Quick answer

An FTIR spectrophotometer in a pharmaceutical QC laboratory is calibrated with a polystyrene film of about 35 µm: you check the wavenumber scale against seven polystyrene bands and the resolution against two band pairs.

For FTIR calibration as per EP (Ph. Eur. 2.2.24), every one of the seven bands must read within ±1.0 cm^-1 on a Fourier-transform instrument, and the absorbance difference must exceed 0.33 between 2870 and 2849.5 cm^-1 and 0.08 between 1589 and 1583 cm^-1 [1]. USP <854>, the Japanese Pharmacopoeia and the Indian Pharmacopoeia use the same film but not identical test sets, and none of the texts reviewed here fixes a calibration frequency. That interval is your laboratory’s decision.

7polystyrene bands in Ph. Eur. Table 2.2.24.-1

±1.0cm^-1 FT tolerance, all seven bands

0.33 / 0.08minimum absorbance differences, FT resolution

Nonecompendial calibration frequency found

Six things most calibration pages get wrong

We checked the figures that circulate on Indian pharma blogs and training decks against the pharmacopoeial texts that could be retrieved in full. The European Pharmacopoeia text is the one quoted most, and also the one misquoted most. These six points are where a record built from a blog fails at audit.

±1.5 and ±2.0 cm^-1 are not FT tolerances. Ph. Eur. Table 2.2.24.-1 has two tolerance columns. The wider values (±1.5 at 3060.0 and 1942.9, ±2.0 at 2849.5 cm^-1) belong to monochromator (dispersive) instruments. For Fourier-transform instruments every band is ±1.0 cm^-1 [1]. Shimadzu’s note on Ph. Eur. 5.0 records that the FT values at those three bands were tightened from ±1.5 to ±1.0 in that edition [6], so older SOPs may still carry the superseded figure.
“Greater than 18” and “greater than 10” are not FT resolution limits. Those are percentage-transmittance differences for monochromator instruments. An FT instrument is judged in absorbance: more than 0.33 (2870 against 2849.5 cm^-1) and more than 0.08 (1589 against 1583 cm^-1) [1]. The Japanese Pharmacopoeia uses a different pair of transmittance limits again: at least 18% and 12% [4].
No pharmacopoeia reviewed here states how often to calibrate. Ph. Eur. 2.2.24 gives no interval [1]. The International Pharmacopoeia says only that instruments should be “checked frequently” [5]. Monthly, quarterly or before-use checks are site policy, and your SOP should say so rather than cite a chapter for them.
USP <854> is not Ph. Eur. with a different number. According to Agilent’s 2020 compliance review, the USP chapter asks for wavenumber accuracy in operational qualification, sets no resolution test, and does not require measurement at more than one wavenumber [4]. The USP text itself is subscription-only, so check this against your current USP–NF before you rely on it.
ATR spectra do not reproduce transmission band positions. The seven reference values are transmission values for a free-standing film. Agilent’s ATR test for Ph. Eur. uses shifted positions (for example 1601.0 and 3059.7 cm^-1) and a ratio-based resolution test that Agilent itself labels as its own recommendation [4]. The ATR paragraph of Ph. Eur. 7.0 2.2.24 sets no performance criteria at all [1].
A library “match score” is not a pharmacopoeial identification result. Ph. Eur. identification requires the bands of the test spectrum to correspond in position and relative size to the reference [1]. A software hit quality such as 922 out of 1000 can support that judgement. It does not replace it.

FTIR calibration as per EP, USP, JP and IP compared

The table puts the four pharmacopoeias side by side for a Fourier-transform instrument in transmission mode. The badge in each row tells you where the figure comes from. compendial means we read the chapter text directly. secondary means it comes from manufacturer compliance documentation because the chapter is paywalled. check source means you must confirm it against the current edition before using it.

