The identification of trimethylsilylated dipeptides with chemical ionization mass spectrometry.

The chemical ionization (CI) and electron impact (EI) mass spectra were compared for over 40 trimethylsilylated (Me₃Si) dipeptides. The dipeptides chosen had all 20 common amino acids represented at amino and carboxyl positions. The CI mass spectra of Me₃Si dipeptides typically contain three ions of high abundance used for dipeptide identification: a sequence-determining ion and two molecular weight-determining ions. The intensity of the molecular weight-determining ions relative to that of the ion that characterizes the N-terminal residue (β-cleavage ion) is greater in the CI mode than in the EI mode. Because the available intensity of the β-cleavage ion is similar in both modes, use of the CI mode will extend the lower limit for Me₃Si dipeptide identification.     

Chemical ionization mass spectrometry of iprodione and its metabolites

One of the cornerstones of an architectural approach adopted by this laboratory in screening vegetables for pesticides and industrial chemicals is the extensive use of element-selective gas chromatographic detectors followed by gas chromatography/mass spectrometry. In this particular case history, a recently introduced European fungicide, iprodione, was thought to be the first reported incidence in mache imported from France. An analytical protocol involving chemical ionization was devised to confirm this finding as well as search for possible potential metabolites.     

Analysis of urinary organic acids by liquid chromatography—atmospheric pressure chemical ionization mass spectrometry

We developed a new method for the rapid determination of urinary organic acids using liquid chromatography—atmospheric pressure chemical ionization mass spectrometry. Mass spectra of authentic organic acids obtained in the negative-ion mode showed intense [M − H]⁻ ions with some fragment ions. Urine samples of patients with methylmalonic aciduria, ornithine transcarbamylase deficiency, and phenylketonuria were extracted using anion-exchange columns. The mass chromatograms of the extracts showed some dominant peaks of abnormal metabolites characteristic of each disorder. This is a useful method for the analysis of urinary organic acids for the diagnosis of organic aciduria, because the sample preparation is simple.     

Comparison of electron and chemical ionization mass spectrometry of sialic acids

Sialic acids were analyzed as per-O-trimethylsilylated compounds by gas-liquid chromatography/mass spectrometry either on electron or chemical ionization by isobutane. Electron ionization mass spectra of these derivatives are very similar to those of the corresponding methyl esters described earlier whereas chemical ionization mass spectra are characterized in the high mass range by loss of the C-2 and the C-4 substituents from the M + 1 ion. Together with other fragment ions the seven different sialic acids analyzed could be clearly identified.     

Localization of water-soluble carbohydrates in wheat stems using imaging matrix-assisted laser desorption ionization mass spectrometry

The pool of endogenous water-soluble oligosaccharides found in the stems of wheat (Triticum aestivum) is being investigated as a potential indicator of grain yield. Techniques such as liquid chromatography with mass spectrometry (LC-MS) can profile these analytes but provide no spatial information regarding their distribution in the wheat stem. The imaging matrix-assisted laser desorption ionization (MALDI) mass spectrometry technique has not been utilized for the analysis of oligosaccharides in plant systems previously. Imaging MALDI mass spectrometry was used to analyse cross and longitudinal sections from the stems of Triticum aestivum. A range of oligosaccharides up to Hex(11) were observed. Water-soluble oligosaccharides were ionized as potassiated molecules, and found to be located in the stem pith that is retained predominantly around the inner stem wall. Imaging MALDI analyses provided spatial information on endogenous oligosaccharides present in wheat stems. The technique was found to offer comparable sensitivities for oligosaccharide detection to those of our established LC-MS method, and has potential for broad application in studying the in situ localization of other compound types in plant material.     

Application of gas chromatography-surface ionization organic mass spectrometry to forensic toxicology

Surface ionization (SI), which consists in the formation of positive and negative ions along the course of thermal desorption of particles from a solid surface, was first applied as a detector for gas chromatography (GC), GC-surface ionization detection (SID); we developed many new sensitive methods for the determination of abused and other drugs by GC-SID. Recently, Fujii has devised a combination of SI and a quadrupole mass spectrometer and named this system a surface ionization organic mass spectrometer (SIOMS), which is highly selective and sensitive for organic compounds containing tertiary amino groups. We have tried to apply this mass spectrometer to forensic toxicological study; so far we have succeeded in determining important drugs-of-abuse and toxic compounds, such as phencyclidine (PCP), pethidine, pentazocine, MPTP and its derivatives from human body fluids with high sensitivity and selectivity. In this review, we describe our recent studies on the application of GC-SIOMS to forensic toxicology.     

