Comparison of the Kjeldahl method and a combustion method for total nitrogen determination in animal feed

The features of the Dumas combustion method (CM) and those of the Kjeldahl method (KM) were compared as they apply to total nitrogen determination in animal feed. Both methods achieved similar repeatability (S.D., 0.11-0.38 from Kjeldahl and 0.15-0.36 from combustion) and similar intra-laboratory reproducibility (S.D., 0.11-0.39 from Kjeldahl and 0.15-0.37 from combustion). R.S.D. is always below 2%. These results show that the CM is suitable for the analysis of protein content in animal feed (5-75% protein content). The CM is recommended owing to its shorter analysis time, its cost and its environmental suitability.     

Repeatability and reproducibility of determination of the nitrogen content of fishmeal by the combustion (Dumas) method and comparison with the Kjeldahl method: interlaboratory study

Ten fishmeal samples (hidden duplicates of 4 meals plus 2 high-protein meals as a Youden pair), tryptophan, and nicotinic acid were analyzed by 18 laboratories using the Dumas method. Thirteen of the laboratories also analyzed the same 12 samples using their current Kjeldahl method. Recoveries (+/-SR) of tryptophan and nicotinic acid were 99.3+/-1.04 and 98.8+/-2.11% by Dumas and 97.1+/-3.03 and 74.6+/-26.76% by Kjeldahl. The Dumas method gave significantly greater values (P < 0.001) than the Kjeldahl method. For fishmeals, Kjeldahl N = 0.989 of Dumas N (P < 0.001). A similar proportionate difference (0.984 of Dumas N) was observed with tryptophan. Most laboratories failed to determine nicotinic acid correctly by Kjeldahl. For fishmeals, the relative standard deviations for repeatability and reproducibility were for Dumas 1.48 and 2.01% and Kjeldahl 1.62 and 2.37%, respectively. A single analysis conducted in 2 laboratories should not differ by more than 5.63% of the mean value when measured by Dumas or by more than 6.64% by Kjeldahl. It is concluded that with fishmeal, Dumas gives a more reliable measure of organic nitrogen than Kjeldahl, and, therefore, Dumas should be the method of choice.     

Rapid method for determination of protein content in cereals and oilseeds: validation, measurement uncertainty and comparison with the Kjeldahl method

The objective of this research was to test suitability of the Dumas combustion method to completely substitute the Kjeldahl method in routine laboratory determination of crude protein content in cereals and oilseeds. The validation of the method demonstrated that it is able to determine crude protein content in cereals and oilseeds in an efficient and accurate manner, with a detection limit w(N) = 0.006%, quantification limit w(N) = 0.019%, repeatability precision RSD _r = 0.41%, intra-laboratory reproducibility precision RSD _R = 0.74%, trueness, expressed in terms of bias b = 0.43%, and linear response between (2.36–19.2) mg N. Measurement uncertainty, expressed as relative expanded uncertainty (coverage factor k = 2, confidence level 95%), was calculated from validation data (U _rel = 2.24%). In order to examine the relationship between two methods, 15 cereal grain and oilseed samples were analyzed using Dumas and Kjeldahl procedure. The Kjeldahl procedure gave slightly lower w(N) values than the Dumas procedure: w _K(N) = 0.9905 w _D(N) = 0.0376 (R ²  = 0.9996). Relative standard deviations and results of homogeneity test obtained during analysis of complex cereal products (cereal breakfast and muesli bars) show that the Dumas combustion method may be less suitable for analysis of such samples compared to Kjeldahl method.     

Kjeldahl nitrogen analysis as a reference method for protein determination in dairy products

Measurement of total nitrogen by Kjeldahl analysis is the historical reference method for determination of the protein content of dairy products and is used for both calibration and validation of alternative methods for protein determination. Accurate evaluation of alternative methods is not possible if there is large uncertainty regarding the reference values. When Kjeldahl analysis is used to establish reference values, the performance of the Kjeldahl testing must be verified and within established expectations. Advice is given for Kjeldahl system optimization, evaluation of test results, and trouble-shooting. Techniques for successful Kjeldahl nitrogen analysis of dairy products other than milk are discussed.     

