Anal. Chem. 1996, 68, 991-996
Isotope Dilution Analysis for Flow Injection ICPMS Determination of Microgram per Gram Levels of Boron in Iron and Steel after Matrix Removal Aurora G. Coedo,* Teresa Dorado, Bernardo J. Fernandez, and Francisco J. Alguacil
CENIM (C.S.I.C.) Avenida Gregorio del Amo 8, 28040 Madrid, Spain
Isotope dilution analysis using flow injection inductively coupled plasma mass spectrometry was applied to determine low boron contents in iron and steel samples. Sample dissolution was carried out in a microwave oven with an HCl, HNO3, and H2SO4 acid mixture and highpressure digestion vessels. A solvent extraction procedure using acetylacetone-chloroform was applied for the separation of the iron matrix as the sulfate (pH ) 1.4 ( 0.1). Test solutions corresponding to 50 mg mL-1 of sample, with the total boron content and iron contents below 1 µg mL-1, were obtained. Flow injection was used as a sampling system to minimize problems arising from the total salt concentration of the test solutions. This method made it possible to analyze boron in iron and steel at submicrogram per gram levels. The detection limit (3σ), in micrograms of boron per gram of sample, was 0.02, and the precision for concentrations above 10 times the detection limit was better than 1%. Accuracy was tested with eight standard reference materials with boron certified values in the 0.5-10 µg g-1 range. The concentration of boron in steels is important due to its influence on certain mechanical properties, such as hardenability, hot workability, creep resistance, etc. Boron suppresses the occurrence of the intergranular fracture and consequently improves resistance to secondary working embrittlement.1 Yurioka et al.2 described the derivation of equations to predict the hardness and martensite content of heat affected zones (HAZs) which can be applied to ferritic steels ranging from low carbon pipe steel to C-Mn and low and high alloy grades. Boron and nitrogen were found to influence HAZ hardenability. The increase in hardenability due to boron (∆H) for basic oxygen converter steels (sulfur, 96% m/m, and ammonium hydroxide 25% m/m NH3) were used to prepare the sample solutions. Nitric acid (0.1% v/v) was employed as the carrier solution. Standard reference material (SRM) 951 from the National Institute of Standards and Technology (NIST, Gaithersburg, MD) (10B/11B ratio of 0.2473; 10B ) 19.827%, 11B ) 80.173%) and [10B]boric acid (a Sigma-Aldrich isotopically enriched biisotopic spike, 97 atom % 10B) were used for isotope dilution analysis. The isotopic composition and the B concentration of the spike solution were determined using the SRM 951 to calculate the instrumental mass discrimination factor and to apply the reverse-ID technique. Chloroform (99.9+% HPLC grade) and acetylacetone (2,4-pentanedione) transition-metal chelating agent were from Aldrich. Eight standard reference materials [ECRMs 097-1 and 281-1 (from Bureau of Analysed Samples Ltd., Middlesbrough, England), 283-1 and 285-1 (from Bundesanstalt fu¨r Materialforschung, Berlin, Germany), SRMs 361, 363, and 365 (from NIST) and JSS 002-2 (from The Iron and Steel Institute of Japan, Tokyo), all with boron contents between 0.5 and 10 µg g-1] were used to examine the validity of the method. Test Sample Preparation Procedure. The procedure is summarized in Figure 1. A 0.25 g test portion of sample, weighed to the nearest 0.1 mg, was introduced into a high-pressure vessel, and 1.0 mL of HCl + 0.5 mL of HNO3 + 0.35 mL of H2SO4 and 5 mL of H2O were added. The vessel was then placed on a carousel,
and a one-step microwave program of 360 W lasting 30 min was applied (the heating program was developed for the simultaneous use of three vessels). A proper amount of the spike 10B solution (250 ng of boron) was added, and the solution was evaporated to solid salt formation. The salts were redissolved with 10 mL of H2O, and the solvent extraction procedure was applied for iron separation. The iron-bearing aqueous solution was transferred to a 50 mL extraction vessel. Twenty milliliters of 1:1 chloroform-acetylacetone was used for rinsing and was also transferred to the extraction vessel. Both phases were shaken for a 3 min mixing period at a constant pH of 1.4 ( 0.1, automatically controlled by adding a 1:2 v/v ammonia solution. After phase separation, the organic phase was rejected, and the extraction procedure was