3. Spectroscopy Experiments 29
3.5. Soil samples
3.5.5. LCF
3.5.5.1. LCF of prepared mixtures
The prepared mixtures can be classified by the complexity of their spectral features and divided up into different categories: standards of identical or similar oxidation states, standards of different oxidation states, different debris components, and debris components and soil samples.
All mixtures were prepared by ratios of 1:1 in weight-%, all used standards were diluted with pure quartz sand by 1:1000. Dilution as well as mixing was done using milligram scales. A list
70 Chapter 3. Spectroscopy Experiments of prepared mixtures is given in table 3.5. All mixtures were measured at KMC-1 at BESSY II with 0.25 eV step size and 1 s dwell time, as described at the beginning of this chapter.
no components 1 Cysteine, Pyrite 2 Pyrite, Marcasite 3 CaSO4, Al2(SO4)3
4 CaSO4, BaSO4 5 CaSO4, CuSO4
6 CaSO4, Al2(SO4)3, FeSO4
7 MgSO4, (NH4)2SO4, CaSO4, BaSO4, CuSO4, FeSO4, Na2SO4, Al2(SO4)3
8 Cysteine, CaSO4, Anthraquinone sulfonic acid 9 dc8, dc11, dc18
10 dc11, dc12, dc18 11 dc11, horizon g1 12 dc12, horizon g1
Table 3.5.: List of prepared mixtures, each mixed by ratio 1:1 in weight-%.
Since a quantitative analysis of XAS data always yields ratios in atom-%, the ratios of the prepared mixtures need to be converted. For the used standards, this can easily be done by their molar mass and number of sulfur atoms. All relevant data is given in table 3.6. For soil and debris components, the conversion is done by their total sulfur concentration in weight-%, given in tables 3.2 and 3.3. The results of linear combination fitting of all mixtures are given in figures 3.32 to 3.43. In each diagram, the measured data, the fit obtained from linear combination fitting, and the compounds present in the specific mixture are plotted on top of each other. In this way, the goodness of the fit can easily be judged qualitatively. For some fits an additional linear function was introduced, which can adjust slight normalization errors. The ratios obtained from linear combination fitting are generally given within each diagram, the actual values are given in the captions. For some mixtures, for which LCF did not yield satisfactory fits, a second diagram is plotted, in which a fit is constructed from the actual values.
Mixtures one and two, displayed in figures 3.32 and 3.33, are both mixtures of two diluted sulfide standards. Cysteine and Pyrite in mixture one (figure 3.32) are separated by a chemical shift of 1.2 eV and LCF yields a perfect fit, which gives the actual ratios of this mixture. Marcasite and Pyrite in mixture two (figure 3.33) are only separated by 0.3 eV. Both substances are partly oxidized, but Marcasite to a much bigger extend. LCF yields a significant deviation from the real values. However, a constructed fit using the actual ratios yields a very good result, except for the sulfate peak.
Chapter 3. Spectroscopy Experiments 71 Mixtures three to seven, shown in figures 3.34 to 3.38, are comprised of a different number of diluted sulfate standards. Therefore, the compounds do not exhibit any chemical shift and can only be distinguished by their pre- and post-edge features and the shape of the white line.
Mixture three (figure 3.34) consists of CaSO4, with distinct and specific post-edge features, and Al2(SO4)3, without characteristic features. LCF gives a good result, close to the actual values.
Mixture four (figure 3.35) comprises CaSO4 and BaSO4, both with characteristic post-edge fea-tures. For this mixture LCF also yields a good result.
Mixture five (figure 3.36) consists of CaSO4and CuSO4, whereat the latter exhibits a character-istic, although small pre-edge peak, but no characteristic post-edge features. Here, LCF yields a very unsatisfying result: pure CaSO4. However, a constructed fit using the actual ratios shows a good result. The deviation in the height of the white line is increased, but the fit is significantly improved in the post-edge region.
Mixture six (figure 3.37) comprises three sulfates, CaSO4, FeSO4, and Al2(SO4)3. The spectral shape of FeSO4 and Al2(SO4)3 is very similar, without any characteristic pre- and post-edge features. Therefore, both are completely interchangeable in LCF analysis. But even assuming that either FeSO4or Al2(SO4)3 could represent the sum of both, LCF gives a bad result. The fit constructed from the actual values, on the other hand, shows similar deviations to that obtained from LCF.
chemical name formula family molar mass [g/mol]
aluminum sulfate Al2(SO4)3 sulfate 342.15
ammonium sulfate (NH4)2SO4 sulfate 132.14
anthraquinone sulfonic acid C14H7NaO5S sulfonate 310.20
barium sulfate BaSO4 sulfate 233.39
calcium sulfate CaSO4 sulfate 136.11
copper sulfate CuSO4 sulfate 159.61
cysteine C3H7NO2S sulfide 121.16
dimethylsulfone (methylsulfonylmethane) C2H6O2S sulfone 94.13
iron sulfate FeSO4 sulfate 151.91
magnesium sulfate MgSO4 sulfate 120.37
marcasite FeS2 sulfide 119.98
pyrite FeS2 sulfide 119.98
sodium sulfate Na2SO4 sulfate 124.04
Table 3.6.: Summary of substances used for mixtures.