Parameter	Ph. Eur. 2.2.24 / BP App. II A	USP <854>	JP 2.25	IP 2.4.6
Reference material	Polystyrene film approx. 35 µm compendial [1]	NIST SRM 1921b-traceable polystyrene film, approx. 35 µm, per Agilent’s summary of all four texts secondary [4]
Wavenumber bands, cm^-1	3060.0, 2849.5, 1942.9, 1601.2, 1583.0, 1154.5, 1028.3	Same seven secondary	Same seven secondary	Same seven check source
Wavenumber tolerance, FT	±1.0 at all seven compendial	±1.0 secondary	±1.0; but ±1.5 at 3060.0, 2849.5, 1942.9 secondary	±1.0 check source
Resolution, FT	ΔA 2870–2849.5 > 0.33 ΔA 1589–1583 > 0.08 compendial	No resolution test in OQ, per Agilent check source	Trough depth 2850→2870 ≥ 18 %T 1583→1589 ≥ 12 %T secondary	> 0.33 and > 0.08 check source
Reproducibility tests	None stated compendial	Not stated in Agilent’s summary	Wavenumber: under 5 cm^-1 near 3060, under 1 cm^-1 near 1028. Transmittance: under 0.5 % secondary	Not stated in Agilent’s summary
Calibration frequency	None stated compendial	Not stated in the sources reviewed. Set it as site policy site policy

Swipe the table sideways on a phone. Ph. Eur. figures are from Ph. Eur. 7.0, chapter 01/2008:20224, and were unchanged in Agilent’s 2020 review. Confirm them against the edition in force at your site.

Two differences matter in daily work. First, JP is the only text in this comparison that adds reproducibility tests. Shimadzu notes that its software keeps them because they come from JP [6]. Second, the USP chapter scopes its compliance range for monograph identification as 3800 to 650 cm^-1 unless a monograph says otherwise [3]. Ph. Eur. records reference spectra between 4000 and 650 cm^-1 [1]. If one method file serves both markets, scan the wider range.

The dispersive column still matters

If your laboratory still runs a grating instrument, Ph. Eur. gives it its own limits. On a monochromator instrument the wavenumber tolerances are ±1.5 (3060.0), ±2.0 (2849.5), ±1.5 (1942.9) and ±1.0 cm^-1 for the remaining four bands. Resolution is a percentage-transmittance difference greater than 18 between 2870 and 2849.5 cm^-1, and greater than 10 between 1589 and 1583 cm^-1 [1]. Do not copy these limits into an FTIR record.

FTIR calibration standards: the polystyrene film and its certificate

Ph. Eur. calls for a polystyrene film of approximately 35 µm and words the wavenumber check permissively: the scale “may be verified” with it [1]. In practice auditors expect a traceable film with its own certificate. Most traceability chains lead back to the NIST SRM 1921 series. The current certificate, SRM 1921b, describes a matte-finish film of approximately 38 µm. Its values are certified at a nominal 23 °C, over 540 to 3125 cm^-1, and valid until 31 December 2026 [2].

Ph. Eur. band, cm^-1	NIST SRM 1921b, band minimum (vacuum), cm^-1	Stated uncertainty, cm^-1	FT tolerance, Ph. Eur.
3060.0	3060.16	0.25	±1.0
2849.5	2849.48	0.49	±1.0
1942.9	1942.97	0.66	±1.0
1601.2	1601.29	0.10	±1.0
1583.0	1582.98	0.09	±1.0
1154.5	1154.50	0.20	±1.0
1028.3	1028.27	0.33	±1.0

NIST values are from Table 4 (band-minimum method) of the SRM 1921b certificate [2]. The 1942.97 cm^-1 band is certified only by the band-minimum method, which is new for 1921b.

FTIR calibration polystyrene reference bands with Ph. Eur., USP, JP and IP wavenumber tolerances and resolution limits — The seven polystyrene reference bands and the resolution pairs, with the FT tolerance each pharmacopoeia applies. Figure: Laafon Galaxy, based on Ph. Eur. 2.2.24 [1], NIST SRM 1921b [2] and Agilent 5994-2339EN [4].

