Fourier transform infrared (FTIR) spectroscopy is widely valued for its speed and accessibility: place the sample, collect a spectrum, compare against a reference. In pharmaceutical environments, however, consistent performance depends on more than the spectrometer alone. The way the sample is presented to the IR beam has a decisive influence on the result.

This is why pharma teams sometimes hit a wall during transfer or validation, and encounter an FTIR method that fails to meet performance criteria. The instrument is qualified, the chemistry is robust, yet spectra drift because the sampling setup fails to adequately control critical variables such as path length, contact pressure, temperature or background conditions. Simply put, an FTIR spectrum is only as reliable as the sampling interface behind it.

This article is a guide for R&D leads, QA/QC managers and research scientists who need repeatable, compliant FTIR data across the (bio)pharmaceutical life cycle. We will focus on three recurring questions:

  • Where does FTIR fit in pharmaceutical control strategy?
  • How do you decide between ATR vs transmission modes in regulated pharma workflows?
  • How do you ensure a sampling interface that makes data robust, transferable and audit-ready?

To answer these questions, we will highlight five key action areas that enable teams to design robust, regulated FTIR workflows. Along the way, typical pharma sample types will be mapped to specialised accessories that make FTIR work reliable in practice.

1. Define the decision: identity, quantitation, structure or process trend

Before selecting ATR or transmission mode (or any accessory), define what the FTIR result is intended to decide, as that determines which variables you must control. Common FTIR decision points in pharma settings include:

  • Identity and verification: confirming raw materials, or excipient and counterfeit screening, typically using compendial mid-IR regions defined in pharmacopoeial methods.1
  • Quantitation and process trending: measuring concentration or monitoring blend uniformity and formulation consistency, where repeatable sampling geometry and path length control are critical.
  • Structural integrity: assessing protein and peptide secondary structure using characteristic amide I and amide II absorbance bands.2
  • Formulation development and troubleshooting: working with complex matrices can obscure weak spectral features, requiring sampling interfaces that minimise variability and improve robustness.

When sampling is engineered appropriately, FTIR can contribute at all stages from discovery and development through to manufacturing and compliance.

2. Choose the mode: ATR or transmission?

Choosing the right sampling interface is central to compliance: ATR and transmission can both be valid if you understand what each mode controls well, along with the potential sources of drift.

Transmission FTIR: defined path length and bulk sampling (strong for quantitation)

Transmission provides a clean, well-defined optical path and interrogates the entire sample volume, which is one reason it’s considered the gold standard when conditions allow (i.e., an optically suitable sample, with sufficient volume and low water content).

Where teams often struggle is operational reality: traditional transmission measurements can be sensitive to assembly variability, bubbles/voids, leakage and inconsistent cleaning practices. All of these factors may directly undermine repeatability.

ATR-FTIR: fast, low prep, tolerant of diverse samples (but contact matters)

Attenuated total reflectance (ATR) is widely used because it works well across solids, gels, pastes, viscous formulations and aqueous systems. It is inherently surface-sensitive as the technique only probes a few micrometres (up to 5 µm) into the sample, at the interface with the ATR crystal,3,4 so contact force and surface condition matter.5,6

Rule of thumb:

  • If you need bulk material quantitation or trend analysis and can reliably control optical path length, lean towards transmission.
  • If you need speed, minimal volume requirements, water tolerance or rugged screening across varied sample types, lean towards ATR, and engineer reproducible contact conditions.

3. Pick the sampling interface that controls your biggest risk

This is crucial for robust methods, and getting it right helps teams avoid downstream pain by making FTIR data defendable in audits. An essential practical point is that every drug has a unique FTIR signature, but this molecular fingerprint is only useful if your sampling interface makes it repeatable. Below are common pharma ‘failure modes’, mapped to accessories that best prevent them.

Microvolume or dilute samples

Multi-reflection ATR is a proven strategy if you’re working with scarce samples, such as early development lots or dilute biologicals, where transmission absorbance disappears into the noise. Specac’s Harrick ConcentratIR2 Multiple Reflection ATR Accessory is designed for micro-liquid samples as low as 2 µL, with optional heated/flow configurations up to 200 °C.

