Solvent-free Sample Preparation Method Research Articles

Advances in solvent-free sample preparation methods for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of polymers and biomolecules

Advances in solvent-free sample preparation methods for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) of polymers and biomolecules

Recent achievements and applications of solid – phase microextraction in pesticide residues analysis in food

Abstract The current trend in sample preparation methods is devoted to minimizing or eliminating the volume of extractive solvent. In this review, the focus on solid phase microextraction (SPME) as a solvent free sample preparation method for the isolation of pesticides residues in different food matrices is given. To achieve satisfactory extraction efficiency, selection of the fiber coating is an important step in the method development. Here, recent trends in new supporting materials and new fiber coatings development are discussed. Finally, applicability of SPME for the pesticide residues analysis in various food matrices using mainly chromatographic methods is also reviewed.

The effects of molecular weight distribution and sample preparation on matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometric analysis of petroleum macromolecules

To date there have been no systematic, quantitative investigations of the effect of sample preparation on the matrix-assisted laser desorption/ionization time-of-flight (MALDI) mass spectrometry response for polydisperse systems. To this end, the interrelationships between sample preparation, analyte molecular weight distribution (MWD) and solubility, and signal response were investigated for mixtures of alkylated polycyclic aromatic hydrocarbon (PAH) oligomers, the constituents of petroleum pitch that serve as precursors for advanced carbon materials. These PAH oligomers served as a useful analyte system for study, as their solvent solubilities decrease significantly with each increasing oligomeric unit. Molecular weight standards consisting of relatively pure dimer and trimer cuts of the starting M-50 petroleum pitch were produced using a dense-gas/supercritical extraction (DGE/SCE) technique and were then used to produce oligomeric mixtures of well-defined composition for study. Both traditional, solvent-based and newer, solvent-free sample preparation methods were evaluated, and their effects on both homogeneity and signal response were determined. While solvent-free sample preparation methods produced homogeneous samples and reproducible results regardless of the MWD of the analyte, solvent-based samples that contained more than one oligomeric cut produced non-homogeneous samples and poor reproducibilities. The differing solubilities of dimer, trimer, and tetramer oligomers in a given solvent (e.g., CS(2) or toluene) were found to be the cause of the inhomogeneities observed in solvent-based sample preparation. A quantitative analysis study performed with dimer/trimer mixtures over a wide range of compositions via solvent-free sample preparation indicates that linear, reproducible calibration curves can be generated and used to calculate the molecular composition of unknown dimer/trimer mixtures with confidence.

Surfactant‐assisted pressurized liquid extraction for determination of flavonoids from Costus speciosus by micellar electrokinetic chromatography

Micellar electrokinetic chromatography (MEKC), a mode of capillary electrophoresis (CE), is considered an efficient analytical technique allowing for the reduction of organic solvent consumption during the experimental procedure. However, during sample preparation of natural products, the usage of large amount of organic solvent is generally unavoidable. In this article, therefore, a fast, simple, efficient, highly automatic and organic solvent-free sample preparation method, namely surfactant-assisted pressurized liquid extraction (PLE), was developed for the extraction of flavonoids in Costus speciosus flowers before MEKC analysis. The various experimental parameters such as the type and concentration of surfactant, and extraction time were evaluated systematically. Under the optimized conditions, the extraction efficiencies of surfactant-assisted PLE methods were comparable with Soxhlet extraction using organic solvent. The combination of surfactant-assisted PLE and MEKC was shown to be a green, rapid and effective approach for extraction and analysis of flavonoids in C. speciosus flowers.

Using solvent‐free sample preparation to promote protonation of poly(ethylene oxide)s with labile end‐groups in matrix‐assisted laser desorption/ionisation

Protonation is usually required to observe intact ions during matrix-assisted laser desorption/ionization (MALDI) of polymers containing fragile end-groups while cation adduction induces chain-end degradation. These polymers, generally obtained via living free radical polymerization techniques, are terminated with a functionality in which a bond is prone to homolytic cleavage, as required by the polymerization process. A solvent-free sample preparation method was used here to avoid salt contaminant from the solvent traditionally used in the dried-droplet MALDI procedure. Solvent-based and solvent-free sample preparations were compared for a series of three poly(ethylene oxide) polymers functionalized with a labile end-group in a nitroxide-mediated polymerization reaction, using 2,4,6-trihydroxyacetophenone (THAP) as the matrix without any added salt. Intact oligomer ions could only be produced as protonated molecules in solvent-free MALDI while sodium adducts of degraded polymers were formed from the dried-droplet samples. Although MALDI analysis was performed at the laser threshold, fragmentation of protonated macromolecules was still observed to occur. However, in contrast to sodiated molecules, dissociation of protonated oligomers does not involve the labile C--ON bond of the end-group. As the macromolecule size increased, protonation appeared to be less efficient and sodium adduction became the dominant ionization process, although no sodium salt was added in the preparation. Formation of sodiated degraded macromolecules would be dictated by increasing cation affinity as the size of the oligomers increases and would reveal the presence of salts at trace levels in the MALDI samples.

