Characterization of Impurities and Degradants Using Mass Spectrometry

Guodong Chen, Birendra Pramanik, Mike S. Lee, Birendra Pramanik, Mike S. Lee

• Publisher: Wiley
• Year: 2011
• ISBN:
  9780470922972
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

The book highlights the current practices and future trends in structural characterization of impurities and degradants. It begins with an overview of mass spectrometry techniques as related to the analysis of impurities and degradants, followed by studies involving characterization of process related impurities (including potential genotoxic impurities), and excipient related impurities in formulated products. Both general practitioners in pharmaceutical research and specialists in analytical chemistry field will benefit from this book that will detail step-by-step approaches and new strategies to solve challenging problems related to pharmaceutical research.

Phase I

Methodology

Chapter 1

Introduction to Mass Spectrometry

Scott A. Smith

Department of Chemistry, Michigan State University, East Lansing, MI 48824

Ruth Waddell Smith

Forensic Science Program, School of Criminal Justice, Michigan State University, East Lansing MI 48824

Yu Xia

Department of Chemistry, Purdue University, West Lafayette, IN 47907

Zheng Ouyang

Weldon School of Biomedical Engineering, Purdue University, West Lafayette, IN 47907

1.1 History

Although mass spectrometry (MS) has aged by about one century, it has never ceased to evolve into an increasingly powerful and important technique for chemical analysis. The development of mass spectrometry can be folded into a few periods, where the capabilities of a particular discipline of science were advanced significantly and steadily due to the introduction of MS into that field. Those periods are, approximately, physics (1890s–1945), chemistry (1945–1975), materials science (1955–1990), and biology/medicine (1990–present) [1]. The history of MS shows that the technique has facilitated many significant scientific achievements, from the discovery of isotopes [2], to purifying the material for the first atomic bombs [3], to space exploration [4, 5], to the mass analysis of whole red blood cells each weighing several tens of picograms [6]. The following is a short account of some of the notable feats that have transpired in this field.

1.1.1 Atomic Physics

The technique now known as MS has its roots in atomic physics at the beginning of the twentieth century, when it was originally applied by physicists toward answering questions on the nature of atoms. Throughout much of the 1800s, the prevailing wisdom held that atoms were indivisible, that all atoms of a given element had the same mass, and that the masses of all elements were multiples of the mass of hydrogen [7–9]. Despite these beliefs, the interrogation of bulk elements through chemical means (gravimetric analyses) demonstrated that some atomic masses were, in fact, not unit integers of that of hydrogen (e.g., chlorine). Furthermore, for much of the century, relatively little was known of the nature and origins of electricity. Hence, the explanations for these phenomena awaited the discovery of electrons and isotopes through physical investigations.

Toward the end of the 1800s, many physicists were interested in unraveling the underlying principles of electricity. To study the properties of electric currents, they would create a potential difference between two electrodes in partially evacuated discharge tubes made of glass and containing various types of gas. Evidence for cathode rays (electron beams) was first observed by Plücker in 1859 when he noticed a green phosphorescence occurring on his discharge chamber at a position adjacent to the cathode [10]. In time, the investigations of other physicists led to an accumulation of clues about the nature of cathode rays, including observations that (1) they are directional, moving from the cathode to the anode, (2) they are energetic, as determined by observing platinum foil becoming white-hot when placed in their path, (3) they conduct negative charge, as determined by measurement with electrometers, (4) they are particles rather than waves, (5) their energy is proportional to the acceleration potential to which they are subjected, (6) they have dimensions that are smaller than those of atomic gases, as determined by considering their penetration depth through media of varying density, and (7) they may be derived from any atom through various means, including heat, X rays, or electrical discharge [10]. Thomson went on to develop the means for measuring electron mass in a discharge chamber evacuated to low pressure (see Figure 1.1) [11]. By applying a magnetic field (B) and an electric field (E), both at right angles to each other as well as to the direction of electron propagation, they could determine the electron velocity (v) by canceling out the deflections of the magnetic and electric forces (i.e., |Bev − Ee| = 0) such that the electrons travel in a straight line, yielding v = E/B. The ratio of electron mass to electron charge (m_e/z) could also be arrived at from experimental measurements as

, where l is the distance traveled by an electron through a uniform electric field and

is the angle through which electrons are deflected as they exit the electric field [11]. From this and other experiments, Thomson demonstrated that the mass of electrons are about

(0.001%) that of the proton (the mass of protons, the ionized form of the smallest known particles at the time, was by then known from electrolysis research) [11]. Thomson was awarded the 1906 Nobel Prize in Physics “in recognition of his theoretical and experimental investigations on the conduction of electricity by gases” [12].