When paired with a 23-bounce extended silicon crystal, therapeutic peptides and proteins can be reliably analysed down to µg/mL levels, as shown in this Application Note. The ConcentratIR2 supports broad stability and folding assessments of high-value peptides, meaning it’s often the core enabler for a microvolume FTIR sampling strategy for biopharmaceuticals.

Viscous or sticky liquids

For many QC labs, the liquid cell is a day-to-day challenge. Poor path length repeatability and messy cleanup can quickly undermine trending and transfer. The Specac Pearl liquid transmission accessory is engineered for precisely controlled path length and easy handling, built around a rapidly interchangeable ‘Oyster Cell’ assembly with fixed path length options from 25–1,000 µm.

If you’ve encountered problems due to variable liquid cells, the Pearl’s reproducibility in transmission FTIR analysis becomes the difference between ‘works in R&D’ and ‘works in QC’, making repeatable liquid transmission more practicable for busy labs.

Reliable high-throughput workflows

For routine QC workflows, where speed, flexibility and reproducible sample pressure are all critical, single-reflection ATR with a consistent loading interface is the ideal setup.

The Quest ATR Accessory provides a reliable workhorse solution, featuring readily interchangeable crystal ‘pucks’ and a pressure tower to ensure repeatable load application.

Demanding solid samples

When working with abrasive, hard, corrosive, or other challenging solids, Specac’s Golden Gate Diamond ATR Accessory provides a highly robust sampling configuration for single-reflection work.

In pharmaceutical analysis, this robustness also enables reliable quantitative FTIR: mixtures of caffeine, ibuprofen and starch gave R2 values of 0.90–0.95 compared with the traditional but far more laborious KBr pellet method (see this Application Note). Exchangeable ‘anvils’ further allow the Golden Gate to be configured for air- or moisture-sensitive samples and volatile liquids.

Temperature-controlled and flow studies

If your challenge is temperature-dependent (e.g. degradation, solubility, melt transitions, conformational stability) or you need flow-through sampling for continuous analysis and cleaning cycles, Specac’s heatable transmission and flow cells are a direct fit. The range is designed for heating up to 250 °C (or cooling), with flow cell variants incorporating Swagelok connections to integrate with pumping systems.

Two high-value QC points to keep in mind:

  • FTIR can help identify residual solvents, degradation products or cross-contamination in relevant workflows, particularly when deployed with a controlled, reproducible interface and a clear decision framework.
  • In solid-state work, subtle spectral differences can distinguish polymorphs, such as peak splitting or small band shifts of 5–10 cm−1. This is crucial given that different crystal forms can significantly alter bioavailability and stability.

Those are the kinds of insights you don’t want masked by variable sample presentation.

4. For proteins and peptides: getting secondary structure data that stands up

If your problem relates to peptide/protein therapeutics, FTIR is well established for probing secondary structure,2,7 especially via the amide I/II bands from 1,700–1,500 cm−1. That’s why biopharma teams are increasingly using it for screening and stability readouts (like unfolding and aggregation), detecting structural changes in proteins using FTIR without extensive sample preparation.

Managing water is part of the method, not an afterthought

For aqueous formulations, the major practical constraint is managing water interference in mid-IR for proteins. Here, ATR’s low sample penetration depth greatly reduces water interference compared with transmission. One defensible approach is to collect a reference/buffer spectrum at the same temperature as the protein spectrum, then apply spectral subtraction and chemometrics to extract protein structural features.7–9

In protein and peptide FTIR, accessory choice intersects with method design:

  • Multiple-reflection ATR maintains signal for low concentrations and small volumes; here, the ConcentratIR2 is ideal for analysing peptide formulations via FTIR, down to µg/mL concentrations with ≤10 µL sample requirements (see our Infographic on GLP-1 therapeutics for a real-word example).
  • Temperature-controlled transmission/flow setups enable controlled ramping for conformational stability studies.

For R&D work, using D2O buffers shifts the O–D stretch away from amide I to reduce spectral overlap and enhance sensitivity, as demonstrated in an Application Note on temperature-driven conformational changes in N-Ras proteins.

