Ion chromatography gradient system offers the user the option to clean up the separation column after injection of heavily loaded samples
With difficult sample matrices, it is sometimes necessary to employ a concentration gradient from a weak to concentrated eluent over the course of the chromatographic run to enable separation of anions that have a wide range of affinities towards the analytical column.
Anions that are weakly retained by the column elute first and as the mobile phase concentration is increased, the more strongly adhering anions can then be eluted with the more powerful eluent.
A gradient system offers the user the option to clean up the separation column after injection of heavily loaded samples.
A typical Metrohm ion chromatography gradient system uses the advanced modules comprising two 818 IC pumps, the 819 IC detector, the 820 IC separation centre, the 830 IC interface and the 828 IC dual suppressor.
The system is controlled through the data acquisition and control software, IC Net 2.3, which is FDA 21 CFR Part 11 compliant when controlled within a Windows 2000 or XP operating system.
Historically the use of gradient systems in ion chromatography has been restricted to the use of hydroxide eluents as any change in the carbon dioxide concentration caused a marked rise in the baseline when a carbonate/bicarbonate eluent was used as the mobile phase.
The eluting power of hydroxide is weak compared with that of carbonate and meant that high alkali concentrations had to be utilised to achieve the desired separation.
The use of strongly alkaline solutions was often undesirable and could cause corrosion and reliability problems with metal pump delivery systems.
The continuous mode of operation of the Metrohm 828 IC dual suppressor allows gradient systems to be realised in a simple way.
The H+ cations required for anion suppression are produced electrochemically and as the 828 suppressor can also eliminate carbon dioxide, this opens up a new field of applications allowing the flexible carbonate/bicarbonate mobile phases to be used without restrictions.
The injection and carbonate peaks are reduced to a minimum and with the 828 suppressor it is also possible to suppress concentrated hydroxide eluents allowing flexibility and freedom of the methodologies used.
Together with the unique electronic suppression of the Metrohm 819 IC detector, extremely low detection limits can be achieved usually without the need for large sample loops or sample preconcentration.
The advantages of gradient separation include higher resolution of fast eluting analytes and an acceleration of the late eluting species.
On the front end it is possible to achieve resolution between fluoride, glycolate, acetate and formate with the Metrosep A Supp 5 separation column. The more strongly retained species like benzoate and molybdate can be seen towards the rear of the chromatograph and a major advantage of gradient chromatography is the clean up of the separation column.
The column is washed with a strong concentration of the mobile phase at the end of the run to effectively regenerate the anion exchange sites increasing the life expectancy of the column.