Determination of geometrical and positional isomers in octadecenoic acids by chemical ionization mass spectrometry

Chemical ionization mass spectra of trimethylsilylated methyl esters of octadecenoic acids were investigated. The position of the original double bond could be deduced from the recognizable fragment ions produced by cleavage of the carbon-carbon bond between the two trimethylsilyl alcohols. The cis- and trans-isomers could be also distinguished from each other by comparing the abundance of the fragment ions given by subsequent loss of the trimethylsilyl residue and/or methoxy group from the characteristic peaks.     

Sensitive determination of pethidine in body fluids by surface ionization organic mass spectrometry

We have presented a simple and sensitive method for determining pethidine, a narcotic analgesic drug in body fluids by gas chromatography (GC)/surface ionization organic mass spectrometry (SIOMS). Good linearity was obtained in the range of 0.625–25 ng/ml of whole blood and urine by mass chromatography, and in the range of 0.05–2 ng/ml of whole blood by selected ion monitoring (SIM). Pethidine and diphenylpyraline (internal standard) were extracted from body fluids with Bond Elut Certify cartridges; their recoveries were above 95%. The detection limits (signal-to-noise ratio=3) were estimated to be 0.2 ng/ml of whole blood or urine by mass chromatography, 0.02 ng/ml of whole blood by SIM.     

High-resolution gas chromatography-mass spectrometry of the methyl esters of organic acids from uremic hemofiltrates

The organic acid fraction of hemofiltrates was investigated in the form of methylates by glass capillary gas chromatography-mass spectrometry. The pattern obtained is similar to that of urinary organic acid methylates from healthy individuals. A marked difference was noted for N-phenylacetyl-α-aminoglutarimide, present in hemofiltrate at levels 50–100 times higher than those in urine. Analysis of hemofiltrate samples taken at different times during a hemofiltration with post-dilution technique revealed that the hemofiltrate concentration of most compounds was drastically reduced during the course of the hemofiltration treatment. Compared to the other compounds, the reduction in hemofiltrate concentration of N-phenylacetyl-α-aminoglutarimide was extremely rapid.     

Chemical-ionization mass spectrometry of carbohydrates: behavior of the antibiotic celesticetin under various conditions of ionization

The chemical-ionization mass spectrum of the polyfunctional carbohydrate antibiotic, celesticetin, is remarkably simple, in contrast to its electron-impact mass spectrum. With ammonia as the ionizing gas the protonated molecular ion is the principal species, and simple fragmentation modes reflect eleavage between principal moietics of the molecule. Substantial modifications of the fragmentation pathway are observed when isobutane is used as the ionizing gas. The different pathways of fragmentation according to the mode of ionization are interpreted in terms of differential reactivity of various heteroatomic centers in the molecule toward each of the ionizing agents. Use of a combination of ionizing modes may provide a useful general method for identification on a micro scale of complex carbohydrates isolated from fermentation or other mixtures.     

Liquid ionization mass spectrometry of phospholipids

Several phosphatidylcholines (PC) and a phosphatidylethanolamine (PE) were subjected to liquid ionization (LI) mass spectrometry, in which a sample is ionized through energy transfer from metastable argon atoms under atmospheric pressure. Commercially available and synthesized, saturated or unsaturated fatty acid containing phospholipids and their mixtures were studied. A sample either as a concentrated chloroform-methanol solution or with glycerol (matrix) gave characteristic peaks such as MH+ and four fragment ions. One of the fragment ions (e.g., m/z 551 of PC 16:0, 16:0) containing both fatty acid residues has been commonly observed with other ionization methods such as CI, FD, and FAB, but the other fragment ions have not been observed in other mass spectra with one exception on desorption CI. Ions b and d (e.g., m/z 464 and 328, respectively, for PC 16:0, 16:0) contain one fatty acyl residue and the other ion containing the phosphorylcholine moiety appears at m/z 196 for PC. Thus the masses of the MH+ ion and these fragment ions provide useful structural information even in the case of a mixture. The ion b (e.g., m/z 488 of PC 18:0, 18:2) observed during an early period of heating was formed mainly by the loss of one acyl group at sn-1 of the glycerol backbone and thus may be used to differentiate the positional specificities of the constituent fatty acids. The temperature of the sample, however, should be controlled precisely, because it has a significant effect on the mass spectrum. The present method (LI) also provided useful information for a mixture of PC and PE.     

