p. 7
The buffering ion sh ould carry the sam e charge as the ion exch ange group
and should have a pKa withi n 0.5 pH units of the pH u sed in the separat ion.
Buffering ions of opp osite charge may ta ke part in the ion ex change process
and cause local disturbances in pH.
Starting pH
Cation exchangers (SP): At least 1 pH unit below t he pI of substance to be
bound.
Anion exchangers (Q): At least 1 pH unit above the pI of s ubstance to be
bound.
Table 2. Buffers for cat ion exchange chrom atography.
pH interval Substance Con c. (mM) Counter-ion pKa (25 ° C)1
1.4–2.4 Maleic acid 20 Na+ 1.92
2.6–3.6 Methyl malonic acid 20 Na+ or Li+ 3.07
2.6–3.6 Citric acid 20 Na+ 3.13
3.3–4.3 Lactic acid 50 Na+ 3.86
3.3–4.3 Formic acid 50 Na+ or Li+ 3.75
3.7–4.7; 5.1–6.1 Succinic acid 50 Na+ 4.21; 5.64
4.3–5.3 Acetic acid 50 Na+ or Li+ 4.75
5.2–6.2 Methyl malonic acid 50 Na+ or Li+ 5.76
5.6–6.6 MES 50 Na+ or Li+ 6.27
6.7–7.7 Phosphate 50 Na+ 7.20
7.0–8.0 HEPES 50 Na+ or Li+ 7. 56
7.8–8 .8 BICIN E 50 Na+ 8.33
1 Ref: Handbook of chem istry and phy sics, 83rd edition, CRC, 2002–2003.