Utility of three types of mass spectrometers for determining elemental compositions of ions formed from chromatographically separated compounds

Concentration factors of 1000 and more reveal dozens of compounds in extracts of water supplies. Library mass spectra for most of these compounds are not available, and alternative means of identification are needed. Determination of the elemental compositions of the ions in mass spectra makes feasible searches of commercial and chemical literature that often lead to compound identification. Instrumental capabilities that constrain the utility of a mass spectrometer for determining ion compositions for compounds that elute from a chromatographic column are scan speed, mass accuracy, linear dynamic range, and resolving power. Mass peak profiling from selected ion recording data (MPPSIRD) performed with a double-focusing mass spectrometer provides the best combination of these capabilities. This technique provides unique ion compositions for ions of higher mass from compounds eluting from a gas chromatograph than can be obtained by orthogonal acceleration time-of-flight (oa-TOF) or Fourier transform ion cyclotron resonance mass spectrometry. Multiple compositions are usually possible for an ion with a mass exceeding 150 Da within the error limits of the mass measurement. The correct composition is selected based on measured exact masses of the mass peak profiles resulting from isotopic ions higher in mass by 1 and 2 Da and accurate measurement of the summed abundances of these isotopic ions relative to the monoisotopic ion. A profile generation model (PGM) automatically determines which compositions are consistent with measured exact masses and relative abundances. The utility of oa-TOF and double-focusing mass spectrometry using ion composition elucidation (MPPSIRD plus the PGM) are considered for determining ion compositions of two compounds found in drinking water extracts and a third compound from a monitoring well at a landfill. Published in 2002 by John Wiley & Sons, Ltd.     

Analysis of volatiles and semivolatiles in drinking water by microextraction and thermal desorption

A new technique to analyze aqueous samples for nanograms per liter levels of volatile and semivolatile compounds using microextraction and thermal desorption into a gas chromatograph/ion trap mass spectrometer (GC/MS) is described. This method is inherently sensitive (50 mL of aqueous sample is extracted prior to each desorption), uses no solvents, and detects volatiles and semivolatiles in the same analysis. Aqueous standards and environmental samples are pumped through a length of porous-layer open-tubular capillary column, which is then thermally desorbed onto a 30 m × 0.25 mm i.d. analytical column interfaced to an ion trap mass spectrometer for subsequent separation and detection. Sharp chromatographic peaks and reproducible retention times (RT) were observed. Replicate injections of surrogates (n = 6) averaged 32.6% R.S.D. Analysis of domestic tap water detected 55 analytes, some at the low-nanograms per liter level, and detected 3 halogenated ethenes, not previously reported in drinking water. Analysis of an aqueous sample from a municipal ground water source detected the presence of numerous semivolatile compounds at trace-levels.     

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.     

Software-based mass spectral enhancement to remove interferences from spectra of unknowns

Gas chromatography-mass spectrometry data from the analysis of complex environmental samples were converted into ASCII text and imported into a personal computer spreadsheet. A macro was written to perform mass spectral enhancement by statistical and mathematical procedures to separate the spectra of compounds of interest from interfering mass spectral responses, such as those of broadly eluting hydrocarbons. The extracted mass spectra were compared to reference spectra, with the result that usually 80–90% of the ions common to those in the reference spectra were successfully extracted by using this method. This procedure improved mass spectral quality and the ability of the data system to perform successful library searches. The fitted quality parameters showed systematic improvements after the data were subjected to the spectral enhancement procedures. These procedures could help to identify unknowns by separating their spectra from other signals, such as those of background aliphatic hydrocarbons.     

