Mass Spectrometry – An Introduction

Mass spectrometry is a laboratory method used to determine the composition of a substance and the molecular weights of its constituents. This is particularly useful when you are working with peptides, as they can have similar structures and molecular weights that are not easily distinguished visually or by simple testing.

Mass spectrometry can not only tell us if a peptide sample is present but also the molecular mass of a peptide sample and, with the right techniques, its sequence and potential impurities: this makes it an important ingredient in analytical testing and research.

What Is Mass Spectrometry?

This technique is called mass spectrometry, or MS for short. What it does is turn molecules into charged particles, called ions. They are then separated according to their mass-to-charge ratio (m/z).

The instrument isn’t weighing a molecule in the same way as a normal set of scales. Instead it measures the behaviour of charged molecules within the instrument. The obtained data can then be used to determine the molecular mass of compounds in the sample.

Researchers can use this information to confirm if the measured mass is consistent with the expected mass of a given peptide in peptide studies.

Usually a mass spectrum will show a series of peaks. The peaks correspond to ions detected at a certain m/z value. The height or intensity of a peak is the correspondence to how strong the ion was detected.

How Does Mass Spectrometry Work?

Modern mass spectrometers can be very sophisticated, but the basic process is easier to understand than it first looks.

A sample is introduced into the instrument first. After that we need to convert molecules into ions so that we can control and measure them. Depending on the sample type and the information you need, you have different ionisation methods at your disposal.

They then pass through the mass analyser. The analyser is used to separate them according to their mass-to-charge ratios. Then it measures all the ions that strike the detector.

The instrument converts those measurements into a mass spectrum. Scientists can examine the pattern of peaks and compare the measured values to expected values or reference data.

The exact process differs between instruments. Electrospray ionisation (ESI) methods are commonly used for peptides because they can transfer relatively large molecules into the gas phase without the harsh conditions associated with some other ionisation methods.

Why Use Mass Spectrometry for Peptides?

A peptide is a linkage of amino acids. A small change in the structure can affect the molecular mass of peptides. This makes mass spectrometry a valuable tool in the confirmation of a peptide identity.

For example, if a molecular mass is predicted, the measured value for a peptide can be compared with the predicted value. Mass alone does not always prove that two molecules have exactly the same structure, but a close match can support the proposed identity.

Mass spectrometry can be further used to identify modifications, truncations or other molecular species. Depending on the technique, it may also give information about degradation products or impurities.

This limitation is one reason why MS is often used in conjunction with other analytical techniques rather than as a single test.

How Does a Mass Spectrometer Work?

Modern mass spectrometers can be very sophisticated, but the basic process is easier to understand than it first looks.

A sample is introduced into the instrument first. After that we need to convert molecules into ions so that we can control and measure them. Depending on the sample type and the information you need, you have a range of ionisation methods at your disposal.

Then they pass through the mass analyser. The analyser separates them according to their mass to charge ratio. The ions that reach the detector are then recorded by the detector.

The instrument then converts these measurements into a mass spectrum. Scientists can observe this pattern of peaks. They can compare measured values to expected values or to reference data.

The exact process differs between instruments. Electrospray ionisation (ESI) methods are commonly used for peptides because they can transfer relatively large molecules into the gas phase without the harsh conditions associated with some other ionisation methods.

Why Use Mass Spectrometry for Peptides?

A peptide is a chain of amino acids. A small change in structure can affect the molecular mass of peptides. This makes mass spectrometry a useful tool in identifying a peptide.

For example, if a molecular mass is predicted, the measured value for a peptide can be compared with the predicted value. Mass alone does not always prove that two molecules have exactly the same structure, but a close match can support the proposed identity.

Mass spectrometry can be further used to identify modifications, truncations or other molecular species. Depending on the method, it may also provide information on degradation products or impurities.

This is one reason why MS is often used in conjunction with other analytical techniques rather than as a single test.

Understanding Mass-to-Charge Ratio

You will see the term ‘m/z’ used a lot in mass spectrometry reports. It is mass/charge ratio.

This matters because molecules can carry more than one charge. A peptide with several positive charges can therefore appear at a different m/z value from the same peptide carrying a single charge.

This is particularly relevant when analysing larger peptides and proteins. The instrument may detect several related peaks representing different charge states of the same molecule.

Specialised software can use these charge states to calculate the molecular mass of the original molecule.

Understanding this point is important when reading an MS result. The number shown on a spectrum is not necessarily the straightforward molecular weight of the peptide.

Mass Spectrometry and Peptide Identification

Mass spectrometry can be used in many ways to help in peptide identification.

One easy way is to compare the measured molecular mass with the expected molecular mass. If the values differ significantly, this may indicate that the sample is not a pure compound, is a modified form, is a degradation product or has some other problem which requires further investigation.

More detailed analysis can break peptide ions down into smaller pieces. The resulting fragments can give information about the amino acid sequence of the peptide.

The approach is commonly referred to as tandem mass spectrometry or MS/MS. But researchers don’t just look at the intact peptide; they also look at the fragments to get more structural information.

The more detailed the question being asked, the more important the choice of instrument and analytical method becomes.

What Can Mass Spectrometry Detect?

Mass spectrometry can give information on a variety of compounds in a sample. In peptide research it may assist researchers to explore:

• Molecular mass
• Charge states
• Peptide identity
• Sequence-related information
• Certain chemical modifications
• Degradation products
• Some impurities or unexpected components

But the technique has its limitations. A mass match alone does not verify all features of a peptide structure. Different molecules may have the same/similar molar masses.

Researchers may therefore couple MS with techniques such as HPLC, which separates out components in a sample before analysis. HPLC and MS answer different questions, and when used together, they can give a better picture of the sample composition.

HPLC and Mass Spectrometry

HPLC and mass spectrometry are often mentioned in the same breath, but they are not the same thing.

HPLC is the method of the physical and chemical separation of the compounds. In the chromatogram thus obtained, different peaks can be obtained, which can be used to evaluate the purity and composition of a sample.

Mass spectrometry, in contrast, gives information on the mass-to-charge ratio of ions.

Together these two techniques are called LC-MS. It combines the separation power of liquid chromatography with the identification and mass information from mass spectrometry. This can be especially useful when a sample contains more than one compound and one technique would not be able to differentiate between them.

What Does an MS Result Tell You?

An MS result should be interpreted in context rather than by looking at one number in isolation.

A mass that is reported close to the expected mass can be used to support the identity of a peptide. Another possibility is that it might be worth looking into. The changes, impurities, degradation or differences in the sample preparation may be the reasons.

The quality of the conclusion also depends on the instrument, ionisation method, sample preparation, calibration and data analysis.

This is why a mass spectrum is more than just a graph. The result needs to be understood alongside the analytical method used to produce it.

The Role of Mass Spectrometry in Peptide Quality Testing

Mass spectrometry is an important analytical tool because it gives researchers information that cannot be obtained from visual inspection alone.

For peptide samples you can look at things like molecular identity, molecular mass, and structural information. It can be used in conjunction with techniques such as HPLC and contribute to a wider assessment of what is present in a sample.

It is worth remembering that no single analytical test answers every question. Mass spectrometry is most useful when its results are considered alongside the method used, the expected characteristics of the peptide and other available analytical data.

For anyone reading peptide testing information, the key point is simple: mass spectrometry helps researchers determine what molecular species are present by measuring their mass-to-charge behaviour. That makes it a valuable part of modern peptide analysis and research.