粒度表征及激光粒度分析技术进展

1、 Mar. 31, 2004, Mar. 31, 2004, 颗粒特性分析测定仪器颗粒特性分析测定仪器n英国马尔文公司英国马尔文公司n激光衍射粒度分析仪激光衍射粒度分析仪 0.02 3500 微米 500/1000 Hz 扫描速率n激光动态光散射分析仪激光动态光散射分析仪 1 5000纳米/ZETA电位n干粉干粉/喷雾粒度分析仪喷雾粒度分析仪n在线粒度分析仪在线粒度分析仪n高浓度超声粒度分析仪高浓度超声粒度分析仪n绝对分子量分布测定仪绝对分子量分布测定仪 100 1 e 12 Daltonsn美国康塔公司美国康塔公司n全自动比表面及孔隙度分析仪全自动比表面及孔隙度分析仪 (分析站数可选 1/2

2、/3/6) 0.005 m2/g 3.5 5000 埃n压汞仪压汞仪 3.6纳米 426 微米孔径n化学吸附仪化学吸附仪 (TPR/TPD)n流动法快速比表面测定仪流动法快速比表面测定仪n全自动真密度计全自动真密度计n自动堆密度分析仪自动堆密度分析仪Malvern Instruments UK Headquarters 英国马尔文仪器有限公司英国马尔文仪器有限公司 _激光粒度分析仪的创始人激光粒度分析仪的创始人- - 世界上最大的激光粒度分析仪专业设计和生产厂家世界上最大的激光粒度分析仪专业设计和生产厂家- - 世界上第一台相关处理器世界上第一台相关处理器- - 世界上第一台激光衍射法粒度分析

3、仪，世界上第一台激光衍射法粒度分析仪，- - 世界上第一台激光世界上第一台激光PCSPCS粒度分析仪粒度分析仪- - 世界上第一台超声粒度分析仪世界上第一台超声粒度分析仪- - 销售量占世界第一，仅中国大陆已有销售量占世界第一，仅中国大陆已有450450台以上台以上- - 已获得已获得ISO9001ISO9001标准标准, , 欧洲欧洲EMCEMC标准认证标准认证, GMP, GMP标准认标准认 证，唯一完全符合美国证，唯一完全符合美国FDA QSpecFDA QSpec要求要求- - 多方位应用支持，在中国设立正式技术服务中心多方位应用支持，在中国设立正式技术服务中心. .First .19

4、77- IEE McRobert Award1977- Queens Award for Technical Achievement1981- Queens Award for Export Achievement1987- TASC Award for Analytical Equipment1988- Queens Award for Technical Achievement1988- Queens Award for Export Achievement1993- Engineering Safe Workers Award1994- British Safety Council Aw

5、ard and?.谁关心粒度分布？石油石化、材料科学、化工、制药、地质、涂料和颜料、陶瓷、磨料、造纸、电池、能源和航空，稀土，非矿，航天，军工墨粉，食品，环保（水处理和沙尘）等领域 Malvern End User Field Distributionin 2000 (* new gain point)4%18%13%17%10%2%4%8%4%4%4%2%8%2%Pharm.*Inst./Univ.Petro./Chem*Chem.(Coat,dye.)*Geo./Soil/sand*Cement*Electronics/BatteryPowder/Mineral*Non-Fe MatalC

6、eramicsAerospace*Coal Slurry*Alumina Factory*Cosmetic激光粒度仪在轻工行业的应用n食品、医药工业：新型天然高吸收率食品、饮料、药品或保健品n1）动物超细粉:超细鳖粉、蛇粉、蚂蚁粉、蛆粉等等。n2）植物超细粉：植物奶、蔬菜汁（粉）、果汁或粉、天然植物色素等等。n3) 超细药物：药物超细后，可提高吸收率和疗效；中药材超细后，不仅可提高吸收率和疗效，而且还是服用方便，避免传统复杂的饮片煎煮。n4) 其他：超细麸皮粉、玉米胚芽渣粉、甘蔗渣粉、粉茶、灵芝粉、花粉、螺旋藻粉、人参粉等等。激光粒度仪在轻工行业的应用n 纺织工业：n加入超细氧化硅、氧化钛可以

