Signal enhancement effect of halogen matrix in electrothermal vaporization-inductively coupled plasma-mass spectrometry

In tungsten furnace electrothermal vaporization(ETV)-inductively coupled plasma mass spectrometry(ICP-MS), the presence of halogen matrices caused a signal enhancement for volatile elements such as Zn, Cd and Pb, whose halides melting and boiling points were relatively low. In order to clarify the mechanism of signal enhancement in ETV-ICP-MS, the effects of chemical interaction between analytes and halogen matrices on the surface of ETV furnace, the transport efficiency of vaporized analytes from the furnace into the ICP and the physical properties of the ICP itself and of the micro plasma (interface plasma) in the interface region between the sampling and the skimmer cones were investigated by atomic absorption and atomic emission spectrometry. Among the effects mentioned above, neither the chemical interaction on the surface of the ETV furnace nor the transport efficiency of vaporized analytes could be related to the analyte signal enhancements. The degree of enhancement was found to depend on the ionization potential of the coexisting halogen and was not caused by a variation in the physical properties of the ICP but rather by a variation of those of the interface plasma. These results suggest that the halogen matrices may affect the physical properties of the interface plasma, contributing to the promotion of the ionization of analytes.     

Analyte transport efficiencies in electrothermal vaporization for inductively coupled plasma mass spectrometry

A modified graphite furnace for solid-sampling atomic absorption spectrometry as an electrothermal vaporizer (ETV) was coupled to a Perkin-Elmer/Sciex ELAN 6000 ICP mass spectrometer. The integrals obtained from electrothermal vaporization of aliquots containing As, Cd, Cu, Co, Fe, Mn, Pb, Se, and Zn were compared with those obtained from pneumatic nebulization of the same aqueous standard solution. The pneumatic nebulizer was calibrated by weighing the mass of aqueous aerosol trapped on a filter. With "wet plasma" conditions maintained also for measurements with the ETV and reference signals for analyte signals obtained with the calibrated pneumatic nebulization, the transport efficiency of the ETV system, e.g. the ratio of the analyte amount introduced into the plasma to that amount dosed into the vaporizer, was determined. The transport efficiency of two different tube and interface designs has been evaluated. Investigations with and without the use of trifluoromethane as reactive gas, with different furnace heating rates, and with varying gas flows were performed. In general, the tube equipped with a nozzle led to generally higher transport efficiency than the standard tube. Without trifluoromethane transport efficiencies ranged from 10% to 35% with the standard tube and from 15% to 50% with the nozzle-type tube. With addition of 2 mL min(-1) trifluoromethane to the argon flow of 400 mL min(-1) through the tube, transport efficiencies from 20% to 70% and from 70% to 100% were achieved with the standard and nozzle-type tubes, respectively.     

Analyte transport efficiencies in electrothermal vaporization for inductively coupled plasma mass spectrometry

A modified graphite furnace for solid-sampling atomic absorption spectrometry as an electrothermal vaporizer (ETV) was coupled to a Perkin–Elmer/Sciex ELAN 6000 ICP mass spectrometer. The integrals obtained from electrothermal vaporization of aliquots containing As, Cd, Cu, Co, Fe, Mn, Pb, Se, and Zn were compared with those obtained from pneumatic nebulization of the same aqueous standard solution. The pneumatic nebulizer was calibrated by weighing the mass of aqueous aerosol trapped on a filter. With “wet plasma” conditions maintained also for measurements with the ETV and reference signals for analyte signals obtained with the calibrated pneumatic nebulization, the transport efficiency of the ETV system, e.g. the ratio of the analyte amount introduced into the plasma to that amount dosed into the vaporizer, was determined. The transport efficiency of two different tube and interface designs has been evaluated. Investigations with and without the use of trifluoromethane as reactive gas, with different furnace heating rates, and with varying gas flows were performed. In general, the tube equipped with a nozzle led to generally higher transport efficiency than the standard tube. Without trifluoromethane transport efficiencies ranged from 10% to 35% with the standard tube and from 15% to 50% with the nozzle-type tube. With addition of 2 mL min^–1 trifluoromethane to the argon flow of 400 mL min^–1 through the tube, transport efficiencies from 20% to 70% and from 70% to100% were achieved with the standard and nozzle-type tubes, respectively.     

