Time-Resolved Mass Spectrometry

From Concept to Applications

Pawel L. Urban, Yu-Chie Chen, Yi-Sheng Wang

• Publisher: Wiley
• Year: 2016
• Print ISBN: 9781118887325
• eBook ISBN: 9781118887356
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Time is an important factor in physical and natural sciences. It characterizes the progress of chemical and biochemical processes. Mass spectrometry provides the means to study molecular structures by detecting gas-phase ions with the unique mass-to-charge ratios. Time-resolved mass spectrometry (TRMS) allows one to differentiate between chemical states that can be observed sequentially at different time points. Real-time mass spectrometric monitoring enables recording data continuously with a specified temporal resolution. The TRMS approaches – introduced during the past few decades – have shown temporal resolutions ranging from hours down to microseconds and beyond.
This text covers the key aspects of TRMS. It introduces ion sources, mass analyzers, and interfaces utilized in time-resolved measurements; discusses the influence of data acquisition and treatment; finally, it reviews most prominent applications of TRMS – in the studies of reaction kinetics and mechanism, physicochemical phenomena, protein structure dynamics, biocatalysis, and metabolic profiling.
It will assist science and engineering students to gain a basic understanding of the TRMS concept, and to recognize its usefulness. In addition, it may benefit scientists who conduct molecular studies in the areas of chemistry, physics and biology.

Chapter 1
Introduction

Time flies over us, but leaves its shadow behind.

Nathaniel Hawthorne (1804–1864)

1.1 Time in Chemistry

According to one definition, time is “the indefinite continued progress of existence and events in the past, present, and future regarded as a whole” [1]. For millennia, time has intrigued philosophers and artists. The inevitability of time flow has triggered frustration and hope. While basic chemical knowledge was gathered in antiquity, modern chemistry has been developed since the 17th century. During the first

years, the notion of time in chemistry was obscure and elusive. It appeared in the spotlight when Peter Waage and Cato Guldberg began to develop the concept of chemical kinetics. Since then, the time dimension was instantly promoted to become an important factor in chemical reactions. In modern chemistry textbooks, potential energy diagrams often represent changes in the energy of reactants along an axis labeled as the reaction path. However, time is the variable that describes the progress of every chemical transition and physical process. Thus, time has always been among the key factors studied in chemical science [2]. Investigating chemical phenomena in relation to time has turned out to be vital for the understanding of fundamental concepts – in particular, reaction kinetics.

Temporal resolution is the ability of a method to discern consecutive transitions in the studied dynamic systems. In the field of analytical chemistry, there exist numerous methods that allow one to measure concentrations of substances in solutions or gaseous mixtures at given time points. However, many conventional methods possess limited temporal resolution. In some cases, samples are obtained from reaction mixtures at specific time points. As a result the temporal characteristics of the studied process can only be described considering the limited frequency of sampling points. The obtained samples can be regarded as zero-dimensional. We live in a four-dimensional world that is described by three spatial dimensions. Time is the fourth elusive dimension that describes happenings in the other three dimensions (change of position). Various novel analytical methods have been developed to grasp the 3D nature of chemistry. For example, optical methods are irreplaceable when it comes to 2D and 3D spatial analysis (imaging) of chemical processes [3, 4]. Frequently these methods also provide superior temporal resolutions. Due to numerous technical obstacles, advancement of these multidimensional analysis tools could only happen because of the developments in physics, optics, photonics, and physical chemistry.

Some physical or chemical processes are so fast that their existence can only be verified using highly refined analytical approaches. For example, using infrared (IR) spectroscopy and computational methods, it was possible to confirm the existence of the simplest Criegee intermediate

which has a lifetime counted in microseconds [5]. Other processes (e.g. radioactive decay of uranium-238 with a half-life of

) are so slow that their progress cannot easily be observed during a human lifetime. However, most reactions occurring in the biological world are accelerated by biocatalysts (enzymes). Such catalytic processes can be observed on timescales of seconds and minutes. Reaction kinetics encompasses experimental methodology and the associated mathematical treatment aiming to describe the progress of chemical reactions in time. Understanding chemical kinetics can help us to optimize important reactions, so that they can be applied in large-scale synthesis, and used by industry. The kinetic profiles of reactions let us gain fundamental insights on the reaction mechanisms. Similarly, to chemical reactions, there exist other processes which serve chemists every day – distillation and extraction are just two examples. Studying kinetic properties of dynamic processes involving molecules requires the use of appropriate analytical methodology – capable of recording molecular events in the time domain.

Several physical techniques were introduced to chemistry laboratories in order to enable monitoring of chemical and physical processes in time. They include such dissimilar platforms as: fluorescence detection [6], IR spectroscopy [7], diffraction [8, 9], nuclear magnetic resonance (NMR) [10–12] as well as crystallography [13]. Since ultrafast phenomena are relevant to many fundamental studies in physics and chemistry [14], various spectroscopic techniques have been developed which enable investigation of molecular events in the time range from 10^-9 to 10^-18 s [15, 16]. Ultrafast IR and Raman spectroscopies enable measurements of phenomena which occur on the pico- and femtosecond timescales

; corresponding to the elementary steps that affect chemical reactivity, including changes in the electron distribution, molecular structure and translocation of chemical moieties [17]. Pulse fluorometry and phase-modulation fluorometry enable the measurement of fluorescence lifetimes, which typically last over

[18, 19]. Time-resolved luminescence methods are routinely used in fundamental and applied sciences. For instance, by monitoring the intensity of light emitted following an excitation pulse in the nano- to millisecond range, one can distinguish contributions of de-excitation of individual fluorophores and/or phosphors. This approach enables sensitive detection of labeled molecules (or supramolecular probes) in complex biological samples which possess intrinsic luminescence (e.g., autofluorescence). Lifetime spectroscopy is nowadays almost routinely used in the analysis and imaging of biological specimens [20].

Optical methods have grounded their place in chemistry. They also have intrinsic limitations: the most prominent one is low molecular selectivity. Monitoring unknown substances and identification of unknown analytes, which are frequently present in complex mixtures, often requires powerful analytical strategies. Two particularly significant ones are NMR and mass spectrometry (MS). These two techniques have dissimilar principles: while MS separates and detects ions in the gas phase, NMR recognizes nuclei based on the characteristic electromagnetic radiation emitted by them at specific conditions in a mag...

Table of contents

• Cover
• Title Page
• Copyright
• Table of Contents
• Author Biographies
• Preface
• Acknowledgments
• List of Acronyms
• Chapter 1: Introduction
• Chapter 2: Ion Sources for Time-resolved Mass Spectrometry
• Chapter 3: Mass Analyzers for Time-resolved Mass Spectrometry
• Chapter 4: Interfaces for Time-resolved Mass Spectrometry
• Chapter 5: Balancing Acquisition Speed and Analytical Performance of Mass Spectrometry
• Chapter 6: Hyphenated Mass Spectrometric Techniques
• Chapter 7: Microfluidics for Time-resolved Mass Spectrometry
• Chapter 8: Quantitative Measurements by Mass Spectrometry
• Chapter 9: Data Treatment in Time-resolved Mass Spectrometry
• Chapter 10: Applications in Fundamental Studies of Physical Chemistry
• Chapter 11: Application of Time-resolved Mass Spectrometry in the Monitoring of Chemical Reactions
• Chapter 12: Applications of Time-resolved Mass Spectrometry in the Studies of Protein Structure Dynamics
• Chapter 13: Applications of Time-resolved Mass Spectrometry in Biochemical Analysis
• Chapter 14: Final Remarks
• Index
• End User License Agreement