High-performance liquid chromatography coupled to direct analysis in real time mass spectrometry: investigations on gradient elution and influence of complex matrices on signal intensities.

Direct analysis in real time (DART) time-of-flight mass spectrometry (TOF-MS) has been tested for its suitability as a detector for gradient elution HPLC. Thereby a strong dependency of signal intensity on the amount of organic solvent present in the eluent could be observed. Adding a make-up liquid (iso-propanol) post-column to the HPLC effluent greatly enhanced detection limits for early eluting compounds. Limits of detection achieved employing this approach were in the range of 7-27 µg L(-1) for the parabene test mixture and 15-87 µg L(-1) for the pharmaceuticals. In further investigations DART ionization was compared to several other widely used atmospheric pressure ionization methods with respect to signal suppression phenomena occurring in when samples with problematic matrices are analyzed. For this purpose extracts from environmental and waste water samples were selected as model matrices which were subsequently spiked with a set of six substances commonly present in personal care products as well as six pharmaceuticals at concentration levels between 100 µg L(-1) and 500 µg L(-1) corresponding to 100 ng L(-1) and 500 ng L(-1) respectively in the original sample. With ionization suppression of less than 11% for most analytes investigated, DART ionization showed similar to even somewhat superior behavior compared to atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) for the Danube river water extract; for the more challenging matrix of the sewage plant effluent extract DART provided better results with ion suppression being less than 11% for 9 out of 12 analytes while values for APCI were lying between 20% and >90%. Electrospray ionization (ESI) was much more affected by suppression effects than DART with values between 26% and 80% for Danube river water; in combination with the sewage plant effluent matrix suppression >50% was observed for all analytes.

Identification and semi-quantitative analysis of parabens and UV filters in cosmetic products by direct-analysis-in-real-time mass spectrometry and gas chromatography with mass spectrometric detection.

A method based on direct-analysis-in-real-time mass spectrometry (DART-MS) for the qualitative and semi-quantitative analysis of eight organic UV filters and four parabens in twelve cosmetic products with substantially different formulations (as cream, milk, lotion, oil, lipstick) was developed. All tested substances could be identified unambiguously in the investigated samples without any sample pre-treatment. Direct analysis of cosmetic products allows semi-quantitative determination of parabens. For UV filters no satisfactory results were obtained by direct analysis but all analytes could be quantified by simply dissolving the samples in methanol, addition of an internal standard and subsequent measurement of the solution by DART-MS without further pre-treatment. The results obtained using DART-MS were confirmed by a more established method namely gas chromatography with mass spectrometric detection (GC-MS).

Investigations on the coupling of high-performance liquid chromatography to direct analysis in real time mass spectrometry.

A novel hyphenated technique namely the combination of HPLC with direct analysis in real time (DART) mass spectrometry (MS) is presented. The coupling of HPLC to DART-MS was achieved by a simple interface design with a capillary end piece transferring the HPLC effluent to the ionization region of the DART. Flow rates (0.3-1.6 mL min(-1)) and inner diameters of the capillary (50-150 microm) were harmonized to provide a stable liquid jet. Ionization-related parameters optimized included positioning of the capillary end piece with respect to He outlet of the ionization source and the MS inlet, He heater temperature and He flow rate. Among DART-MS voltages, the grid electrode voltage proved to have the most pronounced effect on signal intensities. A major benefit of this setup is the possibility to employ (commonly not MS-compatible) HPLC eluents such as phosphate buffers up to a phosphate concentration of 120 mM even at typical HPLC flow rates such as 1 mL min(-1) and above without negative side effects like contamination of the ion source or ion suppression. Experiments evaluating the correlation of signal intensity to mass-flow and concentration revealed that DART-MS can be seen as a mass flow sensitive detector. The usability of this hyphenated technique has been tested on the example of four parabenes (measured in the negative ion mode) as well as a set of pyrazine derivatives (measured in the positive ion mode). For the parabenes limits of detection (LOD) in the range of 20-55 microg L(-1) and linear ranges from at least 200-10000 microg L(-1) with correlation coefficients better than 0.997 were obtained.

Determination of organic UV filters in water by stir bar sorptive extraction and direct analysis in real-time mass spectrometry.

A screening method for analyzing environmental waters contaminated with UV filters using direct analysis in real-time mass spectrometry (DART-MS) was developed. To demonstrate the suitability of DART-MS a test set of seven organic UV filters, namely benzophenone-3 (BP-3), ethylhexyl dimethyl p-aminobenzoate (OD-PABA), 4-t-butyl-4'-methoxydibenzoylmethane (BM-DBM), homomethyl salicylate (HMS), 2-(ethylhexyl) salicylate (EHS), octocrylene (OC), and 4-methylbenzylidene camphor (4-MBC), was defined. In the first step, standard solutions of the analytes prepared in methanol were investigated in order to determine optimum parameters for the DART-MS. Because of the very low concentrations of UV filters expected in environmental water samples, a pre-concentration step using stir bar sorptive extraction was performed. DART-MS allows the direct, simple and rapid semi-quantitative analysis of the analytes enriched on the surface of the polydimethylsiloxane-coated stir bars. The optimized method provided calibration curves with correlation coefficients R > 0.959, repeatability from 5% (for 4-MBC) to 30% (for BM-DBM) relative standard deviation and limits of detection lower than 40 ng L(-1) for all analytes. Finally, real lake water samples from locations with typical leisure activities were analyzed. Results obtained with the developed DART-MS method were cross-checked by confirmatory analysis using thermodesorption gas chromatography mass spectrometry (TD-GC-MS). Thereby, it could be demonstrated that both analytical methods provide comparable concentrations for the UV filters in the lake water samples.

Rapid identification of stabilisers in polypropylene using time-of-flight mass spectrometry and DART as ion source.

A method for analysing plastic samples without any sample pretreatment using direct analysis in real time mass spectrometry (DART-MS) was developed. DART-MS allows the direct, simple and rapid identification of polymer additives in plastic products. To demonstrate the suitability of DART-MS for the detection of a wide range of commonly employed stabilising agents, a test set of 21 stabilisers was selected. In a first step standard solutions of these stabilisers in toluene as well as toluene-extracts from polymer samples were analysed. Subsequently, to prove the applicability of the developed DART-MS method also for the direct analysis of plastic products, samples of polypropylene containing a range of stabilisers were prepared using a lab-scale compounder. Polymer samples were cut into 0.5 cm wide pieces and directly placed into the DART ion source. Focusing on the DART ionisation, several parameters like discharge needle potential, potential of the grid electrode and the discharge electrode, the heater temperature and the gas flow had to be varied to guarantee optimum results. Both positive and negative ionisation was tested, whereby the positive ion mode led to higher signal intensities for all analytes. Determination of accurate masses to improve the certainty in signal assignment could be achieved by using PEG 600 as an internal standard for mass calibration. The developed method allowed the detection of all selected additives (including some of their degradation products) in real polymer samples.