Progress of laser ionization mass spectrometry for

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coupled plasma mass spectrometry (LA-ICPMS) is not discussed, and readers may ... schematic diagram of particle distribution after laser-solid interaction in high vacuum ... (more than 1026 m− 3) at the front of the plume, which meets the pile ... Dissociation of polyatomic ions was found at high laser irradiance and high ...
Spectrochimica Acta Part B 65 (2010) 871–883

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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b

Review

Progress of laser ionization mass spectrometry for elemental analysis — A review of the past decade Yiming Lin a, Quan Yu a, Wei Hang a,b,⁎, Benli Huang a a b

Department of Chemistry, Key Laboratory of Analytical Science, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen, 361005, China

a r t i c l e

i n f o

Article history: Received 2 June 2010 Accepted 28 August 2010 Available online 7 September 2010 Keywords: High irradiance laser ionization Resonance ionization Mass spectrometry Elemental analysis Single particle analysis

a b s t r a c t Mass spectrometry using a laser ionization source has played a significant role in elemental analysis. Three types of techniques are widely used: high irradiance laser ionization mass spectrometry is capable of rapid determination of elements in solids; single particle mass spectrometry is a powerful tool for single particle characterization; and resonance ionization mass spectrometry is applied for isotope ratio measurements with high sensitivity and selectivity. In this review, the main features of the laser ablation process and plasma characterization by mass spectrometry are summarized. Applications of these three techniques for elemental analysis are discussed. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Features of laser ablation in vacuum and low pressure background Laser-induced plasma characterization via mass spectrometry . . 3.1. Ion yield investigation . . . . . . . . . . . . . . . . . . 3.2. Cluster or polyatomic ion formation . . . . . . . . . . . 3.3. Angular distribution investigation . . . . . . . . . . . . 3.4. Velocity (kinetic energy) distribution investigation . . . . 4. Laser ionization mass spectrometry (LIMS) . . . . . . . . . . . 5. Single particle laser ionization mass spectrometry . . . . . . . . 6. Resonance ionization mass spectrometry (RIMS) . . . . . . . . . 7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . gas . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The laser was applied in analytical chemistry as early as the 1960 s. Lasers offer a highly versatile energy source for atomization, excitation, and ionization. Any samples with known light absorptivity can be analyzed by laser ablation and ionization to provide elemental information. The widely accepted technique of laser sampling has ⁎ Corresponding author. Department of Chemistry, Key Laboratory of Analytical Science, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China. E-mail address: [email protected] (W. Hang). 0584-8547/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2010.08.007

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many advantages, e.g. little or even no sample size requirement and sample preparation, low risk of reagent or solution waste, the avoidable introduction of contamination, and high spatial (lateral and in-depth) resolution. The power density deposited on the spot area can easily be controlled by adjustment of laser parameters (e.g., pulse duration, wavelength and energy) in comparison with other ionization source. Significant benefits and capabilities are brought into practical chemical analysis with laser sampling [1–4]. In combination with the proper detection system, it is an attractive technique for the elemental composition analysis of various samples through various spectroscopic methods. Among these methods, the optical spectroscopy, such as laser induced breakdown spectroscopy

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(LIBS) or laser induced atomic fluorescence [5–7], is particularly suited for the study of neutrals and excited species. In contrast to optical spectroscopy, mass spectroscopy and charge collection techniques allow for the investigation of charged particles in the plume and provide useful information about the ion yields of the ablated particles. Mass spectrometry is one of the most promising techniques for its capabilities of sensitive, precise, and accurate multielemental determinations as well as isotope ratio measurements [8]. Laser ionization mass spectrometry (LIMS) uses high laser irradiance for both the ablation and ionization. The time-of-flight (TOF) analyzer is easily coupled with the pulsed source of laser. LIMS can be applied as an absolute technique for solid analysis without the use of any standards in some specific conditions. Single particle mass spectrometry (SPMS) with laser for ionization is capable of particle component as well as particle size distribution characterization. Resonance ionization mass spectrometry (RIMS) is known for its selectivity and sensitivity of isotope ratio measurements. With the progress in elemental mass spectrometry and laser sampling and ionization techniques, efforts to improve laser ionization mass spectrometry are ongoing. This review covers three types of elemental mass spectrometry techniques that utilize a laser as the ionization source: high irradiance LIMS with a single laser for both the ablation and ionization; SPMS with laser for ionization; and RIMS. LIMS is known for its high ionization efficiency and multi-elemental capability; however, the laser–solid interaction is a complicated process that is not yet fully understood. Thus, the main features of the laser ablation process and plasma characterization related to elemental mass spectrometry are summarized. This article focuses on mass spectrometry with a laser ionization source for elemental analysis; laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is not discussed, and readers may refer to Ref. [9–14] for LA-ICPMS topics. The last comprehensive review of LIMS dates back to 1992 [2]. This review covers the period between 2000 and 2010. Although an attempt is made to consider the instrumentation and methodology involved, the selection of topics and references is based on typical and novel applications of the three techniques in elemental analysis. 2. Features of laser ablation in vacuum and low pressure background gas The physical processes of the laser-solid interaction are highly complex and interrelated. They depend on not only the laser parameters, e.g. power density, wavelength, pulse duration, and spot size, but also the material's physical properties, e.g. absorption properties, morphology, homogeneity, and melting and boiling points. The mechanisms involved in the laser-solid interaction have been studied extensively, but they are still not fully understood. However it can be controlled and highly reproducible to enable analytical measurement capabilities. For LIMS, the ion source is operated in vacuum or low pressure ambient gas environment. Many experiments and theoretical simulations have been carried out in the past decade in an effort to understand the processes in more detail. The commonly recognized aspects of laser-solid interaction can be generalized as follows: laser-solid interaction, removal of sample particles, and plasma formation and expansion. The high power laser energy deposited on a metallic target surface is first absorbed by free electrons. The energy is then transferred to the lattice, which leads to electron-lattice coupling. Ablation is initiated with melting, dissociation, and vaporization of materials. At a laser intensity of about 103 W/cm2, the local temperature approaches the boiling point of the material [15,16]. Material removal is controlled by thermal conduction, and at approximately 107–1010 W/cm2, depending on laser wavelength and pulse duration, the plume is partially ionized [16]. Plasma temperature can reach as high as several tens of thousands of Kelvin with 109 W/cm2 laser irradiation flux [17,18].

