Mass Spectrometry (MS)

Mass spectrometry identifies substances by measuring the mass-to-charge ratio (m/z) of ionised particles. It can determine the molecular mass of a compound and its fragmentation pattern.

How a Mass Spectrometer Works

1. Ionisation

Sample molecules are ionised (electrons removed)

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2. Acceleration

Ions are accelerated by electric field

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3. Separation

Ions separated by m/z ratio

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4. Detection

Ions detected; spectrum produced

The molecular ion peak (M⁺): The peak with the highest m/z value (excluding isotope peaks) represents the intact ionised molecule and gives the molecular mass of the compound.

Infrared (IR) Spectroscopy

IR spectroscopy identifies functional groups in a molecule by measuring which infrared frequencies are absorbed. Different bonds absorb at characteristic frequencies called wavenumbers (cm^-1).

Key IR Absorptions

O-H (alcohol)

3200-3550 cm^-1 -- broad absorption

O-H (carboxylic acid)

2500-3300 cm^-1 -- very broad

C=O (carbonyl)

1680-1750 cm^-1 -- strong, sharp

N-H (amine)

3300-3500 cm^-1 -- medium absorption

Fingerprint region: Below 1500 cm^-1 is the fingerprint region -- a complex pattern unique to each molecule. It is used to confirm identity by matching to a reference spectrum.

Chromatography

Chromatography separates mixtures based on differences in how components distribute between a stationary phase and a mobile phase. Components that interact more strongly with the stationary phase move more slowly.

TLC

Thin-Layer Chromatography. Quick identification. Stationary phase: silica gel on a plate. Calculate R_f values.

HPLC

High-Performance Liquid Chromatography. Precise quantitative analysis. Uses high pressure to push liquid through a column.

GC

Gas Chromatography. For volatile compounds. Mobile phase is an inert gas. Components identified by retention time.

R_f Values (TLC)

R_f = distance moved by substance / distance moved by solvent front

• R_f values range from 0 to 1 and are characteristic of a substance under specific conditions.

• Useful for identifying unknown compounds by comparison with known standards.

Key Vocabulary

Mass Spectrometry

An analytical technique that ionises sample molecules and separates them by mass-to-charge ratio to determine molecular mass.

IR Spectroscopy

A technique that identifies functional groups by measuring which infrared frequencies are absorbed by a sample.

Retention Time

The time taken for a component to pass through a chromatography column. Used to identify substances in GC and HPLC.

Molecular Ion Peak (M⁺)

The peak in a mass spectrum corresponding to the intact ionised molecule, giving the molecular mass of the compound.

Worked Examples

1

A mass spectrum shows a molecular ion peak at m/z = 46. Suggest the compound.

Step 1: The molecular ion peak at 46 gives the molecular mass as 46 g/mol.

Step 2: Consider common compounds: ethanol (CH₃CH₂OH) has M_r = 12+3+12+2+16+1 = 46.

Step 3: The compound is likely ethanol (C₂H₅OH). This could be confirmed with IR spectroscopy showing an O-H absorption.

2

An IR spectrum shows a broad absorption at 3300 cm^-1 and a strong peak at 1710 cm^-1. What functional groups are present?

Step 1: Broad absorption at 3300 cm^-1 indicates an O-H group.

Step 2: Strong peak at 1710 cm^-1 indicates a C=O (carbonyl) group.

Step 3: The combination of O-H (broad) and C=O suggests the compound is a carboxylic acid (-COOH).

3

In a TLC experiment, a substance travels 4.2 cm while the solvent front moves 6.0 cm. Calculate the R_f value.

Step 1: R_f = distance moved by substance / distance moved by solvent front

Step 2: R_f = 4.2 / 6.0

Step 3: R_f = 0.70

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

In mass spectrometry, the molecular ion peak tells you the:

Molecular mass of the compound Number of atoms in the molecule Boiling point of the compound Colour of the compound

Question 2

A strong, sharp IR absorption at 1720 cm^-1 most likely indicates:

An O-H bond A C=O bond A C-H bond An N-H bond

Question 3

In chromatography, components are separated based on their:

Colour Electrical charge only Different interactions with stationary and mobile phases Temperature sensitivity

Question 4

Gas chromatography is best suited for analysing substances that are:

Solid at room temperature Large polymers Ionic compounds Volatile (easily vaporised)

Question 5

A TLC spot moves 3.5 cm and the solvent front moves 7.0 cm. The R_f value is:

0.50 0.25 2.0 3.5

Analytical Chemistry Techniques

Mass Spectrometry (MS)

How a Mass Spectrometer Works

Infrared (IR) Spectroscopy

Key IR Absorptions

O-H (alcohol)

O-H (carboxylic acid)

C=O (carbonyl)

N-H (amine)

Chromatography

TLC

HPLC

GC

R_f Values (TLC)

Key Vocabulary

Mass Spectrometry

IR Spectroscopy

Retention Time

Molecular Ion Peak (M⁺)

Worked Examples

A mass spectrum shows a molecular ion peak at m/z = 46. Suggest the compound.

An IR spectrum shows a broad absorption at 3300 cm^-1 and a strong peak at 1710 cm^-1. What functional groups are present?

In a TLC experiment, a substance travels 4.2 cm while the solvent front moves 6.0 cm. Calculate the R_f value.

Knowledge Check

Key Concepts Summary

Analytical Chemistry Techniques

Mass Spectrometry (MS)

How a Mass Spectrometer Works

Infrared (IR) Spectroscopy

Key IR Absorptions

O-H (alcohol)

O-H (carboxylic acid)

C=O (carbonyl)

N-H (amine)

Chromatography

TLC

HPLC

GC

Rf Values (TLC)

Key Vocabulary

Mass Spectrometry

IR Spectroscopy

Retention Time

Molecular Ion Peak (M+)

Worked Examples

A mass spectrum shows a molecular ion peak at m/z = 46. Suggest the compound.

An IR spectrum shows a broad absorption at 3300 cm-1 and a strong peak at 1710 cm-1. What functional groups are present?

In a TLC experiment, a substance travels 4.2 cm while the solvent front moves 6.0 cm. Calculate the Rf value.

Knowledge Check

Key Concepts Summary

R_f Values (TLC)

Molecular Ion Peak (M⁺)

An IR spectrum shows a broad absorption at 3300 cm^-1 and a strong peak at 1710 cm^-1. What functional groups are present?

In a TLC experiment, a substance travels 4.2 cm while the solvent front moves 6.0 cm. Calculate the R_f value.