A continuous flow probe method for on‐line introduction of liquid samples for detection by laser desorption with resonant two‐photon ionization in supersonic beam mass spectrometry

A continuous flow probe is used as a direct inlet source for injection of liquid samples into a time‐of‐flight (TOF) mass spectrometer. The direct liquid inlet is coupled to laser desorption as a means of rapidly vaporizing the nonvolatile sample dissolved in the solvent for entrainment into a supersonic jet expansion. The target analyte is then selectively analyzed by resonance enhanced multiphoton ionization (REMPI) in the TOF device. This method demonstrates the ability to continuously inject thermally labile biological samples such as neurotransmitters and oligopeptides for detection and structural analysis by REMPI. In addition, sensitivity limits in the low ng regime are demonstrated with quantitation over 3 orders of magnitude.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70599/2/RSINAK-62-4-957-1.pd

An ion trap storage/time‐of‐flight mass spectrometer

An ion trap/time‐of‐flight (TOF) mass spectrometer combination has been developed in order to combine the storage capabilities of an ion trap with the speed and resolution of a time‐of‐flight device. The ion trap is an rf quadrupole trap which operates in the total storage mode, i.e., with the dc voltage=0 on the end caps and rf voltage on the ring electrode. The trap has an ion storage time of ≥2 s at an rf potential of 310 Vpp. The stored ions are ejected into the time‐of‐flight device using a −150 V dc pulse on the exit end cap which causes the ion trajectories in the trap to become unstable. The ions are mass analyzed using either a linear or reflectron TOF. In the linear mode the resolution is 240 while in the reflectron mode a resolution of 1300 at m/z 93 is achieved. The storage capabilities of this device may have important applications towards enhancing sensitivity, the study of very slow metastable decay, and photodissociation mass spectrometry and spectroscopy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70469/2/RSINAK-63-10-4277-1.pd

Proteomics in Melanoma Biomarker Discovery: Great Potential, Many Obstacles

The present clinical staging of melanoma stratifies patients into heterogeneous groups, resulting in the application of aggressive therapies to large populations, diluting impact and increasing toxicity. To move to a new era of therapeutic decisions based on highly specific tumor profiling, the discovery and validation of new prognostic and predictive biomarkers in melanoma is critical. Genomic profiling, which is showing promise in other solid tumors, requires fresh tissue from a large number of primary tumors, and thus faces a unique challenge in melanoma. For this and other reasons, proteomics appears to be an ideal choice for the discovery of new melanoma biomarkers. Several approaches to proteomics have been utilized in the search for clinically relevant biomarkers, but to date the results have been relatively limited. This article will review the present work using both tissue and serum proteomics in the search for melanoma biomarkers, highlighting both the relative advantages and disadvantages of each approach. In addition, we review several of the major obstacles that need to be overcome in order to advance the field

Supercritical fluid injection of nonvolatiles with resonant two photon ionization detection in supersonic beam mass spectrometry

Supercritical fluids of N2O and CO2 are used to solubilize nonvolatile compounds for supersonic jet expansions. R2PI is then used to ionize these compounds in a time of flight mass spectrometer. By tuning the dye laser wavelength a cold wavelength ionization spectrum can be obtained that is characteristic of that species.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87289/2/612_1.pd

Selective resonance enhanced multiphoton ionization of aromatic polymers in supersonic beam mass spectrometry

Various polymers have been studied using a pulsed laser evaporation method for volatilizing these species and entraining them into a supersonic jet expansion followed by resonance enhanced multiphoton ionization (REMPI) detection in a reflection time-of-flight mass spectrometer. In most cases the monomer is detected as the dominant ion, although sometimes smaller characteristic fragments are observed instead. Fragmentation of the monomer can also be produced for structural analysis. However, larger oligomers are rarely observed with any intensity unless the average molecular weight distribution is less than 1000. In addition, REMPI at 266 nm is shown to be a means of selectivity detecting absorbing aromatic polymers in polymer blends without interference from aliphatic or non-aromatic polymers. Among the polymers studied are polystyrene, EPON epoxy resins and various polyamides.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29225/1/0000280.pd

Applicability of resonant two photon ionization in supersonic beams to halogenated aromatic hydrocarbons

This work investigates problems encountered in the application of one color resonant two‐photon ionization as an ionization source in supersonic beam mass spectroscopy. Of particular interest is the fact that photons of one color may not provide sufficient energy to cause ionization even when the laser source is tuned to an excited vibronic molecular state. We have therefore correlated trends in ionization potential with molecular structure for simple systems, specifically, halogenated aniline, phenol and toluene derivatives and mono‐ and di‐distributed benzenes. In the case of para‐substituted compounds where there is little substituent group interacting ionization occurs efficiently at the S0 → S1 origin, provided ultrafast processes are not active as in the case of iodo substituted benzenes. Many ortho compounds, however, are found not to ionize efficiently. This is probably due to a combination of coulombic and steric interactions which result in an increase in ionization potential. These types of effects have been qualitatively related to the electron releasing and withdrawing properties of the substitutent groups thereby allowing reasonable predictions to be made regarding those types of subsdtituted benzenes which can be probed with R2PI.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87501/2/620_1.pd

Resonance enhanced multiphoton ionization of nucleosides by using pulsed-laser desorption in supersonic beam mass spectrometry

Pulsed-laser desorption is used as a means of volatilizing nucleosides into the gas phase for entrainment into a supersonic jet expansion. These species are then studied in a time-of-flight mass spectrometer by resonance enhanced multiphoton ionization. Using this combination of techniques either relatively soft ionization or controlled fragmentation can be produced. The degree of molecule ion formation is shown to be relatively strong compared to other soft ionization methods such as FAB/MS, SIMS, FD or FI. In addition, the resulting fragmentation produces ions that are characteristic of the structure of the nucleoside. It is further demonstrated that such fragmentation can be used to discrimination between isomeric nucleosides and also between ribose and deoxyribose sugar groups.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27986/1/0000419.pd

The design of an atmospheric pressure ionization/time‐of‐flight mass spectrometer using a beam deflection method

A novel time‐of‐flight (TOF) mass spectrometer configuration has been designed which can be interfaced to a continuous ion beam source produced by atmospheric pressure ionization. The TOF device uses a beam deflection method to sweep the ion beam past a slit placed near the ionization source in order to generate a start pulse for TOF detection. The beam sweep technique is modeled by computer simulation and optimized for the various experimental parameters. Nonvolatile samples are injected into the TOF device using liquid injection into a glow discharge atmospheric pressure ionization source in helium. A resolution of at least 519 at m/z 311 is obtained, which is limited by the experimental parameters available in our experiment. The mass resolution is computer modeled and it is shown that as the mass increases, the experimental constraints become less important, and the resolution will increase. It is predicted that using the correct experimental conditions and with the addition of an ion reflector that resolution of well over 1000 should be obtained.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70760/2/RSINAK-63-1-139-1.pd

Free-jet spectra and structure of o-, m- and p-dihydroxybenzenes

Analysis of the free-jet spectra of hydroquinone (para), catechol (ortho) and resorcinol (meta) shows the presence of multiple origins in each molecular spectrum which are interpreted as belonging to separate structural isomers.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25500/1/0000041.pd