Ultra-trace determination of Se in sediments by electrothermal vaporizer-inductively coupled plasma-mass spectroscopy: use of the ETV as a thermochemical reactor

The performance of electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) was evaluated for the ultra-trace determination of total selenium in sediment reference materials. Citric acid, when combined with an appropriate thermal program, promoted the early release of Se in a molecular form from the graphite surface, effecting a separation of the analyte from the concomitant matrix, thereby demonstrating the use of the ETV as a thermochemical reactor. No special sample pretreatment is needed and an absolute detection limit of 10 pg was achieved. Concentrations of Se in different sediment CRMs were determined and results obtained by both isotope dilution (ID) and standard addition (SA) methodologies were compared and evaluated. Mass bias effects prevented accurate application of ID techniques.     

电热蒸发-电感耦合等离子体质谱(ETV-ICP-MS)技术中气溶胶传输效率研究进展

蒸发效率和传输效率是影响电热蒸发-电感耦合等离子体质谱(ETV-ICP-MS)技术分析性能的关键因素。综述有关气溶胶传输效率的研究进展,在归纳常用的传输系统评价方法的基础上,重点对影响气溶胶传输效率的电热蒸发装置的改进、蒸发过程的探讨、基体改进剂的选择等研究现状进行评述。有关ETV蒸发过程中基体干扰的作用机制仍有待进一步系统研究,这对于校正策略的优化、新型校正技术的创新与集成具有重要的理论指导意义,可推动ETV-ICP-MS在地质、环境、生物等学科的更广泛应用。     

Development of an electrothermal vaporization ICP-MS method and assessment of its applicability to studies of the homogeneity of reference materials

A method has been developed for measurement of the homogeneity of analyte distribution in powdered materials by use of electrothermal vaporization with inductively coupled plasma mass spectrometric (ETV-ICP-MS) detection. The method enabled the simultaneous determination of As, Cd, Cu, Fe, Mn, Pb, and Zn in milligram amounts of samples of biological origin. The optimized conditions comprised a high plasma power of 1,500 W, reduced aerosol transport flow, and heating ramps below 300 degrees C s(-1). A temperature ramp to 550 degrees C ensured effective pyrolysis of approximately 70% of the organic compounds without losses of analyte. An additional hold stage at 700 degrees C led to separation of most of the analyte signals from the evaporation of carbonaceous matrix compounds. The effect of time resolution of signal acquisition on the precision of the ETV measurements was investigated. An increase in the number of masses monitored up to 20 is possible with not more than 1% additional relative standard deviation of results caused by limited temporal resolution of the transient signals. Recording of signals from the nebulization of aqueous standards in each sample run enabled correction for drift of the sensitivity of the ETV-ICP-MS instrument. The applicability of the developed method to homogeneity studies was assessed by use of four certified reference materials. According to the best repeatability observed in these sample runs, the maximum contribution of the method to the standard deviation is approximately 5% to 6% for all the elements investigated.     

Determination of metals in airborne particulates by LEAFS and ICP-MS after sampling on reusable graphite filters

Laser excited atomic fluorescence spectrometry combined with electrothermal atomization (ETA-LEAFS) and inductively coupled plasma mass spectrometry combined with electrothermal evaporation (ETV-ICP-MS) were used to measure the concentrations of some metals in ambient air sampled at the outskirts of Berlin. Using graphite to collect airborne particulates the contents of lead, palladium, and thallium could be determined in the pg/m(3) range.     

