Standards

2 4 6 5 2 4 7 0 2 4 7 5 2 4 8 0 2 4 8 5 2 4 9 0 2 4 9 5 2 5 0 0 2 5 0 5 2 5 1 0

i n o r g . s u l f i d e ( p y r i t e : F e S O ₄) o r g . d i s u l f i d e ( c y s t i n e : C ₆H _{1 2}N 2O 4S 2) o r g . s u l f i d e ( c y s t e i n e : C ₃H 7N O ₂S ) o r g . s u l f i d e ( m e t h i o n i n e : C ₅H _{1 1}N O ₂S ) s u l f o x i d e

s u l f i t e ( s o d i u m s u l f i t e : N a₂S O ₃) , p a r t l y o x i d i z e d s u l f o n a t e ( a n t h r a q u i n o n e s u l f o n i c a c i d : C _{1 4}H 7N a O ₅S ) s u l f a t e ( s t r o n t i u m s u l f a t e : S r S O ₄)

Countrate [arb. units]

E n e r g y [ e V ] X A N E S

S K - e d g e

Figure 3.2.: Sulfur XANES spectra of diluted standards of different oxidation states; spectra taken at KMC-1 at BESSY II.

Chapter 3. Spectroscopy Experiments 31 Spectra of standard substances of different sulfur oxidation states were taken (fig. 3.2), as well as spectra of different sulfur standard compounds in the same oxidation state (fig. 3.3). A list of all measured standards can be found in the Appendix. Model compounds that cover the whole range of possible oxidation states were chosen. The used standards are also typically abundant in soils, either in the organic or mineral soil fraction. Further standards represent abundant species in building rubble and war debris.

Spectra are shown in the energy range

2 4 6 0 2 4 7 0 2 4 8 0 2 4 9 0 2 5 0 0 2 5 1 0

Figure 3.3.: Sulfur XANES spectra of diluted stan-dards: various sulfates with different pre-and post-edge features; spectra taken at KMC-1 at BESSY II.

2460 eV to 2510 eV, to include the whole normalization range for each sulfur species.

All spectra in figures 3.2 to 3.4 were record-ed at KMC-1 at BESSY II with a step size of 0.25 eV and a dwell time of 1 s. Depend-ing on the signal to noise ratio, each spec-trum was recorded at least twice and at least at two different positions on the sam-ple. As described in section 2.2.3, white line positions vary in energy with oxidation state of the sulfur compound, ranging from

∼2470 eV to 2482.5 eV. Qualitatively, the rise in peak height with increasing oxida-tion state can also be seen.

Figure 3.3 illustrates the differences in pre-and post-edge features pre-and FWHM for sub-stances with the same oxidation state, name-ly sulfates. The measured standard spec-tra are needed for peak fitting procedures as reference for peak positions and FWHM, as well as for deconvolution via linear com-bination fitting. The standard substances were used as they were purchased, but di-luted to approximately the same concen-trations that were found within the soil samples (tab. 3.3). Thus, any concentra-tion dependent effects like self absorpconcentra-tion, examplarily shown for three standards in fig. 3.4, were prevented. In order to re-ceive a composition as similar as possible to the real soils, clean and pure quartz sand (Merck) was used for the dilution.

These mixtures were finely ground in a mor-tar and then applied on tape.

To exclude any influence by the described preparation methods, spectra of the sand used for dilution (fig. 3.5(a)) as well as of the used tapes (fig. 3.5(b)) were taken. The spectra in figure 3.5(a) and that of carbon tape in figure 3.5(b) were taken at KMC-1 at BESSY II, with a step size of 0.25 eV and 1 s dwell time. Kapton tape was measured at BL8 at SLRI

32 Chapter 3. Spectroscopy Experiments with a step size of 0.25 eV and a dwell time of 10 s. The spectra of the tapes in figure 3.5(b) cannot be normalized; they only show minimal traces of sulfur and can therefore be regarded as sulfur free. The visible sulfate traces may even originate in precipitation of sulfate bearing aerosols from building materials, that are known to be pervasive in buildings. Vertical lines in figure 3.5(a) represent averaged peak positions of the different sulfur species. Figure 3.5(a) also includes the spectrum of a fingerprint on carbon tape of the experimenter, to show how sensitive sample preparation for these XANES experiments actually is. The fingerprint yields a detailed spectrum of different sulfur oxidation states very similar to soil spectra. This is true for the general shape of the spectra, implying a similar distribution of oxidation states, as well as for signal strength, implying a similar sulfur concentration (compared to horizon c1, for instance).

