SOP for operation of IR Spectrophotometer

Infrared (IR) spectroscopy is an important analytical technique used in pharmaceutical laboratories, particularly for the identification of pharmaceutical substances and raw materials. An IR spectrophotometer measures how a sample interacts with infrared radiation and produces a characteristic spectrum.

In a pharmaceutical Quality Control (QC) laboratory, IR spectroscopy is commonly used for identification of raw materials, drug substances and, where applicable, finished products. Proper operation of the instrument is important because sample preparation, background measurement, instrument settings and cleanliness can all affect the quality of the spectrum.

This Standard Operating Procedure (SOP) provides a general guide for operating an IR/FTIR spectrophotometer. The actual procedure should always be adapted to the specific instrument manufacturer's instructions, approved analytical method and applicable pharmacopoeial requirements.

Purpose

The purpose of this SOP is to describe the general procedure for safe and proper operation of an IR spectrophotometer used for pharmaceutical analysis.

The procedure helps to ensure:

• Consistent instrument operation
• Proper sample measurement
• Reliable spectral comparison
• Correct documentation
• Reduced risk of analytical errors

Scope

This SOP applies to the operation of IR or FTIR spectrophotometers used in pharmaceutical Quality Control and analytical laboratories.

It may be used for identification and other IR-based analytical procedures where the technique is specified by the approved method.

Because different manufacturers and models use different software, accessories and operating procedures, the instrument-specific manufacturer's manual should always be followed.

Responsibility

The QC Analyst is responsible for operating the instrument according to the approved SOP, preparing samples correctly, performing the analysis and recording the required information.

The QC Head or designee is responsible for ensuring that analysts are properly trained and that the procedure is implemented correctly.

The QA Department is responsible for approval and document control of the SOP and for ensuring that the procedure complies with the laboratory's quality system.

Principle of IR Spectroscopy

IR spectroscopy is based on the interaction between infrared radiation and molecules.

When infrared radiation interacts with a sample, certain frequencies may be absorbed because of molecular vibrations such as stretching and bending of chemical bonds. These absorptions produce characteristic bands in the resulting spectrum.

The spectrum is generally displayed against wavenumber (cm⁻¹). Depending on the instrument and measurement mode, the vertical axis may represent transmittance or absorbance.

Because different substances have different molecular structures, their IR spectra can contain characteristic patterns. For this reason, an IR spectrum is often described as a type of chemical fingerprint.

IR spectroscopy is different from UV-Visible spectroscopy, which measures absorption mainly in the ultraviolet and visible regions. If you are interested in the other commonly used spectroscopic technique in pharmaceutical QC, see our guide to UV-Visible Spectroscopy: Principle, Instrumentation and Applications.

Equipment and Materials

The following equipment and materials may be required:

• IR/FTIR spectrophotometer
• Computer with instrument software
• Appropriate sampling accessory
• Reference standard
• Test sample
• Sample-preparation materials
• Suitable cleaning materials

The exact materials depend on the instrument and analytical procedure.

Modern FTIR instruments may use accessories such as ATR, transmission cells or other sampling systems. The selected technique should be appropriate for the sample and approved analytical procedure.

Precautions Before Operation

Before operating the instrument, check the following:

1. Ensure that the instrument and surrounding area are clean.
2. Check the power supply and instrument connections.
3. Ensure that the instrument is in suitable environmental conditions.
4. Check the condition of the sampling accessory.
5. Make sure the sample compartment is clean.
6. Use appropriate personal protective equipment when handling chemicals.
7. Avoid touching sensitive optical or sampling surfaces unnecessarily.
8. Use only suitable sample holders and accessories.
9. Follow the manufacturer's safety instructions.
10. Do not analyze unsuitable hazardous materials unless the instrument and approved procedure specifically allow it.
11. Check whether the instrument requires a warm-up or stabilization period.
12. Do not change validated operating parameters without authorization.

Good laboratory practices are particularly important when performing instrumental analysis. Proper documentation, equipment handling and cleaning should form part of routine laboratory work. For more information, see our article on SOP for Good Laboratory Practice.

