cu NaCl system (no change in the atomization temperature for copper). The transition metal chlorides are thermally lew stable than NaCl and consequently they may be expected t o decompose predominantly to metal atoms and chlorine, rather than leaving the surface of the furnace w21llas molecules. For the Cu-NaC1 system, carrier distillation may still take place without changing the appearance of the copper atomic absorption peak. As carrier distillation occurs at temperatures below 500 "C, as observed for lead, practically no copper atoms can be expected to appear in the vapor phase owing to their higher atomization temperature. At the same Lime, copper may be lost in molecular form. Slow us. Fast Measurements. Consequences of the signal shift, as described in the third of the observations listed above, for measurements with the conventional detection system are clearly seen when Figures 8 and 1 2 are compared. Overcorrection results with the slow system since the relative time shift between the atomic and molecular peaks is not apparent and the full molecular absorption signal rather than its variable proportion corresponding in time to the atomic absorption peak is substracted. Distortion of the fast atomic absorption peak contributes further 1o the discrepancy. Interference Mechanrsm. The mechanism of chemical interference due to NaCl appears to be more complex than
8
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978 D
A 1 -
T T
0
B
T
E
T
0
0
F
C
T T
Figure 11. Time-resolved measurements for Pb-NaCI system at Pb 217.0 nm line. (1) 0.4 ng Pb; (2) 0.4 ng Pb 4-NaCI; (3) NaCI. NaCl added: (A) 0.2 ng; (B) 2 ng; (C) 20 ng; (D) 200 ng; (E) 2 pg; (F) 10 pg. Average of 4 atomizations. Duration of trace, 2 s
that of chemical interference caused by transition metal chlorides. More experimental evidence is needed to explain the underlying processes in detail; however, using the present data, some conclusions may be drawn. Freedom from interference in the Ni-NaC1 system, similar to the corresponding transition metal chloride systems, can be ascribed to the high atomization temperature for nickel. Sodium chloride (bp 1460 "C) and its decomposition products are dissipated from the furnace before the nickel atomization temperature of 1710 "C is reached. In contrast with severe depression of lead atomic absorption signal by transition metal chlorides, lead atomic population is not greatly affected by the presence of up to 1 pg of NaCl (Figure 12). As discussed previously, the atomization temperature for lead is lowered in the presence of NaC1. The atomic absorption peak appears a t 750 "C and this tem-
perature is not adequate for efficient formation of chlorine by decomposition of NaCl molecules. This fact is confirmed by a time-resolved measurement for atomization of 20 ng NaCl taken a t the Na 589.0 nm line. Total absorption is reached at 840 "C, confirming that high concentration of chlorine capable of interacting with gaseous lead atoms may not be expected below this temperature. I t can be determined from the same NaCl trace, that high concentration of chlorine is maintained in the furnace up to temperatures in excess of copper atomization temperature of 1430 "C. The depression of copper atomic absorption signal (Figure 9) can then be interpreted as the vapor phase chloride formation, although a possible contribution by the carrier distillation to the loss of copper atoms cannot be discounted. Removal of the Alkali Metal Chloride Interference. I t was already demonstrated for the transition metal chloride
ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
9
1-
Figure 14. Time-resolved measurements for Pb-NaCI system at Pb
L.-.
L 10'
11-1
10'
II.
ng
..
(0'
(0,
10'
Its,
Flgure 12. Effect of NaCl on peak absorbance by 0.4 ng lead plotted from Figure 11. ( 0 )Pb atomic absorption; (0)background absorption
due to NaCl 1-
Figure 13. Time-resolved measurements for Pb-NaCI system at Pb 217.0 nm line with excess of nitric acid added. (1) 0.4 ng Pb 20 pg "0,; (2) 0.4 ng Pb 2 pg NaCl 20 pg "0,; (3) 2 pg NaCl 20 pg "0,. Average of 4 atomizations. Duration of trace, 2 s
+
+
+
+
systems that the interferences can be eliminated by chemical treatment with acids. The same approach can be used to remove NaCl interference on lead and copper. In practical analysis, treatment with nitric acid was successfully used to suppress spectral interference in the determination of lead in blood by furnace atomic absorption (17). An excess of nitric acid effectively eliminates the spectral interference in the Pb-NaC1 system, as shown in Figure 13, by converting t h e interferent to N a N 0 3 which causes no perceptible molecular absorption. Comparison with the corresponding trace in Figure 11E reveals that a slight enhancement of the lead signal results while its shift to a lower temperature is maintained. Treatment with phosphoric acid which proved effective for removal of chemical interference in the Cu-NaC1 system, was also applied to the Pb-NaC1 system. The atomization temperature for lead is increased in the presence of phosphoric acid (18) but t h e molecular absorption peak is also affected as can be seen in Figure 14. The trace for NaCl in the presence of phosphoric acid exhibits two maxima. The lower first molecular absorption peak appears a t the same temperature range as the original NaCl peak and is probably due to the residual amount of NaCl in the sample. The higher second peak was identified as being caused by Na2HP04. Lead atomization is obviously retarded since lead atoms are occluded in the resulting matrix and cannot be released until the matrix has decomposed. The experience that phosphoric acid can be used to remove chemical interference in the Cu-NaC1 system would suggest that the formation of the more refractory matrix does not affect copper atomization. This is true since the Na2HP04molecular absorption peak appears
