Ion trap mass analysis at high pressure: an experimental characterization

In recent years, it has become increasingly interesting to understand the performance of mass spectrometers at pressures much higher than those employed with conventional operating conditions. This interest has been driven by several influences, including demand for the development of reduced‐power miniature mass spectrometers, desire for improved ion transfer into and through mass spectrometers, enhanced‐yield preparative mass separations, and mass filtering at the atmospheric pressure interface. In this study, an instrument was configured to allow for the performance characterization of a rectilinear ion trap (RIT) at pressures up to 50 mtorr with air used as the buffer gas. The mass analysis efficiency, mass resolution, isolation efficiency, and collision‐induced dissociation (CID) efficiency were evaluated at pressures ranging from 1 to 50 mtorr. The extent of degradation of mass resolution, isolation efficiency and ion stability as functions of pressure were characterized. Also, the optimal resonance ejection conditions were obtained at various pressures. Operations at 50 mtorr demonstrated improved CID efficiency in addition to peak widths of 2 and 5 m/z units (full width at half‐maximum, FWHM) for protonated caffeine (m/z 195) and Ultramark (m/z 1521) respectively. Copyright © 2009 John Wiley & Sons, Ltd.     

A digital ion trap mass spectrometer coupled with atmospheric pressure ion sources

In a digital ion trap (DIT), the quadrupole trapping and excitation waveforms are generated by the rapid switching between discrete d.c. voltage levels. As the timing of the switch can be controlled precisely by digital circuitry, the approach provides an opportunity to generate mass spectra by means of a frequency scan in contrast to the conventional voltage scan, thus providing a wider mass range of analysis. An instrument has been constructed which employs a 'non-stretched' ion trap and the field fault around the aperture of the end-cap electrode can be corrected electronically using a field-adjusting electrode. The ion trap was coupled with electrospray ionization (ESI) and atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI) sources to demonstrate the capability of the digital method. AP-MALDI mass spectra of singly charged ions with mass-to-charge ratios upto 17 000 Th were generated using a trapping voltage of only 1000 V. Forward and reverse mass scans at resolutions up to 19 000 and precursor ion isolation at resolutions up to 3500 with subsequent tandem mass spectrometric analysis were demonstrated. The method of generating the digital waveforms and period scan is described. Discussion of the issues of mass range, scan speed, ion trapping efficiency and collision-induced dissociation efficiency are also provided.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

Electric modeling and characterization of pulsed high‐voltage nanoelectrospray ionization sources by a miniature ion trap mass spectrometer

A better understanding of nanoelectrospray ionization (nano‐ESI) would be beneficial in further improving the performances of nano‐ESI. In this work, the pulsed high‐voltage (HV) nano‐ESI has been electrically modeled and then systematically characterized by both voltage‐current and mass spectrometry measurements. First, the equivalent resistance of a nano‐ESI source changes with respect to both emitter tip diameter and the HV applied. Increased voltage could improve both spray current and ionization efficiency of the pulsed HV nano‐ESI. Compared with conventional DC HV method, a pulsed HV has less heating effect on the capillary tip and thus allowing the application of a much higher voltage onto a nano‐ESI source. As a result, a pulsed HV nano‐ESI could further boost the ionization efficiency of nano‐ESI by employing even higher voltages than conventional DC nano‐ESI sources.     

A B-TOF mass spectrometer for the analysis of ions with extreme high start-up energies

Weak magnetic deflection is combined with two acceleration stage time-of-flight mass spectrometry and subsequent position-sensitive ion detection. The experimental method, called B-TOF mass spectrometry, is described with respect to its theoretical background and some experimental results. It is demonstrated that the technique has distinct advantages over other approaches, with special respect to the identification and analysis of very highly energetic ions with an initially large energy broadening (up to 1 MeV) and with high charge states (up to 30+). Similar energetic targets are a common case in intense laser-matter interaction processes found during laser ablation, laser-cluster and laser-molecule interaction and fast particle and x-ray generation from laser-heated plasma.     

Characterization of electrode surface roughness and its impact on ion trap mass analysis

With the recent trend towards mass spectrometer miniaturization, the fabrication of mass analyzers and other ion optical components is being performed at scales where critical dimensions range from several millimeters to several micrometers. Depending on the sizes of the objects and the nature of the fabrication method used, electrode surface roughness can become non‐negligible and affect the analytical performance of the mass analyzer. In this work, a method of characterizing surface roughness is introduced through the concept of spatial roughness frequency. The roughness of a given surface is quantitatively described using spatial roughness components at a series of frequencies and with characteristic intensities. Based on this concept, an analytical method has been developed to describe the electromagnetic field inside an electrode assembly including consideration for the electrode roughness. The methodology is applied in simplified form to cylindrical and rectilinear ion trap analyzers. Four types of surface finishes were applied to ion trap electrodes of various sizes to illustrate the surface roughness effects on the high‐order fields and to compare the analytical performance of the ion traps. Application of this method to arrays of large numbers of micro‐scale ion traps has enabled the impact of fabrication methodology to be evaluated in terms of mass resolution for the ion trap arrays. Copyright © 2008 John Wiley & Sons, Ltd.     

