Columns background

Figure 38. Three-electrode plasmajet [225] a) Electrode h) Plume c) Cathode block d) Plasma column (background) continuum e) Argon anode block front f) Argon anode block back...

Every effort was made to standardize the sample size and mass spectrometer tuning to ensure consistent spectra throughout this collection. Where it was necessary to manipulate the spectra, e.g., to remove traces of the injection solvent or column background, due care was exercised to avoid distorting the data. Mass calibration was checked several times a day, and the inertness of the interface was demonstrated daily by the analysis of cholesterol, which produced a 386/368 ion ratio of greater than 2 1. 

Figure 14.5 Distillation column with intermediate condenser. The profile can be designed to fit the background process. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduced by permission of the Institution of Chemical Engineers.)...

The scope for integrating conventional distillation columns into an overall process is often limited. Practical constraints often prevent integration of columns with the rest of the process. If the column cannot be integrated with the rest of the process, or if the potential for integration is limited by the heat flows in the background process, then attention must be turned back to the distillation operation itself and complex arrangements considered. 

Ion-exchange columns can be substituted into the general HPLC instrument shown in Eigure 12.26. The most common detector measures the conductivity of the mobile phase as it elutes from the column. The high concentration of electrolyte in the mobile phase is a problem, however, because the mobile-phase ions dominate the conductivity, for example, if a dilute solution of HCl is used as the mobile phase, the presence of large concentrations of H3O+ and Ck produces a background conductivity that may prevent the detection of analytes eluting from the column. 

The use of an amperometric detector is emphasized in this experiment. Hydrodynamic voltammetry (see Chapter 11) is first performed to identify a potential for the oxidation of 4-aminophenol without an appreciable background current due to the oxidation of the mobile phase. The separation is then carried out using a Cjg column and a mobile phase of 50% v/v pH 5, 20 mM acetate buffer with 0.02 M MgCl2, and 50% v/v methanol. The analysis is easily extended to a mixture of 4-aminophenol, ascorbic acid, and catechol, and to the use of a UV detector. 

Equipment Absorption, stripping, and distiUation operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed. The gas and liquid normally flow countercurrently, and the devices serve to provide the contacting and development of interfacial surface through which mass transfer takes place. Background material on this mass transfer process is given in Sec.. 5. 

An additional limit to the size of a passive array relates to the current which flows in an OLED when it is under reverse bias [189]. When a given pixel is turned on in the array, there are many possible parallel paths for the current, each involving two diodes in reverse bias and one forward. Hence, as the number of rows and columns increases, there is a higher level of background emission from non-selected pixels that limits the contrast ratio of the array. As a result, the contrast degrades as N increases. 

In IC this problem of electrolyte background is overcome by means of eluant suppression. Thus in the above example of sodium and potassium analysis, if the effluent from the separating column is passed through a strong base anion exchange resin in the hydroxide form (suppressor column) the following two processes occur ... 

Suppressor column. Where electrical conductance is used for detection of sample ions in the effluent from the columns, an eluant background of low conductivity is required. The function of the suppressor column is to convert eluant ions... 

Because the vacuum in the mass spectrometer and the cleanliness of the ion source, transfer line, GC column, and so forth are not perfect, a mass spectrum will typically have several peaks that are due to background. All GC/MS spectra, if scanned to low enough mass values, will have peaks associated with air, water, and the carrier gas. Other ions that are observed in GC/MS are associated with column bleed and column contamination. 

Bleeding An appearance of a background signal from a chromatographic system, caused by the stationary phase or contamination of the inlet system. The column bleed usually increases with increasing column temperature. 

Every column (including chemically bonded columns) will have some column bleed. The amount of column bleed will increase with increasing column temperature, film thickness, column diameter, and column length. The base line starts to rise approximately 25-50° below the upper temperature limit of the stationary phase. After a column is installed in a GC/MS system, a background spectrum should be obtained for future reference. 

In contrast to conventional FAB where the analyte is dissolved in the matrix material, it has been found that FAB performance can be obtained when the mobile phase contains as little as 5% of the matrix material, thus reducing the chemical background associated with the technique. It should be noted tliat if the matrix material is added before the column it may have an effect on the separation achieved. 

Ions at m/z 55, 60, 214 and 236 are observed but do some or all of these arise from the background and are present throughout the analysis, or are they present in only a few scans, i.e. are they from a component with insufficient overall intensity to appear as a discrete peak in the TIC trace An examination of reconstructed ion chromatograms (RICs) from these ions generated by the data system may enable the analyst to resolve this dilemma. The TIC shows the variation, with time, of the total number of ions being detected by the mass spectrometer, while an RIC shows the variation, with time, of a single ion with a chosen m/z value. The RICs for the four ions noted above are shown in Figure 3.15. These ions have similar profiles and show a reduction in intensity as analytes elute from the column. The reduction in intensity is a suppression effect. 

What effect will the background have on the analysis While the background, in isolation, may look to be significant, its real importance can only be determined by examination of the mass spectrum of a compound as it elutes from the HPLC column. The mass spectrum recorded at the TIC maximum after 4.65 min is shown in Figure 3.16. 

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