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Simultaneous analysis of trace concentrations of lead and arsenic by energy-dispersive x-ray fluorescence spectrometry
A&. Radiar. Isor. Vol. 44, No. 8, pp. 1101-1104. Printed in Great Britain. AH rights reserved
1993
Copyright
0969-8043193 $6.00 + 0.00 ;cl 1993 Pergamon Press Ltd
Simultaneous Analysis of Trace Concentrations of Lead and Arsenic by Energy-dispersive x-Ray Fluorescence Spectrometry DARIUSZ Faculty
of Physics
WFGRZYNEK
and Nuclear
and
Techniques, University 30-059 Krakbw,
BARBARA of Mining Poland
(Received 6 January 1993; in revised
HOlZYfiSKA
and Metallurgy,
Al. Mickiewicza
30,
form22 January 1993)
A method for the analysis of trace concentrations of lead by Energy-Dispersive x-Ray Fluorescence spectrometry when arsenic is also present in the sample is described. To apply the method, the PbLp-line, emitted from the sample to be analyzed, must be available and it cannot interfere with x-ray peaks from other constituents in the sample. An additional reference sample containing lead and no arsenic is required. The intensity ratio (Pb,,/Pb,,) is used for correction using a reference which is arsenic free. By applying the emission-transmission method (Giauque Ed al., 1973) the corrected intensities of Pb,,- and As,,-peaks are calculated for samples of unknown composition. Based on the corrected intensities the concentrations of Pb and As are determined. To verify the method the IAEA reference material, Marine Sediment
SD-M-Z/TM, was analyzed and the results are presented here.
Introduction The Energy-Dispersive x-Ray Fluorescence (EDXRF) analysis is a convenient method for the determination of trace elements. The method is especially useful in the study of environmental pollution by heavy metals. In the case of analysis of environmental samples such as soil, sediment or biological materials a minimum chemical pretreatment or even no chemical preparation of the material prior to XRF measurement is required. The fundamental parameter approach allows a wide range of materials to be analyzed. However, the EDXRF method has certain limitations. In spite of good energy resolution of modern semiconductor detectors, fully overlapped peaks cannot be resolved even with the aid of spectral deconvolution programs. In some cases, if the KUpeak of a given element is overlapped by a peak of another element, the intensity of the K/?-peak can be used directly for quantitative analysis since the fundamental parameters for KP-lines are tabulated (Krause et al., 1978; McMaster et al., 1968; Storm and Israel, 1970). The problem arises when the intensities of L-series peaks are to be used for quantitative analysis. The values of fundamental parameters are well established for La-lines only. Hence one has only the intensity of the La-peak available for quantification purposes. If this peak interferes with x-ray peaks of other elements in a
sample, serious systematic errors can be introduced in the results of analysis. This is the case in the determination of lead when arsenic is also present in a sample. The Pb,- and As,,-peaks overlap completely in the EDXRF spectrum. When the intensity of AsKp is too low or the peak is hidden under an x-ray peak of an other constituent of a sample, such as Br,,, it cannot be used for the determination of As or for the calculation of the corrected intensities of As,,and Pb,,-peaks. In this paper we describe a method for calculating the corrected intensities of PbL, and As,,-peaks when both elements are present in a sample. To apply this method for the simultaneous determination of Pb and As the intensity of Pb,, must be free of any interferences with x-ray peaks of any other elements. The intensity ratio of Pb,,- and PbLB-peaks measured for a reference sample which does not contain arsenic is utilized for correction of the intensity of the overlapped Pb,,-peak in the analyzed sample.
