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Determination of α-amino nitrogen in pea protein hydrolysates a comparison of three

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水性聚氨酯防水涂料执行标准

水性聚氨酯防水涂料执行标准

水性聚氨酯防水涂料执行标准GB/T 34999-2017Organic light emitting diode display—Part 1：General specification有机发光二极管显示器第1部分：总规范GB/T 34981-2017Determination of nitrate in water—Spectrophotometric method水中硝酸盐的测定分光光度法GB/T 34980-2017Determination of nitrite in water—Spectrophotometric method水中亚硝酸盐的测定分光光度法GB/T 34979-2017Determination of sulfate in water—Spectrophotometric method水中硫酸盐的测定分光光度法GB/T 34976-2017Determination of total nitrogen in water—Ultravioletspectrophotometric method水中总氮的测定紫外分光光度法GB/T 34977-2017Determination of total phosphorus in water—Spectrophotometric method水中总磷的测定分光光度法GB/T 34978-2017Determination of chloride in water—Spectrophotometric method 水中氯化物的测定分光光度法GB/T 34974-2017Test methods of eddy current testing for tubes管材涡流检测试验方法GB/T 34975-2017Test method for the determination of carbon monoxide in liquefied petroleum gas液化石油气中一氧化碳测定方法GB/T 34972-2017Determination of total sulfur in water—Ultraviolet fluorescence spectrometric method水中总硫的测定紫外荧光光谱法GB/T 34973-2017Determination of total phosphorus in water—Molybdenum blue spectrophotometric method水中总磷的测定钼蓝分光光度法GB/T 34971-2017Determination of total nitrogen in water—High temperature catalytic oxidation method水中总氮的测定高温催化氧化法GB/T 34969-2017Determination of total nitrogen in water—Kjeldahl method水中总氮的测定 Kjeldahl法GB/T 34970-2017Determination of total phosphorus in water—Ascorbic acidmolybdenum blue spectrophotometric method水中总磷的测定抗坏血酸钼蓝分光光度法GB/T 34967-2017Determination of total phosphorus in water—Molybdenum blue spectrophotometric method with ashing水中总磷的测定灼烧钼蓝分光光度法GB/T 34968-2017Determination of total phosphorus in water—Molybdenum blue spectrophotometric method with hydrolysis水中总磷的测定水解钼蓝分光光度法GB/T 34966-2017Determination of total phosphorus in water—Colorimetric method 水中总磷的测定比色法GB/T 34964-2017Determination of total nitrogen in water—Semi-micro Kjeldahl method水中总氮的测定半微量Kjeldahl法GB/T 34965-2017Determination of total phosphorus in water—Spectroph。

滴定法测定酱油中氨基酸态氮含量的不确定度评定

滴定法测定酱油中氨基酸态氮含量的不确定度评定

表3 回收率测定结果样品编号加标量/（mg·kg-1）回收率/%酸性红诱惑红11109.70109.652105.00105.35 3109.45103.204298.90100.055101.45100.53 6101.83100.80710103.14100.308104.15102.469103.95100.77 2.4.2 精密度试验取回收试验卤肉加标样品3份（每个水平各1份），连续测定6次，记录峰面积，计算精密度。

结果见表4。

表4 精密度测定结果项目加标量/（mg·kg-1）精密度/%酸性红1.0 1.82.03.3 10.00.7诱惑红1.0 2.82.00.8 10.0 1.32.4.3 检出限确定称取卤肉阴性样品1份，取混合标准储备液（50 µg·mL-1）0.01 mL，按试样分析步骤处理测定，目标物各组分色谱峰信噪比：酸性红为S/N=8.2（＞3），诱惑红为S/N=12.3（＞3）。

由实验结果可知，当称样量为2 g，各目标组分的方法检出限为酸性红：0.3 mg·kg-1，诱惑红：0.3 mg·kg-1。

2.5 盲样考核试验实验室盲样考核能力验证一般以《ISO13528实验室间能力比对试验》经典Z值法评估实验室的检验检测水平，Z值的大小代表实验室的结果与中位值的偏离程度，而符号“+”和“-”代表与中位值的偏离方向，一般|Z|≤1为结果满意[6]。

本次对盲样进行检测，称取两份盲样考核卤肉制品进行试验，采用已建立的方法进行测试，做2次平行试验。

酸性红和诱惑红测定结果分别为1.92 mg·kg-1、5.37 mg·kg-1，盲样真值分别为2.07 mg·kg-1 、5.35 mg·kg-1，Z值分别为-0.65和0.17，|Z|均小于1，结果均为满意。

