Sample intake

In the preparation of many solid state reference materials, reduction of the grain size plays an important role. Usually this reduction is required because of the measurement methods to be used both in the projects and later by the users of the reference material, as well as to come to an acceptable minimum sample intake. The minimum sample intake can be defined as the minimum amount of material needed, so that the heterogeneity of the material does not affect the repeatability of the measurement method. The reduction of the grain size is usually implemented by crushing and/or grinding techniques. The techniques employed and the equipment used must be suitable for the purpose of processing the material. Potential problems of contamination, loss of volatile components, and/or other physical and... 

High sample intakes (reduced homogeneity problems)... 

The first publications on SFE of APEO were discussed in a review on analytical methods for APEO [42]. For the determination of alkylphe-nols in sewage sludge and sediment, a SFE technique was optimised, using C02 at 80°C and 351 atm, at a flow rate of 2 mL min-1. Extraction times were 15 min static and 10 min dynamic, with a sample intake of 0.1-1 g [41]. In this method, in situ acetylation of the alkylphenols using acetic anhydride was performed. The extract was washed with an aqueous K2C03 solution to remove co-extracted acetic acid, and cleaned up using 5% deactivated silica. 

Most existing methods are based on instrumental analysis involving exhaustive sample pretreatment and preconcentration steps, followed by purification and fractionation before final chromatographic separation and detection. For fat and oil samples, dissolving the lipids in an appropriate solvent is usually the first treatment. This has been achieved by melting the fat at 90°C followed by LLE or direct solid liquid extraction (SEE) with an apolar solvent [37], column extraction with a mixture of apolar solvents after drying of the sample with anhydrous Na2S04, Soxhlet extraction and/or sonication with apolar solvents. Typically, sample intake is between 0.5 g and 1 g and quantitative recoveries >60% have been reported. 

Greenfield ef. ai.l l) observed a reduction of signal intensity that correlates with sample intake effects from the modified solution viscosity and/or surface tension of mineral acids. This, coupled with peristaltic pumping of solutions into the nebulizer, considerably reduces physical interferences. Increased salt concentration also has an effect on solution physical properties. In the experience of these authors, the high levels of salt in the matrix also increases the noise from the nebulizer system. This degradation of nebulizer performance, which is not necessarily accompanied by a proportional reduction in sensitivity, is the cause of the observed deterioration of detection limits in real samples as opposed to ideal solutions. 

Another method of probing sulfuric acid aerosols is to heat the sample intake sufficiently to vaporize sulfuric acid-water aerosols but not other particles such as those containing ash minerals the difference between the measured particles with and without intake heating provides a measure of the contribution of sulfuric acid-water. Using this technique, Deshler et al. (1992), for example, have shown that more than 90% of the stratospheric particles above Laramie, Wyoming, after the Mount Pinatubo eruption were composed of sulfuric acid-water mixtures. 

FIGURE 3.8 Total sample intake of KK-Ay mice during the experimental period. For the... 

FIGURE 1.11 Basic components of an automatic sampler 1—sample intake 2—sample transport tube 3—pump controller and 4—sample bottles. (Based on Dick, M.E. 1996. In L.H. Keith (ed.), Principles of Environmental Sampling, 2nd edition, pp. 237-258. American Chemical Society and Dick, E.M. 1994. In B. Markert (ed.), Environmental Sampling for Trace Analysis, pp. 255-278. VCH.)... 

Expressed for 2 g (wet weight) sample intake and 100 mL final volume following the three sigma criterion. 

Two lateral holes allow for the crossing of electrons and another hole at the bottom allows the product ions to pass through. Moreover, there is a reagent gas input tube and an opening for the sample intake. The sample is introduced by means of a probe which will close the opening. 

Seawater samples processed in the manner just stated are placed in a freezer and returned to the laboratory frozen until analyzed. Biological and chemical activity between the time samples are collected and analyzed are greatly reduced. For analysis the flasks are taken from the freezer, placed in a water-bath shaker, and agitated for 30 min until the samples reach 25 °C. The bottie is connected directly to the sample intake line on the instrument via Vi6-in. outside diameter (OD) stainless steel tubing. 

Each laboratory was requested to perform analyses in compliance with a detailed set of instructions and to provide information on sample intake, moisture content, digestion procedure, analytical techniques, calibrants, use of CRMs and anything else that could be relevant to the determinations. Five independent analytical... 

Care must also be taken to ensure that particle size distributions are adequate as selected particle size distributions will achieve greater homogeneity in reference materials especially when very low sample intakes are used for measurements. This is extremely important for environmental reference materials such as soils, sediments, etc., and is optimally achieved by jet-milling with ultra-fine classification of particles. With this technique, which is non-contaminating, fast and well-controlled, the sieved fraction below 2 mm of the material is ground by... 

Thallium measurements were performed on the original sediment on the analytical spectral line at 276.8 nm. The accuracy of the result was estimated at 10% (Is), thus leading to a final result of 0.29 0.03 mg kg. A homogeneity factor He = 11.7% X yfing was derived from these results. This factor is relatively high due to the relative large sample intake. 

The suitability of the quality control tools prepared under the SWIFT-WFD project was proved from the homogeneity and stability studies carried out on all materials. Table 5.1.3 summarizes the tested analytes for each material, the institution responsible for the analysis, and the analytical technique and sample intake used. 

Material Center Analysed elements (Analytical techniques) Sample intake... 

Calibration / Separation techniques Detectors response +1 Work outside linear range Saturation/under detection limit Contamination of detector Lower/increase sample intakes or concentrate Dilute sample or/and calibration samples Run blanks... 

