Name: tlc separation by polarity
Uploaded: 2020-03-26T07:22:28.000Z
Duration: 3 min 7 s
Channel: JoVE

كروماتوغرافيا الطبقة الرقيقة ، أو TLC ، هي تقنية تستخدم لفصل المركبات العضوية بناء على الاختلافات في أقطابها. هناك ثلاثة مكونات لنظام TLC: المرحلة الثابتة ، والمواد المذابة ، ومذيب التطور. 
المرحلة الثابتة هي السطح الذي سيتم فصل المركبات عليه ، والذي يتكون عادة من هلام السيليكا شديد القطبية. المركبات أو المخاليط التي يتم تحليلها هي المواد المذابة. يتم رصد المحاليل المخففة للمواد المذابة على حافة واحدة من اللوحة باستخدام الشعيرات الدموية الرقيقة. 
ثم يتم غمر الجزء السفلي من اللوحة في مذيب التطوير ، والذي عادة ما يكون مذيبا عضويا أقل قطبية من المرحلة الثابتة. ينتقل المذيب إلى أعلى اللوحة عن طريق العمل الشعري. عندما يمر المذيب عبر البقع المذابة ، فإنه يحمل معه بعضا من كل مذاب. هذا الخليط المذيب هو المرحلة المتحركة. 
يسمى الجزء العلوي من المرحلة المتنقلة جبهة المذيب. عندما تنتقل المرحلة المتحركة إلى أعلى اللوحة ، تلتصق جزيئات المذاب والمذيبات مؤقتا بالمرحلة الثابتة بناء على قطبيتها. 
تنجذب المزيد من المواد المذابة القطبية بقوة إلى المرحلة الثابتة ، لذلك تلتصق في كثير من الأحيان وتبقى عالقة لفترة من الوقت. تلتصق المواد المذابة القطبية الأقل في كثير من الأحيان ولا تبقى عالقة لفترة طويلة. هذا يعني أن المواد المذابة القطبية الأقل تقضي وقتا أطول في المرحلة المتنقلة من المواد المذابة القطبية. بمعنى آخر ، تنتقل المواد المذابة القطبية الأقل بشكل أسرع من المواد المذابة القطبية. 
لتحقيق فصل جيد بين المركبات ، يجب أن يكون المذيب قطبيا بدرجة كافية لتحريك المذاب الأكثر قطبية. فيما يلي بعض المذيبات الشائعة بترتيب زيادة القطبية. في بعض الأحيان ، يتم الجمع بين مذيبين لتحقيق القطبية المطلوبة. 
نظرا لأن معظم المركبات العضوية بيضاء أو عديمة اللون ، فعادة ما تحتوي ألواح TLC على مركب فلورسنت. تحت ضوء الأشعة فوق البنفسجية ، ستظهر المركبات العضوية على شكل بقع داكنة على اللوحة المتوهجة. 
بمجرد تصور المركبات ، يمكنك قياس المسافة التي قطعتها وحساب عامل التخلف ، أو R f لكل منها. هذه القيمة هي المسافة التي يقطعها المذاب مقسومة على المسافة الإجمالية التي تقطعها الطور المتحرك. 
إذا تم اختيار المذيب بشكل صحيح ، فسيكون لكل مذاب R f مختلف بشكل واضح. يسمح لنا ذلك بتحديد المركبات المكونة في المخاليط من خلال مطابقة قيم R f مع المركبات المعروفة. 
في هذا التمرين المعملي، ستستخدم TLC لتحديد Rf للعديد من المركبات المعروفة، وبعد ذلك ستستخدم قيم Rf لتحديد هوية مركب غير معروف.

