x ray fluorescence analysis of lanthanide mixtures...
TRANSCRIPT
X‐RAY FLUORESCENCE ANALYSIS OF LANTHANIDE MIXTURES USING PARTIAL LEAST SQUARES REGRESSION
V.V.Panchuk 1,2, D.O.Kirsanov1,3, A.A.Goydenko1, M.M.Khaydukova 1,3, S.M.Irkaev2, А.V.Legin 1,3, V.G.Semenov1,2
1 Institute of Chemistry, St. Petersburg State University, St. Petersburg, Russia 2 Institute for Analytical Instrumentation RAS, St. Petersburg, Russia
3 Laboratory of artificial sensory systems, ITMO University, St. Petersburg, Russia
Chemometrics in XRF studies
Regression models in XRF
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1. Ordinary least squares for low concentration:
C=a0+a*Ianalyte
2. Polynomial Regression for high concentration:
C=a0+a1*Ianalyte+a2*I2analyte
3. Multiple regression for complex matrix effects:
m
analytejjjj
m
analytjjjjanalyteanalyte IeIbIIaIaaC
analyte,1
2
,1
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Primary X‐rayradiation
Some basic principals of X‐Ray fluorescence spectroscopy
X‐ray fluorescenceradiationh = EL‐EK
4
5
X‐Ray fluorescence lines
XRF techniques for liquid sample analysis:
Energy‐dispersive X‐Ray fluorescence (EDX)
Total External Reflection X‐Ray fluorescence technique (TXRF)
Energy dispersive X‐Ray fluorescence technique (EDX)
Sample cup
Sample
Fluorescence radiation
X‐Ray tube
Primaryradiation
Backscattered radiation
Detector
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Inte
nsity
Energy
https://www.bruker.com/7
Total External Reflection X‐Ray fluorescencetechnique (TXRF)
(Reflector)
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TXRF Sample preparation
Sample Addition of internal standard
Taking off some l
Pipettingon reflector
Drying by evaporation
Measuring the spectra
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EDX and TXRF techniquesAdvantages of EDX
‐ Low cost
‐ No sample preparation
‐ High reproducibility spectral line intensity
Advantages of TXRF
‐ Detection limit – up to 1 part per billion
‐Many types and manufacturers of spectrometers
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The purpose of the study
Is it possible to obtain high precision of TXRF with “cheap and dirty” EDX instrumentation?
TXRF = EDX + chemometrics?
Case study – quantification of several similar elements (lanthanides) in complex mixtures at low concentration – a tricky task for EDX
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Samples40 mixtures of six lanthanides : Ce Pr Nd Sm Eu Gd
Lanthanide concentrations in mixtures were varied in a range from 10−6 to 10−3 mol/L.
4f elementsSimilar electronic structureVery similar properties
Solutions of lanthanide nitrates in 0.01 M nitric acidConcentrations in the range 10−6 to 10−3 mol/L. Uniform distribution of samples in a concentration hyperspaceTo obtain denser distribution of experimental points in the low concentration range we used logarithmic concentrations in design.
30 samples for calibration 10 samples in an independent test set
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EDX measurements
EDX spectra of lanthanides' L‐series were obtained using energy dispersive X‐ray fluorescence spectrometer Shimadzu EDX‐800HS.
5ml of sample solution was placed in a sample cup and covered by polypropylene film of 20‐micrometer thickness.
Spectra were obtained with Rh X‐Ray tube at 15 kV voltage and 310 μA current. These conditions allowed for the maximum signal/noise ratio of spectral lines.
Each spectrum was acquired for 600 s.
Spectra were smoothed by Savitzky‐Golay filter, 2 polynomial order, 5 points window.
0 2 4 6 8 10 12 14
CePr
Nd
Sm
Eu
Gd
Inte
nsity
, r.u
.
Energy, keV
0,00
0,02
0,04
0,06
0,08
4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0
CePr
Nd
Sm
Eu
Gd
Inte
nsity
, r.u
.
Energy, keV
0,02
0,04
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Problems:1. low signal/noise in spectra due to small concentrations of lanthanides2. overlapping peaks because of the similar nature of the analyzed elements
EDX spectra of lanthanides
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TXRF measurementsRigaku Nanohunter spectrometer Internal standard – copper. 20 μl of sample solution was dropped onto quartz glass reflector, dried on a heating plate at 50 °C Incidence angle of excitation radiation 0.1°.Mo X‐ray tube at 50 kV voltage and 0.6 mA current.Accumulation time 300 s for each sample.Spectra was normalization on K line copper
4,0 4,5 5,0 5,5 6,0 6,5 7,0 7,5 8,0
0,0
0,5
1,0
1,5
2,0
2,5
Inte
nsity
, a.u
.
Energy, keV
Problems:1. low signal/noise in spectra due to small
concentrations of lanthanides2. overlapping peaks from similar nature of the analyzed elements
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PLSThe spectral range 4–8 keV was employed for calculations 201 variables for EDX and 545 variables for TXRF all (L‐series lines of the analyzed lanthanides)
All models were based on 3LVThis was enough to attain 99–100% of explained variance in Y (lanthanide concentration)
OLSIn case of EDX – the area under the characteristic XRF Lα line
The area values were determined with proprietary Shimadzu software PCEDX‐E version 1.02.
In case of TXRF – Lα line intensity was employed for OLS.
Data processing
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EDX Data processing
0,0000 0,0005 0,0010
0,0000
0,0005
0,0010
Measured, mol/L
Ce
Pred
icte
d, m
ol/L
Measured, mol/L
Calibration set Test set
Ce
0,0000 0,0005 0,0010
0,0000
0,0005
0,0010
Nd
EuEu
Nd
0,0000 0,0005 0,0010
0,0000
0,0005
0,0010
0,0000 0,0005 0,0010
0,0000
0,0005
0,0010
0,0000 0,0002 0,0004 0,0006 0,0008
0,0000
0,0005
0,0010
0,0000 0,0002 0,0004 0,0006 0,0008 0,0010
0,0000
0,0005
0,0010
OLS PLS
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4 , 0 4 , 5 5 , 0 5 , 5 6 , 0 6 , 5 7 , 0 7 , 5 8 , 0
G d
E u
S m
N d
P r
C e
PLS regression coefficients for EDX spectral variables
RMSEP values (mol/L) in determination of lanthanides in mixturesbased on OLS and PLS regression
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Ce Pr Nd Sm Eu Gd
EDX OLS 1.1 × 10−4 1.6 × 10−4 1.7 × 10−4 1.5 × 10−4 2.1 × 10−4 2.5 × 10−4
EDX PLS 5.3 × 10−5 4.2 × 10−5 5.3 × 10−5 6.3 × 10−5 5.1 × 10−5 2.6 × 10−5
TXRF OLS 2.9 × 10−5 2.3 × 10−5 9.1 × 10−5 1.1 × 10−4 1.1 × 10−4 7.6 × 10−5
TXRF PLS 2.9 × 10−5 2.9 × 10−5 5.4 × 10−5 7.7 × 10−5 2.2 × 10−5 1.6 × 10−5
Concluding remarks
• Due to low signal/noise ratio and overlapping signals in EDX measurements OLS fails to provide for reasonable precision in lanthanide determination especially in low concentrations.
• PLS processing of these data allowed for significant improvement of accuracy. Moreover, simple XRF method such as EDX can provide for the same precision as a more sophisticated TXRF, when the measurement's results are fitted by PLS regression.
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