Three certificate details catch laboratories out:

The certificate has an expiry. The SRM 1921b values are stated valid until 31 December 2026 [2]. A commercial film traceable to it carries its own certificate and its own recertification date. Log that date in the instrument file, the same way you would for a qualified working standard.
Resolution and apodisation are part of the certified method. NIST certifies its values at instrument resolutions up to 4 cm^-1, with boxcar (no) apodisation at 4 cm^-1, and asks for at least six transmittance spectra [2]. Ph. Eur. leaves resolution and apodisation to the manufacturer’s prescription [1]. Write the settings you actually use into the SOP, so every check runs under the same conditions.
Temperature is specified. The NIST values are given at 23 °C, and the certificate supplies temperature coefficients for use outside 18 to 28 °C [2]. A laboratory in an Indian summer without air conditioning is outside that window.

How the FTIR works, and what each calibration check actually tests

A Fourier-transform instrument does not scan wavelength by wavelength. A Michelson interferometer splits and recombines the infrared beam while one mirror moves. The detector records an interferogram, and the software converts it to a spectrum by Fourier transformation. This design is why the polystyrene check is a check of the whole optical and software chain, not of one component. Tap a component to see its function and the check that exposes a fault in it.

Tap a component

Each part of the optical path shows its function here, the calibration check that detects a fault in it, and the typical failure.

Component	Function	Check that exposes a fault	Typical failure (engineering, not compendial)
IR source	Emits broadband mid-infrared radiation	Low energy or interferogram amplitude at the start-up diagnostic	Ageing emitter, weak signal, noisy spectra
Beamsplitter	Divides the beam between the two mirrors and recombines it	Energy check; resolution pairs losing depth	Humidity damage to hygroscopic optics
Fixed and moving mirrors	Create the optical path difference that encodes every wavenumber	Resolution test (0.33 and 0.08 absorbance differences)	Misalignment after transport or shock
HeNe reference laser	Tracks moving-mirror position, which sets the wavenumber axis	Wavenumber accuracy at the seven polystyrene bands	Laser ageing, axis shift or failed initialisation
Sample compartment and purge	Holds film, KBr disc or ATR accessory	Water vapour and CO₂ bands in the background	Exhausted desiccant, open lid, dirty ATR crystal
Detector	Converts the modulated beam into an electrical signal	Noise level and energy at start-up diagnostics	Saturation, cooling faults on cooled detectors

Failure modes are general service experience, not pharmacopoeial text. Use your instrument manual’s diagnostics for the numerical energy and noise limits.

Ph. Eur. also provides for compensating water vapour and atmospheric carbon dioxide on FT instruments [1]. Compensation is not a substitute for a dry compartment. A polystyrene check run against a wet background tells you more about the desiccant than about the instrument.

Which test set applies to your instrument?

Pick your sampling mode and the pharmacopoeia your products are released against. The verdict gives you the test set to write into your SOP and the point an auditor will check first.

1. How is the instrument used for release testing?

2. Which pharmacopoeia governs the product?

Choose one option from each group

The verdict names the wavenumber and resolution tests that apply, and the first thing an auditor looks for.

Wavenumber and resolution checker

Enter the band positions your instrument reports for the polystyrene film, and the absorbance values for the two resolution pairs. The checker applies the tolerances of the pharmacopoeia you select. It works entirely in your browser and stores nothing.

Tolerance set

3060.0

2849.5

1942.9

1601.2

1583.0

1154.5

1028.3

Resolution inputs: absorbance for Ph. Eur., USP and IP FT sets; %T trough depth or %T difference for the JP and monochromator sets.

Pair 1 difference (2870 / 2849.5)

Pair 2 difference (1589 / 1583)

Press Check results to evaluate the example values, or enter your own.

The USP row uses ±1.0 cm^-1 as reported by Agilent [4], and USP sets no resolution test there, so the resolution result is informative only for USP. The checker supports your record. It does not replace the instrument software’s validated calculation.

