Interesting Customizations

What's in this page:
I.    Using the Dual Inlet in continuous flow applications
i.      Adding the Dual Inlet configuration to the Gasbench or Conflo configuration
ii.     Using the Dual Inlet as a reference gas source for continuous flow applications
iii.    Changing the acquisition.isl script to reset the bellows pressure at the beginning of each sample analysis
iv.    Studies of isotope ratio of gas in bellows over time (fractionation of gas in bellows)

I.    Using the Dual Inlet in continuous flow applications

Our MAT 253 is equipped with a dual inlet as well as continuous flow inlets. It was of interest in the lab to use the reference bellows of the dual inlet as a reference gas source for continuous flow analyses. This would prove particularly useful when the analyte gas is something expensive or particularly toxic, such as CO. In order to use the dual inlet in conjunction with a continuous flow peripheral it was necessary to change the instrument configuration in Isodat. As a gas bellows will continuously bleed the gas that is in it we decided to write an ISL script in order to reset the bellows pressure at the beginning of each analytical run. This was done with significant help from Peter Weigel at Thermo. After getting all of this to work, it was necessary to carry out some studies to see how the isotope ratio of the reference gas in the bellows would change over time.

Below is the step by step procedure for doing what we did. Advance at your own risk...

i.      Adding the Dual Inlet configuration to the Gasbench or Conflo configuration
First, to use the dual inlet along side of a continuous flow device go to the configurator and add the dual inlet to the "source" tab of the configuration in question. In Acquisition or Workspace, make a backup copy of your specific ISL acqusition file (for example, global\user\GasBench\ISL\acquisition.isl).

After that, from within the method editor just click on the "edit" tab next to the ISL acquisition file and change the contents of the "Main" field. This is what mine looks like (with a few added comments to clarify some of this): (note that it was not necessary to include the DualInlet_Lib.isl)

ii.     Using the Dual Inlet as a reference gas source for continuous flow applications
Our tests of the stability of the isotope ratio of the gas in the bellows over time were inconclusive, perhaps due to temperature variations in the lab at that time. Since those early experiments the stability of our laboratory environment has greatly improved, however, we have not had the resources to return to these tests. Consequently, as of April 2007, we do not use the dual inlet as a source of reference gas for continuous flow applications.

gasbench-DualInlet_panel.JPG

iii.    Changing the acquisition.isl script to reset the bellows pressure at the beginning of each sample analysis
The acquisition ISL script file we used is here. A more detailed explanation of what the script does is here.

Remember that the gas flow from the dual inlet is restricted by crimps so that the sample and reference gases will bleed out at the same rate, thus minimizing fractionation between the two over time. Hence, the isotope ratio of the gas in the bellows will change over time, but the sample gas will follow it so that the difference between the two remains constant.

For continuous flow applications, we typically use a reference gas from a large cylinder. In a 24 hour period the gas from the cylinder should not fractionate in any noticeable way. The stability of the isotope ratio of the gas in the bellows was tested by measuring it against a reference gas pulse from the gasbench. In an 18 hour experiment monitoring H2 we find that its isotope ratio varies too much to be useful for a long sequence of continuous flow samples. See the data below for more info.

Similar experiments with CO2 suggested that there is a strong temperature effect on the isotope ratio of the CO2 originating from either the continuous flow source or the dual inlet. Most likely this is from the temperature-dependent fractionation associated with the phase change from liquid to gaseous CO2 in the pressurized CO2 cylinder. However, we have not yet performed the right set of experiments to verify and qualify this.

iv. Studies of isotope ratio of gas in bellows over time (fractionation of gas in bellows)

In order to use the bellows of the dual inlet as a source of reference gas for a continuous flow application, then the isotope ratio of the gas in the bellows must remain stable or it should vary in a way that can be corrected for. We did some studies over 24 hour periods to see what happens to the isotope ratio of the gas in the bellows. We used the GasBench reference gas as the point of reference and measured the isotope ratio of the bellows gas with respect to it. For more details on the experimental setup, contact the lab manager.

Hydrogen

Hydrogen as a continuous flow reference gas from the bellows. Here are plots showing the variation of the isotope ratio of the H2 in the left and right bellows. The script shown earlier in this page was used to reset the pressure in each bellows to give a 5V signal throughout the day. The bellows reached their minimum volume after around 18 to 19 hours and this can be seen in the plots.

The top two plots show that the H2 in the bellows fractionates as the bellows empty. This is not surprising of course. This indicates that H2 will not be a good continuous flow reference gas for more than a couple of hours as its isotope ratio will drift significantly over time. The other thing to notice is that the bellows of our dual inlet emptied in 18 hours. That also indicates a limit to the amount of time that it can be used unless the bellows is refilled every once in a while.

The second set of plots simply show the beam intensity of the measurements over time. Clearly, as the bellows volume is reduced its pressure drops faster and this is seen between the time that the bellows voltage is set to 5V and the time that the reference gas is actually measured.

The bottom left plot shows the difference in the isotope ratio of the gas in each of the bellows over time. This shows the ingenuity of the design of the dual inlet. Although the gas in both bellows is fractionating, it fractionates at the same rate as both are set in the factory to bleed at about the same rate. Hence, it is possible to make high precision measurements of the gas in one bellows relative to the gas in the other bellows over time. In the data shown here, the standard deviation of the measurement is around 0.35�. Although the same gas was filled into both bellows, we were not careful to prevent fractionation when filling each bellows and this is seen by the difference between the two gases of around -0.4�. We will carry out another study at some point to determine exactly how long one should wait for gas to equilibrate isotopically when opening valves on the dual inlet.

The bottom right plot shows a rolling average of the data in the lower left plot. This indicates a slightly cyclic variation in the difference of isotope ratios of the gas in each bellows.

The raw data used to generate these plots is here.

Dual_Inlet_H2_fixed_pressure 1.gif

CO2

The raw data for the results of a similar experiment with CO2 are here. I think that the left bellows setting was 5000mV whereas the right bellows setting was 5050mV. Regardless, the left bellows pressure at the beginning (bellows volume = 100%) was a little higher than the setting and that is why the beam from this bellows drops in intensity for the first few runs. Another thing to note is that after 26 hours the bellows were both around 60% of their full volume. This reflects on the higher viscosity of CO2.

The second set of plots show δ18O as a function of time and are observed to vary quite a bit over the 26 hour period. This suggests that it is not a good idea to use the bellows as a source of reference gas for continuous flow measurements without prior verfication of the isotope ratio stability. We believe the effect is in fact due to the phase change from liquid to gaseous CO2 in our continuous flow reference gas cylinder. Consequently, the bellows are probably just fine as a source of reference gas for CO2, we just don't have the data yet to confirm this. Our interpretation of the oberved variation would also help to explain why no significant shift was observed for H2.

CO2_bellowsfixedamplitude.gif

The plot below shows the difference in isotope ratio of the gases in the two reference bellows. Again, we were not careful to prevent fractionation when filling the bellows, but this was not the purpose of the experiment. We just want to see what happens to the isotope ratio of the gases in the two bellows. As can be seen below, although there is significant drift in the value of each bellows gas, the relative difference between them does not change significantly. In this case, the standard deviation of the difference is 0.017�. The density of points here is half of that shown in the above plots. This is because in each acquisition there were two peaks from each bellows. The difference of the mean value for each acquisition is what is shown below (hence half the points of the above plots).

CO2_bellowsfixedamplitude_relativediff.gif