Analyte-matrix Interactions Research Articles

Perspective on sample preparation fundamentals

The primary objective of this contribution is to offer insights into fundamentals associated with typical sample preparation, facilitating rational design of new analytical technologies and effective optimization of existing techniques. Sample preparation stands a pivotal stage in the analytical process. Unfortunately, the optimization of associated parameters often relies on trial and error rather than systematic scientific methodologies. If an extraction method provides good recovery of spikes of standards, it is assumed that it works well and no further consideration is given to the underlying principles driving its performance. In the realm of sample preparation, the fundamentals of method optimization are not accorded the same significance as in other technologies, such as chromatography or mass spectrometry. Consequently, the fundamentals of sample preparation are typically overlooked in analytical chemistry curricula.A notable impediment to progress in the sample preparation area is underdeveloped understanding of the fundamentals of extraction, particularly when dealing with natural, often complex samples, where native analyte-matrix interaction is different compared to spikes. This stands in contrast to the physiochemically simpler systems employed in separation and quantification steps, such as chromatography and mass spectrometry.A careful consideration of the underlying principles of sampling and sample preparation can lead to the creation of more efficient and environmentally friendly technologies. Embracing these principles aligns with global shift towards more sustainable future, challenging the perception that sample preparation is solely an artistic endeavour and highlighting its potential as a scientifically grounded discipline.

Identification and quantification of ciprofloxacin, enrofloxacin and difloxacin residues in milk using excitation-emission fluorescence and second-order standard addition methods

In the presence of non-modeled components and analyte-matrix interaction in real samples, a combination of second-order multivariate calibration and standard addition is desired to achieve accurate results via second-order advantage. Enrofloxacin (ENR), Difloxacin (DIF), and Ciprofloxacin (CIP) are Fluoroquinolones widely considered antibiotic residues in milk. Excitation-emission fluorescence spectra of these antibiotics at pH = 9.1 provide second-order data for identifying and quantifying them in a mixture using second-order calibration modeling.Rank annihilation factor analysis (RAFA) and multivariate curve resolution-alternating least squares (MCR-ALS) with trilinearity constraints are applied to determine the CIP, ENR, and DIF residues in synthetic mixtures and milk using second-order excitation-emission fluorescence spectroscopy. In both methods, the trilinearity constraint guarantees a unique true solution to the spectral profiles of analytes. The combination of second-order standard addition and higher-order calibration modeling with RAFA and MCR-ALS supports the second-order advantage in milk analysis.

Hydralazine as a Versatile and Universal Matrix for High-Molecular Coverage and Dual-Polarity Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Imaging.

Few matrices have the potential to be universally applicable for imaging vast endogenous compounds ranging from micro to macromolecules. In this article, we present hydralazine (HZN) as a versatile and universal matrix for matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) of a wide range of endogenous compounds between 50.0 and 20,000.0 Da. HZN was prepared from its hydrochloride by alkalizing HZN·HCl with ammonia to enhance the optical absorptivity at the preferred MALDI UV laser wavelength. To further improve its performance for MALDI MS, HZN was doped with NH4OH or TFA, resulting in matrix superior performance for imaging biologically relevant compounds in the negative and positive-ion modes, respectively. The analyte-matrix interaction was also enhanced by the optimized matrix solvent and the deposition amount. Compared with conventional matrices such as 2,5-dihydroxybenzoic acid, α-cyano-4-hydroxycinnamic acid, and 9-aminoacridine (9-AA), the HZN matrix provided higher sensitivity, broader molecular coverage, and improved signal intensities. Its broad acquisition range makes it versatile for imaging small molecular metabolites and lipids, as well as proteins. In addition, HZN was applied successfully for the visualization of tissue-specific distributions and changes of small molecules, lipids, and proteins in the kidney and liver sections of obese ob/ob and diabetic db/db mice. The use of the HZN matrix shows great potential application in the field of pathological research.

