How Does USP <941> Relate to XRD Services at McCrone Associates?

USP <941> is what is known as a general chapter in USP parlance, the purpose of general chapters is to outline essential practices for analytical procedures, manufacturing processes, and product testing. A review of USP <941> Characterization of Crystalline and Partially Crystalline Solids by X-Ray Powder Diffraction (XRPD) was performed by McCrone Associates and this article explores the relationship between the general chapter and our analytical XRD capabilities and services at McCrone Associates, Inc.

As a general chapter, USP <941> is more descriptive in nature than it is prescriptive. The introduction section advocates for the use of XRPD when characterizing crystalline materials, but it does not state a goal of unifying or homogenizing specific procedures or methods for performing XRD analysis. In this article, we will review each of the sections listed in USP <941> and discuss how they relate to our analytical capabilities and services. We will call out specific recommendations made within the general chapter and the differences between our capabilities and those discussed in the chapter.

SECTION I: PRINCIPLE

This section reviews some of the basic principles behind XRD analysis. It begins with Bragg’s law which explains the relationship between the interplanar spacings (d) of a crystalline lattice and the diffraction angle (2θ). It includes a discussion of polycrystalline samples and provides five examples including an amorphous pattern. It concludes with a discussion of some of the sources of error that contribute to peak broadening and factors that contribute to the background signal in a diffraction pattern.

McCrone scientists maintain a high-level understanding of the guiding principles behind X-ray diffraction, but the specific principles employed by an OEM for a given instrument or analytical method are sometimes proprietary.

SECTION II: INSTRUMENT

USP <941> describes a powder diffractometer as having the five main components listed below. Our XRD instruments have each of these five main components, so they meet the definition of diffractometer:

• An x-ray source
• Incident beam optics
• A goniometer
• Diffracted beam optics
• An x-ray detector

The general chapter acknowledges that different instrument configurations are widely used for different types of analysis. This section goes on to describe in detail the Bragg-Brentano parafocusing configuration, which is a widely used configuration in the field.

However, this is not the configuration used at McCrone Associates. At McCrone, we use a transmission configuration, which the general chapter notes, has the advantage of reducing the effects of preferred orientation in a sample. Another advantage transmission instruments have is collecting data in two dimensions instead of one dimension. This allows the analyst to view the diffraction pattern as both an image and a line profile as opposed to just a line profile.

McCrone selected a transmission configuration because it allows for analysis of both powder samples and individual particles between about 30 and 300 µm in size. Not very many instrument configurations are capable of analyzing a single particle in that size range.

SECTION III: X-RAY RADIATION

This section describes the common ways XRD instruments generate x-rays. The source is initially polychromatic (multiple wavelengths), but it is advantageous to use a monochromatic beam using the characteristic K_α wavelength of copper, molybdenum, iron, cobalt, etc. A monochromator is often used to achieve this end.

There is also a note in this section on radiation safety. McCrone Associates, Inc. is located in the state of Illinois and is therefore regulated and inspected by the IEMA. McCrone Associates also has a radiation safety badge and ring program. A leak test is performed monthly to ensure radiation is contained within the XRD instrument enclosures and not affecting personnel.

SECTION IV: SPECIMEN PREPARATION AND MOUNTING

The discussion in this section is also geared towards an instrument with a Bragg-Brentano configuration, however, many of the recommendations apply generally to all diffractometers.

This section only considers powder diffraction, it does not consider samples in other forms such as particles or suspensions. Our instrument configuration is capable of analyzing all three of these sample types.

Diffraction patterns measure the innate periodicity within a crystalline lattice, which is an intrinsic material property (i.e. a property that changes as the material itself changes); whereas a sample’s shape or morphology is an extrinsic material property, meaning it can change without changing the identity of the material. Therefore, as long as the amount of sample is above the detection limit of the instrument, and it is crystalline, diffraction patterns can be reliably collected and interpreted regardless of shape or morphology.

Preferred orientation is discussed at length as it can be a problem for the Bragg-Brentano configuration, but our transmission configuration allows for sample rotation during data collection. This mitigates the effects of preferred orientation when measuring diffraction patterns.

Mechanical milling is also warned against as a method for breaking down large particles into smaller particles to increase randomness of crystal orientation and homogeneity of a powder. This is a procedure we often employ, but we never use energetic milling instruments; we only use hand grinding with a mortar and pestle.

The last two subsections that appear under the Specimen Mounting heading are particularly relevant to reflection mode instruments, but not as applicable to transmission instruments.

SECTION V: CONTROL OF THE INSTRUMENT PERFORMANCE

This section makes note of the fact that there is always a tradeoff between delivering maximum intensity and maximum resolution when aligning a diffractometer of any configuration. Therefore, the overall performance must be tested and monitored periodically using certified reference materials.

McCrone Associates maintains service contracts on all of our XRD instruments which include an annual realignment of the X-ray optics. After the alignment and preventative maintenance are performed, the calibration method is updated to incorporate any optimizations made to the optical alignment of the system.

