USGS

U.S. Geological Survey Open-File Report 01-257

By Kinga M. Revesz, Jurate M. Landwehr, and Jerry Keybl


METHODOLOGY AND EQUIPMENT

The new method discussed here makes use of a continuous flow isotope ratio mass spectrometer (CFIRMS), the Thermoquest-Finnigan Delta Plus XL. Attached to this mass spectrometer is a preparation device (Thermoquest GasBench II) with a robotic sampling arm by which the sample ultimately is sent to the mass spectrometer (Fig. 1). However, the calcium carbonate samples first must be properly prepared.

The sample initially was dried in an oven at 90C overnight to prevent any moisture from reacting with carbon dioxide and exchanging an oxygen atom. This drying needed to be done only once, as long as the sample was kept in a tightly sealed container when not in use. The sample vessels, 12.5 x 100 mm borosilicate glass, also were dried in a 90C oven for at least 24 hours. Once the vessels were removed from the oven, they were capped immediately to keep out the moisture. Each glass vessel was washed before reuse. Each set of vessels was rinsed eight times with tap water and once with de-ionized (DI) water. An ultrasonic bath then was filled with DI water and all of the vessels were submerged. The vessels were cleaned in the ultrasonic bath for 30 minutes, changing the water three times. The vessels then were removed, emptied and, dried as described previously.

Each dried sample was weighed on a microbalance in an aluminum boat with a target weight of 400 20 g. Each sample then was transferred quantitatively to a clean and dried sample vessel and capped with a rubber septum (Labco Limited, Pierceable Rubber Wad). The rubber septum retains an airtight seal after being punctured with a needle. A sample set, consisting of up to 94 vessels containing calcium carbonate, was loaded into the GasBench II auto sampler. For each sample in the set, two aliquots were run. The carbon and oxygen isotope ratios of these two aliquots were compared later to see if they were within the accepted range (0.1 and 0.2 per mill for symbol13C and symbol18O, respectively); if not, additional aliquots were analyzed. Vessels containing one of three isotopically different kinds of reference materials also were interspaced among the samples. No less than one set of reference materials for every eight unknowns was analyzed. The weights of reference materials were in the same range as that of the samples.

After the set of samples and reference materials were assembled, vessels were loaded into the GasBench II tray. The GasBench II preparation device as well as CFIRMS were controlled by the Isotope Data (ISODAT) computer program. Sample information, including sample ID and weight, were added to the queue of the ISODAT program. Vessels containing acid also were added to the tray. While carbonate and acid were in the GasBench II, the tray was kept at a constant 26C. This way the acid temperature was identical to that of the sample before it was added. The GasBench II then automatically flushed the samples with helium using the flushing needle to inject helium and displace the air contained above the samples. Helium is the carrier gas for the CFIRMS; it is inert and reacts with neither the sample nor the mass spectrometer. Helium has a mass (4), which is substantially different than the mass of CO2 (44; 45; 46).

After the flushing process was complete, acid was added to the calcium carbonate. To prevent any oxygen atoms from exchanging with the carbon dioxide, only 100 percent phosphoric acid (1.906 g/cc) was used. A gastight syringe was used to transfer 0.1 ml of acid into each sample vessel. Care was taken to keep the septum acid free. The samples were left to react with the acid for 24 hours and the resulting gases were analyzed automatically over night in sequential sample order. The "method" in ISODAT was defined in such a way that each set of seven sample gas injections was bracketed by three reference gas injections, before and after each set.

The next day the sample vessels were removed and the data were transferred from ISODAT to a laboratory information management system (LIMS) (Coplen, 2000), where the final sample values were computed. For each sample, LIMS computed the symbolX value relative to a working standard, (symbol13Cwstd or symbol18Owstd) (equation 2), by defining the Rstd as the average of the six independently computed reference gas ratio that bracketed each sample set. A correction factor was applied to symbolX to obtain the symbol13CVPDB and symbol18OVSMOW. This correction factor was computed as the linear fit between the analytic results for the three reference standard materials and their known reference values relative to VPDB and VSMOW, respectively. If the seven computed symbol values of each of the two sample aliquots are not within the accepted ranges (0.1 and 0.2 per mill for symbol13C and symbol18O, respectively), the samples were re-analyzed. A step-by-step procedure and a detailed set up for the CaCO3 method in ISODAT are given in Appendixes A and B, respectively.

The reaction conditions ultimately chosen after a series of lab experiments were to maintain the acid reaction (1) at a constant temperature of 26C and (2) for a duration of 24 hours. Originally the samples were reacted at 65C, approximately the temperature suggested by the equipment manufacturer. At that temperature, reacted overnight, the carbon isotope ratios for the reference materials were reproducible and accurate, but the oxygen isotope ratios for the reference materials were neither accurate nor reproducible. Shortening the reaction time improved the reproducibility and accuracy of the oxygen isotope ratios of the working standard material (Figure 2). Using a lower reaction temperature (R.Yam, private communication, 2000) and a 24-hour reaction time allowed reproduction of isotope standards for both elements, carbon and oxygen (Figure 3). The fact that the length of the reaction affected oxygen isotope ratios, unlike the carbon isotope ratios, suggests that an exchange reaction for the oxygen isotope had been occurring during the overnight acid reaction period. Since the shortened reaction time of 1.5 hours gave acceptable results, this secondary reaction must have a kinetic component. Therefore, lowering the reaction temperature should inhibit or slow down this secondary reaction.


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