A Gravimetric Approach to Sample Preparation at Pfizer

Awareness of the benefits of gravimetric sample preparation has increased significantly over the past couple of years. Recognition of this state-of-the-art technology by industry organizations such as the United States Pharmacopeia (USP) has supported this trend. A recent revision to USP chapter 1251 “Weighing on an Analytical Balance”, which became official on Dec 1, 2013, included a detailed description of the steps involved in gravimetric dosing for sample and standard preparation.

Pfizer’s Analytical Research and Development Group (AR&D) in Groton, Conn., have embraced this new approach to sample preparation using a pioneering fully automated gravimetric sample preparation workstation. Detailed studies have been carried out which compare the differences between preparing samples and standards using a conventional manual volumetric process and the new automated gravimetric method.

“Sample preparation is an analytical workflow focus area. We are delighted to have worked with Mettler-Toledo in the development and preliminary evaluation of the Quantos QX series. A targeted automated (gravimetric) sample preparation approach with the QX has demonstrated improved precision, reduced sample and solvent consumption and less analyst time as compared to manual approaches,” says Gang Xue, associate research fellow, Pfizer ARD.

In this article, data is presented from a reproducibility/precision study performed by the Analytical Group, using the non-proprietary material caffeine. In this experiment sample preparation is performed manually with volumetric flasks and compared with a new fully automated method, where samples are prepared on a gravimetric sample preparation workstation (Quantos QX7 from Mettler Toledo). The QX7 workstation incorporates a six-place microbalance for weighing of the solids and solvents with automated interchange of up to 10 solid dosing heads and five solvent dosing heads.

Enhanced reproducibility and precision
This study investigated the reproducibility and precision of manual sample preparation compared to an automated gravimetric method. For the manual volumetric method, six replicate solutions were prepared by weighing 20 mg of caffeine on an analytical balance and making it up to 50 mL in a volumetric flask using a 30:70 methanol/water mixture. This procedure took a total of 50 min. Dissolving/mixing the samples on a shaker took 15 min of this time, so the manual labor time is 35 min full-time equivalent (FTE)—five minutes per sample for each weighing and dilution plus five minutes for the diluent prep. The relative standard deviation of these samples was 1.67%. Table 1 shows the manual volumetric data.

Sample Set Name:			MW Manual Repeatability
Sample Set Id:			18622
Result Set Id:			18740

	Sample Name	Result ID	Name	*Area (µVsec)**	Calc. Conc. (mg/mL)
1	M1	18744	Caffeine	5568747.28	0.413
2	M2	18745	Caffeine	5694384.72	0.423
3	M3	18746	Caffeine	5646098.85	0.419
4	M4	18747	Caffeine	5450694.28	0.405
5	M5	18748	Caffeine	5501403.13	0.408
6	M6	18749	Caffeine	5632460.66	0.418
Mean					0.41
%RSD					1.67

Table 1: Manual sample preparation using a volumetric flask – reproducibility and precision

Table 2 shows the equivalent data from the automated gravimetric method. Six replicate solutions were prepared by weighing 5 mg of caffeine and adding 12.5 g of the same diluent (30:70 methanol/water) directly into a vial. This procedure took a total of 30 min, consisting of 10 min preparation time—such as filling and installing powder and liquid dosing heads and setting up the sequence—with an additional 20 min running time. The RSD of these samples was 0.49%.

Sample Set Name:			MW Automated Repeatability
Sample Set Id:			17729
Result Set Id:			17808

	Sample Name	Result Id	Name	*Area (µVsec)**	Calc. Conc. (mg/mL)
1	A1	17812	Caffeine	5278355.59	0.377
2	A2	17813	Caffeine	5264847.66	0.376
3	A3	17814	Caffeine	5297039.30	0.378
4	A4	17815	Caffeine	5265959.19	0.376
5	A5	17816	Caffeine	5297020.61	0.378
6	A6	17817	Caffeine	5224532.00	0.373
Mean					0.38
%RSD					0.49

Table 2: Automated gravimetric sample preparation – reproducibility and precision

	Manual Prep:	Automated Prep:	Difference:
Amount of substance	20 mg solid + 50 mL diluent	5 mg solid + 12.5 g diluent	Save 75% substance
Time	50 mins (total) 35 mins (FTE)	30 mins (total) 10 mins (FTE)	Save 70% labour time
Precision	%RSD = 1.67	%RSD = 0.49	Improve precision x 3

Table 3: Comparison between manual and automated sample preparation

This study highlights three key advantages of automated gravimetric sample preparation: 75% less substance and solvent are used to prepare the sample solutions; 70% of labor time is saved in the sample preparation steps; and, most importantly, the precision is improved by a factor of more than three.

Practical advantages of automated gravimetric sample preparation
The reproducibility of the samples prepared is significantly improved, which will have a knock-on effect in the quality of the analytical results generated, which in turn will lead to a reduction in out-of-specification results caused by weighing and dilution errors. The reduced labor time and amount of substance and solvent saved also has the potential to have a dramatic impact on laboratory efficiency and running costs for a typical analytical laboratory.

Revised version of the article first published in q&more 01/13. With friendly permission of Succidia AG