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Potential of Solid Sampling Electrothermal Vaporization for solving spectral interference in Inductively Coupled Plasma Optical Emission Spectrometry
Spectrochimica Acta Part B 64 (2009) 363–368
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Potential of Solid Sampling Electrothermal Vaporization for solving spectral interference in Inductively Coupled Plasma Optical Emission Spectrometry Alemayehu Asfaw ⁎, Grethe Wibetoe Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
a r t i c l e
i n f o
Article history: Received 12 August 2008 Accepted 6 April 2009 Available online 15 April 2009 Keywords: Spectral interference BEC ICP-OES ETV Direct analysis of soil
a b s t r a c t Spectral interference is one of the main causes of erroneous results in Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). This paper describes some cases of spectral interferences with conventional nebulization ICP-OES and the potential of solving them utilizing electrothermal vaporization for volatility-based separation. The cases studied were, the well-known spectral overlap between the As and Cd lines at 228.8 nm that are only 10 pm apart, and the interference of Fe on the main emission lines of As, Cd and Pb. The spectral interferences were studied by monitoring the typical signals of solutions that contain the analytes and the potential interferent, by studying the spectra and calculating Background Equivalent Concentration (BEC)-values. A three step temperature program was developed to be used for direct analysis of solid soil samples by Electrothermal Vaporization (ETV)-ICP-OES: step 1 (760 °C, 40 s), step 2 (1620 °C, 20 s) and a cleaning step (2250 °C, 10 s) where Cd vaporizes in step 1, As, Pb and part of Fe in step 2 and the major part of Fe in the cleaning step. Because As and Cd were time-separated using this program, their prominent lines at 228.8 nm, could be used for determination of each element by ETV-ICP-OES, in spite of the serious wavelength overlap. Selective vaporization was also shown to reduce or eliminate the Fe background emission on As, Cd and Pb lines. To confirm the applicability of the method, a solid soil certified reference materials was analyzed directly without any sample treatment. Good or reasonable accuracy was obtained for the three elements. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Inductively Coupled Plasma Optical Emission Spectrometry (ICPOES) is one of the most important techniques for multi-elements determination; the technique has the potential to determine approximately 70 elements in various types of samples. However, due to the complex spectra generated from the high temperature plasma, spectral interferences are often inevitable. Spectral interference in ICP-OES originates from plasma and sample components that emit radiation at or in the vicinity of the analytical line, giving continuum or structured background emission or a single line overlap with the analyte line. Even with the most skillful selection of lines to be used for measurements, it is often not possible to find analytical lines without contribution from other elements, and the interference needs to be corrected for. Various background correction methods are described in the literature, e.g. in a monograph [1]. In a recent paper [2] some newer mathematical correction methods are reviewed. Even when the background could be corrected for, its presence is a disadvantage since it leads to increased Limit of Detection (LOD).
⁎ Corresponding author. Present Address: Department of chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6. Tel.: +1 613 533 6000x75375; fax: +1 613 533 6669. E-mail address: [email protected] (A. Asfaw). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.04.002
Another way to deal with these interferences is to separate the analyte from the interfering matrix component(s). It could preferably be done in-situ by using an appropriate introduction system like the well-known and often used hydride generation (HG) and cold-vapor (CV) techniques. Unfortunately, these techniques can only be used for elements that form volatile hydrides or elemental vapors (mainly, As, Bi, Ge, Pb, Se, Sb, Sn, Te and Hg). Another alternative is to use an electrothermal vaporization (ETV) system coupled to the ICP-OES to separate the analytes from the potential interfering matrix component by selective volatilization. If the analyte and matrix components have similar volatility, a chemical modifier could be used to increase the difference in volatility between them. The use of ETV for sample introduction for ICP-OES or ICP-MS, instead of the conventional nebulizer, can be advantageous for several reasons in addition to the possibility of selective volatilization and insitu removal of the matrix components. It allows the analysis of liquid, slurry and solid samples; small amount of sample is sufficient for analysis; and it improves sample transport efficiency and has the potential to provide detection limits superior to those of pneumatic nebulization. In a review concerning recent progress in ETV-ICP-OES and ICP-MS by Hu et al. [3] it is stated that ETV-ICP-OES/MS is considered as one of the most versatile methods with respect to its accommodation of different types of samples with complex matrices, especially solid samples.
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Most of the work reported so far utilizing ETV for selective volatilization has been for separation of molecular interference and interfering isobars using ICP-MS [4–8]. The application for ETV-ICPOES is scarcer. However, this technique was used for selective volatilization and improved transport efficiency for As, Cd, Hg, Pb, Se and Zn using sodium thiosulfate as a chemical modifier [9]. In a more recent work, trace concentration of Pd was determined from a Pt matrix using ETV-ICP-OES where the elements were in-situ separated based on their different vaporization temperature [10]. In several of the cases referred to a chemical modifier was used to alter the volatility of the analytes or matrix components. In a paper entitled “The ETV as a thermochemical reactor for ICP-MS sample introduction” by Sturgeon and Lam [11] the use of modifiers to alter the volatility of analytes/matrix and examples of applications for ICP-OES and ICP-MS are reviewed. In the present work, some cases of spectral interferences in ICPOES were studied; i.e. the spectral overlap between the As and Cd lines at 228.8 nm that is only 10 pm apart, and also the spectral interference of Fe on the emission lines of As, Cd and Pb. Owing to the spectral overlap of the prominent lines of As 228.802 nm and Cd 228.812 nm, the lines should not be used in trace analysis of Cd and As [12]. Iron is one of the line-rich elements in ICP-OES and different types of spectral interference from iron occur on the emission lines of many elements including As, Cd, and Pb [12], causing difficulties in the determination of trace amount of these analytes in matrices that contain high concentration of iron. In our study, the potential of selective vaporization with electrothermal vaporization as an alternative mean for solving spectral interference in ICP-OES was investigated. In addition, solid sampling was utilized, i.e. the sample and standard were introduced into the ETV without any sample pretreatment. A soil certified reference material was used to prepare the calibration curves for the analysis of another certified reference soil sample. 2. Experimental 2.1. Equipment and apparatus The instrument used was Varian Vista AX CCD Simultaneous Axial view ICP-OES (Varian Ltd, Australia). For the introduction of the liquid samples using pneumatic nebulization (neb-ICP-OES), a V-groove nebulizer with Sturman–Masters spray chamber was used. For analysis of solid samples, an electrothermal vaporization unit, ETV-4000 from Spectral Systems (Fürstenfeldbruck, Germany) equipped with an autosampler AWD-10 for introduction of the graphite boats containing the samples, was coupled to the ICP-OES with a 1 m long teflon tubing between the outlet of the ETV and the torch inlet. The details of ETV-4000 have been described in reference [13]. A dryer tool, T/IR from the same company was used to dry the samples before analysis. The dryer consists of a thermostated heat plate where the graphite boats with samples are placed and an IR-heater that irradiate the sample from above. This system with heating from below and above ensures a smooth removal of moisture from the sample. The vaporizer enables a rapid analysis of solid samples without need for any further sample pretreatment. The experimental parameters used for neb-ICP-OES and for solid sampling ETV-ICP-OES are given in Table 1. 2.2. Reagents and samples Multi-elements standards containing As, Cd and Pb were prepared from 1000 µg mL− 1 single element standard stock solutions (Teknolab, Kolbotn, Norway). Iron solution of 10 mg mL− 1 was prepared by adding 1.00 g of pure Fe metal and 20 ml of conc. HNO3 to a beaker and heating it on a plate; after dissolving, the solution was diluted to 100 mL. The concentrated acid used was 65% HNO3 (1.39 kg/L) of pro-analysis quality (Merck, Darmstadt, Germany).The
Table 1 Instrumental condition used for neb-ICP-OES and solid sampling-ETV-ICP-OES. ICP-OES Instrument RF (kW) Plasma Ar flow (L min− 1) Auxiliary Ar flow (L min− 1) Nebulizer Ar flow (L min− 1) Instrument stability delay (s) Signal scan mode Scan duration (s) Replicate reading time (s) Element and Analytical lines (nm)
Varian Vista AX CCD simultaneous axial view 1.0a 15.0a 1.5a 0.9b, 0c 15b, 0c Quantitativeb, Transientc 100c 10b As (188.980), As (193.696), As (228.812), Cd (214.439), Cd (226.502), Cd (228.802), Pb (220.353), Pb (283.305), Pb (217.000)
ETVc Vaporizer Transport gas flow rates/mL min− 1 Carrier gas (Argon 1) Bypass gas (Argon 2) Reaction gas (5% Freon-23)
ETV-4000 450 150 1.9
ETV-heating program: Steps
T (°C)
Ramp time (s)
Hold time (s)
Step 1 Step 2 Step 3
760 1620 2250
0 0 0
40 20 10
a b c
Both. neb-ICP-OES. Solid sampling ETV-ICP-OES.
