Column back-pressure monitoring

A typical HPLC system set to a flow rate of 1 ml/min, indicating a column back pressure of 1265 psi and connected to a UV/visible detector which is set to monitor the column effluent at 260 nm. 

These techniques are useful for the separation of complex mixtures. Quantitative analyses have been reported on malathion pesticide in vegetable matter, limonin in grapefruit peel, and additives in compound rubber by means of SEC and RPC126>. SEC was performed with unstabilized THF and monitored at 215 nm. Working so near to the UV cut-off (210 nm) of THF was possible through the use of a detector with a flowing reference installed between the pump and the sample valve. Then the eluent passed the column and finally the analytical cell. Since the column back pressure never exceeded 10 MPa, this eluent line caused no problems. 

Monitor the column back-pressure and efficiency performance and take corrective action immediately don t wait until the column is plugged. If pressure is abnormally high, try back flushing the column as soon as... 

Monitor eluant spectroscopic properties and column back-pressure during th above column conditioning. Table 2 gives a guide to the typical ranges of pressures you should observe. 

During the separation itself some trouble can occur concerning the back pressure. An increasing back pressure indicates contamination of the column and thus should be monitored. If this happens or if a visible contamination of the sorbent is noticed, a regeneration of the column is necessary. However, if the back pressure rises very rapidly the column may be clogged by denatured proteins. As a first attempt, the frits should be replaced by new ones, trying the top adapter first. 

Valve V3 is set to allow gas to exit the selected adsorber through the back pressure regular (BPR) as soon as the desired operating pressure is achieved. For depressurization, V3 is switched to allow the column exit gas to bypass the BPR and be vented through valve V4. V4 is a vacuum/vent selector and V5 allows a single pressure transducer to monitor system pressures at up to four selected points. 

The back pressure (35-45 bar) generated from the thermospray evaporation process in the capillary interface line is monitored by the Spectroflow pump used for the post column addition of the 0.5 M ammonium acetate solution. It should be stable( 1 bar) to insure good reproducibility of the ion signal. An increase in the back pressure would indicate partial clogging of the in-line filter or the thermospray probe tip. The blockage must be eliminated before proceeding with the analysis. 

Reaction pressure was maintained with a dome-loaded back-pressure regulator (Circle Seal Controls). All heated zones were controlled and monitored with a Camile 2500 data acquisition system (Camile Products). Products were analyzed online by gas chromatography with an HP 5890 II GC, equipped with an FID, and a DB-Petro 100 m column (J W Scientific), operated at 35° C for 30 min, ramped at 1.5°/min to 100° C, 5°/min to 250° C for 15 min. An alkylate reference standard (Supelco) allowed identification of the trimethylpentanes (TMP) and dimethylhexanes (DMH). The combined mass of TMP and DMH is referred to hereafter as the alkylate product . As discussed elsewhere [19], propane, an impurity in the isobutane feed, was used as an internal standard for butene conversion calculations. Since isomerization from 1-butene to 2-butene isomers is rapid over acidic catalysts, reported conversion is for all butene isomers to C5 and higher products. Isobutylene formation was not observed under any conditions. 

As is quite evident from interpreting the ehromatograms in Fig. 4.50, RP-HPLC columns have a finite lifetime It is good practice for the analyst to keep a record of N (as calculated using the above equation) versus either time or number of injected samples in an attempt to continuously monitor column performance. Because HPLC reciprocating pumps maintain constant flow rate, a continuous observation of the back-pressure or pressure buildup at the front on the HPLC column is an important parameter to monitor. Making sure that there are no leaks in an operating HPLC is also very important. 

The method (27) describes the measurement of clonidine in human plasma and urine by combined gas chromatography-mass spectrometry with ammonia chemical ionization. Addition of [ 84] clonidine to plasma or urine is followed by ethylacetate extraction of clonidine from alkaline medium, back extraction into acid extraction into ethyl ether from alkaline medium and evaporation of the extract to dryness. Trimethylanilinium hydroxide is added to the residue, and dimethyl derivatives of clonidine are formed by on column methylation with an injection-port temperature of 250 for g.c. -70-eV m.s., the glass column (1.8 m X 2 mm) packed with 3% of OV-17 on Gas-Chrom Q (100 to 120 mesh) is operated at 245 . With He s carrier gas (15 ml min" ) NH3 is admitted to an ion-source pressure of 0.2 Torr, and ions are monitored at m/e 258 and 264. Graphs of peak height ratios (m/e 258 to 264) vs amounts of clonidine in urine (up to 40 ng ml-i) and in plasma (up to 5 ng ml i) are rectilinear. The precision for assay of clonidine in plasma is 11% at 0.25 ng ml" and 5% at 0.5 ng ml" and the lower limit of determination is 0.1 ng ml. ... 

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