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Reduction of matrix interferences in furnace atomic absorption with the L'vov Platform
05&I-8547/81/080773-11$02.00/O 0 1981 Per&mm PESS Ltd.
Specfmhimicn Acta, Vol. 368, No. 8, pp. 773-783, 1981. Printed in Great Britain.
Redaction atomic
of
matrixinterferences
absorption
in furnace
with tbe L’vov Platform
M. L. KAISER, S. R. KOIRTYOHANN, E. J. HINDERBERGER Chemistry
Department
and Environmental Trace Substances Research Columbia, MO 65211, U.S.A.
Center,
University
of Missouri,
and H. E. TAYLOR U.S. Geological Survey, Box 25046 Mailstop 407, Denver Federal Center, Denver, CO 80225, U.S.A. (Receioed 21 January
1981)
Abatraet-Use of a modified L’vov Platform and ammonium phosphate as a matrix modifier greatly reduced matrix interferences in a commercial Massmann-type atomic absorption furnace. Platforms were readily fabricated from furnace tubes and, once positioned in the furnace, caused no inconvenience in operation. Two volatile elements (Pb, Cd), two of intermediate volatility (Co, Cr) and two which form stable oxides (Al, Sn) were tested in natural water and selected synthetic matrices. In every case for which there was a significant matrix effect during atomization from the tube wall, the platform and platform plus modifier gave improved performance. With lead, for example, an average ratio of 0.48*0.11 was found when the slope of the standard additions plot for six different natural water samples was compared to the slope of the standard working curve in dilute acid. The average slope ratio between the natural water matrices and the dilute acid matrix was 0.94*0.03 with the L’vov Platform and 0.96*0.03 with the platform and matrix modifier. In none of the cases studied did the use of the platform or platform plus modifier cause an interference problem where none existed while atomizing from the tube wall. An additional benefit of the platform was a factor of about two improvement in peak height precision.
1. INTR~DU~H~N introduction in 1963, the Massmann furnace design [l] common to most commercial instruments for furnace atomic absorption (AA) has demonstrated a troublesome susceptibility to matrix interferences which can cause severe reductions or enhancements of the analyte signal. Interferences are of three basic types: spectral, physical and chemical. The spectral interferences caused by molecular absorption and light scatter have been successfully eliminated in most cases by the use of background correction systems. Physical interferences tend to alter the shape of the absorption peak by changing the appearance time (time at which absorbance due to the metal vapor first appears) and thus the appearance temperature of the analyte. This results in a change in the signal profile and thus analyte response. Examples of suggested mechanisms for physical interference include covolatilization of the analyte along with a more volatile matrix [2] and occlusion of the analyte in matrix crystals [3, 4, 51. Chemical interferences can be caused by reaction of the analyte with the hot graphite walls of the furnace to form refractory carbides and by formation of stable gaseous molecules which escape without decomposing to atoms. The molecules may be formed as the analyte is vaporized or they may result from gas phase reactions. Methods used to reduce matrix interferences, in addition to optimization of furnace parameters, have included chemical treatment of the graphite tube [6, 71 and introduction of an active gas such as hydrogen or air during a portion of the furnace heating
SINCE rrs
[l] H. -MANN, Spectrochim. Acta 23B, 215 (1968). [2] R. C. HWON, J. M. C~-~AWAYand T. PLATO,Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA., (November 1976). [3] R. B. CRUZ and J. C. VAN LOON, Anal. Chim. Acta 72, 231 (1974). [4] J. SMEYER+VERBEKE,Y. MICHOTQ P. VAN DEN W~NKELand D. L. WANT, Anal. Chem. 48, 125 (1976). [5] D. J. CHURELL.Aand T. R. COPELAND,Anal. Chem. 50, 309 (1978). [6] D. J. HODGES, Analyst 102, 66 (1977). [7] K. C. THOMPSON,K. WAGSTAFF and K. C. WHEATSTONE,Analyst 102,310 (1977). 773
M. L. KAISER etal.
714
cycle [8]. Another method, which was first reported by EDIGER in 1974 [9], involves the addition of chemical modifiers to alter the drying, charring, or atomization properties of the sample. Many different types of matrix modifiers have since been explored and reported in the literature. Unfortunately, matrix interferences are not completely eliminated by these methods and the method of standard additions probably remains the most frequently employed calibration method in furnace AA, particularly for samples of unknown composition. Standard additions is a very time consuming process and is limited by the quality of the blank and the linearity of the working curve. Therefore, convenient methods are needed to reduce or eliminate matrix interferences in the commercial furnaces used for AA. WOODRIFFet al. [lo] and KRASOWSKI and COPELAND[ll] have compared the constant temperature Woodriff furnace to the commercial pulse-type furnace and found the former design to be much less susceptible to interferences from matrix constituents. Consequently, investigators have been exploring methods for modifying the pulse-type design such that it will better approximate the performance of constant temperature furnaces without sacrifice in convenjence. In 1977, L’vov [12] proposed placing-the sample on a thin graphite platform within the tube of a Massmann-type furnace. This platform is heated primarily by radiation from the hot tube walls. Thus the surface temperature of the platform lags behind that of the wall and furnace atmosphere. When vaporization occurs from the platform, the tube and gaseous atmosphere h ve reached a higher and more nearly constant temperature. Matrix-dependent cLn ges in appearance time will result in reduced variation of the furnace temperatures at the time of atomization [12]. Also the fact that the gaseous furnace atmosphere is at a higher temperature will help to decompose molecular species. As a result the platform helps to eliminate both physical and chemical matrix interferences in the pulse-type furnace. GREGOIREand CHAKRABARTI [13] have studied the effects of the L’vov Platform on analyte sensitivities. They reported a general, though slight enhancement of peak height and area measurements. SLAVIN and MANNING[14] found greatly reduced interferences on lead response when the platform was used compared with vaporization from the tube wall. In this paper we describe a relatively simple platform-fabrication method and explore its effects upon interferences for six elements in synthetic and natural water matrices. 2. EXPERIMENTAL 2.1 Apparatus These studies were done on a Perkin-Elmer Model 703 atomic absorption spectrophotometer and a Perkin-Elmer HGA-500 graphite furnace. The spectrophotometer was equipped with deuterium-arc background correction. The microprocessor-controlled furnace power supply could be programmed for up to nine steps in the cycle. Each step had provision for a controlled increase in temperature to assure the smooth drying and charring of the sample. Of particular significance to this study was the provision for heating the graphite tube with maximum power, and thus maximum speed, to a preset atomization temperature. An optical sensor was used to detect when the preset temperature was reached and to trigger a switch to a power level appropriate to maintain that temperature. Read-out was on a Perkin-Elmer Model 56 strip chart recorder. Sample introduction to the furnace was performed by a Perkin-Elmer AS-1 automatic sampler. All experimentation was done with 20 ~1 sample volume and 1.5 ml rinse volume except for cadmium studies which were done using a 10 ~1 sample volume. Argon was used as the sheath and purge gas throughout. Perkin-Elmer electrodeless discharge lamps were used for lead, cadmium and tin; and hollow cathode [S] [9] [lo] [ll] [12] [13] [14]
R. D. BEATY and M. M. COOKSEY,Atom. Absorp. Newsletter 17, 53 (1978). R. D. EDIGER, Atom. Absorp. Newsletter 13, 61 (1974). L. R. ~GEMAN, J. A. NICHOLS,P. VISWANADHAM and R. WOODRIFF,Anal. Chem. 51,1406(1979). J. A. KRASOWSKIand T. R. COPELAND,Anal. Chem. 51,1843 (1979). B. V. L'VOV, Spectrochim. Acta 33B, 153 (1977). D. C. GREGOIREand C. L. CHAKRABARTI,Anal. Chem. 49, 2015 (1977). W. SLAVIN and D. C. MANNING, Anal. Chem. 51, 261 (1979).
