Determination of diquat and paraquat in water by liquid chromatography-(electrospray ionization) mass spectrometry

A method for the determination of the herbicides diquat and paraquat in water was developed using liquid chromatography-(electrospray ionization) mass spectrometry [LC-(ESI)MS]. The analytes were isolated on an ENVI-8 DSK solid phase extraction (SPE) disk and eluted with 5-M trifluoroacetic acid (TFA). The eluate was evaporated to dryness and the analytes were redissolved in the mobile phase (7% methanol/93% water/25-mM TFA). The extract was analyzed by liquid chromatography (C1 column) with postcolumn addition of propionic acid/methanol followed by (ESI)MS. Diquat was detected using the [M²⁺ ? H⁺] ion (M²⁺ = dication) at m/z 183, whereas paraquat was detected using the mono-trifluoroacetate ion pair [M²⁺/^?OOCCF₃] at m/z 299. Quantitation was done by isotope dilution mass spectrometry using d ₄-diquat and d ₈-paraquat and the corresponding ions [M²⁺ ? D⁺] and [M²⁺/^?OOCCF₃] at m/z 186 and m/z 307, respectively. Detection limits of 0. 1 and 0. 2 µg/L, respectively (based on the dications), were adequate to meet the Ontario Drinking Water Objectives of 70 and 10 µg/L, respectively, and the Ontario Provincial Water Quality Objective for diquat of 0. 5 µg/L. Precision and accuracy were 14% and 6% for diquat and 12% and 3% for paraquat.     

Sensitivity of Positive Ion Mode Electrospray Ionization Mass Spectrometry in the Analysis of Thiol Metabolites

High signal intensities of glutathione (GSH), [GSH+H]⁺ (m/z 308), cysteine (CySH), [CySH+H]⁺ (m/z 122), and homocysteine (hCySH), [hCySH+H]⁺ (m/z 136), are observed in ESI MS with on‐line electrochemistry (EC). Dimers formed by H‐bonding, which are not electrochemical products, are detected as [2GSH+H]⁺ (m/z 615), [2CySH+H]⁺ (m/z 243) and [2hCySH+H]⁺ (m/z 271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]⁺ (m/z 613), [CySSCy+H]⁺ (m/z 241) and [hCySSCyh+H]⁺ (m/z 269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.     

Structural determination by atmospheric pressure photoionization tandem mass spectrometry of some compounds isolated from the SARA fractions obtained from bitumen

We have identified compounds obtained from the SARA fractions of bitumen by using atmospheric pressure photoionization mass spectrometry and low‐energy collision tandem mass spectrometric analyses with a QqToF‐MS/MS hybrid instrument. The identified compounds were isolated from the maltene saturated oil and the aromatic fractions of the SARA components of a bitumen. The QqToF instrument had sufficient mass resolution to provide accurate molecular weight information and to enhance the tandem mass spectrometry results. The APPI‐QqToF‐MS analysis of the separated compounds showed a series of protonated molecules [M + H]⁺ and molecular ions [M]^+? of the same mass but having different chemical structures, in the maltene saturated oil and the aromatic SARA fractions. These isobaric ions were a molecular ion [M₂]^+? at m/z 418.2787 and a protonated molecule [M₅ + H]⁺ at m/z 287.1625 in the saturated oil fraction, and molecular ions [M₆]^+? at m/z 418.1584 and [M₇]^+? at m/z 287.1285 in the aromatic fraction. The identification of this series of chemical compounds was achieved by performing CID‐MS/MS analyses of the molecular ions [M]^+? ([M₁]^+? at m/z 446. 2980, [M₂]^+? at m/z 418.2787, [M₃]^+? at m/z 360.3350 and [M₄]^+? at m/z 346.2095) in the saturated oil fraction and of the [M₅ + H]⁺ ion at m/z 287.1625 also in the saturated oil fraction. The observed CID‐MS/MS fragmentation differences were explained by proposed different breakdown processes of the precursor ions. The presented tandem mass spectrometric study shows the capability of MS/MS experiments to differentiate between different classes of chemical compounds of the SARA components of bitumen and to explain the reasons for the observed mass spectrometric differences. However, greater mass resolution than that provided by the QqToF‐MS/MS instrument would be required for the analysis of the asphaltene fraction of bitumen. Copyright © 2011 John Wiley & Sons, Ltd.     

