Mass Spectrometry

How Mass Spectrometry Works

The process involves three main steps:

1. Ionization - Convert molecules to charged ions
2. Mass analysis - Separate ions by mass-to-charge ratio
3. Detection - Count ions at each m/z value

Component	Function	Peptide Application
Ion source	Creates charged molecules	ESI or MALDI
Mass analyzer	Separates by m/z	Quadrupole, TOF, Orbitrap
Detector	Counts ions	Generates spectrum
Data system	Processes results	Calculates molecular weight

Ionization Methods for Peptides

Electrospray Ionization (ESI)

Aspect	Details
Sample state	Liquid solution
Coupling	Directly with HPLC (LC-MS)
Charge states	Multiple (z = +2 to +10+)
Mass range	Excellent for peptides
Advantages	Soft ionization, quantitative

MALDI (Matrix-Assisted Laser Desorption/Ionization)

Aspect	Details
Sample state	Crystallized with matrix
Coupling	Usually offline
Charge states	Typically singly charged
Mass range	Very high (proteins)
Advantages	Tolerates salts, rapid screening

Mass Analyzers Compared

Analyzer	Resolution	Mass Accuracy	Speed	Cost
Quadrupole	Low	0.1 Da	Fast	Low
Time-of-Flight (TOF)	High	5-20 ppm	Fast	Medium
Orbitrap	Very high	Less than 5 ppm	Medium	High
Ion trap	Medium	0.1 Da	Medium	Medium
Q-TOF	High	5 ppm	Fast	High

Peptide Identification by MS

Molecular Weight Confirmation

Peptide	Calculated MW	Measured MW	Match?
BPC-157	1419.53 Da	1419.54 Da	Yes
Semaglutide	4113.58 Da	4113.60 Da	Yes
TB-500	4963.44 Da	4963.45 Da	Yes

Acceptable mass accuracy: within 0.1% or 10 ppm

Detecting Impurities

Impurity Type	MS Detection	Example
Deletion peptide	-MW of missing AA	-131 Da (Met)
Oxidation	+16 Da	Met sulfoxide
Deamidation	+1 Da	Asn to Asp
Truncation	Shorter sequence	Missing terminal AA
Aggregation	2x or 3x MW	Dimer, trimer

Tandem Mass Spectrometry (MS/MS)

For sequence verification:

1. Precursor selection - Isolate peptide ion
2. Fragmentation - Break peptide bonds (CID, HCD, ETD)
3. Product analysis - Detect fragment ions
4. Sequence assembly - Match fragments to sequence

Fragment Type	Cleavage Site	Contains
b-ions	N-terminal	N-terminus
y-ions	C-terminal	C-terminus
a-ions	CO loss from b	N-terminus
c/z-ions	ETD fragments	N/C-terminus

LC-MS Coupling

The combination of HPLC and MS provides:

Capability	Benefit
Separation + identification	Resolve then identify
Quantification	Peak area = amount
Impurity profiling	Identify all species
Stability testing	Monitor degradation

Typical LC-MS Workflow

1. Inject sample onto HPLC column
2. Separate by reverse-phase gradient
3. Elute into ESI source
4. Acquire MS spectra continuously
5. Generate chromatogram with MS data

Quality Control Applications

Test	MS Method	Information
Identity	Full scan	Correct MW?
Purity	LC-MS	Related substances
Sequence	MS/MS	Correct sequence?
Modifications	High-res MS	PTMs, degradants
Quantification	LC-MS/MS	Amount present

Frequently Asked Questions

Why is mass spectrometry essential for peptides?

Mass spectrometry provides unambiguous molecular weight confirmation that no other technique can match. While HPLC can show purity as a percentage, MS proves the peptide is actually the correct molecule. For research peptides, MS data is the definitive identity test.

What do multiple charge states mean?

In ESI, peptides acquire multiple protons, creating ions like [M+2H]2+, [M+3H]3+, etc. Multiple charge states help confirm identity (all should calculate to same MW) and extend the mass range of the analyzer. The pattern also helps distinguish peptides from other compounds.

How accurate does molecular weight need to be?

For peptide identity, mass accuracy within 0.1% is typically acceptable. High-resolution instruments achieve under 5 ppm (0.0005%), enabling detection of modifications like single deamidation (+1 Da) even on large peptides. Research-grade peptides should have MS data within 0.1 Da of calculated mass.