Laser ablation of synthetic geological powders using ICP-AES detection: eVects of the matrix, chemical form of the analyte and laser wavelength M. Motelica-Heino,*a O. F. X. Donarda and J. M. Mermetb aLaboratoire de Chimie Bio-Inorganique & Environnement, EP CNRS 132, France bLaboratoire des Sciences Analytiques, UMR 5619, Universite� Claude Bernard-Lyon I, France Received 19th October 1998, Accepted 24th February 1999 A systematic study of the influence of the physical and chemical properties of analogues of geological samples and of laser wavelength on laser ablation was conducted.DiVerent chemical forms of several test elements were investigated, first as pure elements, then mixed in various matrices. Synthetic model samples were prepared from diVerent crystalline compounds of Mg, Al and Fe spiked in SiO2 or CaCO3 and manufactured in the form of pressed pellets that were analysed by laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES).This study was performed at two laser wavelengths (1064 and 266 nm) with a Nd5YAG laser and using ICP-AES as an elemental detector. These various steps made it possible to demonstrate that the LA-ICP-AES response factors of the analytes are strongly dependent on their chemical form and on the bulk composition of the matrix. These eVects were also found to be laser wavelength dependent and the use of a UV laser did not lead to any improvements to minimise these eVects.In contrast, there was no influence of the grain size or binding pressure of the pressed pellets. However, normalisation to a matrix element could compensate for diVerences in the response factors of the analytes. Implications for direct quantitative elemental analysis of geological materials by laser ablation related techniques are that calibration without matching in terms of the mineralogical and chemical composition of the matrix and of the chemical forms of the analytes can lead to systematic errors in the determination of the element concentration.many other fields of application, LA has been particularly 1 Introduction considered for geochemistry.3,4 The direct trace elemental analysis of solid samples is still a Trace element analysis of powdered or particulate material challenge for the analyst. Plasma spectrochemical methods is often required in geochemistry or environmental monitoring oVer high sensitivity and low detection limits. However, they as many natural samples such as soils and sediments occur as require solid samples to be dissolved prior to analysis, which a mixture of discrete grains.Before analysis, bulk rock samples is often diYcult and time consuming. Therefore, instrumental are also commonly reduced to powders. Powders can be techniques that allow direct analysis of solid samples with introduced into the plasma after digestion or even directly little preparation are particularly interesting in the analysis of through slurry nebulization if a micrometre particle size can diYcult solid samples such as refractory or geological mate- be achieved.5 Little consideration has been given to LA of rials.So far, physical methods such as X-ray fluorescence powders although it provides rapid analysis. Moreover, the (XRF), particle induced X-ray excitation (PIXE), electron fundamental processes involved in the LA of particulate probe microanalysis (EPMA), neutron activation analysis materials are poorly understood.Previous work with LA-ICP- (NAA) and ion microprobe (SIMS) have been extensively AES/MS on powdered samples has been conducted in the used for micro-, bulk and surface analysis. More recently, field of geology and the environment. Since the pioneering lasers have been coupled to plasma-based analytical techniques work of Gray,6 bulk analysis by LA-ICP-MS of geological such as inductively coupled plasma mass spectrometry materials using pressed pellet samples has continued to receive (ICP-MS) or atomic emission spectrometry (AES).Laser attention. Several workers have performed analysis of reference ablation (LA) allows direct introduction into the ICP and silicate rocks7–13 and carbonates.14,15 Samples were prepared expands the scope of ICP-MS/AES to any solid sample. It as pressed pellets using a binder after grinding. For environavoids the fastidious preparation steps and problems that mental monitoring, the same strategy was used for the analysis occur with dissolution such as contamination, loss of volatile of sediments by LA-ICP-MS16 and for the analysis of soils by elements and dilution of the elements.LA is a powerful LA-ICP-AES.17 DiVerent calibration strategies were used for sampling tool for the ICP as it can perform in situ analysis or major, minor and trace element analysis such as calibration be a suitable alternative to dissolution when bulk analysis is with a geological reference material, or calibration against required.LA-ICP-MS/AES is a tandem analytical technique synthetic powders spiked with analytes occurring in solid or where the laser beam is used as a sampling probe and the liquid form. Globally, only semiquantitative analysis could be ICP-MS/AES system as a detector. When targeted on the achieved, i.e., accuracy was within one order of magnitude, sample surface the laser beam generates