Nitro musk metabolites bound to carp hemoglobin: determination by GC with two MS detection modes: EIMS versus electron capture negative ion MS

Nitroaromatic compounds including synthetic nitro musks are important raw materials and intermediates in the synthesis of explosives, dyes, pesticides, and pharmaceutical and personal-care products (PPCPs). The nitro musks such as musk xylene (MX) and musk ketone (MK) are extensively used as fragrance ingredients in PPCPs and other commercial toiletries. Identification and quantification of a bound 4-amino-MX (4-AMX) metabolite as well as a 2-amino-MK (2-AMK) metabolite were carried out by gas chromatography–mass spectrometry (GCMS), with selected ion monitoring (SIM) in both the electron ionization (EIMS) and electron capture (EC) negative ion chemical ionization (NICIMS) modes. Detection of 4-AMX and 2-AMK occurred after the cysteine adducts in carp hemoglobin, derived from the nitroso metabolites, were released by alkaline hydrolysis. The released metabolites were extracted into n-hexane. The extract was preconcentrated by evaporation, and analyzed by GC-SIM-MS. A comparison between the EI and EC approaches was made. EC NICIMS detected both metabolites whereas only 4-AMX was detected by EIMS. The EC NICIMS approach exhibited fewer matrix responses and provided a lower detection limit. Quantitation in both approaches was based on an internal standard and a calibration plot.     

Using a triple-quadrupole mass spectrometer in accurate mass mode and an ion correlation program to identify compounds

Atomic masses and isotopic abundances are independent and complementary properties for discriminating among ion compositions. The number of possible ion compositions is greatly reduced by accurately measuring exact masses of monoisotopic ions and the relative isotopic abundances (RIAs) of the ions greater in mass by +1 Da and +2 Da. When both properties are measured, a mass error limit of 6-10 mDa (< 31 ppm at 320 Da) and an RIA error limit of 10% are generally adequate for determining unique ion compositions for precursor and fragment ions produced from small molecules (less than 320 Da in this study). 'Inherent interferences', i.e., mass peaks seen in the product ion mass spectrum of the monoisotopic [M+H]+ ion of an analyte that are -2, -1, +1, or +2 Da different in mass from monoisotopic fragment ion masses, distort measured RIAs. This problem is overcome using an ion correlation program to compare the numbers of atoms of each element in a precursor ion to the sum of those in each fragment ion and its corresponding neutral loss. Synergy occurs when accurate measurement of only one pair of +1 Da and +2 Da RIAs for the precursor ion or a fragment ion rejects all but one possible ion composition for that ion, thereby indirectly rejecting all but one fragment ion-neutral loss combination for other exact masses. A triple-quadrupole mass spectrometer with accurate mass capability, using atmospheric pressure chemical ionization (APCI), was used to measure masses and RIAs of precursor and fragment ions. Nine chemicals were investigated as simulated unknowns. Mass accuracy and RIA accuracy were sufficient to determine unique compositions for all precursor ions and all but two of 40 fragment ions, and the two corresponding neutral losses. Interrogation of the chemical literature provided between one and three possible compounds for each of the nine analytes. This approach for identifying compounds compensates for the lack of commercial ESI and APCI mass spectral libraries, which precludes making tentative identifications based on spectral matches.     

Automated determination of precursor ion, product ion, and neutral loss compositions and deconvolution of composite mass spectra using ion correlation based on exact masses and relative isotopic abundances

After an accidental, deliberate, or weather-related dispersion of chemicals (dispersive event), rapid determination of elemental compositions of ions in mass spectra is essential for tentatively identifying compounds. A direct analysis in real time (DART)ion source interfaced to a JEOL AccuTOFmass spectrometer provided exact masses accurate to within 2 mDa for most ions in full scan mass spectra and relative isotopic abundances (RIAs) accurate to within 15-20% for abundant isotopic ions. To speed determination of the correct composition for precursor ions and most product ions and neutral losses, a three-part software suite was developed. Starting with text files of m/z ratios and their ion abundances from mass spectra acquired at low, moderate, and high collision energies, the ion extraction program (IEP) compiled lists for the most abundant monoisotopic ions of their exact masses and the RIAs of the +1 and +2 isotopic peaks when abundance thresholds were met; precursor ions; and higher-mass, precursor-related species. The ion correlation program (ICP) determined if a precursor ion composition could yield a product ion and corresponding neutral loss compositions for each product ion in turn. The input and output program (IOP) provided the ICP with each precursor ion:product ion pair for multiple sets of error limits and prepared correlation lists for single or multiple precursor ions. The software determined the correct precursor ion compositions for 21 individual standards and for three- and seven-component mixtures. Partial deconvolution of composite mass spectra was achieved based on exact masses and RIAs, rather than on chromatography.     

