Method of Identification
Sunday 28 July 2013
Now, to identify and quantify these two compounds!
Method of Identification:
High Performance Liquid Chromatography (HPLC)
A chromatographic technique used to separate components in a mixture and to identify & quantify the components. Generally, this method involves liquid sample being passed over a solid adsorbent material (stationary phase) packed in a column using a liquid mobile phase.
A chromatographic technique used to separate components in a mixture and to identify & quantify the components. Generally, this method involves liquid sample being passed over a solid adsorbent material (stationary phase) packed in a column using a liquid mobile phase.
The analyte will interact differently with the stationary phase thus resulting in different retention times or elution rate. These interactions are usually based on the polarity or boiling points of analytes with the stationary phase. As chromatography can be described as a mass transfer process involving adsorption, the HPLC relies on pumps passing a pressurised liquid and sample mixture through the column filled with a sorbent, thus leading to the separation of the sample components. Sorbents are chemicals such as organochlorosilanes that are chemically bonded to silica particles resulting in chemically bonded stationary phase.
The difference between HPLC and normal liquid chromatography is that the operational pressures are significantly higher. As HPLC has the ability to do trace analysis, it’s column is usually 2.1 to 4.6mm wide and 30-250mm long. Also, the particles that are chemically bonded to stationary phase are of an average size of 2-5mm. Due to this property, the HPLC has high resolving power when separating mixtures. Resolving power means the ability to separate components into clear retention time peaks.
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Types of HPLC:
1. Normal phase HPLC-
Stationary Phase: Polar
Separation of analytes: based on ability to engage in polar interactions with polar stationary phase.
Mobile Phase: Non-Polar - works effectively in separating analytes that are soluble in non-polar solvents.
Adsorption strength & Elution Order: Determined by Interaction of different polarity analytes with the stationary phase. Or, the different boiling points between analytes.
Retention time of analytes: Affected by Eluotropic strenght of mobile phase (For example, the more polar the solvents of mobile phase, the more it will interact with the stationary phase, thus hindering the interaction of analytes with stationary phase.)
However, there are disadvantages of normal phase HPLC. It is only applicable to samples which are soluble in non-polar solvent. Also, it is usually not suitable for gradient elution as silica in the chemically bonded stationary phase is slow in establishing a state of equilibrium with the mobile phase. Moreover, because of deactivation of stationary phase by water contaminants, the solvents are expensive to obtain, thus not economically friendly.
However, there are disadvantages of normal phase HPLC. It is only applicable to samples which are soluble in non-polar solvent. Also, it is usually not suitable for gradient elution as silica in the chemically bonded stationary phase is slow in establishing a state of equilibrium with the mobile phase. Moreover, because of deactivation of stationary phase by water contaminants, the solvents are expensive to obtain, thus not economically friendly.
2. Reversed phase HPLC-
Stationary Phase: Non-Polar, consist of re-modified silica where the R group is a straight chain alkyl group.
Separation of analytes: based one their ability to engage in non-polar interactions with the non-polar stationary phase.
Mobile Phase: Polar - works effectively in separating analytes that are soluble in polar solvents.
Adsorption strength & Elution Order: Dependent on the polarity of analytes with stationary phase. Or, the different boiling points between the analytes.
Retention time of analytes: Affected by Polarity of mobile phase. (For example, the non-polar solvents will “fight” with the analytes at the stationary phase because of interaction between like particles. This will therefore interrupt the elution order of analytes.)
However, because of the derivatized silica particles in the stationary phase, aqueous bases should never be used as these will destroy the silica particle. Thus aqueous liquids can be used but this will also damage the metal parts of the HPLC equipment. After experiment, one should always flush the HPLC with clean solvent to remove residual acids or buffers and stored in appropriate composition of solvent. Therefore, reversed phase HPLC has a few more steps to ensure the longevity of the machine.
Steps to quantify for Acetone in acetone nail polish remover:
Step 1: Identity phase of HPLC and mobile phase polarity.