The determination of total nitrogen in plant materials with an automatic nitrogen analyser

An automatic analyser is used for the measurement of total nitrogen in chemical compounds and botanical material after the combustion method of Dumas in vacuum. The analyser is described and results for several chemical compounds and plant materials are given. The results have been statistically evaluated to compare the reproducibility and accuracy of the automatic analyser and the Kjeldahl method.     

Bestimmung von stickstoff in refraktärmetallen—I molybdän und wolfram

This is a report on round-robin tests concerning the determination of nitrogen traces in molybdenum and tungsten, which were carried out by the refractory metals group of the chemistry section of the Gesellschaft Deutscher Metallhütten- und Bergleute. The following methods were used: classical and modified Kieldahl methods (in particular, a newly developed micro-Kjeldahl method), inert-gas fusion, vacuum fusion and ultrahigh-vacuum diffusion extraction. As the nitrogen values obtained by the first round-robin test exhibited considerable scatter, possible shortcomings of the various Kjeldahl methods, as well as the quantitative nitrogen recovery by the extraction methods, were carefully examined. The second round-robin test then gave the following results: the nitrogen content of the investigated molybdenum sheet was 2 ppm, the content of the tungsten sheet 0.5 ppm. Classical Kjeldahl methods are not applicable to the determination of nitrogen contents below 10 ppm. Frequently, detection limits of Kjeldahl methods range considerably above 10 ppm, mainly because of contamination by reagents and/or the laboratory atmosphere. The micro-Kjeldahl method and the hot-extraction and fusion-extraction methods are capable of determining nitrogen contents down to 1 ppm at best. The only method available for nitrogen determinations in metals at sub-ppm levels is by ultrahigh-vacuum diffusion extraction. The good agreement between the nitrogen values obtained for molybdenum by the micro-Kjeldahl method and by the various extraction methods is the most reliable proof of the quantitative nitrogen recovery by the extraction methods.     

Über die Stickstoffbestimmung in hochtemperaturstabilen Metallnitriden und -carbonitriden

A report is given on the determination of nitrogen content in nitrides and carbonitrides of transition metals. In detail several methods are compared:

1. The interference of CuO with the reaction mechanism is examined by the Dumas method. By thermodynamic calculations and experimental investigations as well as by X-ray diffraction analysis it has been verified that the release of nitrogen is only possible by the use of a CO₂ carrier gas. The modified Dumas method is the best suitable method for a precise and reproducable nitrogen analysis in thermally high-stable nitrides and carbonitrides. Especially the high capacity of indication is of interest, so for instance, it is still possible to analyse 0.5 ppm by using samples of 1 g.
2. In decomposing nitrides not easily to dissolve the Kjeldahl method fails because of the oxidation effect of special solvents which at higher temperatures and extended time of analysation leads to a considerable decrease in verification.
3. The method of hot extraction with steel, cobalt, and nickel leads also without a carbon saturated prefusion at a temperature of 2000°C to a decrease in verification. The precipitation of graphite in the melt is clearly responsible for this effect. By means of the electron beam microprobe analysis it could be proved that the remaining nitrogen was in the graphite needles, but not as an undecomposed metallic nitride. In general the results of the platin flux method were in good agreement with the results of the methods compared. An analysis without a bath metal is always successful, if the nitride is easily to dissolve, reacts quickly with the carbon of the graphite crucible, and if the melting point of the nitrides or carbides is near working temperature.

