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A viscous film sample chamber for Laser Ablation Inductively Coupled Plasma – Mass Spectrometry
Author’s Accepted Manuscript A VISCOUS FILM SAMPLE CHAMBER FOR LASER ABLATION INDUCTIVELY COUPLED PLASMA – MASS SPECTROMETRY Damiano Monticelli, Davide Civati, Barbara Giussani, Carlo Dossi, Davide Spanu, Sandro Recchia www.elsevier.com/locate/talanta
PII: DOI: Reference:
S0039-9140(17)31109-8 https://doi.org/10.1016/j.talanta.2017.10.060 TAL18058
To appear in: Talanta Received date: 27 June 2017 Revised date: 26 October 2017 Accepted date: 28 October 2017 Cite this article as: Damiano Monticelli, Davide Civati, Barbara Giussani, Carlo Dossi, Davide Spanu and Sandro Recchia, A VISCOUS FILM SAMPLE CHAMBER FOR LASER ABLATION INDUCTIVELY COUPLED PLASMA – MASS SPECTROMETRY, Talanta, https://doi.org/10.1016/j.talanta.2017.10.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A VISCOUS FILM SAMPLE CHAMBER FOR LASER ABLATION INDUCTIVELY COUPLED PLASMA – MASS SPECTROMETRY
Damiano Monticelli*, Davide Civati, Barbara Giussani, Carlo Dossi, Davide Spanu, Sandro Recchia Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria Via Valleggio 11, 22100 Como (Italy)
*corresponding author: [email protected]; tel: +390312386427
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ABSTRACT Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) is a powerful method to determine the elemental composition of solid-state samples as it combines the high sensitivity and isotope selectivity of ICP-MS detection and the simplicity of laser ablation sampling. This technique enables rapid multiple sampling of the analysed material, such as needed for mapping or in-depth profiling applications. However, the duration of these measurements is practically restricted by the time taken for the particle to be transported from the sampling point to the ICP torch. The ablation cell, i.e. the sample holder, should combine high removal rate, high efficiency (i.e. complete transport of the ablated material) and reduced memory effects. These goals may be achieved by carefully designing the geometry of the cell and its gas flow patterns. A new cell design which enables a homogeneous fast removal (around 210 ms as the time for the transient signal to fall to 10% of its peak value using the 238U signal from a NIST610 glass standard) from a cylindrical chamber with 70 mm diameter is introduced in this paper. This result is achieved by combining a diffused, cylindrical flow pattern with an extraction tube coaxial with the laser beam and fixed to the laser assembly which enables the sampling point to be constantly positioned on the ablation spot. The lower part of the cell is mounted on the x,y stage for sample movement: the cell sealing is warranted by a viscous film junction between the lower and upper cell parts. Optimisation and performances of the apparatus are discussed in detail: performances are compared to existing designs. Graphical Abstract
KEYWORDS: Laser Ablation; Inductively Coupled Plasma – Mass Spectrometry; ablation cell; washout time
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INTRODUCTION Laser ablation has proven an efficient method for the direct introduction of solid samples in inductively coupled plasma sources followed by both spectrophotometric and mass detection, the latter being by far preferred nowadays because of the superior analytical performances. Despite its apparently easy concept and rugged design, four decades of research in this field have evidenced that obtaining reliable results is a matter of finely tuning the entire experimental setup, the critical steps being the laser-solid interaction, the aerosol transport and the aerosol breakdown in the plasma (see the reviews [1-3]). Surface imaging, or even 3D imaging, is the most popular and interesting field of application of Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICPMS). Applications include geology (tephrochronology [4], dating [5], 3D imaging [6]), environmental analysis and paleoclimate reconstruction (giant clam shells [7], otoliths [8], corals [9] and tree rings [10]), biomedical applications (immunoassays [11], tissue imaging [12], selenoprotein quantification [13]; see alsoby Konz et al. [14] for a review of biomedical applications) as well as single cell analysis[15]. This technique offers a high spatial resolution together with the high sensitivity typical of mass spectrometric techniques, making the detection of the spatial distribution of trace elements feasible. Images with pixels in the range of a few tens of square micrometers or even lower are nowadays possible (see e.g. [12]). The resolution is practically limited by the dimension of the ablated spot and the time taken to remove the generated aerosol from the ablation cell (washout time). Submicrometric ablation spot are obtained by non-linear effects typical of ultrashort pulses (see e.g. [16]) or near field enhancement effects [12]. Accordingly, the design of the ablation cell should warrant the complete and fast removal of the aerosol generated by the laser-solid interaction to retain a high enough resolution avoiding the mixing of particles produced by subsequent laser shots. The washout time also strongly impacts the sensitivity of the technique and its limits of detection: together with higher resolution, its minimisation warrants lower detection limits, which are also mandatory when the ablated material is limited due to very small ablation spots. Several efforts have accordingly been dedicated to the development of an efficient ablation cell, aiming at obtaining optimal flow patterns limiting particle dispersion [17-29]: recent advancements in the field were collected in a review paper by van Malderen et al. [30]. The employment of a sampling device to collect the particulate directly at the ablation spot demonstrated a particularly efficient strategy [19-21, 24, 25]. These otherwise efficient designs still show some drawbacks connected to the difficulty in sample movement, the 3
homogeneity of the washout time and the possibility to keep the sampling point at the ablation spot. As a general feature, extremely low washout times were achieved at the expenses of sample size [30]. In the present paper we developed a new cell design in which the sampling tube is kept in the same position as the ablation spot ensuring the cell tightness by a viscous fluid film. Two new gas flow pattern with cylindrical symmetry were developed and tested. As a result, an homogeneous washout times of 210 ms was achieved for a standard cylindrical cell of 70 mm diameter, calculated as the time for the transient signal to fall to 10% of its peak value using the 238U signal from a NIST610 glass standard). This design fills the gap for applications requiring medium size or multiple small size samples to be analysed at moderately high resolution.
EXPERIMENTAL Instrumental setup An Inductively Coupled Plasma-Mass Spectrometry, model X-Series II from Thermo Elemental was used for the mass spectrometric measurements. The dwell time was set to 50 and 20 ms for 238U and 140Ce, respectively (see below). A fourth harmonic, Nd:YAG 266nm laser system from New Wave (UP Series 266) was used for laser ablation. Laser fluence was kept at 4 J/cm2 in all of the experiments, whereas the spot size was changed from 80 to 5 m for NIST 610 and CeO2 samples, respectively (the reduced spot size enabled the acquisition of the 140Ce signal in pure ceria). Helium was used as the transport gas in the ablation chamber and mixed with argon in a Venturi connection before feeding into the ICP torch. After a brief optimization, we decided to use 1.04 L/min of helium and 0.90 L/min of argon makeup gas as the best compromise between signal intensity and washout time. Particle size distribution was measured at the exit of the ablation chamber by an optical particle counter (Lighthouse Handheld 3016). Gas tightness of the cell was checked by a leak detector produced by Ion Science, model GasCheck 3000.
Cell construction A sketch and a picture of the developed ablation cell are reported in Figure 1 and 2, respectively. The two cell parts were lathed from aluminium: the upper one is basically a 140 mm diameter annulus, whereas the lower one is a 19 mm thick disk in which a 70 mm 4
diameter x 14 mm depth cylinder was lathed to host the sample. Gas tightness between the two parts of the sample cell is ensured by a thin viscous film (Edwards Ultra grade 19 mineral oil, 144 cSt at 20°): this configuration allows the lower part of the cell to be fixed onto the x,y stage and be moved with respect to the upper one while ensuring gas tightness. This fluid has excellent chemical inertness and very low vapour pressure (silicon and perfluorinated spray lubricant were also tested but could not warrant cell tightness; on the other hand, higher viscosity fluids like silicon grease hindered the free movement of the two cell sections). The central part of the upper annulus is covered by a transparent silicon foil (1mm thick, 60mm o.d.) which was perforated to accommodate the sampling tube. The latter features a high quality quartz windows transparent to the 266 nm laser radiation. The upper part of the cell is mounted on an aluminium frame integral to the laser ablation body. The sample cell features a tangential gas flow inlet in its standard configuration (see Figure 1). Nevertheless, two additional parts can be fitted at the cell bottom to modify the gas flow pattern, aiming at homogenizing the gas flow in the cell (see Figure 3). The first insert (Figure 3A) is a disk leaving a 1 mm annulus around the cell walls for the helium gas to flow, resulting in an annular vertically diffused flow. The second modification features a smaller disk coupled to an annulus with “L” section (Figure 3B) that changes the flow to horizontally diffused. A smaller cell was also realised to assess the effect of the cell volume on its performances: it has the same design as in Figure 1 but an internal diameter of 50 mm and a height of 9 mm. A tangential flow only was checked with this configuration as introducing flow modifier (such as the ones reported in Figure 3) would strongly reduce the volume available for the sample.
