Electrolyte exclusion effect

Errors observed in the use of ISEs fall into three categories. First are errors caused by lack of selectivity. For instance, many CE electrodes lack selectivity against other halide ions. Second are errors introduced by repeated protein coating of the ion-sensitive membrane, or by contamination of the membrane or salt bridge by ions that compete or react with the selected ion and thus alter electrode response. These necessitate periodic changes of the membrane as part of routine maintenance. Finally, the electrolyte exclusion effect, which applies only to indirect methods and is caused by the solvent-displacing effect of lipid and protein in the sample, results in falsely decreased values (see the section on the electrolyte exclusion effect later in this chapter). 

Spectrophotometric methods fall into three categories those based on enzyme activation, those that detect the spectral shift produced when either Na or binds to a macrocyclic chroraophore, and fluorescent sensors. Both approaches have been applied to smaller automated instruments. However, the high cost of reagents for these methods and the fact that few problems exist with ISE methods has resulted in small niche use of these methods, primarily in some smaller instruments used in physicians offices or clinics. 

Kinetic spectrophotometric assays for Na are based on activation of the enzyme p-galactosidase by Na to hydrolyze o-nitrophenyl-[3-D-galactopyranoside (ONPG). The rate of production of o-nitrophenol (the chromophore) is measured at 420 nm. 

K -specific enzyme activation assays are illustrated by methods using tryptophanase, one of a number of K -enhance.d enzymes. Accuracy and, precision of the method have been reported to compare favorably with those achieved with flame photometry. Bilirubin and hemoglobin and other cations are said to have little effect as inter-ferants, but lipemic samples could not be analyzed.  

Macrocyclic ionophores are molecules whose atoms are organized to form a cavity into which metal ions fit and bind with high affinity. Such compounds are also called polycyclic ethers, crown ethers, cryptands, or cryptahemispherands. Different macrocyclics can be made with cavities tailored to fit the ionic radii of different elements. When chromogenic properties are imparted to these ionophores, spectral shifts 

---

Indirect ISE methods dilute the sample in a diluent of fixed high ionic strength so that for Na, the activity coefficient approaches a value of 1. Under these circumstances, the measurement of activity, a (where a = y x concentration, and y is the activity coefficient), is tantamount to measurement of concentration. Flame photometry measures emission of a specific ion after dilution in solutions of high reference ion concentration so that specific emission is also tantamount to the measurement of concentration of the specific ion in total plasma volume. It is the dilution of total plasma volume and the assumption that plasma water volume is constant that render both indirect ISE and flame photometry methods equally subject to the electrolyte exclusion effect. In certain settings, such as ketoacidosis with severe hyperlipidemia or multiple myeloma with severe hyperproteinemia, the negative exclusion effect may be so large that laboratory results lead clinicians to believe that electrolyte concentrations are normal or low when, in fact, the concentration in the water phase may be high or normal, respectively. 

The most common causes of the electrolyte exclusion effect leading to pseudohyponatremia or pseudohypokalemia are hyperlipidemia and hyperproteinemia. In severe hypoproteinemia, the effect works in reverse, resulting in a falsely high (2% to 4%) Na" or value. Several approaches have been proposed to improve the physiological accuracy of electrolyte values determined by methods... 

The 9mOsmol/kg added to the above equation represents the contribution of other osmoticaUy active substances in plasma, such as K", Ca " ", and proteins, and 1.86 is two times the osmotic coefficient of Na, reflecting the contributions of both Na and CT. The reference interval for plasma osmolality is 275 to 300mOsmol/kg. Comparison of measured osmolality with calculated osmolality can help identify the presence of an osmolal gap, which can be important in determining the presence of exogenous osmotic substances. Comparison of calculated and measured osmolalities can also confirm or rule out suspected pseudohyponatremia caused by the previously discussed electrolyte exclusion effect. 

