Elemental Compositions Of Ions Research Articles

Derivatization to increase the detectability of small peptides in blood serum in the analysis by ESI and MALDI high resolution mass spectrometric methods

This work highlights the efficient approach to highly sensitive determination of dipeptides that can present in biological liquids at very low and trace quantities. The approach involves preliminary derivatization of peptides with tris(2,4,6-trimethoxyphenyl)-methyl carbenium hexafluoroborate followed by ESI and MALDI high-resolution mass spectrometry. Using model dipeptides with various amino acid compositions and sequences, it was shown that the derivatization reaction proceeded smoothly in mild conditions and gave rise to pink-red colored salt derivatives. Ready cations of interest for the analysis are easily desorbed from the salt-derivatives providing strong signals in ESI and MALDI mass spectra and this ensures high sensitivity of the analysis. Another positive aspect is the removal of the target signal from the region of a matrix noise, since the introduced fragment possesses a large mass increment (359 Da). High resolution mass spectrometry, which provides the determination of accurate weights and elemental compositions of ions, was used to reliably detect model dipeptides added to artificial urine and blood serum. A number of these dipeptides was shown to be present in real blood serum collected from volunteers. Collision induced dissociation of precursor cations composed of derivatizing reagent and dipeptide moieties gives rise to characteristic and simple fragmentation mass spectra. A comparison of limits of detection (LOD) measured for non-modified and derivatized dipeptides showed that the latter derivatives provide the highest sensitivity when LOD is determined by using multiple reaction monitoring (MRM) transitions. The suggested derivatization approach was shown to be useful for unambiguous identification of special dipeptides in artificial media and dietary supplements.

Application of electrospray ionization hybrid ion trap/time-of-flight mass spectrometry in the rapid characterization of quinocetone metabolites formed in vitro

The application of electrospray ionization hybrid ion trap/time-of-flight mass spectrometry coupled with high-performance liquid chromatography (LC/MS-IT-TOF) in the rapid characterization of in vitro metabolites of quinocetone was developed. Metabolites formed in rat liver microsomes were separated using a VP-ODS column with gradient elution. Multiple scans of metabolites in MS and MS(2) modes and accurate mass measurements were automatically performed simultaneously through data-dependent acquisition in only a 30-min analysis. Most measured mass errors were less than 10 ppm for both protonated molecules and fragment ions using external mass calibration. The elemental compositions of all fragment ions of quinocetone and its metabolites could be rapidly assigned based upon the known compositional elements of protonated molecules. The structure of metabolites were elucidated based on the combination of three techniques: agreement between their proposed structure, the accurate masses, and the elemental composition of ions in their mass spectra; comparison of their changes in accurate molecular masses and fragment ions with those of parent drug or metabolite; and the elemental compositions of lost mass numbers in proposed fragmentation pathways. Twenty-seven phase I metabolites were identified as 11 reduction metabolites, three direct hydroxylation metabolites, and 13 metabolites with a combination of reduction and hydroxylation. All metabolites except the N-oxide reduction metabolite M6 are new metabolites of quinocetone, which were not previously reported. The ability to conduct expected biotransformation profiling via tandem mass spectrometry coupled with accurate mass measurement, all in a single experimental run, is one of the most attractive features of this methodology. The results demonstrate the use of LC/MS-IT-TOF approach appears to be rapid, efficient, and reliable in structural characterization of drug metabolites.

Discovering Sudan IV in Salty Eggs with LC–TOF-MS

This study evaluated the screening of Sudan dyes in salty egg using liquid chromatography/time-of-flight mass spectrometry (LC–TOF-MS). With this technology, accurate mass measurements were made to generate elemental compositions of ions and to provide complementary structural information. This approach is useful for the elucidation of organic pollutants at trace levels in complex food extracts and in the present study salty egg samples were analyzed for the presence of Sudan IV. TOF accurate mass elemental composition data of Sudan IV were found to be crucial for structural elucidation which was applied to the investigation of nine salty egg samples.

Determination of ion and neutral loss compositions and deconvolution of product ion mass spectra using an orthogonal acceleration time‐of‐flight mass spectrometer and an ion correlation program

Exact masses of monoisotopic ions, and the relative isotopic abundances (RIAs) of ions greater in mass by 1 and 2 Da than the monoisotopic ion, are independent and complementary physical properties useful for distinguishing among elemental compositions of ions possible for a given nominal mass. Using these properties to determine elemental compositions of product ions and neutral losses increases the masses of precursor ions for which unique compositions can be determined. Compositions of the precursor ion, product ion, and neutral loss aid mass spectral interpretation and guide modest chemical literature searches for candidate standards to be obtained for confirmation of tentative compound identifications. This approach is essential for compound characterization or identification due to the absence of commercial libraries of electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) product ion spectra. For a series of 34 exact mass measurements, an orthogonal acceleration time-of-flight mass spectrometer provided 34 and 29 values accurate to within 2 and 1 mDa, respectively, for ions from eight simulated unknowns with [M+H](+) ion masses between 166 and 319 Da. Of 36 RIA measurements for +1 Da or +2 Da ions, 35 were accurate to within 20% of their predicted values (or to within 0.4 RIA % when the RIA value was less than 1%) in the absence of obvious interferences, in cases where the monoisotopic ion peak areas were at least 1.7 x 10(5) counts and the ion masses exceeded 141 Da. An ion correlation program (ICP) provided the unique and correct compositions for all but three of the 34 ions studied. Manual inspection of the data eliminated the incorrect compositions. To test the utility of the ICP for deconvoluting composite product ion spectra, all 34 ions were tested for correlation. Six of eight precursor ions were identified as such, while two were compositional subsets of others and were not properly identified. The six precursor ion compositions were still found by the ICP even though ions with masses less than 158 Da were not considered since they could no longer be correlated with a single precursor ion. Finally, two unidentified analytes were characterized, based on data published by others and using the ICP together with mass spectral interpretation.

