Enhancement of mass resolution in the quadrupole ion trap via resonance ejection

The Finnigan MAT ion trap mass spectrometer (ITMS) has unit mass resolution and a nominal upper mass/charge limit of only 650 when operated in the standard mass-selective instability method for acquiring mass spectra. Mass resolution up to {approx}2500 can be achieved with the use of the axial modulation technique. Ions can also be ejected at a q{sup z} value other than that corresponding to the mass-selective instability boundary by application of an auxiliary signal to the end-cap electrodes. When the frequency of this signal is resonant with the secular frequency for a particular m/z, such ions can become kinetically excited and ejected from the trap via a process known as resonance ejection. We demonstrate the capability to mass-selectively isolate or eject ions over a wide mass range and to generate mass spectra of electrospray-generated ions by scanning the frequency of the resonance ejection signal applied to the end-cap electrodes. A block diagram of the experimental modifications to the ITMS required for frequency-swept resonance ejection is shown. 2 refs., 5 figs

Peptide/MHC Tetramer-Based Sorting of CD8+ T Cells to a Leukemia Antigen Yields Clonotypes Drawn Nonspecifically from an Underlying Restricted Repertoire

Testing of T cell-based cancer therapeutics often involves measuring cancer antigen-specific T-cell populations with the assumption that they arise from in vivo clonal expansion. This analysis, using peptide/MHC tetramers, is often ambiguous

Advances in structure elucidation of small molecules using mass spectrometry

The structural elucidation of small molecules using mass spectrometry plays an important role in modern life sciences and bioanalytical approaches. This review covers different soft and hard ionization techniques and figures of merit for modern mass spectrometers, such as mass resolving power, mass accuracy, isotopic abundance accuracy, accurate mass multiple-stage MS(n) capability, as well as hybrid mass spectrometric and orthogonal chromatographic approaches. The latter part discusses mass spectral data handling strategies, which includes background and noise subtraction, adduct formation and detection, charge state determination, accurate mass measurements, elemental composition determinations, and complex data-dependent setups with ion maps and ion trees. The importance of mass spectral library search algorithms for tandem mass spectra and multiple-stage MS(n) mass spectra as well as mass spectral tree libraries that combine multiple-stage mass spectra are outlined. The successive chapter discusses mass spectral fragmentation pathways, biotransformation reactions and drug metabolism studies, the mass spectral simulation and generation of in silico mass spectra, expert systems for mass spectral interpretation, and the use of computational chemistry to explain gas-phase phenomena. A single chapter discusses data handling for hyphenated approaches including mass spectral deconvolution for clean mass spectra, cheminformatics approaches and structure retention relationships, and retention index predictions for gas and liquid chromatography. The last section reviews the current state of electronic data sharing of mass spectra and discusses the importance of software development for the advancement of structure elucidation of small molecules

Fixed-wavelength R2PI/tandem mass spectrometry for mixture analysis in the quadrupole ion trap

Many aromatic molecules have strong absorption bands in the uv which are rather broad at room temperature; the ionization potentials for many such polyatomics are also often less than 9 eV. Therefore, they can be ionized with high efficiency by resonance enhanced two-photon ionization (R2PI) using the frequency-quadrupled output of the Nd:YAG laser at 266 nm (4.7 eV). Because the degree of fragmentation accompanying R2PI is a function of laser power, mass spectrometric analysis of such fragments also can often improve the specificity of the overall analysis. However, in the case of organic mixture analysis, the complexity of R2PI/fragmentation mass spectra may compromise compound identification tandem mass spectrometry (MS/MS) can also enhance the specificity of analysis over that provided by a single stage of mass spectrometry. Although R2PI/fragmentation and MS/MS can yield complementary data, situations may often occur in which the spectra reveal no unique structural information. Nevertheless, the combination of R2PI and MS/MS can still provide an advantage over the use of R2PI alone

Real-time Detection and Tandem Mass Spectrometry of Secondary Organic Aerosols with a Quadrupole Ion Trap

An aerosol quadrupole ion trap mass spectrometer is reported that is sensitive, has unique capabilities to perform chemical ionization, is operated in real-time, and is able to perform tandem mass spectrometry. The instrument samples particles with an aerodynamic lens and volatilizes them within the heated ion trap electrode assembly. Analyte molecules are ionized within the ion trap by proton transfer from reagent ions, and resultant fragmentation is reduced compared to vacuum UV photoionization. Particle concentrations can be detected linearly over two orders of magnitude and as low as 5 μg/m3. To demonstrate the real-time analysis capability of the instrument, secondary organic aerosol particles were produced by reaction of 100 ppb α-pinene and 200 ppb ozone in an aerosol bag and observed in real-time to monitor the progress of the reaction. Pinic acid and pinonic acid are two of the many components of the secondary aerosol mixture that form and gradually decrease in concentration. Individual concentrations are calculated using pinic acid as an internal standard and vary from 4-36 ppb. The identities of analyte ions from both compounds are confirmed by tandem mass spectrometry in real-time