Matrix-assisted DOSY

The analysis of mixtures by NMR spectroscopy is challenging. Diffusion-ordered NMR spectroscopy enables a pseudo-separation of species based on differences in their translational diffusion coefficients. Under the right circumstances, this is a powerful technique; however, when molecules diffuse at similar rates separation in the diffusion dimension can be poor. In addition, spectral overlap also limits resolution and can make interpretation challenging. Matrix-assisted diffusion NMR seeks to improve resolution in the diffusion dimension by utilising the differential interaction of components in the mixture with an additive to the solvent. Tuning these matrix-analyte interactions allows the diffusion resolution to be optimised. This review presents the background to matrix-assisted diffusion experiments, surveys the wide range of matrices employed, including chromatographic stationary phases, surfactants and polymers, and demonstrates the current state of the art.

Effect of Aging Organochlorine Pesticides in Various Soil Types on Their Extractability Using Selective Pressurized Liquid Extraction

In this study, extractability of 13 organochlorine pesticides aged in six different soil types, was investigated. Acceptable recoveries were achieved for most of the analytes spiked into five of the soil types. No apparent correlation was perceptible between the soil characteristics and extractability of the aged analytes, signifying exhaustiveness of the extraction. However, recovery was considerably minimized (<75%) in the clay ensuring the hypothesis of study stating “extraction efficiency might be matrix dependent”. Therefore, the study demonstrates importance of optimizing extraction conditions even for those believed to be less prone to matrix effect, due to distinct properties of analyte-matrix interactions.

3D ToF-SIMS Analysis of Peptide Incorporation into MALDI Matrix Crystals with Sub-micrometer Resolution.

The analytical sensitivity in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is largely affected by the specific analyte-matrix interaction, in particular by the possible incorporation of the analytes into crystalline MALDI matrices. Here we used time-of-flight secondary ion mass spectrometry (ToF-SIMS) to visualize the incorporation of three peptides with different hydrophobicities, bradykinin, Substance P, and vasopressin, into two classic MALDI matrices, 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA). For depth profiling, an Ar cluster ion beam was used to gradually sputter through the matrix crystals without causing significant degradation of matrix or biomolecules. A pulsed Bi3 ion cluster beam was used to image the lateral analyte distribution in the center of the sputter crater. Using this dual beam technique, the 3D distribution of the analytes and spatial segregation effects within the matrix crystals were imaged with sub-μm resolution. The technique could in the future enable matrix-enhanced (ME)-ToF-SIMS imaging of peptides in tissue slices at ultra-high resolution. Graphical Abstract ᅟ.

Development of an analytical method for the simultaneous analysis of MCPD esters and glycidyl esters in oil-based foodstuffs

Substantial progress has been recently made in the development and optimisation of analytical methods for the quantification of 2-MCPD, 3-MCPD and glycidyl esters in oils and fats, and there are a few methods currently available that allow a reliable quantification of these contaminants in bulk oils and fats. On the other hand, no standard method for the analysis of foodstuffs has yet been established. The aim of this study was the development and validation of a new method for the simultaneous quantification of 2-MCPD, 3-MCPD and glycidyl esters in oil-based food products. The developed protocol includes a first step of liquid–liquid extraction and purification of the lipophilic substances of the sample, followed by the application of a previously developed procedure based on acid transesterification, for the indirect quantification of these contaminants in oils and fats. The method validation was carried out on food products (fat-based spreads, creams, margarine, mayonnaise) manufactured in-house, in order to control the manufacturing process and account for any food matrix–analyte interactions (the sample spiking was carried out on the single components used for the formulations rather than the final products). The method showed good accuracy (the recoveries ranged from 97% to 106% for bound 3-MCPD and 2-MCPD and from 88% to 115% for bound glycidol) and sensitivity (the LOD was 0.04 and 0.05 mg kg−1 for bound MCPD and glycidol, respectively). Repeatability and reproducibility were satisfactory (RSD below 2% and 5%, respectively) for all analytes. The levels of salts and surface-active compounds in the formulation were found to have no impact on the accuracy and the other parameters of the method.