Selective determination of haloperidol in human serum: surface ionization mass spectrometry and gas chromatography with surface ionization detection

Surface ionization organic mass spectrometry (SIOMS) has been performed on the clinically important drug haloperidol using quadrupole mass spectrometry in which the thermal ion source has a rhenium oxide emitter. The surface ionization (SI) mass spectrum is presented, interpreted in a purely empirical way by means of evidence from previous investigations, and then compared to results from conventional electron impact (EI) ionization. An approach to detection of this drug in serum by gas chromatography (GC) with a surface ionization detector (SID) and GC-SIOMS is described. This approach demonstrates that (a) haloperidol is efficiently surface-ionized, giving a unique SI mass spectrum, (b) experimental results rationalize the combined sensitivity and selectivity of the GC-SID for the examined drug, (c) the detection limit for haloperidol in serum is 1.1 ng/ml (S/N = 3) by GC-SID (the coefficients of variation of the assay are generally low, i.e. below 8.5%) and (d) the GC-SIOMS coupling can be used for sensitive and selective detection of haloperidol in serum.     

Determination of tetrabromobisphenol A in human serum by liquid chromatography-electrospray ionization tandem mass spectrometry

A method for the determination of tetrabromobisphenol A (TBBPA) in human serum utilizing solid-phase extractions (SPEs) and liquid chromatography (LC) with electrospray ionization tandem MS (MS/MS) has been developed. After purification and concentration of TBBPA using consecutive SPEs on reversed-phase and normal-phase cartridges, the serum sample was subjected to LC. TBBPA was separated on a C18 reversed-phase column by gradient elution with a mixture of water, methanol, and acetonitrile as the mobile phase, and then detected with electrospray ionization MS/MS in negative ion mode. 13C12-TBBPA was suitable as an internal standard for the reproducible determination of TBBPA in human serum samples (5 g). The method has been validated in TBBPA concentration range of 5-100 pg per g serum, and the recoveries in the concentration range were higher than 83.3%. The repeatabilities of the proposed method of non-spiked control serum (6.3 pg per g serum) and spiked serum (added 5-100 pg per g serum) were within 10.0% as relative standard deviations. The limit of quantification (LOQ) for TBBPA was 4.1 pg per g serum, which was corresponded to 0.63 fmol on column.     

Liquid chromatography-electrospray ionization mass spectrometry for direct identification and quantification of iophenoxic acid in serum

A liquid chromatographic-electrospray ionization mass spectrometric technique was developed for direct quantitation of iophenoxic acid (IA) in serum. IA was spiked into canine, feline, bovine, equine, and porcine sera, extracted, and quantified using negative ion monitoring following chromatographic separation on a Luna C18(2) 3 microm (100 mm x 2.1mm) reversed-phase column. The limit of detection was 25 ng/mL and the limit of quantification was 50 ng/mL. Inter- and intra-assay accuracy (86-113% and 87-115%, respectively) and precision (1.8-7.7%) were calculated. Analysis of serum collected from feral pigs, raccoons, and opossums following ingestion of IA-marked baits confirmed the appropriateness of this method for bait acceptance studies.     

Electrohydrodynamic ionization mass spectrometry of biochemical materials

Electrohydrodynamic ionization mass spectrometry has been applied to a range of biochemical materials dissolved in glycerol with NaI as electrolyte. Sugars (glucose, sucrose, raffinose), nucleosides (adenosine, thymidine, uridine), a tripeptide (glutathione) and an aminocyclitol antibiotic (neomycin) have been analyzed. Unambiguous analysis of a multicomponent solution has been demonstrated. All samples yielded several quasimolecular ions involving either proton or cation attachment to clusters of sample and/or solvent molecules. Unlike other techniques such as field desorption, electrohydrodynamic ionization is not observed to cause fragmentation of sample molecules. The mass spectrometer was operated so as to analyze only those ion clusters which had not undergone decomposition processes; under these conditions, most materials are ionized with similar efficiencies if the total abundance of all characteristic quasimolecular ions is considered. Information regarding the amino acid sequence of glutathione was obtained by thermal pretreatment of the glycerol solution before mass analysis. Positive and negative ion spectra give complementary information which can resolve potential ambiguities regarding the exact composition of quasimolecular ions. Electrohydrodynamic ionization mass spectrometry should be applicable to materials which cannot be ionized by other methods.