Nitrosation Assay Procedure (NAP Test): How to Assess the Nitrosation Potential of Amine-Containing APIs

Introduction:

The Nitrosation Assay Procedure (NAP Test) is an experimental approach for investigating the nitrosation potential of amine-containing active pharmaceutical ingredients (APIs). As manufacturers strengthen their assessment of nitrosamine formation risk in drug products, knowing whether a drug substance can react with nitrosating species has become important for impurity control, process development and regulatory readiness.

Nitrosamines can raise serious toxicological concerns depending on their structure and exposure level. Certain nitrosamines are classified as probable or possible human carcinogens, and regulators have set risk-based approaches for evaluating and controlling them. Not every amine-containing API carries the same risk, because molecular structure, steric accessibility, reaction conditions, nitrite sources and competing reactions all shape the outcome.

A properly designed NAP test investigates this chemical potential experimentally instead of relying only on structural prediction. Combined with manufacturing knowledge, impurity profiling and toxicological assessment, the results support better-informed risk management. This article covers the purpose of the test, its chemistry, the workflow, analytical techniques, interpretation, common challenges and its regulatory role.

Summary:

• The Nitrosation Assay Procedure (NAP Test) is a worst-case, in vitro stress test that shows whether an amine-containing API can be converted into an N-nitrosamine or NDSRI.
• The classic protocol, proposed by a WHO expert group in 1978, uses 10 mM drug, 40 mM sodium nitrite, pH 3–4, 37 °C, for 1–4 hours.
• A negative result under these harsh conditions strongly suggests the compound is unlikely to nitrosate under normal manufacturing and storage conditions.
• Secondary amines react far faster than tertiary amines, so structure drives priority.
• Modified NAP protocols (organic solvent, tert-butyl nitrite, lower nitrite excess) address solubility and artefact limits of the classic test.
• Results are read by LC-MS/MS or HRMS and should feed your formal risk assessment, not replace it.
• Working with an experienced analytical partner reduces false conclusions and speeds regulatory responses.

---

1: What Is the Nitrosation Assay Procedure (NAP Test)?

The Nitrosation Assay Procedure (NAP Test) is a standardized laboratory stress test that exposes an amine-containing compound to excess nitrite under acidic, warm conditions to find out whether it forms N-nitroso products. It was proposed by a WHO expert group in 1978 as a general in vitro system for ranking how readily drugs can be nitrosated, and it has regained importance because of nitrosamine drug substance-related impurities (NDSRIs).

In practice, the NAP test answers one question: can this molecule form a nitrosamine at all, and if so, which one and how much? That answer helps decide whether confirmatory testing, process changes or control strategies are needed.

A NAP study typically investigates:

• The susceptibility of the API’s functional groups to nitrosation.
• The potential formation of API-related N-nitrosamines, also called nitrosamine drug substance-related impurities (NDSRIs).
• The influence of selected reaction conditions on impurity formation.
• The identity and approximate or quantitative level of suspected nitrosation products.
• The need for confirmatory testing, process changes or additional analytical controls.

Important distinction: no single experimental recipe suits every API. The classic conditions below are a useful benchmark, but the design must reflect the compound’s chemistry and the question being asked. FDA guidance recognizes targeted forced degradation under nitrosating conditions as one way to investigate whether a nitrosamine can form from an API or its fragments, and the NAP test is used within a broader risk assessment, not as a standalone compliance test.

---

2: Why Does the NAP Test Matter for Amine-Containing APIs?

The NAP test matters because it replaces theoretical prediction with experimental evidence about a specific API. Structure-based risk assessments can flag a secondary amine, but they cannot always say whether nitrosation actually occurs, which product forms, or whether complex scaffolds react by other pathways instead.

Key reasons teams run it:

• Confirm or exclude predicted nitrosamines. Published work shows NAP data can rule out nitrosamines proposed purely from structure.
• Prioritize risk. Compounds that resist nitrosation under the harshest conditions are unlikely to form nitrosamines under typical processing or dry storage.
• Support regulatory dialogue. Agencies expect a documented, science-based risk evaluation. Acceptable intake limits are very low (for example, EMA’s default of 18 ng/day for compounds without specific data), so evidence matters.
• Generate reference material. Nitrosation products can guide the preparation of analytical standards for NDSRIs.

Note that nitrosation studies are not a universal mandatory requirement. They are a tool inside a broader risk-management workflow.

What Factors Influence Nitrosamine Formation?

