The simultaneous determination of thorium and uranium in mineral fertilizers by inductively coupled plasma optical emission spectrometry (ICP OES) was optimized. The three-level factorial design was applied to optimize the operating conditions of radiofrequency power and nebulizer gas flow rate. The analytical method was specific and selective for both elements.
Keywords:
thorium; uranium; simultaneous determination; experimental design; ICP OES
| Parameter | Condition |
|---|---|
| Radiofrequency power / kW | 1.0 |
| Plasma gas flow rate / (L min-1) | 15.0 |
| Auxiliary gas flow / (L min-1) | 1.5 |
| Nebulizer gas flow rate / (L min-1) | 0.9 |
| Nebulizer type | concentric, SeaSpray |
| Spray chamber type | cyclone |
| Replicates | 3 |
| Injector tube diameter / mm | 2.4 |
| Signal integration time / s | 1.0 |
| Background corrector | automatic mode on |
| Wavelength / nm | Th II: 401.913; U II: 385.957; MgI: 285.208; Mg II: 280.265;CaII: 373.690; Fe II: 259.940 |
| Experiment | RFP / kW | NGF / (L min-1) | Intensity of Th / cps | Intensity of U / cps | Mg II/Mg I ratio | Normalized intensity of Th | Normalized intensity of U | Normalized Mg II/Mg I ratio | Multiple response |
|---|---|---|---|---|---|---|---|---|---|
| 1 | 1.0 | 0.6 | 7673 | 2532 | 10.0 | 0.647 | 0.726 | 0.762 | 2.13 |
| 2 | 1.0 | 0.8 | 9738 | 2691 | 8.4 | 0.822 | 0.771 | 0.639 | 2.23 |
| 3 | 1.0 | 1.0 | 9718 | 2790 | 8.4 | 0.820 | 0.800 | 0.639 | 2.26 |
| 4 | 1.2 | 0.6 | 4403 | 2172 | 10.2 | 0.372 | 0.622 | 0.777 | 1.77 |
| 5 | 1.2 | 0.8 | 9059 | 3144 | 11.7 | 0.764 | 0.901 | 0.890 | 2.56 |
| 6 | 1.2 | 1.0 | 8761 | 2961 | 11.4 | 0.739 | 0.848 | 0.870 | 2.46 |
| 7 | 1.4 | 0.6 | 6180 | 2429 | 13.1 | 0.521 | 0.696 | 1.000 | 2.22 |
| 8 | 1.4 | 0.8 | 10041 | 3211 | 12.2 | 0.847 | 0.920 | 0.927 | 2.69 |
| 9 | 1.4 | 1.0 | 11852 | 3490 | 11.6 | 1.000 | 1.000 | 0.880 | 2.88 |
| 10 | 1.2 | 0.8 | 8897 | 2964 | 11.4 | 0.751 | 0.849 | 0.864 | 2.46 |
| 11 | 1.2 | 0.8 | 8897 | 2964 | 11.6 | 0.751 | 0.849 | 0.879 | 2.48 |
| 12 | 1.2 | 0.8 | 9024 | 2865 | 11.7 | 0.761 | 0.821 | 0.887 | 2.47 |
| 13 | 1.2 | 0.8 | 8102 | 2745 | 11.4 | 0.684 | 0.787 | 0.868 | 2.34 |
| 14 | 1.2 | 0.8 | 9006 | 2593 | 11.3 | 0.760 | 0.743 | 0.862 | 2.37 |
| Parameter | S.S. | d.f. | M.S. | F-value | p-value |
|---|---|---|---|---|---|
| (1) RFP (L + Q) / kW | 2.75 × 10-1 | 2 | 2.75 × 10-1 | 4.30 × 101 | 4.46 × 10-2 |
| (2) NGF (L + Q) / (L min-1) | 4.98 × 10-1 | 2 | 4.98 × 10-1 | 7.80 × 101 | 6.67 × 10-3 |
| 1L by 2L | 7.00 × 10-2 | 1 | 7.00 × 10-2 | 1.10 × 101 | 2.11 × 10-2 |
| Lack of fit | 7.50 × 10-2 | 3 | 2.50 × 10-2 | 3.90 | 8.84 × 10-2 |
| Pure error | 3.20 × 10-2 | 5 | 6.00 × 10-3 | – | – |
| Total sum | 9.08 × 10-1 | 13 | – | – | – |
