Mass Spectrum (Mass Spectrometry) — IAL Edexcel AS Chemistry

Mass spectrometry organic chemistry-র সবচেয়ে important analytical tool। এই chapter ভালোভাবে বুঝলে Mr, molecular ion, fragment ion—সব easy হয়ে যায়। চল casual Bangla তে clean করে দেখি।

---

What is Mass Spectrometry?

Mass spectrometry এমন একটা technique যেটা দিয়ে আমরা জানতে পারি:

• Molecular mass (M_r)
• Isotopes
• Structure related fragment ions

Output হিসেবে যেটা পাই সেটাই mass spectrum।

---

Basic Principle

Ionised particles electric বা magnetic field দিয়ে deflect হয়। Deflection depend করে:

• Mass (m)
• Charge (z)

Key ratio:

$$\frac{m}{z}$$

Most ions have charge +1, তাই AS level-এ: m/z ≈ mass

---

Stages of a Mass Spectrometer

1. Vaporisation
2. Ionisation (electron impact)
3. Acceleration
4. Deflection
5. Detection

Exam-এ steps explain করতে বললে এই order follow করো।

---

Molecular Ion (M⁺)

Molecular ion হলো:

$$M+e-\to M^{+}+2e-$$

Mass spectrum-এ সবচেয়ে বড় m/z peak (except isotopes) normally M⁺ peak।

Bangla tip: M⁺ peak দেখেই Mr পাওয়া যায়।

---

Base Peak

Base peak মানে:

• Highest intensity peak
• Intensity = 100%
• সবচেয়ে stable fragment

Base peak মানেই highest mass—এইটা ভুল।

---

Fragment Ions

Molecular ion ভেঙে fragment ion তৈরি হয়। Example:

$${\mathrm{CH}}^3+{\mathrm{CH}}^2\to {\mathrm{CH}}^3+{\mathrm{CH}}^2$$

Different structure → different fragment pattern।

---

Using Mass Spectrum to Find Mr

Highest m/z value peak (ignoring isotope peaks) = Relative molecular mass

Example:

If highest m/z peak = 46 → M_r = 46

---

Isotopes in Mass Spectra

Isotopes different mass দেয়। Example: Chlorine

• ³⁵Cl
• ³⁷Cl

So Cl-containing compound এ:

• M peak
• M+2 peak

Intensity ratio ≈ 3:1

---

Worked Example (Mr Calculation)

A compound shows a molecular ion peak at m/z = 58.

Therefore:

$$Mr=58$$

Simple but very common exam question।

---

MCQ Practice (AS Level)

Q1. What does the molecular ion peak represent?

A) Most intense peak
B) Highest m/z fragment
C) Fragment with highest stability
D) Ion with charge −1

Answer: B

---

Q2. The base peak in a mass spectrum:

A) Has the greatest mass
B) Represents the molecular ion
C) Has intensity of 100%
D) Is always M+2

Answer: C

---

Exam-Style Structured Question

Q. A compound has the following mass spectrum data:

Molecular ion peak at m/z = 44.

(a) State the relative molecular mass of the compound.
(b) Explain what the molecular ion peak represents.
(c) Define the term base peak.

Worked Answer

(a)

$$Mr=44$$

(b) Molecular ion peak represents the ion formed when the molecule loses one electron but does not fragment.

(c) Base peak is the peak with the highest intensity (100%) and represents the most abundant ion.

---

Common Exam Mistakes

• Base peak = highest mass ভাবা
• Isotope peaks ignore করা
• m/z আর Mr confuse করা

---

Final Revision Tip

Mass spectrum question দেখলেই: M⁺ peak → Mr → base peak → fragments। এই order follow করলে marks আসবেই।