Reactive Intermediates: MS Investigations in Solution.
By Leonardo S. Santos
- Publisher: Wiley-VCH
- Number Of Pages: 341
- Publication : 2010-02-22
- ISBN-10 / ASIN: 3527323511
- ISBN-13 / EAN: 9783527323517
Edited by a top expert, this book represents a powerful tool showing how to successfully apply ESI-MS in revealing, elucidating and helping to consolidate several proposed mechanisms of reactions that need to be emphasized.
With a foreword by Chris Enke
Foreword:
Mass spectrometry could be described, without implying any criticism, as an
example in progress. Each time it appears to be approaching maturity,
another breakthrough occurs to expand its usefulness in new areas of science. As
this volume clearly demonstrates, that process is still going on. In the early twentieth
century, mass spectrometry was principally a tool for physicists to study particles
and petroleum chemists to characterize petroleum mixtures.Wider use by chemists
began with the ability to obtain structural information from the spectra of pure
organic molecules. The analytical application of mass spectrometry truly came of age
with its use as a detector for gas chromatography. Indeed, up to the present, advances
in chromatography and mass spectrometry have leapfrogged each other, combining
to create analytical tools that have steadily advanced in selectivity and detection limit
for over two decades. The power of these tools is such that they have found critical
applications in virtually every area of science,medicine.
From the standpoint of mass spectrometric instrumentation, the story of this
evolution has taken place on four fronts:
(1) Methods of separating ions of different atomic or molecular masses
(2) Methods of obtaining more chemical information by tandem mass spectrometry
(3) Methods of ionizing analyte molecules
(4) Methods that improve sensitivity and throughput.
Spectacular advances in all four of these areas have facilitated the remarkable
expansion of mass spectrometry in diverse areas, including the subject of this
volume. Sometimes new applications are introduced by mass spectrometrists
recognizing an area of opportunity and sometimes by researchers in that area
who have the temerity and opportunity to try a new technique. In any case,
mass spectrometry has become a central tool in scientific investigation, and
facilities for its use have become a critical part of virtually all scientific research
organizations.
Most analytical methods use a bulk property to distinguish, separate, or identify an
analyte. Properties such as chemical reactivity, chromatographic retention time, and
optical absorbance or emission reveal information about an aggregate of analyte
molecules, giving a collective response value. A remarkable thing about a mass
spectrum, and one of the unique attributes of mass spectrometry, is that the analyte
molecules or atoms are separated by mass, and the detector records the mass of each
individual analyte molecule. The isotopic composition of the analyte is revealed as
are mass shifts due to modifications of very large molecules, even when only a
fraction of them have been modified.
To perform a separation of sample molecules or atoms according to their individual
masses, all mass spectrometers rely on the fact that the trajectory of a charged
particle in the presence of electric and magnetic fields is mass-dependent, or, more
exactly, dependent on the mass-to-charge ratio (m/z) of the particle. To avoid
distortion of the differentiating trajectory by collisions with molecules, this separation
must be carried out in a vacuum, though, as we shall see, sometimes such
collisions can be used to advantage.
Chapter 1 in this volume reviews the historical development of mass spectrometers
in some detail. I will here introduce the general concepts of mass separation
in the context of some seminal developments in the instrumentation. Ions
accelerated to a nearly constant kinetic energy (1/2 mv2) will, in a region with
uniform magnetic field, have a curved trajectory dependent on the ion momentum.
Magnetic sector mass spectrometers, based on this principle, held a dominant
position for many years. The addition of an electric sector greatly improved
the mass resolution, and these double-focusing mass spectrometers were the
gold standard into the 1990s. Obtaining mass resolutions in the tens of thousands
enabled the development of exact mass determination, whereby the amount of
the mass defect in the elements could be used to determine the chemical formula
of an analyte based on the measurement of the ion m/z to within some few parts
per million. The champion, however, for mass resolution has been the Fourier
transform ion cyclotron resonance (FTICR) mass spectrometer introduced by
Marshall and Comisarow in 1974. Ions in a very high magnetic field move in a
circle on a plane orthogonal to the magnetic field flux. The frequency of their
rotation is a function of their m/z value. A batch of ions is excited to rotation and
the resulting signal is analyzed by Fourier Transform to obtain the frequencies
and thus the m/zs of the ions in the batch. Mass accuracies in small fractions of
parts per million have been achieved.
Though mass analysis based on ion flight time was an early innovation, the lack
of good high-speed electronics and its poor mass resolution prevented its wide
adoption. What did bring mass spectrometry into the main stream of chemical
analysis was the development of the quadrupole mass spectrometer. This simple
device provided unit resolution mass spectra in a relatively compact, low-cost
format. The quadrupole mass analyzer with gas chromatography
resulted in a powerful analytical tool that continues to see wide use in a variety.
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