Interpretation of Mass Spectra for Elemental Speciation Studies A dissertation submitted to the Division of Research and Advanced Studies of the University of Cincinnati in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry of the College of Arts and Sciences 2005 by Juris Meija B.Sc., Chemistry, University of Latvia, 2001 Committee Chair: Professor Joseph A. Caruso ABSTRACT OF DISSERTATION During the last decade, mass spectrometry has become a powerful tool in understanding the various aspects of molecular processes occurring in biological systems. This paves the way to the understanding of complicated life processes that are among the greatest challenges in the contemporary bioscience. Sample preparation is a critical area in elemental speciation analysis and it is important that techniques used to extract elemental species are efficient yet correctly reflecting the chemical species present in native sample. However, with all respect to sample preparation, data analysis and critical evaluation of experimental observations based on the molecular-level explanation is becoming a major obstacle in transferring the experimental knowledge into valid conclusions. Many problems in the context of mass spectrometry can be solved using techniques of computer sciences, graph theory, and discrete mathematics. The aim of this dissertation is to recollect several essays that demonstrate the power and the need for developing skills in mass spectrometry data interpretation. These include the study of chemical behavior of selenium bio-volatiles using mass spectrometry, element-specific fingerprinting of mass spectra using isotope patterns, data analysis for accurate isotope ratio measurements and isotope pattern analysis as an extension of isotope dilution analysis. PREFACE When Alice stepped through the looking glass, she did and so I sketched the corresponding equations on not know what she will do there nor did she had even a the back of my airplane itinerary. Later we slightest clue what ever will happen to her. My refined the mathematics with Nacho and what we dissertation is just such a journey. When I joined Doc’s called the “general approach to the method of group, he listed about four or so potential project ideas isotope dilution” was presented later that year in and I seem to like the one that he thought was perhaps Plymouth, UK (Chapter 6). This is how the main research projects initiated. the least interesting. When I saw a slide of Maria’s [Montes-Bayon] preliminary work that plants emit I would most like to thank my advisor (whom we CH3SeCH3 it sounded interesting, but then she showed call simply Doc) Professor Joe Caruso not only that plants also can emit CH3SeSeCH3. For a moment I for his guidance throughout my graduate career, thought this was the coolest thing in the world and I I thank him for allowing me to be myself, for immediately became fascinated with the chemistry of allowing me to pursue projects even though selenium volatiles, even though all I knew about volatile sometimes they had not much to with the Se was that these things “exist”. This lead to about four research interests of the group. He supported me publications (Chapters 2-4), including one pirated (!) in all those weird “virtual” studies that I publication (Sepu 2004 (22), 16–19). In autumn of 2003, performed sitting at my laptop computer. I thank shortly before going to Oviedo, I submitted a tutorial him for being such an interesting advisor. This manuscript on isotope pattern reconstruction (J. Am. Soc. had a large impact on my attitude towards the big Mass Spectrom. 2004 (15), 654–658). About two weeks things in the life. It is a great art to present a later Nacho [Garcia Alonso] came to me and told that he research seriously, yet attractive and in a very liked the manuscript (he was one of the reviewers). In a charming manner. Joe is a master of this art. two minute hallway talk later that day I told that I could Anne Vonderheide had tremendous impact on apply the algorithm to explain some odd things my English writing abilities and she encouraged regarding the accuracy of isotope ratio measurements in me so much. Not to mention that without our electron impact mass spectra of tin compounds. About extraordinary extensive library at the University week later we had the general fragmentation mechanism of Cincinnati I could not even pursue a single of these compounds (J. Mass. Spectrom. 