2.6 方法应用随机抽取市内各大市场及商超的卤肉样品共30批次，应用本方法进行合成着色剂的测定。

过硫酸钾氧化4-氨基安替比林分光光度法测定地表水中挥发酚

过硫酸钾氧化4-氨基安替比林分光光度法测定地表水中挥发酚

Vol. 11, No. 226〜3#第11卷第2期2021年4月中国无机分析化学ChineseJournalofInorganicAnalyticalChemistrydoi ：10. 3969". issn. 2095-1035. 2021. 02. 006过硫酸钾氧化4-氨基安替比林 分光光度法测定地表水中挥发酚叶开晓童天詰梁柳玲洪欣"(广西壮族自治区生态环境监测中心，南宁530028)摘 要 在pH 值为10.0 + 0.2的缓冲溶液中，以过硫酸钾溶液作氧化剂，用经乙醇和正己烷(1 + 4)混 合溶液提纯的4-氨基安替比林(4-AAP )作显色剂，采用三氯甲烷分光光度法测定地表水中挥发酚$结果表明，经乙醇和正己烷(1 + 4)混合溶液提纯的4-AAP ,损失较少，且纯度较高，有利于准确配制4-AAP溶液。

此外，稳定、无色的过硫酸钾溶液代替稳定性差、有毒的铁氤化钾溶液作氧化剂，有利于降低空白值，且显色稳定，方法检出限为0. 3 %g/L,用于测定地表水中的挥发酚，平均加标回收率为96.0% $满足标准方法的要求$关键词挥发酚；4-氨基安替比林；提纯；过硫酸钾；地表水中图分类号：O657. 32；TH744. 12文献标志码：A文章编号：2095-1035(2021)02-0026-05Determination of Volatile Phenol by 4-Aminoantipyrine Spectrometric Method with Potassium Persulfate as OxidantYE Kaixiao ，TONG Tianzhe ，LIANG Liuling ，HONG Xin "(Guangxi Ecological and Environmental Monitoring Center ，Nanning ，Guangxi 530028，China )Abs8rac8 Inthebu f ersolution with pH 1#.#+#. 2!potassium persulfate solution wasusedasoxidant !volatilephenolinsurfacewaterwasdeterminedbyspectrophotometricmethodwith4-AAPpurifiedbythemixedsolution ofethanoland hexane (1+4).Theresultsshowedthat4-AAP purified bythe mixed solutionofethanolandhexane (1+4)hadlesslossandhigherpurity !whichwasbeneficialtotheaccurate preparation of 4-AAP solution. In addition , the stable and colourless potassium persulfate solution was usedas oxidant instead of potassium ferricyanide solution !which is favorable to reduce the blank value and the colorisstable.Thedetectionlimit was#.3 %g /L !which mettherequirementsofthenationalstandardmethod. Volatile phenol in surface water was determined by this method with 96. 0% of average standardadditionrecovery.Keywords volatile phenol *4-aminoantipyrine *purification *pota**ium per*ulfate *urface water收稿日期：2020-08-11 修回日期：20200906基金项目：广西自然科学基金重大项目(2013GXNSFEA053001) $作者简介：叶幵晓，女，工程师，主要从事环境监测研究$ E-mail ： yekaixiao_001@ 163. com"通信作者：洪欣，女，高级工程师，主要从事环境监测研究$ E-mail :hxanna@ 163. com引用格式：叶开晓，童天詰，梁柳玲，等•过硫酸钾氧化4-氨基安替比林分光光度法测定地表水中挥发酚中国无机分析 化学 2021 11 2):26-30FYE Kaixiao !TONG Tianzhe !LIANG Liuling etalFDeterminationofVolatilePhenolby4-AminoantipyrineSpectrometricMethod with Potassium Persulfate as Oxidant[J'. Chinese Journal of Inorganic Analytical Chemistry,2021,11(2) ：26-30.第2期叶开晓，等：过硫酸钾氧化4-氨基安替比林分光光度法测定地表水中挥发酚27■>I—1—刖_酚在苯环或稠环上连接一个径基则为一元酚,连接多个径基则为多元酚，通常认为能与水蒸气一起蒸出、沸点在230C以下的一元酚为挥发酚&1'。

