Geoscience Reference
In-Depth Information
One cycle freeze-thaw
40
20
0
-20
-40
-60
-80
-100
T1
T2
C
A
1
2
345
Sample Reference Number
678910 11 12 13
Figure 4.4. The percent change in fluorescence intensity after one cycle of freezing/thawing in 13
freshwater samples. T1 and T2 = tryptophan-like fluorescence in the regions of
λ
ex/em 280/350 nm
and 215-220/340 nm respectively; C and A = humic-like fluorescence in the regions
λ
ex/em 380/420-
480 nm and 260/380-460 nm respectively. (Reproduced from Hudson et al.
2009
.)
absorbance properties for over 1 year (C. Swan, University of California, Santa Barbara,
unpublished data). Likewise long-term (up to 1 year) frozen storage has been routinely
used for CDOM fluorescence, absorbance and DOC analyses on open ocean samples with
no observable changes outside of analytical error (P. Coble, unpublished data).
Freezing of filtered water samples to -20°C is also a commonly used storage method for
DOM analyses (e.g., Coble et al.,
1998
; Murphy et al.,
2008
; Conmy et al.,
2009
; Walker
et al.,
2009
; Gao et al.,
2010
; Spencer et al.,
2010
;Yamashita et al.,
2010a
). Clearly the quan-
tity and quality of DOM in a sample will impact on its response to freezing and as a “gen-
eral rule of thumb” it seems that highly colored allochthonous dominated DOM samples
see much greater effects as a result of the freeze/thaw process than optically clear autoch-
thonous dominated DOM samples (e.g., marine waters), and a number of marine DOM
studies have shown minimal effects of freeze/thaw on DOM optical properties (Conmy
et al.,
2009
; Yamashita et al.,
2010b
; P. Coble, unpublished data). Spencer et al. (
2007c
)
and Hudson et al. (
2009
) found for a range of freshwater samples on freeze/thaw that
fluorescence intensities and absorption coefficients exhibited both increases and decreases
although on the whole a loss of CDOM was observed (e.g., for
a
340
the majority of samples
in Spencer et al. [
2007c
] showed a decrease and 77% of the samples exhibited a change
outside of the analytical reproducibility). As
Figure 4.4
(Hudson et al.,
2009
) highlights,
typically both protein-like fluorophores and humic and fulvic-like fluorophores intensities
decline after one freeze/thaw cycle. Spencer et al. (
2007c
) also examined the influence
of freeze/thaw on the excitation/emission properties of a number of fluorophores and for
both the fulvic and humic-like fluorophores examined (excitation maxima 320-350 and
340-390 nm and emission maxima 400-450 and 440-500 nm respectively) mean changes
across the samples were within the analytical error but individual samples exhibited up
to ± 20 nm shifts. For the humic and fulvic-like fluorophores the greatest proportion of
change was a blue shift potentially indicating a breakdown in aromatic moieties or loss due