After the phase transition, opaque plasma is formed with a duration of several picoseconds [19,20]. Thus, the subsequent major fraction of laser energy is absorbed by the plume plasma, which attenuates the interaction between the solid surface and laser beam. The primary photon absorption process in laser-induced plasma is inverse bremsstrahlung (IB). During collisions with neutral and ionized atoms, the free electrons absorb the photons of the laser beam, which generates excitation and ionization processes in the plume [21,22]. The reported fraction of ionization in the plasma is typically between approximately 10% to 80% at a high laser fluence regime [22–24]. Normally, the laser-emitted plasma consists of three ion groups. The main group is thermal ions, which are thermalized after the adiabatic expansion; the slow group is ascribed to X-rays reabsorbed outside the laser focus; and the fast group is due to the electric field built by the escape of hot electrons [16,25]. During laser irradiation, a dense layer in contact with the target surface is formed where frequent collisions between particles happen. This so-called Knudsen layer modifies the velocity distribution of ions from a half-space Maxwellian to a full-range Maxwellian distribution with a shifted center-of-mass velocity [21,26,27]. Ions generated in the nascent erosion plasma expand freely into a vacuum with kinetic energies ranging from several eV to thousands of eV; the energies are typically much lower for neutral species [22,27]. Angular distributions of the ejected particles have been fit to an approximated cosnθ function [16,21,27,28]. Generally, n depends on the laser fluence and ambient conditions; n is lower when the plasma propagates into an ambient gas where multiple collisions occur, which results in a broad angular distribution [27]. The processes of plume expansion into an ambient gas are quite complicated and elusive. These processes involve the scattering and cooling of ions [29], the formation of shock waves [4,30–32] and recombination processes [16,33]. The plasma suffers from impedance of background gas during adiabatic expansion into ambient gas. A shock wave is formed on the contact surface where the compressed gas layer meets the ablated plume, and the ablated species propagation is slowed; the plasma temperature also decreases, which sustains the recombination process. Three-body recombination is present mainly in high density plasma [34,35]. The schematic diagram of particle distribution after laser-solid interaction in high vacuum and low-pressure helium ambient is shown Fig. 1. Numerical simulation studies of the plume expansion can be obtained either by hydrodynamic models [34,36], by Monte Carlo simulations [37] or by a hybrid model [38,39]. A series of numerical simulations based on laser ablation processes and plume expansion were performed by Bogaerts et al. In the first step, particle formation is considered. For micro-sampling at high laser intensity, phase explosion and melt splashing cause the ejection of droplets. The heterogeneous sampling is due to the spatial distribution of the laser beam energy profile [40]. The model describes the laser-solid interaction processes, including heating, melting, vaporization, plume expansion in vacuum or a background gas, the plasma formation, and laser-plasma interaction. Thus, the temperature of the target surface and the crater depth in the target can be obtained [41]. Moreover, the plume temperature is calculated, which reaches up to about 6 × 104 K with irradiance on the order of 109 W/cm2 in vacuum conditions at about 10 to 20 ns and then drops drastically [41–43]. The maximum plume temperature changes very slightly for ambient pressures ranging from 10− 3 to 5 atm [42]. At 1 atm of ambient gas pressure, the plume density reaches the highest value (more than 1026 m− 3) at the front of the plume, which meets the pile of the background gas [43,44]. Using the Saha–Eggert equation, the fraction of atoms, singly- and multiply-charged ions, and electrons is calculated with temporal and spatial distribution [41,43,44]. The plume velocity increases near the front of the plume. In vacuum conditions, this is a free expansion with a velocity of 2 × 104 to 2.5 × 104 m/s; however, in a background gas, the plume expansion is retarded to a much lower velocity (1–2 × 103 m/s) and shorter plume

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Fig. 1. Schematic diagram of particle distribution after laser–solid interaction in (a) high vacuum and (b) low-pressure helium ambient.

length [42,45]. The expansion dynamics in a vacuum were confirmed later by experimental results in which the angular distributions of ions were also determined [46]. The angular distribution study found that ablated ions were highly directional, and a heuristic validity of the Lorentzian distribution function was performed as follows for the fitting of the angular distribution instead of the cosnθ function that is normally used: f ðθÞ = f0 ðθÞ +

  A φ π ðθ−θc Þ + φ2

ð1Þ

where θ is the off-axis angle, θc is the location of the peak, φ is the spread of angular distribution, f0(θ) is the background offset and finally A is the integral of the distribution. Simulations on the effect of laser parameters, including laser irradiance, pulse duration, and wavelength, were also carried out [47]. Good agreement was reached with experimental data from other literatures [48–50]. 3. Laser-induced plasma characterization via mass spectrometry In contrast to optical spectroscopy, mass spectrometry techniques allow for the investigation of charged particles in the plume. The analysis of charged species is paramount for understanding both the elementary and the macroscopic processes involved in laser ablation and ionization, especially when an ambient gas is present. In particular, mass spectrometry techniques not only allow for the identification of the masses but can also provide ion yield, angular distribution, velocity distribution, and cluster formation data from each laser shot. 3.1. Ion yield investigation Several concurrent mechanisms, including laser-solid and laserplume interactions and consequent plume dynamic expansion, yield charged particles that are markedly dependent on laser fluence. When laser fluence increases, more material is ablated from the solid target, and more laser energy is absorbed by the vapor, which in turn causes more efficient vaporization, higher plume temperature, and higher ionization efficiency. Torrisi et al. [51] utilized a quadrupole mass

spectrometer (QMS) to investigate the energy threshold at which the etching occurs and the etching rates as a function of laser fluence in vacuum. A theoretical energy threshold was deduced to compare with the experimental data, and the good agreement that was observed suggests that the threshold mainly depends on the cohesive energy of the metal. Above the energy threshold, there is a proportional relationship between the ejected mass yield and the energy of the laser irradiation. QMS was later used to measure the fractional ionization for Al, Au, and Ta at different laser fluence [23]. A linear increase in the fractional ionization with the laser fluence up to 8.5 × 109 W/cm2 was obtained. The fractional ionization is in very good agreement with the data from the ion collector. The nearly linear relationship of ion yield and the laser energy was also confirmed by Wang et al., who used a time-of-flight mass spectrometer (TOFMS) with borocarbide targets [52,53] and metallic samples [54]; our group used a much higher fluence laser (low 1011 W/cm2) to examine this relationship [29]. Moreover, the group of Amoruso reported a few studies on the characterization of charged-species in laser ablation and ionization of borocarbide targets by TOFMS [53–56]. The ion yields as a function of laser fluence, the delay time between the ablation and the extraction field pulse, and the sampling distance were investigated. Effects of laser wavelength as well as laser irradiance and buffer gas pressure were investigated by our group [57] using laser ionization orthogonal time-of-flight mass spectrometry (LI-OTOFMS). This study found that the dependence of ion signals on the gas pressure and sampling distance is insensitive to different laser wavelengths (1064, 532, 355, and 266 nm) for metal targets. Ion signals are governed primarily by elemental properties, such as the first ionization potential, electron affinity, and mass number. Dissociation of polyatomic ions was found at high laser irradiance and high ambient gas pressure. Collisional dissociation effects between the background gas and ablated species was also suggested. A thermal process was used to explain the different relative sensitivity coefficient (RSC) values of elements at various laser wavelengths. Gunster et al. [58] measured two different types of samples (dense sintered AlN and AlN powder samples) at five different wavelengths (280, 300, 560, 600, and 1000 nm). For dense sintered AlN ceramic, a nearly linear relationship between ablation rate and pulse energy

