Lipophilic Analytes Research Articles

Intralipid as a matrix additive for evaluating hyperlipidemic postmortem blood.

Postmortem whole blood samples can differ greatly in quality where hyperlipemia is a frequent variable that can influence the results of analytical methods. The aim of this study was to investigate the influence of lipemia on postmortem analysis as well as demonstrate the usage of Intralipid in comparison to pooled postmortem lipids as matrix additives for meaningful evaluation and validation of hyperlipidemic postmortem samples. Hyperlipidemic blood samples were simulated by adding different concentrations of Intralipid or pooled authentic postmortem lipids to bovine whole blood. The hyperlipidemic blood samples were spiked with 14 benzodiazepines and five sedative and antianxiety drugs (alprazolam, clonazepam, 7-aminoclonazepam, diazepam, flunitrazepam, 7-aminoflunitrazepam, hydroxyzine, lorazepam, midazolam, nitrazepam, 7-aminonitrazepam, nordazepam, oxazepam, propiomazine, dihydropropiomazine, temazepam, triazolam, zolpidem and zopiclone). Samples were prepared with liquid-liquid extraction followed by ultra-high performance liquid chromatography-mass spectrometry. The effects of lipemia on the recovery of analytes and internal standards (ISs) were evaluated to determine the effect of, and any differences between, the two additives. Lipemia was found to cause major interference when quantifying the analytes. For most analytes, the ISs could compensate for analyte losses. However, the most hydrophilic analytes (7-amino metabolites), together with the most lipophilic analytes (propiomazine and dihydropropiomazine), were greatly affected by lipemia (<50% recovery), and the IS could not compensate for analyte losses. In general, lower analyte recoveries were observed for samples with Intralipid as a lipemic additive in comparison to those containing pooled postmortem lipids. Both Intralipid and pooled postmortem lipids showed marked effects on the analytical results. Intralipid gave a good indication of the effects of lipemia and could be a useful tool for making a meaningful evaluation of hyperlipidemic postmortem samples during the method development and validation.

Polymeric membrane-modified voltammetric sensors for lipophilic analytes with nanomolar detection limit: Key parameters influencing the response characteristics

Thin plasticized PVC membrane-coated glassy carbon working electrodes have been used for the voltammetric measurement of highly lipophilic, electroactive drugs. Compared to conventional working electrodes, these membrane-coated electrodes exhibit remarkable detection limit and selectivity and are less prone to electrode fouling. The unique performance characteristics of these sensors are related to the large partition coefficient of the analyte in the membrane coating where it is oxidized in a non-aqueous membrane phase. To analyze the influence of the key parameters of the response of membrane-coated sensors, we derived theoretical expressions on the voltammetric response of the sensors. In our analysis we considered 1) the partition coefficient (Pmw) of the analyte between the aqueous sample and the organic membrane, 2) the membrane volume to sample volume ratio (Vm/Vw), and 3) the binding constant of constituents in the sample that bind the analyte (K). The results of our theoretical analysis have been tested through voltammetric measurement of highly lipophilic analytes with logPow values (logarithm of the partition coefficient between octanol and water) ranging between 0.3 and 7.5. By understanding of the influence of the sensor design parameters on the overall sensor response, these parameters can be tuned for optimized response slope, detection limit, etc., for solving specific analytical tasks.

Nanofiber polymers as novel sorbents for on-line solid phase extraction in chromatographic system: A comparison with monolithic reversed phase C18 sorbent

A novel application of nanofiber polymers in the wide area of a sample preparation techniques known as solid phase extraction has been studied. We demonstrated application of nanofibers as sorbents for use in a system including on-line extraction coupled with column-switching high performance liquid chromatography. Four types of nanofibers including polyamide 6 with two different surface densities, poly(ε-caprolactone), and polystyrene were tested. We found that three of them were very efficient extraction sorbents sufficiently stable for application in the on-line system. Our results confirmed that the extraction efficiency of the nanofibers depended on the type and chemistry of the spun polymer as well as on the fabrication process of the nanofibrous mats that affected their density, structure, surface density, and mechanical functionality. We also compared performance of all four nanofibers with a conventional monolithic reversed-phase C18 sorbent in terms of extraction efficiency using on-line solid-phase extraction-HPLC system. Polyamide 6 was found to be the best sorbent for lipophilic analytes with a retention and extraction efficiency for the target analytes comparable with that of the C18 monolith.