Cell characterisation Gas tightness is clearly the first requirement of any newly devised sample cell and particularly critical when a viscous film is used. It was assessed by comparing the gas flow exiting the central channel of the ICP torch with the set values of the mass flow controller regulating the helium and argon flows. The measurement was performed at the inner channel of the ICP torch to account for possible issues (i.e. counterpressures) in the entire transfer line. Gas flows were measured by a manual bubble flowmeter. The washout time was calculated from the transient signals as the time taken by the signal to fall to 10% of its peak value. 5
Washout times as a function of different parameters (sampling height, flow pattern and sampling tube diameter) were assessed by analysing the cerium signal in a commercial, 5 cm diameter glass disk. The latter allowed the washout time to be assessed for analytes at trace concentration: cerium concentration is 2.8 mg/kg in this sample as determined using NIST SRM 610 that contains 449.4 ± 1.0 ppm of Ce [31]. Moreover, one sample only could be used to assess the washout time on the entire sample surface. Sampling heights, i.e. the distance between the sample surface and the collecting tube, were changed by raising the latter with respect to the cell upper surface (see Figure 2 where small polytetrafluoroethylene parts increase the distance between the sampling tube and the cell). Distances of 1 and 4 mm were assessed. Analysis of variance (ANOVA) was performed by traditional statistical packages running in the open-source R software environment RStudio. The optimal performances were achieved using a sampling height of 4 mm, a sampling tube diameter of 8 mm and a horizontal flux geometry: this configuration was used for the experiments reported in the following sections. Two more samples were also used for evaluating the washout time in the optimised cell: the standard NIST 610 glass (several elements at around 500 mg/kg in a glass matrix) and pressed pellets of cerium dioxide (pressed at 150 bar in 30 mm pellets). The washout time was determined in a number of points uniformly distributed on the cell surface by locating the samples in the desired position (the commercial glass sample occupies the entire cell surface). In particular, when Ce pellet and NIST 610 glass were used, 21 points were investigated for the tangential and annular vertical flows, 17 for the annular horizontal flow, whereas 13 points were probed when the commercial glass disk was analysed. Five transient signals were collected at five close, 100 m distance, locations to obtain a reliable estimation of the washout time and an esteem of its precision.
A portion of a random access memory (RAM) board was analysed by a line scan to demonstrate the resolution achievable with the proposed ablation cell employing the following instrumental parameters: fluence 0.70 J/cm2, scan speed 20 μm/s, spot size 12 μm, laser frequency 20 Hz and a dwell time of 5 ms for 63Cu and 1 ms for 197Au.
RESULTS AND DISCUSSION Cell design
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As above mentioned, there are mainly two correlated factors that prompt for the development of novel designs for laser ablation chambers. The first one relates to the achievement of short washout times which leads increased sensitivities (the generated aerosol is less diluted by the carrier gas), higher spatial resolution (the mixing of the aerosol generated by subsequent shots decreases) and increased scan speed (see e.g. [30]). The second factor relates to the achievement of homogeneous washout times through the entire sampled surface: this issue is particularly important to obtain reliable surface elemental distributions. The results obtained by some of us [25] clearly indicate that very short washout times can be obtained through a careful control of the gas flow pattern inside the ablation chamber. The limit of this design is that low washout times (30 ms) were achieved only in the centre of the ablation cell, whereas 50 time higher figures were measured close to the cell walls. The so called “two-volume cell”, initially introduced by Arrowsmith and Huges [32], is a good compromise between low and homogeneous washout times, as it physically confines the sample holder volume (first volume) from the ablated plume sampling volume (second volume). The novel cell presented here (see Figure 1) is a further evolution of the two-volume strategy [32] and its latest evolution [24]. The major innovation is that the sampling gas pipe (the second volume) is always kept at the ablation site as it is located in the upper part of the cell that is integral to the laser ablation equipment (see Figure 2). The lower part is instead used to hold the sample and is moved by an x, y stage: contrary to existing designs [20], we decided not to include the stage into the sample holder, enabling the full exploitation of the scanning capabilities of the built-in x, y stage. This choice actually implies that the cell is divided in two distinct parts. To ensure the gas tightness of the whole assembly the utilisation of bellows was firstly considered, but no suitable solution could be found. Therefore we decided to evaluate the potential utilisation of a viscous film. This unusual design immediately appeared to be a good choice, even if a careful optimisation of the downstream carrier flow is necessary to ensure the complete gas tightness. An oil with an appropriate viscosity (Edwards Ultra grade 19 mineral oil, 144 cSt at 20°, commonly used for rotary pumps) is placed between the upper and the lower disks of the chamber. The lower disk can be freely moved by the x, y stage, while the upper disk weights on the lower one (without constrains on the z axis) and is maintained in a fixed x, y position by a fixed framework (see Figure 2). 7
Finally, the lower cell disk is designed to accommodate some additional parts (see Figure 3) which allow changing the flow pattern from cyclonic to vertically diffused or horizontally diffused. It should be noticed that the pipe dimensions cannot be reduced to an arbitrarily small volume, since the after-shot plume expansion is non negligible (see e.g. [33] and references therein). Moreover, the second volume must be sufficiently large to collect the whole ablated material, thus avoiding sample losses toward the first volume which may cause decreased sensitivities and memory effects. With this design we reached a low second cell volume, together with a flexible gas inlet system, without posing limitations on the scanning capabilities of the moving stage and thus on the analysable sample surface.
Air tightness Air tightness is clearly the first issue to be addressed if this cell design has to be adopted. If unwanted overpressures are generated by gas flows, gas leaks may occur by the formation of preferential channels through the viscous film. In this respect, when only the He carrier flow is used, the developed cell is fully gas tight, i.e. helium inflow and outflow are comparable within the measurement error. However, the outflow was 10% lower than the inlet under standard working conditions, when the argon makeup flow downstream the cell was turned on (as reported in the Experimental section, the outlet flow was measured at the internal channel of the ICP torch to take into account the entire transfer line). Two critical points in the line were identified causing over pressure: i) the Y connection where helium and the makeup gas are mixed; ii) the connection between the line and the torch which in the starting configuration is made with a 13mm Rotulex connection with a 2 mm i.d.. Both caused a restriction of the line from 4 to 2 mm inner diameter. The connection to the torch was simply changed to a 4 mm i.d. Rotulex connection. The mixing between helium and argon is a more difficult issue as no counterpressure and as low as possible turbulence should be generated. A Venturi connection was adopted similar to the one described by Lindner et al [24]. This mixing system ensures minimum turbulence and dead volumes, but also a limited Venturi effect which contributes in extracting helium and the transported aerosol out of the cell. This configuration assured the complete transport of helium out of the cell, with no statistically differences between the inlet and outlet flow (differences below 0.5%). Two further tests were performed to ensure no leakage of helium transport gas and no variations in the 8
presence of atmospheric gases (i.e. that no gas enters the system from outside the cell). Firstly, a gas leak detector indicated no helium leaks from the viscous film and the conjunction between the silicon foil and the sampling gas pipe during cell movimentation. Secondly, we checked that background signals due to atmospheric gases do not change during cell movimentation. Data reported in Figure S1 clearly show the constancy of the signals related to atmospheric gases when the cell is moved through its maximum x-y limits, demonstrating that the entire system (cell plus transfer line) is air tight under experimental conditions.
