Determination of paraffins in food simulants and packaging materials by liquid chromatography with evaporative mass detection and identification of paraffin type by liquid chromatography/gas chromatography and fourier transform infrared spectroscopy

A liquid chromatographic method with evaporative mass detection (EMD) is described for the determination of paraffins in food contact materials that do not contain polyolefin oligomers, or paraffins migrating from these materials into fatty food simulants or certain simple foods. A normal-phase column operating at maximum column efficiency separates nonparaffinic and paraffinic materials without resolving the latter into individual components, and EMD is used to quantitate the paraffins. An on-line qualitative method that uses liquid chromatography/gas chromatography with flame ionization detection discriminates between paraffin waxes and oils in food contact materials, food simulants, and certain simple foods; a Fourier transform infrared spectrophotometric qualitative method also discriminates between waxes and oils, but is usually restricted to food contact materials that do not contain polyolefins and to migration experiments with organic solvents as fatty food simulants (with some other fatty food simulants, paraffin type must then be identified in the food contact material).     

Laser desorption/ionization time-of-flight mass spectrometry of triacylglycerols in oils

Selected triacylglycerols (TAGs) were directly analyzed on a standard stainless steel target plate by laser desorption/ionization time-of-flight mass spectrometry (LDI-TOFMS). Sodium and potassium ion adducts of TAGs were produced, and the thermal desorption/ionization mechanism was invoked to rationalize the experimental observations. The method permits a simple and fast qualitative analysis of TAGs. Advantages of this approach relative to matrix-assisted laser desorption/ionization (MALDI) are simpler sample preparation, lack of need to use a matrix with consequent absence of matrix interference peaks in the spectra, and potential improvements in shot-to-shot reproducibility due to the absence of the crystallization step resulting in a more homogenously deposited sample. The procedure was successfully applied to the determination of TAGs in whole oils, yielding very fast TAG fingerprints.     

In-situ probing of the biotic-abiotic boundary of plants by laser desorption/ionization time-of-flight mass spectrometry

Laser desorption/ionization time-of-flight (LDI-TOF) mass spectrometry was applied for the direct analysis of cuticular waxes on intact plant tissues. Cuticular wax compounds were ionized by laser desorption in the presence of colloidal silver. Silver-adduct ions were detected on samples from Arabidopsis thaliana and from maize. Good spot-to-spot reproducibility indicated homogeneous coverage of the sample by the fine colloidal material. The results were consistent with GC-MS analyses of cuticular extracts, thus confirming the feasibility of direct analysis based on this protocol. Molecular masses of the adduct ions correspond well with the known composition of cuticular waxes. Moreover, LDI-TOF gave good estimates of the relative local abundances of a given compound. However, bias was found in cases where compounds with different ionization efficiencies were analyzed.     

Direct surface analysis of fungal species by matrix-assisted laser desorption/ionization mass spectrometry

In this study various methods of sample preparation and matrices were investigated to determine optimum collection and analysis criteria for fungal analysis by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Intact spores and/or hyphae of Aspergillus niger, Rhizopus oryzae, Trichoderma reesei and Phanerochaete chrysosporium were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). The fungal samples were applied to the MALDI sample target as untreated, sonicated, or acid/heat treated samples, or blotted directly from the fungal culture with double-stick tape. Ferulic acid or sinapinic acid matrix solution was layered over the dried samples and analyzed by MALDI-MS. Statistical analysis showed that simply using double-stick tape to collect and transfer to a MALDI sample plate typically worked as well as the other preparation methods, and required the least sample handling.     

Laser desorption ionization and MALDI time-of-flight mass spectrometry for low molecular mass polyethylene analysis

Polyethylene's inert nature and difficulty to dissolve in conventional solvents at room temperature present special problems for sample preparation and ionization in mass spectrometric analysis. We present a study of ionization behavior of several polyethylene samples with molecular masses up to 4000 Da in laser desorption ionization (LDI) time-of-flight mass spectrometers equipped with a 337 nm laser beam. We demonstrate unequivocally that silver or copper ion attachment to saturated polyethylene can occur in the gas phase during the UV LDI process. In LDI spectra of polyethylene with molecular masses above approximately 1000 Da, low mass ions corresponding to metal-alkene structures are observed in addition to the principal distribution. By interrogating a well-characterized polyethylene sample and a long chain alkane, C94H190, these low mass ions are determined to be the fragmentation products of the intact metal-polyethylene adduct ions. It is further illustrated that fragmentation can be reduced by adding matrix molecules to the sample preparation.     

