Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics.     

Depth profiling of tin-coated glass by laser ablation inductively coupled plasma emission spectrometry with acoustic signal measurement

A pulsed, frequency-quadrupled Nd:YAG laser (266 nm, 10 Hz) coupled to an inductively coupled plasma atomic emission spectrometer (ICP-AES) was employed for depth profiling by ablation of a pyrolytically deposited Sn layer (300 nm) on float glass. The procedure consisted of performing individual ablation cycles (layer-by-layer). A raster with stroke distance of either 50 μm or 200 μm (the raster density) was used as an ablation pattern. The ablation was stopped after each cycle and the peak area of the resulting transient optical signal of the ICP discharge was plotted against the cycle number. The ablation rate of 90 to 20 nm per cycle at a low-energy pulse (6 mJ to 1 mJ) was determined by profilometry. A beam masking was employed to attenuate the laser shot energy and to eliminate the peripheral irregularity of the beam profile. Almost uniform removal of the square area (1 mm × 1 mm) of the coating by ablation was achieved by combining the fitted raster density, beam masking, focusing and beam energy. Different ablation processes were distinguished in cases of the tin coating and the uncoated glass surface. While the coating was mainly evaporated, the uncoated glass surface exhibited a crumbling associated with production of glass powder. This was confirmed by electron microscopy observations. The measured acoustic signal followed the behavior of the emission intensity of the Sn line and was supposed to be proportional to the amount of Sn vapors. The emission intensity depth profile of the Sn coating with graded structure was obtained, which qualitatively corresponded with the depth profile measured by secondary ion mass spectrometry.     

Depth profiling of tin-coated glass by laser ablation inductively coupled plasma emission spectrometry with acoustic signal measurement

A pulsed, frequency-quadrupled Nd:YAG laser (266 nm, 10 Hz) coupled to an inductively coupled plasma atomic emission spectrometer (ICP-AES) was employed for depth profiling by ablation of a pyrolytically deposited Sn layer (300 nm) on float glass. The procedure consisted of performing individual ablation cycles (layer-by-layer). A raster with stroke distance of either 50 microm or 200 microm (the raster density) was used as an ablation pattern. The ablation was stopped after each cycle and the peak area of the resulting transient optical signal of the ICP discharge was plotted against the cycle number. The ablation rate of 90 to 20 nm per cycle at a low-energy pulse (6 mJ to 1 mJ) was determined by profilometry. A beam masking was employed to attenuate the laser shot energy and to eliminate the peripheral irregularity of the beam profile. Almost uniform removal of the square area (1 mm x 1 mm) of the coating by ablation was achieved by combining the fitted raster density, beam masking, focusing and beam energy. Different ablation processes were distinguished in cases of the tin coating and the uncoated glass surface. While the coating was mainly evaporated, the uncoated glass surface exhibited a crumbling associated with production of glass powder. This was confirmed by electron microscopy observations. The measured acoustic signal followed the behavior of the emission intensity of the Sn line and was supposed to be proportional to the amount of Sn vapors. The emission intensity depth profile of the Sn coating with graded structure was obtained, which qualitatively corresponded with the depth profile measured by secondary ion mass spectrometry.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd : YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999     

Evaluation of the analytical capability of NIR femtosecond laser ablation-inductively coupled plasma mass spectrometry.

A laser ablation-inductively coupled plasma-mass spectrometric (LA-ICPMS) technique utilizing a titanium-sapphire (TiS) femtosecond laser (fs-laser) has been developed for elemental and isotopic analysis. The signal intensity profile, depth of the ablation pit and level of elemental fractionation were investigated in order to evaluate the analytical capability of the present fs-laser ablation-ICPMS technique. The signal intensity profile of (57)Fe, obtained from iron sulfide (FeS(2)), demonstrated that the resulting signal intensity of (57)Fe achieved by the fs-laser ablation was almost 4-times higher than that obtained by ArF excimer laser ablation under a similar energy fluence (5 J/cm(2)). In fs-laser ablation, there is no significant difference in a depth of the ablation pit between glass and zircon material, while in ArF laser ablation, the resulting crater depth on the zircon crystal was almost half the level than that obtained for glass material. Both the thermal-induced and particle size-related elemental fractionations, which have been thought to be main sources of analytical error in the LA-ICPMS analysis, were measured on a Harvard 91500 zircon crystal. The resulting fractionation indexes on the (206)Pb/(238)U (f(Pb/U)) and (238)U/(232)Th (f(U/Th)) ratios obtained by the present fs-laser ablation system were significantly smaller than those obtained by a conventional ArF excimer laser ablation system, demonstrative of smaller elemental fractionation. Using the present fs-laser ablation technique, the time profile of the signal intensity of (56)Fe and the isotopic ratios ((57)Fe/(54)Fe and (56)Fe/(54)Fe) have been measured on a natural pyrite (FeS(2)) sample. Repeatability in signal intensity of (56)Fe achieved by the fs-laser ablation system was significantly better than that obtained by ArF excimer laser ablation. Moreover, the resulting precision in (57)Fe/(54)Fe and (56)Fe/(54)Fe ratio measurements could be improved by the fs-laser ablation system. The data obtained here clearly demonstrate that, even with the fundamental wavelength (NIR operating at 780 nm), the fs-laser ablation system has the potential to become a significant tool for in-situ elemental and isotopic analysis of geochemical samples including heavy minerals and metallic materials.     