SOP: calibration and operation of FTIR spectrophotometer

Two linked documents follow. The first is the performance verification (calibration) SOP. The second is the routine operation SOP that this page originally carried, rewritten so it no longer depends on one software version. Adapt the header fields to your document control system. Instrument-specific steps should follow your manufacturer’s manual, for example the Shimadzu IRSolution workflow or its equivalent. For the UV-Vis equivalent, see our UV-Visible spectrophotometer SOP and qualification guide.

SOP No.: QC/INS/___ Version: 01 Effective: DD-MMM-YYYY Review: DD-MMM-YYYY Department: Quality Control Supersedes: ___

1. Purpose

To verify, at defined intervals, that the FTIR spectrophotometer meets pharmacopoeial requirements for wavenumber accuracy and resolution before use for identification testing.

2. Scope

Applies to FTIR spectrophotometer ID ___ in the QC laboratory, in transmission mode, used for identification of APIs, excipients and finished products. It does not cover near-infrared (NIR) instruments, quantitative IR methods, or ATR-specific performance tests. ATR criteria must be defined separately from the manufacturer’s specification.

3. Responsibility

Analyst, QC: performs the verification and records raw data.
Officer / Executive, QC: reviews the record, and labels the instrument “Calibrated” or “Out of calibration”.
Head, Quality Assurance: approves this SOP and any deviation investigation.

4. Materials and equipment

Certified polystyrene film, approx. 35 µm, traceable to NIST SRM 1921 series, within its certificate validity
FTIR spectrophotometer with validated software and an audit trail enabled
Desiccant or dry purge supply for the sample compartment
Lint-free tissue; powder-free gloves (do not touch the film surface)

5. Procedure

5.1Switch on the instrument and allow the warm-up time stated by the manufacturer. Record the time of switch-on.

5.2Confirm the desiccant indicator or purge flow is acceptable and the laboratory temperature is recorded. Note any reading outside 18 to 28 °C [2].

5.3Run the instrument’s start-up diagnostic (energy, laser, interferogram) and attach the printout.

5.4Set the resolution and apodisation prescribed by the manufacturer for pharmacopoeial checks, and record both. Use the same settings at every verification [1].

5.5With the sample compartment empty and closed, record a background spectrum.

5.6Check the film certificate number and expiry. Place the film in the holder without touching the measurement area, then close the compartment.

5.7Record the spectrum over 4000 to 650 cm^-1 (Ph. Eur.) or at least 3800 to 650 cm^-1 (USP) [1] [3].

5.8Using the peak-pick function, record the observed positions of the transmission minima near 3060.0, 2849.5, 1942.9, 1601.2, 1583.0, 1154.5 and 1028.3 cm^-1 in Annexure-I.

5.9Switch the ordinate to absorbance. Record the absorbance at the minimum near 2870 cm^-1 and the maximum at 2849.5 cm^-1, then at the minimum near 1589 cm^-1 and the maximum at 1583 cm^-1. Calculate both differences.

5.10For JP release, record the additional reproducibility measurements from two consecutive spectra, as summarised in section 6.

5.11Save the spectrum with a file name that carries instrument ID and date. Print, sign and attach it to Annexure-I. Return the film to its case.

5.12Compare results with section 6. Affix the calibration status label showing date done and date due.

6. Acceptance criteria

Parameter	Limit (FT, transmission)	Basis
Wavenumber, 7 polystyrene bands	±1.0 cm^-1 each	compendial Ph. Eur. 2.2.24, Table 2.2.24.-1 [1]
Resolution, 2870 / 2849.5 cm^-1	ΔA > 0.33	compendial Ph. Eur. 2.2.24 [1]
Resolution, 1589 / 1583 cm^-1	ΔA > 0.08	compendial Ph. Eur. 2.2.24 [1]
JP: 3060.0, 2849.5, 1942.9 cm^-1	±1.5 cm^-1	secondary JP 2.25 as summarised by Agilent [4]
JP resolution (trough depth)	≥ 18 %T; ≥ 12 %T	secondary JP 2.25 via Agilent [4]
JP reproducibility	Wavenumber under 5 (3060), under 1 (1028) cm^-1; %T under 0.5	secondary JP 2.25 via Agilent [4]
IP 2.4.6	Apply current IP text	check source Agilent reports Ph. Eur.-equivalent limits [4]; the IP text was not available for review
ATR accessory	Manufacturer specification	site policy no compendial ATR limits in Ph. Eur. 7.0 [1]