The application of gas chromatography/atmospheric pressure chemical ionisation time‐of‐flight mass spectrometry to impurity identification in Pharmaceutical Development

Accurate mass measurement (used to determine elemental formulae) is an essential tool for impurity identification in pharmaceutical development for process understanding. Accurate mass liquid chromatography/mass spectrometry (LC/MS) is used widely for these types of analyses; however, there are still many occasions when gas chromatography (GC)/MS is the appropriate technique. Therefore, the provision of robust technology to provide accurate mass GC/MS (and GC/MS/MS) for this type of activity is essential. In this report we describe the optimisation and application of a newly available atmospheric pressure chemical ionisation (APCI) interface to couple GC to time‐of‐flight (TOF) MS. To fully test the potential of the new interface the APCI source conditions were optimised, using a number of standard compounds, with a variety of structures, as used in synthesis at AstraZeneca. These compounds were subsequently analysed by GC/APCI‐TOF MS. This study was carried out to evaluate the range of compounds that are amenable to analysis using this technique. The range of compounds that can be detected and characterised using the technique was found to be extremely broad and include apolar hydrocarbons such as toluene. Both protonated molecules ([M + H]⁺) and radical cations (M^+.) were observed in the mass spectra produced by APCI, along with additional ion signals such as [M + H + O]⁺. The technique has been successfully applied to the identification of impurities in reaction mixtures from organic synthesis in process development. A typical mass accuracy of 1–2 mm/zunits (m/z 80–500) was achieved allowing the reaction impurities to be identified based on their elemental formulae. These results clearly demonstrate the potential of the technique as a tool for problem solving and process understanding in pharmaceutical development. The reaction mixtures were also analysed by GC/electron ionisation (EI)‐MS and GC/chemical ionisation (CI)‐MS to understand the capability of GC/APCI‐MS relative to these two firmly established techniques. Copyright © 2010 John Wiley & Sons, Ltd.     

Isotope abundance analysis for improved sample identification with tandem mass spectrometry

Tandem mass spectrometry (MS/MS) is widely used for trace level sample analysis in complex mixtures. However, sample identification in MS/MS is challenging and not as trustworthy as with electron ionization (EI) mass spectral libraries. This paper presents a novel method for the combination of isotope abundance analysis (IAA) and EI‐MS/MS for improved sample identification even at trace level in complex matrices. Accordingly, the first quadrupole is scanned in a narrow range around the molecular ion group of isotopomers such as M⁺, [M+1]⁺ and [M+2]⁺, Q2 serves for collision‐induced dissociation to produce product ions while Q3 transfers the major sample product ions with low resolution, thus encompassing and uniformly transmitting all the product ion isotopomers. IAA can then be used to derive elemental formula information from the cleansed experimental data. IAA‐MS/MS was experimentally tested with perfluorotributylamine and a very good matching factor of 995 (out of 1000) was obtained for IAA on m/z 502, 503 and 504 (fragment ion isotopomers) while Q3 transmitted the m/z 264 product ion with a mass window of 6 m/z units. The IAA‐MS/MS method was further tested with the pesticide diazinon on its molecular ions m/z 304, 305 and 306 while Q3 was locked on its m/z 179 product ion with a mass window of 6 m/z units. Again, very good matching factors were obtained, even for 40 pg diazinon on‐column during its GC/MS analysis (match = 981). IAA‐MS/MS combines the traditional benefits of MS/MS in the removal of matrix interferences with the IAA power of elemental analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

Antioxidant Pathways and One‐Electron Oxidation of Dopamine and Cysteine in Electrospray and On‐Line Electrochemistry Electrospray Ionization Mass Spectrometry

Dopamine [DA]⁺ (m/z 154), DA dimer [2DA‐H]⁺ (m/z 307) and DA quinone [DAQ]⁺ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e^?, 1H⁺ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e^?, 2H⁺ oxidation reaction. On‐line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone‐cysteine adduct [DAQ+Cys]⁺ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone‐cysteine adduct by 1e^? pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.     

