UCL Bloomsbury Environmental Isotope Facility

Gas Bench II

Material (change to what was analyzed….) was analyzed at the Bloomsbury Environmental Isotope Facility (BEIF) at University College London on a ThermoFinnigan DeltaPLUS XP stable isotope mass spectrometer attached to a ThermoScientific Gas Bench II device. 

Standard and unknown sample material (normally >100µg) was loaded into glass vials, methanol rinsed and kept overnight in a 70°C oven.

For analysis, each vial was manually acidified with 100% Phosphoric acid (0.1 ml) using a syringe injection via the screw cap septa.

Precision of all internal (BDH, IAEA & IFC) and external standards (NBS19) greater then 1000mv is ±0.03 for δ13C and  ±0.08 for δ18O. For all smaller standards 200-1000mv, the precision is ±0.04 for δ13C and  ±0.13 for δ18O. 

All values are reported in the Vienna Pee Dee Belemnite notation (VPDB) relative to NBS19.

• Full set of Gas Bench Protocols (external link)

Flash EA Methods

Bulk dry powdered samples are loaded into tin capsules that are placed into an autosampler and dropped into a furnace with a pulse of oxygen where they are burned. The gases are carried by the He carrier gas through an oxidant to complete the burning, chemical traps (if present) to trap sulfur and halogens, a copper reduction furnace to trap excess O₂ and convert the NOx's formed to N₂ for analysis, a drying tube (typically magnesium perchlorate) to remove water, a GC column to separate the gases (N₂ from CO₂) before flowing to the EA detector and finally via a CONFLO IV interface into the mass spectrometer.

The “Zero Blank” autosampler uses a “sealed carousel” design. All samples are purged simultaneously with He in an enclosed chamber. This eliminates any nitrogen background (from atmosphere) that is introduced in the open carousel sampler when it moves to drop the sample into the reactor. The microstepper motor that drives the autosampler can switch from a 50 position carousel to a 32 position carousel for large or bulky samples.

δ¹³C, δ¹⁵N and δ²H of organic matter by high temperature combustion in oxygen.

BEIF routinely achieve precisions better than ±0.1‰ for δ¹³C using laboratory and international standards.

TC/EA

A publication by Gehre et al. describes the experimental setup of the TC/EA for the analysis of waters in which precisions of < 0.1 ‰ for d18O and < 0.5 ‰ for d²H are obtained.

It is necessary to have a liquid injection port on the TC/EA. Thermo provides some squishy silicon septa with the liquid injection port but the thermogreen septa from Supleco seem to be more appropriate for the application. They have an optimal temperature range of 50 - 350 °C. A special adapter must be used for the reversed flow of the helium as described in the aforementioned publication. This adapter is provided by several companies including (IVA analytical and Elementar Americas). Similarly, a glassy carbon tube is needed that runs the length of the ceramic tube and fits onto the adapter. For waters, the 7 mm ID tube gives rise to peaks that are more narrow than with the 8.5 mm ID tube. The top of the glassy carbon tube should ideally have a narrow opening to prevent water vapor from backstreaming into the dead space between the aluminum injection port and the glassy carbon tube. Greater peak tailing will be observed without the narrow opening. If the glassy carbon tube does not have a narrow opening (the ones provided by the above mentioned manufacturers do not) then a graphite cylinder with a narrow (3-4 mm) opening can be used.

Some Pitfalls:
The original "liquid injection kit" came with a Hamilton Company 1.2uL syringe. These small volume syringes have a wire plunger that runs inside of the syringe needle. They need to be very well matched to prevent leaking or seizing of the plunger in the needle. It has been the experience of at least one lab that the Hamilton Co syringe gets scratched fairly easily. This can be felt as resistance to moving the plunger up and down. When this happens the seal between the plunger and needle is lost and some of the sample water will be pushed back through the syringe body during injection resulting in poor reproducibility of the injection. As far as the analytical precision of the measurement goes, the reproducibility of the injection is key. Poor injection reproducibility will result in poor precision and accuracy. The SGE 0.5uL syringe with the 23 gauge needle is available with a needle/plunger replacement kit.

δ¹⁸O and δ²H by reductive pyrolysis of organic matter on glassy carbon

δ²H by reduction of water on Cr metal