Diastereomeric differentiation of peptides with CuII and FeII complexation in an ion trap mass spectrometer

Complexation by transition metal ions (CuII and FeII) was successfully used to differentiate the diastereomeric YAGFL, YDAGFL and Y(D)AGF(D)L pentapeptides by electrospray ionization-ion trap mass spectrometry in the positive ion mode using low-energy collision conditions. This distinction was allowed by the stereochemical effects due to the (D)Leu/(L)Leu and the (D)Ala/(L)Ala residues yielding various steric interactions which direct relative dissociation rate constants of the binary [(M - H) + MeII]+ complexes (Me = Cu or Fe) subjected to low-energy, collision-induced dissociation processes. The interpretation of the collision-induced dissociation spectra obtained from the diastereomeric cationized peptides allowed the location of the deprotonated site(s), leading to the postulation of ion structures and fragmentation pathways for both the [(M - H) + CuII]+ and [(M - H) + FeII]+ complexes, which differed significantly. With CuII, consecutive fragmentations, initiated by the decarboxylation at C-terminus, were favored relative to sequence product ions. On the other hand, with FeII, competitive fragmentations resulting in abundant sequence product ions and significant internal losses were preferred. This could be explained by different localizations of the negative charge, which directs the orientation of both the [(M - H) + CuII]+ and [(M - H) + FeII]+ binary complexes fragmentations. Indeed, the free negative charge of the [(M - H) + CuII]+ ions was mainly located at one oxygen atom: either at the C-terminal carboxylic group or, to a minor extent, at the Tyr phenol group (i.e. zwitterionic forms). On the other hand, the negative charge of the [(M - H) + FeII]+ ions was mainly located at one of the nitrogen atoms of the peptide backbone and coordinated to FeII (i.e. salt non-zwitterionic form).Moreover, this study reveals the particular behavior of CuII reduced to CuI, which promotes radical losses not observed from the peptide-FeII complexes. Finally, this study shows the analytical potentialities of the complexation of transition metal ions with peptides providing structural information complementary to that obtained from low-energy, collision-induced dissociation processes of protonated or deprotonated peptides.     

Quantification and evaluation of ion transmission efficiency in two-stage vacuum chamber miniature mass spectrometer

Miniature mass spectrometer is more compact and portable than traditional commercial mass spectrometry, with more potential for application outside the laboratory. However, a miniature mass spectrometer is less sensitive than a commercial instrument, limiting its application scenarios. The ion transmission efficiency of the instrument is an essential factor affecting the sensitivity. Still, there are few works of literature on the quantitative study of the ion transmission efficiency of each component from a systematic perspective. In this paper, the Faraday cup coupled with a microcurrent signal testing instrument was used to measure the ions generated by nanoelectrospray ionization (nano-ESI), which have successfully gone through several components. Then the ion transmission efficiency of each component was quantified. Results showed that the front lens had the highest ion transmission efficiency of 39.7%, whereas the inlet and skimmer had the lowest ion transfer efficiency of 0.8% and 17.1%. Next, the influence of control parameters on ion transmission efficiency of critical components was investigated. If optimized, the ion funnel and the skimmer had the potential to improve their transmission efficiency by 120% and 79%, respectively. This paper shows the decreasing intensity distribution of ions in the whole transmission process and the transmission efficiency of each component, which can guide for improving the sensitivity of the miniature mass spectrometer.     

Rapid screening and characterization of drug metabolites using a new quadrupole-linear ion trap mass spectrometer

The application of a new hybrid RF/DC quadrupole-linear ion trap mass spectrometer to support drug metabolism and pharmacokinetic studies is described. The instrument is based on a quadrupole ion path and is capable of conventional tandem mass spectrometry (MS/MS) as well as several high-sensitivity ion trap MS scans using the final quadrupole as a linear ion trap. Several pharmaceutical compounds, including trocade, remikiren and tolcapone, were used to evaluate the capabilities of the system with positive and negative turbo ionspray, using either information-dependent data acquisition (IDA) or targeted analysis for the screening, identification and quantification of metabolites. Owing to the MS/MS in-space configuration, quadrupole-like CID spectra with ion trap sensitivity can be obtained without the classical low mass cutoff of 3D ion traps. The system also has MS(3) capability which allows fragmentation cascades to be followed. The combination of constant neutral loss or precursor ion scan with the enhanced product ion scan was found to be very selective for identifying metabolites at the picogram level in very complex matrices. Owing to the very high cycle time and, depending on the mass range, up to eight different MS experiments could be performed simultaneously without compromising chromatographic performance. Targeted product ion analysis was found to be complementary to IDA, in particular for very low concentrations. Comparable sensitivity was found in enhanced product ion scan and selected reaction monitoring modes. The instrument is particularly suitable for both qualitative and quantitative analysis.     

