Determination of alpha-tocopherol in infant foods by liquid chromatography combined with atmospheric pressure chemical ionisation mass spectrometry

A novel, sensitive and specific method for the quantification of alpha-tocopherol in two infant foods (milk and cereals) using liquid chromatography on-line with positive atmospheric pressure chemical ionisation mass spectrometry detection (LC/APCI-MS) has been developed. The samples were first saponified in order to eliminate fats and to transform tocopherol esters into free tocopherol, followed up by a liquid-liquid extraction of the analyte in petroleum benzine/diisopropyl ether (75:25, v/v) prior to injection onto the LC system. For the quantification, deuterium-labelled tocopherol was used as internal standard and the samples were monitored in selected ion monitoring (SIM) mode. Calibration curves between 1-40 microg/mL of alpha-tocopherol showed a good linear correlation (r(2) = 0.99994), and the detection limit was determined to be 2.5 ng/mL. The within-day and between-day precision were determined for several dietetic infant formulae and certified reference samples, and found to be below 3.5%. The accuracy determined on a Nestlé reference sample (milk powder) was calculated to be 115.2 +/- 1.2%, which confirms the robustness of the proposed method. This study shows that single quadrupole LC/MS can be applied for the quantification of vitamins in food and the method offers better sensitivity and selectivity than traditional method such as LC-UV. This would simplify the preparation of the food samples and consequently enhance the vitamin analysis throughput in the food area.     

Characterisation of sulphoxides by atmospheric pressure ionisation mass spectrometry

An observation that a series of proprietary compounds containing a methyl thiophenyl group all underwent metabolic S-oxidation, and that the product ion spectra of the resulting S-oxides showed methyl radical loss under low-energy atmospheric pressure ionisation tandem mass spectrometry (API-MS/MS) conditions, has led to an investigation of the fragmentation of commercially available sulphoxides. The phenyl methyl sulphoxides studied do lose methyl radicals under MS/MS conditions on triple quadrupole mass spectrometers. In addition, the phenyl sulphoxides, with simple substituents other than a methyl group, also showed a tendency to lose the substituent as a radical. It was concluded that radical loss from these simple sulphoxides was characteristic of S-oxidation of these molecules. Radical losses, such as those reported here, are used in-house to distinguish S-oxidation from N- and C-oxidation in metabolism studies.     

Direct quantitation of volatile organic compounds in packaging materials by headspace solid-phase microextraction-gas chromatography-mass spectrometry

The quantification of volatile organic compounds (VOCs) in flexible multilayer packaging materials using headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was studied. The analytes imclude 22 compounds such as aldehydes. ketones, carboxylic acids and hydrocarbons formed by thermooxidative degradation of polyethylene during the extrusion coating process in the manufacture of the packaging, and many of them are involved in the unpleasant and undesirable odour of these materials. External standard calibration using a solution of the analytes in an appropriate solvent was the first approach studied. Aqueous solutions of the analytes provided low reproducibility and the reduction of aldehydes to alcohols under the HS-SPME conditions. Hexadecane was chosen as the solvent since its polarity is similar to that of polyethylene and its volatility is lower than that of the analytes. However, hexadecane should be added to the sample before the analysis as it modifies the absorption capacity of the fibre. A 75-microm Carboxen-poly(dimethylsiloxane) fibre was used to extract the VOCs from the headspace above the packaging in a 15-ml sealed vial at 100 degrees C after 5 min of preincubation. The influence of the extraction time on the amount extracted was studied for a standard solution of the analytes in hexadecane, together with the influence of the volume of the standard solution and the amount of the sample placed in the vial. Standard addition and multiple HS-SPME were also studied as calibration methods and the results obtained in the quantitative analysis of a packaging material were compared.     

Ion suppression in the determination of clenbuterol in urine by solid-phase extraction atmospheric pressure chemical ionisation ion-trap mass spectrometry

Ion suppression effects were observed during the determination of clenbuterol in urine with solid-phase extraction/multiple-stage ion-trap mass spectrometry (SPE/MS(3)), despite the use of atmospheric pressure chemical ionisation. During SPE, a polymeric stationary phase (polydivinylbenzene) was applied. Post-cartridge infusion of analyte to the SPE eluate after the extraction of blank urine was performed to obtain a profile of the suppression. Single and multiple-stage MS were performed to provide insight in the suppressing compounds. The ion suppression was mainly ascribed to two m/z values, but still no identification of the compounds was achieved from the multiple-stage MS data. No ionisable and non-ionisable complexes and/or precipitation of clenbuterol with matrix compounds were observed. A concentration dependence of the percentage of suppression was observed. Up to 70% of the signal was suppressed upon post-cartridge infusion of 0.22 microg/mL (at 5 microL/min) clenbuterol into the eluate, and this decreased to about 4% at infusion of 22 microg/mL clenbuterol. Molecularly imprinted polymers were used to enhance the selectivity of the extraction. Although matrix components were still present after extraction, no interference of these compounds with the analyte was observed. However, the bleeding of the imprint from the polymer (brombuterol) caused significant ion suppression.     

