Post-Processing

On the old MAT252, we have measured differences in isotopic composition between carbonate standards measured on the two parallel extraction lines of the Kiel II Device before correction to VPDB. These differences changed as a function of sample size and isotopic composition. For all standards greater than 0.8 volts, the difference between the A line and B line was 0.03 for d¹⁸O and 0.02 for d¹³C with the B line producing the more enriched values. As a result, we treated each extraction line separately for converting to VPDB, using only the standards analyzed on each line to make the conversion.

Two new Finnigan MAT253 mass spectrometer systems for the analysis of calcium carbonate samples were purchased by the Paleoceanography research group at WHOI in 2003 and 2004. They were installed by Finnigan in March 2004 and have been in operation since July 2004. The purpose of this page is to document the procedures and capabilities of this instrument. During the interval between March and July 2004, we analyzed more than 1200 weighed carbonate standards (NBS18, NBS19, Carrara, B1 Q, AtlantisII, B1). The precision of 600 NBS19 analyses (all sizes from 15 to 300µg) was ±0.08 for d¹⁸O and ±0.03 for d¹³O. Small sample precision (<20µg and under all laboratory conditions) is ±0.10 for d¹⁸O and ±0.05 for d¹³C. Based on the standard analyses, we have developed an empirically-based calibration of our measurements to PDB which includes corrections for the following analytical bias:

• Differences in the mean value of our standards through time
  
• These differences are caused by variations in our analytical procedures (length of reaction, trap temperatures during reactions and transfers, duration of transfers) and caused by failures of some important components during sample analyses (traps that did not achieve cold enough temperatures for compete gas transfers). The magnitude of these variations is 0.15 ^o/oo for d¹⁸O and 0.04 for d¹³C.
  

After correcting for these biases, the precision for all analyses of NBS19 is ±0.07 for d¹⁸O and ±0.03 for d¹³C. For samples between 10 and 20 µg, the precision is poorer, but still an acceptable ±0.117 for d¹⁸O and ±0.05 for d¹³C.