Optimisation of cell parameters In the attempt to define the best cell design, four major cell configurations were assessed, namely the particle sampling height, the gas flow pattern, the diameter of the sampling tube and the cell dimensions (see Experimental section for technical details). The sampling height, i.e. the distance between the sample surface and the entrance of the sampling volume, is expected to play a major role in fast and complete particle transport. A steady decrease in washout times was observed with increasing distances from the sample surface, from 1500 ms to 300 ms for 1 and 8 mm distance, respectively (time for the transient signal to fall to 10% of its peak value). Nevertheless, the decrease in washout time was paralleled by a decline in the signal integral, evidencing a reduced transport efficiency. The latter was assessed by measuring the particle size distribution by an optical particle counter (see Figure S2). These data show that fractions with nominal diameter higher than 1 μm are not removed from the cell when a 8 mm distance is used. Accordingly, 1 and 4 mm sampling height only were employed in all of the following experiments. It should be highlighted that such a behaviour may be distinct of a 266 nm wavelength radiation which is known to produce larger particles than shorter wavelength systems (5th harmonic Nd:YAG at 213 nm or excimer ArF at 193 nm) or ultrashort pulses (see e.g. [1-3] and references therein). Three different flow patterns were assessed: a tangential flow [25] and two different annular, diffused flows (vertical and horizontal, see Figures 1 & 3). Preliminary experiments conducted with the tangential, cyclonic flux evidenced inhomogeneous washout times (range 200 – 460 ms): this feature is typical of the cyclonic flux inlet [25], although different patterns in washout times were registered with the present and past cell. Accordingly, this gas flow configuration was abandoned.
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Following these preliminary results, the effect of sampling height, sampling tube diameter and inlet gas flow were systematically investigated. All of the three factors were investigated at two different level according to a full factorial 23 design: sampling height: 1 and 4 mm; sampling cell diameter: 4 and 8 mm; flux geometry: horizontal and vertical. Washout values obtained during the eight experiments are illustrated in Figure 4 as contour plots, whereas numerical values are reported in Table 1. Washout times are included in the range 240-317 ms, whereas the standard deviations of the values measured in the 13 different positions are in the 15-27 ms interval (5 replicate measurements of the washout time were performed at each sampling site, see Experimental). Accordingly, the viscous film cell features a low washout time and a variable degree of homogeneity depending on the adopted configuration: the effect of the instrumental configurations will be presented in the current section, whereas a comparison with existing designs is discussed in the following section. A first exploratory analysis highlights a non-negligible difference among the groups (i.e. the eight different configurations): the one-way analysis of variance (ANOVA) resulted in an F value of 10.7 meaning a p value lower than 0.01 for df1=7 (between groups degrees of freedom) and df2=95 (within groups degrees of freedom). This means that there exist systematic differences in washout time values between the eight configurations. The among group difference becomes negligible at a 0.05 significance level when configurations e and g (see Table 1) are excluded from calculations (df1=5, df2=92, F=1.93 to be compared with a critical F value of 2.34 for p=.05): these configurations show the highest washout times and are both based on a vertical flux plus a 4 mm sampling volume (see Table 1). Six out of eight configurations feature undistinguishable values of washout times according to the ANOVA analysis. The results of the factorial design were also investigated by ANOVA analysis to assess the effect of the three investigated cell parameters. As a result, neither the main factors (sampling height, sampling chamber diameter and gas flux), nor the second order interactions show a significant effect on the response (the washout time) at the 0.05 level (the most significant factor is gas flux geometry with a p value equal to 0.08). Such an absence of effects may partially stem from the preliminary work aimed at narrowing the investigated sampling heights and the flux geometries (see the beginning of this section). Nevertheless, this feature is a strong indication of the robustness of the ablation cell: substantial changes in the in/out flows and in the positioning of the sampling volume has a limited effect on the performances of the ablation cell in term of washout times. 10
In conclusion, we suggest to use configuration d in Figure 4 (sampling height 4 mm, sampling tube diameter 8 mm and a horizontal flux geometry): the value of the washout times is not statistically lower than the other configurations (see previous discussion), but it is significantly more uniform over the cell (see Figure 4d and Table 1). Figure S3 in the Supplementary Material shows that the integral of transient signals are homogeneous over the entire cell surface (see Figure S3). The optimal configuration was adopted for the measurements reported in the following sections. The washout times were also calculated using higher concentration samples to assess whether the analyte concentration affected the washout time. Moreover, literature data are usually calculated with more concentrated samples: an unbiased comparison relies on the evaluation of the washout times vs concentration trend. NIST610 (uranium concentration 461.5 ± 1.1 mg/kg) and pressed cerium oxide pellets (Ce concentration 81% or 810000 mg/kg) were analysed in 17 positions covering the entire cell surface. As a result, washout times of 206±79 and 138±34 ms were measured: comparing with the reported 240-260 ms for 2.8 mg/kg of cerium (see Table 1), a decreasing trend in washout time values vs concentration is clear. Such a trend deserves further investigation: an increase in signal to noise ratio is surely a first factor as well as the different materials analysed (glasses vs an oxide pellet) . As an example of application to a inhomogeneous material, we ablated by a line scan a section of a Random Access Memory (RAM) board (Figure 5), analysing the signals of gold and copper. Three different areas are visible at the optical microscope, i.e. the gold contact which is 300 µm wide, the body of the board and the gold track on the PCB. As a result, the cell enables the ready identification of the three different features of the memory board. The correlation between gold and copper signals points to the use of a gold alloy rather than pure gold with non-negligible amounts of copper.
Comparison with existing designs Optimal configurations of the viscous film cell feature washout times in the range 240-260 ms (see entries a-d in Table 1). Washout times and investigable sample surface are reported in Figure 6 for literature data and the ablation cell introduced in the present work (data in numerical form are reported in Table S1 in supplementary information). This comparison may only be semi-quantitative as the homogeneity of the data is limited, hindering the application of a fully quantitative approach. Washout times have been calculated in different ways, employing thresholds set to the 0.1%, 1% or 10% of the signal 11
maximum. Secondly, different samples were used to estimate the washout times, typically the NIST 610 and 612 standard glasses with concentrations of trace elements of 500 and 50 mg/kg, respectively. Thirdly, different laser sources were employed, including pulse duration in the ns and fs range as well as different laser wavelengths: these factors are known to generate ablated particles of different dimensions and, accordingly, different aerodynamic properties. Fourthly, a limited number of authors [18, 21, 25, 29] systematically evaluated the washout times over the entire sample surface or a significant portion of it. Finally, information on sample size and analyzable sample surface are not always explicitly reported or even missing and had to be estimated from reported data (cell geometry, etc.). As a general feature, Figure 6 seems to suggest an increase in washout times with increasing analysable sample surface, although the linear correlation is not significant (n=20, r=0.27, p=0.24): it should also be mentioned that data at high washout time/analysable surface show high leverage, i.e. they are highly influent in the linear regression. Regarding the performances of the proposed cell, Figure 6 evidences that it is situated in the target region of the plot washout time vs analysable surface, i.e. the optimal region situated at low washout times and high analysable surface. As a matter of fact, it features the lowest reported washout time for an analysable surface of 20 cm2. Five cell designs are situated in this optimal area and will be compared to the proposed cell. The commercial ablation cell produced by ESI (TruLineTM) should firstly be mentioned (available data reported in Table S1): it shows washout times similar to the ones reported here (around 150 ms at the 10% level and 250 ms at the 1%) but it could not be included in Figure 6 as the analysable area could not be found (reported data seems to suggest a diameter around 6 cm, very similar to the viscous film cell). The machinery behind this cell are evidently undisclosed but the cell also features a pinch valve to minimise pulse broadening. Another commercially available cell (HelEx II with CleanShotTM, labelled 13 in Figure 6) shows similar performances with a lower washout time and a smaller analysable area, performances overlapping the ones of the cell recently developed by Douglas et al. [34] (the latter cell includes mofications of the ICP torch). Finally, the cells developed by Feldman et al. [18] and ESI (labelled 3 and 12 in Figure 6, respectively) show analysable areas over 100 cm2 with washout times in the 0.5-0.7 range (the cell described in [18] is limited to the analysis of blotted membranes). For the sake of completeness, two more
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large volume cells produced by RESOlution (S155) and GeoLas should be mentioned although no data on washout times could be found. Homogeneity of washout times is even more difficult to be compared as data are not reported in the literature. Figures are reported by Douglas et al. only [34], whereas plots are merely found in [18]: data are comparable to the ones reported in the present paper (see Table 1), although a truly systematic investigation is missing in these two papers. A very good homogeneity in signals with RSD% of 2% is claimed for the three commercial cells, with no assessment of washout time values in different points of the cell.
