3.1 Optimization of instrumental condition
The sensitivity of an instrument is closely linked to its detection limit (L = kSb/S, where Sb represents the standard deviation of multiple blank measurements, S denotes the sensitivity of the test method, and k is the coefficient determined based on a given confidence level) [5]. The signal sensitivity and stability of the instrument can be adjusted by various parameters, including the flow rate of the nebulizer, RF voltage, and others, to ensure that the Mn strength (5ppb) is greater than 50000cps, the signal deviation is less than 1%, and the long-term stability signal deviation is less than 2%, which meets the test requirements for constant elements.
When the total amount of soluble solids is below 0.5% (w/V), the matrix effect will not impact the test signal [9]. Given that the salinity level is approximately 35‰, the matrix effect after dilution is less than 0.035%, which is below the threshold of 0.5%. After dilution, the matrix effect will not significantly impact the change of the test signal. Therefore, the sample pretreatment method is suitable for ICP-OES testing.
3.2 Optimization of spectral lines
The high energy excitation generated by emission spectrum leads to the production of excited states of the element. As electrons transition from the excited state to the ground state, they release different energies, resulting in the formation of distinct characteristic spectral lines. According to statistics, there are over 10 million spectral lines, with each element usually possessing multiple emission spectral lines that may overlap and interfere with each other.
In this study, elements of K, Ca, Na, Mg, S, Sr, and B have multiple characteristic spectral lines that differ greatly due to differences in their nature. For instance, K has two strong spectral lines at 766 nm and 769 nm, and two weak ones at 404 nm and 693 nm (Fig. 1).
The spectral lines observed at 404nm and 693nm display low intensities that are in close proximity to the background, thereby limiting their ability to accurately depict the relationship between intensity and concentration. Furthermore, the 769nm spectral line presents a double-peak structure with strong interference from an additional spectrum line. While it is possible to extract the emission intensity of the K element through correction, this process introduces a level of error and uncertainty. In contrast, the 766nm spectral line exhibits a clear background, a well-defined peak shape, and a superior signal-to-noise ratio, establishing it as the preferred spectral line for analysis.
The emission lines of Na occur at 589 nm, 588 nm, 568 nm, and 330 nm (Fig. 2), with significant variation in intensity. The spectral lines at 330nm and 568nm demonstrate low intensity that is in close proximity to the background, thereby failing to establish a clear relationship between intensity and concentration. Furthermore, the 588nm spectral line displays interference peaks within its background. In contrast, the 589nm spectral line exhibits a clear background, a complete peak shape, and a substantial signal-to-noise ratio. Thus, it is deemed the optimal choice for spectral analysis. The Calcium exhibits more than 20 spectral lines, and its relatively strong ones are at 396 nm, 393 nm, 422 nm, 317 nm, 315 nm, and 373 nm, and the intensity of these spectral lines differs by 2 orders of magnitude. Strontium presents complex emission spectral lines, mainly at 407 nm, 421 nm, 216 nm, 460 nm, and 215 nm. The emission spectral line intensity of B is relatively low, mainly at 249.7 nm and 249.6 nm, with similar emission intensity. Spectral lines of magnesium mainly appear at 279 nm, 280 nm, 285 nm, and 383 nm. In contrast, S has only a few spectral lines, mostly at 181 nm, 180 nm, and 182 nm, with generally low intensity. According to the content of elements and the intensity of their spectral lines, the optimal spectral lines chosen were 766 nm for K, 317 nm for Ca, 589 nm for Na, 285 nm for Mg, 181 nm for S, 407 nm for Sr, and 249.7 nm for B, respectively.
