APCI sulfonic acids

This ESI(+) TIC, however, is dominated by strong and broad signals that eluted between 17 and 31 min, neither observable under APCI(+/—) nor ESI(-) conditions. Even under gradient RP-C18 conditions a strong tailing effect was observed while isocratic RP-C18 failed. The information obtained by ESI—LC—MS(+) was that the compounds could be ionised in the form of [M]+ ions at m/z 230, 258 and 286. ESI-LC-MS-MS(+) resulted in product ion spectra which, by means of a MS-MS library, were found to be characteristic for the amphoteric amine oxide surfactants. These compounds not yet observed in household formulations will be presented later on with the RIC of LC separation (cf. Fig. 2.5.11(d)). After identification as amine oxides, the separation and detection of this compound mixture now could be achieved by an isocratic elution using a PLRP-column and methane sulfonic acid and ESI(+) ionisation with the result of sharp signals (RT = 4-6 min) as presented in Fig. 2.5.11(d). 

As expected, LC separation of the dichloromethane/acetone SPE eluate in the RP-mode, presented as FIA-APCI-MS(+) in Fig. 2.12.13, was impossible because the alkyl ethoxy amines as cationic surfactants could not be eluted under conventional RP-separation conditions [37, 53]. The use of methane sulfonic acid for ion-pairing resulted in the separation of the compounds in the methanol eluate as shown as TIC (d) and selected ion trace masses (m/z 504 (a), 670 (b), and 802 (c)) in Fig. 2.12.14. Here, the short-chain ethoxy amines were eluted later than the more polar long-chain homologues [39]. 

Storm et al. [74] evaluated various trialkylamines as ion-pairing agents for the LC separation of aromatic sulfonates. Tributylamine was preferred. In time-scheduled SRM, 19 aromatic sulfonic acids could be determined with detection limits of 3-74 pg/1. Socher et al. [75] demonstrated the applicability of ion-exchange chromatography with an ammonium acetate salt gradient up to 500 mmol/1 in combination with negative-ion ESI as well as APCI LC-MS. 

The analyses of environmental samples confirmed the ubiquitious presence of surfactants in surface and sea water as a result of the surfactants discharged with STP effluents. Analysis of River Elbe (Germany) water samples by GC-MS and APCl-LC-MS and MS/MS confirmed qualitatively the presence of nonpolar and polar organic pollutants of AEO, NPEO, CDEA and aromatic sulfonic acid type, respectively [226], After Cjg and/or SAX SPE anionic and non-ionic surfactants were qualitatively and quantitatively analysed in surface water samples by APCI-LC-MS in the negative or positive mode, respectively. Alkylphenol ethoxylates (APEOs) could be confirmed in river water at levels of 5.6 pg L [331]. 

Most of the modern pesticides and their degradation products are characterized by medium to high polarity and thermal lability. Neutral and basic compounds (phenylureas, triazines) are more sensitive using APCI (especially in positive ion mode), while cationic and anionic herbicides (e.g., bipyridylium ions, sulfonic acids) are more sensitive using ESI (especially in negative ion mode). Based on comparative analysis of 75 pesticides, the so-called ionization-continuum diagram illustrates the relationship between compound acidity and appropriate ionization modes (Figure 2). 

Figure 6. Reconstructed ion current LC/APCI/MS (positive ion) chromatogram of a mixture of 19 analytes (each 10 J.g/ml in water). l.MPA, 2. TDGO, 3. triethanolamine, 4. jV-methyldiethanolaminc, 5. EPA, 6. iV-ethyldietha-nolamine, 7. thiodiglycol sulfone, 8. 3-quinuclidinol, 9. EMPA, 10. TDG, 11. n-PrPA, 12. diisopropylaminoethanol, 13. EEPA, 14. r -PrMPA, 15. tert-BuPA, 16. w-BuPA, 17. cHexMPA, 18. Pin MPA, 19. benzilic acid. (Reprinted from Journal of Chromatography A, 759, R.M. Black and R.W. Read, Application of liquid chromatography-atmospheric pressure chemical ionisation mass spectrometry, and tandem mass spectrometry, to the analysis and identification of degradation products of chemical warfare agents, pp. 79-92 (1997), with permission from Elsevier)...

ESI is the most common interface since IPC and MS were coupled initially. By 2008, most applications IPC-MS used the ESI interface [58,68-82] because analytes amenable to IPC are usually already ionic in the column effluent that enters the interface. Examples of APCI-MS applications [83,84] include two-fold use of both interfaces [85] they gave similar results in the determination of polyunsatured fatty acid monoepoxides [86]. For determining mono- and di-sulfonated azo dyes, ESI proved to give the best performance in terms of sensitivity and reproducibility [83]. Joining negative APCI-MS and ESI-MS unambiguously identified several acidic oxidation products of 2,4,6-trinitrotoluene in ammunition, wastewater, and soil extracts [61]. 

In APCI mass spectra of carbamates, fragment ions are observed, which are most likely due to thermal decomposition in the heated nebulizer interface and snbseqnent ionization of the thermal decomposition products [11, 14, 20-23]. For example, base peaks were observed at m/z 163 for oxamyl, due to the loss of methyl isocyanate, at m/z 168 for propoxur, dne to the loss of propylene, and at m/z 157 for aldicarb, due to the loss of HjS. The APCI mass spectra of aldicaib and two of its metabolites, aldicarb sulfoxide and aldicarb snlfone, showed significant fragmentation. Major fragments for aldicarb were dne to the loss of carbamic acid (to m/z 116) and due to charge retention at [CH3-S-C(CH3)2]. For aldicarb sulfoxide and aldicarb sulfone, the loss of carbamic acid resnlted in the base peaks of the spectra (at m/z 132 and 148, respectively). 

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