Purity

Understanding Purity Grades

Research peptides are available at different purity levels:

Grade	Purity	Typical Applications
Pharmaceutical	99%+	Clinical trials, drug products
Research Plus	98%+	In vivo studies, publications
Research	95%+	Standard research, in vitro
Economy	90%+	Screening, preliminary work
Crude	70-85%	Bulk/cost-sensitive applications

Why Purity Matters

Research Type	Minimum Recommended	Reason
Cell culture	95%	Impurities may affect cells
Animal studies	98%	Safety and reproducibility
Binding assays	95%	Accurate affinity data
Structural studies	98%+	Clean NMR/crystallography
Screening	90%	Cost efficiency

Types of Impurities

Synthesis-Related

Impurity	Cause	Detection
Deletion peptides	Incomplete coupling	MS (-AA mass)
Insertion peptides	Double coupling	MS (+AA mass)
Truncated sequences	Premature cleavage	MS, HPLC
Protecting groups	Incomplete deprotection	MS (+group mass)
Epimerization	Racemization	Chiral HPLC

Degradation Products

Impurity	Cause	Detection
Oxidized forms	Met, Cys, Trp oxidation	MS (+16 Da)
Deamidated forms	Asn, Gln hydrolysis	MS (+1 Da)
Aggregates	Peptide clumping	SEC, MS
Fragments	Bond cleavage	MS, HPLC

Other Contaminants

Contaminant	Source	Concern
Salts	Purification buffers	Weight accuracy
Solvents	Incomplete drying	Stability
Water	Hygroscopic uptake	Weight accuracy
Endotoxin	Bacterial contamination	In vivo use

Purity Testing Methods

Method	What It Measures	Limitations
HPLC (UV 214)	Peptide-related substances	Doesn’t detect salts
Mass spectrometry	Molecular identity	Semi-quantitative
Amino acid analysis	Composition	Averages all species
Elemental analysis	C, H, N percentages	Indirect purity
Water content	Karl Fischer	Only water
Salt content	Ion chromatography	Specific ions

HPLC Purity Calculation

Component	Peak Area	Percentage
Target peptide	9,700,000	97.0%
Related impurity A	150,000	1.5%
Related impurity B	100,000	1.0%
Related impurity C	50,000	0.5%
Total	10,000,000	100%

Purity vs Potency

Important distinction:

Term	Definition	Example
Purity	% of sample that is target compound	98% purity
Potency	Amount of active peptide per unit weight	85% peptide content
Net peptide content	Actual peptide in sample	Purity x Potency

Example Calculation

A 10 mg vial with 98% purity and 85% peptide content:

• Actual peptide = 10 mg x 0.98 x 0.85 = 8.33 mg
• The rest: salts, water, counterions

Certificate of Analysis

A quality CoA should include:

Parameter	Acceptable Range
HPLC purity	Meets specification
MS identity	Within 0.1% of calculated
Appearance	White to off-white powder
Peptide content	75-95% typical
Water content	Under 10% typical
Counterion	TFA or acetate identified

Red Flags

Warning Sign	Concern
No HPLC chromatogram	Data may be fabricated
No MS data	Identity not confirmed
Generic method	May not be optimized
Missing lot number	Traceability issues

Improving Purity

Purification methods to increase purity:

Starting Purity	Method	Expected Result
70-85%	Preparative HPLC	95%+
90-95%	Optimized prep HPLC	98%+
95-98%	Multiple purifications	99%+

Frequently Asked Questions

Is HPLC purity the same as overall purity?

No. HPLC purity measures peptide-related substances but doesn’t account for non-peptide impurities like salts, water, or residual solvents. A peptide can have 98% HPLC purity but only 80% net peptide content due to salt and water content.

Why do purity requirements vary by application?

Impurities can affect research differently depending on the application. Cell-based assays may be sensitive to certain impurities. Quantitative studies need accurate dosing. In vivo work requires higher purity for safety. Preliminary screening can tolerate more impurities since it’s just initial evaluation.

How much does higher purity cost?

Each 1% increase in purity above 95% typically adds 10-20% to the cost because additional purification steps are needed and yield decreases. Going from 95% to 99% purity might double or triple the price while significantly reducing available quantity.