     

Simultaneous Determination of Total Nitrogen and Metal Elements in Tobaccos by High Performance Ion Chromatography

A simple method, developed primarily for simultaneous determination of total nitrogen and inorganic cations by high performance ion chromatographic (HPIC), was optimized for digestion of flue‐cured tobaccos, and compared with the traditional Kjeldahl method and atomic absorption spectrometry (AAS). Nitrogen determination by either Dumas method or Kjeldahl method is time‐consuming and tedious. Metal elements determination by either inductively coupled plasma‐mass spectrometry (ICP‐MS) or AAS may be more expensive and requires specialist equipment. The use of HPIC to simultaneous determine total nitrogen as ammonium ion and metal elements as inorganic cations after sample digestion significantly improves the speed of the analysis compared with the conventional methods. The cation‐exchange column and suppressed conductivity detector was used for determination of ammonium and inorganic cations in the presence of the elevated levels of sulfuric acid found in digested sample. The propsoed digestion method was accurate and precise, and required little investment. The determination of ammonium and inorganic cations was linear from 15 pg·L^?1 up to 25 ng·L^?1. The results obtained by the HPIC method were compared with those for the conventional methods approach for the determination of total nitrogen and metal elements. The application of the HPIC method is also demonstrated for a variety of other plant samples matrices.     

Determination of the total nitrogen content of hard, semihard, and processed cheese by the Kjeldahl method: collaborative study

The objective of this collaborative study was to determine interlaboratory performance statistics for a modified and optimized version of AOAC Method 920.123 for the determination of the total nitrogen content of hard, semihard, and processed cheese by Kjeldahl analysis. Details included addressing the issues of material homogeneity, test portion size (1 g), quantitative transfer (weighing on to filter paper), ensuring system suitability (nitrogen recoveries), and using AOAC Method 991.20 as the basis for nitrogen analysis. Fifteen laboratories tested 18 pairs of blind duplicate cheese materials with a crude protein content between 18 and 36%. Materials represented hard, semihard, and processed commercial cheeses with a wide range of composition. Statistical performance parameters expressed as crude protein (nitrogen x 6.38), g/100 g, with invalid and outlier data removed were mean = 26.461, repeatability standard deviation (Sr) 0.111, reproducibility standard deviation (S(R)) = 0.153, repeatability relative standard deviation (RSDr) = 0.42%, reproducibility relative standard deviation (RSDR) = 0.58%, repeatability (r) = 0.312, and reproducibility (R) = 0.428. The interlaboratory study results were acceptable and comparable to those for the milk Kjeldahl nitrogen method on a relative nitrogen basis. The Study Directors recommend that this modified method for the determination of total nitrogen in hard, semihard, and processed cheese by Kjeldahl analysis be adopted First Action as an improved method to replace Method 920.123.     

全自动凯氏定氮仪测定化肥中的含氮量

建立全自动凯氏定氮仪检测化肥中氮含量的分析方法。称取总氮量在0.03~0.3 g范围的样品,经消解仪消解及全自动凯氏定氮仪碱化蒸馏,以硼酸为接收液,用0.15~0.20 mol/L的盐酸标准溶液进行自动滴定。该方法加标回收率为99.0%~101.5%,测定结果的相对标准偏差小于1.0%(n=6)。与国标GB/T 8527–2010法测定结果无显著性差异,全自动凯式定氮仪法的准确度、精密度均优于国标方法,而且所用试剂少、分析时间短,满足实验室快速检测大批量化肥中含氮量的需要。     

磷酸铵镁沉淀法测定聚马来酸酐氨化体系中的铵离子

通过甲醛法、四苯硼钠沉淀法以及磷酸铵镁沉淀法(MAP)测定模拟体系中铵离子含量,分析比较确定了聚马来酸酐(PMA)与氨气反应产物中铵离子的测定方法,即利用磷酸氢二钠和氯化镁结合铵离子形成磷酸铵镁沉淀,并通过凯式定氮法确定体系中铵离子含量。结果表明,pH值为9.0时,铵离子测定得到了较好的结果,测定平均值为5.50mmol/g,测定平均误差为2.1%,方法有很好的精密度和准确度,能够满足实际测试要求。     

Comparative studies on some analytical methods

This work present comparative results on powder milk storage quality, obtained from analytical methods. Protein content was determined conventional (Kjeldahl) and colorimetric with biuret reagent at 540 nm and integral quality by thermogravimetric and biological methods. A method was developed for the protein separation of powder milk. Powder milk was submitted to degradation processes at 45, 60 and 80°C for 20 days. The results indicated that protein content values were inconsistent if determinations by Kjeldahl and colorimetric methods and biological tests were compared. There is evidence of thermal decomposition of powder milk as detected by biological and thermogravimetric methods.The authors thanks CNP_q/PADCT for financial support.     