microparticulate mate- while the use of an internal standard significantly improved rial that is brought with an argon stream into the ICP where results.However, quantitative analysis using external caliit is atomised. LA has been extensively reviewed for its bration proved possible only if close matching of the matrices of the sample and the standard could be achieved in terms of fundamental aspects1 and analytical applications.2 Among J. Anal. At. Spectrom., 1999, 14, 675–682 675bulk chemistry and, moreover, in terms of mineralogical translational x–y stage displacement with a translation rate of 10 mm s-1.The sampling pattern was a 1 mm long raster line. composition. Variability of the elemental response factors between diVerent matrices was thought to be controlled by The ablated material was transported by an argon carrier gas into the injector tube of the ICP-AES system. the physical properties of the individual minerals that host the elements. A Perkin-Elmer (Norwalk, CT, USA) Optima 3000 DV ICP-AES instrument was used for elemental analysis.The LA is a complex process that depends on both laser parameters and physico-chemical parameters of the materials. The detector is a custom segmented-array charge coupled device allowing simultaneous multi-element measurements of the line influence of the laser wavelength on elemental response factors was not investigated in previous studies using infrared (IR) intensity and adjacent background.All measurements were performed with the axial plasma viewing mode. Acquisition laser ablation (with Nd5YAG lasers operating at their fundamental wavelength). As the general trend in LA is to move started after a pre-ablation of 30 s to reach a steady signal. Elemental signals were then averaged over the acquisition towards shorter wavelengths in the ultraviolet (UV) region (working with quadrupled Nd5YAG systems or with excimer period. Each measurement was performed five times on diVerent sample locations and the mean calculated.Experimental lasers), it would be interesting to know if there are any significant improvements in terms of matrix eVects when settings for LA-ICP-AES are detailed in Table 1, together with spectral lines and integration parameters. shorter wavelengths are used. Since elements can be present in diVerent chemical and physical (e.g., grain size) forms in geological samples, it is also crucial to verify whether LA 2.3 Method eYciency is similar, regardless of the form of the element.The 2.3.1 Model samples. Synthetic powders of diVerent com- aim of this work was, therefore, to investigate the ablation position as previously described were used to prepare simple behaviour of model geological samples at two laser waveor complex matrices with controlled physical and chemical lengths using a Nd5YAG laser operating at 266 and 1064 nm. properties. Samples were prepared as 12 mm diameter pellets Synthetic mol samples were prepared as pressed pellets from with a press using a pressure of 10 tonnes without any binder.diVerent crystalline compounds of Mg, Al and Fe spiked in SiO2 and CaCO3. The aim was to move from pure compounds Single phase samples. Silica was used to prepare model to complex matrices, with controlled physical and chemical matrices with controlled physical (grain size, binding pressure) matrix properties. The eVects of matrix properties and also of properties.SiO2 powders of diVerent grain sizes (5, 10, 10–40 the chemical and physical form of the analytes on the elemental and 100–200 mm) were prepared under two pressures, 5 and response factors were investigated by using ICP-AES as an 10 tonnes, respectively. Pure compounds of Mg, Al and Fe elemental detector. Finally, the analytical matrix eVects of were used to prepare model phases with diVerent chemical LA-ICP-MS/AES were highlighted and the potential of these forms.Pure chemical synthetic powders of Mg (oxide, sulfate, techniques for direct quantitative analysis of real geological metaphosphate, acetate and fluoride), Al (oxide, phosphate, materials was evaluated. sulfate and fluoride) and Fe (reduced, oxide and sulfate) were pressed under a pressure of 10 tonnes. 2 Experimental Samples with complex matrix. Pure compounds of Al, Mg 2.1 Samples and Fe were spiked in SiO2 and CaCO3 at diVerent concen- Synthetic crystalline compounds of Mg, Al and Fe occurring trations to prepare dual phase matrices with diVerent bulk in chemical forms were selected: magnesium oxide (Fluka, composition and chemical form of the minor phases.Sigma-Aldrich, St. Quentin Fallavier, France), sulfate Magnesium oxide, sulfate, metaphosphate, acetate and fluoride (Janssen, Beerse, Belgium), metaphosphate, fluoride (Prolabo, Fontenay sous Bois, France) and acetate (Aldrich); aluminium Table 1 Instrumental operating conditions oxide (Merck, Darmstadt, Germany), sulfate, phosphate (Prolabo) and fluoride (Riedel-de Hae�n, Hannover, Germany; Laser ablation device— Sigma-Aldrich); iron oxide (Merck), reduced and sulfate Laser type Surelite Nd5YAG (Prolabo).The grain size of these powders was in the Wavelength 1064 nm 266 nm (quadrupled) 10–200 mm range. Four pure SiO2 powders (Prolabo) used for Energy 300 mJ per pulse (1064 nm) chromatography with