CONCLUSIONS A large cylindrical cell with a uniform, low washout time was realised and its performances assessed. As a result, a washout time around 250 ms was achieved for a cylindrical cell with an inner diameter of 70 mm for trace amounts of cerium (2.8 mg/kg) in a commercial glass. This value lowers to around 210 ms for the ablation of the NIST 610 standard (461.5 ± 1.1 mg/kg U). The cell may be fitted on the stage of standard instrumentations for laser ablation. The limited dependence of the washout times from cell dimensions and gas flow patterns, as far as they have a cylindrical symmetry, further supports the robustness of this configuration. These results are achieved by keeping the sampling point at the ablation site as is true for most recent cell designs. Modelling of flow pattern and particle trajectories would greatly help in increasing the performances of the proposed cell. Although clear indications already emerged from the experimental data, e.g. dimensional fractionation as a function of sampling height, numerical modelling would provide a sound ground for the theoretical interpretation of the results and suggest future directions. The comparison with existing designs highlighted that the viscous film cell offers a good compromise between a moderately low washout time (around 210 ms evaluated by uranium signal in NIST 610 standard) and the possibility to analyse a large surface area (20 cm2). It is an instrument added to the analytical chemist toolbox for the investigation of medium sized samples with low washout times and constant sensitivity all over the sample surface. The cell may be proficiently used also to investigate multiple small samples. Regarding quantitative applications, adequate homogenisation of the signal should be considered, as is true for any cell showing fast washout times when low repetition rates are employed (see e.g. [26]).
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Progression of this design may evolve in two directions. In principle, larger analysable surfaces may be achieved developing larger diameter cells: as shown for 5 and 7 cm I.D. cells, the cell diameter do not affect performances because of the adopted extraction strategy. Reduction in washout times could instead be attained by adopting a single straight transfer line from the cell to the ICP torch analogous to the “sniffer” adopted by Douglas et al. [34]. The sampling tube could be inserted directly in the lateral wall of the upper part of the cell avoiding the 90 degree turn in the particle trajectory of the present design.
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[17] D. Bleiner, H. Altorfer, A novel gas inlet system for improved aerosol entrainment in laser ablation inductively coupled plasma mass spectrometry, J. Anal. Atomic Spectrom. 20 (2005) 754-756. [18] I. Feldmann, C.U. Koehler, P.H. Roos, N. Jakubowski, Optimisation of a laser ablation cell for detection of hetero-elements in proteins blotted onto membranes by use of inductively coupled plasma mass spectrometry, J. Anal. Atomic Spectrom. 21 (2006) 10061015. [19] J. Pisonero, D. Fliegel, D. Günther, High efficiency aerosol dispersion cell for laser ablation-ICP-MS, J. Anal. Atomic Spectrom. 21 (2006) 922-931. [20] C.C. Garcia, H. Lindner, K. Niemax, Transport efficiency in femtosecond laser ablation inductively coupled plasma mass spectrometry applying ablation cells with short and long washout times, Spectrochim. Acta B 62 (2007) 13-19. [21] Y. Liu, Z. Hu, H. Yuan, S. Hu, H. Cheng, Volume-optional and low-memory (VOLM) chamber for laser ablation-ICP-MS: application to fiber analyses, J. Anal. Atomic Spectrom. 22 (2007) 582-585. [22] M. Tanner, D. Günther, Signal acquisition in ms time resolution for in-torch LA-ICPMS, J. Anal. Atomic Spectrom. 22 (2007) 1189-1192. [23] D. Asogan, B.L. Sharp, C.J.P. O' Connor, D.A. Green, R.W. Hutchinson, An open, non-contact cell for laser ablation-inductively coupled plasma-mass spectrometry, J. Anal. Atomic Spectrom. 24 (2009) 917-923. [24] H. Lindner, D. Autrique, C.C. Garcia, K. Niemax, A. Bogaerts, Optimized transport setup for high repetition rate pulse-separated analysis in laser ablation-inductively coupled plasma mass spectrometry, Anal. Chem. 81 (2009) 4241-4248. [25] D. Monticelli, E.L. Gurevich, R. Hergenroder, Design and performances of a cyclonic flux cell for laser ablation, J. Anal. Atomic Spectrom. 24 (2009) 328-335. [26] W. Muller, M. Shelley, P. Miller, S. Broude, Initial performance metrics of a new custom-designed ArF excimer LA-ICPMS system coupled to a two-volume laser-ablation cell, J. Anal. Atomic Spectrom. 24 (2009) 209-214. [27] G. Carugati, S. Rauch, M.E. Kylander, Experimental assessment of a large sample cell for laser ablation-ICP-MS, and its application to sediment core micro-analysis, Microchim. Acta 170 (2010) 39-45. [28] H. Lindner, D. Autrique, J. Pisonero, D. Günther, A. Bogaerts, Numerical simulation analysis of flow patterns and particle transport in the HEAD laser ablation cell with respect to inductively coupled plasma spectrometry, J. Anal. Atomic Spectrom. 25 (2010) 295-304. [29] M.B. Fricker, D. Kutscher, B. Aeschlimann, J. Frommer, R. Dietiker, J. Bettmer, D. Günther, High spatial resolution trace element analysis by LA-ICP-MS using a novel ablation cell for multiple or large samples, Int. J. Mass Spectrom., 307 (2011) 39-45. [30] S.J.M. van Malderen, A.J. Managh, B.L. Sharp, F. Vanhaecke, Recent developments in the design of rapid response cells for laser ablation-inductively coupled plasma-mass spectrometry and their impact on bioimaging applications, J. Anal. Atomic Spectrom. 31 (2016) 423-439. [31] K.P. Jochum, U. Weis, B. Stoll, D. Kuzmin, Q. Yang, I. Raczek, D.E. Jacob, A. Stracke, K. Birbaum, D.A. Frick, D. Günther, J. Enzweiler, Determination of Reference Values for NIST SRM 610–617 Glasses Following ISO Guidelines, Geostan. Geoanal. Res. 35 (2011) 397-429. [32] Arrowsmith P., Huges S.K., Entrainment and Transport of Laser Ablated Plumes for Subsequent Elemental Analysis, Applied Spectrosc. 42 (1988) 1231 - 1239. [33] D. Bleiner, Z. Chen, D. Autrique, A. Bogaerts, Role of laser-induced melting and vaporization of metals during ICP-MS and LIBS analysis, investigated with computer simulations and experiments, J. Anal. Atomic Spectrom. 21 (2006) 910-921. 16
[34] D.N. Douglas, A.J. Managh, H.J. Reid, B.L. Sharp, High-Speed, Integrated Ablation Cell and Dual Concentric Injector Plasma Torch for Laser Ablation-Inductively Coupled Plasma Mass Spectrometry, Anal. Chem. 87 (2015) 11285-11294.
Table 1. Average washout times and standard deviation over the entire sample surface.
Name
Sampling
Sampling
Height
Flux geometry
Mean
Standard
tube diameter
Washout
Deviation
(mm)
(mm)
time (ms)
(ms)
a
1
4
Horizontal
238
23
b
1
8
Horizontal
250
25
c
4
4
Horizontal
260
27
d
4
8
Horizontal
243
15
e
1
4
Vertical
281
47
f
1
8
Vertical
253
18
g
4
4
Vertical
317
32
h
4
8
Vertical
252
22
17
CAPTION TO FIGURES
Figure 1. Basic scheme of the developed cell (quoted dimensions in mm): orange line = location of the viscous film; green line = flow path; brown rectangle= schematic representation of the sample specimen. Figure 2. Final prototype mounted onto the laser ablation apparatus. Note that samples can be efficiently illuminated through the transparent silicon foil. Figure 3. Additional parts to be used for: A vertically diffused and B horizontally diffused gas flow patterns (quoted dimensions in mm). The green lines schematically depict flow paths. Figure 4. Contouring map of washout evaluated using the signal of 140Ce generated by the ablation of the commercial glass disk employing different configurations (see labels a-h reported in Table 1). Figure 5. Line scan analysis of a random-access memory (RAM) board, using a 12 µm spot size, a 20 Hz frequency with a 20 µm/s scan rate. Figure 6. Summary of literature data reported in plot washout time vs. analysable surface (see labels 1-20 reported in Table S1, label 19 indicates the results reported in this work).