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
775
lamps for cobalt, chromium and aluminum. A 500 W line-voltage regulator was also provided to help stabilize the line voltage. Pyrolytically coated graphite tubes (Perk&Elmer No. 290-1766) were used for most of the work though some experiments were done with uncoated tubes (P.E. No. 290-1633). Both tube types were tantalum treated (tantalized) using the method of ZATKA [15] to extend tube life. Eppendorf microliter pipeta with polypropylene tips were used for preparing standards and samples. Polyethylene sample cups were used with the AS-l automatic sampler. 2.2 Reagents Standard solutions were made using appropriate dilutions of 1000 pg ml-’ standard solutions obtained from VWR Scientific Inc. (Pb, Cd, Co and Cr) or Fisher Scientific Co. (Al and Sn). Synthetic matrix solutions for Ca, Mg, K and Na were prepared from 1000 pg ml-’ Fisher standards. Phosphate matrix solutions were prepared from reagent grade KH,PO, and also from H,PO,. The sulfate matrix solution was prepared from reagent grade Fe(NH,),(SO,), and from H,SO_,. Perchlorate matrix solutions were prepared from 70% HClO,. All standards, synthetic solutions and water samples were acidified to 0.16 M in HNO,. All solutions injected into the furnace had nitrate as the dominant anion. The effect of matrix ions except perchlorate was tested at a concentration of 500 +gml-’ because this was regarded as the maximum concentration likely to be encountered in natural water systems. Perchlorate was used at 0.12~ because this is the approximate concentration in sample solutions following perchloric acid digestions. In a few cases significant blank signals were seen but none were more than 5% of the highest standard and appropriate corrections were applied. The 5% matrix-modifier solution was prepared from reagent-grade ammonium phosphate. Impurities were removed by extraction into methylisobutyl-ketone after complexing with ammonium pyrrolidine carbodithioate. Matrix-modifier solution was diluted ten-fold to give 0.5% NH,HzPO, in the injected sample. Natural water samples from the U.S. Geological Survey (USGS) Standard Reference Water Collection were also tested. Sample numbers 57, 59, 61,63,65 and 67 were used. The major constituent concentrations were measured by inductively coupled plasma emission spectroscopy. Calcium was in the range lO45 Fg ml-‘; potassium 0.4-3.0 Fg ml-‘; magnesium 1.5-28.0 pg ml-‘; sodium 2-61 pg ml-‘. Sulfate values were in the range 13-120 pgml-’ [16]. 2.2.1. Platform design and fabrication. Platforms were fabricated from tantalized pyrolytically coated graphite tubes. Originally the platforms were made by hand. A graphite tube was broken and a piece which included the grooved-end section was filed into a curved rectangular form approximately 5 mm x 7 mm with a mass of about 58 mg. At a later date, the University of Missouri machine shop produced the platforms for this research. It was possible by this method to produce eight (4 from each end of the tube) curved, grooved graphite platforms from each tube. The platform was placed within the graphite tube and centered directly under the tube sample port. This was accomplished through the use of a locally fabricated positioning tool, constructed from an aluminum plate (25 mm x 40 mm X 3 mm) with a threaded rod (50 mm X 2.5 mm) through the center of the plate and a lock nut which allowed for correct positioning of the platform. The right quartz optical window was removed and the platform was properly positioned and the automatic sampler was adjusted to assure that the sample solution was deposited in the center of the platform. 2.2.2. Furnace optimization. As each new element was studied, optimization experiments were run to find the best temperatures and times for the three steps of dry, char and atomization. The tantalum treatment did not produce any appreciable change in tube behavior for the elements studied. Optimum conditions were close to those recommended by the manufacturer for the pyrolytically coated tubes when atomizing from the tube wall. Optimum drying conditions were set to provide a smooth, even evaporation of the solvent with no spattering. These conditions were continuously checked visually, with the aid of a dental mirror, throughout the lifetime of the graphite tubes. As the tubes aged the resistance increased and thus the actual temperatures within the furnace increased and altered the drying pattern. The best char temperatures were found by holding the atomization temperature constant and varying the char temperature. Likewise, the optimum atomization temperature was found by fixing the char temperature and varying the temperature of firing. The optimum char temperature was the maximum value which did not show loss of analyte. The higher this temperature, the more likely the matrix constituents could be decomposed. Optimized atomization temperature was the lowest temperature (to prolong tube life) which still gave a maximum peak-height signal. The maximum power mode was used for atomization and the purge gas flow was reduced from the normal flow of 300 ml min-’ to 50 ml min-’ during atomization. Charring and atomization behavior indicated that the platform temperature lagged behind tube-wall temperature by about 200°C when the wall was above 500°C. Optimum temperatures given by the manufacturer for atomization from the tube wall were increased by that approximate amount in order to retain the same analysis time per sample. The cool-down step between firings was lengthened by 10 seconds to allow the platform to return to room temperature. Table 1 provides a summary of the optimum furnace parameters for the elements studied. [15] V. J. ZATKA, Anal. Chem. 50, 538 (1978). [16] M. FISHMAN, U.S. Geological Survey, personal communication
(1980).
716
M. L.
KAISER
etal.
Table 1. Optimum furnace parameters
Wavelength
Spectral Bandpass
Element
(nm)
(nmf
Cd
228.8
0.7
Pb
283.3
Char Temp. (“C) Wall
Atomize Temp. CC)
Platform
Wall
Platform
Burn-out (“C) Wall
Platform
500
1400
1600
-
0.7
300 (lo/lo*) 500
(10/15) 700
(O/5) 2000
(O/7) 2000
-
(10/15) 1200
(O/5) 2700
(O/7) 2800
2900
2900
-
Cr
357.9
0.7
(15/15) 1000
co
240.7
0.2
(10/15) 1000
(10/15) 1200
(O/6) 2400
(O/8) 2600
(l/2) 2600
(l/2) 2700
Al
309.3
0.7
(lo/lo) 1200
(10/15) 1400
(O/6) 2700
(O/8) 2800
(l/2) 2800
(l/2) 2900
Sn
224.6
0.7
(lo/lo) 700
(10/15) 900
(O/6) 2400
(O/S) 2700
(l/2)
(l/2) -
(10/15)
(10/15)
(015)
(O/S)
* (lo/lo) indicates a ramp time of 10 s to reach the desired temperature and a hold time of 10 s at that temperature. Zero ramp time indicates use of maximum power. Drying was at 110°C (15/15) from the tube wall and 250°C (10/15) from the platform.