controlled grain size were chosen to 4 mJ per pulse (266 nm) represent the silicate matrix. The diVerent grain sizes were, Laser repetition rate 10 Hz respectively, 5, 10, 10–40 and 100–200 mm.Pure CaCO3 Sampling pattern 1 mm straight line (Prolabo) was used for the carbonate matrix. Translation rate 10 mm s-1 Focus position 0 2.2 Instrumentation ICP-AES— A Nd5 YAG Continuum Surelite laser (Continuum Lasers, Type Perkin-Elmer Optima DV Santa Clara, CA, USA) was used under Q-switched operation. ICP parameters: This device can generate laser light at the fundamental wave- Rf power 1100 W length (1064 nm) or at the fourth harmonic (266 nm), pro- Nebuliser gas flow 0.65 l min-1 cessing the fundamental laser beam with harmonic generators.Integration parameters: The laser had a maximum laser energy delivery per pulse of Processing mode Area 6 mJ at 266 nm and 350 mJ at 1064 nm. The laser repetition Integration time 2 ms Sampling time 1 s rate could be adjusted to between 1 and 20 Hz. A UV beam Replicates 50 expander (BXGU-10–3x-266) from CVI Laser Corporation (Albuquerque, NM, USA) was positioned after the output of Spectral lines: Si 251 nm the laser to limit divergence of the laser beam when working Mg 279 nm with the 266 nm wavelength. The laser beam was focused Al 396 nm perpendicularly onto the sample surface via a plano-convex Fe 238 nm 200 mm focal length lens.The sample was mounted in a Ca 422 nm movable ablation cell controlled by step motors, allowing 676 J. Anal. At.Spectrom., 1999, 14, 675–682powders were spiked in a SiO2 matrix at relative phase proportions of 10, 20, 30 and 40%, respectively. MgO was spiked in a CaCO3 matrix at 10, 20, 30 and 40%. Aluminium oxide, phosphate, sulfate and fluoride were spiked in a SiO2 matrix at phase proportions of 10, 20, 30 and 40%, respectively. Al2O3 was spiked in a CaCO3 matrix at 10, 20, 30 and 40%. Fe (reduced, oxide and sulfate) was spiked in a SiO2 matrix at 10, 20, 30 and 40%. Fe was spiked in a CaCO3 matrix at 10, 20, 30 and 40%.Powders were mixed and ground with a pestle before being pressed. 2.3.2 Analysis by LA-ICP-AES. LA-ICP-AES analysis of the SiO2 samples. The SiO2 samples were analysed for Si by LA-ICP-AES with the laser working at 266 nm. Each analysis was performed for laser pulse energies of 1 and 4 mJ. LA-ICP-AES analysis of the single phase samples. The samples of Mg (oxide, sulfate, metaphosphate, acetate and fluoride), Al (oxide, phosphate, sulfate and fluoride) and Fe (reduced, oxide and sulfate) were analysed by LA-ICP-AES at 1064 and 266 nm using the standard conditions previously defined.Mg, Al and Fe were measured for the three diVerent families of samples, respectively. LA-ICP-AES response factors Fig. 1 EVects of grain size (a) and binding pressure (b) on the response for Mg, Al and Fe were obtained for each sample in the of Si in the SiO2 matrix using the laser at 266 nm. following way: the response factor for element E, R(E), was calculated as the AES elemental mean signal I(E) divided by the elemental concentration in the matrix C(E), i.e., R(E)= deviation) between five diVerent analyses. Within each set of I(E)/C(E).experiments the signal appears to be proportional to the energy. Although a slight increase can be observed in the LA-ICP-AES analysis of the complex matrices. The samples 10–40 mm size range at 4 mJ, the signal is almost independent of Al, Mg and Fe spiked in SiO2 or CaCO3 were analysed by of the grain size.Results show that the ICP-AES response is LA-ICP-AES at 1064 and 266 nm, respectively, using the mainly influenced by the energy level. In contrast, the eVect operating conditions previously defined. Mg, Al and Fe were of the grain size is almost negligible. There is no significant measured together with Si or Ca. LA-ICP-AES response eVect of the binding pressure on the signal when the binding factors were calculated as described above.For the samples pressure changes from 5 to 10 tonnes regardless of the laser with a SiO2 matrix, response factors relative to Si were energy and the grain size. calculated as response factors divided by the response factor The laser spot size was of the order of 20 mm and the grain of Si: I(E)/I(Si) C(Si)/C(E). sizes were above and below the laser spot size. Therefore, results show that the laser–powder interaction corresponds to a true ablation process and not to a dislocation of the 3 Results and discussion compressed grains, regardless of the grain size.A binding 3.1 LA-ICP-AES response factors for single phase matrices pressure of 10 tonnes was suYcient to neutralise the eVect of the grain size. The response factors were indeed independent First, LA was investigated for pure chemical compounds. The of the structural characteristics of the powder such as the aim was to understand the eVects of the physical and chemical grain size and binding pressure.These results clearly suggest form of the analytes on LA-ICP-AES response factors together that the eVects of the physical properties of the matrix in with the role of laser wavelength. terms of grain size do not significantly influence the LA processes when powders are prepared as pressed pellets, at 3.1.1 EVects of the physical properties of the matrix. First, it is