Mass Spectrometry,Review of the Basics: Ionization

Abstract: Mass spectrometry (MS) has become an integral tool in life sciences. The first step in MS analysis is ion formation (ionization). Many ionization methods currently exist; electrospray ionization (ESI) and matrix-assisted laser desorption ionization (MALDI) are the most commonly used. ESI relies on the formation of charged droplets releasing ions from the surface (ion evaporation model) or via complete solvent evaporation (charge residual model). MALDI ionization, however, is facilitated via laser energy and the use of a matrix. Despite wide use, ESI cannot efficiently ionize nonpolar compounds. Atmospheric pressure chemical ionization (APCI) and atmospheric pressure photo ionization (APPI) are better suited for such tasks. APPI requires photon energy and a dopant, whereas APCI is similar to chemical ionization. In 2004, ambient MS was introduced in which ionization occurs at the sample in its native form. Desorption electrospray ionization (DESI) and direct analysis in real time (DART) are the most widely used methods. In this mini-review, we provide an overview of the main ionization methods and the mechanisms of ion formation. This article is educational and intended for students/researchers who are not very familiar with MS and would like to learn the basics; it is not for MS experts.     

Secondary processes in atmospheric pressure chemical ionization-ion trap mass spectrometry: a case study of orotic acid

Atmospheric pressure chemical ionization is known for producing unusual artifacts of the ionization process in some cases. In this work, processes occuring in atmospheric pressure chemical ionization/MS of orotic acid that afforded ions accompanying protonated and deprotonated orotic acid molecules in the spectra were studied. Two processes ran in parallel in the ion source: decarboxylation of neutral orotic acid and collision-induced dissociation of its protonated or deprotonated form. A procedure discerning pre-ionization decomposition and post-ionization dissociation by manipulating ion source parameters was proposed. Experiments with isotopically labeled solvents confirmed ion-molecule reactions of the product of collision-induced dissociation of protonated orotic acid with solvent molecules in the ion source and even under vacuum in the ion trap.     

电弧及辉光微等离子体大气压取样的发射光谱研究

在线化学分析需要实现开放环境下的样品取样和电离/激发。相比于激光切削或者激光诱导击穿,大气压微等离子体系统结构简单,更利于小型化。因而基于大气压微等离子体的在线化学分析技术引起行业的广泛关注。为了确定合适的微等离子体源进行样品的在线元素检测,需要进一步了解各放电模式及工作参数下微等离子体的自身特性以及取样效果。该工作主要研究了电弧及辉光放电微等离子体在大气压下对样品铁取样发射光谱的特性。实现了在开放环境下对高熔点金属样品的在线检测,并发现电弧放电微等离子体与光谱分析源联用具有更高的取样效率。高采样效率的电弧放电微等离子体源为实现金属及难解离样品的检测提供了一种新的方法。同时,相较于传统的取样装置,避免了复杂的样品制备、样品传输过程。实验装置采取简单的针对板放电结构,分别利用高压脉冲电源、直流电源获得电弧放电和辉光放电。实验的结果表明,在放电功率近似相等的条件下,电弧放电产生的微等离子体对样品铁取样的光学发射谱中,样品元素的特征谱线占据主导地位,同时伴随有空气中氮气的谱线,而且铁离子(FeⅡ)谱线的相对强度显著高于氮气分子谱线的相对强度。而在直流辉光放电中,样品铁原子(FeⅠ)谱线相对强度非常不明显。由此说明,电弧放电产生的微等离子体具有更高的采样效率。放电在样品表面留下的溅射坑也得出了相同的结论。增加辉光放电电流到25 mA,发现样品元素铁的谱线仍然没有明显的增强。同时,也研究了采样间距对两种采样模式的影响。实验结果表明,间距对两种模式的采样光谱没有显著的影响。采用主要成分为铝的合金铝箔进行了上述对比实验,得出相同的结论,即电弧放电微等离子体更适合作为光谱分析源来实现对金属样品的实时快速检测。     