repeated on the aqueous phase using 10 mL of the extractant mixture. After separation, the aqueous layer was transferred to a 50 mL Teflon beaker, 1 mL of HNO3 was added, and the solution was evaporated to a volume of about 2 mL. After cooling, the solution was diluted to 5.0 mL with water. The resulting test solution corresponds to a sample proportion of 5 g/100 mL. Blank solutions were prepared according to the same preparation procedure but without iron. As long as iron was not present in the extraction solution, the pH value did not change through the extraction process, and the pHs of the blank solutions were adjusted to 1.4 ( 0.1 before being transferred to the extraction vessel. DISCUSSION AND RESULTS Matrix Interferences. Some of the earlier works carried out with ICPMS clearly showed that the system was not tolerant to solutions containing significant amounts of dissolved solids. Solid deposition on the sampling apertures usually limits the total dissolved solids content of the solutions to 0.1 or 0.2% w/v when conventional nebulization is used; otherwise, deposition of material will occur on the sampler and skimmer cones, and this will cause serious drift problems. The signal loss is not simply a result of a reduction in the number of ions entering the ICPMS system, but more likely is a modification of the ion extraction process. In practice, matrix effects can be difficult to measure and quantify. In general, the lower the atomic mass of the analyte, the greater the effect of the matrix element will be; consequently, the boron signal (mass number 10-11) is heavily decreased. Using a continuous nebulization sampling system, Ekstroem and Gustavsson5 found a suppression effect of about 50%, the concentration of boron being 5 ng mL-1 in the 0.1 g/100 mL iron matrix. Coedo and Dorado,9 using flow injection analysis, found a suppression effect of about 40%, the concentration of boron being 100 ng mL-1 in the 0.5 g/100 mL iron matrix. In addition, high matrix concentrations tend to have a deleterious effect on the precision of the results. A number of methods may be used to overcome this matrix effect: dilution of samples, use of internal standard, instrumental optimization, matrix matching of standards and samples, standard addition calibration, etc. However, the most satisfactory method may be to separate the analyte from the matrix, as, in this case, the matrix may be totally removed with the additional benefit of analyte concentration. Acetylacetone can be used as both a solvent and a reagent for iron extraction; as an illustration of the effectiveness of acetylacetone for trace quantities of iron, the quantitative extraction of 59Fe produced (in less than
spectrographic limits) in neutron-irradiated Co18 may be quoted. Acetylacetone functions as a reagent by first forming a metal chelate with the iron ion. The neutral metal chelate, being more of an organic nature than inorganic, is then extracted by the excess acetylacetone in the organic layer. A 1:1 mixture of chloroform and acetylacetone was chosen as an extraction reagent. This mixture decreased the tendency toward emulsion formation and has the advantage of having a density greater than 1. As the percentage of metal ion extracted is principally a function of the pH and of the oxidation state of the metal ion, tests were carried out to optimize this parameter. Preliminary tests to determine the effect of pH on the extraction of iron(III) as the sulfate were performed. The data produced indicate that 1.3-1.5 is the optimal pH interval; at pH values below 1.3, the iron extraction is not quantitative, and at pH values of more than 1.5, iron hydroxide precipitation takes place. Consequently, the extraction process was carried out at a continuously controlled pH ) 1.4 ( 0.1. Tests were performed varying the shaking time from 1 to 10 min, and 3 min was found to be enough to equilibrate both phases. The reagent-organic phase volume was varied from 10 to 40 mL; 20 mL in the first extraction and 10 mL in the second proved to be a good proportion. Under these operating conditions, iron(III) was extracted >99.8%; this was verified by analyzing the remaining iron in the aqueous layer. In contrast, it was proved that boron was not coextracted. Recovery tests were performed to validate that no loss of boron occurs throughout the test sample preparation processes. For this purpose, iron samples (0.250 g portions of Johnson & Matthey high-purity iron) spiked with boron contents between 25 ng and 2.5 µg were processed as the test samples (using the complete proposed method), and quantitative boron recoveries (>99.9%) were obtained. After iron matrix separation, the source of salt concentration in the test solutions resulted from the H2SO4 added in the acid digestion step and from the NH4OH to adjust the pH. Vanhaecke et al.19 found that the signals from light analytes showed a high suppression in a 0.5 mol L-1 H2SO4 matrix compared with a 0.14 mol L-1 HNO3 matrix. To overcome the problems caused by the salt concentration in the samples after iron extraction, flow injection was used as a microsampling system. Since only discrete volumes are introduced into a carrier stream which is fed into the nebulizer, the instrument is not exposed to massive amounts of dissolved solids; thus, minimal buildup of deposition on the cones occurs, thereby improving the repeatability of the measurements compared with continuous flow. The ratio of FI transient peak height signal to the signal level obtained during continuous aspiration is a measure of the dispersion coefficient (D). In association with the analysis of solutions with high amounts of dissolved solids, this coefficient is an important parameter. The dispersion of the discrete sample in the carrier flow is controlled by the volume injected and the length of tubing between the FI valve and the nebulizer. To achieve the best possible stability and sensitivity and the shortest wash time, the valve has been position as close as possible to the nebulizer. In a previous work,9 some fundamental parameters of FI were optimized for ICPMS boron determination in steel samples. These parameter settings were used in this study. In spite of the fact that the dispersion coefficient (D) approaches unity at injection volumes of >600 µL; (18) Morrison, G. H.; Freiser, H. Solvent Extraction in Analytical Chemistry; John Wiley & Sons, Inc.: New York, 1957; pp 157-159. (19) Vanhaecke, F.; Vanhoe, H.; Vandecasteele, C.; Dams, R. Anal. Chim. Acta 1991, 244, 115-22.
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Figure 2. Effect of salt concentration. Signal depression of a 50 ng mL-1 boron solution.
the injection volume selected for the sample loop was 250 µL, a D value of ∼0.80 being achieved. The aim was to minimize the deposition of salts on the interface while obtaining a good level of sensitivity. Signal depression caused by the presence of H2SO4 and NH4OH resulting from the iron extraction process was still observed during microsampling. The boron signal depressions caused by different concentrations of these two reagents were estimated. Measurements were performed with 50 ng mL-1 boron solutions in 0.25, 0.50, 1.0, 1.5, and 2.0 mol L-1 (NH4)2SO4. The results of peak area measurements, expressed as relative signal, are shown in Figure 2. From the 0.35 mL of sulfuric acid added for sample dissolution, 1.2 mol L-1 is the maximum (NH4)2SO4 concentration present in the test solutions, which produces a signal depression of about 30%. Isotope Dilution Analysis. In any case, although FI avoids the potential instabilities in aspirating high salt concentration solutions, systematic errors may result if the salt concentrations of sample and standard solutions are not exactly adjusted. To overcome this problem, the ID technique was applied. The principle of the method is that, by altering the natural ratio between two isotopes in the sample, adding an accurately known quantity of an isotopic spike, and measuring the ratio of the mixture, the concentration (C) of analyte present in the original sample can be deduced according to the general equation
C)
KMs(As - RBs) W(RB - A)
(1)
where K is the ratio of the natural atomic weight and the atomic weight of the enriched material, Ms is the mass of the spike, W is the weight of the sample, A and B are the natural isotopic abundances of both isotopes, As and Bs are the isotopic abundances of both isotopes in the spike, and R is the measured isotope ratio of the mixture of the sample and the spike. 994
Analytical Chemistry, Vol. 68, No. 6, March 15, 1996