18
Figure 1
19
Figure 2
20
Figure 3
21
Figure 4
22
Figure 5
23
Figure 6
HIGHLIGHTS
A sample chamber for laser ablation based on a viscous film has been designed and tested Several configurations were evaluated measuring the washout times A uniform and low (210 ms) washout time was achieved over a 20 cm2 surface Performances are compared to existing designs
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PII: DOI: Reference:
S0039-9140(17)31109-8 https://doi.org/10.1016/j.talanta.2017.10.060 TAL18058
To appear in: Talanta Received date: 27 June 2017 Revised date: 26 October 2017 Accepted date: 28 October 2017 Cite this article as: Damiano Monticelli, Davide Civati, Barbara Giussani, Carlo Dossi, Davide Spanu and Sandro Recchia, A VISCOUS FILM SAMPLE CHAMBER FOR LASER ABLATION INDUCTIVELY COUPLED PLASMA – MASS SPECTROMETRY, Talanta, https://doi.org/10.1016/j.talanta.2017.10.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A VISCOUS FILM SAMPLE CHAMBER FOR LASER ABLATION INDUCTIVELY COUPLED PLASMA – MASS SPECTROMETRY
Damiano Monticelli*, Davide Civati, Barbara Giussani, Carlo Dossi, Davide Spanu, Sandro Recchia Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell’Insubria Via Valleggio 11, 22100 Como (Italy)
*corresponding author: [email protected]; tel: +390312386427
1
ABSTRACT Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) is a powerful method to determine the elemental composition of solid-state samples as it combines the high sensitivity and isotope selectivity of ICP-MS detection and the simplicity of laser ablation sampling. This technique enables rapid multiple sampling of the analysed material, such as needed for mapping or in-depth profiling applications. However, the duration of these measurements is practically restricted by the time taken for the particle to be transported from the sampling point to the ICP torch. The ablation cell, i.e. the sample holder, should combine high removal rate, high efficiency (i.e. complete transport of the ablated material) and reduced memory effects. These goals may be achieved by carefully designing the geometry of the cell and its gas flow patterns. A new cell design which enables a homogeneous fast removal (around 210 ms as the time for the transient signal to fall to 10% of its peak value using the 238U signal from a NIST610 glass standard) from a cylindrical chamber with 70 mm diameter is introduced in this paper. This result is achieved by combining a diffused, cylindrical flow pattern with an extraction tube coaxial with the laser beam and fixed to the laser assembly which enables the sampling point to be constantly positioned on the ablation spot. The lower part of the cell is mounted on the x,y stage for sample movement: the cell sealing is warranted by a viscous film junction between the lower and upper cell parts. Optimisation and performances of the apparatus are discussed in detail: performances are compared to existing designs. Graphical Abstract
KEYWORDS: Laser Ablation; Inductively Coupled Plasma – Mass Spectrometry; ablation cell; washout time
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INTRODUCTION Laser ablation has proven an efficient method for the direct introduction of solid samples in inductively coupled plasma sources followed by both spectrophotometric and mass detection, the latter being by far preferred nowadays because of the superior analytical performances. Despite its apparently easy concept and rugged design, four decades of research in this field have evidenced that obtaining reliable results is a matter of finely tuning the entire experimental setup, the critical steps being the laser-solid interaction, the aerosol transport and the aerosol breakdown in the plasma (see the reviews [1-3]). Surface imaging, or even 3D imaging, is the most popular and interesting field of application of Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICPMS). Applications include geology (tephrochronology [4], dating [5], 3D imaging [6]), environmental analysis and paleoclimate reconstruction (giant clam shells [7], otoliths [8], corals [9] and tree rings [10]), biomedical applications (immunoassays [11], tissue imaging [12], selenoprotein quantification [13]; see alsoby Konz et al. [14] for a review of biomedical applications) as well as single cell analysis[15]. This technique offers a high spatial resolution together with the high sensitivity typical of mass spectrometric techniques, making the detection of the spatial distribution of trace elements feasible. Images with pixels in the range of a few tens of square micrometers or even lower are nowadays possible (see e.g. [12]). The resolution is practically limited by the dimension of the ablated spot and the time taken to remove the generated aerosol from the ablation cell (washout time). Submicrometric ablation spot are obtained by non-linear effects typical of ultrashort pulses (see e.g. [16]) or near field enhancement effects [12]. Accordingly, the design of the ablation cell should warrant the complete and fast removal of the aerosol generated by the laser-solid interaction to retain a high enough resolution avoiding the mixing of particles produced by subsequent laser shots. The washout time also strongly impacts the sensitivity of the technique and its limits of detection: together with higher resolution, its minimisation warrants lower detection limits, which are also mandatory when the ablated material is limited due to very small ablation spots. Several efforts have accordingly been dedicated to the development of an efficient ablation cell, aiming at obtaining optimal flow patterns limiting particle dispersion [17-29]: recent advancements in the field were collected in a review paper by van Malderen et al. [30]. The employment of a sampling device to collect the particulate directly at the ablation spot demonstrated a particularly efficient strategy [19-21, 24, 25]. These otherwise efficient designs still show some drawbacks connected to the difficulty in sample movement, the 3
homogeneity of the washout time and the possibility to keep the sampling point at the ablation spot. As a general feature, extremely low washout times were achieved at the expenses of sample size [30]. In the present paper we developed a new cell design in which the sampling tube is kept in the same position as the ablation spot ensuring the cell tightness by a viscous fluid film. Two new gas flow pattern with cylindrical symmetry were developed and tested. As a result, an homogeneous washout times of 210 ms was achieved for a standard cylindrical cell of 70 mm diameter, calculated as the time for the transient signal to fall to 10% of its peak value using the 238U signal from a NIST610 glass standard). This design fills the gap for applications requiring medium size or multiple small size samples to be analysed at moderately high resolution.
EXPERIMENTAL Instrumental setup An Inductively Coupled Plasma-Mass Spectrometry, model X-Series II from Thermo Elemental was used for the mass spectrometric measurements. The dwell time was set to 50 and 20 ms for 238U and 140Ce, respectively (see below). A fourth harmonic, Nd:YAG 266nm laser system from New Wave (UP Series 266) was used for laser ablation. Laser fluence was kept at 4 J/cm2 in all of the experiments, whereas the spot size was changed from 80 to 5 m for NIST 610 and CeO2 samples, respectively (the reduced spot size enabled the acquisition of the 140Ce signal in pure ceria). Helium was used as the transport gas in the ablation chamber and mixed with argon in a Venturi connection before feeding into the ICP torch. After a brief optimization, we decided to use 1.04 L/min of helium and 0.90 L/min of argon makeup gas as the best compromise between signal intensity and washout time. Particle size distribution was measured at the exit of the ablation chamber by an optical particle counter (Lighthouse Handheld 3016). Gas tightness of the cell was checked by a leak detector produced by Ion Science, model GasCheck 3000.
Cell construction A sketch and a picture of the developed ablation cell are reported in Figure 1 and 2, respectively. The two cell parts were lathed from aluminium: the upper one is basically a 140 mm diameter annulus, whereas the lower one is a 19 mm thick disk in which a 70 mm 4
diameter x 14 mm depth cylinder was lathed to host the sample. Gas tightness between the two parts of the sample cell is ensured by a thin viscous film (Edwards Ultra grade 19 mineral oil, 144 cSt at 20°): this configuration allows the lower part of the cell to be fixed onto the x,y stage and be moved with respect to the upper one while ensuring gas tightness. This fluid has excellent chemical inertness and very low vapour pressure (silicon and perfluorinated spray lubricant were also tested but could not warrant cell tightness; on the other hand, higher viscosity fluids like silicon grease hindered the free movement of the two cell sections). The central part of the upper annulus is covered by a transparent silicon foil (1mm thick, 60mm o.d.) which was perforated to accommodate the sampling tube. The latter features a high quality quartz windows transparent to the 266 nm laser radiation. The upper part of the cell is mounted on an aluminium frame integral to the laser ablation body. The sample cell features a tangential gas flow inlet in its standard configuration (see Figure 1). Nevertheless, two additional parts can be fitted at the cell bottom to modify the gas flow pattern, aiming at homogenizing the gas flow in the cell (see Figure 3). The first insert (Figure 3A) is a disk leaving a 1 mm annulus around the cell walls for the helium gas to flow, resulting in an annular vertically diffused flow. The second modification features a smaller disk coupled to an annulus with “L” section (Figure 3B) that changes the flow to horizontally diffused. A smaller cell was also realised to assess the effect of the cell volume on its performances: it has the same design as in Figure 1 but an internal diameter of 50 mm and a height of 9 mm. A tangential flow only was checked with this configuration as introducing flow modifier (such as the ones reported in Figure 3) would strongly reduce the volume available for the sample.
