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Photoionization MS


General Properties of PI
The hallmark of threshold photoionization (PI) is its ability to choose a narrow band ionization energy sufficiently high to ionize and detect most molecules of interest, yet sufficiently low to avoid detection of the most common constituents present in air. The figure illustrates the principle behind photoionization’s benefits. Syagen PI sources impart an energy that is higher than the ionization potential (IP) 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 PI ionizes molecules of interest 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 as the PhotoMate® product line. The LPPI method was developed for Syagen’s QitTof™ MS system, and is utilized in the the mobile FieldMate™ and FieldTracker™ systems.



Atmospheric Pressure Photoionization (APPI)
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 provides an important complement to the well known ESI and APCI methods by expanding the range and classes of compounds that can be analyzed with Mass Spectrometry, including nonpolar molecules not easily ionized by ESI or APCI. APPI sensitivity is also excellent. Detection limits of about 1 pg have been measured, comparable to APC and ESII. The APPI source is linear over 5 orders of magnitude.

Highest Performance
  • Widest range of ionizable compounds
  • Large linear dynamic range means best source for quantitative analysis
  • Minimal ion suppression
Greatest Versatility
  • Dopants can supercharge performance
  • Works for positive and negative ionization
  • Does as much and more than APCI


 Excellent quantitation of polar and non-polar lipids:



Steroids are detected loud and clear:



Chromatography conditions for chiral separations are no problem for APPI:



Any many other troublesome compounds:

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