Sample preparation, generally quality control

Sample quality is generally assessed by the determination of /J-galactosidase activity. For MPS type IVB, the a-mannosidase activity is chosen as an indicator enzyme. Leukocyte homogenates that are sufficient for at least 20 separate runs are prepared from one source and aliquots are kept frozen. These samples serve as quality controls for each run. Heat-inactivated leukocyte homogenates may serve as a positive control (patient-mimics). 

Limits of detectability for the desired elemental analyses vary depending upon the matrix, elements, methods of sample preparation, and quality of instrumentation applied. Generally, these are on the order of 1 to 100 parts per million. The limit of detectability, however, is only one criterion in evaluating methods of analysis. The liiue of analysis is important, particularly in production and process control laboratories, in multi element spectrometers, it is possible to perform as many as 30 simultaneous elemental determinations in from 20 to 120 seconds, depending upon the material being analyzed. 

The design of an assay is, in large measure, prospective quality assurance. The factors that are likely to affect the results of the assay must be defined and controlled to the greatest extent possible. Once the general outlines of an assay have been established, key features should be examined, including optimization of sample preparation, sample stability, choice of standards, assay range, assay repeatability, optimization of separation, and optimization of detection. 

As far as the preparation of the stock solutions for calibration standards and quality control samples is concerned, different labs use different approaches. In general, the calibrators used for the validation should be prepared from... 

In this section a concise overview of the most widely used analytical procedures for the determination of PCBs in environmental matrices (namely, air, sea water, snow/firn/ice, sediment/soil and biota) is given. Regardless of the nature of the sample, the following steps are generally included in an analytical procedure i) sample collection and storage ii) sample preparation (extraction of the analytes and cleanup of the extract) iii) instrumental analysis iv) data evaluation, including analytical quality control. 

At present there is no universally accepted Standard Method for Preparation of Samples from Contaminated Sites and few laboratories operate strictly comparable sample preparation methods. So it is important that the methods used for initial sample preparation are verifiably/// for their analytical purpose and applied by trained staff under an appropriate level of quality control. The design of sample preparation methods is generally constrained by the need to produce powders or finished samples with uniform physical characteristics by the most rapid method, whilst preventing the loss or alteration of any part of the sample by contamination, cross-contamination or chemical reaction. 

Some forensic laboratories will rim what is known as a QC (quality control) sample ahead of the analytical sequence to assess whether all of the system suitability criteria are met. The QC sample is a simulated sample of known concentration and is prepared independently by one or several members of staff within the laboratory. Generally, the QC sample is run and assessed against the system suitabiUty criteria before the start of the analytical sequence and again at the end. The concentration of the sample is calculated and assessed against the true value. A Umit is set and this must be met in order for the data to be considered vaUd. 

CRMs can be either analytical standards (Section 2.2.1), or calibration solutions that are solutions of an analytical standard at a certified concentration in a clean solvent, or natural material (matrix) CRMs that are prepared from material found in nature, i.e., are surrogates for the real samples to be analyzed. Matrix CRMs are invaluable as tools in quality assurance and quality control (QA/QC) in analytical laboratories, as discussed in Chapter 10. However, CRMs are generally too expensive to be used as everyday QCs and are generally used as method development tools or in occasional checks of the laboratory QCs (Section 2.5.3). 

Quality Control samples (QCs) can be generally defined as samples that are prepared from control matrix fortified with the analyte(s) of interest and are used to demonstrate that a method is performing according to the established uncertainty targets within a particular run. They... 

---