Precursor ion approach for simultaneous determination of nonethoxylated and ethoxylated alkylsulfate surfactants

We present a new liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for simultaneous determination of sodium lauryl sulfate and sodium laureth sulfate homologues in the range of alkyl chain length C12–C16 with 0–5 ethoxy groups. The method is based on scanning the precursor ions fragmenting to m/z 80 and 97 (Precursor Ion Scanning mode), which makes it specific for species with easily cleavable sulfate groups.

Journal of Chromatography A 1653 (2021) 462421 Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma Precursor ion approach for simultaneous determination of nonethoxylated and ethoxylated alkylsulfate surfactants Katarzyna Pawlak a, Kamil Wojciechowski a,b,∗ a b Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, Warsaw 00-664, Poland SaponLabs Ltd, Noakowskiego 3, Warsaw 00-664, Poland a r t i c l e i n f o Article history: Received 29 March 2021 Revised 16 July 2021 Accepted 16 July 2021 Available online 22 July 2021 a b s t r a c t We present a new liquid chromatography–tandem mass spectrometry (LC-MS/MS) method for simultaneous determination of sodium lauryl sulfate and sodium laureth sulfate homologues in the range of alkyl chain length C12 –C16 with 0–5 ethoxy groups The method is based on scanning the precursor ions fragmenting to m/z 80 and 97 (Precursor Ion Scanning mode), which makes it specific for species with easily cleavable sulfate groups By monitoring fragmentation of thus discovered quasi-molecular ions we were able to unequivocally identify all sulfate species present in complex mixtures of alkyl and alkyl-ether sulfates with molecular weight ranging from 200 to 600 m/z Because of the intrinsic sulfate-sensitivity, the presented method can be also applied to non-sodium salts of alkyl- and alkyl-ether sulfates (e.g ammonium, mono- or triethanolamine, etc.), which are often used by cosmetic manufacturers to justify the misleading SLS- and SLES-free claims (where SLS and SLES refer to sodium lauryl sulfate and sodium laureth sulfate, respectively) The use of reversed phase liquid chromatography (RPLC) column with C4 instead of C18 shortened significantly the overall analysis time and allowed us to use a semiquantitative method (based on single standard for Quantitative Analysis of Multi-component System, QAMS) to determine several SLS and SLES homologues in one run with the limit of quantification (LOQ) = 0.4 μg/mL and of detection (LOD) in the range 0.12–0.97 μg/mL The method was successfully applied to 17 commercially available cosmetic/household products allowing verification of their manufacturers’ declarations © 2021 The Author(s) Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Introduction Anionic surfactants are major constituents of most detergents and cosmetic cleaning/washing products but are also ubiquitous in many other formulations, where wetting, dispersing, emulsifying or foaming activities are required [1] Most of the currently available shampoos, shower gels, liquid soaps and dishwashing liquids are based on alkyl and alkyl-ether sulfates produced by sulfonation of alkyl alcohols and ethoxylated alkyl alcohols In industrial practice pure alcohols are very rarely used, typically their mixtures are employed instead (e.g a mixture obtained from hydrolyzed and hydrogenated coconut and palm oils, as in the case of the most popular alkyl sulfate – sodium lauryl sulfate (SLS)) Its ethoxylated analogue (sodium laureth sulfate, SLES) is produced analogously, with an additional intermediate alcohol ethoxylation step [2] The ∗ Corresponding author at: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, Warsaw 00-664, Poland E-mail addresses: kpawlak@ch.pw.edu.pl (K Pawlak), kamil.wojciechowski@ch.pw.edu.pl (K Wojciechowski) “lauryl” name and its derivative “laureth” were introduced to highlight the ill-defined structures of the resulting mixtures, in contrast to single species, e.g sodium dodecylsulfate (SDS) The ethoxylation step introduces another heterogeneity of chemical structures of SLES (number of the ethoxy units), in addition to the variable length of the alkyl chain present in SLS Consequently, the commercially available alkyl and alkyl-ether sulfates show wide distribution of chemical structures and surfactant properties [3,4] Despite their great efficacity in lowering surface tension and sustaining foams, SLS and SLES may pose some environmental hazards caused by their limited biodegradability and persistent foam formation [5] Their high detergent power may also lead to excessive lipid and protein removal when used on a daily basis [6–8] This prompts some consumers to avoid products with sodium lauryl and laureth sulfates and look for their “milder” alternatives In response, numerous cosmetic producers replace the sodium salts with those of ammonium, lithium, ethanolamine, etc., or replace SLS and SLES with their homologous mixtures (e.g “sodium coco sulfate”) to misguide the consumer with a different INCI (International Nomenclature of Cosmetic Ingredients) name [9] Given https://doi.org/10.1016/j.chroma.2021.462421 0021-9673/© 2021 The Author(s) Published by Elsevier B.V This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 the variety of homologous alkyl and alkyl-ether sulfate surfactants, there is an urgent need for a robust and universal analytical method capable of differentiation between not only the alkyl chain length