7、提高产品的质量和光洁度；n利用纳米氧化锌吸收紫外线的能力，制成防紫外线服装化纤；n利用纳米氧化锌对远红外发射性能，制成内衣、护腰、护腕，可以改善血液循环和组织供氧能力，加强机体水分子共振，促进新陈代谢，从而起到辅助理疗保健功效；也可利用其抑菌、杀菌作用制成抗菌纤维。n皮革：超细蚕丝的加入可制出高性能、高光洁度的皮革。n化妆品和日用品：n超细钛白粉、碳酸钙、蚕丝和色素等是口红、粉饼、护肤膏、肥皂、牙膏、洗发液的重要组成原料。另外，利用纳米TiO2吸收紫外线的能力，可制成防晒剂、抗菌剂等。激光粒度仪在轻工行业的应用n油漆、涂料：n加入超细粉后，可制成高性能、高附着力的新型产品；加入TiO2达到抗菌

8、、消臭的目的；加入超微粉体香味剂制成自然香型涂料。n造纸、橡胶、塑料等工业：n加入固体填料如重质碳酸钙、氧化钛、氧化硅等超细粉后，可制成高性能铜板纸、塑料及橡胶制品；纳米陶瓷粉与高分子材料复合可显著的提高工程塑料的力学性能。n抗菌防臭的餐具、卫生洁具，包装材料等等。n其他：超细碳黑可制成高质量的复印或打印粉墨。 激光粒度仪在乳品及相关行业的应用 粒度与食品色泽、口感、稳定性密切相关n脱脂奶及奶制品的稳定性：光明乳业n植物油脱色：白土n香精香料等原料：苏州丹尼斯克，国际香精香料公司n巧克力：可可粉和蔗糖粒度作为品质及口感的指标： 确保巧克力只溶在口：Cadbury Ltd.n高品质奶油、冰激淋、

9、人造黄油和低脂黄油片需要粒度控制：Unilever 。n环境保护：水质监测和废水处理：粒度和zeta电位n面粉粒度控制：机械化生产面条及其他面食所必需。Story.Story.来自一个懊悔者的倾诉来自一个懊悔者的倾诉- 2002- 2002年年3 3月月1919日日面粉乌冬面还需要粒度控制？No￥3000万投资万投资60车皮废品车皮废品 ！ 粉体颗粒粒度是产品的主要质量指标，它可用来预测产品稳定性、纸张涂层特性、颜料覆盖能力、水泥凝固时间、药物活性，食品色泽及口感等等，也是选择分离设备，预测滤饼层的渗透性或比阻等的依据。 粉体颗粒粒度在各个环节的实时监控是现代化生产的重要手段。为什么他们关心粒

10、度分布？为什么他们关心粒度分布？粒度分布控制有什么意义？由粗放型向集约型经营转变的必备手段（1）晶粒：晶粒：指单晶颗粒，即颗粒内为单相，无晶界。（2）一次颗粒：一次颗粒：指含有低孔隙率的一种独立的粒子。它能被电子显微镜观察到。（3）团聚体：团聚体：是由一次颗粒为降低表面势能而通过范德华力或固定的桥键作用形成的更大颗粒。团聚体内含有相互连接的孔隙网络，它能被电子显微镜观察到。（4）二次颗粒：二次颗粒：指人为制造的粉料团聚粒子。 目前所谓“纳米材料”的功能绝大多数体现为 团聚体的功能，其粒度能被马尔文激光粒度仪 精确测出。若能将团聚体分散成一次颗粒，则 将表现出纳米颗粒更多的特性。 关于颗粒的基本

