Direct quantification of inorganic iodine in seawater by mixed-mode liquid chromatography-electrospray ionization-mass spectrometry.

Atmospheric iodine plays a relevant role in climate change. Bearing in mind that most of this iodine comes from the oceans, analytical methods capable of determining iodine in a challenging matrix as seawater are necessary. In this work, the first method capable of direct determination of total inorganic iodine in seawater at subnanomolar level based on mixed-mode liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) without any sample treatment is presented. Analytical characteristics of the developed method were studied in terms of linear range, limits of detection and quantification, precision, trueness, matrix effect, and robustness. The detection limit for iodide was as low as 0.16 nM, injecting 5 µL of seawater without any sample treatment and the working linear range of four orders of magnitude was wide enough to cover the broad concentration range observed in seawater samples. Average values for repeatability and intermediate precision were 4.1% and 8.1%, respectively. The suitability of the method was demonstrated through its application to the analysis of several types of samples, including seawater samples taken at different locations along the Spanish Mediterranean coast and some domestic iodized salts. According to the results obtained, the method developed is rapid, easy to apply and to be automated, avoids sample treatment and requires only few microliters of sample. Furthermore, it has a low detection limit and allows the quantification of inorganic iodine over a wide concentration range.

A comprehensive profiling of sulfatides in myelin from mouse brain using liquid chromatography coupled to high-resolution accurate tandem mass spectrometry.

Sulfatides are sulfoglycolipids found in the myelin sheath. The composition ratio of sulfatide molecular species changes with age, and it has also been associated with the pathogenesis of various human central nervous system diseases. However, profiling sulfatides in biological samples is difficult, due to the great variety of molecular species. In this work, a new, easy and reliable liquid chromatography-electrospray tandem mass spectrometry (LC-ESI(+)-MS/MS) method has been developed to profile sulfatide content in biological samples of myelin. The 'wrong-way-round' ionization effect has been described for this type of molecules for the first time, making it possible to correctly identify as many as 37 different sulfatides in mouse brain myelin samples, including molecules with different fatty acid chain lengths and varying degrees of unsaturation and hydroxylation. A chemometric analysis of their relative abundances showed that the main difference among individuals of different ages was the content of sulfatides with odd-numbered fatty acid chains, in addition to hydroxylated species.

Comprehensive evaluation of direct injection mass spectrometry for the quantitative profiling of volatiles in food samples.

Although qualitative strategies based on direct injection mass spectrometry (DIMS) have recently emerged as an alternative for the rapid classification of food samples, the potential of these approaches in quantitative tasks has scarcely been addressed to date. In this paper, the applicability of different multivariate regression procedures to data collected by DIMS from simulated mixtures has been evaluated. The most relevant factors affecting quantitation, such as random noise, the number of calibration samples, type of validation, mixture complexity and similarity of mass spectra, were also considered and comprehensively discussed. Based on the conclusions drawn from simulated data, and as an example of application, experimental mass spectral fingerprints collected by direct thermal desorption coupled to mass spectrometry were used for the quantitation of major volatiles in Thymus zygis subsp. zygis chemotypes. The results obtained, validated with the direct thermal desorption coupled to gas chromatography-mass spectrometry method here used as a reference, show the potential of DIMS approaches for the fast and precise quantitative profiling of volatiles in foods.This article is part of the themed issue 'Quantitative mass spectrometry'.

Fast and simultaneous determination of endocrine disrupting compounds by ultra-high performance liquid chromatography-tandem mass spectrometry.

