Ultraviolet (UV) absorbance is the most widely used detection method for high-performance liquid chromatography (HPLC) separations. In pharmaceutical analysis, purity determinations often include quantitation of related impurities based on relative HPLC peak areas obtained at a specific wavelength. In order for this quantitation to accurately reflect weight percentages of impurities, the relative UV response factors (absorptivities) at the given wavelength must be known. In this work, we present a convenient method for determining relative UV response factors on-line, without isolation or purification of impurities, without standards, and without requiring known analyte concentrations. The procedure described makes use of a chemiluminescent nitrogen-specific HPLC detector (CLND) in conjunction with a UV detector. The CLND response is directly proportional to the number of moles of nitrogen in each eluting peak, and can, therefore, be used to determine relative amounts of each nitrogen-containing impurity present in the sample, provided the molecular formulas are known (e.g. from exact mass LC-MS). It is a simple matter, then, to determine the relative UV response factors from the UV area ratios obtained for the same sample. The feasibility and accuracy of this method is demonstrated for gradient HPLC separations of commercially available compounds of widely varying structures. Finally, the method's utility in obtaining accurate mass balance is demonstrated by application to p

Operation of ion-selective electrode detectors in the sub-Nernstian/linear response range: application to flow-injection/enzymatic determination of L-glutamine in bioreactor media. A novel approach for eliminating positive errors from endogenous ionic interferences when using ion-selective electrodes as detectors in flow-injection enzyme-based blosensing configurations is described. The method involves using a high background level of interfering ions in the sample diluent/carrier stream to convert the normally logarithmic potentiometric sensor into a linear detector over a given concentration range of primary ions. A split-stream single-detector arrangement provides a convenient means to compensate for varying levels of background interferent ions in the injected samples. One portion of the split stream passes directly to the ion-electrode detector, yielding a signal linearly related to the concentration of endogenous primary ions in the sample. The second portion of the split sample is delayed while passing through an immobilized enzyme that generates electrode detectable primary ions in proportion to the concentration of the substrate analyte in the sample.

In the study of enzyme catalyzed kinetic resolution of racemates, it is imperative to assay how the optical yield varies with chemical conversion. In this paper, a method using one-time injection to determine enantiomeric excess and conversion of the stereoselective esterification of racemic ibuprofen with n-butanol catalyzed by lipase was developed with a commercially available HPLC CSP column Regis(S, S) Whelk-01. In the linear range of detector, all peak areas of products and substrates are proportional to their concentrations. Because the total mole concentration remains unchanged (equal to the initial value of ibuprofen) in the reaction process, the conversion could be calculated from the peak areas, provided the ratio of response factors was known. The calibration curves of two ibuprofen enantiomers with racemic ibuprofen as external standard were overlapped, indicating fiR = fiS. By investigating the variation of peak areas of products and substrates against conversion (determined by external standard), the ratio of peak area-concentration response factor of ibuprofen butyl ester to that of unreacted ibuprofen was determined to be 1 through linear regressions, from which the conversion could be directly determined by the self normalization of the peak areas. With a mobile phase of IPA/hexane/HAc/triethylamine (15/85/0.2/0.05, V/V, flow rate 0.4 mL/min), the resolution of ibuprofen enantiomers was sufficient for precise enantiomeric purity determination.

to generate a response proportional to the integral of the analyte concentration over time and/or space … very broad linear range. For solid sorbents, such as Carbowax/DVB or PDMS/DVB, the linear range is narrower … analyte n is not only directly proportional to Co, but also directly proportional to the SPME sampling

show well-separated peaks, one per analyte. Each analyte appears in its respective time (retention time) having area proportional to its amount. Chromatography High-performance liquid chromatography (HPLC), formerly referred to as high-pressure liquid chromatography, is a chromatography technique in analytical chemistry used to separate, identify, and quantify specific components (analytes) in mixtures. The mixtures can originate from food, chemicals, pharmaceuticals, biological, environmental and agriculture, etc., in which the sample analyzed is either Hi…

a response assumed to be proportional to the concentration. For accurate results, the instrument's response to the analyte in the unknown should be compared Ultraviolet–visible spectrophotometry (UV–Vis or UV-VIS) refers to absorption spectroscopy or reflectance spectroscopy in part of the ultraviolet and the full, adjacent visible regions of the electromagnetic spectrum. Being relatively inexpensive and easily implemented, this methodology is widely used in diverse applied and fundamental applications. The only requirement is that the sample absorb Organic compounds, especially those with a high degree of conjugation, also absorb light in the UV or visible regions of the electromagnetic spectrum. The solvents for these determinations are often water for water-soluble compounds, or ethanol for organic-soluble compounds. (Organic solvents may have significant UV absorption; not all solvents are suitable for use in UV spectroscopy. Ethanol absorbs very weakly at most wavelengths.) Solvent polarity and pH can affect the absorption spectrum of an organic compound. Tyrosine, for example, increases in absorption maxima and molar extinction coefficient when pH increases from 6 to 13 or when solvent polarity decreases. While charge transfer complexes also give rise to colors, the colors are often too intense to be used for quantitative measurement. The Beer–Lambert law states that the absorbance of a solution is directly proportional to the concentration of the absorbing species in the solution and the path length. Thus, for a fixed path length, UV–Vis spectroscopy can be used to determine the concentration of the absorber in a solution. It is necessary to know how quickly the absorbance changes with concentration. This can be taken from references (tables of molar extinction coefficients), or more accurately, determined from a calibration curve. A UV–Vis spectrophotometer may be used as a detector for HPLC. The presence of an analyte gives a response assumed to be proportional to the concentration. For accurate results, the instrument's response to the analyte in the unknown should be compared with the response to a standard; this is very similar to the use of calibration curves. The response (e.g., peak height) for a particular concentration is known as the response factor. The wavelengths of absorption peaks can be correlated with the types of bonds in a given molecule and are valuable in determining the functional groups within a molecule. The Woodward–Fieser rules, for instance, are a set of empirical observations used to predict λmax, the wavelength of the most intense UV–Vis absorption, for conjugated organic compounds such as dienes and ketones. The spectrum alone is not, however, a specific test for any…

radiation is directly proportional to its frequency and inversely proportional to its wavelength … from the peak area. The total area of an absorption peak is directly proportional to the number … form, the peak height is directly proportional to the peak area. However, as the peak shape