Determination, micro-Kjeldahl

Protein content was determined by a semi-automated micro-Kjeldahl method [4], The conversion factor used was 6.25. 

The micro Kjeldahl method involves reducing all of the organic nitrogen to ammonia, then distilling the ammonia into an absorbing solution and determining it colorimetrically [11,12,22],... 

Mertens etal. [11] and Stevens [12] designed semiautomated versions of the micro Kjeldahl which avoided the distillation step altogether. In their versions, after the digestion step the digestion solution was diluted and the ammonia determined with an ammonia probe. The limitation on the sensitivity, then, is the sensitivity of the ammonia probe. This limits the method to the more productive oceanic waters. 

Another approach to the organic nitrogen problem is to use persulfate wet oxidation to convert the nitrogen to nitrate or nitrite, in place of the reduction to ammonia [13,14,24,25]. Results are fully comparable with those from the micro Kjeldahl digestion but the technique is far simpler. The precision should also be higher, since the final step in the measurement, the colorimetric determination of nitrite, is much more precise than any of the ammonia methods. 

Protein solubilities of soy flour and extrudates in the following solvent systems were determined by the micro-Kjeldahl method ( 9). A portion (0.1 g) of finely-ground sample (No. 60 sieve) was extracted with 9.9 ml of solvent for 1 hr at room temperature followed by centrifugation and filtration. An aliquot of the supernatant was used for nitrogen determination. 

Carbon and hydrogen were determined by a microcombustion method and nitrogen by a micro-Kjeldahl method. The oxygen in the total samples was determined by neutron activation and that in the fractions by... 

DIN 15722. Testing of Solid Fuels Determination of Nitrogen Content Dumas Method. DIN 51722. Testing of Solid Fuels Determination of Nitrogen Content Semi-Micro Kjeldahl Method. 

ISO 333. Determination of Nitrogen Semi-micro Kjeldahl Method. 

The question of the limiting degree of substitution (usually established by a bulk nitrogen determination using a micro Kjeldahl technique) in nitrocelluloses could a priori be rationalized in terms of two extreme models. 

Figure 5-3 Absolute error in the micro-Kjeldahl determination of nitrogen. Each dot represents the error associated with a single determination.

Carbamovlethvlation and CarbQxvethvlation. The purified cotton fibers were carbamoylethylated and carboxyethylated in aqueous solution containing acrylamide and 20 sodium hydroxide for lOh at 20 C. Then the samples were washed with distilled water, 0.1 acetic acid, and distilled water and air-dried. Degree of carbamoylethylation and carboxyethylation per anhydroglucose residue of the samples was determined by micro Kjeldahl method and back-titration technique (l), respectively. 

Spectral methods (spark source mass spectrometry SSMS, secondary ion mass spectrometry SIMS, inductively coupled argon plasma for emission spectroscopy ICAP-ES) which avoid separation steps are increasingly applied for multi-element analysis. Hot extraction is used for 0, N, H determinations. Oxygen is also determined by activation analysis, nitrogen after adaptation of classical methods (micro-Kjeldahl). Combination and comparison of different, independent methods are desirable, but hampered by the often limited availability of samples of actinides. 

Protein was determined by the method of Lowry (7) after hydrolysis with 0.2N NaOH (100°C, 15 min). Total nitrogen was measured by the micro-Kjeldahl method with sulfuric acid/hydrogen peroxide reagentj the ammonia was detected with Nessler s reagent. Glucose was measured by standard colorimetric assay using dinitrosalicylic acid. Starch was hydrolyzed with concentrated HC1 and then determined as sugar. 

Many physicochemical assays are established to quantify the protein mass. It is determined by exploiting the extinction coefficient in optical density measurements or by colorimetric assays such as the Bradford, Lowry, bicinchoninic (BCA), and biuret assay [13, 14]. Albeit easy to perform, these colorimetric assays suffer from inaccuracies that are due to the use of inappropriate standards like bovine serum albumin. If relevant standards are not available, quantitative amino acid analysis [6], the (micro-)Kjeldahl nitrogen method [14, 15] or gravimetry as very accurate but time-consuming alternatives can be applied. 

Total nitrogen of the AIS was determined by a semi-micro Kjeldahl procedure on a 0.50 g sample and the protein content calculated using the factor 6.25. The ash content was determined on a 0.5 g sample of the AIS ignited In a muffle furnace at 550 C for 16 hours. 

Micro-Kjeldahl digestion method The classical method has been adapted for determination of nitrogen in plastics. 

