MALD/I TOF PSD and CID: Understanding precision, resolution, and mass accuracy and MALD/I TOFMS: Investigation of discrimination issues related to solubility

Abstract

Part I of this thesis describes details of MS/MS experiments conducted on a TOFMS instrument and begins with an optimization of the CID conditions for poly(ethylene glycol) (PEG). Optimum conditions were found using lithium cation, helium collision gas, and a gas pressure that attenuated the parent-ion signal to between 25–50%. With these conditions, the use of PEG as a mass calibrant for PSD and CID was investigated, using a design of experiments to investigate the sources of variability related to precision and accuracy. There is a mass dependence with respect to precision within segments; however, the variability for a particular ion was not statistically significant for any of the variables tested. The variability associated with mass accuracy was demonstrated to be plate-position dependent, and most accurate results were obtained when the calibrant and analyte were close together on the sample plate. Several instrument-operating parameters were optimized, resulting in improvements to overall product-ion resolution and mass accuracy. Testing with different classes of polymers, as well as peptide examples, demonstrated that the PEG calibration method is robust for a variety of analyte types. Part II of the thesis focused on furthering the understanding of two types of discrimination observed in the MALD/I TOF analysis of synthetic polymers: 1) molecular weight distribution, and 2) polymer structure. Molecular weight discrimination was investigated with a series of narrow PMMA reference standards that demonstrated the importance of comparing the distribution, rather than simply average MW values, which also demonstrated limitations of the dynamic range of the detection system on the MALD/I TOF instrument. HPLC conditions were developed to relate the solubility of the matrix and analyte, demonstrating the importance of relative solubility in MALD/I sample preparation. The roles of salt-to-analyte and matrix-to-analyte ratios were investigated for polymers, which further demonstrated the importance of having good compatibility of matrix, analyte, and cationization reagent (i.e., solubility). The importance of solubility was exemplified with the development of conditions to analyze PET samples by MALD/I. Initial investigations of supporting analytical techniques were shown to have the potential to provide information to support the investigation of the role of solubility.%%%%Ph.D., Analytical Chemistry – Drexel University, 2004