Ultra-pressure liquid flow rates

The measurement of partial pressures over a liquid or solid mixture of two metals is not as simple. Mostly, it is restricted to higher temperatures or even to the molten phase. The direct measurement can be done, for example, in high or ultra high vacuum, using a Knudsen cell and a mass spectrometer for selective pressure determination. Dynamic measurements were developed, e.g., transportation methods. A steady stream of a reactive gas is passed over the sample transporting the reactive component to a cooled region of the apparatus. From the measured mass of the transported metal and the flow rate the vapor pressure can be calculated. Kubaschewski et alP have given a detailed description of the experimental possibilities. 

The penalty for using fine particles is resistance to solvent flow. Until recently, HPLC operated at pressures of 70-4(X) bar to attain flow rates of 0.5-5 mL/min. In 2004, equipment became available to employ 1.5- to 2-p.m-diameter particles at pressures up to 1 000 bar. These instruments provide increased resolution or decreased run time. Chromatography with 1.5- to 2-p,m-diameter particles at high pressure is commonly called UPLC (Ultra-Performance Liquid Chromatography). 

Also, new-generation HPLC instrument which can exceed the pressure limitation of conventional equipment has been developed. This technique is ultra performance liquid chromatography (UPLC), where small particle size columns with 1.7 pm particle size in combination with high flow rates offers increased speed, resolution, and sensitivity [15]. Ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) has shown to be well suited for support of PK studies [16-19], Often, there is a need to improve the sensitivity of the assay for the determination of PK parameters, especially for very potent compounds, since lower amount is administrated, and also for compounds with low bioavailability. UPLC-MS/MS has shown decreased ion suppression and superior sensitivity, and has been successfully applied to PK studies for compounds with low bioavailability [20-22]. 

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