Spectroscopy and Spectroscopic Analytical Techniques in the United States Pharmacopeia–National Formulary (USP–NF)

The U.S. Pharmacopeia (USP) is a non-profit scientific organization whose mission is to improve global public health through public standards and related programs that ensure the quality, safety and benefits of medicines and foods. Vibrational spectroscopy is an analytical tool that is used in the United States Pharmacopeia and National Formulary (USP–NF) and has been since the 1950s. We explain the organization of the documentation within the USP–NF and how vibrational spectroscopy plays an important role in this regulatory document collection.

The U.S. Pharmacopeia (USP) is a nonprofit scientific organization established over 200 years ago to “improve global health through public standards and related programs that ensure the quality, safety and benefits of medicines and foods” (1). It was formed in 1820 by a small group of physicians who were concerned about the dangers of poor-quality medicines. When it was founded, Dr. Lyman Spalding noted, “the value of the pharmacopeia depends on the fidelity with which it conforms to the best state of medical knowledge of the day” (2).

Since its inception, the USP has worked collaboratively with Federal and State policymakers and other stakeholders. In 1848, Congress passed the Drug Importation Act to stop importation of poor-quality drugs from Europe that were deemed “good enough for America” (3). This legislation recognized the USP as the authority on the standards to be used to determine drug strength and purity.

The second regulatory milestone that tightened the relationship between the USP and the federal government was the passage of the Pure Food and Drug Act of 1906. This act was the start of the U.S. Food and Drug Administration (FDA), and it also officially required that drugs marketed in the United States meet USP standards.

In the 1990s, the USP recognized the need for similar standards for dietary supplements. Later, the USP expanded its scope to include food quality; ensuring food identity, purity, and quality was added to the mission of the USP in 2006 when it acquired the Food Chemical Codex.

Throughout the history of the USP, expertise and scientific advancements have been key to its mission. Spectroscopy, in particular vibrational spectroscopy, has played a key role since the early days of the techniques. This summary explains how vibrational spectroscopy fits into the USP Compendia and how these techniques are used to ensure drug quality.

USP–NF Overview

The organization of the documentation within the USP and National Formulary (USP–NF) may seem unusual for someone concentrating on a particular technique. However, by remembering the mission of USP, the strategy becomes clear.

The USP Compendia starts with General Notices, which apply to official standards recognized by USP. The heart of the USP–NF lies in the monographs. Each monograph defines a medicine, including its specifications and its details related to packaging, storage, and labeling. A mono- graph also defines the tests, procedures, and acceptance criteria for the strength, quality, and purity of a material—in other words, the compendial identity standards.

The General Chapters describe tests, techniques, and procedures that are important to the contents of the monographs. They also contain information on pharmaceutical compounding, how to interpret the requirements in the compendia, and general guidance to drug manufacturers. The General Chapters are applicable to a material through references in the General Notices, the monographs, or other chapters.

Spectroscopy in the USP

A monograph starts with general chemical information about a drug, such as the name, molecular formula, added substances, physical description, and solubility. The next section of the monograph describes identification tests. These tests may be spectroscopic in nature. Another place spectroscopy comes into play is the assay section of a monograph.

Spectroscopy with commercial instrumentation started to be used in analytical chemistry laboratories in the 1940s. Vibrational spectroscopy, in particular infrared (IR) spectroscopy, had similar beginnings and, as noted by Rabkin (4), American Cyanamid contracted with PerkinElmer to produce the first commercial instruments. By the 1950s, laboratories were well-equipped with spectrophotometers (5).

This expansion of spectroscopy went hand-in-hand with developments that were underway in the USP–NF. In 1950, USP 14 had the first entries referring to spectrophotometry. By 1960, IR spectroscopy was mentioned as distinct from UV spectroscopy. The applications grew and by the 2022 version of the USP–NF, spectroscopic techniques, including mid-IR, near-IR (NIR), UV-vis, and Raman spectroscopy, appear in over 250 general chapters and monographs. The number of general chapters and monographs do not include references via Chapter <197> “Spectroscopic Identification Tests.”