大气压基体辅助激光解析离子源发展及其应用

介绍了国际上新出现的一种离子源——大气压基体辅助激光解析离子源(atmospheric pressure matrix-assisted laser desorption/ionization,AP-MALDI)。该离子源是在真空MALDI的基础上研究发展起来的,广泛应用于生物大分子及其相关领域的研究。该离子源较真空MALDI电离更加柔和,产生的碎片离子更少,又由于其工作在大气压下,可作为外接离子源方便地与各类质谱分析器联用,大大扩展了基体辅助激光解析离子源的应用范围。文章详细介绍了AP-MALDI的基本原理、结构及技术进展。此外,还对该离子源在生物高聚物分析方面的最新应用进行了介绍。     

On the use of atmospheric pressure plasmas as electromagneticreflectors and absorbers

Tenuous man-made plasmas in the Earth's atmosphere from sea level to 100 km are discussed. An ionization source generates a tenuous plasma with an electron number density n_e that is high near the source and diminishes with distance from the source. After the source shuts off, n_e decreases as a function of time as electrons recombine with positive ions or attach to negative ions. The electromagnetic properties that are essential to an understanding of these plasmas, which can be modeled as cold collisional plasmas, is discussed. Gas and plasma characteristics, such as momentum-transfer collision rate, plasma lifetime, recombination kinetics, and the effect of noble gases, are presented. Typical collision rates and plasma lifetimes at atmospheric pressure are quantified. Applications for a plasma with a gradient are discussed. They include a high-altitude plasma that can reflect or absorb from HF to VHF and a broadband atmospheric pressure absorber. The generation and use of plasma, including electron impact ionization with a high-energy electron-beam source and UV photoionization of an alkali vapor or an organic vapor such as tetrakis (dimethylamino)ethylene (TMAE), is described. The power required to sustain a plasma is quantified, and properties such as maximum absorption and bandwidth are discussed. Tradeoffs among maximum absorption, absorption bandwidth, duty ratio, and applied power permit optimization of absorption primarily at VHF     

Analysis of lipids with desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS) and desorption electrospray ionization-mass spectrometry (DESI-MS)

In this article, the effect of spray solvent on the analysis of selected lipids including fatty acids, fat-soluble vitamins, triacylglycerols, steroids, phospholipids, and sphingolipids has been studied by two different ambient mass spectrometry (MS) methods, desorption electrospray ionization-MS (DESI-MS) and desorption atmospheric pressure photoionization-MS (DAPPI-MS). The ionization of the lipids with DESI and DAPPI was strongly dependent on the spray solvent. In most cases, the lipids were detected as protonated or deprotonated molecules; however, other ions were also formed, such as adduct ions (in DESI), [M-H](+) ions (in DESI and DAPPI), radical ions (in DAPPI), and abundant oxidation products (in DESI and DAPPI). DAPPI provided efficient desorption and ionization for neutral and less polar as well as for ionic lipids but caused extensive fragmentation for larger and more labile compounds because of a thermal desorption process. DESI was more suitable for the analysis of the large and labile lipids, but the ionization efficiency for less polar lipids was poor. Both methods were successfully applied to the direct analysis of lipids from pharmaceutical and food products. Although DESI and DAPPI provide efficient analysis of lipids, the multiple and largely unpredictable ionization reactions may set challenges for routine lipid analysis with these methods.     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Collison nebulizer as a new soft ionization source for mass spectrometry

We have proposed that a Collison-type nebulizer be used as an ionization source for mass spectrometry with ionization under atmospheric pressure. This source does not require the use of electric voltage, radioactive sources, heaters, or liquid pumps. It has been shown that the number of ions produced by the ⁶³Ni radioactive source is three to four times larger than the number of ions produced by acoustic ionization sources. We have considered the possibility of using a Collison-type nebulizer in combination with a vortex focusing system as an ion source for extractive ionization of compounds under atmospheric pressure. The ionization of volatile substances in crossflows of a charged aerosol and an analyte (for model compounds of the amine class, viz., diethylaniline, triamylamine, and cocaine) has been investigated. It has been shown that the limit of detecting cocaine vapor by this method is on the level of 4.6 × 10^–14 g/cm³.     

