Detector Concentrations

Figure 7.1. Effect of stopping the flow on two types of detector concentration and mass flow rate.

The importance of the extra column dispersion now becomes apparent, as equation (26) shows that the minimum detectable mass Increases linearly with the extra column dispersion. It Is also becomes obvious that it is of little use designing a detector for increased sensitivity (Xp) if this is achieved (as is often the case) at the expense of increased extra column dispersion (oe). Conversely, if the chromatographic system is designed to have very low extra column dispersion, a proportional reduction in the minimum detectable mass will be achieved even if the actual detector concentration sensitivity remains the same. It follows, that in the design of an optimized column for a particular analysis, the extra column dispersion will determine both the radius of the column and the mass sensitivity that will be available. 

On-line MS detectors for SEC offer great promise for serving simultaneously as an absolute detector, concentration-sensitive detector (i.e., total ion current), and composition detector, especially for oligomers and low-MW polymers. Successful developments in the use of matrix-assisted laser desorption ionization (MALDI) MS for high-MW polymers are extremely encouraging, and on-line MALDI MS should... 

The spectra were similar with the LC-SAX column in line. The minor changes in ion relative intensity when FIA spectra were compared to LC/MS spectra are not uncommon in TSP mass spectrometry. TSP spectra are concentration dependent and with the column in line detector concentration is reduced due to band broadening. Thermospray mass spectrometry exhibits limited day-to-day reproducibility of ionization efficiency and fragmentation patterns, and a dependence of ion intensity on flow rate (21). TSP spectra also are affected by pressure, temperature, and vapor composition (22.231 and apparently also the design of the TSP source (24). 

Figure 17.18 Internal Concentration Profiles of the Troger s base enantiomers measured with (a) a polarimeter and (b) a UV detector. Concentrations of each enantiomer. Solid line experimental data at port 6. Dotted line experimental data at port 5, ) Calculated profiles using the 3-layer isotherm model. K. Mihlbachler, A. Seidel-Morgenstem, G. Guiochon, AIChE J., 50 (2004) 611 (Figs. 8 and 9). Reproduced by permission of the American Institute of Chemical Engineers. 1997 AIChE. All rights reserved.

There is a large variety of analytical problems that require the separation of radioactive species by column liquid chromatography. This need arises, for instance, in activation analyses, the separation of fission products or the separation of radioactively labelled compounds. Examples of special importance come from the biosciences, where labelled molecules are used in research on metabolism. Whenever a sample contains radioactive species, it is possible to follow their elution from a separation column specifically by radiometric detection. This review outlines briefly the principles of radiometric detection and radiometric detectors, concentrating on problems that arise from the combination of separations by high-performance column liquid chromatography with radiometric detection. 

The difficulty in connecting CZE with other than on-line UV detectors is that most of the other detection systems require a certain fiow. This is generally done with a sheath flow between the outlet of the capillary and the nebulizer of the detector. Concentric nebulizers used in ICP-MS are self-aspirating and they induce a suction flow in the CE capillary, which causes dispersion. The employed sheath flow rates should not be too high because they result in dilution of the analyte ions. 

FIGURE 13.1 An eleciropherogram of 13 mobility markers run on a MegaBACE 4000. Injection voltage 8 kV, 20 s, run voltage 8 kV, 60 min, and oven temperature is 44°C. Mobilities calculated using a 60 cm long capillary, 43 cm to the detector. Concentration of each marker is 5 nM. 

D = GPC detector (concentration) A = Micraborc tube B = Microbore gd cohimn... 

Appendix 4.1 Responses of Chromatographic Detectors Concentration vs Mass-Flux Dependence... 

It is seen from equation (20) that the minimum detectable mass, or mass sensitivity of a chromatographic system, where the column has been designed to have the optimum radius for the detector employed, is directly proportional to the extra column dispersion and the detector concentration sensitivity. It follows that detector dispersion is as important as detector sensitivity in its influence on the overall chromatographic mass sensitivity where the chromatographic system has been optimized with respect to the radius of the column. The effect of extra column dispersion and in particular, detector dispersion on the overall mass sensitivity of the chromatogaphic system is not generally appreciated or completely understood. As the total extra column dispersion is the integral of a variety of sources, the distribution and nature of the various sources of dispersion will now be considered in some detail. 

Where Ac is the detector concentration sensitivity or the "Minimum Detectable Concentration" which is the parameter of importance to chromatographers. 

As SIMS measures the number of secondary ions striking the detector, concentrations are most typically reported in units of atoms per some spatial or volumetric dimension, i.e. atoms/cm (at/cm ) in Static SIMS or atoms/cm (at/cm or at/cc) in Dynamic SIMS. These can be converted into percentage concentration values if the spatial or volume density of the substrate analyzed is known ( 1 x 10 at/cm and 5 X 10 at/cm for Silicon). Likewise, weight percent values can be defined by accounting for the mass of the respective signals. 

Figure 2 Spectrum of lichen sample. Pressed pellet, measured with a standard EDXRF spectrometer, Rh tube 30 kV, 0.2 mA, 500 s, Pd filter, Si(Li) detector concentration values of respective elements given in rg g"h...

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