Benefits of a Dual-View ICP-OES for the Determination of Boron, Phosphorus, and Sulfur in Low Alloy Steels Michael Duffy and Robert Thomas The Perkin-Elmer Corporation, 761 Main Avenue, Norwalk, CT 06859-0219 USA INTRODUCTION Boron, phosphorus, and sulfur are critical elements in defining the physical properties of steel. While boron is intentionally added to the molten ingot produced by the steelmaking process to improve the steel's hardenability characteristics, phosphorus and sulfur are contaminants from the blast furnace ironmaking process. Although their levels can be reduced by the addition of various chemicals during the refining of the molten metal, phosphorus and sulfur are generally not desired at high levels in the steel. In controlled amounts, they can improve the machinability of the steel, but in higher concentration they will have a dramatic impact on the toughness and ductility of the steel. Considering they are such critical elements, it is important that B, P, and S are monitored very closely during the steel-making process. Because speed and turnaround of analysis are the most important criteria in the quality control of this process, solid sampling techniques like XRF or arc/spark emission are the preferred analytical tools. However, there are times when solid sampling techniques are not suitable for the determination of these elements in steel samples. For example, when the concentration of B, P, and S is at the ppm level in the steel, these techniques are just not sensitive enough. In addition, some steel samples like thin strip or wire are difficult to analyze by a solid sampling technique. Finally, it may be desirable to have another technique available to check the XRF or arc/spark emission for quality control purposes. Whenever any of these situations arise, the laboratory has to use alternative trace element techniques. ABSTRACT This study investigates the use of inductively coupled plasma-optical emission spectrometry (ICP-OES) for the determination of boron, phosphorus, and sulfur in steel. Historically, this analysis has been performed by either X-ray fluorescence or arc/spark emission or by traditional wet chemistry when sample size restricted the use of solid sampling techniques. A series of NIST low alloy SRMs were analyzed using a dual-view ICP-OES that was configured to view the plasma both radially (side-on) and axially (end-on). The axial plasma offers an approximately 5 10 times improvement in detection limit, while the radial plasma maintains superior linear dynamic range and chemical/matrix performance. The benefit of the dual-view design over traditional technology is that if the elemental concentrations are too low for the radial-view configuration, they can be determined using the axial-view configuration. Accuracy and precision data will be presented for B, P, and S in the NIST SRM 361 365 series by both radial and axial viewing. Traditionally, wet chemistry has filled this role. Boron and phosphorus in the past have been determined by colorometric and/or gravimetric techniques, while sulfur has been done by either volumetric or infrared absorption techniques. Unfortunately, wet techniques are very labor-intensive when the sample workload is high. The advent of atomic absorption (AA) in the mid sixties dramatically reduced the analysis time of many trace elements in steel. However, some of the critical trace elements like B, P, and S could not be determined 1 successfully by atomic absorption because AA sources are generally not hot enough to produce enough ground state atoms of these elements. The emergence of inductively coupled plasma-optical emission spectrometry (ICP-OES) in the late seventies appeared to fill a void in trace metal analysis created by the limitations of AA. Besides being a multielement technique, ICP-OES was recognized as producing a much higher temperature source with far superior detection limits for the "difficult elements" like B, P, and S. For this reason, it was decided to evaluate the possibility of using ICP-OES for the determination of these elements in steel. METHODOLOGY The SRM 361 365 series of NIST (National Institute of Standards and Technology) Standard Reference Materials (SRMs) were chosen for this evaluation. SRMs 361 364 are a general purpose series of low alloy steels that contain a wide concentration range of B, P, and S, while SRM 365 is an electrolytic iron containing very low concentrations of these elements. It was decided to use three of the series for calibration purposes and then read all of them back as samples, including the other members of the series. The basic premise of this evaluation was that if good accuracy and precision can be obtained when members of the series are read off the original calibration, then it can be concluded that unknown steels of similar composition will give as good results, assuming the calibration is linear. The values of the SRM 361 365 standards are given in Table I; the ones used for calibration are marked with an *. ES-009