Reactions of atomic transition-metal ions with long-chain alkanes

Understanding metal ion interactions with long-chain alkanes not only is of fundamental importance in the areas of organometallic chemistry, surface chemistry, and catalysis, but also has significant implication in mass spectrometry method development for the analysis of polyethylene. Polyethylene represents one of the most challenging classes of polymers to be analyzed by mass spectrometry. In this work, reactions of several transition-metal ions including Cr+, Mn+, Fe+, Co+, Ni+, Cu+, and Ag+ with long-chain alkanes, C28H58 and C36H74, are reported. A metal powder and the nonvolatile alkane are co-deposited onto a sample target of a laser desorption/ionization (LDI) time-of-flight mass spectrometer. The metal ions generated by LDI react with the vaporized alkane during desorption. It is found that all these metal ions can form adduct ions with the long-chain alkanes. Fe+, Co+, and Ni+ produce in-source fragment ions resulting from dehydrogenation and dealkylation of the adduct ions. The post-source decay (PSD) spectra of the metal-alkane adduct ions are recorded. It is shown that PSD of Ag+ alkane adduct ions produces bare metal ions only, suggesting weak binding between this metal ion and alkane. The PSD spectra of the Fe+, Co+, and Ni+ alkane adduct ions display extensive fragmentation. Fragment ions are also observed in the PSD spectra of Cr+, Mn+, and Cu+ alkane adduct ions. The high reactivity of Fe+, Co+, and Ni+ is consistent with that observed in small alkane systems. The unusually high reactivity of Cr+, Mn+, and Cu+ is rationalized by a reaction scheme where a long-chain alkane first forms a complex with a metal ion via ion/induced dipole interactions. If sufficient internal energy is gained during the complex formation, metal ions can be inserted into C-H and C-C bonds of the alkane, followed by fragmentation. The thermal energy of the neutral alkane is believed to be the main source of the internal energy acquired in the complex. Finally, the implication of this work on mass spectrometry method development for polyethylene analysis is discussed.     

Molecular weight analysis of water‐soluble poly(phenylene ethynylene)s using MALDI‐TOF MS

Molecular weights of seven poly(phenylene ethynylene)‐based water‐soluble conjugated polyelectrolytes (CPEs) obtained through Sonogashira coupling are determined by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS). A standard sample preparation protocol is developed to characterize the seven CPEs using 2,5‐dihydroxybenzoic acid as the matrix (M) and AgTFA as the cationization reagent (CR). High‐quality MALDI mass spectra are obtained at volume mixing ratios (CPE/M/CR) of 5/5/1 for anionic polymers (P1–P4) and 5/50/1 for cationic polymers (P5–P7). Molecular weight, molecular weight distribution, and end‐group information are analyzed. The effects of molecular weight of CPEs on optical and quenching properties are also studied. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2537–2543     

Analysis of deprotonated acids with silicon nanoparticle-assisted laser desorption/ ionization mass spectrometry

Chemically modified silicon nanoparticles were applied for the laser desorption/negative ionization of small acids. A series of substituted sulfonic acids and fatty acids was studied. Compared to desorption ionization on porous silicon (DIOS) and other matrix-less laser desorption/ionization techniques, silicon nanoparticle-assisted laser desorption/ionization (SPALDI) mass spectrometry allows for the analysis of acids in the negative ion mode without the observation of multimers or cation adducts. Using SPALDI, detection limits of many acids reached levels down to 50 pmol/μl. SPALDI of fatty acids with unmodified silicon nanoparticles was compared to SPALDI using the fluoroalkyl silylated silicon powder, with the unmodified particles showing better sensitivity for fatty acids, but with more low-mass background due to impurities and surfactants in the untreated silicon powder. The fatty acids exhibited a size-dependent response in both SPALDI and unmodified SPALDI, showing a signal intensity increase with the chain length of the fatty acids (C12-C18), leveling off at chain lengths of C18-C22. The size effect may be due to the crystallization of long chain fatty acids on the silicon. This hypothesis was further explored and supported by SPALDI of several, similar sized, unsaturated fatty acids with various crystallinities. Fatty acids in milk lipids and tick nymph samples were directly detected and their concentration ratios were determined by SPALDI mass spectrometry without complicated and time-consuming purification and esterification required in the traditional analysis of fatty acids by gas chromatography (GC). These results suggest that SPALDI mass spectrometry has the potential application in fast screening for small acids in crude samples with minimal sample preparation.     