11、概念关于颗粒的基本概念n什么是颗粒？什么是颗粒？nFeret 直径 - 平行切面之间的距离. nMartin 直径 - 等分线直径n最长直径n最短直径n等效周长直径 - 同等周长的圆圈直径n等效投影面积直径 - 与投影面积相同的圆面直径n等效表面积直径n等效体积直径各种方法对球体粒径平均值的不同表征各种方法对球体粒径平均值的不同表征对于三个直径分别为1、2、3单位的球体，它们的平均值是多少呢？n电子显微镜法：取长度平均值 D1，0 平均直径=（1+2+3）/3 = 2.00 =d/nn 图像分析仪： 取面积平均值 D2，0 平均直径=Sq. rt. (12+22+32)/3 = 2.16 =

12、Sq. rt.(d2/n)n 电场感应法： 取颗粒的体积平均值 D3，0 平均直径=Cube rt. (13+23+43)/3 = 2.20 = Cube rt.(d3/n)n激光衍射法： 取平均当量体积值 D4，3，因为它不需要颗粒数 如果颗粒的密度不变，这个值与平均当量重量值是一致的。 D4,3 = (14+24+34)/(13+23+33) = 2.72 =d4/ d3 D3,2 = (13+23+33)/(12+22+32) = 2.57 = d3/d2 D3，2称作当量面积平均值，或称作索尔特平均直径（SMD）， 它通常应用于颗粒的表面活性或表面积显得非常重要的领域（如 催化作用或燃

13、烧作用）。 哪种表征或结果是哪种表征或结果是“正确正确”的？的？都正确，都反映了颗粒的某种特性即使是球体，不同的方法也给出不同的平均值在描述粒径大小时，必须同时指明测定方法才有意义不同粒度测定方法之间的比较没有意义不同方法之间的比较必须转化成 同一粒径的定义 常见粒度分析方法统计方法代表性强, 动态范围宽分辨率低n筛分方法 38微米- n沉降方法0.01-300微米n光学方法0.001-3500微米非统计方法分辨率高代表性差, 动态范围窄n显微镜方法重复性差光学 1微米-电子0.001微米-n电域敏感法0.5-1200微米优点n统计量大, 代表性强n便宜n重量分布缺点n下限38微米n人为因素影

14、响大n重复性差n非规则形状粒子误差n速度慢显微镜方法优点n可直接观察粒子形状n可直接观察粒子是否团聚n光学显微镜便宜缺点n代表性差n重复性差n测量投影面积直径n速度慢stsfugd218沉降法优点n测量重量分布n代表性强n经典理论, 不 同 厂 家仪器结果对比性好n价格比激光衍射法便宜缺点n对于小粒子测试速度慢, 重复性差n非球型粒子误差大n不适应于混合物料n动态范围比激光衍射法窄光学方法n激光衍射方法( 0.02-3500微米)nPCS 光子相关光谱方法( 0.001-3微米)n光阻方法(0.01-250微米) 英国马尔文仪器公司英国马尔文仪器公司 世界最大的专业设计及制造厂家世界最大的专业

15、设计及制造厂家 Small particles scatter light at wide anglesLaserParticlesDetectorFourierLensBeamLarge particles scatter light at narrow anglesParticle CharacterizationTypical Elements of InstrumentsBeamLaser Fourier LensParticle SampleChamber Detector ArraySample Particles Light Scattered from Particles Me

16、asured Scattered Light Intensity Profile0 .3 /3 /3 0 /3 0 0角 度散射光强粒子直径减少粒子直径减少10倍，散射光强减弱一百万倍倍，散射光强减弱一百万倍 !根据瑞利散射定律：根据瑞利散射定律： I D6 / 4 散射光强 粒子直径 光源波长HeNeLaser2mw Power0. 63 m wavel engt hSpat i alFi l t erPar t i cl essuspendedi nGasorl i qui dFour i erTr ansf or mLensUnscat t er edLi ghtScat t er ed

17、Li ghtMul t iEl ementDet ect orObscur at i onDet ect or技术难点：检测器面积有限；小粒子散射光弱；杂散光影响选择哪类激光光源？选择哪类激光光源？氦-氖气体激光器(633nm):无背景噪音，对0.3nm颗粒响应 两类激光器的差异氦氖激光器：n 光源稳定性好n 单色性好n 相位一致固体二极管激光器：n 光源稳定性差n 分散性高n 相位相关性差，导致数据的重复性低Mastersizer 2000 blue light enhancement检检 测测 器器 系系 统统nThe first Malvern systems developed mor

