Errors in Sample Preparation

Avoidance of errors in sample preparation (extraction, derivatization) could be minimized by rigorous training of laboratory personnel, including appreciation of the patient behind each anonymous test tube. An environment free of noise and distractions is required to minimize the risk of serial solvent extractions being pooled in the wrong tube redundant labeling of glassware and step-by-step checklists are also critical elements of error prevention and detection. 

New methods for non-destructive quantitative analysis of additives based on MIR spectra and multivariate calibration have been presented [67, 68], One of the limitations in the determination of additive levels by MIR spectroscopy is encountered in the detection limit of this technique, which is usually above the low concentration of additive present, due to their heavy dilution in the polymer matrix. The samples are thin polymer films with small variations in thickness (due to errors in sample preparation). The differences in thickness cause a shift in spectra and if not eliminated or reduced they may produce non-reliable results. Methods for spectral normalisation become necessary. These methods were reviewed and compared by Karstang et al. [68]. MIR is more specific than UV but the antioxidant content may be too low to give a suitable spectrum [69]. However, this difficulty can be overcome by using an additive-free polymer in the reference beam [67, 68, 69, 70]. On the other hand, UV and MIR have been successfully applied to quantify additives in polymer extracts [71, 72, 66]. 

Internal standards are used to correct for errors in sample preparation or sample introduction and to help determine solute recoveries. They are added to the sample at the earliest possible introduction point in the analytical process. Standards solutions containing the solutes of interest are prepared, preferably at two or three different, known concentrations but with a constant concentration of internal standard. The same concentration of internal standard is also added to each sample. All the standards and samples receive the same treatment from sample preparation, through the sample introduction and separation processes, to detection. 

One important practical aspect of PLS is that it takes into account errors both in the concentration estimates and spectra. A method such as PCR will assume that the concentration estimates are error free. Much traditional statistics rest on this assumption, that all errors are in the dependent variables (spectra). If in medicine it is decided to determine the concentration of a compound in the urine of patients as a function of age, it is assumed that age can be estimated exactly, the statistical variation being in the concentration of a compound and the nature of the urine sample. Yet in chemistry there are often significant errors in sample preparation, for example, accuracy of weighings and dilutions and so the independent variable (c) in itself also contains errors. With modem spectrometers, these are sometimes larger than spectroscopic errors. One way of overcoming this difficulty is to try to minimise the covariance between both types of variables, namely the x (spectroscopic) and c (concentration) variables. 

Table 19-1. Typical. sources of error in sample preparation.

Other common error sources that are included are technician error in sample preparation, temperature differences in standards or instrument while taking data, cahbration standard instability, instrument noise and drift, changes in instrument wavelength sethng, nonlinearity, stray light effects, particle size differences, color differences with concentrahon, solvent interaction differences with change in concentrahon, and the reference method does not measure the same component as a spectroscopic method. An example of the last problem would typically be found if measuring the percent... 

Another advantage of the reference element technique is that dilution errors can be recognized and eliminated when the reference element is added at an early stage of sample preparation before the final dilution. Moreover, using the reference element technique, quantitative analysis could actually be done without accurate dilution and without using volumetric flasks etc., resulting in additional timesaving and elimination of potential errors in sample preparation. 

Industrial analytical laboratories search for methodologies that allow high quality analysis with enhanced sensitivity, short overall analysis times through significant reductions in sample preparation, reduced cost per analysis through fewer man-hours per sample, reduced solvent usage and disposal costs, and minimisation of errors due to analyte loss and contamination during evaporation. The experience and criticism of analysts influence the economical aspects of analysis methods very substantially. 

In bioanalysis, High-Performance Liquid Chromatography (HPLC) is the analytical technique most frequently used. Often, extended sample preparation is required to make a biological sample (the matrix) suitable for HPLC-analysis. The compound of interest, the analyte, has to be isolated from the matrix as selective and quantitative as possible. The quality of the sample preparation largely determines the quality of the total analysis procedure. In a survey Majors [2] showed that approximately 30% of an error generated during sample analysis was due to sample preparation, which indicates the need for error reduction and quality improvement in sample preparation. 