but also the extent of ethoxylation of SLES One of the major obstacles in quantitative analysis of complex mixtures of natural or synthetic compounds is the lack of analytical standards In contrast to many well-defined compounds with distinct structures, no reference materials are available for most of the complex molecules (like proteins, lipids or alkaloids) or mixtures of homologues (such as alkyl and alkyl-ether sulfates in the present case) This problem can be circumvented using computational methods based on Quantitative Structure and Ionization Intensity Relationship (QSIIR) The method was successfully applied to predict relative levels of 29 organic acids in complex matrices, registered by product ion scanning It offered accurate results (in the range of 80–120%) for 16 organic acids for which the absolute concentrations were quantified and used as reference For the remaining organic acids, such accuracy could not be achieved due to lack of standards or too low concentration of the acid in the samples [10] Such approach enables development of methods relying on a single standard for Quantitative es of Multi-component System (QAMS) [11,12] Currently, most analytical methods for determination of alkyl and alkyl-ether sulfate are based on spectrophotometric, electrochemical and chromatographic techniques [13–15] The official EU method for determination of anionic surfactants in detergents described in Regulation (EC) No 648/2004 of the European Parliament and of the Council and in ISO 7875-1(1996) standard is based on formation of blue-colored salts of anionic surfactants with the methylene blue dye, which are determined spectrophotometrically after extraction to chloroform (Methylene Blue Active Substance test, MBAS) Although the method is continuously improved [16,17], the ion-pair formation reaction does not provide sufficient selectivity to enable distinction not only between the nature of the anionic group (phosphate, carboxylic, sulfate or sulfonate) but also the alkyl chain structure Another group of non-chain-length-selective methods is based on formation of ionic associates between anionic surfactants and cationic species using potentiometric sensors [18] or suppression of ionic conductivity using ion chromatography [19] Using a similar approach, Levine et al were capable of determining concentration of ammonium lauryl sulfate, sodium laureth sulfate and sodium alkyl (C10 SO4 − – C16 SO4 − ) ether sulfates present in commercially available detergents (dynamic linear ranges: 1.0–500, 2.5–550 and 3.0–630 mg/L, respectively) [20] After chromatographic separation, a mixture of four linear alkylbenzene sulfonates (C10 SO4 − - C13 SO4 − ) was detected by UV spectroscopy [21] and a mixture of anionic and nonionic surfactants (including sodium lauryl sulfate and α -olefin sulfonate) - using an evaporative light scattering detector (ELSD) [22] Nevertheless, selectivity is often when such detectors are employed [21,23,24] Regardless of the detection method, anionic surfactants can be separated using ion exchange chromatography [19] However, most commonly the separation is achieved thanks to differences in the affinity of aliphatic chains to a hydrophobic stationary phase For this purpose, deprotonated anionic surfactants (pH > 7) can be separated as neutral ion pairs formed with quaternary ammonium cations (ion pair liquid chromatography, IPLC [25]) or in a non-dissociated form (pH < 7), using reversed-phase liquid chromatography, RPLC The former method is, however, not suitable for mass spectrometry because of significant signal suppression [26] The advantage of lowering pH of the mobile phase is a reduction of hydrophilic interactions interfering with the chromatographic process On the other hand, reduced pH may lower ionization efficiency in mass spectrometry [26] Matthus and colleagues [27] separated the alkylbenzene sulfonate (LAS), alcohol ethoxy- lates (AE), and alcohol ethoxylated sulfates (AES) by reversedphase chromatography using a C18 column and ammonium acetate to stabilize pH of the mobile phase The partially separated compounds were detected using a fluorescence detection (FLD) and a thermospray mass spectrometer working in the scanning mode (m/z range 20 0–80 0) This provided more detailed structural information at the expense of sensitivity The potential for quantitative analy was exemplified by determination of the total concentration of active substances in sewage in relation to the AES and LAS commercial mixtures Dufour et al employed ultra-high-performance liquid chromatography with high-resolution mass spectrometry (UPLC-HR-MS) to separate four homologues of alkylbenzene sulfonate (4-dodecylbenzenesulfonic acid, DBSA) The homologues differed in their alkyl chain lengths (decylbenzenesulfonate, undecylbenzenesulfonate, dodecylbenzenesulfonate, tridecylbenzenesulfonate) and could be separated in the RPLC mode using C4, C18 and C30 columns [28] Levine et al employed RPLC coupled with electrospray ionization quadrupole ion trap mass spectrometry (ESI-Q-IT-MS) to simultaneously determine three common surfactants: an amphoteric cocoamphoacetate, a nonionic alcohol ethoxylate and an anionic SLES (dynamic linear range 1.5–40 mg/L for total amount of SLES normalized against commercial mixture) [29] Four alkyl sulfate and ethoxymers of alkyl-ether sulfate homologues were determined in the SPE-preconcentrated wastewater samples using a liquid chromatography–tandem mass spectrometry (LC–MS/MS) with electrospray ionization (ESI) in negative ion mode Based on the commercial mixture producer’s declarations, the limits of quantification (LOQ) in the range 0.3–0.4 μg/L and 0.5–1.5 μg/L were reported for the SLS and SLES homologues, respectively [30] Another interesting approach to separate surfactants present in commercial mixtures of SLS and SLES employed the ion mobility mass spectrometry [31] The ionized compounds were distinguished based on their drift time in an electric field, which depended on their molecular weight The authors obtained six peaks for the SLS mixture and twelve for the SLES one, although their identity was not determined due to lack of standards Nevertheless, the developed method could be applied to determine the total SLS and SLES content (with respect to commercially available SLS and SLES mixtures) adsorbed by different dish surfaces In this contribution we further extend the analytical capabilities of LC-MS/MS technique by employing for the first time an MS/MS scanning of the precursor ions to follow the sulfatebearing species We propose a novel method for determination of individual alkyl and alkyl-ether sulfates in mixtures of cosmetic/household ingredients and products using a single and easily available standard (SDS) for quantitative analy of multi-component system (QAMS) We show how the experimentally observed dependency of signal intensity on the retention, number of fragmentation ions obtained in a collision cell and recovery from a stationary phase can be accounted for in QAMS methods to obtain good accuracy SDS could be employed as a universal and sole standard, as it was present in every sample The new method allowed us to detect non-ethoxylated and 20 ethoxylated sulfates in commercial SLES products To show the unprecedented application potential of the new method for real-life samples analy we verified the manufacturers’ declarations about the presence/absence of SLS and SLES ingredients in 17 cosmetic/household commercial products Experimental Separation of the alkyl and alkyl ether sulfates was carried out using a 1220 Infinity II LC Systems (Agilent Technologies, USA), whereas their identification and quantitation was achieved with a 6460 Triple Quad tandem mass spectrometric detector with a Jet Stream ion source (Agilent Technologies, USA) Analytes were separated by an Aeris Widepore C4 column (2.1 × 150 mm, 3.6 μm, K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Fig Product ion mass spectra registered for dodecylsulfate anion (DS− , C12 SO4 − , m/z 265) and a selected anion present in SLES with their fragmentation ions ˚ Phenomenex) The chromatographic separation employed a 300 A, gradient elution from (40% acetonitrile: 60% water) to (95% acetonitrile: 5% water), both containing 0.15% (v/v) formic acid, over at a flow rate of 0.2 mL/min Both the column and mobile phase were thermostated at 45 °C The mass spectrometer was operated in negative ion mode established by ionization voltage of 30 0 V and nozzle voltage V Heated (300 °C) nebulization gas flow of L/min applied at 30 psi was selected as the most appropriate to enhance the ionization of low molecular weight compounds The Precursor Ion (PrecI) scanning was applied for the discovery of SLS and SLES homologues A tandem mass spectrometer operating in the PrecI mode finds the parent ions (the first quadrupole, Q1, operates in the m/z 20 0–80 scanning mode) for the m/z values of characteristic fragment ions (the second quadrupole, Q3, operates in the selected ion monitoring mode) The negative fragmentation ions SO3 − at m/z 80 and HSO4 − at m/z 97 were selected as characteristic of the sulfatebearing compounds (Fig 1, Table SM1) Structurally similar sulfonate ions would produce the m/z 81 signals of HSO3 − ions in addition to m/z 80 The m/z values of the parent ions were selectively discovered by the mass spectrometer The analyses were controlled and processed by a MassHunter Workstation software (Agilent Technologies, USA) The employed chromatographic and MS conditions are collected in Table SM1 The commercial shampoos, hair conditioner and liquid soap were purchased in a local cosmetic store in Warsaw (Poland) The reference shampoo without SLS and SLES was provided by Saponlabs Ltd (Poland) Three SLES and four SLS-type mixtures with the specified amount of active substances (27–70% w/w) and ethoxylation degree were obtained from an industrial supplier All SLES were declared as sodium salts of ethoxylated sulfates of predominantly C12 -C14 alcohols with different ethoxylation degree and average molecular weights (Mav ∼ = 340 g/mol, ethoxylation degree 1–2.5: SLES-340; Mav ∼ = 384 g/mol, ethoxylation degree 1– 2.5: SLES-384; Mav ∼ = 432 g/mol, ethoxylation degree > 2.5: SLES432) The following SLS-type products were used: sodium lauryl sulfate (SLS), ammonium lauryl sulfate (ALS), triethanolamine lauryl sulfate (TEALS), monoethanolamine lauryl sulfate (MEALS) Sodium dodecylsulfate, SDS (puriss ACS reagent, ≥ 99.0%) was pur- chased from Sigma Aldrich (Poland) Acetonitrile (LC/MS purity) from POCH (Gliwice, Poland); formic acid (LC/MS purity), from Fisher Scientific (Fair Lawn, NJ, USA) Demineralized water from a Milli-Q system Model Millipore Elix (Molsheim, France) was employed Standard solution of SDS (1.0 mg/mL) was prepared in Milli Qwater The surfactant SLES mixtures (0.2–0.5 mg/mL) and commercial cosmetic formulations (0.7–1.4 mg of cosmetic/mL) were prepared by dissolving