important to understand whether the ablation of powders least under the conditions used in this work.In fact, laser sampling of pressed pellets was found to be true ablation and consists of either grain removal or dislocation of the matrix. Therefore, the eVect of the physical properties of the pressed not dislocation of the grains, and the amount of ablated material was not dependent on the grain size of the powders. pellets on the ablation behaviour was investigated for the SiO2 model samples. Si was chosen for this study because it is a However, for bulk alysis of trace elements in powdered materials prepared as pressed pellets, the grain size would major geochemical component.The Si emission signal was used to monitor the amount of material removed from the certainly play a significant role. The distribution of trace elements within the matrix is likely to be dependent on the matrix by the laser. The relative dimensions of the laser spot and the grains are likely to be important in the sampling grain size. Therefore, the sample has to appear homogeneous within the sampling volume probed by the laser beam to be process.The laser spot size was of the order of 20 mm when using the UV wavelength and the grain sizes (respectively, 5, representative of the distribution of trace elements. This would certainly depend on the grain size of the trace elements and 10, 10–40 and 100–200 mm) were selected above and below the laser spot size. The laser sampling area was a 1 mm long on their concentration. Thus, to eliminate the influence of grain size on the distribution of trace elements, the sampling straight line.The width of this line was governed by the laser spot size, i.e., 20mm. Therefore, the area of the sampling area has to be larger than the matrix grain size. This should be the subject of another study. However, for all the experi- pattern was about 0.2 mm2. The length of the sampling area was set far above the grain sizes to be representative of the ments presented in this work, the dimensions of the sampling area were selected far above the grain sizes.bulk of the pellets. Fig. 1 displays the ICP-AES response factors of Si using the laser at 266 nm as a function of grain size, binding pressure 3.1.2 EVect of the chemical form of the element. The influence of the chemical form of an element on its ablation behaviour and laser energy. Error bars express the dispersion (standard J. Anal. At. Spectrom., 1999, 14, 675–682 677was studied for Mg, Al and Fe with the pure compounds dent on the laser wavelength, the response factor being generally higher in the UV mode.described above for both UV and IR laser ablation. Fig. 2 shows the ICP-AES response factors for Al, Mg and Fe for the single-phase matrices as a function of the chemical form 3.2 LA-ICP-AES response factors for complex matrices of these elements and the laser wavelength. It can be seen that After studying the eVect of the chemical form of an element the response factors obtained for Mg are much higher (at least on elemental response factors with pure compounds, the next five times) for the UV wavelength than for IR in all samples.step was to repeat the experiments with analytes spiked in a Response factors also vary with the chemical form of Mg. matrix. SiO2 and CaCO3 matrices were used for this study as This is most pronounced for the IR. The organic form of Mg being representative of two important geochemical poles, displays a higher signal response than the inorganic forms for silicates and carbonates.The combined influence of the laser both laser modes. These results clearly show that the response wavelength, chemical form, the nature of the matrix and the factor of Mg is significantly dependent on the laser wavelength concentration of the analyte in the matrix on the ICP-AES and also on the chemical form of Mg. The IR ablation mode response factor was evaluated. Samples with diVerent com- is less sensitive than UV to the chemical form.This trend can pounds of Mg, Al and Fe spiked in SiO2 or CaCO3 at diVerent also be observed with Al. There are also response factor concentrations, as described above, were used for this study. variations between the diVerent compounds of Al. The larger For both UV and IR laser ablation modes, ICP-AES variations are obtained with UV. The response factor is also responses of Mg, Al and Fe versus their concentration in the much higher (at least three times) in the UV mode than for complex matrices are presented in Fig. 3. The variations of IR. A slightly diVerent trend can be observed with Fe. the ICP-AES signal of Mg with its concentration in the matrix Response factors are similar for UV and IR ablation modes show diVerent patterns according to the chemical form of Mg, regardless of the chemical form of Fe but vary between the the nature of the matrix and the laser wavelength. For most diVerent forms.In this case, results obtained show that the experiments, the signal for Mg varies linearly with the concen- response factor of Fe is dependent on the chemical form of tration from 0 to 15%. The response of the organic form is Fe, regardless of the laser wavelength. In summary, for the non-linear for both UV and IR modes. In the CaCO3 matrix, three families of model samples tested, the LA-ICP-AES the response factors appear to reach a plateau after 15%. The response factor was strongly dependent on the chemical form results show that the slope of Mg signal versus concentration of the element for both UV and IR ablation.This trend was significantly more pronounced in the UV laser ablation mode but it was not a general rule. IR laser ablation appeared to be less sensitive than UV ablation to the chemical form of the compound. The LA-ICP-AES response factor was also depen- Fig. 3 LA-ICP-AES responses versus concentration for pure com- Fig. 2 LA-ICP-AES response factors calculated for Mg (a), Al (b) pounds spiked in SiO2 and CaCO3 matrices for Mg (a), Al (b) and Fe (c).and Fe (c) in pure compounds. 