Hydrophilic interaction matrix effects

Hydrophilic interaction liquid chromatography (HILIC) can handle the matrix effects nonvolatiles, which may reduce the evaporation of volatile ions during the electrospray process. Ion-exchange methods that require the use of salts could otherwise interfere with MS. 

A selective, sensitive, and rapid hydrophilic interaction liquid chromatography with electrospray ionization tandem mass spectrometry was developed for the determination of donepezil in human plasma [32], Donepezil was twice extracted from human plasma using methyl-ferf-butyl ether at basic pH. The analytes were separated on an Atlantis HILIC Silica column with the mobile phase of acetonitrile ammonium formate (50 mM, pH 4.0) (85 15, v/v) and detected by tandem mass spectrometry in the selective reaction monitoring mode. The calibration curve was linear (r = 0.9994) over the concentration range of 0.10-50.0 ng/ ml and the lower limit of quantification was 0.1 ng/ml using 200 /d plasma sample. The CV and relative error for intra- and inter-assay at four quality control levels were 2.7% to 10.5% and —10.0% to 0.0%, respectively. There was no matrix effect for donepezil and cisapride. The present method was successfully applied to the pharmacokinetic study of donepezil after oral dose of donepezil hydrochloride (10 mg tablet) to male healthy volunteers. 

Solvent selection depends largely on the nature of the analytes and the matrix. Although the discussions in Chapter 2 can be used as a guideline to account for the solvent-analyte interactions, the matrix effects are often unpredictable. There is no single solvent that works universally for all analytes and all matrices. Sometimes, a mixture of water-miscible solvents (such as acetone) with nonmiscible ones (such as hexane or methylene chloride) are used. The water-miscible solvents can penetrate the layer of moisture on the surface of the solid particles, facilitating the extraction of hydrophilic organics. The hydrophobic solvents then extract organic compounds of like polarity. For instance, hexane is efficient in the extraction of nonpolar analytes, and methylene chloride extracts the polar ones. 

From Fig. 10, peptides were, of course, eluted from the RPC column (top) in order of increasing hydrophobicity. Under characteristic cation-exchange conditions (middle) [the presence of 10% (v/v) acetonitrile helps to eliminate unwanted hydrophobic interactions between solutes and the column matrix (see earlier)], the four peptides were very poorly resolved, as expected given the identical net charge on the peptides. Interestingly, the low concentration of acetonitrile (10%) has already induced hydrophilic interactions with the matrix in that the elution order is already opposite to that of RPC (the most hydrophobic peptide was eluted first and the most hydrophilic last). In contrast, under HILIC/CEC conditions (bottom), the elution order remains the same but the peptides are now well resolved. Clearly, to effect a separation of these peptides on the cation-exchange column, an increased concentration (80%, v/v) of acetonitrile was required in the mobile phase in order to promote hydrophilic interactions with the column matrix to complement the ionic interactions. 

Acronyms d SPE dispersive solid-phase extraction HILIC hydrophilic interaction liquid chromatography IT ion trap LLE liquid—liquid extraction MSPD matrix solid-phase dispersion PGC porous graphitized carbon PLE pressurized-liquid extraction PHWE pressurized hot-water extraction QqQ triple quadrupole QLIT quadrupole linear ion trap QuEChERS quick, easy, cheap, effective, rugged, and safe TCA trichloroacetic acid UHPLC ultraperformance liquid chromatography ZIC ttILIC zwitterionic hydrophilic-interaction liquid chromatography. 

Extensions of BCS beyond the oral IR area has also been suggested, for example to apply BCS in the extended-release area. However, this will provide a major challenge since the release from different formulations will interact in different ways with in vitro test conditions and the physiological milieu in the gastrointestinal tract. For example, the plasma concentration-time profile differed for two felodipine ER tablets for which very similar in vitro profiles had been obtained, despite the fact that both tablets were of the hydrophilic matrix type based on cellulose derivates [70], This misleading result in vitro was due to interactions between the gel strength of the matrix and components in the dissolution test medium of no in vivo relevance. The situation for ER formulations would be further complicated by the need to predict potential food effects on the drug release in vivo. 