As ketone is a polar analyte, the phased used for HPLC will be the reversed phase and mobile phase solvent will be polar.
Step 2: Preparation of Mobile phase and Stock solution.
Mobile phase constituents will be 80% acetic acid and 20% acetonitrile buffered to pH4.2 with 50% sodium hydroxide.
Using Acetone (minimum technical grade) of containing 500mg/L of mobile phase, pipette 10ml out into a 50ml volumetric flask and add mobile phase till 50ml. this will give a concentration of 10mg/100ml of stock solution.
Step 3: Preparation of standard solutions
Flask 1- Pipette 1.0ml of stock solution into 10ml volumetric flask. Top it up with mobile phase.
Flask 2 –Pipette 3.0ml of stock solution into 10ml volumetric flask. Top it up with mobile phase.
Flask 3 – Pipette 5.0ml of stock solution into 10ml volumetric flask. Top it up with mobile phase.
Step 4: Preparation of sample solution
Dilute 1.0ml of solution with mobile phase in 10ml volumetric flask. Then filter with a disposable 0.2µm nylon filter cartridge into a sample vial.
Step 5: Sample Analysis
Set up HPLC according to Standard Operational Instructions in the laboratory. Place the Standard solutions and Sample solution onto the rack of the HPLC. Adjust flow rate to 0.7ml/min. Program using autosampler to inject 5µl of the solutions in separate runs into the HPLC column.
Results:
Once analysis is done, collect data to calculate concentration of acetone in flask 1-3. After that, use similar retention times of acetone in flask 1-3 to identify the correct peak area of sample. Next plot calibration graph of peak area against concentration of acetone in ppm. Then deduce the concentration of acetone in the nail polish remover sample solution.
As many non-acetone nail polish remover has this particular compound, Ethyl Acetate, that companies which manufacture these nail polish removers tout does the same job as acetone nail polish remover, we decided to do an analysis on it too.
Steps to quantify for Ethyl Acetate in non-acetone nail polish remover:
Step 1: Identification of HPLC phase and polarity of mobile phase:
As ethyl acetate is gently polar, it will be soluble in polar mobile phase, hence reversed phase HPLC is used in this case.
Step 2: Preparation of mobile phase and stock solutions
Mobile phase constituents will be 80% acetic acid and 20% acetonitrile buffered to pH4.2 with 50% sodium hydroxide and make it in two pyrex bottles of 2L each.
Use Ethyl acetate ACS reagent with density of 0.90g/ml and solubility of 80g/L. Pipette 1.25ml of the reagent and dilute it with mobile phase in a 1L pyrex bottle. Transfer 10ml into a 10ml volumetric flask for easier handling. This will give a concentration of 10mg/100ml.
Repeat steps 3-5 from above.
Results:
Once analysis is done, collect data to calculate concentration of ethyl acetate in flask 1-3. After that, use similar retention times of acetone in flask 1-3 to identify the correct peak area of sample. Next plot calibration graph of peak area against concentration of ethyl acetate in ppm. Then deduce the concentration of ethyl acetate in the non-acetone nail polish remover sample solution.
Conclusion:
Due to the fact that there is no concrete evidence of high concentration of acetone (ppm) in acetone nail polish removers, so long as one does not use or dip fingernails into acetone nail polish removers for over 30 mins, the only physical discomfort will be the stench of acetone. As for the use of non-acetone nail polish removers which contain ethyl acetate, it has not much of a difference. Not only that, non-acetone nail polish removers do not function as well as acetone nail polish removers because it is not as strong in getting rid of nail polish off nails.
Thus, please choose your nail polish removers according to the strength of your nail polish. If one decides to use gel type nail polish, it will be wiser to choose acetone nail polish remover for faster and cleaner removal of nail polish. You may even pay much lesser than those non-acetone nail polish removers!
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Name: Glennis Ong Yu Xuan
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