678 J. Anal. At. Spectrom., 1999, 14, 675–682curve is constant for all the inorganic forms of Mg spiked in ship between the ICP-AES response of Mg, Al and Fe and the elemental concentrations was linear up to at least 10%, SiO2 whereas it is constant for a lower concentration range (under 15%) except in the CaCO3 matrix.Therefore, the ICP- which is a large range. For both UV and IR laser ablation, this trend was noticed for the SiO2 and CaCO3 matrices and AES response factor of Mg is independent of the concentration for these conditions. The relationship between the signal and regardless of the chemical form of Mg, Al and Fe. Thus, this result indicates that the response factor of an analyte is the concentration of Al in the diVerent matrices is also linear up to 20% except in the IR mode for the sulfate and fluoride constant when its concentration in the matrix changes.However, this might not be true over larger concentration compounds where the linear concentration range is limited to 5%. Similar conclusions to those for Mg can be obtained. The ranges as the composition of the host matrix changes signifi- cantly. Normalisation to a matrix element can compensate for relationship between the signal of Fe and the concentration is also linear up to 40% except for the experiment with the changes in matrix composition.At this stage, the ICP-AES response factor appeared to be significantly dependent on the CaCO3 matrix where the signal behaves linearly in a smaller range (up to 30%). The response factor of Fe appears to be chemical form of the element and laser wavelength and to a lesser extent on the matrix composition. The concentration of independent of the concentration over a large concentration range. The ICP-AES responses normalised to Si versus the an analyte in the matrix occurring as a minor element does not have an eVect on the response factor.relative concentration (i.e., elemental concentration divided by the concentration of Si) for Mg, Al and Fe in SiO2 are Response factors for Mg, Al and Fe in SiO2 and CaCO3 matrices were derived from previous data using a simple linear presented in Fig. 4. First, it can be noted that the relative signal for Mg varies linearly with the relative concentration regression in the concentration range where the ICP-AES response was linear.The aim was to compare elemental within the whole concentration range. Al and Fe present a similar trend. The slopes of the diVerent curves are diVerent response factors according to the chemical form of the element, the nature of the matrix (SiO2 or CaCO3) and the LA mode. with respect to the chemical form of the analyte, the nature of the matrix and the laser wavelength. However, at least for First, response factors calculated for Mg are higher for the UV laser mode than for the IR mode.For the UV mode, the Mg and Al, the diVerences between the experiments are less important than the simple response factors. In contrast to response factor for MgO is diVerent with respect to the matrix in which it was included (2.0×107±0.72×107 and what was noticed for simple response factors, the relationship between the relative response and the relative concentration is 9.5×107±2.1×107, respectively, for SiO2 and CaCO3).In contrast, for IR the response factor is similar regardless of the always linear. In this set of experiments, the eVects of matrix composition matrix (7.2×106±1.2×106 for SiO2 and 5.3×106±0.63×106 for CaCO3). Response factors show the same dependence on and concentration of the analyte in the matrix on the response factors were evaluated. For the complex matrices, the relation- the chemical form similarly to what was previously observed for the pure compounds. Response factors calculated for Al are slightly higher or similar for UV than for IR in contrast to what was observed with pure compounds of Al.The response factors for Al2O3 in SiO2 and CaCO3 are not significantly diVerent (respectively, 2.1×105±6.3×105 and 1.9×105±0.23×105). They show the same dependence on the chemical form of Al as for the pure compounds. The response factors calculated for Fe show the same general trend for the chemical form of Fe as for the pure compounds except for Fe2O3.In the UV the response factor is lower in SiO2 than in CaCO3 (respectively, 8.4×103±2.9×103 and 2.6×104±0.72×104). The same trend occurs in the IR mode but with less variations (6.5×103±0.67×103 for SiO2 and 1.6×104±0.44×104 for CaCO3). Because the eVects of