300 400 500 600 m/z
300 400 500 600 m/z
Fig. 1.13 Negative mode single quadrupole MS spectra of sulfuric acid monoester of 3-hydroxy retinoic acid: (A) electrospray, (B) atmospheric pressure chemical ionization.
induced dissociation but by thermal degradation. The product ion spectrum of the precursor ion at m/z 395 shows only a strong ion at m/z 97, corresponding to the HSO4~ ion (data not shown). The ion at m/z 315 corresponds to 3-hydroxy retinoic acid generated in the source by the loss of SO3. The second ion at m/z 297 corresponds to the loss of an additional 18 units (H2O). At a first glance thermal degradation in APCI sounds detrimental, but because it is quite reproducible it can provide further structural information in qualitative analysis.
The setup for atmospheric pressure photoionization (APPI) [32-34] is very similar to that for APCI. Only the corona discharge is replaced by a gas discharge lamp (krypton, 10.0 eV) that generates vacuum ultraviolet photons. The liquid phase is also vaporized by a pneumatic nebulizer. Most analytes have ionization potentials below 10 eV while HPLC solvents have higher ionization potentials (water 12.6 eV, methanol 10.8 eV, acetonitrile 12.2 eV). The absorption of a photon by the molecule and the ejection of an electron forms a radical cation. Better sensitivities have been reported with the addition of dopants such as toluene or acetone. The mechanism of ionization is not fully understood but two different mechanisms can occur: (i) dopant radical cations react with the analyte by charge transfer or (ii) the dopant radical cation ionize the solvent molecules by proton transfer which can then ionize the analyte. APPI can also be performed in the negative mode. Like APCI, APPI can handle a large range of analytes. The performance of APPI is flow rate-dependent; and better sensitivities, compared to APCI, have been reported at lower flow rates. It appears also that APPI is less sensitive to matrix suppression and source contamination. Atmospheric pressure photoionization proves to be particularly attractive for the analysis of steroids and quinones.
With atmospheric pressure ionization the signal response is strongly analyte-dependent. To combine more than one ionization source (ESI, APCI, APPI) is particularly attractive to extend the range of compounds that can be analyzed simultaneously. Most pharmaceutical compounds can be analyzed automatically with positive or negative ESI mode using standard conditions . Those compounds which give no signal require special attention, such as optimized solvent conditions or a change in ionization method resulting in a significant loss in time. Gallagher et al.  have developed a combined ESI-APCI (ESCi) source for high speed online LC-MS analysis. The combined source allows alternate online ESI and APCI scans with polarity switching within a single analysis. During the LC-MS run the high voltage power supply can be switched within 100 ms from the electrospray capillary to the APCI discharge needle. Figure 1.14 shows the LC-MS analysis of a mixture of daidzein and acetophenone with the ESCi source. In this case daidzein shows the best response with ESI while acetophe-none gives a strong signal with APCI.
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