David C. Schriemer

Micro‐Scale Frontal Affinity Chromatography with Mass Spectrometric Detection: A New Method for the Screening of Compound Libraries

In one run the binding constants Kd for all the active components of a ligand library at sub-microgram quantities can be determined. A mixture of ligands is continuously infused through a column of immobilized receptor, and the eluent analyzed by electrospray mass spectrometry. From the affinity chromatogram produced (see picture) the breakthrough volume of a single compound and hence its Kd value can be determined.

Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry for the Analysis of Polydienes

An analytical method based on matrix-assisted laser desorption ionization (MALDI) time-of-flight mass spectrometry has been developed to provide information on oligomer structure, average molecular weight, and molecular weight distributions of polydienes (e.g., polybutadiene and polyisoprene), an important class of industrial polymers. This MALDI method involves the use of all-trans-retinoic acid as the matrix, copper (II) nitrate as the cationization reagent, and tetrahydrofuran as the solvent. The incorporation of this copper salt generates Cu+ adducts with the polymer chains. It also improves the signal strength and extends the upper mass range when used with all-trans-retinoic acid, as compared to silver nitrate. With this formulation, it is demonstrated that polybutadienes of narrow polydispersity with masses up to 300,000 u and polyisoprenes of narrow polydispersity with masses up to 150,000 u can be analyzed. The upper molecular weight limit is set by the requirement of using higher matrix-to-polymer ratios with increasing polymer molecular weight, to the point where the instrument can no longer detect the small quantity of polymer present in the matrix host. It is also shown that this sample preparation generates previously unreported adduction behavior. The practical implications of this adduction behavior on polymer structural analysis, accuracy of molecular weight determination, and the upper molecular weight limit of oligomer resolution are discussed. It is illustrated that, in a linear time-lag focusing MALDI instrument, oligomer resolution can be obtained for polydienes with molecular weights up to 24,000, providing structural confirmation of the end-groups and the repeat unit. The average molecular weights of a number of polydienes of narrow polydispersity determined by MALDI are compared to those obtained by gel permeation chromatography, and discrepancies are noted.

LASER-INDUCED SURFACE IONIZATION IN A TIME-OF-FLIGHT MASS SPECTROMETER

A laser‐induced ionization scheme based on laser‐metal interaction is described. It is found that when a low power, pulsed UV laser beam is directed to the repeller plate of a Wiley–McLaren time‐of‐flight mass spectrometer (TOFMS), which is held at much higher positive potential than the extraction grid, extensive ionization of gas phase species can take place. For monatomic species such as Ar and Xe, multiply charged ions up to Ar6+ and Xe9+ are obtained. For both aromatic and aliphatic organic molecules, classical electron‐impact‐type mass spectra are observed. Mass resolution is in the range of 300–400, comparable to that obtained using laser‐induced resonant two‐photon ionization (R2PI) in the same linear TOFMS. It is shown that this technique can be quite efficient in ionizing organic species. A detection limit of 5 ng and a linear dynamic range of four orders of magnitude for benzene are demonstrated. While not as sensitive as R2PI, it has a much wider applicability and more uniform ionization effic...

A Dual Ionization Source with Laser Desorption Sample Introduction in a Time-of-Flight Mass Spectrometer

Abstract A new mass spectrometric detection system has been developed for the analysis of non-volatile chemical species such as biochemicals. This system involves the use of a novel dual ionization source, consisting of laser-induced electron ionization (EI) and multiphoton ionization (MPI), to provide versatile ionization for a wide range of chemicals in a time-of-flight mass spectrometer. Laser desorption is incorporated into the system for sample vaporization. The experimental design of this system is described. It is demonstrated that two ionization modes can be interchanged very readily by simply adjusting the position and focus of the laser beam. EI and MPI mass spectra of a number of amino acids and small peptides are shown to illustrate the detection capability of the system. For chemicals amenable to both ionization methods, EI and MPI can provide sensitive detection with different fragmentation patterns. Thus, the operation in a dual ionization mode provides enhanced structural information. In a...

Effect of sample compositions on chemical analysis using matrix-assisted laser desorption ionization mass spectrometry

Matrix-assisted laser desorption ionization (MALDI) is an effective ionization technique for mass spectrometry. It take advantages of some unique properties of certain organic chemicals to provide entrapment, isolation, vaporization, and ionization of the analyte of interest. While the main application of the MALDI technique is currently in the area of biological molecule analysis, it is possible to use this technique for monitoring polymer chemistry such as degradation processes. This is potentially important for studying and developing environmentally degradable polymers. Direct analysis of the analyte in real-world samples is possible with MALDI. However, there is a significant effect of the overall composition of a sample on the detectability and performance of MALDI. Two examples are given to illustrate the positive and negative effects of buffers, salts, and additives on the MALDI sample preparation.