Determination of Phosphorus in Soil by ICP-OES Using an Improved Standard Addition Method

In this study, an improved standard addition method (ISAM) was developed for the determination of phosphorus in soil by ICP-OES based on the conventional standard addition method (CSAM) and calibration curve method (CCM). Certified standard soils were analyzed by the proposed ISAM method. The values obtained by ISAM method agreed with the certified values. Additionally, the results obtained by ISAM method were compared with those determined by the other two methods (CSAM and CCM). All the values obtained by the ISAM agreed with those from the other two methods. The detection limit, quantification limit, and recovery rate of each method were calculated, and the recovery rates of soil samples and the blank were all within the range of 90%–110%. Finally, the proposed method was applied to determine phosphorous in soil samples from Guangnan County, Yunnan Province, China, and the meadow soil from Qinghai-Tibet Plateau, China. The relative errors between the results from ISAM and CCM were all within 10%, and t-test showed that the results between ISAM and CCM had no significant difference (P > 0.05). Therefore, the proposed method overcame the matrix effect in some extent and was an acceptable method for the rapid and accurate batch analysis of P content in soil sample, especially batch samples with obvious matrix effect.


Introduction
Determination of total phosphorus (P) in soils is very important in agriculture as the amount of phosphorus is related to crop production. Recently, research on the biogeochemical cycle has become a significant focus in the fields of soil science, ecology, and geology. However, some of the research has focused on the phosphorus cycle [1][2][3][4][5]. erefore, a reliable and rapid method is essential for the determination of phosphorus in soil.
Molybdenum blue method is the traditional technique for determination of phosphorus in soil [6][7][8]. is method has some disadvantages including complicated steps and procedures, being influenced by human operation, and low analytical efficiency. It is not suitable for the rapid and batch determination of soil samples. Inductively coupled plasma optical emission spectroscopy (ICP-OES) is an essential, modern analytical tool that has a high sensitivity and a broad linear range. In recent years, ICP-OES has been used in determination of phosphorous in geological samples [9], high-purity nickel [10], and biodiesel B100 from feedstock [11]. ere are several quantitative methods used in ICP-OES mainly included calibration curve method (CCM) [12][13][14][15] and conventional standard addition method (CSAM) [16,17]. e CCM is simple, but it is the susceptible to errors by fluctuations in operating conditions and the matrix effect. However, internal standard has been used to minimize errors caused by instrumental drift and to reduce chemical matrix effect [18][19][20]. CSAM is useful when the sample has obvious matrix effect and the matrix-matching method cannot be used. However, it is time-consuming and required a complex procedure. erefore, an improved standard addition method (ISAM), based on the CSAM and CCM, was proposed in this study for the determination of total phosphorus in soil. In this method, soil mixture solution was determined by the CSAM. e value of slope was calculated from the obtained calibration curve. All the samples were measured directly by ICP-OES for the determination of P, and the response signal values were obtained. en, the total P value of each test sample could be calculated. Be similar to CSAM, the improved method overcomes the matrix effect. And be similar to CCM, ISAM was simple and rapid. So it is suitable for the rapid and accurate analysis of batch samples, especially the large number samples with obvious matrix effect. e difference between each analysis strategy is summarized in Table 1.
To verify the reliability of the novel method, 4 types of certified standard soils were analyzed, and the results were compared with those determined by the CCM and the CSAM. e values obtained were in close agreement with the certified values and the results from CCM and the CSAM. e detection limit and quantification limit were measured. Meanwhile, the proposed method was applied to determine phosphorous in soil samples from Guangnan County of Yunnan Province, China, and the meadow soil from Qinghai-Tibet Plateau, China.

Materials.
Concentrated nitric acid (GR, guarantee reagent) and hydrofluoric acid (GR, guarantee reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. A stock standard solution of phosphorus (P) (1000 μg/mL) was purchased from the National Center of Analysis and Testing for Nonferrous Metals and Electronic Materials, and a stock standard solution of rhenium (Re) (1000 μg/mL) was purchased from Shanghai Macklin Biochemical Co., Ltd., China. Working standard solutions were freshly prepared from those stock solutions as required.
Four certified standard soils-GBW (E) 070041 (Institute of Soil Science, Chinese Academy of Science), GSS-5 and GSS-8 (Institute of Geophysical and Geochemical Exploration, Ministry of Geology and Mineral Resources), and GSS-14 (Institute of Geophysical and Geochemical Exploration, IGGE)-were obtained for the determination of total phosphorus. Soil samples were from Guangnan County, Yunnan Province, China, and Qinghai-Tibet Plateau, China. e soils were air-dried and passed through a 2 mm sieve, and subsamples were further ground to pass through a 0.25 mm sieve.

