The Investigation of Fertilizer Analyses Using Microwave Digestion and the Agilent 720-ES Application Note Inductively Coupled Plasma-Optical Emission Spectrometers Authors Christine M. Rivera Doug Shrader Introduction Fertilizers play a vital role in sustaining crop yields by supplying essential plant nutrients such as macronutrients nitrogen (N), phosphorus (P 2 ) and potassium (K 2 O). The optimum fertilizer ratio of these three elements can vary according to the type of plant material being fertilized. Secondary nutrients such as calcium, magnesium and sulfur and micronutrients boron, copper, iron, manganese, molybdenum and zinc also play important roles in plant growth. The purpose of these investigations was to evaluate microwave digestion procedures for two analyses of fertilizer samples. The experiment was divided into two phases. Phase 1 was to evaluate using microwave digestion to prepare fertilizers for the determination of As, Ca, Cd, Cu, Cr, Fe, K, Na, Mg, Mn, P, Pb, Se and Zn by simultaneous ICP-OES. Phase 2 was to evaluate using microwave digestion to prepare fertilizers for the determination of available phosphorus (P 2 ) and potassium (K 2 O) by simultaneous ICP-OES. Traditionally, the preparation of samples for phosphorus and potassium is done by extraction with ammonium citrate EDTA. The samples are placed in a heated water bath and shaken for 1 hour. The reagents are added to warmed samples and the shaking must be continuous. The number of samples typically required for preparation is large and this process is extremely time-consuming. In this field, phosphorus is traditionally determined using an auto-analyzer, which can be tedious to set up and run. Potassium is typically determined by flame photometry. To simplify sample preparation and analysis, the CEM MARS Xpress microwave digestion system with stirring option and Agilent 720-ES simultaneous ICP-OES were used for both analysis phases.
Instrumentation Agilent 720-ES The Agilent 720-ES with axial torch configuration is a truly simultaneous ICP-OES with solid-state, Charge Couple Device (CCD) detection system. The custom-designed and patented CCD detector incorporates IMAP technology, whereby pixels are arranged in continuous angled arrays matched exactly to the image produced by the echelle optics. This provides true simultaneous measurement and full wavelength coverage from 167 nm to 785 nm. Microwave Digestion System CEM, Corp. MARSXpress is an ultra-high throughput microwave digestion system designed to make high-throughput sample preparation and research applications quick and easy. Forty high-pressure vessels, available in 55 ml or 75 ml sizes, can be processed per run with temperature control of every vessel. Sample Preparation Phase 1 To verify the method, two Magruder fertilizer check standards (200204 and 200206), which form part of the association of American Plant Feed Control Officials (AAPFCO) round robin laboratory checks, and a certified reference material, Industrial Sludge (CRM-S-I) from High Purity Standards, were prepared for analysis. Approximately 0.5 g of sample was accurately weighed and transferred to 55 ml MARSXpress vessels. Then, 9 ml of HNO 3 and 1 ml of HCl was added to each sample vessel. Samples were digested in duplicate. The microwave digestion method is summarized in Table 1. Table 1. Microwave Digestion Settings Max % Ramp Pressure Temp. Hold Stages power (W) power (min.) (PSI) ( C) (min) 1 1200 100 15:00 120 200 15:00 The total digestion time was 30 minutes. Figure 1 represents the average digestion temperature over time in the vessels during sample preparation. 2 Figure 1. MarsXpress digestion temperature profile. All samples were diluted to 50.00 ml in plastic disposable tubes and filtered with 2 micron Teflon FilterMate. This high dirt trapping FilterMate is especially suitable for trace level analysis and is supplied with lot certification for trace metals. The filtered samples were transferred directly to the Agilent SPS3 autosampler for analysis on the Agilent 720-ES. Calibration Solution Preparation The calibration summary for Phase 1 is listed in Table 2. Table 2. Phase 1 Calibration Summary Elements Concentration (mg/l) As, Cd, Cr, Cu, Mn, P, Pb, Se, Zn 0.5, 5, 10 and 50 Ca, Fe, K, Na, Mg 5, 10, 100 and 1000 Conditions Instrument operating conditions are shown in Table 3. Table 3. Instrument Operating Conditions Parameter Setting Spraychamber type Double-pass, glass cyclonic Nebulizer type SeaSpray Nebulizer flow 0.75 L/min RF Power 1.25 kw Plasma gas flow 15 L/min Auxiliary gas flow 1.5 L/min Uptake delay 20 s Stabilization delay 10 s Rinse time 15 s Internal standard 2 mg/l yttrium Ionization buffer 0.4% caesium Integration time* 60 s Replicates 2 Total analysis time# 165 s *If Se is not required at detection limit concentrations, a 5 s integration time is adequate. # 55 s for sample sets not requiring low concentrations of Se to be measured.
