Mass Spectrometry - Principles and Operation — AQA A-level Chemistry

SPEC 3.1.1.2 - ATOMIC STRUCTURE

Mass Number, Isotopes and Mass Spectrometry

Understanding isotopes and how the time of flight mass spectrometer identifies them

Mass Number and Atomic Number

Atomic Number Z

The number of protons in the nucleus. Also called the proton number.

The atomic number defines the chemical identity of an element.

Mass Number A

The total number of protons + neutrons in the nucleus (the total number of nucleons).

Nucleons are responsible for almost all of the mass because electrons weigh virtually nothing.

mass number A = number of protons + number of neutrons

Isotopes

Every single atom of any particular element has the same number of protons in its nucleus and therefore the same number of electrons. But the number of neutrons may vary.

Definition

Atoms with the same number of protons but different numbers of neutrons are called isotopes.

Different isotopes of the same element react chemically in exactly the same way as they have the same electron configuration.

Atoms of different isotopes of the same element vary in mass number because of the different number of neutrons in their nuclei.

Example

All atoms of carbon have atomic number 6. That is what makes them carbon rather than any other element. However, carbon has three isotopes with mass numbers 12, 13, and 14. All three isotopes react in the same way, for example, burning in oxygen to form carbon dioxide.

Isotopes of Carbon

Name of isotopecarbon-12carbon-13carbon-14

Symbol¹²₆C¹³₆C¹⁴₆C

Number of protons666

Number of neutrons678

Abundance98.89%1.11%trace

Key Point

All three isotopes have the same chemical properties because they all have 6 electrons in the same configuration. But they have different physical properties (e.g., mass, density) because they have different masses.

Isotopes of Hydrogen

Isotopes are usually identified by the name of the element and the mass number, as in carbon-13. However, isotopes of hydrogen have their own names:

NameSymbolProtonsNeutrons

Hydrogen (protium)¹H10

Deuterium²H11

Tritium³H12

Why Special Names?

Hydrogen isotopes have special names because the mass differences are proportionally huge — tritium is 3× heavier than protium! However, all these isotopes behave chemically just like the most common isotope, hydrogen-1.

The Mass Spectrometer

The mass spectrometer is an analytical technique used to identify different isotopes and determine relative atomic masses.

Principle

There are several types of mass spectrometer but all work on the principle of forming ions from the sample and then separating the ions according to the ratio of their charge to their mass (m/z).

Time of Flight (TOF)

The type described here is called a time of flight (TOF) instrument. It records the time it takes for ions of each isotope to reach a detector. Lighter ions travel faster and arrive first.

TOF Mass Spectrometer: Overview

Vacuum

The whole apparatus is kept under a high vacuum to prevent the ions that are produced colliding with molecules from the air.

5 Stages

1. Ionisation → 2. Acceleration → 3. Ion Drift → 4. Detection → 5. Data Analysis

Stage 1: Ionisation

Two types of ionisation are used:

Electron Impact

The sample is vaporised and high energy electrons are fired at it from an electron gun (a hot wire filament with a current running through it).

This knocks off one electron from each particle forming a 1+ ion:X(g) + e⁻ → X⁺(g) + 2e⁻

Electrospray

The sample is dissolved in a volatile solvent and forced through a fine hollow needle connected to a high voltage supply.

This produces tiny positively charged droplets that have gained a proton from the solvent. The solvent evaporates leaving single ions.

Stage 2: Acceleration

2

ACCELERATION

The positive ions are attracted towards a negatively charged plate and accelerate towards it. Lighter ions and more highly charged ions achieve a higher speed.

Key Principle

All ions with the same charge gain the same kinetic energy (KE = ½mv²). Since KE is constant, lighter ions travel faster than heavier ions.

Stage 3: Ion Drift

3

ION DRIFT

The ions pass through a hole in the negatively charged plate, forming a beam and travel along a tube, called the flight tube, to a detector.

Time of Flight

The time taken to travel through the flight tube depends on the mass. t ∝ √m

Stage 4: Detection

4

DETECTION

When ions with the same charge arrive at the detector, the lighter ones are first as they have higher velocities. The flight times are recorded.

How Detection Works

The positive ions pick up an electron from the detector, which causes a current to flow. The greater the abundance of an isotope, the greater the current produced.

What Is Measured

Time of arrival → determines the m/z value (mass)

Size of current → determines the relative abundance

Stage 5: Data Analysis

5

DATA ANALYSIS

The signal from the detector is passed to a computer which generates a mass spectrum.

Reading the Spectrum

Peak position (m/z) = mass of isotope | Peak height = relative abundance

Interpreting Mass Spectra

The mass spectrometer can be used to identify the different isotopes that make up an element. It detects individual ions, so different isotopes are detected separately because they have different masses.