concentration were not significant for relative ICP-AES responses, relative response factors for Mg, Al and Fe in the SiO2 matrix, i.e., response factors normalised to Si, were derived from previous data using simple linear regression.Relative response factors allow one to compensate for variations in the amount of ablated material between the diVerent samples and thus compare the role of chemical form and laser wavelength on the ablation eYciency. In contrast to simple response factors, normalised response factors calculated for Mg, Al and Fe are similar in the UV and IR laser modes. There seems to be no significant variations for the normalised response factors calculated for Mg between diVerent forms of Mg for both UV and IR laser modes.For Al similar conclusions can be obtained except for the phosphate form which has a higher response. In contrast to Mg and Al, relative response factors calculated for Fe are dependent on the chemical form. They show the same trend according to the chemical form as for simple response factors but with lower fluctuations. These results suggest that response factors normalised to Si are less dependent on wavelength and also far less sensitive to the chemical form than the simple response factors.Globally, response factors presented the same dependence on the chemical form as for pure compounds. The LA-ICP- Fig. 4 Relative response versus concentration ratio for pure compounds spiked in SiO2 for Mg (a), Al (b) and Fe (c). AES response factors were also dependent on laser wavelength, J. Anal. At. Spectrom., 1999, 14, 675–682 679the response factor being generally higher in the UV, but sediments were analysed by LA-ICP-MS (Nd5YAG 1064 nm) by Williams and Jarvis.9 Variations of the response factors diVerences between UV and IR for a given compound spiked in a matrix were lower than for pure compounds.The response normalised to Ba for major and trace elements varied according to the host matrix. Major elements gave similar factors were also dependent on the nature of the matrix and this influence was found to be higher in the UV than in the responses only for matrices with similar mineralogical composition.LA was thought to be controlled by the physical IR. These results underline the occurrence of two factors: an important contribution of the chemical form of the element properties of individual minerals in which the elements occur rather than by the bulk rock chemistry. For silicate geostand- and an influence of the nature of the matrix. It appeared that the eVect of the chemical form of the element and laser ards, LA-ICP-MS response factors for major, minor and trace elements showed the same dependence on matrix wavelength are predominant eVects, even if the nature of the matrix plays a role.Clearly, the influence of the chemical form properties in the UV and IR modes using a Nd5YAG laser.19 Use of shorter wavelengths led to improvements in terms of of the element is complex and is laser wavelength dependent. However, the eVects of the chemical form and laser modes sensitivity for LA but not in terms of matrix dependence.In summary, the response factors of analytes in geological were lower for the normalised response factors than for the simple response factors. Moreover, for some elements normal- materials are influenced by matrix properties such as bulk chemical composition and mineralogy. Moreover, the elemen- isation to Si could overcome the influence of the chemical form and laser wavelength. tal response factor is also likely to be dependent on the individual mineralogical phase including the element.Indeed, diVerences in the LA eYciency of diVerent mineralogical 3.3 Influence of the matrix on the ablation eYciency of the phases, related to their specific physical properties, will analytes influence the response factors of the elements included in the phases. Response factors are, therefore, likely to exhibit a The influence of the nature of the matrix on LA was studied for diVerent compounds of Mg, Al and Fe spiked in SiO2 or complex dependence on global matrix properties and on individual properties of the host mineral. Thus, variability in CaCO3 matrices (cf.Section 3.2). It appeared that the nature of the matrix had an influence on the elemental responses elemental response factors will occur between diVerent families of materials because of the dependence on global matrix factors of the analytes spiked in the matrix. EVects of the composition of the matrix were investigated with elements properties. However, diVerent elemental distribution patterns between the mineralogical phases within the same matrix spiked in the SiO2 or CaCO3 at high concentration; changes in the composition of the matrix were able to influence the would also generate variations in response factors.These matrix eVects can be overcome by using normalisation to a response factors of the analytes, although changes were limited and normalisation to a matrix element could overcome matrix element, at least for matrices of a similar nature.For more complex matrices, from a mineralogical point of view, these eVects. Although only two diVerent matrices were used in the experiments, the nature of the matrix was thought to normalisation