激光剥蚀电感耦合等离子体质谱法中生物样品的元素分馏效应研究

采用213 nm-纳秒激光剥蚀系统对生物基体样品的剥蚀颗粒进行研究,优化了激光剥蚀条件.在剥蚀能量为25%,束斑直径为200 μm,剥蚀速率为20 μm/s,频率为20 Hz,载气为700 mL He + 700 mL Ar时,信号强度及稳定性最佳.以31P为内标元素,最佳剥蚀条件下,考察了56个元素的相对分馏因子.结果表明,生物基体的剥蚀颗粒相较于NIST 610 玻璃标样更大,达到3 μm;生物基体中元素分馏效应相较于玻璃基体小,大多数元素的相对分馏因子达到1.0 ±0.1.探讨了生物基体中元素分馏机理,分析了生物基体相较于玻璃基体剥蚀颗粒大,而相对分馏因子未明显增大的原因.一方面可能是粒径3 μm的颗粒进入电感耦合等离子体后能原子化;另一方面,大的剥蚀颗粒的富集效应相对较小.进一步对分馏效应的影响因素进行研究,发现分馏效应与激光剥蚀能量、激光频率和扫描速率相关,并且与元素的氧化物沸点负相关,与氧化物键能和电离能正相关.     

Industrial applications of SAW-sensors: on-line control of preparative HPLC

Results of on-line process control with an SAW-sensor based gas-sensor microsystem (SAGAS PC) are presented for industrial applications. The control of the solvent mixer of an HPLC-system was chosen as a typical example. All applications dealt with the detection and subsequent qualification as well as quantification of volatile organic compounds (VOC’s), e.g. organic solvents or even water by investigating their vapor. Mass-sensitive transducers, especially surface acoustic wave (SAW)-devices, have become a feasible tool for VOC-analytics. The measurement principle is based on frequency changes which are directly correlated with mass changes on the sensor surface and thus provide a particularly suitable signal transduction. The data analysis of all applications is based on pattern recognition methods, and focused on artificial neural networks for quantitative determination of these solvents and solvent-mixtures. The results obtained clearly showed the suitability of such a sensor system for on-line process-control purposes. Received: 25 September 1998 / Revised: 18 November 1998 / Accepted: 26 November 1998     

Industrial applications of SAW-sensors: on-line control of preparative HPLC

Results of on-line process control with an SAW-sensor based gas-sensor microsystem (SAGAS PC) are presented for industrial applications. The control of the solvent mixer of an HPLC-system was chosen as a typical example. All applications dealt with the detection and subsequent qualification as well as quantification of volatile organic compounds (VOC’s), e.g. organic solvents or even water by investigating their vapor. Mass-sensitive transducers, especially surface acoustic wave (SAW)-devices, have become a feasible tool for VOC-analytics. The measurement principle is based on frequency changes which are directly correlated with mass changes on the sensor surface and thus provide a particularly suitable signal transduction. The data analysis of all applications is based on pattern recognition methods, and focused on artificial neural networks for quantitative determination of these solvents and solvent-mixtures. The results obtained clearly showed the suitability of such a sensor system for on-line process-control purposes. Received: 25 September 1998 / Revised: 18 November 1998 / Accepted: 26 November 1998     

Determination of trace elements in quartz glass by use of LINA-Spark-ICP-MS as a new method for bulk analysis of solid samples

