Furnace tube oxidation strategies

The GC-C ‘high capacity’ furnace tube is supplied with copper oxide packing and does not require oxidation prior to use. The GC-C ‘natural gas’ and GC-N furnace tubes are supplied with braided metal wire packing (NiCr, Cu and Pt), with the NiCr and Cu requiring to be oxidized prior to use. Depending on usage and the sample loading on the furnace tubes it is likely that short periods of re-oxidation may also be required with time.

There are many oxidation strategies you could employ, but all involve oxygen being bled into the sample line helium flow.

Warning! To prevent Liquid Oxygen formation - Please do not oxidize the furnace tube when the cold trap is submersed in Liquid Nitrogen.

Setting the oxygen flow

The first stage in any oxidation protocol is to set the oxygen gas flow.

  1. You should ensure that the whole GC setup is leak free and the furnace is at room temperature.
  2. Tune the stable isotope analyzer so you are detecting m/z 32 in the major Faraday collector (this can simply be done by peak jumping to m/z 33).
  3. Ensure that you have oxygen gas connected and then manually open the oxygen vent valve on the GC interface. With a flow meter measure the vent flow and initially set it between 3 and 5 ml/min.
  4. Using lyticOS, open the O2 valve and either just look at the signal from the major collector or record the signal using time scan method. Oxygen will soon elute through the system and the m/z 32 signal should rise. Adjust this flow manually until you get a signal of about 10nA (200uA trap current). Record the flow out of the vent so that this step does not need to be repeated in the future.

Oxidizing a new furnace tube (GC-C ‘natural gas’ or GC-N furnace tube configuration)

Oxidation of a new furnace tube can be achieved using the following steps:

  1. Raise the temperature of the furnace to 450°C. If you are monitoring m/z 32, you should see the oxygen levels reaching the isoprime visION Mass Spectrometer drop considerably – the oxygen is being consumed by the furnace packing. Eventually the oxygen levels detected will recover, i.e. the oxygen passing over the metals are not being consumed in the oxidation process. After this, the process is repeated at 550°C; again, the observed oxygen levels will drop and then recover.
  2. The process of raising the temperature should be repeated further at increasing temperature intervals of 650°C, 750°C, 850°C, 900°C and 950°C. Please note that when you raise the furnace temperature to around 800°C you will see the oxygen levels initially increase, rather than decrease – this is normal and is due to O2 being released when Cu2O is being produced.
  3. The whole oxidation procedure can be monitored as described above, or, to reduce the O2 loading on the ion source, and our recommendation, you can isolate the ion source (either set the source inlet to ‘All off’ in the software or manually shut the isolation valve) and increase your temperature at set times. For example, 450°C for 60 mins, 550°C for 60 mins, 650°C for 120 mins, 750°C for 60mins, 850°C for 60mins, 900°C for 30mins and 950°C for 30mins (if you are planning to oxidize overnight, we would recommend setting the furnace to 650°C and oxidizing before you leave and then heating up stepwise the next morning).
  4. After the final step, switch off the O2 valve and wait for at least 1-2 hours before starting analysis.

Re-oxidizing a furnace tube (GC-C ‘high capacity’, ‘natural gas’ or GC-N)

Although less frequent with the GC-C ‘high capacity’ furnace tube configuration, with time, and very much dependent on the sample loading, the furnace tube may require re-oxidation.

Similar to the initial oxidation there are many strategies that can be employed, e.g. 5 to 10 mins of oxygen dosing with the furnace at operating temperature or cool down the furnace to 650°C and oxidize stepwise again to the operating temperature, e.g. 850°C for GC-C ‘high capacity’ and 950°C for GC-C ‘natural gas’ and GC-N configurations.

Automation of the oxidation process using lyticOS

Oxidation strategies can be controlled by lyticOS through the CAL controllers – the O2 flow cannot be controlled except to switch it on or off. These built in methods can be customized to suit oxidation strategies.

It is also possible to use an lyticOS method to analyze samples with a constant ‘trickle’ of O2 being bled into the sample line helium flow. This enables a relatively consistent oxidation environment and can be a useful approach to aid combustion efficiency when analyzing very complex mixtures that put heavy loads onto the furnace tube. Please note that extended high O2 backgrounds may reduce the lifetime of your ion source filament, therefore it is recommended to reduce the O2 flow that would ordinarily be used for initial oxidation or re-oxidation protocols. While monitoring m/z 32 the flow should be adjusted manually so the signal response is less than 1nA. (See ‘Setting the oxygen flow’ for details). Analyzing samples with a constant flow of O2 is not suitable for the GC-N furnace configuration as the background oxygen levels are likely to be too high and thus prevent the 100% conversion of NOx to N2.