Questions tagged [chromatography]

collective term for a set of laboratory techniques for the separation of mixtures

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2answers
8k views

Gas chromatography/mass spectrometry: Can I use peak ratios and molecular mass to derive quantity?

Can derive the quantity of a substance from a GC/MS report if I know: the ratio to another substance in the data, the quantity of that second substance, and the molecular mass of both substances. ...
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2answers
573 views

How to separate two diastereomeric amines?

I am developing a method to synthesize amines using organometallic catalysts and my reaction yield these two diastereomeric amines ($\ce{R2N}$ is, in this case, a morpholine group). With both being ...
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1answer
787 views

Silica turning transparent during column chromatography with chloroform

Initially, my column had been loaded with n-hexane and ethyl acetate, however I ended up having to use a more polar solvent to elute my product. It was my first time running chloroform through a ...
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1answer
9k views

What is the Kovat's Index used for?

As mentioned in the title, why do we use the Kovat's Index in gas phase chromatography? It seems a little arbitrary to compare everything to the order that n-alkanes (such as n-pentane, n-hexane, and ...
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363 views

How to translate the mobile phase from thin layer chromatography to column?

I recently was told that once you find a good solvent mixture for separation on thin layer chromatography (TLC), you should only use 10% of the more polar solvent (for silica) when you run your column....
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If the polarity of the solvent increases, will the Rf value increase or decrease?

True or False: A sample of aspirin has an $R_f$ value of 0.20 when measured on a silica gel TLC plate with a certain solvent mixture. If the polarity of the solvent is increased, the $R_f$ value of ...
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1answer
591 views

What is the purpose of trimming the bottom corners of TLC plates?

During my chemistry lab course, I was told by my instructor to make 2 small cuts on the bottom corners of the TLC (Thin-Layer Chromatography) plate. Does this make the solvent rise uniformly on the ...
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2answers
3k views

Removal of tetrabutylammonium from an organic reaction

I need to synthesize some organic compound for my Ph.D. research. Everything went OK during the synthetic route but got messed up at the very end. In my final step (once the final compound has been ...
7
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1answer
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What is the behavior of a Zwitterion during ion exchange chromatography?

We're interested in purifying a protein that has an azide moiety using ion exchange chromatography. Due to its unique structure, the moiety is a Zwitterion with a novel chemical behavior. My question ...
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484 views

In High Performance Liquid Chromatography, why are ratios of solvents used?

How do they allow for better separation? For example, why isn't 100% hexane used?
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1answer
604 views

How to choose a proper chromatography way to separate substrates?

I'm a freshman and I recently learned how to do TLC and column chromatography. I also know a little about other ways to separate things by chromatography, such as affinity chromatography, gel ...
6
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1answer
759 views

Different elution profiles of carvacrol and thymol

I have identified carvacrol and thymol via thin-layer-chromatography and via gas chromatography On a normal silica plate the thymol has a higher Rf-value then carvacrol. The explanation for this would ...
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54 views

Can you convert smells to variables? [closed]

Is there a way to map smells to numerical values of different variables? I.e. With RGB colours, each colour is made up of numerical values from 0-255. For example, red is (255,0,0), and yellow is (...
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20k views

Normal phase vs reverse phase HPLC

What is the difference between normal phase and reverse phase high-performance liquid chromatography?
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1answer
4k views

What is 'Retention period' in chromatography and what is its importance?

I heard people quite often speak about retention time/retention period in Chromatography and Mass Spectrometry. I see somewhere that it is the time duration between the injection time and the target ...
5
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1answer
5k views

What does 'preconcentration techniques' mean?

In the scope of the analysis of organic compounds in column-switching liquid chromatography, what does the following statement mean? sample preconcentration techniques with high enrichment factor(...
5
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1answer
145 views

How to quantify amount of pigments in plant leaves?

Using thin-layer chromatography, it is possible to see what pigments plant leaves contain. For example, I can test whether there are chlorophyll a or b, carotenoids, or anthocyanins. What kind of ...
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2answers
190 views

Is is possible to do mass spectrometry without coupled liquid chromatography?

