Flash chromatography - Principle, Instrumentation and Application - Pharmacognosy and Phytochemistry II B. Pharma 5th semester PDF Notes

Flash chromatography

Objectives

At the end of the lecture, student will be able to

Discuss the principle of flash chromatography

Explain the instrumentation of flash chromatography

Flash chromatography

Most of the sophisticated analytical techniques like NMR, MS and FT-IR need sample in the extreme pure form which is usually achieved by Preparative column chromatography but it is time consuming

From past few decades, technique called as Flash chromatography is developed which is a modification of Preparative column chromatography

Flash chromatography is basically an air pressure driven hybrid of medium pressure and shorter column chromatography which has been optimized for particularly rapid separation

Flash chromatography is a technique used to separate mixtures of molecules into their individual constituents, frequently used in the drug discovery process 

Column chromatography is separated into two categories, depending on how the solvent flows down the column

If the solvent is allowed to flow down the column by gravity, or percolation, it is called gravity column chromatography.

If the solvent is forced down the column by positive air pressure, it is called flash chromatography

Flash chromatography is basically an air pressure driven hybrid of medium pressure and shorter column chromatography which has been optimized for particularly rapid separation

Flash chromatography is a technique used to separate mixtures of molecules into their individual constituents, frequently used in the drug discovery process 

Flash chromatography differs from the conventional technique in two ways: first, slightly smaller silica gel particles (250-400 mesh) are used

Second, due to restricted flow of solvent caused by the small gel particles, pressurized gas (ca. 10-15 psi) is used to drive the solvent through the column of stationary phase.

The net result is a rapid (“over in a flash”) and high resolution chromatography

The absorbent has a much smaller particle size

Different kinds of chromatographic columns- If the column contains a porous plate to support packing, no additional support such as cotton, glasswool and sand is necessary 

Components:

1. Sorbent Selection: 

The basic prerequisite for successful separations is the choice of the proper adsorbent

Silica gel and alumina

These adsorbents are sold in different mesh sizes, “silica gel 60” or “silica gel 230-400” etc

This number refers to the mesh of the sieve used to size the silica, specifically, the number of holes in the mesh or sieve through which the crude silica particle mixture is passed

Adsorbent particle size affects how the solvent flows through the column

 Smaller particles (higher mesh values) are used for flash chromatography; larger particles (lower mesh values) are used for gravity chromatography

For example, 70-230 silica gels are used for gravity columns and 230-400 mesh for flash columns

Adsorbents used in flash chromatography:

Silica: Slightly acidic medium. Best for ordinary compounds, good separation is achieved

Florisil: Mild, neutral medium. 200 mesh can be effective for easy separations. Less than 200 mesh best for purification by filtration

Alumina: Basic or neutral medium, effective for easy separations, and purification of amines

2. Solvent Systems: 

Flash column chromatography is usually carried out with a mixture of two solvents, with a polar and a nonpolar component. Occasionally, just one solvent can be used

One component solvent system:

Hydrocarbons: pentane, petroleum ether, hexane

Ether , dichloromethane (very similar polarity)

Ethyl acetate

Two-component solvent systems

Ether/Petroleum Ether, Ether/Hexane, and Ether/Pentane

Ethyl Acetate/Hexane: The standard, good for ordinary compounds and best for difficult separations

Methanol/Dichloromethane: For polar compounds

For basic compounds- Add a small amount of triethylamine or pyridine to the solvent mixture (about 0.1%).

For acidic compounds, a small amount of acetic acid is sometimes useful

In this case, be very careful in concentrating the solvent as trace amounts of acids can be very dangerous when they are concentrated with a product

Acetic acid can often be safely removed away by adding portions of toluene

Column Selection

Select a column that is 10, 20, 40 mm ID based upon preparative requirements

SINGLE Step Flash Columns - purification of organic compounds

 Thomson flash columns - wide variety of sizes ranging from 4g to 300g silica-based for easy scalability of synthetic reactions

C18 flash columns which enable the end-user to utilize these flash columns for a broad range of reactions 

TYPICAL VOLUME OF ELUANT REQUIRED FOR PACKING AND ELUTION

Column Diameter (mm)

Volume of eluent (ml)

Sample Load (mg)

Rf > 0.2

Rf >0.1

10

100

100

40

20

200

400

160

30

400

900

360

40

600

1600

600

50

1000

2500

1000

Packing of Column: 

Obtain a glass column and make sure that it has either a glass frit or a plug of cotton wool directly above the stopcock to prevent the silica gel from escaping from the column through the stopcock

Next, put a 1/2 inch layer of clean sand above the plug of glass wool to obtain a flat surface, with the same diameter as that of the body of the column.

Add dry silica gel adsorbent, 230-400 mesh   One way to fill the column is to invert it into the jar of silica gel and scoop it out & then tamps it down before scooping more out 

Another way to fill the column is to pour the gel into the column using a 10 mL beaker  and tamp it down on the bench top to pack the silica gel

Tap gently and evenly the sides of the column with a piece of rubber tubing to settle the silica gel

Pour a good amount of elution solvent onto the silica gel. Use pressurized gas to force the solvent through the silica

Continue to flush solvent through the silica gel until the entire silica plug becomes homogeneous in appearance

Recycle the solvent coming through the column onto the top of the column several times before all the silica gel is solvated

Loading of sample onto the silica gel column: Two different methods are used to load the column: the wet method and the dry method

Wet Loading Method: In the wet method, the sample to be purified is dissolved in a small amount of solvent, such as hexanes, acetone, or other solvent and loaded onto the column.

