Nitrite and Nitrate Analysis in Pharmaceutical Excipients and Drug Products
Trace-level analysis to support nitrosamine risk assessment and excipient control
Nitrite impurities in pharmaceutical excipients have become an important consideration in the assessment and control of N-nitrosamine impurities in medicinal products. Even when present at very low concentrations, nitrite can act as a nitrosating precursor and react with susceptible amines in an active pharmaceutical ingredient (API), API impurity or degradation product. This can result in the formation of small-molecule nitrosamines or nitrosamine drug substance-related impurities (NDSRIs) during drug-product manufacture or storage.
Excipia provides sensitive quantitative analysis of nitrite and nitrate in pharmaceutical excipients, raw materials and, where appropriate, drug products. Methods can be developed and qualified for trace-level determination down to the low ppb range where required by the matrix and project objectives.
The analysis can be used to support nitrosamine risk assessments, compare excipient suppliers and batches, investigate potential sources of nitrosating species, establish raw-material controls and support formulation or process changes intended to reduce nitrosamine formation.
Why is nitrite in pharmaceutical excipients important?
Nitrosamine formation generally requires both a nitrosating species and a susceptible amine. In drug products, residual nitrite present in excipients can provide the nitrosating component.
The US FDA specifically identifies nitrite impurities in excipients as a potential source of nitrosamine formation. Its current guidance states that nitrites have been reported in many excipients at parts-per-million levels and that concentrations may vary between excipient lots and suppliers. Consequently, FDA recommends that supplier qualification programmes take potential nitrite impurities into account.
This is particularly relevant for APIs or API-related compounds containing secondary amines or other structural features susceptible to nitrosation. Nitrosamine formation does not necessarily occur during manufacture alone. Reactions between residual nitrite and vulnerable amines may continue during storage, meaning that apparently small differences in excipient nitrite content can become relevant to the final drug-product impurity profile.
Nitrite determination therefore provides information about nitrosamine formation potential, rather than simply measuring the nitrosamine after it has formed.
Nitrite, nitrate and nitrosamine formation
Nitrite (NO₂⁻) and nitrate (NO₃⁻) are related inorganic nitrogen species, but their relevance to nitrosamine formation is different. Nitrite is the principal concern because, particularly under acidic conditions, it can generate nitrous acid and other reactive nitrosating species. These can react with susceptible secondary amines and certain other nitrogen-containing compounds to form N-nitrosamines.
A simplified representation is:
Nitrite + acid → nitrosating species
Nitrosating species + susceptible amine → N-nitrosamine
The actual reaction in a pharmaceutical formulation is considerably more complex. The rate and extent of nitrosamine formation can depend on nitrite concentration, API structure, amine concentration, water activity, formulation microenvironment, pH, temperature, manufacturing conditions and storage time.
Nitrate is generally much less reactive as a direct nitrosating agent. Nevertheless, determining nitrate alongside nitrite can provide useful information on the origin and overall profile of inorganic nitrogen species in an excipient. It may help identify raw-material or manufacturing-process differences and can be useful during comparative supplier investigations.
How much nitrite is present in pharmaceutical excipients?
There is no single representative nitrite concentration that can be assigned to an excipient solely from its compendial name. Published pharmaceutical-industry data demonstrate substantial differences between excipient types, individual manufacturers and batches. The collaborative Nitrites in Excipients database, described by Boetzel and co-workers, was developed specifically to provide experimental information for pharmaceutical nitrosamine risk assessment. The published dataset demonstrated that both the average nitrite concentration and batch-to-batch variability differ between excipients and that supplier-to-supplier differences can be significant. (Boetzel et al., Journal of Pharmaceutical Sciences, 2023)
This distinction is important. A statement that an excipient complies with the Ph. Eur., BP, USP or another pharmacopoeial monograph does not necessarily establish its suitability with respect to nitrite for a particular nitrosamine-sensitive formulation. Conventional pharmacopoeial compliance and nitrosamine risk are different questions.
