L4 - Mass Spectrometry

objectives

Understandings

• Mass spectrometry (MS), proton nuclear magnetic resonance spectroscopy (¹H NMR) and infrared spectroscopy (IR) are techniques that can be used to help identify compounds and to determine their structure.

Applications and Skills

• Deduction of information about the structural features of a compound from percentage composition data, MS, ¹H NMR or IR.

Guidance

• The operating principles are not required for any of these methods.

Mass Spectrometry - Development

Particles with a large m/z value are deflected least

those with a low m/z value are deflected most.

Uses

• Mass spectrometry was initially used to show the identity of isotopes.
• It is now used to calculate molecular masses and characterise new compounds

How it Works helps us to explain the Mass Spectrums we receive

• A mass spectrometer consists of ... an ion source, an analyser and a detector

IONISATION

• gaseous atoms are bombarded by electrons from an electron gun and are IONISED

• sufficient energy is given to form ions of 1+ charge

ACCELERATION

• ions are charged so can be ACCELERATED by an electric field

DEFLECTION

• charged particles will be DEFLECTED by a magnetic or electric field

DETECTION

• by electric or photographic methods

HOW DOES IT WORK? - Deflection

• the radius of the path depends on the value of the mass/charge ratio (m/z)
• ions of heavier isotopes have larger m/z values so follow a larger radius curve as most ions are 1+charged, the amount of separation depends on their mass
• if an ion acquires a 2+ charge it will be deflected more; its m/z value is halved

WHAT IS A MASS SPECTRUM?

• In early research with a mass spectrograph, Aston (Nobel Prize, 1922) demonstrated that naturally occurring neon consisted of three isotopes ... ²⁰Ne, ²¹Ne and ²²Ne.

• positions of the peaks gives atomic mass

• peak intensity gives the relative abundance

• highest abundance is scaled to 100% and other values are adjusted accordingly

Example Calculation 1

Example Calculation 2

Fragmentation

• Spectra obtained for organic molecules have many peaks. Each peak is due to a particular fragment with a certain m/z value.
• highest m/z value usually corresponds to the molecular ion
• its position provides information about the molecular mass of a substance
• the tallest peaks come from the most stable species

Interpretation of thousands of spectra has shown that many classes of organic compound show characteristic fragmentation patterns due to their functional groups.

It is possible to identify the type of compound from its spectrum by looking at the ...

• position of peaks
• differences between major peaks

The Molecular Ion

• In the spectrum of octane, a signal occurs at 114 due to the species C₈H₁₈⁺
• The species due to the final signal is known as the molecular ion and is usually corresponds to the molecular mass of the compound.

Fragmentation

• The rest of the spectrum provides additional information of the molecule’s structure.
• Peaks appear due to characteristic fragments (e.g. 29 due to C₂H₅⁺) and differences between two peaks also indicates the loss of certain units (18 for H₂O, 28 for CO).

Fragmentation - Alkanes

• The mass spectra of simple hydrocarbons have peaks at m/z values corresponding to the ions produced by breaking C-C bonds. Peaks can occur at ...

• the stability of the carbocation formed affects its abundance
• the more stable the cation the higher the peak
• the more alkyl groups attached to the carbocation the more stable it is

most stable tertiary 3° > secondary 2° > primary 1° least stable

• alkyl groups are electron releasing and stabilise the cation

Fragmentation - Halogenoalkanes

• Multiple peaks occur in the molecular ion region due to different halogen isotopes.
• There are two peaks for the molecular ion of C₂H₅Br, one for the molecule containing the isotope ⁷⁹Br and the other for the one with the ⁸¹Br isotope.
• Because the two isotopes are of similar abundance, the peaks are of similar height.

Fragmentation - Aldehydes and Ketones

• Cleavage of bonds next to the carbonyl group (C=O) is a characteristic fragmentation of aldehydes and ketones.
• A common fragment is carbon monoxide (CO) but as it is a molecule and thus uncharged it will not produce a peak of its own.
• However, it will produce an m/z drop of 28 somewhere in the spectrum.
• The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group

• the more stable the acylium ion RCO⁺, the more abundant it will be and the more abundant the species the taller its peak in the mass spectrum

• The position of the carbonyl group influences the fragmentation pattern because the molecular ion fragments either side of the carbonyl group.