Investigation of non-specific signals in nanoporous flow-through and flow-over based sensors

Porous materials are ideal hosts to fabricate high sensitivity devices. Their large specific area and the possibility to modify the type and the strength of the matrix-analyte interactions allow the realization of sensors with finely tailored characteristics. In this article, we investigate how mass transport across the nanoporous structure influences the response due to the non-specific signal by comparing flow-through versus flow-over geometries. We observed a systematic overestimation of the sensitivity for porous substrate devices made of closed-ended pores compared with open-ended pore ones. Our analysis shows that such an effect is due to (unbound) analytes or contaminants that remain trapped within the pores and are not removed by rinsing of the sample. This result was verified by measuring similar samples in both flow through and flow over configurations, as well as their residual response after blockage of all their active sites. We also notice that sensors based on free-standing membranes show similar results independent of the fact that mass transport is induced by either an external pressure source or simply by Brownian motions.

Z‐sinapinic acid: the change of the stereochemistry of cinnamic acids as rational synthesis of a new matrix for carbohydrate MALDI‐MS analysis

Successful application of matrix-assisted laser desorption/ionization (MALDI) MS started with the introduction of efficient matrices such as cinnamic acid derivatives (i.e. 3,5-dimethoxy-4-hydroxycinnamic acid, SA; α-cyano-4-hydroxycinnamic acid). Since the empirical founding of these matrices, other commercial available cinnamic acids with different nature and location of substituents at benzene ring were attempted. Rational design and synthesis of new cinnamic acids have been recently described too. Because the presence of a rigid double bond in its molecule structure, cinnamic acids can exist as two different geometric isomers, the E-form and Z-form. Commercial available cinnamic acids currently used as matrices are the geometric isomers trans or E (E-cinnamic and trans-cinnamic acids). As a new rational design of MALDI matrices, Z-cinnamic acids were synthesized, and their properties as matrices were studied. Their performance was compared with that of the corresponding E-isomer and classical crystalline matrices (3,5-dihydroxybenzoic acid; norharmane) in the analysis of neutral/sulfated carbohydrates. Herein, we demonstrate the outstanding performance for Z-SA. Sulfated oligosaccharides were detected in negative ion mode, and the dissociation of sulfate groups was almost suppressed. Additionally, to better understand the quite different performance of each geometric isomer as matrix, the physical and morphological properties as well as the photochemical stability in solid state were studied. The influence of the E/Z photoisomerization of the matrix during MALDI was evaluated. Finally, molecular modeling (density functional theory study) of the optimized geometry and stereochemistry of E-cinnamic and Z-cinnamic acids revealed some factors governing the analyte-matrix interaction.

SORPTION OF POLYBROMINATED DIPHENYL ETHERS IN BIOSOLID MODEL SAMPLES

Biosolid model samples (BMSs) were developed to study the capacity of various matrixes to adsorb polybrominated diphenyl ethers (PBDEs) as a function of their composition. A commercial standard reference samples for biosolids containing a certified amount of PBDEs is currently not available for extraction evaluation. Therefore, this study will allow for the selection of an appropriate concentration of PBDEs to produce a reference material for analytes associated with the matrix. The sorption of PBDEs increased clearly concomitantly with the amount of organic matter present in the model, revealing that organic matter rather than the inorganic fraction is mainly responsible for the analyte-matrix interaction. Hydrophobic interactions are predominant in the retention of PBDEs in the biosolid matrix because more hydrophobic congeners are generally sorbed to a higher extent than those that are less hydrophobic. In order to know the probable PBDE sorption mechanism on the model sample of biosolid, the Gibbs free energies were calculated using the Freundlich model, giving values between -8 and -10 kJ/mol. These results suggest that the sorption mechanism of the PBDEs is through physiosorption. Therefore, the Freundlich model should be satisfactory for describing the analyte behavior.