Theory For the determination of trace elements by EDXRF analysis (i.e. the so-called emissiontransmission method) a monochromatic excitation source was used (Giauque et al., 1973, 1979). In this method transparent samples of intermediate thicknesses are measured. The method is based on the
1101
1102
DARIUSZWFGRZYNEK and BARBARA HOLY~SKA
fundamental parameter approach except that the mass attenuation coefficients of an unknown sample are determined experimentally. This is accomplished by measuring the transmission of x-ray energies excited in a thick multielement target through the sample to be analyzed. Neglecting the enhancement effects (Wegrzynek et nl., 1993) the intensity of the j-th x-ray line of a given element emitted from an intermediate thickness sample can be described as follows: I, = I,G,e(E,)K,c,mF,
F
=
1-
J
z(E,)p,
’ Php
(4)
(la)
where: K,=w
concentration of Pb does not need to be known); unk, indicates an unknown sample to be analyzed containing As and Pb. 4fter rearrangement of equation (3) the expected intensity of Pb,,-line emitted from an unknown sample, can be expressed as:
(lb)
where Cor is a correction factor. From the above equation and equation (la) the concentration of lead in an unknown sample is given by: pnk
ew{-MEoYsin
cp+ .+J/sin ILlm) h-4%)/sin cp+ p(E,)/sin $lm
unk _
cpb - r,G~t(E,b,=)~p:,,F~~:,~,~YIIk
and I,, intensity of the primary radiation; G,, geometrical factor; c(E,), detection efficiency for the characteristic line of energy E,; w, fluorescence yield; J, jump ratio of the mass-attenuation coefficient at the absorption edge; T(&), photoelectric massattenuation coefficient of a given element for the primary radiation; p,, probability of the emission for thejth characteristic line of a given element; c, weight fraction of a given element in a sample; m, mass per unit area of a sample; p(E,), p(E,), mass-attenuation coefficients of a sample for primary radiation and for the characteristic radiation of a given element, respectively; 4, $, the angle of incidence for primary radiation and the exit angle for fluorescent radiation, respectively. The total mass-attenuation coefficient of a sample, for the primary and characteristic radiation, is calculated according to the formula:
The expected intensity of As,,-peak from the following equation: Iunk
_
ASK= =
pk Pb,., f ASK, -
cp + p(E,)/sin
$]m = In
(2)
where: I:, intensity of thejth x-ray line emitted from a thick multielement target sample; I: + ‘, intensity of the jth x-ray line emitted from the target attenuated by the sample; Is, intensity of the jth x-ray line emitted from the sample. Next, the absorption factor, F,, can be obtained. Based on equation (1 a) a system of equations can be written for calculation of the ratios of the intensities of Pb,, and PbLB-peaks for a reference sample, and similarly for an unknown sample:
(5)
can be calculated
Gt:,
ynk PbL, + ASK,
(6)
where: I%:nb:, +ASK,,the measured
intensity of the overlapped Pb,,- and As,,-peak. Based on equations (la) and (6) the concentration of arsenic in an unknown sample can be calculated: Funk [unk PbL, + AsKo -
Ibl:fl
x --!?!!2 F”“k
x
Cor
P
c rsk = hd%c(E~,,~
(7)
)K,,Kz~~~~mU”’
Because the energies of As,,- and Pb,, -peaks are very close (10.532 and 10.54 keV, respectively) we can assume that: 6 (EARN, 1=
[p(E,)/sin
x Cor
(Ic)
(84
t (Epb,,)
and
According to equation (7) and the made in equation (8) the concentration unknown sample can be expressed as:
assumptions of As in an
Funk pnk PbLz + Awa
-
c ysk =
And finally by combining
&Ffl
x
--!% x Funk PbLp
equations
Cor
(5) and (9):
K C$ = cgk x - Ph. K ASK.
Iunk X
where: ref, indicates a reference sample which contains Pb but is free of As (in this sample the
PbLn+Aw. I
Pbu
Funk 1 Pbu X__, Funk PbL. Car
(10)
In the following section we will show the experimental results of analysis using the proposed method.
Simultaneous
analysis
1103
of Pb and As
. . .
6
. . . . . .
+AsK a9
Bra
.
P
.
. .
J
:
l
. =i
w
14
13
12 Energy
15
[keV]
Fig. 1. A region of the x-ray spectrum of the reference material, IAEA Marine Sediments SD-M-2/TM
(counting time = 20,000 s).