2.2.3. Element selection. One goal of the research was to survey interference behavior using elements with a range of properties. Therefore, two volatile elements were chosen (Pb, Cd), two of intermediate volatility (Co, Cr) and two which form rather stable oxides (Al, Sn). Molybdenum was originally chosen as one of the oxide-forming elements but the signal was suppressed so severely in the tantalized tubes that its study was abandoned. Matrix elements were selected from those likely to be encountered in natural water systems and also from common elements or ions which were known or suspected of causing troublesome interferences. 2.2.4. Measurement procedure. The parameter chosen to compare interference behavior was the variation in analyte sensitivity (working curve slope) in the presence of the matrix and in 0.16 M HNO,. In case of water samples, the slope of the standard additions curve was compared with the slope of the working curve in 0.16 M HNO,. In cases where the matrix modifier was used, the slope comparison was made with the modifier present in both samples and standards. Accepted values for most elements examined in the natural waters were available from the USGS. The results were from round-robin comparisons and in most cases the standard deviations were too large to make direct comparison of values very useful. The analyte concentration was selected to fall well within the linear response range and peak absorbances were used to construct all working curves. Peak heights were originally chosen because most users of furnace AA employ that mode rather than peak area. We learned very recently [17] that peak area measurements may have additional advantages when using the L’vov Platform but have not yet verified that observation in our own laboratory. The deuterium arc background corrector was used for all measurements and background levels were carefully monitored to assure both that the amount of background absorption was within the accurate correction range and that background problems were not made more severe by use of the L’vov Platform and the matrix modifier.
3. 3.1 Tantalum
REXJLTS
AND DISCUSSION
tube treatment
The primary advantage to tantalizing the tubes by ZATKA’S [15] method was to extend tube life. Tube life was measured by repeatedly atomizing aluminum until the sensitivity dropped to 50% of the maximum value. The life of ordinary graphite tubes was found to be extended by a factor of 8-10 as previously reported [15]. The life of pyrolytically coated tubes was approximately doubled when tested under conditions optimum for aluminum. The greater life was sufficient justification for continuation of the tantalum treatment. 3.2 Platform geometry The very simple platform fabrication procedure described earlier produced a platform with many desirable properties. Fig. 1 shows the arrangement of the platform within the tube. The grooves on the platform aid in reducing the spread of the deposited sample. Physical contact between the tube and platform is restricted to the [17] W. SI X\‘IN,Pcrkin--Elmer Corporation,
personal communication
(1980).
Reduction
of matrix interferences
in furnace atomic absorption
with the L’vov Platform
777
Tube Dimensions = 28 X 6(ID) mm Platform Dimensions = 7 X 5 nun Fig. la. The modified
L’vov Platform-side
view of platform position
within the graphite tube.
Platform
Fig. lb. The modified
L’vov Platform-end
view of platform position
within the graphite tube.
edges of the platform because of the difference between inside and outside radii of curvature of the tube. The minimal contact assures the desired lag between the wall and platform temperature during heating. At the same time, the curved platform geometry minimizes obstruction of the optical path. On our instrument the platform caused no detectable light loss from either the hollow-cathode or deuterium-arc beams. This is a significant improvement over the approximate 10% light loss reported by SLAVINand MANNING[14] for their platform design. The platform introduces no inconvenience in furnace operation. The total time increase for each analysis is a negligible 10 seconds added to the cooling time. 3.3 Sensitivity, precision and background Peak-height sensitivities were not affected significantly by the platform. The ratio of sensitivities with and without platform ranged from 0.9 for Cd to 1.3 for Co and Cr. Sensitivities were similar to those reported by others [18]. Precision was improved by the tantalum treatment and even more for some elements by the addition of the platform. Typical data for two elements are given in Table 2. Similar precision was obtained for the other four elements. There are probably two reasons for the improved precision. The platform limits the spread of the sample within the furnace and results in a more reproducible sample location. Also, because the Table 2. Precision study using pyrolytically Element Cadmium* (228.8 nm) untreated tube tantalized tube with platform Chromiumt (357.9 nm) untreated tube tantalized tube with platform
0.16~HN0,
coated graphite tubes
0.16 ~HN0~+500
pgml-’
5.44% RSD 3.33 1.85
7.24% RSD 4.09 2.86
1.14 1.17 1.32
3.57 1.62 1.80
Na
* 10 t.~l aliquot of 0.008 pg ml-’ Cd standard. t 20 ~1 aliquot of 0.06 fig ml-’ Cr standard. [18] Analytical Methods Using the HGA Graphite Furnace, Perkin-Elmer Perk&Elmer, Norwalk, CT (19781.
Service Manual for the HGA 500,
M. L. KAISERet al.
778
sample is vaporized under more nearly isothermal conditions, small variations in furnace operation had less effect on absorbance. Day-to-day variations in working curve slopes seldom exceeded 5%. Many of the samples tested showed background absorbances as high as 0.2 A. Background readings were unaffected for the numerous samples tested by either the tantalum treatment or the L’vov Platform. Reduced background due to more efficient decomposition of absorbing molecules at the higher temperature had been anticipated but was not observed. 3.4 Matrix interferences in natural waters Each element was studied on the basis of matrix interferences when vaporized from the wall of the tantalized tube, from the platform and from the platform with the added matrix modifier (0.5% NH4H,P04). As discussed earlier, the slope ratios between samples and standards were compared. A ratio larger than 1.00 showed an enhancement by the matrix, and a ratio less than 1.00 revealed a suppression. 3.4.1. Lead. The results for lead in the six USGS water samples are shown in Fig. 2. The slope ratios for all samples showed about a factor of two suppression when atomization was from the tube wall. The situation was greatly improved by the L’vov Platform and in most cases a further modest improvement was achieved by addition of the matrix modifier. The full magnitude of the effect of the modifier was not revealed by these experiments. In most cases use of the platform left little interference to be corrected and experiments with the modifier alone were not run. The bars on the far right give the average slope ratio for the six water samples, and the brackets beside each bar indicate the standard deviation of the ratio. It is obvious that the slopes are approaching the desired ratio of 1.00 and that slope variations among the samples are greatly reduced. The average slope ratio was 0.48*0.11 for atomization from the wall, 0.94*0.03 with the platform alone, and 0.96&0.03 with platform plus modifier. As mentioned earlier, the large standard deviations on the USGS values for these waters made direct comparison of values obtained an unsatisfactory way to assess matrix behavior. However, the data given in Table 3 indicate that, at least for lead, values for the water samples read directly from a working curve showed good agreement with expected values when the platform and platform plus modifier were used. 3.4.2. Cadmium. Results for cadmium are summarized in Fig. 3. Data are presented for only three samples because the cadmium concentration in the others was too high for the standard additions plot to be linear. We could have diluted the samples, of ,-
)
)_
Fig. 2. Fresh water matrix effects on lead response
in pyrolytically
coated
graphite
tubes.
