During the analysis of NH3 in H2O gas streams, a big issue when trying to detect low NH3 levels arises due to the spectral overlap between the m/z 17 of both NH3 and water. The water background can be subtracted, but that results in an increase in uncertainty of the NH3 concentration results.
Soft ionization can be used to deconvolute the two species by allowing selective ionization of different gases through setting the ionization energy for a given mass. The energy of excited electrons in the electron impact ionization is typically in the order of 70 eV.
It is possible to alter the electron energy between 4 and 150 eV at 0.1 eV increments, using Hiden gas analysis systems (Figure 1).
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Figure 1. The Hiden QGA Gas Analyzer
Soft Ionization of NH3/H2O
The results of water vapor monitoring in air, with scanning between masses 15 and 19 during progressive scanning of the electron energy between 10 and 30 eV at 1 eV increments, are shown in Figure 2. Using a multi-variant scan, the Hiden QGA Gas Analyzer can easily perform this type of scan (Figure 3).
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Figure 2. m/z vs electron energy-H2O/Air
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Figure 3. MASsoft Multi-variant Scan
As can be seen in Figure 2, the occurrence of minimal ionization of water is observed below 15 eV. The ionization of H2O to H2O+ takes place above this level. As can be observed, the only ionization product (m/z 18) that can be detected at 18 eV (red) while sampling water is H2O+.
H2O does not ionize to OH+ until around 20 eV. Hence, OH+ and NH3+ can be separated using the deviation in ionization energy when the ionization threshold of NH3 to NH3+ is less than this level.
At 11 eV, ionization to NH3+ occurs. The results of scanning from masses 15 to 19 during progressive scanning of the electron energy between 10 and 30 eV are shown in Figure 4. Here, a H20/NH3 vapor mix is sampled by the Hiden QGA. The figure depicts ionization occurring at m/z 17 from around 13 eV.
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Figure 4. m/z vs electron energy-NH3/H2O/Air mix
Since Figure 2 depicts the absence of ionization at m/z 17 until around 20eV, the species being generated in the ionization process is NH3+, showing the possibility of separating H20 and NH3 with the Hiden QGA through the soft ionization technique.
The electron energy scan is continued to demonstrate the possibility of detecting both m/z 17 and 18 at 18eV (red). This means that NH3 can be detected and measured in the presence of high water levels, using electron energy of 18eV.
Detection Limits
The detection limits of this method were studied using 18 eV as the ionization energy. In this case, the Hiden QGA was coupled to a vapor/gas mixing manifold that can mix different concentrations of NH3 in a 2% H2O stream.
Argon was used as the carrier gas. The results of altering the concentration of NH3 from 10 to 100 ppm during the analysis are presented in Figure 5, clearly showing the possibility of achieving detection limits of better than 10 ppm NH3 in water rich gas.
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Figure 5. Simultaneous measurement of low ppm levels of ammonia, nitric oxide and oxygen in percentage (2%) concentrations of water.
Conclusion
The results clearly show the ability of the Hiden QGA to efficiently separate overlapping species using soft ionization techniques.
They also demonstrate the high sensitivity of the instrument even when reduced electron energy was used. With Hiden QGA software packages, soft ionization scans can be performed in many different ways:
- Local setting of electron energy to allow setting different electron energies for individual masses to optimize sensitivity and minimize fragmentation of overlapping species
- Global setting of electron energy to allow scanning all masses using the same energy
- Electron energy scans to determine ionization threshold for individual species
- Multi-variant scans for analysis of varying cracking patterns with electron energy
This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.
For more information on this source, please visit Hiden Analytical.