Important

There is no peak at 20.2 (the relative atomic mass) because no neon atoms actually have this mass. Individual atoms only have whole number masses — the Aᵣ is just an average.

2+ Ions in Mass Spectrometry

During the ionisation process, a 2+ charged ion may be produced. This means it will be affected more by the electric field.

Important

A 2+ ion has its mass to charge ratio (m/z) halved.

This can be seen on spectra as a trace at half the expected m/z value.

Example

A ²⁰Ne²⁺ ion would appear at m/z = 20/2 = 10, not 20.

Mass Spectrum of Chlorine

Chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%), occurring in a ratio of almost exactly 3:1.

Atomic Ions

³⁵Cl⁺ at m/z 35 | ³⁷Cl⁺ at m/z 37

Molecular Ions

³⁵Cl-³⁵Cl = 70 | ³⁵Cl-³⁷Cl = 72 | ³⁷Cl-³⁷Cl = 74

Calculating Relative Atomic Mass (Aᵣ)

Mass spectra give us the information needed to calculate the relative atomic mass of an element from its isotopic abundances.

Formula

Aᵣ = Σ (m/z × abundance)Total abundance

Method

Step 1: Multiply each m/z value by its abundance

Step 2: Add all the products together

Step 3: Divide by the total abundance

Note

If abundances are given as percentages, the total abundance = 100

01Worked Example

An element has two isotopes with the following mass spectrum data. Calculate the relative atomic mass.

m/zAbundance

1075

1225

1

Multiply each m/z by abundance:

(10 × 75) = 750

(12 × 25) = 300

2

Add the products:

750 + 300 = 1050

3

Divide by total abundance:

Aᵣ = 1050 ÷ 100 = 10.5

Worked Example02

Calculate the relative atomic mass of chlorine given that ³⁵Cl has 75% abundance and ³⁷Cl has 25% abundance.

1

Multiply each m/z by abundance:

(35 × 75) = 2625

(37 × 25) = 925

2

Add the products:

2625 + 925 = 3550

3

Divide by total abundance:

Aᵣ = 3550 ÷ 100 = 35.5

03Worked Example

Magnesium has three isotopes. Calculate its relative atomic mass from the mass spectrum data below.

m/zAbundance (%)

2479

2510

2611

1

Multiply each m/z by abundance:

(24 × 79) + (25 × 10) + (26 × 11)

= 1896 + 250 + 286 = 2432

2

Divide by total abundance:

Aᵣ = 2432 ÷ 100 = 24.32

Practice Question 1

Question: Copper has two isotopes: ⁶³Cu (69.2%) and ⁶⁵Cu (30.8%). Calculate the relative atomic mass of copper to 1 decimal place.

Aᵣ = [(63 × 69.2) + (65 × 30.8)] ÷ 100

Aᵣ = (4359.6 + 2002) ÷ 100

Aᵣ = 6361.6 ÷ 100 = 63.6

Practice Question 2

Question: A mass spectrum shows peaks at m/z = 20 (abundance 90.5) and m/z = 22 (abundance 9.5). Identify the element and calculate its Aᵣ.

Aᵣ = [(20 × 90.5) + (22 × 9.5)] ÷ 100

Aᵣ = (1810 + 209) ÷ 100

Aᵣ = 2019 ÷ 100 = 20.19

The element is Neon (Ne)

Practice Question 3

Question: Boron has two isotopes: ¹⁰B and ¹¹B. The relative atomic mass of boron is 10.8. Calculate the percentage abundance of each isotope.

Let ¹⁰B = x%, then ¹¹B = (100 − x)%

10.8 = [10x + 11(100 − x)] ÷ 100

1080 = 10x + 1100 − 11x

1080 = 1100 − x

x = 20

¹⁰B = 20%, ¹¹B = 80%

Summary

Key Definitions

TermDefinition

Atomic number (Z)Number of protons — defines the element

Mass number (A)Number of protons + neutrons

IsotopesAtoms with same protons, different neutrons

TOF Mass Spectrometer (5 Stages)

1

Ionisation — Electron impact or electrospray → X⁺ ions

2

Acceleration — Electric field gives all ions same KE

3

Ion drift — Lighter ions travel faster through flight tube

4

Detection — Ions pick up electron → current flows

5

Data analysis — Computer generates mass spectrum

Key Exam Skills

Calculate Aᵣ from isotopic abundances using: Aᵣ = Σ(m/z × abundance) ÷ total

Interpret mass spectra: peak position = mass, peak height = abundance

Remember: 2+ ions appear at half the expected m/z value