Chapter <197> Spectroscopic Identification Tests

General notices 5.40 of the USP explains that the identification tests in the USP monographs must be used to ensure the identity of a material—to establish whether it is the material named in the USP–NF. Failure of an article (material) to meet all the requirements of a prescribed Identification test indicates that the article is mislabeled, adulterated, or both.

Chapter <197> of the USP defines the spectroscopic identification tests that can be used in a monograph. These include NIR, mid-IR, and Raman spectroscopy. For mid-IR spectroscopy, the chapter de- scribes at least seven different sampling approaches, both classical and modern, including:

• pellets
• mulls
• solutions
• thin film from neat liquids, solutions or melts between alkyl halide plates
• attenuated total reflection (ATR)
• diffuse reflection
• microsamples

In total, more than 1700 uses of mid-IR spectroscopy are referenced in the monographs; for UV-vis, there are approximately 350 references. Chapter <197> also contains information about applying alternative and harmonized procedures.

Chapters Below and Above 1000

In addition to Chapter <197>, which defines spectroscopic identification tests, there are two more sets of paired chapters in the USP that describe and define vibrational spectroscopy–which respectively belong to the chapters numbered below 1000 and the chapters numbered above 1000 (see USP–NF General Notices 3.10. Applicability of Standards). The chapters numbered below 1000 describe specific techniques that are mandatory requirements for compliance when referenced in a monograph. These chapters include an introduction and brief description of the technique, a section on the qualification of the spectrometer that is the topic of the chapter, and procedures for using the technique. In addition, there are sections in the chapter describing validation of the method, if necessary, and verification to ensure that it is being executed with suitable accuracy, sensitivity, and precision.

These spectroscopy chapters numbered below 1000 are:

• <854> “Mid-IR Spectroscopy”
• <856> “NIR Spectroscopy”
• <858> “Raman Spectroscopy”

Associated with those three chapters numbered below 1000 are three spectroscopy chapters numbered above 1000. The numbers for these chapters corresponds to the scheme seen for those below 1000:

• <1854> “Mid-IR Spectroscopy”
• <1856> “NIR Spectroscopy”
• <1858> “Raman Spectroscopy”

These chapters provide more in-depth descriptions of each technique. They start with the theory, explaining the principles and theoretical background of the technique, including modes of measurement. Each of these chapters includes a section that provides guidance and precautions on the factors that impact measurement, such as spectral resolution, wavenumber accuracy, photometric accuracy, sensitivity, environmental factors, and sampling for individual spectroscopies, because these guidelines are important for ensuring quality data. Another section discusses instruments, configurations, and components, while also highlighting calibration considerations. Finally, these in-depth chapters may include information about applications that may not be covered in their sub-1000 counterparts or in other general chapters or monographs.

The contents of these chapters above 1000 build on the contents of the corresponding chapter numbered below 1000. Using the mid-IR chapters as an example, the contents in Chapter <854> are:

1. Introduction: A summary of what mid-IR spectroscopy is.
2. Qualification of IR Spectrometers:
   1. Installation Qualification
   2. Operational Qualification: Wavenumber Accuracy
   3. Performance Qualification
3. Procedures
   1. Potassium Bromide (KBr) Disks
   2. Mineral Oil Mulls
   3. Self-Supported Polymer Films
   4. Capillary Films
   5. Liquids and Solutions in Transmission Cells
   6. Gases
   7. Attenuated Total Reflection
   8. Diffuse Reflection
   9. Microscope Sampling
4. Validation and Verification

The information contained in this chapter provides a practical approach to mid-IR spectroscopy for someone who is practicing the analysis.

