Useful references for Elemental Analyzer samples
Bowen G. J., Chesson L., Nielson K., Cerling T. E. and Ehleringer J. R. (2005) Treatment methods for the determination of 2H and 18O of hair keratin by continuous-flow isotope-ratio mass spectrometry. Rapid Communications in Mass Spectrometry, 19, 2371-2378, doi:10.1002/rcm.2069.
Water extraction from soils
K. Revesz and P. Wood, A method to extract soil water for stable isotope analysis: Journal of Hydrology, 115 (1990) 397-406.
CO reference gas supplier
Oxygen Service in Orange, California makes CO for regular use with the isotope value in the natural range. Probably, they use sea water as the starting material. The price is just a fraction of other expensive suppliers. We have been using their CO for oxygen isotope reference for years.
Dachun Zhang, Lab manager
We at Menlo Park have also been struggling with d18O standards for use with pyrolysis, especially phosphates. We used two in-house standards at 11.3 and 20 per mil, both secondarily calibrated from VSMOW, but we found we run a lot of samples outside this range - so we custom-made our own standards. I started with two calibrated waters, -16.5 and +253.0 per mil, and mixed them in 5 varying proportions (0.2, 0.4, 0.6, 0.8, 1.0) to make a linear range of waters, measured them for d18O and then added to each a scoop of P2O5. I figured since P2O5 + 3H2O --> 2H3PO4, 3 out of 8 phosphate oxygens should be from the water. I neutralized the samples using NaOH and then did a fast precipitation using AgNO3. Filtered, washed 3X, dried, ground, etc... Results below:
d18O water d18O Ag3PO4
Roughly y = 0.45x + 16. Since these weren't exactly what I was looking for I repeated the process using waters of -55 and +30 per mil, and got:
I performed two calibration runs, including all 5 from the second set and #s 2 and 3 from the first set, and added NBS-18, NBS-19, USGS-34, USGS-35, IAEA-N3, NBS-127, L-SVEC, TU-1, TU-2 (phosphates from Torsten Vennemann), YR1a (low phosphate standard from Ruth Blake), and our in-house High and Low standards. I used our in-house standards for drift and linearity corrections and ran everything else as unknowns (n=4 each). With the exceptions of NBS-18 and 19, which fell slightly below the line, we got an equation (true vs. measured) of y = 0.985x + 0.164, with an R2 of 0.9993. We did notice a slight shift between nitrate standards and phosphate standards. Although they had identical slopes, the phosphate intercept was +0.5 and the nitrate intercept was -0.5. Certainly this deserves a closer look, but it appears that running mixed oxyanions may work for some labs, at least in short runs. Phosphates seem to be quite stable and combust nicely, and the fact that they can easily be customized to fit the isotopic needs of individual labs is a bonus. Our analyses were run on a Eurovector EuroEA (1270C) feeding a VG Optima mass spec, in case anyone is interested... I should also mention we are currently working on a similar method for synthesizing a range of barium sulfate standards. I'll post this method when we have the kinks worked out.
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