Bestimmung von stickstoff in refraktärmetallen—II Titan

This is a report on round-robin tests concerning the determination of nitrogen in titanium, which were carried out by the refractory-metals group of the chemical board of the Gesellschaft Deutscher Metallhütten- und Bergleute. The following methods were used: classical and modified Kjeldahl methods, inert gas fusion and vacuum fusion extraction. With the platinum (or palladium) flux technique (noble metal to sample ratio ～ 20 : 1, temperature ? 2000°) the reductive fusion methods are capable of determining nitrogen contents ? 20 ppm. Under special conditions, contents higher than 10–40 ppm can be determined by the Kjeldahl method (the lower limit varies from laboratory to laboratory). With a micro circulating distillation and determination apparatus, and dissolution of the sample within the pressure vessel, the detection limit can be lowered to 1 ppm. Because of possible interferences by reducible nitrogen compounds from the laboratory air, blanks must be determined with high-purity aluminium. The results from 7 laboratories (Kjeldahl 4; reductive fusion 3) for a titanium sample containing 138 ppm nitrogen were in good agreement (standard deviation 5 ppm).     

On the application of the Dumas technique for the determination of nitrogen in refractory nitrides

The classical Dumas apparatus has been modified. The generally recommended oxidants CuO and V(2)O(5), were found to give relatively high blank values and were therefore replaced by Cu(2)O. The preparation of the Cu(2)O is described. It is an exceptionally suitable oxidant for the Dumas analysis of the Group Va nitrides as well as manganese nitrides and molybdenum nitrides because it does not release oxygen into the gas phase. The reaction of the Group IVa nitrides as well as of CrN with CuO and Cu(2)O is, however, not completely quantitative at 1000-1150 degrees in reasonable reaction times. The results of the Dumas analysis of a Group IVa nitride should be corrected by measuring the nitrogen content of the residue (e.g., by hot extraction). The relative standard deviations do not exceed 0.38%. The nitrogen contents (% w/w) of the residues range from <0.001% for Mo(2)N + MoN to 0.54% for Cr(2)N + CrN. BN, Si(3)N(4) and AlN could not be decomposed with copper oxides under the conditions used. AlN could be analysed by adding V(2)O(5), but the rsd was 0.55% and the nitrogen content in the residue was 0.52% w/w. Neither BN nor Si(3)N(4) could be analysed by addition of V(2)O(5).     

The determination of the nitrogen content and nitrogen-15 abundance by means of the nitrogen gas generated from inorganic and organic materials

A method is described for the determination of the total nitrogen content, after conversion to nitrogen gas, of various types of samples before the mass-spectrometric analysis of the ¹⁵N abundance. The sample is burned with a mixture of copper oxide, copper and lime in a closed tube under vacuum (modified Dumas method). The gas is released into an evacuated chamber of known volume and the nitrogen pressure is used to determine the nitrogen content of the sample. The gas is then admitted into a mass spectrometer. In order to obtain complete combustion of the material and to generate pure nitrogen gas, various combustion temperatures, combustion times and oxidizing agents were compared.     

Optimization of a nitrogen analyser based on the Dumas method

The objectives of this work was to obtain a total nitrogen concentration in milk with the Dumas method for industrial and research applications. This method was faster than the Kjheldal method (5 min against 2 h or 3 h) but less precise. A three factor experimental design was performed to optimize the instrument, a FP-2000 supplied by LECO. The first interpretation of the experimental design was disappointing and no conclusion could be done. It was not a failure of experimental design but a lack of thinking on the physical and chemical aspect of the problem. Selection and construction of composite responses based on chemical and physical consideration were the keys to optimize the Dumas method. This method is now as precise as the Kjheldal method, but considerably faster.     