Efficient Extraction of Fuel Oil Hydrocarbons from Concrete

A strategy is presented for the efficient extraction of hydrocarbons from concrete to enable their monitoring. Both polar and non-polar solvents were examined with the application of several techniques: cold solvent extraction with shaking, Soxhlet, and ultrasound-assisted extraction. The impacts of sample aging and inundation with water on the extraction efficiency were investigated to elucidate the nature of analyte-matrix interactions. Organic matrix-destructive solvents that partially disintegrate concrete (ethyl acetate, methanol, and acetonitrile) were most efficient for all considered scenarios, yielding recoveries in a range of 95-100% upon a two-step, four-day, cold solvent extraction or 24-h Soxhlet extraction even for highly contaminated and aged samples.

Molecular dynamics simulations of matrix assisted laser desorption ionization: Matrix–analyte interactions

There is synergy between matrix assisted laser desorption ionization (MALDI) experiments and molecular dynamics (MD) simulations. To understand analyte ejection from the matrix, MD simulations have been employed. Prior calculations show that the ejected analyte molecules remain solvated by the matrix molecules in the ablated plume. In contrast, the experimental data show free analyte ions. The main idea of this work is that analyte molecule ejection may depend on the microscopic details of analyte interaction with the matrix. Intermolecular matrix–analyte interactions have been studied by focusing on 2,5-dihydroxybenzoic acid (DHB; matrix) and amino acids (AA; analyte) using Chemistry at HARvard Molecular Mechanics (CHARMM) force field. A series of AA molecules have been studied to analyze the DHB–AA interaction. A relative scale of AA molecule affinity towards DHB has been developed.

Selective desorption/ionization of sulfatides by MALDI-MS facilitated using 9-aminoacridine as matrix

Recently, we used the favorable properties of 9-aminoacridine (9-AA) as matrix for the quantitative analysis of acidic metabolites and glycerophospholipids from extracts of biological materials [Sun, G., Yang, K., Zhao, Z., Guan, S., Han, X., and Gross, R.W. (2007) A shotgun metabolomics approach for rapid analysis of negatively-charged water-soluble cellular metabolites from mouse heart tissue. Anal. Chem. 79: 6629-6640; Sun, G., Yang, K., Zhao, Z., Guan, S., Han, X., and Gross, R.W. (2008) Matrix-assisted laser desorption/ionization-time of flight mass spectrometric analysis of cellular glycerophospholipids enabled by multiplexed solvent dependent analyte-matrix interactions. Anal. Chem. 80: 7576-7585.] by MALDI-MS. Herein, we extend this discovery and identified the selective desorption/ionization of sulfatides over other examined anionic lipids present in lipid extracts of biological samples by MALDI-MS using 9-AA as matrix. Through this approach, a high throughput method for the quantitative analysis of low to very low abundance sulfatide molecular species directly from crude lipid extracts has been developed. This method possessed a linear dynamic range of over 1,000-fold, a detection limit at the high attomole level, and a reproducibility of approximately 10% deviation. Many potential factors that might affect the quantitation of sulfatide species employing the method were examined and their effects were found to be negligible within experimental error. Collectively, these results demonstrate a powerful high throughput method for the measurement of sulfatides directly from extracts of biological samples, facilitating the study of sulfatide metabolism, trafficking, and homeostasis in health and disease.

Critical factors in chemical characterization for the evaluation of decontamination in solids using advanced oxidation

Advanced oxidation technologies (AOT) have been applied to the treatment of numerous organic pollutants embedded in solid matrices (e.g., soil, sediments, sludge, etc.). Given potentially strong matrix-analyte interactions in solids, chemical characterization of both the target contaminants and their oxidation products is critical for the evaluation of any decontamination method. The success of AOT applications has been evaluated either directly (based on the removal of original contaminants, extent of mineralization, and/or formation of by-products), or indirectly, e.g., based on toxicity or chemical oxygen demand. Since indirect methods do not provide comprehensive understanding of the pollutants' fate, direct analytical approaches are covered in this review while focusing on sample preparation and detailed chromatographic characterization, assessing the strengths and weaknesses of these methods. The significance of sample preparation, in particular extraction, is discussed with respect to the nature of matrix-analyte interactions, as those may also affect the selection of the remediation method. The ultimate goal of this review is the presentation of methods employed to achieve mass balance closure, which is essential to ensure the full understanding of degradation pathways.

