APCI surfactants

Applications APCI-MS is often more widely applicable than ESI-MS to the analysis of classes of compounds with a low molecular weight, such as basic drugs and their metabolites, antibiotics, steroids, oestrogens, benzodiazepines, pesticides, surfactants, and most other organic compounds amenable to El. LC-APCI-MS has been used to analyse PET extracts obtained by a disso-lution/precipitation procedure [147]. Other applications of hyphenated APCI mass spectrometric techniques are described elsewhere LC-APCI-MS (Section 7.33.2) and packed column SFC-APCI-MS (Section 73.2.2) for polar nonvolatile organics. 

In off-line coupling of LC and MS for the analysis of surfactants in water samples, the suitability of desorption techniques such as Fast Atom Bombardment (FAB) and Desorption Chemical Ionisation was well established early on. In rapid succession, new interfaces like Atmospheric Pressure Chemical Ionisation (APCI) and Electrospray Ionisation (ESI) were applied successfully to solve a large number of analytical problems with these substance classes. In order to perform structure analysis on the metabolites and to improve sensitivity for the detection of the various surfactants and their metabolites in the environment, the use of various MS-MS techniques has also proven very useful, if not necessary, and in some cases even high-resolution MS is required. 

The FIA-MS screening approach using soft ionisation interfaces prior to any CID procedure provides an overview of the MS separation procedure, which is based on the different m/z ratios of the molecular or cluster ions generated. With the help of this very fast screening method—positive or negative FIA-MS by-passing the analytical column—the surfactant chemist is able to characterise complex blends and formulations without difficulty (Fig. 2.5.1) while the experienced analyst is able to make initial statements about the presence of frequently used and therefore most important surfactants in environmental samples (Fig. 2.5.2) despite the presence of complex matrices. The information provided by ESI or APCI—FIA—MS overview spectra for a first characterisation [8,17-19], which were also available with non-API soft ionising interfaces such as FAB [20] or TSI [9] in industrial blends as well as environmental samples, were obtained from ... 

Equidistant or clustered signals, characteristic of some anionic, nonionic or cationic surfactants (cf. Fig. 2.5.1(a) and (b). So the presence of non-ionic surfactants of alkylpolyglycolether (alcohol ethoxylate) type (AE) (structural formula C H2 i i-0-(CH2-CH2-0)x-H) could be confirmed in the formulation (Fig. 2.5.1(a)) applying APCI-FIA-MS in positive mode. AE compounds with high probability could also be assumed in the heavily loaded environmental sample because of the patterns of A m/z 44 equally spaced ammonium adduct ions ([M + NH4]+) shown in its FIA-MS spectrum in Fig. 2.5.1(b). 

Either in ESI or APCI ionisation mode. Differences in the ionisation yields of both API techniques as observed with some surfactants later on can be taken into account to get additional information to the substance-specific information by fragmentation. So the negative ionisation of an alkylether sulfate blend (AES) using either ESI or... 

As an example of an anionic surfactant mixture frequently contained in detergent formulations, an AES blend with the general formula C H2 i i—O—(CH2—CH2—O) —SO3 was examined in the negative FLAMS mode. Because of the considerable differences observed between both API ionisation mode overview spectra, the ESI—FIA—MS(—) and the APCI—FIA—MS(—) spectra are reproduced in Fig. 2.5.3(a) and (b), respectively. Ionisation of this blend in the positive APCI—FIA—MS mode, not presented here, leads to the destruction of the AES molecules by scission of the O—SO3 bond. Instead of the ions of the anionic surfactant mixture of AES, ions of AE can then be observed imaging the presence of non-ionic surfactants of AE type. 

First a screening in the APCI—FIA—MS(+) and APCI—FIA—MS(—) mode was carried out. From the positively generated overview spectrum as presented in Fig. 2.5.7(a) for identification, characteristic parent ions already known from the FIA—MS spectra of the pure blends and examined by MS—MS now were selected for MS—MS examination of the mixture. From the composed surfactant mixture, the ions at m/z 380, 556 and 670 were submitted to CID in positive APCI—FIA—MS—MS mode. Product ion spectra of these ions are presented in Fig. 2.5.7(b)—(d). 

Fig. 2.5.7. (a) APCI-FIA-MS(+) screening recording an artificial formulation mixed from surfactant blends as presented in Figs. 2.5.3 (AES), 2.5.5 (AE) and 2.5.6 (polyglycol amine blend). Product ion spectra of selected parent ions m/z 380, 556 and 670 of surfactant formulation as in (a) obtained by APCI-FIA-MS-MS(+). 