purge gas, argon of purity 99.99%, and reaction gas, 5.0% (mol/mol) Freon-23(CHF3) in argon, were from AGA (Oslo, Norway). The water used was purified (18 MΩ cm) using Millipore Elix-5/MilliQ water purification system (Millipore, Bedford, USA). The certified reference materials (CRMs) used were GBW7411 Chinese soil (China National Analysis Center for Iron and Steel, Beijing, China) and SRM2710 Montana soil (National Institute of Standards and Technology (NIST), Gaithersburg, USA). 2.3. Samples for the study of spectral interferences Two sets of liquid samples were used for the study of the spectral interference: 1) solutions that contain 5 µg mL− 1 of the analytes (As, Cd and Pb) and 0, 1.0, 2.0 or 3.0 mg mL− 1 of Fe; 2) solutions that contain 0.1, 1.0 and 5.0 µg mL− 1 of the analytes and 2.0 mg mL− 1 of Fe. The concentrations were selected based on the natural abundance of the analytes and the interferent (Fe), in the environmental samples, i.e. the concentration ratio of Fe to that of each analyte (As, Cd and Pb) ranges from about 10–2000 in environmental reference materials such as Chinese Soil (GBW7411), Montana soil (SRM2710) and Urban particulate matter (SRM 1648). In some other environmental CRM samples such as San Joaquin soil, the concentration ratio is as high as 9.104 for Fe/Cd. 2.4. Preparation of solid calibration standards Calibration curves for As, Cd, and Pb were prepared using Montana soil (SRM2710) which has the following certified concentrations and uncertainties (±95%) of the analytes: As (626 ± 38 µg/g), Cd (21.8 ± 0.2 µg/g), and Pb (5532 ± 80 µg/g). Four calibration standards (1–5 mg of the Montana soil) plus a blank (i.e. an empty ETV boat) were used for making the calibration curves, which were based on peak areas versus the absolute mass of the analyte in µg. Calculated from the certified values, the absolute mass ranges for the analytes in the calibration curve were: As (0.626–3.130 µg), Cd (0.0218– 0.109 µg) and Pb (5.532–27.66 µg). Three replicates of Chinese soil
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were analyzed as sample using appropriate sample amount (mass 3– 5 mg) to ensure that the absolute amounts of the analytes were within the range of the calibration curves. The concentration of iron in the Chinese soil (5.57% Fe) is higher than in the Montana soil (3.38% Fe), which helps to investigate the potential of the method in solving interference effect.
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cell the evaporated analytes and the gas mixture were mixed with the bypass gas (Argon 2). The bypass gas flows from outside of the evaporating cell and leads to a fast cooling of sample vapors, which is known to support the formation of nuclei in ETV-ICP procedures, as discussed in reference [14].
2.5. Sample introduction
2.6. Analysis of the data and calculating of Background Equivalent Concentration (BEC) values
For neb-ICP-OES, sample propulsion was done by the instrument's peristaltic pump using a sample uptake rate of 2.0 mL min− 1, through the nebulizer. For solid sampling ETV-ICP-OES, a small amount of SRM sample were weighed into graphite boats and dried using the T/IR dryer for about 30 s (temperature just above 100 °C). The sample boat was inserted into the ETV-furnace by the auto-sampler. Time synchronization between the ETV furnace system and the ICP spectrometer was performed with the aid of the instrument's PC through an electrical circuit, where the start of the ETV program triggers the ICP-OES transient signal acquisition. Based on the set temperature program, the analytes were vaporized from the ETV and transported into the plasma with the carrier gas flow (Argon 1) in combination with halogen containing reaction gas (i.e. 5.0% (mol/mol) CHF3 in argon). After leaving the evaporation
Spectra (as shown in Fig. 1) for neb-ICP-OES were obtained from the instrument software (Vista PRO version v 4.0). For solid sampling ETV-ICP-OES, the transient signal from the instrument was integrated using the home made excel macro. The macro was made to selectively integrate the signal over a set time interval. Background correction of the transient signals was not performed. ETV-ICP-OES transient signals obtained from the instrument transient mode (as shown in Figs. 2–4) were used for the signal comparison. Background Equivalent Concentration (BEC) is defined as the analyte concentration that produces a net signal (peak height) equal to the background [15]. BEC values were calculated for all the selected emission lines after measurements (using neb-ICP-OES) of solutions containing 5 µg mL− 1 of As, Cd and Pb in the presence of 0, 1.0, 2.0, and 3.0 mg mL− 1 of Fe. It was calculated using the formula:
Fig. 1. ICP-OES spectra at prominent emission lines of As, Cd and Pb in the presence and absence of Fe. Concentration of solutions measured: for As and Pb lines: a) 5 µg mL− 1 of As, Cd and Pb; b) same solution as in a) plus 2 mg mL− 1 of Fe; for Cd lines: a) 1 µg mL− 1 of Cd, As and Pb); b) same solution as in a) plus 2 mg mL− 1 of Fe; experimental condition given in Table 1 was used. The dotted lines are for the background spectra.
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Fig. 4. Typical solid sampling ETV-ICP-OES transient signal of Fe 261.382 line for 5 mg of Montana soil and the corresponding temperature program.
Fig. 2. Typical solid sampling ETV-ICP-OES transient signal for 5 mg of Montana soil at As 228.812 nm and As 188.980 nm lines; temperature program shown in Table 1 was used. IB BEC = Ia − IB Ca , where IB is the background intensity, Ia is the analyte signal intensity and Ca is the concentration of the analyte.