by means of an internal standard should be performed with an element representative of the mineral. have an eVect on the response factors of the elements spiked in the matrix, whereas eVects of the matrix composition were more limited.The same trend was also noticed for LA-ICP- 3.4 Influence of the chemical form AES determination of REE and refractory elements (W, N, Zr) in model geological samples with a CO2 laser at The influence of chemical form on the LA eYciency of the analytes was studied for diVerent compounds of Mg, Al and 10.6 mm.18 Model matrices were prepared by mixing diVerent oxides (Si, Ca, Fe, Al and Mg) and were then spiked with Fe prepared as pure samples (cf.Section 3.2.1). The LA-ICPAES response factors were strongly dependent on the the elements of interest. Variations in the composition of the matrix had no significant eVect on the response factors of the chemical form of the element. The same study was performed for complex matrices using diVerent compounds of Mg, Al analytes. Other studies from the literature suggest that elemental response factors in geological materials are influ- and Fe spiked in SiO2 or CaCO3 matrices (Section 3.1.2).In general, response factors displayed the same dependence on enced by matrix properties such as bulk chemical composition and mineralogy. Indeed, the influence of the physico- the chemical form as for pure compounds. Therefore, ablation eYciency depends significantly on the chemical form of the chemistry of the matrix on the response factors for major, minor and trace elements was confirmed for a large range of element. These results are, to a first approximation, related to the melting temperatures of the compound or to its ability geochemical materials.Several workers have investigated LA of geological or synthetic materials prepared as pressed pellets to be easily decomposed. For instance, the response factor is lowest for the MgO form, which exhibits a melting tempera- with LA-ICP-MS or LA-ICP-AES but these earlier studies were mostly based only on IR lasers. Jarvis and Williams7 ture of 2826 °C, while MgSO4 and MgF2 have melting temperatures of 1127 and 1263 °C, respectively.The highest analysed basaltic rocks and sediments with ICP-MS coupled to a Nd5YAG laser operating at 1064 nm. Significant response factor corresponds to Mg(CH3CO2)2, which is not stable. A similar conclusion can be obtained for Al, where diVerences between rock types for elemental response factors relative to Ba for REE were noted. These workers concluded the lowest response factor corresponds to the most stable compound, i.e., Al2O3, with a melting temperature of 2054 °C.that samples with diVerent chemistry and mineralogy gave diVerent responses for trace elements because of the specific The role of the chemical form of an element on its response factor has not been well studied. However, the influence of location of the REE in mineral phases that had diVerent behaviour during the LA process. Morrison et al.10 performed the chemical form of the elements on their LA eYciency has also been described for some other types of matrices.LA-ICP-MS analysis of geostandards (igneous rocks and shale) also with a Nd5YAG laser at 1064 nm. One reference Similarly to what was previously observed, response factors for metallic additives in polymers were dependent on their material was selected as the standard and the other materials were analysed as samples using Y as an internal standard. chemical form within the matrix.20 Goodall and Johnson21 also noticed a compound-specific response for uranium when Relative responses for trace elements varied with the diVerent matrices but these variations were similar for elements within determining this element in lithium chloride from fuel reconditioning materials by LA-ICP-AES.A Nd5YAG laser selected geochemical groups such as REE. DiVerences in the interaction of the laser with these matrices were thought to operating at 1064, 532 or 355 nm was used and Li was chosen as the internal standard. A compound-specific be the cause of such a dispersion.Silicates, basic rocks and 680 J. Anal. At. Spectrom., 1999, 14, 675–682response of uranium(III) chloride or uranium(IV) chloride was elements in polymers.20 It was also noted that the ablation process was dependent on the chemical environment, i.e., the observed and calibration using uranium compounds other than the specific compound was unsuccessful. Eppler et al.22 structures of the other additives. Because normalisation to a matrix element can compensate for diVerences between the investigated the eVects of matrix composition and chemical form of Pb and Ba spiked in soil and sand matrices with chemical forms and also laser modes, diVerences in the ablation eYciency are related to the amount and nature of the ablated laser-induced breakdown spectroscopy using a Nd5YAG laser at 1064 nm. Emission signals of Pb and Ba present in material.DiVerent mechanisms for LA have been proposed.1 The two major mechanisms significant for solid sampling are the samples as crystalline compounds were influenced by compound speciation (oxide, sulfate, chloride or carbonate).vaporisation and ablation itself, i.e., explosion. The LA process is usually