Procedure for Operation of IR Spectrophotometer

The exact operating sequence varies between instruments. The following is a general procedure.

1. Switch On the Instrument

Connect the instrument to the appropriate power supply and switch it on according to the manufacturer's instructions.

If the instrument requires a stabilization period, allow sufficient time before starting the analysis.

Switch on the computer and open the instrument software after the instrument is ready.

2. Open the Instrument Software

Start the IR/FTIR software.

If user login is required, log in using the authorized account.

Check the instrument status and make sure there are no active errors or warnings that could affect the measurement.

3. Initialize the Instrument

If the software provides an initialization or instrument-check function, perform it according to the manufacturer's instructions.

Check the status of the relevant instrument components.

Depending on the instrument, this may include the light source, laser, detector, beam splitter and sampling accessory.

If a significant instrument error is displayed, do not continue routine analysis until the problem has been investigated according to the laboratory procedure.

4. Select the Analytical Method

Select the approved analytical method or measurement program.

The method may specify parameters such as:

• Spectral range
• Resolution
• Number of scans
• Measurement mode
• Detector
• Sampling technique
• Apodization

Do not assume that one set of parameters is suitable for every IR analysis.

For example, a range around 4000–400 cm⁻¹ is commonly used for mid-infrared measurements, but the actual range should be based on the approved analytical procedure and instrument capability.

Similarly, a resolution of 4 cm⁻¹ may be appropriate for some routine applications, but it should not be treated as a universal requirement.

5. Collect the Background Spectrum

Before measuring the sample, collect the background spectrum using the appropriate procedure.

Background measurement helps compensate for contributions from the instrument and surrounding environment.

The exact background procedure depends on the sampling technique.

For example, an ATR system may require background collection with the ATR crystal clean and without the sample.

Follow the manufacturer's instructions for the specific accessory.

6. Prepare the Sample

Prepare the sample according to the approved analytical procedure.

Depending on the method and instrument, the sample may be analyzed using:

• ATR
• Transmission
• Solution method
• Pellet method
• Other validated sampling techniques

The sample should be representative and free from avoidable contamination.

Sample preparation is particularly important because an incorrect preparation technique can produce a spectrum that does not properly represent the material.

7. Place the Sample

Place the sample correctly in the selected sampling accessory.

For ATR measurements, ensure adequate and consistent contact between the sample and ATR crystal according to the manufacturer's instructions.

Do not apply excessive force to the accessory.

For transmission measurements, ensure that the sample holder or cell is correctly positioned.

8. Enter Sample Information

Enter the required information into the software.

This may include:

• Sample name
• Sample identification number
• Batch or lot number
• Test name
• Analyst ID
• Date
• Other information required by the laboratory

Use the laboratory's approved data-management and documentation practices.

9. Scan the Sample

Start the measurement after confirming that the method and sample information are correct.

The instrument will collect the programmed number of scans and generate the IR spectrum.

Do not disturb the instrument or sampling accessory during measurement.

10. Review the Spectrum

After the scan is completed, review the spectrum.

Check for:

• Unexpected peaks
• Excessive noise
• Baseline abnormalities
• Poor sample contact
• Saturated peaks
• Weak signal
• Other unusual spectral features

If the spectrum appears abnormal, investigate the possible cause according to the approved laboratory procedure.

Avoid simply repeating an analysis without understanding the reason for the abnormal result.

11. Compare with Reference

For identification testing, compare the sample spectrum with the appropriate reference standard or reference spectrum.

The comparison may be performed manually or using validated software, depending on the laboratory procedure.

The reference material should be appropriate for the intended test.

The final acceptance decision should follow the applicable monograph or approved analytical procedure.

12. Save the Result

Save the spectrum and associated analytical information in the approved electronic data system.

Ensure that the data are properly identified and traceable to the sample.

Where required, generate and print the approved analytical report.

13. Complete the Instrument Logbook

Record instrument usage according to the laboratory procedure.