Application of atmospheric pressure chemical ionisation mass spectrometry in the analysis of barbiturates by high-speed analytical countercurrent chromatography

Four barbiturates (barbital, allobarbital, phenobarbital and butalbital) were analysed using high-speed analytical countercurrent chromatography (HSACCC) and high-performance liquid chromatography (HPLC) interfaced with mass spectrometry, using negative mode atmospheric pressure chemical ionisation (APCI). The polar biphasic solvent system of butyronitrile/acetonitrile/water (1:1:1) was used, in the upper-stationary, lower-mobile mode of operation, at a flow rate of 1 mL/min and a rotational speed of 1200 rpm, equating to an applied "g"-field of 177 g. The fractional stationary phase retention (S(F)) was 0.58. Representative mass spectral data are presented from the HPLC and the HSACCC analyses. Structural information was obtained using source-induced fragmentation at increased source block voltages. The effect of increasing g-field on chromatographic resolution is illustrated using the binary base system of butyronitrile/water (1:1), under electrospray ionisation.     

Capillary column supercritical fluid chromatography-atmospheric pressure ionisation mass spectrometry interface performance of atmospheric pressure chemical ionisation and electrospray ionisation.

A supercritical fluid chromatography interface probe for atmospheric pressure ionisation mass spectrometry (API-MS) with the advantage of convenient switch between ionisation modes [atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI)] has recently been reported [P.J.R. Sj?berg, K.E. Markides, J. Chromatogr. A, 785 (1997) 101]. In order to obtain a stable ion signal and a low minimum detectable quantity, the design of the spray devise has to be optimised. For easy optimisation in the APCI mode, the corona needle was mounted directly on the interface probe. To compensate for the adiabatic cooling of the expanding mobile phase in the APCI mode, a heated region around the restrictor tip was used. In comparison, ESI required no additional heat, which might also prevent fragmentation for thermolabile compounds. As the mobile phase used was neat CO2, a low flow of make-up liquid was utilised in the ESI mode for transfer of the analytes from the expanding CO2 gas to the liquid phase before ionisation. The low make-up liquid flow in the ESI mode was sufficient for preventing the restrictor from becoming blocked. Factors that influence the ion signal intensity and stability have been studied. In APCI mode, corona needle position, nebuliser gas flow and gas additives were studied and in ESI mode, spray capillary assembly dimension and position, liquid flow-rate and composition were studied. The achievable detection limits were in the 50-0.1 pg (i.e., 290 fmol-140 amol) range. The detection limit in APCI mode was improved by a factor of about 20-25 compared to an earlier design [L.N. Tyrefors, R.X. Moulder, K.E. Markides, Anal. Chem. 65 (1993) 2835].     

Analysis of volatile organic compounds in polymers by dynamic headspace and gas chromatography/mass spectrometry

An experimental method for the analysis of volatile organic compounds in polymers is described. The technique involves dynamic headspace sampling, collection, and concentration of the volatiles in a cold trap, followed by capillary column gas chromatography/mass spectometry. Flow switching is carried out by the Deans switching technique. Four technical polymers used as pharmaceutical packaging materials have been analyzed in order to demonstrate the method.     

Detection of volatile organic sulfur compounds in water by headspace gas chromatography and membrane inlet mass spectrometry

Two gas chromatographic methods, GC-FID (flame ionization detection) and GC-ELCD (electrolytic conductivity detector) are compared in tlie analysis of volatile organic sulfur compounds (VOSCs) in water samples with a membrane inlet mass spectrometry (MIMS) technique. Carbon disulfide, ethanethiol, dimethyl sulfide, ethyl-methyl sulfide, thiophene, and dimethyl disulfide were used as test compounds. Linear dynamic ranges were found to be two decades with the GC-ELCD method and four decades with the GC-FID and MIMS methods. Detection limits were at low (μg/1 levels with the two gas chromatographic methods and clearly below μg/1 level with the MIMS method. Analysis of one sample takes 40 min with the gas chromatographic methods and five minutes with the MIMS method. The selectivity was good, especially with the GC-ELCD and the MIMS method. In addition, quantitative results obtained with spiked water samples by the three methods are compared.     

