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Optimization of X-ray Fluorescence Spectrometry for Environmental Analysis of Arsenic at Low Concentrations in Sediment and Soil Materials
Pickering, Jennifer; Tonish-Prince, Jessica; Cares, James; Cribb, Warner – Dept. of Geoscienece, Middle Tennessee State University, Murfreesboro, Tennessee
Abstract
The metal arsenic (As) is a health threat to people living in regions where As-bearing minerals are present in rocks and soils. According to U.S. EPA, consumption of As at concentrations greater than 60 parts per million (ppm) can cause death. Lower doses can cause health problems ranging from cardiovascular ailments to skin diseases. As is widely distributed throughout Earth’s crust in sulfide minerals. When eroded from rock, those minerals become concentrated in soils and stream sediments. People living in underdeveloped countries are at particular risk for As poisoning, as their agricultural and domestic activities are more likely to result in consumption of As-bearing soil and sediment. This research focuses on optimization of x-ray fluorescence spectrometry (XRF) methods for measurement of As at concentrations less than 20 ppm. Measurement of As by XRF at low concentrations is difficult because the most intense As XRF energy level is very close to two XRF energy levels of lead (Pb), a metal commonly present with As in minerals. The proximity and intensities of the Pb energy levels to that of As effectively ‘absorb’ the As XRF energy, making its measurement at low concentrations unreliable. A secondary As energy level with no Pb interference can instead be measured, but with decreased analytical sensitivity. For environmental applications, it is important that the analytical sensitivity be high. Our research shows that sensitivity of As analysis is optimized by independent measurement of both Pb energy levels and the primary As energy level, followed by mathematical corrections for the Pb intensity absorption effects. This model was calibrated and tested using 10 USGS and NIST standards containing between 0.12 and 18.9 ppm As. Results of this research are applicable to a wide range of environmental studies focusing on the origin, concentration, and distribution of As in soils and sediment.
Hypothesis
Based upon observation of regression curve errors for a low As concentration USGS standard (RGM-1 = 3.0 ppm), an intensity correction for the PbLα XRF peak reduces the analytical uncertainty for As when measured as AsKα, as compared to: a) analysis of the same peak with no correction for Pb interference, b) correction for the intensity of PbLα, c) correction for the PbLα + PbLβ peaks, d) correction for (PbLα + PbLβ) + PbLα peaks.
The Importance of Accurate Measurement of Arsenic Concentrations in Rock and Soil Material
According to the Agency for Toxic Substances and Disease Registry, ingesting Arsenic (As) can cause health problems ranging from nausea to death. Smaller amounts will cause vomiting, reduced red and white blood cell counts, damaged blood vessels, irregular heartbeats, or a “pins and needles” feeling in the hands and feet. The skin may also turn black, and warts and sores may appear on the hands, feet, and torso. Ingesting larger amounts of As may cause cancer in the liver, lungs, and bladder. Some studies have shown that As exposure leads to high blood pressure and diabetes. In 1975, 50 ppb (parts per billion) As in drinking water was deemed an acceptable amount to safely ingest. After more studies had been conducted on the issue, the Clinton administration proposed a reduction from 50 ppb to 10 ppb. That proposal was suspended by the Bush administration after much criticism from mining and wood-treating industries, as well as criticism from the National Rural Water Association, which expressed concerns about a price increase in drinking water. Though there have been some cases of arsenic poisoning in North America, mostly in the western United States, it is much more common in the poorer countries of Asia. A 2007 study found that over 137 million people in more than 70 countries are drinking arsenic-contaminated water.
Problems Associated with X-ray Fluorescence Analysis of Arsenic
XRF analysis of As is complicated by the close proximity of a strong Pb Lα emission wavelengths to the highest energy emission wavelength of As: Kα energy (keV) λ (nm) Arsenic Kα 0.1176 10.54Lead Lα 0.1175 10.55
The effect of the high intensity Pb Lα emission line is to ‘absorb’ the lower intensity As Kα line. This reduces the accuracy of XRF measurement of As at low concentrations. Accordingly, most XRF analysis of As utilizes the As Kβ emission line (11.73 keV) which is not influenced by any Pb emission line. However, the As Kβ emission line has a much lower intensity than the Kα line also reducing the accuracy of As measurement at low concentrations.
As Ka Regression Curves As Kb Regression Curves
Lead Interference Correction
A mathematical correction for the influence of the Pb Lα emission line on the As Kα emission line:Xu = Ao + m Iu (K1I1 + K2I2 +
K3I3 + .......KzYz)
Xu = concentration of As in the unknownAo = intercept of As regression curve with calculated As concentration axis (y-axis)m = slope of As regression curveIu = intensity of As photon measurement in unknownK1,2,3...x = known concentration of Pb in standard #1, #2, #3, etc.I1,2,3...x = intenstity of Pb photon measurement in standard #1, #2, #3, etc.