18、e than 20 years ago featured a detector design shown herenThis comprised 32 concentric photodiode rings.Live detector elementsInsulating spacesParticle CharacterizationTypical Elements of InstrumentsBeamLaser Fourier LensParticle SampleChamber Detector ArraySample Particles Light Scattered from Part

19、icles Measured Scattered Light Intensity ProfileDetector DesignIn the new design, not only was the dead space completelyeliminated, but the detector dimensions were changedenabling us to measure a much greater angular range, and hence a wider size distribution.Low Angle Laser Light ScatteringThe Com

20、plete no Compromise SolutionParticles in Liquid suspension 0.05-900 micronsReverse Fourier modeExtra Large Angle DetectorsSample cellMain Detector Backscatter detectorConcentrationDetectorMastersizer 2000 光路设计 1998年10月问世FA: 前倾角检测器； LA: 大角检测器； BS: 背散射光检测器Mastersizer 2000有哪些性能突破有哪些性能突破?- - 动态测定范围加宽:0.

21、02-2000微米（一次完成）- 粒度分布的分辨率提高，特别是在亚微米区- 准确性达 1%; 重复性达0.5%- 扫描速度达1000次/秒; 一次测量仅需2秒- 高度智能化,不需更换镜头，没有两种测试混合-“插拔式”: 分散器快速更换- SOP 操作: ISO 标准化要求- 开放式软件设计及数据库: 无需用户编程Carbon black ink.在0.03um 的碳黑样品中逐步加入100um的透明玻璃球，结果显示：- 不会对碳黑样品测量造成影响大庆滤后水中颗粒分布测定（灵敏度实验：颗粒浓度：4ppm,遮光度=1）趋 势 图2 3 4 5 6 记 录 编号135791113151719Param

22、eterd (0.1)d (0.5)d (0.9)Mastersizer 2000喷射式干法测定喷射式干法测定钕铁硼粒度分布钕铁硼粒度分布 5次结果粒度分布趋势变化分析次结果粒度分布趋势变化分析钕铁硼：湿法制备钕铁硼：湿法制备 费氏粒度费氏粒度=4.7um MS2000激光粒度分析结果激光粒度分析结果: D(3,2)=4.9umContinuous SprayTimeConcentrationSizespray1.xlcNasal Spray FIG 3Latex MeasurementsParticle Diameter (m.)%0 10 20 30 40 50 60 70 80 010

23、20 30 40 50 60 70 80 90 100 0.1 1.0 10.0 100.01000.0496nm Latex-32.019um Latex-39.870um Latex-431.1um Latex-3301um Latex-3100um Latex-7Latex Mixture( single measurement)Particle Diameter (m.)%0 10 20 010 20 30 40 50 60 70 80 90 100 0.1 1.0 10.0 100.01000.0.25/2/9.9/31/100/301-11与跟踪发展起来的同类产品相比-马尔文激光粒

24、度分析仪具有以下特点： 1. 30秒内完成从加样到打印全部操作，一次得到全范围粒度 分布图（例如，0.02 2000微米）。n2. 全范围统一的激光衍射法，保证背景一致，不会漏检，无假峰。n3. 速度快，重现性高。每次测试可在1秒钟内达到500次扫描，重现性优于0.5%，在线分析可达每秒2000次扫描。n4. 动态范围宽，可以达到20000.02微米或35004微米一个量程。n5. 直接测量体积分布，无需标定，无需校准，无需人工调整。n6. 可有多种分散方式，可进行干法和喷雾测量。n7. 湿法测量可直接用自来水，无需使用超纯水(纳米粒子除外)。n8. 适用于非球形和混合物料粒子测试。n9. 新

25、型Mastersizer2000具有高度智能化及标准化SOP操作特点， 1m下分辨率极高。n10. 全中文操作及说明应用软件，MS2000中文操作说明书 The Mastersizer Family马尔文光子相关光谱仪马尔文光子相关光谱仪- Zetasizer (HS) 系列: 2 3000 nm- Autosizer 4800: 1- 5000 nmSize distribution(s)51050 1005001000Diameter (nm)510% in classZave = 3.6nmZetasizer系列 纳米粒度及Zeta电位分析仪和MPT-1 自动滴定系统The MPT-1