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
779
Table 3. Concentration of lead in USGS standard reference water samples read directly from a working curve (values and standard deviations based on four runs over an S-month period) Atomization from Sample number
USGS value (round-robin results)
Wall
57 59 61 63 65 67
2O.Ok7.3 16.8+4.8 10.6jz3.9 4.9k3.5 38.4jz9.9 5.1*3.7
5.8kO.4 12.7ztl.6 3.3*0.4 co.1 22.2h4.8 co.1
platform 17.2bO.5 20.8kO.8 10.7*0.9 2.2~1~0.5 39.3zt3.7 2.5 kO.5
platform plus modifier 19.9+ 1.4 21.7h1.5 ll.lztO.8 2.1 ztO.3 43.2k2.1 2.7*0.2
course, but this would have probably diluted out the matrix effects we were trying to observe. Two of the three samples showed a significant suppression when atomized from the tube wall. Addition of the platform improved the results and the platform plus modifier brought the ratio of slopes very close to the desired 1.00 for all samples. Again, the bars on the far right show the average and standard deviation of the slope ratios. The improvement, while less dramatic than for lead, is still quite significant. Unlike lead, the matrix modifier offers further advantage over the platform alone for the cadmium analyses in these natural waters. 3.4.3. Cob& Results for cobalt are summarized in Fig. 4. Here enhancements relative to standards in 0.16 M HNO, are noted and in all samples except no. 57, the platform offers improvement. The platform plus modifier bring all slope ratios quite close to 1.00. 3.4.4. Chromium. Chromium results are summarized in Fig. 5. Since no severe matrix effects were seen in these samples when atomizing from the tube wall, dramatic improvements would not be expected. The improvement in the standard deviation of the slopes is probably at least as significant as the somewhat closer approach to the desired ratio. 3.4.5. Tin. Fig. 6 summarizes the tin results. The solid bars actually over-represent the response for atomization from the tube wall. The signal was essentially lost in the baseline noise when tin was added to these waters. A rather dramatic improvement was
Fig. 3. Fresh water matrix effects on cadmium response in pyrolytically coated graphite tubes.
M. L.
780
KAISER
et al.
Fig. 4. Fresh water matrix effects on cobalt response in pyrolytically coated graphite tubes.
61
63
Fig. 5. Fresh water matrix effects on chromium response in pyrolytically
57
59
USGS
61
63
Standard
Reference
water
Sample
coated
graphite
tubes.
Number
Fig. 6. Fresh water matrix effects on tin response in pyrolytically coated graphite tubes.
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
781
achieved with the platform and the platform plus modifier which brought all slope ratios very close to the desired value. Since tin solutions are unstable in nitric acid, one might ask if loss of tin by precipitation could account for the low signals in the water samples. The same solutions were used for all of these tests. Atomization from the wall was studied first, followed by platform atomization, and then platform plus modifier. Therefore, the suppressions noted must have been from the atomization process, not from reduced tin concentrations in solutions. 3.4.6. Aluminum. Only two of the water samples were sufficiently low in aluminum to give linear standard additions plots and only one of these, No. 61, showed significant deviation in the slope ratios. The ratios obtained were 0.83 with atomization from the tube wall, 0.80 with the platform, and 0.99 with platform plus modifier.
3.5 Ejrects in synthetic matrices Relative response values for the six elements in synthetic matrices are given in Table 4. The only cases of significant deterioration in performance due to the platform is for lead in the sodium and potassium matrices. This may be due to the use of peak heights instead of areas. There is some indication that the ammonium phosphate matrix modifier caused tin response to deteriorate slightly in the magnesium, sodium and potassium matrices. On the other hand, in every case where there was a large deviation from unit slope (>15%) the platform and modifier gave improved performance. Examples are lead, cobalt and aluminum in perchlorate matrix; chromium in potassium, and sulfate matrices, and tin in phosphate, sulfate and perchlorate matrices. The data given for phosphate and sulfate are from solutions prepared by dissolving KH2P0, and Fe(NH,),(SO,),, respectively. Similar data were obtained when H3P0, and H2S04 were used to prepare the matrix solutions. Therefore, the effects reported appear to be primarily due to the test anion, not the accompanying metal. Another striking aspect of the data in Table 4 is the number of matrix-analyte combinations which give relative response near one with atomization from the tube wall. That is, the number of combinations where little matrix effect was noted. There are probably two reasons for this. In all cases the dominant volatile anion was nitrate in this work. Many of the severe inter-element effects reported in the literature are in solutions high in chloride and other halogens. Another reason may be the use of tantalized tubes for the wall atomizations. For example, tin showed slope ratios of 2.48, 2.39, 1.93 and 1.29 for Ca, Mg, K and Na matrices, respectively, when atomization was from the wall of an untreated pyrolytic graphite tube.
4. %JMMARY Use of the modified L’vov Platform and matrix modification reduced matrix interferences in natural water samples. The largest improvements were for the volatile and oxide-forming elements. The test elements of intermediate volatility showed less dramatic improvement but the original matrix effects were not severe so there was little room for improvement. It is important to note, however, that there was no deterioration in performance for any analyte tested in natural waters due to the platform or matrix modifier. Therefore, a single procedure would be applicable to all elements tested which is an important consideration in an applications laboratory. With synthetic matrices, addition of the L’vov Platform and matrix modifier improved all cases where a severe matrix interference was noted. The only case where either the platform or platform plus matrix modifier did not restore the analyte response to within 10% of the response in 0.16 M HN03 was for aluminum in the presence of perchloric acid. There the improvement of about a factor of four compared with atomization from the wall still gave a slope ratio of about 0.25 which indicates that a severe suppression mechanism was still present.
0.92 1.02 1.03
1.00 1.00 1.00
wall platform platform + modifier
0.86 1.12 0.99 1.09 1.05 0.93
1.00 1.00 1.00 1.00 1.00 1.00
wall platform platform + modifier
wall platform platform + modifier
Al 0.08
Sn 0.06 kg ml-’
ml-’
1.02 1.05 1.03
1.00 1.00 1.00
wall platform platform + modifier
co 0.05 kgrnl-’
fig
1.03 0.92 1.05
ml-l
ml-’
0.90 1.03 1.04
1.00 1.00 1.00
wall platform platform + modifier
1.00 1.00 1.00
pg
pg
pg
ml-’
Ca2+ 500 pgml-’
HNO, 0.16~
Atomization condition
wall platform platform + modifier
Cr 0.06
Cd 0.008
Pb 0.03
Analyte and concentration
1.02 1.02 0.84
0.85 1.01 1.12
0.98 1.03 1.00
0.94 0.97 1.05
0.92 0.92 1.07
1.11 0.92 0.92
Mg*+ 500 kgn~-l
1.02 1.03 1.02 0.99 1.01 0.98 1.03 0.99 0.85
0.97 1.05 1.02 0.94 1.00 0.94 1.03 0.97 0.87
0.92 0.99 0.98
0.95 0.94 0.95 1.16 0.92 1.07
0.87 0.82 0.85
0.94 0.89 0.89
1.25 0.89 0.94
Na* 500 Ngml-’
Kf 500 pgrn-l
Table 4. Relative response in synthetic matrices
0.88 1.04 0.99 1.06 1.06 1.05 0.09 0.97 1.04
1.00 1.03 0.99 0.98 1.01 1.04 0.80 0.92 0.93
0.97 0.96 0.97
0.99 0.94 1.02
1.15 0.99 1.00
0.87 0.98 1.02
0.86 0.94 0.97
0.94 0.98 0.93
SW,500 pgml-’
PO:500 kgrn-’
0.35 1.07 1.04
0.08 0.26 0.24
0.80 1.01 0.91
0.92 1.00 1.01
1.07 0.98 1.05
0.86 0.82 0.92
c10; 0.12~
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
783
5. CONCLUSIONS The L’vov Platform is successful in causing the cyclically heated commercial furnaces to behave more like the constant-temperature type without loss in convenience. The combination of the platform with a simple and easily purified matrix modifier yields standard additions slopes which are within 10% of the calibration curve slopes. This method was shown to greatly reduce interference behavior in natural waters and in selected synthetic matrices with a simultaneous improvement in precision. The benefits appear to be general, which suggests that the method will be applicable in many practical analytical situations.