considered to be qualitatively diVerent for UV and Results presented in this work on synthetic analogues of geological materials and also from other workers showed IR. For IR ablation this process results from laser light– plasma–material interactions where fusion and vaporisation that the variability in LA-ICP-AES response factors was related not only to diVerences in matrix properties but also are predominant.In contrast, UV laser beams couple directly onto the sample. Explosion is the predominant mechanism to the chemical form of the analytes, at least for elements occurring as minor discrete phases in the matrix, and this and removal of material from the sample is mainly caused by mechanical fragmentation. As the UV laser interacts directly influence of the chemical form of the analytes on the response to LA was very important.This suggests that even if the with the sample, it is likely to be more sensitive to matrix properties and physical properties of the analytes spiked in response to LA is controlled by matrix properties and by the location in mineralogical phases as previously suggested, the matrix than IR ablation. elements present in the matrix have a specific response to the laser, depending on their chemical form.In geological materials, location in a mineral and chemical form of the 4 Conclusion element are generally interdependent in the sense that an element is enclosed in a given mineral in a specific form. Geological materials such as rocks, sediments or soils have However, these results go further and suggest that elements complex matrices, and are a mixture of several mineralogical can have a specific response to LA depending on their phases.Trace elements occur as major elements of minor chemical form. This is very important in environmental individual minerals or are included or associated with major analytical chemistry where metallic pollutants can occur in mineralogical phases in diVerent chemical forms. This work diVerent forms in natural matrices. dealt with metals spiked in solid form in synthetic analogues of geological materials. Results showed that analytical matrix eVects in LA-ICP-AES were related to diVerences in global 3.5 Influence of the laser wavelength matrix properties and individual properties of the analytes The laser wavelength appeared to be a very important such as chemical form.With a Nd5YAG laser, the use of UV parameter in the diVerent sets of experiments. For pure instead of IR wavelengths did not overcome these matrix compounds, LA-ICP-AES response factors were strongly eVects. Quantitative analysis using external calibration thereinfluenced by the chemical form of the element but this trend fore requires matching of the unknown sample and the stanwas also laser wavelength dependent as IR ablation was less dard in terms of matrix properties such as chemical and sensitive than UV to the chemical form of the compound (cf.mineralogical composition and also in terms of the chemical Section 3.1.2). The LA-ICP-AES response factors were also form of the analytes. Thus, even under consistent laser conhigher in the UV mode than for IR.For analytes spiked in ditions, the non-matching of the standard and the sample will SiO2 or CaCO3 matrices, similar conclusions as for pure lead to systematic analytical errors. Internal standardisation compounds were obtained even if the nature of the matrix with a matrix element may compensate for matrix eVects due also plays a role (cf. Section 3.2). However, diVerences to diVerences between the host matrix and chemical forms for between UV and IR were less important for some elements some elements.in complex matrices than for pure compounds. It should also be noted that working with relative responses, i.e., response factors normalised to a matrix element, reduced the eVects of References both the laser wavelength and chemical form of the analyte for some elements. 1 R. Russo, Appl. Spectrosc., 1995, 49, 14A. EVects of the chemical form of the elements and of the 2 S. A. Darke and J. F. Tyson, J. Anal. At. Spectrom., 1993, 8, 145. 3 C.Neal, Rev. Geophys., 1995, 33, Suppl. nature of the matrix on response factors could be assigned 4 W. T. Perkins, in Microprobe Techniques in the Earth Sciences, either to the ablation process itself or to the transport of the ed. P. J. Potts, J. F. W. Bowls and S. J. B. Reed, Chapman and ablated material. It seems that the laser plays a key role, Hall, London, 1995, ch. 7, pp. 291–325. because the results were significantly dependent on the laser 5 K. E. Jarvis, Chem. Geol., 1992, 53, 335. wavelength. Clearly, the LA eYciency of an element is directly 6 A. L. Gray, Analyst, 1985, 110, 551. dependent on the laser wavelength. Because elemental response 7 K. E. Jarvis and J. G. Williams, Chem. Geol., 1993, 106, 251. 8 N. Imai, Anal. Chim. Acta, 1990, 235, 381. factors were related to the physical properties of the diVerent 9 J. G.Williams and K. E. Jarvis, J. Anal. At. Spectrom., 1993, 8, 25. compounds such as melting temperature, ablation eYciency 10 C. A. Morrison, D. D. Lambert, R. J. S. Morrison, W. W. Ahlers depends on chemical form. It was also found to be dependent and I. A. 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