For example, published data for microcrystalline cellulose (MCC) have demonstrated nitrite concentrations varying between suppliers and batches. Because MCC may constitute a substantial fraction of an oral solid dosage form, even relatively low nitrite concentrations can make it an important contributor to the total nitrite burden of the formulation.
The same principle applies to other high-load excipients. The risk associated with an excipient is determined not only by its nitrite concentration but also by how much of that excipient is present in the formulation.
Why is ppb-level nitrite analysis important?
Nitrite concentrations in excipients are frequently discussed in ppm or µg/g. However, a sufficiently sensitive analytical method is needed to distinguish genuinely low-nitrite materials and to support supplier or batch selection.
For example:
1 ppm = 1 µg/g = 1,000 ng/g
Consequently, an analytical capability in the low ppb range can provide considerably more information than a conventional limit test when materials with very low residual nitrite concentrations need to be differentiated.
This is especially useful when the objective is to select a low-nitrite excipient grade, investigate batch-to-batch variability or establish whether a formulation change has meaningfully reduced the available nitrite burden.
Excipia therefore develops the analytical approach according to the required decision limit rather than applying a single generic detection limit to every matrix.
Nitrite and nitrate analysis at Excipia
Excipia uses an in-house developed HPLC method for the quantitative determination of nitrite and nitrate in pharmaceutical materials. The sample preparation and analytical conditions can be adapted to the excipient or drug-product matrix and the sensitivity required for the investigation.
Projects may include analysis of a single excipient, comparison of multiple excipient suppliers, batch-to-batch variability studies, analysis of complete formulations or investigations supporting an established nitrosamine risk.
The analytical procedure is selected and optimised according to the sample matrix and required sensitivity. Particular attention is paid to sample preparation because extraction recovery, matrix effects, reagent background and contamination can become significant when measurements are performed at trace concentrations.
Where required, method performance can be demonstrated through assessment of specificity, calibration behaviour, recovery, repeatability, limit of detection (LOD) and limit of quantification (LOQ).
Results can be reported as µg/g (ppm), ng/g (ppb), µg/kg or another appropriate concentration unit, together with the applicable analytical reporting limit.
Analytical scope
Analytes:
Analytical technique:
Sample types:
Sensitivity:
Applications:
Nitrite (NO₂⁻) and nitrate (NO₃⁻)
In-house developed HPLC method
Pharmaceutical excipients, raw materials and selected drug products
Low-ppb determination where required and demonstrated for the specific matrix
Nitrosamine risk assessment, supplier and batch comparison, excipient qualification and formulation investigations
Analysis of nitrite in excipients
Excipient analysis is one of the principal applications of the service.
A nitrite investigation can be performed on materials such as cellulose derivatives, microcrystalline cellulose, lactose and other sugars, starches, superdisintegrants, inorganic excipients, polymers, lubricants and other formulation components. Because nitrite levels can depend on raw materials and manufacturing processes, testing can be particularly valuable when qualifying a new supplier or comparing nominally equivalent grades.
Rather than asking only:
“Does this excipient contain nitrite?”
a pharmaceutical development programme can determine:
“How much nitrite does each excipient contribute to the drug product, and which supplier or grade provides the lowest practical nitrite burden?”
This provides a much stronger basis for formulation and supplier decisions.
Comparing excipient suppliers and batches
Regulatory guidance increasingly recognises that excipient nitrite content cannot always be treated as a fixed material property. Published datasets show differences between suppliers as well as batch-to-batch variability. Manufacturing route, raw materials, water quality, drying conditions and other process parameters may influence the residual nitrite level. A comparative Excipia study can therefore analyse several suppliers or batches under the same analytical procedure and sample-preparation conditions.
This allows direct comparison of the materials and can support:
- supplier qualification
- selection of a lower-nitrite excipient grade
- evaluation of alternative sources
- investigation of batch variability and
- development of an appropriate incoming-material control strategy.
