# Tag Info

15

Building on my comment, marking the distance that the solvent traveled allows us to calculate the retention factor $R_f$ (or apparently retardation factor, according to IUPAC meddlers who need to rename things with perfectly good names that everyone else uses). The retardation factor is the ratio of the distance traveled by the spot to the distance traveled ...

9

Why do we use the Kováts index? The Kováts index is used to normalize GC data. This Wikipedia page(though not very long), sums up why you would want to do so. Retention times of the same compound on even two different versions of identical instrument with the same temperature and pressure program using the same column from the same manufacturer might not ...

9

I wish we would stop teaching chromatography in terms of "polar" and "nonpolar." The aspirin will interact fairly strongly with the silica due to hydrogen bonding/electrostatic interactions of the carboxylic acid and the ester with the silica. If you increased the "polar" component of the mobile phase, it would travel further due to the mobile phase ...

8

Question 1 Yes, of course! Something that is "relatively" non-polar would travel further with a non-polar mobile phase because it would have less attraction to the stationary phase. While a polar substance would not travel as far with the mobile phase because it would have a greater attraction to the stationary phase. Question 2 Now, that depends. Are you ...

8

Normal phase Normal phase HPLC systems are similar to the flash-column chromatography that you might be familiar with. A silica stationary phase is eluted with a non-polar solvent such as hexane, or a fairly non-polar solvent mixture such as 2-propanol in hexanes. In normal phase chromatography, only organic solvents are used. In the normal phase, polar ...

8

Yes, your question contains the answer. Think about capillarity. Without the little cuts the eluant would be "sucked" and flow from both the bottom and the left ( and right) edges of the thin plate, resulting in a tilted flow that tends to move the elute faster along the vertical edges while clumping the spots to the center; decrease the resolution at the ...

7

Is this even how GC/MS results work? As cbeleites said the method you described is a proper technique but not likely to be appropriate given the information you cited. In GC/MS you should have two sets of information. The first is the GC Total Ion Chromatograph (TIC) which will have time as the x-axis and response (abundance) as the y-axis. For each ...

7

Since aspirin has a carboxylic acid group on it, it would be considered polar. Silica gel, consisting of $\ce{SiO2}$, is also polar. Since polar molecules attract other polar molecules, the aspirin molecules will tend to bind to the silica and not move up the TLC plate in a nonpolar eluent, resulting in a low $R_f$ value. When the polarity of the eluent is ...

6

Here are some similarities: There is always a mobile (e. g. Gas - GC, Liquid - HPLC; GPC) and a stationary phase (liquid; gel - GC, solid - LCs, GPC). Compounds in the sample interact different with both phases and are therefore held back stronger or lesser. This results in the accumulation of compounds that interact similar at some point in the system $-$ ...

6

Dead time ($t_{\bf M}$, also called holdup time) is the time it takes for the mobile phase (eluent) to traverse one length of the column, for a given flow rate. Because moieties in the eluent do not interact chemically with the stationary phase, $t_{\bf M}$ is therefore a lower bound on retention times for the species that do interact (chemically) with the ...

6

The stationary phase in chromatography is the one that doesn’t move according to the eyes of a macroscopic (i.e. human researcher) observer. (That complicated way to put it was to prevent anybody raising any relativism arguments.) Obviously, the paper does not move through the water but the water does through the paper. You should discard the five-ish ...

6

The key to the answer is understanding how FID works. The hydrogen flame has a minimal flame ionisation, what is needed for the low signal baseline. Incoming organic molecules from the HPGC column create in the flame a lot of ions and increase the flame electric conductivity. Using alternatives causing higher ionisation would decrease FID sensitivity that ...

6

Not sure if I can fulfill Ed's "real answer". I like the word you used for tuning solvent polarity- a knob. Modern students may understand this better. There is no theoretical restriction in chromatography to use multiple solvents or a single solvent. For example in gas chromatography, you always use a pure gas. The reason for using a mixture of solvents in ...

5

I would expect it to depend on the pH of the mobile phase. The charge of your stationary phase and whether you want the azide to elute will determine what charge you want on the azide. Given that, you should set the pH of the mobile phase either acidic or basic enough to ensure that the azide is fully charged in the direction that you want. From Wikipedia: ...

5

Why, of course you can, but most of the time one of them would be "too strong" and the other "too weak", so neither would work well for you. That's why you have to mix them to the right proportion (which depends on your system, and is found by trial and error).

