An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part I: Optimization and characteristics

An inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS) has been constructed and evaluated for elemental analysis. The instrument produces analog spectra similar to those from quadrupole inductively coupled plasma mass spectrometers. The large abundance of Ar ions is deflected away from the microchannel plate detector to reduce detector dead time and space-charge complications. The ICP-TOFMS, operated in a linear (nonreflecting) mode, currently has a resolving power of 500 (full width at half maximum). Present ion optics employed in the instrument require a trade-off between signal-to-noise ratio and resolving power. In addition, mass-dependent kinetic energies in the supersonic beam created in the ICP mass spectrometer interface cause a mass bias in the right-angle TOFMS because the ions must be steered to the detector to compensate for their velocity in the supersonic beam direction. In the current design the sampling duty cycle is only approximately 3%, thereby limiting sensitivity. However, positive potentials applied to the right-angle extraction region can increase sensitivity by a factor of 2–4 by slowing down the ions that enter the extraction zone. The transmission efficiency of the TOFMS is approximately 20% and is limited by divergence of the ion packet in the drift tube.     

Optimization of a laser ablation-inductively coupled plasma "time of flight" mass spectrometry system for short transient signal acquisition

Simultaneous ion sampling and sequential detection offered by inductively coupled plasma 'time of flight' mass spectrometry (ICP-TOFMS) provides advantages for the analysis of short transient concentration-variable signals as produced in laser ablation. In order to investigate the capabilities of ICP-TOFMS in combination with an excimer laser ablation system, ablation studies on reference materials and geological samples were carried out. Various ICP-TOFMS parameters were optimized for laser-induced aerosols. Transverse rejection ion pulse was used to extend the dynamic range in concentration. A reduced volume ablation cell was designed and used in order to increase the sample density in the ICP. Results for 63 simultaneously measured isotopes (SRM 610 from NIST) lead to limits of detection in the 1-100 microg/g range for a 80 microm crater diameter (10 Hz, 1.2 mJ pulse energy). The reproducibility of signal ratios was determined to be better than 2% RSD for transient signals using 102 ms integration time. These optimized parameters were then used for the analysis of tin-rich fluid inclusions. Preliminary results of multielement analysis and isotopic ratio determinations on individual fluid inclusions (63 isotopes, 102 ms integration time) demonstrate the capabilities of ICP-TOFMS in combination with laser ablation.     

Optimization of a laser ablation-inductively coupled plasma “time of flight” mass spectrometry system for short transient signal acquisition

Simultaneous ion sampling and sequential detection offered by inductively coupled plasma ‘time of flight’ mass spectrometry (ICP-TOFMS) provides advantages for the analysis of short transient concentration-variable signals as produced in laser ablation. In order to investigate the capabilities of ICP-TOFMS in combination with an excimer laser ablation system, ablation studies on reference materials and geological samples were carried out. Various ICP-TOFMS parameters were optimized for laser-induced aerosols. Transverse rejection ion pulse was used to extend the dynamic range in concentration. A reduced volume ablation cell was designed and used in order to increase the sample density in the ICP. Results for 63 simultaneously measured isotopes (SRM 610 from NIST) lead to limits of detection in the 1–100 μg/g range for a 80 μm crater diameter (10 Hz, 1.2 mJ pulse energy). The reproducibility of signal ratios was determined to be better than 2% RSD for transient signals using 102 ms integration time. These optimized parameters were then used for the analysis of tin-rich fluid inclusions. Preliminary results of multielement analysis and isotopic ratio determinations on individual fluid inclusions (63 isotopes, 102 ms integration time) demonstrate the capabilities of ICP-TOFMS in combination with laser ablation. Received: 6 March 2000 / Revised: 11 May 2000 / Accepted: 14 May 2000     