While the surfactant mixture composed by mixing the different blends could be cleared up by FIA-MS and MS-MS to a great extent, these methods failed in the identification of most constituents contained in a commercially available household detergent formulation. The limitations of mixture analysis became obvious with the application of the API methods such as ESI and APCI in FIA-MS-MS mode and are described here by means of examples. 

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). 

The results of this stock-taking of the surfactants present in a household detergent formulation demonstrated that the peak shapes of negatively recorded TICs (e) and (f) look quite similar, while the APCI(+) and ESI(+) TICs recorded under the same chromatographic conditions (Fig. 2.5.11(a) and (b)) are quite different. As expected, the use of different chromatographic conditions resulted in considerable variations, but APCI or ESI applied under the same LC conditions proved the selectivity of both interface types for specific compounds. This effect sometimes will be supported by the selection and application of highly specific and selective LC conditions. 

To recognise ion suppression reactions, the AE blend was mixed together either (Fig. 2.5.13(a) and (b)) with the cationic quaternary ammonium surfactant, (c, d) the alkylamido betaine compound, or (e, f) the non-ionic FADA, respectively. Then the homologues of the pure blends and the constituents of the mixtures were quantified as presented in Fig. 2.5.13. Ionisation of their methanolic solutions was performed by APCI(+) in FIA-MS mode. The concentrations of the surfactants in the mixtures were identical with the surfactant concentrations of the blends in the methanolic solutions. Repeated injections of the pure AE blend (A 0-4.0 min), the selected compounds in the form of pure blends (B 4.0—8.8 min) and their mixtures (C 8.8— 14.0 min) were ionised and compounds were recorded in MID mode. For recognition and documentation of interferences, the results obtained were plotted as selected mass traces of AE blend (A b, d, f) and as selected mass traces of surfactant blends (B a, c, e). The comparison of signal heights (B vs. C and A vs. C) provides the information if a suppression or promotion has taken place and the areas under the signals allow semi-quantitative estimations of these effects. In this way the ionisation efficiencies for the pure blends and for the mixture of blends that had been determined by selected ion mass trace analysis as reproduced in Fig. 2.5.13, could be compared and estimated quite easily. 

Fig. 2.5.12. APCI-FIA-MS(+) overview spectra of industrial surfactant blends used as pure blends or mixtures in the examination of ionisation interferences, (a) C13-AE, (b) cationic (alkyl benzyl dimethyl ammonium quat) surfactant, (c) amphoteric C12-alkylamido betaine, and (d) non-ionic FADA all recorded from methanolic solutions. 

In the qualitative analyses of surfactants, the FIA-MS screening method applying both soft ionising API interface types, APCI and ESI, provides the overview spectra that contain the molecular ions or adduct... 

For confirmation of low concentrations of NPEO homologues in complex samples from the Elbe river, APCI—LC—MS—MS(+) was applied to record the substance-characteristic ion mass trace of m/z 291. The SPE isolates contained complex mixtures of different surfactants. The presence of NPEOs in these complex samples was confirmed by generating the precursor ion mass spectrum of m/z 291 applying MS— MS in the FIA-APCI(+) mode. This spectrum, showed in Fig. 2.6.5, presents the characteristic series of ions of NPEOs at m/z 458, 502,...,678, all equally spaced with A m/z 44 u. Besides the NPEOs, small amounts of impurities could be observed because of the very low concentrations of NPEOs in the water sample [25]. In the foam sample, the identity of NPEOs could be easily confirmed by APCI-FIA-MS-MS(+) because of their high concentrations in this matrix. The LC-... 

Fig. 2.6.6. APCI-LC-MS-MS(+) (CID) daughter ion mass spectrum of [M + NH4]+ ion at m/z 678 generated from Cig-SPE of foam sample. Compound could be identified as non-ionic surfactant NPEO (CgHi9-C6H4-0-(CH2-CH2-0)m-H) (inset)... 

Studies on an industrial blend of AG applying FIA—MS analysis demonstrated that the non-ionic surfactant was ionised under both (+/—)-ESI and APCI conditions [5]. In negative ionisation, the APCI produced deprotonated molecular ions, whereas the ESI produced acetate adduct ions. 

Flow injection analysis (FIA) ESI-MS and APCI-MS spectra for an EO/PO polyether modified silicone surfactant (PEMS) used as a personal care product have been obtained in positive and negative ionisation modes with the positive ionisation mode yielding the best results [41]. The spectra obtained in both modes were highly complicated, and thus no assignment was given. Significant differences in the ionisation results were obtained from the two interfaces, with those ions observed in the ESI-MS spectrum appearing in the lower... 

