Ashing step

Modem catalysts produce a much higher percentage of isotactic polypropylene than ia the past, eliminating the need for a cosdy extraction step to remove an atactic fraction. Yields ate high enough (>10,000 g polymer/g catalyst) so that a catalyst removal (de-ashing) step is no longer requited. 

A general procedure is to place the weighed sample into a platinum or silica glass crucible and heat it in a muffle furnace to a white ash. The temperature should be kept at 400 to 450°C if any of the more volatile metals are being determined. Salts or sulfuric acid may be added, if needed, and a final ashing step can be done with hydrofluoric acid if required. The residue is then dissolved in concentrated nitric acid and warm water, and diluted to volume. The final concentration of acid should be between 1 and 5%. 

For more complex solutions and samples where it is not possible to remove the matrix during the ashing step it may be necessary to use the method of standard additions. Some workers advocate that this method should always be run initially to check for interference effects so that the best calibration procedure can be selected. If the standard additions graph and the direct calibration graph were parallel, freedom from interferences in the sample would be indicated, i.e. the element is in the same form in sample and standard immediately before atomisation, or the two forms give the same absorption response. 

Boron was removed from distilled water by using a column of Dowex-1 (OH -form). For the photometric determination as a boron-curcumin complex, interfering ions were removed on small columns of Dowex-1 (formiate) and Dowex-50 (Na ) For the evaporation and ashing step, calcium hydroxide was used (recovery of one ng ca. 80%). By complexation of bivalent ions with 1,2-diamino-cyclohexane-NJ, N, N -tetra-acetic acid, water samples at pH 5.5 passed through a colunm of Dowex 50 W-X 8 (NH4) for the determination of barium Elution followed with 4 M nitric acid. Particular references for the separation of barium from sodium, calcium and sulfate are given. 

The ashing step is performed in a silica oven containing integrally molded heating elements, further minimizing the risk of contamination. 

Flame atomic absorption is sensitive enough to measure part-per-billion levels of nickel in aqueous solution, but it is not that sensitive for vanadium. Heated vaporization atomic absorption is more sensitive, permitting detection of vanadium down to 20 ng/ml in aqueous solution. Therefore, for the practical quantitative determination of nanogram/gram concentrations of both nickel and vanadium in petroleum, the combination of HVAA with a preconcentration ashing step was selected for detailed study. 

Metal salts The metal salts that originated from the de-ashing step required disposal. Since de-ashing is no longer needed, their quantity has been diminished by a factor of >100. The only source of metal salts remaining is equipment flushing where small amounts are found. 

The effect of sample preservation on determination of F in healthy and pathological human thyroids has also been studied (Blazewicz et al., 2011). It was pointed out that the way of tissue preservation (either in formalin or by freezing) had no significant effect on the iodine determination result (a = 0.1) by ion chromatography combined with the pulsed amparometic detection method (IC-PAD). Sample decomposition is a critical step in iodides analysis as well. All reported methods have a digestion or ashing step prior to the final determination of... 

In this case, NH4CI is removed during the ashing step and the residual NaN03 does not interfere during the atomisation. 

Fluoride is present in normal serum at about 0.01 ppm. Before the technology of fluoride ion-selective electrodes became available, procedures for the determination of this ion were so tedious and time-consuming that few were performed in the clinical lab. Presently, inorganic fluoride and, with an ashing step, total bound fluoride can be quickly and accurately assayed. 

Preparation of all types of organic samples usually requires a preliminary ashing step. This includes a variety of substances, such as plant or animal tissue and fluids, soils, foods, grain, and organic compounds synthesized for some specific purpose. Spectroscopic analysis is applied to such samples to determine their inorganic constituents. 

Poly(4-methyl pentene) is produced by the same process and equipment as polypropylene. A post finishing de-ashing step, however, is required. In addition, aseptic conditions are maintained during manufacture to prevent contamination that may affect clarity. 

The AOCS method Ca 12-55 (AOCS, 1978) is based on the molybdenum blue method but uses zinc oxide instead of magnesium carbonate in the ashing step. The method differs somewhat from that described under Section 6.2.15 with respect to the details of the procedure. 

Atomization is accomplished by quickly raising the temperature of the graphite furnace to the maximum allowable level (or a preselected level) after completion of the ashing step. 

There are certain situations in which sample preparation can be completely or largely omitted, as in the case of graphite-furnace AAS analysis, which permits the direct introduction of solid samples. Here the ashing step has been incorporated into the determination itself, which is then referred to as a direct process. This integration permits an analysis to be carried out more quickly, and the chance of errors is reduced by limiting the number of individual operations [136]. On the other hand, it is essential that the accuracy of such an analysis be confirmed by comparisons with standard reference materials. 

The atomization and ashing steps both depend on the chemical form of the sample. Extensive studies by Ebdon et al indicated that the chemical interferences with carbon atomizers can lead to serious errors. With commercial atomizers, the chemical interferences can be overcome to some extent by rigidly controlling the atomization step, with regard to timing, rate of heating and temperatures. 

---