As we move to Chapter <1854>, we now approach more of the theory and the rationale behind the practices described in Chapter <854>. The contents in <1854> are:

1. Principles of Mid-IR Spectroscopy
   1. A theoretical discussion of the principles of mid-IR spectroscopy, which includes a discussion of molecular vibration fundamentals and different vibrational modes.
2. Sampling Procedures: An expanded discussion on the sampling procedures presented in <854> and separated into:
   1. Bulk Sample Analysis
      1. Potassium Bromide (KBr) Disks
      2. Mineral Oil Mulls
      3. Self-Supported Polymer Films
      4. Capillary Films
      5. Liquids and Solutions in Transmission Cells
      6. Gases
      7. Attenuated Total Reflection
      8. Diffuse Reflection
   2. Microspectroscopy and Imaging
      1. Transmission Microscopy
      2. Reflection Microscopy
      3. Hyperspectral Imaging
3. Instrumentation
4. Factors that Impact Measurement Performance
   1. Spectral Resolution
   2. Wavenumber Accuracy
   3. Photometric Accuracy
   4. Sensitivity
   5. Beer’s Law Linearity

This more in-depth treatment provides the theory and background to ensure that the spectroscopic techniques are applied properly. One of the most important reasons for thoroughly understanding a technique is to address the application of alternative and harmonized methods and procedures. An alternative method can be used, but it must be fully validated and provide results comparable to those of the compendial method. Alternative methods can be developed to simplify sample preparation, enhance precision and accuracy, shorten analysis time, or be better suited to automation. Without thoroughly understanding the original method, developing and applying alternative methods properly would be difficult.

Legal Recognition of the USP–NF

Under the Federal Food, Drug and Cosmetic Act of 1938, the USP and NF are recognized as official compendia, and a drug with a name recognized in the USP–NF must comply with compendial identity standards or be deemed adulterated, misbranded, or both.

Chapter 21 of the U.S. Code of Federal Regulations (21 CFR) deals with matters related to the FDA. Parts 210 and 211 of 21 CFR describe current good manufacturing practices for finished pharmaceuticals, and indicate that if USP methods are used, reference to them will suffice for meeting requirements of the FDA; 21 CFR Part 211.194(a)(2) - Addressing Laboratory Records states:

“A statement of each method used in the testing of the sample. The statement shall indicate the location of data that establish that the methods used in the testing of the sample meet proper standards of accuracy and reliability as applied to the product tested. (If the method employed is in the current revision of the United States Pharmacopeia, National Formulary, AOAC International Book of Methods, or in other recognized standard references, or is detailed in an approved new drug application and the referenced method is not modified, a statement indicating the method and reference will suffice). The suitability of all testing methods used shall be verified under actual conditions of use.”

Thus, the information in the USP is a recognized approach to characterizing a material. The chapters on spectroscopy are essential for such characterization. Thus, the information compiled and provided in the USP has important regulatory standing.

Summary

USP’s standards contribute to ensuring the quality of medicines, dietary supplements, and foods, thus helping protect our drug, dietary supplement, and food supplies from substandard and adulterated materials. The USP–NF has served that purpose for over 200 years. Spectroscopic analysis plays an important role in the identification tests in USP–NF, and by extension, in ensuring the quality of pharmaceutical products. As technology changes the approaches change, but these changes are always with the aim to protect health and safety.

References

(1) “USP-NF Mission and Preface.” https://online.uspnf.com/uspnf/document/1_ GUID-E09DC39F-1D28-4D90-8D76- 0ED109FFB640_8_en-US?source=TOC (accessed 2022-11-30)

(2) Spalding, L. The Pharmacopoeia of the United States of America: 1820; By the Authority of the Medical Societies and Colleges. Boston: Printed by Wells and Lilly for Charles Ewer; Dec. 1820 Library Catalog; MMS ID 992564553406676; NLM ID 2567001R

(3) Scientific American. The Little-known History and Global Future of Quality Medicines. Michael Eisenstein, 2018. https://www.scientificamerican.com/custom-media/little-known-history-of-quality-medicines/ (accessed 2022-11-30)

(4) Rabkin, Y. M. Technological Innovation in Science: The Adoption of Infrared Spectroscopy by Chemists. Isis 1987, 78 (1), 31–54.

(5) Mukhopadhyay, R. The Rise of Instruments during World War II. Anal. Chem. 2008, 80 (15), 5684–5691. DOI: 10.1021/ac801205u.

About the Authors

Edmond Biba is a Principal Scientist in General Chapters Department, and Horacio Pappa is a Senior Director of General Chapters Department of Science Division of United States Pharmacopeia (USP). Direct correspondence to: exb@usp.org