Reversed-phase HPLC sample concentration effects

The method for chloroacetanilide soil metabolites in water determines concentrations of ethanesulfonic acid (ESA) and oxanilic acid (OXA) metabolites of alachlor, acetochlor, and metolachlor in surface water and groundwater samples by direct aqueous injection LC/MS/MS. After injection, compounds are separated by reversed-phase HPLC and introduced into the mass spectrometer with a TurboIonSpray atmospheric pressure ionization (API) interface. Using direct aqueous injection without prior SPE and/or concentration minimizes losses and greatly simplifies the analytical procedure. Standard addition experiments can be used to check for matrix effects. With multiple-reaction monitoring in the negative electrospray ionization mode, LC/MS/MS provides superior specificity and sensitivity compared with conventional liquid chromatography/mass spectrometry (LC/MS) or liquid chromatography/ultraviolet detection (LC/UV), and the need for a confirmatory method is eliminated. In summary,... 

A sensitive reverse-phase HPLC method has been developed for the analysis of etodolac in tablet formulation [22]. The chromatographic separation was achieved using a reverse-phase Cu column, having dimensions of 3.3 cm x 0.46 cm i.d. (3 pm particles) and which was maintained at 30°C. The mobile phase consisted of pH 6.0 phosphate buffer / methanol (60 40 v/v), and was eluted at 1 mL/min. Analyte detection was effected on the basis of UV detection at 230 nm. Diazepam was used as an internal standard. The sample preparation entailed grinding the etodolac tablets, followed by extraction with methanol (using sonication). A retention time of 1.46 min was obtained for etodolac under these conditions, and the method was found to be linear, precise, and accurate over the concentration range of 0.01 to 0.1 mg/mL. 

Figure 10.5 Effects of Na2S04 on fate of ATP. A 30 fig sample of the extract was added to reaction mixtures containing, in a total volume of 200 fiL, 100 fiM ATP, 0.4 mM Mn2+, 50 mM Tris-HCl (pH 7.4), and 0 to 200 mM sodium sulfate. The reaction was incubated at 31°C for 2 hours and terminated by heating to 155°C. Samples were analyzed by reversed-phase HPLC. Chromatograms represent an analysis of each reaction mixture 2 hours after the start of the reaction at four sulfate concentrations. Inset Initial rate of AMP and ADP formation from ATP. (From Rossomando and Jahngen, 1983.)... 

A possible source of error when using reversed-phase systems is the adsorption of surfactants on the surfaces of the HPLC apparatus. For example, if the sample is injected as a purely aqueous solution, surfactant will build up in the metal injection valve depending on the volume of sample used to flush the loop. When the HPLC mobile phase is diverted through the valve, it will remove the adsorbed surfactant along with the measured sample volume. Thus, if varying volumes of sample are used to flush the injector, the peak size observed for a specific surfactant concentration will vary, even with the same loop size (93). This effect is minimized by dissolving the sample in the mobile phase. 

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