The two major advantages of this method over conventional analysis against a set of standards stem from the fact that ratios rather than absolute sensitivities are measured. First, once the spike has been added to the sample and allowed to come to equilibrium, losses of analyte become unimportant, since the isotopic ratio of the mixture would not be altered. Second, the determination is largely unaffected by changes in instrumental sensitivity and matrix effects since both isotopes should be affected to the same extent, and thus the ratio will remain constant. Boron has two naturally occurring isotopes, 10B (19.78%) and 11B (80.22%), both of which are free from isobaric overlap and polyatomic ion interferences. When measuring boron isotopes (mass difference of 10% between 10B and 11B), a high instrumental mass bias occurs from mass discrimination from concomitant elements.7 Mass discrimination means the discrepancy between the natural isotope ratio and the measured isotope ratio. To correct for the mass discrimination, the blank-corrected isotope ratio of 50 ng mL-1 boron standard solution (NIST SRM 951) was measured. The natural isotope ratio was divided by the measured isotope ratio, and a discrimination factor of 1.087 was found. The isotope ratio measured in the sample analysis was corrected by multiplied by the mass discrimination factor; the boron concentration can then be calculated according to the corrected ratio by applying eq 1. Isotopic Composition and Concentration of B in the Spike. One of the factors contributing most to the accuracy of ID determinations is the uncertainty in the spike concentration. To determine boron in unknowns, first the boron isotopic composition of the enriched spike must be characterized, and then the total boron concentration of the spike solution must be determined. A major drawback in the implementation of ID in routine analysis is the uncertainty in ratio measurement that contributes significantly to the uncertainty in determined concentration. It was established from the literature20 that optimum performance of the ID technique could be achieved over a range of spiked ratios from 0.1 to 10. In this study, the boron spike concentration was determined by the reverse-ID technique with the certified standard sample NIST SRM 951. A series of standards with 10B/11B ratios ranging from 0.4 to 9 were prepared from different concentrations of natural boron (NIST SRM 951) and a constant concentration (50 ng mL-1) of Aldrich enriched B. The B spike concentration was calculated from the measured 10B/11B ratios (corrected for background and fractionation). Table 2 shows the analytical results; the isotopic spike composition found was 96.77% 10B and 3.23% 11B, and the calculated concentration of B, in a nominal 50 ng mL-1 solution, was 49.43 ng mL-1. As it can be seen, the RSD value of the calculated 10B/11B ratios was nearly constant with changes in isotope ratio from 0.4 to 9. Even at solution concentrations in the 55-550 ng mL-1 range, it was possible to measure the above-mentioned ratios with a precision of 0.6 ( 0.15. According to Longerich,15 that means that the optimum value of 10B/11B is given by the geometric mean of the measured isotope ratios of the sample and the tracer; this optimum ratio value [(0.23) × (27.58)1/2] makes the error magnification factor a minimum. Using the reverse-ID technique, other factors, such as the errors in the discrimination factor and in the ratio of the spike, play a minor role, as both parameters are neutralized and become negligible. (20) Thomson, M.; Walsh, J. N. Handbook of Inductively Coupled Plasma Spectrometry; Blackie, Chapman and Hall: New York, 1989; p 249.
Table 2. Analytical Results of the Spike 10B/11B
standard (NIST SRM 951) isotope ratio measured (50 ng mL-1) discrimination factor spike (nominal spike composition) isotope ratio measured (50 ng mL-1) isotope ratio corrected 10B atomic abundance 11B atomic abundance
) 0.2473 0.2275 ( 0.0020 (n ) 6) 1.0870 97% 10B-3% 11B 27.53 ( 0.85 (n ) 6) 29.93 ( 0.85 96.77 ( 0.10% 3.23 ( 0.10%
Concentration of B in the Spike B standarda (ng mL-1) 500.00 250.00 100.00 50.00 25.00 10.00 5.00 av SD 95% C.I.
B spike (ng mL-1) 50 50 50 50 50 50 50
10B/11B
nominalb
ratios calcc (RSD)
calcd B in spike (ng mL-1)
0.366 0.484 0.833 1.396 2.461 5.226 8.776
0.364 (0.73) 0.479 (0.50) 0.827 (0.48) 1.385 (0.62) 2.445 (0.50) 5.184 (0.44) 8.697 (0.65)
49.27 49.37 49.48 49.60 49.60 49.49 49.35
49.43 0.22% 0.54%
a B from the standard: NIST SRM 951. b Nominal B from the spike: Sigma-Aldrich. c Corrected for background and fractionation. RSD reported for n ) 4.