电感耦合等离子体质谱法测定硼同位素丰度

以硼同位素标准物质NIST SRM 951配制标准溶液,在优化的仪器操作条件下对电感耦合等离子体质谱(ICP-MS)测定的硼同位素质量进行校正,求出校正因子,确定了样品的线性浓度范围,选定样品浓度为1.1 mg/L。在同样的仪器条件下首先测定了硼标准物质的硼同位素丰度比,测量误差为0.2%,然后测定了硼同位素浓缩过程中硼样品的硼同位素丰度比,测定结果的相对标准偏差为1.1%。此外考察了仪器的稳定性。实验结果表明本方法“记忆效应”小,结果可靠,测量精度高。     

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part III: Analytical performance

A time-of-flight mass spectrometer (TOFMS) was evaluated as a mass analyzer for inductively coupled plasma mass spectrometry (ICP-MS). The long-term drift of signals was in the range of 7–8% relative standard deviation, whereas the short-term precision was between 5 and 20%, somewhat worse than is typically reported for commercial ICP-MS instruments (5%). However, precision can be improved considerably in the TOFMS by ratioing isotopic peaks or through internal standardization, a consequence of its ability to extract all measured ions simultaneously from the inductively coupled plasma. This feature was demonstrated by monitoring the ²⁰⁶Pb/²⁰⁸Pb ratio with boxcar averagers. In this ratioing mode, precision was improved to approximately 0. 5%. Detection limits were measured with two alternative signal processing systems: (1) discriminator-gated integration and (2) integration of digitized spectra. Both methods improved the signal-to-noise ratio by a factor of from 10 to 100, although detection limits were still 1–2 orders of magnitude poorer for most elements than from the best commercial ICP-MS instruments. The dynamic range of the discriminator-gated integration system is over 4 orders of magnitude, but can be extended to 10⁶ with planned increases in primary ion-beam current, which is currently 10–100 times lower than is found in other instruments. Virtually simultaneous multielement and multiisotope analysis is possible for masses from ⁷Li to ²⁰⁹Bi with minimal mass bias and detection limits on the 0. 4–2-ppb level.     

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part II: Direct current quadrupole lens system for improved performance

An electrostatic quadrupole lens has been substituted for a cylindrical lens system used in the original inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS). With an improved vacuum system also installed, the cylindrical and quadrupole lenses are compared to each other and to the performance of the prototype ICP-TOFMS. The quadrupole lens requires no tradeoff between ion throughput and resolving power as was encountered with cylindrical lenses. The background noise in both ion-optical systems is within the same order of magnitude. Images of the ion beam formed by each ion-optical system have been obtained on a microchannel plate-phosphor screen. The quadrupole lens shows a higher ion-beam flux and produces a slitlike focus required in the orthogonal ICP-TOFMS instrument. Signal-to-noise ratios in the ICP-TOFMS can be improved by using a technique called pulsed-ion injection that is particularly convenient with the quadrupole lens. In this technique, one quadrupole electrode is pulsed to prevent ions from entering the extraction zone except when an ion packet is to be extracted for mass analysis. This technique significantly reduces the noise over continuous ion injection. In the orthogonal ICP-TOFMS with pulsed-ion injection, 0.5 frnol of analyte could be detected in 1.4 ms with a proper data acquisition system. Overall, the combination of a quadrupole lens and pulsed-ion injection may provide detection limits for the ICP-TOFMS that are competitive with those of quadrupole inductively coupled plasma-mass spectrometry instruments.     

Species-specific isotope-ratio measurements of volatile tin and antimony compounds using capillary GC-ICP-time-of-flight MS