The sand, however, yields a very noisy spectrum, indicating very low sulfur concentration. The peak height results from normalization errors typical for very noisy spectra with only traces of sulfur, cp. section 2.3. To emphasize the low sulfur concentration within the sand used for dilution, its total fluorescence signal was compared to one of the very lowly concentrated soil samples - horizon c1 (0.06 weight-%). The result is shown in figure 3.6. Si obviously originates from sand, Fe and Cu from the sample holder, all other elements are typical in soils.

2 4 6 5 2 4 7 0 2 4 7 5 2 4 8 0 2 4 8 5 2 4 9 0 2 4 9 5 2 5 0 0 2 5 0 5 2 5 1 0

o r g . s u l f i d e ( c y s t e i n e ) 0 . 1 % o r g . s u l f i d e 1 % o r g . s u l f i d e 1 0 % o r g . s u l f i d e 1 0 0 %

s u l f o n a t e ( a n t h r a q u i n o n e s u l f o n i c a c i d ) 0 . 1 % s u l f o n a t e 1 %

s u l f o n a t e 1 0 % s u l f o n a t e 1 0 0 % s u l f a t e ( C a S O ₄) 0 . 1 % s u l f a t e 1 % s u l f a t e 1 0 % s u l f a t e 1 0 0 %

Countrate [arb. units]

E n e r g y [ e V ] X A N E S

S K - e d g e

Figure 3.4.: Concentration effects on sulfur XANES spectra of standards of different oxidation states;

spectra taken at KMC-1 at BESSY II; concentrations are given in weight-% standard substance to sand used for dilution.

Chapter 3. Spectroscopy Experiments 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

org. sulfide sulfoxide sulfonate

s a n d f i n g e r p r i n t

Countrate [arb. units]

E n e r g y [ e V ] X A N E S

S K - e d g e

sulfate

(a) Sand used for dilution (Merck) and finger-print

2 4 7 0 2 4 8 0 2 4 9 0

C a r b o n T a p e K a p t o n T a p e

Countrate [arb. units]

E n e r g y [ e V ] X A N E S

S K - e d g e

(b) Used tapes

Figure 3.5.: Sulfur XANES spectra of used sample preparation equipment; spectra taken at beamline KMC-1 at BESSY II and BL8 at SLRI (Kapton Tape). The sand used for dilution yields a very noisy spectrum; the peak height results from normalization errors as described in section 2.3. The fingerprint on carbon tape yields a spectrum very similar to soil spectra.

The tapes show only traces of sulfur and are regarded as sulfur free.

34 Chapter 3. Spectroscopy Experiments

0 5 1 0

C a_Kα F e _Kβ C u _Kβ

C u_Kα

F e _Kα S Kα

P Kα

S i_Kα h o r i z o n c 1

s a n d

Countrate [arb. units]

E n e r g y [ k e V ] A l_Kα

( b )

(a) Total fluorescence spectrum of the sand used for dilution in comparison to horizon c1, both on carbon tape; Fe and Cu from sample holder spectra measured at KMC-1 at BESSY II. A zoom into the area marked by the orange box can be found in figure (b).

1 , 0 1 , 2 1 , 4 1 , 6 1 , 8 2 , 0 2 , 2 2 , 4 2 , 6 2 , 8 3 , 0

S Kα

P Kα

S i_Kα

A l_Kα

h o r i z o n c 1 s a n d

Countrate [arb. units]

E n e r g y [ k e V ]

(b) Zoom into region of interest

Figure 3.6.: Total sulfur content of the sand used for dilution in comparison to horizon c1 (0.06

weight-%); spectra measured at KMC-1 at BESSY II.

Chapter 3. Spectroscopy Experiments 35