Accuracy combustion analysis

Ag, Cl, and N to six-figure accuracy.1 This Nobel Prize-winning research allowed the accurate determination of atomic masses of many elements. In combustion analysis, a sample is burned in excess oxygen and products are measured. Combustion is typically used to measure C, H, N, S, and halogens in organic compounds. To measure other elements in food, organic matter is burned in a closed system, the products and ash (unburned material) are dissolved in acid or base, and measured by inductively coupled plasma with atomic emission or mass spectrometry. 

In another combustion analysis the sample is flash-heated at temperatures of up to 1800°C. The evolved N2 is either measured volumetrically in an azotometer or via a gas chromatograph [28,29] equipped with a thermoconductivity detector. N2, CO2, and H2O can also be determined simultaneously and hence the C and the H contents. The accuracy of the method is better than 1 rel-% N for finely powdered samples. 

Table 7-5 Accuracy and precision of combustion analysis of pure compounds" ... 

Table 7-5 shows results for two of seven compounds sent to many labs to compare their performance in combustion analysis. For each compound, the first row gives the theoretical wt% for each element and the second row shows the measured wt%. Accuracy is excellent Mean wt% C, H, N, and S are usually within 0.1 wt% of theoretical values. The 95% confidence intervals for uncertainty for C for the first compound is 0.63 wt% and the uncertainty for the second compound is 0.33 wt%. The mean uncertainty for C listed in the bottom row of the table for all seven compounds in the study was 0.47 wt%. Mean 95% confidence intervals for H, N, and S are 0.24, 0.31, and 0.76 wt%, respectively. Chemists consider a result within 0.3 wt% of theoretical to be good evidence that the compound has the expected formula. This criterion can be difficult to meet for C and S with a single analysis because the 95% confidence intervals are larger than 0.3. 

Applications Basic methods for the determination of halogens in polymers are fusion with sodium carbonate (followed by determination of the sodium halide), oxygen flask combustion and XRF. Crompton [21] has reported fusion with sodium bicarbonate for the determination of traces of chlorine in PE (down to 5 ppm), fusion with sodium bisulfate for the analysis of titanium, iron and aluminium in low-pressure polyolefins (at 1 ppm level), and fusion with sodium peroxide for the complexometric determination using EDTA of traces of bromine in PS (down to 100ppm). Determination of halogens in plastics by ICP-MS can be achieved using a carbonate fusion procedure, but this will result in poor recoveries for a number of elements [88]. A sodium peroxide fusion-titration procedure is capable of determining total sulfur in polymers in amounts down to 500 ppm with an accuracy of 5% [89]. 

By analysis and by measurement, we can determine H2 — H and express it in terms of values at 25 °C and 1 atm. Since Q is large for combustion reactions, these pressure corrections usually have a small effect on the accuracy of the heat of combustion determined in this way. 

Design practices stem from standard fire test procedures in which the temperature history of the test furnace is regarded as an index of the destructive potential of a fire. Thus, the practice of describing the expected effects and damage mechanism is based on temperature histories. This standard design practice is convenient but lacks accuracy in terms of structural performance. The severity of a fire should address the expected intensity of the heat flux that will impact the structure and the duration of heat penetration. A simple analysis of the expect nature of an unwanted fire can be based on the heats of combustion and pyrolysis of the principal contents in the facility. The heat of combustion will identify the destructive nature of the fire, while the heat of pyrolysis will identify the severity of the fire within the compartment itself and will also identify the destructive potential of the fire in adjacent spaces. 

In flame calorimetry, it is not easy to measure directly with good accuracy the mass of reactants consumed in the combustion. Therefore, the results are always based on the quantitative analysis of the products and the stoichiometry of the combustion process. In the case of reaction 7.73, the H20 produced was determined from the increase in mass of absorption tubes such as M, containing anhydrous magnesium perchlorate and phosphorus pentoxide [54,99], When organic compounds are studied by flame combustion calorimetry, the mass of C02 formed is also determined. As in bomb calorimetry, this is done by using absorption tubes containing Ascarite [54,90]. 

