APCI phenolic compounds

Liquid chromatography, coupled to the different ionization sources, is generally the technique most used to characterize the phenolic profile in fruit and vegetable products. With regard to the source ionization, it seems that ESI is used more frequently than other sources, such as APCI or APPI. Another important aspect of this technique is the ionization of phenolic compounds. Negative ionization seems to be more suitable... 

Recent advances in electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), thermospray, and particle beam LC-MS have advanced the analyst toward the universal HPLC detector, but price and complexity are still the primary stumbling blocks. Thus, HPLC-MS remains expensive and the technology has only recently been described. Early commercial LC-MS uses particle beam and thermospray sources, but ESI and APCI interfaces now dominate. Liquid chromatography MS can represent a fast and reliable method for structural analyses of nonvolatile compounds such as phenolic compounds (36,37), especially for low-molecular-weight plant phenolics (38), but the limited resolving power of LC hinders the widespread use of its application for phenolics as compared to GC-MS. 

M.A. Aramendia, V. Borau, 1. Garcia, C. Jimenez, F. Lafont, J.M. Marinas, F.J. Urbano, Qualitative and Quantitative analyses of phenolic compounds by LC and detection with APCI-MS, Rapid Commun. Mass Spectrom., 10 (1996) 1585. 

O. Jauregui, E. Moyano, M.T. Galceran, LC-APCI-MS for chlorinated phenolic compounds. Application to the analysis of polluted soil, J. Chromatogr. A, 823 (1998) 241. 

The detection and identification of phenolic compounds, including phenolic acids, have also been simph-fied using mass spectrometry (MS) techniques on-hne, coupled to the HPLC equipment. The electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) interfaces dominate the analysis of phenohcs in herbs, fmits, vegetables, peels, seeds, and other plants. In some cases, HPLC, with different sensitivity detectors (UV, electrochemical, fluorescence), and HPLC-MS are simultaneously used for the identification and determination of phenolic acids in natural plants and related food products.In some papers, other spectroscopic instmmental techniques (IR, H NMR, and C NMR) have also been apphed for the identification of isolated phenolic compounds. 

Phenolic compounds Vaccinium angustifolium (lowbush blueberry) HPLC-DAD-APCI-MS 87... 

Atmospheric pressure ionization (API) has been mainly used for the ionization of phenolic compounds, applying either electrospray (ESI) or atmospheric pressure chemical ionization (APCI), as it can be observed in Table 16.7. However, ESI is more often used to ionize the different families of phenolic compounds. Moreover, APCI and ESI can be operated under both negative and positive ion modes. Although positive ionization mode [47,93,102,103] was used for detection of various phenolic compounds, it was found that negative ionization mode [94,95,101,104,107] was excellent for phenolic compounds analysis. In this sense, the combination of both polarities in the same method provided good results [75,92,99,100] for the simultaneous determination of several families of compounds. Phenolic acids [75,94,95,101,103,107], flavonols [75,94,95,99,101,103,107], flavonones [99,103] flavanols [92,101,103,104], and flavones [75,103] were often detected in negative ion mode, although some of these families were also detected in positive mode [75,99,100,102]. Anthocyanidins [47,100], coumarins [102], and isoflavones [75,93,102] were detected in positive mode. 

The main sources used to analyze phenolic compounds are electron impact (El), chemical ionization (Cl), fast atom bombardment (FAB,) atmospheric pressure ionization (API) including atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo-ionization (APPI), electrospray ionization (ESI), and in parallel to the advent of electrospray ionization, matrix-assisted laser desorption ionization (MALDI) [104—107, 112]. 

Much data on the structure of flavonoids in crude or semipurified plant extracts have been obtained by HPLC coupled with MS, in order to obtain information on sugar and acyl moieties not revealed by ultraviolet spectrum, without the need to isolate and hydrolyze the compounds. In the last decade, soft ionization MS techniques have been used in this respect, e.g., thermospray (TSP) and atmospheric pressure ionization (API). However, the most used methods for the determination of phenols in crude plant extracts were the coupling of liquid chromatography (LC) and MS with API techniques such as electrospray ionization (ESI) MS and atmospheric pressure chemical ionization (APCI) MS. ESI and APCI are soft ionization techniques that generate mainly protonated molecules for relatively small metabolites such as flavonoids. 

Note that palmitic acid does not have a chromophore and would, therefore, not be visible in the UV trace. In general, negative ion APCI and ESI are sensitive to those compounds that can easily deprotonate (e.g., acids and phenols) or attach a negative ion either in the gas phase or in solution, while positive ion APCI and ESI are sensitive to compounds with relatively high proton affinity (e.g., amines and low oxidation state sulfur compounds). As ESI and APCI can be complementary, so positive ion and negative ion can be as well. Consider Fig. 11 which shows a more... 

The determination of phenols was preferentially performed using GC-MS with analytes in underivatized or derivatized form, but LC-MS methods were also developed. API methods for the analysis of phenols in aqueous matrices were applied [315, 316, 317]. APCI-LC-MS was found to be more sensitive than ESI application despite the possibility of improving ESI-sensitivity by a post-column addition of diethyla-mine [317]. Detection limits were observed with 0.02-20 ng injected onto the column. The determination of alkylphenols and bisphenol A as compounds with endocrine disrupter potential was also performed by ESI-LC-MS from aqueous [318, 319] and sediment samples with detection limits in the low pg L range [346]. 

Developments in LC-MS have probably been greater than in GC-MS though that too has seen significant advances. LC-atmospheric pressure chemical ionization (APCI)-mass spectrometry has become particularly important and has found numerous applications in the determination of, for example, industrial mass chemicals such as dyes, aromatic sulfonates, surfactants, and complexing agents, and also trace compounds such as drugs, endocrine-disrupting compounds, toxins, phenols. 

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