2005 (40), in manuscript. And last, I thank Maria so much press). This scheme also explained the origin of about 2- again and again about those two wonderful 3% systematic error observed by few other research autumns that I could spend in Oviedo research groups. Few months later I was listening to some talk in group. There were so many other nice people the winter conference on plasma spectrochemistry (Jan involved in my research projects that I simply 2004, Ft. Lauderdale, FL) and it just occurred to me that cannot list them here. I just want to say that I the isotope pattern reconstruction algorithm is essentially was very delighted to collaborate with all of a reverse method of isotope dilution. I was quite excited them. juris TABLE OF CONTENTS CHAPTER 1 | ELEMENTAL SPECIATION………………………………………………… 3 1.1. Elemental Speciation: The concept and the need 1.2. Conceptual approaches to elemental speciation 1.3. Instrumental approaches CHAPTER 2 | MASS SPECTRA OF SELENIUM BIOVOLATILES…………………………… 13 2.1. Abstract 2.2. Introduction to selenium biovolatiles 2.3. Preparation of selenium standards 2.4. Mass spectra of diselenides and selenosulfenates i. EI+ ionization ii. EI– ionization iii. CI+ ionization iv. CI– ionization 2.5. Conclusions 2.6. Experimental CHAPTER 3 | POLYCHALCOGENIDES AND THEIR MASS SPECTRA……………………… 38 3.1. Abstract 3.2. Chalcogenide exchange reaction i. Formation of polychalcogenides 3.3. Mass spectra of trichalcogenides i. Triselenides ii. Branched trichalcogenides 3.4. Experimental 1 CHAPTER 4 | THEORY OF ISOTOPE PATTERN RECONSTRUCTION……………………… 58 4.1. Abstract 4.2. Introduction 4.3. Isotope pattern reconstruction 4.4. Signal deconvolution in the mass domain i. Signal peak shape analysis ii. Peak centroid mass analysis 4.5. Application to dimethyl diselenide mass spectra i. Deconvolution in the intensity domain ii. Deconvolution in the mass domain ii. Peak centroid mass shift analysis 4.6. Aspects of isobaric interferences in mass spectra CHAPTER 5 | INTERPRETATION OF BUTYLTIN MASS SPECTRA FOR ISOTOPE RATIO MEASUREMENTS…………………………………. 73 5.1. Abstract 5.2. Introduction 5.3. Mass spectra of butyltin compounds i. Interpretation of mass spectra using isotope pattern reconstruction ii. Fragmentation behavior of butyltin compounds 5.4. Isotope ratio measurements in fragment ions 5.5. Conclusions 5.6. Experimental CHAPTER 6 | ALTERNATIVE LOOK AT THE METHOD OF ISOTOPE DILUTION…………… 91 6.1. Abstract 6.2. Introduction 6.3. Theory 6.4. Results and Discussion i. Effects of carbon isotope variations ii. Application to butyltin quantitation iii. Uncertainty of isotope pattern reconstruction iv. Applications to multiple spike isotope dilution 6.5. Conclusions 6.6. Experimental CHAPTER 7 | PROSPECTIVES……………………………………………………………. 110 Mathematical speciation CHAPTER 7 | REFERENCES……………………………………………………………… 114 2 “Perhaps Looking-glass milk isn't good to drink…” /Lewis Carroll, Through the Looking Glass/ CHAPTER 1 | ELEMENTAL SPECIATION 3 1.1. Elemental speciation: the concept and the need For more than 450 years, toxicologists have relied on an idea expressed by Paracelsus in the fifteenth century: "Alle Dinge sind Gift und nichts ohn Gift; alein die Dosis macht das ein Ding kein Gift ist” [all things are poison and not without poison; only the dose makes a thing not a poison”]. With the exception of E = mc2, perhaps no other single statement has wielded such force in establishing the popular notoriety and the professional stature of an individual in the history of science as the words just quoted. The statement of Paracelsus is clearly one of the main driving forces in modern elemental speciation studies. Nowadays it is known that molecular structure governs properties of compounds and not the individual constituents of that molecule. For example, from two different arsenic-containing compounds one might be toxic th (such as Me2AsOH) and the other - harmless (such as arsenobetaine). In the middle of the 18 century Louis Pasteur established the fact that the biological properties of organic compounds are structure specific and soon after the very same idea was advertised by English mathematician/writer Lutwidge Dodgson (known as Lewis Carroll). In his work “Through the looking glass” he incorporates the monologue “perhaps Looking-glass milk isn't good to drink…”, most likely referring to the properties of lactic acid mirror isomers. Elemental speciation refers to the characterization of the species of one or more particular elements. The ultimate goal for this analytical activity is to identify and possibly quantify one or more species associated with a certain sample. Speciation (speciation analysis) is driven by the need for better risk/benefit assessments than total elemental analyses can provide. The toxicity of an element depends on its physical-chemical form present in the sample and on its capacity to move through the intestinal barrier. However, elemental species identification is not always an easy task, especially for high molecular weight species. Speciation includes the elucidation 4 of the oxidation state, total charge or molecular weight of the species, binding strength of the elements, etc. In all the cases, the information obtained leads to the ultimate structural identification of the particular species of interest and, as necessary, its quantification. Sample preparation, analyte extraction, pre-concentration or enrichment play important roles in trace element speciation and sometimes they are the key factors, since often speciation analyses are in sub-ppb range.1,2 Elemental speciation in foods is of particular interest because of their high consumption.