The analytical performance of an axial inductively-coupled-plasma time-of-flight mass spectrometer (ICP-TOFMS) as a detector for fast transient chromatographic signals resulting from the coupling to capillary gas chromatography (CGC) was investigated. A cryotrapping GC-ICP-TOFMS method for the determination of volatile metal(loid) compounds (VOMs) in gases was used and the suitability of the TOF mass analyzer for multielemental speciation analysis and multi-isotope ratio determinations was studied in terms of accuracy and precision. Isotope ratios 118Sn/120Sn and 121Sb/123Sb have been determined in in-house gas standard atmospheres in Tedlar bags at two different levels (100 pg and 1 ng) for different elemental species (SnH4, MeSnH3, Me2SnH2, Me3SnH, BuSnH3, SbH3, and MeSbH2). A limitation arising from counting statistics in both detection modes could be shown. A solution containing rhodium (10 ng mL(-1)) and cadmium (40 ng mL(-1)) was introduced simultaneously to the GC outlet. Rhodium acts as a continuous internal standard and Cd is used for mass-bias correction (by measuring the 111Cd/113Cd ratio). The detection system in both pulse counting and analog mode was examined. The best attainable precision was established for Me2SnH2 (analog mode, 12 replicates, 1 ng, RSD 0.34%, accuracy 0.31%) whereas most other species ranged between 0.4 and 0.5% RSD if higher concentrations were used. The limitations of the pulse counting system are clearly seen, with peak heights of more than 2000 counts reaching saturation (for an integration time of 100 ms), which reduces the accuracy of isotope ratio determinations. A dozen VOM could be detected in an aged landfill gas sample; several unidentified Sn compounds were present. Although their isotope ratios are within the confidence value of the standards, it is not yet clear if the acquired precision is good enough to identify isotopic fractionation of metal(loid)s through biovolatilization processes. With the precision achieved, the combination of cryotrapping GC and ICP-TOFMS is a powerful tool for monitoring volatile multi-element species in multi-tracer experiments and isotope dilution methodology.     

Precise and accurate isotope ratio measurements by ICP-MS

The precise and accurate determination of isotope ratios by inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) is important for quite different application fields (e.g. for isotope ratio measurements of stable isotopes in nature, especially for the investigation of isotope variation in nature or age dating, for determining isotope ratios of radiogenic elements in the nuclear industry, quality assurance of fuel material, for reprocessing plants, nuclear material accounting and radioactive waste control, for tracer experiments using stable isotopes or long-lived radionuclides in biological or medical studies). Thermal ionization mass spectrometry (TIMS), which used to be the dominant analytical technique for precise isotope ratio measurements, is being increasingly replaced for isotope ratio measurements by ICP-MS due to its excellent sensitivity, precision and good accuracy. Instrumental progress in ICP-MS was achieved by the introduction of the collision cell interface in order to dissociate many disturbing argon-based molecular ions, thermalize the ions and neutralize the disturbing argon ions of plasma gas (Ar+). The application of the collision cell in ICP-QMS results in a higher ion transmission, improved sensitivity and better precision of isotope ratio measurements compared to quadrupole ICP-MS without the collision cell [e.g., for 235U/238U approximately 1 (10 microg x L(-1) uranium) 0.07% relative standard deviation (RSD) vs. 0.2% RSD in short-term measurements (n = 5)]. A significant instrumental improvement for ICP-MS is the multicollector device (MC-ICP-MS) in order to obtain a better precision of isotope ratio measurements (with a precision of up to 0.002%, RSD). CE- and HPLC-ICP-MS are used for the separation of isobaric interferences of long-lived radionuclides and stable isotopes by determination of spallation nuclide abundances in an irradiated tantalum target.     