Vaporization and atomization of uranium in a graphite tube electrothermal vaporizer: a mechanistic study using electrothermal vaporization inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry

The mechanism of vaporization and atomization of U in a graphite tube electrothermal vaporizer was studied using graphite furnace atomic absorption spectrometry (GFAAS) and electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS). Graphite furnace AAS studies indicate U atoms are formed at temperatures above 2400°C. Using ETV-ICP-MS, an appearance temperature of 1100°C was obtained indicating that some U vaporizes as U oxide. Although U carbides form at temperatures above 2000°C, ETV-ICP-MS studies show that they do not vaporize until 2600°C. In the temperature range between 2200°C and 2600°C, U atoms in GFAAS are likely formed by thermal dissociation of U oxide, whereas at higher temperatures, U atoms are formed via thermal dissociation of U carbide.The origin of U signal suppression in ETV-ICP-MS by NaCl was also investigated. At temperatures above 2000°C, signal suppression may be caused by the accelerated rate of formation of carbide species while at temperatures below 2000°C, the presence of NaCl may cause intercalation of the U in the graphite layers resulting in partial retention of U during the vaporization step. The use of 0.3% freon-23 (CHF₃) mixed with the argon carrier gas was effective in preventing the intercalation of U in graphite and U carbide formation at 2700°C.     

Efficiency of sample introduction into inductively coupled plasma by graphite furnace electrothermal vaporization

A laboratory constructed graphite furnace electrothermal vaporizer (GF-ETV) was used for studying transport efficiencies. This device enables collection of the vaporization products that exit the central sampling hole of the horizontal graphite tube. For determination of the transport efficiency between the GF-ETV and the ICP-torch three methods were tested: (1) deposition of the aerosol particles and the vapour of certain elements by mixing the vaporization product with supersaturated steam and subsequent condensation (direct method); (2) dissolution of the deposited sample fraction from the interface components (indirect method); and (3) calculation from line intensities when applying GF-ETV and pneumatic nebulization sample introduction methods using mercury as a reference element. The latter, `mercury reference method' required 100% transport efficiency for mercury with the ETV, which could be approximated with the use of argon as carrier gas (without halocarbon addition). With a 200 cm³/min flow rate of internal argon in the graphite tube, the transport efficiency was between 67 and 76% for medium volatility elements (Cu, Mn and Mg) and between 32 and 38% for volatile elements (Cd and Zn). By adding carbon tetrachloride vapour to the internal argon flow, the transport efficiency increased to 67–73% for the five elements studied.     

Mechanism of vaporization of yttrium and rare earth elements in electrothermal vaporization inductively coupled plasma mass spectrometry

The mechanism of vaporization of yttrium and the rare earth elements (REEs) has been studied using graphite furnace atomic absorption spectrometry (GFAAS) and inductively-coupled plasma mass spectrometry (ICP-MS). The appearance temperatures for Y and the REEs obtained by GFAAS were generally identical to the appearance temperatures obtained using ETV-ICP-MS. At lower temperatures, Y and the REEs are predominantly vaporized in atomic form or as oxides, while at temperatures above 2500°C, the elements are vaporized as oxides and/or carbides. This accounts for the very high sensitivity of ETV-ICP-MS compared to GFAAS for the determination of these elements. Absolute limits of detection for Y and all of the REEs using ETV-ICP-MS ranged from 0.002 pg for Tm to 0.2 pg for Ce. The use of freon as a chemical modifier was effective in controlling analyte carbide formation and reducing memory effects.     

Modifier effects from palladium and iridium in the determination of arsenic and antimony using electrothermal vaporisation inductively coupled plasma mass spectrometry

A study was made of the stabilising and carrying effects of palladium and iridium as modifiers in the determination of arsenic and antimony by electrothermal vaporisation inductively coupled plasma mass spectrometry. The signal intensities of arsenic and antimony were found to increase with decreasing volume (50–10 μl) and concentration (40–5 μg ml⁻¹) of the palladium modifier solution. Similar effects were not observed for iridium. Palladium and iridium had about the same stabilising effect on the analytes; significant loss of the analyte occurred at pyrolysis temperatures above 900°C. The two modifiers gave rise to about the same increase in signal intensity, but the transport mechanism is probably different. The results indicate that iridium interacts with the graphite surface such as to make carbon the main carrier. In contrast, palladium probably acts as an active carrier.     