Poly(ethylene glycol) cationization with alkali metals in matrix-assisted laser desorption ionization investigated with the solvent-free method

Matrix-assisted laser desorption ionization (MALDI) was applied to determine the relative efficiencies for attachment of alkali cations to poly(ethylene glycol) polymers. The solvent-free sample preparation method was employed in order to discriminate the gas-phase polymer-cation complex formation mechanism from solubility and co-crystallization effects. We show that the cohesion energy of the cation precursor has a strong influence on the outcome of the MALDI measurements. This potential drawback is circumvented by the simultaneous use of alkali salts of similar lattice energy. The experiments include chain polymers of different length (PEG600 and PEG1000) and alkali cations of different size (Na+, K+, Cs+). We found that the attachment efficiencies decrease with growing cation size. This decrease is significantly more rapid for the shorter PEG600 polymers. These results corroborate the general trends observed in earlier studies, but correct the dependence on alkali cation size assessed in previous MALDI experiments with the conventional dried-droplet sample preparation method.

Imaging the Morphology of Solvent-Free Prepared MALDI Samples

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. The development of solvent-free sample preparation methods has enabled MALDI to analyze insoluble materials and, interestingly, can provide higher-quality mass spectral data. Although the utility of solvent-free sample preparation for MALDI has been demonstrated, the reasons for its success are only now being discovered. In this study, we use microscopy tools to image samples prepared using solvent-free methods to examine the morphology of these samples. The samples are prepared using a simple vortex method. Our results show that the average particle size of typical MALDI matrices is reduced from their original tens to hundreds of micrometers to hundreds of nanometers. This size reduction of the matrix occurs in one minute using the vortex method. We also observe remarkably smooth and homogeneous sample morphologies for the laser to interrogate, especially considering the relatively crude methods used to prepare our samples.

Molecular-Weight Distributions of Coal and Petroleum Asphaltenes from Laser Desorption/Ionization Experiments

Molecular-weight distributions (MWDs) of asphaltenes extracted from coal and petroleum have been measured in laser desorption/ionization (LDI) mass spectrometric experiments. The dried-droplet and solvent-free sample preparation methods are compared. The coal asphaltenes have a relatively narrow MWD (full width ≈ 150 amu) with an average molecular weight of ≈340 amu. The petroleum asphaltenes display a broader MWD (full width ≈ 300 amu) and are heavier on average (≈680 amu). The LDI spectra also provide evidence for the formation of noncovalent clusters of the two types of asphaltenes during the desorption process. Petroleum and coal asphaltenes exhibit aggregation as do large model polycyclic aromatic hydrocarbons (PAHs) with five or more fused rings also included in the study. Smaller PAHs (pyrene) exhibit less aggregation, especially when alkane-chain substituents are incorporated to the molecular structure. This indicates that asphaltenes possess large PAHs and, according to the relatively small molec...

Clustering of polycyclic aromatic hydrocarbons in matrix‐assisted laser desorption/ionization and laser desorption mass spectrometry

The desorption/ionization behaviour of polycyclic aromatic hydrocarbons (PAHs) in matrix-assisted laser desorption/ionization (MALDI) and laser desorption (LD) mass spectrometry was studied by the solvent-free sample preparation method. As the understanding of the desorption/ionization mechanism in MALDI is normally hampered by the different ionization and desorption efficiencies of the analytes, this work was focused on the analyses of a homologous series of four hexabenzocoronenes (HBCs) possessing virtually the same ionization efficiency: HBC parent, hexamethyl-hexabenzocoronene (HBC-C1), hexapropyl-hexabenzocoronene (HBC-C3) and hexakis(dodecyl)-hexabenzocoronene (HBC-C12). The different signal intensities obtained in their mass spectra can be related to differences in their desorption efficiencies, which are attributed to the different strengths of the intermolecular interactions between unsubstituted and alkylated HBCs in the solid state. The influence of the aromatic structure of PAHs on their photoionization/desorption probability was investigated. As a model system, an equimolar mixture composed of HBC-C12 and hexakis(dodecyl)-hexaphenylbenzene (HPB-C12) was chosen. The aromatic structures of both molecules and thus their absorption coefficients at the laser wavelength differ substantially and have a huge influence on their photoionization efficiency. The combined effect of laser light absorption and intermolecular interactions on the desorption/ionization behaviour of giant PAHs was further studied by using an equimolar mixture composed of a larger PAH (C(222)H(42)) and its dendritic precursor (C(222)H(150)). This mixture shows the opposite behaviour to that of the former example, because the balance between desorption and ionization efficiency has changed significantly. The present investigation should be of interest for providing a better understanding of MALDI and LD spectra obtained from natural PAH-containing samples, such as heavy oils, asphaltenes or pitches, for which our artificial mixtures represent suitable model systems.

