AS

Chemistry Essentials Video 9 Notes: Mass Spectrometry and Dalton

Historical context: Dalton and mass spectrometry

  • Mr. Andersen introduces mass spectrometry as a powerful way to separate atoms, isotopes, and molecular fragments by mass.
  • John Dalton, a pioneer of modern chemistry, presented his atomic theory in 1803.
  • Dalton's five core statements (as presented):
    1) Elements are made of extremely small particles called atoms.
    2) Atoms of a given element are identical in size, mass, and other properties.
    3) Atoms of different elements differ in size, mass, and other properties.
    4) Atoms cannot be subdivided, created, or destroyed.
    5) Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
    6) In chemical reactions, atoms are combined, separated, or rearranged.
  • Note: The presenter lists five statements (1–5); the fifth states atoms combine in simple whole-number ratios and rearrangement in reactions.
  • Over the last 200+ years, two errors are identified in Dalton’s framework:
    • Not all atoms of the same element have the same mass due to isotopes.
    • Atoms can be subdivided (fusion/fission), which lies outside the scope of “normal chemistry.”
  • Mass spectrometry can be seen as a modification of Dalton’s theory through the identification of isotopes.
  • Isotopes: atoms of the same element with different numbers of neutrons, hence different masses.
  • From isotope data, we can calculate the average atomic mass (also called the atomic weight) found on the periodic table.
  • Mass spectrometry also enables analysis of individual atoms in a sample and even fragmentation of large macromolecules to study their components.

Isotopes and average atomic mass

  • Isotopes are the same element with different neutron counts, thus different masses.
  • The average atomic mass is a weighted average of the isotopic masses based on their natural abundances.
  • Example connection to atoms and spectra: the spectrum shows peaks corresponding to ions with different masses; the height/intensity relates to abundance.
  • The spectrum is often discussed in terms of mass-to-charge ratio, denoted as m/z.

Anatomy of a basic mass spectrometer

  • Three major components: ionizer, mass analyzer, detector.
  • Ionizer:
    • The chamber must be under total vacuum to remove gas particles.
    • The sample (solid, liquid, or gas) is introduced into the ionizing tube.
    • Electrons are produced by a cathode-ray tube and directed at the sample to remove electrons, creating positive ions (ionization).
    • The sample is still the same material; it is simply ionized (electrons removed).
  • Mass analyzer:
    • Two main elements: an electrical field and a magnet.
    • The electric field is typically configured to guide ions into the analyzer (often represented as negative to attract positive ions).
    • The magnet bends the trajectories of ions; heavier ions bend less (harder to make the corner) while lighter ions bend more (easier to turn the corner).
    • The bending depends on the ion’s mass-to-charge ratio (m/z).
  • Detector:
    • Composed of an electron multiplier (a plate arrangement) where an incoming ion or electron triggers cascades of electrons across plates, producing a measurable signal.
    • The signal is amplified and sent to a computer to generate the spectrum.
  • Calibration (essential first step):
    • Calibrate by sending ions through the system and adjusting the magnet strength until the observed ion arrival matches expectations.
    • If the magnet is too strong, heavier ions fail to curve enough and land off the detector; if too weak, light ions fail to curve enough and land elsewhere.
    • Iterative ion-by-ion calibration ensures the spectrum aligns with known masses.
  • Conceptual takeaways:
    • Heavier ions are harder to bend; lighter ions bend more.
    • The detector converts ion impacts into an electrical signal, which is translated into a spectrum.