Identification of diastereomeric chlorophyll allomers by atmospheric pressure chemical ionisation liquid chromatography/tandem mass spectrometry

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry (APCI-LC/MS) has been used for identification of the epimers of hydroxy, methoxy and methoxylactone allomers of chlorophyll a (13(2)-HO-chl a, 13(2)-MeO-chl a and 15(1)-MeO-lact-chl a), the hydroxy allomer of bacteriochlorophyll a (13(2)-HO-bchl a) and the hydroxy and methoxylactone allomers of bacterioviridin a (13(2)-HO-bvir a and 15(1)-MeO-lact-bvir a). The APCI mass spectra show that facile fragmentations involve the methoxyl or hydroxyl groups at the C-13(2) or C-15(1) chiral centres. Losses involving the C-13(2) or C-15(1) hydroxyl or methoxyl groups occur more easily from the S-epimer than from the R-epimer due to the greater relief of the steric strain associated with interaction with the bulky C-17 substituent. The differences in mass spectrometric fragmentation can be used as a diagnostic tool for the assignment of the stereochemical configuration at the C-13(2) or C-15(1) chiral centres.     

顶空气相色谱-质谱法测定玩具中的10种挥发性有机物

建立了检测玩具中10种挥发性有机物(VOC)残留量的顶空气相色谱-质谱(HS-GC-MS)方法。样品经140 ℃、45 min静态顶空后,通过DB-624色谱柱分离和质谱检测,外标法定量。该方法对于不同VOC的定量限(LOQ)均在0.66 mg/kg以下,线性范围为0.001～2.0 μg,平均回收率在79%～106%之间,相对标准偏差(RSD)在0.4%～5.6%之间。该方法具有准确灵敏、简单快速等特点,将其应用于实际玩具样品的检测取得了良好效果。     

Characterisation of humic substances using atmospheric pressure chemical ionisation and electrospray ionisation mass spectrometry combined with size-exclusion chromatography

Humic substances were analysed by atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) mass spectrometry in positive and negative modes. Using APCI the average m/z range of humic substances was reduced 5-fold compared to ESI. High-resolution time-of-flight mass spectrometry revealed the formation of multiply charged molecules in the ESI mode. Moreover, it was possible to obtain daughter ion mass spectra of humic substances by nanospray tandem mass spectrometry. The size-exclusion chromatography elution profile of humic substances was highly influenced by the pH of the analyte solution. By contrast, the pH had no significant influence on the observed mass spectra of humic substances.     

Hyphenation of atmospheric pressure chemical ionisation mass spectrometry to supercritical fluid chromatography for polar car lubricant additives analysis

Car lubricant additives are added to mineral or synthetic base stocks to improve viscosity and resistance to oxidation of the lubricant and to limit wear of engines. As they belong to various chemical classes and are added to a very complex medium, the base stock, their detailed chromatographic analysis is very difficult and time consuming. In a previous paper, it was demonstrated that supercritical fluid chromatography (SFC) allows the elution of common low-molecular-weight additives. Since their total resolution could not be achieved owing to the limited peak capacity of packed columns, the hyphenation of selective and informative detection methods such as atomic emission detection (AED) was required. Further to results obtained in SFC-AED, this work describes the hyphenation of SFC to atmospheric pressure chemical ionisation ion trap mass spectrometry (MS). SFC–MS hyphenation is detailed: temperature, flow rates of gas and mobile phase introduced in the source, position of the restrictor, ionisation additives and conditions of autotune are studied. Car lubricant monitoring requires negative and positive ionisation modes with or without the addition of ionisation auxiliary solvent according to the nature of additives. Moreover, when sensitivity is of major concern for a selected additive, the autotuning routine of the MS has to be performed in conditions as close as possible to analytical conditions, i.e. under subcritical conditions. Unambiguous identification and structure elucidation of several additives in formulated car lubricants are also presented.     

Determination of volatile organic compounds by headspace trap

A new analytical method for the determination of halogenated and aromatic volatile organic compounds in groundwater, mineral water, and drinking water at concentrations ranging between 1-10000 ng/L is developed. A new type of headspace sampler that combines static headspace sampling with a trap is used, yielding very low detection limits and good repeatability without carryover effects. An unexpected transformation of 1,1,2,2-tetrachloroethane into trichloroethene is observed and explained.     

