Electron Ionization Mass Spectrometry: How Many Fragments Are Accelerated Toward the Analyzer Tube?

Correct Answer

Correct Answer: 2 fragments

The decisive principle in this question is that a mass spectrometer does not accelerate and analyze every chemical species produced during electron ionization. The analyzer responds to charged particles. Neutral molecules and neutral radicals are not accelerated toward the analyzer because they do not experience the required electrostatic force.

Among the five species shown, only the third fragment carries a positive charge together with an unpaired electron, while the fifth fragment carries a positive charge. Therefore, only these two positively charged species are accelerated toward the analyzer tube.

Hence, the total number of fragments accelerated toward the analyzer tube is:

2

Understanding the Basic Principle of Electron Ionization Mass Spectrometry

Why Charge Is the Most Important Feature

In electron ionization mass spectrometry, also called EI mass spectrometry, molecules in the gas phase are bombarded with high-energy electrons. A commonly used electron energy is approximately 70 eV. This collision can remove an electron from a neutral molecule and produce a positively charged species.

The general process can be represented as:

M + e⁻ → M⁺• + 2e⁻

Here, M represents the neutral molecule, while M⁺• is the molecular radical cation. The symbol + represents a positive charge, and the dot • represents an unpaired electron.

After ionization and fragmentation, the mass spectrometer uses electric fields to accelerate charged particles. The analyzer then separates the ions according to their mass-to-charge ratio (m/z).

A neutral species has no net electrical charge. Therefore, an electric field cannot accelerate it in the same way as an ion. This means that the first task in this question is not to determine whether a species is a radical, stable molecule or resonance structure. The first task is simply to identify whether it carries a charge.

Detailed Analysis of All Five Fragments

Fragment 1: Neutral Radical

The first species contains an unpaired electron, represented by a dot. However, no positive or negative charge is shown.

An unpaired electron makes a species a radical, but a radical is not necessarily an ion. A neutral radical still has an overall electrical charge of zero.

Because this fragment has no net charge, it is not accelerated toward the analyzer tube by the electric field.

Therefore, Fragment 1 is not counted.

Fragment 2: Neutral Radical

The second species also contains an unpaired electron, but it does not carry a positive charge.

This distinction is fundamental in mass spectrometry. The presence of a dot indicates a radical, whereas the presence of a + sign indicates a positively charged ion. A dot alone does not make a fragment detectable by the mass analyzer.

Since the second species is electrically neutral, it will not be accelerated toward the analyzer tube.

Therefore, Fragment 2 is not counted.

Fragment 3: Positively Charged Radical Ion

The third species contains both an unpaired electron and a positive charge. Such a species is called a radical cation or odd-electron positive ion.

The positive charge is the important feature for this question. Because the fragment is positively charged, it responds to the electric field inside the mass spectrometer and can be accelerated toward the analyzer.

The unpaired electron does not prevent its detection. In fact, radical cations are commonly produced during electron ionization.

Therefore, Fragment 3 is counted.

Fragment 4: Neutral Radical

The fourth species contains a carbon-carbon double bond and an unpaired electron. However, no positive charge is indicated.

The presence of a double bond changes the structure and possible stability of the fragment, but it does not change the basic mass spectrometric requirement. The fragment must possess a net charge to be accelerated by an electric field.

Since this species is a neutral radical, it is not accelerated toward the analyzer tube.

Therefore, Fragment 4 is not counted.

Fragment 5: Positively Charged Ion

The fifth species is shown inside brackets with a positive charge. This clearly identifies it as a cation.

Because it possesses a net positive charge, the fragment responds to the electric field and is accelerated toward the analyzer tube. Whether the positive charge is localized on one carbon atom or delocalized over the fragment does not change the answer to this question.

Therefore, Fragment 5 is counted.

Which Fragments Reach the Analyzer?

The five species can be classified according to their electrical nature:

Thus, the species accelerated toward the analyzer tube are:

Fragment 3 and Fragment 5

Therefore, the number of fragments is:

2

Why Neutral Radicals Are Not Accelerated

A Radical Is Not Always an Ion

A major conceptual point in this question is the difference between a radical and an ion. A radical contains at least one unpaired electron, while an ion possesses a net electrical charge.

These two properties are independent. A species may be a neutral radical, a radical cation or even a radical anion. Therefore, simply seeing a dot in a chemical structure is not sufficient to conclude that the species will be analyzed in a mass spectrometer.

The electric force acting on a particle can be represented as:

F = qE

where F is the electrostatic force, q is the charge on the particle and E is the electric field.

For a neutral species:

q = 0

Therefore:

F = 0

This explains why neutral fragments are not accelerated by the electric field toward the mass analyzer.

Difference Between a Neutral Radical and a Radical Cation

Neutral Radical

A neutral radical contains an unpaired electron but has no overall charge. It may be chemically reactive, but it does not respond to the accelerating electric field as an ion.

The first, second and fourth fragments in the question belong to this category.

Radical Cation

A radical cation contains both an unpaired electron and a positive charge. Such species are particularly important in electron ionization mass spectrometry because electron removal from a neutral molecule commonly produces an odd-electron molecular ion.

The third fragment belongs to this category and is therefore accelerated toward the analyzer.

What Happens Inside an Electron Ionization Mass Spectrometer?

Ionization of the Sample

The sample is first converted into the gas phase and introduced into the ion source. A beam of energetic electrons collides with the gaseous molecules and can remove electrons from them.

This produces positive molecular ions, often in the form of radical cations.

Fragmentation of the Molecular Ion

The molecular ion may contain excess internal energy and undergo bond cleavage. This process can generate smaller fragments.

A typical fragmentation may produce one charged fragment and one neutral fragment. Only the charged fragment continues through the mass spectrometric analysis.

Acceleration of Positive Ions

The positive ions are accelerated by an electric potential. Because they possess electrical charge, they gain kinetic energy and move toward the analyzer.

Neutral molecules and neutral radicals do not undergo the same electrostatic acceleration.

Separation According to Mass-to-Charge Ratio

The analyzer separates ions according to their mass-to-charge ratio, written as m/z.

The detector records these ions and produces the mass spectrum. Since neutral species do not possess a meaningful ionic m/z value for this detection process, they do not generate corresponding peaks in the conventional mass spectrum.

Final Answer

In an electron ionization mass spectrometer, only charged fragments are accelerated toward the analyzer tube. Among the five species shown in the question, the third fragment is a positively charged radical cation, and the fifth fragment is a positively charged cation.

The remaining three species are neutral radicals and are not accelerated by the electric field.

Therefore, the total number of fragments accelerated toward the analyzer tube is:

Correct Answer: 2