7. Frequency

Start-up diagnostic and background: each day of use site policy
Polystyrene wavenumber and resolution check: at the interval fixed by QA from usage and trend history, commonly monthly or quarterly site policy
Additionally: after any service, laser or source replacement, relocation, or failed system suitability site policy

No compendial frequency exists in the texts reviewed [1] [5]. Record the rationale for the interval you choose in the instrument qualification file.

8. Precautions

Never clean the polystyrene film with solvent, and never touch the measurement area. Scratched or creased films shift bands.
Do not run the check with an exhausted desiccant. Water vapour bands near the 1583 to 1601 cm^-1 region can distort peak picking.
Do not use a film beyond its certificate validity. That is an audit finding even when the numbers pass.
Do not edit the spectrum (baseline, smoothing) before peak picking unless the SOP specifies it. Audit trail entries will show it.

9. Deviation handling

If any criterion fails, repeat once after re-running the background and checking the film seating. Record both results; do not overwrite the first.
If the repeat fails, label the instrument “Out of calibration – do not use” and inform the QC Officer.
Raise an incident report. Assess the impact on results generated since the last passing check, and list the batches tested.
Call the service engineer. After repair, repeat the full verification before release to use.

10. Annexures

Annexure-I: FTIR calibration record (see the third tab). Annexure-II: film certificate copy. Annexure-III: start-up diagnostic printout.

11. Revision history

Version	Date	Change	Approved by
00	DD-MMM-YYYY	New SOP	Head QA
01	DD-MMM-YYYY	FT tolerances aligned to Ph. Eur. Table 2.2.24.-1; frequency marked as site policy	Head QA

SOP No.: QC/INS/___ Version: 01 Effective: DD-MMM-YYYY Department: Quality Control

1. Purpose

To lay down the procedure for operating the FTIR spectrophotometer for identification of materials by comparison with a reference substance or a reference spectrum.

2. Scope

Applies to routine identification testing on FTIR ID ___ in the QC department. It does not cover performance verification, which is covered by the calibration SOP.

3. Responsibility

QC Chemist: operation and data recording.
QC Manager: review, and accountability for the result.

4. Procedure

4.1Confirm the instrument calibration label is within date. Log in with your own user ID; never use a shared login.

4.2Switch on and complete the warm-up and automatic adjustment (auto-align or equivalent) the software requests.

4.3Select the method with the specified range, resolution and number of scans. Enter the sample name and A.R. number.

4.4Record the background: empty compartment for transmission, or clean and dry crystal for ATR.

4.5Prepare the sample as the monograph directs. For a halide disc, Ph. Eur. uses 1 to 2 mg of substance with 300 to 400 mg of dried potassium bromide or potassium chloride, pressed at about 800 MPa. Reject the disc if transmittance at about 2000 cm^-1 is below 60 per cent without compensation [1].

4.6Place the disc or sample, close the compartment and acquire the spectrum.

4.7Record the reference substance (CRS or qualified working standard) under the same conditions, or retrieve the reference spectrum.

4.8Overlay the two spectra. Conclude that the sample complies only when the transmission minima correspond in position and relative size [1]. A library match score may be reported as supporting data.

4.9If spectra differ in the solid state, follow the monograph’s instructions for recrystallisation before concluding non-compliance. The International Pharmacopoeia notes that such differences may come from crystalline form [5].

4.10Save the data in the controlled folder, print, sign and attach to the analytical record. Do not export and delete original files.

4.11Clean the die, holder or ATR crystal, remove the sample, close the software, switch off if the SOP requires it, and log off. Complete the instrument usage log.

5. Recommended distribution

Quality Control (controlled copy); Quality Assurance (master copy).