API-MS methods have been successfully applied to the quantification of M2D-C3-0-(E0)n-Me, with reliable and reproducible results obtained after online HPLC separation [29,30]. The method was used to quantify recoveries of the surfactant from the surface of plant foliage and from solid substrates under controlled laboratory conditions. Extension of the method to environmental samples has not been investigated. The entire linear dynamic range for HPLC-APCI-MS was not determined, but linearity was observed within the required... 

Surfactant solution HPLC-LSD %uptake HPLC-APCI-MS %uptake... 

An industrial blend of ethylene oxide (EO) PEMS marketed as a personal care product was examined by positive ion FIA-APCI-MS and LC-APCI-MS-MS (Fig. 2.8.8) [41]. The FIA-APCI-MS spectrum without LC separation (Fig. 2.8.8(a)) is dominated by ions corresponding to unreacted PEG (m/z 520, 564, 608, 652,...), whilst the ions corresponding to the PEMS (m/z 516, 560, 604, 648,...) could only be clearly observed following LC separation (Fig. 2.8.8(b)). Comparison of the TIC chromatograms of PEMS and PEG (Fig. 2.8.8(c) and (h)) demonstrates the dominance of the PEG by-products in the commercial formulation. It is unclear whether the observed relative intensities are representative of the actual amounts or of the different ionisation efficiencies, due to the confidential nature of the product composition. However, the spectra indicate a trisiloxane surfactant structure of that shown in Fig. 2.8.2 (R = Ac) and FIA-MS analysis of another commercial formulation of this product showed good spectra dominated by the silicone surfactants [48], indicating that the PEG by-product composition can vary significantly in commercially available PEMS formulations. 

API—MS methods have also been applied to qualitative and quantitative determinations of the degradation of silicone surfactants [29]. Comparison of the APCI—MS spectrum of M2D—C3—O—(EO) —Me with that following 24 h equilibration in the presence of halloysite clay is shown in Fig. 2.8.9. 

API-MS techniques (ESI and APCI) have been shown to be informative methods for the qualitative and quantitative analysis of organosilicone surfactants. The use of HPLC-API-MS has enabled improvements in... 

An industrial blend of AE surfactants with the general formula (CnH2n+iO-(CH2-CH2-0)xH n = 12, 14, 16 and 18) was examined using APCI-FIA-MS(-I-) for screening purposes (see Fig. 2.9.2(a)). According to the number of glycol units and the number of alkyl chain links, a series of homologue ammonium adduct ions ([M + NH4]+) equally spaced either with Am/z 44 (-CH2-CH2-0-) or Am/z 28... 

Non-ionic surfactants of a commercial washing powder were separated by supercritical fluid chromatography (SFC) and determined by APCI-MS. The constituents were first extracted by supercritical fluid extraction (SFE) using C02 with or without methanol as a modifier. Variations of the conditions resulted in a selective extraction of the analytes, which could be determined without further purification. Six groups of surfactants were observed, four of which are alkyl-polyethoxylates. The presence of APEO could be excluded by identification recording SFC-FTIR (Fourier transform infrared) spectra [31]. 

In a screening approach, non-ionic surfactants were monitored in the form of their [M + NH4]+ ions, equally spaced with Am/z 44 and identified by FIA-MS-MS(+) in combination with APCI or ESI interface [34,35]. Ci8-SPE was performed prior to selective elution by diethyl ether [35]. Ions of the non-ionics of AE type at m/z 350-570 (Am/z 44) were identified as surfactants with the general formula Ci3H27-0(CH2CH20)mH (m = 3-7). The complexity of the mixture confirmed the results using the diagnostic parent scans m/z 89 for aliphatic non-ionic surfactants of ethoxylate type necessary [35]. 

APCI-FIA- and APCI-LC-MS(+) analyses were performed to characterise non-ionic surfactants in complex mixtures. Product ion and parent ion spectra were recorded to confirm the characterised nonionics. MS-MSC+) results proved that AE compounds (C H2 +1-0(CH2CH20)mH) with different alkyl and ethoxylate chain lengths were present in all samples [35,36]. 

One of the most observed degradation pathways of non-ionic surfactants of ethoxylate type in the biochemical wastewater treatment process is the bond scission between the lipophilic alkyl chain and the hydrophilic ethoxylate moieties. The resulting ethoxylate compounds, PEG or PPG, are highly polar and are not quite easy to degrade, therefore often they can be observed in wastewater discharges. So, APCI— FIA-MS(+) product ion spectra of selected [M + NH4]+ ions, which were under suspicion as PEG (general formula HO—(CH2—CH2—0) H)... 

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