Amount of solvent used is critical, too much solvent, the loading solvent will interfere with the elution and hence the purification or separation of the mixture

Dry Loading Method: 

First dissolve the sample to be analyzed in the minimum amount of solvent and add about 100 mg of silica gel

Swirl the mixture until the solvent evaporates and only a dry powder remains

Place the dry powder on a folded piece of weighing paper and transfer it to the top of the prepared column

Add fresh eluting solvent to the top and now it is ready to begin the elution process

Elution of the column 

 Add a good part of elution solvent to the column

Apply pressure to force the solvent through the column by pressing on the top of the Pasteur pipette with a pipette bulb

Only force the solvent to the very top of the silica: do not let the silica go dry and add fresh solvent as necessary

Pressure should be the minimum necessary to keep a steady stream coming out of the column

Analysis of fractions: If the fractions are colored, simply combine like-colored fractions, although TLC before combination is usually advisable.

If the fractions are not colored, they are analyzed by TLC (usually)

Once the composition of each fraction is known, the fractions containing the desired compound(s) are combined.

Cleaning the Column: 

Flush all the remaining solvent out of the column using pressurized gas

Flowing air through the column for 2 hours will give dry, free flowing silica gel.

In most cases, washing the column with water and acetone is sufficient. If necessary, a small amount of liquid soap can be used

When all liquid solvent has been removed from the reservoir, remove the last remnants of solvent by applying a vacuum (from aspirator) to the bottom of the column

Try to avoid scratching the columns with abrasive brushes or soaps

When all liquid solvent has been removed from the reservoir, remove the last remnants of solvent by applying a vacuum (from aspirator) to the bottom of the column

Try to avoid scratching the columns with abrasive brushes or soaps

Advantages:

Fast and economic methods for the synthesis laboratory

Ideal for the separation of compounds up to gram quantities

No expensive equipment required

Automated changes between normal phase and reversed phase chromatography

Applications:

It is used for Purification of Peptides

It is used for Separation of Closely Related Organic Compounds

Flash systems are powerful tools for purification of trace compounds from organic mixtures.

It is used as a tool to monitor the reaction progress and to isolate and identify a mixture's compounds.

It is used for High Speed Flash Fractionation of Natural Products – Tocopherols

To purify, identify and collect the isomers of an aqueous modified antibiotic precursor

Continuous Gradient Purification of Closely Related Drug Intermediates Using Flash Chromatography

It has received increased attention as a lead investigation and optimization tool in drug discovery

INSTRUMENTATION OF FLASH CHROMATOGRAPHY

1. Pump Systems

2. Sample Injection Systems

3. Glass Columns, Filling Sets

4. Pre columns, Column Valves

5. Fraction Collector

6. Detectors and Chart Recorders

7. Computerized LCD Display

Pump system - Pump Controller C610, Control Unit C620, Pump Manager C615.

The pump modules can be controlled by three different units. A pressure range up to either 10 bar or 50 bar gives optimum separation results for a broad range of applications

Pump Controller - Delivered with a overpressure sensor for maximum safety and the flow rate can be easily adjusted by turning a knob and is indicated by a large illuminated LCD-display

The Pump Controller C-610 is designed for isocratic separations

The unit has Input/Outputs for 2 solvent valves and level sensors and includes a pressure sensor and mixing chamber

 The Pump Manager C-615 is designed for both isocratic and gradient separations

SAMPLE INJECTION SYSTEMS

Injection systems are designed to facilitate column loading with liquids and low solubility oils and solids

COLUMNS:

Glass Columns: Depending on the nature and the quantity of the sample offers a series of column types which vary in form, size and performance

A wide range of columns offer maximum flexibility for every situation

Plastic+Glass-coated Glass Columns are available for larger amounts of samples and higher pressure applications on a high safety level

 

PRECOLUMNS:

The large Pre column, fits to Glass Columns of ID 70 and 100 mm inner diameter. The small Pre column, fits to Glass Columns of inner diameter of ID 15, 26, 36 and 49 mm

Pre column are minimizing dead volumes and enhance the life time of the main column by trapping contaminants

FILLING SETS:

Dry Filling Set : Silica gel in the size range of 25 –200micro meters can be packed with this method

The Dry Filling Set is employed for filling glass columns with silica gel using compressed gas.

Slurry Filling Set is used for wet filling and conditioning of glass columns with silica gel particles smaller than25 micro meter

Modern cartridges:

DETECTORS AND RECORDERS/SOFTWARE:

UV Photometer C-635 and UV Monitor C-630

Both detectors are delivered in combination with a preparative flow cell

For most applications one of the robust UV/Vis detectors would be sufficient for the systems detection needs

RI detector

FRACTION COLLECTOR:

Fraction of known quantities are collected in a fraction collector

Computerized LCD Display

Summary

Solvent is forced down the column by positive air pressure - flash chromatography

Flash chromatography - An air pressure driven hybrid of medium pressure and shorter column chromatography

Adsorbents – Silica, florisil, alumina

Solvent – single component and two component

Column - SINGLE Step Flash Columns and Thomson flash column

Solvent is forced down the column by positive air pressure - flash chromatography

Packing of column

Loading of column – wet packing and dry packing

Elution of the column

Cleaning of column

Advantages and applications

Pump Systems

 Sample Injection Systems

 Glass Columns, Filling Sets

 Pre columns, Column Valves

Fraction Collector

 Detectors and Chart Recorders

Computerized LCD Display

 For Detailed PDF Notes Click on Download Button 

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