Test of Hypothesis
The XRF spectrometer was calibrated using seven USGS and NIST standards, each with a known concentration of As.
U.S. Geological Survey Standard RGM-1 (As = 3.0 ppm) was analyzed 50 times by XRF. Each analysis independently measured the following peak positions: AsKα, AsKβ, PbLα, and PbLβ.
Linear regression of calibration data was carried out in four separate Pb correction models.
1) No correction for Pb interference.2) Correction for the intensity of PbLα. 3) Correction of the intensities of both the PbLα and PbLβ peaks. 4) Correction for intensity for the PbLα + PbLβ peaks plus an added correction for the PbLα peak
The measured RGM-1 As concentration was recalculated for each of the four methods.
Standards Used to Test Regression Model
Standard As (ppm) Pb (ppm)NIST 1944 18.9 330NIST 2586 8.7 432USGS W-2 1.2 9.3USGS QLO-1 3.5 20USGS STM-1 4.6 18USGS CLB-1 13 5.1USGS RGM-1 3 24
RGM-1 Regression Errors
Emission Line None PbLα PbLα + Lβ PbLα + Lβ + PbLαAsKα 48.57% 0.90% 23.15% 2.57%
AsKβ 104.04% 92.07% 90.09% 35.48%
% Error = (Corrected Concentration/3.0) x 100RGM-1 As Reference Value = 3.0 ppm
Model Results for AsKα Intensities
Model Results for AsKβ Intensities
Correction Med. (ppm)
Mean (ppm)
Ave. Error
St. Dev. Conf. (95%)
None 4.55 4.54 13.6 0.097 0.027
PbLα 3.52 3.58 10.44 0.408 0.113
PbLα + PbLβ 4.24 4.24 6.02 0.398 0.11
(PbLα + PbLβ) + PbLα 3.39 3.38 15.5 0.415 0.115
Correction Med. (ppm)
Mean (ppm)
Ave. Error St. Dev. Conf. (95%)
None 4.52 4.42 10.46 1.953 0.541
PbLα 4.02 4.63 15.66 6.103 1.691
PbLα + PbLβ 5.18 5.05 26.18 0.74 0.205
(PbLα + PbLβ) + PbLα 4.57 4.63 15.87 1.588 0.44
Acknowledgements
The MTSU x-ray fluoresce and ICPMS labs are funded by the National Science Foundation. We gratefully acknowledge the assistance of Beth Weinman, Vanderbilt University Dept. of Earth and Environment Sciences.
References: http://www.essortment.com/all/arsenicpoisonin_rljn.htmhttp://www.organicconsumers.org/corp/arsenic.cfm
Correction: None
As STM-1As QLO-1
As W-2
As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
m
Correction: None
As STM-1
As QLO-1
As W-2
As RGM-1As CLB-1
As NIST 2586
As NIST1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
m
Correction: PbLa
As STM-1
As QLO-1
As W-2As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
m
Correction: PbLa
As STM-1As QLO-1
As W-2
As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
mCorrection: PbLa+b
As STM-1As QLO-1
As W-2
As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
mCorrection: PbLa+b
As STM-1
As QLO-1
As W-2
As RGM-1As CLB-1As NIST
2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Calc
ula
ted
Co
ncen
trati
on
pp
m
Correction: Pbla & Pbla+b
As STM-1As QLO-1As W-2
As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Known Concentration ppm
Calc
ula
ted
Co
ncen
trati
on
Correction: PbLa & PbLa+b
As STM-1
As QLO-1As W-2
As RGM-1
As CLB-1
As NIST 2586
As NIST 1944
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
16.00
18.00
20.00
0.00 5.00 10.00 15.00 20.00
Known Concentration ppm
Calc
ula
ted
Co
ncen
trati
on
pp
m
Conclusion
1. Measurement of AsKα intensities result in lower average analytical uncertainty than measurement of AsKβ intensities. 2. Measurement of AsKα intensities result in lower analytical error deviations than
measurement of AsKβ intensities.3. Corrections for interference of the PbLα and/or (PbLα + PbLβ) intensities with the AsKα peak result in significantly lower analytical errors as compared to analysis of the AsKα peak with no correction for Pb interference.4. The Pb interference correction model allows more accurate environmental analysis of As, and is applicable to geological and toxicological studies on the origin, concentration, and distribution of As in soils and sediments.