Nitrosamine formation depends on the API’s structure, the nitrosating species, pH, temperature, time, the surrounding matrix and competing reactions. A useful NAP study considers these variables systematically.

• Chemical structure: electronic effects, steric hindrance and the location of nitrogen atoms affect susceptibility.
• Nitrosating species: nitrite and other agents behave differently depending on the environment.
• pH: acidity controls the formation and reactivity of nitrosating species.
• Temperature and reaction time: both influence kinetics and product stability.
• Solvent and matrix: water content, excipients and other components can alter the reaction.
• Competing reactions: oxidation, hydrolysis and degradation can consume reactants or create other products.
• API concentration and reagent ratio: these set the conditions under which potential formation is judged.

A good design separates deliberately harsh conditions from those that represent real manufacturing or storage.

---

3: What Are the Standard NAP Test Conditions?

The standard NAP test uses 10 mM API, 40 mM sodium nitrite, pH 3–4, 37 °C and a 1–4 hour reaction time. These conditions are deliberately aggressive: nitrous acid forms readily at low pH, the nitrite excess is large, and the temperature speeds the reaction.

Parameter	Classic NAP test condition
API concentration	10 mM
Sodium nitrite	40 mM (about 4-fold molar excess)
pH	3–4 (often adjusted to about 3.0–3.5 with HCl)
Temperature	37 °C
Duration	1–4 hours
Medium	Aqueous acidic solution
Analysis	LC-MS/MS or LC-HRMS (GC-MS for volatile nitrosamines)

Published implementations monitor and re-adjust pH after adding the sample, because the API itself can shift it. Control reactions without nitrite are also essential.

---

4: How Do You Perform a NAP Test Step by Step?

A NAP test is performed by preparing a buffered API solution, adding nitrite, incubating, quenching, and analyzing for nitroso products against controls. A typical workflow looks like this:

1. Define the target. List the amine sites and the nitrosamines they could form, including any NDSRI predicted by structure.
2. Prepare solutions. Dissolve the API at 10 mM in acidified water. Prepare a nitrite stock separately.
3. Adjust and verify pH. Bring the mixture to the target range and re-check after the API is added.
4. Initiate the reaction. Add sodium nitrite to reach 40 mM and incubate at 37 °C.
5. Sample over time. Pull aliquots at set points within 1–4 hours to see kinetics, not just an endpoint.
6. Quench. Stop the reaction promptly so results reflect the intended time point and no further nitrosation occurs during analysis.
7. Analyze. Use a validated, sensitive LC-MS/MS or HRMS method with suitable standards or accurate-mass confirmation.
8. Include controls. Run an API-only blank, a nitrite-only blank and, where possible, a known reactive amine as a positive control.
9. Interpret and document. Report formation, yield, identity confirmation and limitations.

Which Controls and Design Choices Make a NAP Test Defensible?

A defensible NAP test pairs a documented hypothesis with justified conditions and a full set of controls. Recommended controls include:

• An API control without the nitrosating challenge.
• A reagent or matrix blank.
• A suitable positive control or reference compound, where scientifically appropriate.
• Replicate preparations to measure experimental variability.
• Stability or recovery controls where degradation, adsorption or sample preparation could affect interpretation.

A harsh challenge reveals chemical susceptibility, but it should not alone be used to estimate how much impurity forms in routine manufacturing or over shelf life. Where useful, run a stringent challenge and a process-representative experiment side by side. Evaluate sampling and quenching carefully: if nitrosation continues after sampling, or a product decomposes in storage, the data will not reflect the intended time point.

---

5: Which Amines Are Most Likely to Nitrosate?

Secondary amines are the most likely to nitrosate, and acidic pH around 3–4 is the optimum range for the reaction. Tertiary amines can form nitrosamines only through slower dealkylation pathways. One modeling and experimental study found roughly two orders of magnitude lower nitrosamine formation from a tertiary amine than from its secondary analogue under matched conditions.

Amine type	Relative nitrosation risk	Comment
Secondary amine	High	Direct N-nitrosation; main NDSRI concern
Tertiary amine	Lower	Needs dealkylation first; often slower
Primary amine	Different pathway	Usually unstable diazonium products, not stable nitrosamines
Hindered or electron-poor amines	Variable	Steric and electronic effects can block reaction

Some complex amines never form the expected N-nitrosamine and instead undergo oxidation or C-nitrosation. This is why experiments matter.

This classification is a screening aid, not a definitive risk ranking. A tertiary amine should not automatically be treated as safe, and the presence of an amine alone does not prove that a problematic nitrosamine will form. APIs with several nitrogen sites may need each plausible pathway and product evaluated.