Figure 3. Peak shape of the transient signals for 10B and 11B: (a) blank, (b) 50 ng mL-1 B standard, (c) 50 ng mL-1 B spike, and (d) 100 ng mL-1 B (50 ng mL-1 B standard + 50 ng mL-1 B spike).
Figure 3 shows the 11B and 10B peak shape transient signals from (a) blank solution, (b) 50 ng mL-1 B standard solution (NIST SRM 951), (c) 50 ng mL-1 B spike solution (Sigma-Aldrich), and (d) 100 ng mL-1 B, 1:1 v/v standard + spike solution. Rapid washout of the sample in the FI sampling introduction system is illustrated in this figure, which shows that the boron signal returns to background in about 20 s after having reached the maximum value. In ID, the blank and background corrections are very important, as their difference from both isotopes can seriously influence the ratio. A “procedural” blank was used to correct for
Table 3. Boron Results in Standard Reference Materials (µg g-1)a sample
certified
found (ID-ICPMS)
ref 21 (ICP-OES)
BAS 097-1 BAS 281-1 BAM 283-1 BAM 285-1 JSS 002-2 NIST SRM 361 NIST SRM 365 NIST SRM 363b
3 (1) 12 (2) 3 (1) 6.0 (1.5) 0.5 3.7 1.2 7.8
2.54 (0.05) 10.31 (0.10) 2.05 (0.07) 5.38 (0.10) 0.49 (0.02) 4.78 (0.05) 1.35 (0.02) 14.00 (0.12)
1.93 ( 0.15 10.24 ( 0.20 2.03 ( 0.19 5.05 ( 0.43 0.54 ( 0.07 4.52 ( 0.14 1.60 ( 0.09 12.61 ( 0.24
a Standard deviation given in parentheses (n ) 3). b Results of investigations by AG der Dillinger Hu´ttemwerke:3 13.7 ( 0.6.
interferences which occur as a result of the reagents used for sample preparation, for contamination, and, in addition, for plasma background species due to solvent and the argon ICP. The error was minimized by measuring the blank after each determination and subtracting these succeeding blank counts from the previous sample determination. Isotope Dilution Measurement on an Unknown. The first step in isotope dilution analysis is to prepare an unspiked solution of the sample. This solution is required for two purposes. First, it can be used to obtain a rough estimate of the concentration of the analyte for establishing appropriate values for spike addition. Second, it can be used to measure the ratio for the proposed pair of isotopes. These data provide a very sensitive diagnosis of isobaric interferences. Since the method was applied to the analysis of reference materials, with certified boron content, the spike addition was determined from a knowledge of the quantity of boron present in the sample taken. No significant deviation of the measured ratio from the value calculated from the natural abundance of the two boron isotopes was found, and no evidence that the boron isotope ratio is dependent on the presence of other elements in unspiked solutions of steel samples has been observed. Taking the discrimination factor into account, the natural ratio of 0.2300 ( 0.0025 was found for unspiked solutions of the reference materials analyzed. To analyze steel samples with boron contents between 0.1 and 10 µg g-1 from test samples prepared according to the developed solvent extraction method (0.250 g/5 mL) and having 10B/11B isotope ratios between 0.3 and 9, a spike (enriched 10B) quantity of 250 ng must be added to the test sample portion (0.250 g). Limit of Detection, Precision, and Accuracy. The limit of detection was calculated as the standard deviation of the blank (3σ) of six measurements. A similar amount of spike was added to the blank solution and to the test samples to evaluate the blank values. A detection limit of 0.02 µg g-1 was obtained. This value is 10 times lower than any calculated previously9 with external calibration from 0.5% m/v sample solutions. Precision was evaluated at a concentration level of 10 times the detection limit. This solution was prepared in triplicate, and four consecutive peak area measurements were made for each. The RSD values obtained were