3. Results and discussion 3.1. Study of spectral interference in pneumatic nebulization ICP-OES Due to the proximity of the emission lines of As and Cd at 228.8 nm, with a wavelength gap of only 10 pm, these lines are not resolved with most spectrometers. The spectra obtained for a solution containing both As and Cd, were nearly the same regarding position and sensitivity, for the solution measured at the two respective
Fig. 3. Typical solid sampling ETV-ICP-OES transient signal for 5 mg of Montana soil measured at 283.305 nm and 220.353 nm Pb lines; temperature program shown in Table 1 was used.
emission lines. Thus, the 228.8 nm lines could not be used for determination of As and Cd when both elements are present. However, the degree of spectral interference depends on the relative concentration of the two elements, and the interference on As from Cd is more severe than ad vice versa, since the 228.802 nm Cd line is more sensitive than the 228.812 nm As line. The extent of the interference of Fe on the analytical lines of As, Cd and Pb was studied by observing the spectra at various prominent lines of the analytes with and without Fe, and in addition by calculating background corrected signal to background ratio (SBR= (Signal–Background)/Background) and BEC values for different concentrations of Fe. Fig. 1 shows typical ICP-OES spectra at different prominent emission lines of the analytes in the presence and absence of 2 mg mL− 1 of Fe. Data for SBR and BEC values are given in Table 2. The data for As 228.802 nm and Cd 228.812 nm are the same in the presence and absence of Fe (Table 2, Fig. 1); that is because the data are not corrected for the mutual spectral interference of As and Cd at the 228.8 nm lines (the solutions measured contains both As and Cd). This confirms again the spectral overlap between these two lines. The BEC is a useful term in ICP-OES as it provides a quantitative measure of the background signal for a matrix at a specified wavelength. The significant importance of BEC is also its relation to the limit of detection; LOD is defined as 3 RSDB BEC/100 where RSDB is the relative standard deviation of the blank given in % [15]. The results show that all the lines studied are influenced by the presence of iron. The background signal increases as seen from the spectra (Fig. 1) and from the increased BEC values when the sample contains 2 mg mL− 1 of Fe (Table 2). Other experiments showed that the BEC values for solutions that contain 5 µg mL− 1 of the analytes (As, Cd and Pb) increased with increasing Fe concentration (0, 1.0, 2.0 or 3.0 mg mL− 1 of Fe) for all the emission lines of the analytes. The BEC values were significantly higher at all the three emission lines of Pb followed by As 193.696 nm and As 188.980 nm lines; the values at emission lines of Cd were relatively lower compared to the others. In most of the cases the background increases observed in the presence of Fe could be explained by a side wing background interference, since most of the Fe lines in the vicinity of the analytical lines, found in the literature and/or observed, are positioned more than 30 pm apart from all the analytical lines (Table 2). In addition, in some cases the Fe lines are positioned less than 10 pm apart from the analytical lines that result in more severe spectral overlapping. For As, all the three emission lines are affected by Fe background. For the As 193.696 nm and 228.812 nm lines the background is nearly constant in the vicinity of the analytical lines (see Fig. 1) and it could therefore probably be corrected for by a simple off-line background correction. However, the use of As 228.812 nm line is hindered by the spectral interference of Cd.
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Table 2 Comparison of signal and background intensities, SBR and the corresponding BEC for prominent emission lines of As, Cd and Pb in the absence and presence of Fe. Intensity (counts per s) Emission Linesa (nm)
Fe (mg mL− 1)
Signal
Background
SBRc
BEC (µg mL− 1)
Interfering line (nm)d, e
As 188.980
0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0
155 221 162 238 7789 7258 7789 7258 4393 4285 4290 3685 424 1382 162 550 352 871
8 75 6 122 123 992 123 992 79 980 69 310 119 1016 33 387 45 645
18 2 26 1 62 6 62 6 55 3.4 61 11 3 0.4 4 0.4 7 0.4
0.3 2.6 0.2 5.3 0.08 0.8 0.08 0.8 0.09 1.5 0.08 0.5 2.0 14 1.3 12 0.7 14
188.970 (− 10)f, g
As 193.696 As 228.812b Cd 228.802b Cd 226.502 Cd 214.439 Pb 283.305 Pb 217.000 Pb 220.353 a b c d e f g
Fe193.672 (− 25)f, g Fe 228.725 (− 87); Fe 228.868 (+56); Fe 228.763 (− 49)g, Cd 228.802 (− 10) Fe 228.725 (− 77); Fe 228.868 (+66); Fe 228.763 (− 39)g, As 228.812(+ 10) Fe 226.559(+ 57); Fe 226.459(− 43) Fe 226.439 (− 63); Fe 226.465(− 37)f,g Fe 214.519(+ 80); Fe 214.445 (+ 6) Fe 214.390 (− 49) Fe 283.340 (+ 35); Fe 283.310 (+ 5) Fe 283.244 (− 61); Fe 283.258 (−47)f,g Fe 216.955 (− 5); Fe 217.054 (+ 55) Fe 220.408 (+ 55); Fe 220.346 (− 7)
The concentration of As, Cd and Pb is 5 µg mL− 1. The data for As using the 228.802 nm and Cd 228.812 nm are not corrected for the mutual spectral interference of As and Cd. SBR = (background corrected Signal to Background Ratio, i.e. Signal–Background/Background). From Ref [12] unless stated otherwise. The values in parenthesis are Δλ in pm, i.e. the interfering line minus the analytical line. Not in Ref [12]. Observed lines in Fig. 1.