The determination of trace elements in pure quartz glass samples has been performed by coupling an ICP quadrupole mass spectrometer with the LINA-Spark-Atomizer, an IR laser ablation system dedicated to direct bulk and surface analysis of solid samples. Linear calibration curves were obtained for nine elements (Na, Al, Ca, Ti, Cr, Mn, Zr, Ba, and Pb) in the ng g(-1) range with detection limits of less than 10 ng g(-1) for Ca, Cr, Mn, Zr, Ba, and Pb and in the range of 120-220 ng g(-1) for Na, Al, and Ti. The distance between the laser focal point and the sample surface has a significant influence on signal intensity and precision, both of which can be improved by a factor of approximately two by focusing the laser 15 mm behind the sample surface. Aerosol moistening reduced the standard deviation of the signal intensity by a factor of 2-4. Signal instability, which resulted from different ablation rates or variations in the transmission of the mass spectrometer, were compensated by use of the simultaneously measured SiAr+ ion as an internal standard. Under these conditions precision was usually better than 5% RSD. The results were compared with those obtained by use of a commercial LA-ICP-MS system. With this instrumentation linear calibration curves were achieved for three elements only (Al, Ti, and Pb), showing that LA-ICP-MS is less appropriate for bulk analysis in the ng g(-1) range.     

Analysis of glass by UV laser ablation inductively coupled plasma atomic emission spectrometry. Part 1. Effects of the laser parameters on the amount of ablated material and the temporal behaviour of the signal for different types of laser

Two lasers working in the UV part of the spectrum have been used for the analysis of glass samples. An XeCl excimer laser (308 nm) and a Nd:YAG laser operating at the third harmonic (355 nm) and the fourth harmonic (266 nm) have been selected. The energy was 100 mJ and 5 mJ for the excimer laser and the Nd:YAG laser, respectively. Because of different spot sizes, the fluence was of the same magnitude for both lasers. Crater characterization indicated that the laser ablation efficiency was similar for the two lasers when normalized to the same energy. However, the XeCl was found to be more efficient when the results were normalized to irradiance unit. The amount of probed material and ablated material was measured, leading to an efficiency higher than 80%. The influence of the glass colour and the laser wavelength was evaluated. The XeCl laser provided the largest amount of material but was sensitive to the glass colour. This laser was mainly suitable for bulk analysis. In contrast, the Nd:YAG, particularly at 266 nm, was insensitive to the glass colour and was appropriate for localized analysis. Inductively coupled plasma atomic emission spectrometry was used for atomization and excitation of the ablated material. A good agreement was found between the temporal behaviour of the amount of ablated material and the analyte signal.     

Studies of LA-ICP-MS on quartz glasses at different wavelengths of a Nd:YAG laser

The capability of LA-ICP-MS for determination of trace impurities in transparent quartz glasses was investigated. Due to low or completely lacking absorption of laser radiation, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) proves difficult on transparent solids, and in particular the quantification of measurement results is problematic in these circumstances. Quartz glass reference materials of various compositions were studied by using a Nd:YAG laser system with focused laser radiation of wavelengths of 1064 nm, 532 nm and 266 nm, and an ICP-QMS (Elan 6000, Perkin Elmer). The influence of ICP and laser ablation conditions in the analysis of quartz glasses of different compositions was investigated, with the laser power density in the region of interaction between laser radiation and solid surface determining the ablation process. The trace element concentration was determined via calibration curves recorded with the aid of quartz glass reference materials. Under optimized measuring conditions the correlation coefficients of the calibration curves are in the range of 0.9–1. The relative sensitivity factors of the trace elements determined in the quartz glass matrix are 0.1–10 for most of the trace elements studied by LA-ICP-MS. The detection limits of the trace elements in quartz glass are in the low ng/g to pg/g range.     

Studies of LA-ICP-MS on quartz glasses at different wavelengths of a Nd:YAG laser

The capability of LA-ICP-MS for determination of trace impurities in transparent quartz glasses was investigated. Due to low or completely lacking absorption of laser radiation, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) proves difficult on transparent solids, and in particular the quantification of measurement results is problematic in these circumstances. Quartz glass reference materials of various compositions were studied by using a Nd:YAG laser system with focused laser radiation of wavelengths of 1064 nm, 532 nm and 266 nm, and an ICP-QMS (Elan 6000, Perkin Elmer). The influence of ICP and laser ablation conditions in the analysis of quartz glasses of different compositions was investigated, with the laser power density in the region of interaction between laser radiation and solid surface determining the ablation process. The trace element concentration was determined via calibration curves recorded with the aid of quartz glass reference materials. Under optimized measuring conditions the correlation coefficients of the calibration curves are in the range of 0.9-1. The relative sensitivity factors of the trace elements determined in the quartz glass matrix are 0.1-10 for most of the trace elements studied by LA-ICP-MS. The detection limits of the trace elements in quartz glass are in the low ng/g to pg/g range.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

