Advances in HPLC detection—towards universal detection

High-performance liquid chromatography has already become one of themost widely applied analytical separation techniques because of its superior performance and reliability, especially in the pharmaceutical, environmental, forensic, clinical, food and flavor sciences. However, the relatively slow progress made in the field of HPLC detection has influenced the general progress and the extent of application of HPLC technology. Due to the lack of a universal detector that can be used for quantitative analysis without the need for an authentic standard, the information obtained from analytical HPLC is heavily limited. However, the needs of pharmaceutical companies (e.g. modern drug discovery research using highthroughput quantification methods for lead discovery) and other industries demand the development of a generic quantitative detection technique that can quickly determine the quantity of a chemical without the need to use an authentic standard and the tedious calibration procedure. Researchers are therefore focusing much of their activity into developing a truly “universal” detector for HPLC [1]. The HPLC detectors reported in peer-reviewed publications so far can be classified into [2]: elemental detectors (atomic absorption/emission, inductively coupled plasma–mass spectrometry and microwave-induced plasma); optical detectors (UV/visible, IR/Raman, optical activity, evaporative light scattering and refractive index); luminescent detectors (fluorescence/phosphorescence, chemiluminescence/bioluminescence); electrochemical detectors (potentiometry, novel material/modified electrodes, array electrodes and pulsed and oscillometric techniques); mass spectrometric detectors (time-of-flight/MALDI, Fourier transform ion cyclotron resonance mass spectrometry, electrospray/thermospray, atmospheric pressure ionization and particle beam); and other detection systems (nuclear magnetic resonance, radioactivity detectors, surface plasmon resonance). It is well known that NMR is a spectroscopic techniquewith a very high information content. Although the LC–NMR technique has been successfully applied to the characterization of drug impurities and synthetic compounds in the form of combinatorial libraries [3], analysts are aware of the very high cost of NMR instrumentation. For this reason, the LC–NMR technique has not gained widespread usage [4]. Attention has also focused on the design of LC–FTIR systems, which are either of the flow-cell or the solvent-elimination type. Problems facing this technique include the limited compatibility of the optimized separation and detection conditions, and the rather complicated interface required. Extensive investigations into the use of RI (refractive index) detectors [5–8], evaporative light scattering detectors (ELSDs) [9–12] and chemiluminescence nitrogen detectors (CLNDs) [13–31] as universal detectors for HPLC have been performed. The RI detector, which measures the change in the refractive index of the column effluent as it passes through the flow-cell, could be a universal detector in theory. The greater the difference in RIs between the sample and the mobile phase, the higher the sensitivity of the system. On the other hand, when analyzing complex mixtures, the sample components may cover a wide range of refractive index values, and the RIs of some of the sample components may closely match that of the mobile phase, making them “invisible” to the detector. The RI detector is a pure differential instrument, and any changes in the eluent composition require the rebalancing of the detector. This factor Anal Bioanal Chem (2008) 390:299–301 DOI 10.1007/s00216-007-1633-0