كروماتوغرافيا طبقة رقيقة

الكروماتوغرافيا
الكروماتوغرافيا هي تقنية تستخدم في الكيمياء العضوية لفصل المركبات في خليط بناء على اختلافاتها في الذوبان بين مرحلتين مختلفتين. يشبه المفهوم الاستخراج السائل السائل ، باستثناء الكروماتوغرافيا ، تتكون المرحلتان من المرحلة الثابتة والمرحلة المتنقلة. المرحلة الثابتة عبارة عن مادة صلبة - عادة ما تكون حبة هيدروجيل دقيقة - بينما المرحلة المتنقلة عبارة عن مذيب ناقل. 
في الكروماتوغرافيا التقليدية ، يتم تعبئة المرحلة الثابتة في عمود رأسي ، ويتم إدخال خليط المحلول في الجزء العلوي من العمود. عندما يتدفق الخليط عبر المرحلة الثابتة ، تنقسم المركبات بين المرحلة الثابتة والمرحلة المتحركة بناء على هياكلها وأقطابها ، وتشكل نطاقات منفصلة. تتحرك المواد المذابة التي تتفاعل بشكل ضعيف مع المرحلة الثابتة بسرعة أكبر عبر العمود وتخرج - أو تتخلص - أولا. تتحرك المواد المذابة التي تتفاعل بقوة أكبر مع المرحلة الثابتة ببطء عبر العمود وتتخلص لاحقا. يمكن جمع العصابات بشكل فردي من أجل عزل وتنقية المركبات في خليط. 
هناك عدة أنواع من الكروماتوغرافيا ، كل منها يسخر خاصية كيميائية مختلفة لتحقيق الفصل. على سبيل المثال ، في كروماتوغرافيا التبادل الأيوني ، يمكن أن تكون حبات الطور الثابتة موجبة أو سالبة الشحنة ، مما يجذب جزيئات ذات شحنة معاكسة فقط. في كروماتوغرافيا استبعاد الحجم ، تتكون المرحلة الثابتة المسامية من حبات مثل بوليمرات الاغاروز أو ديكستران. يمكن للجزيئات الأصغر أن تدخل المسام بسهولة أكبر ، بينما تتدفق الجزيئات الأكبر عبر المسام وتتخلص بشكل أسرع. 
<h4>كروماتوغرافيا الطبقة الرقيقة</h4>
كروماتوغرافيا الطبقة الرقيقة (TLC) هي نوع من تقنيات الكروماتوغرافيا التي تفصل المركبات بناء على قطبتها. مثل الكروماتوغرافيا التقليدية ، هناك ثلاثة مكونات لنظام TLC: المرحلة الثابتة ، والمرحلة المتحركة ، والمذاب. ومع ذلك ، على عكس الكروماتوغرافيا التقليدية ، يتم ترتيب المرحلة الثابتة في طبقة رقيقة على لوحة بدلا من تعبئتها في عمود. غالبا ما يستخدم TLC هلام السيليكا القطبي ، وهو شكل من أشكال ثاني أكسيد السيليكون ، كمرحلة ثابتة. تشكل المرحلة الثابتة روابط هيدروجينية بسبب مجموعات OH على سطحها. 
أولا ، يتم رسم خط البداية في الجزء السفلي من لوحة TLC باستخدام قلم رصاص. يتم رصد المركبات أو الخليط الذي يتم تحليله على خط البداية باستخدام شعرية رفيعة. بعد ذلك ، يتم غمر الجزء السفلي من اللوحة في المرحلة المتحركة ، والتي عادة ما تكون مذيبا عضويا أقل قطبية من المرحلة الثابتة. ينتقل المذيب إلى أعلى اللوحة عن طريق العمل الشعري ، ويمرر البقع المذابة ويحمل بعضا من كل مكون معه. 
عندما ينتقل المذيب إلى أعلى اللوحة ، تنقسم المكونات إما إلى المرحلة المتحركة أو المرحلة الثابتة. إذا كان المكون قطبيا ، فإنه يتفاعل أكثر مع المرحلة القطبية الثابتة. يسافر ببطء ويتحرك فقط لمسافة قصيرة على لوحة TLC. إذا كان مكون العينة أقل قطبية - وأكثر قابلية للذوبان في الطور المتحرك مما هو عليه في المرحلة الثابتة - فإنه يتفاعل أكثر مع الطور المتحرك ويسافر أبعد على لوحة TLC. يعد مدى قطبية المكون والمرحلة المتنقلة ضروريين لفهم الفصل والتنبؤ به. 
تحتوي ألواح TLC عادة على صبغة فلورية تفاعلية للأشعة فوق البنفسجية تتوهج تحت مصدر للأشعة فوق البنفسجية يبلغ 254 نانومتر. لذلك ، يمكن تحليل لوحات TLC من خلال مراقبتها تحت ضوء الأشعة فوق البنفسجية. ستظهر المركبات الموجودة داخل لوحة TLC ، مثل المواد المذابة ذات الأهمية ، على شكل بقع داكنة مقارنة بالخلفية الخضراء. من خلال تدوير البقع بقلم رصاص من الجرافيت ، يمكن قياس المسافة التي قطعتها المركبات بالنسبة إلى مقدمة المذيب. بقعة المركب العضوي ، إن لم تكن الفلورسنت نفسها ، تخفي مضان اللوحة وتظهر كبقعة داكنة. بعض المركبات العضوية نشطة للأشعة فوق البنفسجية وتنبعث منها الضوء عند تعرضها للأشعة فوق البنفسجية. عادة ما تكون هذه مركبات مترافقة ، أي تلك التي لها روابط مزدوجة ومفردة بالتناوب ، ويمكن التعرف عليها من خلال الطول الموجي المنبعث. 
<h4>عامل التخلف</h4>
من خلال تحليل عامل التخلف (R f ) لمكون بمذيب معين ، يمكن تحديد مذاب غير معروف باستخدام TLC. عامل التأخر هو نسبة المسافة التي يقطعها المكون إلى المسافة التي تقطعها الطور المتحرك. 
<img src="/CDNSource/lm/labs/62/62_Concepts_1.jpg">
يتم قياس المسافة التي يقطعها المذاب من خط البداية إلى نقطة مركز البقعة ، ويتم قياس المسافة التي تقطعها المرحلة المتحركة من نفس خط البداية إلى مقدمة المذيب. يعتمد عامل التخلف للمركب على المرحلة المتنقلة المستخدمة. عامل التخلف كبير بالنسبة للمركبات غير القطبية للغاية مع طور متحرك غير قطبي. تظهر قيم عامل التخلف المنخفضة للمكونات القطبية ذات الطور المتحرك غير القطبي. 
في حالة الطور المتنقل غير القطبي للغاية ، قد لا تتحرك بعض المكونات القطبية على الإطلاق. ينتج عن هذا عامل تأخر منخفض للغاية وفصل غير كاف. تتسبب المرحلة المتحركة شديدة القطبية في تحرك المركب مع المذيب وتنتج عامل تأخر مرتفع للغاية. ينتج عن هذا القليل جدا من الفصل بين المكونات. 
لكي يكون الفصل فعالا ، يجب أن تكون عوامل التخلف للمكونات حوالي 0.3 - 0.7. للعثور على مرحلة الهاتف المحمول الفعالة ، يتم استخدام التجربة والخطأ. في كثير من الأحيان ، يثبت مزيج من مذيبين أنه الأكثر فعالية. 
<h4>المراجع</h4>
<ol>
	<li style="font-size:17px">هاريس ، العاصمة (2015). التحليل الكيميائي الكمي. نيويورك ، نيويورك: دبليو إتش فريمان وشركاه. </li>
</ol>