Inorganic trace analysis by mass spectrometry

Mass spectrometric methods for the trace analysis of inorganic materials with their ability to provide a very sensitive multielemental analysis have been established for the determination of trace and ultratrace elements in high-purity materials (metals, semiconductors and insulators), in different technical samples (e.g. alloys, pure chemicals, ceramics, thin films, ion-implanted semiconductors), in environmental samples (waters, soils, biological and medical materials) and geological samples. Whereas such techniques as spark source mass spectrometry (SSMS), laser ionization mass spectrometry (LIMS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), glow discharge mass spectrometry (GDMS), secondary ion mass spectrometry (SIMS) and inductively coupled plasma mass spectrometry (ICP-MS) have multielemental capability, other methods such as thermal ionization mass spectrometry (TIMS), accelerator mass spectrometry (AMS) and resonance ionization mass spectrometry (RIMS) have been used for sensitive mono- or oligoelemental ultratrace analysis (and precise determination of isotopic ratios) in solid samples. The limits of detection for chemical elements using these mass spectrometric techniques are in the low ng g⁻¹ concentration range. The quantification of the analytical results of mass spectrometric methods is sometimes difficult due to a lack of matrix-fitted multielement standard reference materials (SRMs) for many solid samples. Therefore, owing to the simple quantification procedure of the aqueous solution, inductively coupled plasma mass spectrometry (ICP-MS) is being increasingly used for the characterization of solid samples after sample dissolution. ICP-MS is often combined with special sample introduction equipment (e.g. flow injection, hydride generation, high performance liquid chromatography (HPLC) or electrothermal vaporization) or an off-line matrix separation and enrichment of trace impurities (especially for characterization of high-purity materials and environmental samples) is used in order to improve the detection limits of trace elements. Furthermore, the determination of chemical elements in the trace and ultratrace concentration range is often difficult and can be disturbed through mass interferences of analyte ions by molecular ions at the same nominal mass. By applying double-focusing sector field mass spectrometry at the required mass resolution—by the mass spectrometric separation of molecular ions from the analyte ions—it is often possible to overcome these interference problems. Commercial instrumental equipment, the capability (detection limits, accuracy, precision) and the analytical application fields of mass spectrometric methods for the determination of trace and ultratrace elements and for surface analysis are discussed.     

The potential of inductively coupled plasma mass spectrometry detection for high-performance liquid chromatography combined with accurate mass measurement of organic pharmaceutical compounds

Quantification of unknown components in pharmaceutical, metabolic and environmental samples is an important but difficult task. Most commonly used detectors (like UV, RI or MS) require standards of each analyte for accurate quantification. Even if the chemical structure or elemental composition is known, the response from these detectors is difficult to predict with any accuracy. In inductively coupled plasma mass spectrometry (ICP-MS) compounds are atomised and ionised irrespective of the chemical structure(s) incorporating the element of interest. Liquid chromatography coupled with inductively coupled plasma mass spectrometry (LC/ICP-MS) has been shown to provide a generic detection for structurally non-correlated compounds with common elements like phosphorus and iodine. Detection of selected elements gives a better quantification of tested 'unknowns' than UV and organic mass spectrometric detection. It was shown that the ultrasonic nebuliser did not introduce any measurable dead volume and preserves the separation efficiency of the system. ICP-MS can be used in combination with many different mobile phases ranging from 0-100% organic modifier. The dynamic range was found to exceed 2.5 orders of magnitude. The application of LC/ICP-MS to pharmaceutical drugs and formulations has shown that impurities can be quantified below the 0.1 mol-% level.     

Determination of trace-level haloacetic acids in drinking water by ion chromatography-inductively coupled plasma mass spectrometry

A new method for the determination of nine haloacetic acids (HAAs) with ion chromatography (IC) coupled to inductively coupled plasma mass spectrometry (ICP-MS) was developed. With the very hydrophilic anion-exchange column and steep gradient of sodium hydroxide, the nine HAAs could be well separated in 15 min. After suppression with an ASRS suppressor that was introduced in between IC and ICP-MS, the background was much decreased, the interference caused by sodium ion present in eluent was removed, and the sensitivities of HAAs were greatly improved. The chlorinated and brominated HAAs could be detected as 35ClO and 79Br without interference of the matrix due to the elemental selective ICP-MS. The detection limits for mono-, di-, trichloroacetic acids were between 15.6 and 23.6 microg/l. For the other six bromine-containing HAAs, the detection limits were between 0.34 and 0.99 microg/l. With the pretreatment of OnGuard Ag cartridge to remove high concentration of chloride in sample, the developed method could be applied to the determination of HAAs in many drinking water matrices.     

