Electrochemical detectors mass sensitivity

The detection principles for biosensors integrated on microfluidic chips are classified into several types, including optical, electrochemical, and mass-sensitive methods. The trend in the development of detectors has been to constantly pursue two key virtues sensitivity and selectivity. In this regard, the research issues on the detectors for microfluidic systems are not significantly different to those for conventional biosensors. 

Specifications for modem detectors in HPLC are given by Hanai [538] and comprise spectroscopic detectors (UV, F, FUR, Raman, RID, ICP, AAS, AES), electrochemical detectors (polarography, coulometry, (pulsed) amperometry, conductivity), mass spectromet-ric and other devices (FID, ECD, ELSD, ESR, NMR). None of these detectors meets all the requirement criteria of Table 4.40. The four most commonly used HPLC detectors are UV (80%), electrochemical, fluorescence and refractive index detectors. As these detectors are several orders of magnitude less sensitive than their GC counterparts, sensor contamination is not so severe, and... 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

There are two basic approaches used to characterize seawater DOM (Benner, 2002). The first of these is to directly analyze bulk compositions (e.g., elemental or isotopic compositions) or individual compounds in the sample without concentration. This approach requires high-sensitivity methods for either broad biochemical types (e.g., total amino acids or carbohydrates) or individual compounds, often by either spectroscopic or chromatographic methods coupled to electrochemical or mass spectro-metric detectors. The latter type of molecular-level analyses are now feasible for measuring individual amino acids (Lindroth and Mopper, 1979), sugars (Skoog et al., 1999), and amino sugars (Kaiser and Benner,... 

CE detection is similar to detectors in, and include absorbance, fluorescence, electrochemical, and mass spectrometric detectors. The capillary can also be filled with a gel, which eliminates the electroosmotic flow. Separation is accomplished as in conventional gel electrophoresis but the capillary allows higher resolution, greater sensitivity, and on-line detection. In CE, low picogram amounts of analytes can be detected using glass fiber optics. However, this does not mean low limits of detection since only a few nanoliters can be injected. 

Current IPC detectors are on-stream monitors. HPLC detectors range from (1) non selective or universal (bulk property detectors such as the refractive index (RI) detector), characterized by limited sensitivity, (2) selective (discriminating solute property detectors such as UV-Vis detectors) to (3) specific (specific solute property detectors such as fluorescence detectors). Traditional detection techniques are based on analyte architecture that gives rise to high absorbance, fluorescence, or electrochemical activity. Mass spectrometry (MS) and evaporative light scattering detectors (ELSDs), can be considered universal types in their own right... 

Sensitive Optical Detectors. More sensitive optical techniques that have been used with CE include fluorescence, refractive index, chemiluminescence, Raman spectrophotometry, and circular dichroism. The most sensitive optical detection method used in CE is laser-induced fluorescence (LIE), which is capable of detection limits in the 10 to 10" mol (or better) range. This detection mode is easily accomplished with analytes that are either easily labeled with a fluorescent substrate (e.g., intercalators for double-stranded DNA) or are naturally fluorescent (e.g., proteins or peptides containing tryptophan). CE systems have also been interfaced with mass spectrometers, and electrochemical detection methods have been developed, although such detectors must be isolated electrically from the electrophoretic voltages. 

In the normal-phase HPLC, compounds to be separated are adsorbed to microparticulate silica gel and eluted in the order of least polar to most polar. Acceptable separation and quantitative yields of neutral and charged retinoids are obtained. Reverse-phase HPLC is preferable for acid-sensitive compounds such as 5,6-epoxyretinoic acid. Photometric, electrochemical, and mass spectrophotometric detectors have been used. 

With the exception of mass-sensitive electrochemical, light-scattering, conductivity and photoconductivity detectors, all those described in Sections 6.2-6.7 are concentration-sensitive. 

The detector is another of the critical components of a high pressure liquid chromatograph, and in fact, the practical application of liquid chromatography had to await a good detector system. Many types of detectors are now on the market. The four most common, the ultraviolet absorption (uv), fluorescence, refractive index (RI), and electrochemical (EC) detectors, will be discussed as well as the newer light scattering mass sensitive detector. 

Procedures for the production of large quantities of fused silica capillaries were developed to fill the growing demand for capillary gas chromatography columns. The advent of laser induced fluorescence (LIF) and electrochemical (EC) detectors provided the high mass sensitivity necessary for detection in nanoliter volumes. Jorgenson and Lucas published several key papers on modern CE in 1981. Since then CE has experienced exponential growth to the point where there are well over 2000 articles published annually involving CE. 

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