217.0 nm line with excess of phosphoric acid added. (1) 0.4 ng Pb 10 pg H,PO,; (2) 0.4 pg Pb 2 pg NaCI -t 10 ug H,PO,; (3) 2 pg NaCl 10 pg H,PO,. Average of 4 atomizations, Duration of trace,
+
+
+
5 s
a t 1250 "C, indicating the decomposition of the matrix well below the atomization temperature for copper (1430 "C). Ashirzg Losses. The effect of ashing on the Pb-NaC1 system in the presence of nitric acid was briefly investigated. In Figure 13, a slight increase in the lead atomic absorption signal was observed following treatment with nitric acid. When the same system was subjected to 15-s ashing reaching the final temperature of 715 "C, no change was observed for the lead solution containing only nitric acid while the signal for the lead solution contaifiing both NaCl and nitric acid was depressed t,o 30% of its original value. In a receat study on the determination of lead in strong NaCl solutions (19),similar behavior was observed with lead ashing losses from solutions containing NaCl and nitric acid occurring a t lower temperatures than those from nitric acid medium only. The difference can be easily explained by using observations on lead atomization temperatures reported earlier in this study. The lowered atomization temperature for lead in the presence of NaCl is obviously reflected in higher ashing losses a t a given ashing temperature.
CONCLUSIONS From the experimental evidence I I general conclusion can be made that the interference in the -transition metal chloride systems is due to a time overlap in the furnace between the analyte atoms and the interfering species. The extent of the overlap depends, on one hand, on the decomposition temperature of the interferent, its amount and the boiling point and, on the other hand, on the atomization temperature of the analyte. A similar interpretation applies to the alkali metal chloride systems, although carrier distillation has to be considered as a contributing factor. Apart from the properties of the imalyte and interferent, the extent of the overlap is also influenced by the furnace construction and by the heating program used. For a short furnace, fast diffusion combined with fast heating during atomization makes it possible to minimize this overlap. While it is recognized that increased residence time of atoms in a longer furnace results in improved detection limits, this advantage should be considered in the context of increased probability of interference due to greater overlap. ACKNOWLEDGMENT The authors thank L. E. Smythe and B. J. Orr for valuable discussions and are grateful to Varian Techtrori Pty. Ltd. for the grant of the atomic absorption spectrometer. LITERATURE CITED (1) G. Baudin, M. Chaput. and L. Feve, Spectrochim. Acta, P a r t 6 , 28, 425 (1971). (2) J. P. Matousek, Am. Lab., 3 (6),45 (1971). (3) J. P. Matousek. "Proceedings of the XVI1 Colloquium Spectroscopicum Internationale", Florence, Vol. I , 1973, p 57. (4) R . B. Cruz and J. C. Van Loon, Anal. Chim. Acta, 72, 231 (1974). (5) J. Smeyers-Verbeke, Y. Michotte, P. Van den Winkel, and D. L. Massart. Anal. Chem.. 48, 125 (1976).
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 1, JANUARY 1978
W. Frech and A. Cedergren, Anal. Chim. Acta, 82, 83 (1976). W. Frech and A. Cedergren, Anal. Chim. Acta, 82, 93 (1976). J. Aggett and A. J. Sprott, Anal. Chim. Acta, 72, 49 (1974). K. Ohta and M. Suzuki. Talanta, 23, 560 (1976). F. J. M. J. Maessen and F. D. Posma, Anal. Chem., 46, 1439 (1974). F. D. Posma, H. C. Smit, and A. F. Rwze, Anal. Chem., 47, 2087 (1975). C. HendrikxJongeriusand L. de Gabn, Anal. Chim. Acta, 87, 259 (1976). E. H. Piepmeier and L. de Galan, Specfrochim. Acta, Part B , 3 1 , 163 (1976). (14) "Handbook of Chemistry and physics", 55th ed.,R. C. Weast, Ed., Chemical Rubber Co., Cleveland, Ohio, 1975. (15) B. V. L'vov, "Atomic Absorption Speckochemical Analysis", Adam Hilger, London, 1970, p 288. (16) E. Schroll, Z . Anal. Chem., 198, 40 (1963). (6) (7) (8) (9) (10) (1 1) (12) (13)
(17) V. P. Garnys and J. P. Matousek, Clin. Chem. ( Winston-Salem, N . C . ) , 21., 891 .~119751. ~, (18) E. J. Czobik and J. P. Matousek, Talanta, in press. (19) W. Frech and A. Cedergren, Anal. Chim. Acta. 88, 57 (1977). I
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RECEIVED for review July 13, 1977. Accepted September 26, 1977. This work was presented in part at the 5th International Conference on Atomic Spectroscopy, Melbourne, Victoria, Australia, August 25-29,1975, and at the 3rd FACSS meeting and the 6th International Conference on Atomic Spectroscopy, Philadelphia, Pa., November 15-19, 1976.
