Why people use different TFA conc in MP A and MP B?

[wangge]

Hi, All,
I wonder why people use 0.05%TFA in H2O as MP A, while using 0.04% TFA in ACN as MP B? What is the rational behind using the different TFA conc. in MP A and MP B?
Your answers are appreciated.

[Alex Buske]

Hi,

if people use different TFA concs in some methods there might be a reason. The best thing is to ask the original authors.
Maybe at at specific wavelength the absorption is equal, thuse avoiding a "gradient ramp" in the chromatogramm.
Maybe someone thought about pH in a way that compares H+ and OH- ratios. As ACN is aprotic smaller amount of TFA could have same effect.

Alex

[WK]

Wangge
I agree with Alex first comment - that they are probably using a low wavelength close to the cut-off for TFA to attempt to produce a straight baseline.
WK

[skunked_once]

wangge,

Alex was correct. The reason is to produce a flat baseline in the 210-220 nm range during the gradient. When I prepare mobile phases like this I use 0.1% TFA in 95:5 water/acetonitrile for A and 0.08% TFA in 5:95 water/acetonitrile for B. After preparing the mobile phases, I measure the absorbance of each mobile phase at 215 nm and add a few drops of TFA if necessary so that the absorbance of A is 0.03-0.04 AU greater than B.

[Mark Tracy]

The absorbance spectrum and extinction coefficient of TFA changes with the concentration of acetonitrile. At 214 nm, the shift is minimized. The different concentrations in A and B compensate for that. This trick works well as long as the gradient is slow, but sharp gradients interact with the column to give nonlinear behavior.