Mass spectra of some α-substituted phenyl-α,α′-dimethoxyl ketones and their derivatives

The mass spectra of some α-substituted phenyl-α,α′-dimethoxyl ketones (compounds 1) and their 2,4-dinitrophenylhydrazones (compounds 2) and semicarbazones (compounds 3) have been studied. The characteristic fragments at m/z (M ? 73) from compounds 1, m/z (M ? 253) from compounds 2 and m/z (M ? 130) from compounds 3 are abundant and proposed to be [ArCROCH₃]⁺. Fragmentations yielding [M⁺ ? 49] from compounds 2 are abnormal and probably involve the methoxyl and nitro groups. The intense peak at m/z 130 due to [CH₃OCH₂CNNHCONH₂]⁺ from compounds 3 corresponds to α-cleavage of the molecular ion. Some other fragments from these new compounds are interpreted in this paper.     

Gas‐phase fluorine migration reactions in the radical cations of pentafluorosulfanylbenzene (Aryl―SF5) and benzenesulfonyl fluoride (Aryl―SO2F) derivatives and in the 2,5‐xylylfluoroiodonium ion

The gas‐phase reactions of Aryl―SF₅^·+ and Aryl―SO₂F^·+ have been studied with the electron ionization tandem mass spectrometry. Such reactions involve F‐atom migration from the S‐atom to the aryl group affording the product ion Aryl―F^·+ by subsequent expulsion of SF₄ or SO₂, respectively. Especially, the 4‐pentafluorosulfanylphenyl cation 4‐SF₅C₆H₄⁺ (m/z 203) from 4‐NO₂C₆H₄SF₅^·+ by loss of ·NO₂ could occur multiple F‐atom migration reactions to the product ion C₆H₄F₃⁺ (m/z 133) by loss of SF₂ in the MS/MS process. The gas‐phase reactions of 2,5‐xylylfluoroiodonium (pXyl―I⁺F, m/z 251) have also been studied using the electrospray tandem mass spectrometry, which involve a similar F‐atom migration process from the I‐atom to the aryl group giving the radical cation of 2‐fluoro‐p‐xylene (or its isomer 4‐fluoro‐m‐xylene, m/z 124) by reductive elimination of an iodine atom. All these gas‐phase F‐atom migration reactions from the heteroatom to the aryl group led to the aryl―F coupling product ions with a new formed C_Aryl―F bond. Density functional theory calculations were performed to shed light on the mechanisms of these reactions. Copyright © 2014 John Wiley & Sons, Ltd.     

Development and application of a LC–MS/MS assay for the simultaneous quantification of edaravone and taurine in beagle plasma

An LC–MS/MS method was developed and validated for the simultaneous quantification of edaravone and taurine in beagle plasma. The plasma sample was deproteinized using acetonitrile containing formic acid. Chromatographic separations were achieved on an Agilent Zorbax SB‐Aq (100 × 2.1 mm, 3.5 μm) column, with a gradient of water (containing 0.03% formic acid) and methanol as the mobile phase at a flow rate of 0.3 mL/min. The analyte detection was carried out in multiple reaction monitoring mode and the optimized precursor‐to‐product transitions of m/z [M+H]⁺ 175.1 → 133.0 (edaravone), m/z [M+H]⁺ 189.1 → 147.0 (3‐methyl‐1‐p‐tolyl‐5‐pyrazolone, internal standard, IS), m/z [M–H]^? 124.1→80.0 (taurine), and m/z [M–H]^? 172.0 → 80.0 (sulfanilic acid, IS) were employed to quantify edaravone, taurine, and their corresponding ISs, respectively. The LOD and the lower LOQ were 0.01 and 0.05 μg/mL for edaravone and 0.66 and 2 μg/mL for taurine, respectively. The calibration curves of these two analytes demonstrated good linearity (r ＞ 0.99). All the validation data including the specificity, precision, recovery, and stability conformed to the acceptable requirements. This validated method has successfully been applied in the pharmacokinetic study of edaravone and taurine mixture in beagle dogs.     