It's possible to identify a solution with several compounds* without the coupled liquid chromatography (LC) before to isolate the compounds? On the university where I study, the LC is broken, so all ...
5
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1answer
89 views

What mechanisms might make a substance travel different distances on the same TLC plate?

When doing TLCs to monitor reactions, it is considered good common practice to not only spot the reactants and the reaction mixture (likely after some mini-workup), but also to include a ‘cross-spot’: ...
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2answers
14k views

How does the Rf value change when changing the eluent composition?

A non-polar molecule X is analyzed by TLC chromatography on silica to determine its $R_f$ value. The first eluent was hexane:EtOAc = $9:1$ and the $R_f$ value was $0.5$. In the second run, the ...
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1answer
572 views

A good book for learning paper and thin-layer chromatography

What book would you recommend for learning paper and thin-layer chromatography? I don't have access to fancy equipment (just to these two basic techniques that are relatively cheap), so it would be ...
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2answers
206 views

Gas chromatography flame ionization detector (FID) - why hydrogen gas?

Why is $\ce{H2}$ used in an FID, apart from the fact its combustion does not contaminate the flame? In other words, is the temperature of the flame important or critical? Is the stability of the ...
4
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1answer
363 views

What inference can be made from the fact that there is no reversal in how three components eluate?

I gave an answer this question about TLC. I now realize that I am blundering my way through this question. The problem statement doesn't specify if the compounds to be eluted are polar or non-polar. ...
4
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1answer
982 views

Reasoning retention factor of benzoic acid

In my organic chemistry lab, we ran a TLC plate on silica to determine the identity of an unknown compound. The two possibilities were benzoic acid and acetanilide. I know that the general rule is, ...
4
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1answer
51 views

What happens to the “leftover” Ions in the Mass Analyzer

I am very intrigued by what happens to the ions which do not make it to the detector in mass spectrometers, do they just pile up? I have never seen anyone clean our quadrupole, is this done? or do ...
4
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1answer
12k views

HPLC: peak area vs concentration

Many text books say that there is a proportionality between the peak area of the chromatogram and the concentration. However, in my opinion, there should be a proportionality between the concentration ...
4
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1answer
80 views

Could chromatography induce a conformational change?

We'd been studying chromatography in our instrumental analysis course and I was wondering if chromatography could induce a conformational change. For example, you have a straight chain with a ...
4
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1answer
235 views

Reverse Phase Chromatography

Please help me figuring out what is going on with my RP-HPLC protocol. I am purifying a small peptide out of E.coli Culture, heat stable enterotoxin, its amino acid sequence is: N-T-F-Y-C-C-E-L-C-...
4
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1answer
1k views

What does the pressure in the HPLC(High performance liquid chromatography) do?

Well I have to do research about the HPLC. It says, that because of the pressure the stationary phase can be made out of smaller particles and the pillars can be shorter. Also the seperation of the ...
4
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2answers
610 views

What is the relationship between the molecular weight of a polymer and that of polystyrene?

So, I read in a similar chemistry thread that you can get Mn and Mw of the polymer of the interest by comparing the result of the sample obtained from GPC/SEC with the standard known molecule weight ...
4
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1answer
958 views

How do I find Rf values & length size of paper in chromatography?

A student is designing an experiment using paper chromatography. From previous results, she knows that the Rf values for two dyes that she wishes to investigate are 0.72 and 0.80. It is proposed ...
4
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1answer
421 views

HPLC method for salicylic acid

Salicylic acid is a relatively polar compound hence I thought that normal phase would be preferred as the stationary phase of normal phase is polar. However, it is said that reverse phase would be the ...
4
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1answer
61 views

Protein purification with Cobalt

Cobalt exhibits a more specific interaction with histidine tags, resulting in less nonspecific interaction than nickel. For this reason, cobalt is the preferred divalent cation for purifying His-...
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0answers
107 views

How to make a sephadex slurry

I want to purify crude alpha-amylase with column chromatography. I am using a spehadex 75, But for some reason I can't find any information on how to make the slurry. I can quickly find tons of ...
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0answers
405 views

Difference between neutral and basic activated alumina

Sigma Aldrich sells two different types of activated alumina for column chromatography: neutral and basic. Apparently basic alumina is more active and better at adsorbing anions. Why is this the ...
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1answer
1k views

Why note the solvent level on chromatography paper when it is removed from the chamber?