Analysis of wax ester molecular species by high performance liquid chromatography/atmospheric pressure chemical ionisation mass spectrometry

High chromatographic resolution of wax esters (WEs) was achieved by non-aqueous reversed-phase liquid chromatography on a Nova-Pak C18 column by optimising the acetonitrile/ethyl acetate mobile phase gradient. The retention behaviour of WEs was studied in this chromatographic system. The WEs eluted according to their equivalent carbon number (ECN) values; within the group of WEs with the identical ECN, the most unsaturated species tended to elute first. The isobaric WEs with different positions of the ester moiety were separated from each other whenever the lengths of the chains were sufficiently different. The methyl-branched esters eluted at shorter retention times than the straight-chained analogues, and the resolution among methyl-branched WEs depended on the position of the branching. The analytes were detected by atmospheric pressure chemical ionisation mass spectrometry (APCI-MS) using data-dependent scanning. WEs provided simple full-scan spectra with abundant protonated molecules and low-intensity fragments. Collision-induced dissociation (CID) promoted identification of the WE molecular species. The responses of WEs were found to be dependent on the number of double bonds and on the alkyl-chain length; the limits of the detection ranged from 20 μmol/L to 200 nmol/L. The HPLC/APCI-MS was applied for the analysis of the WEs isolated from honeycomb beeswax, jojoba oil and human hair. Good agreement between reported results and the literature data was achieved, with several novel polyunsaturated WEs also being found.     

Structural characterisation of unsaturated bacterial hopanoids by atmospheric pressure chemical ionisation liquid chromatography/ion trap mass spectrometry

The production of bacteriohopanepolyols (BHPs) is widespread in many different groups of prokaryotes; however, unsaturated components are less common except amongst the acetic acid bacteria. Here we describe the characterisation of mono- (Delta(6) or Delta(11)) and diunsaturated (Delta(6,11)) bacteriohopanetetrols isolated from the acetic acid bacterium Gluconacetobacter xylinus (formerly Acetobacter aceti ssp. xylinum) by atmospheric pressure chemical ionisation ion trap mass spectrometry (APCI-MS(n)). APCI-MS(2) spectra are compared with equivalent electron ionisation (EI) spectra and differences in fragmentation pathways are discussed. Having established characteristic spectral features for a range of unsaturated BHPs we now have the ability to rapidly detect the presence of unsaturated BHPs in both natural environmental samples (soils, sediments, water columns) as well as in microbial cultures.     

Characteristic fragmentation of bacteriohopanepolyols during atmospheric pressure chemical ionisation liquid chromatography/ion trap mass spectrometry

Bacteriohopanepolyols (BHPs) fragment via characteristic pathways during atmospheric pressure chemical ionisation liquid chromatography/ion trap mass spectrometry (APCI-LC/MS(n)). Comparison of the MS(2) spectra of bacteriohopane-32,33,34,35-tetrol (BHT) and 2 beta-methylbacteriohopane-32,33,34,35-tetrol has confirmed the previously proposed ring-C cleavage occurring between C-9 and 11 and C-8 and 14. This fragmentation, diagnostic of all hopanoids, also occurs in BHPs containing an amino group (-NH(2)) at C-35 although the higher relative stability of the ion limits this fragmentation to a minor process after protonation of the basic nitrogen function. Studies of a number of cell cultures including a prochlorophyte (Prochlorothrix hollandica) and a cyanobacterium (Chlorogloeopsis LA) demonstrate the power of this technique to detect composite BHPs with a complex biological functionality at C-35. We also report the first observation of intact pentafunctionalised bacteriohopanepolyols using this method.     

On-line flow injection analysis of volatile organic compounds in seawater by membrane introduction mass spectrometry

The combination of flow injection analysis with membrane introduction mass spectrometry for analysis of volatile organic compounds (VOCs) in seawater is examined and is compared to measurements made in water. Membrane introduction mass spectrometry is performed using a benchtop ion trap mass spectrometer, and characterization of various aspects of the flow injection and ion trap combination for the analysis of volatile organic compounds (including anthropogenic halocarbons) in seawater is carried out. The analyte responses are shown to be linear over several orders of magnitude (e.g. for methylene chloride), independent of seawater pH (e.g. for chlorobenzene) and independent of matrix effects for the VOCs studied. A comparison of the performance of a microporous (Teflon) membrane with that of an amorphous silicone membrane is made, and the former is shown to provide lower detection limits which are in the parts-per-trillion range (300 ppt for chlorobenzene, 190 ppt for trans-1,2-dichloroethene). The microporous membrane provides faster response times by a factor of four to five for relatively more polar compounds, such as chlorobenzene. An analysis of a seven-component mixture demonstrates the ability of this on-line combination to allow multicomponent analysis of mixtures of some complexity.