2.2.
Apparatus. An Optima 8300 ICP-OES (PerkinElmer, USA) was applied for the analysis, and the instrumental specifications are given in Table 2. And an 18.2 MΩ deionized pure water treatment system (Millipore, USA) was used for preparing deionized water. A closed-vessel microwave digestion system (Mars 6, CEM, US) was applied to perform microwave-assisted digestion procedures for the soil samples. It is equipped with a 40-position rotor. A heating apparatus (BHW-09C, Shanghai Botong Chemical Technology Co. LTD, China) was employed to evaporate the residual acid.

Microwave Digestion.
Approximately 0.15 g of a soil sample was weighed (with an accuracy of 0.0001 g) and placed in 50 mL Teflon vessel. Concentrated nitric acid (8 mL) and hydrofluoric acid (4 mL) were added. e closed vessels were introduced in a microwave oven-assisted sample digestion system. For complete digestion of the samples, a heating program comprising three steps was used. e program of the microwave dissolution is presented in Table 3. After cooling to room temperature, the digestion solution in Teflon vessel was evaporated to about 1 mL in a heating apparatus at 175°C. Evaporation was a necessary step since acid concentrations would have been too high for the ICP-OES. Careful operation was needed to avoid drying of the evaporation residues. en, all solutions were transferred to a 100 mL volumetric flask and accurately diluted to the mark with deionized water. Run blanks with all the chemicals and process except the soil sample.

Calibration Working Standards for the ISAM.
Soil mixture solution was obtained from the mixture of digested soil solutions.
A stock calibration standard of P (200 mg/L) was prepared by diluting 10 mL 1000 mg/L P calibration standard solution to 50 mL with deionized water. A stock calibration standard solution of P (500 mg/L) was prepared by diluting 12.5 mL 1000 mg/L P standard calibration solution to 25 mL with deionized water. A series of calibration working standards of P for the ISAM were diluted by soil mixture solution.
e concentrations of the calibration working standards are listed in Table 4.
As element Re is rare in soil, element Re was selected as an internal standard to minimize errors caused by instrumental drift and was added in situ. A 10 mg/L Re internal standard solution was prepared by diluting 5 mL of a 1000 mg/L Re standard solution to 500 mL with deionized water.

ISAM
Rapid and simple, overcome the matrix effect in some extent e detection limit and the quantitation limit were higher than the other two methods  All calibration standards were then stored in the freezer at a temperature of 5°C prior to analysis and were shaken before measurement to ensure proper mixing of P in solutions.

Calibration Working Standards for CCM and CSAM.
Calibration working standards of P in the 0-3 mg/L range were prepared by diluting the stock calibration standards with deionized water. Re (10 mg/L) was selected as an internal standard to minimize errors caused by instrumental drift and was added in situ.
All calibration standards were then stored in the freezer at a temperature of 5°C prior to analysis and were shaken before measurement to ensure proper mixing of P in solutions.

Calculation of Detection Limit (L D ) and Quantification
Limit (L Q ). Once the method was set up, the detection limit (L D ) and quantification limit (L Q ) were calculated. e L D was calculated from the measurement of the blank sample. e blank sample was measured ten times under reproducibility conditions. e detection limit was obtained from the following expression [18]: where S � value of standard deviation of the measurements. e L Q was calculated according to the IUPAC guidelines as ten times the standard deviation of the measurement, for a number of the measurements equal to ten [21]: 2.6. Recovery Test. Duplicate samples of 4 certified standard soils (GSS-5, GSS-8, GSS-14, and GBW (E) 070041) and blank were spiked with 50 μL of a 500 mg/L P standard solution. e spiked samples were digested and measured in the same manner as described in Section 2.3. Run blanks with all the chemicals and process except the soil sample.
Recovery rate was calculated using the following equation: recovery rate � ((P content in spiked sample) − (P content in unspiked sample)) known P content added to spiked sample .

Results and Discussion
3.1. eory. Generally, the CSAM was used for the determination of total P in soil, and multiple standard curves were obtained as y 1 � m 1 x 1 + b 1 , y 2 � m 2 x 2 + b 2 , . . ., y n � m n x n + b n , corresponding to multiple test samples. e content of each test sample can be obtained from the intercept of each standard curve with the X-axis, which was b n /m n . When the matrix of the batch sample was the same or similar, the values of m were close to each other and could be approximated as m 1 � m 2 � · · · � m n � m. erefore, for a series of samples with the same or similar matrix, the slopes were assumed to be the same.
In this study, an ISAM based on the CSAM and CCM was proposed. One sample was determined by CSAM. e value of slope, m, was calculated from the obtained calibration curve. en, all the samples were measured directly by ICP-OES, and the response signal values, b n, were obtained. Hence, the concentration of total P (C n ) in the each test sample was calculated according to the following equation: e procedure of analysis by ISAM is shown in Figure 1. Actually, the matrix of the soil samples was not completely the same. To further reduce the error, the soil mixture solution prepared according to Section 2.4.1 was treated as sample and used to obtain the value of m using the CSAM.  Table 5, and the ICP-OES spectrograms are shown in Figures 2 and 3. As shown in Table 5, the differences between the obtained value and certified value (△) were all with the error of the certified value. e average of △ 1 was a little smaller than the average of △ 2 . is indicated that the results determined by ISAM with standard solutions diluted by soil mixture solution were more close to the certified values. us, to some extent, the use of mixture of soil digestion solution could correct the analytical accuracy.
Operate conditions were as shown in Table 2 Operate conditions were as shown in Table 2 Furthermore, as compared to Figure 3, the baselines of the spectrogram in Figure 2 were closer to each other than that in Figure 3. It indicated that the matrix of standard solutions diluted by soil mixture solution was more consistent with the matrix of soil digestion solution. From this perspective, the matrix effect between the standard solution and the digestion solution could be reduced by using the soil mixture solution. us, in this study, the soil mixture solution was applied to prepare the calibration working standard solutions.