Experimental Testing of the method was performed with an XRF system consisting of a Si(Li) detector with the resolution of 180 eV at 5.9 keV coupled to Canberra S- 100 System integrated with an IBM PS/2 microcomputer. As the excitation source ‘@‘Cdof activity of 0.37GBq was used. The net x-ray peak intensities were calculated using the AXIL software (Van Espen et al., 1986). All the samples were prepared in the form of pressed pellets. The mass of a sample was obtained by weighing. Single element standard samples, composed of an appropriate element oxide mixed with cellulose were used for the determination of the combined geometry and detection efficiency factors, (I,G,+(E,)). As a target, a thick multielement sample containing Ca, Ti, Mn, Zn, Pb and Zr, in oxide forms, mixed with cellulose was used. The reference sample consisting of about 1% of Pb (as lead oxide) and 99% of cellulose was prepared as a pressed pellet having the mass equal to about 100 mg. The samples to be analyzed were prepared from the IAEA referTable 1. The calculated
Sample
I 2 3 4 5
concentrations
* t + f f
2.st 2.7 2.6 2.9 2.6
Results and Discussion In Fig. 1 a region of the x-ray spectrum of a sample containing the reference material is shown. As can be noticed the only evidence of the presence of arsenic in the sample is that the intensity of Pb,,-peak is much higher than the intensity of the Pb,,. In the absence of arsenic the intensities of both Pb,,- and PbrB-peaks should be comparable. The AsKB-peak is masked by Br,,-peak. The results of the determination of lead and arsenic, by the proposed method, are listed in Table 1.
of Pb and As in the IAEA
Calculated concentrations of Pb without taking into account the presence of As in the sample (ppm) 58.7 56.1 54.2 58.9 55.2
ence material, Marine Sediments SD-M-Z/TM, by mixing 300 mg of the reference material with 300 mg of cellulose. The mixture was homogenized in an agate mortar. From this mixture 5 pellets, having a known mass of about 100mg each, were prepared. The reference sample was measured for 1000 s. The samples containing the reference material were measured for 20,000 s each.
Reference Material
Calculated concentrations of Pb taking into account the presence of As in the sample (ppm) 22.2 23.7 21.8 22.2 25.1
f 2.9 + 2.9 k 2.8 + 2.9 & 2.7
SD-M-2/TM*
Concentration of As calculated according to equation (IO) (ppm) 21.0 18.7 18.6 21.2 17.3
* f + f f
3.0 2.9 2.9 3.1 2.8
*The certified values of concentrations of Pb and As at 95% confidence level: Pb, 22.8 ppm (confidence interval: 21.2-25.6 ppm); As, 18.3 ppm (confidence interval: 17.4-19.3 ppm). tThe errors in the table are due to counting statistics. AR1 4418-C
DARIUSZ W~GRZYNEK and BARBARAHOCYP~SKA
1104
As can he seen, the concentrations
obtained
for
Pb are in good agreement with the certified values. Also the results for As, calculated according to equation (lo), shows reasonable agreement with the reference data. However. the standard deviations. for concentrations of arsenic, are higher than those for Pb because the intensities of As,,-peaks were calculated from the difference of the intensities according to equation (6). The presented data clearly show that serious systematic errors can be introduced in the analysis of Pb if one does not take into account the presence of As. This method is especially useful when the concentration of As is much lower than that of Pb and the As,,-peak is hardly noticeable in the spectrum.
References Giauque R. D., Goulding F. S., Jaklevic J. M. and Pehl R. H. (1973) Trace element determination with semiconductor detector x-ray spectrometers. Anal. Chem. 45, 67 1.
Giauque R. D., Garret R. B. and Coda L. Y. (1979) Determination of trace elements in light element matrices by x-ray fluorescence spectrometry with incoherent scattered radiation as an internal standard. Anal. Cbem. 51. 511. Krause M. O., Nestor C. W. Jr., Sparks C. J. Jr and Ricci E. (1978) X-ray fluorescence cross sections for K and L x-rays of the elements. Oak Ridge National Laboratory ORNL-5399. McMaster W. H.. Delarand N. K. and Hubbel J. H. (1968) Compilation of x-ray cross sections. Lawrence Radiation Laboratory Report UCRL50174. Storm E. and Israel H. I. (1970) Photon cross sections from 1 keV to 100 MeV for elements Z = 1 to Z = 100. Nucl. Data Tables Al, 565.
Van Espen P., Janssens K. and Nobels J. (1986) AXIL-PC software for the analysis of complex x-ray spectra. Chemo. Lob. 1, 109. Wegrzynek D., Holyriska B. and Pilarski T. (1993) The fundamental parameter method for energydispersive x-ray fluorescence analysis of intermediate thickness samples with the use of monochromatic excitation. X-Ray Spectromelry 22 (accepted for publication).