| Parameter | BEC / (μg L-1) | LODa a Limits established using 0.20 g of sample in a final volume of 25.0 mL. BEC: Background equivalent concentration; LOD: limit of detection; LOQ: limit of quantification. / (μg L-1) | LOQa a Limits established using 0.20 g of sample in a final volume of 25.0 mL. BEC: Background equivalent concentration; LOD: limit of detection; LOQ: limit of quantification. / (μg L-1) | LODa a Limits established using 0.20 g of sample in a final volume of 25.0 mL. BEC: Background equivalent concentration; LOD: limit of detection; LOQ: limit of quantification. / (mg kg-1) | LOQa a Limits established using 0.20 g of sample in a final volume of 25.0 mL. BEC: Background equivalent concentration; LOD: limit of detection; LOQ: limit of quantification. / (mg kg-1) |
|---|---|---|---|---|---|
| Th (401.913 nm) | 13.0 | 5.0 | 18.0 | 0.6 | 2.2 |
| U (385.957 nm) | 15.0 | 6.0 | 20.0 | 0.8 | 2.5 |
| NIST SRM | Certified value / % | Found valuea a Mean ± confidence interval at 95% (n = 3); / % | RSD / % | Agreementb b agreement = [(found value / certified value) × 100]. NIST standard reference material (SRM) 694: Western Phosphate Rock; NIST SRM 120c: Florida Phosphate Rock; RSD: relative standard deviation. / % |
|---|---|---|---|---|
| 694 (as U) | 0.01414 ± 0.00006 | 0.01252 ± 0.00057 | 1.8 | 89 ± 4 |
| 120c (as U3O8) | 0.0135 ± 0.0002 | 0.0122 ± 0.0013 | 4.3 | 90 ± 10 |
| Sample | Potential interfering element | Th | U | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Ca / (mg kg-1) | Fe / (mg kg-1) | Found value / (mg kg-1) | Added concentration / (mg kg-1) | Found value after added / (mg kg-1) | RSD / % | Recoverya a Recovery = [(found value after added – found value) / added concentration) × 100] / % | Found value / (mg kg-1) | Added concentration / (mg kg-1) | Found value after added / (mg kg-1) | RSD / % | Recoverya a Recovery = [(found value after added – found value) / added concentration) × 100] / % | |||
| NIST 694 | 25.20 ± 0.59b b concentration expressed in % m/m ,c c results expressed as concentration in % m/m of CaO (NIST 694: found value = 35.3 ± 0.8%, certified value = 43.6 ± 0.4%, agreement = 81 ± 2%; NIST 120c: found value = 40.86 ± 3.47%, certified value = 48.02 ± 0.17%, agreement = 85 ± 7%) and Fe2O3 (NIST 694: found value = 0.67 ± 0.03%, certified value = 0.79 ± 0.06%, agreement = 85 ± 4%; NIST 120c: found value = 1.03 ± 0.10%, certified value = 1.08 ± 0.03%, agreement = 95 ± 9%). NIST 694: Western Phosphate Rock; NIST 120c: Florida Phosphate Rock; results expressed as mean ± 95% confidence interval (n = 3) | 0.47 ± 0.02b b concentration expressed in % m/m ,c c results expressed as concentration in % m/m of CaO (NIST 694: found value = 35.3 ± 0.8%, certified value = 43.6 ± 0.4%, agreement = 81 ± 2%; NIST 120c: found value = 40.86 ± 3.47%, certified value = 48.02 ± 0.17%, agreement = 85 ± 7%) and Fe2O3 (NIST 694: found value = 0.67 ± 0.03%, certified value = 0.79 ± 0.06%, agreement = 85 ± 4%; NIST 120c: found value = 1.03 ± 0.10%, certified value = 1.08 ± 0.03%, agreement = 95 ± 9%). NIST 694: Western Phosphate Rock; NIST 120c: Florida Phosphate Rock; results expressed as mean ± 95% confidence interval (n = 3) | 9.6 ± 0.6 | 125.0 | 127.1 ± 4.2 | 3.0 | 94 ± 7 | 125.2 ± 5.7 | 125.0 | 248.0 ± 11.0 | 4.4 | 98 ± 11 | ||
| NIST 120c | 29.20 ± 2.48b b concentration expressed in % m/m ,c c results expressed as concentration in % m/m of CaO (NIST 694: found value = 35.3 ± 0.8%, certified value = 43.6 ± 0.4%, agreement = 81 ± 2%; NIST 120c: found value = 40.86 ± 3.47%, certified value = 48.02 ± 0.17%, agreement = 85 ± 7%) and Fe2O3 (NIST 694: found value = 0.67 ± 0.03%, certified value = 0.79 ± 0.06%, agreement = 85 ± 4%; NIST 120c: found value = 1.03 ± 0.10%, certified value = 1.08 ± 0.03%, agreement = 95 ± 9%). NIST 694: Western Phosphate Rock; NIST 120c: Florida Phosphate Rock; results expressed as mean ± 95% confidence interval (n = 3) | 0.72 ± 0.07b b concentration expressed in % m/m ,c c results expressed as concentration in % m/m of CaO (NIST 694: found value = 35.3 ± 0.8%, certified value = 43.6 ± 0.4%, agreement = 81 ± 2%; NIST 120c: found value = 40.86 ± 3.47%, certified value = 48.02 ± 0.17%, agreement = 85 ± 7%) and Fe2O3 (NIST 694: found value = 0.67 ± 0.03%, certified value = 0.79 ± 0.06%, agreement = 85 ± 4%; NIST 120c: found value = 1.03 ± 0.10%, certified value = 1.08 ± 0.03%, agreement = 95 ± 9%). NIST 694: Western Phosphate Rock; NIST 120c: Florida Phosphate Rock; results expressed as mean ± 95% confidence interval (n = 3) | 15.7 ± 3.3 | 125.0 | 136.5 ± 6.0 | 2.1 | 97 ± 5 | 103.4 ± 11.0 | 125.0 | 217.5 ± 5.0 | 2.1 | 91 ± 5 | ||
| N:P:K (1:56:0) | < 72.0 | 246.7 ± 8.4 | < 2.2 | 125.0 | 126.6 ± 9.5 | 3.0 | 101 ± 8 | < 2.5 | 125.0 | 124.4 ± 6.6 | 2.1 | 99 ± 5 | ||
| N:P:K (45:2:0) | 0.18 ± 0.03b b concentration expressed in % m/m | 481.6 ± 108.6 | < 2.2 | 125.0 | 104.9 ± 2.7 | 1.0 | 84 ± 2 | < 2.5 | 125.0 | 110.5 ± 5.9 | 2.2 | 88 ± 5 | ||
| N:P:K (46:0:0) | < 72.0 | < 47.7 | < 2.2 | 125.0 | 144.3 ± 8.8 | 2.4 | 115 ± 7 | < 2.5 | 125.0 | 143.7 ± 17.1 | 4.8 | 115 ± 14 | ||
| N:P:K (28:0:22) | < 72.0 | < 47.7 | < 2.2 | 125.0 | 145.8 ± 18.5 | 5.1 | 117 ± 15 | < 2.5 | 125.0 | 147.2 ± 6.6 | 1.8 | 118 ± 5 | ||
| N:P:K (10:10:10) | 152.8 ± 8.0 | < 47.7 | < 2.2 | 125.0 | 137.6 ± 2.4 | 0.7 | 110 ± 2 | < 2.5 | 125.0 | 131.2 ± 10.6 | 3.3 | 105 ± 8 | ||