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exists below 8.8 × 109 W/cm2 that is independent of the wavelength. At pulse energies over 8.8 × 109 W/cm2, saturation of the Al yield is noticeable, which suggests plasma shielding at high laser fluence. For the powder sample, the Al yield does not level off, indicating that the ablation plume characteristics are different for the powder sample. In the case of powder samples, the laser-substrate coupling is changed due to the formation of a closed surface by the fusion of powder particles. Theoretical approaches for an evaluation of the threshold in infrared, visible, and ultraviolet radiation were also made by Torrisi et al. for metals, semiconductors, and insulators [59]. Experimental data were used to evaluate the validity of the method. Two different thermal and photochemical processes, which are induced by IR and UV irradiation, respectively, were discussed to explain the departure of the theoretical results from the experimental data and for the explanation of crater formation and the QMS spectra. The ablation yield depends mainly on the mass of irradiated material and increases with the laser wavelength. The ion emission directly from the target in UV irradiation is much higher than that in IR irradiation [60]. 3.2. Cluster or polyatomic ion formation The existence of clusters or polyatomic ions is well known in laser ionization mass spectrometry. Their abundance is a function of the laser parameters used. The formation of clusters or polyatomic ions disturbs the elemental analysis. Thus, knowledge of cluster formation is important for understanding the chemical and physical processes in the ablated plume. Cluster or polyatomic ion yield is affected by the laser wavelength. Short wavelength laser irradiation generates mostly elemental and monoxide ions, whereas IR laser irradiation leads to the formation of clusters as well as elements, due to their different photon energies [61]. There are two different ablation mechanisms (photochemical and thermal) that operate at the short and long wavelengths. Photodissociation of larger species in the gas phase occurs during UV laser irradiation [62]. Giapintzakis et al. [61] used TOFMS to investigate the positive ions generated by infrared and ultraviolet laser ablation in vacuum and oxygen environments. The inlet of oxygen was controlled by a pulsed nozzle aligned either perpendicular or parallel to the spectrometer axis. Experimental data showed that a distinct portion of the plume generated by an IR laser consisted of stable large-mass clusters, whereas metal ions and their oxides comprised the main portion of the plasma in UV irradiance. Pulsed buffer gas was utilized as a powerful tool by Canulescu et al. [63] and Willmott et al. [27] for the study of the kinetic energy distribution of specific ions. Negative cluster species have also been investigated [63]. The results indicated that the kinetic energies of negative cluster species, which were mainly formed in the presence of the gas pulse, were lower than the neutral species. Laser fluence exerts a competitive effect on cluster formation. Increasing the laser energy results in an increase in the collision frequency and favors the aggregation processes that support the formation of clusters. Conversely, higher fluence leads to an increase of the plume temperature, which can reach beyond the thermal stability limit of the clusters, and thus inhibits larger cluster formation [64]. Polyatomic ion yields from excimer laser ablation of several lanthanide transition metals were investigated by Gibson [65]. The results demonstrate that the ablation process determines the composition and abundance of ions. Thermochemical consideration related to dissociation enthalpy was a valid method used to predict the approximate ion yields. Aggregation processes of small species in the ablation plume were responsible for polyatomic ion formation, resulting in polyatomic ions in the tail of the plume. The same result was obtained by the work of Nam et al. [66], in which a silicon carbide plume produced by laser ablation and ionization was investigated using QMS. Time-of-flight mass spectrometry was used for analysis of Pb and Se cluster formation [67]. More signals from the bigger clusters

were observed with an increase in the distance between the nozzle and the target. Kinetic energies of clusters were estimated through the shift of spectral peaks, which depended on electrical voltage between the target and the intake aperture of the spectrometer. Houska et al. [68,69] utilized a TOFMS operated in negative and positive ion modes for laser desorption/ionization of AgSbS2 and PpSqSer. Recently, the same methodology was applied by the same group to ternary As–S–Se glasses (As33S33.5Se33.5, As33S17Se50, and As33S50Se17) [70]. The stoichiometry of the clusters was determined through so-called “isotopic envelope” analysis and computer modeling. The structures of the clusters were also discussed, and the structures of some binary and ternary clusters were suggested to be heterocyclic. The chemical structure appeared to be quite complex with possible isomers. 3.3. Angular distribution investigation The non-isotropic plasma plume formation caused by the collisional processes in the Knudsen layer results in a strong forward-peaked expansion of the emitted species with a maximum emission according to the target normal [71]. The angular distribution of ablated species can be described by a cosnθ distribution based on a free jet expansion theory, where n is the sharpness parameter that is higher than 1 and depends on the atomic mass [21,23,24,72,73]. Other effects, such as laser fluence, background gas, and laser spot dimensions, modify the angular distribution [72,74,75]. If the ablated material expands into the ambient gas, the angular distributions are extremely sensitive to the properties and pressure of the background gas [27,76,77]. Mass spectrometry can be used for detection of ion intensity from any oblique angle by changing the angle between the target normal and mass analyzer through target surface rotation. A series of studies have been made by the Torrisi group using QMS for the angular distribution analysis [24,60,78–81]. Good agreement between the theoretical and experimental data of n values was obtained by Torrisi et al. [28]. The relative low and high n values indicate that there is a relatively broad angular distribution for light (Ni, Cu) elements and narrow angular distribution for heavy (W, Pb) elements, respectively. This result was confirmed later by the same group [16]. Normally, angular distribution of the neutral species emission is broader than that of the ion species emission. With respect to the ion emission, increasing the charge state leads to an increase in the directivity of the plasma plume around the target normal. In other words, the higher the charge states, the higher the kinetic energies and the more narrow the angular distributions (Fig. 2). 3.4. Velocity (kinetic energy) distribution investigation The investigation of velocity or kinetic energy distributions of ions is an important area in which mass spectrometry is applied for plasma characterization. In particular, mass spectrometry techniques have

Fig. 2. Angular distribution of emitted ions.