them in Milli Q-water For semi-quantitative analyzes of alkyl and alkyl-ethoxy sulfates, to assure that the concentration of all sulfates is within the linear response range of calibration curve, all samples were diluted at two or more levels All solutions were filtered through 0.45 μm syringe filters prior RPLC analy Results and discussion 3.1 Optimization of the detection conditions In order to simultaneously quantify all alkyl and alkyl-ether sulfate homologues, the separation method should allow for selection of all species yielding hydrogen sulfate ion (HSO4 − , m/z = 97) and/or radical sulfate anion (•SO3 − , m/z = 80) upon fragmentation These species, resulting from dissociation of the C-O-S bond in the sulfate group, can be conveniently selected by scanning ions with m/z being reduced to 80 and 97 in the Precursor Ion Scanning (PrecI) Mode Although the latter offers additional intrinsic specificity in MS analy, until now the PrecI mode has been employed almost solely in lipidomic and proteomics analyses [32] and its potential in alkyl sulfate analy has been largely unexploited Therefore, in the first part of the study, the detection conditions for electrospray tandem mass spectrometer (ESI-MS/MS) were optimized using sodium dodecylsulfate (SDS) and a cosmetic/household ingredient Sodium Laureth Sulfate with declared average molecular weight of 384 g/mol and average ethoxylation degree 1–2.5 (SLES384) To this aim, an SDS solution of μg/ml was introduced (5 μL) into the mobile phase stream (acetonitrile:water, 50:50 (v/v)) at a flow rate of 0.2 ml/min (FIA-ESI-MS) Using SDS, the ionization voltage, gas flow rate and temperature, as well as ion K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 transmission voltage (fragmentor voltage) providing the highest signals observed at m/z 265 (corresponding to dodecyl sulfate anion, CH3 -(CH2 )11 -SO4 − , (“C12 SO4 − ” or “DS−“ ), were optimized (see Table SM1 in Supplementary Materials) As alkyl and alkyl-ether sulfates tend to form stable anions also in acidic solutions due to high dissociation constant of sulfate group (pKa = ~2.4) the mobile phase was acidified with formic acid (0.15% (v/v)) to sensitivity by elimination of adducts formation and to reduce the noise of the mass spectrum [26] Analogous analy employing the optimized conditions was performed for SLES-384, where the highest signals were observed at m/z 265, 309, 381 and 441 They corresponded to the C12 anion (CH3 -(CH2 )11 -SO4 − ) previously found in SDS) and to different laureth sulfate anions predominant in the mixture: CH3 -(CH2 )11 -(OCH2 )1 -SO4 − , CH3 -(CH2 )13 -(OCH2 )2 -SO4 − , CH3 (CH2 )11 -(OCH2 )4 -SO4 − , respectively (C12 EO1 SO4 − , C14 EO2 SO4 − and C12 EO4 SO4 − , respectively) The selected dodecyl and laureth sulfate ions were subjected to fragmentation (Product Ion Scanning) using the 10, 20, 30, 40 eV collision energy All resulting spectra featured the m/z 97 signal, corresponding to HSO4 − anion, confirming that all selected ions indeed contained the sulfate group In some cases, additional signal at m/z 80, corresponding to a radical anion SO3 − , was also observed Other signals, if present, corresponded to the breakdown of the C-O or C-C bonds within the ethoxylated part of SLES molecules (Fig 1) The highest signals were obtained for the collision energy of 20 and 40 eV, and these conditions were employed in subsequent optimization of separation conditions temperature of the mobile phase and column, sample volume and sample dilution (Fig SM2) In order to validate selectivity of the MS detection in PrecI mode under optimized conditions, two other SLES mixtures (SLES340 with declared average molecular weight of 340 g/mol and average ethoxylation degree 1–2.5, and SLES-432 with declared average molecular weight of 432 g/mol and average ethoxylation degree > 2.5) were analyzed using the method developed with SLES-384 The chemical identity of the anions discovered by PrecI was assigned using the Product Ion Scanning mode Fragmentation ions m/z 80 and 97, confirmed the presence of the sulfate group in all cases The difference between theoretical and experimentally established monoisotopic mass - M parameter (defined as M =|Mtheoretical -Mexperimental |/Mtheoretical ·106 ) varied between 71 and 639 ppm (Table SM2), which is typical for low-molecularweight-compounds analyzed with quadrupole analyzers, due to the intrinsic low resolution of the latter [36] All tested SLES mixtures show similar chromatograms with the proportion of more lipophilic derivatives increasing in order: SLES340 < SLES-384 < SLES-432 A more detailed analy showed that retention time with the number of both alkyl and alkoxyl groups (Fig SM3), which was used to additionally confirm identity of the homologues This was especially useful when the compound signal was too small to obtain a rich Product Ion spectrum All tested SLES samples were abundant in ethoxylated species spanning the alkyl chain lengths from 12 to 16 carbon atoms, and the number of ethoxy groups from to (Fig 2, Table SM2) Surprisingly, however, all samples featured also signals that could be assigned to non-ethoxylated sulfates (C12 SO4 − - C16 SO4 − ), typical for SLS-type products 3.2 Optimization of the separation conditions by RPLC Having selected the optimum conditions for detecting the species releasing SO3 − and HSO4 − (m/z 80 and 97) in PrecI mode, we optimized the chromatographic method, starting with a selection of