Geoscience Reference
In-Depth Information
seems it can be a suitable storage method and should be examined with suitable testing of
samples relative to those encountered in the study.
4.4.3 Poisoning - Acidification
The addition of chemicals to poison organisms that may alter analytes of interest (e.g., dis-
solved organic matter optical properties) has been applied in a range of studies. Poisoning
of samples can be very useful in preserving sample integrity in situations where no refrig-
eration or frozen storage is possible (e.g., remote field locations without access to power).
Four poisoning chemicals are commonly used to preserve natural water samples; acidifi-
cation to pH ~ 2-3 (typically HCl or H
3
PO
4
), chloroform (CHCl
3
), sodium azide (NaN
3
),
and mercuric chloride (HgCl
2
) (Kaplan,
1992
; Kirkwood,
1992
; Benner and Hedges,
1993
;
Ferrari et al.,
1996
; Wiebinga and de Baar,
1998
; Kattner,
1999
; Gardolinski et al.,
2001
;
Aufdenkampe et al.,
2007
; Hur et al.,
2007
; Bouillon et al.,
2009
; Stubbins et al.,
2010
).
When adding any chemicals as potential preservatives conduct necessary blanks and utilize
high purity grade chemicals (e.g., reagent or A.C.S. grade). Of the four commonly described
poisoning chemicals only the effects of acidification have been examined extensively on
DOM optical properties. Chloroform is viewed as problematic owing to issues with volatil-
ity that can result in losses from poorly fitting seals and even directly through some types of
plastic bottles (Kremling and Brugmann,
1999
). Mercuric chloride has lost favor recently
owing to issues with contamination of concurrent measurements of mercury when any
form of mercury is used in restricted areas (e.g., on board a research vessel) (Kremling and
Brugmann,
1999
). Furthermore, both mercuric chloride and sodium azide are very toxic to
aquatic organisms and may cause long-term adverse effects in the aquatic environment, and
as such any water containing them should be treated as hazardous waste.
A number of studies have shown addition of mercuric chloride to inhibit microbial
growth in samples without any effect on CDOM absorption spectra (Kratzer et al.,
2000
;
Helms et al.,
2008
; Spencer et al.,
2009
), whereas Hg(II) has been shown to quench DOM
fluorescence, particularly protein-like fluorescence (Fu et al.,
2007
; Yamashita and Jaffe,
2008
; Osburn et al.,
Chapter 7
, this volume). Sodium azide has been utilized to prevent
degradation of CDOM absorption during storage and has been shown to have no effect
in some studies (Ferrari et al.,
1996
; Astoreca et al.,
2009
) but also to cause up to a 10%
increase in
a
442
(Tilstone et al.,
2002
). Patel-Sorrentino et al. (2002) examined the effect of
sodium azide addition to fluorescence excitation-emission matrices (EEMs) from a range
of rivers (black and white waters) in the Amazon Basin and reported no effect on the fluo-
rescence intensity of the two humic-like fluorophores defined in their study (excitation
maxima 220-260 and 320-350 nm and emission maxima 420-450 and 420-500 nm).
Acidification of CDOM samples is often carried out to avoid microbial degradation during
storage and because low pH reduces the potential complexation between DOM and metals
(Westerhoff et al.,
2001
; Chen et al.,
2003
; Hur et al.,
2007
; Hiriart-Baer et al.,
2008
). A
classic response in freshwater DOM fluorescence to pH was observed by Patel-Sorrentino
et al. (2002), who reported an increase in fluorescence intensity with increasing pH over the