Cell characterisation Gas tightness is clearly the first requirement of any newly devised sample cell and particularly critical when a viscous film is used. It was assessed by comparing the gas flow exiting the central channel of the ICP torch with the set values of the mass flow controller regulating the helium and argon flows. The measurement was performed at the inner channel of the ICP torch to account for possible issues (i.e. counterpressures) in the entire transfer line. Gas flows were measured by a manual bubble flowmeter. The washout time was calculated from the transient signals as the time taken by the signal to fall to 10% of its peak value. 5
Washout times as a function of different parameters (sampling height, flow pattern and sampling tube diameter) were assessed by analysing the cerium signal in a commercial, 5 cm diameter glass disk. The latter allowed the washout time to be assessed for analytes at trace concentration: cerium concentration is 2.8 mg/kg in this sample as determined using NIST SRM 610 that contains 449.4 ± 1.0 ppm of Ce [31]. Moreover, one sample only could be used to assess the washout time on the entire sample surface. Sampling heights, i.e. the distance between the sample surface and the collecting tube, were changed by raising the latter with respect to the cell upper surface (see Figure 2 where small polytetrafluoroethylene parts increase the distance between the sampling tube and the cell). Distances of 1 and 4 mm were assessed. Analysis of variance (ANOVA) was performed by traditional statistical packages running in the open-source R software environment RStudio. The optimal performances were achieved using a sampling height of 4 mm, a sampling tube diameter of 8 mm and a horizontal flux geometry: this configuration was used for the experiments reported in the following sections. Two more samples were also used for evaluating the washout time in the optimised cell: the standard NIST 610 glass (several elements at around 500 mg/kg in a glass matrix) and pressed pellets of cerium dioxide (pressed at 150 bar in 30 mm pellets). The washout time was determined in a number of points uniformly distributed on the cell surface by locating the samples in the desired position (the commercial glass sample occupies the entire cell surface). In particular, when Ce pellet and NIST 610 glass were used, 21 points were investigated for the tangential and annular vertical flows, 17 for the annular horizontal flow, whereas 13 points were probed when the commercial glass disk was analysed. Five transient signals were collected at five close, 100 m distance, locations to obtain a reliable estimation of the washout time and an esteem of its precision.
A portion of a random access memory (RAM) board was analysed by a line scan to demonstrate the resolution achievable with the proposed ablation cell employing the following instrumental parameters: fluence 0.70 J/cm2, scan speed 20 μm/s, spot size 12 μm, laser frequency 20 Hz and a dwell time of 5 ms for 63Cu and 1 ms for 197Au.
RESULTS AND DISCUSSION Cell design
6
As above mentioned, there are mainly two correlated factors that prompt for the development of novel designs for laser ablation chambers. The first one relates to the achievement of short washout times which leads increased sensitivities (the generated aerosol is less diluted by the carrier gas), higher spatial resolution (the mixing of the aerosol generated by subsequent shots decreases) and increased scan speed (see e.g. [30]). The second factor relates to the achievement of homogeneous washout times through the entire sampled surface: this issue is particularly important to obtain reliable surface elemental distributions. The results obtained by some of us [25] clearly indicate that very short washout times can be obtained through a careful control of the gas flow pattern inside the ablation chamber. The limit of this design is that low washout times (30 ms) were achieved only in the centre of the ablation cell, whereas 50 time higher figures were measured close to the cell walls. The so called “two-volume cell”, initially introduced by Arrowsmith and Huges [32], is a good compromise between low and homogeneous washout times, as it physically confines the sample holder volume (first volume) from the ablated plume sampling volume (second volume). The novel cell presented here (see Figure 1) is a further evolution of the two-volume strategy [32] and its latest evolution [24]. The major innovation is that the sampling gas pipe (the second volume) is always kept at the ablation site as it is located in the upper part of the cell that is integral to the laser ablation equipment (see Figure 2). The lower part is instead used to hold the sample and is moved by an x, y stage: contrary to existing designs [20], we decided not to include the stage into the sample holder, enabling the full exploitation of the scanning capabilities of the built-in x, y stage. This choice actually implies that the cell is divided in two distinct parts. To ensure the gas tightness of the whole assembly the utilisation of bellows was firstly considered, but no suitable solution could be found. Therefore we decided to evaluate the potential utilisation of a viscous film. This unusual design immediately appeared to be a good choice, even if a careful optimisation of the downstream carrier flow is necessary to ensure the complete gas tightness. An oil with an appropriate viscosity (Edwards Ultra grade 19 mineral oil, 144 cSt at 20°, commonly used for rotary pumps) is placed between the upper and the lower disks of the chamber. The lower disk can be freely moved by the x, y stage, while the upper disk weights on the lower one (without constrains on the z axis) and is maintained in a fixed x, y position by a fixed framework (see Figure 2). 7
Finally, the lower cell disk is designed to accommodate some additional parts (see Figure 3) which allow changing the flow pattern from cyclonic to vertically diffused or horizontally diffused. It should be noticed that the pipe dimensions cannot be reduced to an arbitrarily small volume, since the after-shot plume expansion is non negligible (see e.g. [33] and references therein). Moreover, the second volume must be sufficiently large to collect the whole ablated material, thus avoiding sample losses toward the first volume which may cause decreased sensitivities and memory effects. With this design we reached a low second cell volume, together with a flexible gas inlet system, without posing limitations on the scanning capabilities of the moving stage and thus on the analysable sample surface.
Air tightness Air tightness is clearly the first issue to be addressed if this cell design has to be adopted. If unwanted overpressures are generated by gas flows, gas leaks may occur by the formation of preferential channels through the viscous film. In this respect, when only the He carrier flow is used, the developed cell is fully gas tight, i.e. helium inflow and outflow are comparable within the measurement error. However, the outflow was 10% lower than the inlet under standard working conditions, when the argon makeup flow downstream the cell was turned on (as reported in the Experimental section, the outlet flow was measured at the internal channel of the ICP torch to take into account the entire transfer line). Two critical points in the line were identified causing over pressure: i) the Y connection where helium and the makeup gas are mixed; ii) the connection between the line and the torch which in the starting configuration is made with a 13mm Rotulex connection with a 2 mm i.d.. Both caused a restriction of the line from 4 to 2 mm inner diameter. The connection to the torch was simply changed to a 4 mm i.d. Rotulex connection. The mixing between helium and argon is a more difficult issue as no counterpressure and as low as possible turbulence should be generated. A Venturi connection was adopted similar to the one described by Lindner et al [24]. This mixing system ensures minimum turbulence and dead volumes, but also a limited Venturi effect which contributes in extracting helium and the transported aerosol out of the cell. This configuration assured the complete transport of helium out of the cell, with no statistically differences between the inlet and outlet flow (differences below 0.5%). Two further tests were performed to ensure no leakage of helium transport gas and no variations in the 8
presence of atmospheric gases (i.e. that no gas enters the system from outside the cell). Firstly, a gas leak detector indicated no helium leaks from the viscous film and the conjunction between the silicon foil and the sampling gas pipe during cell movimentation. Secondly, we checked that background signals due to atmospheric gases do not change during cell movimentation. Data reported in Figure S1 clearly show the constancy of the signals related to atmospheric gases when the cell is moved through its maximum x-y limits, demonstrating that the entire system (cell plus transfer line) is air tight under experimental conditions.