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the power of simultaneous identification of the specific ionized species and analysis of their kinetic energies from each laser shot. High intensity laser energy generates hot plasma that expands adiabatically in a vacuum along the target surface normal with kinetic energy ranging from several eV to thousands of eV. Three components contribute to the ion velocity: thermal velocity, plasma expansion velocity, and velocity due to coulomb acceleration. Neutral species have an energy that depends mainly on plasma expansion while ions are subject to Coulomb acceleration due to the electrical field generated inside the non-equilibrium plasma. The energy distribution of ions exhibits a shifted Maxwellian–Coulomb distribution, in which the peak value is shifted toward higher velocities:

Thus, the kinetic energies of different metal ions (Al, Cu, Mo, W) were the same. When Ar was used as the buffer gas at a lower pressure, the ions near the nozzle were hydrodynamically controlled and had higher kinetic energies than ions in helium. The temporal distributions were much narrower, and the delay time needed to reach the maximum intensity was much shorter due to the higher kinetic energy of the ions in an argon background. Both the diagnosis and mathematical modeling efforts are critical for a better understanding of the processes involved in laser ablation and ionization. They are essential towards the solutions for matrix effect, elemental fractionation, and spectral interference problems, which is important for the real-world sample analysis by LIMS.

h  m  i 3 Fðvx Þ e vx exp − × ðvx −uk −uC Þ 2kT

4. Laser ionization mass spectrometry (LIMS)

ð2Þ

The vx is the ion velocity according to target normal, k is the Boltzmann constant, m is the ion mass, and T is a temperature determined by fitting the distribution. The shifted Maxwellian– Coulomb distribution introduces a “flow velocity (uk)” plus “Coulomb velocity (uC)” to describe the velocity of the center of mass of the expanding plume [82,83]. Theoretical calculations were consistent with the experimental data obtained using a QMS and a cylindrical electrostatic energy analyzer, which were mounted at different angles with respect to the laser direction [82]. The QMS with electrostatic ion deflection was employed again by Torrisi et al. [79] for kinetic energy measurement. The velocity of neutral species was also measured with a filament in “off” and “on” mode. Although the plasma is in a nonequilibrium condition, an “equivalent temperature” [84] that represented the approximate temperature of the ions expanding along the emission direction was calculated as high as 5.8 × 105 K for the plasma temperature. Inside the non-equilibrium plasma, an “equivalent accelerating voltage” is generated that increases with the laser energy. The “equivalent voltage” results in ion acceleration along the normal of the target surface, which has a more significant effect on ions with higher charge states. Other investigations have been made by laser ablation and ionization of different targets, such as aluminum [78], germanium [80], carbon [85], copper [24], tin [81], zinc, tantalum, and lead [86], with kinetic energies ranging from less than 100 eV to thousands of eV. Velocity and kinetic energy distributions of metal ions in vacuum conditions have been determined using an electrostatic energy analyzer combined with a time-of-flight analyzer for plasma characterization [33]. Kinetic energies of multiply charged ions follow the shifted Maxwellian–Coulomb distribution. There are three types of ions with different kinetic energy distributions that comprise the total distribution: the recombined ions; the surviving ions, and the ionized ions. The combined ion, which initially has a charge higher than the surviving ion, is ejected with a high coulomb velocity, and these ions appear to be the high sideband in the energy distribution. The ionized ion initially has a charge lower than the surviving ion, and the coulomb velocity is small, appearing as a shoulder at lower energy side of the energy distribution. Ion velocities are related to the local electric field built-up by the emission of the photoelectrons following the laser ablation. Plasma characterization by mass spectrometry approaches is always carried out in high vacuum conditions without any ambient gas. Recently, our group has made a comparative study of laser ablation in He and Ar background gases using LI-O-TOFMS [29]. Our instrument is capable of measuring the temporal distribution of specific ion species by varying the delay times between the laser shots and the time when ions are repelled into the TOF analyzer. The velocities of different ion species can be compared qualitatively by the temporal distributions. Because the plume length in helium was estimated to be shorter than the sampling distance between the target and nozzle, only thermalized ions could be extracted into the nozzle.

In LIMS, only one laser source without frequency tunability is required. Using high laser irradiance, atomization as well as the consequent ionization can be accomplished without any other ionization approaches. Thus, the LIMS system is rather compact. The laser induced plasma can be introduced into any kind of mass analyzer, but a TOFMS is usually preferred because it is easily coupled with the pulsed source (Fig. 3). Due to the efficient vaporization and ionization, LIMS has become an attractive technique for multielement and isotope analysis with high sensitivity and low limits of detection (ppm or sub-ppm range) [2]. LIMS can be performed with little or no prior chemical treatment of the sample, and it is suitable for nearly all elements in the periodic table. In 1975, the first laser microprobe mass analyzer (LAMMA) was constructed by Hillenkamp et al. [87]. Since then, the commercial LAMMA instruments rapidly expanded the applications of LI-TOFMS; however, the application is limited due to the large kinetic energy distributions that arise when high irradiance is applied. When the energy distribution exceeds the focusing capability of the reflectron, peak broadening occurs, which results in loss of resolution. These instruments are therefore rarely applied for quantitative analysis because of poor resolution at irradiance levels of 109 to 1010 W/cm2 that are ideal for elemental quantitation. Laser irradiance in LAMMA can only be operated at 108 W/cm2 or below; elemental sensitivity in the LAMMA depends specifically on experimental conditions, e.g. laser flux and matrix composition, which significantly limits its analytical capabilities. According to the literature, the most recent application of LAMMA was the analysis of paint defects with a LAMMA® 1000 by Wolff et al. [88] in 2004, in which LAMMA provided qualitative elemental information on the inorganic composition of the sample and was powerful in the analysis of particles and pimples in cured coating layers. The primary application of LIMS is the trace analysis of metals and alloys. A small, compact high-irradiance laser ionization orthogonal time-of-flight mass spectrometer (LI-O-TOFMS) has been developed by our group [89]. Unlike traditional on-axis geometry instruments, the orthogonal extraction technique, as shown in Fig. 3b, can achieve high resolution because the broad kinetic energy distribution of the ions from the ion source has little effect on the mass analyzer. Relative sensitivity coefficient (RSC) which is a good indicator of the atomization, ionization, and detection efficiency of each element is determined as follows: mes

RSCi = ðci

st

mes

= ci Þ = ðc0

st

= c0 Þ

ð3Þ

is the concentration of an impurity element derived from where Cmes i the relative mass peak intensity, Cmes is the concentration of the 0 reference element derived from the relative mass peak intensity, Cst i is the concentration of an impurity element for the reference sample and Cst 0 is the concentration of the reference element for the reference sample. Nearly uniform RSCs independent of laser energy were achieved for most metal elements. The system was later used for

