Peptide Purity Testing: HPLC & Mass Spectrometry

Every QSC compound batch undergoes two independent analytical tests before release: reverse-phase HPLC for purity quantification and mass spectrometry for identity confirmation. A third-party COA from Janoshik Analytical Laboratories is then issued, providing independent verification of both results.

HPLC Purity Analysis

High-Performance Liquid Chromatography (HPLC) is the primary quantitative method for measuring peptide purity. It separates the target compound from impurities based on differential interaction with a stationary phase, then quantifies each species by UV absorbance.

Reverse-Phase HPLC (RP-HPLC)

QSC uses reverse-phase HPLC with a hydrophobic stationary phase (C18, octadecylsilane bonded silica). Peptides are eluted using a gradient of acetonitrile in water (both containing 0.1% TFA). The hydrophobic components of each peptide determine its retention time — more hydrophobic sequences elute later.

Detection and Quantification

UV detection at 220nm is used for peptide quantification. At this wavelength, the peptide bond itself absorbs, making detection of all peptide species (including truncated sequences lacking aromatic residues) more universal than detection at 254nm, which primarily detects aromatic-containing compounds.

Purity is calculated as the percentage area of the target compound peak relative to the total integrated area of all detected peaks:

Purity (%) = [Target peak area / Total area of all peaks] × 100

QSC's specification for all compounds is target peak area ≥99%. This means all synthesis by-products, truncated sequences, and oxidation products combined account for less than 1% of the UV-absorbing material.

What Impurities Does HPLC Detect?

Key impurities that would appear in a peptide HPLC chromatogram include:

• Deletion sequences — peptides missing one or more internal residues due to incomplete deprotection or coupling
• Truncated sequences — incomplete chains (N-terminal truncations) from suboptimal coupling efficiency
• Oxidised species — particularly methionine oxidation (Met→Met(O)) or cysteine disulfide products
• Racemisation products — D-amino acid incorporation from aggressive coupling conditions
• Residual reagents — traces of protecting group fragments or solvent adducts

A ≥99% purity specification means all of these categories combined are below the 1% detection threshold.

Mass Spectrometry Identity Confirmation

HPLC measures purity percentage but cannot definitively confirm what the main peak is. A different compound with similar hydrophobicity could co-elute with the target. Mass spectrometry solves this by confirming molecular identity.

ESI-MS (Electrospray Ionisation)

QSC primarily uses ESI-MS (electrospray ionisation mass spectrometry) for peptide identity confirmation. In ESI, the peptide is sprayed through a charged needle, forming multiply-charged ions. Peptides typically appear as [M+2H]²⁺, [M+3H]³⁺, etc. The molecular weight is then calculated from the measured m/z values and charge states.

The measured monoisotopic mass is compared to the theoretical molecular weight calculated from the amino acid sequence and molecular formula. QSC's acceptance criterion is agreement within ±1 Da (mass spectrometer instrument tolerance), which unambiguously confirms molecular identity.

What MS Confirms

Independent COA: Janoshik Analytical Laboratories

QSC's own internal testing establishes the baseline for every batch. But independent verification is the critical differentiator between suppliers who self-report purity and those who submit to external scrutiny.

All QSC batches are tested by Janoshik Analytical Laboratories, a well-known independent testing provider in the research peptide community. Janoshik issues a Certificate of Analysis (COA) that includes:

• The HPLC chromatogram with peak integration and purity percentage
• The MS spectrum with identified m/z peaks and calculated molecular weight
• Batch number, compound name, testing date, and analyst signature
• A unique verification code for online authentication

Critically, Janoshik COAs are publicly verifiable. Any customer who receives a QSC product can enter the batch number on the Janoshik website and see the original testing data. This prevents falsification of COAs — a significant problem with less rigorous suppliers who self-issue documents.

How QSC Purity Compares