What mass should an oxytocin certificate show?

PubChem lists oxytocin, C43H66N12O12S2, at an average molecular weight of 1007.2 g/mol and a monoisotopic mass of 1006.44 Da, and the FDA label for Pitocin gives 1007.19. These values describe the disulfide-bonded ring. A main mass about 2 Da higher suggests the ring has opened, and masses near double suggest disulfide-linked dimers.

Does oxytocin really degrade in heat?

Yes. A review of oxytocin quality in low- and middle-income countries describes every oxytocin formulation as heat-sensitive, and a 2019 WHO, UNFPA and UNICEF statement calls for 2 to 8 °C storage and distribution. A forced-degradation study found loss sped up with temperature and was fastest at pH 9.0, so shipping conditions show up in the result.

Is a purity below 95% always a bad sign for oxytocin?

For a powder, it is worth questioning. Pharmacopoeia monographs set total impurities for oxytocin active ingredient at not more than 5%. The FDA label for the Pitocin solution allows up to 16%, but that covers a finished liquid product. What matters next is which impurities are present: deamidated forms, sulfur-rearranged rings or dimers point to degradation or poor storage.

Can a lab create oxytocin impurities while testing it?

It can. Disulfide bonds can rearrange during sample preparation, especially at alkaline pH and high temperature, which is why published disulfide analysis methods prefer neutral or slightly acidic conditions. A careful laboratory prepares oxytocin samples to avoid this, and a good report states its conditions so that any rearranged species can be attributed to the vial.

Why is my oxytocin labelled in units instead of milligrams?

Approved oxytocin products are standardised by activity and labelled in units per mL. A chemical test measures milligrams of peptide instead. The two scales are not interchangeable without a stated basis, so a mass-based amount result compares directly only with a milligram label. If your product is labelled in units, ask the laboratory how it will report the result.

One of the 30 compound testing profiles

Oxytocin

Oxytocin is a 9-amino-acid peptide (C43H66N12O12S2, average mass 1007.2 g/mol) held in a ring by a disulfide between Cys1 and Cys6; a test should confirm that mass, check for deamidated and disulfide-rearranged forms, and keep in mind how readily it degrades with heat.

Oxytocin is a nonapeptide hormone found in mammalian pituitary extracts; a synthetic version is approved by the US FDA as Pitocin, whose label describes the hormone as prepared synthetically (FDA prescribing information).

Test your oxytocin

Identity card

Formula: C43H66N12O12S2
Average mass: 1,007.2 g/mol
Monoisotopic mass: 1,006.44 Da
Length: 9 residues
Modifications: Disulfide bond Cys1-Cys6; C-terminal amide (Gly9-NH2); Free N-terminus
Also sold as: OT, OXT
Sequence: CYIQNCPLG
Note: PubChem lists the line notation H-Cys(1)-Tyr-Ile-Gln-Asn-Cys(1)-Pro-Leu-Gly-NH2, the (1) marking the disulfide-linked cysteines. The FDA label gives the same formula with a molecular weight of 1007.19.

Source: Oxytocin compound record (CID 439302), Pitocin (oxytocin) prescribing information.

Check a certificate's mass against these values with the mass checker, prefilled for Oxytocin.

What usually goes wrong with oxytocin. Documented failure modes, with sources.

Heat damage along the supply chain

A 2020 review of oxytocin quality in low- and middle-income countries describes all oxytocin formulations as heat-sensitive, and lists threats at manufacture (poor active ingredient or processes) and during storage and distribution (lack of temperature control) (Lambert et al., 2020). It cites a 2019 WHO, UNFPA and UNICEF joint statement calling for a 2 to 8 °C cold chain, and notes that cumulative heat exposure is generally not tracked. A vial's shipping history matters as much as its origin.

Deamidation at Gln4, Asn5 and the Gly9 amide

Forced degradation work identified oxytocin's breakdown products by ESI-MS/MS. At pH 2.0, deamidation of Gln4, Asn5 and the C-terminal Gly9-NH2, singly and in combination, was found; at pH 9.0, mono-deamidation at the same three sites also occurred (Hawe et al., 2009). Each deamidation adds 0.985 Da, and three sites mean several near-isobaric species at once, so accurate mass and good chromatographic separation are both needed to see them.

Rearranged sulfur bridges and covalent dimers

At pH 4.5, 7.0 and 9.0, the same study found tri- and tetrasulfide versions of the Cys1-Cys6 ring and disulfide- and dityrosine-linked dimers, with beta-elimination products and larger aggregates also observed (Hawe et al., 2009). Disulfide scrambling can also be created during sample preparation, as it is favoured at alkaline pH and high temperature (Lakbub et al., 2018), so a laboratory has to control its own conditions before attributing such species to the vial.