Isotopic analysis of amino acids containing natural abundance levels of 15N was performed by derivatization, GC separation, on-line combustion and direct analysis of the combustion products by isotope-ratio MS. The N2 gas showed RSD better than 0.1%c for samples larger than 400 pmol and better than 0.5%o for samples larger than 25 pmol. After on-column injection of 2 nmol of each amino acid and delivery of 20% of the combustion products to the mass spectrometer, accuracy was 0.04%e and RSD 0.23%o19. 

Until recently the major problems in the study of combustion have been analytical since it is essential to determine products in the earliest stages of reaction when secondary reactions involving products can be shown to be unimportant. Generally this implies reactant consumptions below 0.1 or 1%. Only gas chromatography is capable of adequate sensitivity, selectivity, and quantitative accuracy under these conditions. However, even gas chromatography has not been able to deal effectively with the analysis of peroxides, and there is need for more work in this field. 

Carbon-14 content is measured by specially designed gas proportional counters (7. Aerosol samples are first converted to CO2 by combustion in a macroscale version of the thermal evolution technique. A clam shell oven was used to heat the sample for sequential evolution of organic and elemental carbon under equivalent conditions. Due to the possibility of thermal gradients, conditions in the macroscale apparatus were adjusted to produce the same recoveries of total carbon (yg C per cm of filter area) as for the microscale apparatus. Carbon-14 data are reported as % contemporary carbon based on the 1978 1 C02 content in the atmosphere. Aldehyde data referred to in this paper were obtained by impinger sampling in dinitrophenylhydrazine/acetonitrile solution and analysis of the derivatives by HPLC with UV detection (12). Olefin measurements were made by a specially designed ozone-chemiluminescence apparatus (13) difficulties in calibration accuracy and background drift with temperature limit its use to inferences of relative reactive hydrocarbon levels. 

Sentences are often found meaning explicitly or implicitly 5 ppm accuracy is sufficient to deduce the elemental composition . This is absolutely not true and is at the origin of many exaggerations. This value comes from a rule of the American Chemical Society that states For most new compounds, HRMS data accurate within 5 ppm or combustion elemental analysis accurate within 0.4% should be reported to support the molecular formula assignment [11]. Thus, this rule does not tell that 5ppm accuracy can be used to deduce an elemental composition, but that it can be used in support of a proposed formula, but not as a proof of that formula. 

The accuracy of the method has been assessed by analysis of a standard sample of bovine liver from the National Bureau of Standards and a standard sulfonated hydrocarbon. For the bovine liver the sulfur content by combustion/ ion chromatography was about 1.2% below the accepted value whereas for the standard hydrocarbon, with a reported sulfur content of 0.97%, combustion/ion... 

A quantitative analysis is not always required. In particular, when searching for the source of poisoning of a catalyst, a qualitative analysis, followed by a semi-quantitative estimate is sometimes sufficient. If the data base from which the semi-quantitative program takes its standards includes many standards of a composition similar to that of the sample, the accuracy of the measurement may be of the order of a percent. On the other hand, if the library does not contain standards of a similar composition, the error may be around 50% or even more. As the problem of catalyst poisoning is frequently encountered, the accuracy of the measurements is generally fairly good. The table below is a comparison of the results of semi-quantitative and quantitative analyses conducted on an used post-combustion catalyst. 

The determination of the total carbon and organic carbon contents is an important part of the analysis of bituminous shale. Normally, the organic carbon content is determined indirectly, as the difference between the total and inorganic carbon contents [50]. One paper [51] described a method for the direct determination of organic carbon in rock samples. The method presupposes the use of standard equipment, namely, the Perkin-Elmer Model 240 analyser, but the combustion takes place at 450 10°C, bearing in mind that inorganic carbonates do not decompose at this temperature. The combustion takes 5 min. The accuracy of the determination of organic carbon is 0.10%. 

Chumachenko et al. [71] developed a GC method for determining nitrogen in poorly combustible organic compounds, consisting in oxidation of a sample in a layer of mixed nickel oxide in a sealed vessel maintained at 950—1000°C. The oxidation products were carried with a flow of helium to a GC column containing SKT carbon, followed by detection with a TCD. It takes 20 min to complete an analysis the accuracy is 0.3%. 

Simek and Tesafik [78] used GC to determine the optimum conditions for elemental analysis. They improved the accuracy of the Dumas micro-method by slowing down the combustion of samples and gradually increasing the oxidation temperature. 

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