Evaluation and correction of isotope ratio inaccuracy on inductively coupled plasma time-of-flight mass spectrometry

Isotope ratio measurements are found to have systematic bias when using the analog detection mode on an inductively coupled plasma time-of-flight (TOF) mass spectrometer. This bias is dependent upon the value of the ratio, the intensity of the signal, and the gain of the electron multiplier tube. The error should not appear if ion counting is employed instead of analog detection, although analog detection with time-of-flight has other distinct advantages. The cause of this isotope ratio inaccuracy is rooted in disproportionate recording of the analog signal because of the need to filter out noise by blocking analog signals below a threshold voltage. This attenuates smaller signals to a greater degree than larger signals. This variable “detection efficiency” causes a larger systematic error in the isotopic ratio as the isotopic abundances become more disparate. Ratios close to unity are generally accurate within the precision of the measurement. The use of an increased gain on the detector leads to improved ratio accuracy, but at the cost of decreased detector lifetime. This research presents a method of analyzing solutions using natural, known isotopic ratios to produce an efficiency correction curve. The average error of several isotope ratios for a 500 ng/mL solution of various elements with ratios between 3.4 and 10 was found to be 6.5% without correction, 3.0% with increased detector gain, 1.1% with efficiency correction and 0.6% with both increased gain and efficiency correction.     

Validating a Single-Particle ICP-MS Method to Measure Nanoparticles in Human Whole Blood for Nanotoxicology

With the rising use of nanomaterials in everyday objects, an uptake of those materials by humans is very likely. To assess their effect on the body, reliable detection of nanoparticles even in very low amounts in human blood becomes increasingly important. Single-particle (SP) inductively coupled plasma–mass spectrometry (ICP-MS) is a fast and reliable method for counting and sizing particles at lowest concentrations, while simultaneously distinguishing between dissolved and particulate analytes. For the first time to our knowledge, a dedicated method validation approach for silver and gold nanoparticle analysis in human whole blood has been performed. With only little sample preparation, trueness expressed as bias with values below or equal to 5% and imprecision values below 6% for particle size as well as an effective limit of detection of around 20?nm are possible. The concentration working range throughout all measured samples was in the nanogram per liter range, making SP-ICP-MS a powerful tool in future nanotoxicological applications and trace analysis of nanoparticles.     

微波消解-ICP-MS法测定药用明胶空心胶囊中微量重金属元素

采用微波消解技术,建立了一种电感耦合等离子体质谱法测定明胶空心胶囊中Cr,Co,Ni,Cu,Zn,As,Cd,Hg和Pb 9种微量重金属元素的方法。确定了微波消解仪和等离子体质谱仪的最佳操作参数,研究共存离子的干扰和消除方法,选择了各元素的测定同位素,以Ge,Rh和Tl为内标补偿基体效应,建立了样品测定方法。应用拟定的方法测定了不同生产厂家、不同批次的空心胶囊中微量重金属的含量。方法对试样中各元素测定的相对标准偏差为1.5%~14.1%,加标回收率在90.0%~102.0%。结果表明,方法简便、快速、灵敏,满足于空心胶囊中9种重金属元素的测定要求。     

Comparison of chloride- and hydride-generation for quantitation of germanium by headspace solid-phase microextraction-inductively coupled plasma-mass spectrometry

Chloride- and hydride-generation headspace solid-phase microextraction (SPME) techniques have been compared for the detection of trace amounts of germanium by inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS). The conditions for generation of germanium chloride, including acid type and concentration, effect of sodium chloride and extraction time, were investigated. Detection limits of 20 and 92 pg mL(-1) and precisions of 18% (n=11) and 9.7% (n=11) were achieved for chloride and hydride generation, respectively, at a concentration of 10 ng mL(-1). The generated germanium chloride and hydride species identities were characterized and confirmed as GeCl(4) and GeH(4) by use of electron-impact ionization mass spectrometry. Chloride generation coupled with SPME sampling and ICP-TOFMS detection resulted in twofold enhancement of sensitivity compared to GeH(4)and detection limits for continuous hydride generation were 20-fold better than reported atomic fluorescence data.     

