Speciation analysis in on-line aerosol mass spectrometry

Ambient aerosol particles affect both the earth's climate and human health. Both effects depend on the chemical composition of the particles including the binding state of specific elements. Modern on-line aerosol mass spectrometry is capable of measuring aerosol composition with high temporal resolution, avoiding artifacts often introduced by classical off-line methods. The two most common types of on-line aerosol mass spectrometers, laser desorption/ionization mass spectrometers (LDI-MS) and thermal desorption electron impact ionization mass spectrometers (TD-EI-MS) provide reliable information on the most common ambient inorganic and organic aerosol species with high temporal resolution. However, for less common aerosol species identification with both types of instrument is frequently associated with large uncertainties. Here, we provide an overview of the element speciation capabilities of current on-line aerosol mass spectrometry for both carbonaceous and non-carbon-containing aerosol species. We describe limitations and other issues for this type of on-line aerosol analysis.     

Direct qualitative analysis of triacylglycerols by electrospray mass spectrometry using a linear ion trap

Triacylglycerols (TAGs) isolated from a biological sample provide a challenge for mass spectrometric analysis because of the complexity of naturally occurring TAGs, which may contain different fatty acyl substituents resulting in a large number of molecular species having the identical elemental composition. We have investigated the use of mass spectrometry to obtain unambiguous information as to the individual TAG molecular species present in a complex mixture of triacylglycerols using a linear ion trap mass spectrometer. Ammonium adducts of TAGs, [M+NH4]+, were generated by electrospray ionization, which permitted the molecular weight of each TAG molecular species to be determined. The mechanisms involved in the decomposition of the [M+NH4]+ and subsequent fragment ions were investigated using deuterium labeling, MS/MS, and MS3 experiments. Collision induced decomposition of [M+NH4]+ ions resulted in the neutral loss of NH3 and an acyl side-chain (as a carboxylic acid) to generate a diacyl product ion. MS/MS data were used to identify each acyl group present for a given [M+NH4]+ ion, and this information could be combined with molecular weight data to identify possible TAG molecular species present in a biological extract. Subsequent MS3 experiments on the resultant diacyl product ions, which gave rise to acylium (RCO+) and related ions, enabled unambiguous TAG molecular assignments. These strategies of MS, MS/MS, and MS3 experiments were applied to identify components within a complex mixture of neutral lipids extracted from RAW 264.7 cells.     

Structural elucidation of monoterpene oxidation products by ion trap fragmentation using on-line atmospheric pressure chemical ionisation mass spectrometry in the negative ion mode.

Based on ion trap mass spectrometry, an on-line method is described which provides valuable information on the molecular composition of structurally complex organic aerosols. The investigated aerosols were generated from the gas-phase ozonolysis of various C(10)H(16)-terpenes (alpha-pinene, beta-pinene, 3-carene, sabinene, limonene), and directly introduced into the ion source of the mass spectrometer. Negative ion chemical ionisation at atmospheric pressure (APCI(-)) enabled the detection of multifunctional carboxylic acid products by combining inherent sensitivity and molecular weight information. Sequential low-energy collision-induced product ion fragmentation experiments (MS(n)) were performed in order to elucidate characteristic decomposition pathways of the compounds. Dicarboxylic acids, oxocarboxylic acids and hydroxyketocarboxylic acid products could be clearly distinguished by multistage on-line MS. Furthermore, sabinonic acid and two C(9)-ether compounds were tentatively identified for the first time by applying on-line APCI(-)-MS(n).     

Direct analysis of catalysts immobilised in ionic liquids using electrospray ionisation ion trap mass spectrometry

Catalysts immobilised in ionic liquids have been analysed using electrospray ionisation ion trap mass spectrometry, which allows identification of the catalyst without contamination from the ionic liquid; additionally, a new ionic liquid containing the Monsanto catalyst as the anionic component, [Rh(CO)2I2]-, has been characterised using the same method.     