mobile and stationary phases Formic acid allowed separation of SLS and SLES homologues with a selectivity comparable to that achieved in the presence of trifluoroacetic acid (TFA) in the mobile phase and better than that for ammonium acetate [23,24] The presence of formic acid in the mobile phase helps to reduce the hydrophilic and enhance the hydrophobic interactions of separated compounds with the alkylated silica stationary phase It should be noted that formic acid, in contrast to TFA, is not an ion-pairing agent, hence its presence does not deteriorate the sensitivity of MS-based method The ability of the mobile phase (0.15% aqueous solution of formic acid with a linearly increasing amount of acetonitrile from to 98% in 20 min) to recover SDS and the SLES-384 components from the surface of a hydrophobic stationary phase was tested using two Phenomenex 150 × 2,1 mm columns One was loaded with fully porous silica particles modified with C18 aliphatic chains (Luna) and the other - with core-shell silica particles of wide pores modified with C4 aliphatic chains (Aeris) Chromatographic performance during gradient elution was better for the C4-bed core-shell column which produced significantly better shape of the peaks (intensity to 10 times higher and lower width, see Fig SM1) The shape of the peak can be by: (1) too strong interactions of the compound with the stationary phase not balanced by the composition of the mobile phase solution, (2) the rate of movement of the compounds in the column depending on the temperature, (3) particle or pore size in the stationary phase [33–35] Considering that the column size, particle size and gradient elution method were the same in both cases, the observed reduction of the peak width must be related to the size of the particles’ pores Due to better chromatographic efficiency (peaks’ shape) without selectivity in comparison to the fully porous C18 column, the core-shell C4 column was chosen for all subsequent experiments Then, optimization of the chromatographic separation process was carried out by examining the gradient elution program, 3.3 LC-MS calibration using SDS In order to allow for a semi-quantitative analy of the alkyl and alkyl-ether sulfates in cosmetic/household ingredients and products, the LC-MS system was calibrated using SDS as a standard (note that in contrast to SLES, SDS is a single molecule with known and defined structure) As shown in Fig (inset), the calibration curve for the m/z 265 signal in PrecI mode is linear up to at least 10 μg/mL of SDS, confirming that the proposed method can be used for quantitative determination of individual alkyl sulfates Nevertheless, an analogous quantitative analy of alkyl-ether sulfates (SLES-type) is more demanding for two reasons: (1) pure reference substances are not easily available, (2) the homologues may differ not only in the alkyl chain length (Cn ) but also in ethoxylation degree (EOm ) Especially the latter fact may complicate quantitative analy since the alkyl and ethoxy chains contribute differently to partitioning and fragmentation behavior of SLES molecules (see Fig SM3) As the concentrations of individual components of the analyzed SLES mixtures were not known, calibration was performed by a series of dilutions to obtain concentrations ranging from 0.1 to 10 μg of raw material/SLES mixture in ml of water SLES-432 was used for this purpose, as it contains the highest number of different SLS- and SLES-type molecules (Fig 2) The diluted solutions were analyzed using the protocol described above (PrecI mode) For each compound detected in the mixture, a good correlation was achieved between the peak area and the amount of SLES-432 in solution (Fig 3, Table SM3) For comparison, also the curve for SDS is included in the same graph (m/z 265) The slope of the resulting curves is proportional to the retention time for all compounds detected in SLES-432, which is clearly a consequence of different contributions of the -CH2 - and -OC2 H5 - groups to the solubility and consequently – to the partition coefficient of different SLS and SLES homologues The observed reduction of the sen4 K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Fig Extracted ion chromatograms (EIC) of SLES-384 (A) and SLES-432 (B) obtained for the precursor ions (indicated in A) discovered for the fragmentation ion m/z 97 (PrecI: ∗ ∗ →97 (40 eV)), see Table SM1 for experimental details) The colors visible in the on-line version correspond to the calibration curves in Fig sitivity for the components eluted after longer time may be caused by several reasons: • • • ture was selected for its highest number of detected SLS and SLES homologues) were acquired by serial dilutions The dependence of signal height on concentration was then established using a linear curve y = ax + b, where y is the peak area and x is the concentration of the sample (ingredient, cosmetic product, etc.) in the analyzed solution (expressed in μg/mL, see Fig SM4) The sensitivity coefficient, fi, for any given alkyl or alkyl-ether sulfate ion (i) in the sample is calculated by normalizing the slope (a) for the i-th ion to that for DS− (Eq (1)): reduced chromatographic recovery of analytes [37] lower ion transport efficiency from the ionization to the vacuum chamber for higher molecular weight ions [38] suppression of ion signal intensity by strong ion paring agents and lower stability of ions in ESI chamber or collision cell for highly ethoxylated homologues [26] Without the use of analytical standards, the observed dependence of sensitivity on retention time excludes a direct quantitative determination of multiple components To circumvent this problem, we propose a simple correction scheme using the calibration data for SDS As shown in Fig 2, the latter is present in all tested SLES mixtures, and its amount can be reliably quantified using the calibration curve from Fig Note that in a more general case, for samples devoid of SDS, it can always be added to the sample in known amount as an internal standard In the proposed scheme, each anion signal is normalized to that of DS− (dodecylsulfate, C12 SO4 − ) First, a calibration curve for DS− anion was obtained using solutions with the SDS standard Next, changes of signals for each ion detected in the mixture SLES-482 (this mix- fi = ai, sample aDS, sample (1) The dependence of fi on retention time is shown in Fig and the numerical values are collected in Table SM4 The retention time significantly with the length of the alkyl chain for both SLS and SLES The sensitivity of the method is primarily dependent on the number of -CH2 - groups in the SLS homologues and ethoxy groups present in the SLES homologues It has already been reported that the length of alkyl chain influences the chromatographic recovery [37] as well as the efficiency of ionization process and ion transmission from the electrospray to vacuum chamber [38] However, for SLES homologues, the decrease in sensitivity is more notable K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Fig Dilution correlation for SLES-432 obtained by LC-MS/MS method using Precursor Ion Scanning mode Colors of points and trend lines in an electronic version correspond to the peaks in Fig The inset shows a calibration graph for DS− obtained for SDS Fig Dependence of sensitivity coefficient on retention time depending on the number of -CH2 - (n) and ethoxy groups (m) in different SLS (Cn ) and SLES (Cn EOm SO4 − ) derivatives K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Table Comparison of the declared and semi-quantitatively (LC-MS/MS) determined amount of active ingredients and the average molecular weight for cosmetic/household products ingredients Product name SLES-340 SLES-384 SLES-432 ALS SLS MEALS TEALS Sum of active ingredients Average molecular weight and ethoxylation degree (in brackets) Declared by manufacturer (%) Determined by LC-MS/MS (%) 69.1 70.0 69.8 27.6 29.0 27.3 38.8 60.1 67.9 68.4 29.1 31.1 21.1 29.5 ± ± ± ± ± ± ± 9.0 10.2 13.7 5.8 6.2 5.2 7.1 This is most likely related to the number of fragmentation products for SLES homologues containing higher number of ethoxy groups, which lowers the impact of the m/z 97 ion (Fig 1) The change of sensitivity with the number of ethoxy groups is linear for the same length of alkyl chain and consequently a linear regression could be applied The latter provides a convenient statistical description of the agreement between the fit and the experimental data, allowing calculations of standard deviation of relative concentrations of SLS and SLES homologues The concentration of each surfactant ion (i) can be then estimated using the sensitivity coefficient (Eq (2)): Ci = ( peak area )i − bDS, SDS aDS, SDS · fi ·d SDb,DS,SDS · fi aDS,SDS Determined by LC-MS/MS 340 (1–2.5) 384 (1–2.5) 432 (> 2.5) 294 296 334 415 344 387 400 289 296 333 425 ± ± ± ± ± ± ± 29 (1.1 ± 0.2) 35 (2.1 ± 0.2) 52 (2.5 ± 0.3) 14 11 58 83 ALS, TEALS and MEALS) are devoid of any ethoxylated impurities (Fig 5B) as expected, the SLES-type ingredients contain significant amounts of SLS-type derivatives (Fig 5A), as already noted during the qualitative analy (see Fig 2) Their presence can be probably explained by incomplete ethoxylation of the raw materials used for their production When such a mixture is subjected to sulfonation reaction, the corresponding mixture of alkyl and alkyl-ether sulfates is obtained The concentration of each alkyl sulfate and alkyl ethoxy sulfate anion was calculated using the semi-quantitative method described in the preceding section and their sum in each of the products was compared with the active component content declared by the manufacturer (Table 1) The agreement between the declared and determined sums is satisfactory, especially taking into account the semi-quantitative nature of our method Only for TEALS and MEALS the difference is more significant, probably due to the lower dissociation degree of triethanolamine and monoethanolamine salts of the alkyl sulfates which may affect both the chromatographic separation and subsequent MS detection The average ethoxylation degree (see Supporting Materials) of the three SLES mixtures with increasing their average molecular weight and agrees with the manufacturer’s declaration The average molecular weight of the alkyl and alkyl ether sulfates was also established on the basis of their composition (Table 1) Also, in this case the agreement with the manufacturer declarations is satisfactory, further validating our semi-quantitative method The present method has been developed and validated for detection of sulfate-based surfactants but can be extended to other ionic surfactants It offers good sensitivity and allows the determination of SLS and SLES homologues using a cheap and widely available standard substance – sodium dodecyl sulfate (SDS) The employed PrecI mode allows even for detection of (ethoxylated) alkyl sulfates not yet described in the literature In