Mass spectrometry — molecular ion, fragmentation patterns, m - z

WHY does this technique exist?

You have an unknown organic compound. You want two things: its molecular mass and clues about its structure. Mass spec gives you both:

The heaviest important peak (the intact ionised molecule) ⇒ the molecular mass.
The pattern of smaller peaks (fragments) ⇒ which bonds broke, hence which groups are present.

HOW a mass spectrometer works (5 steps)

Formula The physics behind

$m/z$ separation WHAT we derive: why ions sort by $m/z$ in a magnetic sector instrument.

After ionisation, an ion of charge $q = z e$ is accelerated through a voltage $V$ . All its electrical potential energy becomes kinetic energy: $zeV = \tfrac{1}{2} m v^2$ Why this step? Energy conservation: work done by the field $= q V$ goes entirely into motion.
The moving ion enters a magnetic field $B$ perpendicular to its velocity. The magnetic (Lorentz) force bends it into a circle of radius $r$ , providing the centripetal force: $zevB = \frac{m v^2}{r}$ Why this step? A charged particle in $B$ feels force $F = zevB$ , and circular motion needs $F = mv^2/r$ .
From step 2: $v = \dfrac{zeBr}{m}$ . Substitute into step 1: $zeV = \tfrac12 m\left(\frac{zeBr}{m}\right)^2 = \frac{z^2e^2B^2r^2}{2m}$
Cancel one $ze$ and rearrange: $\boxed{\dfrac{m}{z} = \dfrac{e B^2 r^2}{2V}}$

Reading it: for fixed $B, r, V$ , only one $m/z$ reaches the detector. Scanning $B$ (or $V$ ) sweeps different $m/z$ to the detector ⇒ a full spectrum.

Figure — Mass spectrometry — molecular ion, fragmentation patterns, m - z

Reading a spectrum

Worked example Example 3 — Why fragmentation reveals structure (favoured cleavages)

Bonds break to give the most stable cation. Stability order of carbocations: $3^\circ > 2^\circ > 1^\circ > CH_3^+$ ; allylic/benzylic and acylium ( $R\text{–}C\equiv O^+$ ) are extra-stable.

A ketone like propanone (CH₃COCH₃, $M=58$ ) loses •CH₃ to give the acylium ion $CH_3CO^+$ at $m/z = 43$ — a strong, characteristic peak.
Why this step? The C–C bond next to C=O breaks because the resulting $CH_3CO^+$ is resonance-stabilised ( $CH_3\text{–}C\equiv O^+ \leftrightarrow CH_3\text{–}\overset{+}{C}=O$ ). Stable cation ⇒ favoured fragmentation ⇒ tall peak.

Isotope peaks — the M+1 and M+2 clues

Common mistakes (Steel-man + fix)

Active recall

Recall Quick self-test (cover the answers)

What does the $x$ -axis of a mass spectrum represent? → $m/z$ (mass-to-charge ratio).
Which peak gives molecular mass? → highest $m/z$ real peak ( $M^{+\bullet}$ ).
Why is $M^{+\bullet}$ a radical cation? → it lost ONE electron, leaving an unpaired electron + a positive charge.
Cl pattern? → M : M+2 ≈ 3 : 1. Br pattern? → M : M+2 ≈ 1 : 1.
Neutral lost giving a peak 15 below M? → •CH₃ (methyl radical).

Recall Feynman: explain to a 12-year-old

Imagine throwing magnets of different weights past a big magnet. Light ones curve a lot, heavy ones curve only a little. By seeing where each lands you figure out how heavy it is. In mass spec we first zap a molecule so it carries a charge (now the big magnet can grab it), then we throw it through. The molecule also often shatters into pieces — and the shape of the broken pieces tells us what the molecule was built from, like guessing a Lego model from the bricks lying on the floor.

Flashcards

What is the molecular ion

M^{+\bullet}

The intact molecule minus one electron — a radical cation (positive charge + one unpaired electron) appearing at the highest real

m/z

What does

m/z

stand for and why is it usually ≈ mass?