Random copolymer synthesis Poly(acrylamide-co-sodium acrylates) (1) were prepared (reaction 5) by ammonium persulfate initiation in distilled water at a 10% monomers concentration at 25°C. The copolymers were isolated by precipitation into methanol, followed by freeze drying. The reactivity ratios were determined and the predicted copolymer composition was in excellent agreement with that experimentally determined by the micro Kjeldahl nitrogen analysis. Molar feed ratios of acrylamide to sodium acrylate varied from 96/4 to 55/45. 

The blue colour of indophenol formed by phenol and hypochlorite in the presence of NH3 was first reported by Berthelot (1859). About 30 applications of this reaction have been adopted for the determination of ammonia in various media. To be reasonably sensitive, the reaction requires elevated temperatiue or a catalyst. A number of transition metals ions have been used as catalyst, including Mn % Ag% Fe Cu% [Fe(CN )] [Fe(CN)5NO] The last was suggested by Lubochinsky and Zalta (1954) and seems to yield the highest sensitivity. In this ion NO has a positive charge and Fe is divalent. Mann, Jr. (1963) has applied the Lubochinsky-Zalta technique for NH3 determination after a micro Kjeldahl digestion but experienced difficulties with the Hg ion used as a catalyst in the digestitm. 

The determination of ammonia after the regular or modified Kjeldahl digestion presents rather less serious problems than those already dis cussed. The advantages of the micro-Kjeldahl distillation (69, 80, 81, 82, 83) as compared with the macro>method, or even the semimicro-method are now generally recognized. A comparative study of the macro-and microscale determination in the analysis of flour, wheat and com for their protein content was made by Robinson and Shellenberger (27). The micro-Kjeldahl method has been used for systematic plasma protein analysis (84, 85), saliva proteins (86), milk proteins (87), and cerebrospinal fluid protein (88),... 

Nitrogen can be determined by micro Kjeldahl digestion techniques. 

These are the substances in blood, other than proteins, which contain nitrogen. They include urea, creatinine, uric acid, amino acids and ammonia. The non-protein nitrogen fraction can be determined by a micro-Kjeldahl method, followed by Nesslerization. In most laboratories, however, blood urea is determined and the other non-protein nitrogen compounds are measured only where clinically indicated, e.g. uric acid. 

This requirement is met for almost all the important elements by use of optical emission spectroscopy and x-ray fluorescence spectrometry and other classical methods discussed in Chapter 3. X-ray fluorescence spectrometry is applicable to all elements with an atomic number greater than 12. Using these two techniques, all metals and non-metals down to an atomic number of 15 (phosphorus) can be determined in polymers at the required concentration (Cook et al, Hous and Silverman, Mitchell and O Hear and Bergmann et al ). Nitrogen is detominable at these levels by micro Kjeldahl digestion techniques. 

The repeatability of the micro Kjeldahl method was studied by Rodgers and Harter (10). In this method 1 g of zirconium is dissolved by heating in a 100 ml distillation flask with 25 ml hydrochloric acid (1+1) to which 15 drops hydrofluoric acid (48 %) have been added. After the addition of 15 to 20 ml sodium hydroxide (60 %) to the extraction solution, the ammonia formed by dissolving the sample is steam-distilled into a 50 ml Messier cylinder. The distillation takes 6 minutes. One ml Messier reagent is then added, and the solution is diluted to 50 ml with ammonia-free water. The yellow colour of the solution is determined photometrically. Unfortunately, no mention of blank values is made, which very much diminishes the value of the results. 

The joint study referred to under 1.2.1. confirms what was said for zirconium, concerning the accuracy of macro and micro Kjeldahl and concerning the accurate determination of the blank value. This is illustrated in Table VI-3, which contains some results obtained for the determination of blank values of the dissolution acid by the micro process. 

In these investigations it was also shown that the Kjeldahl process is suitable for the determination of nitrogen in titanium, both in its conventional form - dissolution in hydrofluoric acid, conventional distillation and photometric determination - if certain experimental conditions are observed, and in the modified form of pressure digestion, micro-Kjeldahl process, circulatory distillation and coulometric determination. These results can also be applied to the analysis of titanium alloys. 

The detection limit differs from laboratory to laboratory and is situated between 10 and 40 Mg/g. Using pressure digestion and the micro Kjeldahl method concentrations as low as 1 ug/g can however be determined. The essential conditions for obtaining accurate analytical results in the Kjeldahl process are complete control of the blank value and rigid exclusion of contamination from the atmosphere of the laboratory. It is therefore advisable to determine the blank value by the use of very pure aluminium, and to carry out the determination in a separate room in an atmosphere free from ammonia and nitric acid". 

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