Probing the mechanisms of ambient ionization by laser-induced fluorescence spectroscopy

The ionization mechanisms of several atmospheric pressure ion sources based on desorption and ionization of samples deposited on a surface were studied. Home-built desorption electrospray ionization (DESI), laserspray ionization (LSI), and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) sources were characterized using low-molecular-weight compounds, in particular fluorescent dyes. Detection of the desorbed and ionized species was performed by laser-induced fluorescence and ion cyclotron resonance mass spectrometry. The dependences of the signal intensities on various experimental parameters were studied. The data obtained reveals common features, such as formation of solvated species and clusters in the ionization processes, in all of the techniques considered.     

GMAW电弧等离子体平衡成分计算及其在光谱学中的应用

计算了常压下3 000～25 000 K范围内熔化极气体保护焊(GMAW)保护气体Ar,CO₂,82%Ar-18%CO₂及其与Fe蒸汽的混合物的平衡成分。上述气体被看作一种Ar-CO₂-Fe等离子体,等离子体中的39种粒子被分为5种主元粒子和34种非主元粒子。根据化学方程,非主元粒子由主元粒子表示以减少未知数的个数和求解量,再利用牛顿迭代法对平衡方程进行求解,最终实现了成分求解。计算结果表明,Ar气随着温度升高依次发生一次电离和二次电离,CO₂气体除了在高温时发生原子电离外,在低温时(T<8 000 K)还存在CO₂,O₂,CO等分子的解离,82%Ar-18%CO₂混合气则既有解离又有电离。Fe的加入会增加等离子体的电子密度,特别是在15 000 K以下。等离子体成分的确定为GMAW电弧等离子体辐射属性计算以及电弧中Fe蒸汽浓度的光谱测定奠定了基础。     

腔增强吸收光谱技术在大气环境研究中的应用进展

大气污染是全球性环境问题,针对大气环境污染物检测技术的发展对大气环境研究至关重要。光谱分析方法因其具有特异性选择、高精度、高时间分辨等优势,已被广泛应用于大气污染物检测领域。腔增强吸收光谱(CEAS)技术由腔衰荡光谱(CRDS)技术发展而来,是通过测量透过高精细度谐振腔的光强获得分子吸收信息的高灵敏度探测技术。该技术自提出至今二十余年来,因为成本低、操作简单、灵敏度高、适应性强等优势,成为大气环境研究中痕量气体检测的重要手段。介绍了基于相干和非相干光源的两种CEAS技术原理和基本装置构成,其中,采用LED作为光源的非相干宽带腔增强吸收光谱(IBBCEAS),因其成本更低且能够检测几十纳米带宽的光谱范围,在大气研究领域中应用更加广泛。综述了国内外应用CEAS技术针对大气环境中氮氧化物(NO₂,NO₃,N₂O₅,HONO)、挥发性有机物（甲醛,乙二醛,甲基乙二醛,甲烷,乙烯）、卤素单质(I₂,Br₂)、含卤素化物(OIO,IO,BrO)、臭氧(O₃)及气溶胶等污染物的检测工作。同时,从光源、检测器、光腔结构改进、仪器检测灵敏度优化等多个方面对已有工作进行了全面的归纳总结,重点阐述了其在实验室条件下的检测能力,以及实际大气环境下应用的表现。最后从CEAS技术的系统优化和未来应用趋势方面做出了一定的展望。     

傅里叶变换红外光谱在大气遥感监测中的应用

傅里叶变换红外光谱遥感技术是一门综合探测技术，它可以对气体组分分进行远距离，实时，多组分同时监测。本文介绍了遥感傅里叶变换红外谱光在遥测大气层中痕量气体，污染源排放的废气，灭火过程产生的有毒气体、气体空间分布等方面的应用现状。     

差分光学吸收光谱(DOAS)技术在烟气SO2监测中的应用

本文将差分光学吸收光谱(Differential Optical Absorption Spectroscopy,DOAS)技术中的浓度反演方法运用到烟气SO₂监测中.利用SO₂分子在300nm附近具有强吸收的特性,测量出烟气SO₂的吸收光谱,用DOAS方法反演出SO₂分子的浓度,消除了烟气中烟尘、水汽和其它成分的影响,使在线实时测量成为可能,在实验室内对几种SO₂分子浓度样气进行了测量和浓度反演.     