毛细管电泳-电感耦合等离子质谱联用的接口设计

描述了毛细管电泳电感耦合等离子体质谱(CE-ICP-MS)联用技术的单T型接口,自行设计了双T型接口,并对两接口的分析性能作了比较。解决了接口中的常见问题,使用节流阀减小自吸作用并降低了CE分离物的稀释倍数,排气阀使提升量保持稳定。经考察得知,采用自吸作用提升液流流量稳定,其重现性RSD小于5％;双T型接口较单T型接口对CE分离更有利。采用双T型接口联用时,CE分离La、Ce、Nd混合离子迁移时间RSD小于2％,MS信号RSD小于15％,且不同浓度样品经CE分离后其MS信号基本呈线性关系。     

Use of a novel array detector for the direct analysis of solid samples by laser ablation inductively coupled plasma sector-field mass spectrometry

The use of laser ablation (LA) as a sample-introduction method for inductively coupled plasma mass spectrometry (ICP-MS) creates a powerful tool for trace elemental analysis. With this type of instrument, high analyte spatial resolution is possible in three dimensions with ng/g limits of detection and minimal sample consumption. Here, simultaneous detection is used to eliminate the correlated noise that plagues the ablation process. This benefit allows analyses to be performed with single laser pulses, resulting in improved depth resolution, even less sample consumption, and improved measurement precision. The new instrument includes an LA sample-introduction system coupled to an ICP ionization source and a Mattauch-Herzog mass spectrograph (MHMS) fitted with a novel array detector. With this instrument, absolute limits of detection are in the tens to hundreds of fg regime and isotope-ratio precision is better than 0.02% RSD with a one-hour integration period. Finally, depth-profile analysis has been performed with a depth resolution of 5 nm per ablation event.     

Inductively coupled plasma mass spectrometry with an enlarged sampling orifice and offset Ion Lens. I. Ion trajectories and detector performance

A new inductively coupled plasma mass spectrometer (ICP-MS) with four stages of differential pumping is described. The relatively large sampling orifice (1.31-mm dia.) improves signals for metal ions and resists plugging from deposited solids. A new ion lens is described that deflects ions off center and then back on center into the differential pumping orifice; there is no photon stop in the center of the beam. Calculations of ion trajectories using SIMIOn show that only those ions that leave the skimmer on center are transmitted, whereas most other lenses used in ICP-MS transmit only ions that leave the skimmer off axis. The performance of a Channeltron electron multiplier is compared to that of a Daly detector. Both detectors yield similar sensitivities of ≈ 10⁶ counts s^?1 per ppm and detection limits of ≈ 1 pptr. The background with a Channeltron electron multiplier is only 0.4 counts s^?1 and is only slightly higher than the dark current count rate. Presumably the offset ion lens used in the present work efficiently screens the detector from photons emitted by the plasma. The background with the Daly detector is 4 counts s^?1, which represents a substantial improvement over the background obtained in previous use of the Daly detector with JCP-MS.     

氧化镁烧结-电感耦合等离子质谱法测定冰铜中的铼

本文建立了一种利用电感耦合等离子体质谱法（ICP-MS）测定冰铜中铼的方法。样品经氧化镁烧结,热水浸取法处理,溶液经732强酸型阳离子交换树脂处理交换去除干扰离子,实现了ICP-MS直接测定冰铜中的铼。实验结果表明,在最佳试验条件下,方法的检出限可低至0.010mg/t,加标回收率为92.00%~104.00%,测定结果的相对标准偏差（RSD,n=7）小于5%,测定结果令人满意。由此可见,该方法精密度高,准确度高,可用于冰铜中铼的测定。     

Investigations on cluster and molecular ion formation by plasma mass spectrometry

The formation of molecular and cluster ions of different inorganic materials in plasma mass spectrometry – spark source mass spectrometry (SSMS), radiofrequency glow discharge mass spectrometry (rf GDMS), laser ionization mass spectrometry (LIMS), inductively coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS (LA-ICP-MS) – was investigated and compared. Similar abundance distributions of cluster ions were observed for a graphite sample, for boron nitride/ graphite and for metal oxide/graphite mixtures using different plasma mass spectrometric methods. A correlation of intensities of metal argide ions in ICP-MS with their bond dissociation energies was used to estimate unknown dissociation energies of molecular ionic species. For the elements of the 2nd or 3rd period in the periodic table, the intensities of most argon molecular ions (ArX⁺) measured by ICP-MS rise with increasing atomic number in a similar manner to the theoretically calculated bond dissociation energies of argon molecular ions.