A rapid and sensitive analytical method for the simultaneous determination of thirteen endocrine disruptors (five phthalates, seven parabens, and bisphenol A) in a single chromatographic run has been developed for the first time. The separation method, based on ultra-high performance liquid chromatography (UHPLC), allows the separation of all compounds (including isobaric pairs) in less than 4.1 min. The fast polarity switching mode of the triple quadrupole mass spectrometer used enables the registration of positive (phthalates) and negative (parabens and BPA) ions in the same acquisition run. A Response Surface Methodology was used for the optimization of the method. The optimum elution program starts with 0.2 min in isocratic conditions (79.8% water; 20% acetonitrile, 0.2% ammonium formate 5mM at pH 10.2), then the content of acetonitrile is linearly increased in 2 min up to 42%, and later up to 98% in 1.1 min. The analytical characteristics of the developed method were satisfactory. The method is robust and showed a linear response with determination coefficients (R(2)) higher than 0.991 in the range 5.0-2000 pg on column (or higher) for all the compounds investigated. Instrumental intra- and inter-day precision (expressed as relative standard deviation) were lower than 12% for parabens and bisphenol A, and between 5.9% and 27% for phthalates. Instrumental detection and quantification limits (iLODs and iLOQs) were in the range of medium-high femtograms (270-1300 pg on column for iLODs). Finally, the suitability of the developed method was demonstrated through its application to the analysis of commercial personal care products (shower gels) without any sample treatment, only a simple dilution, being possible to determine the simultaneous presence of phthalates, parabens, and bisphenol A in almost all the gels analyzed.

Analysis of iminosugars and other low molecular weight carbohydrates in Aglaonema sp. extracts by hydrophilic interaction liquid chromatography coupled to mass spectrometry.

A method by hydrophilic interaction liquid chromatography coupled to tandem mass spectrometry (HILIC-MS(2)) has been successfully developed for the simultaneous analysis of bioactive iminosugars and other low molecular weight carbohydrates in Aglaonema leaf extracts. Among other experimental chromatographic conditions, mobile phase eluents, additives and column temperature were evaluated in terms of retention time, resolution, peak width and symmetry provided for target carbohydrates. In general, narrow peaks (wh: 0.2-0.6min) with good symmetry (As: 0.9-1.3) and excellent resolution (Rs>1.8) were obtained for iminosugars using an acetonitrile:water gradient with 5mM ammonium acetate in both eluents at 55°C. Tandem mass spectra were used to confirm the presence of previously detected iminosugars in Aglaonema extracts and to tentatively identify for the first time others such as miglitol isomer, glycosyl-miglitol isomers and glycosyl-DMDP isomers. Concentration of total iminosugars varied from 1.35 to 2.84mgg(-1) in the extracts of the different Aglaonema samples analyzed. To the best of our knowledge, this is the first time that a HILIC-MS(2) method has been proposed for the simultaneous analysis of iminosugars and other low molecular weight carbohydrates of Aglaonema sp. extracts.

Insight into the retention processes of phthalate metabolites on different liquid chromatography stationary phases for the development of improved separation methods.

The retention behavior of nine MPAEs has been studied, using commercial LC columns with octadecylsilane (ODS), phenyl, and amide-type SPs. First, it was found that the use of methanol in the mobile phase is not advisable, because induce a transesterification reaction of MPAEs in the electrospray ion source, regardless of the SP used. On the other hand, different responses were observed when representing the logarithm of retention factors (k) vs. the volume fraction of ACN (φ) in the mobile phase, for the three SPs tested. A quite linear trend was obtained for ODS (at φ values below 0.80) and Phenyl columns. On the contrary, the Amide column shows a striking U-shape trend, typical of both hydrophobic and hydrophilic retention mechanisms. Therefore, the separation process was mainly hydrophobic in the ODS and phenyl SPs, but in the amide-type a dual retention mechanism was found, showing zones with predominant hydrophobic or hydrophilic interactions, depending on both the compound and the experimental conditions. A high content of acetonitrile (>75%) and low concentration of formic acid in the mobile phase promote the hydrophilic separation mechanism for MPAEs on the amide SP. So, this dual separation mechanism can be modulated modifying the pH and content of organic modifier in the mobile phase, allowing greater flexibility to develop improved methods. Taking advantage of this, a separation method was optimized in this amide column using a Box-Wilson Central Composite experimental design, which allows separating the studied MPAEs with a time-saving of around 40% comparing to the conventional phenyl SP.