酸性氯酸钾处理-水浴加热全自动凯氏定氮仪测定土壤矿物中的固定态铵

固定态铵作为一种重要的潜在氮源,准确测定其含量,对于认识土壤肥力的形成机理、生态系统氮循环和保障国家粮食安全具有十分重要的意义。固定态铵最常用的方法是Silva-Bremner法,但该方法试剂配制要求严苛,前处理过程操作繁琐、费时。为了实现快速、准确、安全地测定土壤固定态铵的含量,对前处理过程进行了研究,用酸性氯酸钾溶液处理土壤样品,用水浴代替震荡,并与Silva-Bremner法的结果进行了对比。试验了自动凯氏定氮仪工作参数、酸性氯酸钾的浓度和用量、水浴加热的温度和时间,确定了测定土壤固定态铵的最佳条件。结果表明,称取1克样品加入20毫升氯酸钾-盐酸混合溶液（3％-10％）预处理,用0.5mol/L氯化钾溶液交换、洗涤,残渣加入加5mL氢氟酸-盐酸溶液（5mol/L-1mol/L）,在沸水浴加热1.5-2h,用自动凯氏定氮仪蒸馏滴定。对5种类型土壤固定态铵7次测定的结果进行了数据分析,相对标准偏差（RSD）均小于3.87%,最大G值1.49,最大F检验值4.12,最大t检验值0.72,均小于临界值,两种方法测定结果无显著差异。该方法的操作安全性和分析效率得到大幅提高,方法的精密度、准确度满足要求。     

Sequential injection system for on-line analysis of total nitrogen with UV-mineralization

An automatic method for the determination of total nitrogen in wastewater by sequential injection analysis and mineralization with UV radiation has been developed. The method is based on the mineralization of the samples with sodium persulphate in basic medium under UV radiation. Small volumes of sample and reagents are firstly aspirated into a single channel and then propelled by flow reversal to the UV reactor and then to the detector. The organic and inorganic nitrogen compounds are oxidized to nitrate that is then measured at 226 nm. The sequential injection procedure has been optimized and the factors affecting the efficiency of the oxidation have been studied with a number of test substances with different chemical structures and properties. Solutions in the concentration range 1-56 g l⁻¹ of nitrogen can be analyzed with the described procedure. The sample rate is of 30-40 samples h⁻¹. The LOD is 0.6 mg l⁻¹ N and the reproducibility is 1.8% (28 mg l⁻¹ N). Organic carbon in the form of glucose was added to a number of test solutions to study the potential interference of organic matter.The method was compared with the Kjeldahl digestion method by analyzing 15 wastewater samples with both methods. The nitrate and nitrite content of the non-oxidized samples were subtracted from the corresponding nitrogen content determined after photo-oxidation and the value compared with the Kjeldahl nitrogen content.     

Flow injection analysis : Part III. Comparison of continuous flow spectrophotometry and potentiometry for the rapid determination of the total nitrogen content in plant digests

The Berthelot reaction for the determination of ammonia by formation of indophenol blue, has been adapted to continuous flow spectrophotometry and the effects of acidity, temperature, ethanol content and sampling rate have been investigated; optimal conditions are described for the determination of nitrogen in Kjeldahl digests. Comparison of results obtained by this method, the air-gap electrode and classical distillation show no statistical difference at the 1% level, but the flow injection technique can be run at a much higher sampling rate (110–180 samples per h) than any other method and uses as little as 0.3 ml of digest per analysis.     

Dosage de l'azote dans l'uranium : Définition d'un échantillon de référence

Traces of nitrogen can be determined in uranium by reductive fusion in vacuum or in a current of helium. The two methods gave similar results, and these were confirmed by the Kjeldahl method. The excellent agreement between the values obtained made it possible to establish a reference standard.