Reliability of sampling

The reliability of any environmental analytical data depends upon the reliability of sample quality. To generalize from analytical results on a small portion of material to a larger population requires careful planning and execution if bias is to be avoided. This article considers the general problems involved in sampling heterogeneous bulk populations such as soil, air, and natural waters specific details for particular types of materials are not included. These problems include the heterogeneity of most environmental materials the costs in time, manpower, and effort required for collection of real samples and the need to avoid contamination or decomposition of samples after collection. 

For a method to be useful it must provide reliable results. Unfortunately, methods are subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis. When a method is relatively free from chemical interferences, it can be applied to the determination of analytes in a wide variety of sample matrices. Such methods are considered robust. 

A considerable amount of time is necessary to reach the point at which sample analyses can commence, and it is essential that the stability and reliability of the mass spectrometer be high to ensure maximum sample throughput during the limited time available between calibration checta. 

Instrumental Interfaces. The basic objective for any coupling between a gas chromatograph (gc) and a mass spectrometer (ms) is to reduce the atmospheric operating pressure of the gc effluent to the operating pressure in the ms which is about 10 kPa (10 torr). Essential interface features include the capability to transmit the maximum amount of sample from the gc without losses from condensation or active sites promoting decomposition no restrictions or compromises placed on either the ms or the gc with regard to resolution of the components and reliability. The interface should also be mechanically simple and as low in cost as possible. 

This states that the sample standard deviation will be at least 72 percent and not more than 128 percent of the population variance 90 percent of the time. Conversely, 10 percent of the time the standard deviation will underestimate or overestimate the population standard deviation by the corresponding amount. Even for samples as large as 25, the relative reliability of a sample standard deviation is poor. 

It is known that the reliability of analytical information obtained depends particularly on the range of reference materials (RM) used. The most of RMs developed by the Institute of Geochemistry, SB RAS are included in the State Register of certified types of National Certified Reference Materials of Russian Federation. The reference materials are routinely analyzed for the stability and their life durations are timely prolonged. Developed RMs (27 samples) characterize mainly mineral substances. 

Analysis of soils is an important task in the environmental researches. Reliability of ICP-MS results of soil analysis mainly depends on chemical sampling. Recently microwave systems are widely used for preparation of different samples. Influence of microwave radiation on sample ensures a complete decomposition of sample, greatly increases the mineralization, and allows possible losses of volatile elements to be minimized. In the given study to intensify decomposition of soils we applied the microwave sample preparation system MULTIWAVE (Anton Paar, Austria and Perkin-Elmer, USA) equipped with rotor from 6 autoclaves with TEM reaction chambers of 50 ml volume. 

With the best observing conditions, it is possible for the trained observer to compete with photoelectric colorimeters for detection of small color differences in samples which can be observed simultaneously. However, the human observer cannot ordinarily make accurate color comparisons over a period of time if memory of sample color is involved. This factor and others, such as variability among observers and color blindness, make it important to control or eliminate the subjective factor in color grading. In this respect, objective methods, which make use of instruments such as spectrophotometers or carefully calibrated colorimeters with conditions of observation carefully standardized, provide the most reliable means of obtaining precise color measurements. 

We have said that every time the calibration analyzes a new unknown sample, this amounts to an additional validation test of the calibration. It can be a major mistake to believe that, just because a calibration worked well when it was being developed, it will continue to produce reliable results from that point on. When we discussed the requirements for a training set, we said that collection of samples in the training set must, as a group, be representative in all ways of the unknowns that will be analyzed by the calibration. If this condition is not met, then the calibration is invalid and cannot be expected to produce reliable results. Any change in the process, the instrument, or the measurement procedure which introduces changes into the data measured on an unknown will violate this condition and invalidate the method If this occurs, the concentration values that the calibration predicts for unknown samples are completely unreliable We must therefore have a plan and procedures in place that will insure that we are alerted if such a condition should arise. 

