Sample preparation methods plastics

The choice of which method to use may be decided by the type of polymer/plastic material and the metal required for analysis. With the analysis of a completely unknown sample it may be necessary to carry out trial and error tests of different methods with and without spiking before accepting the final sample preparation method. Once confidence is achieved in the method of analysis, the procedure is noted for future reference. 

Solid-phase extraction is a relatively new sample preparation method based on chemically modified silica gel or other sorbent type packed into a small plastic disposable cartridge or column. The cartridges are usually coimected to the end of a syringe and samples passed through by pressure, and the columns and disks are normally used in a vacuum manifold. Sorbents with various functional groups can be obtained in different configurations and sizes from commercial suppliers, which can supply procedures for analyses and bibliographies of applications. 

Most methods for the determination of additives in plastics come essentially under two headings, namely with or without sample preparation. The following eight analytical categories are thus distinguished ... 

Analytical techniques for the quantitative determination of additives in polymers generally fall into two classes indirect (or destructive) and direct (or nondestructive). Destructive methods require an irreversible alteration to the sample so that the additive can be removed from the plastic material for subsequent detention. This chapter separates the additive wheat from the polymer chaff , and deals with sample preparation techniques for indirect analysis. 

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organisations (ASTM, AOAC International, EPA, etc.), see ref. [64]. Examples are standard test methods for carbon black/ash content (ASTM Method D 1506-97), lead analysis in direct paint samples (ASTM Method E 1645-94), etc. Table 8.15 shows some microwave ashing references (detection weight). A French AFNOR method utilises the atmospheric pressure single-mode microwave method as an alternative sample preparation procedure for Kjeldahl nitrogen determination [84], The performance of a microwave-assisted decomposition for rapid determination of glass fibre content in plastics for QC has been described [85]. 

All reagents and solvents that are used to prepare the sample for analysis should be ultrapure to prevent contamination of the sample with impurities. Plastic ware should be avoided since these materials may contain ultratrace elements that can be leached into the analyte solutions. Chemically cleaned glassware is recommended for all sample preparation procedures. Liquid samples can be analyzed directly or after dilution when the concentrations are too high. Remember, all analytical errors are multiplied by dilution factors therefore, using atomic spectroscopy to determine high concentrations of elements may be less accurate than classical gravimetric methods. 

In relation to sample preparation, Raman spectra can be obtained from pure complexes in the bulk state, seeing that for better performance the careful grinding of samples is required. Contrary to FTIR spectroscopy, where samples are mixed with mineral oil (Nujol) or KBr pellets, in Raman spectroscopy a pure substance is used. For this reason, the Raman spectroscopy is called a nondestructive measurement method. Additionally, analysis can be carried out through many containers such as glass, Pyrex reaction vessels, plastic containers, and so on. 

Because of the favorable sorptive properties of the reversed-phase supports, batch adsorption and desorption can be a very effective way to desalt a chromatographed sample or to partially fractionate a peptide mixture during a purification procedure. For example, 1-2 gm of an oc-tadecyl silica packed into a silanized glass or plastic pipette can be used for the batch fractionation of small amounts of a crude peptide extract from tissues, such as the pancreas or pituitary, or from a synthetic experiment. A number of commercial products, such as the Waters Sep-Pak, have found use in this manner 10) as a purification or sample preparation aid. Protocols for batch extraction procedures on alkyl silicas have been discussed 17a,b) and applied to neuropeptides 10, 158, 166) and other hormonal peptides 88, 162, 167, 168). With these methods recoveries of peptides present in a tissue extract are generally higher than those found with classical fractionation techniques due in part to the fact that proteolytic degradation is minimized. 

Any RNA preparation method can be used that provides intact, undegraded RNA. Biopsy samples can be flash frozen, pulverized, and suspended in guanidinium thiocyanate prior to extraction by the Chomczynski method (24). Small, fresh samples may also be suspended in guanidinium thiocyanate. In order to shear the sample for improved extraction, the samples should be passed several times through a 22-g needle, attached to a plastic syringe. The extraction steps can be repeated several times in order to get a clean RNA preparation prior to precipitation. 

Methods of Sample Preparation for Metal Content of Plastics,... 

The first step in analysing plastics for metals content in polymers by ICP-AES technique is that they must be prepared in solutions that are suitable for nebulization. There are four general methods applicable for sample preparation for metal analysis by ICP-AES and they are solvent dissolution of some plastics dry ashing using a muffle furnace acid digestion using a microwave oven and oxygen bomb combustion. 

The metal salts are added to plastic products as organometallic, metallocenes or inorganic salts. The ashing method of sample preparation is only suitable for elements that do not volatilise during the heating cycle. The results obtained and recorded in Table 4.10 are reproducible for most elements with the exception of Pb and Sb in which some loss had taken place using the opened dry ashing method even in the presence of PTSA. 

In developing a method for metal analysis of plastic samples it may be necessary to perform sample preparation by dissolution and three combustion methods to decide which one is the most suitable. The metal, level and the sample determine the most suitable method. 

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