1993
Copyright
0969-8043193 $6.00 + 0.00 ;cl 1993 Pergamon Press Ltd
Simultaneous Analysis of Trace Concentrations of Lead and Arsenic by Energy-dispersive x-Ray Fluorescence Spectrometry DARIUSZ Faculty
of Physics
WFGRZYNEK
and Nuclear
and
Techniques, University 30-059 Krakbw,
BARBARA of Mining Poland
(Received 6 January 1993; in revised
HOlZYfiSKA
and Metallurgy,
Al. Mickiewicza
30,
form22 January 1993)
A method for the analysis of trace concentrations of lead by Energy-Dispersive x-Ray Fluorescence spectrometry when arsenic is also present in the sample is described. To apply the method, the PbLp-line, emitted from the sample to be analyzed, must be available and it cannot interfere with x-ray peaks from other constituents in the sample. An additional reference sample containing lead and no arsenic is required. The intensity ratio (Pb,,/Pb,,) is used for correction using a reference which is arsenic free. By applying the emission-transmission method (Giauque Ed al., 1973) the corrected intensities of Pb,,- and As,,-peaks are calculated for samples of unknown composition. Based on the corrected intensities the concentrations of Pb and As are determined. To verify the method the IAEA reference material, Marine Sediment
SD-M-Z/TM, was analyzed and the results are presented here.
Introduction The Energy-Dispersive x-Ray Fluorescence (EDXRF) analysis is a convenient method for the determination of trace elements. The method is especially useful in the study of environmental pollution by heavy metals. In the case of analysis of environmental samples such as soil, sediment or biological materials a minimum chemical pretreatment or even no chemical preparation of the material prior to XRF measurement is required. The fundamental parameter approach allows a wide range of materials to be analyzed. However, the EDXRF method has certain limitations. In spite of good energy resolution of modern semiconductor detectors, fully overlapped peaks cannot be resolved even with the aid of spectral deconvolution programs. In some cases, if the KUpeak of a given element is overlapped by a peak of another element, the intensity of the K/?-peak can be used directly for quantitative analysis since the fundamental parameters for KP-lines are tabulated (Krause et al., 1978; McMaster et al., 1968; Storm and Israel, 1970). The problem arises when the intensities of L-series peaks are to be used for quantitative analysis. The values of fundamental parameters are well established for La-lines only. Hence one has only the intensity of the La-peak available for quantification purposes. If this peak interferes with x-ray peaks of other elements in a
sample, serious systematic errors can be introduced in the results of analysis. This is the case in the determination of lead when arsenic is also present in a sample. The Pb,- and As,,-peaks overlap completely in the EDXRF spectrum. When the intensity of AsKp is too low or the peak is hidden under an x-ray peak of an other constituent of a sample, such as Br,,, it cannot be used for the determination of As or for the calculation of the corrected intensities of As,,and Pb,,-peaks. In this paper we describe a method for calculating the corrected intensities of PbL, and As,,-peaks when both elements are present in a sample. To apply this method for the simultaneous determination of Pb and As the intensity of Pb,, must be free of any interferences with x-ray peaks of any other elements. The intensity ratio of Pb,,- and PbLB-peaks measured for a reference sample which does not contain arsenic is utilized for correction of the intensity of the overlapped Pb,,-peak in the analyzed sample.