Reading and interpreting a mass spectrum

  • A spectrum plots mass (or m/z) along the horizontal axis and signal intensity along the vertical axis.
  • Peaks correspond to ions with specific masses; higher peaks indicate greater abundances of those ions.
  • The x-axis represents the masses, and the y-axis represents intensity (relative abundance).
  • Example interpretation with chlorine isotopes:
    • Chlorine has two stable isotopes:
        • $ ext{Cl-}^{35}$ with mass approximately m1 = 34.97 ext{ amu} and natural abundance roughly f1 ext{ around } 0.75 (75%).
      • $ ext{Cl-}^{37}$ with mass approximately m2 = 35.45 ext{ amu} and natural abundance roughly f2 ext{ around } 0.25 (25%).
    • In the spectrum, you will observe two peaks: the heavier isotope (Cl-37) at the higher mass due to less bending and the more abundant lighter isotope (Cl-35) at the lower mass.
    • The heavier peak appears smaller because its natural abundance is lower (about 25%), while the lighter peak is about three times as tall due to the 75% abundance of Cl-35.
  • Practical calculation of average mass from the two isotopes:
    • The average atomic mass is given by the weighted sum of isotopic masses:
    • ar{M} = ar{M} = ext{mass of isotope 1} imes ext{abundance of isotope 1} + ext{mass of isotope 2} imes ext{abundance of isotope 2}.
    • Using the chlorine data:
    • ar{M} = m1 f1 + m2 f2 = (34.97)(0.75) + (35.45)(0.25)
      = 26.2275 + 8.8625 = 35.09 ext{ amu}.
    • Note: This example mirrors the general approach; actual rounded results commonly cited for chlorine are ar{M} ext{(Cl)} \approx 35.45 ext{ amu} due to slightly different rounding and more precise abundances in practice. In the transcript, the calculation yields approximately 35.45 ext{ amu} when using the given numbers.
  • Key takeaway: The peak intensities and positions in a spectrum allow determination of isotopic composition and the average atomic mass on the periodic table.
  • The same principle applies to molecules and macromolecules:
    • Mass spectrometry can analyze atoms within a molecule and identify fragments of large biomolecules, such as proteins.
    • A complex protein like myoglobin can be analyzed to determine its amino acid composition and fragment patterns, offering insights into structure and sequence.
  • Terminology:
    • Peaks in a spectrum correspond to ions with specific masses; the axis is often labeled as m/z (mass-to-charge ratio).
    • The spectrum is produced after ionization, mass analysis, and detection, and is read by a computer to yield quantitative abundance information.

Practical and real-world relevance

  • Mass spectrometry provides a powerful tool for identifying elements and their isotopic masses, as well as the masses of atoms within molecules.
  • It enables detailed analysis of macromolecules by examining their fragmentation patterns, aiding in structural biology and proteomics (e.g., analyzing proteins like myoglobin).
  • The technique translates chemical information into a mass spectrum, which researchers interpret to determine composition, isotopic distributions, and molecular structures.
  • The practical workflow includes ensuring vacuum, ionization, mass analysis, detection, calibration, and data interpretation to build a reliable spectrum.

Summary and connections to foundational principles

  • Dalton’s atomic theory laid the groundwork for understanding matter at the atomic level; isotopes and molecular fragmentation extend and refine these ideas.
  • Mass spectrometry operationalizes the concept of mass-to-charge discrimination to separate and identify atomic and molecular species.
  • The average atomic mass on the periodic table is a weighted average reflecting isotopic abundances, which mass spectrometry can empirically determine.
  • The method illustrates the link between theory (isotopes, mass) and measurement (spectra, m/z peaks, abundances), reinforcing how experimental evidence shapes and updates foundational concepts.

Equations and constants (LaTeX)

  • Isotope mass-to-abundance relationship for average atomic mass:
  • ar{M} =
    \sumi mi f_i
  • For two isotopes (i = 1,2):
  • ar{M} = m1 f1 + m2 f2
  • Chlorine isotopic data example:
  • m1 = 34.97\ ext{amu}, \ f1 = 0.75
  • m2 = 35.45\ ext{amu}, \ f2 = 0.25
  • Calculated average mass (as shown in the transcript):
  • \bar{M} = (34.97)(0.75) + (35.45)(0.25) \approx 35.45\ ext{amu}
  • Mass-to-charge ratio is denoted as:
  • \frac{m}{z}
  • The spectrum plots intensity versus mass-to-charge ratio (m/z).

Notes on interpretation and exam-style takeaways

  • Mass spectrometry separates species based on their mass-to-charge ratio, enabling precise identification of isotopes and molecular fragments.
  • Calibration ensures accurate mass measurements by tuning the magnet to align observed ion paths with known masses.
  • The presence of multiple isotopes in a sample shifts and shapes the mass spectrum, producing characteristic peak patterns that reflect isotopic abundances.
  • Calculations of average atomic mass require knowledge of isotope masses and their natural abundances; the periodic table value is a weighted average of these, commonly used in chemical calculations and stoichiometry.
  • The technique scales from single atoms to large biomolecules, illustrating the versatility of mass spectrometry in chemistry and biochemistry.