Acid fractionation during carbonate digestion with phosphoric acid –
Assessment of two different techniques applied for clumped and stable
isotope analysis using a Tuneable Infrared Laser Differential Absorption
Spectrometer (TILDAS)
Abstract
Rationale: Aerodyne Tunable Infrared Laser Differential Absorption
Spectrometer (TILDAS) allows for the measurement of carbonate-clumped (∆
638) and stable oxygen isotope ratios (δ
628) without the mass interference of
17O, and understanding acid fractionation factors
during phosphoric acid digestion is essential for inter-laboratory data
comparison. Here, we present δ 628 and ∆
638 ratios in CO 2 generated during
phosphoric acid digestion of a reference carbonate at different
temperatures. Methods: Carrara Marble (MAR-J1) calcite is digested with
⁓104% phosphoric acid using a) Break Seal method is a modified version
of a McCrea-type reaction vessel where complete equilibration is
achieved between product CO 2 and H 2O
generated from the digestion and b) Individual Acid Bath (IAB) where the
samples are digested with fresh aliquot of acid each time and the
product CO 2 and H 2O simultaneously
frozen in a U-trap connected to the reaction chamber by liquid N
2. Results: The regression equations for acid
fractionation of δ 628 are: a) Break Seal Method: 1000
ln α (dig. temp- 25℃) = (0.532 ± 0.030) × 10
6/T 2 + (-6.185 ± 0.308) ; R
2 = 0.98. b) IAB: 1000 ln α (dig. temp-
25℃) = (0.359 ± 0.027) × 10 6/T 2 +
(-4.114 ± 0.247) ; R 2 = 0.97. The regression
equations for acid fractionation of ∆ 638 are: a) Break
Seal Method: 1000 ln α (dig. temp - 25℃) = (0.0189 ±
0.0017) × 10 6/T 2 + (-0.2151 ±
0.0174) ; R 2 = 0.95. b) IAB: 1000 ln α
(dig. temp - 25℃) = (0.0529 ± 0.0041) × 10
6/T 2 + (-0.6072 ± 0.0356) ; R
2=0.97. Conclusion: The shallow slope of the IAB
method for δ 628 and the steeper slope for ∆
638 indicate minimal resetting of the isotope
composition of product CO 2 due to re-equilibration or
interaction with free H 2O molecules. This observation
enables inter-laboratory comparisons of δ 628 and ∆
638 in calcite.