Results and Discussion Phase 1 The results for Phase 1 are summarized in Table 4. Many of the samples encountered in feed and fertilizer laboratories consist of high concentrations of nutrients. Some of these elements cause ionization interferences while others cause spectral overlap problems, for example, iron. The use of an ionization buffer, for example, 0.4 % caesium minimized the ionization interferences. The spectral overlap can be overcome by the advanced background correction techniques of the ICP Expert II software, such as fitted background correction and FACT (Fast Automated Curve-Fitting Technique) spectral deconvolution. Table 4. Results Summary Sample labels As 188.980 Expected Ca 370.602 Expected Cd 214.439 Expected Cr 267.716 Expected units mg/kg mg/kg % % mg/kg % mg/kg mg/kg Sludge B 141 141 0.0233 0.0242 0.64 NA 110 111 Magruder 4B 2.05 1.75 2.71 2.48 12.31 NA 125.2 132.6 Magruder 6B 5.75 5.66 4.93 5.94 1.51 NA 50.88 51.08 Sample labels Cu 327.395 Expected Fe 261.382 Expected K 2 O 404.721 Expected Mg 279.078 Expected units % % % % % % K 2 O % % Sludge B 0.0407 0.0398 0.012 0.014 NA NA 12.6 12.2 Magruder 4B 0.0461 0.0307 0.350 0.400 11.02 10.54 1.62 1.64 Magruder 6B 1.010 0.976 0.500 0.500 21.37 20.54 0.53 0.62 Sample labels Mn 294.921 Expected Na 589.592 Expected P 214.914 Expected units % % % % % P 2 % P 2 Sludge B 0.51 0.48 0.94 0.94 0.51 0.50 Magruder 4B 0.036 0.039 0.31 0.29 8.1 9.1 Magruder 6B 0.014 0.015 0.57 0.58 9.1 9.9 Sample labels Pb 220.353 Expected Se 196.026 Expected Zn 213.857 Expected units mg/kg mg/kg mg/kg mg/kg % % Sludge B 6.8 5.7 NA NA 0.0244 0.0249 Magruder 4B 1.16 2.18 0.43 0.44 0.043 0.048 Magruder 6B 1.88 2.15 0.13 0.12 0.003 0.003 Phase 2 Available K 2 O and P 2 The extraction reagent preparation requires 325 g EDTA and 650 g dibasic ammonium citrate dissolved in 19.5 L distilled water. With mixing, 390 ml of a 1:1 solution of NH 4 OH:H 2 0 is then added. When the solution is cooled to room temperature, the ph is carefully adjusted to 7.0 with additional 1:1 solution of NH 4 OH:H 2 0 and the final solution diluted to 26.0 L. The traditional method requires that 0.25 g of sample undergo extraction in 100 ml ammonium citrate reagent in a Wheaton bath stabilized to 65 C (the extraction solution is added to the warmed sample). Upon completion of the extraction procedure, the solutions are cooled and diluted to 250 ml with ammonium citrate/edta reagent. The samples are then typically run on the flame photometer for available K 2 O and the auto-analyzer for P 2. The first part of the Phase 2 experiment was to determine if ICP- OES is a viable alternative technique for the determination of K and P. A set of ten fertilizer samples were extracted per the defined method and analyzed by the traditional techniques and ICP-OES. With the ICP-OES technique, beryllium was selected as the internal standard and 0.8% caesium as an ionization buffer. Optimum instrument conditions were found to be at a power of 1.1 kw and a nebulizer flow of 0.65 L/min. The total sample analysis time was 55 s/sample. A comparison of the results by technique for the determination of K and P are summarized in Table 5. 3
