Column operation guard columns

Valproic acid was determined in tablets and plasma using ion-chromatography [29], The extract was injected onto a column (6.5 cm x 6 mm) of Dionex ICE separator resin fitted with a guard column of Aminex cation exchange resin and operated with aq. 0.5 mM C02 as mobile phase (0.7 mL/min) and conductivity detection. For tablets, the calibration graph was rectilinear for 0.2-25 pg/mL with limit of detection of 50 pg/mL. For plasma, the response was linear for 50-200 pg/mL and limit of detection was 2 pg/mL. 

Nimodipine, tarcine, and their metabolites were determined by HPLC in the plasma of patients with Alzheimer s disease [23], After extraction and evaporation under nitrogen to dryness, the residue was reconstituted into acetonitrile/H20 (1 3), and analyzed on a 5-pm Hypersil phenyl column (25 cm x 4.6 mm i.d.) equipped with a Nova Pak phenyl guard column and operated with acetonitrile/4.4 mM KH2P04 buffer (39 61) as the mobile phase (flow rate of 1.5 mL/min). Detection was effected either fluorimetrically at 330 nm excitation and 360 nm emission, or by its UV... 

Standard analytical separation columns probably operate at too high a backpressure to be used in this type of system. However, guard columns often contain the same separation material as an analytical column and have much lower backpressure. To a certain extent, the flow rate can be reduced to lower column backpressure. 

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