The record may include:

• Date
• Instrument identification
• Sample ID
• Test performed
• Analyst name or ID
• Start/end time, where required
• Observations
• Cleaning or maintenance performed

Important IR Instrument Parameters

Spectral Range

Spectral range defines the portion of the infrared region measured by the instrument.

The appropriate range depends on the analytical purpose and approved method.

A broad mid-IR range may be used for general identification, but the exact range should always follow the method.

Resolution

Resolution determines how well the instrument can distinguish closely spaced spectral features.

Higher resolution can provide more spectral detail but may require additional measurement time.

The method should specify the required resolution where applicable.

Number of Scans

The number of scans can affect the signal-to-noise ratio.

Increasing the number of scans generally improves the signal-to-noise performance but also increases analysis time.

The number of scans should therefore follow the validated or approved procedure.

Measurement Mode

The instrument may display the result as absorbance or transmittance.

The selected mode should be consistent with the analytical procedure and reporting requirements.

IR Identification Test

One of the most common pharmaceutical applications of IR spectroscopy is identification.

A test sample is analyzed and its spectrum is compared with an appropriate reference spectrum.

Identification may be based on characteristic absorption bands or an overall spectral comparison, depending on the analytical procedure.

The use of a suitable reference standard is important because the spectrum can be affected by factors such as:

• Sample preparation
• Moisture
• Instrument conditions
• Sampling technique
• Contamination
• Physical form of the sample

For analysts working in pharmaceutical QC, understanding identification tests is only one part of analytical testing. Other techniques such as HPLC are routinely used when quantitative analysis, assay or impurity determination is required. You can read more about this in HPLC: Principle, Instrumentation, Applications and System Suitability.

Cleaning of IR Instrument

After completing the analysis:

1. Remove the sample carefully.
2. Clean the sampling accessory according to the manufacturer's instructions.
3. Use only compatible cleaning materials.
4. Do not scratch or damage the ATR crystal or other sensitive surfaces.
5. Ensure that no sample residue remains.
6. Close the sample compartment when appropriate.
7. Save all required analytical data.
8. Complete the instrument logbook.

Cleaning requirements depend on the accessory. For example, ATR crystals require careful cleaning because improper cleaning can damage the surface.

Shutdown Procedure

The shutdown procedure depends on the manufacturer's instructions and laboratory practice.

If the instrument is designed to remain powered on between analyses, follow the approved laboratory procedure rather than switching it off after every test.

When shutdown is required:

1. Complete and save the analysis.
2. Close the software as instructed.
3. Shut down the computer if required.
4. Switch off the instrument according to the manufacturer's instructions.
5. Leave the instrument and surrounding area clean.

Common Problems During IR Analysis

Weak Spectrum

Possible causes include:

• Insufficient sample
• Poor sample contact
• Incorrect sample preparation
• Dirty sampling accessory
• Incorrect measurement parameters

Excessive Noise

Possible causes include:

• Insufficient scans
• Environmental interference
• Instrument problems
• Poor sample preparation

Unexpected Peaks

Possible causes include:

• Sample contamination
• Solvent interference
• Incorrect background
• Sample impurities
• Dirty sampling accessory

Poor Matching with Reference Spectrum

Possible causes include:

• Incorrect reference
• Wrong sample preparation
• Different sampling technique
• Contamination
• Instrument or accessory problems

The cause should be investigated before repeating the test.

Maintenance and Performance Checks

The IR spectrophotometer should be maintained according to the manufacturer's recommendations and the laboratory's approved maintenance program.

Depending on the instrument and application, routine checks may include:

• Instrument performance verification
• Wavenumber accuracy checks
• Resolution checks
• Background checks
• Detector or source checks
• Accessory inspection
• Cleaning

The frequency and acceptance criteria should be defined by the approved procedure, manufacturer's instructions or applicable pharmacopoeial requirements.

Data Integrity and Documentation

All IR analytical data should be properly recorded and maintained.