Extractable Organic Carbon and its Differentiation by Polarity in Diesel Exhaust, Wood Smoke, and Urban Particulate Matter

In order to determine whether characterization of organic carbon (OC) in particulate matter (PM) is limited due to the extraction or analysis, we have evaluated the OC extractability from three model PM matrices (wood smoke, diesel exhaust, and urban PM). We have compared hot pressurized water (HPW) fractionation and sequential organic solvent Soxhlet extraction. The evaluation of extraction efficiency was based on three methods: thermal optical transmittance (TOT), reflectance, and total organic carbon analyses. For all three PM samples, comparable OC yields were obtained with both extraction methods accounting for ∼ 45–60% of the total OC found in PM (based on TOT). These recoveries also suggest that a significant portion of OC in all PM materials remains unextractable. Further investigation of OC distribution in different polarity fractions using spectrometric techniques (including gas chromatography with mass spectrometry, carbon 1s near-edge X-ray absorption fine structure, and proton nuclear magnetic resonance spectroscopies) revealed that sequential extraction may be useful approach to differentiate the constituents attributed to primary emission sources and to secondary aerosol formation, respectively. In contrast to the common expectation that polarity of constituents corresponds to the polarity of the extracting solvent, distribution of OC species (among the different polarity fractions) also depended on their availability (i.e., analyte-matrix interactions). It appears that secondary OC species (e.g., diacids) are not deeply embedded into the matrix, and thus were recovered in the solvent of a corresponding polarity. By contrast, primary OC constituents are strongly bound to the matrix and thus required harsh conditions to be recovered.

Direct Determination of the Enantiomeric Purity or Enantiomeric Composition of Methylpropionates Using a Single Capacitive Microsensor

Capacitive enantioselective sensors have been demonstrated to provide antipodal signals upon dosage of, e.g., the enantiomers of methyl lactate or methyl-2-chloropropionate. In a next step, these sensors have been used to not only qualitatively determine the nature of the respective enantiomer or to quantitatively measure its concentration upon dosage in the pure form but to also assess the enantiomeric composition of mixtures by using only a single capacitive-type sensor. The enantioselective coating material consisted of a modified gamma-cyclodextrin. It was shown that the absorption and desorption kinetics of the two enantiomers of, e.g., the methyl-2-chloropropionate, are sufficiently different and produce sensor signal features that enable an accurate determination of the enantiomeric purity and composition of the chiral analyte or mixture under investigation. The method even allows for detecting small impurities in commercially available samples labeled as 99% enantiomerically pure. Moreover, the results disclosed here show that sensor techniques can be used to reveal details of enantioselective analyte-receptor and analyte-matrix interactions.

Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometric Analysis of Cellular Glycerophospholipids Enabled by Multiplexed Solvent Dependent Analyte−Matrix Interactions

A matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) based approach was developed for the rapid analyses of cellular glycerophospholipids. Through multiplexed solvent-enabled optimization of analyte-matrix interactions during the crystallization process, over a 30-fold increase in S/N was achieved using 9-aminoacridine as the matrix. The linearity of response (r(2) = 0.99) and dynamic range of this method (over 2 orders of magnitude) were excellent. Moreover, through multiplexing ionization conditions by generating suites of different analyte-matrix interactions in the absence or presence of different alkali metal cations in the matrix, discrete lipid classes were highly and selectively ionized under different conditions resulting in the de facto resolution of lipid classes without chromatography. The resultant decreases in spectral complexity facilitated tandem mass spectrometric analysis through high energy fragmentation of lithiated molecular ions that typically resulted in informative fragment ions. Anionic phospholipids were also detected as singly negatively charged species that could be fragmented using MALDI tandem mass spectrometry leading to structural assignments. Collectively, these results identify a rapid, sensitive, and highly informative MALDI-TOF MS approach for analysis of cellular glycerophospholipids directly from extracts of mammalian tissues without the need for prior chromatographic separation.