Annexure-I: FTIR calibration record

Instrument ID: ______ Film certificate No. / valid until: ______ Resolution / apodisation: ______ Lab temperature: ______

Band (cm-1)	Tolerance (cm-1)	Observed (cm-1)	Result
3060.0	±1.0
2849.5	±1.0
1942.9	±1.0
1601.2	±1.0
1583.0	±1.0
1154.5	±1.0
1028.3	±1.0
Resolution A(2870) – A(2849.5)	greater than 0.33
Resolution A(1589) – A(1583)	greater than 0.08
Done by / date		Checked by / date	Next due

Copies as tab-separated text that pastes straight into Excel or Google Sheets.

Equipping or upgrading a QC laboratory?

An FTIR is one line in a QC instrument list that also has to satisfy Schedule M, your target markets’ pharmacopoeias and a qualification plan. Laafon Galaxy’s plant setup cost calculator gives an indicative budget for the QC laboratory alongside the rest of the facility, so you can scope instruments before you request quotations.

Estimate plant and QC lab setup cost

Frequently asked questions

What are the FTIR calibration parameters as per Ph. Eur.?

How often should an FTIR spectrophotometer be calibrated?

Is FTIR calibration as per USP the same as Ph. Eur.?

Can the polystyrene tolerances be used to check an ATR accessory?

Which FTIR calibration standards are accepted in India?

What should I do if the FTIR fails the polystyrene check?

References

European Directorate for the Quality of Medicines and HealthCare. 2.2.24. Absorption spectrophotometry, infrared (01/2008:20224). In: European Pharmacopoeia. 7th ed. Strasbourg: Council of Europe; 2010. Available from: https://www.drugfuture.com/Pharmacopoeia/EP7/DATA/20224E.PDF. Also reproduced as British Pharmacopoeia 2012, Appendix II A: https://www.drugfuture.com/Pharmacopoeia/BP2012/data/1016.html. Accessed September 2026.
National Institute of Standards and Technology. Certificate of Analysis, Standard Reference Material 1921b: infrared transmission wavelength/wavenumber standard (polystyrene film). Gaithersburg (MD): NIST; 2025. Available from: https://tsapps.nist.gov/srmext/certificates/1921b.pdf. Accessed September 2026.
United States Pharmacopeia. General Chapter <854> Mid-Infrared Spectroscopy. In: USP–NF. Rockville (MD): USP. doi:10.31003/USPNF_M3208_04_01. Available from: https://doi.usp.org/USPNF/USPNF_M3208_04_01.html (public preview of the introduction). Accessed September 2026.
Agilent Technologies. Pharmaceutical analysis using FTIR: compliance with European, US, Indian, and Japanese Pharmacopoeia. White paper 5994-2339EN. Santa Clara (CA): Agilent; 2020. Available from: https://www.agilent.com/cs/library/whitepaper/public/whitepaper-pharmacopeia-ftir-5994-2339en-agilent.pdf. Accessed September 2026.
World Health Organization. 1.7 Spectrophotometry in the infrared region. In: The International Pharmacopoeia. 13th ed. Geneva: WHO; 2025. Available from: https://digicollections.net/phint/pdf/b/7.1.7.1.7-Spectrophotometry-in-the-infrared-region.pdf. Accessed September 2026.
Shimadzu Corporation. European Pharmacopoeia 5.0: FTIR technical support note. Kyoto: Shimadzu; 2006. Available from: https://www.shimadzu.com/an/service-support/technical-support/ftir/tips_and_tricks/euro_pharmacopoeia5.html. Accessed September 2026.

This procedure is a template for adaptation. It requires local qualification, validation and Quality Assurance approval before use, and every acceptance criterion must be verified against the current pharmacopoeial edition applicable at your site. Ph. Eur. figures above were read from the 7th edition text; USP, JP and IP figures marked secondary or check source were not read from the chapter itself. Pharmacopoeial texts and Indian statutory instruments change between editions. Technical and educational content only, not medical, legal or investment advice.

FTIR Calibration as per EP, USP & IP: 7 Polystyrene Bands, Limits

Six things most calibration pages get wrong