Determination of metals in airborne particulates by LEAFS and ICP-MS after sampling on reusable graphite filters

Laser excited atomic fluorescence spectrometry combined with electrothermal atomization (ETA-LEAFS) and inductively coupled plasma mass spectrometry combined with electrothermal evaporation (ETV-ICP-MS) were used to measure the concentrations of some metals in ambient air sampled at the outskirts of Berlin. Using graphite to collect airborne particulates the contents of lead, palladium, and thallium could be determined in the pg/m³ range.     

Vaporization and atomization of the platinum group elements in the graphite furnace investigated by electrothermal vaporization-inductively coupled plasma-mass spectrometry

The mechanisms by which the platinum group elements (PGEs) are vaporized in the graphite furnace have been investigated using electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS). The results suggest that live of these elements (Ru, Rh, Pd, Ir and Pt) are reduced to their metallic state in the graphite furnace and then vaporized by direct sublimation of the metal. For Os, the vaporization mechanism is different. In the presence of HNO₃, two distinct vaporization processes are observed. Volatile oxides of Os are released at low temperatures, but some of this oxide is reduced to relatively involatile Os metal which is then vaporized when the temperature is increased above 2000°C. The addition of TeCl₂ chemical modifier was found to have minimal effect on the vaporization mechanism and sensitivity for determination for five of the PGEs. For Os, however, the analytical sensitivity and limit of detection was improved when Te modifier was used in conjunction with a lower vaporization temperature of 1400°C. Optimum conditions for the determination of the PGEs by ETV-ICP-MS are reported, along with their absolute limits of detection; these range from 0.015 pg for Ir to 0.25 pg for Os.     

An investigation of different modifiers in electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS)

The suitability of eleven modifiers (Pd-, Mg-, K-, Ca- and NH₄-salts) for electrothermal vaporization coupled to inductively coupled plasma mass spectrometry (ETV-ICP-MS) for the determination of Mn, Cu, Zn, Cd and Pb has been studied. Solutions containing varying quantities (10–2000 ng absolute) of these salts have been added to four different amounts of analyte to study their suitability as modifier and their mass dependent influence. The best sensitivity enhancement for all elements tested was achieved with IrCl₃ and PdCl₂. From a comparison of the effect of PdCl₂ vs. Pd(NO₃)₂ it could be concluded that the mechanism of matrix modification also depends on the chemical form of the modifier. Particularly, for the volatile elements Cd and Zn differences in the behavior of the different chemical compounds of one metal (e.g. Pd) is evident, which shows that the enhancement effect is a result of the stabilization of the analytes in the graphite tube prior to vaporization and the improvement of the transport efficiency after vaporization.     

Vaporization of Pb,As and Ga alone and in the presence of Pd modifier studied by electrothermal vaporization-inductively coupled mass spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) with electrothermal vaporization (ETV) was used to study the processes taking place in a graphite furnace for atomic spectroscopy. Monitoring of carbon release during the pyrolysis stage provided information on the solid-state reduction processes. Among three carbon species studied (¹²C, ¹³C and ²⁸CO), ¹³C was found to be the most suitable. Gallium and arsenic oxides practically do not reduce during the pyrolysis stage. According to the data on carbon release, two reduction processes of lead species (at about 690–915 and > 1000 °C) were found to take place in the furnace. Two separate peaks of Pb (at the end of the pyrolysis stage and in the vaporization stage) were observed, probably related to vaporization of unreduced lead oxide and elemental Pb, respectively. A pre-reduced palladium modifier suppresses the low-temperature Pb losses so that the high-temperature Pb peak is increased. In the absence of modifier, a gaussian-shaped As signal was formed only if small arsenic masses were introduced into the vaporizer. Increase of the arsenic mass resulted in formation of a pronounced plateau after the peak, in spite of the very high vaporization temperature applied (2500 °C). In the presence of pre-reduced palladium modifier, a gaussian-shaped As signal was formed already at 1700 °C with a 4–15-fold increase in sensitivity depending on the analyte mass. The palladium modifier apparently prevents strong interaction of arsenic with graphite. The obtained data support very high potential of ETV-ICP-MS for detailed investigation of processes occurring in graphite furnaces used in analytical atomic spectroscopy, especially during the pyrolysis stage.     