氨基酸样品中铵盐的去除赵春雷1

氨基酸样品中铵盐的去除赵春雷1

氨基酸样品中铵盐的去除赵春雷1发布时间：2021-09-06T05:54:32.276Z 来源：《中国科技人才》2021年第15期作者：赵春雷1 翁楠2 唐湘华2 [导读] 氨基酸是酱油风味的主要来源，在提高氨基酸含量的同时，或多或少都会导致铵盐的增加，容易造成酱油中铵盐含量的超标。

赵春雷1 翁楠2 唐湘华2 1：云南创新生物产业孵化器管理有限公司 2：云南师范大学生命科学学院云南省昆明市 650117摘要：氨基酸是酱油风味的主要来源，在提高氨基酸含量的同时，或多或少都会导致铵盐的增加，容易造成酱油中铵盐含量的超标。

本文通过对酱油厂家生产的高浓度氨基酸溶液采用物理高温蒸馏法去除铵盐进行可行性分析。

结果表明：采用物理高温蒸馏法可达到去除铵盐的目的，但氨基酸受损率较高，达到了33%，此方法并不适合用于铵盐的去除。

关键词：酱油；铵盐；氨基酸态氮Removal of ammonium salt in high concentration amino acid Abstract：Amino acids in soy sauce are the main source of soy sauce flavor.While increasing the amino acid content，it will more or less increase the ammonium salt content，which is likely to cause the ammonium salt content in soy sauce to exceed the standard.This article is for soy sauce manufacturers to study the feasibility of using physical high-temperature distillation to remove excessive ammonium salts in soy sauce.The results show that the physical high-temperature distillation method can achieve the purpose of removing ammonium salts，but the amino acid damage rate is higher，reaching 33%.This method is not suitable for the removal of ammonium salts. Keywords：soy sauce；ammonium salt；amino acid nitrogen酱油是中国传统烹饪中常用的调味品，它是由豆粕、谷类等富含植物性蛋白质和淀粉质的原料，在水、食盐、酶等的参与下通过发酵熟成，经多道工序浸滤提取出的液体调味品。

两种测定酱油中氨基酸态氮方法的探讨

两种测定酱油中氨基酸态氮方法的探讨

两种测定酱油中氨基酸态氮方法的探讨章银珠;赵春玲;张书芬【摘要】Two methods (pH meter and colorimetric method) for the determination of amino acid nitrogen in soy sauce were discussed.It is not clear whether there is any difference between the two methods.According to the national standard GB 5009.235-2016 "The determination of amino acid nitrogen in food",the key factors influencing the results of determination and operation cautions to ensure the accuracy of the method were discussed,and then the content of amino acid nitrogen in 10 commercial soy sauce samples were determined by the two methods.The results showed that there was no significant difference between test results of two methods (P＞ 0.05).%该研究探讨了酱油中氨基酸态氮测定的两种方法:酸度计法和比色法.因两种方法间有无差异尚未有定论,依据GB 5009.235-2016《食品中氨基酸态氮的测定》标准,通过探讨影响测定结果的关键因素和操作注意事项来保证方法的准确性,再抽取市售10种酱油,分别用两种方法测定其氨基酸态氮含量,试验结果显示,两种方法检测结果无显著性差异(P＞0.05).【期刊名称】《中国酿造》【年(卷),期】2017(036)006【总页数】3页(P163-165)【关键词】酱油;氨基酸态氮;酸度计法;比色法【作者】章银珠;赵春玲;张书芬【作者单位】宁波市食品检验检测研究院,浙江宁波315000;宁波市食品检验检测研究院,浙江宁波315000;宁波市食品检验检测研究院,浙江宁波315000【正文语种】中文【中图分类】TS264.2酱油是我国传统的调味品，具有悠久的历史，其味道鲜美，富含多种营养物质，集调色、调味于一体。

黄豆酱中氨基酸态氮的测定及其影响因素

黄豆酱中氨基酸态氮的测定及其影响因素
氨 基 酸 态 氮 的 检 测 有 目 测 法、 酸 度 计 法、 比 色 法 [1]。比色法比较繁琐，目测法中指示剂的变色范围 的精准度较低，酸度计法是利用氨基酸的两性性质， 加入甲醛以固定氨基的碱性，使羧基显示出酸性，用 氢氧化钠标准溶液滴定，测试出氨基酸态氮的含量， 简单快捷，使用试剂少，准确度高。所以本法中用酸 度计指示滴定终点。
指捏住滴定管上部无刻度部分，右手两指捏住玻璃球 部分，避免手与有刻度部分的滴定管接触，若溶液温 度发生改变，将使管内溶液发生热胀冷缩，影响滴定 结果；滴定速度不能太快，更不能让溶液成股流下， 这样容易因为酸度计反应迟缓造成滴定过量。一般保 持在 30 滴 /min 的速度。
关键词：黄豆酱；氨基酸态氮；酸度计法
Abstract：In the determination of amino acid state nitrogen in soybean paste, the operation of the personnel, the controlled state of the instrument, the uniformity of the sample, the environmental factors, and the effectiveness of the reagent all have a great influence on the test results. Only these factors are fully taken into account, to get accurate test results.
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Analysis and Testing 分析检测
液并入容量瓶，并加水至刻度，混匀后过滤，滤液备用。 1.2.2 样品测试

