External control samples

This method consists of producing control samples that contain the same phases as the original sample, bnt in known amonnts. An abacus is then drawn with the percentage of a phase as the x-coordinate and the intensity ratio of two peaks specific to each of the phases as the y-coordinate. In the same experimental conditions, the diffraction diagram of the sample we wish to study is then measured. The intensity ratio of the two peaks in question makes it possible to directly read, by using the abacus, the proportions of the different phases. 

This method is efficient, and does not require an extensive knowledge of the crystal structures of the phases. It is, however, imperative for each of the phases to be pure. This is sometimes difficult to achieve, particirlarly if one of the phases is metastable. In that case, the first method is used. 

When the study reqitires a high accitracy, a variation of the second method can be used. In each of the control samples, a known percentage of an internal control sample with well-defined absorption characteristics is added. This time, the intensity measurements for the peaks of the analyzed phases are brought to scale with the intensity of a peak of the control sample. 

As we saw above, this method makes it possible to take into account the absorption effects. Various samples are sold by the National Institute of Standard 

Regardless of the calcrrlation method the acctrracy of quantitative phase analyses by X-ray diHraction strongly depends on the qrrality of the measurements. In particrrlar, and as we have alreaily seen, it is imperative to eliminate any preferential orientation effects. When the study is conducted rmder good conditions, the detection threshold is close to 1% and the accuracy of the analysis is in the range of a few percent. 

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Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

Methodology appropriate for the measuring of DTA profiles has been extensively reviewed [12,13]. A schematic diagram illustrating the essential aspects of the DTA technique is shown in Fig. 3. Both the sample and reference materials are contained within the same furnace, whose temperature program is externally controlled. The outputs of the sensing thermocouples are amplified, electronically subtracted, and finally shown on a suitable display device. 

A control sample is a sample for which the concentrations of the test analyte is known and which is treated in an identical manner to the test samples. It should ideally be of a similar overall composition to the test samples in order to show similar physical and analytical features. For instance, if serum samples are being analysed for their glucose content, the control sample should also be serum with a known concentration of glucose. A control sample will be one of many aliquots of a larger sample, stored under suitable conditions and for which the between batch mean and standard deviation of many replicates have been determined. It may be prepared within the laboratory or purchased from an external supplier. Although values are often stated for commercially available control samples, it is essential that the mean and standard deviation are determined from replicate analyses within each particular laboratory. 

The qnality control (QC) of the laboratory lesnlts can be either internal or external. The appropriate level of QC depends on the lehabihty of the method, the criticality of the work etc. It is widely accepted that for rontine analysis a level of internal QC of at least 5% (i.e. 1 control sample every 20 rontine samples) is reasonable. 

The control samples for the target control charts and their measures are the same as for the classical control charts. The limits are given by different external quality criterion, described in the slide. 

Neither AdoMet nor AdoHcy have so far been included in any external quality control scheme. Control samples are not commercially available. A pooled plasma sample is included in each batch as internal quality control. 

A control plasma is analyzed in each series and serves as the internal quality control sample. Analysis of plasmas and urines of the ERNDIM special assays schemes (eight samples per year, four pairs at four concentration levels) serves as the external quality control program. 

External quality control samples are provided by European Research Network for the evaluation and improvement of screening, Diagnosis and treatment of Inherited disorders of Metabolism in its Special Assays Serum scheme. Eight lyophilised samples are distributed each year, consisting of four pairs of samples with increasing... 

Biotinidase has not been included in external quality control programs, and no control samples are commercially available. Assessment of quality is performed by including a plasma sample of an individual with normal biotinidase activity in each assay set. Such a sample is ideally stored at - 70°C in aliquots and thawed just once (see Pitfalls and Limitations , below). 

Quality control samples are run in duplicate for each assay. Currently, there is no official external quality control program available. 

GTPCH activity measurements are not available in external control programs and can only be compared by exchanging results from other laboratories (for examples see www.bh4.org). Control biopterin and neopterin samples are commercially available from Dr. Schircks Laboratories, Jona, Switzerland (www.schircks.com). We recommend using internal controls (e.g., normal control fibroblasts in each enzyme... 

Analytichem International has taken trace enrichment a step further with the development of the Analytichem Automated Sample Preparation LC Module, A ASP LCM. This unit uses derivatized silica in specially configured cassettes on which the sample is off-line enriched by using an inexpensive vacuum manifold. After enrichment, the cassette is placed in the unit, which holds up to 10 cassettes containing 10 samples each, and the enriched samples are eluted directly on the analytical column. Provisions allow for early- and late-eluting bands to be switched to waste. The unit also provides for external control of... 

Examples of quality assurance protocols that are considered standard practice in any data collection scheme include the use of both internal control samples (e.g. use of field blanks and spikes6) and external quality assurance samples (e.g. duplicate samples of known concentrations sent to different laboratories) to determine the extent of intra- and interlaboratory variation. Ensuring that the data have not been compromised or corrupted may also require setting up accessible data archives of original paper or electronic records so that the accuracy of summaries of the data published in documents and articles can be verified. 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

Quality control - independent department or external laboratory responsible for all aspects of quality control. Samples from each batch must be retained for one year, unless not practicable. 

Sample dispersion in multi-commuted flow systems is governed by the same parameters as in other flow systems. However, the manifold status can be modified by the operation of these discretely operated devices. Consequently, external timing of the various committed devices is an important aspect of controlled sample dispersion, as emphasised below. 

Every laboratory takes great pains to ensure that the methods in use continue to produce reliable results. Laboratory staff monitor performance of assays using quality control samples to give rcassunince that the method is performing satisfactorily with the patients. specimens. The.se are internal quality controls which are analysed every day or every time an assay is run. The expected values are known and the actual results obtained are compared with previous values to monitor performance. In external quality assurance schemes, identical samples arc distributed to laboratories results are then compared. 

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