26、是为是为Zetasizer分析仪特制的高效专用自动滴定装置分析仪特制的高效专用自动滴定装置 粒径范围：2 nm 3000 nm 专利高灵敏测量系统,下限测量无须高功率激光器。 2000年最新M3专利技术，具有高分辨高准确性，可分辨混合粒子1mV的Zeta电位差异。 突出的高重现性，由精确的内部温控技术和在线pH测量加以保证。 全自动三元滴定系统：精确控制、实时测量pH，盐浓度和电导率的连续变化。 全自动结果自诊断系统：结果判断因子(RQF) 保证了每次测量质量。 内置ISO13321标准指导操作。PHOTONCORRELATION SPECTROSCOPYPCSWHAT DOES IT STA

27、ND FOR?PCSWHAT DOES IT DO?It measures the size of small particlesHOW SMALL?VERYSMALL!HOW SMALL IS SMALL?The smallest sized particle we can measure with PCS is 2 nanometres (nm)What is a nanometre?It is 10-7 of a centimetre5 million!How many 2nm particles will fit end to end into 1 cm?02143Lets consi

28、der a rulerSOME PARTICLE SIZES.nRed Blood Cell.7000nmnBacteria/Yeast Cells.2000-3000nmnSmallest particle seen through a light microscope.200nmnViruses10-100nmnMad Cow Disease Prions.10-20nmnProtein (lysozyme).3nmWHAT DOES A PCS INSTRUMENT CONSIST OF?PinholeDetector(Photomultiplier/AvalanchePhoto Dio

29、de)LaserSample containing particlesComputercontainingcorrelator card (7132)Scattered lightZetasizer HS (High Sensitivity) instruments contain an APD detectorTHE LASER AND THE WAVELENGTHThe laser used in the Zetasizer instruments is a Helium-Neon laser(He-Ne)Its wavelength is 633nm (red light)What do

30、 you mean by wavelength?A laser produces lightof one wavelengthTHE DARK SIDE OF THE MOONREDORANGEYELLOWGREENBLUEINDIGOVIOLETWAVELENGTHEach colour has a different wavelengthRED633nmBLUE488nm633nm488nmTHE INTENSITY OF SCATTERED LIGHTThe intensity of scattered light obtained from a dispersion of partic

31、les is dependent uponLaser wavelength Particle size SCATTERED LIGHT ANDLASER WAVELENGTHGreen light will give approximately 2 times the intensity of scattered light compared to red lightThis is the reason why some people buy a different laser to use with a ZetasizerIf you are trying to measure very s

32、mall particles or not many particles, the intensity of scattered light will be very lowSCATTERED LIGHT AND PARTICLE SIZEDispersion of smallparticlesLaserThe intensity of scattered light from small particlesis equal in all directionsSCATTERED LIGHT AND PARTICLE SIZEDispersion of largeparticlesThe int

33、ensity of scattered light from large particlesis not equal in all directionsLaserINTENSITY OF SCATTERED LIGHT VERSUS ANGLE40608010012005101520253035AngleRelative Intensity1000nm800nm600nm400nm200nmMULTI-ANGLE INSTRUMENTSPeople measuring particles of 300nm and smallerwill only require a 90o fixed ang

34、le instrument (Zetasizer 1000/3000)People measuring particles of 300nm and largerwill require a multi-angle instrument (Autosizer 4800)Laser diffraction (Mastersizer)simultaneouslymeasures the intensity of scattered light at various anglesHOW DOES PCS WORK?All small particles in waterwill move rando

35、mlyThis process is called Brownian motionBecause the water molecules surrounding them are constantly hitting themWhy is that?BROWNIAN MOTION OFLARGE PARTICLESHHOHHOHHOHHOHHOHHOHHOHHOHHOLarge Particles - Slow Brownian MotionBROWNIAN MOTION OFSMALL PARTICLESHHOHHOHHOHHOHHOHHOHHOHHOHHOSmall Particles -

