Geoscience Reference
In-Depth Information
recommended that investigators then document their storage procedures and any tests they
conducted on storage effects in the methods sections of reports and publications. As there
is no general consensus on a preferred storage method,
Section 4.4.2
highlights a number
of current common practices and describes the effects of the storage method used for each
study on specific DOM types.
4.4.2 Refrigeration and Freezing
Refrigerators and freezers used to store DOM samples should not be used to store other
biological samples, as some volatile organic compounds can pose a contamination risk.
Caps on bottles should be screwed on tightly to prevent leakage, evaporation and contam-
ination and bottles are recommended to be stored in an upright position. Bottle caps can
be wrapped with Teflon tape after filling to provide a more secure seal. To avoid break-
ing bottles on freezing, do not overfill (i.e., allow room for expansion on freezing) and if
possible cool the samples first via refrigeration to avoid breakage due to thermal shock.
Storage blanks should be carried out to examine any potential artefacts. Although glass
is a preferred storage container due to its ease of cleaning (see
Section 4.2.3
) Teflon and
some “aged” plastic containers when cleaned appropriately (see
Section 4.2.3
) are more
suitable for frozen storage owing to the greatly reduced chance of breakage in comparison
to glass.
Refrigeration in the dark at approximately 4°C is commonly used for short-term stor-
age of filtered DOM samples (e.g., Coble et al.,
1998
; Baker,
2002
; Stedmon et al.,
2003
;
Wickland et al.,
2007
; Fellman et al.,
2009
; Hood et al.,
2009
; Lapworth et al.,
2009
)
and a NASA study showed no changes in CDOM absorption for samples refrigerated for
less than 24 hours (Mitchell et al.,
2000
).
Figure 4.3
shows that in a range of freshwa-
ter samples the protein-like fluorophore (tryptophan-like fluorophore; typical excitation
maxima 270-280 nm and emission maxima 335-360 nm) and the prominent fulvic-like
fluorophore (typical excitation maxima 310-370 nm and emission maxima 410-460 nm)
(Coble et al.,
1998
; McKnight et al.,
2001
; Baker,
2002
; Stedmon et al.,
2003
; Spencer
et al.,
2007b
) showed no change within analytical reproducibility in the first 7 days of stor-
age (
Figure 4.3
). After 7 days of storage all of the freshwater samples exhibited a decrease
in both fluorophores outside analytical reproducibility and at the 2-month stage show a
decrease of 10-35% in relation to initial fluorescence intensity dependent on sample type
and fluorophore under investigation (
Figure 4.2
) with typically a greater decrease in the
tryptophan-like fluorophore relative to the fulvic-like fluorophore. This is similar to results
reported by Hudson et al. (
2009
), who also observed a decline in fluorescence intensity
over time in refrigerated samples and found greater removal of protein-like fluorescence
compared to humic and fulvic-like fluorescence. It is therefore suggested that refrigerated
samples be analyzed within 1 week. This guideline though will be very dependent on water
type, and relevant hold times should be determined for each study specific to the range of
DOM types under investigation. For example, open-ocean 0.2 μm filtered seawater sam-
ples from the Equatorial Pacific have been stored at 4°C with no measurable change in