All the three emission lines of Cd are probably affected by wing background interference of Fe, as several Fe lines are situated in the vicinity of the analytical lines. For the 214.439 nm Cd line there is also an Fe line only 6 pm apart, that can lead to severe interferences in the presence of high amounts of Fe. The main problem with the 228.802 nm Cd line is the interference from As. For Pb, the interference from Fe was more severe. In addition to wing interference from Fe at all the analyte lines there were some closer Fe lines that could give more severe interference as the Fe lines are positioned only 7 pm or less from the respective Pb lines (see Table 2). The inapplicability of the Pb 283.305 nm line for the determination of Pb in solutions containing higher concentration of Fe was demonstrated: for example, the same spectra were obtained at the Pb 283.305 nm and Fe 283.309 nm for samples that contain only Fe (10 mg mL− 1). The potential interferences of Fe on As, Cd and Pb using neb-ICP-OES were investigated by analyzing aqueous solutions containing 1.0 and 5.0 µg mL− 1 of the analytes with 2 and 1 mg mL− 1 of Fe, respectively (the concentrations ratios of analyte/Fe were either 200 or 2000) using multi-element standard solutions containing only the analytes (no Fe). The As and Cd lines at 228.28 were omitted because of their mutual interference. The best accuracy was obtained for Cd (86–106% recovery), and also for As the results were reasonable, when the Fe/As concentration ratio was 200 (85–91% recovery). The recovery for As became worse when the Fe/As concentration ratio was 2000 (72–134% recovery). The results for Pb were poor. The % recovery of Pb was too high (120–375%) when Pb 283.305 nm and Pb 217.000 nm lines were used, but too low (61–79%) when the Pb 220.353 nm line was used. Both fitted and off-peak background corrections were tried, but the accuracy was not improved. The lower % recoveries can be due to the background overcorrection and the higher % recoveries are probably due to uncorrected direct spectral overlap. 3.2. Use of selective volatilization with ETV for solving spectral interference The possibility to solve the spectral interference by using selective volatilization with ETV for sample introduction was studied utilizing direct introduction of solid samples. The use of aqueous standards for analyses of solid samples has in many cases shown to give accurate
results in Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) but, when ETV is coupled to ICP-OES/ICP-MS more effort is needed to get accurate results if aqueous standards are used. The reason is mainly the difference in transport efficiency (from the outlet of the ETV to the ICP) between the analyte from aqueous solution and from the solid powdered material. A suitable modifier could be added to act as a carrier help to reduce this interference or a reference material of similar composition to the solid sample could be used. The latter approach was chosen in the present case. However, selective vaporization is still needed to solve the spectral interferences as the concentration of interfering elements varies in sample and CRM. One argument often stated against using CRM for calibration is that CRMs are expensive. But, with ETV-ICP only a small amount of reference material is needed for calibration keeping the calibration cost low. 3.2.1. Optimization of temperature programs For the purpose of separately vaporize the analytes and the potential interferent, various temperature programs were tested. Experiences from GFAAS were helpful in designing of the initial heating program. During optimization, a three-step program was used, varying the temperature of the first step from 450 to 760 °C and the second step from 1200 to 1750 °C. The third step was a cleaning step. The optimal program found is given in Table 1; i.e. step 1 (760 °C, 40 s), step 2 (1620 °C, 20 s) and step 3 (2250 °C, 10 s). Typical signals obtained for solid soil samples at different emission lines using this program are shown in Figs. 2–4. As can be seen from the figures, Cd vaporizes in step 1, As and Pb in step 2 and Fe in steps 2 and 3. Because Cd was the only element that vaporized in the first step, the mutual interference of As and Cd can be eliminated by selective vaporization. The effective separation of As and Cd can be seen from Fig. 2. The transient signal at the 228.812 nm As line has two peaks; the first one that appears in heating step 1 must be from Cd as it is not present when measurements are done at the As 188.980 nm line. The second is from As and appears in step 2 at the same position as when measuring at the As 188.980 nm line. Because Cd is also separated from Fe, also the potential interference of this element on Cd can be eliminated.
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Table 3 Results (mean ± SD, n = 3) for the direct analysis of Chinese soil (GBW7411).a,b Analytical line (nm)
Integrationc interval
Results (µg g− 1)
Certified values (mean ± SD) Chinese soil (GBW7411)
Cd 228.802 As 228.812 As 188.980 Pb 283.305 Pb 220.353
Step 1 Step 2 Step 2 Step 2 Step 2
20 ± 3.6 219 ± 39 170 ± 26 0.22 ± 0.07 0.24 ± 0.04
Cd: 28.2 ± 1.3 µg g− 1 As: 205 ± 11 µg g− 1
a b c
Pb: 0.27 ± 0.01% m/m
Montana soil SRM (2710) was used as calibration standard. Preparation of standard is given in Section 2.4. The ETV temperature program is given in Table 1.
As can be seen from Fig. 4, when Montana soil was measured at the 261.382 Fe line, about 25% of the Fe vaporizes in step 2 and about 75% in step 3. For Fe, the signal lasts over the whole step 2 (20 s), while for As and Pb (Figs. 2 and 3) the signals last only for about 10 s or less. Thus, any spectral interference from Fe can be reduced by selectively integrating the signal in the time-range that As and Pb vaporizes. Whether Fe will give a significant interference on the quantitative determination of As and Pb depends on the relative concentrations of the analyte and interferent, the BEC values and the integration time used for the measurements. 3.2.2. Analytical results The result obtained for three replicate of the Chinese soil (GBW7411) is given in Table 3. The experimental condition given in Table 1 was used for the analysis. Solid standards were prepared using the Montana soil SRM. In the program used, the integration time needed to be the same as the step time. Based on the heating step where the analyte vaporizes, the transient signals were integrated in different intervals (step 1 or step 2). For As and Cd the most sensitive lines at 228.812 and 228.802 nm, respectively could be used for analyses. Good or reasonable accuracy was obtained for the three elements as seen in Table 3. With more advanced software for data handling including smoothing of the signal, background correction of transient signals and integration over a smaller time-range, the accuracy of the analysis may be further improved for some of the elements. 4. Conclusion The use of pneumatic nebulization for the determination of trace amount of As, Cd and Pb in samples that contains higher concentration of Fe is affected by spectral interference from Fe; it is also observed that the signals of As and Cd at the As 228.812 nm and Cd 228.802 nm lines are indistinguishable, showing the direct spectral overlap between the lines. Volatility-based separation using solid sampling ETVICP-OES was found to be an alternative means for solving spectral interference in ICP-OES. We believe that the results and the convenience of the method can be further improved with the use of an ICP-OES with an integrated ETV and appropriate software included.