Determination of polybrominated biphenyls in Tasmanian devils (Sarcophilus harrisii) by gas chromatography coupled to electron capture negative ion tandem mass spectrometry or electron ionization high‐resolution mass spectrometry

Two gas chromatography/mass spectrometry (GC/MS) methods for the determination of polybrominated biphenyls (PBBs) by isotope dilution analysis (IDA) using ¹³C₁₂‐PBB 153 in the presence of polybrominated diphenyl ethers (PBDEs) were compared. Recovery of ¹³C₁₂‐PBB 153 which was added to the extracted lipids before sample purification was commenced ranged from 88–117% (mean value 98.2 ± 8.9%). Nevertheless, IDA analysis of PBBs using ¹³C₁₂‐labelled congeners is limited by the potential co‐elution of PBBs with polybrominated diphenyl ethers (PBDEs). The pair PBB 153 and BDE 154 was inspected since M⁺ and [M–2Br]⁺ ions of ¹³C₁₂‐PBB 153 and BDE 154 were only separated by 4 u. Gas chromatography/electron ionization high‐resolution mass spectrometry with selected ion monitoring (GC/EI‐HRMS‐SIM) was suitable when m/z 475.7449 and m/z 477.7429 were used for ¹³C₁₂‐PBB 153 because they are below the monoisotopic peak of the [M–2Br]⁺ fragment ion of hexaBDEs at m/z 479.7. Gas chromatography/electron capture negative ion tandem mass spectrometry selected reaction monitoring (GC/ECNI‐MS/MS‐SRM) measurements could be applied because ¹³C₁₂‐PBB 153 and BDE 154 were separated by GC on a 25‐m Factor Four CP‐Sil 8MS column. Comparative measurements with GC/EI‐HRMS‐SIM and GC/ECNI‐MSMS‐SRM were carried out with samples of Tasmanian devils from Tasmania (Australia), an endangered species due to a virus epidemy which has already proved fatal for half of the population. Both techniques verified concentrations of PBB 153 in the range 0.3–11 ng/g lipids with excellent agreement of the levels in all but two samples. The PBB residue pattern demonstrated that PBB pollution originated from the previous discharge with technical hexabromobiphenyl which is dominated by PBB 153. Other congeners such as PBB 132 and PBB 138 were detected in the Tasmanian devils but the proportions relative to PBB 153 were lower than in the technical product. Samples of healthy and affected Tasmanian devils showed no significant difference in the PBB pollution level. The PBB concentrations in the Tasmanian devils were significantly below those causing toxic effects. On the other hand, PBB concentrations were one level or even higher than PBDEs. Copyright © 2008 John Wiley & Sons, Ltd.     

Tandem mass spectrometric approaches for the analysis of alkylguanines in human urine

Human exposure to carcinogenic alkylating agents can lead to the formation of covalently bound adducts in DNA, some of which are excreted in urine as alkylated purines following DNA degradation and repair. Tandem mass spectrometric methods have been developed for the qualitative and quantitative determination of such alkylpurines in human urine. Short-chain alkyl- and hydroxyalkylguanines have been synthesized with the substituents at the N-7-, O⁶- and N2-positions of guanine. Examination of the product ion scans of their molecular ions (electron impact (EI) ionization) revealed that the ion at m/z 151, [guanine]⁺, was common to all of the alkylguanines studied, with the exception of the methylated analogues. Precursor ion scans of this ion on partially purified human urine extracts showed the presence of several ions (e.g. m/z 179, 195) which were consistent with molecular ions for alkylguanines. The presence of these and other constituents was confirmed by product ion spectra of molecular ions (EI and fast atom bombardment), and by high-performance liquid chromatographic separation prior to tandem mass spectrometry (MS/MS). Evidence was obtained for the presence of N-7-methyl-, N²-dimethyl-, N²-dimethyl-, N²-ethyl- and N-7-(2-hydroxyethyl)guanine. Quantitative methods were established for these five alkyl guanines using gas chromatography mass spectrometry (GC/MS) and GC/MS/MS. Deuterated internal standards were synthesized and added to the urine prior to extraction of alkylpurines by Sep-Pak cartridge chromatography. The products were converted into their tert-butyldimethylsilyl derivatives and analysed by selected ion monitoring (SIM) of [M – 57]⁺ or by multiple reaction monitoring (MRM) of the fragmentation M^+˙ → [M – 57]⁺. The MRM method yielded values for N-7-methylguanine of 2.57 ± S.D. 1.32 mg day^?1 (n = 6), N2-methylguanine of 0.31 ± 0.10 mg day^?1 (n = 10) and N²-dimethylguanine of 0.21 ± 0.23 mg day^?1 (n = 10). N²-Ethyl- and N-7-(2-hydroxyethyl)guanine could only be detected by SIM at levels of ～0.5 and 2 μg day^?1, respectively. The MRM analyses, although inherently less sensitive than the SIM analyses, exhibit greater selectivity and consequently fewer contaminant ions.     

