Mass Spectrometry

Mass spectrometry

A technique used to identify and analyze particles based on their mass-to-charge ratio (m/z).

Purpose of mass spectrometry

To separate ions according to mass-to-charge ratio and determine their relative amounts.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons, giving them different masses.

Evidence for isotopes

Mass spectrometry can separate atoms of the same element into different masses, showing that multiple isotopes exist.

Mass-to-charge ratio (m/z)

The mass of an ion divided by its charge; this determines how strongly the ion is affected by the electromagnetic field.

Ionization

The process of removing electrons from atoms or molecules to form positive ions.

Positive ion formation

An atom becomes a positive ion when one or more electrons are removed.

Mass spectrometer process

The main stages are ionization, acceleration, deflection, and detection.

Acceleration

Ions are accelerated so they have approximately the same kinetic energy before entering the deflection region.

Why ions are accelerated

Equalizing the ions' kinetic energy allows their different mass-to-charge ratios to cause different amounts of deflection.

Deflection

Ions are bent by an electromagnetic field; the amount of bending depends on their mass-to-charge ratio.

Deflection and mass

At the same charge, lighter ions are deflected more while heavier ions are deflected less.

Deflection and charge

At the same mass, ions with greater positive charge are deflected more strongly.

Detection

Ions hit a detector, causing a signal that can be measured and converted into a mass spectrum.

Vacuum in a mass spectrometer

The chamber is kept at very low pressure so ions do not collide with air or other particles and change direction.

Mass spectrum

A graph showing the abundance or relative intensity of ions as a function of their mass-to-charge ratio.

Mass spectrum x-axis

The mass-to-charge ratio (m/z) of the detected ions.

Mass spectrum y-axis

The abundance or relative intensity of each ion.

Percent abundance

The percentage of particles in a sample that are a particular isotope or ion; all isotope percent abundances add to 100%.

Relative intensity

A scale in which the tallest peak is assigned an intensity of 100 and all other peaks are scaled relative to it.

Percent abundance vs relative intensity

Percent abundance describes the actual percentage of each isotope, while relative intensity compares peaks to the most abundant peak.

Mass spectrum peak

A peak represents ions with a particular mass-to-charge ratio.

Peak height

A taller peak means that more ions of that mass-to-charge ratio were detected relative to other ions.

Isotope identification

Isotopes appear as separate peaks at different mass-to-charge values.

Most abundant isotope

The isotope corresponding to the tallest peak on a percent-abundance mass spectrum.

Average atomic mass

The weighted average of the masses of an element's naturally occurring isotopes.

Weighted average atomic mass

The average atomic mass calculated by multiplying each isotope's mass by its fractional abundance and adding the results.

Average atomic mass formula

Average atomic mass = Σ(isotope mass × fractional abundance).

Fractional abundance

Percent abundance written as a decimal; divide percent abundance by 100.

Why atomic mass is not usually a whole number

The periodic-table atomic mass is a weighted average of the naturally occurring isotopes, not the mass of one individual atom.

Mass number

The total number of protons and neutrons in an isotope.

Mass-to-charge ratio for a +1 ion

For a +1 ion, the m/z value is approximately equal to its mass.

Diatomic molecule

A molecule made of two atoms of the same or different elements bonded together.

Diatomic isotope combinations

If an element has multiple isotopes, its diatomic molecules can contain two light atoms, two heavy atoms, or one of each.

Diatomic molecule mass

The mass of a diatomic molecule is the sum of the masses of its two atoms.

Molecular ion

The intact molecule that has been ionized without breaking apart; its m/z can correspond to the molecular mass when it has a +1 charge.

Fragment ion

A smaller charged piece produced when an ionized molecule breaks apart.

Fragmentation

The breaking of a molecule into smaller pieces during ionization.

Why fragmentation is useful

Different molecules produce characteristic fragment patterns that can help identify their structures.

Mass spectrum of a molecule

A molecular ion peak may show the mass of the whole molecule, while additional peaks can represent fragments.

Reading molecular mass spectra

Match peaks to possible molecular or fragment masses and use the pattern of peaks to infer the molecule's structure.

Successive ionization

The removal of more than one electron from the same atom, producing ions with charges such as 1+ and 2+.

First ionization energy

The energy required to remove the first electron from a gaseous atom.

Second ionization energy

The energy required to remove an electron from a gaseous 1+ ion.

First vs second ionization energy

The second ionization energy is generally greater because the electron is being removed from an already positively charged ion.

Why successive ionization requires more energy

After electrons are removed, the remaining electrons experience stronger attraction to the positively charged nucleus.

2+ ion

An ion formed when two electrons have been removed from a neutral atom.

Why few 2+ ions form

Removing a second electron requires additional energy, so fewer particles undergo a second ionization.

Effect of 2+ charge on m/z

A 2+ ion has a lower m/z than the same particle with a 1+ charge because m/z = mass/charge.

Effect of 2+ charge on deflection

For ions of the same mass, a 2+ ion is deflected more strongly than a 1+ ion because it has a larger charge-to-mass relationship.

Mass-to-charge ranking

When comparing ions, consider both mass and charge; lighter mass and greater positive charge generally produce greater deflection.

How to rank deflection

Compare m/z values: lower m/z means greater deflection, while higher m/z means less deflection.

Isotope abundance from a spectrum

The relative sizes of isotope peaks can be used to determine how common each isotope is.

Why relative intensities do not add to 100

Relative intensity is scaled so the largest peak equals 100; it is not a percentage distribution.

Why percent abundances add to 100

Percent abundance represents the complete distribution of isotopes in the sample.

Diatomic mass spectrum

Multiple peaks can occur because different isotope combinations produce molecules with different masses.

Predicting diatomic peak sizes

The most abundant isotope combination produces the largest peak, while less common combinations produce smaller peaks.

Fragment peak

An MS peak caused by a molecule breaking into a smaller charged fragment.

Molecular ion peak

The peak corresponding to the intact ionized molecule; it can help determine the molecule's molecular mass.

Fragmentation pattern

The collection of fragment peaks produced by a molecule; it can provide information about molecular structure.

Using mass spectra to identify molecules

Use the molecular ion mass and characteristic fragment masses/patterns together to determine the likely identity or structure.

Key relationship for mass spectrometry

Lower m/z → greater deflection; higher m/z → less deflection.

Key relationship for isotopes

Same element → same number of protons; different number of neutrons → different masses.

Key relationship for abundance

Taller isotope peak → greater abundance of that isotope.

Key relationship for average atomic mass

More abundant isotopes have a greater influence on the average atomic mass.