Different Approaches for Linearity Determination. The first approach is to weigh different amounts of authentic sample directly to prepare linearity solutions of different concentrations. Since solutions of different concentration are prepared separately from different weights, if the related substances reach their solubility limit, they will not be completely dissolved and will be shown as a nonlinear response in the plot. However, this is not suitable to prepare solutions of very low concentration, as the weighing error will be relatively high at such a low concentration. In general, this approach will be affected significantly by weighing error in the preparation. 

Sample preparation in NLC and NCE is the most important step in analysis due to the nano nature of these modalities. The sampling should be carried out in such a way as to avoid changes in the chemical composition of the sample. The quantitative values of species depend on the strategy adopted in sample preparation. Extraction recoveries may vary from one species to another and they should, consequently, be assessed independently for each compound as well as for the compounds together. Materials with an integral analyte, that is, bound to the matrix in the same way as the unknown, which is preferably labeled (radioactive labeling) would be necessary, which is called method validation. As discussed above few papers described off- and online sample preparation methods on microfluidic devices. Of course, online methods are superior due to lower risk of contamination and error of methods. Not much work been carried out on online nanosample preparation devices, which need more research. Briefly, to get maximum extraction of analytes, sample preparation should be handled very carefully. 

These errors with the exception of missed holding time are correctable provided that the sample still exists in a quantity sufficient for re-extraction and reanalysis. The short holding time for organic analysis extraction (7 days for water samples and 14 days for soil samples) is always a limiting factor in sample preparation. (Organic analyses have two different holding times as shown in Appendices 12 and 13, one for extraction and the other one for analysis). 

This equation does not have an unique solution. The same value of error, Et, can be obtained by using different combinations of ns and na. Combinations of ns and na should be chosen based on scientific judgment and the cost involved in sample preparation and analysis. 

There are several other factors that are important when it comes to the selection of equipment in a measurement process. These parameters are items 7 to 13 in Table 1.2. They may be more relevant in sample preparation than in analysis. As mentioned before, very often the bottleneck is the sample preparation rather than the analysis. The former tends to be slower consequently, both measurement speed and sample throughput are determined by the discrete steps within the sample preparation. Modern analytical instruments tend to have a high degree of automation in terms of autoinjectors, autosamplers, and automated control/data acquisition. On the other hand, many sample preparation methods continue to be labor-intensive, requiring manual intervention. This prolongs analysis time and introduces random/systematic errors. 

Errors in standard and sample preparation. Gravimetric and volumetric errors in the preparation of the standard and the sample are... 

Most mills control the cooking cycle by automatic time-temperature controllers and recorders. The rate of temperature rise to the conversion plateau must be slow to prevent hot pockets or cold areas. The rate of temperature increase to the inactivation plateau must be rapid to prevent excessive depolymerization in the intermediate temperature range. The viscometers operate according to different mechanisms time to expel paste from a sample device (Norcross) vibration of a probe in the paste (Dynatrol) torque readings (Brookfield) or pressure drop on passage through an orifice (Escher Wyss). Potential errors in viscosity can result from variations in starch solids due to differences in moisture content of the starch, errors in slurry preparation and the quantity of condensate added by the steam. The process yields a maximum paste concentration of about 32%. 

Sample preparation represents a formidable challenge in the chemical analysis of the real-world samples. Not only is the majority of total analysis time spent in sample preparation, but also it is the most error-prone, least glamorous, and the most labor-intensive task in the laboratory. The components to be separated from the matrix are usually taken up with an auxiliary substance such as a carrier gas, an organic solvent, or an adsorbent. These separation processes can be regarded as extraction procedures (i.e., liquid-liquid extraction, liquid-solid extraction, Soxhlet extraction, solid-phase extraction, supercritical fluid extraction, solid-phase microextraction, etc.). 

From previous statistical studies on similar systems which Include the combination of errors Involved in sample preparation and measurement of static compressive stress-strain characteristics, the following reproducibility results were obtained (4,5, ) ... 

Because initial sample preparation is essentially practical in nature and because the processes utilised are often considered to be well understood, the potential for serious error during sample preparation is frequently underestimated. The provision of a sufficient supply of representative sub-samples for analysis irrespective of sample type involves applying a fit for purpose method for each individual sample prepared. The following chapter describes some of the most widely used initial sample preparation methods and proposes practical solutions to problems encountered in the preparation laboratory. 

Behne, D. (1981). Sources of error in sampling and sample preparation for trace element analysis in medicine. J. Clin. Chem. Clin. Biochem., 19,115. 

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