Mass Spectrometry

Mass Spectrometry

tells you:

• molecular weight

• possible molecular formula

• fragments of the molecules

How MS works

1. A beam of electrons hits the molecule

2. An electron is knocked off, now the molecule has one less electron, and becomes positively charged. (ionization)

Ionization

Making an ion

neutral molecule —> lose one electron —> positive ion

EX: CH₄ —> CH₄^+(dot)

Cation Radical

the molecule with a lost electron now has a positive charge and one unpaired electron, this is called a molecular ion, or, a cation radical

Molecular Ion (Molecule im)

heaviest & most important peak. (notice how peak 98 is the molecular ion, it is the heaviest peak). If M⁺ = 98 m/z , the molecule weighs approximately 98 g/mol (formula mass).

X-axis for MS

m/z: Mass-to-Charge Ratio

• m: mass

• z: charge

EX:

Y-axis for MS

Relative abundance (or relative intensity). The y-axis tells you how many ions of a particular m/z were detected. "How common is this fragment?" The taller the peak, the more molecules broke apart to make that fragment.

Fragments

A molecular ion is unstable, it breaks apart. Each fragment makes its own peak. If a molecule loses even a Hydrogen, it is still a fragment.

EX: CH₃-CH₂-CH₃ , 15+14+15= 44 m/z. If a signal is found at 29 m/z, that is a CH₃CH₂⁺ fragment

Base Peak

Tallest Peak is the base peak. It is assigned a relative abundance of 100%. It does NOT necessarily represent the whole molecule, the base peak is the most abundant ion detected. It is the fragment that forms most often when the molecule breaks apart.

EX: Suppose after smashing 100 houses, you find:

• 70 roofs

• 50 windows

• 20 doors

• 10 chimneys

The roof is the most common piece.

The roof is like the base peak.

Nitrogen Rule

• Even # of Nitrogens = Even # molecular mass

• Odd # of Nitrogens = Odd # of Nitrogens

EX: M⁺ = 73, there’s most likely 1 Nitrogen

M⁺ = 86, most likely 0 or 2 Nitrogens.

Isotopes

Atoms have naturally occurring isotopes. For example: Carbon:

• ¹²C = about 98.9% (most common)

• ¹³C = about 1.1%

If your molecule contains one ¹³C instead of ¹²C, it weighs 1 amu more, giving the M+1 peak.

EX: Ethanol:

• M = 46

• M+1 = 47 (because some ethanol molecules contain one ¹³C)

What about M+2? Some elements have isotopes that are 2 amu heavier.

Ratio

The ratio is simply comparing the peak heights.

• 3:1 = the first peak is 3× taller than the second.

• 1:1 = both peaks are the same height.

• 10:1 = the first peak is 10× taller than the second.

EX: If the heights are: M = 90, M+2 = 30

The ratio is:

90:30= 3:1 The M peak is 3 times taller than the M+2 peak.

Common Isotopes of Carbon

• Carbon-12

• Carbon-13

Small M + 1 peak,

M = molecule containing only ¹²C

• M+1 = molecule containing one ¹³C

• M+1 gets larger as the number of carbons increases.

Common Isotopes of Chlorine

• Chlorine-35

• Chlorine-

M and M + 2, ratio 3:1.

• 75% of molecules contain ³⁵Cl

• 25% contain ³⁷Cl

Common Isotopes of Bromine

• Bromine-79

• Bromine-81

M and M + 2, ratio 1:1.

• About half contain ⁷⁹Br

• About half contain ⁸¹Br

M + 1

One heavier amu than regular M

M + 2

Two heavier amu’s than regular M