CHAPTER 12

In progress (last updated Oct 20)

Mass Spectrometry (MS) Instrument

Gives:

• exact molecular weight of compound

• exact molecular weight of some fragments → provides structural information

Who built the first ever MS instrument? When?

• JJ Thomson

• 1912

Who built the first fully functional MS instrument? Whenb?

• Francis Aston (JJ Thomson’s student)

• 1919

3 Major Components of a Mass Spectrometer

(1) Ionization Source

• Where sample molecules are injected

• Ionization: An electrical charge is applied to sample molecules, producing ions (usually positive)

• Ions may fragment

(2) Mass Analyzer

• Separates ions according to their mass-to-charge ratio (m/z).

• Middle part of instrument

(3) Detector

• Detects and counts the separated ions

• Number of ions detected determines the relative abundance of each peak in mass spectrum

Define “Ionizataion”

Process by which neutral molecules are converted into charged ions by applying energy.

Why are molecules ionized in mass spectrometry?

Molecules are ionized because only charged particles can be manipulated by the electric/magnetic fields of the mass spectrometer and separated according to their mass-to-charge ratio (m/z). Neutral species are not detected. [Chapter 12 Slides | PDF]

ANSWER THIS IN YOUR OWN WORDS, COME BACK TO THIS

T or F: The detector in a MS instrument cannot detect neutral fragments.

True.

T or F: During mass spectrometry, after ionization occurs, every single molecule has been converted into a charged ion.

False.

INCLUDE BRIEF/SIMPLE EXPLAANTION

Hard v.s. Soft Ionization Sources

• Provide example of each

Hard Ionization Sources

• Uses high energy

• Causes extensive fragmentation of the molecule

• Useful for obtaining structural information from fragment patterns

• E.g: Electron Impact (EI) ionization

Soft Ionization Sources

• Uses lower energy

• Preserves the intact parent molecule

• Produces less fragmentation

• Useful for determining molecular weight, especially for large molecules

• E.g.

  • Electrospray Ionization (ESI)

  • Matrix-Assisted Laser Desorption Ionization (MALDI

Memory Trick: Hard = Hits Hard → Lots of fragments (EI)

(a) Is hard ionization or soft ionization more commonly used nowadays.

(b) What is the main reason why someone would chose to use the less common option

(a) Soft ionization is more commonly used nowadays.

(b) Because hard ionization sources produce extensive fragmentation, which provides…

• valuable structural information about unknown compound

• fragmentation pattern which can help determine how the atoms are connected

Which came first:

(a) Hard ionization sources

(b) Soft ionization sources

___________________ was the first ionization technique used in mass spectrometry. Since then, newer ionization methods have been developed, including ___________________ and ___________________, which provide alternative ways of ionizing molecules while often producing less fragmentation (i.e. preserving parent molecule better).

Electron Impact (EI) ionization was the first ionization technique used in mass spectrometry. Since then, newer ionization methods have been developed, including Electrospray Ionization (ESI) and Matrix-Assisted Laser Desorption/Ionization (MALDI), which provide alternative ways of ionizing molecules while often producing less fragmentation (i.e. preserving parent molecule better).

T or F: Both and hard ionization can give information about a parent compound’s molecular weight.

Briefly justify your answer.

True.

Diff b/w them is that…

• soft ionization → better at preserving parent ion

• hard ionization → better for generating fragments

Soft ionization sources minimize fragmentation & help preserve the molecular ion, making determination of the parent compound's molecular weight easier. Hard ionization methods (EI) produce extensive fragmentation, providing structural information about the molecule.

Describe process of Electron Impact (EI) ionization and why its beneficial.

• Hard ionization technique.

• Molecules bombarded w/ high-energy e^- → some are ionized

• Collisions b/w molecules & e^- causes e^- (usually 1) to be knocked off from the molecule → forming a radical cation

  • Radical cation = Ion with an unpaired e^-

  • Radical cation is AKA a molecular ion (M⁺)

• Extensive fragmentation often occurs, producing many fragment ions.

• Fragmentation patterns → Provide useful info in determining molecular structure of compound

Describe role of Magnetic Sector Mass Analyzer

• Type of mass analyzer

• Charged ions pass through a magnetic field

• Neutral fragments are not affected and are not detected

• Ions are separated according to m/z

• Only positive ions reach the detector

• Mass of ion is the m/z value

Most cation radicals have a charge of ___, so what differentiates them is their _____.

Most cation radicals have a charge of 1+, so what differentiates them is their mass.

Since the charge of most fragments/molecular ions are ___________, their _____________ are essentially what determines their masses.

Since the charge of most fragments/molecular ions are the same, their mass-to-charge ratios (m/z) are essentially what determines their masses.

X-Axis of Mass Spectrum

Y-axis of Mass Spectrum