36、 Fast Brownian MotionBROWNIAN MOTION AND SCATTERED LIGHTBecause the scattered waves of lightare interfering with one anotherWhy does this happen?The Brownian motion of the particles causes the scattered light tofluctuate in intensityBROWNIAN MOTION AND SCATTERED LIGHTScreenTwo beams interfere andcan

37、cel each other outresulting in a decreased intensity in the scattered lightLet us consider 2 stationary particlesBROWNIAN MOTION AND SCATTERED LIGHTScreenTwo beams interfere andenhance each otherresulting in an increased intensity in the scattered lightBROWNIAN MOTION AND SCATTERED LIGHTMany scatter

38、ed beams interfere with one anotherresulting in a very complex intensity pattern of specklesScreenConsider many particlesHOW DO WE MEASUREHOW QUICKLY THE SCATTERED LIGHT IS CHANGING?We use a device called a correlatorWe were the firstmanufacturers of correlatorsin the world HOW DO WE MEASUREHOW QUIC

39、KLY THE SCATTERED LIGHT IS CHANGING?Today the correlator is a card which fits into a personal computer(7132)THE CORRELATORThe intensity of thescattered light is measuredat different times by the correlatorThe correlator then calculates how quicklythe signal is changing and thisgives us the particle

40、sizeHow does a correlator work?The 7132 correlator can look at the intensity of scattered light every 50 nanosecondsi.e. 20 million times a second!雪崩式光子电子计数检测器n固态二极管检测器(Solid state diode detectors)n当光子撞击时，产生了电子空穴对n产生的高压加速了电子运动n被加速的电子获的足够的能量进一步导致电离度的增加，如同雪崩一样。n最初的光子能雪崩式产生大量的约106 电子这个数量级远大于普通的光电备增器( p

41、hotomultiplier detector)n因此有了新一代的雪崩式光子电子计数器检测器应用实例: Absorbing systemsz- 平均粒径平均粒径 (nm)多分散度多分散度测定时间测定时间分分计数率计数率(KCPS)18.418.418.4345.4346.0346.1180.4179.3181.2177.3177.5178.40.2460.2210.2240.1980.2120.212151515222RUN123456检测器检测器PMPMPMAPDAPDAPD10mW He-Ne laser 测碳黑样品测碳黑样品计数率越高越好，结果重复性越好计数率越高越好，结果重复性越好应用

42、实例: 溶液中的蛋白质51050 100500Diameter (nm)102030% in classIntensity Size distributionz- 平均粒径平均粒径 (nm)多分散度多分散度RUN123MEAN10.311.411.010.9 0.50.2240.2430.2440.237 0.011 测量时间测量时间 = 15 分分/次次0.5% 牛血清蛋白牛血清蛋白 (BSA)Stern layerZETA POTENTIALWithin the diffuse layer is a notional boundary (the slipping plane) within

43、 which the particle acts as a single entityThe potential at this boundary is theZETA POTENTIALSlipping planeDiffuse layer-1000mVDistance from particle surfaceSurface potentialZeta potentialZETA 电位与散射体系的稳定性Negative zeta potentialPositive zeta potential+30mV-30mV0mV稳定稳定不稳定ZETA 电位与 pH 关系2468101-60-40-2

44、00204060等电点等电点Zeta 电位电位 (mV)pH2稳定稳定不稳定HOW DO WE MEASUREZETA POTENTIAL?Zeta potential is measuredby measuring how quicklyparticles move in an applied electrical fieldHow do we do that?MEASURING PARTICLE MOBILITY IN AN ELECTRIC FIELD+-THE CAPILLARY CELL IN A ZETASIZERGlass Capillary CellPositiveElectr

45、odeNegativeElectrodeCROSSING OF TWO LASER BEAMSInterferenceFringes高分辨Zeta电位技术 M3专利 （2000年7月）常规电泳法得到的常规电泳法得到的ZETA电位图电位图M3技术得到的技术得到的ZETA电位图电位图 图1、四种标准物质混合的Zeta电位图 图2、M3专利技术特征 (4种标准品分别为-50mV,-60mV,-84mV,-108mV) Theory of use of the stationary layer methodnElectrophoresisnElectroosmosisnThe stationary l

