Fluorescence Indices and Their Interpretation - Aquatic Organic Matter Fluorescence - page 332

higher than 10 mg L -1 when comparing samples between instruments and Zsolnay et al.
( 1999 ) kept absorbance at 254 below 0.1 cm -1 to avoid inner filter effects when developing
the humification index. Miller et al. ( 2010 ) discussed concentration effects, showing the
change in the ex 370 nm peak (used to calculate FI) at A 254 values of 0.1 cm -1 , 0.3 cm -1 , and
1.0 cm -1 . They demonstrated the unequal impact of concentration across emission inten-
sities and that at high A 254 values inner-filer corrections do not fully remove concentration
effects. Whereas the 0.1 cm -1 and 0.3 cm -1 absorbance samples were similar, the 1.0 cm -1
sample had a noticeably different peak shape.
9.4.3 pH Effect on Fluorescence
The chemical character of the matrix in which the organic matter is dissolved can affect the
overall fluorescence of the sample. Laane ( 1982 ) identified a change in DOM fluorescence
due to pH effects and Miano and Senesi ( 1992 ) demonstrated the effect of pH on fulvic and
humic acid standards, showing that although the fluorescence emission intensity increases
with more acidic pH, it does not increase evenly across the scan. Because fluorescence
indices involve the ratio of points or areas in two different regions of the scan this means
pH can affect the value of an index. McKnight et al. ( 2001 ) collected fluorescence spectra
and calculated fluorescence index values (see Section 9.2.4 ) for solutions of fulvic acids
from 7.5 m depth in Lake Fryxell in Antarctica (microbially derived end member) and the
Suwannee River in Georgia (terrestrially derived end member provided by IHSS) at pH 2.0
and pH 7.5, to evaluate the potential for minimizing the effects of metal binding in quench-
ing fluorescence by acidifying natural water samples. At pH 2.0, the fluorescence index of
the Lake Fryxell fulvic acid sample was 1.8 and the peak emission location at an excitation
wavelength of 370 nm was 442 nm. In contrast, at pH 7.5, the fluorescence index increased
to 1.9, and the emission peak location shifted to 448 nm. At pH 2.0, the fluorescence index
of the Suwannee River Fulvic acid was 1.3 and the emission peak was at 460 nm. At pH
7.5, the fluorescence index of the Suwannee River fulvic acid increased to 1.4 and the emis-
sion peak increased to 461 nm. McKnight et al. ( 2001 ) point out that a difference in FI of ±
0.1 may be indicative of a change in source. Thus the differences at pH 7.5 and 2 approach
the difference that could represent a biogeochemical difference. Although there is not a
specific recommended pH for index calculation, it is important to consistently be aware
of pH effects and keep pH values consistent for sample comparison. Chapter 7 includes a
more detailed discussion of the effect of pH on DOM fluorescence.
9.5 Conclusions
The literature associated with fluorescence indices shows that over a period of just over a
decade, researchers in the fields of soil science, marine science, hydrogeology, and envir-
onmental science independently derived a series of fluorescence indices for characterizing
organic matter. In general, indices track either the position or intensity of the humic-like