Spectroscopic characterization of some chlorinated tricyclic hydrocarbons of the C10 series: Chlordanes,chlordenes and nonachlors

Carbon-13 and proton nuclear magnetic resonance (n.m.r.) spectra of ten chlorinated hydrocarbons, which are components of technical chlordane and one chlordane metabolite, were examined. For chlordene, dichlorochlordene, cis- and trans-nonachlor, and cis- and trans-chlordane, whose chemical structures are well-established, the relationships between the n.m.r. parameters and these structures were investigated. The results allow confirmation of proposed structures for α-, β-, and γ-chlordene. A structure, corresponding to 2,4,5,5,6,7,8,8-octachloro-2,3,3a,4,5,7a-hexahydro-1,4-methano-1-H-indene, is proposed for another chlordane component, previously known only as Compound K.     

Capillary electrophoresis and capillary electrochromatography of organic pollutants

Capillary electrophoresis (CE) has a unique capability for separation of analytes of environmental concern, particularly those that are more polar and ionic, based on the complementary separation principle of electrophoresis. In the past few years, CE has been selectively used to analyze various classes of compounds having current or potential environmental relevance. This review outlines the current status of CE for the determination of environmental pollutants, based predominantly on research results published from the beginning of 1997 to early 1999. Covered are environmental pollutants of all types except pesticides and inorganics. Certain naturally produced toxins are also covered because of their significant impacts upon human health and the environment. CE methods, as with all methods, must be judged on their ability to provide approaches that are reliable, sensitive, selective, and rapid, while meeting "green chemistry" initiatives for pollution prevention. We also compare CE methods to benchmark environmental techniques involving gas chromatography-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and high performance liquid chromatography (HPLC).     

Detection of illicit drugs on surfaces using direct analysis in real time (DART) time-of-flight mass spectrometry

Methamphetamine (meth) from meth syntheses or habitual meth smoking deposited on household surfaces poses human health hazards. The U.S. State Departments of Health require decontamination of sites where meth was synthesized (meth labs) before they are sold. National Institute for Occupational Safety and Health (NIOSH) methods for meth analysis require wipe sampling, extraction, clean‐up, solvent exchange, derivatization, and/or mass spectral analysis using selected ion monitoring. Rapid and inexpensive analyses could screen for drug‐contamination within structures with greater spatial resolution, provide real‐time analyses during decontamination, and provide thorough documentation of successful clean ups. Herein an autosampler/open‐air ion source time‐of‐flight mass spectrometric technique is described that required only direct sampling using cotton‐swab wipes. Each wipe sample collection required 2 min and data acquisition required only 13 s per sample. Optimum collision‐induced dissociation voltages, desorption gas temperatures, and wipe sample solvents were determined for 11 drugs. Peaks were observed in analyte‐ion traces for 0.025 µg/100 cm² of meth and seven other drugs. This level is half the detection limit of NIOSH methods and one‐fourth of the lowest U.S. state decontamination limit for meth. Dynamic ranges of 100 in concentration were demonstrated for eight drugs, which is sufficient for a screening technique. The volatilities of 11 drugs deposited on glass were determined. The pick up of the drugs by solvent‐soaked cotton‐swab wipes from glass relative to acrylic latex paint was also compared. Published in 2011 by John Wiley & Sons, Ltd.