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Fig. 3. Schematic diagram of (a) on-axis and (b) orthogonal geometry laser ionization TOFMS.

standardless semi-quantitative analysis of alloy samples [90]. A pulse train, whose pulse number and period can be adjusted, was used as the repelling pulse to reduce the interference from clusters or polyatomic ions. For the metallic elements in steel, isotopic abundance ratios were in reasonable agreement with their natural values. Quite good correlation of element composition and signal intensity from six steel standards was achieved, which covered a concentration range of approximately 6 orders of magnitude. The limit of detection (LOD) value was as low as 10− 8 mol/g. It appears that this technique is independent of matrix composition, which was also investigated in our later work [91]. Matrix effects and elemental fractionation were alleviated with an increase in the laser energy. Laser microprobe Fourier-transform ion cyclotron resonance mass spectrometry (FTICRMS) was developed by Carre et al. [92]. By means of an internal lens and an external adjustable telescope, laser spots could be adjusted to as small as 5 μm. The characterization of dust particles from stainless steel factories was presented using nonresonant laser ionization. The ion abundance ratio of Cr(III)/Cr(VI) was used to distinguish the chromium-containing compounds. The inorganic pollutants as well as the organic compounds (polycyclic aromatic hydrocarbons (PAHs)) of these samples were investigated. The Klunder group developed laser ablation ion-storage time-offlight mass spectrometry (LA-ISTOF-MS) for direct analysis of a solid [93–95]. This hybrid technique took advantage of the storage ability of the ion trap and the high speed and resolution of time-of-flight mass spectrometry [96]. A selective range of masses were stored in the ion trap, thus reducing background interferences from sample matrices. The mass resolution and ejected ions were affected by different spatial positions of the trapped ions caused by the variable phase of the RFtrapping cycle. The resolution approached a maximum of 1500, and a LOD near 10 pg was obtained in a single laser pulse. The system has been successfully used in the accurate determination of ultra-trace elements with a reasonable isotope ratio. Geological samples are difficult to analyze due to complex phase compositions and high inhomogeneity. Laser ablation and ionization are considered to be one of the most versatile techniques for the analysis of geological materials, which circumvent cumbersome preparation and decomposition procedures that would overshadow the actual analytical measurements as the number of samples and elements to be analyzed increases. In our group, Tong et al. utilized an in-house-built LIMS with a collisional cooling cell for geological

sample analysis [97]. The results demonstrated that there was very little interference; however, for light elements, such as Na, Mg, Al, and Si, the RSC values were quite small due to the mass discrimination effect of the collisional cooling cell. The instrument was later modified by replacing the cooling cell with an Einzel lens [98]. Powdered geological samples were also analyzed by Sysoev using a laser ionization time-of-flight mass spectrometer, known as LAMAS-10 M. The spectrometer was most notable for its unique mass analyzer, in which a sectorial axisymmetric electric field was used instead of the traditional reflectrons [99]. Reliable determinations could be obtained with LOD of 2–5 × 10−10 mol/g for noble metals (Ru, Rh, Pd, Ir, Pt, and Au) in sulfide ores. Another geological application of laser mass spectrometry is in planetary science. The defining feature of planetary missions is in situ analysis, which necessitates a mass spectrometry system in miniature [100,101]. Brinckerhoff et al. developed a LIMS that weighed about 2 kg and was less than 2 × 103 cm3 in size [102]. A series of energy windows were used to sample the emitted ions with different kinetic energy distributions. The in-depth analysis of the basalt demonstrates the ability of LIMS for bulk, unweathered material. Sufficient mass resolving power (about 250) was achieved for distinguishing isotopic peaks. A flexible TOF-MS was later developed by the same group [103,104]. Not only the elemental but also the molecular information for solid samples could be obtained. Using UV laser desorption (106 to 107 W/cm2), novel curved-potential reflectrons, and a gridless ion source, the ionization efficiency, reflectron and transmission efficiency were improved. In-depth analysis of multilayer coatings is widely used in material science. Secondary ion mass spectrometry (SIMS) and glow discharge mass spectrometry (GDMS) are the typical methods; however, SIMS is limited to an analytical depth of only a few micrometers, and GDMS suffers from a poor lateral resolution. Using the advantages of laser ablation, the problems mentioned above can be overcome by femtosecond laser ablation and ionization time-of-flight mass spectrometry, which was constructed by Margetic et al. [105]. The laser fluence was conducted at the ablation threshold regime to achieve a high depth resolution. Ablation rates of 4.5 nm/pulse and 2 nm/pulse were estimated for Co-implanted semiconductor and thin film metal standard NIST 2135c, respectively; however, the precision was poor due to pulse-to-pulse energy stability of the laser system. Isotope ratio determination using LI-TOFMS has proven to be very rapid without sample pretreatment. Song et al. utilized a home-made

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LI-TOFMS system to determine the isotope ratio in metal samples [106]. After a series of parameter optimizations, less than 10% deviation between the measured isotope ratio and the natural abundance was estimated for various samples. Another approach by the same group examined the fast analysis of swipe samples for environmental monitoring and nuclear safety applications by isotope analysis [107]. One-step laser ionization was also used for analysis of microparticles deposited on a metal sheet. Defocusing of the laser energy allowed the reuse of a single particle for several replicates to improve the accuracy of the data. Isotope abundances were measured that were within 5% to 10% agreement with the natural abundance, excluding minor isotopes with larger deviations. A reflection time-offlight mass spectrometer was utilized by Joseph et al. for determination of boron isotopes in boron carbide (B4C) [108] and boric acid [109]. Good agreements were obtained with the results by thermal ionization mass spectrometric (TIMS) method. The LIMS was also used for isotope ratio measurements of 235U and 238U, which illustrates the power of LIMS for the application of nuclear reactor control [110]. In our LI-O-TOFMS system, less than 4% error was observed for 7 molybdenum isotopes at low 10− 5 mol/g [90]. The determination of nonmetallic elements in solid samples is difficult owing to their physical and chemical properties, such as their relatively high ionization potentials. Traditional methods have proven to be insufficient for the determination of nonmetals due to low source temperatures (ICPMS, GDMS), insensitivity to light elements (XRF), or ultraviolet light emission of nonmetals (optical spectrometry). We applied LI-O-TOFMS for the determination of nonmetallic elements in solids [111]. Helium at 250 Pa was used as the buffer gas in the ion source, and the laser irradiance approached 7×1010 W/cm2. Low-discrepancy RSCs for different elements were obtained for a series of artificial samples containing B, C, N, O, F, Si, P, S, Cl, As, Br, Se, I, and Te. High correlations between the elemental concentrations and the signal intensities were achieved with LODs in the 10− 7–10− 8 mol/g range and a dynamic range of 6 orders of magnitude for most nonmetallic elements. Semi-quantitative multi-elemental analyses of biological samples, including tea leaf standard, Laminaria japonica, and pig skin, were made by our group with acceptable RSCs for most elements [112]. The biological applications in tissue and subcellular imaging of elements takes advantage of the high lateral resolution of SIMS and high sensitivity of laser postionization. Studies by the Arlinghaus group of boron location in cell cultures [113] and recent work on the mapping of copper in micro-scale biopsies [114] revealed the potential of this technology in bioscience applications. If the resonant or femtosecond laser postionization is used, the elemental selectivity and sensitivity will be further improved. With regard to speciation analysis using LIMS, several excellent studies have been conducted over the past decade. Aubriet et al. evaluated the capabilities of laser ionization Fourier transform mass spectrometry for the speciation of chromium compounds [115]. The conclusion was that trivalent, hydrated trivalent, and hexavalent chromium compounds could be distinguished unambiguously with the help of the ratios of small cluster pairs. Most of the first-row transition-metal oxides were also investigated, and a strong correlation between the oxides and the cluster ions was found to exist in both positive and negative ions [116]. This work also showed that the number of metal atoms in a higher valence state increases with the increase in oxygen/metal, which could be used for the differentiation of several oxides, such as vanadium, titanium, chromium oxides, etc. Ignatova et al. examined molecular speciation of inorganic mixtures based on the peak intensity ratio determined using Fourier transform laser microprobe mass spectrometry [117]. The studies described above were primarily conducted with laser irradiation of less than 108 W/cm2, which indicates that the laser-target interaction mechanism probably involves a desorption-ionization process accompanied by the emission of isolated neutral species, ions, and electrons. In a recent study, we used LI-O-TOFMS to conduct direct speciation

