Xiaqing Xu

Date of Graduation

2017

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Fred L King

Committee Co-Chair

Patrick Callery

Committee Member

Glen P Jackson

Committee Member

Justin A Legleiter

Committee Member

Stephen J Valentine

Abstract

Glow discharge mass spectrometry (GDMS) is widely used for trace elemental analysis of solid samples. A glow discharge source employs cathodic sputtering to release cathode species to the gas phase and subsequent collisional ionization of the sputtered species in the plasma. Ions representing the cathode material are differentiated by mass-to-charge values and then recorded by mass spectrometry. A pulsed radio frequency (RF) glow discharge time-of-flight mass spectrometry (GD-TOF MS) is employed in this study for its unique advantages, including the capability of direct analysis of nonconducting solids, temporal separation of ionization mechanisms, and the ability to acquire a complete spectrum within a single pulse cycle.;Although GD-TOF MS exhibits superior performance in direct solid analysis, its application to solution samples faces several problems arising from the solvent. In this study, analyte solutions are converted to dry residues on a conducting cathode surface involving the deposition of microliter volumes of analyte solution and subsequent drying in vacuum desiccator before conventional GDMS analysis. By this means, analytes including cysteine, glycyl-L-histidyl-L-lysine copper (GHK-Cu) complex and iodinated organic compounds are successfully sampled by GD plasma for mass spectrometric measurement.;Cysteine analysis is a follow-up study based on previous findings in this lab. Cysteine residues analyzed by the current GDMS system reveals a characteristic ion at m/z 76 in the afterpeak regime. However, the cysteine residue prepared previously in a Cu reservoir only produces a very weak characteristic ion signal and this ion signal dissipates quickly over time, indicating significant sample loss in the sputtering process. So, a new sample preparation method was introduced, involving adding a layer of paper material on top of the cathode surface. The layer is designed to provide a better support to the solution residue. In particular, hydrophilic membrane filter as the layer material allows even distribution of the cysteine solution added, and is expected to trap the residue inside the pores as well as on the membrane surface. Residue prepared on the membrane shows superior performance in that an intense and long-lasting cysteine fragment ion signal can be obtained. However, the reproducibility of this method needs improvements, as can be seen from the quantitative results. Comparison of GD results between cysteine and deuterated cysteine fails to prove that the cysteine fragment at m/z 76 is produced from loss of carboxylic acid group.;GDMS analysis of GHK-Cu complex was completed with dry residues formed on a silver cathode. Residue analysis in a pulsed plasma only produces a distinct Cu+ ion signal regarding the analyte in the afterpeak regime. No information regarding the peptide structure was detected. However, the stoichiometric ratio of Cu element to the peptide is known (1:1), enabling the quantification of copper peptide to be completed with Cu measurement. Before quantification, factors including power, pressure, pulse width, duty cycle and sampling distance were evaluated for their influences on the Cu+ ion signal and optimal conditions were located for the best sensitivity. Subsequently, quantification was completed with GHK-Cu residues prepared from five concentrations and the calibration curve obtained shows good linearity.;Iodinated organic compounds, specifically three thyroid hormones (THs) were analyzed by GDMS as well. Because iodine has a high ionization potential (10.45 eV), a helium plasma was established within the GDMS system and its efficiency in sputtering and ionization was proved in a comparison with argon plasma for the analysis of blank Cu and triiodothyronine (T3). Analysis of THs residues in a pulsed helium plasma reveals a similar pattern and only an I+ ion signal found in the afterpeak is of sufficient intensity for further analysis. Quantification of diiodothyronine was demonstrated as an example and the calibration curve shows good linearity. Cross comparison among the three THs reveals that iodine atoms found at different positions within organic molecules are ionized to the same extent in GDMS.

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