72 Chapter 3. Spectroscopy Experiments
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
C 3H 7N O 2S ( 0 . 6 7 )
F e S2 ( 0 . 3 3 )
l i n e a r
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
Figure 3.32.: Mixture 1: Cysteine (0.67), Pyrite (0.33)
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
F e S2 / P y r i t e ( 0 . 6 8 ) F e S2 / M a r c a s i t e ( 0 . 3 2 )
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
(a) LCF
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
F e S2 / P y r i t e ( 0 . 5 ) F e S2 / M a r c a s i t e ( 0 . 5 ) l i n e a r
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
(b) constructed with actual ratios Figure 3.33.:Mixture 2: Pyrite (0.5), Marcasite (0.5).
Chapter 3. Spectroscopy Experiments 73 Mixture 7 (figure 3.38) comprises eight different sulfates, some with and some without charac-teristic pre- and / or post-edge features. Obviously, this system is over-determined. By LCF, different, equally good fits can be obtained, with varying ratios of the different compounds, simply depending on their succession in the data input mask (of SIXPACK). A constructed fit using the actual ratios naturally also yields a good result.
Mixture eight, displayed in figure 3.39, consists of three sulfur standards of different oxidation states: Cysteine, an organic sulfide, Anthraquinone sulfonic acid, a sulfonate, and CaSO4, a sulfate. The compounds are well separated in energy by at least 1.2 eV. The fit obtained from LCF represents the data almost perfectly and the obtained ratios are quite close to the actual ones.
Mixtures nine and ten, shown in figures 3.40 and 3.41, comprise different debris components.
For both mixtures, LCF yields perfect fits, except for the emerging shoulder in mixture nine, which has already been discussed in section 3.3. The resulting ratios, however, significantly deviate from the actual values.
Mixture 11 (figure 3.42) consists of debris component dc11, and soil horizon g1. LCF yields very good results, in terms of a good fit and output of the exact ratios.
Mixture 12 (figure 3.43) comprises clay brick dc 12 and horizon g1. LCF yields a good fit, but ratios deviating from the actual ones. A constructed fit using the actual ratios also reproduces the data nicely, yielding an improved fit in the sulfide energy range, but increased deviation for the sulfate peak. The pronounced sulfoxide peak, however, cannot be fitted. Its origin is unclear.
Figure 3.34.: Mixture 3: CaSO4 (0.91), Al2(SO4)3 (0.09).
Figure 3.35.:Mixture 4: CaSO4 (0.59), BaSO4
(0.41).
74 Chapter 3. Spectroscopy Experiments Figure 3.36.: Mixture 5: CaSO4 (0.59), CuSO4 (0.41).
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
Chapter 3. Spectroscopy Experiments 75
Figure 3.39.: Mixture 8: Cysteine (0.72), Anthraquinone Sulfonic Acid (0.0.19), CaSO4(0.09).
76 Chapter 3. Spectroscopy Experiments
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
d c 1 8 ( 0 . 4 5 )
d c 8 ( 0 . 0 2 )
d c 1 1 ( 0 . 5 3 )
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
Figure 3.40.: Mixture 9: dc11 (0.75), dc18 (0.23), dc8 (0.02).
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
d c 1 8 ( 0 . 4 0 )
d c 1 1 ( 0 . 4 5 )
d c 1 2 ( 0 . 1 5 )
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
Figure 3.41.:Mixture 10: dc11 n(0.72), dc18 (0.21), dc12 (0.07).
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
d c 1 1 ( 0 . 9 5 )
h o r i z o n g 1 ( 0 . 0 5 )
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
Figure 3.42.:Mixture 11: dc11 (0.95), horizon g1 (0.05).
Chapter 3. Spectroscopy Experiments 77
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
d c 1 2 ( 0 . 5 4 )
h o r i z o n g 1 ( 0 . 4 6 )
l i n e a r
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
(a) LCF
2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0
d a t a f i t
d c 1 2 ( 0 . 6 7 )
h o r i z o n g 1 ( 0 . 3 3 )
Countrate [arb. units]
E n e r g y [ e V ] X A N E S
S K - e d g e
(b) constructed with actual ratios Figure 3.43.:Mixture 12: dc12 (0.67), horizon g1 (0.33).
In conclusion, it can be stated that LCF yields good results if the different species in a mixture are clearly distinguishable by either energy position of the white line (mixtures 1 and 8) or characteristic spectral features (mixtures 3, 4 and 11). The results become inaccurate (mixture 2) or even arbitrary (mixtures 6 and 7), if the involved components are too similar. However, if the goal is to rather extract the total ratio of the occurring oxidation states than to extract the ratios of each compound (e.g. each sulfate), LCF will still work (mixture 8). Small deviations of the fitted from the actual ratios are certainly due to the preparation method and experimental setup. Neither the dilution nor the mixing using a spattle and milligram scales is absolutely exact. Moreover, due to the small spotsize at KMC-1, the measurements are sensitive to local variations in sample composition and concentration. The LCF routine, on the other hand, seems to be especially sensitive to white line peak heights, as can explicitly be seen in the fitting results of mixture 5. Constructed fits using the actual ratios of the mixtures always lead to improved fits for a wide energy range, but to increased deviations in the white line peaks (and therefore to a reduced total goodness of fit).
Errors in mixtures 9 and 10 could be due to wrong values of the total sulfur concentration within the involved components.