Chromatographic compromise

Quantitative mass spectrometry, also used for pharmaceutical appHcations, involves the use of isotopicaHy labeled internal standards for method calibration and the calculation of percent recoveries (9). Maximum sensitivity is obtained when the mass spectrometer is set to monitor only a few ions, which are characteristic of the target compounds to be quantified, a procedure known as the selected ion monitoring mode (sim). When chlorinated species are to be detected, then two ions from the isotopic envelope can be monitored, and confirmation of the target compound can be based not only on the gc retention time and the mass, but on the ratio of the two ion abundances being close to the theoretically expected value. The spectrometer cycles through the ions in the shortest possible time. This avoids compromising the chromatographic resolution of the gc, because even after extraction the sample contains many compounds in addition to the analyte. To increase sensitivity, some methods use sample concentration techniques. 

Instrumental Interfaces. The basic objective for any coupling between a gas chromatograph (gc) and a mass spectrometer (ms) is to reduce the atmospheric operating pressure of the gc effluent to the operating pressure in the ms which is about 10 kPa (10 torr). Essential interface features include the capability to transmit the maximum amount of sample from the gc without losses from condensation or active sites promoting decomposition no restrictions or compromises placed on either the ms or the gc with regard to resolution of the components and reliability. The interface should also be mechanically simple and as low in cost as possible. 

Probably the best compromise for silica based stationary phases is to use corrected retention volume data for solutes eluted at a (k ) of greater than 5 and only compare chromatographic data for solutes of approximately the same molecular size. 

The optimization of chromatographic separations can generally be seen as a compromise between speed, i.e., to produce the largest possible amount of data or substance per unit time, and resolution, i.e., to produce the highest possible quality of data or purity of substance. Obviously the goal for optimization differs according to the purpose of the separation and also between scale of operation. Therefore, different parameters are critical for different situations. Still, some basic rules for optimization may be applied. 

The use of both sub- and supercritical fluids as eluents yields mobile phases with increased diffusivity and decreased viscosity relative to liquid eluents [23]. These properties enhance chromatographic efficiency and improve resolution. Higher efficiency in SFC shifts the optimum flowrate to higher values so that analysis time can be reduced without compromising resolution [12]. The low viscosity of the eluent also reduces the pressure-drop across the chromatographic column and facilitates the... 

A compromise between coloration and economics in commercial ionic liquid production is therefore necessary. Since chromatographic decoloration steps are known and relatively easy to perform (see Section 2.2.3), we would not expect there to be a market for a colorless ionic liquid, if the same substance can be made in a slightly colored state, but at a much lower price. 

The number of cycles that is attainable is also a function of the chromatographic peak width - the narrower the peak, then the faster the cycle rate required to define that peak accurately. The peak widths encountered in HPLC, which are relatively wide compared to GC, are such that a compromise between scan speed and sensitivity is less likely to be required. 

The optimization of preparative and even micropreparative chromatography depends on the choice of an appropriate chromatographic system (adsorbent and eluent), sample application and development mode to ensure high purity, and yield of desirable compounds isolated from the layer. For the so-called difficult separations, it is necessary to perform rechromatography by using a system with a different selectivity. But it should be taken into account that achievement of satisfactory results frequently depends on a compromise between yield and the purity of the mixture component that is being isolated. 

The MRM experiments do not require chromatographic separation of the metabolites. Therefore, other LC conditions, columns, gradient, and injection volumes may be used provided that there is adequate sensitivity and specificity, and the chromatographic quality is not compromised. 

The most common types of preparative-scale gas chromatographic instruments are based on pacXed column technology [489-491]. The primary objective in preparative-scale gas chromatography is to obtain a high sample throughput. An inevitable result of this goal is that either resolution or separation time, or both, must be compromised. The primary method... 

Both flow-cell and solvent elimination SFC-FTTR are useful, in particular for thermolabile components. This hyphenated technique requires a compromise between chromatographic and spectroscopic requirements. Its use... 

In both cases, either conventional FTIR transmission or diffuse reflection detection may be used. Because TLC and the postspectroscopic evaluation are not linked directly, few compromises have to be made with regard to the choice of the solvent system employed for separation. Chromatographic selectivity and efficiency are not influenced by the needs of the detector. The TLC plate allows the separation to be made in a different site from the laboratory where the separated analytes are evaluated. The fact that the sample is static on the plate, rather than moving with the flow of a mobile phase, also puts less demand on the spectrometer. The popularity of TLC-IR derives in part from its low cost. 

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

An ideal SPE cartridge should have enough capacity and retain sufficient analytes to achieve good recovery while providing good adsorption so that chromatographic separation is not compromised. Other modifications such as online dilution may be needed to offset certain disadvantages. 

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