addition, the method does not require any a priori knowledge of m/z values specific for each parent and fragmentation ion, which is an essential requirement of the MRM method Although the latter offers the highest sensitivity [39], the PrecI mode still offers good sensitivity and facilitates interpretation of the data as compared to the mass spectra and chromatograms obtained in MS scanning mode (2) where: aDS, SDS and bDS, SDS are the calibration curve parameters for DS− ion in a standard SDS solution (external standard), and d is the sample dilution The detection limits can be calculated using the sensitivity coefficients, fi , and calibration parameters for DS− (Eq (3)) LODi = 3.3 · Declared by manufacturer (3) where SDb,DS, SDS is the standard deviation of calibration curve parameter b for DS− ion in a standard SDS solution Additionally, standard deviation (SD) for the peak areas in blank samples (the shampoos devoid of SLS and SLES) was established (SDSh , n = 5) and it was found significantly lower than SDb The LOD was in the range 0.12–0.97 mg/L, which is satisfactory for determination of SLS and SLES homologues in cosmetic products Moreover, the sensitivity of the method could be further improved by switching to a Multiple Reaction Monitoring (MRM) mode and monitoring the specified pairs of parent/fragmentation ions 3.4 Validation of the semi-quantitative method for alkyl and alkyl ether sulfates determination The method described above allows to correct for the experimentally observed dependence of sensitivity on retention time in mixtures of practically unlimited number of SLS and SLES homologues and enables their semi-quantitative determination In the absence of reliable analytical standards for any other than SDS homologue of SLS and SLES, the analytical validity of the method was critically assessed by comparing the sum of all semi-quantitatively determined components with the total content of active substance (manufacturer declaration) in seven cosmetic/household ingredient products Three SLES mixtures (SLES-340, SLES-384, SLES-432) and four SLS-type derivatives: sodium lauryl sulfate (SLS), ammonium lauryl sulfate (ALS), triethanolamine lauryl sulfate (TEALS) and monoethanolamine lauryl sulfate (MEALS) were employed for this purpose Figs and SM5 collect the extracted ion chromatograms (EIC) showing the signal intensity from the non-ethoxylated alkyl sulfate anions (SLS-type, Fig 5A) and alkyl ethoxy sulfate anions (SLES-type, Fig 5B) species (all selected in PrecI mode) The first striking observation is that while the SLS-type ingredients (SLS, 3.5 Alkyl (SLS-type) and alkyl ether sulfate (SLES-type) anions determination in cosmetic/household products Having established and validated the semi-quantitative method for determination of SLS and SLES homologues, we analyzed 17 cosmetic/household products and compared their content of alkyl and alkyl ethoxy sulfates with the declarations provided by manufacturers in the INCI (International Nomenclature of Cosmetic Ingredients) lists The products were chosen in a way to represent formulations with declared presence and absence of SLS- and SLEStype derivatives K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Fig Extracted multi-ion chromatograms (EIC) of: A (left panel) - alkyl (non-ethoxylated, SLS-type) and B (right panel) - alkyl ethoxy (ethoxylated, SLES-type) sulfates detected in cosmetic/household ingredients using Precursor ion mode (PrecI: ∗ ∗ →97 (40 eV)) would not be able to produce both m/z 80 and 97 signals Therefore, they could not interfere with determination of the sulfatebased surfactants using our method based on PrecI mode It should be noted that the fragmentation ion observed at m/z 97 can also be obtained for H2 PO4 − anion produced during fragmentation of phospholipids Nevertheless, such signals could be excluded on the basis of different retention times due to longer aliphatic chains in According to declarations, some of the selected products contain other anionic surfactants similar to SLS and SLES: sulfonate, sulfosuccinate, sulfoacetate, taurate or isethionate, where the sulfur atom in the headgroup is bound directly to C-atom (see Table 2) Because of much higher energy required to dissociate an S-C bond, under the presently employed fragmentation condition and due to lack of the forth oxygen present in sulfate group, these moieties K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Table SLS and SLES homologues content determined semi-quantitatively in cosmetic and household products compared with the producer’s declarations in the INCI lists (names in italics indicate alkyl (SLS-type) and alkyl-ether (SLES-type) sulfates) No Product description Moisturizing shampoo for dry hair Nutrifying shampoo for dry hair Smoothing cleansing conditioner Moisturizing shampoo for dry hair Shampoo for dry and damaged hair Shampoo for dyed hair Moisturizing shampoo for normal and dry hair Strengthening shampoo for greasy and falling out hair Repair-shampoo Shampoo without SLES Anti-dandruff shampoo 10 11 12 Hypoallergenic shampoo for fair, dyed and bleached hair 13 14 15 16 Hair-repairing shampoo Liquid soap Anti-dandruff shampoo Shampoo for delicate and damaged hair Bath and shower gel 17 Sulfur-containing surfactants declared in INCI list Sodium Coco-Sulfate Sodium Laureth Sulfate, Disodium Laureth Sulfosuccinate, Sodium Lauryl Sulfoacetate None Sodium Methyl Cocoyl Taurate, Sodium C14-16 Olefin Sulfonate Sodium Lauryl