The analyst should ensure that:

• Data are attributable to the correct analyst.
• Sample information is correct.
• Original spectra are retained where required.
• Changes to electronic data are controlled.
• Records are stored in the approved location.
• Unexpected results are properly investigated.

The principles of good documentation are an important part of pharmaceutical QC. Analysts should therefore follow the laboratory's data-integrity and documentation procedures.

Analytical Method Validation

When an IR-based analytical procedure requires validation, its performance should be evaluated according to its intended purpose.

Depending on the application, characteristics such as specificity/selectivity, precision, accuracy or other relevant characteristics may need to be demonstrated.

ICH Q2(R2) provides current guidance for validation of analytical procedures and explains how analytical procedures should be demonstrated to be fit for their intended purpose.

For a detailed explanation of analytical validation, see Analytical Method Validation Parameters.

Good Practices for Reliable IR Results

For consistent and reliable results:

• Keep the instrument clean.
• Keep the sampling accessory clean.
• Use suitable reference standards.
• Prepare samples carefully.
• Collect an appropriate background.
• Follow the approved analytical procedure.
• Do not change method parameters without authorization.
• Handle the ATR crystal or other accessories carefully.
• Perform required performance checks.
• Maintain complete records.
• Investigate abnormal spectra properly.
• Follow the manufacturer's instructions.

Frequently Asked Questions

What is an IR spectrophotometer?

An IR spectrophotometer is an analytical instrument that measures the interaction of infrared radiation with a sample and produces an infrared spectrum.

What is IR spectroscopy used for in pharmaceutical QC?

It is commonly used for identification of pharmaceutical substances and raw materials and for other applications where an IR-based analytical procedure is specified.

What is the principle of IR spectroscopy?

IR spectroscopy is based on the absorption of infrared radiation by molecular vibrations. Different chemical structures produce characteristic absorption patterns.

What is FTIR?

FTIR stands for Fourier Transform Infrared Spectroscopy. It is a modern form of infrared spectroscopy that uses Fourier-transform processing to obtain the spectrum.

What is ATR in FTIR?

ATR stands for Attenuated Total Reflectance. It is a sampling technique that allows many samples to be analyzed with minimal preparation.

Why is background measurement performed?

Background measurement helps account for contributions from the instrument and surrounding environment before the sample spectrum is collected.

What is the commonly used IR range?

A broad mid-infrared region such as approximately 4000–400 cm⁻¹ is commonly used for many routine measurements, but the actual range should follow the approved analytical method and instrument capability.

Why is sample preparation important in IR spectroscopy?

Incorrect sample preparation can change the appearance of the spectrum and may lead to an incorrect interpretation or identification.

References

1. United States Pharmacopeia (USP-NF). General chapters and applicable monographs relating to infrared spectroscopy and pharmaceutical identification.
2. International Council for Harmonisation (ICH). ICH Q2(R2): Validation of Analytical Procedures.
3. U.S. Food and Drug Administration (FDA). Q2(R2) Validation of Analytical Procedures: Guidance for Industry, March 2024.
4. Skoog DA, Holler FJ, Crouch SR. Principles of Instrumental Analysis. Cengage Learning.
5. Stuart B. Infrared Spectroscopy: Fundamentals and Applications. Wiley.
6. Manufacturer's User and Maintenance Manual for the specific IR/FTIR spectrophotometer used in the laboratory.

Conclusion

IR spectrophotometry is an important technique in pharmaceutical analysis, particularly for identification of pharmaceutical materials. However, obtaining a reliable IR spectrum requires proper instrument operation, suitable sample preparation, correct background measurement, appropriate reference comparison and good documentation.

The exact operating parameters should not be assumed to be the same for every instrument or method. Settings such as spectral range, resolution and number of scans should follow the approved analytical procedure and manufacturer's instructions.

For QC analysts, understanding both the practical operation and the basic theory of IR spectroscopy is important. When combined with knowledge of techniques such as HPLC, UV-Visible spectroscopy and analytical method validation, it provides a stronger foundation for pharmaceutical laboratory work.