Laboratories have traditionally relied on volumetric methods for preparing samples, standards and solutions. This manual process has changed very little over the years, and has considerable risk of variability and errors. New approaches and accessories, including state-of-the-art gravimetric methods, can address these errors and make sample preparation much more efficient and accurate.
Efficient Sample and Solution Preparation
In a typical analytical laboratory, sample and solution preparation is a crucial part of many processes, lying at the heart of accurate, precise data generation. Errors in sample prep steps have various consequences, particularly in regulated environments, where any non-compliance may require time-consuming and labour-intensive correction. If repeat analyses are necessary, consuming additional precious – often expensive – materials, confidence in laboratory data can fall.
Generally, samples for analysis need to be in solution, and this requires accurate, precise preparation of samples, standards and reagents. Standards of known concentrations must be accurately prepared for instrument calibration, as well as any other solutions or reagents, such as buffers, that are required for the analytical protocol.
Out-of-Specification Results
All laboratories − regardless of industry or field − must generate reliable, accurate data. This may or may not be regulation driven, and the impact of an OOS result will vary accordingly. In research laboratories, for example, the key driver is the need for method reproducibility and accurate results, whilst production, testing and QC laboratories are likely to be strictly regulated, in compliance with stringent GMP/GLP regulations. However, in striving to ensure highly accurate experimental data, the general principles applying to out-of-specification results are equally relevant to both regulated and non-regulated laboratories.
It is widely accepted that sample processing is the most time-consuming aspect of an analytical workflow (61%), from research published more than a decade ago. If you also consider that sample processing is the largest single source of OOS errors (30%) in an analytical workflow, followed by operator error (19%), then you quickly realise that addressing (improving) the manual sample processing steps can have a significant positive impact on both resources and reducing OOS errors (see Fig. 1).
Whilst enhanced analytical instrumentation and software have provided dramatic improvements and considerable time savings in sample analysis and data processing over the last decade, surprisingly sample processing techniques have changed very little during the last century, so it seems fair to assume that the percentage of time spent on sample processing is even higher now.
Volumetric Sample Preparation
Typically the sample is weighed directly into a volumetric flask−or alternatively onto a weighing paper and subsequently transferred into the flask−and then the flask is filled with solvent. Frequently, results are handwritten in a laboratory journal, with a risk of transcription errors, making any errors hard to identify and correct. The problem with errors at the sample preparation stage is that they will simply be perpetuated, or even magnified, in downstream processing.
Sources of variability and error
Even for experienced professionals, the steps involved in volumetric sample preparation can take a lot of manual time and effort and have a high risk of human error, as well as being prone to day-to-day and operator-to-operator variability.
Sources of variability and error in a typical volumetric workflow include:
How can I improve sample preparation?
Weighing an exact amount into a volumetric flask is challenging. It is easy to put too much or too little sample in the flask and the narrow neck makes it tricky to avoid spillage.
Although weighing the sample onto a weigh paper (or equivalent) might seem more straightforward, there is still a risk of sample loss during transfer to the volumetric flask (or destination vessel).
Sample loss during transfer can be reduced or avoided altogether with these helpful accessories.
Alternatively, gravimetric sample preparation does not require weigh papers, which eliminates any risk of sample loss during transfer to the volumetric flask. Only an approximate amount of sample needs to be weighed in− making it easier and much faster−because the variable amount of solvent added compensates for over or under-target samples, to always achieve the precise concentration.
With volumetric flasks, reading the meniscus correctly is a subjective action and therefore subject to variability and human error.
Gravimetric sample preparation does not require volumetric flasks, which eliminates any errors associated with subjective meniscus reading.
Up to 50% of new volumetric glassware fails to meet Class A specifications according to NIST.
Gravimetric sample preparation does not require volumetric glassware, so is not dependant on the glassware tolerances.
Volumetric flasks are usually calibrated at 20 °C. However, the contents of the flask are rarely measured at this temperature. Temperature variations cause by reaction of the contents or sonication of the flask can cause variability in the prepared concentration.
Gravimetric sample preparation does not require volumetric flasks, and so there is no variability of the prepared concentration due to temperature effects.
Mistakes in selection of the right size of volumetric flask or pipette can lead to errors in the prepared concentration.
Gravimetric sample preparation does not require specific glassware or volumetric equipment.
With volumetric flasks, as with any reusable glassware, there is a risk of cross-contamination, so the cleaning procedure used is critical.
Gravimetric sample preparation has no risk of cross contamination, as reusable glassware is not required.
OOS results can easily arise from labelling of the flasks incorrectly and transcription errors during manual recording of the data.
Gravimetric sample preparation eliminates transcription and labelling errors, as data is automatically recorded and transferred electronically. Labels can be printed, as required.
With volumetric sample preparation, weighing errors are typically minimized by preparing larger volumes of solution, because it is easier to accurately manually weigh out 10 mg of substance than 1 mg. This results in volumes far in excess of what is actually required being prepared, with only small proportions used for analysis, consuming additional precious (and often limited) sample and incurring additional costs relating to waste disposal.
With gravimetric sample preparation, laboratories are no longer constrained by the availability of appropriate volumetric glassware, only the required amount of solution that needs to be prepared; the need to round up the volume to the nearest flask size is unnecessary. The amount of substance and solvent used can be reduced by up to 90%.