Table 5. Traditional Methodology Verses ICP OES (1 Hour Water Bath Extraction) Results ICP-OES Flame photometer ICP-OES Auto-analyzer Calibration solutions K 2 O (769.897 nm) K 2 O P 2 O5 (214.914 nm) P 2 (units) (% w/v) (% w/v) (% w/v) (% w/v) Blank 0 0 Std 1 0.1145 Std 2 0.06023 0.4581 Std 3 0.68709 Std 4 0.24038 0.91612 Std 5 1.3742 ICP-OES Flame photometer ICP-OES Auto-analyzer Calibration solutions K 2 O (769.897 nm) K 2 O P 2 (214.914 nm) P 2 (units) (% w/v) (% w/v) (% w/v) (% w/v) NIST SRM 200a 34.81 34.64 52.62 52.11 Sample 1 4.51 4.59 21.6 21.59 Sample 2 4.50 4.29, 5.11, 5.19 8.10 8.59 Sample 3 9.22 9.22, 9.30 10.04 10.09 Sample 4 9.89 9.80, 9.71 10.04 9.75, 9.95 Sample 5 1.80 1.48, 2.46, 2.16 19.87 20.07 Sample 6 2.50 2.53, 3.03, 3.15 22.47 22.49, 22.69 Sample 7 10.17 9.88 10.58 10.47 Sample 8 8.56 8.58 4.17 4.06 Sample 9 5.47 5.45 9.71 9.73 Sample 10 18.49 18.00 16.56 16.70, 16.45 Note: No internal standard for K 2 O. The final step of the Phase 2 study was to mimic the extraction process using microwave digestion. The microwave is not used for total digestion of the fertilizer sample, but as a way to consistently heat the extraction to 65 C. Three Magruder fertilizers and NIST SRM 200a potassium dihydrogen phosphate (KH 2 PO 4 ) were carefully weighed to 0.1 g into HP 5000 vessels. Using the 75 ml vessels, 75 ml of ammonium citrate/edta extraction fluid was added. Stirring bars were added to each sample to simulate the shaking process and the contents were heated to 65 C for 1 hour. Table 6 summarizes the results collected by ICP-OES for available K 2 O and P 2. 4
Table 6. Summary of Results for Microwave Extraction and Determination of K 2 O and P 2 by ICP OES K 2 O 404.721 K 2 O 769.897 P 2 185.878 P 2 214.914 Calibration solutions(units) (% w/w) (% w/w) (% w/w) (% w/w) Calibration Blank 0 0 0 0 Std 1 0.01139 0.01139 Std 2 0.012 Std 3 0.03038 0.05785 0.05785 Std 4 0.06022 0.06022 0.11449 0.11449 Std 5 0.1223 0.1223 0.22881 0.22881 Std 6 0.2431 0.2431 Std 7 0.49789 K 2 O 404.721 K 2 O 769.897 Expected P 2 185.878 P 2 214.914 Expected Samples(units) (% w/w) (% w/w) (% w/w) (% w/w) (% w/w) (% w/w) NIST_SRM 200a 34.42 34.58 34.61 52.44 52.56 52.11 NIST SRM duplicate 34.93 34.34 52.70 52.44 MAG # 4 11.66 10.45 10.54 20.71 20.36 20.82 MAG # 4 duplicate 10.04 10.48 20.90 20.75 MAG # 6 26.65 25.41 25.42 22.33 22.07 22.54 MAG # 6 duplicate 23.99 24.88 22.25 22.10 MAG # 9 12.57 13.66 13.00 14.04 13.85 13.00 MAG # 9 duplicate 12.65 13.66 14.09 13.86 Conclusion The preparation of fertilizer samples by microwave digestion/extraction for the determination of macro, secondary and micro nutrients by simultaneous ICP-OES was evaluated and found to compare well with more traditional methods. The combination of microwave and ICP-OES techniques resulted in significantly faster and simpler sample preparation and analysis, requiring only a single analytical system to measure all elements of interest. Acknowledgements Elaine Hasty, CEM Corporation, Matthews, NC Peter Kane, Office of Indiana State Chemist, West Lafayette, IN 5
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