Optimisation of cell parameters In the attempt to define the best cell design, four major cell configurations were assessed, namely the particle sampling height, the gas flow pattern, the diameter of the sampling tube and the cell dimensions (see Experimental section for technical details). The sampling height, i.e. the distance between the sample surface and the entrance of the sampling volume, is expected to play a major role in fast and complete particle transport. A steady decrease in washout times was observed with increasing distances from the sample surface, from 1500 ms to 300 ms for 1 and 8 mm distance, respectively (time for the transient signal to fall to 10% of its peak value). Nevertheless, the decrease in washout time was paralleled by a decline in the signal integral, evidencing a reduced transport efficiency. The latter was assessed by measuring the particle size distribution by an optical particle counter (see Figure S2). These data show that fractions with nominal diameter higher than 1 μm are not removed from the cell when a 8 mm distance is used. Accordingly, 1 and 4 mm sampling height only were employed in all of the following experiments. It should be highlighted that such a behaviour may be distinct of a 266 nm wavelength radiation which is known to produce larger particles than shorter wavelength systems (5th harmonic Nd:YAG at 213 nm or excimer ArF at 193 nm) or ultrashort pulses (see e.g. [1-3] and references therein). Three different flow patterns were assessed: a tangential flow [25] and two different annular, diffused flows (vertical and horizontal, see Figures 1 & 3). Preliminary experiments conducted with the tangential, cyclonic flux evidenced inhomogeneous washout times (range 200 – 460 ms): this feature is typical of the cyclonic flux inlet [25], although different patterns in washout times were registered with the present and past cell. Accordingly, this gas flow configuration was abandoned.
9
Following these preliminary results, the effect of sampling height, sampling tube diameter and inlet gas flow were systematically investigated. All of the three factors were investigated at two different level according to a full factorial 23 design: sampling height: 1 and 4 mm; sampling cell diameter: 4 and 8 mm; flux geometry: horizontal and vertical. Washout values obtained during the eight experiments are illustrated in Figure 4 as contour plots, whereas numerical values are reported in Table 1. Washout times are included in the range 240-317 ms, whereas the standard deviations of the values measured in the 13 different positions are in the 15-27 ms interval (5 replicate measurements of the washout time were performed at each sampling site, see Experimental). Accordingly, the viscous film cell features a low washout time and a variable degree of homogeneity depending on the adopted configuration: the effect of the instrumental configurations will be presented in the current section, whereas a comparison with existing designs is discussed in the following section. A first exploratory analysis highlights a non-negligible difference among the groups (i.e. the eight different configurations): the one-way analysis of variance (ANOVA) resulted in an F value of 10.7 meaning a p value lower than 0.01 for df1=7 (between groups degrees of freedom) and df2=95 (within groups degrees of freedom). This means that there exist systematic differences in washout time values between the eight configurations. The among group difference becomes negligible at a 0.05 significance level when configurations e and g (see Table 1) are excluded from calculations (df1=5, df2=92, F=1.93 to be compared with a critical F value of 2.34 for p=.05): these configurations show the highest washout times and are both based on a vertical flux plus a 4 mm sampling volume (see Table 1). Six out of eight configurations feature undistinguishable values of washout times according to the ANOVA analysis. The results of the factorial design were also investigated by ANOVA analysis to assess the effect of the three investigated cell parameters. As a result, neither the main factors (sampling height, sampling chamber diameter and gas flux), nor the second order interactions show a significant effect on the response (the washout time) at the 0.05 level (the most significant factor is gas flux geometry with a p value equal to 0.08). Such an absence of effects may partially stem from the preliminary work aimed at narrowing the investigated sampling heights and the flux geometries (see the beginning of this section). Nevertheless, this feature is a strong indication of the robustness of the ablation cell: substantial changes in the in/out flows and in the positioning of the sampling volume has a limited effect on the performances of the ablation cell in term of washout times. 10
In conclusion, we suggest to use configuration d in Figure 4 (sampling height 4 mm, sampling tube diameter 8 mm and a horizontal flux geometry): the value of the washout times is not statistically lower than the other configurations (see previous discussion), but it is significantly more uniform over the cell (see Figure 4d and Table 1). Figure S3 in the Supplementary Material shows that the integral of transient signals are homogeneous over the entire cell surface (see Figure S3). The optimal configuration was adopted for the measurements reported in the following sections. The washout times were also calculated using higher concentration samples to assess whether the analyte concentration affected the washout time. Moreover, literature data are usually calculated with more concentrated samples: an unbiased comparison relies on the evaluation of the washout times vs concentration trend. NIST610 (uranium concentration 461.5 ± 1.1 mg/kg) and pressed cerium oxide pellets (Ce concentration 81% or 810000 mg/kg) were analysed in 17 positions covering the entire cell surface. As a result, washout times of 206±79 and 138±34 ms were measured: comparing with the reported 240-260 ms for 2.8 mg/kg of cerium (see Table 1), a decreasing trend in washout time values vs concentration is clear. Such a trend deserves further investigation: an increase in signal to noise ratio is surely a first factor as well as the different materials analysed (glasses vs an oxide pellet) . As an example of application to a inhomogeneous material, we ablated by a line scan a section of a Random Access Memory (RAM) board (Figure 5), analysing the signals of gold and copper. Three different areas are visible at the optical microscope, i.e. the gold contact which is 300 µm wide, the body of the board and the gold track on the PCB. As a result, the cell enables the ready identification of the three different features of the memory board. The correlation between gold and copper signals points to the use of a gold alloy rather than pure gold with non-negligible amounts of copper.
Comparison with existing designs Optimal configurations of the viscous film cell feature washout times in the range 240-260 ms (see entries a-d in Table 1). Washout times and investigable sample surface are reported in Figure 6 for literature data and the ablation cell introduced in the present work (data in numerical form are reported in Table S1 in supplementary information). This comparison may only be semi-quantitative as the homogeneity of the data is limited, hindering the application of a fully quantitative approach. Washout times have been calculated in different ways, employing thresholds set to the 0.1%, 1% or 10% of the signal 11
maximum. Secondly, different samples were used to estimate the washout times, typically the NIST 610 and 612 standard glasses with concentrations of trace elements of 500 and 50 mg/kg, respectively. Thirdly, different laser sources were employed, including pulse duration in the ns and fs range as well as different laser wavelengths: these factors are known to generate ablated particles of different dimensions and, accordingly, different aerodynamic properties. Fourthly, a limited number of authors [18, 21, 25, 29] systematically evaluated the washout times over the entire sample surface or a significant portion of it. Finally, information on sample size and analyzable sample surface are not always explicitly reported or even missing and had to be estimated from reported data (cell geometry, etc.). As a general feature, Figure 6 seems to suggest an increase in washout times with increasing analysable sample surface, although the linear correlation is not significant (n=20, r=0.27, p=0.24): it should also be mentioned that data at high washout time/analysable surface show high leverage, i.e. they are highly influent in the linear regression. Regarding the performances of the proposed cell, Figure 6 evidences that it is situated in the target region of the plot washout time vs analysable surface, i.e. the optimal region situated at low washout times and high analysable surface. As a matter of fact, it features the lowest reported washout time for an analysable surface of 20 cm2. Five cell designs are situated in this optimal area and will be compared to the proposed cell. The commercial ablation cell produced by ESI (TruLineTM) should firstly be mentioned (available data reported in Table S1): it shows washout times similar to the ones reported here (around 150 ms at the 10% level and 250 ms at the 1%) but it could not be included in Figure 6 as the analysable area could not be found (reported data seems to suggest a diameter around 6 cm, very similar to the viscous film cell). The machinery behind this cell are evidently undisclosed but the cell also features a pinch valve to minimise pulse broadening. Another commercially available cell (HelEx II with CleanShotTM, labelled 13 in Figure 6) shows similar performances with a lower washout time and a smaller analysable area, performances overlapping the ones of the cell recently developed by Douglas et al. [34] (the latter cell includes mofications of the ICP torch). Finally, the cells developed by Feldman et al. [18] and ESI (labelled 3 and 12 in Figure 6, respectively) show analysable areas over 100 cm2 with washout times in the 0.5-0.7 range (the cell described in [18] is limited to the analysis of blotted membranes). For the sake of completeness, two more
12
large volume cells produced by RESOlution (S155) and GeoLas should be mentioned although no data on washout times could be found. Homogeneity of washout times is even more difficult to be compared as data are not reported in the literature. Figures are reported by Douglas et al. only [34], whereas plots are merely found in [18]: data are comparable to the ones reported in the present paper (see Table 1), although a truly systematic investigation is missing in these two papers. A very good homogeneity in signals with RSD% of 2% is claimed for the three commercial cells, with no assessment of washout time values in different points of the cell.
