How to Determine Molecular Weight?

1. What is Molecular Weight?

Molecular Weight (MW), also known asRelative Molecular Mass, is defined as the ratio of the mass of a molecule to 1/12th the mass of a carbon-12 atom, with units inDalton (Da) or g/mol. In practical applications, we often focus onExact Mass, which is the number calculated from the precise masses of isotopes of each atom, especially critical in mass spectrometry analysis.

 

2. Summary of Common Methods for Determining Molecular Weight

 

Method	Technical Principle	Applicable Objects	Remarks
Mass Spectrometry (MS)	Based on the mass-to-charge ratio (m/z) of charged particles in an electromagnetic field	Small molecules, proteins, peptides, etc.	Can measure exact mass, most widely applicable
Gel Permeation Chromatography (GPC/SEC)	Based on molecular size exclusion behavior in porous media	Polymers, proteins, high molecular weight substances	Can estimate relative molecular weight, suitable for degree of polymerization analysis
SDS-PAGE Electrophoresis	Proteins migrate according to size when wrapped in SDS	Proteins	Rough estimation, greatly affected by structure
Nuclear Magnetic Resonance (NMR)	Resonance frequency of nuclear spins in molecules related to chemical environment	Small molecules	Can deduce structure and hence calculate molecular weight
Elemental Analysis	Deduction of molecular formula from the mass percentage of constituent elements	Organic compounds	Requires pure samples, suitable for verification
Vapor Pressure/Osmotic Pressure Measurement	Thermodynamic principles applicable for average molecular weight measurement of polymers	High molecular weight substances	Frequently used in the polymer industry

  

3. Determining Molecular Weight by Mass Spectrometry

1. Principle of Mass Spectrometry

Mass Spectrometry (MS) ionizes molecules into charged ions and separates and detects them based on their mass-to-charge ratio (m/z) as they migrate through electric/magnetic fields.

 

2. Common Applicable Platforms

(1) ESI-MS (Electrospray Ionization): Suitable for proteins, peptides, metabolites, etc.

（2）MALDI-TOF-MS (Matrix-Assisted Laser Desorption/Ionization): Commonly used for proteins and polymers

（3）HRMS (High-Resolution Mass Spectrometry, such as Orbitrap, Q-TOF): Can obtainExact Mass, useful for inferring molecular formula

 

4. Determining Protein Molecular Weight by SDS-PAGE (Suitable for Routine Laboratory Verification)

SDS-PAGE is based onthe principle of proteins migrating according to size in an electric fieldand is estimated using standard protein molecular weight markers.

1. Advantages

Simple operation, low cost. Quickly estimates protein purity and size.

 

2. Limitations

Limited resolution, especially for proteins of similar size. Modifications such as glycosylation and aggregation may affect migration behavior.

 

V. How to choose the appropriate molecular weight determination method?

  

Research purpose	Recommended method	Description
Small molecule compound structure confirmation	High-resolution mass spectrometry (HRMS)	Accurate to four decimal places, combined with isotope peak determination.
Protein expression analysis	SDS-PAGE + MALDI-TOF-MS	PAGE for preliminary screening, MALDI to confirm size.
Peptide/oligosaccharide detection	ESI-MS/MS	Can be combined with tandem mass spectrometry for structural information.
High molecular weight polymer distribution	GPC	Obtain distribution range (Mw, Mn, PDI).
Requires high throughput and sensitive detection.	LC-MS	Suitable for complex samples when combined with chromatography.

Biotech Park provides advanced mass spectrometry platforms (including Orbitrap Exploris, TripleTOF, QqQ, etc.) to offer professional molecular weight determination services for different types of molecules:

（1）Protein/peptide/metaboliteOne-stop molecular weight analysis

(2) ProvidesAccurate molecular formula prediction and structural annotation.

(3) CustomizableLC-MS quantitative + qualitative combined solution.

(4) Includes complete spectra, raw data, and analysis reports, supporting scientific publication.

Whether it's basic chemical synthesis, protein expression validation, or quality control in new drug discovery, accurate molecular weight determination is an indispensable step. Choosing the right method will lay a solid foundation for subsequent research.

 

Biotech Park - Characterization of Bioproducts, Multi-group Biomass Spectrometry Detection Quality Service Provider

 

Related services:

High-resolution mass spectrometry molecular weight identification