Mass spectrometry explained for peptides

Mass spectrometry for peptides: how is molecular identity investigated?

In the quality control of research peptides, multiple analytical techniques are often combined. HPLC can provide information about chromatographic purity, but a high main peak does not automatically prove that the correct peptide is present.

To investigate molecular identity further, mass spectrometry is often used.

Mass spectrometry, usually abbreviated as MS, is an analytical technique that measures the mass-to-charge ratio of ionized molecules. In peptide analysis, the measured molecular mass can be compared with the theoretically expected mass of the amino acid sequence.

When these values correspond, this supports the identification of the peptide under investigation. However, mass spectrometry alone also does not provide a complete picture of quality.

In this blog, we discuss how mass spectrometry works, why peptides can produce multiple mass signals, and how MS results are interpreted together with HPLC and other analytical methods.

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What is mass spectrometry?

Mass spectrometry is an analytical technique in which molecules are converted into charged particles, or ions.

These ions are then separated and measured based on their mass-to-charge ratio.

This ratio is represented as:

m/z

This denotes:

• m for mass;
• z for electrical charge.

A mass spectrometer therefore does not always directly measure the complete molecular mass. The instrument measures the mass-to-charge ratio.

In larger molecules, including many peptides, multiple charges can occur. As a result, the same peptide may appear at different positions in the mass spectrum.

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Why is mass spectrometry used in peptide analysis?

A peptide consists of a specific sequence of amino acids.

Every amino acid combination has a theoretically calculable molecular mass.

Mass spectrometry can be used to investigate whether the measured mass corresponds to this theoretical value.

MS analysis can contribute to:

• confirmation of molecular identity;
• comparison with the theoretical molecular mass;
• detection of certain chemical modifications;
• analysis of degradation products;
• investigation of peptide variants;
• comparison between production batches.

A corresponding mass supports the identity of a peptide.

However, the result must always be assessed in combination with the analytical method and other quality data.

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What is the theoretical molecular mass?

The theoretical molecular mass is calculated based on:

• the amino acid sequence;
• the number of amino acids;
• any chemical modifications;
• the molecular form;
• specific terminal groups.

When amino acids form a peptide chain together, peptide bonds are formed.

During the formation of each peptide bond, a water molecule is split off. Therefore, the mass of a complete peptide is not simply the sum of all the individual amino acids.

Modifications can also affect the mass.

Examples include:

• amidation;
• acetylation;
• phosphorylation;
• oxidation;
• addition of fatty acid chains;
• formation of disulfide bridges.

The theoretical mass must therefore be calculated for the exact molecular structure.

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What does molecular weight mean?

The terms molecular mass and molecular weight are often used interchangeably.

In laboratory reports, the value is usually expressed in:

• dalton, abbreviated as Da;
• kilodalton, abbreviated as kDa.

One dalton corresponds approximately to the mass of one hydrogen atom.

A peptide with a molecular mass of 3,000 Da therefore has a mass of approximately 3 kDa.

The molecular mass of peptides can vary considerably, depending on:

• chain length;
• amino acid composition;
• chemical modifications.

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How does a mass spectrometer work?

A mass spectrometer usually consists of several components.

1. Sample introduction

The peptide is first prepared and introduced into the instrument.

Sample preparation can affect:

• signal intensity;
• detectability;
• formation of ions;
• background signals.

2. Ionization

The molecules are electrically charged.

Commonly used techniques for peptide analysis include:

• electrospray ionization;
• matrix-assisted laser desorption/ionization.

3. Mass analysis

The ions formed are separated based on their mass-to-charge ratio.

4. Detection

The detector records the ions.

5. Data processing

The measured signals are converted into a mass spectrum.

This spectrum contains peaks corresponding to detected ions.

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What is electrospray ionization?

Electrospray ionization, abbreviated as ESI, is a commonly used ionization technique for peptides.

During ESI, a liquid sample is passed through a fine opening while an electrical voltage is applied.

This results in electrically charged droplets.

When the solvent evaporates, charged molecules remain.

An important characteristic of ESI is that larger molecules can acquire multiple electrical charges.

A peptide may, for example, occur as:

• [M+H]⁺;
• [M+2H]²⁺;
• [M+3H]³⁺.

Here, M represents the molecular mass of the peptide.

Multiple charges produce different m/z signals for the same molecule.

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Why does one peptide have multiple peaks?

When a peptide takes up multiple protons, different charge states are formed.

Suppose a peptide has a molecular mass of approximately 3,000 Da.