The aim of this paper is to demonstrate that, depending on the type of silica (hydrophilic or hydrophobic), different kinds of colloidal interaction forces are effective, to show how these forces are influenced by the composition of the liquid matrix, and finally to elucidate the influence of the colloidal forces on the rheological properties of silica-resin dispersions. For this purpose, experimental results obtained from steady-shear and dynamic rheological experiments together with theoretical calculations will be presented. 

The behavior of immobilized enzymes differs from that of dissolved enzymes because of the effects of the support material, or matrix, as well as conformational changes in the enzyme that result from interactions with the support and covalent modification of amino acid residues. Properties observed to change significantly upon immobilization include specific activity, pH optimum, Km, selectivity, and stability.23 Physical immobilization methods, especially entrapment and encapsulation, yield less dramatic changes in an enzyme s catalytic behavior than chemical immobilization methods or adsorption. The reason is that entrapment and encapsulation result in the enzyme remaining essentially in its native conformation, in a hydrophilic environment, with no covalent modification. 

CFractionation of aquatic humic substances by ion-exchance mechanisms s been limited severely by undesirable matrix interactions of the exchange medium. Hydrophobic matrix-exchange resins also interact with aquatic humic substances by hydrophobic effects (Abrams and Breslin, 1965) and hydrophilic matrix-exchange gels also interact with polyfunctional solutes by... 

Penicillin G acylase (PGA) has pivotal role in industry for the synthesis of penicillin antibiotics. PGA catalyzes the hydrolysis of peniciUm and other P-lactam antibiotics to produce 6-amino penicillinic acid [53, 54]. Stability of PGA was investigated by assaying the enzyme activity at different time points after incubation of PGA in various ILs [53]. In the absence of substrate, about 2,000-fold increase in t, was observed in a hydrophobic IL, [EMIM][Tf2N] with respect to twpropanol. Whereas in the presence of substrate, PGA showed less stability in [Tf N]" containing ILs. The reverse trend was found for PGA in a water miscible IL [BMIM][PFg], i.e., PGA showed 9 times increase in tj in [BMIM][PF ] in the presence of substrate even at an elevated temperature of 40°C [54]. These altered t, values in hydrophobic ILs in the presence of substrates were explained on the basis of cumulative outcome of specific interaction of substrates with the active site of enzyme and inhibitory effect of the hydrolytic products formed in the reaction medium. It can be speculated that in a more hydrophilic IL like [BMIM][PF ], substrates shield the enzyme active site from direct interaction with the ionic matrix and thus imparts a stabilizing effect whereas in hydrophobic IL [EMIM][TfjN], the inhibitory effect of hydrolytic products predominates and destabilizes the enzyme. 

Both of the types of polymer mentioned above can be modified by the incorporation of hydrophobic monomers onto the essentially hydrophilic acrylate backbone. The effect of this is to modify their characteristics by giving them so-called associative properties. These hydrophobes can interact or associate with other hydrophobes in the formulation (e.g., surfactants, oils, or hydrophobic particles) and thus build additional structures in the matrix [3-11]. These associative polymers are termed cross-polymers when they are based on carbomer-type chemistry [12] and hydrophobically modified alkali-soluble emulsions (HASEs) when based on ASE technology. 

The resemblance of the under-water stress relaxation curves and the dissimilarity of the stress relaxation behavior of the Nafion acid and the salt in the dry state may be explained as follows. The phase separated hydrophilic regions are expected to contain a substantial fraction of the ether side chains which are anchored in the ionic domains by their polar end groups. In the dry state, the coulombic interactions within the ionic aggregates are so strong that these domains probably serve as effective crosslinks. This would not only reduce the mobility of molecules within the domains but would also control the mobility of the fluorocarbon matrix through the side chains this, in turn, leads to the rise in the primary relaxation temperature. 

---