Sample Preparation One-gram amounts of the samples were weighed and transferred to acid-rinsed poly-methylpentene (PMP) volumetric flasks. The chips were rinsed to the bottom of the flasks with a spray of deionized water from a rinse bottle to a volume of about 20 ml. Addition of HCl to iron results in a chemically reductive state and sulfur may be lost through the formation of hydrogen sulfide (H 2 S) gas. To avoid this and to maintain an oxidative state, one drop (about 20 µl) of bromine was added to the water in each flask followed by mixing, and then the addition of about 5 ml of high-purity (Fisher Optima) HNO 3. The reaction and dissolution occurs very quickly. In some cases, small amounts of water were added during the reaction to cool and slow down its progress. After the initial reaction subsided, 1 ml of electronic-grade HF was added, followed by about 5 ml of HCl. The next step was to dilute the samples to a final volume of 100 ml. The samples were mixed and a portion of each solution was transferred to separate 50-mL polypropylene autosampler tubes. The higher concentration carbon samples will appear dark and murky from suspended carbonaceous material (probably graphite) that will not settle out very quickly. However, because a GemCone high-solids nebulizer was used for this evaluation, it was decided to analyze the samples directly and not to filter out the carbon particulates. This type of nebulizer has been described in the literature, but it is basically a conespray nebulizer designed for high-dissolved and suspended solids (1). One word of caution: Deionized and distilled water invariably contains sulfur, so it is important to use the same batch through all steps of the analysis. INSTRUMENTATION All work was performed using a Perkin-Elmer Model Optima Table I NIST SRM Calibration Standards and Concentrations in Wt % Element SRM 361 SRM 362 SRM 363 SRM 364 SRM 365 Boron 0.0003 7 * 0.0025 * 0.0007 8 0.0106 * Not certified Phosphorus 0.014 * 0.041 * 0.02 9 * 0.01 8 0.0025 Sulfur 0.015 * 0.038 * 0.009 * 0.02 9 0.0056 Note: According to NIST, the values listed for these SRMs are not expected to deviate from the true value by ±1 in the last significant figure. For subscript values, the deviation is not expected to be greater than ±5. It must also be noted that because of the uncertainty (deviation) associated with many SRMs (especially the ones with subscript values), NIST recommends caution when using them for calibration purposes. 3000DV inductively coupled plasma-optical emission spectrometer. The Optima 3000 has been described in the literature, but is basically a high-resolution Echelle optical system, utilizing two separate channels, one optimized for the UV and one optimized for the visible region. The two-dimensional image of the plasma produced by the Echelle grating is projected onto a 19-mm x 15-mm segmented array charge-coupled device (SCD) solid state detector. This solid state detector was designed specifically for the Echelle spectrometer and contains 224 optically sensitive subarrays on its surface, positioned to take advantage of 3 4 of the most sensitive emission lines of every element detectable by ICP emission (2, 3). The low UV capability, which is required for the determination of B, P, and S in steel, is achieved by purging the spectrometer and transfer optics with about 2 L/min of nitrogen. The only feature different on the DV version of the Optima 3000 is that the plasma torch is positioned horizontally. By incorporating modified transfer optics and an air shear gas to cut off the cool tail plume of the plasma, the DV can view the plasma both axially (end-on) and radially (side-on). This means that the analyst has the ability to use the instrument axially to achieve the best detection limits or radially to get the widest dynamic range and minimum chemical interferences. The benefits of the axial technology have been described in the literature (4). The benefit of a dual-view optical design is that if the desired detection limits cannot be achieved with the radial view, the analysis can be repeated using the axial view. Instrument Operating Parameters Tables II, III, and IV contain information on the relevant instrument parameters used in this study. For the first part of the evaluation, the instrument was viewed radially. TABLE II Emission Lines Used Element Boron Phosphorus Sulfur TABLE III Radial-View Plasma Conditions Wavelength 182.527 nm 178.221 nm 180.669 nm Plasma Auxiliary Nebulizer RF Viewing Elements gas flow gas flow gas flow power height B, S, P 15 L/min 0.4 L/min 0.72 L/min 1440 W 15 mm 2
RESULTS AND DISCUSSION Figures 1, 2, and 3 show the spectral scans of B, P, and S in the SRMs used for the calibration. 10 000 ppm of iron was also evaluated to get an understanding of the spectral contribution of pure iron with no analytes present. Figure 4 shows calibration curves, with correlation coefficients, for the three elements. Tables V, VI, and VII show the concentration and precision values measured in the standard reference materials, run as samples after the initial calibration using radial viewing. Discussion The wavelength scan information for the three elements shows that spectrally-clean emission lines were being used for the evaluation. The 10 000 ppm Fe confirms that there was no serious spectral contribution from the Fe, although it was clear that the solution was contaminated with all three of the analytes. As a result of using these relatively interferencefree emission lines, all three wavelengths showed good linearity as was confirmed by the excellent correlation coefficients. It can be seen from the data, that most of the experimental results agree very favorably with the NIST certified values. If one keeps in mind that the reference values have an uncertainty of ±1 and the subscript data has an uncertainty of ±5 in the last significant figure, then the conclusion can be made that the results are very encouraging. The only results that could be considered questionable are the boron result for SRM 361 and the phosphorus result for SRM 364. The SRM 361 boron certified value of 0.0003 7 % ± 0.00005 % is the lowest boron value of the whole series. Based on the sample weight of 1 g/100 ml, this represents about 30 ppb in solution. Fig. 1. boron at 182.527 Fig. 2. phosphorus at 178.221 Fig. 3. sulfur at 180.669 TABLE IV Peak Processing and Background Correction Points Integration Points/ BGC BGC Element time Processing Peak left right B, P, S Auto-integration Peak area 2 None +0.010 nm B, P, S Min. 10 seconds Peak area 1 0.021 nm +0.017 nm B, P, S Max. 50 seconds Peak area 3 0.017 nm +0.014 nm 3