氨基态氮

中华人民共和国国家标准果蔬汁饮料中氨基态氮的测定方法甲醛值法 GB/T 12143.2-89Determination method for amino nitrogen contentin fruit and vegetable juice beverages-Formol number───────────────────────────────────────1 主题内容与适用范围本标准规定了用甲醛值滴定法测定果蔬汁饮料中氨基态氮含量的方法。

本标准适用于浓缩果蔬汁、果蔬原汁、果蔬汁饮料及果蔬汁固体饮料。

2 引用标准GB 601 化学试剂标准溶液制备方法GB 604 化学试剂指示剂pH变色域制定法GB 685 化学试剂甲醛溶液HG 3-1082 30%过氧化氢3 原理氨基酸为两性电解质。

在接近中性的水溶液中,全部解离为双极离子。

当甲醛溶液加入后,与中性的氨基酸中的非解离型氨基反应,生成单羟甲基和二羟甲基诱导体,此反应完全定量进行。

此时放出氢离子可用标准碱液滴定,根据碱液的消耗量,计算出氨基态氮的含量。

其离子反应式如下：NH2 NH2││R─CH─COO→←R─CH─COO-+H+ ……………………………………(1)NH2 NHCH2OH││R─CH─COO-+HCHO　R─CH─COO- …………………………………(2)NHCH2OH HOH2CNCH2OH││R─CH─COO-+HCHO→←R─CH─COO- (3)4 试剂所用试剂均为分析纯,使用的水为蒸馏水或同等纯度的水。

4.1 0.1mol/L氢氧化钠标准溶液：按GB 601配制与标定。

4.2 0.05mol/L氢氧化钠标准滴定溶液：用0.1mol/L的氢氧化钠标准溶液当天稀释。

4.3 中性甲醛溶液：量取200mL甲醛溶液(GB 685)于400mL烧杯中,置于电磁搅拌器上, 边搅拌边用0.05mol/L氢氧化钠溶液调至pH8.1.4.4 30%过氧化氢(HG 3-1082).4.5 pH 6.8缓冲溶液：按GB 604中“缓冲溶液的制备”配制。

自酿原浆啤酒氨基酸态氮含量和总酸的测定

酿酒科技2019年第9期（总第303期）·LIQUOR -MAKING SCIENCE &TECHNOLOGY 2019No .9(Tol .303)DOI ：10.13746/j.njkj.2019043基金项目：山西大同大学博士科研启动费（2014-B -13；2016-B -13）；大同市工业重点研发计划项目（2018014）。

收稿日期：2019-03-05作者简介：米智（1985-），男，讲师，博士，主要从事食品发酵技术研究，E -mail ：mizhi775@ 。

通讯作者：刘荔贞（1986-），女，讲师，博士，从事生物化学研究，E -mail ：liulizhen2009@ 。

优先数字出版时间：2019-04-18；地址：/kcms/detail/52.1051.TS.20190418.1506.006.html 。

自酿原浆啤酒氨基酸态氮含量和总酸的测定米智1，刘荔贞2，武晓红1，骈亚男1（1.山西大同大学生命科学学院，山西大同037009； 2.山西大同大学化学与环境工程学院，山西大同037009）摘要：为了探究自酿原浆啤酒中氨基酸态氮含量和总酸，本实验采用山西大同大学生物工程试验中心酿造的啤酒为原料，以甲醛值法和氢氧化钠滴定法测定其氨基酸态氮含量和总酸。

结果表明，麦芽醪在整个发酵过程中，氨基酸态氮前4天含量升高，5～13d 在0.0072g/100mL 上下波动，在临近出酒期间，含量再次升高；而总酸在第3天是最大值，约为1.68mL/100mL ，其余发酵阶段一直在1.02mL/100mL 上下波动。