Possible signals are then approximately:

• one positive charge: m/z around 3.001;
• two positive charges: m/z around 1.501;
• three positive charges: m/z around 1.001.

These signals may all belong to the same peptide.

The software can use this charge distribution to calculate the original molecular mass.

This process is called deconvolution.

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What is a deconvoluted mass spectrum?

A raw ESI measurement may contain multiple charge peaks.

This can make interpretation complex.

Deconvolution software converts the different charge states into one estimated molecular mass.

The deconvoluted spectrum can therefore be compared more easily with:

• the theoretical mass;
• a reference standard;
• product specifications.

The quality of the calculation depends on:

• signal intensity;
• resolution;
• background noise;
• software settings;
• presence of overlapping components.

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What is MALDI?

Matrix-Assisted Laser Desorption/Ionization, abbreviated as MALDI, is another ionization technique.

The peptide is mixed with a matrix material and applied to a sample plate.

A laser activates the matrix, producing peptide ions.

MALDI often produces predominantly singly charged ions.

This can make the spectrum appear simpler than an ESI spectrum.

MALDI is often combined with a time-of-flight mass analyzer.

This combination is referred to as:

MALDI-TOF

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What is time-of-flight?

In a time-of-flight mass analyzer, ions are accelerated by an electric field.

They then travel through a flight tube.

Ions with a lower mass-to-charge ratio generally reach the detector faster than ions with a higher ratio.

The time of flight is used to calculate the m/z value.

TOF systems can be used for:

• peptide identification;
• mass determination;
• comparison of molecular variants.

The final accuracy depends on calibration, instrument settings, and sample preparation.

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What is LC-MS?

Liquid chromatography-mass spectrometry, abbreviated as LC-MS, combines liquid chromatography with mass spectrometry.

The components are first separated chromatographically.

The individual signals are then analyzed by the mass spectrometer.

LC-MS can combine information about:

• retention time;
• chromatographic profile;
• molecular mass;
• possible impurities;
• degradation products.

This allows LC-MS to provide more insight than a separate HPLC or MS analysis.

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What is the difference between HPLC and mass spectrometry?

HPLC and mass spectrometry answer different analytical questions.

HPLC

HPLC can provide information about:

• chromatographic purity;
• relative amount of detected components;
• retention times;
• changes in the chromatographic profile.

Mass spectrometry

Mass spectrometry can provide information about:

• molecular mass;
• possible identity;
• chemical modifications;
• certain degradation products.

A simplified comparison:

Therefore, both techniques are often combined.

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Can the same molecular mass occur in different peptides?

Yes.

Different amino acid sequences can have the same or nearly the same total mass.

These are sometimes called isobaric structures.

A matching mass therefore does not always prove that the complete amino acid sequence is correct.

More extensive identification may require additional techniques, such as:

• tandem mass spectrometry;
• fragmentation analysis;
• amino acid analysis;
• sequence analysis;
• comparison with a reference standard.

The analytical method required depends on the desired level of certainty.

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What is tandem mass spectrometry?

Tandem mass spectrometry is usually written as:

MS/MS

During MS/MS, a selected peptide ion is further fragmented.

The resulting fragments are then measured again.

The fragmentation pattern can provide information about:

• amino acid sequence;
• location of modifications;
• peptide identity;
• structure of degradation products.

MS/MS can therefore provide more structural information than just the intact molecular mass.

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What are peptide fragments?

When a peptide chain breaks at specific sites during MS/MS, fragment ions are produced.

Common fragment types include:

• b ions;
• y ions.

The masses of these fragments can be used to reconstruct parts of the amino acid sequence.

The interpretation can be complex.

Not every peptide fragments in the same way.

Factors that may have an influence include:

• amino acid composition;
• electrical charge;
• peptide size;
• fragmentation method;
• chemical modifications.

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Can mass spectrometry detect oxidation?

Certain oxidative changes cause a measurable mass change.

For example, the addition of one oxygen atom often results in a mass increase of approximately 16 Da.

As a result, mass spectrometry can help identify certain oxidation products.

Detection depends on:

• concentration;
• instrument resolution;
• analytical method;
• separation of components.

Not every oxidative change can be fully localized solely on the basis of a single mass signal.

Additional fragmentation analysis may be required for this.

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Can mass spectrometry determine the amount of peptide?

Not automatically.

The intensity of a mass signal is influenced by:

• ionization efficiency;
• molecular properties;
• solvent;
• instrument settings;
• presence of other components.

Therefore, a signal that is twice as strong does not automatically mean that twice as much peptide is present.