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analysis of iron oxides [118]. With high laser irradiance (1010 W/cm2), ion abundance distributions were similar at different wavelengths. Parametric adjustments in the ion source will not change the variety of cluster ions and their relative ratios, which is important for the speciation analysis because it relies on cluster ion ratios. In addition to + + + using the ratios of the Fe+ 2 /Fe2O and Fe2O /Fe2O2 small cluster ions, + + + the ratios of Fe3O+ 2 /Fe3O3 and Fe4O3 /Fe4O4 can also be used for speciation with high laser irradiance. Based on the published studies, at low irradiance (≤108 W/cm2) in LIMS, spectra are wavelengthdependent [119], but at high irradiance (≥109 W/cm2) in LIMS, the cluster ion ratios are independent of wavelength [118]. Moreover, the cluster distribution under low irradiance relies heavily on the neutral FeO, which is directly generated through laser desorption. In high irradiance LIMS, apart from electron ionization of ablated neutral species, clusters were also formed by aggregation from the atomized iron oxides when the plasma cooled down, which could induce the stabilization of cluster ion ratios. Mahajan et al. [120] developed a laser microprobe mass spectrometer capable of analyzing nitrogen and noble gases, such as Ar, Kr, and Xe, in chondrule from Dhajala meteorite. An ultra high vacuum (UHV) cleanup system was designed to generate low system blanks, and stainless steel mesh (SSM) was used to collect noble gases and nitrogen by cooling them to liquid nitrogen temperature. Noble gas isotope records were deciphered for individual presolar grains. Another novel application of the LI-O-TOFMS system in our group is for the detection and quantitation of residues, which were prepared by evaporating mixed salt solutions [121]. All of the elements in the residues were clearly observed with reasonable isotope ratios. The absolute limit of detection which represents the lowest ablated mass of a certain element in a single shot is determined by using the following equation: mi = ðSc = Sr Þ × mt

ð4Þ

where mi is the lowest mass ablated for a specific element in a single shot, Sc and Sr are the area of the crater and the residue, and mt is the lowest mass of the specific element which can be just right detected in the residue. By calculating the individual masses from the ablated area due to a single laser shot, the femtogram absolute limit of detection was achieved. RSC values for different elements were within 1 order of magnitude in different laser fluence, indicating the capability for semi-quantitative analysis. Consecutive analyses were performed with less than 30% RSD of the total ion current. 5. Single particle laser ionization mass spectrometry Atmospheric particles with sizes ranging from less than 10 nm to greater than 10 μm are often mixtures that include constituents such as heavy metals, sea salt, mineral dust, sand and a confusing array of organic molecules [122]. The single particle mass spectrometer (SPMS) is capable of simultaneously determining the particle size and chemical composition in real time for a single particle or an ensemble of particles. The designs of an SPMS were addressed in recent articles [123– 127]. A SPMS with a laser for an ionization source can also be considered as a type of LIMS that is used for single particle analysis, whereas the traditional LIMS is used for bulk analysis. The SPMS is generally composed of the vacuum inlet, the particle sizing, laser ionization source and the mass spectrometer (Fig. 4). Orifices, capillaries, and aerodynamic lenses are usually designed for sample inlets that only allow the passage of particles with a particular size or a range of sizes [124,125]. When the particles expand through a nozzle into a vacuum, they are accelerated by numerous gas-particle collisions to form a high speed particle beam. The exerted force on each particle is strongly forward-directed, and the inlet particles are focused to form a particle beam with very low divergency.

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Fig. 4. Schematic diagram of single particle laser ionization mass spectrometry.

Undesirable evaporation and condensation are minimized. Particles are detected by the scattering of light that occurs when a particle passes across a continuous laser beam, which acts like an optical particle counter. Two ellipsoidal reflectors are used to collect the scattered light. Continuous lasers, such as the He–Ne (633 nm) and doubled Nd-YAG (532 nm) lasers, are optimal for particle detection that includes the smallest particles (about 100 nm). An SPMS can measure the velocity of each particle based on the transit time between two continuous laser beams. This velocity measurement is not only used for timing the pulsed ionization laser but also for precise particle size determination [123,124]. For elemental analysis of saltand metal-containing particles, a strong UV laser pulse of 1010 to 1011 W/cm2 irradiance is used for ionization. The laser is triggered at the appropriate delay time to impact each particle as it enters the ionization region, resulting in the ionization of elements in each particle. The ions that are generated are then extracted into the mass spectrometry system. Three major mass analyzers that have been applied in single particle mass spectrometers are time-of-flight (TOF), quadrupole mass filter, and the quadrupole ion trap [128]. TOFMS is convenient to analyze the natural ion source from the pulsed laser ionization. An entire mass spectrum of a single particle can be obtained using a TOF mass spectrometer. Another advantage of TOFMS is the wide dynamic range for which trace informative species in a particle can by analyzed. Both the singly and multiply charged ions are obtained to yield a semi-quantitative measurement of a single particle; however, quantitative determination suffers from severe matrix effects [122,129]. Real-time SPMS has been used to study aerosols from various fields, such as marine, continent, urban, troposphere, fireworks or a metered dose inhaler. Examples cited below are not meant to be exhaustive but are selected to reflect the application of SPMS for elemental analysis of the aerosols. The main limitation of aerosol mass spectrometry is quantitative measurement. Only individual particles having a few chemical components, and a homogeneous surrounding matrix can be quantified [122,125]. The relative sensitivity factor (RSF) has been used to obtain quantitative results from multicomponent samples [130,131]: A