Unfortunately such systems are not commercially available at the moment. The developed method can be extended to other elements and used for liquid as well as solid samples. However, the most important step is careful selection of the temperature program and chemical modifiers, if needed. Use of chemical modifier could change the vaporization temperature of both the analyte and the potential interfering species. Acknowledgements The authors would like to acknowledge Anne-Marie Skramstad for her technical support with the ETV and to Torfinn Fongen at Holger Teknologi (Oslo, Norway) for making the Excel macros needed for integration of the transient signals. References [1] A.T. Zander, in: A. Montaser, D.W. Dolightly (Eds.), Inductively Coupled Plasma in Analytical Atomic Spectroscopy, 2nd edition, VCH Publishers, USA,1992, pp. 329–339. [2] Z. Zhang, X. Ma, Methods for correction of spectral interferences in inductively coupled plasma atomic emission spectrometry, Curr. Top. Anal. Chem. 3 (2002) 105–114. [3] B. Hu, S. Li, G. Xiang, M. He, Z. Jiang, Recent progress in electrothermal vaporization— inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, Appl. Spectrosc. Rev. 42 (2007) 203–234. [4] P.P. Mahoney, S.J. Ray, G. Li, G.M. Hieftje, Preliminary investigation of electrothermal vaporization sample introduction for inductively coupled plasma time-offlight mass spectrometry, Anal. Chem. 71 (1999) 1378–1383. [5] M. Song, T.U. Probst, Rapid determination of technetium-99 by electrothermal vaporization-inductively coupled plasma-mass spectrometry with sodium chlorate and nitric acids as modifiers, Anal. Chim. Acta 413 (2000) 207–215. [6] M. Song, T.U. Probst, N.G. Berryman, Rapid and sensitive determination of radiocesium (Cs-135, Cs-137) in the presence of excess barium by electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) with potassium thiocyanate as modifier, Fresenius J. Anal. Chem. 370 (2001) 744–751. [7] G. Ertas, J.A. Holcombe, Optimization of ETV-ICP(TOF)MS and transient signal profiles for reducing isobaric interferences, J. Anal. At. Spectrom. 20 (2005) 687–695. [8] A. Rowland, T.B. Housh, J.A. Holcombe, Use of electrothermal vaporization for volatility-based separation of Rb–Sr isobars for determination of isotopic ratios by ICP-MS, J. Anal. At. Spectrom. 23 (2008) 167–172. [9] T. Kantor, Optimization of the electrothermal vaporization conditions for inductively coupled plasma excitation spectrometry: selective volatilization versus covolatilization approaches, Fresenius J. Anal. Chem. 355 (1996) 606–614. [10] Y. Wu, Z. Jiang, B. Hu, C. Xiong, Y. Li, In-situ separation and determination of palladium from platinum based on different vaporization temperatures by electrothermal vaporization inductively coupled plasma optical emisssion spectrometry with YPA4 resin acting both as adsorption material and chemical modifier, Microchim. Acta 148 (2004) 279–284. [11] R.E. Sturgeon, J.W. Lam, The ETV as a thermochemical reactor for ICP-MS sample introduction, J. Anal. At. Spectrom. 14 (1999) 785–791. [12] N. Daskalova, Iv Boevski, Spectral interferences in the determination of trace elements in environmental materials by inductively coupled plasma atomic emission spectrometry, Spectrochim. Acta Part B 54 (1999) 1099–1122. [13] J. Hassler, A. Detcheva, O. Förster, P.R. Perzl, K. Florian, Working with a modern ETV-device and an ICP-CID-spectrometer, Ann. Chim. 89 (1999) 827–836. [14] U. Schäffer, V. Krivan, A graphite furnace electrothermal vaporization system for inductively coupled plasma atomic emission spectrometry, Anal. Chem. 70 (1998) 482–490. [15] V. Thomsen, G. Roberts, K. Burgess, The concept of background equivalent concentration in spectrochemistry, Spectroscopy 15 (2000) 33–36.
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Spectrochimica Acta Part B j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s a b
Potential of Solid Sampling Electrothermal Vaporization for solving spectral interference in Inductively Coupled Plasma Optical Emission Spectrometry Alemayehu Asfaw ⁎, Grethe Wibetoe Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, Norway
a r t i c l e
i n f o
Article history: Received 12 August 2008 Accepted 6 April 2009 Available online 15 April 2009 Keywords: Spectral interference BEC ICP-OES ETV Direct analysis of soil
a b s t r a c t Spectral interference is one of the main causes of erroneous results in Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). This paper describes some cases of spectral interferences with conventional nebulization ICP-OES and the potential of solving them utilizing electrothermal vaporization for volatility-based separation. The cases studied were, the well-known spectral overlap between the As and Cd lines at 228.8 nm that are only 10 pm apart, and the interference of Fe on the main emission lines of As, Cd and Pb. The spectral interferences were studied by monitoring the typical signals of solutions that contain the analytes and the potential interferent, by studying the spectra and calculating Background Equivalent Concentration (BEC)-values. A three step temperature program was developed to be used for direct analysis of solid soil samples by Electrothermal Vaporization (ETV)-ICP-OES: step 1 (760 °C, 40 s), step 2 (1620 °C, 20 s) and a cleaning step (2250 °C, 10 s) where Cd vaporizes in step 1, As, Pb and part of Fe in step 2 and the major part of Fe in the cleaning step. Because As and Cd were time-separated using this program, their prominent lines at 228.8 nm, could be used for determination of each element by ETV-ICP-OES, in spite of the serious wavelength overlap. Selective vaporization was also shown to reduce or eliminate the Fe background emission on As, Cd and Pb lines. To confirm the applicability of the method, a solid soil certified reference materials was analyzed directly without any sample treatment. Good or reasonable accuracy was obtained for the three elements. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Inductively Coupled Plasma Optical Emission Spectrometry (ICPOES) is one of the most important techniques for multi-elements determination; the technique has the potential to determine approximately 70 elements in various types of samples. However, due to the complex spectra generated from the high temperature plasma, spectral interferences are often inevitable. Spectral interference in ICP-OES originates from plasma and sample components that emit radiation at or in the vicinity of the analytical line, giving continuum or structured background emission or a single line overlap with the analyte line. Even with the most skillful selection of lines to be used for measurements, it is often not possible to find analytical lines without contribution from other elements, and the interference needs to be corrected for. Various background correction methods are described in the literature, e.g. in a monograph [1]. In a recent paper [2] some newer mathematical correction methods are reviewed. Even when the background could be corrected for, its presence is a disadvantage since it leads to increased Limit of Detection (LOD).
⁎ Corresponding author. Present Address: Department of chemistry, Queen's University, Kingston, Ontario, Canada K7L 3N6. Tel.: +1 613 533 6000x75375; fax: +1 613 533 6669. E-mail address: [email protected] (A. Asfaw). 0584-8547/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2009.04.002
Another way to deal with these interferences is to separate the analyte from the interfering matrix component(s). It could preferably be done in-situ by using an appropriate introduction system like the well-known and often used hydride generation (HG) and cold-vapor (CV) techniques. Unfortunately, these techniques can only be used for elements that form volatile hydrides or elemental vapors (mainly, As, Bi, Ge, Pb, Se, Sb, Sn, Te and Hg). Another alternative is to use an electrothermal vaporization (ETV) system coupled to the ICP-OES to separate the analytes from the potential interfering matrix component by selective volatilization. If the analyte and matrix components have similar volatility, a chemical modifier could be used to increase the difference in volatility between them. The use of ETV for sample introduction for ICP-OES or ICP-MS, instead of the conventional nebulizer, can be advantageous for several reasons in addition to the possibility of selective volatilization and insitu removal of the matrix components. It allows the analysis of liquid, slurry and solid samples; small amount of sample is sufficient for analysis; and it improves sample transport efficiency and has the potential to provide detection limits superior to those of pneumatic nebulization. In a review concerning recent progress in ETV-ICP-OES and ICP-MS by Hu et al. [3] it is stated that ETV-ICP-OES/MS is considered as one of the most versatile methods with respect to its accommodation of different types of samples with complex matrices, especially solid samples.
