Thermo Scientific VH-D20 Operating Manual page 100

6 • Operation
Page 100
Additive/Buffer pKa
Ammonium 3.75
formate
Formic acid 3.75
Trifluoroacetic acid 0.3
(TFA)
• Acetic acid (AA) and especially formic acid (FA) are recommended as
primary choices of acidic modifier for use with the detector. They
are relatively volatile and the pH at typical concentrations is well
within the compatibility range of most columns.
• Trifluoroacetic acid (TFA) is frequently used as acidic modifier with
the detector. However, as indicated in the table above, the pH of
0.1% v/v TFA is beyond the range of many columns. Use a lower
concentration, if possible, to minimize column bleed.
• TFA and other perfluorocarboxylic acids, such as heptafluorobutyric
acid (HFBA), can function both as acidic modifiers and anionic ion
pairing reagents. These additives are less volatile than AA and FA
and will more readily form non-volatile salts with other additives or
sample components. If you use these additives, do not use basic
additives, such as ammonium salts or amines.
• TFA is known to be unstable after exposure to air. Single-use
ampoules of TFA are therefore recommended if choosing this
additive.
• If using cationic ion pairing reagents, such as TEA, practice the same
precautions as with anionic pairing reagents to avoid formation of
non-volatile salts with other additives and to stay within the range of
column and detector pH compatibility.
• AA and FA and their corresponding ammonium salts are
recommended as primary choices to control pH within the above
indicated buffer range.
• Ammonium bicarbonate may be used to control pH at higher levels
but requires use of an elevated evaporation temperature, such as
50 °C. Although the buffer range extends to pH 11, make sure to
keep the pH below the detector maximum of 12. More importantly,
make sure to keep the pH well below the compatibility range of your
column.
• If using a pH electrode to adjust buffer pH, only do so in aqueous
solutions without organic modifiers. Always make pH measurements
in separate aliquots of the aqueous solution to avoid cross-
contaminating the mobile phase buffers with non-volatile salts from
the pH electrode calibrations.
Buffer Recommended
range max.
concentration
2.8 – 4.8 10 mM
2.8 – 4.8 0.3% v/v
-- 0.1% v/v
Charged Aerosol Detectors (VH-D20, VF-D20)
pH at max.
concentration
in water
2.5
1.9
Operating Manual