Cortisol extraction through human skin by reverse iontophoresis

Continuous monitoring of cortisol at the surface of the skin would advance the diagnosis and treatment of cortisol-related diseases, or of elevated cortisol levels related to stress in otherwise healthy populations. Reliable and accurate detection of cortisol at the skin surface remains a limiting factor in real-time monitoring of cortisol. To address this limitation, cortisol extraction through excised human skin by reverse iontophoresis was studied in vitro in side-by-side diffusion cells using a radiolabeled probe. The skin was subjected to four direct current regimens (0, 28, 56, 113μAcm−2) with the anode in the donor chamber and the cumulative cortisol concentrations recorded in the receiver chamber. The 56 and 113μAcm−2 regimens significantly increased transport of 3H-cortisol through the skin, and current density correlated directly with transcutaneous transport of 3H-cortisol. The threshold of detection of electroosmotic versus passive diffusion of cortisol through the skin was between 28 and 56μAcm−2. The results of this study are significant in examining how lipophilic analytes found in the bloodstream respond to reverse iontophoresis across the skin. In addition, a device integration technique is presented which illustrates how continuous cortisol extraction and sensing could potentially be achieved in a conventional wearable format.

Capillary Electrophoresis with Organic Solvents in Pharmaceutical Analysis: A Systematic Guide through the Background

Capillary electrophoresis (CE) with organic solvents has gained special interest in pharmaceutical analysis as it offers an alternative to overcome problems with the solubility especially of lipophilic analytes in aqueous solutions. However, beside the solubility of the compounds (typical pharmaceuticals are weak acids and bases, that can be present in ionic and molecular form as well) other properties and variables relevant for capillary electrophoretic separations are influenced by the solvent. In the present paper the role of the solvent on these quantities is discussed from a more general point of view. This discussion includes the influence on the ionic mobility and its different concentration dependence for solvents like methanol, acetonitrile, propylene carbonate, formamide, N,N-dimethylformamide, N,N-dimethylacetamide and nitromethane in comparison with water. It also includes the explanation of the partially extreme change in the pK values of the weak acids and bases when water is replaced by the organic solvent, by taking into account the stabilization of the individual charged and uncharged particles involved in the protolysis equilibria. For a better comprehension of the role of the solvent on the resolution of the analytes, also its influence on the separation efficiency has to be elucidated. For this purpose, the function of the solvent concerning the inevitable peak broadening due to diffusion and the limitations of the achievable plate number is explicated based on a sound background. As the understanding of the effect of the solvent on the relevant properties of the analytes and the constituents of the background electrolyte, and on the properties of the separation system is a prerequisite for a systematic planning of the nonaqueous CE experiment and the critical evaluation of the result obtained, the present paper tries to contribute to this goal. Keywords: Capillary electrophoresis, organic solvents, nonaqueous, NACE, pharmaceutical, solubility, migration, dispersion, efficiency, plate number, analysis time.

On‐stage liquid‐phase lipid microextraction coupled to nanospray mass spectrometry for detailed, nano‐scale lipid analysis

Developments in instrumentation aimed at microscopic sampling have led to an emphasis on applications analyzing small volumes and molecular concentrations within biological, chemical, and industrial samples. Simultaneous improvements in the sensitivity and versatility of nanospray mass spectrometers have made it possible to directly couple these sampling and analysis processes. We developed a versatile liquid-phase lipid microextraction (LPME) technique for nanoliter to microliter volumes that is amenable to direct nanospray mass spectrometry (NMS). Lipophilic analytes within several types of biological samples were extracted and analyzed by partitioning and concentrating the analytes based on their solubility within two immiscible or partially miscible liquid phases. The utility of LPME-NMS is demonstrated by extracting and analyzing molecules in four different types of applications: (1) visualization of an extracted neutral lipid-specific fluorescent dye from an aqueous solvent; (2) identification of controlled acid-catalyzed hydrolysis of triacylglycerols within nanospray capillaries; (3) reproducible sampling of a fatty acid emulsion; and (4) profiling of diverse lipids in a complex biological matrix of rabbit serum. The modified instrumentation of a multi-port, on-stage bioworkstation shows considerable versatility by combining nanomanipulation, microextraction and direct NMS for a variety of chemical, biological, industrial, and clinical applications.

