Q.45 Among the five fragments given below,
the number of fragment(s) accelerated to the analyzer tube in mass spectrometer
with electron ionization is/are _______.
In electron ionization mass spectrometry, only positively charged fragments (molecular ion or fragment cations/radical cations) are accelerated into the analyzer tube; neutral radicals are not.
Interpreting the given fragments
The question lists five fragments (· denotes an unpaired electron; ⁺ denotes positive charge):
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·CH₂–CH₃
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CH₃–CH₃
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CH₃–CH₂–CH₂·
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CH₂=CH–CH₂⁺
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[CH₃–CH₂–CH₃]·⁺
In EI, fragmentation produces a mixture of neutral molecules, neutral radicals, cations, and radical cations. Only the cationic species are accelerated by the electric field into the mass analyzer. Neutral and purely radical species are not affected and are pumped away in the vacuum system.
Which fragments are accelerated?
Fragment 1: ·CH₂–CH₃ (ethyl radical)
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Contains only a radical (odd electron) and no positive charge.
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Neutrals and radicals are not accelerated or detected in a standard positive‑ion mass spectrum.
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Therefore, this fragment is not accelerated.
Fragment 2: CH₃–CH₃ (ethane molecule)
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Completely neutral, no unpaired electron indicated.
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Neutral molecules are unaffected by the accelerating plates and do not enter the mass analyzer.
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Hence, this fragment is not accelerated.
Fragment 3: CH₃–CH₂–CH₂· (propyl radical)
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Neutral radical, again uncharged.
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Like fragment 1, it is lost in the vacuum system and not analyzed.
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So, this fragment is not accelerated.
Fragment 4: CH₂=CH–CH₂⁺ (allyl cation)
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Carries a positive charge (⁺) and no explicit radical mark, so it is an even‑electron cation.
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In EI, such fragment cations are exactly the species that are focused and accelerated into the analyzer tube.
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Therefore, this fragment is accelerated.
Fragment 5: [CH₃–CH₂–CH₃]·⁺ (propyl radical cation)
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This is a radical cation: it has both a positive charge and an unpaired electron.
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The molecular ion in EI is typically a radical cation (M·⁺), and any fragment radical cations are likewise accelerated and detected.
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Hence, this fragment is accelerated.
Thus, among the five fragments, only 4 and 5 are accelerated into the analyzer tube.
Answer: Number of fragments accelerated = 2.
Concept recap: electron ionization and analyzer tube
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In electron ionization (EI), a high‑energy electron beam knocks an electron out of a neutral molecule to give a molecular ion, usually a radical cation M·⁺.
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This molecular ion can further fragment to give:
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Neutral radicals or neutral molecules (no charge)
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Even‑electron cations
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Radical cations
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In the ion source, an electric field accelerates only positively charged ions (M⁺, M·⁺ and fragment ions) into the mass analyzer; neutral fragments (including radicals) are not influenced and are removed by pumping.
So, only charged species, irrespective of being radical cations or simple cations, travel down the analyzer tube, which is why fragments 4 and 5—and not 1, 2, or 3—are counted.
Introduction
Understanding which fragments are accelerated to the analyzer tube in mass spectrometry with electron ionization is a frequent exam question in analytical chemistry and CSIR‑NET‑type tests. In EI, the key rule is that only positively charged fragments—whether simple cations or radical cations—are driven into the mass analyzer, while neutral radicals and molecules are not detected. This article explains that principle in depth using the given five fragments, and shows clearly why only two of them reach the analyzer tube.