---

6: What Are the Limitations of the Classic NAP Test?

The main limitations are poor solubility of many modern APIs in water, artefacts from extreme conditions, and over-prediction relative to real-world risk. Because the test is intentionally harsh, a positive result does not mean a drug product will contain the nitrosamine; it means the molecule is capable of forming it.

• Solubility. Lipophilic APIs may not dissolve at 10 mM in aqueous acid.
• Artefacts. Nitrous acid can oxidize as well as nitrosate, producing degradants that complicate interpretation.
• Relevance. Early studies found that drug ranking in the NAP test can differ from ranking under conditions closer to the stomach.
• False reassurance risk. Some molecules fail to react under one condition but react under another, so one negative run should be interpreted in context.

What Other Challenges Arise in NAP Test Development?

The main additional challenges are separating real products from artifacts, achieving adequate sensitivity, confirming identity and demonstrating method suitability.

• Real products versus artifacts: a signal may come from an impurity already in the API, contamination, carryover, matrix interference or in-source fragmentation. Blanks, untreated controls, carryover checks and confirmatory experiments resolve these.
• Sensitivity: the method must reach a level suited to the risk assessment. Justify the target using the acceptable intake, maximum daily dose, sample preparation and dilution factors, not an arbitrary detection limit.
• Identity confirmation: accurate mass or a single mass transition can support an identity without proving it. Reference standards, fragmentation evidence and chromatographic behavior may be needed.
• Method suitability: when data support a regulatory quality assessment, the procedure should be validated for its intended use (specificity, accuracy, precision, linearity or response model, range, limit of quantitation, recovery and robustness, as applicable). FDA notes that its published nitrosamine methods must be validated by the user when results support a required quality assessment or submission.

---

7: What Is the Modified NAP Test and When Should You Use It?

The modified NAP test adapts the classic procedure for poorly soluble or sensitive APIs, typically by using an organic solvent and an organic nitrosating agent such as tert-butyl nitrite. In published work, the drug is dissolved in solvent and incubated at 37 °C with the nitrosating agent at about a 1:10 molar ratio, giving faster and cleaner conversion for many NDSRI cases.

An industry consortium also described three practical condition sets for challenging amines:

Condition set	Medium	Nitrite	Temperature
1	Amine with acetic acid (about one third of volume) in water	1.5 equivalents	20–25 °C
2	Amine with dilute HCl to pH 3–4	1.5 equivalents	20–25 °C
3	Amine free base in a suitable organic solvent	Organic nitrite	Per protocol

For molecules with several amine sites, sub-stoichiometric nitrite (0.1 or 0.5 equivalents) can reveal which site reacts first and avoid over-reaction. If no nitrosamine can be formed or isolated under these varied conditions, the resulting chemical understanding can help justify why the nitrosamine is not expected to arise.

---

Which Analytical Techniques Are Used in the NAP Test?

LC-MS/MS and high-resolution mass spectrometry (HRMS) are the most frequently useful techniques, because they give sensitive detection, structural information and targeted or untargeted screening. The best choice depends on the expected impurity, API properties, matrix and required reporting limit.

Technique	Role in NAP testing
LC-MS/MS	Targeted detection and quantification of known or suspected nitrosamine products
LC-HRMS	Accurate-mass screening and investigation of unexpected API-related impurities
GC-MS or GC-MS/MS	Analysis of suitable volatile or thermally stable nitrosamines
HPLC or UHPLC	Chromatographic separation before detection, usually coupled with mass spectrometry
NMR spectroscopy	Added structural characterization when enough material is available
Reference-standard comparison	Confirms identity through retention time, mass-spectral behavior and other characteristics

Why Is LC-MS/MS Useful for Nitrosamine Analysis?

LC-MS/MS selectively monitors a suspected impurity through precursor-ion and product-ion transitions, so with good separation, suitable standards and a fit-for-purpose method it supports sensitive, quantitative analysis. A mass transition alone does not establish identity, however: interferences, isobaric compounds and in-source fragmentation can mislead, so retention time, ion ratios, accurate mass or fragmentation evidence and reference-standard comparison should be weighed as appropriate.

When Should HRMS Be Considered?