Quantitative analysis of crude plasma extracts by ion trap tandem mass spectrometry

The ion trap mass spectrometer is a tandem-in-time instrument that has promise as an extremely sensitive device for practical tandem mass spectrometry assays. An approach for the quantitative analysis of unknown drug levels in crude extracts, using combined capillary gas chromatography and the ion trap mass spectrometer in the tandem mode, is described. One-gram plasma samples were spiked with an anti-inflammatory drug at levels of 1–100 ng, and with 50 ng of a chemical analog internal standard. Crude extracts of the plasma samples are analyzed by using scan functions that utilize combined radiofrequency (rf) and de voltages. The need for combined rf- and de-voltage sequences for analysis of such extracts is demonstrated by comparison to attempted analyses using only rf voltages. Limitations of the method are: (1) the need for accurate calibration of ionization times to obtain linear calibration lines, and (2) the lack of automatic gain control for scans using combined rf and dc voltages to control and optimize parent ion populations and to allow a simpler analysis of “unknowns. ”     

Fragmentation study of hexanitrostilbene by ion trap multiple mass spectrometry and analysis by liquid chromatography/mass spectrometry

The fragmentation pathways of three explosive compounds with similar structures, hexanitrostilbene (HNS), cyclotrimethylene trinitramine (RDX), and 2,4,6-trinitrotoluene (TNT), have been investigated by multiple mass spectrometry (MSn, n = 1, 2, 3) with electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) sources. The electron capture mechanism for these compounds in negative ion APCI and ESI mode differs from the usual negative ion mechanism, deprotonation or addition of other species. This was shown for HNS and TNT, which both gave a [M]- anion but not a [M-H]- ion in APCI, and the [M]- anion of HNS was observed in ESI. The quantitative analysis of HNS was performed by liquid chromatography (LC)/ESI-MS, and the results obtained by the internal standard (ISTD) method were compared with those from the external standard (ESTD) method, demonstrating that both quantitation approaches are useful, with good sensitivity, reproducibility and linearity, and ESTD is preferable in routine applications.     

Direct chemical analysis of uv laser ablation products of organic polymers by using selective ion monitoring mode in gas chromatography/mass spectrometry

Trace quantities of laser ablated organic polymers were analyzed by using commercial capillary column gas chromatography/mass spectrometry; the instrument was modified so that the iaser ablation products could be introduced into the capillary column directly and the constituents of each peak in the chromatogram were identified by using a mass spectrometer. The present study takes advantage of the selective ion monitoring mode for significantly improving the sensitivity of the mass spectrometer as a detector, which is critical in anatyzing the trace quantities and confirming the presence or absence of the species of interest in laser ablated polymers. The initial composition of the laser ablated polymers was obtained by using an electron impact reflectron time-of-flight mass spectrometer and the possible structure of the fragments observed in the spectra was proposed based on the structure of the polymers.     

Direct determination of serotonin in gut lavage fluid by liquid chromatographic ion trap tandem mass spectrometry

Direct determination of serotonin (5-HT) in gut lavage fluid from patients examined due to various gastrointestinal complaints has been achieved. The method involves addition of 5-methoxytryptamine (5-CH3O-HT) internal standard, centrifugation, filtration and injection of the sample supernatant in a liquid chromatographic system coupled to an ion trap tandem mass detector. Electro-spray in positive mode was used to isolate and fragment the protonated ions [5-HT+H]+ and [5-CH3O-HT+H]+ signals 177 and 191 m/z, respectively. Quantification was carried out by extracting the ion fragment chromatograms at 160 and 174 m/z for 5-HT and 5-CH3O-HT, respectively. The relationship 5-HT/5-CH3O-HT was modelled by using a simultaneous design in order to estimate the optimal amount of internal standard to be added to the samples prior to quantification.     

离子阱颗粒质谱研究进展

随着质谱技术的不断发展,对超高质量颗粒物质的分析已经成为质谱领域研究的一个重要方向.离子阱颗粒质谱(particle ion trap mass spectrometry)作为用于完整颗粒质量分析的有利工具,拓展了质谱技术在巨大颗粒物质量分析中的应用范围.本文对离子阱颗粒质谱仪器的研究进展及其在各个领域的应用进行了综述,并展望了离子阱颗粒质谱未来的发展趋势.     