Specfmhimicn Acta, Vol. 368, No. 8, pp. 773-783, 1981. Printed in Great Britain.
Redaction atomic
of
matrixinterferences
absorption
in furnace
with tbe L’vov Platform
M. L. KAISER, S. R. KOIRTYOHANN, E. J. HINDERBERGER Chemistry
Department
and Environmental Trace Substances Research Columbia, MO 65211, U.S.A.
Center,
University
of Missouri,
and H. E. TAYLOR U.S. Geological Survey, Box 25046 Mailstop 407, Denver Federal Center, Denver, CO 80225, U.S.A. (Receioed 21 January
1981)
Abatraet-Use of a modified L’vov Platform and ammonium phosphate as a matrix modifier greatly reduced matrix interferences in a commercial Massmann-type atomic absorption furnace. Platforms were readily fabricated from furnace tubes and, once positioned in the furnace, caused no inconvenience in operation. Two volatile elements (Pb, Cd), two of intermediate volatility (Co, Cr) and two which form stable oxides (Al, Sn) were tested in natural water and selected synthetic matrices. In every case for which there was a significant matrix effect during atomization from the tube wall, the platform and platform plus modifier gave improved performance. With lead, for example, an average ratio of 0.48*0.11 was found when the slope of the standard additions plot for six different natural water samples was compared to the slope of the standard working curve in dilute acid. The average slope ratio between the natural water matrices and the dilute acid matrix was 0.94*0.03 with the L’vov Platform and 0.96*0.03 with the platform and matrix modifier. In none of the cases studied did the use of the platform or platform plus modifier cause an interference problem where none existed while atomizing from the tube wall. An additional benefit of the platform was a factor of about two improvement in peak height precision.
1. INTR~DU~H~N introduction in 1963, the Massmann furnace design [l] common to most commercial instruments for furnace atomic absorption (AA) has demonstrated a troublesome susceptibility to matrix interferences which can cause severe reductions or enhancements of the analyte signal. Interferences are of three basic types: spectral, physical and chemical. The spectral interferences caused by molecular absorption and light scatter have been successfully eliminated in most cases by the use of background correction systems. Physical interferences tend to alter the shape of the absorption peak by changing the appearance time (time at which absorbance due to the metal vapor first appears) and thus the appearance temperature of the analyte. This results in a change in the signal profile and thus analyte response. Examples of suggested mechanisms for physical interference include covolatilization of the analyte along with a more volatile matrix [2] and occlusion of the analyte in matrix crystals [3, 4, 51. Chemical interferences can be caused by reaction of the analyte with the hot graphite walls of the furnace to form refractory carbides and by formation of stable gaseous molecules which escape without decomposing to atoms. The molecules may be formed as the analyte is vaporized or they may result from gas phase reactions. Methods used to reduce matrix interferences, in addition to optimization of furnace parameters, have included chemical treatment of the graphite tube [6, 71 and introduction of an active gas such as hydrogen or air during a portion of the furnace heating
SINCE rrs
[l] H. -MANN, Spectrochim. Acta 23B, 215 (1968). [2] R. C. HWON, J. M. C~-~AWAYand T. PLATO,Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, PA., (November 1976). [3] R. B. CRUZ and J. C. VAN LOON, Anal. Chim. Acta 72, 231 (1974). [4] J. SMEYER+VERBEKE,Y. MICHOTQ P. VAN DEN W~NKELand D. L. WANT, Anal. Chem. 48, 125 (1976). [5] D. J. CHURELL.Aand T. R. COPELAND,Anal. Chem. 50, 309 (1978). [6] D. J. HODGES, Analyst 102, 66 (1977). [7] K. C. THOMPSON,K. WAGSTAFF and K. C. WHEATSTONE,Analyst 102,310 (1977). 773
M. L. KAISER etal.
714
cycle [8]. Another method, which was first reported by EDIGER in 1974 [9], involves the addition of chemical modifiers to alter the drying, charring, or atomization properties of the sample. Many different types of matrix modifiers have since been explored and reported in the literature. Unfortunately, matrix interferences are not completely eliminated by these methods and the method of standard additions probably remains the most frequently employed calibration method in furnace AA, particularly for samples of unknown composition. Standard additions is a very time consuming process and is limited by the quality of the blank and the linearity of the working curve. Therefore, convenient methods are needed to reduce or eliminate matrix interferences in the commercial furnaces used for AA. WOODRIFFet al. [lo] and KRASOWSKI and COPELAND[ll] have compared the constant temperature Woodriff furnace to the commercial pulse-type furnace and found the former design to be much less susceptible to interferences from matrix constituents. Consequently, investigators have been exploring methods for modifying the pulse-type design such that it will better approximate the performance of constant temperature furnaces without sacrifice in convenjence. In 1977, L’vov [12] proposed placing-the sample on a thin graphite platform within the tube of a Massmann-type furnace. This platform is heated primarily by radiation from the hot tube walls. Thus the surface temperature of the platform lags behind that of the wall and furnace atmosphere. When vaporization occurs from the platform, the tube and gaseous atmosphere h ve reached a higher and more nearly constant temperature. Matrix-dependent cLn ges in appearance time will result in reduced variation of the furnace temperatures at the time of atomization [12]. Also the fact that the gaseous furnace atmosphere is at a higher temperature will help to decompose molecular species. As a result the platform helps to eliminate both physical and chemical matrix interferences in the pulse-type furnace. GREGOIREand CHAKRABARTI [13] have studied the effects of the L’vov Platform on analyte sensitivities. They reported a general, though slight enhancement of peak height and area measurements. SLAVIN and MANNING[14] found greatly reduced interferences on lead response when the platform was used compared with vaporization from the tube wall. In this paper we describe a relatively simple platform-fabrication method and explore its effects upon interferences for six elements in synthetic and natural water matrices. 2. EXPERIMENTAL 2.1 Apparatus These studies were done on a Perkin-Elmer Model 703 atomic absorption spectrophotometer and a Perkin-Elmer HGA-500 graphite furnace. The spectrophotometer was equipped with deuterium-arc background correction. The microprocessor-controlled furnace power supply could be programmed for up to nine steps in the cycle. Each step had provision for a controlled increase in temperature to assure the smooth drying and charring of the sample. Of particular significance to this study was the provision for heating the graphite tube with maximum power, and thus maximum speed, to a preset atomization temperature. An optical sensor was used to detect when the preset temperature was reached and to trigger a switch to a power level appropriate to maintain that temperature. Read-out was on a Perkin-Elmer Model 56 strip chart recorder. Sample introduction to the furnace was performed by a Perkin-Elmer AS-1 automatic sampler. All experimentation was done with 20 ~1 sample volume and 1.5 ml rinse volume except for cadmium studies which were done using a 10 ~1 sample volume. Argon was used as the sheath and purge gas throughout. Perkin-Elmer electrodeless discharge lamps were used for lead, cadmium and tin; and hollow cathode [S] [9] [lo] [ll] [12] [13] [14]
R. D. BEATY and M. M. COOKSEY,Atom. Absorp. Newsletter 17, 53 (1978). R. D. EDIGER, Atom. Absorp. Newsletter 13, 61 (1974). L. R. ~GEMAN, J. A. NICHOLS,P. VISWANADHAM and R. WOODRIFF,Anal. Chem. 51,1406(1979). J. A. KRASOWSKIand T. R. COPELAND,Anal. Chem. 51,1843 (1979). B. V. L'VOV, Spectrochim. Acta 33B, 153 (1977). D. C. GREGOIREand C. L. CHAKRABARTI,Anal. Chem. 49, 2015 (1977). W. SLAVIN and D. C. MANNING, Anal. Chem. 51, 261 (1979).