Reliable quantification requires validated methods and suitable standards.

Possible techniques include:

• quantitative LC-MS;
• internal standards;
• isotopically labeled references;
• calibrated HPLC assays.

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Can mass spectrometry demonstrate sterility?

No.

Mass spectrometry for peptide identity is not a sterility test.

The result provides no direct information about:

• bacteria;
• fungi;
• microbial growth;
• sterility of the product.

Sterility requires specific microbiological test methods.

Endotoxins are also not automatically determined during a standard peptide MS analysis.

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Can mass spectrometry measure endotoxins?

Not through a standard identity analysis.

Endotoxins are complex bacterial components.

Specific methods are used for endotoxin testing, such as:

• Limulus Amebocyte Lysate test;
• recombinant factor C methods.

Therefore, a correct mass spectrum of a peptide does not prove that the material has a low endotoxin level.

Identity, purity, sterility, and endotoxins are separate quality parameters.

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Why can laboratory results differ?

Different laboratories may report slightly different mass values.

Possible causes include:

• instrument type;
• calibration;
• ionization method;
• sample preparation;
• salt adducts;
• software used;
• resolution;
• interpretation method.

Small differences do not automatically mean that a result is incorrect.

The measured value must be assessed within:

• instrument accuracy;
• expected molecular form;
• analytical method used.

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What are adducts?

During mass spectrometry, peptide ions can bind to other charged particles.

Examples include:

• sodium;
• potassium;
• ammonium.

This can result in additional peaks.

For example, a sodium adduct has a different mass from a simply protonated peptide ion.

Adducts do not automatically mean that a different peptide is present.

Interpretation requires knowledge of:

• sample preparation;
• buffers used;
• expected ion forms.

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What information belongs in an MS report?

A professional analysis report may include, among other things:

• name of the peptide;
• batch or lot number;
• theoretical molecular mass;
• measured molecular mass;
• ionization method;
• type of mass spectrometer;
• mass spectrum;
• date of analysis;
• name of the laboratory.

More extensive analyses may also include:

• measurement range;
• resolution;
• mass accuracy;
• fragmentation data;
• used software.

The required reporting depends on the research objective.

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Mass Spectrometry and Batch Control

Each production batch can be examined separately.

Batch-specific MS analysis supports:

• identity verification;
• comparison between batches;
• traceability;
• investigation of deviations.

The batch number on the analysis report must match the identification on the vial.

A mass spectrometry result without a clear batch link has limited value for traceability.

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Scientific Limitations

Mass spectrometry is a powerful analytical technique, but not a complete quality assessment.

A matching molecular mass does not automatically prove:

• complete amino acid sequence;
• high chromatographic purity;
• correct peptide quantity;
• sterility;
• low endotoxin levels;
• biological activity;
• long-term stability.

For a broader assessment, multiple methods may be needed:

• HPLC;
• LC-MS;
• MS/MS;
• quantitative assay;
• stability studies;
• moisture analysis;
• additional product-specific tests.

The results must be interpreted together.

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Summary

Mass spectrometry is used to investigate the molecular mass of research peptides.

The technique measures the mass-to-charge ratio of ionized molecules.

In peptide analysis, the measured mass can be compared with the theoretically expected mass.

A match supports molecular identification.

Important analytical techniques include:

• ESI-MS;
• MALDI-TOF;
• LC-MS;
• MS/MS.

However, mass spectrometry does not automatically provide information about:

• exact quantity;
• chromatographic purity;
• sterility;
• endotoxins;
• biological activity.

Therefore, MS is often combined with HPLC and additional quality analyses.

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Research Disclaimer

Peptidera's research peptides are offered exclusively for Research Use Only (RUO) and are intended for laboratory research and analytical applications.

Not intended for human or animal consumption, diagnostic use, therapeutic application, or self-administration.

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Category:
Peptide Knowledge & Quality Control

Related Peptidera products:
Lyophilized research peptides with batch identification and available analytical quality data

Related internal blogs:

• PB-0227 — Lyophilized Peptides and Lyophilization
• PB-0228 — Peptide Vials and Packaging Integrity
• PB-0229 — Batch Numbers and Traceability
• PB-0230 — Peptide Purity and HPLC
• What is a Certificate of Analysis?

Internal link suggestions:

• Link to PB-0230 for the difference between HPLC and MS
• Link to PB-0229 for batch-specific analysis
• Link to the COA page for mass spectrometry test results
• Link to the research peptide collection
• Link to the page about independent laboratory analysis