HRMS should be considered when the reaction product is not known in advance. Accurate mass and isotope patterns help assess proposed formulas, and fragmentation studies support structural hypotheses. For an unknown NAP product, a typical workflow is:

1. Screen for features that increase after nitrosating exposure.
2. Compare treated samples with appropriate controls.
3. Review accurate-mass and isotope-pattern data.
4. Interpret fragmentation against plausible structures.
5. Confirm the leading hypothesis with an authentic standard or other orthogonal evidence.

Report a suspected product at an appropriate confidence level until the evidence supports definitive identification.

How Should NAP Test Results Feed the Risk Assessment?

NAP results should be one evidence line in a documented risk assessment that also considers nitrite sources, process conditions, excipients, packaging and acceptable intake. Regulators across regions expect confirmatory testing where risk is identified, using validated methods with limits of quantitation at or below the acceptable intake.

A practical decision path:

• No nitrosamine formed under NAP and modified conditions: document the evidence and justify low risk.
• Nitrosamine formed: identify it, quantify it, determine its acceptable intake (including CPCA-based categorization where applicable) and test real batches.
• Ambiguous or complex result: repeat under alternative conditions or with isotopic confirmation.

How Should NAP Test Results Be Interpreted?

Interpret NAP results using both the analytical evidence and the conditions under which the impurity formed. Detection under a forced challenge shows potential chemical susceptibility; it does not by itself show that the same impurity will occur in a marketed medicine or exceed an acceptable intake limit.

Observation	Possible interpretation	Recommended follow-up
No suspected product detected	Nothing detected within the method’s demonstrated capability under the tested conditions	Review sensitivity, target selection, recovery and whether the conditions addressed the risk hypothesis
Product detected only after forced exposure	The API may have a nitrosation pathway under challenge conditions	Confirm identity and assess relevance to manufacturing, formulation and storage
Confirmed nitrosamine at low levels	Formation demonstrated under the tested conditions	Quantify with a suitable method and compare exposure with the applicable limit
Product rises with time or challenge severity	Formation may be condition-dependent	Study kinetics and check whether conditions are process-representative
Unexpected peaks	Other reaction products or analytical artifacts	Investigate with controls, HRMS, fragmentation and further characterization

How Is the Significance of a Detected Nitrosamine Assessed?

Significance depends on the impurity’s identity, concentration, daily product dose, toxicological assessment and the applicable regulatory recommendation. For a confirmed impurity, estimated daily intake is:

Daily intake (ng/day) = Concentration (ng/g) × Daily product mass (g/day)

For example, a hypothetical nitrosamine at 20 ng/g with a daily product mass of 2 g gives 40 ng/day. This is an illustration, not a safety conclusion: the result must be compared with the compound-specific acceptable intake under the applicable regulatory framework.

How Does the NAP Test Support a Regulatory Risk Assessment?

The NAP test supplies experimental evidence for a nitrosamine risk assessment but does not replace the wider evaluation of manufacturing, formulation, degradation, contamination and exposure. FDA recommends a risk-based approach of risk assessment, confirmatory testing when a risk is identified, and mitigation. Health Canada likewise stresses evaluating potential sources across APIs, drug products, process materials and storage conditions. Check the current versions of each guidance before acting.

Results can help teams to:

• Support or refine a structural nitrosamine risk hypothesis.
• Identify potential API-related nitrosamines and degradation pathways.
• Decide whether targeted confirmatory testing is warranted.
• Establish or refine analytical controls and specifications when justified.
• Investigate sources of impurity formation, including through a nitrosamine root cause investigation for injectable drugs.
• Document the scientific rationale in the pharmaceutical quality system.

How Can Nitrosamine Formation Be Mitigated?

Mitigation should target the demonstrated or reasonably plausible root cause while preserving API quality, process performance and product stability. Common strategies are:

• Controlling nitrite and other precursors in raw materials, excipients, water and processing inputs.
• Reviewing reagent selection, reaction conditions and manufacturing sequence.
• Assessing recovered solvents and recycled materials where relevant.
• Evaluating formulation and packaging contributions, for example through nitrosamine leachables testing.
• Optimizing process parameters where evidence shows they influence formation.
• Adding analytical monitoring and acceptance criteria where justified.
• Reassessing risk after changes to materials, suppliers, processes or formulation.

No single strategy fits every product, so each change should be supported by evidence and checked for unintended effects on quality and stability.

How Does NAP Testing Apply Across Different Drug Classes?

NAP logic applies to any amine-containing API, but the practical questions differ by drug class and product type. These ResolveMass resources show how nitrosamine evaluation is approached in specific settings:

• Nitrosamine risk assessment for a proton pump inhibitor
• Nitrosamine testing for antidepressants and SSRIs
• Nitrosamine testing for antibiotics and beta-lactam APIs

---

Why Work With ResolveMass Laboratories Inc.?