Theory For the determination of trace elements by EDXRF analysis (i.e. the so-called emissiontransmission method) a monochromatic excitation source was used (Giauque et al., 1973, 1979). In this method transparent samples of intermediate thicknesses are measured. The method is based on the
1101
1102
DARIUSZWFGRZYNEK and BARBARA HOLY~SKA
fundamental parameter approach except that the mass attenuation coefficients of an unknown sample are determined experimentally. This is accomplished by measuring the transmission of x-ray energies excited in a thick multielement target through the sample to be analyzed. Neglecting the enhancement effects (Wegrzynek et nl., 1993) the intensity of the j-th x-ray line of a given element emitted from an intermediate thickness sample can be described as follows: I, = I,G,e(E,)K,c,mF,
F
=
1-
J
z(E,)p,
’ Php
(4)
(la)
where: K,=w
concentration of Pb does not need to be known); unk, indicates an unknown sample to be analyzed containing As and Pb. 4fter rearrangement of equation (3) the expected intensity of Pb,,-line emitted from an unknown sample, can be expressed as:
(lb)
where Cor is a correction factor. From the above equation and equation (la) the concentration of lead in an unknown sample is given by: pnk
ew{-MEoYsin
cp+ .+J/sin ILlm) h-4%)/sin cp+ p(E,)/sin $lm
unk _
cpb - r,G~t(E,b,=)~p:,,F~~:,~,~YIIk
and I,, intensity of the primary radiation; G,, geometrical factor; c(E,), detection efficiency for the characteristic line of energy E,; w, fluorescence yield; J, jump ratio of the mass-attenuation coefficient at the absorption edge; T(&), photoelectric massattenuation coefficient of a given element for the primary radiation; p,, probability of the emission for thejth characteristic line of a given element; c, weight fraction of a given element in a sample; m, mass per unit area of a sample; p(E,), p(E,), mass-attenuation coefficients of a sample for primary radiation and for the characteristic radiation of a given element, respectively; 4, $, the angle of incidence for primary radiation and the exit angle for fluorescent radiation, respectively. The total mass-attenuation coefficient of a sample, for the primary and characteristic radiation, is calculated according to the formula:
The expected intensity of As,,-peak from the following equation: Iunk
_
ASK= =
pk Pb,., f ASK, -
cp + p(E,)/sin
$]m = In
(2)
where: I:, intensity of thejth x-ray line emitted from a thick multielement target sample; I: + ‘, intensity of the jth x-ray line emitted from the target attenuated by the sample; Is, intensity of the jth x-ray line emitted from the sample. Next, the absorption factor, F,, can be obtained. Based on equation (1 a) a system of equations can be written for calculation of the ratios of the intensities of Pb,, and PbLB-peaks for a reference sample, and similarly for an unknown sample:
(5)
can be calculated
Gt:,
ynk PbL, + ASK,
(6)
where: I%:nb:, +ASK,,the measured
intensity of the overlapped Pb,,- and As,,-peak. Based on equations (la) and (6) the concentration of arsenic in an unknown sample can be calculated: Funk [unk PbL, + AsKo -
Ibl:fl
x --!?!!2 F”“k
x
Cor
P
c rsk = hd%c(E~,,~
(7)
)K,,Kz~~~~mU”’
Because the energies of As,,- and Pb,, -peaks are very close (10.532 and 10.54 keV, respectively) we can assume that: 6 (EARN, 1=
[p(E,)/sin
x Cor
(Ic)
(84
t (Epb,,)
and
According to equation (7) and the made in equation (8) the concentration unknown sample can be expressed as:
assumptions of As in an
Funk pnk PbLz + Awa
-
c ysk =
And finally by combining
&Ffl
x
--!% x Funk PbLp
equations
Cor
(5) and (9):
K C$ = cgk x - Ph. K ASK.
Iunk X
where: ref, indicates a reference sample which contains Pb but is free of As (in this sample the
PbLn+Aw. I
Pbu
Funk 1 Pbu X__, Funk PbL. Car
(10)
In the following section we will show the experimental results of analysis using the proposed method.
Simultaneous
analysis
1103
of Pb and As
. . .
6
. . . . . .
+AsK a9
Bra
.
P
.
. .
J
:
l
. =i
w
14
13
12 Energy
15
[keV]
Fig. 1. A region of the x-ray spectrum of the reference material, IAEA Marine Sediments SD-M-2/TM
(counting time = 20,000 s).
