grade of TFA???

[rick1112]

Hi,
which Grade of TFA is commonly used in case of RP?? has any one of you observed any difference in baseline/peak profile with respect to different grade of TFA ( in terms of baseline noise, drift...etc...)

thanks

[tom jupille]

The absolute best you can obtain! Practice in labs I have been involved with is to buy in 1 mL ampoules and store in the refrigerator.

[LC_labrat]

1ml Ampoules-I've seen bad baseline noise with 50mL bottles that have been partially used. The ampoules are more expensive but so is trobuleshooting and repeating an analysis.

[Bruce Hamilton]

In response to the question, the usual grades used are HPLC grade or spectrophotometric grade.

However, finding the most suitable product can be difficult.

1. You need a supplier who has a reasonable turnover of stock to maximise product shelf life.
2. You need to ensure the quality will be consistent - usually by purchasing from a reputable manufacturer, rather than a local rebrander ( who are becoming more common chemical suppliers - around here, anyway ).
3. You need to be able to compare product specifications - not a trivial task for UV properties.
Purity is easy, I tend to go for grades with >99.5%.
Some UV absorption specifications are for the neat TFA ( usually obvious by wavelengths 260 - 300 nm ) but some brands offer data for 0.1% in water ( usually quoting wavelengths around 250 to 280 nm ). A few manufacturers provide both.

Containers?. Well if your employer likes paying 5 times the price, then use 1 ml ampoules, but most competent analysts should be able to purchase a 100ml bottle as a stock solution, and decant aliquots into smaller, convenient, working solution containers. Just make sure the savings are passed to you, not your boss, as a salary increase.

I currently choose based on % purity, and find that reputable brand spectrophotometric grades are high turnover locally, and work well for me.

Interestingly, here the HPLC grades tend to be higher-priced, even though they should be higher usage. The higher cost is sometimes hidden by selling in 100g ( 70ml ) containers, rather than 100ml.

Please keep having fun,

Bruce Hamilton

[rick1112]

Bruce,

but most competent analysts should be able to purchase a 100ml bottle as a stock solution, and decant aliquots into smaller, convenient, working solution containers

can i know what type of container are used to make the aliquotes???

Tom,

The absolute best you can obtain! Practice in labs I have been involved with is to buy in 1 mL ampoules and store in the refrigerator.

why keep TFA in refrigerator?? we usual keep TFA at R.t


How about sequence grade TFA?? is that a better or lower than HPLC grade or spectrophotometric grade???..some one told me that sequence grade TFA has anti-oxidants, is it true?

[Bruce Hamilton]

I use 4 ml screw cap vials with a thick white PTFE-faced cap ( not the thin PTFE present in caps intended to be injected through). Reagents last for months in fridge.

I use a positive displacement pipette, with argon purge of the vial before filling and, with the stock bottle under gentle argon blanket, dispense into vials and close the caps. I tend to prefer argon when available, because it's heavier than air, but nitrogen is OK.

The reason for using an inert gas blanket is to maintain UV transparency and avoid moisture issues.

I used to store all solutions in the fridge - to slow degradation, however I found it convenient to store the working bottle at ambient.

I find storing rarely-used unstable reagents, such as TFA in the fridge or freezer is a good idea, regardless of the storage conditions manufacturers suggest, provided you ALWAYS allow sufficient time for the whole container to return to ambient temperatures before opening.

That way, you can be assured that expiry dates will be valid. If you don't wait, one opening can kill the contents for some purposes. I find that I can confidently use "expired" stock stored in the fridge or freezer for research and development purposes, however YMMV.

Re Sequence and other grades, the purpose is different, so the TFA may not be tested for, or even batch consistent for the critical property of UV transparency. Hence, most analysts I know use either UV grade or HPLC grade TFA.

[DJ]

In Syder's latest book he has an anecdote about TFA-- (paraphrasing) Bottles of TFA, once opened, go bad, and storage under N2 did not prevent this.

What "happens" to TFA? It is hard for me to imagine an oxidation product.

How stable are solutions of TFA in water? MeCN?

[Bruce Hamilton]

A comment about degradation on standing was also in his 1997 book.
It's possible that some analyses might be compromised, but TFA is usually considered stable.