اللوني
الكروماتوغرافيا هي تقنية تستخدم في الكيمياء العضوية لفصل المركبات في خليط بناء على اختلافاتها في الذوبان بين ...

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Thin-Layer Chromatography

tlc separation by polarity

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column.

The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a thin capillary. Then the bottom of the plate is placed in the mobile phase, which is usually an organic solvent that is less polar than the stationary phase.

As the solvent moves up the plate by capillary action, it carries the sample components with it. Each compound then partitions between the mobile phase and the stationary phase. A polar component interacts more with the polar stationary phase, so it moves more slowly and stays near the start. A less polar component is more soluble in the mobile phase, so it travels farther up the plate.

TLC plates often include a UV-reactive fluorescent dye that glows under 254 nm UV light. The compounds appear as dark spots against the green background, and the spots can be circled with a graphite pencil so their movement can be measured. Some organic compounds are UV-active and emit light when exposed to UV light. These are often conjugated compounds, which have alternating double and single bonds, and they can be identified by the wavelength they emit.

The retardation factor, or Rf, helps identify an unknown solute from TLC results. It is the ratio of the distance traveled by the compound to the distance traveled by the solvent front. The compound distance is measured from the start line to the center of the spot, and the solvent distance is measured from the same start line to the solvent front. Rf depends on the mobile phase used.

Rf values are high for highly non-polar compounds with a non-polar mobile phase, and low for polar compounds with a non-polar mobile phase. If the mobile phase is too non-polar, some polar compounds may not move at all. If it is too polar, the compound may move with the solvent and the separation may be poor. For an effective separation, the Rf values of the components should be about 0.3 to 0.7 apart. Choosing the best mobile phase usually takes trial and error, and a mixture of two solvents often works well.