The use of the CRM, the information it bears and the quality of the material it represents, tells the operator that his s value is due to the measurement precision of his method. If relevant, the CRM should tell him which parts of the uncertainty of the measurements are not due to the analytical method but could come from inhomogeneity of the material. Unless very small sample intakes of solid materials are used (solid sampling AAS, INAA), this inhomogeneity should be negligible. At least in using CRMs for environmental monitoring, if the analyst compares his value of. v with a stated. 

Multiple phases. Natural or artificial (manufactured) solids can be composed in several phases. The quality of the dispersion of these phases one in the other will allow the analyst to benefit from similar test samples within the entire batch of material. Material sciences teach us that the degree of homogeneity of the mixture will depend on the nature of the mixed phases and, in particular, the size of the solid particles, the particle size distribution and the respective density. The more similar the density and the particle size, the better is the achieved homogeneity. To achieve similar test samples, even for very small particle sizes, requires a narrow particle size distribution. Muntau et al. [29-30], Griepink et al. [31] and Pauwels et al. [32] have studied the relation between particle size and minimum sample intake. They have demonstrated that a major factor for homogeneity of small sample intakes is the low particle size of the solid material and their size distribution. This aspect will be discussed in more details in the section on homogeneity testing. 

When properly ground and mixed, the material can be brought to a stage where very small portions are representative of the whole batch of material. When no difference between these small portions can be detected the material is called homogeneous for the studied test portion or sample intake. The smaller this portion is, the better the homogeneity. 

The real difficulties remain in the determination of U. It is relatively simple to determine the method uncertainty of nondestructive analysis as repeated measurements can be performed on the same sample [39]. It is far more difficult with destructive methods and in particular in organic trace analysis. In the latter case, all the steps in the procedure rarely allow one to achieve a repeatability with a relative standard deviation of less than several percent. The methods often require a large sample intake as samples of a few milligrams are not easy to handle in extraction systems. 

In general, when trace substances or elements have to be studied, the sensitivity of the method can be a limiting factor. The analyst cannot establish at which level of sample intake inhomogeneity of the material is detectable because the sensitivity of the... 

A reference material is produced so that each unit allows repeated measurements. It is necessary to verify that all test portions in the unit of (C)RM are identical at least for the parameter to be measured. The within-vial inhomogeneity is the basic investigation on the validity of the homogenisation procedure. It will also have the objective to determine, if possible, the minimum sample intake. As already discussed above the minimum sample size often corresponds to the limit of determination of the method rather than the real inhomogeneity level of the material. The determination of the minimum sample size and a first view on the homogeneity can be obtained already on the bulk material before packaging. The final assessment must be performed on the packed material. 

The minimum sample intake for which homogeneity becomes a significant uncertainty factor of the measurement is (often) however debatable. The CRM is intended to validate methods used for routine measurements. In daily practice of environmental monitoring, the prerequisite is to sub-sample representative samples (see Chapter 2). The problem lies more in having the possibility of handling large samples rather than extremely low sample sizes. It is another debate when forensic analyses are performed as methods may have to be validated on extremely small samples. 

For materials destroyed during analysis, it is not possible to measure all individual samples. If the trend in preparation is continuous and known, an extrapolation on the uncertainty of the certified value can be attempted but this would also decrease the analytical value of the CRM. In the majority of situations drifts or trends are not foreseen and known. Therefore, it will be necessary to measure samples all over the packaging procedure. The between-vial homogeneity study has the objective to verify that no difference exists in the parameter of interest (concentration of the element or substance) between sets. It may also verify that the matrix composition remains similar. The objective of the within-vial study was to verify the homogenisation procedure and estimate the minimum sample intake if necessary. The between-vial homogeneity assesses that the material is suitable for comparability between laboratories. 

All tests must be performed in the most repeatable way. Again experiments should be done in parallel all extractions in parallel, if possible one calibration. If necessary and possible (stability problems), all extracts produced in parallel should be stored in the dark at low temperatures. Purification and final determination should be done also in parallel. It is preferable to study the between-vial homogeneity with the same sample intake as for the within-vial test. This should allow more direct comparisons of results. 

Long-term reproducibility is the prime quality of an analytical method used for the study of stability. Stability must be performed on the element or substance to be certified. Extrapolations on the stability of tracers are of little interest. If the uncertainty of the method for an analyte is poor, even for large sample intakes, the analyst must refer to the experience on similar materials with higher contents or to the general chemical properties of the substance or element. In any other situations certification may be impossible. Such difficulties are mainly encountered in organic or organo-metallic analysis. Experience has shown that some substances may be stable in a matrix but unstable in another even similar one [46-47]. All analytical methods suffer from long term reproducibility. In some cases analysts have developed tools and tricks to minimise this effect. 

The distribution of a limited number of microbes within a solid matrix is essentially inhomogeneous, i.e. for a certain level of intake, differences due to the distribution of the microbes will appear. The more microbes that have been introduced into the matrix, the less these differences will be important and the lower the sample intake will be where differences appear. The theoretical distribution of the microbes in the sample is described by a Poisson distribution [29,30]. The RIVM group, which developed these materials, used a modified Cochran s dispersion test to evaluate the variation within a single capsule and between capsules. In fact overdispersion (more inhomogeneity than theoretically expected) has been noticed for nearly all RMs and CRMs produced [31]. The certification trial revealed that it had no influence on the outcome of the interlaboratory certification study as the between-laboratory reproducibility largely covered this overdispersion factor. 

The minimirai sample intake for the determination of the contents of the six analytes in the contaminated wood sample as well in the blank wood sample and also for the recovery experiments is 1,0 g of wood. 

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