Conductivity background

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. 

Metal Detection (MD) MD measures instrument responses to deposits of ferrous and nonferrous metals up to 10 to 20 ft deep. Detection of high-density deposits in shallow depths. Good inexpensive preliminary survey tool. Background conductivities greater than 40 millimhos/meter impairs results. Wet clay soils impair resluts. 

The concentration of electrolytes which do not take part in the titration reaction should be small in order to keep the background conductance of the solution sufficiently low. 

Flow through conductance cells are useful detectors in ion-exchange chromatographic separations where the analytes are ionic when they enter the detector cell. At the low concentrations encountered, conductivity is proportional to the mobility of the ions involved as well as their concentration, and this, together with the background conductivity of the solvent, demands that standards are used. Conductivity increases with an increase in temperature and it is important in such measurements that temperature is monitored and appropriate corrections made when calculating the results. 

On the contrary, eluents with high background conductance (e.g., NaOH) are amenable for indirect conductivity detection, where analyte elution is accompanied by a negative conductance change [247]. 

Suppressed conductivity detection is the most common mode of detection and differs from the previous approach for the use of an additional device, called suppressor, whose function is to reduce the background conductivity of the eluent prior to the conductivity cell and to increase the signal of the analyte. 

Conductivity detector is the most common and useful detector in ion exchange chromatography. However UV and other detectors can also be useful [10]. Conductivity detection gives excellent sensitivity when the conductance of the eluted solute ion is measured in an eluent of low background conductance. Therefore when conductivity detection is used dilute eluents should be preferred and in order for such eluents, to act as effective competing ions, the ion exchange capacity of the column should be low [1]. 

Figure 26-27 Irregular peak shapes arise when the conductivity of the analyte band, kq, is not the same as the background conductivity, Kb.

A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The A1 electrode (500 nm Al/100 nm Ti) was deposited in a 600-nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with Si3 N4 (160 nm). To avoid gas bubble formation after dielectric breakdown, the electric field for separation was limited to 50 V/cm [145]. This four-electrode configuration allows for sensitive detection at different background conductivities without the need of adjusting the measurement frequency [328]. 

A simpler and technologically superior approach is the measurement of the direct electrical conductance. The background conductivity of the mobile phase is electronically subtracted, not requiring a suppressor device. One example of direct conductivity detection is the simultaneous determination of potassium nitrate and sodium monofluorophosphate in dentrifices [76]. Alendronate, a bisphonate, can be directly detected in intravenous solutions and tablets using an anion-exchange column and conductivity detection [77]. Another example, from one of the author s (JA) laboratory is shown in Figure 5.3. Direct conductivity detection makes it possible to selectively detect choline in the presence of an equal molar amount of an antibiotic which is not detected. 

The first approach uses a suppressor device which is located between the analytical column and the detector cell. This device chemically removes the mobile-phase buffer counterions, thus reducing the background conductivity. This type of detector increases postcolumn dead volume and puts... 

Although the ionic strength of the eluant may remain the same throughout the run, the background conductivity can decrease due to the changing dielectric constant. These baseline changes can be compensated either by chemical means or by computer baseline subtraction. Often, mobile phase ion chromatography is used to elute ions which are very... 

Okada [1] has described a redox suppression for the ion exclusion chromatography of carboxylic acids with conductiometric detection. The reaction between hydriodic acid (the eluent) and hydrogen peroxide (the precolumn reagent) is used as the redox suppressor for ion exclusion chromatography of carboxylic acids. The suppressor is useful with highly acidic eluents and reduces background conductance more effectively than a conventional ion exchange suppressor. 

Interestingly, in addition to mitochondrial membrane differences, the properties of the plasma membranes in ectotherms compared to endotherms show similar differences in degrees of unsaturation of the phospholipid constituents. The higher proportion of polyunsaturated fatty acyl chains in endotherms in turn correlates with three- to four-fold higher Na+ and K+ background conductance and hence-higher... 

Another method widely used is based on conductometric measurements at short times after an irradiation pulse. This method determines the electrical charge of the species studied and is thus useful for radicals where several dissociations can take place. This technique can be complicated by buffering effects if the parent compound itself undergoes acid-base equilibria in the region of interest for the study of the radical (see e.g. Bhatia and Schuler, 1973b). It is therefore, a prerequisite to know all the other p/ -values which may be involved. Another limitation of this technique lies in the fact that only p f-values between about 2 and 12 can be studied and high background conductivity decreases the sensitivity of the measurements. 

An electrochemical detector is destructive. It requires (1) a working electrode (where oxidation or reduction takes place), (2) an auxiliary electrode, and (3) a reference electrode (to regulate voltage and compensate for changes in background conductivity of the eluent). When an active substance flows into the electrochemical cell and a potential difference is applied between the working and reference electrodes, the electrolysis of the analyte yields a current (detector signal) that is a function of the applied potential. The three steps in the process are ... 

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