TLC Separation by Polarity

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steam distillation

Steam Distillation and Low-Boiling Separation

Steam distillation is a separation method for temperature-sensitive organic compounds. It works best when the organic compound is immiscible in water, meaning the two liquids do not mix. The method helps remove the organic molecule from a non-volatile contaminant without using the higher temperatures needed for many other separations. In steam distillation, the immiscible mixture is heated until both water and the volatile organic compound distill together. The vapor rises to a condenser,...

thin-layer chromatography

Thin-layer chromatography (TLC) separates compounds in a mixture by polarity. It uses the same basic parts as other chromatography methods: a stationary phase, a mobile phase, and the solute. In TLC, the stationary phase is a thin coating on a plate instead of material packed into a column. The stationary phase in most TLC systems is polar silica gel, a form of silicon dioxide. Its surface has OH groups that can form hydrogen bonds. The sample is first spotted on a pencil start line with a...

column chromatography

Separating Mixtures by Polarity

Column chromatography separates compounds in an organic chemistry lab by using differences in physical properties. It is especially useful for microscale separations when less than 10 grams of material is available for analysis. The method can separate a mixture based on solubility, polarity, hydrophobicity, size, and charge. Any chromatography system has three main parts: the stationary phase, the mobile phase, and the sample mixture. In column chromatography, the stationary phase is usually...

hydrolysis of an ester

Soap Formation from Triglycerides

Triglycerides are lipids made of a glycerol backbone linked to three fatty acids. They are common in vegetable oils and animal fats. Plant lipids are the basis of vegetable oils, while animal lipids are used as lard or general sources of fats. A fatty acid has a long carbon chain, usually between 12 and 20 carbons, with a weak organic acid at one end. Some fatty acids contain double bonds. These are unsaturated fats. The double bond usually gives the chain a bent, cis shape. As a result,...

synthesis of luminol

Luminol Chemiluminescence in Forensics

Luminol chemiluminescence shows how a chemical reaction can produce light and help with blood detection. Chemical reactions may be endothermic or exothermic, depending on whether they absorb or release heat as reactants become products. The enthalpy of a reaction is the energy difference between reactants and products, or ΔH. A positive ΔH means the reaction is endothermic. A negative ΔH means it is exothermic. Many reactions do not happen in a single step. They can pass through intermediates...

esterification

Ester Formation and Everyday Fragrances

Esters are organic compounds with a carbonyl group attached to an oxygen that connects to another alkyl or aryl group. Their general formula is RCOOR'. They are often made by reacting a carboxylic acid with an alcohol in an esterification reaction. In this process, the hydroxyl group of the acid is replaced by an alkyl or aryl group. Many simple esters have familiar fruit and flower smells because their structures are small and less complex. Formic acid and methanol can combine to form methyl...

identification of unknown aldehydes and ketones

identification-of-unknown-aldehydes-and-ketones

Carbonyl Tests for Aldehydes and Ketones

Aldehydes and ketones are carbonyl compounds, which means they contain a C=O functional group. A ketone has two alkyl or aryl groups attached to the carbonyl carbon. Acetone is the simplest ketone, with two methyl groups. An aldehyde is similar, but it has at least one hydrogen attached to the carbonyl carbon. Formaldehyde is the simplest aldehyde, and acetaldehyde has one hydrogen and one methyl group. The carbonyl carbon in both aldehydes and ketones is electrophilic because the oxygen atom...

uv-vis spectroscopy of dyes

How Dyes Absorb Visible Light

Dyes absorb visible light because their molecular structure lets them interact with specific wavelengths. When light hits a substance, some light is absorbed and the rest is reflected or transmitted. The color we see depends on the wavelength that is reflected. A blue substance reflects blue light, around 430 to 480 nm, and absorbs the complementary orange region, around 590 to 630 nm. Compounds that do not absorb in the visible range appear colorless to the human eye. Light also has energy and...

identifying alcohols

Alcohol Tests for Primary, Secondary, and Tertiary

Alcohol tests can help sort alcohols by structure and reactivity. Alcohols are organic molecules with a hydroxyl group attached to an alkyl or aryl group, written as ROH. If the carbon with the hydroxyl group has one R group, the alcohol is primary. If it has two R groups, it is secondary. If it has three R groups, it is tertiary. Alcohols can also be aromatic when they contain a benzene ring. Phenol is the simplest aromatic alcohol. To identify an alcohol, chemists can use its chemical...

chemistry

Comprehensive, curriculum-focused videos for both instructors and students in introductory chemistry lab courses.

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Learn how thin-layer chromatography separates compounds by polarity, how spots are seen under UV light, and how to use Rf values to compare samples.

TLC Separation by Polarity | JoVE K12

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TLC Separation by Polarity

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