b

A

b

RSFðA = BÞ = ðNi = Ni Þ = ðNa = Na Þ

ð5Þ

where nAi and nBi refer to the ion number of species A and B, nAa and nBa refer to atom number of species A and B. The differences in the response of the various species to the method used in addition to the changes in the response of a particular species due to changes in the sample matrix could be corrected by the RSF. The accurate determination of Na+/K+ in sea-salt particles has shown that the RSF is useful for ambient particles [130]. Reents et al. described a procedure for quantitative determination of the major and minor

compositions in individual particles [132]. The problem of matrix effects was solved without previous assumptions of aerosol composition and calibration. An accuracy of 20% was achieved. Due to the high sensitivity and time resolution of SPMS, many studies have been performed to increase our understanding of particle compositions and sources, to monitor temporal changes of particle number concentrations and to classify these changes [133–140]. The characterization of long-range transport of forest fire particles and mineral dust was performed by Owega et al. [141,142]. Other approaches for on-line particle analysis for atmospheric research have contributed to a better understanding of atmospheric aging of particles [143]; characterization of particle ice nucleation [144,145] and atmospheric aerosol transformation processes [146]. Detection of ultrafine particles with diameters less than 100 nm by light scattering is difficult due to the low intensity of scattered light. Consequently, instruments for the analysis of ultrafine particles have been constructed that are based on a UV laser pulsed at a high frequency for ionization [147–149]. The chance of one particle coinciding with a laser pulse is rather small, and the mass spectrum related to an individual particle can be obtained. A direct quantitative correlation of the elemental composition of the particle to the ion signals has been found to provide a particle analysis with a size distribution of 20 to 900 nm [150]. On the other hand, optimization of the experimental conditions on the composition biases is required due to the decrease of ion formation with the decreasing particle size [151]. In 2006, Wang et al. investigated airborne nanoparticles with diameters of 7 to 25 nm using a reflectron time-of-flight mass spectrometer coupled with an ion trap [152]. The size cut-off of the instrument had to be low to improve the performance for ultrafine particle analysis. Attempts have been made to measure bio-aerosols because of their possible health effects [153]. Aerosol mass spectrometry, which provides mono-atomic and polyatomic cluster ion information, is potentially capable of on-line differentiation of individual biological particles, including species from bio-hazardous materials, generated by the inorganic and organic background material from the natural or anthropogenic activity. Several research groups are currently working on real-time, single-aerosol particle analysis to improve mass spectrometric performance for the fast, sensitive discrimination of bacteria, spores and viruses. A commercial TSI3800 ATOFMS (aerosol time-of-flight mass spectrometer) instrument has been utilized by Beddows et al. to study background aerosols. LIBS was used for comparison of elemental analysis, and Raman spectroscopy was used to provide complementary molecular information [154]. In the ATOFMS, the negative and positive ions that are formed are recorded by the dual time-of-flight mass analyzer. The size distribution and time evolution of detected particle species abundance were obtained as well. Discrimination between biological aerosols and the

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background atmospheric particles was made. The ATOFMS was found to have a higher likelihood of characterizing a bio-aerosol through the accessible mono-atomic and polyatomic cluster ion signals. Mass spectra from individual Bacillus endospores were obtained by Steele et al. with a bipolar aerosol time-of-flight mass spectrometer [155]. The laser energy was found to significantly influence the mass spectra from Bacillus spores. Thus, the variability of the desorption/ionization (DI) laser beam profile had attenuated the reproducibility of singleparticle mass spectra. The results were confirmed by the same author in later research in which the laser energy was decreased to the threshold [156]. The flattened laser profile allowed the minimum laser fluence for the ionization of Bacillus spores and increased the reproducibility of single-spore identification. Fergenson et al. reported the real-time and reagentless characterization of individual airborne cells with ATOFMS in seconds [157]. Two species of Bacillus spores were distinguished from one another in a variety of biological and non-biological background materials, such as soil, fungal spores, and powders, by matching the mass spectra with fingerprint taken from pure samples. 6. Resonance ionization mass spectrometry (RIMS) Theoretically, the RIMS can also be called as LIMS because ions are generated with laser energy. Different from the traditional LIMS and SPMS, the RIMS employs one or more tunable lasers for step-wise excitation and finally ionization of the evaporated atoms in the gas phase to obtain highly selective ultratrace analysis [8,158–160]. RIMS is known for its selectivity and sensitivity due to the dual isotope specific properties (mass spectrometer and optical isotope selectivity through isotope shifts in the optical excitation process), large optical cross-sections of the optical excitation and ionization processes, complete isobaric suppression and a LOD in the fg to ag range [160,161]. Applications of the RIMS include rare isotope determination, trace gas detection, solid state surface analysis, explosives and chemical warfare agent determination [161]; however, RIMS is not suitable for multi-element analysis, application to a new element requires elaborate development and commercial support is not available for routine operation. A tunable laser-based detection system is very desirable but is expensive. Several methods, including sample sublimation by thermal techniques, ion sputtering, and laser ablation, can be used to generate a source of neutral target atoms in the gas phase [162]. The gas atoms are ‘stored’ in a gas cell, a hot cavity, or on a cold surface to assure good overlap with the incident laser [163]. Through single- or multistep excitation of the gas atoms by the lasers, which are tuned to the precise wavelength required for atomic resonance, ionization of the impurities in the analyte is carried out. Ionization is performed nonresonantly into the continuum, resonantly to an autoionizing state or electric field- or infrared-ionization from the high-lying Rydberg states [160,161]. The ions produced are then accelerated into the mass spectrometer (Fig. 5).

Fig. 5. Schematic diagram of RIMS source.

