The hallmark of threshold photoionization (PI) is the ability to choose a narrow band of ionization energy that is sufficiently high to ionize and detect most molecules of interest, yet sufficiently low to avoid detection of the most common constituents of air. The figure illustrates the principle behind these benefits.

Syagen PI sources impart an energy that is higher than the ionization potentials (IPs) of most target molecules, yet is lower than the IPs of the most common atmospheric constituents as well as most common solvents. Hence, these potential interferents go undetected, enabling significantly greater dynamic range for detecting and measuring very low concentration molecules. Furthermore, because molecules of interest are ionized near their IP thresholds, there is minimal fragmentation to clutter a mass spectrum. These performance features provide tremendous benefits for analyzing mixtures and samples in complex matrices.

Key benefits of photoionization mass spectrometry are:

• Detection of a wide range of compounds including non-polars
  
  
• Minimal fragmentation and predominant parent ion signal
  
  
• Minimum air and solvent signal due to the high ionization potentials of all common air molecules (e.g., N₂, O₂, H₂O, CO₂, CO, Ar, etc.) and many common solvents (CH₃OH, H₂O, CH₃CN, chloroalkanes, etc.)
  
  
• Minimum ion suppression
  
  
• Large linear dynamic range
  
  

Syagen has developed two patented PI technologies: atmospheric pressure photoionization (APPI) and low pressure photoionization (LPPI). The APPI method was developed primarily for use with LC/MS instrumentation and is sold under the PhotoMate product line. The LPPI method was developed for GC/MS and for direct injection analysis and is included in Syagen’s Radiance Pro high-throughput molecular analyzer and in new field-portable GC/MS technology (FieldMate) in development.

PI has been shown to have important advantages over the conventional methods of atmospheric pressure chemical ionization (APCI) and electrospray ionization (ESI). The latter methods are based on attaching a charge, such as a proton, to molecules. Because the affinity for charge can vary widely for different molecules, the ionization efficiencies can likewise vary widely. Competition for charge can also lead to ion suppression. The mechanism of photoionization — ejection of an electron following photon absorption by a molecule — is independent of the surrounding molecules, thereby reducing ion suppression effects.

APPI is an important complement to ESI and APCI by expanding the range and classes of compounds that can be analyzed, including nonpolar molecules that are not easily ionized by ESI or APCI. Sensitivity is also excellent. Detection limits of about 1 pg have been measured, comparable to APCI and ESI. The APPI source is linear over at least 5 orders of magnitude.

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