High performance capillary electrophoresis - T.E.A.M.

Modes
Name
pK a
Phosphate 2.12 (pK a1
)
Citrate 3.06 (pK a1
)
Formate 3.75
Succinate 4.19 (pK a 1
)
Citrate 4.74 (pK a2
)
Acetate 4.75
Citrate 5.40 (pK a3
)
Succinate 5.57 (pK a2
)
MES 6.15
ADA 6.60
BIS-TRIS propane 6.80
PIPES 6.80
ACES 6.90
MOPSO 6.90
Imidazole 7.00
MOPS 7.20
Phosphate 7.21 (pK a2
)
TES 7.50
HEPES 7.55
HEPPS 8.00
TRICINE 8.15
Glycine amide, 8.20
hydrochloride
Glycylglycine 8.25
TRIS 8.30
BICINE 8.35
Morpholine 8.49
Borate 9.24
CHES 9.50
CHAPSO 9.60
CAPS 10.40
Phosphate 12.32 (pK a 3
)
Table 10
Commonly used buffers
A number of commonly used buffers and their useful pH
ranges are given in table 10. The so-called biological ‘Good
buffers’ (that is Tris, borate, histidine, CAPS,...) are especially
useful. These buffer ions are generally large and can
be used in high concentrations without generating significant
currents. A potential disadvantage of these large buffer
ions is their strong UV absorbance characteristics.
Matching buffer ion mobility to solute mobility is important
for minimizing peak shape distortions, as mentioned in
section 2.3.4.5. Furthermore, it is necessary to select
leading and trailing buffer ions for on-capillary sample
focusing by isotachophoresis, as described in this section
3.5.
Buffer ions can also be used to complex with solutes and
alter selectivity. Tetraborate is one notable example. This
ion has been used to improve separations of catechols and
carbohydrates.
3.1.1.2 Buffer pH
Alterations in pH are particularly useful when solutes have
accessible pI values, such as peptides and proteins. Working
above and below the pI value will change the solute
charge and cause the solute to migrate either before or
after the EOF. Below its pI a solute possesses a net positive
charge and migrates toward the cathode, ahead of the EOF.
Above the pI the opposite occurs. Due to the high chemical
stability of the fused silica capillary, the accessible pH
range can vary from below 2 to more than 12, but is usually
limited by the pH stability of the solute.
In addition to affecting solute charge, changing the pH will
also cause a concomitant change in EOF. This may necessitate
re-optimization of a separation. For instance, adequate
resolution may be obtained at low pH, but when increased
50