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Internal Energy02:00

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The total of all possible kinds of energy present in a substance is called the internal energy (U), sometimes symbolized as E. Suppose a system with initial internal energy, Uinitial, undergoes a change in energy (transfer of work or heat), and the final internal energy of the system is Ufinal. Change in internal energy equals the difference between Ufinal and Uinitial.
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The internal energy of a thermodynamic system is the sum of the kinetic and potential energies of all the molecules or entities in the system. The kinetic energy of an individual molecule includes contributions due to its rotation and vibration, as well as its translational energy. The potential energy is associated only with the interactions between one molecule and the other molecules of the system. Neither the system's location nor its motion is of any consequence as far as the internal...
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Enthalpy changes are typically tabulated for reactions in which both the reactants and products are at the same conditions. A standard state is a commonly accepted set of conditions used as a reference point for the determination of properties under other different conditions. For chemists, the IUPAC standard state refers to materials under a pressure of 1 bar and solutions at 1 M and does not specify a temperature. Many thermochemical tables list values with a standard state of 1 atm. Because...
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With or Without Internal Standard in HPLC Bioanalysis. A Case Study.

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Journal of Chromatographic Science
|January 8, 2019
PubMed
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Using an internal standard in High-Performance Liquid Chromatography (HPLC) for meloxicam (MXC) quantification in human plasma did not significantly improve accuracy or precision. External standard quantification may be adequate in certain situations.

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Area of Science:

  • Analytical Chemistry
  • Pharmacokinetics

Background:

  • The use of internal standards in High-Performance Liquid Chromatography (HPLC) is debated.
  • While intended to enhance accuracy and precision, its necessity varies with application.

Purpose of the Study:

  • To compare HPLC quantification of meloxicam (MXC) in human plasma with and without an internal standard.
  • To evaluate if internal standardization reduces errors, particularly with low injection volumes.

Main Methods:

  • HPLC analysis on reversed-phase with UV detection.
  • Meloxicam (MXC) determination in human plasma after protein precipitation.
  • Piroxicam (PXC) utilized as the internal standard for comparison.

Main Results:

  • Analyte stability was confirmed.
  • No significant improvement in accuracy or precision was observed when using PXC as an internal standard for MXC quantification.
  • Simple protein precipitation was sufficient.

Conclusions:

  • Internal standardization with PXC offers no significant advantage for MXC determination in human plasma via HPLC post-protein precipitation.
  • External standard quantification can be sufficient, challenging the mandatory use of internal standards.