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| We are expanding our elemental analysis capability. In addition to
metals analysis by ICPMS, ICPOES,
GFAA, and XRF,
we now offer total chlorine, bromine, fluorine, sulfur, and phosphorus
after oxygen combustion.
The combustion process releases covalently bound, typically organic, forms of these elements. The combustion is performed in an oxygen pressurized Parr bomb or the glass apparatus required in USP<471>. The elements are converted to their anions or oxy-anions such as chloride, sulfate, or phosphate. The anions may then be analyzed by titration or other wet chemical techniques. For example, the USP assay for sulfur is an Oxygen Flask Combustion followed by a titration. The assay is very accurate with specifications of 99.5-100.5%. The anions may also be trapped in a dilute sodium bicarbonate solution and analyzed by ion chromatography. Recovery of the elements at trace levels from standards is normally 90-110%. Detection limits are generally 10 ppm. Several methods have been published using this approach: USP<471>, EPA Method 9076, EP (2.5.10), and various FCC, ASTM, and AOAC Methods. |
USP<471> Oxygen Flask Combustion
The Parr combustion apparatus consists of a sealed steel reactor that is pressurized to 40 atm with oxygen. The bomb is made from a corrosion resistant alloy that is required for determining halogens (chlorine, etc.). The sample is ignited by a platinum filament.
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Pros and ConsEach of these has advantages and disadvantages for analysis of certain samples and elements. The oxygen flask is used with samples usually between 10 and 50 mg. The sample is placed on a special halide-free filter paper which is folded around it and placed in a platinum basket inside the flask. The flask contains an absorbing liquid, and is flushed with oxygen prior to ignition of the paper. After ignition, the sealed flask is swirled and the contents rinsed out and collected for analysis. The small sample size for this technique is an advantage for samples which are expensive. The Parr bomb is used with samples between 100 and 500 mg. The sample is placed in a small metal dish which sits under a fuse wire connected to the top plate of the bomb by a holder. A volume of absorbing liquid is placed inside the bomb, and the bomb is sealed with the sample inside, and pressurized to 30 psi with pure oxygen. The bomb is then ignited with an electric current to the fuse wire, swirled and cooled, and then vented. The trapping solutions are then rinsed out and collected for analysis. The collected rinse volumes for both techniques are similar. Accelerants such as mineral oil or ashless filter paper may be used to aid combustion in the Parr bomb. |
Overly wet samples may not burn well in either
apparatus, and overly dry samples may explode too violently in the Parr
bomb, giving incomplete combustion. An experienced operator will use the
appropriate combination of accelerants and added moisture to ensure good
combustion of a sample. Generally speaking, dry samples that are ground to powder will give the best combustion results. Liquid samples can be placed in appropriate capsules for combustion, or may be directly combusted in the Parr bomb holding cup. A sample must be combustible to be analyzed, but most carbon containing compounds or polymers will combust in an oxygen atmosphere under the right circumstances. USP31-NF26 specifies the oxygen flask in monographs for determination of bromine, chlorine, iodine, selenium and sulfur. We have successfully analyzed samples for fluorine (F) by this method, also. Sulfur is measured as sulfate by ion chromatography-suppressed conductivity (IC-SC). The halogen elements are measured as the halide anion by IC-SC or for F by selective electrode. The same techniques are used for Parr bomb rinses. For metal elements, we have a quartz liner for our Parr bomb to prevent contamination by the apparatus itself. We have the ability to analyze for a number of elements in absorbing liquids by ICP-MS including Br, Cd, Cr, Hg, Pb and Se. |
Metals in PlasticsIn recent years, there is a rising concern about traces of toxic elements, such as Cd, Cr, Pb, and Hg in plastics and other organic type samples. Some plastics cannot be dissolved by the routine use of acids, and many samples will lose elements such as Hg during digestion. To circumvent the loss of mercury, an oxygen bomb methodology was developed . The method includes the use of platinum ignition electrodes and a quartz jacket to prevent contamination from the stainless steel walls of the oxygen bomb vessel. The closed vessel environment of the oxygen prevents the loss of mercury. |
Once the sample is combusted, the residue is dissolved in acid, and the solution is analyzed for trace elements by
ICPMS.
Use of the oxygen bomb to prepare the sample prior to analysis by ICPMS proved to be successful, with matrix spike recoveries of 70 - 130%. One drawback in using this procedure is the time needed to prepare the samples. Each sample can take as long as one hour to prepare. Also see Pb and Cd in PVC. |
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9240 Santa Fe Springs
Rd |
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562.948.2225 Fax 562.948.5850 |
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Formerly West Coast Analytical Service (WCAS) and Bodycote Testing Group |