Quadrupole ion trap mass spectrometry of fullerenes

The fullerenes C₆₀ and C₇₀ can be ionized by desorption from a liquid matrix upon bombardment by Cs⁺ ions of 7 keV kinetic energy. The resulting radical cations, when activated in the ion trap by collisions with Xe target, in the presence of helium, undergo extensive dissociation by loss of multiple C₂ units. Large internal energies are deposited into these molecular ions and the dissociation efficiency is in excess of 60%.     

Synthesis of heptapeptides and analysis of sequence by tandem ion trap mass spectrometry

Two heptapeptides have been prepared by Fmoc methodology using Wang resin as solid support. For attachment of the first amino acid, several coupling systems were evaluated, and DIC/DMAP system could give yields of >99% and low levels of racemization. The selection of scavenger combination to deprotect side chains revealed that H₂O/p-cresol was good at scavenging trityl and 1,2-ethanedithiol was highly efficient for scavenging t-butyl. Through shortening the preactivation time to 5 min, the racemization which occurred during formation of amide bonds coupled by HBTU was minimized. The crude peptides were characterized by RP-HPLC and MS, and sequenced by MS/MS to acquire reliable amino acid sequence information.     

Recent developments in analytical ion trap mass spectrometry

The current status of quadrupole ion trap mass spectrometry is reviewed, with particular emphasis on liquid chromatographic coupling, membrane inlet introduction, laser desorption/ionisation and selective chemical ionisation. The flexibility, high sensitivity and multi-stage tandem mass spectrometric capability of the quadrupole ion trap are all illustrated.     

Sequence analysis of alginate-derived oligosaccharides by negative-ion electrospray tandem mass spectrometry

Negative-ion electrospray tandem mass spectrometry (ES-MS/MS) with collision-induced dissociation (CID) is attempted for sequence determination of alginate oligosaccharides, derived from polyanionic alginic acid, polymannuronate, and polyguluronate by partial depolymerization using either alginate lyase or mild acid hydrolysis. Sixteen homo- and hetero-oligomeric fragments were obtained after fractionation by gel-filtration and strong anion exchange high performance liquid chromatography. The product-ion spectra of these alginate oligosaccharides were dominated by intense B-, C-, Y-, and Z-type ions together with (0,2)A- and (2,5)A-ions of lower intensities. Internal mannuronate residues (M) produce weak but specific decarboxylated Z(int)-ions (Z(int) - 44 Da; int: denotes internal), which can be used for distinction of M and a guluronate residue (G) at an internal position. A reducing terminal M or G, although neither gives rise to a specific ion, can be identified by differences in the intensity ratio of fragment ions of the reducing terminal residue [(2,5)A(red)]/[(0,4)A(red)] (red: denotes reducing terminal).     

Determination of polar organic compounds in atmospheric aerosols by gas chromatography with ion trap tandem mass spectrometry

A gas chromatography with ion trap mass spectrometry method has been developed and validated for the analysis of 27 polar organic compounds in atmospheric aerosols. The target analytes were low‐molecular‐weight carboxylic acids and methoxyphenols, as relevant markers of source emissions and photochemical processes of organic aerosols. The operative parameters were optimized in order to achieve the best sensitivity and selectivity for the analysis. In comparison with the previous gas chromatography with mass spectrometry procedure based on single ion monitoring detection, the tandem mass spectrometry technique increased the analytical sensitivity by reducing detection limits for standard solutions from 1–2.6 to 0.1–0.4 ng/μL ranges (concentrations in the injected solution). In addition, it enhanced selectivity by reducing matrix interferences and chemical noise in the chromatogram. The applicability of the developed method in air quality monitoring campaigns was effectively checked by analyzing environmental samples collected in the Po Valley (Northern Italy) in different seasons. The obtained results indicate that the ion trap mass spectrometer may be an ideal alternative to high‐resolution mass spectrometers for the user‐friendly and cost‐effective determination of a wide range of molecular tracers in airborne particulate matter.     

