API Particle Size Analysis: Chemical Identification by Confocal Raman and TERS
API particle size analysis by confocal Raman and TERS is the determination of particle and domain size with associated chemical identification, at micro and nanometer scale. Unlike a conventional particle-size distribution, which measures the size of a population without confirming what each particle is made of, Percevia chemically identifies each domain, resolving below the optical diffraction limit when needed.
What is API particle size analysis by confocal Raman and TERS?
A conventional particle-size distribution, by laser diffraction, sieving or dynamic light scattering, measures the size of a population of particles, but does not report what each particle is made of. In a formulation with an API and multiple excipients, that measurement does not chemically separate which fraction of the result corresponds to the active and which to each excipient.
Confocal Raman microscopy answers that question by pairing size with chemical identity: each observed particle or domain is compared against reference vibrational signatures of the API and excipients, allowing size to be measured specifically for particles confirmed as the active. TERS extends that capability to the nanometer scale, below the optical diffraction limit of conventional confocal Raman, when the domains of interest are too small to be resolved by optical microscopy.
Feasibility depends on the components' Raman response, matrix fluorescence, the size and concentration of the domains of interest and sample preparation. Before promising a particle-by-particle measurement, Percevia evaluates the analytical question, the matrix and the ability to distinguish the relevant signatures. The report separates observation, processing and interpretation so the conclusion does not imply greater precision than the data support.
What chemically identified particle size reveals, and why it matters
An aggregate size distribution can hide which fraction of the result is actually API, particularly when the active and an excipient share a similar size range.
| What is observed | What it can indicate | Practical consequence |
|---|---|---|
| Particle size confirmed as API, not excipient | Chemical identity associated with each measured particle, separating what is active from what is excipient within the same size range. | Avoids attributing to the API a size result that actually corresponds to an excipient particle. |
| Domains below the optical diffraction limit | The presence of nanometer-scale domains not resolved by conventional confocal Raman or by laser-diffraction techniques. | Can indicate that a bulk particle-size technique underestimates or misses a fine fraction relevant to dissolution. |
| Correlation between particle size and dissolution | API domains larger or smaller than expected change the surface area exposed to the dissolution medium. | Can explain a dissolution-profile deviation that does not show up in the aggregate particle-size distribution. |
| Micronization verification | Confirmation that a micronization process actually reduced the size of the API particles, not just of excipient aggregates. | Supports a process decision without relying solely on a physical size measurement with no chemical identity attached. |
| Size consistency across batches or products | Comparison of particle size and chemical identity across batches, or between a reference product and a generic. | Can signal an equivalence risk when API particle size differs even with a comparable bulk distribution. |
Particle-by-particle measurement by confocal Raman and TERS does not replace a statistical particle-size distribution over a large population; it answers a different question, about the chemical identity associated with the size of specific particles or domains.
From individual particle to decision
Method choice depends on the components' Raman response, the scale of the particles or domains of interest and the analytical question. No technique enters the scope without answering a question the feasibility screening raised.
| Technique | What it measures | What decision it supports |
|---|---|---|
| Reference spectra | The vibrational signature specific to the API and the excipients. | Whether particles of different components can be chemically distinguished under the study conditions. |
| Particle-by-particle confocal Raman microscopy | The size, shape and chemical identity of individual particles and domains, at micron scale. | Which fraction of the observed distribution corresponds to the API and which to each excipient. |
| TERS · Tip-Enhanced Raman Spectroscopy | Chemical identity of domains at nanometer scale, below the optical diffraction limit. | Whether particles or domains too small to be resolved by conventional confocal Raman are in fact API. |
| Correlation with optical or electron microscopy | The morphology and physical dimension of the observed particle, correlated to the position of each collected spectrum. | Whether the measured physical size matches the chemically identified particle, rather than a sample artifact. |
The analytical workflow
Every sample runs through the same logical sequence, with resolution, scale and the need for TERS adjusted to the question and the matrix; not every sample requires a nanometer-scale measurement.
Feasibility screening
Assessment of the matrix, known components, fluorescence and the expected size range, and of the analytical question, to define whether a particle-by-particle measurement can answer it, and whether TERS is needed.
Feasibility opinion and study design
Reference spectra characterization
Collection of spectra from the API and the excipients, to confirm the relevant signatures can be distinguished under the real sample conditions, before any size measurement.
A reference spectral library for the sample
Particle-by-particle acquisition, with TERS when needed
Collection of spectra from individual particles and domains across the defined area and resolution, with TERS when the domains of interest fall below the optical diffraction limit of conventional confocal Raman.
