Question

33:Why does vacuum ultraviolet photoionization with |120-nmwavelength radiation produce less molecular fragmentation than

does 70 eV electron ionization?

Short answer

See the answer below.

Step-by-step solution

Single-photon ionization:

Single-photon ionization (SPI) with VUV radiation is capable of soft ionization of all organic compound classes, including the saturated alkanes.

 Answering the above mentioned question

Single-photon ionization (SPI) with VUV radiation is capable of soft ionization of all organic compound classes, including the saturated alkanes. This article focuses on SPI because its ability to detect all organics makes it a candidate to become the long-sought universal soft ionization method. SPI has been used for decades for fundamental studies and increasingly is used for applied work. More recently, the simplicity of SPI and the increasing availability of reliable, high-intensity VUV light sources have been motivating researchers to use this method for many new applications.

The performance of VUV light sources is crucial for successful SPI-MS applications because the typical cross section of SPI, σspi, is relatively small: 2–20 megabarn (Mb; i.e., 10−18 cm2 ), which is only ~1/100 of the average cross section obtained with 70-eV EI ionization for organic compounds (18). In addition, the typical electron flux in an EI ion source is significantly higher than the VUV photon flux achievable with current light sources, so one could assume a comparably low sensitivity of SPI-MS.

Nevertheless, relatively low LODs (mid to low parts-per-billion range) have been demonstrated for organic compounds in direct-inlet MS experiments conducted with laser-generated VUV radiation or novel VUV lamps known as electron-beam-pumped rare-gas excimer lamps. For example, an SPI-MS LOD of 35 ppb was achieved for toluene with a measurement time of 0.65 seconds (s) by a TOF mass spectrometer equipped with an EBEL VUV light source. In a laser SPI-MS experiment (frequency-multiplied Nd:YAG laser at 118 nm), a LOD of 2 ppb was achieved for toluene for an average of 100 laser shots at 10-Hz repetition rate (measurement time 10 s).