Sample preparation, generally instrumental methods

Human biological materials to be investigated include (a) hard calcified tissues, e.g. bone, teeth, other calcified formations (b) semi-hard tissue, e.g. hair, nails (c) soft body tissues and (d) various biological fluids and secretions in the human body. The treatment of each of these materials varies from one material to another and, as stated earlier, is often determined by the instrumental method to be employed for measuring the analytical signal, the elements to be determined and the concentration levels at which these are present. For the purposes of this discussion, it shall be generally assumed that the analytical techniques employed include atomic absorption spectrometry both with (F-AAS) as well as with a furnace (GF-AAS), neutron activation analysis (NAA), flame emission spectrometry (FES) voltammetric methods and the three inductively coupled plasma spec-trometric methods viz. ICP-atomic emission spectrometry, ICP-mass spectrometry and ICP-atomic fluorescence spectrometry. The sample preparation of biological methods for all ICP techniques is usually similar (Guo, 1989). 

Validation of methods for quantitative determination of impurities includes precision studies. Repeatability is generally assessed by analysis of the same sample or samples prepared by the same analyst in replicate assays within a short duration of time. Repeatability should be assessed using (i) a minimum of nine determinations covering the specified range for the procedure (e.g., three concentrations/three replicates each) or (ii) a minimum of six determinations at 100% of the test concentration. Repeatability is evaluated by averaging the mean results from replicate assays and calculating the standard deviation (SD) and RSD. Repeatability of the method can be stated as either SD or RSD values. If an instrument is required for assay performance, then the same instrument should be used for the replicate assays. 

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. 

Because preparation involves specialized procedures and instruments, most of analytical laboratories have Sample Preparation Sections, such as Organic Extraction and Metal Digestion shown in Figure 4.2. (The General Chemistry Section does not have a separate preparation group, as sample preparation is usually part of the analytical procedure). Because laboratory accuracy and precision strongly depend on the individual s technique, sample preparation personnel must be trained in each procedure, and their proficiency be documented. The laboratory must have a set of SOPs for preparation methods performed and must ensure that the Sample Preparation Group personnel are trained to follow them to the letter. 

In a modern laboratory, automated computer software for data acquisition and processing performs most of data reduction. Raw data for organic compound and trace element analyses comprise standardized calibration and quantitation reports from various instruments, mass spectra, and chromatograms. Laboratory data reduction for these instrumental analytical methods is computerized. Contrary to instrumental analyses, most general chemistry analyses and sample preparation methods are not sufficiently automated, and their data are recorded and reduced manually in laboratory notebooks and bench sheets. The SOP for every analytical method performed by the laboratory should contain a section that details calculations used in the method s data reduction. 

Even in relatively large programs, few laboratories will justify the initial expense and calibration effort required for development of the emission spectrographic method. As reported by Scott etal. [3], sample preparation will generally not differ significantly from that required for AAS. Instrumental neutron activation analysis (INAA) is only attractive where a reactor is already available, and multielement analysis by this technique requires the use of high resolution Ge(Li) crystals and multiple irradiations for elements with differing activation product half-lives. The key elements, cadmium, nickel and lead still require analysis by AAS because of limitations of the INAA method [4]. 

In practice an instrumental detection limit is of limited use because in analytical chemistry it is rare that no other procedural steps are involved. Normally a limit of detection for the whole analytical method is required. The terminology used in this area is confusing. In general, limit of detection and detection limit are synonymous. The detection limit will encompass factors such as (a) sample matrix effects (b) loss of the analyte during sample preparation etc. The detection limit for the analytical procedure is defined as The minimum single result which, with a stated prohahility, can be distinguished from a suitable blank value . ... 

This book intends to supply the basic information necessary to apply the methods of vibrational spectroscopy, to design experimental procedures, to perform and evaluate experiments. It does not intend to provide a market survey of the instruments which are available at present, because such information would very soon be outdated. However, the general principles of the instruments and their accessories, which remain valid, are discussed. Details concerning sample preparation and the recording of the spectra, which is the subject of introductory courses, are assumed to be known. Special procedures which are described in monographs, such as Fourier transformation or chemometric methods, are also not exhaustively described. This book has been written for graduate students as well as for experienced scientists who intend to update their knowledge. 

---