The calibration verification is performed to check that the system meets or exceeds the acceptance criteria for peak position accuracy for a quartz standard reference material. This procedure is repeated monthly throughout the year to demonstrate continued performance until the next preventative maintenance visit.

SECTION VI: QUALITATIVE PHASE ANALYSIS (IDENTIFICATION OF PHASES)

This section is one of the most prescriptive in the general chapter. It states that the identification of an unknown phase is usually based on the visual or computer-assisted comparison of key features of its diffraction pattern to the experimental or calculated pattern of a single-phase reference diffraction pattern. The key features are the 2θ diffraction angles (or d-spacings) and the relative intensities of the diffraction peaks (or reflections).

For most organic crystals, when using Cu-K_α radiation, it is appropriate to record the diffraction pattern in a 2θ range from as near 0° as possible to at least 30°. The agreement in the 2θ diffraction angles between specimen and reference is expected to be within 0.2° for the same crystalline form, whereas relative intensities between specimen and reference may vary considerably due to the size, number, and orientation of measured crystals.

Hydrates and solvates are recognized to have varying unit cell dimensions, which can result in larger shifts in peak position than other classes of materials. In this case, the differences in 2θ can be > 0.2° between experimental and reference pattern of the same crystalline form.

For inorganic materials it is often necessary to extend the 2θ scan range well beyond 30°. However, when comparing to a reference pattern, it is generally sufficient for a scan to include the 10 strongest reflections.

The general chapter warns that it is sometimes difficult or even impossible to identify phases in the following cases:

• Non crystalline or amorphous substances
• The component is less than 10 wt% of the specimen
• Pronounced preferred orientation effects
• The phase has not been filed in the database used
• The formation of solid solutions
• The presence of disordered structures that alter the unit cell
• The specimen comprises too many phases
• The presence of lattice deformations
• The structural similarity of different phases

McCrone Associates maintains a subscription to the ICDD database that includes approximately one million diffraction patterns for the inorganic and organic compounds often analyzed by our clients. We also maintain a subscription to JADE Pro – a software package that links to the ICDD database and provides advanced searching algorithms and other analytical tools.

SECTION VII: QUANTITATIVE PHASE ANALYSIS

This section discusses phase quantification, which can be used on mixtures of two or more known phases to answer the question of how much material is present by mass or volume of each component in the mixture. Amounts of crystalline phases as small as 10% may be determined using XRPD and perhaps less under favorable conditions.

Quantitative phase analysis can be based on the integrated intensities, on the peak heights of several individual diffraction lines, or on the full diffraction pattern. Whichever method is used, the data points are compared to the corresponding values of reference materials. These reference materials must be single phase or a mixture of known phases.

With access to the JADE Pro software linked to the ICDD database, McCrone Associates favors a whole pattern fitting approach to phase quantification. This method is generally regarded as the most advanced and reliable approach to phase quantification, but the tools available in JADE Pro make implementing this approach relatively straightforward for a trained scientist. Therefore, the other methods listed in this section of the general chapter are unnecessary.

SECTION VIII: ESTIMATE OF THE AMORPHOUS AND CRYSTALLINE FRACTIONS

This section discusses a method for estimating the fraction of amorphous material in a single sample with an amorphous and crystalline component. The general chapter makes clear that this is a rough estimate and does not yield an absolute degree of crystallinity and should be restricted to comparative studies only. It also mentions other more sophisticated methods such as the Ruland method.

Estimates of amorphous phase fractions are traditionally outside the scope of McCrone’s services. That is because we typically use an amorphous glass capillary to hold powder samples in place during data collection. This sample holder contributes to the amorphous phase fraction in an unpredictable way depending on the thickness of the capillary wall at the point where it bisects the x-ray beam. Capillary wall thickness is not uniform throughout the capillary.

Mounting the powder in a sample cup and analyzing the powder directly would be a better option, but this introduces sample safety concerns, particularly when it comes to active pharmaceutical ingredients (drug substances).

SECTION IX: SINGLE CRYSTAL STRUCTURE

Theoretical diffraction patterns can be calculated for any known phase and can be used to index the peak positions of an unknown phase. This can be useful for phase identification.

The ICDD database and JADE software packages are both capable of calculating theoretical diffraction patterns. For example, whole pattern fitting in JADE Pro provides an automated and integrated method for comparing theoretical patterns to experimental patterns.

However, determination of an unknown crystal structure is an advanced XRD application that is usually performed on a single crystal sample of high purity. This type of request is outside the scope of our analytical services at McCrone Associates.

SUMMARY

In this article, we reviewed each of the sections listed in USP <941> and discuss how they relate to our analytical XRD capabilities and services at McCrone Associates. We examined and discussed several differences between our capabilities and procedures and those outlined in the general chapter. We also made note of any recommendations or best practices listed, and discussed how some of our best practices align with the guidance. Through this detailed discussion, we hope to demonstrate to our clients in the pharmaceutical industry how our XRD capabilities and services meet the requirements detailed within the USP <941> general chapter.

Contact us to learn more about how we can assist you with particle identification/analysis.