Extractability testing analytical methods

Insufficient testing is one of the major causes of method failure. The amount of data needed to publish a new procedure in a peer-reviewed journal and the procedural detail supplied therein are often insufficient to allow a different user to validate a method rapidly. The developer should evaluate if the method will work using chemicals, reagents, solid-phase extraction columns, analytical columns, and equipment from various vendors. Separate lots of specific supplies within a vendor should be evaluated to determine if lot-to-lot variation significantly impacts method performance. Sufficient numbers of samples should be assayed to estimate the lifetime of the analytical column and to determine the effects of long-term use on the equipment. 

Brandt [200] has extracted tri(nonylphenyl) phosphite (TNPP) from a styrene-butadiene polymer using iso-octane. Brown [211] has reported US extraction of acrylic acid monomer from polyacrylates. Ultrasonication was also shown to be a fast and efficient extraction method for organophosphate ester flame retardants and plasticisers [212]. Greenpeace [213] has recently reported the concentration of phthalate esters in 72 toys (mostly made in China) using shaking and sonication extraction methods. Extraction and analytical procedures were carefully quality controlled. QC procedures and acceptance criteria were based on USEPA method 606 for the analysis of phthalates in water samples [214]. Extraction efficiency was tested by spiking blank matrix and by standard addition to phthalate-containing samples. For removal of fatty acids from the surface of EVA pellets a lmin ultrasonic bath treatment in isopropanol is sufficient [215]. It has been noticed that the experimental ultrasonic extraction conditions are often ill defined and do not allow independent verification. 

Apart from differences between muscle tissues from various parts of an animal, there are qualitative and quantitative differences in composition between animal species. Therefore, analytical methods will always have lo be tested on material from each individual species, since differences in fat composition, in the presence of species-specific proteins, and in colored components such as in the case of myoglobin in poultry and beef may influence both the extraction and the separation of the analytes. As an example, a recovery higher than 70% was obtained for furazolidone after spiking chicken and veal calf muscle tissue but only 10% after spiking pork tissue (16). In this study, the recovery from pork meat could markedly be improved by addition to the aqueous exfraction solvent of about 25% acetonitrile, an observation indicating binding of furazolidone to pork-specific proteins. 

A new analytical method was based on the treatment of SAs with p-aminobenzoic acid, forming derivatives suitable for UV detection. The SAs were extracted from the kidney and liver samples, with recoveries ranging from 55% to 100%. The reaction yield was tested by microbial inhibition tests sensitive to low concentrations of SAs. After the incubation with / -aminobenzoic acid, none of eight SAs produced an inhibition zone on the assay medium, which showed 100% conversion to the appropriate derivatives (162). 

Detection of other Esters of Fixed Acids [oxalates, tartrates, succinates, citrates).—A certain quantity of the oil (if possible 10-20 c.c. or more) is saponified in the usual way, the excess of alkali being neutralised with hydrochloric acid in presence of phenolphthalein and the alcohol expelled on a water-bath. The residue is diluted with water and extracted with ether, the aqueous solution being tested for oxalic, tartaric, succinic and citric acids by the ordinary analytical methods. 

Arsenical or mercury compounds are detected by evaporating a quarter of a litre of the ink and heating the extract with 1-2 c.c. of concentrated sulphuric acid and 5-10 c.c. of fuming nitric acid until nitrous vapours are eliminated, the addition of nitric acid and the heating being repeated until a perfectly colourless liquid is obtained (Rothe). The sulphuric add is then expelled and the residue tested for arsenic and mercury by the ordinary analytical methods. 

The USP XX (30) and the Official Methods of Analysis of the Association of Official Analytical Chemists (AOAC) (44) describe in detail the curative method more commonly known as the rat line method. This method is not applicable to products offered for poultry feeding. For poultry feeds, and fish liver oils and their extracts, the AOAC (44) also describes in detail the chick bone ash method. In addition to the above two methods, there is a method based on the comparative measurement of serum calcium in rats referred to as Intestinal Calcium Transport Assay and another method based on the comparative amounts of calcium absorbed by control and test chicken referred to as Calcium Absorption Test. These methods are described by DeLuca and Blunt (45). 

There are several future trends for the development of passive sampling techniques. The first is the development of devices that can be used to monitor emerging environmental pollutants. Recently, attention has shifted from hydrophobic persistent organic pollutants to compounds with a medium-to-high polarity, for example, polar pesticides, pharmaceuticals, and personal care products.82 147148 Novel materials will need to be tested as selective receiving phases (e.g., ionic liquids, molecularly imprinted polymers, and immunoadsorbents), together with membrane materials that permit the selective diffusion of these chemicals. The sample extraction and preconcentration methods used for these devices will need to be compatible with LC-MS analytical techniques. 

For all analytical methods the quality of the results ultimately relates back to the chemical purity of the very best available SRM and to the linearity of the correlation curve for the experimentally measured property vs. the SRM concentration. For substances that are naturally chiral there is the additional very serious concern about enantiomeric purity. The determination of an enantiomer whether for an enantiomeric purity test, or for an enantiomeric ratio or excess test in the study of a partial racemic mixture, is one of the more difficult analytical problems. To actually report the enantiomeric purity of an enantiomer as better than 99% is truly beyond the capability of current analytical methodology [31], for after all few substances ever have a chemical purity that is guaranteed to be greater than 99%. So, as mentioned earlier, one has to accept the fact that the results are measured relative to an enantiopurity of an SRM that is defined to be 100%. This limitation of course impacts on the true meaning of a calculated enantioexcess, and to a much lesser degree perhaps, in assays of chiral substances extracted from plant materials using calibration data that were obtained for synthetic SRM s. 

---