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
775
lamps for cobalt, chromium and aluminum. A 500 W line-voltage regulator was also provided to help stabilize the line voltage. Pyrolytically coated graphite tubes (Perk&Elmer No. 290-1766) were used for most of the work though some experiments were done with uncoated tubes (P.E. No. 290-1633). Both tube types were tantalum treated (tantalized) using the method of ZATKA [15] to extend tube life. Eppendorf microliter pipeta with polypropylene tips were used for preparing standards and samples. Polyethylene sample cups were used with the AS-l automatic sampler. 2.2 Reagents Standard solutions were made using appropriate dilutions of 1000 pg ml-’ standard solutions obtained from VWR Scientific Inc. (Pb, Cd, Co and Cr) or Fisher Scientific Co. (Al and Sn). Synthetic matrix solutions for Ca, Mg, K and Na were prepared from 1000 pg ml-’ Fisher standards. Phosphate matrix solutions were prepared from reagent grade KH,PO, and also from H,PO,. The sulfate matrix solution was prepared from reagent grade Fe(NH,),(SO,), and from H,SO_,. Perchlorate matrix solutions were prepared from 70% HClO,. All standards, synthetic solutions and water samples were acidified to 0.16 M in HNO,. All solutions injected into the furnace had nitrate as the dominant anion. The effect of matrix ions except perchlorate was tested at a concentration of 500 +gml-’ because this was regarded as the maximum concentration likely to be encountered in natural water systems. Perchlorate was used at 0.12~ because this is the approximate concentration in sample solutions following perchloric acid digestions. In a few cases significant blank signals were seen but none were more than 5% of the highest standard and appropriate corrections were applied. The 5% matrix-modifier solution was prepared from reagent-grade ammonium phosphate. Impurities were removed by extraction into methylisobutyl-ketone after complexing with ammonium pyrrolidine carbodithioate. Matrix-modifier solution was diluted ten-fold to give 0.5% NH,HzPO, in the injected sample. Natural water samples from the U.S. Geological Survey (USGS) Standard Reference Water Collection were also tested. Sample numbers 57, 59, 61,63,65 and 67 were used. The major constituent concentrations were measured by inductively coupled plasma emission spectroscopy. Calcium was in the range lO45 Fg ml-‘; potassium 0.4-3.0 Fg ml-‘; magnesium 1.5-28.0 pg ml-‘; sodium 2-61 pg ml-‘. Sulfate values were in the range 13-120 pgml-’ [16]. 2.2.1. Platform design and fabrication. Platforms were fabricated from tantalized pyrolytically coated graphite tubes. Originally the platforms were made by hand. A graphite tube was broken and a piece which included the grooved-end section was filed into a curved rectangular form approximately 5 mm x 7 mm with a mass of about 58 mg. At a later date, the University of Missouri machine shop produced the platforms for this research. It was possible by this method to produce eight (4 from each end of the tube) curved, grooved graphite platforms from each tube. The platform was placed within the graphite tube and centered directly under the tube sample port. This was accomplished through the use of a locally fabricated positioning tool, constructed from an aluminum plate (25 mm x 40 mm X 3 mm) with a threaded rod (50 mm X 2.5 mm) through the center of the plate and a lock nut which allowed for correct positioning of the platform. The right quartz optical window was removed and the platform was properly positioned and the automatic sampler was adjusted to assure that the sample solution was deposited in the center of the platform. 2.2.2. Furnace optimization. As each new element was studied, optimization experiments were run to find the best temperatures and times for the three steps of dry, char and atomization. The tantalum treatment did not produce any appreciable change in tube behavior for the elements studied. Optimum conditions were close to those recommended by the manufacturer for the pyrolytically coated tubes when atomizing from the tube wall. Optimum drying conditions were set to provide a smooth, even evaporation of the solvent with no spattering. These conditions were continuously checked visually, with the aid of a dental mirror, throughout the lifetime of the graphite tubes. As the tubes aged the resistance increased and thus the actual temperatures within the furnace increased and altered the drying pattern. The best char temperatures were found by holding the atomization temperature constant and varying the char temperature. Likewise, the optimum atomization temperature was found by fixing the char temperature and varying the temperature of firing. The optimum char temperature was the maximum value which did not show loss of analyte. The higher this temperature, the more likely the matrix constituents could be decomposed. Optimized atomization temperature was the lowest temperature (to prolong tube life) which still gave a maximum peak-height signal. The maximum power mode was used for atomization and the purge gas flow was reduced from the normal flow of 300 ml min-’ to 50 ml min-’ during atomization. Charring and atomization behavior indicated that the platform temperature lagged behind tube-wall temperature by about 200°C when the wall was above 500°C. Optimum temperatures given by the manufacturer for atomization from the tube wall were increased by that approximate amount in order to retain the same analysis time per sample. The cool-down step between firings was lengthened by 10 seconds to allow the platform to return to room temperature. Table 1 provides a summary of the optimum furnace parameters for the elements studied. [15] V. J. ZATKA, Anal. Chem. 50, 538 (1978). [16] M. FISHMAN, U.S. Geological Survey, personal communication
(1980).
716
M. L.
KAISER
etal.
Table 1. Optimum furnace parameters
Wavelength
Spectral Bandpass
Element
(nm)
(nmf
Cd
228.8
0.7
Pb
283.3
Char Temp. (“C) Wall
Atomize Temp. CC)
Platform
Wall
Platform
Burn-out (“C) Wall
Platform
500
1400
1600
-
0.7
300 (lo/lo*) 500
(10/15) 700
(O/5) 2000
(O/7) 2000
-
(10/15) 1200
(O/5) 2700
(O/7) 2800
2900
2900
-
Cr
357.9
0.7
(15/15) 1000
co
240.7
0.2
(10/15) 1000
(10/15) 1200
(O/6) 2400
(O/8) 2600
(l/2) 2600
(l/2) 2700
Al
309.3
0.7
(lo/lo) 1200
(10/15) 1400
(O/6) 2700
(O/8) 2800
(l/2) 2800
(l/2) 2900
Sn
224.6
0.7
(lo/lo) 700
(10/15) 900
(O/6) 2400
(O/S) 2700
(l/2)
(l/2) -
(10/15)
(10/15)
(015)
(O/S)
* (lo/lo) indicates a ramp time of 10 s to reach the desired temperature and a hold time of 10 s at that temperature. Zero ramp time indicates use of maximum power. Drying was at 110°C (15/15) from the tube wall and 250°C (10/15) from the platform.