Characterization of new metabolites from in vivo biotransformation of 2‐amino‐ 3‐methylimidazo[4,5‐f]quinoline in mouse by mass spectrometry

In studying the metabolic pathways underlying the mechanism of carcinogenesis of the heterocyclic amine of 2‐amino‐3‐methylimidazo[4,5‐f]quinoline (IQ), we recently found a new metabolite which gave an [M + H]⁺ ion of m/z 217 when subjected to electrospray ionization (ESI) in positive‐ion mode. Following ip injection of this metabolite of m/z 217 (designated as m/z 217) to beta‐naphthoflavone‐treated mice, 57% of the total radioactivity was recovered in a 24‐h mouse urine sample. HPLC separation followed by MS analysis indicates that the urine sample contained m/z 217 (36 ± 3% of total recovered radioactivity) and two other peaks that gave rise to the [M + H]⁺ ions of m/z 393 (31 ± 4%, designated as m/z 393) and m/z 233 (14 ± 1%, designated as m/z 233). Beta‐glucuronidase treatment of m/z 393 resulted in a radioactive peak corresponding to m/z 217. ESI in combination with various mass spectrometry techniques, including multiple‐stage mass spectrometry, exact mass measurements and H/D exchange followed by tandem mass spectrometry, was used for structural characterization. The urinary metabolites of m/z 217, 393 and 233 were identified as 1,2‐dihydro‐2‐amino‐5‐hydroxy‐3‐methylimidazo[4,5‐f]quinoline, 1,2‐dihydro‐2‐amino‐5‐O‐glucuronide‐3‐methylimidazo[4,5‐f]quinoline and 1,2‐dihydro‐2‐amino‐5,7‐dihydroxy‐3‐methylimidazo[4,5‐f]quinoline, respectively. Our results demonstrated that m/z 217 is biotransformed in vivo to m/z 393 by O‐glucuronidation and to m/z 233 by oxidation. The observation of these more polar metabolites relative to IQ suggests that they may arise from a previously undescribed detoxicification pathway. Copyright © 2009 John Wiley & Sons, Ltd.     

Chiral detection of entecavir stereoisomeric impurities through coordination with R‐besivance and ZnII using mass spectrometry

In this study, a mass spectrometry (MS)‐based kinetic method (KM) is shown to be successful at analyzing a multichiral center drug stereoisomer, entecavir (ETV), both qualitatively and quantitatively. On the basis of the KM, the bivalent complex ion [M^II(A)(ref*)₂]²⁺ (M^II = divalent metal ion, A = analyte, and ref* = chiral reference) was set as precursor ion in MS/MS. The experiment results suggest strong chiral selectivity between ETV and its isomers when using Zn^II coordinated with the chiral reference R‐besivance (R‐B). The logarithm of the fragment ion abundance ratio and the enantiomeric percentage (%) exhibits a strong linear relation because of the competitive loss of the reference and analyte. The product ion pair [Zn^II(R‐B)A‐H]⁺ (m/z 733) and [Zn^II(R‐B)₂‐H]⁺ (m/z 849), together with [R‐B + H]⁺ (m/z 394) and [A + H]⁺ (m/z 278), can realize the identification of ETV and all of its chiral isomers. Theoretical calculation were also performed using the B3LYP functional with the 6‐31G* and LanL2DZ basis set to clarify the mechanism of structural difference of these bivalent complex ions. The results reveal that MS‐KM can be used to detect optical impurities without a chiral chromatographic column and fussy sample pretreatment. The established method has been used to determine stereoisomeric impurities of less than 0.1% in ETV crude drug, a demonstration of its simple and effective nature for rapid detection of stereoisomeric impurities.     