Effect of Tetrabutylammonium Cation on Solid-Phase Analytical Derivatization as a Function of Analyte Lipophilicity

Analytical derivatizations (AD) can increase the sensitivity of analyses—including those with mass spectrometric detection—by as much as three orders of magnitude. The extra steps required, however, are a possible impediment to their use. To simplify AD we investigated solid-phase analytical derivatization (SPAD) of compounds with diverse structures by using pentafluorobenzyl bromide (PFBBr) as the reagent. Model compounds were organic acids (e.g. phenols, chlorophenols and carboxylic acids) which were simultaneously extracted and derivatized from 0.1 M NaOH onto a polystyrene–divinylbenzene resin (XAD-4) as their pentafluorobenzyl (PFB) derivatives. Test analytes ranged in molecular weight from 94 for phenol to 266 for pentachlorophenol and octanol–water partition coefficients (log P) values ranged from 1.48 for phenol to 7.15 for hexadecanoic acid. Under SPAD conditions, reaction rates rapidly increased with log P, but yields for less lipophilic compounds, although precise, were unacceptably low. Use of the tetrabutylammonium (TBA) cation as a phase transfer catalyst increased the yield of compounds with low log P; but, unexpectedly, as the log P of the analyte increased, the phase transfer catalyst caused a decrease in yield. The data from this study define the log P range of compounds that require TBA for optimal yield and the log P range of compounds for which TBA compromises yield. This insight led to a simple, two-step, one-pot technique that gave high yields of the PFB derivatives for the entire range of analytes studied. SPAD first extracted/derivatized the lipophilic analytes from aqueous solution onto the solid phase. Extraction/derivatization of the polar analytes followed upon addition of TBA to the reaction mixture. The PFB ethers and esters of the entire range of analytes were then eluted from the XAD-4. The two-step procedure was faster, used less reagent and required lower temperature than comparable methods in the literature. With the two-step procedure, pentafluorobenzylation of phenols and carboxylic acids from water gave yields in excess of 88% with the exception of phenol and pentachlorophenol which were recovered in 57 and 44%, respectively. For all analytes, relative standard deviations were below 15%. The effect of matrix on yield varied between zero and a decline in recoveries of approximately 20–30% depending on the analyte and concentrations of NaCl or humic acid. In the presence of matrix components relative standard deviations remained below 20%.

Chocolate HILIC phases: development and characterization of novel saccharide-based stationary phases by applying non-enzymatic browning (Maillard reaction) on amino-modified silica surfaces

Novel saccharide-based stationary phases were developed by applying non-enzymatic browning (Maillard Reaction) on aminopropyl silica material. During this process, the reducing sugars glucose, lactose, maltose, and cellobiose served as "ligand primers". The reaction cascade using cellobiose resulted in an efficient chromatographic material which further served as our model Chocolate HILIC column. (Chocolate refers to the fact that these phases are brownish.) In this way, an amine backbone was introduced to facilitate convenient manipulation of selectivity by additional attractive or repulsive ionic solute-ligand interactions in addition to the typical HILIC retention mechanism. In total, six different test sets and five different mobile phase compositions were investigated, allowing a comprehensive evaluation of the new polar column. It became evident that, besides the so-called HILIC retention mechanism based on partition phenomena, additional adsorption mechanisms, including ionic interactions, take place. Thus, the new column is another example of a HILIC-type column characterized by mixed-modal retention increments. The glucose-modified materials exhibited the relative highest overall hydrophobicity of all grafted Chocolate HILIC columns which enabled retention of lipophilic analytes with high water content mobile phases.

Label-Free Biosensing with Lipid-Functionalized Gold Nanorods

The fabrication of a label-free mass spectrometry and optical detection-based biosensor platform for the detection of low-abundance lipophilic analytes in complex mixtures is described. The biosensor consists of a lipid layer partially tethered to the surface of a gold nanorod. The effectiveness of the biosensor is demonstrated for the label-free detection of a lipophilic drug in aqueous solution and a lipopeptide in serum.

Challenges in Urine Bioanalytical Assays: Overcoming Nonspecific Binding

Dr Allena Ji is the Director of Bioanalytical Services, XenoBiotic Laboratories, Inc., NJ, USA. She has worked in the bioanalytical field for many years and accumulated rich experience in LC-MS/MS method development, method validation and sample analysis under GLP compliance in large pharmaceutical company and contract laboratory settings. In the past 10 years, Allena worked at Pfizer (Legacy of Wyeth) and investigated many small-molecule drug candidates for their nonspecific binding in urine assays. Nonspecific binding of compounds results in a severe underestimation of the compounds' concentrations and poor precision and accuracy in urine bioanalytical assays. To overcome nonspecific binding in urine assays, Allena and her colleagues developed a series of practical approaches for urine method development. By adding an appropriate anti-adsorptive agent at its optimum concentration to the urine collection containers, the nonspecific binding can be blocked. Urine assays have much higher hurdles than plasma assays due to nonspecific binding and variability of urine pH, salt concentration, volume and solubility of drug(s) in urine. A simple and systematic approach for urine method development is emphasized in this paper. Nonspecific binding is a very serious issue in bioanalytical urine assays where a compound(s) adsorbs to the container wall. The adsorption happens frequently in urine assays because urine lacks proteins and lipids that can bind to the analytes or solubilize lipophilic analytes. Therefore, urine bioanalytical assays tend to suffer from analyte losses more often than plasma assays. In the past decade, there have been many methods described to overcome nonspecific adsorption in urine assays based on individual analyte characteristics. However, a common and simple method development approach for various analytes has not been discussed and summarized. In this article we demonstrate, discuss and summarize a common approach to urine method development with a focus on overcoming adsorption issues. The advantages and limitations of commonly used anti-adsorptive agents, such as bovine serum albumin, zwitterionic detergents such as CHAPS, sodium dodecyl benzene sulfonate, β-cyclodextrin, Tween 80 and Tween 20 are discussed.