自酿原浆啤酒氨基酸态氮含量和总酸与常见市售啤酒的含量相当。

关键词：原浆啤酒；甲醛值；滴定；氨基酸态氮；总酸中图分类号：TS262.5；TS261.4；TS261.7文献标识码：A文章编号：1001-9286（2019）09-0098-06Determination of the Content of Amino Acid Nitrogen and Total Acids of Self-Made Draft BeerMI Zhi 1,LIU Lizhen 2,WU Xiaohong 1and PIAN Yanan 1(1.College of Life Science,Datong University,Datong,Shanxi 037009;2.College of Chemistry andEnvironmental Science,Datong University,Datong,Shanxi 037009,China)Abstract :In this study,self-made beer from Bioengineering Experiment Lab of Datong University was used as the research object,and the content of amino acid nitrogen (AAN)and total acids in beer was determined by formaldehyde method and sodium hydroxide titration.The results suggested that,AAN content in wort increased sharply in the first four days in the brewing process,then fluctu-ated around 0.0072g/100mL on the 5th to 13th day,and increased again at the final fermentation period;the content of total acids reached the maximum value on the third day (about 1.68mL/100mL),then fluctuated around 1.02mL/100mL in the rest of the time;the content of AAN and total acids in self-made draft beer was almost the same as that in commercially available beer.Key words :draft beer;formaldehyde method;titration;amino acid nitrogen;total acids啤酒是以大麦芽为主要原料，添加颗粒酒花，经酵母菌发酵酿制而成的含二氧化碳、起泡、低酒精浓度的饮料酒。

液相色谱法测定氮氧化物吸收液中尿素的含量

当代化工研究^■Chenmical I ntermediate科研开发2019•01液相色谱法测定戴氧化物吸收液中尿素的含量*叶琴娣韩磊魏辉荣赵鑫伟陈华顾奇龙(浙江安诺芳胺化学品有限公司浙江312300)摘要：建立了液相色谱法测定氮氧化物吸收液中尿素含量的方法。

色谱柱为邢2柱（250mmx 4.6匪x5Wi)，采用乙腈（95 : 5)流动相实 现尿素与杂质峰的完全分离。

尿素的浓度在0.0040-0. 3000g/L范围内与色谱峰面积呈良好的线性。

加标回收率为99. 4-103. 4%，测定结果 的相对标准偏差为0_5M (n=10)。

关键词：氮氧化物；展素含量；液相色谱法中图分类号：0 文献标识码：ADetermination of Urea Content in Nitrogen Oxides Absorption Liquid by LiquidChromatographyYe Qindi,Han Lei,Wei Huirong,Zhao Xinwei,Chen Hua,Gu Qilong(Zhejiang Amino-chem CO.,LTD.,Zhejiang,312300)A bstract: Liquid c hromatography was applied t o the determination o f u rea content in Nitrogen oxides absorption liquid. The chromatographic column is a NH2 column (250mm'x4.6mm'^5fim) with acetonitrile (95:5) f lowing phase to achieve complete separation o f u rea and impurity peaks. When urea content is in the range o f0.0040-0.3 O OOg/L, it had g ood linear with chromatographic p eak a rea. The recovery rate was f rom 99.4 to 103.4%, and the relative standard deviation o f t he measured result was 0.50% (n=10).K ey w ords：nitrogen oxides\ urea content' liquid chromatography引言 2.实验过程在化学工业中，针对不同尾气中的氮氧化物，企业需 要采用相应的尾气处理装置对放空尾气处理至合格后才能放 空。