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Most of the work reported so far utilizing ETV for selective volatilization has been for separation of molecular interference and interfering isobars using ICP-MS [4–8]. The application for ETV-ICPOES is scarcer. However, this technique was used for selective volatilization and improved transport efficiency for As, Cd, Hg, Pb, Se and Zn using sodium thiosulfate as a chemical modifier [9]. In a more recent work, trace concentration of Pd was determined from a Pt matrix using ETV-ICP-OES where the elements were in-situ separated based on their different vaporization temperature [10]. In several of the cases referred to a chemical modifier was used to alter the volatility of the analytes or matrix components. In a paper entitled “The ETV as a thermochemical reactor for ICP-MS sample introduction” by Sturgeon and Lam [11] the use of modifiers to alter the volatility of analytes/matrix and examples of applications for ICP-OES and ICP-MS are reviewed. In the present work, some cases of spectral interferences in ICPOES were studied; i.e. the spectral overlap between the As and Cd lines at 228.8 nm that is only 10 pm apart, and also the spectral interference of Fe on the emission lines of As, Cd and Pb. Owing to the spectral overlap of the prominent lines of As 228.802 nm and Cd 228.812 nm, the lines should not be used in trace analysis of Cd and As [12]. Iron is one of the line-rich elements in ICP-OES and different types of spectral interference from iron occur on the emission lines of many elements including As, Cd, and Pb [12], causing difficulties in the determination of trace amount of these analytes in matrices that contain high concentration of iron. In our study, the potential of selective vaporization with electrothermal vaporization as an alternative mean for solving spectral interference in ICP-OES was investigated. In addition, solid sampling was utilized, i.e. the sample and standard were introduced into the ETV without any sample pretreatment. A soil certified reference material was used to prepare the calibration curves for the analysis of another certified reference soil sample. 2. Experimental 2.1. Equipment and apparatus The instrument used was Varian Vista AX CCD Simultaneous Axial view ICP-OES (Varian Ltd, Australia). For the introduction of the liquid samples using pneumatic nebulization (neb-ICP-OES), a V-groove nebulizer with Sturman–Masters spray chamber was used. For analysis of solid samples, an electrothermal vaporization unit, ETV-4000 from Spectral Systems (Fürstenfeldbruck, Germany) equipped with an autosampler AWD-10 for introduction of the graphite boats containing the samples, was coupled to the ICP-OES with a 1 m long teflon tubing between the outlet of the ETV and the torch inlet. The details of ETV-4000 have been described in reference [13]. A dryer tool, T/IR from the same company was used to dry the samples before analysis. The dryer consists of a thermostated heat plate where the graphite boats with samples are placed and an IR-heater that irradiate the sample from above. This system with heating from below and above ensures a smooth removal of moisture from the sample. The vaporizer enables a rapid analysis of solid samples without need for any further sample pretreatment. The experimental parameters used for neb-ICP-OES and for solid sampling ETV-ICP-OES are given in Table 1. 2.2. Reagents and samples Multi-elements standards containing As, Cd and Pb were prepared from 1000 µg mL− 1 single element standard stock solutions (Teknolab, Kolbotn, Norway). Iron solution of 10 mg mL− 1 was prepared by adding 1.00 g of pure Fe metal and 20 ml of conc. HNO3 to a beaker and heating it on a plate; after dissolving, the solution was diluted to 100 mL. The concentrated acid used was 65% HNO3 (1.39 kg/L) of pro-analysis quality (Merck, Darmstadt, Germany).The
Table 1 Instrumental condition used for neb-ICP-OES and solid sampling-ETV-ICP-OES. ICP-OES Instrument RF (kW) Plasma Ar flow (L min− 1) Auxiliary Ar flow (L min− 1) Nebulizer Ar flow (L min− 1) Instrument stability delay (s) Signal scan mode Scan duration (s) Replicate reading time (s) Element and Analytical lines (nm)
Varian Vista AX CCD simultaneous axial view 1.0a 15.0a 1.5a 0.9b, 0c 15b, 0c Quantitativeb, Transientc 100c 10b As (188.980), As (193.696), As (228.812), Cd (214.439), Cd (226.502), Cd (228.802), Pb (220.353), Pb (283.305), Pb (217.000)
ETVc Vaporizer Transport gas flow rates/mL min− 1 Carrier gas (Argon 1) Bypass gas (Argon 2) Reaction gas (5% Freon-23)
ETV-4000 450 150 1.9
ETV-heating program: Steps
T (°C)
Ramp time (s)
Hold time (s)
Step 1 Step 2 Step 3
760 1620 2250
0 0 0
40 20 10
a b c
Both. neb-ICP-OES. Solid sampling ETV-ICP-OES.
purge gas, argon of purity 99.99%, and reaction gas, 5.0% (mol/mol) Freon-23(CHF3) in argon, were from AGA (Oslo, Norway). The water used was purified (18 MΩ cm) using Millipore Elix-5/MilliQ water purification system (Millipore, Bedford, USA). The certified reference materials (CRMs) used were GBW7411 Chinese soil (China National Analysis Center for Iron and Steel, Beijing, China) and SRM2710 Montana soil (National Institute of Standards and Technology (NIST), Gaithersburg, USA). 2.3. Samples for the study of spectral interferences Two sets of liquid samples were used for the study of the spectral interference: 1) solutions that contain 5 µg mL− 1 of the analytes (As, Cd and Pb) and 0, 1.0, 2.0 or 3.0 mg mL− 1 of Fe; 2) solutions that contain 0.1, 1.0 and 5.0 µg mL− 1 of the analytes and 2.0 mg mL− 1 of Fe. The concentrations were selected based on the natural abundance of the analytes and the interferent (Fe), in the environmental samples, i.e. the concentration ratio of Fe to that of each analyte (As, Cd and Pb) ranges from about 10–2000 in environmental reference materials such as Chinese Soil (GBW7411), Montana soil (SRM2710) and Urban particulate matter (SRM 1648). In some other environmental CRM samples such as San Joaquin soil, the concentration ratio is as high as 9.104 for Fe/Cd. 2.4. Preparation of solid calibration standards Calibration curves for As, Cd, and Pb were prepared using Montana soil (SRM2710) which has the following certified concentrations and uncertainties (±95%) of the analytes: As (626 ± 38 µg/g), Cd (21.8 ± 0.2 µg/g), and Pb (5532 ± 80 µg/g). Four calibration standards (1–5 mg of the Montana soil) plus a blank (i.e. an empty ETV boat) were used for making the calibration curves, which were based on peak areas versus the absolute mass of the analyte in µg. Calculated from the certified values, the absolute mass ranges for the analytes in the calibration curve were: As (0.626–3.130 µg), Cd (0.0218– 0.109 µg) and Pb (5.532–27.66 µg). Three replicates of Chinese soil
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were analyzed as sample using appropriate sample amount (mass 3– 5 mg) to ensure that the absolute amounts of the analytes were within the range of the calibration curves. The concentration of iron in the Chinese soil (5.57% Fe) is higher than in the Montana soil (3.38% Fe), which helps to investigate the potential of the method in solving interference effect.
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cell the evaporated analytes and the gas mixture were mixed with the bypass gas (Argon 2). The bypass gas flows from outside of the evaporating cell and leads to a fast cooling of sample vapors, which is known to support the formation of nuclei in ETV-ICP procedures, as discussed in reference [14].
