LibrarianMeg said:They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.
Thank you for spelling out the reasoning rather than just the conclusion. Sending this to two other people who asked me the same thing last week.
Adding the clinical framing, because it changes how the question reads.
Vendor quality matters when discussing certificates of analysis. I've tested my last 5 batches through Janoshik and purity has ranged from 97.0% to 99.6%. Consistency is key.
For anyone new: always request the COA, and consider independent verification. The $300 for a Janoshik test is worth the peace of mind.
Dr.NateNeph said:Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$133, 10-14 day turnaround…
My vendor review relevant to certificates of analysis: ordered 4 times from my current compounder. Consistency has been excellent.
Shipping: 2-day priority with gel ice
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 98.5% average across orders (Janoshik verified)
Customer service: Responsive within 4 hours
Price: $149/month for tirzepatide 10mg equivalent
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsHPLC_Greg said:The provenance questions are more useful than the numbers.
I tested my certificates of analysis batch through Janoshik — 98.4% purity. That's within acceptable range for compounded peptides. Turnaround was 10 business days.
The test costs ~$148 but gives you actual HPLC chromatography data plus mass spec confirmation. For the peace of mind alone, it's worth every penny. I test every new batch from a new vendor.