Geoscience Reference
In-Depth Information
environmental conditions, such as changing salinity, pH, or sunlight. These processes are
considered in detail in
Chapters 7
and
8
(this volume) on chemical and biological effects on
OM fluorescence, and are reviewed briefly in the context of fluorescence indices in
Section
9.4
. In the rest of this section we will consider each commonly used index and its interpre-
tation separately, along with a brief explanation of how and why it was developed.
9.2.1 A “Humification Index” to Track Chemical Properties
Developed by Kalbitz and Colleagues (HIX
SYN
)
Kalbitz el al. (
1999
) utilized spectroscopic techniques to distinguish differences in dissolved
humic substances due to variations in land use in a fen area in Germany. Groundwater
samples as well as topsoil aqueous extracts were collected from regions of arable land,
grassland, and woodland in various stages of succession. UV, fluorescence, and IR spec-
troscopic properties were tested for their suitability as an indicator of land use and modifi-
cation. They measured the fluorescent signature using synchronous scan fluorescence with
an excitation range from 260 to 560 nm and an offset of 18 nm. Scans of resin-extracted
humic and fulvic acids at 10 mg L
-1
DOC were normalized to carbon content and charac-
teristic peaks were identified at ex 360/em 378 nm and ex 400/em 418 nm excitation/emis-
sion wavelength pairs, and a shoulder at ex 470/em 488 nm. Kalbitz et al. (
2000
) compared
the peak locations in the original whole water samples to the peak locations in the fulvic
acid fractions and found that the peak locations in the whole water samples had shifted to
shorter wavelengths after isolation, which was associated with fewer highly substituted
aromatic nuclei.
Kalbitz et al. (
1999
) used fluorescence intensity ratios to calculate two humification
(HIX
syn
) indices for the extracted humic material. Each index ratios the fluorescence emis-
sion intensity at ex 360/em 378 nm against the emission intensity at ex 400/em 418 nm or
ex 470/em 488 nm. The (ex 400/em 418)/(ex 360/em 378) and (ex 470/em 488)/(ex 360/em
378) indices were shown to correlate closely with each other (
r
= 0.91,
n
= 46) and could be
reproducibly measured because they are independent of total fluorescence intensity. Kalbitz
et al. (
1999
) argued that these indices are humification ratios, whereby a higher index value
indicates a higher degree of humification, inferred from a higher degree of polycondensa-
tion and a shift to longer fluorescence wavelengths. The higher wavelength (ex 470/em
488)/(ex 360/em 378) humification index correlated inversely with the C/N ratio of the
samples (
r
= -0.63) and positively with the infrared spectra at 1620 cm
-1
(which is related
to absorbance from aromatic C=C structures;
r
= 0.71). It also correlated inversely with the
infrared absorbance ratio at 1725 cm
-1
/1620 cm
-1
(which is related to the ratio of COO- to
C=C structures;
r
= -0.69), strongly suggestive that this index is indeed a measure of poly-
condensation. For the whole water samples Kalbitz et al. (
2000
) calculated an index with a
(ex 390/em 408)/(ex 355/em 373) ratio to account for the shift to shorter wavelengths in the
spectrum. As shown in
Figure 9.2
, this was found to strongly correlate to the index for the
associated humic material (
r
= 0.89,
n
= 46,
α
< 0.001), the FTIR absorbance at 1620 cm
-1
(
r
= 0.66,
n
= 44,
α
< 0.001) and the C/N ratio (
r
= -0.48,
n
= 46,
α
< 0.01), indicating the