Determination of allergenic hydroperoxides in essential oils using gas chromatography with electron ionization mass spectrometry

Fragrance monoterpenes are widely used commercially due to their pleasant scent. In previous studies, we have shown that air‐exposed monoterpenes form hydroperoxides that are strong skin sensitizers. Methods for detection and quantification of the hydroperoxides in essential oils and scented products are thus desirable. Due to thermolability and low UV absorbance, this is a complicated task. We have recently developed a sensitive LC–ESI‐MS method, but with limited structural information and separation efficiency for positional isomers and stereoisomers. In the present study, we investigated derivatization with a trimethyl silyl reagent and subsequent GC with electron ionization MS for the determination of monoterpene hydroperoxides. All investigated monoterpene hydroperoxides could be chromatographed as thermostable trimethyl silyl derivatives and yielded the fragment m/z 89 ([OSi(CH₃)₃]⁺) at a higher extent compared to corresponding alcohols. Limonene‐2‐hydroperoxide and four other hydroperoxide isomers of limonene were separated and detected in sweet orange oil autoxidized for two months. The concentration of limonene‐2‐hydroperoxide isomers was found to be 19 μg/mg in total. Also isomers of linalyl acetate hydroperoxide and linalool hydroperoxide were detected in autoxidized petitgrain oil (two months). The presented GC–MS method showed concentrations in the same order as previous LC–MS/MS analysis of the same type of oils.     

Development of a new electron ionization/field ionization ion source for gas chromatography/time‐of‐flight mass spectrometry

We have developed a combined EI/FI source for gas chromatography/orthogonal acceleration time‐of‐flight mass spectrometry (GC/oaTOFMS). In general, EI (electron ionization) and FI (field ionization) mass spectra are complementary: the EI mass spectrum contains information about fragment ions, while the FI mass spectrum contains information about molecular ions. Thus, the comparative study of EI and FI mass spectra is useful for GC/MS analyses. Unlike the conventional ion sources for FI and EI measurements, the newly developed source can be used for both measurements without breaking the ion source vacuum or changing the ion source. Therefore, the combined EI/FI source is more preferable than the conventional EI or FI ion source from the viewpoint of the reliability of measurements and facility of operation. Using the combined EI/FI source, the complementarity between EI and FI mass spectra is demonstrated experimentally with n‐hexadecane (100 pg): characteristic fragment ions for the n‐alkane such as m/z 43, 57, 71, and 85 are obtained in the EI mass spectrum, while only the parent peak of m/z 226 (M⁺) without any fragment ions is observed in the FI mass spectrum. Moreover, the field desorption (FD) measurement is also demonstrated with poly(ethylene glycol)s M600 (10 ng) and M1000 (15 ng). Signals of [M+H]⁺, [M+Na]⁺ and [M+K]⁺ are clearly detected in the FD mass spectra. Copyright © 2009 John Wiley & Sons, Ltd.     

Mass Spectrometric Identification of Multihydroxy Phenolic Compounds in Tibetan Herbal Medicines

A highly selective and accurate method based on derivatization with dansyl chloride coupled with liquid chromatography–mass spectrometry has been developed for identification of natural pharmacologically active phenolic compounds in extracts of Lomatogonium rotatum plants (Tibetan herbal medicine) obtained by solid-phase extraction. The number of hydroxyl groups on the dansylated phenols was estimated by LC–MS–MS analysis in positive-ion mode. Dansyl derivatization of the compounds introduced basic secondary nitrogen into the phenolic core structures and this was readily ionized when acidic HPLC mobile phases were used. MS fragmentation of the derivatives generated intense protonated molecular ions of m/z [MH]⁺ (phenol aglycones were transformed into the corresponding free phenols by cleavage of an aglycone bond). Collision-induced dissociation of the protonated molecule generated characteristic product ions of m/z 234 and 171 corresponding to the protonated 5-(dimethylamino)naphthalene sulfoxide and 5-(dimethylamino)naphthalene moieties, respectively. Selected reaction monitoring based on the m/z [MH]⁺ to 234 and 171 transitions was highly specific for these phenolic compounds. Characteristic ions with m/z values of [MH – 234]⁺, [MH – 2 × 234]⁺, and [MH – 3 × 234]⁺ were of great importance for estimation of the presence of multihydroxyl groups on the phenolic backbone.     