Towards automated, miniaturized and solvent-free sample preparation methods

Towards automated, miniaturized and solvent-free sample preparation methods

Analysis of volatiles in water using headspace solid-phase microcolumn extraction

A rapid solvent-free sample preparation method is presented in which volatilized organic substances are extracted via a solid-phase microcolumn mounted on a gas-tight syringe by the aspiration of an appropriate volume of headspace (tens of millilitres) of the sample under investigation. The microcolumn with the analytes concentrated on an adsorbent material is then transferred to the analytical instrument for desorption and analysis. The instrumental set-up consists of a Twister desorption unit (Gerstel) mounted on an Agilent 6890 GC system equipped with a CIS 4 programmed temperature vaporiser injector and coupled with an Agilent 5973 MS system. The instrumentation is slightly modified in comparison with the analysis made with a twister (stir bar coated with a layer of polydimethylsiloxane) after stir bar sorptive extraction and, thus, its utilization has been extended. For volatiles in water, the method is applicable to the qualitative screening at the μg/l to sub-μg/l level and to the quantitative determination at the ng/l level.

Extending the solvent-free MALDI sample preparation method

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. One of the key issues in traditional MALDI sample preparation is making good solutions of the analyte and the matrix. Solvent-free sample preparation methods have been developed to address these issues. Previous results of solvent-free or dry prepared samples show some advantages over traditional wet sample preparation methods. Although the results of the published solvent-free sample preparation methods produced excellent mass spectra, we found the method to be very time-consuming, with significant tool cleaning, which presents a significant possibility of cross contamination. To address these issues, we developed an extension of the solvent-free method that replaces the mortar and pestle grinding with ball milling the sample in a glass vial with two small steel balls. This new method generates mass spectra with equal quality of the previous methods, but has significant advantages in productivity, eliminates cross contamination, and is applicable to liquid and soft or waxy analytes.

Solid-phase microextraction (SPME) of permethrin residues from cucumber using a silica-bonded phase-coated stainless steel fibre

Solid-phase microextraction (SPME) is a rapid, economic and solvent-free sample preparation method for the isolation of an analyte from its matrix. The technique uses a few centimetres of some adsorptive materials such as activated charcoal, polydimethylsiloxane or octadecyl silane coated onto fused silica optical fibres or, more recently, stainless steel fibres mounted into a microsyringe. The proposed method allows the extraction of permethrin from cucumber matrix into the coating, avoiding sample handling and saving evaporation of solvents and concentration steps. Adsorbed permethrin was desorbed in the split/splitless injection port of the gas chromatograph. The time required for each run was about 1.5 h, which gave a preconcentration of several orders of magnitude. The method could separate and quantify cis- and trans-isomers of permethrin. The calibration curve showed linearity in the range 1–9 μg ml−1, with detection limits of 0.03 and 0.05 μg ml−1 for the cis- and trans-isomers of permethrin, respectively. The method had a recovery rate of about 70% and a relative standard deviation of less than 13%. Results suggest that this procedure provides a rapid and sensitive alternative method to those currently available.

Characterization of low molecular weight hydrocarbon oligomers by laser desorption/ionization time‐of‐flight mass spectrometry using a solvent‐free sample preparation method

A new solvent-free sample preparation method using silver trifluoroacetate (AgTFA) was developed for the analysis of low molecular weight paraffins and microcrystalline waxes by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Experiments show that spectral quality can be enhanced by dispersing AgTFA directly in liquid paraffins without the use of additional solvents. This preparation mixture is applied directly to the MALDI probe. Solid waxes could be examined by melting prior to analysis. The method also provides sufficiently reproducible spectra that peak area ratios between mono- and bicyclic alkane peaks indicated variations in the cycloalkane content of paraffin samples. Dehydrogenation of hydrocarbons observed during the desorption/ionization process was studied by analysis of alkane standards.

Solid phase microextraction combined with HPLC for determination of metal ions using crown ether as selective extracting reagent

A new solid phase microextraction (SPME) method for the analysis of metal ions in aqueous samples has been developed. Fibers doped with crown ether were used to extract mercury ions from aqueous solution through the formation of a stable complex. The complex is then desorbed from the fiber using high-performance liquid chromatography (HPLC) for separation and UV detection. This newly developed SPME method for the extraction of metal ions from aqueous samples is possible to combine in-line with other more sensitive elemental detection methods such as Atomic Absorption Spectrometry (AAS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), etc. The potential of SPME as a portable field sampling tool makes it a promising solvent-free sample preparation method for environmental field analysis. © 1998 John Wiley & Sons, Inc. J Micro Sep 10: 167–173, 1998

New directions in sample preparation for analysis of organic compounds

Recently emphasis has been placed on developing solvent-free sample preparation methods because of new regulatory restrictions on solvents. New and established solvent-free techniques that use gas, membrane or sorbent extracting phases are being investigated for various applications. The need to validate and optimize these new methods has also sparked research into fundamental extraction processes. In-depth investigations are being conducted on the interactions between native analytes and sample matrices to find conditions that quantitatively release the analytes. The development of sample preparation is a significant challenge and an opportunity for the contemporary analytical chemist. The results of the current research will have a profound effect on future analytical technology.