Direct laser desorption/ionization time-of-flight mass spectrometry of conjugated polymers

Two conjugated polymers (CPs), poly(9,9-dioctylfluorene) (PF) and poly(3-octylthiophene) (PT) were analyzed by direct laser desorption/ionization time-of-flight mass spectrometry (LDI-ToF MS). Because of their strong absorption near the wavelength of the laser (337 nm), easy and transient energy transfer properties and sufficient thermal stability, CPs can be desorbed and ionized directly without a matrix. For comparison, these two polymers were also analyzed using matrix-assisted laser desorption/ionization (MALDI)-ToF MS in the positive reflectron mode. The results revealed that they are very similar in terms of quality and resolution. All results demonstrate that LDI-ToF MS is an alternative method for the mass characterization of some conjugated systems, thereby simplifying the process of sample preparation and result analysis.     

Laser desorption/ionization mass spectrometry on porous silica and alumina for peptide mass fingerprinting

We investigated a variant of desorption/ionization on porous silicon (DIOS) mass spectrometry utilizing an aqueous suspension of either porous silica gel or porous alumina (pore size of 60 and 90 A, respectively). Laser desorption/ionization (LDI) from samples directly deposited on a stainless steel surface without any inorganic substrates was also achieved. Synthetic peptides designed to cover large sequence diversity constituted our model compounds. Sample preparation, including material conditioning, peptide solubilization, and deposition protocol onto standard matrix-assisted laser desorption/ionization (MALDI) probe, as well as ionization source tuning were optimized to perform sensitive reproducible LDI analyses. The addition of either a cationizing agent or an alkali metal scavenger to the sample suspension allowed modification of the ionization output. Comparing hydrophilic silica gel to hydrophobic reversed-phase silica gel as well as increasing material pore size provided further insights into desorption/ionization processes. Furthermore, mixtures of peptides were analyzed to probe the spectral suppression phenomenon when no interfering organic matrix was present. The results gathered from synthetic peptide cocktails indicated that LDI mass spectrometry on silica gel or alumina constitutes a promising complementary method to MALDI in proteomics for peptide mass fingerprinting.     

气相色谱-场电离飞行时间质谱测定中间馏分油中链烷烃的形态分布

利用气相色谱-场电离飞行时间质谱技术( GC-FI TOF MS)建立了中间馏分油中不同异构程度链烷烃的碳数分布表征方法。首先利用GC-FI TOF MS技术对不同异构程度的链烷烃进行分离、鉴别,然后建立了中间馏分油沸点范围内正构烷烃、异构烷烃在GC-FI TOF MS测定时的相对响应因子算法及不同异构程度链烷烃的碳数分布算法,最后考察此定量方法的精密度和准确度。结果表明,GC-FI TOF MS可以将同碳数链烷烃区分为异构程度不同的3部分：多取代基异构烷烃、单取代基异构烷烃和正构烷烃;可利用GC-FI TOF MS对异构程度不同的链烷烃进行定量分析,精密度较好,相对偏差小于15%。此方法无需进行样品预分离,可直接进样分析,缩短了分析时间,且首次提供了不同异构程度异构烷烃的碳数分布信息。     

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of some conducting polymers

The application of laser desorption/ionization and matrix-assisted laser desorption/ionization mass spectrometry to the study of the structure of some conducting polymers was investigated. A methodology was developed and the experimental conditions for the characterization of these polymers were established. The parameters chosen to be changed for the method optimization are the matrix material (seven different matrices were tested depending on the polymer studied) and the solvent used in the preparation of the samples. The method seems to be suitable for the study of the structure of two oxidation states of polyaniline: emeraldine and pernigraniline. The analysis of the polymers derived from m-aminophenylbenzothiazole, benzothiazole and tetrathiopentalenes was also performed.} Copyright 2000 John Wiley & Sons, Ltd.     

