Measurement of Moisture in HBr

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Introduction

Moisture in corrosive gases will accelerate the rate of metal corrosion in gas supply systems which, in turn, can lead to their premature failure. For this reason, it is important to monitor moisture levels in these gases. FTIR is an effective way to monitor for moisture in these corrosive gases. In order to determine low levels of moisture in corrosive gases, it is necessary to use long path gas cells such as the "White Cell" design. The key to using this design is the ability of the mirrors to maintain their optical characteristics even in the presence of corrosive gases. The CIC Photonics 4Runner™, with its protected, gold-coated mirrors is an ideal solution to this problem. The 4Runner™ is a 4-meter gas cell built with a Ni-coated 316L stainless steel body and 304 stainless steel, protected gold mirrors. The plenum design of the low-volume 4Runner™ (0.6 liters) enhances the gas exchange through the gas cell, resulting in a more rapid response to changes in the gas system.

Experiment and Results

The data were collected with a nitrogen-purged FTIR spectrometer using a LN2-cooled MCT detector. The gas cell temperature was held at 50 degrees Celsius while the spectra were collected at a slightly elevated pressure. Once the system was sufficiently dried to eliminate background moisture, moisture calibrations were generated using a moisture generator with diluent nitrogen. A subsequent moisture calibration was then performed in the same manner by adding moisture in nitrogen into the HBr gas stream.

To the left are two FTIR spectra from a high purity HBr gas stream that was further purified using a point-of-use getter. The before-spectra, in blue, clearly show the moisture at less than 20 parts-per-million in the high purity HBr gas. Note the moisture is well-resolved from the HBr and HCl spectral regions. The after-spectra, in red below, is that of the high purity HBr product after being passed through the point-of-use purifier. The moisture has been removed to less than 100 part per-billion levels as indicated by the absence of the spectral bands around both the 1500 cm-1 band and 3800 cm-1 bands. Once the moisture is removed, the CO2 clearly stands out as an impurity, as does the presence of the deuterium bromide, clearly resolved from the moisture bands.

Moisture in HBr before and after point-of-use cleanup

One key to the effective collection of this data is that the spectrometer be well-purged with very dry nitrogen, both within the spectrometer and inside the optical couplers and the transfer mirror box. This purging is easily accomplished using the simple and robust transfer mirror box standard with CIC Photonics 4Runner™ gas cell as well as the aluminum optical couplers produced by CIC Photonics.

Conclusion

This application note demonstrates the ability of the CIC Photonics 4Runner™ gas cell to operate in corrosive environments for the measurement of analytes such as CO, CO2, and moisture.