Revolutionizing Sample Preparation in Analytical Laboratories

In pharmaceutical analysis, precision and error reduction are paramount for high-quality results. Traditional volumetric methods often introduce variability, leading to Out of Specification (OOS) results that disrupt analytical laboratory workflows. Gravimetric sample preparation offers a transformative solution, enhancing weighing accuracy in labs, reducing laboratory errors, and improving efficiency. This blog explores how the gravimetric method addresses these challenges, with insights from key figures and tables in the original document.

Understanding Gravimetric Sample Preparation

Gravimetric sample preparation involves weighing both the solid sample and the solvent to achieve precise concentrations, unlike the volumetric method, which relies on less accurate volume measurements using volumetric glassware. By leveraging gravimetric dosing mixing systems like Quantos powder, this approach minimizes variability, reduces OOS results, and streamlines laboratory processes.

The Impact of Out-of-Specification Results

OOS results have long challenged the pharmaceutical industry, notably since a 1993 court ruling involving Barr Labs, which clarified that OOS results may stem from laboratory errors rather than batch failures. The FDA’s 2006 guidelines emphasized thorough laboratory investigations to identify root causes. Figure 1 highlights sample processing and human error as the primary sources of OOS results, with sample processing consuming over 61% of laboratory time. Figure 2 illustrates the formal OOS investigation process, which can take days or months, costing thousands of dollars and generating complex Corrective and Preventative Actions (CAPAs) that complicate SOPs.

Figure 1: Sources of OOS results and time spent in laboratory

Figure 2: illustrates the accepted formal process for how to investigate OOS results

Figure: 2 Formal process for an OOS investigation.

Good Weighing Practice: Ensuring Accuracy

The Good Weighing Practice (GWP) standard provides a scientific foundation for weighing accuracy in labs, addressing measurement uncertainty and minimum weight requirements per USP General Chapter <41>. Figure 3 shows how measurement uncertainty increases with smaller sample weights, emphasizing the need for precise calibration balance procedures. A common misconception is that tare weight contributes to minimum weight compliance, but USP General Chapter <1251> clarifies that only the sample weight matters.

Figure 3: Measurement Uncertainty: Absolute (green line) and relative (blue line) measurement uncertainty of a weighing instrument. The accuracy limit of the balance, the so-called minimum weight, is the intersection point between relative measurement uncertainty and the required weighing accuracy.

To account for types of laboratory variability, such as environmental factors and operator inconsistencies, a safety factor is applied—typically 2 for manual weighing but reduced to 1.5 for automated sample preparation systems like Quantos. Figure 4 likely illustrates the application of this safety factor, showing how it compensates for variability in weighing processes to ensure reliable results.

Figure 4: Safety Factor: Variability of the relative measurement uncertainty due to changing environmental conditions and influences introduced by the operator. Weighing in the green area guarantees adherence to the weighing accuracy requirements (application of a safety factor).

Challenges with Volumetric Methods

The volumetric method relies on volumetric glassware, introducing several error sources:

Figure 5 details a typical volumetric method workflow, outlining steps like gathering equipment and material, weighing, labeling, sonicating, and cleaning. This multi-step process, often exceeding 40 steps for multiple samples, is prone to laboratory errors. Figure 6 contrasts tuned and untuned sonicators, highlighting inconsistent energy distribution in mixing, a key source of variability. Manual labeling further risksNums OOS results due to identification errors, requiring solvent-based removal that adds inefficiencies.

Figure 5: Simple sample preparation workflow

Table 1: Relative percent errors for Class A glassware

Figure 6: Foils from a tuned and untuned sonicator

The Advantages of Gravimetric Sample Preparation

Gravimetric sample preparation, as per USP General Chapter <1251>, uses automated sample preparation systems like the Quantos powder gravimetric dosing mixing system, depicted in Figure 7 . These systems, equipped with RFID (radio frequency identification) for process security, weigh solids and liquids into disposable vials, eliminating cross-contamination risk. volumetric glassware and reducing

Figure 7: Gravimetric dispensing systems: XPE balances with Quantos powder and liquid dosing modules

Key Benefits of Gravimetric Sample Preparation

* = Automated powder dispensing for free-flowing powders; Manual dispensing for sticky powders, pastes, gels, liquid samples, etc.

** = typical value

Table 2: Gravimetric dispensing system specifications

These automated gravimetric systems are being adopted by analytical and QA/QC laboratories in the pharmaceutical industry.

Figure 8: New simplified sample prep workflow using a gravimetric approach

Figure 9: Comparison of volumetric and gravimetric sample preparation on the amount of solvent used

* = typical value

** = The safety factor quoted is that recommended for stable environments and trained operators. For unstable environmental conditions
or insufficiently trained operators higher safety factors should be used.

Table 3: Substance saving based on automated sample preparation methods

Volumetric vs. Gravimetric: A Direct Comparison

Table 4 highlights how gravimetric sample preparation reduces determinate and indeterminate errors compared to the volumetric method. For a 0.25 mg/mL solution, the volumetric method requires a 200 mL flask and 50 mg of material, while the gravimetric method uses significantly less. Table 5 details an experiment where a 0.603 mg/g standard was prepared with high precision using Quantos, contrasted with Table 6, showing higher variability (0.60% RSD) in the volumetric method versus 0.40% for the gravimetric method. Table 7 summarizes these advantages, emphasizing substance savings, reduced cross-contamination risk, and improved accuracy.

Table 4 : Comparing errors in volumetric and gravimetric processes

Table 5: Automated gravimetric sample preparation data

Table 6: Manual (volumetric) sample preparation

Table 7: Comparison of volumetric vs. gravimetric methods

Conclusion

Gravimetric sample preparation revolutionizes analytical laboratory workflows by addressing the root causes of OOS results. Using automated sample preparation systems like Quantos, laboratories achieve superior weighing accuracy, eliminate volumetric glassware errors, and enhance efficiency. These systems reduce laboratory errors, minimize waste, and improve compliance through automated data management. For laboratories seeking precision and reliability, the gravimetric method is a critical advancement.

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References:

1. United States of America v. Barr Laboratories, Inc., 812 F Supp 458 (DNJ 1993).

2. U.S. Food and Drug Administration. Guidance for Industry: Investigating out-of-specification (OOS) test results for pharmaceutical production. FDA. Available at: www.fda.gov/downloads/Drugs/GuidanceCompli-anceRegulatoryInformation/Guida nces/UCM070287.pdf.

3. Majors, R.E. LC/GC Magazine, 1991, 1997, 2002

4. General Chapter <41> “Weights and Balances”, US Pharmacopeia USP34 – NF29, Rockville, Maryland, 2013, Online-Edition

5. GWP® - Good Weighing PracticeTM – A Risk-Based Approach to Select and Test Weighing Instruments, White Paper, Mettler-Toledo AG, Greifensee, Switzerland, July 2009.

6. Reichmuth A., Fritsch K., Good Weighing Practices in the Pharmaceutical Industry – Risk-Based Qualifica- tion and Life Cycle Management of Weighing Systems, Pharmaceutical Engineering, Volume 29, Number 6, ISPE, Tampa FL, USA, 2009.

7. Fritsch K., Quenot J.-L., Good Weighing Practices – Avoid OOS Results with Proper Weighing, Pharmaceuti- cal Formulation & Quality, Volume 14, Number 1, Wiley-Blackwell, Hoboken, NJ, USA, 2012.

8. Evaluation of Measurement Data – Guide to the Expression of Uncertainty in Measurement (GUM), JCGM 100:2008, Bureau International des Poids et Mesures, Sèvres, France, 2008. Available at www.bipm.org.