Investigation of microdialysis sampling calibration approaches for lipophilic analytes: Doxorubicin

Microdialysis is an important sampling technique in many pharmacokinetics and pharmacological studies. Applying microdialysis to lipophilic analytes can be difficult as low extraction efficiencies are generally obtained with these types of analytes. In this investigation, the effects of applying microdialysis to the lipophilic compound, doxorubicin are discussed. Using varying concentrations of doxorubicin (DOX) from 1 to 20 μM, in vitro probe calibrations were performed by delivery, recovery and no-net flux. Any changes in the extraction efficiencies calculated were monitored through the addition of an internal standard, antipyrine. DOX was chosen as a representative lipophilic analyte because its red color could be visibly monitored on the probe. At higher concentrations the probe membrane became redder. For delivery experiments, the inlet of the probe was more highly colored than the outlet. The opposite was true for recovery experiments, in which the outlet was more highly colored than the inlet. It was observed that while antipyrine was well-behaved in these experiments, for DOX the extraction efficiency determined by recovery was not the same as the extraction efficiency determined by delivery ( p < 0.005, 0.05). It was further observed that for DOX the extraction efficiency determined by a no-net flux experiment was in good agreement with the value determined by delivery and not that determined by recovery. However, the only point in which no DOX was present in the perfusate was not on the no-net flux line. In addition, the transport of DOX across the microdialysis membrane was considerably slower than the transport of antipyrine.

Nitromethane as solvent in capillary electrophoresis

Nitromethane has several properties that make it an interesting solvent for capillary electrophoresis especially for lipophilic analytes that are not sufficiently soluble in water: freezing and boiling points are suitable for laboratory conditions, low viscosity leads to favourable electrophoretic mobilities, or an intermediate dielectric constant enables dissolution of electrolytes. In the present work we investigate the change of electrophoretically relevant analyte properties – mobilities and p K a values – in nitromethane in dependence on the most important experimental conditions determined by the background electrolyte: the ionic strength, I, and the pH. It was found that the mobility decreases with increasing ionic strength (by, e.g. up to 30% from I = 0 to 50 mmol/L) according to theory. An appropriate pH scale is established by the aid of applying different concentration ratios of a buffer acid with known p K a and its conjugate base. The mobility of the anionic analytes (from weak neutral acids) depends on the pH with the typical sigmoidal curve in accordance with theory. The p K a of neutral acids derived from these curves is shifted by as much as 14 p K units in nitromethane compared to water. Both findings confirm the agreement of the electrophoretic behaviour of the analytes with theories of electrolyte solutions. Separation of several neutral analytes was demonstrated upon formation of charged complexes due to heteroconjugation with chloride as ionic constituent of the background electrolyte.

Spacer length effect of a chiral stationary phase based on (+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid.

A new chiral stationary phase (CSP) containing 11 methylene-unit spacer was prepared by bonding (+)-(18-crown-6)-2,3,11,12-carboxylic acid to aminoundecylsilica gel. The new CSP was superior to the one containing three methylene-unit spacer in the resolution of alpha-amino acids, beta-amino acids, amines, and amino alcohols in terms of both the separation (alpha) and the resolution factors (R(S)). In the resolution of alpha-amino acids on the new CSP containing a long spacer, the retention factors (k(1)) were quite small compared to those on the CSP containing a short spacer. However, in the resolution of relatively more lipophilic beta-amino acids, amines, and amino alcohols, the retention factors (k(1)) were generally greater on the CSP containing a long spacer than on the CSP containing a short spacer. All of these resolution behaviors have been rationalized by the effective competition of the ammonium ions (R-NH(3)(+)) generated by the residual undecylamino groups of the new CSP under acidic condition with the ammonium ions (R-NH(3)(+)) of analytes for the complexation inside the cavity of the crown ether ring of the CSP and the effective lipophilic interaction between the CSP and the relatively more lipophilic analytes.