Evaluation of different hydrophilic stationary phases for the simultaneous determination of iminosugars and other low molecular weight carbohydrates in vegetable extracts by liquid chromatography tandem mass spectrometry.

Iminosugars are considered potential drug candidates for the treatment of several diseases, mainly as a result of their α-glycosidase inhibition properties. A method by hydrophilic interaction liquid chromatography tandem mass spectrometry has been optimized for the first time for the simultaneous determination of complex mixtures of bioactive iminosugars and other low molecular weight carbohydrates (LMWC) in vegetable extracts. Three hydrophilic stationary phases (sulfoalkylbetaine zwitterionic, polyhydroxyethyl aspartamide and ethylene bridge hybrid (BEH) with trifunctionally bonded amide) were compared under both basic and acidic conditions. The best sensitivity (limits of detection between 0.025 and 0.28ngmL-1) and overall chromatographic performance in terms of resolution, peak width and analysis time were obtained with the BEH amide column using 0.1% ammonium hydroxide as a mobile phase additive. The optimized method was applied to the analysis of extracts of hyacinth bulbs, buckwheat seeds and mulberry leaves. Iminosugar and other LMWC structures were tentatively assigned by their high resolution daughter ions mass spectra. Several iminosugars such as glycosyl-fagomine in mulberry extract were also described for the first time. Among the extracts analysed, mulberry showed the widest diversity of iminosugars, whereas the highest content of them was found in hyacinth bulb (2.5mgg-1) followed by mulberry (1.95 mgg-1).

On the influence of column temperature on the isothermal retention indices of structurally different solutes on a poly(dimethylsiloxane) capillary column.

The dependence of isothermal retention indices (I) on column temperature over a wide temperature range has been studied for solutes belonging to nine chemical functions on a capillary column coated with poly(100% dimethyl siloxane). I values for some solutes are reported for the first time on capillary columns. I values increased with increasing column temperature, with the exception of the linear alcohols and the esters, which decreased with increasing temperature, and of cyclobutanol, 2-butanone, 2-pentanone, 1-butylamine and 1-pentylamine that showed a well-defined minimum in the 358-377K range. Moreover, a minimum at the higher temperature range for longer and less polar solutes such as 1-nonanol was observed for the first time. The three trends of I vs. T were perfectly described by the extended model (I=a+bT(-1)+clnT). On the other hand, the dependence of I on the carbon atom number (z) of the solute was linear and with a slope of similar magnitude for all homologous series studied, except for the alicyclic compounds. For the latter, higher slope values and worse correlations were obtained, owing to their larger surface area and to the different conformations that they adopt in order to minimize the ring strain. In addition, due to its higher chain stiffness, an important influence of the column temperature on these slopes was observed.

Characterization by the solvation parameter model of the retention properties of commercial ionic liquid columns for gas chromatography.

For the first time, four commercial ionic liquid columns (SLB-IL59, SLB-IL76, SLB-IL82 and SLB-IL100) for gas chromatography have been comprehensively evaluated in terms of efficiency, polarity and solvation properties. Grob tests and McReynolds constants showed that they were all high-efficiency columns of high polarity, but with low inertness to compounds with hydrogen bonding capabilities. The solvation parameter model was used to characterize the solvation interactions of the four columns in the 80-160°C temperature range. Results revealed that all the ionic liquids studied can be considered moderately hydrogen-bond acid and highly cohesive stationary phases, on which the dominant contributions to retention were the dipolar-type and hydrogen-bond base interactions, while π-π and n-π interactions were barely significant. The SLB-IL59 column provided the best separation of homologs, while the SLB-IL76 and SLB-IL100 columns had the most basic and the most acidic phases, respectively. A principal component analysis for the commonly used stationary phases in capillary GC showed that these commercial ionic liquid columns fill an empty area of the available selectivity space, which clearly enhances the separation capacity of this technique.

Application of the solvation parameter model to poly(methylcyanopropylsiloxane) stationary phases.