2.5. Sample introduction
2.6. Analysis of the data and calculating of Background Equivalent Concentration (BEC) values
For neb-ICP-OES, sample propulsion was done by the instrument's peristaltic pump using a sample uptake rate of 2.0 mL min− 1, through the nebulizer. For solid sampling ETV-ICP-OES, a small amount of SRM sample were weighed into graphite boats and dried using the T/IR dryer for about 30 s (temperature just above 100 °C). The sample boat was inserted into the ETV-furnace by the auto-sampler. Time synchronization between the ETV furnace system and the ICP spectrometer was performed with the aid of the instrument's PC through an electrical circuit, where the start of the ETV program triggers the ICP-OES transient signal acquisition. Based on the set temperature program, the analytes were vaporized from the ETV and transported into the plasma with the carrier gas flow (Argon 1) in combination with halogen containing reaction gas (i.e. 5.0% (mol/mol) CHF3 in argon). After leaving the evaporation
Spectra (as shown in Fig. 1) for neb-ICP-OES were obtained from the instrument software (Vista PRO version v 4.0). For solid sampling ETV-ICP-OES, the transient signal from the instrument was integrated using the home made excel macro. The macro was made to selectively integrate the signal over a set time interval. Background correction of the transient signals was not performed. ETV-ICP-OES transient signals obtained from the instrument transient mode (as shown in Figs. 2–4) were used for the signal comparison. Background Equivalent Concentration (BEC) is defined as the analyte concentration that produces a net signal (peak height) equal to the background [15]. BEC values were calculated for all the selected emission lines after measurements (using neb-ICP-OES) of solutions containing 5 µg mL− 1 of As, Cd and Pb in the presence of 0, 1.0, 2.0, and 3.0 mg mL− 1 of Fe. It was calculated using the formula:
Fig. 1. ICP-OES spectra at prominent emission lines of As, Cd and Pb in the presence and absence of Fe. Concentration of solutions measured: for As and Pb lines: a) 5 µg mL− 1 of As, Cd and Pb; b) same solution as in a) plus 2 mg mL− 1 of Fe; for Cd lines: a) 1 µg mL− 1 of Cd, As and Pb); b) same solution as in a) plus 2 mg mL− 1 of Fe; experimental condition given in Table 1 was used. The dotted lines are for the background spectra.
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Fig. 4. Typical solid sampling ETV-ICP-OES transient signal of Fe 261.382 line for 5 mg of Montana soil and the corresponding temperature program.
Fig. 2. Typical solid sampling ETV-ICP-OES transient signal for 5 mg of Montana soil at As 228.812 nm and As 188.980 nm lines; temperature program shown in Table 1 was used. IB BEC = Ia − IB Ca , where IB is the background intensity, Ia is the analyte signal intensity and Ca is the concentration of the analyte.
3. Results and discussion 3.1. Study of spectral interference in pneumatic nebulization ICP-OES Due to the proximity of the emission lines of As and Cd at 228.8 nm, with a wavelength gap of only 10 pm, these lines are not resolved with most spectrometers. The spectra obtained for a solution containing both As and Cd, were nearly the same regarding position and sensitivity, for the solution measured at the two respective
Fig. 3. Typical solid sampling ETV-ICP-OES transient signal for 5 mg of Montana soil measured at 283.305 nm and 220.353 nm Pb lines; temperature program shown in Table 1 was used.
emission lines. Thus, the 228.8 nm lines could not be used for determination of As and Cd when both elements are present. However, the degree of spectral interference depends on the relative concentration of the two elements, and the interference on As from Cd is more severe than ad vice versa, since the 228.802 nm Cd line is more sensitive than the 228.812 nm As line. The extent of the interference of Fe on the analytical lines of As, Cd and Pb was studied by observing the spectra at various prominent lines of the analytes with and without Fe, and in addition by calculating background corrected signal to background ratio (SBR= (Signal–Background)/Background) and BEC values for different concentrations of Fe. Fig. 1 shows typical ICP-OES spectra at different prominent emission lines of the analytes in the presence and absence of 2 mg mL− 1 of Fe. Data for SBR and BEC values are given in Table 2. The data for As 228.802 nm and Cd 228.812 nm are the same in the presence and absence of Fe (Table 2, Fig. 1); that is because the data are not corrected for the mutual spectral interference of As and Cd at the 228.8 nm lines (the solutions measured contains both As and Cd). This confirms again the spectral overlap between these two lines. The BEC is a useful term in ICP-OES as it provides a quantitative measure of the background signal for a matrix at a specified wavelength. The significant importance of BEC is also its relation to the limit of detection; LOD is defined as 3 RSDB BEC/100 where RSDB is the relative standard deviation of the blank given in % [15]. The results show that all the lines studied are influenced by the presence of iron. The background signal increases as seen from the spectra (Fig. 1) and from the increased BEC values when the sample contains 2 mg mL− 1 of Fe (Table 2). Other experiments showed that the BEC values for solutions that contain 5 µg mL− 1 of the analytes (As, Cd and Pb) increased with increasing Fe concentration (0, 1.0, 2.0 or 3.0 mg mL− 1 of Fe) for all the emission lines of the analytes. The BEC values were significantly higher at all the three emission lines of Pb followed by As 193.696 nm and As 188.980 nm lines; the values at emission lines of Cd were relatively lower compared to the others. In most of the cases the background increases observed in the presence of Fe could be explained by a side wing background interference, since most of the Fe lines in the vicinity of the analytical lines, found in the literature and/or observed, are positioned more than 30 pm apart from all the analytical lines (Table 2). In addition, in some cases the Fe lines are positioned less than 10 pm apart from the analytical lines that result in more severe spectral overlapping. For As, all the three emission lines are affected by Fe background. For the As 193.696 nm and 228.812 nm lines the background is nearly constant in the vicinity of the analytical lines (see Fig. 1) and it could therefore probably be corrected for by a simple off-line background correction. However, the use of As 228.812 nm line is hindered by the spectral interference of Cd.
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Table 2 Comparison of signal and background intensities, SBR and the corresponding BEC for prominent emission lines of As, Cd and Pb in the absence and presence of Fe. Intensity (counts per s) Emission Linesa (nm)
Fe (mg mL− 1)
Signal
Background
SBRc
BEC (µg mL− 1)
Interfering line (nm)d, e
As 188.980
0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0 0.0 2.0
155 221 162 238 7789 7258 7789 7258 4393 4285 4290 3685 424 1382 162 550 352 871
8 75 6 122 123 992 123 992 79 980 69 310 119 1016 33 387 45 645
18 2 26 1 62 6 62 6 55 3.4 61 11 3 0.4 4 0.4 7 0.4
0.3 2.6 0.2 5.3 0.08 0.8 0.08 0.8 0.09 1.5 0.08 0.5 2.0 14 1.3 12 0.7 14
188.970 (− 10)f, g
As 193.696 As 228.812b Cd 228.802b Cd 226.502 Cd 214.439 Pb 283.305 Pb 217.000 Pb 220.353 a b c d e f g
Fe193.672 (− 25)f, g Fe 228.725 (− 87); Fe 228.868 (+56); Fe 228.763 (− 49)g, Cd 228.802 (− 10) Fe 228.725 (− 77); Fe 228.868 (+66); Fe 228.763 (− 39)g, As 228.812(+ 10) Fe 226.559(+ 57); Fe 226.459(− 43) Fe 226.439 (− 63); Fe 226.465(− 37)f,g Fe 214.519(+ 80); Fe 214.445 (+ 6) Fe 214.390 (− 49) Fe 283.340 (+ 35); Fe 283.310 (+ 5) Fe 283.244 (− 61); Fe 283.258 (−47)f,g Fe 216.955 (− 5); Fe 217.054 (+ 55) Fe 220.408 (+ 55); Fe 220.346 (− 7)
The concentration of As, Cd and Pb is 5 µg mL− 1. The data for As using the 228.802 nm and Cd 228.812 nm are not corrected for the mutual spectral interference of As and Cd. SBR = (background corrected Signal to Background Ratio, i.e. Signal–Background/Background). From Ref [12] unless stated otherwise. The values in parenthesis are Δλ in pm, i.e. the interfering line minus the analytical line. Not in Ref [12]. Observed lines in Fig. 1.