What impurity level is normal

The FDA label for Pitocin, a finished solution, states that it may contain up to 16% of total impurities, and that the hormone is made synthetically to avoid contamination with vasopressin and other small polypeptides (FDA prescribing information). The raw active ingredient is held tighter: the US, European, International, British and Japanese pharmacopoeia monographs summarised by Lambert and colleagues set total impurities for oxytocin API at not more than 5% (Lambert et al., 2020). A dry powder sold as oxytocin should be judged against the API figure, not the solution allowance.

Which tests matter most

LC-MSConfirms the 1006.44 Da monoisotopic mass and identifies deamidated forms, trisulfides and dimers by their masses.
HPLC-UVPubChem lists liquid chromatography with UV detection at 220 nm against standards as a chemical purity procedure for oxytocin.
Peptide mappingComparing reducing and non-reducing conditions confirms that Cys1 and Cys6 are joined to each other, not to another chain.
HRMSDeamidation at three possible sites produces small mass shifts that accurate mass resolves.

Reading oxytocin results

PubChem's 1007.2 g/mol average and 1006.44 Da monoisotopic mass (PubChem) describe the disulfide-bonded ring. In mass spectrometry a disulfide-linked species weighs 2 Da less than the sum of its free-thiol parts (Lakbub et al., 2018), so a main mass about 2 Da high suggests the ring is open.

Approved oxytocin is labelled by activity: the Pitocin label describes a standardised content of oxytocic hormone in units per mL (FDA prescribing information). A chemical amount test reports milligrams of peptide, so it compares directly with a weight label but not with a unit label.

Oxytocin broke down fastest at pH 9.0, then pH 7.0, pH 2.0 and pH 4.5, following (pseudo) first-order kinetics described by the Arrhenius equation (Hawe et al., 2009). For a dissolved product, formulation pH is part of what a result reflects. A dry, sealed powder kept cool is the cleanest sample to send.

Public Tested.ph results

No public oxytocin records yet

Each public oxytocin record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing oxytocin

What mass should an oxytocin certificate show?: PubChem lists oxytocin, C43H66N12O12S2, at an average molecular weight of 1007.2 g/mol and a monoisotopic mass of 1006.44 Da, and the FDA label for Pitocin gives 1007.19. These values describe the disulfide-bonded ring. A main mass about 2 Da higher suggests the ring has opened, and masses near double suggest disulfide-linked dimers.
Does oxytocin really degrade in heat?: Yes. A review of oxytocin quality in low- and middle-income countries describes every oxytocin formulation as heat-sensitive, and a 2019 WHO, UNFPA and UNICEF statement calls for 2 to 8 °C storage and distribution. A forced-degradation study found loss sped up with temperature and was fastest at pH 9.0, so shipping conditions show up in the result.
Is a purity below 95% always a bad sign for oxytocin?: For a powder, it is worth questioning. Pharmacopoeia monographs set total impurities for oxytocin active ingredient at not more than 5%. The FDA label for the Pitocin solution allows up to 16%, but that covers a finished liquid product. What matters next is which impurities are present: deamidated forms, sulfur-rearranged rings or dimers point to degradation or poor storage.
Can a lab create oxytocin impurities while testing it?: It can. Disulfide bonds can rearrange during sample preparation, especially at alkaline pH and high temperature, which is why published disulfide analysis methods prefer neutral or slightly acidic conditions. A careful laboratory prepares oxytocin samples to avoid this, and a good report states its conditions so that any rearranged species can be attributed to the vial.
Why is my oxytocin labelled in units instead of milligrams?: Approved oxytocin products are standardised by activity and labelled in units per mL. A chemical test measures milligrams of peptide instead. The two scales are not interchangeable without a stated basis, so a mass-based amount result compares directly only with a milligram label. If your product is labelled in units, ask the laboratory how it will report the result.

Sources

Oxytocin compound record (CID 439302), PubChem, US National Library of Medicine
Pitocin (oxytocin) prescribing information, US Food and Drug Administration
Hawe A et al. Towards heat-stable oxytocin formulations: analysis of degradation kinetics and identification of degradation products. Pharm Res. 2009;26(7):1679-88, Pharmaceutical Research (PMC)
Lambert P et al. Oxytocin quality: evidence to support updated global recommendations. J Pharm Policy Pract. 2020;13:14, Journal of Pharmaceutical Policy and Practice (PMC)
Lakbub JC et al. Recent mass spectrometry-based techniques and considerations for disulfide bond characterization in proteins. Anal Bioanal Chem. 2018;410(10):2467-2484, Analytical and Bioanalytical Chemistry (PMC)

Last reviewed 6 Oct 2026.