Mass-to-charge ratio; since most ions carry charge

z=1

m/z

equals the ion's mass.

Derive

m/z

for a magnetic sector instrument.

From

zeV=\tfrac12 mv^2

and

zevB=mv^2/r

m/z = eB^2r^2/(2V)

Base peak vs molecular ion?

Base peak = tallest (most stable/abundant ion, intensity set to 100%); molecular ion = highest

m/z

giving molecular mass. They can differ.

Why does losing •CH₃ give a peak 15 below M, not 16?

A methyl radical (•CH₃, mass 15) leaves via homolytic C–C cleavage; no extra hydrogen is removed.

What isotope ratio reveals chlorine?

M : M+2 ≈ 3 : 1, from

^{35}

Cl:

^{37}

Cl ≈ 3:1.

What isotope ratio reveals bromine?

M : M+2 ≈ 1 : 1, from

^{79}

Br:

^{81}

Br ≈ 1:1.

How do you estimate number of carbons from M+1?

M+1/M intensity ≈ 1.1% × (number of carbons), from

^{13}

C abundance.

Why is the acylium ion

RCO^+

a common strong peak?

It is resonance-stabilised (

R\text{–}C\equiv O^+ \leftrightarrow R\text{–}\overset{+}{C}=O

), so cleavage next to C=O is favoured.

Why must molecules be ionised before analysis?

Only charged particles can be accelerated and deflected by electric/magnetic fields to be sorted.

Connections

Isotopes and Relative Atomic Mass — basis of M+1/M+2 patterns.
Carbocation Stability — predicts favoured fragmentation.
Infrared Spectroscopy & NMR Spectroscopy — complementary structure tools.
Lorentz Force & Circular Motion — physics of ion deflection.
Radical Reactions — homolytic bond cleavage in fragmentation.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Mass spectrometry ka core idea simple hai: hum molecule ko tolte hain. Pehle ek electron nikaal ke molecule ko charged bana dete hain (isko ionisation kehte hain), kyunki sirf charged particle hi electric aur magnetic field se control ho sakta hai. Ye jo charged whole molecule banta hai use molecular ion ( $M^{+\bullet}$ ) kehte hain — ye ek radical cation hai, matlab uske paas charge bhi hai aur ek akela electron bhi. Spectrum me ye sabse zyada $m/z$ wala asli peak hota hai, aur yahi humein molecule ka mass batata hai.

$m/z$ ka matlab hai mass-to-charge ratio. Kyunki zyadatar ions ka charge $z=1$ hota hai, $m/z$ practically mass hi ho jaata hai. Magnet ke andar halke ion zyada mudte (bend) hain aur heavy ion kam, isliye magnetic field scan karke hum har $m/z$ ko detector tak laate hain — iss se poora spectrum milta hai. Formula $m/z = eB^2r^2/2V$ bas energy conservation aur circular motion se nikal jaata hai, ratta maarne ki zaroorat nahi.

Asli maza hai fragmentation me. Molecule tukdo me tootta hai, aur hamesha wahi bond tootta hai jisse sabse stable cation bane (jaise acylium $RCO^+$ , ya $3^\circ$ carbocation). Tujhe ye yaad rakhna: •CH₃ nikalne par mass 15 ghatti hai (16 nahi!), kyunki methane nahi, methyl radical nikalta hai. Aur ek pro tip — Cl ka pattern 3:1 (M aur M+2) hota hai, Br ka 1:1. Isotope se Cl/Br pakadna exam me free marks hai.

Ek common galti: sabse tall peak (base peak) ko molecular mass samajh lena. Galat! Base peak sabse stable/abundant ion hai, mass to sabse right wale peak se milta hai. Dono ko alag rakho aur tu spectrum aaram se padh lega.

Test yourself — Spectroscopy & Analysis (Intro)

Connections