All the three emission lines of Cd are probably affected by wing background interference of Fe, as several Fe lines are situated in the vicinity of the analytical lines. For the 214.439 nm Cd line there is also an Fe line only 6 pm apart, that can lead to severe interferences in the presence of high amounts of Fe. The main problem with the 228.802 nm Cd line is the interference from As. For Pb, the interference from Fe was more severe. In addition to wing interference from Fe at all the analyte lines there were some closer Fe lines that could give more severe interference as the Fe lines are positioned only 7 pm or less from the respective Pb lines (see Table 2). The inapplicability of the Pb 283.305 nm line for the determination of Pb in solutions containing higher concentration of Fe was demonstrated: for example, the same spectra were obtained at the Pb 283.305 nm and Fe 283.309 nm for samples that contain only Fe (10 mg mL− 1). The potential interferences of Fe on As, Cd and Pb using neb-ICP-OES were investigated by analyzing aqueous solutions containing 1.0 and 5.0 µg mL− 1 of the analytes with 2 and 1 mg mL− 1 of Fe, respectively (the concentrations ratios of analyte/Fe were either 200 or 2000) using multi-element standard solutions containing only the analytes (no Fe). The As and Cd lines at 228.28 were omitted because of their mutual interference. The best accuracy was obtained for Cd (86–106% recovery), and also for As the results were reasonable, when the Fe/As concentration ratio was 200 (85–91% recovery). The recovery for As became worse when the Fe/As concentration ratio was 2000 (72–134% recovery). The results for Pb were poor. The % recovery of Pb was too high (120–375%) when Pb 283.305 nm and Pb 217.000 nm lines were used, but too low (61–79%) when the Pb 220.353 nm line was used. Both fitted and off-peak background corrections were tried, but the accuracy was not improved. The lower % recoveries can be due to the background overcorrection and the higher % recoveries are probably due to uncorrected direct spectral overlap. 3.2. Use of selective volatilization with ETV for solving spectral interference The possibility to solve the spectral interference by using selective volatilization with ETV for sample introduction was studied utilizing direct introduction of solid samples. The use of aqueous standards for analyses of solid samples has in many cases shown to give accurate
results in Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) but, when ETV is coupled to ICP-OES/ICP-MS more effort is needed to get accurate results if aqueous standards are used. The reason is mainly the difference in transport efficiency (from the outlet of the ETV to the ICP) between the analyte from aqueous solution and from the solid powdered material. A suitable modifier could be added to act as a carrier help to reduce this interference or a reference material of similar composition to the solid sample could be used. The latter approach was chosen in the present case. However, selective vaporization is still needed to solve the spectral interferences as the concentration of interfering elements varies in sample and CRM. One argument often stated against using CRM for calibration is that CRMs are expensive. But, with ETV-ICP only a small amount of reference material is needed for calibration keeping the calibration cost low. 3.2.1. Optimization of temperature programs For the purpose of separately vaporize the analytes and the potential interferent, various temperature programs were tested. Experiences from GFAAS were helpful in designing of the initial heating program. During optimization, a three-step program was used, varying the temperature of the first step from 450 to 760 °C and the second step from 1200 to 1750 °C. The third step was a cleaning step. The optimal program found is given in Table 1; i.e. step 1 (760 °C, 40 s), step 2 (1620 °C, 20 s) and step 3 (2250 °C, 10 s). Typical signals obtained for solid soil samples at different emission lines using this program are shown in Figs. 2–4. As can be seen from the figures, Cd vaporizes in step 1, As and Pb in step 2 and Fe in steps 2 and 3. Because Cd was the only element that vaporized in the first step, the mutual interference of As and Cd can be eliminated by selective vaporization. The effective separation of As and Cd can be seen from Fig. 2. The transient signal at the 228.812 nm As line has two peaks; the first one that appears in heating step 1 must be from Cd as it is not present when measurements are done at the As 188.980 nm line. The second is from As and appears in step 2 at the same position as when measuring at the As 188.980 nm line. Because Cd is also separated from Fe, also the potential interference of this element on Cd can be eliminated.
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Table 3 Results (mean ± SD, n = 3) for the direct analysis of Chinese soil (GBW7411).a,b Analytical line (nm)
Integrationc interval
Results (µg g− 1)
Certified values (mean ± SD) Chinese soil (GBW7411)
Cd 228.802 As 228.812 As 188.980 Pb 283.305 Pb 220.353
Step 1 Step 2 Step 2 Step 2 Step 2
20 ± 3.6 219 ± 39 170 ± 26 0.22 ± 0.07 0.24 ± 0.04
Cd: 28.2 ± 1.3 µg g− 1 As: 205 ± 11 µg g− 1
a b c
Pb: 0.27 ± 0.01% m/m
Montana soil SRM (2710) was used as calibration standard. Preparation of standard is given in Section 2.4. The ETV temperature program is given in Table 1.
As can be seen from Fig. 4, when Montana soil was measured at the 261.382 Fe line, about 25% of the Fe vaporizes in step 2 and about 75% in step 3. For Fe, the signal lasts over the whole step 2 (20 s), while for As and Pb (Figs. 2 and 3) the signals last only for about 10 s or less. Thus, any spectral interference from Fe can be reduced by selectively integrating the signal in the time-range that As and Pb vaporizes. Whether Fe will give a significant interference on the quantitative determination of As and Pb depends on the relative concentrations of the analyte and interferent, the BEC values and the integration time used for the measurements. 3.2.2. Analytical results The result obtained for three replicate of the Chinese soil (GBW7411) is given in Table 3. The experimental condition given in Table 1 was used for the analysis. Solid standards were prepared using the Montana soil SRM. In the program used, the integration time needed to be the same as the step time. Based on the heating step where the analyte vaporizes, the transient signals were integrated in different intervals (step 1 or step 2). For As and Cd the most sensitive lines at 228.812 and 228.802 nm, respectively could be used for analyses. Good or reasonable accuracy was obtained for the three elements as seen in Table 3. With more advanced software for data handling including smoothing of the signal, background correction of transient signals and integration over a smaller time-range, the accuracy of the analysis may be further improved for some of the elements. 4. Conclusion The use of pneumatic nebulization for the determination of trace amount of As, Cd and Pb in samples that contains higher concentration of Fe is affected by spectral interference from Fe; it is also observed that the signals of As and Cd at the As 228.812 nm and Cd 228.802 nm lines are indistinguishable, showing the direct spectral overlap between the lines. Volatility-based separation using solid sampling ETVICP-OES was found to be an alternative means for solving spectral interference in ICP-OES. We believe that the results and the convenience of the method can be further improved with the use of an ICP-OES with an integrated ETV and appropriate software included.
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