5. Make it QC-ready: robust development, validation and audit defence

In practice, FTIR delivers most value in pharmaceutical control strategies where the workflow is engineered for control, rather than where uncontrolled sampling variables undermine the decision. If your goal is robust FTIR method development for QC release, expectations should be aligned with current life cycle guidance:

  • ICH Q14 describes science- and risk-based approaches to developing and maintaining analytical procedures across the pharmaceutical life cycle.10
  • ICH Q2(R2) sets expectations for validation characteristics and evaluation.11
  • USP 854 describes techniques and typical operating regions used for compliance.1

Why FTIR methods struggle in validation

When teams find an FTIR method isn’t holding up, the root causes are often practical and fixable:

  • Transmission with uncontrolled path length (e.g., cell assembly variability, bubbles, leakage).
  • ATR with variable contact pressure or inconsistent sample presentation.
  • Temperature mismatch (especially for biologicals).
  • Background and subtraction rules not written tightly enough for QC execution.

Building defensible FTIR methods

Making FTIR data defendable in audits relies on defining and controlling, in both the method and the SOP:

  • Sampling configuration (accessory model, crystal/window materials, path length for transmission, pressure mechanism for ATR).
  • Sample handling (mixing rules, volume window, fill/contact steps, cleaning procedures).
  • Temperature (set point, equilibration time, matched reference conditions).
  • Background strategy (‘blank’ definition, when collected and how often updated).
  • System suitability (reference material checks, acceptance criteria).
  • Cleaning verification and carryover controls.

The sample picks the accessory

A consistent theme to remember is that the sample picks the accessory: the right interface transforms raw spectra into usable, defensible information, while the choice of method picks the appropriate controls.

For teams with non-standard samples or persistent challenges, Specac’s highly experienced application scientists can help. We regularly collaborate on customised designs, whether it be for unusual sample formats, extreme conditions or specific material compatibility needs.

For a deeper look at accessory selection across the pharmaceutical development life cycle, including microvolume peptide/protein workflows, practical transmission setups for viscous liquids, rugged ATR for QC and temperature-controlled studies, watch our on-demand, expert-led webinar: Formulation to final product.

Need help selecting the right accessory for your most pressing pharmaceutical analysis challenge? Book a short technical consultation today: https://specac.com/book-application-appointment/

References

1        United States Pharmacopeia, <854> Mid-infrared spectroscopy, https://doi.org/10.31003/uspnf_m3208_04_01, (accessed January 2026).

2        A. Barth, Biochim. Biophys. Acta, Bioenerg., 2007, 1767, 1073–1101.

3        S. E. Glassford, B. Byrne and S. G. Kazarian, Biochim. Biophys. Acta, Proteins and Proteomics, 2013, 1834, 2849–2858.

4        H. Tiernan, B. Byrne and S. G. Kazarian, Spectrochim. Acta A, 2020, 241, 118636.

5        M. J. Jafari, F. G. Bäcklund, T. Arndt, B. Schmuck, G. Greco, A. Rising, A. Barth and T. Ederth, ACS Appl. Polym. Mater., 2023, 5, 9433–9444.

6        M. S. Bradley, Spectroscopy, 2025, 40, 27–29.

7        H. Yang, S. Yang, J. Kong, A. Dong and S. Yu, Nat. Protoc., 2015, 10, 382–396.

8        J. Kong and S. Yu, Acta Biochim. Biophys. Sin., 2007, 39, 549–559.

9        J. Titus, H. Ghimire, E. Viennois, D. Merlin and A. G. U. Perera, J. Biophotonics, 2018, 11, e201700057.

10      ICH, Analytical procedure development Q14, https://database.ich.org/sites/default/files/ICH_Q14_Guideline_2023_1130_ErrorCorrection_2025.pdf, (accessed January 2026).

11      ICH, Validation of analytical procedures Q2(R2), https://database.ich.org/sites/default/files/ICH_Q2%28R2%29_Guideline_2023_1130_ErrorCorrection_2025.pdf, (accessed January 2026).