The ionization source is a crucial component of a mass spectrometer. The characteristics of the ion source, including the method of ionization, pressure, temperature, and other factors play a decisive role in determining the response, sensitivity, and quantitative performance. This article explains how a complex mixture of volatile organic compounds (VOCs) would appear when measured directly with three common ionization methods for mobile labs and other ambient monitoring: electron ionization (EI), single photon ionization (SPI), and chemical ionization (CI).
Electron Ionization (EI)
Many substances can be ionized by EI. This ‘hard ionization’ (high energy) technique produces characteristic fragments that can be accurately identified in conjunction with the NIST database. Usually, the ionization is preceded by a column separation of the material to be measured. A series of individual compounds elute from the column and is ionized by EI. However, in order to achieve rapid response during online monitoring, some manufacturers do not use a chromatographic column, allowing atmospheric samples to be ionized directly by the electron ionization source.
If a complex sample is measured without a column, the EI mass spectra of all substances in the sample will be superimposed. This can lead to an extremely complex spectrum that is difficult to interpret. Figure 1 shows an example of superimposed spectra from a mixture of ten substances. The spectra were taken from the NIST EI-MS database. Due to the high fragmentation caused by EI, the signals of only ten substances create hundreds of ion peaks over a wide mass range. Further, the spectra from toluene, xylene, and trimethylbenzene interfere with one another: the signal on any one ion peak may come from multiple compounds. If the hundreds or thousands of VOCs that may exist in the atmosphere were to be measured simultaneously, the spectrum would become unimaginably complex. Obviously, it is very difficult to accurately ‘decode’ the spectra of complex constituent gases from direct injection EI detection spectra, and the accuracy of the reported concentrations is questionable.
Single Photon Ionization (SPI)
Single photon ionization uses light of a precise energy to directly ionize VOCs. A particular VOC can only be ionized if its ionization energy is less than the photon energy (typically 10.6 eV)1.
The ionization energies of some oxygen- and chlorine-containing substances are higher than 10.6 eV and cannot be detected by SPI. Other substances have ionization energies only slightly lower than 10.6 eV. This leads to a relatively low sensitivity, sometimes more than a factor of 10 lower than the sensitivity to benzene.1 SPI also cannot ionize acetonitrile, methanol, acrylonitrile, or formic and acetic acids, which are pollutants commonly encountered during online monitoring. Figure 2 shows the simulated spectra of ten common substances, all at the same concentration, as would be measured by SPI. While sensitivities are similar among compounds with similar structures, such as alkyl aromatics, they can differ by several orders of magnitude between compounds with different functional groups. To achieve highly accurate analytical results, the SPI mass spectrometer needs to be externally calibrated for each target substance.
Chemical Ionization (CI)
Chemical ionization uses a two-stage ionization process. First, reagent ions are produced in a source ionization region, then mixed with VOC analytes in an ion-molecule reaction chamber, where analyte ions are ionized via chemical ionization with the reagent ion. This achieves “soft” (low-energy) ionization of the VOC analytes.
Proton transfer reaction (PTR) ionization is one of the most commonly used and well-established CI techniques. PTR uses hydronium (H3O+) as the reagent ion. To be ionized by PTR, the VOC analyte must have a proton affinity greater than that of water (691 kJ/mol).
The vast majority of common VOC target analytes in ambient air have proton affinities much larger than that of water, and so are detected efficiently, with similar sensitivities. Additionally, many species that are of emerging interest or are less commonly reported, including PAHs and highly oxidized molecules, also have high proton affinities, and are detected sensitively. The proton transfer reaction with these molecules is very fast.
An important advantage of PTR ionization is that sensitivities fall within a narrow range (Figure 3). This allows the ‘semi-quantitative’ analysis of some substances that cannot be quantified by external standards: their sensitivities can be estimated fairly accurately, based on their chemical structure and the instrument operating conditions. The ‘semi-quantitative’ analysis method is broadly accepted by the scientific research community and used in many publications. Therefore, an unknown or unexpected pollutant can be quantified with a reasonable error. This is not possible with direct injection EI and SPI ionization techniques.