Evaluation of tungsten coil electrothermal vaporization-Ar/H2 flame atomic fluorescence spectrometry for determination of eight traditional hydride-forming elements and cadmium without chemical vapor generation

A tungsten coil electrothermal vaporizer (W-coil ETV) was coupled to an Ar/H(2) flame atomic fluorescence spectrometer for the determination of eight traditional hydride-forming elements (i.e., As, Bi, Ge, Pb, Sb, Se, Sn, and Te) as well as cadmium without chemical vapor generation. A small sample volume, typically 20muL, was manually pipetted onto the W-coil and followed by a fixed electric heating program. During the vaporization step, analyte was vaporized off the coil surface and swept into the quartz tube atomizer of AFS for further atomization and excitation of atomic fluorescence by a flow of Ar/H(2) gas, which was ignited to produce the Ar/H(2) flame. The tungsten coil electrothermal vaporizer and Ar/H(2) flame formed a tandem atomizer to produce reliable atomic fluorescence signals. Under the optimal instrumental conditions, limits of detection (LODs) were found to be better than those by flame atomic absorption spectrometry (FAAS) or inductively coupled plasma optical emission spectrometry (ICP-OES) for all the nine elements investigated. The absolute LODs are better or equivalent to those by hydride generation atomic fluorescence spectrometry (HG-AFS). Possible scattering interferences were studied and preliminary application of the proposed method was also reported.     

Transport efficiencies and analytical determinations with electrothermal vaporization employing electrostatic precipitation and electrothermal atomic spectroscopy

Analyte transport efficiencies of solid as well as liquid samples were determined for electrothermal vaporization (ETV). A graphite furnace of the boat-in-tube type was employed for ETV. The generated aerosol was transported by an argon flow via a tubing into an external precipitator and deposited on a L’vov platform with a corona-like discharge. The sample on the secondary platform was analysed with a laboratory-constructed coherent forward scattering multielement spectrometer. For determining the analyte transport efficiencies, the comparative measurements were carried out with standard solutions dosed directly on the platform in the spectrometer furnace. The simultaneously investigated elements were Cu, Fe and Mn in the standard reference material BCR CRM 189 wholemeal flour and in a multielement standard solution containing approximately the same element ratio as certified for the solid sample. ETV boat-to-L’vov platform transport efficiencies of approximately 19% for Cu, 24% for Fe and 19% for Mn were calculated for both solid samples and multielement standard solutions. Cu, Fe and Mn in wholemeal flour were determined simultaneously by calibrating against aqueous multielement standard solutions injected into the boat as well as by the standard addition method. The results agree satisfactorily, the deviations from the certified values are below 10% and the relative standard deviations are typically 5–8%. The limits of detection are 250 pg for Cu (λ=324.8 nm), 230 pg for Fe (λ=248.3 nm) and 90 pg for Mn (λ=279.8 nm), loaded into the ETV boat.     

Determination of refractory elements in atmospheric particulates using slurry sampling electrothermal vaporization inductively coupled plasma optical emission spectrometry and inductively coupled plasma mass spectrometry with polyvinylidene fluoride as chemical modifier

