Evidence for bias in C and N concentrations and δ13C composition of terrestrial and aquatic organic materials due to pre-analysis acid preparation methods

We investigated the effects of sample acidification on the stable carbon and nitrogen isotopic composition (delta13C and delta15N), as well as the organic carbon (OC) and total nitrogen (TN) composition, of an algal culture and a marine sediment. Replicate measurements of untreated and acid-treated samples were made using 1 M, 2 M and 6 M HCl, 6% H2SO3 and 1 M H3PO4. For all treatments the precision of the analysis for the acid-treated sample was equal to or less than that in the non-acidified sample. For the algae, analysis of variance (ANOVA) indicated no significant differences in the mean OC and TN concentration, or delta13C and delta15N composition, between any acid treatment and non-acidified samples. For the sediment sample a comparison could only be made between the different acid treatments because the untreated contained significant amounts ( approximately 30%) of carbonate carbon. ANOVA indicated that the mean OC determined in sediment samples after the 1 M HCl treatment and the mean delta13C values after the 6% H2SO3 and 1 M H3PO4 treatments were significantly different (p < 0.013 and < .05, respectively) from all other treatments. Mass balance calculations indicate that in some instances delta13C values were biased due to a contribution from unreacted carbonate carbon. There were no significant differences in the mean TN between any acid-treated and non-acidified samples. The mean delta15N values after 6 M HCl, 6% H2SO3 and 1 M H3PO4 treatments were significantly different from the untreated sediment sample (p < 0.044). Based on the significant bias observed for the delta15N and delta13C values, a weak (1-2 M) HCl solution is confirmed as the most appropriate acid for the removal of inorganic carbon from natural materials requiring elemental and isotopic analysis.

The effect of acidification on the determination of organic carbon, total nitrogen and their stable isotopic composition in algae and marine sediment.

Acidification of a solid sample to separate inorganic carbon (IC) from organic carbon (OC) is a widely encountered procedure in limnology and oceanography. Traditionally, OC was isolated to determine the weight-% of OC (%OC), but it is becoming increasingly common to determine the isotopic signatures of the OC (δ13COC and Δ14COC). This raises a need for a closer scrutiny of the currently used acidification methods. First, because IC and OC typically have distinct carbon-isotopic signatures, δ13COC and Δ14COC values can be compromised if IC is not completely removed. Second, it is possible to isotopically fractionate a sample if a small portion of OC is lost during acidification, because OC itself is both chemically and isotopically heterogeneous. This study evaluated two acidification methods by HCl—in the vaporous (HClvap) and aqueous (HClaq) phases—to determine %OC, δ13COC, and Δ14COC in coastal sediments. Each method was assessed according to the criteria that it (1) has low blank levels, (2) is able to remove dolomite, (3) yields accurate %OC, and (4) yields accurate δ13COC and Δ14COC values. HClvap fulfilled all criteria, given that the samples were not overexposed to acid. Overexposure led to underestimation of δ13COC and Δ14COC values. HClaq gave similar results but was less reliable in that it consistently underestimated %OC and yielded inaccurate δ13COC value for one test sample. It is recommended that an optimal acid exposure is carefully determined for each sample type to obtain most accurate δ13COC and Δ14COC values.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lom.2008.6.254

Carbonate removal from coastal sediments for the determination of organic carbon and its isotopic signatures, δ13C and Δ14C: comparison of fumigation and direct acidification by hydrochloric acid

1. Stable isotopes of carbon and nitrogen are mainstay tracers in diverse fields of ecology, particularly in studies of food webs. 2. Investigators are generally interested in tracing dietary C and N, and hence routinely remove non-dietary, inorganic C contained in calcified structures (e.g. shells, bones) by chemical dissolution of the carbonates. Acid treatment can, however, isotopically fractionate samples if part of the organic matter is lost or chemically modified, resulting in potentially altered d13C and d15Nvalues. 3. Here, we synthesize the effects of acid treatments on stable isotope analysis reported in the literature, showing that: (i) the method can change both d13C and d15N values; (ii) shifts in d13C are generally, but not always, consistent with expectations of more depleted carbon ratios after the removal of the isotopically heavier inorganic carbonates; (iii) nitrogen ratios either decrease or increase in 15N content; and (iv) the majority (74-79% of comparisons) of reported changes to d13C and d15N values attributable to acid treatment are <1%, but larger acid effects can occur. 4. Acidification is needed if mechanical removal of calcified structures is unfeasible in carbonate-rich sample matrices containing low organic C and N, but should otherwise be very carefully considered before its use as a routine pre-treatment step of biological samples in isotope ratio mass spectrometry.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1111/2041-210X.12183

Effects of acid treatment on carbon and nitrogen stable isotope ratios in ecological samples: a review and synthesis

Fluvial bed sediments represent an important sink and source for a variety of organic and inorganic compounds. Their most important constituent is organic matter (OM) and its primary component organic carbon (OC). Few studies have been conducted in fluvial environments examining bed-associated OM or OC. This is surprising given the recent interest in global carbon cycling and the importance of bed-associated organics as ecosystem energy sources. The objective of this study was to examine the relationship between OM, determined by loss-on-ignition (LOI), and OC in fluvial bed sediments determined by a dry combustion analyzer. The wide adoption of the LOI method in soil science reflects its ease of use, it is inexpensive, it is rapid, requires no specialized training, and strong statistical relationships commonly exist between OM and OC estimated by standard dry combustion procedures. Regression models were developed between OC and OM for six bed sediment size fractions (≤2.0 mm) for 113 sample sites in a tropical stream on Oahu, Hawaii. All models were highly significant (p < 0.0001), with coefficients of determination ranging from 35 to 79%. Measurement of LOI explained 64% of the variation in OC for all grouped data. The black-box LOI approach may be useful for rapid reconnaissance surveys of drainage systems. Examination of OM to OC conversion factors for Manoa bed sediments indicates that values typically observed in the soils literature (1.7–2.2) are far too low. Values of OM/OC were found to increase with increasing grain size, and decrease with increasing LOI percentage. Conversion factors obtained for grouped data had a mean of 14.9, a coefficient of variation of 21%, and a range of values between 6.2 and 27.4. It is suggested that these high conversion factors reflect significant water loss by dehydration of Fe, Al, and Mn oxides at a muffle furnace temperature of 450 °C. Therefore, the blind application of conversion factors developed from soils should be avoided when converting from OM to OC for fluvial bed sediments.

Loss-on-ignition estimates of organic matter and relationships to organic carbon in fluvial bed sediments

Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment: an interlaboratory comparison

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Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment: an interlaboratory comparison