2.2.3. Element selection. One goal of the research was to survey interference behavior using elements with a range of properties. Therefore, two volatile elements were chosen (Pb, Cd), two of intermediate volatility (Co, Cr) and two which form rather stable oxides (Al, Sn). Molybdenum was originally chosen as one of the oxide-forming elements but the signal was suppressed so severely in the tantalized tubes that its study was abandoned. Matrix elements were selected from those likely to be encountered in natural water systems and also from common elements or ions which were known or suspected of causing troublesome interferences. 2.2.4. Measurement procedure. The parameter chosen to compare interference behavior was the variation in analyte sensitivity (working curve slope) in the presence of the matrix and in 0.16 M HNO,. In case of water samples, the slope of the standard additions curve was compared with the slope of the working curve in 0.16 M HNO,. In cases where the matrix modifier was used, the slope comparison was made with the modifier present in both samples and standards. Accepted values for most elements examined in the natural waters were available from the USGS. The results were from round-robin comparisons and in most cases the standard deviations were too large to make direct comparison of values very useful. The analyte concentration was selected to fall well within the linear response range and peak absorbances were used to construct all working curves. Peak heights were originally chosen because most users of furnace AA employ that mode rather than peak area. We learned very recently [17] that peak area measurements may have additional advantages when using the L’vov Platform but have not yet verified that observation in our own laboratory. The deuterium arc background corrector was used for all measurements and background levels were carefully monitored to assure both that the amount of background absorption was within the accurate correction range and that background problems were not made more severe by use of the L’vov Platform and the matrix modifier.
3. 3.1 Tantalum
REXJLTS
AND DISCUSSION
tube treatment
The primary advantage to tantalizing the tubes by ZATKA’S [15] method was to extend tube life. Tube life was measured by repeatedly atomizing aluminum until the sensitivity dropped to 50% of the maximum value. The life of ordinary graphite tubes was found to be extended by a factor of 8-10 as previously reported [15]. The life of pyrolytically coated tubes was approximately doubled when tested under conditions optimum for aluminum. The greater life was sufficient justification for continuation of the tantalum treatment. 3.2 Platform geometry The very simple platform fabrication procedure described earlier produced a platform with many desirable properties. Fig. 1 shows the arrangement of the platform within the tube. The grooves on the platform aid in reducing the spread of the deposited sample. Physical contact between the tube and platform is restricted to the [17] W. SI X\‘IN,Pcrkin--Elmer Corporation,
personal communication
(1980).
Reduction
of matrix interferences
in furnace atomic absorption
with the L’vov Platform
777
Tube Dimensions = 28 X 6(ID) mm Platform Dimensions = 7 X 5 nun Fig. la. The modified
L’vov Platform-side
view of platform position
within the graphite tube.
Platform
Fig. lb. The modified
L’vov Platform-end
view of platform position
within the graphite tube.
edges of the platform because of the difference between inside and outside radii of curvature of the tube. The minimal contact assures the desired lag between the wall and platform temperature during heating. At the same time, the curved platform geometry minimizes obstruction of the optical path. On our instrument the platform caused no detectable light loss from either the hollow-cathode or deuterium-arc beams. This is a significant improvement over the approximate 10% light loss reported by SLAVINand MANNING[14] for their platform design. The platform introduces no inconvenience in furnace operation. The total time increase for each analysis is a negligible 10 seconds added to the cooling time. 3.3 Sensitivity, precision and background Peak-height sensitivities were not affected significantly by the platform. The ratio of sensitivities with and without platform ranged from 0.9 for Cd to 1.3 for Co and Cr. Sensitivities were similar to those reported by others [18]. Precision was improved by the tantalum treatment and even more for some elements by the addition of the platform. Typical data for two elements are given in Table 2. Similar precision was obtained for the other four elements. There are probably two reasons for the improved precision. The platform limits the spread of the sample within the furnace and results in a more reproducible sample location. Also, because the Table 2. Precision study using pyrolytically Element Cadmium* (228.8 nm) untreated tube tantalized tube with platform Chromiumt (357.9 nm) untreated tube tantalized tube with platform
0.16~HN0,
coated graphite tubes
0.16 ~HN0~+500
pgml-’
5.44% RSD 3.33 1.85
7.24% RSD 4.09 2.86
1.14 1.17 1.32
3.57 1.62 1.80
Na
* 10 t.~l aliquot of 0.008 pg ml-’ Cd standard. t 20 ~1 aliquot of 0.06 fig ml-’ Cr standard. [18] Analytical Methods Using the HGA Graphite Furnace, Perkin-Elmer Perk&Elmer, Norwalk, CT (19781.
Service Manual for the HGA 500,
M. L. KAISERet al.
778
sample is vaporized under more nearly isothermal conditions, small variations in furnace operation had less effect on absorbance. Day-to-day variations in working curve slopes seldom exceeded 5%. Many of the samples tested showed background absorbances as high as 0.2 A. Background readings were unaffected for the numerous samples tested by either the tantalum treatment or the L’vov Platform. Reduced background due to more efficient decomposition of absorbing molecules at the higher temperature had been anticipated but was not observed. 3.4 Matrix interferences in natural waters Each element was studied on the basis of matrix interferences when vaporized from the wall of the tantalized tube, from the platform and from the platform with the added matrix modifier (0.5% NH4H,P04). As discussed earlier, the slope ratios between samples and standards were compared. A ratio larger than 1.00 showed an enhancement by the matrix, and a ratio less than 1.00 revealed a suppression. 3.4.1. Lead. The results for lead in the six USGS water samples are shown in Fig. 2. The slope ratios for all samples showed about a factor of two suppression when atomization was from the tube wall. The situation was greatly improved by the L’vov Platform and in most cases a further modest improvement was achieved by addition of the matrix modifier. The full magnitude of the effect of the modifier was not revealed by these experiments. In most cases use of the platform left little interference to be corrected and experiments with the modifier alone were not run. The bars on the far right give the average slope ratio for the six water samples, and the brackets beside each bar indicate the standard deviation of the ratio. It is obvious that the slopes are approaching the desired ratio of 1.00 and that slope variations among the samples are greatly reduced. The average slope ratio was 0.48*0.11 for atomization from the wall, 0.94*0.03 with the platform alone, and 0.96&0.03 with platform plus modifier. As mentioned earlier, the large standard deviations on the USGS values for these waters made direct comparison of values obtained an unsatisfactory way to assess matrix behavior. However, the data given in Table 3 indicate that, at least for lead, values for the water samples read directly from a working curve showed good agreement with expected values when the platform and platform plus modifier were used. 3.4.2. Cadmium. Results for cadmium are summarized in Fig. 3. Data are presented for only three samples because the cadmium concentration in the others was too high for the standard additions plot to be linear. We could have diluted the samples, of ,-
)
)_
Fig. 2. Fresh water matrix effects on lead response
in pyrolytically
coated
graphite
tubes.