5

I know it is a relative old issue but I had the same problem with nucleosides during my PhD. I tried a wide range of reagents but all of them lead to the decomposition of compounds or no reaction occurred. The only one that worked was TBAF 1 M in THF. I could not extract them because they are too polar. FCC did not work at all. Solution (at least it worked ...

5

Slightly askew of what was asked, but still relevant. TLC is not dependent on conjugation. That reasoning suggests to me You are using UV light to visualize the TLC. UV light is used because it is clean, quick and non-destructive. There are destructive visualization techniques, such as vanillin stain. In this case, after elution one dips the tlc slide ...

5

The answer is the dimer formation of the benzoic acid. This is why higher concentrations lead to more extended dimer formation, thus higher $\mathrm{R_f}$ values as the carboxyl group "gets shielded." One thing of note is that $\mathrm{R_f}$ values are not directly dependent on the polarity. They also depend for example on the size of the molecule (Oswald ...

5

Mixture of solvents is used for 2 main reasons: Elution time For given HPLC column and set of analytes, the mobile phase must have the proper degree of general polarity, what is often called "solvent strength". For polar sorbents like silica or alumina, more polar solvents have generally bigger strength with shorter elution times. Note that the solvent ...

5

Who was responsible for this naming system and how can we change it? Michael Faraday was responsible for the terms anode and cathode more than hundred years ago. All the confusion regarding the nomenclature will vanish if you do not associate electrostatic signs with these two terms. One should identify the electrode labels with the redox processes rather ...

4

On a reverse phase C18 column (which itself is hydrophobic, it is an octadecyl hydrocarbon chain bound to the support material) the more hydrophobic the molecule, the more strongly it will bind to the stationary phase [like dissolves (or in this case binds to) like], and the longer it will take to elute with an organic solvent. Below are the structures of ...

4

One approach would be to use various solvents. You would require solvents in which your various stains were soluble in. Alcohols / water would be suitable to dissolve the soda. Whereas an organic solvent would be suitable for washing away the gasoline. Though gasoline is typically colourless. Another approach would be to expose the paper to copious UV ...

4

This sounds like A being an internal standard for the determination of B. Internal standards are used to get rid of (small) multiplicative variation due to certain influences, e.g. in GC, the injected volume in optical spectroscopy, the optical path length or the illuminated volume. Sometimes the calculation part of the idea is called normalization. The ...

4

Preconcentration means to increase the concentration of a sample prior to analysis or detection. For example you can do preconcentration for an non-volatile organic sample with evaporation its solvent. An operation (process) as the result of which microcomponents are transferred from the sample of larger mass into the sample of smaller mass, so that the ...

4

To answer your three questions in series: 1) The time is a measure of how strong the interaction of a compound is with the column used. The stronger the interaction the longer the compound will stay on the column and this is therefore a way to separate two compounds. For example one molecule with a weak interaction spending a minute on the column, the other ...

4

The OH group of phenol is indeed more polarized - and a stronger acid - than the OH group of an aliphatic alcohol - maybe that is what you are asking. There are two contributions to this: (1) inductive: the sp2 hybridized carbon participating in the sigma bond to the OH group is more electron-withdrawing than the sp3-hybridized carbon of an alcohol bonded ...

4

However, in my opinion, there should be a proportionality between the concentration and the peak intensity, not the peak area. There is a proportionality between both peak area vs. concentration and peak height vs. concentration. Peak height is proportional to the instantaneous amount of analyte that is transiting the detector. Peak area is proportional ...

4

Chromatography only works when the affinity for the paper vs. affinity for the water is in a dynamic equilibrium. Suppose there are three compounds in the ink, called $\ce{I}$, $\ce{N}$, and $\ce{K}$. Lets call the spots on the paper fiber that bind ink molecules as $\ce{P}$. Then let's write the dyanmic equilibria this way: \ce{I(aq) + P <=>>...

4

TLC in alkaloid research Both analytical and preparative TLC have been used to separate and identify alkaloids from plant material. Pretreatment of TLC plates and adsorbents If you are using silica plates, it is often advantageous to wave the plate above an open bottle with aqueous ammonia or pretreat the plates with an organic solvent containing some ...

4

When you mentioned column chromatography I had to assume you meant flash chromatography or biotage, which by implication is normal phase. When people mention HPLC they are normally defaulting to reverse phase. Now, if you can get reverse phase TLC plates (C18 are available) , you could make some advances. The issue though is then getting the silica and you ...

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