TABLE V for Boron in NIST SRM 361 365 B certifd. B found 361 0.0003 7 <Det.limit --- 362 0.0025 0.0024 5.0% 363 0.0007 8 0.00076 34.1% 364 0.0106 0.0108 2.2% 365 Not certifd. --- --- Fig. 4. Calibration curves for B, P, and S viewed radially. When one considers that the theoretical detection limit of boron in an aqueous solution is only 3 ppb, it is understandable why the experimental result for this SRM is below the detection limit. The experimental phosphorus result for SRM 364 of 0.009% appears not to be in good agreement with the certified value of 0.01 8 %. However, the precision value of 2.3% shows it to be a reasonably precise figure. Also keep in mind that it has an error associated with it of ±0.005 %, so this value could in fact be 0.013 % phosphorus. In an attempt to improve the boron and phosphorus results, it was decided to repeat the analysis of these two SRMs using the axial configuration of the instrument. The aim was to improve the detection limit for boron in SRM 361 and to confirm the phosphorus result in SRM 364. Now instead of viewing the plasma from the side, the computer-controlled transfer optics were configured to view it axially. The axial-view operating parameters were the same as the radialview, except that the plasma was viewed directly down its central channel instead of 15 mm above the load coil when viewed radially. The analytical methodology was identical for both experiments. The calibration plots are shown in Figure 5. It can be seen that the sensitivity of the three elements viewed axially is 5 6 times higher than the radial-view sensitivity. This increased sensitivity is reflected in the comparison of boron results by radial and axial viewing in SRM 361, shown in Table VIII. These results clearly demonstrate that the level of boron in SRM 361 is below the detection limit when viewed radially, but is achievable with good precision when the plasma is viewed axially. TABLE VI for Phosphorus in NIST SRM 361 365 P certifd. P found 361 0.014 0.014 1.4% 362 0.041 0.042 0.9% 363 0.02 9 0.028 1.4% 364 0.01 8 0.009 2.3% 365 0.0025 0.0025 6.8% TABLE VII for Sulfur in NIST SRM 361 365 S certifd. S found 361 0.015 0.015 2.3% 362 0.038 0.037 0.5% 363 0.009 0.009 1.2% 364 0.02 9 0.025 1.1% 365 0.0056 0.0059 4.9% TABLE VIII Comparison of Boron in SRM 361 by Radial and Axial Viewing in Wt % Certified value Radial result Radial % RSD Axial result Axial % RSD 0.00037 < Det. Limit ---- 0.00031 8.7 % 4
when analyzing a set of samples, the radial-viewing configuration should always be the first option. A final comment: Boron is renowned for its poor sample introduction wash-out characteristics. It sticks to the walls of the spray chamber, nebulizer, and sample injector, which makes it very prone to memory effects. For this reason, when determining boron, much longer than normal washout and read delay times must be built into the method. CONCLUSION Fig. 5. Calibration plots of B, P, and S viewed axially. The comparison of phosphorus results by axial and radial viewing in Table IX confirms that the original radial result was good. The initial reaction is that the agreement between the certified and test results is questionable. However, because there is an uncertainty of +/- 0.005 % associated with the certified value of 0.018 %, the agreement can be considered acceptable. A word of caution: It is clear that the axial technology gives better detection limits than a traditional radial-view ICP. However, it does suffer from certain limitations. The linear dynamic range of an axial ICP is shifted down an order of magnitude. This means that even though it spans approximately five orders of magnitude, the improved detection limits in an axial ICP are achieved with a sacrifice in linearity at the top end. In addition, the axial technology also suffers from enhanced chemical interferences. Because the plasma is being viewed end-on, all the chemical effects associated with the different temperature gradients within the plasma are being seen. This means that in an axial ICP, more chemical/matrix-induced interferences will be observed with samples that contain high concentrations of matrix elements.that is why much thought must be given when deciding what viewing configuration to use when analyzing more complicated samples. A general rule of thumb is that unless detection limit is the major criterion TABLE IX Comparison of Phosphorus in SRM 364 by Radial and Axial Viewing in Wt % Certified value Radial result Radial % RSD Axial result Axial % RSD 0.018 0.009 2.3 % 0.010 3.2 % Because of certain limitations in the traditional solid sampling XRF or arc/spark emission techniques, there are times when boron, phosphorus, and sulfur need to be determined by another technique. ICP optical emission spectrometry has shown it can successfully fill that role. It has been clearly demonstrated that the Optima 3000DV ICP-OES, with its radial and axial viewing capabilities, can routinely determine ultratrace levels of these critical elements in low alloy and plain carbon steels. The major benefit of an instrument with both axial and radial technology, when determining trace elements in steel, is that the optimum elemental viewing configuration can be chosen, depending on the analytical challenge. REFERENCES 1. J.C. Ivaldi, J. Vollmer, and W. Slavin, Spectrochim. Acta 46B, No 67, 1063 (1991). 2. T.W. Barnard, M.J. Crockett, J.C. Ivaldi, and P.L. Lundberg, Anal. Chem. 65, No. 9, 1225 (1993). 3. T.W. Barnard, M.J. Crockett, J.C. Ivaldi, P.L. Lundberg, D.A. Yates, P.A. Levine, and D.J. Sauer, Anal. Chem. 65, No. 9, 1231(1993). 4. J.C. Ivaldi and J.F. Tyson, Spectrochim. Acta 50B, 1207 (1993). 5