Influence of Some Matrix Elements on the Determination of Copper and Manganese by Furnace Atomic Absorption Spectrometry Johanna Smeyers-Verbeke, Yvette Michotte, and Desire L. Massart Vrije Universiteit Brussel, Farmaceutisch Instituut, Paardenstraat 67, B- 1640 Sint-Genesius Rode, Belgium
An investigation of the effect of Mg(NO&, MgS04, MgCI2, Ca(N03),, CaCI,, and NaCl on the pulse characterization times of Cu and Mn carbon furnace atomic absorption signals is 7peak,T,, performed. The pulse characterization times 7Tdelay, and 7 2 were determined from the oscilloscope traces obtained by means of a memory oscilloscope. The results of this study Indicate that the time characterlstlcs of the carbon furnace atomic absorption signals of Cu and Mn in the presence of the different salts are not comparable. Consequently, it is probable that different atomization mechanisms should be considered to explain the different interference phenomena studied here. I t seems unlikely that the existing models, describing atomization processes in nonflame atomizers, could contribute much to the elucidation of the interference problem.
In a previous paper dealing with matrix effects in the determination of Cu and Mn using carbon furnace atomic absorption spectrometry ( I ) , a preliminary study of the peak form of the recorded absorption signals was carried out. I n the first instance, a comparison was made between the extent of some of the interferences by application of peak integration and peak height measurements. It was concluded that for Cu as well as for Mn, measurement by integration yields the same results as peak height measurements for those interferences which cause a suppression of the absorption signal. Positive interferences were shown to persist but to be reduced when the integration method is used. This implies that the peak becomes smaller and consequently this allows one to assume that the atomization of the analyte takes less time. This should mean that the curve form of the recorded absorption signals has changed. These findings were supported by registration of the absorption signals by means of a memory oscilloscope. The study of the peak form of atomic absorption signals generated during heating of flameless atomizers is predominantly limited to the investigation of metal solutions in pure water ( 2 , 3 ) .Few applications are found in the studies concerning matrix effects in flameless devices. T o our knowledge, only Ebdon and co-workers ( 4 ) give some oscilloscope traces in their interference study concerning Mn, 0003-2700/78/0350-0010$01 .OO/O
atomized with a carbon filament atom reservoir. The present article describes more detailed investigations into the influence of some matrix elements on the curve form of Cu and Mn carbon furnace atomic absorption signals. Especially interferences resulting in an increase of the absorption signals were investigated.
EXPERIMENTAL Instrumentation. A double beam atomic absorption spectrometer, Perkin-Elmer Model 305, equipped with a deuterium background corrector and a graphite furnace Perkin-Elmer HGA 72 were used. The experimental conditions used are given in a previous paper (1). Registration of the absorption signals at the amplifier output was performed by means of a memory oscilloscope, Tektronix 7613. The time base of the oscilloscope was triggered at the beginning of the atomization cycle. In this way, the shift of absorption signals can be observed. From the oscilloscope traces, photographically recorded by means of a Polaroid camera, the pulse characterization times were determined. The integration unit used has been described in a previous paper ( 1 ) . Preparation of Solutions. Solutions of 50 pg/L Mn (as the chloride) containing, respectively, 500 mg/L Mg as Mg(NO&, MgS04,and MgC12;500 mg/L Ca as Ca(NO&; and solutions of 100 pg/L Cu (as the chloride) containing, respectively, 500 mg/L Mg as MgS04,500 mg/L Na as NaCl, and 500 mg/L Ca as CaC1, were prepared. Fifty-microliter samples were used for the investigations.
RESULTS AND DISCUSSION In a simplified mathematical model for the transport of a sample through an analytical cell, L'vov ( 5 ) defines some characteristics of an analytical signal. The most important are: T~ t h e atomization time average time of residence of a n atom in t h e cell. I t T~ is defined as t h e time taken for t h e absorbance t o decrease from its maximum value, A,,, to a value e times smaller t h e length of time during which t h e signal is reT~ corded. On the basis of these parameters, equations are obtained for both methods of measuring absorption signals, the peak C 1977 American Chemical Society
