I am performing reverse phase HPLC and using TFA as solvent A. I want to know why m I using it ? why not any other acid ?
For a detailed explanation, please review one of the many basic books on liquid chromatography. TFA has several different uses in HPLC.
Quick answers:
The pH of your mobile phase may effect the ionization state of your samples. If the pH and/or pKa are close to or the same as the mobile phase, the sample could exist in two forms (ionized/neutral). We want it at least 2 pH units away, in one form, to retain it on the column (and improve RSD). TFA can be used in low concentrations to get the pH to ~ 2.0 (good for mildly acidic, neutral or basic compounds). See link. "Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development" (at bottom).
TFA is also a very strong ion pairing agent which may be used to improve retention on the column.
When and where appropriate, we try to NOT use TFA in liquid chromatography and instead use other acids. Acetic, Formic, Propionic acid, Hydrochloric, Phosphoric, Sulfuric and others are used for many types of HPLC applications (if a buffer is used, consider the acid based on the salt used). *Ideally, the acid should be available in an ultra-pure grade (high purity) and as clear as possible in the UV region. Caution: Be careful that the acid you choose is not corrosive to the HPLC system (tubing and parts). BTW: In modern chromatography applications, TFA is over used in HPLC. Many chromatographers have not received training in HPLC and only use it because the "other" person did, they saw it in an application note or paper, or just use it without actually asking WHY or if they would be better off using another acid. Ignorance is a dangerous thing. There are reasons for choosing one over another. Often TFA is used in too high a concentration (> 0.1%) or in systems where it may result in system contamination (in-line vacuum degassers) or ion suppression (e.g. ESI).
"Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development";
http://hplctips.blogspot.com/2015/12/common-pka-values-for-acids-bases-used.html
For a detailed explanation, please review one of the many basic books on liquid chromatography. TFA has several different uses in HPLC.
Quick answers:
The pH of your mobile phase may effect the ionization state of your samples. If the pH and/or pKa are close to or the same as the mobile phase, the sample could exist in two forms (ionized/neutral). We want it at least 2 pH units away, in one form, to retain it on the column (and improve RSD). TFA can be used in low concentrations to get the pH to ~ 2.0 (good for mildly acidic, neutral or basic compounds). See link. "Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development" (at bottom).
TFA is also a very strong ion pairing agent which may be used to improve retention on the column.
When and where appropriate, we try to NOT use TFA in liquid chromatography and instead use other acids. Acetic, Formic, Propionic acid, Hydrochloric, Phosphoric, Sulfuric and others are used for many types of HPLC applications (if a buffer is used, consider the acid based on the salt used). *Ideally, the acid should be available in an ultra-pure grade (high purity) and as clear as possible in the UV region. Caution: Be careful that the acid you choose is not corrosive to the HPLC system (tubing and parts). BTW: In modern chromatography applications, TFA is over used in HPLC. Many chromatographers have not received training in HPLC and only use it because the "other" person did, they saw it in an application note or paper, or just use it without actually asking WHY or if they would be better off using another acid. Ignorance is a dangerous thing. There are reasons for choosing one over another. Often TFA is used in too high a concentration (> 0.1%) or in systems where it may result in system contamination (in-line vacuum degassers) or ion suppression (e.g. ESI).
"Common pKa Values for ACIDS & BASES used in HPLC and LC/MS Method Development";
http://hplctips.blogspot.com/2015/12/common-pka-values-for-acids-bases-used.html
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