Size and chemical identity per particle or domain observed
Interpretation and comparison
Comparison across regions, samples or batches included in scope, with an explicit discussion of the resolution, depth and identification limitations the method imposes.
An interpreted report, with limitations stated
Applications
The same particle-level chemical-identification capability supports distinct investigations, from development through production troubleshooting.
Micronization
Post-micronization particle-size verification
Confirming that a micronization process actually reduced the size of API particles, not just of excipient aggregates present in the same size range.
Dissolution
Dissolution-profile deviation investigation
Correlating chemically confirmed API particle size with an unexpected dissolution result.
Batch comparison
Reference product, prototypes and batches
Comparing particle size and chemical identity between a reference product and a generic, prototype or development batch.
Analytical complement
Support for conventional particle-size distribution
A particle-level chemical-identification layer that complements laser-diffraction or sieving results, when the question is which fraction is API, not only the size of the population.
What you receive
An interpreted technical report, not a set of loose spectra and images.
Data and evidence
- Size and chemical identity per particle or domain analyzed
- Representative spectra per component and per particle of interest
- Reference spectra used to differentiate components
- A description of sample preparation, acquisition parameters and TERS use, where applicable
Interpretation and conclusions
- A distinction between particles confirmed as API and excipient particles within the same size range
- Comparison across regions, samples or batches included in scope
- Discussion of spatial-resolution, nanometer-scale and component-identification limitations
- Practical implications for dissolution, micronization or process investigation, where applicable
Scope and turnaround
Scope is set by the analytical question: a feasibility screening is enough for some samples, while a nanometer-scale measurement with TERS requires a broader acquisition design.
- Feasibility screening before any particle-by-particle measurement is promised
- Resolution scale and the need for TERS set in the analytical plan, not from a fixed parameter list
- Results expressed as size and chemical identity per particle or domain, not as a statistical distribution over a population
- Comparison across regions, samples or batches as defined in scope
- Suited to micronization verification, dissolution investigation and product comparison
Frequently asked questions
What is the difference between this analysis and a conventional particle-size distribution?
- A conventional particle-size distribution, by laser diffraction, sieving or dynamic light scattering, measures the size of a population of particles without confirming what each particle is made of. This analysis chemically identifies each measured particle or domain, specifically answering which fraction of the observed size corresponds to the API.
Does this analysis replace laser-diffraction particle-size characterization?
- No. The two are complementary. Laser diffraction is more efficient for characterizing the statistical size distribution of a large particle population. Confocal Raman and TERS analysis is indicated when the question requires chemically confirming which fraction of that distribution is API, or measuring domains below the resolution limit of laser diffraction.
What is the difference between this analysis and the API spatial distribution page?
- Spatial distribution answers where the API is located within a formulation, producing a chemical map of an area. Particle size answers how big individual particles or domains are and whether each one is API or excipient. The two questions use similar instrumentation and often complement each other within the same study.
What is TERS, and when is it needed?
- TERS (Tip-Enhanced Raman Spectroscopy) extends the technique's spatial resolution to the nanometer scale, below the optical diffraction limit of conventional confocal Raman. It is indicated when the particles or domains of interest are too small to be resolved and chemically identified by standard confocal mapping.
Does this technique measure the size of every particle in a sample?
- No. It is a particle-by-particle measurement, applied to a number of particles or domains defined in the study scope, not a statistical scan of an entire population. The study design defines how many particles and regions are analyzed, based on the question.
How does the analysis help verify a micronization process?
- By confirming that the particles effectively reduced in size are indeed API, not excipient aggregates that fall within the same size range measured by a bulk technique. This avoids concluding that micronization worked based on a measurement that does not distinguish components.
Which samples can be evaluated?
- Solid formulations, powders and other compatible matrices may be considered. Geometry, surface, fluorescence, laser stability and sample preparation must be assessed before method definition, which happens at the feasibility-screening stage.
How long does a particle size study by confocal Raman and TERS take?
- It depends on the number of particles analyzed, whether TERS is needed and the number of samples compared, set after the feasibility screening. Turnaround is set in the proposal, after that initial stage.
Is the information submitted handled confidentially?
- Yes. Scope, samples, results and the existence of the study are treated as client confidential information, under a confidentiality agreement signed before samples are sent.
Services that complement particle size analysis
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Scope is set by the analytical question: a feasibility screening is enough for some samples, while a nanometer-scale measurement with TERS requires a broader acquisition design.
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