Years ago, I tested TFA-containing mobile phases out to 6 months with no difference in assay results.

I did find an initial difference in UV transparency and late-eluting gradient peaks which seemed consistent when TFA from different suppliers was used, so I selected the best UV for that analysis and used their product until the local agent stopped importing it because of low usage.

There's a earlier thread here discussing shelf life of TFA-containing phases. Somebody else reported assay stability out to 6 months as well. If stability of solutions wasn't fairly long, then the shelf-life of pre-formulated mobile phases would be very short.

Some vendors of TFA have a shelf life for unopened bottles of two years or more, changing to 3 months after opening - presumably because they have no control over the opening conditions.

As I don't use ampoules, I can't comment on their use, but if you are concerned, you could use both types of TFA containers and report back after a year or so.

All I can offer is based on my experience of using TFA, and I find that the quality of the main mobile phase solvent and amine modifier instability are the major issues affecting UV transparency in my assays. YMMV.

[DJ]

We get our TFA from a large stock bottle. I usually pour a little bit into a glass vial before pipetting the desired amount into my MeCN, H2O.

Do you use disposable pipette tips (polypropylene?) or glass?

Invariably once I pour out a little TFA and use what I'm going to use, there's always some left. I have a separate bottle I pour the "used" TFA back into. Once it gets full (or I have to buy more TFA), I'll distill and re-use.

[Bruce Hamilton]

I'd recommend splitting the large stock ( 100 ml? ) into at least two or three well-sealed, inert gas blanketed, stock bottles, and use them one by one to fill your vials. That way you don't kill the whole container if you contaminate it.

I use positive displacement polypropylene tips, and don't immerse them in the mobile phase solvent, but dispense very quickly under inert gas. I'd be concerned about moisture condensation on glass pipettes. or on vials opened without an inert shield.

Although it sounds complicated, it just the same system used to evaporate small sample fractions, and is very quick, except when bottles have to warm to ambient before opening..

I'm not sure I'd bother to redistill the TFA dregs.

[rick1112]

hi

I use 4 ml screw cap vials with a thick white PTFE-faced cap ( not the thin PTFE present in caps intended to be injected through). Reagents last for months in fridge.

can i know which brand of screw cap vial and thick white PTFE-faced cap are you using ??

[Bruce Hamilton]

Screw cap 4 ml vials are from standard suppliers, eg Wheaton or Chromacol.

Closures I'm not sure about. The body is green with loose inserts of 50 thou of polyolefin foam and 10 thou thick white PTFE. The insert is not glued to the cap and can be replaced. I just checked Wheaton catalogue and their equivalent had only 5 thou of PTFE, which may be adequate. I'd avoid any with glued closure inserts.

You can also use a cork borer to cut disks from 10 - 20 thou PTFE sheet and use those on top of other forms of inserts. Just remember that PTFE sheet usually is coated with invisible rolling lubricant, and needs to be washed with solvent ( Ultrasonication in warm DCM or Chloroform is effective ).

[shaun78]

In a former life a lab I worked in was asked to assign a true stability date to all our LC mobile phases that we had actually generated data for and could support. We used combinations of TFA in water and organic for just about everything, so it was in our best interest to be able to assign absurdly long expiration dates to these chemicals.

I believe we sat 0.1% TFA in water for a year to a year and a half in the corner of a lab and then pulled it out again for assay work. System suitability passed, assay went fine, we collected full spectra on our run and did not notice anything different across the wavelength range (210-400 nm, I believe).

[kplc]

(Emdchemicals.com tx1276) 99.9% min purity trifluoroacetic acid eliminated ALL of my TFA related baseline issues.

[DJ]

What about TFA stability in MeCN?

Bases and I believe acids catalyze addition of water to nitriles, yielding amides. Subsequent hydrolysis ( --> HOAc in the case of MeCN) can occur. However, in synthesis, these reactions are heated to reflux. I don't know how facile this is at ambient temperature, however.

The jury might out on MeCN/H2O/TFA solution stability, however, a TFA/alcohol mobile phase begins to take on the sweet odor of TFA-esters within days.


Aside- What about HFBA? What causes the notoriously noisy baselines at 220nm and below?