TC-EA_waters

Analysis of waters on the 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.

Here is an explanation of how one/some labs have set this up.

Hardware:
 * Thermo Electron TC/EA with liquid injection port
 * CTC GC/PAL autosampler
 * SGE 0.5uL syringe
 * Isotope Ratio Mass Spectrometer with interface

TC/EA: 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.