Why is it important to mark solvent level on chromatography paper when you remove it from the chamber?
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2answers
398 views

What dictates cathode vs anode nomenclature use?

what was the justification for the use the cathode and anode terminology in IEF? There is no redox taking place in IEF, just proteins interacting with an electric field
3
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1answer
106 views

Does every solute spend at least $t_M$ in the mobile phase?

In 'Fundamentals of Analytical Chemistry' 9th edition by Skoog, West, Holler and Crouch, there is the following passage (page 865). The dead time (void time), $t_M$, is the time it takes for an ...
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1answer
94 views

Choosing LC-MS modifier - why only acids?

During method selection our chromatographists have options like NH4, TFA, Formic Acid. What's the point of choosing only between acid? Why not use the strongest that guarantees ionization? The sample ...
3
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3answers
62 views

Applying vacuum at the end of a flash chromatography column

I know that flash chromatography uses a positive or pushing pressure to achieve the goal which is separation. Would the same goal be achieved if you had a vacuum on the other end instead of a ...
3
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1answer
402 views

Chromatography: Why does the insoluble compound get pulled along?

In paper chromatography, we see that a paper with an ink spot on it get dissolved in the liquid and travels up with water due to capillary action. Then the compounds with more affinity to the paper ...
3
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1answer
1k views

What's the difference between Carvacrol and Thymol?

What is the difference when it comes to separating Carvacrol and Thymol on HPPLC with reverse phase - C18 column? Carvacrol elutes first, is that because it is ortho?
3
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1answer
435 views

Diffusion/Streaky bands on TLC plate

These days I am working on a research project where I am supposed to run TLC for samples which are wax extracts from coconut butten nuts using chloroform as the solvent. All the samples have been ...
3
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1answer
241 views

Advantages of Travelling wave IMS and Drift tube IMS

I am learning Ion Mobility spectrometry. I am sorry for a very fundamental question. After reading different articles, I really did not understand what is the advantage of Travelling wave IMS over ...
3
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2answers
951 views

Chromatography Separation Quantifying

This is a textbook question: Suppose you are using chromatography to separate a mixture of two substances. Can you propose a means of quantifying how good or how poor the separation is? So I got ...
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2answers
1k views

Plotting a calibration curve for Gas-Liquid Chromatography

Textbook solution: Why aren't the points like this: $(20,5) (40,10) (60,15) (a,7)$ then to plot the curve we use $y=mx+c$ and then find the value of '$a$'
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1answer
38 views

How appropriate/adaptable is HPLC for in situ sample analysis on Titan?

I have a limited background in chemistry (nothing beyond general Chem 2), so I am asking for help with a group project. We have two chemists on our team, but communication is limited at the time and I ...
3
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1answer
59 views

Is the pressure in an HPLC system the same throughout?

I'm not sure if I'm articulating myself well enough, but here's my problem: I have a current HPLC system, where we typically have backpressure readouts of around 15 MPa. We are looking to get a new ...
3
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2answers
265 views

Is it possible to calculate the concentration percentage using chromatography paper?

I'm trying to calculate the concentrations of specific compounds using paper chromatography. Would I be able to calculate a rough estimate by comparing the size of each concentrated spot? If not is ...
3
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1answer
14k views

How to determine the order of elution in silica gel column chromatography?

Benzofuranoids ustusoranes A (1), B (2), and E (3) were isolated from Aspergillus ustus 094102. If these compounds were separated by column chromatography on silica gel, what would be the elution ...