Electrolytic Suppression and Ion Chromatography

Electrolytic Suppression and Ion Chromatography

The conductivity detector’s response is saturated when the mobile phase has an intensive conductivity, thus, ion suppression is employed (Levin). In doing this, a suppressor is added into the system between the detector and the column. Depending on the properties of the mobile phase, the suppressor produces hydroxyl or hydronium ions to transform their corresponding ions into non-ionized state resulting to reduced conductivity (Levin). While the conductivity of the mobile phases is lessened, the conductance of the solute is improved that enhances detection (Levin). For continuous operation, the suppressor contains scavenger or regenerant (Levin). For an acidic mobile phase, the suppression is done through the cation-exchange column in the hydrogen form. For instance, the elution of the Chloride solutes by Sodium bicarbonate mobile phase can be described by the following equations (Levin):

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Resin-H+  + Na+ HCO32-                       Resin-Na+   +   H2CO3  (eluent)
Resin-H+  + Na+ Cl-                               Resin-Na+  +   HCl         (solute)

A similar process is applied to cation-exchange chromatography where the anion-exchange column is the suppressor in hydroxyl form (Levin). For example, the separation of sodium ions by means of HCl mobile phase is possible by transforming eluent into water as the conductivity of the solute is improved. This can be shown by the following equations (Levin):

Resin-OH-   +   H+ Cl-                       Resin-Cl-   +   H2 O (eluent)
Resin-OH-  +   Na+ Cl-                      Resin-Cl-  +  Na+ OH- (solute)

Conductivity Detector

Conductivity detection works best for inorganic ions since they are naturally electrical conductors, thus, the detector has a universal response (Levin). Also, detectors for conduction are much easier to construct and operate. In this mode of detection, the mobile phase, an electrolyte, passes through the detector across two electrodes with applied potential (Levin). The more current passes through the solution, the higher the conductivity becomes. While, the conductivity of the solution is largely influenced by the type of ionic species, temperature, and ionic strength, the specific conductivity is dependent on the cross-sectional area of the electrode, concentration of the solution, and the distance between electrodes (Levin). As a rule of thumb, the conductivity increases as surface area increases while distance between electrodes decreases (Levin).

Most ions have 30-100 S.cm2.eq-1 limiting equivalent ionic conductance (Levin). The conductivity of ions increases with increasing charge density and with decreasing viscosity (Weiss 3). While the hydronium ion is the most conducting cation, the hydroxyl group has the higher value of conductance in anions (Levin). In line with this, for as long as difference in the conductance between mobile phase and constituent ions exist either positive or negative, detection sensitivity is possible (Levin). This is known as direct mode of detection which is useful for the separation of anions by means of ion chromatographic methods. When the constituent ions have low ionic conductivity the conductance increases as they reached the detection cell (Weiss 3). In the same manner, the mobile phase decreases its conductance as it goes through the detection cell if it is highly conducting (Levin). This is called as the indirect conduction that is applied to anions and cations with hydroxide and mineral acid eluents respectively (Levin).

Fluorophosphate Lewis Dot Structure

In the Lewis electron dot structure of the fluorophosphate ion, the phosphorus as the less electronegative element served as the central atom where the three oxygen atoms and a single fluorine atom are bonded. While one oxygen atom exhibited a double covalent bond with the central atom, the other two are singly bonded but each has a negative net electrical charge. As all bonded atoms satisfied the octet rule, the central atom exhibited expanded octet. Also, by summing up the net electrical charges, the formal charge of the polyatomic ion is -2.

Deionized or D.I. Water

Pure water is an essential component of laboratory solutions like in the preparation of blanks, samples, standards, and other reagents (“Purified Water”). The naturally dissolved inorganic constituents like magnesium, calcium, and chlorides, and other impurities in tap water may interfere on the desired outcome of a chemical reaction. Specifically, water impurities may interact with the analyte, deter reactions in the column, shift background signals, and inhibits detection (“Purified Water”). As such, dissolved inorganic ions are controlled, removed, or reduce to tolerable level, thus, it is called “deionized or D.I.” (“Purified Water”). In connection to this, a type I reagent grade water has 18.3 M?-cm resistivity and 0.055 microsiemens conductivity (“Purified Water”). This type is suitable for chemical analysis such as in electrochemical determination of metals, Gas Chromatography-Mass Spectroscopy or GC-MS, High Performance Liquid Chromatography or HPLC, Ion Chromatography, Atomic Absorption Spectroscopy, Induced Coupled Plasma Spectroscopy, Total Organic Carbon, and Gas Chromatography-Mass Spectroscopy or GC-MS (“Purified Water”). Hence, D.I. water means that the water was deionized at a certain level while D.I. “18M?” water specifies the resistivity value of water and reflects a high degree of deionization.

Works Cited

Levin, Shulamit. “Analysis of Ions Using High Performance Liquid Chromatography: Ion-Chromatography.” 2002. Bioforum. 27 January 2009 <http://www.forumsci.co.il/HPLC/ion_chrm.html>.

“Purified Water.” 8 March 2007. Laboratory in Electrochemistry. 3 February 2009 <http://www.tau.ac.il/~advanal/PurifiedWater.htm>.

Weiss, Joachim. Handbook of Ion Chromatography, 3rd Completely Revised and Enlarged Edition. Weinheim: Wiley-VCH Verlag GmBh, 2004.

 

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