The solvation parameter model has been applied to the specific retention volumes of 65 solutes of varied polarity on glass capillary columns coated with commercial and synthesized poly(methylcyanopropyl)siloxanes (CNPXX) with eight different percentages of cyanopropyl group (CNP). Their system constants were determined at 75, 90, 105 and 120 degrees C. The polymers examined do not either show any acidity (b = 0) or interact with solute pi/n electrons (e = 0); the prominent constants, dipolarity/polarizability and hydrogen-bond basicity, are of the same order (s approximately a), and the cavity formation/dispersive forces have normal values. Constants s, l and a decrease linearly with temperature for each cyanopropyl percentage. At each temperature, the constants s and a increase with polarity of polymer according to a curve, while the constant l decreases slightly. Cluster analysis shows that six CNPXX with medium to high cyanopropyl substitution integrate into a group with other high-polarity cyano-containing stationary phases taken from the literature, while the other three CNPXX with low CNP percentage form a group with other low-polarity stationary phases of different chemical nature. These clusters are supported by the dendrogram of 52 stationary phases made with the nine polymers presented here and other 43 taken from the literature.

System constants of synthesized poly(methyl-3,3,3-trifluoropropyl) siloxanes.

The method of solvation model has been applied to five poly (methyl-trifluoropropyl) siloxanes (TFPSXX) prepared in our laboratories, at five trifluoropropyl (TFP) group contents, XX = 0, 11.5, 26.3, 35.5 and 50.0%, at 80, 100, 120 and 140 degrees C. Previously, specific retention volumes of 60-odd solutes of varied polarities were measured upon each of these stationary phases within the above temperature range. Constant s prevails over all other constants, TFPSXX stationary phases showing strong dipole/induced dipole forces with the solutes, moderate acidity and no basicity at all. Constant e is zero in the stationary phase without TFP groups, but has negative low-medium values for the other fluorine contents, XX from 11.5 to 50.0%, hinting at repulsive forces, as expected. Normal values for constant l, decreasing from the less cohesive TFPS00 to the more cohesive TFPS50, were found. For each TFP content constants s, a and l show a negative temperature dependence, while constant e increases as temperature increases. Constant c also decreases with increasing temperature. At each temperature, constants s and a increase with increasing %TFP (or increasing stationary phase polarity), whereas constants e and l show the opposite trend, diminishing with increasing polarity of the stationary phase. Principal component analysis shows that the five stationary phases presented in this work conform a group with other earlier synthesized trifluoropropyl siloxanes and other fluorinated stationary phases taken from literature: VB-210, QF-1, DB-200, DB-210 and PFS6, showing the same selectivity which only the fluorine atom confers. A dendrogram of 38 stationary phases supports these results.

Hold-up time in gas chromatography. V. Dependence of the retention of n-alkanes on the chromatographic variables in isothermal gas chromatography.

The chromatographic behaviour of n-alkanes and other homologous series in isothermal gas chromatography has been shown to depart from the "linear" representation of the logarithms of the adjusted retention times vs. carbon number. One of the expressions proposed to describe this behaviour is tR(z)=A+exp(B+CzD). In this paper, a regression analysis shows that three of the parameters of the equation depend on different chromatographic variables such as hold-up time, average linear gas velocity, volume and polarity of the stationary phase and temperature of the column. The fourth parameter (D), responsible for the departure from the "linearity", does not depend on any chromatographic variable, and represents a gradual decrease of the contribution of the methylene groups to the general properties of n-alkanes, with no relation to the chromatographic phenomenon.

Comparative study of clean-up and fractionation methods for the determination of organochlorine pesticides in lipids by gas chromatography.

Three of the methods most often used for the clean-up and fractionation of organochlorine pesticides in lipid residue analysis by gas chromatography with electron-capture detection were compared. The overall recoveries of twenty pesticides from spiked samples were higher than 88%, the relative standard deviation being in the range 3-11% (n = 6) at the 36-80 ppb (10(9) level. The three methods were compared by analysis of variance, with no differences in precision at the 0.05 significance level. Differences in recoveries appeared in only two instances. None of the three methods seems to be significantly better than the others for the determination of the pesticides studied.