Figure 1.1 Thomson's apparatus for measuring electron mass-to-charge ratio (m/z). Components are as follows: (A, B) anodes with pinhole apertures to guide and narrow the beam; (C) cathode; (P, P′) electric field deflection electrodes; (S) detection screen. The magnetic field, when applied, was directed orthogonally to both the electron beam and the electric field (indicated by the tickmarks x). (Reprinted from Ref. [10], with permission of John Wiley & Sons, Inc.)

While progressing toward an understanding of electrons, physicists also became interested in understanding the positively charged particles (cations) that were present in discharges [13]. During studies of the effects of weak magnetic fields on cathode rays in 1886, Goldstein discovered positively charged anode rays that traveled in the opposite direction of electrons; unlike cathode rays, these anode rays were not susceptible to deflection by the weak magnetic fields used in Goldstein's experiments [14]. However, in 1898, Wein determined that anode rays in fact could be influenced by the presence of magnetic fields, provided the fields were relatively strong; with this knowledge, he determined that their masses were on the order of atoms rather than the substance of which cathode rays were composed [14]. Building on such early observations, Thomson created a device called the parabolic mass spectrograph (see Figure 1.2), in which he exposed anode rays to parallel magnetic and electric fields in such a way that, while propagating through the field region the rays were influenced vertically by the electric field and horizontally by the magnetic field, with the result that the ions impinged on a photographic plate positioned transverse to the direction of particle propagation [14]. The images on the plate were of parabolas, in which each particular parabola was specific for mass-to-charge ratio (m/z) and the occurrence of parabolic lines was attributed to distributions in kinetic energy [14]. Thomson's device was capable of identifying the presence of ionized gases, and he demonstrated its capabilities by acquiring a mass spectrograph of the mixture of gases constituting the atmosphere [14]. Notably, Thomson's atmospheric data showed the first instance of the rare isotope

(neon-22) adjacent to the predominant

; since he believed that stable elements could have only a single mass (a then widely held belief), he assumed that what was conventionally considered neon was actually a mixture of two elements, with that at mass 22 being previously unknown [2, 14]. Shortly before this time, Rutherford and Soddy discovered nuclear transmutation, whereby fission products from radioactive elements produce as products chemically distinguishable elements of abnormal mass (i.e., isotopes) [15]; however, given the unusual nature of radioactive matter at the time of Thomson's observation, the link was not obvious that neon atoms could occur as distributions of varying mass. It wasn't until 1919, when Aston built an improved mass spectrograph and discovered the isotopes of dozens of elements, that isotope theory became widely accepted by the scientific community [16]. When he published the results of the measurements of the first 18 elements that he investigated, Aston demonstrated that they all were within

of whole-number units, with the exception of hydrogen, which has a very slight deviation from the whole-number trend [16]. For his efforts toward proving the existence of isotopes, Aston won the 1922 Nobel Prize in Chemistry.

Figure 1.2 Ion separations on Thomson's parabolic mass spectrograph. Components are as follows: (I) insulator; (M, N) magnet poles; (P, P′) electric field deflection electrodes; (S) detection screen. The position of ion impact (shown here for two species labeled m₁ and m₂) on the screen was dependent on ion charge and kinetic energy, the electric and magnetic field s...

Table of contents

• Cover
• Wiley Series on Pharmaceutical Science and Biotechnology: Practices,Applications, and Methods
• Title Page
• Copyright
• Preface
• Contributors
• Acronyms
• Part I: Methodology
• Part II: Application
• Index