Determination of trace elements in quartz glass by use of LINA-Spark–ICP–MS as a new method for bulk analysis of solid samples

The determination of trace elements in pure quartz glass samples has been performed by coupling an ICP quadrupole mass spectrometer with the LINA-Spark-Atomizer, an IR laser ablation system dedicated to direct bulk and surface analysis of solid samples. Linear calibration curves were obtained for nine elements (Na, Al, Ca, Ti, Cr, Mn, Zr, Ba, and Pb) in the ng g^–1 range with detection limits of less than 10 ng g^–1 for Ca, Cr, Mn, Zr, Ba, and Pb and in the range of 120–220 ng g^–1 for Na, Al, and Ti. The distance between the laser focal point and the sample surface has a significant influence on signal intensity and precision, both of which can be improved by a factor of approximately two by focusing the laser 15 mm behind the sample surface. Aerosol moistening reduced the standard deviation of the signal intensity by a factor of 2–4. Signal instability, which resulted from different ablation rates or variations in the transmission of the mass spectrometer, were compensated by use of the simultaneously measured SiAr⁺ ion as an internal standard. Under these conditions precision was usually better than 5% RSD. The results were compared with those obtained by use of a commercial LA–ICP–MS system. With this instrumentation linear calibration curves were achieved for three elements only (Al, Ti, and Pb), showing that LA–ICP–MS is less appropriate for bulk analysis in the ng g^–1 range.     

Inductively coupled plasma mass spectrometric study of non-linear calibration behavior during laser ablation of binary Cu–Zn Alloys

Laser ablation behavior of a suite of 10 Cu–Zn binary alloys was studied using inductively coupled plasma mass spectrometry. Three laser systems (20 ns KrF excimer, 6 ns and 35 ps Nd:YAG) were used for ablation. Non-linear calibration plots for both Cu and Zn were observed using all three lasers, despite significant differences in laser ablation mechanisms and good stoichiometry of ablated mass. Crater volume measurements were used to determine the amount of mass removed during repetitive laser ablation from each sample. A change in mass ablation rate for samples with different composition explains observed phenomena. Despite the differences in ablation behavior of these alloys, linear calibration curves were obtained when Zn signal intensity was normalized to signal intensity of Cu or to crater volume.     

Calibration of laser-ablation ICP-MS. Can we use synthetic standards with pneumatic nebulization?

An approach for the determination of trace element concentrations in high purity metals, using an inductively coupled plasma mass spectrometer (ICP-MS) with a laser-ablation system for direct solid sample introduction after calibration with nebulized liquid standards was made. Due to the inherent differences in the rate of sample introduction with laser-ablation and pneumatic nebulization, a matrix element must be used as an internal standard. This is problematical for elements that have no isotope with a relative abundance of less than 0.1 %, since the ion signals would be too high for direct measurement, and reduction of the ablation rate would compromise the sensitivity for trace elements. Due to the high stability of ICP-unit and mass filter of the instrument used, it was found that the tail of a mass-peak of the matrix element could be used as an internal standard. Therefore, a position at –0.5 amu from the matrix-isotope (e.g. ^62.5Cu in copper samples) was used for internal standardization. The standard deviation of this signal in a period of 2.5 h was 3.6% RSD with no notable drift when the laser ablation was used for sample introduction. The calibration of the matrix-element by nebulizing liquid standards showed that the ion signal measured on the peak-tail is directly proportional to the element concentration in the ICP. This indicates that the peak shape is not only stable, but also independent of the peak height. The advantages of this method lie in the easy preparation of calibration standards for quantitative measurements with a laser-ablation system and access to homogeneous standards for materials, that are difficult to homogenize in the solid state. The calibration of the traces is performed relatively to a fixed concentration of the matrix element. Calibrations were carried out for trace concentrations in high purity copper and good recoveries were obtained for high-purity reference standards. Received: 23 February 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

Calibration of laser-ablation ICP-MS. Can we use synthetic standards with pneumatic nebulization?