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A treatise on the state-of-the-art in RIMS was addressed by Wendt et al. concerning the question whether RIMS can be an alternative to accelerator mass spectrometry (AMS) [160]. Through some examples, it was concluded that RIMS was not comparable with AMS; however, RIMS was considered to be an alternative technique to AMS in the case of strong backgrounds or isobaric interferences. Another review focusing on developments in isotope ratio measurements of longlived radioisotopes by RIMS was published by the same author [161]. RIMS with high repetition lasers or continuous wave lasers have been widely used in analytical fields, such as geosciences, environment, cosmochemistry, and bio-medical research. Comparative measurements between AMS and RIMS for determination of long-lived 41Ca with isotopic abundances in the range of 10− 11 to 10− 10 were performed by Geppert et al. [164]. Three-step resonant ionization was used for the artificially produced calcium oxide samples. Selectivity of approximately 109 was achieved in the experiment, which was sufficient for routine measurements. The new perpendicular geometry of the atomic beam and mass spectrometer axes reduced the background from the atomization process. Temperature instabilities resulted in poor precision (5% to 30%) for Ca isotope ratios below 10− 10. Good agreement was obtained with the AMS results at the one-sigma uncertainty for the 10− 11 isotopic abundances. Single-, double-, and triple-resonance ionization of 90Sr has also been investigated [165], and 2 × 104 optical selectivity was reached using the double-resonance method; however, in the triple-resonance process, unfavorable isotope shifts reduced the selectivity by a factor of 100. In the double-resonance scheme, greater than 10− 5 overall detection efficiency and 1010 selectivity were realized. A LOD approaching 0.7 fg was achieved for 90 Sr in samples at 10− 10 relative isotopic abundance. The three-step resonant ionization scheme has been utilized successfully for selective ultra-trace determination of B [166], Gd [167], Pu [168–170], U [171] and Cs [172,173]. A review by Amari et al. addressed the recent development of isotopic ratio determinations in presolar grains using RIMS [174]. Isotope analysis of Mo, Zr, and Ba in single SiC grains has been carried out to assess the application of RIMS. The results suggest that a supernova origin of the grains agrees with patterns predicted for a neutron burst. A LA-RIMS technique has been applied to the isotope analysis of trace elements in agricultural products [175]. A Nd:YAG laser (532 or 355 nm) was used to initiate the atomization process. A precision of 0.3% was achieved for the 25 Mg/26 Mg isotope ratio, which could be used for the identification of the originating areas of agricultural products. The precision of direct measurements of Sr (at sub ppm level) isotope ratio was 14%. Improved performance could be achieved by increasing the ablation power or laser spot size. Resonant laser ionization coupled with laser microprobe fourier-transform ion cyclotron resonance mass spectrometry (RLA-FTICRMS) was developed by the Aubriet group for material science [162]. The system benefits from the selectivity and sensitivity of resonant ionization and the high mass resolution of FTICRMS. Compared with non-resonant FTICRMS, the RLA-FTICRMS increased selectivity by a factor of 2 to 7 in various matrices (metallic, vitreous, and cement). The investigation of atomic properties of heavy elements (proton number larger than 100) is a significant challenge. Sewtz et al. developed a novel ion guide laser ion source [176,177]. Atoms were stored in the buffer gas cell at 35 mbar argon pressure for about 40 ms. The generated ions were separated from the gas cell, underwent mass selection in a quadrupole mass spectrometer and were counted by a channeltron detector. This system has facilitated the investigation of the structure of the element fermium with 1010 atoms of 255Fm. The atomic levels have been predicted by multi-configuration Dirac–Fock calculations, and two resonant transitions have been found. Diode laser-based ionization of gadolinium in biomedical samples was performed by the Wendt group [178,179]. Using triple-resonance ionization, a detection limit of 1.5 × 109 atoms for a specific isotope was obtained with Ti as a sample carrier foil to increase the Gd atomization yield. The dynamic range of the linear response was over

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six orders of magnitude. This method was applied for the determination of Gd that was artificially introduced into normal and tumor tissue samples. The RIMS methodology was found to have good accuracy in determining the Gd content in microsamples of 20 to 50 mg. Combination with a high resolution primary ion source for atomization, which would allow a Cs+ gun to focus on a spot as small as 30 nm in diameter, isotope imaging with sub-μm resolution using RIMS may be possible. A laser resonance ionization source equipped with a gas cell filled with a noble gas (Ar or He) for thermalizing, storing and transporting trace radioactive ions and atoms has been developed by the Kudryavtsev group [180,181]. Off-line and on-line measurements have indicated that the new laser ion source was capable of performing β-decay studies of neutron-deficient and neutron-rich isotopes. Precise measurements of isotope ratios require identification and elimination of any major instability. The interactions of lasers with the gas atoms must be robust and resistant to variations. Any wavelength or intensity change, spatial or temporal changes of the resonance lasers, or fluctuation in the voltages on any electrode in the mass spectrometer system may in principle affect the yield of isotopes. Restraining isotopic fractionations and sustaining useful yields can be achieved through power broadening by the resonance lasers [182]. Through the feedback controls and power broadening, less than 1% precision was obtained for the statistically limited measurements of Cr isotope ratios. This improved measurement was experimentally performed for the detection of the chromium isotopic abundance in the presolar SiC grains. 7. Conclusions Direct analysis of solids by laser ionization mass spectrometry has been widely used in materials research, the semiconductor industry, geology, biomedicine, and environmental research for its excellent ability to determine trace elements. Lasers offer a highly versatile energy source for atomization, excitation, and ionization. The laser parameters (e.g., pulse duration, wavelength, and energy) can be easily controlled to adjust the condition of ionization source. Elemental mass spectrometry is one of the most promising techniques for its capabilities of sensitive, precise, and accurate multi-elemental determinations as well as isotope ratio measurements. The combination of laser ionization and mass spectrometry makes it a powerful tool for elemental analysis. Mass spectrometry is capable of characterizing the laser plasma to determine the ion yield, cluster formation, and angular and velocity distribution. The high laser irradiance, one-step ionization LIMS, is specialized for rapid multi-elemental analysis, and this system shows great potential for the analysis of bulk solids, e.g. metal, alloy, and geological samples. The SPMS with a laser for ionization is adept in determining elemental composition at the single particle level. The capability of on-line aerosol analysis using SPMS has played an important role in nanoparticle analysis, bio-aerosol identification and climate modeling. The RIMS has a significant role in precise isotope ratio measurements at ultratrace levels; the high sensitivity and elemental selectivity make RIMS very suitable for trace isotope analysis in the small sample sizes that are common in radioactive dating, environmental and biological sciences, and cosmochemistry. As a consequence of the efforts of many researchers, the techniques of mass spectrometry with laser ionization source will continue to make progress and become more powerful and versatile for elemental analysis in the future. Acknowledgements We gratefully acknowledge financial support from the Natural Science Foundation of China Financial (No. 20775063), National 863 program (No. 2009AA06Z109), and Fujian Province Department of Science & Technology (No. 2008H0035).

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