Dithia[3.3 ]phanes. 4—mass spectrometry of some dithia[3.3]-naphthalenophanes1

The mass spectral fragmentation of 23 dithia [3.3]naphthalenophanes is discussed. The most characteristic species for these compounds are the M⁺ ion and ions at m/z 115, 135 (or 136), 141, 155 (or 154) and 185.     

One‐Electron Oxidation and Sensitivity of Uric Acid in On‐Line Electrochemistry and in Electrospray Ionization Mass Spectrometry

The sensitivity of detection of uric acid (H₂U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pK_a1=5.4 of H₂U, protonated unoxidized uric acid [H₂U+H]⁺ (m/z 169) was detected together with the protonated uric acid dimer [2H₂U+H]⁺ (m/z 337). The dimer likely forms by 1e^? oxidation of urate (HU^?) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e^?, 2H⁺ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on‐line electrochemical cell voltage is floated on the high voltage of the ES in on‐line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pK_a2=9.8 of H₂U, allantoin (Allnt) (MW 158.04), the final 2e^?, 2H⁺ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]⁺ (m/z 235) and the [2H₂U+H]⁺ dimer was not detected. In direct ESI MS analysis of 1000‐fold diluted urine [NaHU+H]⁺ (pK_sp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH₄Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone‐shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.     

Substituent effects in the mass spectra of diethyl phenyl phosphates

The mass spectra of diethyl phenyl phosphates show substituent effects with electron-donating groups favouring the molecular ion M⁺˙, and the [M? C₂H₄]⁺˙, [M – 2C₂H₄]⁺˙ and [XPhOH]⁺˙ ions. The [PO₃C₂H₆]⁺ (m/z 109) and [PO₃H₂]⁺ (m/z 81) ions are favoured by electron-withdrawing groups. Results suggest that the formation of the [XPhC₂H₃]⁺˙ ion involves rearrangement of C₂H₃ to the position ortho to the phosphate group. Ortho effects are also observed.     

Pharmacokinetic studies of a novel tubulin inhibitor SK1326 in rat plasma by UPLC–MS/MS

A sensitive method for quantitation of SK1326 in rat plasma has been established using ultra-performance liquid chromatography–electrospray ionization tandem mass spectrometry (UPLC–ESI/MS/MS). SK1326 and the internal standard (tramadol) in plasma sample were extracted using acetonitrile. A centrifuged upper layer was then evaporated and reconstituted with a mobile phase of 0.5% formic acid–acetonitrile (35:65, v/v). The reconstituted samples were injected into a C₁₈ reversed-phase column. Using MS/MS in the multiple reaction monitoring mode, SK1326 and tramadol were detected without severe interference from the rat plasma matrix. SK1326 produced a protonated precursor ion ([M + H]⁺) at m/z 432.3 and a corresponding product ion at m/z 114.4. The internal standard produced a protonated precursor ion ([M + H]⁺) at m/z 264.4 and a corresponding product ion at m/z 58.1. Detection of SK1326 in rat plasma by the UPLC–ESI/MS/MS method was accurate and precise with a quantitation limit of 1.0 ng/mL. The validation, reproducibility, stability and recovery of the method were evaluated. The method has been successfully applied to pharmacokinetic studies of SK1326 in rat plasma. The pharmacokinetic parameters of SK1326 were evaluated after intravenous (at a dose of 10 mg/kg) and oral (at a dose of 20 mg/kg) administration of SK1326 in rats. After oral administration (20 mg/kg) of SK1326, the F (fraction absorbed) value was ~77.1%.