Calculating the nitrite contribution of a formulation
The concentration of nitrite in an excipient should be interpreted together with the amount of that excipient in the formulation. For an excipient containing C ppm nitrite and present at M mg per dosage unit, its theoretical nitrite contribution can be estimated from:
Nitrite contribution (ng/dosage unit) = C (µg/g) × M (mg)
For example, an excipient containing 0.5 µg/g nitrite and used at 200 mg per tablet contributes approximately:
0.5 × 200 = 100 ng nitrite per tablet
This does not mean that 100 ng of a nitrosamine will form. Nitrosamine formation depends on reaction chemistry, conversion efficiency, availability of a susceptible amine and the formulation and storage environment. The calculation is nevertheless useful for identifying which excipients dominate the potential nitrite burden.
Excipia can combine analytical results with the quantitative formulation composition to produce an excipient-by-excipient nitrite contribution assessment.
Nitrite analysis in finished drug products
In many projects it is more informative to analyse the individual excipients because this directly identifies potential sources of nitrite and facilitates supplier selection. However, analysis of the finished drug product can also be relevant. Drug-product testing may be useful when investigating an observed NDSRI, comparing formulations, studying nitrite availability after processing, evaluating development or stability batches, or investigating whether manufacturing and storage influence the nitrite/nitrate profile.
Drug products represent more complex analytical matrices than individual excipients. Method suitability therefore has to be established for the particular formulation. Nitrite analysis of the drug product can also be combined with direct nitrosamine analysis or other targeted investigations to obtain a more complete understanding of the formation pathway.
From nitrite concentration to nitrosamine risk
A nitrite result should not normally be interpreted in isolation. A scientifically useful nitrosamine risk assessment considers at least three components:
Nitrosating agent availability × susceptible amine availability × conditions permitting nitrosation
Nitrite analysis addresses the first component. If an API contains a nitrosatable amine, or if a susceptible amine is present as an API impurity or degradation product, the measured nitrite concentrations can be used to refine the theoretical risk assessment and determine which excipients make the greatest contribution. This is particularly relevant for NDSRIs, which share structural similarity with the API and may form directly from nitrosation of the active substance or API-related species.
Regulatory relevance
FDA guidance
The FDA’s Control of Nitrosamine Impurities in Human Drugs guidance identifies nitrite in excipients as a potential source of nitrosamine formation in drug products. FDA notes that nitrite impurities have been reported in commonly used excipients and can vary across suppliers and lots. The agency specifically recommends consideration of potential nitrite impurities within supplier qualification programmes. For products in which NDSRIs are detected, FDA identifies screening excipients and, where appropriate, reformulation using excipients with lower nitrite concentrations as potential mitigation strategies.
FDA has also highlighted formulation strategies such as antioxidants and modification of the formulation microenvironment where scientifically appropriate.
EMA and European regulatory expectations
The European regulatory framework similarly requires marketing authorisation holders to evaluate the potential presence of nitrosamines and to perform confirmatory testing when a risk is identified. EMA’s current nitrosamine guidance covers risk evaluation, confirmatory testing and implementation of appropriate controls. Nitrosamine control therefore extends beyond direct analysis of the nitrosamine itself and requires understanding of potential sources and formation mechanisms within the API, excipients, manufacturing process and drug product.
European Pharmacopoeia
The European Pharmacopoeia has recognised the increasing importance of excipients in nitrosamine risk. The European Pharmacopoeia Commission has added general chapter 2.5.48, Nitrites in excipients, to its work programme. The objective is to provide standard analytical procedures for determining nitrite in excipients and thereby improve the reliability and comparability of these measurements. This development is particularly significant because it confirms that residual nitrite is increasingly being treated as an important excipient quality attribute when relevant to nitrosamine formation. The Ph. Eur. general monographs Substances for pharmaceutical use (2034) and Pharmaceutical preparations (2619) also address the broader issue of N-nitrosamine contamination.