ResolveMass Laboratories Inc. is a Canadian analytical CRO/CDMO whose scientists work daily with mass spectrometry, nitrosamine testing, biosimilar characterization and complex drug-delivery systems. Our approach to the Nitrosation Assay Procedure (NAP Test) is built on three principles:

• Scientific rigor. Every study includes controls, kinetic sampling, quenching verification and orthogonal identity confirmation.
• Transparent reporting. Reports state conditions, limits of detection, assumptions and limitations so reviewers can follow the logic.
• Regulatory awareness. Study design is aligned with current expectations of FDA, EMA and Health Canada for nitrosamine risk evaluation.

Depending on project requirements, our support for amine-containing APIs can include:

• LC-MS/MS analysis: targeted investigation of suspected nitrosamine impurities.
• High-resolution mass spectrometry: accurate-mass screening and characterization of unexpected reaction products.
• Analytical method development: separation, detection and quantification approaches for specific impurities.
• Impurity characterization: investigation of suspected API-related impurities using complementary evidence.
• Method validation and verification: evaluation of method performance for its intended quality-assessment purpose.
• Nitrosamine investigation support: analytical evidence that contributes to a scientifically justified risk assessment and mitigation strategy.

The right scope depends on the API’s chemistry, the suspected nitrosamine, available reference standards, target reporting limits and regulatory objectives. Project-specific capabilities and deliverables should be confirmed with the laboratory before a study starts.

---

Conclusion

A reliable conclusion needs more than detecting or missing a signal. The study must account for analytical specificity, product stability, reaction conditions, actual manufacturing and formulation risks and potential patient exposure, and confirmatory testing and mitigation decisions should follow the applicable regulatory framework using fit-for-purpose methods.

The Nitrosation Assay Procedure (NAP Test) remains one of the most practical ways to learn whether an amine-containing API can form an N-nitrosamine. Run correctly, with controls, kinetic sampling, sensitive mass spectrometric detection and appropriate modified conditions, it turns theoretical concern into usable evidence for your risk assessment and regulatory filings.

---

Frequently Asked Questions:

Reference

• Reiss K, Saiakhov R, Chakravarti S. (Q) SAR Approaches to Predict the Extent of Nitrosation in Pharmaceutical Compounds. Chemical Research in Toxicology. 2025 Feb 27;38(3):433.https://pmc.ncbi.nlm.nih.gov/articles/PMC11921035/
• Simonetto A, Monzani E, Razzetti G, Mantegazza S. Hunt for NDSRIs: Unveiling Hidden Threats with the Novel 15N-Enriched NAP Test. Organic Process Research & Development. 2025 Nov 14;29(12):3102-14.https://pubs.acs.org/doi/abs/10.1021/acs.oprd.5c00223
• Painginkar T, Sonawane TS, Bavadia R, Vasantharaju S, Sathyanarayana MB, Ravi GU. Navigating nitrosamines: origin, detecting, analysing and regulating impurities in pharmaceuticals. Int J Appl Phar. 2024;16:1-1.https://www.academia.edu/download/118196900/51376_1_.pdf
• Vikram PR H, Kandula DR, Gunta U, Kumar G, Deka R, Chiriki DS, Chethan KS, Bannimath N, Yadav T, Beeraka NM, Gurupadayya BM. NDSRIs crisis in pharmaceuticals; insights on Formation pathways, root causes, risk management, and novel analytical techniques. Current medicinal chemistry. 2025 Feb;32(6):1065-81.https://www.benthamdirect.com/content/journals/cmc/10.2174/0109298673322023240829081220
• Wichitnithad W, Angsuwattana P, Yu B, Thitikornpong W, Rojsitthisak P. Managing NDSRIs in Pharmaceuticals: Integrating Structural Risk Assessment and Mitigation Strategies. ACS Chemical Health & Safety. 2026 Apr 22;33(3):350-70.https://pubs.acs.org/doi/abs/10.1021/acs.chas.6c00012
• Mukherjee P, Yao X, Sitaraman S, Castelli J, Brudvig J, Ramdas S. Risk assessment and management strategy of two new NDSRIs in a pharmaceutical drug product for the treatment of a rare disease: From prediction to control. Journal of Pharmaceutical Sciences. 2025 Mar 1;114(3):1572-82.https://www.sciencedirect.com/science/article/pii/S0022354925000346

About the Author