Determination of Total P Value by ISAM and Comparison with CCM and CSAM.
According to the principles discussed in Section 3.1, the concentration of P in soil mixture solution was determined by CSAM. e corresponding linear correlation relationship was obtained as follows:   was 396 mg/kg. e contents of P in other certified standard soils (GSS-8, GSS-14, and GBW (E) 070041) were obtained by the same procedure, and the results are listed in Table 6. As shown in Table 6, the differences between the obtained and certified values (△ 2 ) were within the error of the certified value, indicating that the proposed method was of acceptable accuracy. Relative standard deviation (RSD) values of 1.23%-3.94% revealed relatively good precision. e ISAM combines the advantages of the CSAM and CCM, and it overcomes the matrix effect to some extent.

Journal of Analytical Methods in Chemistry
From this perspective, the ISAM is a useful and potential method for the batch analysis of P content in soil with a complex matrix.
Meanwhile, to further verify the suitability of the improved method for the determination of total P in soil, the P contents of the above certified standard soils were also determined by the CCM and CSAM. All samples were measured 3 times in parallel, and the average results are presented in Table 6. Table 6 shows that all the values obtained by ISAM, CCM, and CSAM agreed with each other. Additionally, △ 2 , △ 3 , and △ 4 were all within the error of the certified values. And the average of △ 2 is smaller than the average of △ 3 and △ 4 . e RSDs of obtained values were all less than 5%.
Furthermore, the determination of the P in 4 certified standard soils using CSAM spent more than 108 min and that using the ISAM only spent nearly 30 min. Compared with CSAM, the proposed ISAM was an simple and timesaving method. erefore, the proposed ISAM is a rapid and accurate method for the determination of P in batch samples.

Calibration Curve, Detection Limit, and Quantification
Limit.
e concentration of P in the blank solution was measured 10 times continuously using ICP-OES. e L D and L Q were calculated as described in Section 2.5. Table 7 presents the calibration curve, correlation coefficient, L D , and L Q .
As shown in Table 7, the L D and L Q of the ISAM were higher than that achieved by other methodologies. However, be similar to CSAM, the proposed ISAM method could overcome the matrix effect in some extent. On the other side, be similar to CCM, the ISAM is simple and rapid. us, the proposed ISAM is suitable for the P determination of the batch sample with matrix effect.
3.5. Recovery Test. As described in Section 2.6, the results of the recovery test are listed in Table 8.
As shown in Table 8, the recovery rates of soil samples and the blank were all within the range of 90%-110%. is indicated that the ISAM was acceptable for the determination of the total P content in soil.

Test of Soil Samples.
e P content of soil samples from Guangnan County of Yunnan Province, China, and the meadow soil from Qinghai-Tibet Plateau, China, were determined by the proposed ISAM in this report and conventional CCM. e results and relative errors (RE) are presented in Table 9.
e soil samples 1-20 in Table 9 were from Qinghai-Tibet Plateau, China, and the soil samples 21-40 were from Guangnan County of Yunnan Province. As shown in Table 9, the P content of soil samples from Guangnan County of Yunnan Province, China, were higher the that from Qinghai-Tibet Plateau, China. e results from the ISAM and CCM were close to each other, and the RE values between 2 methods were all below 6.23%. e difference of results from the 2 methods was compared with t-test. T-test showed that the results between ISAM and CCM had no significant difference (P > 0.05). erefore, the improved method ISAM was an acceptable method to determine P content in soil sample.

Conclusions
A novel method ISAM, based on the CCM and CSAM, was developed to measure the content of total P in soil. e results obtained by the proposed method were compared with the certified values and those determined by conventional methods. It was justified that the ISAM was an acceptable method for evaluation of P content in soil. e developed method, combining the advantages of the CCM and CSAM, overcomes the matrix effect and is suitable for the rapid and accurate analysis of batch samples. e proposed ISAM   method is a potential method for the determination of P content in soil and offers a new choice for the determination.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.