British Pharmacopoeia
The British Pharmacopoeia incorporates Ph. Eur. requirements where applicable and contains compendial procedures and individual monograph tests relating to nitrate and nitrite. Traditional pharmacopoeial nitrate or nitrite limit tests, however, were generally developed for demonstrating compliance with a predefined specification rather than for modern trace-level nitrosamine risk assessment. A project intended to distinguish very low nitrite concentrations between excipient suppliers may therefore require a substantially more sensitive quantitative method than a conventional pharmacopoeial limit test.
Key regulatory sources: FDA, Control of Nitrosamine Impurities in Human Drugs; EMA, Nitrosamine impurities: guidance for marketing authorisation holders; European Pharmacopoeia, General Chapter 2.5.48 Nitrites in excipients (work programme).
Pharmacopoeial compliance does not necessarily mean low nitrite
This distinction is important for both excipient manufacturers and drug-product developers. An excipient can comply fully with its pharmacopoeial monograph while batches from different manufacturers contain different trace concentrations of nitrite. The absence of a nitrite specification in an individual excipient monograph should therefore not automatically be interpreted as evidence that nitrite is absent or irrelevant. The appropriate question is whether the nitrite concentration is sufficiently controlled for the specific formulation and its nitrosamine risk.
When should nitrite testing be considered?
Nitrite analysis can provide particularly useful information when an API contains a potentially nitrosatable amine, an NDSRI has been detected during development or stability studies, excipients are identified as potential nitrite sources during a nitrosamine risk assessment, suppliers or grades need to be compared, or a manufacturer wants to demonstrate that a formulation change reduces the nitrite burden. Testing may also be valuable when published excipient data are being used in a risk assessment. Literature databases are excellent tools for establishing whether an excipient class is potentially relevant, but they cannot establish the concentration in the specific supplier, grade and batch used in a medicinal product.
Actual analytical data provide a stronger basis for product-specific assessment.
Excipient screening for nitrosamine risk
A practical investigation often starts with the quantitative composition of the formulation. Excipia can review the formulation and identify the excipients most likely to contribute meaningfully to the total nitrite burden. High-load excipients are generally particularly important because a relatively low nitrite concentration multiplied by a large amount of material can result in a greater contribution than a higher concentration in a minor formulation component.
Selected excipients can then be analysed individually. The measured concentrations are combined with formulation loading to calculate the approximate nitrite contribution of each material. This creates a nitrite mass-balance or contribution profile for the formulation and allows analytical effort to be focused on the components that matter most.
Supporting excipient selection and formulation development
Once the major nitrite contributors have been identified, alternative suppliers, grades or batches can be evaluated. This approach can support formulation optimisation without necessarily changing the qualitative composition of the drug product. In some cases, selecting a functionally equivalent excipient with a consistently lower nitrite concentration may reduce the theoretical nitrosamine formation potential while minimising the impact on product performance. Because Excipia specialises in excipient characterisation and pharmaceutical reverse engineering, chemical nitrite data can also be considered together with relevant functional properties of the excipient. This is important when a lower-nitrite alternative must remain comparable in properties such as particle size, molecular weight, viscosity, substitution, moisture behaviour or compaction performance.
Typical study design
A nitrite/nitrate investigation can be tailored to the development question. For supplier qualification, several suppliers and multiple batches per supplier can be analysed to determine both average concentrations and batch variability. For formulation risk assessment, the individual excipients can first be screened and their measured nitrite concentrations converted into a theoretical contribution per dosage unit. For root-cause investigations, excipients, API, intermediate blends and finished drug products can be compared to determine where the relevant nitrogen species enter or change within the process.
Where stability-related nitrosamine formation is suspected, nitrite/nitrate results can be interpreted alongside nitrosamine concentrations from initial and stability samples.
Why work with Excipia?
Excipia focuses specifically on the analytical characterisation of pharmaceutical excipients and complex drug-product matrices.