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.     

Sample purification and preparation technique based on nano-scale reversed-phase columns for the sensitive analysis of complex peptide mixtures by matrix-assisted laser desorption/ionization mass spectrometry

A simple reversed-phase nano-column purification and sample preparation technique is described, which markedly improves the mass spectrometric analysis of complex and contaminated peptide mixtures by matrix-assisted laser desorption/ionization (MALDI). The method is simple, fast and utilizes only low-cost disposables. After loading the sample on the column and a subsequent washing step, the analyte molecules are eluted with 50-100 nl of matrix solution directly on to the MALDI/MS target. The washing step ensures removal of a wide range of contaminants. The small bed volume of the column allows efficient sample concentration and the elution process yields very small sample spots. This simplifies the analysis and minimizes discrimination effects due to sample heterogeneity, because the desorption/ionization laser simultaneously irradiates a large portion of the sample. Taken together, these features of the method significantly improve the sensitivity for MALDI/MS analysis of contaminated peptide samples compared with the commonly used sample preparation procedures. This is demonstrated with in-gel tryptic digests of proteins from human brain that were separated by 2D gel electrophoresis. Furthermore, it is shown that with this method 2,5-dihydroxybenzoic acid (DHB) acts as an efficient matrix for peptide mapping. Both detection sensitivity and sequence coverage are comparable to those obtained with the currently preferred matrix alpha-cyano-4-hydroxycinnamic acid (CHCA). The higher stability of peptide ions generated with DHB compared with CHCA is advantageous when analyzing fragile sample molecules. Therefore, the method described here is also of interest for the use of Fourier transform ion cyclotron resonance (FT-ICR) or ion-trap mass analyzers.     

PSMA@Ag复合微球的制备及其在MALDI-MS分析小分子化合物中的应用研究

基质辅助激光解吸电离质谱(MALDI-MS)作为一种有力的分析手段,在生物分子分析中有着广泛的应用,但很难应用于分子量小于500的待测物的分析。该文利用聚多巴胺修饰还原法制备了核壳结构的聚苯乙烯-马来酸酐共聚物@银纳米壳层(PSMA@Ag)复合微球。采用傅立叶红外光谱法验证了聚多巴胺(PDA)的成功修饰。结合扫描电子显微镜(SEM)和紫外-可见光谱(UV-Vis)分析结果,发现Ag纳米壳层成功地包覆在PSMA微球的表面。将制备的PSMA@Ag复合微球作为辅助基质直接应用于MALDI-MS,成功地从0.5μL待测物样品中检测到2 pmol脯氨酸(M_w=115)和1 pmol丝氨酸(M_w=105)。研究结果证明PSMA@Ag微球对MALDI的离子化过程有促进作用,为解决MALDI-MS在分析小分子待测物时背景噪声过大,信号无法分辨的问题提供了一个有效途径。     

Electrode‐assisted desorption electrospray ionization mass spectrometry

A new ion source has been developed for rapid, noncontact analysis of materials at ambient conditions. The method provides desorption of analytes under ambient conditions directly from different surfaces with little or no sample preparation. The new method, termed electrode‐assisted desorption electrospray ionization (EADESI), is on the basis of the ionization of molecules on different surfaces by highly charged droplets produced on a sharp‐edged high voltage tip, and ions produced are introduced into the mass spectrometer through a capillary. The EADESI technique can be applied to various samples including amino acids, peptides, proteins, drugs and human fluids such as urine and blood. EADESI is promising for routine analyses in different fields such as forensic, environmental and material sciences. EADESI interface can be fit to a conventional ion‐trap mass spectrometer. It can be used for various types of samples with a broad mass range. EADESI can also provide real‐time analysis which is very valuable for biomedical applications. Copyright © 2010 John Wiley & Sons, Ltd.     