CONCLUSIONS A large cylindrical cell with a uniform, low washout time was realised and its performances assessed. As a result, a washout time around 250 ms was achieved for a cylindrical cell with an inner diameter of 70 mm for trace amounts of cerium (2.8 mg/kg) in a commercial glass. This value lowers to around 210 ms for the ablation of the NIST 610 standard (461.5 ± 1.1 mg/kg U). The cell may be fitted on the stage of standard instrumentations for laser ablation. The limited dependence of the washout times from cell dimensions and gas flow patterns, as far as they have a cylindrical symmetry, further supports the robustness of this configuration. These results are achieved by keeping the sampling point at the ablation site as is true for most recent cell designs. Modelling of flow pattern and particle trajectories would greatly help in increasing the performances of the proposed cell. Although clear indications already emerged from the experimental data, e.g. dimensional fractionation as a function of sampling height, numerical modelling would provide a sound ground for the theoretical interpretation of the results and suggest future directions. The comparison with existing designs highlighted that the viscous film cell offers a good compromise between a moderately low washout time (around 210 ms evaluated by uranium signal in NIST 610 standard) and the possibility to analyse a large surface area (20 cm2). It is an instrument added to the analytical chemist toolbox for the investigation of medium sized samples with low washout times and constant sensitivity all over the sample surface. The cell may be proficiently used also to investigate multiple small samples. Regarding quantitative applications, adequate homogenisation of the signal should be considered, as is true for any cell showing fast washout times when low repetition rates are employed (see e.g. [26]).
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Progression of this design may evolve in two directions. In principle, larger analysable surfaces may be achieved developing larger diameter cells: as shown for 5 and 7 cm I.D. cells, the cell diameter do not affect performances because of the adopted extraction strategy. Reduction in washout times could instead be attained by adopting a single straight transfer line from the cell to the ICP torch analogous to the “sniffer” adopted by Douglas et al. [34]. The sampling tube could be inserted directly in the lateral wall of the upper part of the cell avoiding the 90 degree turn in the particle trajectory of the present design.
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[17] D. Bleiner, H. Altorfer, A novel gas inlet system for improved aerosol entrainment in laser ablation inductively coupled plasma mass spectrometry, J. Anal. Atomic Spectrom. 20 (2005) 754-756. [18] I. Feldmann, C.U. Koehler, P.H. Roos, N. Jakubowski, Optimisation of a laser ablation cell for detection of hetero-elements in proteins blotted onto membranes by use of inductively coupled plasma mass spectrometry, J. Anal. Atomic Spectrom. 21 (2006) 10061015. [19] J. Pisonero, D. Fliegel, D. Günther, High efficiency aerosol dispersion cell for laser ablation-ICP-MS, J. Anal. Atomic Spectrom. 21 (2006) 922-931. [20] C.C. Garcia, H. Lindner, K. Niemax, Transport efficiency in femtosecond laser ablation inductively coupled plasma mass spectrometry applying ablation cells with short and long washout times, Spectrochim. Acta B 62 (2007) 13-19. [21] Y. Liu, Z. Hu, H. Yuan, S. Hu, H. Cheng, Volume-optional and low-memory (VOLM) chamber for laser ablation-ICP-MS: application to fiber analyses, J. Anal. Atomic Spectrom. 22 (2007) 582-585. [22] M. Tanner, D. Günther, Signal acquisition in ms time resolution for in-torch LA-ICPMS, J. Anal. Atomic Spectrom. 22 (2007) 1189-1192. [23] D. Asogan, B.L. Sharp, C.J.P. O' Connor, D.A. Green, R.W. Hutchinson, An open, non-contact cell for laser ablation-inductively coupled plasma-mass spectrometry, J. Anal. Atomic Spectrom. 24 (2009) 917-923. [24] H. Lindner, D. Autrique, C.C. Garcia, K. Niemax, A. Bogaerts, Optimized transport setup for high repetition rate pulse-separated analysis in laser ablation-inductively coupled plasma mass spectrometry, Anal. Chem. 81 (2009) 4241-4248. [25] D. Monticelli, E.L. Gurevich, R. Hergenroder, Design and performances of a cyclonic flux cell for laser ablation, J. Anal. Atomic Spectrom. 24 (2009) 328-335. [26] W. Muller, M. Shelley, P. Miller, S. Broude, Initial performance metrics of a new custom-designed ArF excimer LA-ICPMS system coupled to a two-volume laser-ablation cell, J. Anal. Atomic Spectrom. 24 (2009) 209-214. [27] G. Carugati, S. Rauch, M.E. Kylander, Experimental assessment of a large sample cell for laser ablation-ICP-MS, and its application to sediment core micro-analysis, Microchim. Acta 170 (2010) 39-45. [28] H. Lindner, D. Autrique, J. Pisonero, D. Günther, A. Bogaerts, Numerical simulation analysis of flow patterns and particle transport in the HEAD laser ablation cell with respect to inductively coupled plasma spectrometry, J. Anal. Atomic Spectrom. 25 (2010) 295-304. [29] M.B. Fricker, D. Kutscher, B. Aeschlimann, J. Frommer, R. Dietiker, J. Bettmer, D. Günther, High spatial resolution trace element analysis by LA-ICP-MS using a novel ablation cell for multiple or large samples, Int. J. Mass Spectrom., 307 (2011) 39-45. [30] S.J.M. van Malderen, A.J. Managh, B.L. Sharp, F. Vanhaecke, Recent developments in the design of rapid response cells for laser ablation-inductively coupled plasma-mass spectrometry and their impact on bioimaging applications, J. Anal. Atomic Spectrom. 31 (2016) 423-439. [31] K.P. Jochum, U. Weis, B. Stoll, D. Kuzmin, Q. Yang, I. Raczek, D.E. Jacob, A. Stracke, K. Birbaum, D.A. Frick, D. Günther, J. Enzweiler, Determination of Reference Values for NIST SRM 610–617 Glasses Following ISO Guidelines, Geostan. Geoanal. Res. 35 (2011) 397-429. [32] Arrowsmith P., Huges S.K., Entrainment and Transport of Laser Ablated Plumes for Subsequent Elemental Analysis, Applied Spectrosc. 42 (1988) 1231 - 1239. [33] D. Bleiner, Z. Chen, D. Autrique, A. Bogaerts, Role of laser-induced melting and vaporization of metals during ICP-MS and LIBS analysis, investigated with computer simulations and experiments, J. Anal. Atomic Spectrom. 21 (2006) 910-921. 16
[34] D.N. Douglas, A.J. Managh, H.J. Reid, B.L. Sharp, High-Speed, Integrated Ablation Cell and Dual Concentric Injector Plasma Torch for Laser Ablation-Inductively Coupled Plasma Mass Spectrometry, Anal. Chem. 87 (2015) 11285-11294.
Table 1. Average washout times and standard deviation over the entire sample surface.
Name
Sampling
Sampling
Height
Flux geometry
Mean
Standard
tube diameter
Washout
Deviation
(mm)
(mm)
time (ms)
(ms)
a
1
4
Horizontal
238
23
b
1
8
Horizontal
250
25
c
4
4
Horizontal
260
27
d
4
8
Horizontal
243
15
e
1
4
Vertical
281
47
f
1
8
Vertical
253
18
g
4
4
Vertical
317
32
h
4
8
Vertical
252
22
17
CAPTION TO FIGURES
Figure 1. Basic scheme of the developed cell (quoted dimensions in mm): orange line = location of the viscous film; green line = flow path; brown rectangle= schematic representation of the sample specimen. Figure 2. Final prototype mounted onto the laser ablation apparatus. Note that samples can be efficiently illuminated through the transparent silicon foil. Figure 3. Additional parts to be used for: A vertically diffused and B horizontally diffused gas flow patterns (quoted dimensions in mm). The green lines schematically depict flow paths. Figure 4. Contouring map of washout evaluated using the signal of 140Ce generated by the ablation of the commercial glass disk employing different configurations (see labels a-h reported in Table 1). Figure 5. Line scan analysis of a random-access memory (RAM) board, using a 12 µm spot size, a 20 Hz frequency with a 20 µm/s scan rate. Figure 6. Summary of literature data reported in plot washout time vs. analysable surface (see labels 1-20 reported in Table S1, label 19 indicates the results reported in this work).
18
Figure 1
19
Figure 2
20
Figure 3
21
Figure 4
22
Figure 5
23
Figure 6
HIGHLIGHTS
A sample chamber for laser ablation based on a viscous film has been designed and tested Several configurations were evaluated measuring the washout times A uniform and low (210 ms) washout time was achieved over a 20 cm2 surface Performances are compared to existing designs
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