46、ayer ElectrophoresisnElectrophoresis is the effect where particles with a net charge when placed in an electric field will migrate towards the electrode with opposite charge.nBecause micron size particles have very low inertia, terminal velocity is reached in microseconds. The actual velocity is det

47、ermined by the charge associated with the particle (zeta potential), the viscosity of the medium and the applied field.Cause -Application of an electric fieldEffect -Particle movementstationary - liquid phasemobile - solid phaseElectroosmosisnElectroosmosis is the movement of a liquid containing ion

48、s, next to a charged surface when a field is applied along the surface.nIn a quartz cell the velocity of the electroosmotic flow is the same order of magnitude as the particle mobilities we wish to measure. This flow takes much longer to establish than electrophoresis, in the order of 10s of millise

49、condsCause -Application of an electric fieldEffect -Liquid movementstationary - solid phasemobile - liquid phaseThe stationary layernThere is only one position in a capillary cell where we can measure the true particle mobility.nIn a capillary which is closed at both ends the liquid returns along th

50、e centre of the cell. We measure where the fluid flow along the cell wall cancels out with the return flow in the centre of the cell.StationaryLayerAvoiding electroosmosisnThe best solution would be to avoid electroosmosis altogether. The mobility of the particles at any point in the cell would then

51、 be the true mobilitynUsing a dip cell is one way to remove the effects of electroosmosis, but there are a lot of disadvantages to this techniquenAnother approach is to neutralise the charge on the cell walls with a coating, but this always wears off and is application specificnA recently described

52、alternative is to reverse the applied field rapidly enough that electroosmosis is insignificant (FFR), this gives an accurate mean, but is low resolutionReversing the applied fieldnAll systems that measure mobilities using LDV (laser Doppler velocimetry) reverse the applied field periodically.nThis

53、is to reduce the polarisation of the electrodes that is inevitable in a conductive solution. This is usually set to about 1 second to allow stabilisation of the fluid flow.Electroosmosis in a slowly reversing fieldStationaryPlaneFFR (Fast field reversal)nIf the field is reversed much more rapidly, i

54、t is possible to show that the particles reach terminal velocity, while the fluid flow due to electroosmosis is insignificant.nThe mean zeta potential calculated by this technique is therefore very robust as the measurement position in the cell is not critical.nHowever, as the velocity of the partic

55、les is sampled for such a short period of time, information about the distribution is degraded. This is what is addressed by the M3 techniqueElectroosmotic flow in a fast reversing fieldStationaryPlaneWhat is M3?nA new method of making zeta potential measurements that uses the best features of both

56、stationary layer and FFR techniquesnNo cell alignmentANDnImproved resolutionThis method requires:nA new cell typenNew software (version 1.51)The best of both worldsnM3 uses measurements of both slow field reversal and fast field reversal, hence the name Mixed Mode Measurement.nIn addition to the zer

57、o field measurement, which gives a better measurement of the actual distribution width, two measurements are done for each zeta potential determination.1. A fast field reversal measurement provides the accuracy and stability of the result2. Information from the slow field reversal improves the resol

58、ution.nBecause we now use the fast field reversal to calculate the mean, we can extract more resolution from the slow field reversal mode, without affecting the accuracy or repeatability of the mean.The best of both worldsnAn M3 measurement consists of:nAn FFR measurement at 1000Hz at the cell centr

59、e. This gives an accurate determination of the mean.nA zero field measurement is performed to measure the effect of Brownian motion broadeningnA slow field reversal measurement at the cell centre. This gives better resolution, however the mobility values will be shifted by the effect of electroosmos

60、is.nThe means calculated from the FFR and slow field measurements are subtracted to give the electroosmotic flow. nThis value is used to normalise the slow field reversal distributionnThe value for electroosmosis is used to calculate the zeta potential of the cell wallZero field correctionnThis meas