Experimental Testing of the method was performed with an XRF system consisting of a Si(Li) detector with the resolution of 180 eV at 5.9 keV coupled to Canberra S- 100 System integrated with an IBM PS/2 microcomputer. As the excitation source ‘@‘Cdof activity of 0.37GBq was used. The net x-ray peak intensities were calculated using the AXIL software (Van Espen et al., 1986). All the samples were prepared in the form of pressed pellets. The mass of a sample was obtained by weighing. Single element standard samples, composed of an appropriate element oxide mixed with cellulose were used for the determination of the combined geometry and detection efficiency factors, (I,G,+(E,)). As a target, a thick multielement sample containing Ca, Ti, Mn, Zn, Pb and Zr, in oxide forms, mixed with cellulose was used. The reference sample consisting of about 1% of Pb (as lead oxide) and 99% of cellulose was prepared as a pressed pellet having the mass equal to about 100 mg. The samples to be analyzed were prepared from the IAEA referTable 1. The calculated
Sample
I 2 3 4 5
concentrations
* t + f f
2.st 2.7 2.6 2.9 2.6
Results and Discussion In Fig. 1 a region of the x-ray spectrum of a sample containing the reference material is shown. As can be noticed the only evidence of the presence of arsenic in the sample is that the intensity of Pb,,-peak is much higher than the intensity of the Pb,,. In the absence of arsenic the intensities of both Pb,,- and PbrB-peaks should be comparable. The AsKB-peak is masked by Br,,-peak. The results of the determination of lead and arsenic, by the proposed method, are listed in Table 1.
of Pb and As in the IAEA
Calculated concentrations of Pb without taking into account the presence of As in the sample (ppm) 58.7 56.1 54.2 58.9 55.2
ence material, Marine Sediments SD-M-Z/TM, by mixing 300 mg of the reference material with 300 mg of cellulose. The mixture was homogenized in an agate mortar. From this mixture 5 pellets, having a known mass of about 100mg each, were prepared. The reference sample was measured for 1000 s. The samples containing the reference material were measured for 20,000 s each.
Reference Material
Calculated concentrations of Pb taking into account the presence of As in the sample (ppm) 22.2 23.7 21.8 22.2 25.1
f 2.9 + 2.9 k 2.8 + 2.9 & 2.7
SD-M-2/TM*
Concentration of As calculated according to equation (IO) (ppm) 21.0 18.7 18.6 21.2 17.3
* f + f f
3.0 2.9 2.9 3.1 2.8
*The certified values of concentrations of Pb and As at 95% confidence level: Pb, 22.8 ppm (confidence interval: 21.2-25.6 ppm); As, 18.3 ppm (confidence interval: 17.4-19.3 ppm). tThe errors in the table are due to counting statistics. AR1 4418-C
DARIUSZ W~GRZYNEK and BARBARAHOCYP~SKA
1104
As can he seen, the concentrations
obtained
for
Pb are in good agreement with the certified values. Also the results for As, calculated according to equation (lo), shows reasonable agreement with the reference data. However. the standard deviations. for concentrations of arsenic, are higher than those for Pb because the intensities of As,,-peaks were calculated from the difference of the intensities according to equation (6). The presented data clearly show that serious systematic errors can be introduced in the analysis of Pb if one does not take into account the presence of As. This method is especially useful when the concentration of As is much lower than that of Pb and the As,,-peak is hardly noticeable in the spectrum.
References Giauque R. D., Goulding F. S., Jaklevic J. M. and Pehl R. H. (1973) Trace element determination with semiconductor detector x-ray spectrometers. Anal. Chem. 45, 67 1.
Giauque R. D., Garret R. B. and Coda L. Y. (1979) Determination of trace elements in light element matrices by x-ray fluorescence spectrometry with incoherent scattered radiation as an internal standard. Anal. Cbem. 51. 511. Krause M. O., Nestor C. W. Jr., Sparks C. J. Jr and Ricci E. (1978) X-ray fluorescence cross sections for K and L x-rays of the elements. Oak Ridge National Laboratory ORNL-5399. McMaster W. H.. Delarand N. K. and Hubbel J. H. (1968) Compilation of x-ray cross sections. Lawrence Radiation Laboratory Report UCRL50174. Storm E. and Israel H. I. (1970) Photon cross sections from 1 keV to 100 MeV for elements Z = 1 to Z = 100. Nucl. Data Tables Al, 565.
Van Espen P., Janssens K. and Nobels J. (1986) AXIL-PC software for the analysis of complex x-ray spectra. Chemo. Lob. 1, 109. Wegrzynek D., Holyriska B. and Pilarski T. (1993) The fundamental parameter method for energydispersive x-ray fluorescence analysis of intermediate thickness samples with the use of monochromatic excitation. X-Ray Spectromelry 22 (accepted for publication).