Direct analysis of thin-layer chromatograms and electrophoretograms by secondary ion mass spectrometry

Mixtures separated by thin-layer chromatography and electrophoresis can be analyzed in situ by secondary ion mass spectrometry. The paramount advantage of such a combination is the selectivity inherent to the mass spectrum, which can provide information sufficient for the identification of an unknown compound as well as precise quantification of known compounds. Separation of the time frames of chromatographic development and mass spectrometer operation relaxes the constraints on each, and unique analytical advantages result.     

High-sensitivity analysis and sequencing of peptides and proteins by quadrupole ion trap mass spectrometry

This paper describes experience with the commercially available LCQ quadrupole ion trap mass spectrometer applied to the off-line analysis of peptides and proteins. The standard front end of the electrospray probe was replaced with a micromanipulator which, with the aid of a magnifying device, allowed the use of a variety of miniaturized spraying interfaces. The low sample consumption and extended analysis times of these devices were ideally suitable to obtain improved results in terms of sensitivity and mass accuracy. This needed a careful optimization of the number of ions stored inside the trap (ion target parameter) and required spectrum averaging of many scans. A method is presented for the mathematical fitting of ZoomScan spectra to theoretical isotopic distributions, which allowed the mass determination of large peptides with more accuracy than that achieved by conventional deconvolution algorithms. A very simple on-line desalting configuration is also described which needed no external micro-high-performance liquid chromatographic pumps, and can be easily mounted using the built-in syringe delivery system of the LCQ. This set-up allowed extended analysis times of 'in-gel' protein digests in subpicomole amounts. Finally, the multiple fragmentation capabilities of the ion trap were found to be extremely useful for the analysis of peptide modifications such as phosphorylation and for sequencing individual peptides from highly complex MHC-bound peptide pools.     

超高效液相色谱-线性离子阱/静电场轨道阱高分辨质谱直接分析葡萄酒中38种多酚类化合物

建立了超高效液相色谱-线性离子阱/静电场轨道阱高分辨质谱检测葡萄酒中38种多酚化合物的检测方法。样品过聚醚砜(PES)滤膜后直接上样分析,Hypersil Gold C18色谱柱分离,以乙腈(含0.1%甲酸)和0.1%甲酸水作为流动相梯度洗脱。在m/z 50~1000范围内进行一级质谱全扫描。以准分子离子峰的精确质量数和提取的色谱图峰面积进行筛查分析和定量,以保留时间和数据依赖扫描(data-dependent scan)模式获得的子离子质谱图进行定性确证。38种多酚化合物的质量偏差不大于5×10^-6(5 ppm),浓度与特征离子峰面积的线性关系良好(浓度线性范围为两个数量级),相关系数(R²)大于0.99,方法检出限为0.002~0.50 mg/kg。3个添加水平的回收率范围为90%~102%,相对标准偏差为0.51%~2.56%。应用该方法检测了葡萄酒中38种多酚化合物的含量,该方法准确、可靠。     

基于气溶胶质谱的二次有机气溶胶识别

二次有机气溶胶(SOA)是大气气溶胶十分重要的组成部分,也是目前人们认识最为薄弱的气溶胶组分.由于有机气溶胶化学组成的复杂性,对SOA进行有效的识别和估算一直是国际气溶胶研究领域的热点和难点问题.本研究尝试使用一种新方法来定量识别深圳冬季大气中的SOA:利用气溶胶质谱仪在线观测的高时间分辨率优势和质谱中的特征碎片离子,应用正定矩阵因子解析(PMF)模型对细粒子组分的主要来源进行解析,识别出其中的二次有机物.结果表明:深圳冬季大气细粒子中SOA浓度平均为9.41±6.33μg/m3,占总有机物质量的39.9±21.8%;相比于一次有机气溶胶(POA),SOA浓度水平变化较为平缓,体现了区域性二次污染物的特征.SOA/BC比值具有鲜明的日变化规律,且与Ox(O3+NO2)的日变化规律相似,说明SOA的生成过程显著地受控于大气光化学活性.深圳冬季大气SOA生成最活跃的时段约为9～15时,期间SOA/BC比值增长了122%.本文为研究我国大气二次有机气溶胶提供了一种新的技术方法和思路.