Electrothermal vaporization (ETV) inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) with polyvinylidene fluoride (PVDF) as chemical modifier are critically compared for the determination of refractory elements in coal fly ash and airborne particulates. The atmospheric particulates that collected on a PVDF filter were introduced into the graphite furnace in the form of a slurry by dissolving the filter in dimethylformamide, and the dissolved filter PVDF, along with additional added PVDF powder, was used as a chemical modifier for subsequent ETV-ICP-OES and ETV-ICP-MS determination. The vaporization behaviors of analytes (Ti, Zr, V, Mo, Cr, La) in ETV-ICP-OES/MS were studied in detail, and the optimal ETV operating parameters were obtained. Under the optimized operating conditions, the detection limits of target elements were 0.08-2.7 ng m(-3) for ETV-ICP-OES and 0.5-50 pg m(-3) for ETV-ICP-MS, respectively, with analytical precisions of 3.5-7.3% for ETV-ICP-OES and 3.9-9.6% for ETV-ICP-MS, respectively. The tolerable amounts of matrix elements for ETV-ICP-OES are higher than for ETV-ICP-MS. Both ETV-ICP-OES and ETV-ICP-MS were used to directly determine the trace refractory elements in coal fly ash and airborne particulates and the analytical results are comparable.     

Determination of some refractory elements and Pb by fluorination assisted electrothermal vaporization inductively coupled plasma mass spectrometry with platform and wall vaporization

Platform and wall vaporization for electrothermal vaporization (ETV)-inductively coupled plasma mass spectrometry (ICP-MS) determination of some refractory elements (Ti, V, Cr, Mo, La and Zr) and Pb were comparatively studied with the use of poly (tetrafluoroethylene) (PTFE) as fluorinating reagent. The factors affecting the vaporization behaviors of the target analytes in the platform and tube wall vaporization including vaporization temperature and time, pyrolytic temperature and time were studied in detail, and the flow rates of carrier gas/auxiliary carrier gas, were carefully optimized. Under the optimal conditions, the signal profiles, signal intensity, interferences of coexisting ions and analytical reproducibility for wall and platform vaporization ETV-ICP-MS were compared. It was found that both wall and platform vaporization could give very similar detection limits, but the platform vaporization provided higher signal intensity and better precision for some refractory elements and Pb than the wall vaporization. Especially for La, the signal intensity obtained by platform vaporization was 3 times higher than that obtained by wall vaporization. For platform vaporization ETV-ICP-MS, the limits of detection (LODs) of 0.001 μg L⁻¹ (La) ~ 0.09 μg L^− 1 (Ti) with the relative standard deviations (RSDs) of 1.5% (Pb) ~ 15.5% (Zr) were obtained. While for wall vaporization ETV-ICP-MS, LODs of 0.005 μg L^− 1 (La) ~ 0.4 μg L^− 1 (Pb) with RSDs of 3.2% (Mo) ~ 12.8% (Zr) were obtained. Both platform and tube wall vaporization techniques have been used for slurry sampling fluorination assisted ETV-ICP-MS direct determination of Ti, V, Cr, Mo, La, Zr and Pb in certified reference materials of NIES No. 8 vehicle exhaust particulates and GBW07401 soil, and the analytical results obtained are in good agreement with the certified values.     

Development of an electrothermal vaporization ICP–MS method and assessment of its applicability to studies of the homogeneity of reference materials

A method has been developed for measurement of the homogeneity of analyte distribution in powdered materials by use of electrothermal vaporization with inductively coupled plasma mass spectrometric (ETV–ICP–MS) detection. The method enabled the simultaneous determination of As, Cd, Cu, Fe, Mn, Pb, and Zn in milligram amounts of samples of biological origin. The optimized conditions comprised a high plasma power of 1500 W, reduced aerosol transport flow, and heating ramps below 300?°C s^–1. A temperature ramp to 550?°C ensured effective pyrolysis of approximately 70% of the organic compounds without losses of analyte. An additional hold stage at 700?°C led to separation of most of the analyte signals from the evaporation of carbonaceous matrix compounds. The effect of time resolution of signal acquisition on the precision of the ETV measurements was investigated. An increase in the number of masses monitored up to 20 is possible with not more than 1% additional relative standard deviation of results caused by limited temporal resolution of the transient signals. Recording of signals from the nebulization of aqueous standards in each sample run enabled correction for drift of the sensitivity of the ETV–ICP–MS instrument. The applicability of the developed method to homogeneity studies was assessed by use of four certified reference materials. According to the best repeatability observed in these sample runs, the maximum contribution of the method to the standard deviation is approximately 5% to 6% for all the elements investigated.