Matrix effect of aluminium, calcium and magnesium in axially viewing inductively coupled plasma atomic emission spectrometry

The matrix effect due to Al, Ca and Mg in axial view mode ICP-AES was investigated over analyte lines with total excitation energy from 1.62 to 16.51 eV. A global qualitative explanation of the action of the matrix is proposed. The energy is directly transferred between matrix and analyte specimens (atoms or ions) during inelastic collisions, which are particularly relevant in the 1.62–8 eV excitation energy range, where the direct action with argon specimens is less probable to occur. Concrete applications of this global qualitative explanation are given, taking into account the resonance energy effect and the spin conservation rule. The characteristic matrix effect of Al and Ca for lines excited by charge transference mechanism was observed. In the presence of Mg, other possible matrix–analyte interactions may reduce the efficiency of this mechanism. The particular behavior of the matrix effect for lines in the 10.5–11.5 eV energy range can be considered as experimental evidence of the Penning ionization–excitation mechanism, which probably actuates along other matrix–analyte interactions.

Matrix–analyte-interaction in MALDI-MS: Pellet and nano-electrospray preparations

The incorporation of analytes into matrix crystals and even more so its mechanistic aspects as a prerequisite for a successful MALDI-MS has been discussed controversially in the literature. Solventless sample preparation techniques can shed new light on this question. In order to investigate some crucial aspects of these preparation techniques, lyophylized peptides and proteins were ground or milled with the powder of two different matrices, 2,5-DHB as incorporating matrix and 2,6-DHB for which protein incorporation was definitely excluded in a prior study, and pressed into pellets. The dependence of the quality of the UV-MALDI-spectra on the mass (up to 12,360 Da) and the milling time in a ball mill is reported. For mellitin different initial axial ion velocities were found, when desorbed from 2,5-DHB-pellets as prepared and after wetting and re-drying. Velocities of 150 and 580 m s −1 for dry and wetted pellets are taken as representative for hard desorption from a surface and soft desorption of matrix-incorporated analytes, respectively. Proteins labeled with either fluorescein isothiocyanate (FITC) or Texas Red (TR) were nano-electrosprayed onto a bed of ferulic acid in a ‘dry’ or ‘wet’ mode. All ‘dry’ deposits exhibit strong fluorescence but do not yield MALDI-ion signals. All ‘wet’ deposits yield MALDI-signals of the proteins; the fluorescence of FITC is quenched in ‘wet’ deposits because of the low matrix pH.

Protein incorporation into MALDI-matrix crystals investigated by high resolution field emission scanning electron microscopy

In this study, high resolution field emission scanning electron microscopy (FE-SEM) was tested for its feasibility to investigate analyte incorporation into and analyte distribution in slowly grown crystals of 2,5-dihydroxybenzoic acid (2,5-DHB) and 2,6-dihydroxybenzoic acid (2,6-DHB); both compounds function as a matrix for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). To investigate matrix–analyte interaction in 2,5-DHB crystals two Au-labels (20 nm colloidal gold and Nanogold) were examined as potential protein markers to visualize proteins in matrix solids by SEM. For this purpose, analyte-doped 2,5-DHB crystals were mechanically cleaved and SEM-micrographs of the opened inner faces were recorded. During the course of the Au-label study, crystal defects became apparent for crystals grown from analyte-free and analyte-doped matrix solutions; these defects are interpreted as ‘fluid’ or ‘liquid inclusions.’ The size and amount of liquid inclusions depend on the individual cooling rate during crystal growth and on the size of analyte molecules added to the matrix solution prior to crystal growth. Considering the results obtained for both matrix compounds, we assume that analyte incorporation occurs via a ‘solid solution’ in the case of 2,5-DHB as earlier proposed and not via phase defects such as liquid inclusions. Whether liquid inclusions—presumably filled with saturated mother solution—help to facilitate the desorption/ionization event remains open.