银离子高效液相色谱-质谱法分析血清中甘油三酯类化合物的组成

针对甘油三酯(TAG)类化合物的复杂性,建立了分析小鼠血清中TAG类化合物的方法。采用经典的氯仿-甲醇溶剂体系对血中的TAG类化合物进行提取。脂质提取物经Varian ChromSpher 5 Lipids柱分离,在0.75 mL/min的流速下以乙腈-正己烷(1:99, v/v)为流动相进行等度洗脱,采用大气压化学电离源正离子模式电离,质谱增强型全扫描、增强型子离子扫描和中性丢失扫描模式检测。根据银离子色谱对双键的保留规律以及质谱所给出的碎片离子信息,对血清中TAG类化合物进行了结构鉴定。结果表明采用该方法可以从小鼠血清中鉴定到66个TAG类化合物以及5个胆固醇酯。该方法简单,重现性好,可通用于其他样品中TAG类化合物的检测。     

高效液相色谱-线性离子阱/静电场轨道阱质谱快速筛查确证生态纺织品中22种致敏性分散染料

建立了基于高效液相色谱-线性离子阱/静电场轨道阱高分辨质谱(HPLC-LTQ/Orbitrap MS)快速筛查确证生态纺织品中22种禁用分散染料的分析方法。样品在95 ℃水浴条件下经吡啶/水(1:1, v/v)振荡提取后,以CAPCELL PAK C₁₈色谱柱分离,以乙腈和5 mmol/L乙酸铵(含体积分数为0.01%的甲酸)作为色谱流动相进行梯度洗脱,采用正、负电喷雾(ESI)离子化模式,利用一级母离子的精确质量数和保留时间对22种分散染料进行快速筛查,利用碰撞诱导解离(CID)下得到的二级碎片离子进行确证。22种分散染料在各自浓度范围内线性关系良好(r²> 0.99),方法的定量限(LOQ)为0.125~2.5 mg/kg。除分散黄49外,绝大多数染料在涤纶布、棉涤混纺布两种纺织品基质中的加标回收率在65%~120%之间,相对标准偏差小于15%。应用本方法对涉及多种纤维类型的40余件纺织品样品进行了筛查,其中一个样品检出分散橙37/76。该方法简便、快速,其选择性高,抗干扰性能好,结果准确可靠,可用于纺织品中分散染料的检测。     

Targeted biomarker detection via whole protein ion trap tandem mass spectrometry: thymosin beta4 in a human lung cancer cell line

N-Terminally acetylated thymosin beta4, a species implicated for use as a cancer biomarker, was identified in a human lung cancer cell line using ion trap tandem mass spectrometry at the whole protein level. Ion-ion proton transfer reactions were used for parent ion concentration/manipulation and to simplify interpretation of product ion spectra. Dissociation data for the +6 to +3 charge states are reported. As is usually the case, structural information available from the ion trap collisional activation of the protein is sensitive to parent ion charge state. Each parent ion charge state selected, however, provided sufficient information to make a confident identification. Furthermore, each charge state provided relatively rich fragmentation. Therefore, any of the charge states can be used to detect with high specificity thymosin beta(4) in a complex protein mixture. There are advantages associated with the rapid detection of protein biomarkers at the whole protein level, as opposed to the peptide level following protein digestion, particularly for relatively small protein and polypeptide biomarkers. Having identified and characterized the protein, product ion spectra obtained directly, without recourse to ion-ion proton transfer reactions, can be used for library matching. However, ion-ion proton transfer reactions for parent ion concentration and charge state purification are advantageous in addressing relatively complex mixtures.     

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。     

Thermal desorption counter‐flow introduction atmospheric pressure chemical ionization for direct mass spectrometry of ecstasy tablets

A novel approach to the analysis of ecstasy tablets by direct mass spectrometry coupled with thermal desorption (TD) and counter‐flow introduction atmospheric pressure chemical ionization (CFI‐APCI) is described. Analytes were thermally desorbed with a metal block heater and introduced to a CFI‐APCI source with ambient air by a diaphragm pump. Water in the air was sufficient to act as the reactive reagent responsible for the generation of ions in the positive corona discharge. TD‐CFI‐APCI required neither a nebulizing gas nor solvent flow and the accompanying laborious optimizations. Ions generated were sent in the direction opposite to the air flow by an electric field and introduced into an ion trap mass spectrometer. The major ions corresponding to the protonated molecules ([M + H]⁺) were observed with several fragment ions in full scan mass spectrometry (MS) mode. Collision‐induced dissociation of protonated molecules gave characteristic product‐ion mass spectra and provided identification of the analytes within 5 s. The method required neither sample pretreatment nor a chromatographic separation step. The effectiveness of the combination of TD and CFI‐APCI was demonstrated by application to the direct mass spectrometric analysis of ecstasy tablets and legal pharmaceutical products. Copyright © 2009 John Wiley & Sons, Ltd.     