Sulfate, Sodium Laureth Sulfate, Sodium Xylenesulfonate None Sodium Coco-Sulfate Alkyl sulfates [%] Alkyl ethoxy sulfates [%] Compliance with INCI (SLS/SLES) 14.69 ± 1.31 11.59 ± 0.53 < LOD 11.25 ± 0.50 +/+ -/+ < LOD 1.51 ± 0.79 < LOD < LOD +/+ -/+ 28.75 ± 1.39 4.54 ± 0.91 +/+ 23.16 ± 1.08 22.88 ± 1.03 19.18 ± 2.41 < LOD -/+/+ 12.14 ± 0.52 0.62 ± 0.14 +/+ 30.51 ± 1.22 < LOD 9.22 ± 1.04 < LOD < LOD 1.77 ± 0.42 +/+ +/+ +/+ Ammonium Lauryl Sulfate, Sodium Laureth Sulfate Sodium Coco-Sulfate None Sodium Laureth Sulfate, Sodium Lauryl Sulfate, Sodium Xylenesulfonate, TEA-Dodecylbenzenesulfonate Sodium Laureth Sulfate, PEG-2 Dimeadowfoamamido-ethylmonium methosulfate Sodium Coco-Sulfate Sodium Laureth Sulfate Sodium Cocoyl Isethionate Disodium Laureth Sulfosuccinate 5.10 ± 1.06 4.31 ± 0.49 -/+ 42.10 ± 1.87 1.48 ± 0.43 < LOD < LOD < LOD 0.83 ± 0.19 < LOD < LOD +/+ -/+ +/+ +/+ Sodium Laureth Sulfate 5.42 ± 0.32 5.97 ± 0.62 -/+ phospholipids An additional verification could be done by monitoring the PO3 − signal (m/z 79) [40,41] The tested cosmetic/household products were diluted with water and to ensure that the resulting concentration lies within the linear range, each sample was additionally diluted twice more The total SLS and SLES homologues concentrations determined semiquantitatively are collected in Table and the respective chromatograms are shown in Fig SM6 The majority of tested products indeed conforms to declarations and only in one case both the alkyl and alkyl ethoxy sulfate anions were detected in significant amounts in a shampoo declared as devoid of any sulfurbased surfactants (shampoo 6) Two other products declared as free from SLS- or SLES-type ingredients (a hair conditioner and a shampoo prepared in our lab with no SLS or SLES added) did conform to their INCI declarations It is worth stressing that one of the products (shampoo 12) according to the producer’s declaration contained PEG-2 dimeadowfoamamidoethylmonium methosulfate (Meadowquat), where a sulfate group is present in the counterion (methosulfate) The latter would fragment producing the m/z 80 and m/z 97 ions However, our method is insensitive to false positive results of this type, since scanning for the PrecI-selected ions is performed only for ions with m/z above 200 Thus, the amount of alkyl sulfates determined using our method is not biased by the presence of Meadowquat, and the product most likely indeed contains non-ethoxylated SLS-type surfactants not declared in the INCI list In some cases (e.g shampoo no 4) the low amount of undeclared alkyl sulfates could be detected, suggesting their unintentional use – for example as an impurity present in other ingredients Given the abundance of alkyl sulfates in SLES-type cosmetic/household ingredients depicted in Fig 5, the undeclared quantities of SLS homologues could have been even unintentionally introduced into final formulations (see e.g shampoos no 2, 12, 14, 17) Conclusions In this contribution we have developed a new MS/MS method for selective detection of alkyl and alkyl ether sulfates based on Precursor Ion Scanning (PrecI) Mode following their separation on a C4 reversed-phase LC column For this purpose, only ions releasing the SO3 − and HSO4 − moieties (m/z 80 and 97, respectively) are selected and further monitored, providing selectivity to the organic sulfate species characteristic to homologues of sodium lauryl sulfate (SLS) and sodium laureth sulfate (SLES) with m/z in the range 20 0–60 Other sulfur-bearing species commonly found in SLS- and SLES-free products (sulfonate, sulfosuccinate, sulfoacetate, taurate or isethionate), where the sulfur atom in the headgroup is bound directly to C-atom, are excluded based on the absence of HSO4 − signal (m/z 97) Further we have developed a quantitative method for determination of dodecylsulfate ions and a semiquantitative method for determination of any ethoxylated or nonethoxylated alkyl sulfates The latter method is based on series of dilutions providing the sensitivity coefficient (fi ) for each signal, which enables subsequent estimation of concentration of the given species in the sample (all in one run using single standard – easily accessible sodium dodecylsulfate (SDS)) The proposed LCMS/MS method allows simultaneous determination of SLS and SLES homologues, confirmation of their identity, determination of average molecular weight of surfactants and degree of ethoxylation, as well as discovery of new sulfate compounds Conventionally, these tasks require the use of three different methods, which can now be replaced by a single one developed within this work The overall accuracy of the method strongly depends on the quality of correlation between fi and retention time for each group of homologues established under the same detection conditions For this reason, we recommend to calibrate the method using mixtures with as many SLS and SLES homologues as possible Extension of the presented method to the environmental or food analy is also possible, although because of higher required sensitivity, the Precursor Ion K Pawlak and K Wojciechowski Journal of Chromatography A 1653 (2021) 462421 Scanning (PrecI) should preferentially be replaced by the Multiple Reaction Monitoring (MRM) mode Using the newly developed method we have assayed four SLStype and three SLES-type cosmetic/household ingredients for the presence of Cn EOm SO4 − species in the range of n (alkyl chain 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