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

4.8.6 · Chemistry › Spectroscopy & Analysis (Intro)

YEH technique exist kyun karti hai?

Tumhare paas ek unknown organic compound hai. Tumhe do cheezein chahiye: uski molecular mass aur uski structure ke baare mein clues. Mass spec dono deta hai:

Sabse bhaari important peak (intact ionised molecule) ⇒ molecular mass.
Chhote peaks ka pattern (fragments) ⇒ kaunse bonds tute, isliye kaunse groups present hain.

Mass spectrometer kaise kaam karta hai (5 steps)

Formula

$m / z$ separation ke peeche ki physics HUM KYA DERIVE KARTE HAIN: kyun ions magnetic sector instrument mein $m / z$ ke hisaab se sort hote hain.

Ionisation ke baad, charge $q = z e$ wala ion voltage $V$ se accelerate hota hai. Uski saari electrical potential energy kinetic energy ban jaati hai: $z e V = \frac{1}{2} m v^{2}$ Yeh step kyun? Energy conservation: field dwara kiya gaya kaam $= q V$ poori tarah motion mein jaata hai.
Chal raha ion ek magnetic field $B$ mein enter karta hai jo uski velocity ke perpendicular hai. Magnetic (Lorentz) force usse radius $r$ ke circle mein modta hai, centripetal force provide karte hue: $z e v B = \frac{m v ^{2}}{r}$ Yeh step kyun? $B$ mein ek charged particle force $F = z e v B$ feel karta hai, aur circular motion ke liye $F = m v^{2} / r$ chahiye.
Step 2 se: $v = \frac{z e B r}{m}$ . Step 1 mein substitute karo: $z e V = \frac{1}{2} m (\frac{z e B r}{m})^{2} = \frac{z ^{2} e ^{2} B ^{2} r ^{2}}{2 m}$
Ek $z e$ cancel karo aur rearrange karo: $\frac{m}{z} = \frac{e B ^{2} r ^{2}}{2 V}$

Ise padhna: fixed $B, r, V$ ke liye, sirf ek $m / z$ detector tak pahunchti hai. $B$ (ya $V$ ) ko scan karne se alag-alag $m / z$ detector tak jaati hain ⇒ ek poora spectrum milta hai.

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

Isotope peaks — M+1 aur M+2 ke clues

Common mistakes (Steel-man + fix)

Active recall

Recall Quick self-test (answers cover karo)

Mass spectrum ki $x$ -axis kya represent karti hai? → $m / z$ (mass-to-charge ratio).
Kaunsi peak molecular mass deti hai? → sabse zyada $m / z$ wali real peak ( $M^{+∙}$ ).
$M^{+∙}$ radical cation kyun hai? → usne ek electron khoya, jo ek unpaired electron + positive charge chhod gaya.
Cl pattern? → M : M+2 ≈ 3 : 1. Br pattern? → M : M+2 ≈ 1 : 1.
M se 15 neeche peak dene wala neutral loss? → •CH₃ (methyl radical).

Recall Feynman: ek 12-saal ke bacche ko explain karo

Socho ki tum alag-alag weights ke magnets ek bade magnet ke paas se phenkते ho. Halke wale zyada curve karte hain, bhaare wale bahut kam. Yeh dekhke ki har ek kahan land karta hai tum pata laga sakte ho ki kitna bhaari hai. Mass spec mein hum pehle ek molecule ko zap karte hain taaki usmein charge aa jaaye (ab bada magnet usse pakad sakta hai), phir hum usse phenkते hain. Molecule aksar chhote pieces mein bhi toot jaata hai — aur tute hue pieces ka shape batata hai ki molecule kisse bana tha, jaise floor par bikre Lego bricks se Lego model guess karna.

Flashcards

Molecular ion

M^{+∙}

kya hai?

Intact molecule minus ek electron — ek radical cation (positive charge + ek unpaired electron) jo sabse zyada real

m / z

par appear karta hai.

m / z

ka matlab kya hai aur yeh usually ≈ mass kyun hota hai?

Mass-to-charge ratio; kyunki zyataar ions charge

z = 1

carry karte hain,

m / z

ion ki mass ke barabar hota hai.

Magnetic sector instrument ke liye

m / z

derive karo.

z e V = \frac{1}{2} m v^{2}

aur

z e v B = m v^{2} / r

se:

m / z = e B^{2} r^{2} / (2 V)

Base peak vs molecular ion?

Base peak = sabse unchi (sabse stable/abundant ion, intensity 100% set); molecular ion = sabse zyada

m / z

jo molecular mass deta hai. Dono alag ho sakte hain.

•CH₃ lose karne se M se 15 neeche peak kyun milti hai, 16 nahi?

Ek methyl radical (•CH₃, mass 15) homolytic C–C cleavage se jata hai; koi extra hydrogen nahi nikalta.

Chlorine kaunsa isotope ratio reveal karta hai?

M : M+2 ≈ 3 : 1,

^{35}

Cl:

^{37}

Cl ≈ 3:1 se.

Bromine kaunsa isotope ratio reveal karta hai?

M : M+2 ≈ 1 : 1,

^{79}

Br:

^{81}

Br ≈ 1:1 se.

M+1 se carbons ki sankhya kaise estimate karte hain?

M+1/M intensity ≈ 1.1% × (carbons ki sankhya),

^{13}

C abundance se.

Acylium ion

R C O^{+}

ek common strong peak kyun hai?

Yeh resonance-stabilised hai (

R - C \equiv O^{+} \leftrightarrow R - C + = O

), isliye C=O ke paas cleavage favoured hoti hai.

Analysis se pehle molecules ko ionise karna kyun zaroori hai?

Sirf charged particles ko electric/magnetic fields accelerate aur deflect kar sakti hain taaki unhe sort kiya ja sake.

Connections

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