Ultrasensitive laser spectroscopy for breath analysis

At present there are many reasons for seeking new methods and technologies that aim to develop new and more perfect sensors for different chemical compounds. However, the main reasons are safety ensuring and health care. In the paper, recent advances in the human breath analysis by the use of different techniques are presented. We have selected non-invasive ones ensuring detection of pathogenic changes at a molecular level. The presence of certain molecules in the human breath is used as an indicator of a specific disease. Thus, the analysis of the human breath is very useful for health monitoring. We have shown some examples of diseases’ biomarkers and various methods capable of detecting them. Described methods have been divided into non-optical and optical methods. The former ones are the following: gas chromatography, flame ionization detection, mass spectrometry, ion mobility spectrometry, proton transfer reaction mass spectrometry, selected ion flow tube mass spectrometry. In recent twenty years, the optical methods have become more popular, especially the laser techniques. They have a great potential for detection and monitoring of the components in the gas phase. These methods are characterized by high sensitivity and good selectivity. The spectroscopic sensors provide the opportunity to detect specific gases and to measure their concentration either in a sampling place or a remote one. Multipass spectroscopy, cavity ring-down spectroscopy, and photo-acoustic spectroscopy were characterised in the paper as well.     

Application of Mn nano-flower sculptured thin films produced on interdigitated pattern as cathode and anode electrodes in field ionization gas sensor

The photolitography method was used for producing interdigitated configurations for cathode and anode electrodes of a field ionization gas sensor in which Mn helical nano-flowers with 3-fold symmetry were deposited using oblique angle deposition together with rotation of the substrate about its surface normal, with each rotation divided into six sections. These sections were alternately rotated at high and low speeds. Three different distances were chosen in the design between anode and cathode electrodes, namely 40, 100 and 200 μm. Physical structure and morphology of electrodes were studied by field emission scanning electron microscope and atomic force microscope analyses.The breakdown voltage of the system was studied for nitrogen, oxygen, argon, air and carbon mono-oxide gases. Investigations with these gases at different distances between anode and cathode and different gas pressures confirmed Paschen's Law. Results showed that at low pressures, decreasing the gap between electrodes increases the breakdown voltage. With fewer gas molecules between the electrodes the number of interactions between particles is reduced and higher energies are required for ionization of gas molecules. At high pressures, the breakdown voltage is decreased because of an increased number of molecular interactions. The sensor demonstrated good selectivity between the different gases and selectivity was enhanced with increasing gas pressure. A direct relationship was found at low pressures (e.g., 0.1 mbar) between the breakdown voltage and the gas ionization energy while at high pressures (e.g., 1000 mbar) this relationship was reversed.     