An approach for the determination of trace element concentrations in high purity metals, using an inductively coupled plasma mass spectrometer (ICP-MS) with a laser-ablation system for direct solid sample introduction after calibration with nebulized liquid standards was made. Due to the inherent differences in the rate of sample introduction with laser-ablation and pneumatic nebulization, a matrix element must be used as an internal standard. This is problematical for elements that have no isotope with a relative abundance of less than 0.1 %, since the ion signals would be too high for direct measurement, and reduction of the ablation rate would compromise the sensitivity for trace elements. Due to the high stability of ICP-unit and mass filter of the instrument used, it was found that the tail of a mass-peak of the matrix element could be used as an internal standard. Therefore, a position at –0.5 amu from the matrix-isotope (e.g. ^62.5Cu in copper samples) was used for internal standardization. The standard deviation of this signal in a period of 2.5 h was 3.6% RSD with no notable drift when the laser ablation was used for sample introduction. The calibration of the matrix-element by nebulizing liquid standards showed that the ion signal measured on the peak-tail is directly proportional to the element concentration in the ICP. This indicates that the peak shape is not only stable, but also independent of the peak height. The advantages of this method lie in the easy preparation of calibration standards for quantitative measurements with a laser-ablation system and access to homogeneous standards for materials, that are difficult to homogenize in the solid state. The calibration of the traces is performed relatively to a fixed concentration of the matrix element. Calibrations were carried out for trace concentrations in high purity copper and good recoveries were obtained for high-purity reference standards. Received: 23 February 1998 / Revised: 20 July 1998 / Accepted: 25 July 1998     

Correlation of acoustic and optical emission signals produced at 1064 and 532 nm laser-induced breakdown spectroscopy (LIBS) of glazed wall tiles

An acoustic signal was used for the internal standardization of laser-induced breakdown spectroscopy (LIBS) of a glazed wall tile. For the LIBS analyses, 1064 nm and 532 nm wavelengths of the Nd:YAG laser were utilized. The tile was depth profiled by a single-spot ablation from the glaze into the substrate. Some lines of major elements Si(I) 252.418, Si(I) 252.851, Al(I) 257.509, Cr(I) 295.368, Al(I) 309.271 nm and Ti(II) 334.904 nm were monitored. The decrease in the optical emissions during the ablation was successfully compensated for by normalization to the square power of the acoustic signal in the interval of 290–340 nm. This approach failed for the lines between 250–270 nm. The results were the same for both lasing wavelengths despite different irradiances. The acquired profiles are in good agreement with the reference X-ray fluorescence measurement.     

Use of a single calibration graph for the determination of major elements in geological materials by laser ablation inductively coupled plasma atomic emission spectrometry with added internal standards

The possibility of internal standardisation in laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) of geological materials by added Sc₂O₃ and Y₂O₃ has been examined to cover the wide range of concentrations of major and minor constituents both in silicate rocks and limestones. A Nd : YAG laser (355 nm, 10 Hz, 10 mJ per shot) was used for the ablation of discs obtained by fusion of the mixture of samples and oxides of Sc and Y with lithium tetraborate. The flux/sample ratio was in the range from 6 to 10. The contents of analytes were within the concentration range from hundredths to tens of percentage. The trace elements copper and nickel were studied, too. An ICP emission spectrometer OPTIMA 3000 DV was used for the measurement of Si, Al, Ca, Mg, Sr, Ba, Fe, Ti, Mn, Ni, Cu, Na and K analyte lines and Y and Sc reference lines in the axial observation mode. The long-term and the short-term repeatability of measurement were improved by employing scandium or yttrium internal references lines for the above analytes from 6% to 1.5% of RSD and from 2.4% to 1.0% of RSD, respectively. The correlation of signals with concentrations was improved in terms of the correlation coefficient r from 0.90–0.97 to 0.98–0.998 and the relative uncertainity on the centroid of concentrations was improved 2–3 times. A single calibration graph covering the concentration range both in silicates and carbonates is possible for each of elements, as the matrix effects are compensated for by internal standards and the excess of Li₂B₄O₇. Received: 20 June 1998 / Revised: 20 July 1998 / Accepted: 21 September 1998     

Scanning vs. single spot laser ablation (λ=213 nm) inductively coupled plasma mass spectrometry

Sampling strategy is defined in this work as the interaction of a repetitively pulsed laser beam with a fixed position on a sample (single spot) or with a moving sample (scan). Analytical performance of these sampling strategies was compared by using 213 nm laser ablation ICP-MS. A geological rock (Tuff) was quantitatively analyzed based on NIST series 610-616 glass standard reference materials. Laser ablation data were compared to ICP-MS analysis of the dissolved samples. The scan strategy (50 μm/s) produced a flat, steady temporal ICP-MS response whereas the single spot strategy produced a signal that decayed with time (after 60 s). Single-spot sampling provided better accuracy and precision than the scan strategy when the first 15 s of the sampling time was eliminated from the data analysis. In addition, the single spot strategy showed less matrix dependence among the four NIST glasses.