『Abstract
This study is the first systematic comparison of the effect of
acid treatment methods on the reliability of organic carbon [C]
and nitrogen [N], and carbon isotope (δ13C) values
on a range of terrestrial and aquatic, modern and geological environmental
materials. We investigated the 3 most common methods; (i) acidification
followed by sequential deionised water rinses (“rinse method”);
(ii) acidification in silver capsules (“capsule method”); and
(iii) acidification by exposure to an acid vapour (“fumigation
method”). We also investigated the effect of sample size and capsule
type (silver and tin) on C/N ratio and δ13C values.
We find (i) that %C, %N, C/N and δ13C showed significant
within and between method variability; (ii) disproportionate and
non-linear offsets of %C, %N and C/N values after acidification
within and between methods and within and between sample materials;
(iii) that alteration in %C did not necessarily manifest themselves
in shifts in δ13C, and vice-versa; (iv) that small
(〜90μg C) sample sizes showed consistent overestimations and inaccuracies
after acidification; (v) that the effect of capsule type was not
significant on most samples, but did show a notable effect on
our aquatic materials, generally increasing %C and %N, and producing
depleted δ13C values. These findings raise cause for
concern on the interpretative nature of C/N ratios and their support
for carbon isotope values. The comparability between laboratories
(different preparation methods) and environmental settings (amount,
type and nature of OM) is also likely to be problematic. We conclude
that the response of C and n concentrations in organic matter
to acid treatment in environmental materials is neither negligible
nor systematic.
Keywords: δ13C; C/N ratio; Organic matter; Acid treatment;
Method comparison; Environmental reconstruction』
1. Introduction
2. Sample materials
3. Sample preparation methods
3.1. Cleaning protocol
3.2. Acid reagents
3.3. Acidification methods
3.3.1. Rinse method
3.3.2. Capsule method
3.3.3. Fumigation method
4. Analytical methods
4.1. C/N ratio and δ13C
4.2. Supernatant analysis
5. Data analysis
6. Results
6.1. Determination of carbon blank
6.2. Effect of sample size on %C and %N
6.3. Small versus normal sample sizes in our sample materials
6.4. Sample material results
6.4.1. Materials from known values
6.5. Other sample materials
7. Discussion
7.1. Method offsets
7.2. Effect of capsules
7.3. Small versus normal sample size
7.4. Proportionality
7.5. Dissolution of IC
7.6. C/N ratio
8. Summary and recommendations
8.1. Recommendations
Acknowledgements
References