Determination of elemental compositions by high resolution mass spectrometry without mass calibrants

Perfluorokerosene can almost always be used as the mass calibrant for ions produced through electron impact ionization of compounds introduced into a mass spectrometer in the gas phase. Unfortunately, no completely universal calibrant is available for ions created by electrospray ionization (ESI) or atmospheric pressure chemical ionization (APCI) of analytes introduced into a mass spectrometer in the liquid phase. ESI and APCI generally provide less sensitivity than electron impact ionization of compounds introduced in the gas phase and a portion of the weaker total signal must arise from calibrant ions. Solvent conditions must be found that provide ions from both the calibrant and analytes, or an alternative flow to the ionization region must be provided for the calibrant solution. These problems were avoided by developing a methodology to determine elemental compositions of ions without using mass calibrants. The methodology utilizes the ability of double focusing mass spectrometers to accurately measure relative abundances of ions and exact mass differences between ions. This approach should simplify analyses of environmental samples that contain mixtures of compounds not amenable to gas chromatography or volatilization from a probe. From one to five steps were used. First, from mass peak profiles of the molecular ion or protonated molecular ion, [M]+, and the [M + 1]+ and [M + 2]+ ions, abundances for the [M + 1]+ and [M + 2]+ profiles relative to the [M]+ profile were determined. The [M + 1]+ and [M + 2]+ profiles resulted from the heavier isotopes of the elements in [M]+, and the profile abundances limited the number of possible elemental compositions for [M]+. Then, to determine if [M]+ contained N atoms, the [M + 1]+ profile was observed with sufficiently high mass resolution to at least partially resolve the profiles of ions containing a 15N or a 13C atom. Next, for a prominent fragment ion, [F]+, relative abundances of the [F + 1]+ and [F + 2]+ profiles were also determined, and the [F + 1]+ profile was inspected for a profile due to 15N atoms to provide a shorter list of possible compositions for [F]+. The lists for [M]+ and [F]+ were compared, and [M]+ compositions that could not produce any possible compositions of [F]+ were rejected, as were [F]+ compositions that could not arise from any possible composition of [M]+. Fourth, exact mass differences between ions were obtained from three mass peak profiles by referencing an unknown exact mass difference against a known exact mass difference. Exact mass differences between [M]+ and [F]+ ions provided compositions of neutral losses from [M]+. Only compositions of [M]+ that could lose the observed neutral loss to provide possible compositions of [F]+ remained viable. Finally, if multiple compositions of [M]+ were still possible, profiles were obtained for [M]+ and two fragment ions resulting from known neutral losses using theoretical exact masses based on each [M]+ composition as the center masses of the three profiles. When the calibration mass option was not used in the multiple ion detector (MID) descriptor, three centered profiles were obtained only for the correct composition. This methodology was demonstrated for seven compounds with molecular weights between 159 and 318 Da. For the lowest-mass compound, only the first step was required to obtain the correct composition of [M]+ while, for the other compounds, two or more steps were needed. Copyright © 1999 John Wiley & Sons, Ltd.

New method of isotope pattern analysis

Abstract A program, called AC, has been developed in Turbo Pascal to calculate the elemental compositions of ions produced in low resolution mass spectra by analysis of the isotope patterns. The program compares calculated intensities with experimentally determined intensities and lists the best matches. It also allows for subtraction of overlapping patterns.

Elemental compositions from daughter ion spectra of m1+ and [m1 + 1]+: Some applications of the method

AbstractThe elemental compositions of ions can be determined in tandem mass spectrometry by comparing the daughter ion spectra of the m1+ and [m1 + 1]+ ions. The method is demonstrated for mass‐analyzed ion kinetic energy spectra but is applicable to all types of daughter ion spectra, including complex collisionally activated dissociation spectra. In this work, the method is applied to compounds that produce daughter ions of known elemental compositions, and the errors and limitations are evaluated. Following that test, the procedure is applied to a compound that may produce daughters of more than one possible elemental composition. The method is sometimes useful even if the formula of the parent is not known; that is, the formulae of unknown parent and daughter ions may be found. Locating a specific atom in an isotopically labeled molecule is another capability of the method. The basic equation of the method was generalized and incorporated into a computer program for performing the calculations.

Degradation of steroid derivatives at a gas chromatograph mass spectrometer interface

Methyloxime trimethylsilyl ether derivatives are frequently employed in gas chromatographic and mass spectrometric analysis of steroids. Under certain instrumental conditions, unusual mass spectra were recorded from some corticosteroid methyloxime trimethylsilyl ether derivatives, reflecting degradation by contact of the steroids with active surfaces in the gas chromatograph mass spectrometer interface. This occurred prior to electron impact ionization. Fragmentations of the degradation products of methyloxime trimethylsilyl ether derivatives of tetrahydrocortisol and tetrahydrodeoxycorticosterone are described, as representatives of C21 steroids with dihydroxyacetone and α-ketol side chains. Elemental compositions of ions were confirmed by high resolution mass spectrometric analysis. Derivatives of steroids with glycerol side chains, e.g. cortolone, showed no evidence of degradation. The problem was overcome by ensuring inertness of the glass restrictor interface and by preventing, as far as posssible, contact of sample with metal surfaces in the source housing.