Effect of analyte lipophilicity on the resolution of α‐ and β‐amino acids on liquid chromatographic ligand exchange chiral stationary phases

AbstractSeparation of the two enantiomers of racemic α‐ and β‐amino acids on two ligand exchange chiral stationary phases (CSPs) prepared previously by covalently bonding sodium N‐((S)‐1‐hydroxymethy‐3‐methylbutyl)‐N‐undecylaminoacetate or sodium N‐((R)‐2‐hydroxy‐1‐phenylethyl)‐N‐undecylaminoacetate on silica gel was studied with variation of the organic modifier (methanol) concentration in the aqueous mobile phase. In particular, the variation of retention factors with changing organic modifier concentration in the aqueous mobile phase was found to be strongly dependent on both the analyte lipophilicity and the stationary phase lipophilicity. In general, the retention factors of relatively lipophilic analytes on relatively lipophilic CSPs tend to increase with increasing organic modifier concentration in the aqueous mobile phases while those of less lipophilic or hydrophilic analytes tend to increase. However, only highly lipophilic analytes show decreasing retention factors with increasing organic modifier concentration in the aqueous mobile phase on less lipophilic CSPs. The contrasting retention behaviors on the two CSPs were rationalized by the balance of the two competing interactions, viz. hydrophilic interaction of analytes with polar aqueous mobile phase and the lipophilic interaction of analytes with the stationary phase.

Improving microdialysis extraction efficiency of lipophilic eicosanoids

Microdialysis recovery of the lipophilic analytes prostaglandin B 2, leukotriene B 4 and C 4 was studied in vitro. Relative recovery (RR) through different commercially-available microdialysis probes for prostaglandin B 2 and leukotrienes was examined using different flow rates. The enhancing effect at different concentrations of binding agents such as α, β, γ-cyclodextrins (α, β, γ-CD) on the microdialysis RR for different eicosanoids was evaluated. Small organic molecules such as ethanol, propylene glycol and dimethyl sulfoxide (DMSO) were studied in terms of their effect on enhancing RR. Inclusion of arachidonic acid in either the perfusion fluid or the sample medium caused the microdialysis RR for these hydrophobic analytes to be increased.

Recent innovation in capillary electrokinetic chromatography with replaceable charged pseudostationary phases or additives.

Recent developments of separation of neutral analytes in capillary systems with the mobile phase driven by the electroosmotic flow (EOF) and charged additives acting as a pseudostationary phase are reviewed. As pseudostationary phases a number of additives are used. Soluble polymers, either anionic or cationic, were applied as alternatives to micelles. Monomeric charged additives are also intended to form associates with the analytes, leading to selective retention and separation in a similar way as the polymeric pseudostationary phases. Dendrimers, spherical macromolecules with highly branched chains and charged terminal groups, are successfully applied for the separation of lipophilic analytes. Polymers with covalently stabilized structures are introduced in the form of permanent micelles and are therefore insensitive to the mobile phase composition, enlarging the applicability of micellar electrokinetic capillary chromatography (MEKC).

Porous zirconia and titania as packing materials for high-performance liquid chromatography

Porous amorphous zirconia (ZrO 2) and titania (TiO 2) packings were synthesized as rigid microparticulate beads by means of a sol—gel process. Sufficient rigidity and desired mesoporosity of the ZrO 2 and TiO 2 particles were achieved only by a gel hardening process, followed by heat treatment. The mean pore diameter, p d, the specific surface area, a s, and the specific pore volume, ν p, were controlled by the heat treatment. Typical values were p d = 8 nm, a s = 80 m 2/g and ν p = 0.23 ml/g. ZrO 2- and TiO 2-based revesed-phase packings were prepared by subjecting the native materials to a specific activation process and reaction with octadecyltrimethoxysilane. Native ZrO 2 and TiO 2 of graduated polarity, tested under normal-phase conditions, yielded a solvent strength series similar to those observed with silica and alumina. Retention of lipophilic analytes in n-heptane and dichloromethane was scarcely affected by the relative water content of the mobile phase. Native ZrO 2 and TiO 2 were found to be suitable for the separation of structural isomers. From calcined ZrO 2 and TiO 2 amines were eluted with totally symmetrical peaks when water-modified dichloromethane was used as the mobile phase. The retention and selectivity of n-octadecyl-bonded ZrO 2 and TiO 2 packings were identical with those of silica-based materials, except for showing a pronounced pH stability up to pH 12 in aqueous—organic mobile phases.