分光光度法测定调味品中氨基酸态氮的研究

取同一样品按照上述方法平行测定 次，测定结
表 2 方法的准确度试验（n=6） Table 2 Accuracy test (n=6)
样品 本底值/μg 加标量/μg 测定值/μg 平均回收率/% RSD/%
样品 1 7.78
10.0
17.23
94.5
5.24
样品 2 11.75
20.0
31.92
该溶液含氮 1.0 mg/mL,贮存于 4 ℃冰箱中，使用时用 移液管精密吸取 10 mL 定容至 100 mL 容量瓶中，此溶 液含氮 100 μg/mL，即 100 mg/L。 1.1.2.2 显色剂
取 15 mL 37 %甲醛和 7.8 mL 乙酰丙酮混合后加 水至 100 mL，剧烈振摇混匀。 1.1.2.3 乙酸铵缓冲溶液
154
2010 年 2 月 第 31 卷第 2 期
食品研究与开发
检测分析
2 结果与讨论 2.1 标准曲线线性分析
含量范围在 1.0 μg/mL～100 μg/mL 呈线性关系， 回归方程为 A=0.004 6 c-0.004 8（c 为 μg/mL），相关系 数为 r=0.998 4。空白溶液平行测定 20 次，方法的检出 限为 0.33 μg/mL。 2.2 显色剂加入量试验
虾油
3 结论
0.64
试 验 结 果 表 明 ，本 方 法 操 作 简 单 、快 速 、重 现 性
0.58
0.63
好，精密度和准确度较高，与甲醛滴定法等方法相比大
0.64
大降低了甲醛的使用量，从而降低了对检验人员身体
0.54
健康和环境的危害，为测定调味品中氨基酸态氮提供
0.60
了一种更为的理想的方法。
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good accuracy (SE l-3%) with recovery values of the standard (L-leucine) from the protein matrix of 91, 111 and 75% for the OPA, ninhydrin and TNBS
method, respectively. 0 1998 Elsevier Science Ltd. All rights reserved.
Analytical. Nutritional and Clinical Methods Section
Determination of a-amino nitrogen in pea protein hydrolysates: a comparison of three analytical methods
PII:
ELSEVIER
SO308-8146(97)00164-7
Food Chemisrry, Vol. 62, No. 3, pp. 363-367, 1998 0 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 030878146/98 $19.00+0.00
$cYi -YZ2
MATERIALS AND METHODS Scope of experiments
where xi is the result obtained using method X; yi is the result obtained using method Y; and yl is the result calculated from the regression equation. Enzymatic hydrolysis
364
R. Panasiuk et al.
ethanethiol (Frister et al., 1986) or N-acetyl+cysteine (Alvarez-Coque et al., 1989). Although the methods for o-amino nitrogen determination as described above have been used individually frequently, only few comparative studies have been reported (Bertrand-Harb et al., 1993). In this study the TNBS, OPA and ninhydrin methods have been compared to establish accurate and reliable DH measurements for pea protein hydrolysis. Colour stability, reproducibility and recovery of a standard from the hydrolysed protein matrix have been determined and the correlation between the data obtained by the three methods is presented.

The absorbance, at given wavelength, obtained after reactions of I,-leucine or amino groups of protein hydrolysate with TNBS, OPA or ninhydrin was recorded for 30min at 30-s intervals. The amounts of standard or hydrolysate were selected in such a way that the absorbance value was between 0.5 and 0.8. For the OPA assay the absorbance was measured directly after the reagents were mixed and for the TNBS and ninhydrin assays after the period required for colour developement, as described in the assay description. Recovery of L-leucine from the hydrolysed protein matrix was determined by standard addition of L-leucine to the protein hydrolysate with a DH of 4%. Two levels of standard addition were used, resulting in an increase in the absorbance values of 20 and 50% as compared to the values found for unfortified samples. The number of samples in each series was 10. The accuracy of particular analytical methods, expressed as the relative standard error (n = 15), was determined with the protein hydrolysate with a DH of 4%. The correlation between the results of the three methods was calculated and presented as linear regression equations. Also, the results were compared using Student’s t-test for the correlated samples (Boiyk and Rudzki, 1977). The presence of systematic constant and variable errors between the methods was tested using Student’s t-test (eqns l-4) for the hypothesis verification that a and b in regression equations do not differ from A = 1 and B = 0 (Boiyk and Rudzki, 1977).
*To whom correspondence should be addressed.
Alternatively, ninhydrin (Wiewi6rowski et al., 1958) and o-phthaldialdehyde (OPA) (Church et al., 1983), which are more selective for a-amino groups than TNBS, may be used as these reagents only react with aminogroups or its derivatives. The nitrogen atom is the essential factor for colour development in these colour reactions. A further advantage of the ninhydrin method is its specific reaction with a-amino groups (analytical wavelength, )cmax 570 nm). Its reaction with = E-amino groups and ammonium ions gives products that absorb at another wavelength, ALmax= 440nm (Yemm and Cocking, 1955; Schilling et al., 1963; Samejima et al., 1971). According to reports by Benson and Hare (1975) and Goyal et al. (1988), OPA also forms a fluorescent adduct with ammonium ions. Although the authors did not indicate that the complex formed absorbs at analytical wavelength, it is likely. Furthermore, fi-mercaptoethanol, widely used as a thiol compound in the OPA assay, initiates a secondary reaction leading to decomposition of the coloured amino-OPA complex (Frister et al., 1986). They reported that as early as 2min from the start of the reaction the absorbance markedly decreases, causing serious errors in the readings. This reaction, however, may be eliminated and thereby the measuring stability improved, by using 363