Extensive study on physicochemical properties of polychlorinated biphenyls in a commercial ion trap mass spectrometer,relevance in analytical and environmental chemistry

Polychlorinated biphenyls (PCBs) exist as 209 congeners, consisting of biphenyl molecules, where the number and substitution positions of halogen atoms are known to affect industrial uses, environmental transport mechanisms, distribution, fate, and toxicity. The complexity of the problem requires accurate physicochemical studies of an increasing number of congeners in order to understand the environmental and biological processes at play. This work presents a systematic study on the thermodynamic and kinetic properties of PCBs by quadrupole ion trap mass spectrometry. A clear relationship between structure and behavior of PCBs in mass spectrometry experiments has been observed. Overall data demonstrate that di‐ortho congeners show lower thermodynamic stability and higher fragmentation rate than non/mono‐ortho. Congeners follow different fragmentation mechanisms according to the number of chlorine atoms in ortho position of the biphenyl system. Experimental kinetic curves of mono/non‐ortho and di‐ortho congeners show a strong similarity with classical first‐order kinetics curves; in particular, di‐ortho congeners follow a first‐order consecutive reaction, while mono/non‐ortho follow a first‐order parallel reaction. For each studied congener, the kinetic constant of reaction (fragmentation) has been determined. Data support environmental levels and biochemical transformations described in literature. The general picture of the PCB behavior inside a quadrupole ion trap provides the basis for the development of reliable and cost‐effective analytical methods to the determination of ultra‐low level trace of PCB congeners.     

Analyzing slowly exchanging protein conformations by ion mobility mass spectrometry: study of the dynamic equilibrium of prolyl oligopeptidase

Ion mobility mass spectrometry (IMMS) is a biophysical technique that allows the separation of isobaric species on the basis of their size and shape. The high separation capacity, sensitivity and relatively fast time scale measurements confer IMMS great potential for the study of proteins in slow (µs–ms) conformational equilibrium in solution. However, the use of this technique for examining dynamic proteins is still not generalized. One of the major limitations is the instability of protein ions in the gas phase, which raises the question as to what extent the structures detected reflect those in solution. Here, we addressed this issue by analyzing the conformational landscape of prolyl oligopeptidase (POP) – a model of a large dynamic enzyme in the µs–ms range – by native IMMS and compared the results obtained in the gas phase with those obtained in solution. In order to interpret the experimental results, we used theoretical simulations. In addition, the stability of POP gaseous ions was explored by charge reduction and collision‐induced unfolding experiments. Our experiments disclosed two species of POP in the gas phase, which correlated well with the open and closed conformations in equilibrium in solution; moreover, a gas‐phase collapsed form of POP was also detected. Therefore, our findings not only support the potential of IMMS for the study of multiple co‐existing conformations of large proteins in slow dynamic equilibrium in solution but also stress the need for careful data analysis to avoid artifacts. Copyright © 2016 John Wiley & Sons, Ltd.     

Ion manipulation and enrichment mass spectrometer for cleavage of disulfide bond via ion/ion reaction

A novel mass spectrometer with the capability of ion manipulation and enrichment was developed to perform gas‐phase ion/ion reactions followed by product ions accumulation. The development of this apparatus opens opportunities for more complex sequences of ion manipulations, thus offers the potential on extensive application involving ion/ion reaction. Here, cleavage of disulfide bond in peptide was demonstrated based upon this ion manipulation and enrichment mass spectrometer. Two typical peptides including S‐glutathionylated ARACAKA with an intermolecular disulfide bond, and oxytocin with an intramolecular disulfide bond were chosen as typical samples to demonstrate the ability of the apparatus. After ion/ion reaction between selected peptide cations and periodate ions (IO₄^?), two kinds of product ions (eg, [M + O + H]⁺ and [M + H + Na + IO₄]⁺) were enriched with a number of accumulation events. Afterwards, the enriched ions were subjected to activation, and the disulfide bond cleavage was clearly observed from the tandem mass spectra. These results illustrate the potential of this apparatus for ion manipulation performing ion/ion reaction, and low abundance product ion enrichment.