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
779
Table 3. Concentration of lead in USGS standard reference water samples read directly from a working curve (values and standard deviations based on four runs over an S-month period) Atomization from Sample number
USGS value (round-robin results)
Wall
57 59 61 63 65 67
2O.Ok7.3 16.8+4.8 10.6jz3.9 4.9k3.5 38.4jz9.9 5.1*3.7
5.8kO.4 12.7ztl.6 3.3*0.4 co.1 22.2h4.8 co.1
platform 17.2bO.5 20.8kO.8 10.7*0.9 2.2~1~0.5 39.3zt3.7 2.5 kO.5
platform plus modifier 19.9+ 1.4 21.7h1.5 ll.lztO.8 2.1 ztO.3 43.2k2.1 2.7*0.2
course, but this would have probably diluted out the matrix effects we were trying to observe. Two of the three samples showed a significant suppression when atomized from the tube wall. Addition of the platform improved the results and the platform plus modifier brought the ratio of slopes very close to the desired 1.00 for all samples. Again, the bars on the far right show the average and standard deviation of the slope ratios. The improvement, while less dramatic than for lead, is still quite significant. Unlike lead, the matrix modifier offers further advantage over the platform alone for the cadmium analyses in these natural waters. 3.4.3. Cob& Results for cobalt are summarized in Fig. 4. Here enhancements relative to standards in 0.16 M HNO, are noted and in all samples except no. 57, the platform offers improvement. The platform plus modifier bring all slope ratios quite close to 1.00. 3.4.4. Chromium. Chromium results are summarized in Fig. 5. Since no severe matrix effects were seen in these samples when atomizing from the tube wall, dramatic improvements would not be expected. The improvement in the standard deviation of the slopes is probably at least as significant as the somewhat closer approach to the desired ratio. 3.4.5. Tin. Fig. 6 summarizes the tin results. The solid bars actually over-represent the response for atomization from the tube wall. The signal was essentially lost in the baseline noise when tin was added to these waters. A rather dramatic improvement was
Fig. 3. Fresh water matrix effects on cadmium response in pyrolytically coated graphite tubes.
M. L.
780
KAISER
et al.
Fig. 4. Fresh water matrix effects on cobalt response in pyrolytically coated graphite tubes.
61
63
Fig. 5. Fresh water matrix effects on chromium response in pyrolytically
57
59
USGS
61
63
Standard
Reference
water
Sample
coated
graphite
tubes.
Number
Fig. 6. Fresh water matrix effects on tin response in pyrolytically coated graphite tubes.
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
781
achieved with the platform and the platform plus modifier which brought all slope ratios very close to the desired value. Since tin solutions are unstable in nitric acid, one might ask if loss of tin by precipitation could account for the low signals in the water samples. The same solutions were used for all of these tests. Atomization from the wall was studied first, followed by platform atomization, and then platform plus modifier. Therefore, the suppressions noted must have been from the atomization process, not from reduced tin concentrations in solutions. 3.4.6. Aluminum. Only two of the water samples were sufficiently low in aluminum to give linear standard additions plots and only one of these, No. 61, showed significant deviation in the slope ratios. The ratios obtained were 0.83 with atomization from the tube wall, 0.80 with the platform, and 0.99 with platform plus modifier.
3.5 Ejrects in synthetic matrices Relative response values for the six elements in synthetic matrices are given in Table 4. The only cases of significant deterioration in performance due to the platform is for lead in the sodium and potassium matrices. This may be due to the use of peak heights instead of areas. There is some indication that the ammonium phosphate matrix modifier caused tin response to deteriorate slightly in the magnesium, sodium and potassium matrices. On the other hand, in every case where there was a large deviation from unit slope (>15%) the platform and modifier gave improved performance. Examples are lead, cobalt and aluminum in perchlorate matrix; chromium in potassium, and sulfate matrices, and tin in phosphate, sulfate and perchlorate matrices. The data given for phosphate and sulfate are from solutions prepared by dissolving KH2P0, and Fe(NH,),(SO,),, respectively. Similar data were obtained when H3P0, and H2S04 were used to prepare the matrix solutions. Therefore, the effects reported appear to be primarily due to the test anion, not the accompanying metal. Another striking aspect of the data in Table 4 is the number of matrix-analyte combinations which give relative response near one with atomization from the tube wall. That is, the number of combinations where little matrix effect was noted. There are probably two reasons for this. In all cases the dominant volatile anion was nitrate in this work. Many of the severe inter-element effects reported in the literature are in solutions high in chloride and other halogens. Another reason may be the use of tantalized tubes for the wall atomizations. For example, tin showed slope ratios of 2.48, 2.39, 1.93 and 1.29 for Ca, Mg, K and Na matrices, respectively, when atomization was from the wall of an untreated pyrolytic graphite tube.
4. %JMMARY Use of the modified L’vov Platform and matrix modification reduced matrix interferences in natural water samples. The largest improvements were for the volatile and oxide-forming elements. The test elements of intermediate volatility showed less dramatic improvement but the original matrix effects were not severe so there was little room for improvement. It is important to note, however, that there was no deterioration in performance for any analyte tested in natural waters due to the platform or matrix modifier. Therefore, a single procedure would be applicable to all elements tested which is an important consideration in an applications laboratory. With synthetic matrices, addition of the L’vov Platform and matrix modifier improved all cases where a severe matrix interference was noted. The only case where either the platform or platform plus matrix modifier did not restore the analyte response to within 10% of the response in 0.16 M HN03 was for aluminum in the presence of perchloric acid. There the improvement of about a factor of four compared with atomization from the wall still gave a slope ratio of about 0.25 which indicates that a severe suppression mechanism was still present.
0.92 1.02 1.03
1.00 1.00 1.00
wall platform platform + modifier
0.86 1.12 0.99 1.09 1.05 0.93
1.00 1.00 1.00 1.00 1.00 1.00
wall platform platform + modifier
wall platform platform + modifier
Al 0.08
Sn 0.06 kg ml-’
ml-’
1.02 1.05 1.03
1.00 1.00 1.00
wall platform platform + modifier
co 0.05 kgrnl-’
fig
1.03 0.92 1.05
ml-l
ml-’
0.90 1.03 1.04
1.00 1.00 1.00
wall platform platform + modifier
1.00 1.00 1.00
pg
pg
pg
ml-’
Ca2+ 500 pgml-’
HNO, 0.16~
Atomization condition
wall platform platform + modifier
Cr 0.06
Cd 0.008
Pb 0.03
Analyte and concentration
1.02 1.02 0.84
0.85 1.01 1.12
0.98 1.03 1.00
0.94 0.97 1.05
0.92 0.92 1.07
1.11 0.92 0.92
Mg*+ 500 kgn~-l
1.02 1.03 1.02 0.99 1.01 0.98 1.03 0.99 0.85
0.97 1.05 1.02 0.94 1.00 0.94 1.03 0.97 0.87
0.92 0.99 0.98
0.95 0.94 0.95 1.16 0.92 1.07
0.87 0.82 0.85
0.94 0.89 0.89
1.25 0.89 0.94
Na* 500 Ngml-’
Kf 500 pgrn-l
Table 4. Relative response in synthetic matrices
0.88 1.04 0.99 1.06 1.06 1.05 0.09 0.97 1.04
1.00 1.03 0.99 0.98 1.01 1.04 0.80 0.92 0.93
0.97 0.96 0.97
0.99 0.94 1.02
1.15 0.99 1.00
0.87 0.98 1.02
0.86 0.94 0.97
0.94 0.98 0.93
SW,500 pgml-’
PO:500 kgrn-’
0.35 1.07 1.04
0.08 0.26 0.24
0.80 1.01 0.91
0.92 1.00 1.01
1.07 0.98 1.05
0.86 0.82 0.92
c10; 0.12~
Reduction of matrix interferences
in furnace atomic absorption with the L’vov Platform
783
5. CONCLUSIONS The L’vov Platform is successful in causing the cyclically heated commercial furnaces to behave more like the constant-temperature type without loss in convenience. The combination of the platform with a simple and easily purified matrix modifier yields standard additions slopes which are within 10% of the calibration curve slopes. This method was shown to greatly reduce interference behavior in natural waters and in selected synthetic matrices with a simultaneous improvement in precision. The benefits appear to be general, which suggests that the method will be applicable in many practical analytical situations.