If the measured Na concentration in plasma is decreased, but measured plasma osmolahty, glucose, and urea are normal, the only explanation is pseudohyponatremia caused by the electrolyte exclusion effect see Chapter 27). This occurs when Na" is measured by either flame emission spectrophotometry or by an indirect ion-selective electrode in patients with severe hyperlipidemia or in states of hyperproteinemia (e.g., paraproteinemia of multiple myeloma). 

The PVA/PSSNa membranes evidence a high permselectivity, comparable with the one of commercial ion exchange membrane as it can see in table 14, where were presented the permeability coefficient (P) and the ratio P to D (diffusion coefficient) that express the effect of porosity and of the electrolyte exclusion. 

A lattice model for an electrolyte solution is proposed, which assumes that the hydrated ion occupies ti (i = 1, 2) sites on a water lattice. A lattice site is available to an ion i only if it is free (it is occupied by a water molecule, which does not hydrate an ion) and has also at least (i, - 1) first-neighbors free. The model accounts for the correlations between the probabilities of occupancy of adjacent sites and is used to calculate the excluded volume (lattice site exclusion) effect on the double layer interactions. It is shown that at high surface potentials the thickness of the double layer generated near a charged surface is increased, when compared to that predicted by the Poisson-Boltzmann treatment. However, at low surface potentials, the diffuse double layer can be slightly compressed, if the hydrated co-ions are larger than the hydrated counterions. The finite sizes of the ions can lead to either an increase or even a small decrease of the double layer repulsion. The effect can be strongly dependent on the hydration numbers of the two species of ions. 

Another mechanism for the hydration repulsion between lipid bilayers was more recently proposed by Marcelja.22 It is based on the fact that in water the ions are hydrated and hence occupy a larger volume. The volume exclusion effects ofthe ions are important corrections to the Poisson— Boltzmann equation and modify substantially the doublelayer interaction at low separation distances. The same conclusion was reached earlier by Ruckenstein and Schiby,28 and there is little doubt that the hydration of individual ions modifies the double-layer interaction, providing an excess repulsion force.28 However, while the hydration of ions affects the double-layer interactions, the hydration repulsion is strong even in the absence of an electrolyte, or double-layer repulsion. 

Spheres in Electrolyte Solutions Induced Dipole and Counterion Exclusion Effects. 

The ion exclusion effect has also been observed on silica gel [ref. 45-48], which appears to be due to silanol groups [ref. 51]. Rinaudo and his coworkers [ref. 46-48] have investigated the exclusion chromatographic behaviour of simple electrolytes and polyelectrolytes on Spherosil (a silica gel), and discussed the dependence of the elution volumes of the sample electrolytes as a function of the sample concentration and the ionic strength of the eluent. The elution volume of a sample electrolyte, V, is dependent not only on the concentration of the electrolyte injected, c, but the sample volume, V. In addition, the sample concentration in the column effluent is no longer the same as that injected, because of the sample band broadening in the elution process. To correlate the different series of experiments, they have thus proposed to plot the dependence of average salt concentration, c = VC /V (v is the volume of solution in... 

The eluent counterion and coion effects contribute the distribution coefficient of sample ion in the opposite direction. In general, the greater the of the eluent anion, the greater the of the sample cation is (counterion effect), while that of the sample anion is smaller (coion effect). Similar counterion and coion effects are also exhibited by the eluent cation. This background electrolyte effect, which appears to be essentially independent of the type of gel materials, suggests that the distribution of ionic solutes between the internal gel phase and the external liquid phase is governed by a mechanism which does not involve the steric exclusion effect as a main factor. 

In their paper concerning electrolyte effects in aqueous exclusion chromatography of inorganic salts, Neddermeyer and Rogers [ref. 12] rationalized the phenomena observed by invoking the existence of a Donnan diffusion [ref. 78], involving an internal volume of the gel penetrable to some, but not all, ionic solutes in solution. The peak of the eluent salt was attributed to the Donnan exclusion effect, on the assumption that the sample ion and the eluent coion were able to penetrate into the gel interior to different degrees. 

---