Analysis of dammar resin with MALDI-FT-ICR-MS and APCI-FT-ICR-MS

Comprehensive analysis of high-resolution mass spectra of aged natural dammar resin obtained with Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) using matrix-assisted laser desorption/ionization (MALDI) and atmospheric pressure chemical ionization (APCI) is presented. Dammar resin is one of the most important components of painting varnishes. Dammar resin is a terpenoid resin (dominated by triterpenoids) with intrinsically very complex composition. This complexity further increases with aging. Ten different solvents and two-component solvent mixtures were tested for sample preparation. The most suitable solvent mixtures for the MALDI-FT-ICR-MS analysis were dichloromethane-acetone and dichloromethane-ethanol. The obtained MALDI-FTMS mass spectrum contains nine clusters of peaks in the m/z range of 420-2200, and the obtained APCI-FTMS mass spectrum contains three clusters of peaks in the m/z range of 380-910. The peaks in the clusters correspond to the oxygenated derivatives of terpenoids differing by the number of C(15)H(24) units. The clusters, in turn, are composed of subclusters differing by the number of oxygen atoms in the molecules. Thorough analysis and identification of the components (or groups of components) by their accurate m/z ratios was carried out, and molecular formulas (elemental compositions) of all major peaks in the MALDI-FTMS and APCI-FTMS spectra were identified (and groups of possible isomeric compounds were proposed). In the MALDI-FTMS and APCI-FTMS mass spectrum, besides the oxidized C(30), triterpenoids also peaks corresponding to C(29) and C(31) derivatives of triterpenoids (demethylated and methylated, correspondingly) were detected. MALDI and APCI are complementary ionization sources for the analysis of natural dammar resin. In the MALDI source, preferably polar (extensively oxidized) components of the resin are ionized (mostly as Na(+) adducts), whereas in the APCI source, preferably nonpolar (hydrocarbon and slightly oxidized) compounds are ionized (by protonation). Either of the two ionization methods, when used alone, gives an incomplete picture of the dammar resin composition.     

Continuous field measurements of delta(13)C-CO(2) and trace gases by FTIR spectroscopy

Continuous analysis of the (13)C/(12)C ratio of atmospheric CO(2) (delta(13)C-CO(2)) is a powerful tool to quantify CO(2) flux strengths of the two major ecosystem processes assimilation and respiration. Traditional laboratory techniques such as isotope ratio mass spectrometry (IRMS) in combination with flask sampling are subject to technical limitations that do not allow to fully characterising variations of atmospheric delta(13)C-CO(2) at all relevant timescales. In our study, we demonstrate the strength of Fourier transform infrared (FTIR) spectroscopy in combination with a PLS-based calibration strategy for online analysis of delta(13)C-CO(2) in ambient air. The ability of the instrument to measure delta(13)C-CO(2) was tested on a grassland field-site and compared with standard laboratory-based IRMS measurements made on field-collected flask samples. Both methods were in excellent agreement, with an average difference of 0.4 per thousand (n=81). Simultaneously, other important trace gases such as CO, N(2)O and CH(4) were analysed by FTIR spectroscopy.     

用于原子能级结构研究的激光共振电离光谱系统

激光共振电离光谱技术是一种利用一路或多路激光将待测原子选择性共振激发与电离,通过测量离子信号来研究原子能级结构的光谱技术。研建了一套激光共振电离光谱装置,用于原子高激发态能级结构参数的测量。分别从该装置的总体结构、关键技术和应用实例等方面进行了详细介绍。该套装置主要包括高调谐精度的染料激光器系统、高效的激光离子源系统和高分辨率的飞行时间质量分析器。染料激光器系统包括3台多纵模可调谐染料激光器和1台单纵模可调谐染料激光器,均为脉冲工作方式,重复频率为10 kHz,泵浦源均为532 nm的Nd∶YAG固体激光器。激光离子源系统包括原子化源、激光与原子相互作用区和离子光学透镜组三部分组成,样品在原子化源中被电加热实现原子化,喷射出的原子被激光选择性激发、电离,产生的离子被离子传输透镜整形成能量分散小、束窄的离子束。飞行时间质量分析器采用了反射式结构设计、脉冲垂直推斥技术和偏转板调节技术。利用此装置,实验测定了U原子的自电离态光谱,获得了U原子一条较佳的三色三光子共振电离路径,对应激光的波长分别为591.7,565.0和632.4 nm。此系统还可用于测量同位素位移和原子超精细结构等参数。另外,由于此系统中联用了质量分析器,因此可用于样品多元素分析、痕量元素分析、同位素丰度分析。     

Mass-spectrometric investigation of coal gases from samples with a low and high degree of coalification

The methods and results of experimental investigation into the composition of gases by mass spectrometry with electron impact ionization are described. Experiments are performed on a crossed-beams setup with ion separation and detection using a monopole mass spectrometer with an upgraded ion source. Emphasis is on determination of the molecular composition of gases, including methane, liberating from the coal material as a function of temperature.