The reliability of the results depends in large measure on how well deviations from the (ideal) linear relationship between log / and dry weight per unit area can be eliminated or allowed for. As is well known, this can be accomplished by the comparative method (3.10), provided that standard (reference system) and unknown, identical in mass, shape, and elementary composition, are exposed to the same x-ray beam. In the cytological investigations, these conditions are difficult to meet, not only because the samples are complex in composition, but also because they are very small, as is clear from the units employed (micromicrograms per square micron or 10 12 gram per 10 8 sq cm). 

The reliability of the results may be judged from the following. In sections of biological materials, about 10 pt thick, the absolute errors in the determination of calcium ranged between 0.15 and 0.2 g per square micron those for phosphorus and sulfur between 0.06 and 0.1 /igg per square micron. These errors often corresponded to less than 5% of the amount of element present in the sample. 

The most reliable way to control the chelant program is to base it on FW chelant demand, rather than to aim at maintaining a chelant residual in the boiler. The fact is that most chelant residual tests in the field are widely inaccurate, and EDTA begins to break down at pressures over 400 psig. Nevertheless, it is useful to conduct these tests on a purely qualitative basis, simply to gain some confidence that a small residual exists. Consequently, recommended chelant residuals are provided in the appropriate tables. Needless to say, it is imperative that the chelant demand is made on a truly representative FW sample or series of samples. 

As is well known the difficulty of analysis of a sample increases as its complexity increases. Analysis usually commences with a rather nonspecific clean-up step and requires that the separation step that follows be highly selective and depends on a detection step that is as specific as possible. As the selectivity of detection increases there is also an increase in the reliability of the identification and it is possible to reduce the demands made on the selectivity of the preceding separation method. This is the case for radiometric and enzymatic methods and also explains the popularity of fluorescence measurements. The latter obtain their selectivity from the freedom to choose excitation and measurement wavelengths. 

Analysis of methyl parathion in sediments, soils, foods, and plant and animal tissues poses problems with extraction from the sample matrix, cleanup of samples, and selective detection. Sediments and soils have been analyzed primarily by GC/ECD or GC/FPD. Food, plant, and animal tissues have been analyzed primarily by GC/thermionic detector or GC/FPD, the recommended methods of the Association of Official Analytical Chemists (AOAC). Various extraction and cleanup methods (AOAC 1984 Belisle and Swineford 1988 Capriel et al. 1986 Kadoum 1968) and separation and detection techniques (Alak and Vo-Dinh 1987 Betowski and Jones 1988 Clark et al. 1985 Gillespie and Walters 1986 Koen and Huber 1970 Stan 1989 Stan and Mrowetz 1983 Udaya and Nanda 1981) have been used in an attempt to simplify sample preparation and improve sensitivity, reliability, and selectivity. A detection limit in the low-ppb range and recoveries of 100% were achieved in soil and plant and animal tissue by Kadoum (1968). GC/ECD analysis following extraction, cleanup, and partitioning with a hexane-acetonitrile system was used. 

Carey et al. 1979 Kadoum 1968). Some problems still exist with sample preparation and separation, which affect the precision, accuracy, and specificity of analyses. Further studies to improve sample preparation and selectivity of detection might be beneficial in improving the reliability of existing methods. 

Research use of analytical results in the framework of a nonanalytical setting, such as a governmental investigation into the spread of pollution here, a strict protocol might exist for the collection of samples (number, locations, time, etc.) and the interpretation of results, as provided by various consultants (biologists, regulators, lawyers, statisticians, etc.) the analytical laboratory would only play the role of a black box that transforms chemistry into numbers in the perspective of the laboratory worker, calibration, validation, quality control, and interpolation are the foremost problems. Once the reliability and plausibility of the numbers is established, the statisticians take over. 

Reliable evaluation of the potential for human exposure to endosulfan depends in part on the reliability of supporting analytical data from environmental samples and biological specimens. In reviewing data on endosulfan levels monitored or estimated in the environment, it should also be noted that the amount of chemical identified analytically is not necessarily equivalent to the amount that is bioavailable. 

---