Samples contaminated with matrix

GC/MSD instrument detection limit is typically 1 to 5 ig/mL (ppm) operated in scan mode. For a 1-g sample extracted into 10 mL, the method detection limit is about 10 /ig/g (ppm) which is sufficient for screening purposes. Table 16.1 summarizes typical recovery figures using this simplified extraction procedure. For soils fortified with 100 ppm of diesel with matrix ranging from sand to peat moss, recovery varies from 68 to 83%. To test the rapid extraction further, an actual sample contaminated with well-aged diesel was analyzed and showed a value of 54% of that obtained by more vigorous extraction (2,254 ppm versus 4,200 ppm). 

The type of data produced in a f.a.b. experiment is affected by the pH and ionic strength of the matrix. The former may be controlled either by the addition of acids or bases, although, in practice, it is usually preferable to keep the matrix acidic. The ionic strength is partly dictated by the purity of the sample (many biological compounds are still contaminated with salts, even after extensive purification) and partly by exogenous additives. Three additives are especially useful for carbohydrate work. They are as follows. 

After collection, samples were divided into 5-cm sections or into sections determined by changes in sample matrix. Only sediment from the interior of the core (not in contact with the corer) was used to avoid possible contamination from the polycarbonate and from sediment fines forced along the wall of the tube. Sectioned samples were stored in solvent-rinsed bottles at 4 °C until analyzed. Some samples used in this study were collected from reaches of the submerged riverbed (where PCBs were deposited before the reservoir was filled). Sample locations (Figure 1) were determined in relation to buoys placed and maintained by the U.S. Army Corps of Engineers. Samples used in this study were limited to those found to be contaminated with at least 1 xg/g of total PCB to minimize quantification error during weight-percent calculations. 

The possibility of detecting a phosphotionate pesticide in durum wheat has been also investigated. The determination was accomplished via chemical oxidation of the phosphothionate molecule both in buffer and in matrix extract solution optimizing the experimental parameters (reagents concentration and reaction time). The procedure was then applied to standardize the pirimiphos methyl detection obtaining the calibration curves under different conditions. The LOD with matrix extract was 50 or 100 ng/mL, depending on the extract % addition, which allowed the detection of samples contaminated at the MRL — 5 mg/kg. The samples mean recovery was 70.3% and no false positive samples were detected [49]. 

Modifiers are, however, used in a rather indiscriminate way in many laboratories. If used carelessly they can contaminate the sample solution with the element that is being determined and they can actually add to the background interference which one intends to reduce. By carefully optimizing the ashing and atomization temperatures for specific food matrices, as described above, the use of matrix modifiers can be reduced to the cases when they are really necessary. An additional benefit of matrix modification is that the sample and standard matrix are made very similar, this often making the standard addition method unnecessary. How this is carried out is described in detail in most instrument manuals and in specific textbooks. Commonly used modifiers are ammonium nitrate, ammonium phosphate, Mg nitrate, Pd nitrate, and ascorbic acid. 

To examine the ability of membranes to prepare samples with known contaminants, we contaminated the above peptide and protein solution with 5% glycerol and 500 mM NaCl. In addition to preventing effective crystallization of analyte samples with matrix on conventional stainless steel surfaces, glycerol and sodium contaminants are frequently present in biological samples. Doped samples were prepared for MALDI-TOF analysis by saturating the membrane with MeOH, immediately followed by the addition of 1 ul of the sample. The membrane was washed 3 times with 3-6 ml 70% methanol in water and allowed to dry after each wash. Once dry, lul saturated matrix solution was added to the sample spot. 

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