Instant Notes: Analytical Chemistry

As yet, the number of applications is limited but is likely to grow as instru-

mentation, mostly based on existing CE systems, and columns are improved

and the theory of CECdevelops. Current examples include mixtures of poly-

aromatic hydrocarbons, peptides, proteins, DNA fragments, pharmaceuticals

and dyes. Chiral separations are possible using chiral stationary phases or by

the addition of cyclodextrins to the buffer. In theory, the very high efficiencies

attainable in CECprovides high peak capacities, and therefore the possibility

of separating complex mixtures of hundreds of components. A typical CEC

separation is shown in Figure 3.

D9 – Electrophoresis and electrochromatography: modes, procedures and applications 187

02

1 2

4

Time (min)

68

UV absorbance (254 nm)

(^34)
5
6 7
8 9
(^1011)
12
(^1314)
Column: 30 cm × 75 μm, 17 cm packed with 1.5 μm dp non-porous ODS-II (Micra Scientific);
15% methanol–85% 10 mM MES; applied voltage: 12 kV; injection: 1 s at 2 kV. Peaks: 1 = oct
tetranitro-1,3,5,7-tetrazocine; 2 = hexahydro-1,3,5-trinitro-1,3,5-triazine; 3 = 1,3-dinitrobenzen
trinitrobenzene; 5 = nitrobenzene; 6 = 2,4,6-trinitrotoluene; 7 = 2,4-dinitrotoluene; 8 = methyl-
trinitrophenylnitramine; 9 = 2,6-dinitrotoluene; 10 = 2-amino-4,6-dinitrotoluene; 11 = 2-nitrotol
nitrotoluene; 13 = 4-amino-2,6-dinitrotoluene; 14 = 3-nitrotoluene.
Fig. 3. Separation of 14 explosive compounds by capillary electrochromatography. Column:
30 cm ¥75 μm, 17 cm packed with 1.5 μm dp non-porous ODS-II (Micra Scientific); mobile
phase: 15% methanol–85% 10 mM MES; applied voltage: 12 kV; injection: 1 s at 2 kV.
Peaks: 1 = octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine; 2 = hexahydro-1,3,5-trinitro-
1,3,5-triazine; 3 = 1,3-dinitrobenzene; 4 = 1,3,5-trinitrobenzene; 5 = nitrobenzene;
6 = 2,4,6-trinitrotoluene; 7 = 2,4-dinitrotoluene; 8 = methyl-2,4,6-trinitrophenylnitramine;
9 = 2,6-dinitrotoluene; 10 = 2-amino-4,6-dinitrotoluene; 11 = 2-nitrotoluene;
12 = 4-nitrotoluene; 13 = 4-amino-2,6-dinitrotoluene; 14 = 3-nitrotoluene.
Classical gel electrophoresis is primarily a qualitative technique, the identifica-
tion of unknown solutes being accomplished by comparisons of migration
distances with those of standards run simultaneously or sequentially under
identical conditions. Migration distances can be expressed as Rf values
analogous to those used in TLC(Topic D3).
Solute migration times in CE and CEC separations can be equated to
chromatographic retention times, tR, and similar methods are used to identify
unknowns (Topic D2), that is:
● comparisons of retention timeswith those of known solutes under identical
conditions;
● comparisons of electropherogramsof samples spikedwith known solutes
with the electropherogram of the unspiked sample;
● comparisons of UV-visible spectra recorded by a diode-array detector with
those of known solutes;
● interfacingof a CEor CECsystem with a mass spectrometer. Identifications
are facilitated by searching libraries of computerized spectra (Topics F3 and
F4).
Qualitative
analysis