New aspects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: a universal solvent-free sample preparation

A method of solvent-free sample preparation is shown to be of universal applicability for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). Results obtained were compared with those of traditional solvent-based sample preparation for MALDI-MS in order to demonstrate their similarities with respect to accuracy, sensitivity and resolution for polymers such as polystyrene and poly(methyl methacrylate) in a mass range from 2 to 100 kDa. The results revealed that there is fundamentally no difference in the quality of the obtained mass spectra, and we conclude that the mechanism of desorption and ionization remains unchanged. However, the solvent-free sample preparation turned out to have some advantages over the traditional method in certain cases: quick and easy applicability is shown for polyetherimide avoiding time-consuming optimization procedures. In particular, industrial pigments that are insoluble in common solvents were characterized without interfering signals from fragments. The method even showed improvements with respect to reproducibility and mass discrimination effects in comparison to traditional sample preparation. Additionally, this contribution provides new insight regarding the analyte/matrix preorganization for the desorption step which now appears to be independent of crystallinity.     

基质辅助激光解吸电离飞行时间质谱技术分析三七和海兔神经连索内的皂甙组分

基质辅助激光解吸电离飞行时间(MALDI-TOF)质谱技术能高效解吸三七提取液(Panaxnotoginseng Extraction, PNE)中的混合皂甙分子为皂甙离子,并供质量分析器检测与分析.选用MALDI-TOF 质谱技术直接分析色谱纯皂甙样品的纯度,其检测灵敏度优于反相高效液相色谱法(RP-HPLC).优化提取中药三七(Panax notoginseng, PN)的混合皂甙, 选用MALDI-TOF质谱技术直接分析PNE中的皂甙种类和相对含量,发现PNE至少含有20种不同分子结构的皂甙组分,其中人参皂甙(Ginsenoside) Rg1和三七皂甙(Notoginsenoside)R1含量相对较高.选用薄板层析法(TLC)制备PNE中的R1皂甙.MALDI-TOF质谱技术研究蓝斑背肛海兔(Notarcus leachiicirrosus Stimpson, NLCS)神经连索内的超微量R1的组成与分布.建立PNE皂甙的指纹图谱,并适合于评价中药三七的品质和分析体内超微量皂甙的代谢过程与机理.     

Ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of intact hemoglobin complex from whole human blood

Ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOFMS) was applied to the analysis of intact human hemoglobin complex directly from whole human blood. The less acidic matrix substance 2,6-dihydroxyacetophenone provided sufficient insensitivity to the impurities present in this crude biological matrix to make any sample pretreatment except dilution dispensable. This matrix facilitated exact molecular mass determination of the non-assembled hemoglobin alpha- and beta-chain (SD < or = +/-0.28 Da and Deltam < or =6.4 ppm for n = 10) if trifluoroacetic acid was used as an additive. Replacement of the denaturing additive trifluoroacetic acid by the neutral salt ammonium acetate allowed the detection of the intact hemoglobin alpha(2)beta(2)H(n)-assembly (n = 0-4) and the alphabeta-subassembly. A prerequisite for the detection of the noncovalent complex was the application of a very soft sample preparation procedure. Crystal morphology, sample concentration and laser pulse energy were also found to be important parameters for the analysis of the intact protein complexes. However, comparison with published electrospray ionization (ESI)-MS results on mammalian hemoglobin molecules shows that, even under the applied gentle conditions, MALDI does not provide a completely reliable picture of the solution-phase equilibrium. In contrast to ESI, especially extensive loss of the heme b (H) groups was noticed. The disruption of the rather stable heme b-globin interaction is assumed to be induced by photo-excitation during the desorption/ionization process.     

A novel sample preparation method of matrix-assisted laser desorption/ionization time of flight mass spectrometry for polystyrene

A novel sample preparation method of matrix-assisted laser desorption/ionization mass spectrometry for polystyrene was reported. Compared to the conventional dried-droplet method, the efficiency of ionization and signal intensity of mass spectra were improved. The mechanism was also analyzed.