Nitrite and nitrate analysis can therefore be integrated into a broader investigation rather than treated as an isolated analytical test. Depending on the project, this may include excipient characterisation, supplier comparison, deformulation, trace impurity analysis, formulation assessment and investigation of nitrosamine formation mechanisms.
Our objective is not simply to report a concentration, but to generate analytical data that can be used to answer the underlying pharmaceutical question:
Where does the nitrite come from, how much does each material contribute, and what can be changed to reduce the risk?
Frequently asked questions
Why is nitrite measured in pharmaceutical excipients?
Nitrite can act as a nitrosating precursor. In formulations containing susceptible amines, residual nitrite from excipients may contribute to the formation of N-nitrosamines, including NDSRIs, during manufacture or storage.
Is nitrite the same as a nitrosamine?
No. Nitrite is an inorganic ion and a potential precursor to nitrosamine formation. Nitrosamines are separate chemical compounds formed through nitrosation reactions involving suitable nitrogen-containing compounds.
Why measure nitrate as well as nitrite?
Nitrite is normally the more direct nitrosamine risk factor. Nitrate measurement can nevertheless provide additional information about raw materials, manufacturing processes and the overall inorganic nitrogen profile of an excipient.
Are nitrite levels specified in pharmacopoeial excipient monographs?
Not universally. The absence of a specific nitrite limit does not demonstrate that an excipient contains no nitrite. The European Pharmacopoeia has recognised this analytical need and has added general chapter 2.5.48 Nitrites in excipients to its work programme.
Can two pharmacopoeial-grade excipients have different nitrite concentrations?
Yes. Published pharmaceutical-industry data demonstrate differences between excipient types, suppliers and batches. Both materials may comply with the same pharmacopoeial monograph while having different trace nitrite profiles.
Can Excipia measure nitrite at ppb levels?
Methods can be developed and optimised for trace-level determination, including the low ppb range where required and technically achievable for the sample matrix. The achievable LOD and LOQ are established for the specific analytical method and material rather than assumed from a generic instrument sensitivity.
Should every excipient in a formulation be tested?
Not necessarily. A risk-based approach can first consider excipient type, published nitrite data, supplier information and the amount of each excipient present in the formulation. Analytical testing can then focus on the materials that are most likely to contribute significantly to the overall nitrite burden.
Can nitrite testing replace nitrosamine testing?
No. Nitrite analysis and nitrosamine analysis answer different questions. Nitrite analysis evaluates an important risk factor and potential source, whereas direct nitrosamine analysis determines whether a particular nitrosamine is actually present. The two approaches can be complementary.
Can nitrite analysis support supplier qualification?
Yes. Comparing multiple batches from different suppliers under identical analytical conditions can provide product-specific evidence for selection and qualification of lower-nitrite excipients.
Regulatory Guidance and Scientific References
FDA – Control of Nitrosamine Impurities in Human Drugs
EMA – Nitrosamine impurities: guidance for marketing authorisation holders
European Pharmacopoeia – Work Programme, General Chapter 2.5.48 Nitrites in excipients
Boetzel R. et al. – A Nitrite Excipient Database: A Useful Tool to Support N-Nitrosamine Risk Assessments for Drug Products, Journal of Pharmaceutical Sciences, 2022.
Nitrite and nitrate testing for your formulation
Excipia can support projects ranging from the analysis of a single excipient to a complete investigation of the nitrite contribution of a pharmaceutical formulation.
A study can include excipient screening, supplier and batch comparison, quantitative nitrite and nitrate analysis, calculation of the nitrite contribution per dosage unit and interpretation of the results within a wider nitrosamine risk assessment.
Contact Excipia to discuss nitrite and nitrate analysis of your excipients or pharmaceutical formulation.
Welcome to Excipia, a service platform of Avivia, dedicated to the composition, quality and quantity of excipients in medicinal products.
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Menno Wiltink
Founder of Excipia.