3.3 Matrix effect
The high salinity of seawater poses a risk of instrument damage, including blockage, signal suppression, and breakdown of the torches. To mitigate these issues, a dilution method is employed for testing purposes. This approach involves establishing a standard curve using variations in the concentration of K, Ca, Na, Mg, S, Sr, and B in artificial seawater. Dilution is achieved by pipetting accurately measured volumes of 0.05mL, 0.1mL, 0.2mL, 0.4mL, 0.6mL, 0.8mL, and 1.0mL of seawater, which are then diluted to a final volume of 10.0mL before analysis. The recovery and dilution factor characteristics of K, Na, Mg, Sr, and B (Fig. 3) indicated that the recovery rate of the elements increased gradually with an increase in the dilution factor. At a dilution factor of 100, the recovery rate exceeded 90%, effectively eliminating the matrix effect and generating high-quality test data. ICP-OES analysis has low requirements for soluble solids, and when the total amount is less than 0.5% (w/V), the matrix effect is not expected to influence test signal changes [9]. The salinity level of seawater is approximately 35‰, and a 100-fold dilution will result in a dilution of 0.035%, which is lower than the aforementioned threshold. Consequently, the matrix effect is eliminated after dilution, rendering this sample preparation method suitable for meeting ICP-OES testing requirements.
3.4 Detection limits
The detection limit of the method was defined as the 3-times standard deviation (σ) equivalent concentration multiplied by the dilution factor of the corresponding value of 0.5% nitric acid background, tested over 10 consecutive times.The detection limits of K, Ca, Na, Mg, S, Sr, and B were found to be different, with optimal wavelength detection limits of 1.34mg/L for K, 1.0 mg/L for Ca, 1.1 mg/L for Na, 1.0 mg/L for Mg, 2.1mg/L for S, 0.80µg/L for Sr, and 0.5 mg/L for B. These values were lower than the concentrations of these elements in the diluted seawater, resulting in a higher signal-to-noise ratio and enabling the acquisition of optimal test signals.
3.4 Results
The data results were verified based on precision, relatively standard deviation (RSDs), and recovery.
Table 1
Determination Results of major and trace elements in seawater (n = 10)
| | Ca (mg/L) | Mg(mg/L) | K (mg/L) | Na (mg/L) | S (mg/L) | Sr (µg/L) | B (µg/L) |
RGHS | Average | 406 | 1212 | 398 | 11211 | 948 | 7993 | 4309 |
RSD (%) | 0.61 | 0.89 | 0.53 | 0.52 | 3.06 | 0.32 | 1.81 |
RGHS | Average | 508 | 1509 | 496 | 13704 | 1166 | 10038 | 5409 |
RSD (%) | 1.26 | 0.94 | 0.44 | 0.57 | 1.86 | 0.97 | 1.85 |
ADD | 100 | 300 | 100 | 2500 | 225 | 2000 | 1000 |
Recovery(%) | 102.6 | 99.0 | 97.9 | 99.7 | 97.0 | 102.2 | 110.0 |
RGHS | Average | 607 | 1806 | 593 | 16229 | 1415 | 12021 | 6119 |
RSD (%) | 1.30 | 0.76 | 0.25 | 0.56 | 0.65 | 0.47 | 1.75 |
ADD | 200 | 600 | 200 | 5000 | 450 | 4000 | 2000 |
Recovery(%) | 100.5 | 99.0 | 97.7 | 100.4 | 103.9 | 100.7 | 90.5 |
JAHS | Average | 423 | 1283 | 403 | 12196 | 1125 | 8496 | 4870 |
RSD (%) | 0.54 | 0.37 | 0.60 | 0.30 | 1.74 | 0.56 | 0.87 |
JAHS | Average | 521 | 1601 | 510 | 14741 | 1348 | 10514 | 5981 |
RSD (%) | 0.67 | 0.51 | 0.52 | 0.38 | 1.12 | 0.65 | 1.09 |
ADD | 100 | 300 | 100 | 2500 | 225 | 2000 | 1000 |
Recovery(%) | 98.2 | 106.0 | 107.3 | 101.8 | 99.0 | 100.9 | 111.1 |
JAHS | Average | 626 | 1896 | 604 | 17239 | 1600 | 12573 | 6801 |
RSD (%) | 1.20 | 1.13 | 1.20 | 1.07 | 2.13 | 0.94 | 1.03 |
ADD | 200 | 600 | 200 | 5000 | 450 | 4000 | 2000 |
Recovery(%) | 101.5 | 102.2 | 100.4 | 100.9 | 105.5 | 101.9 | 96.6 |
From Table 1, the RSDs of K, Ca, Na, Mg, S, Sr, and B in the artificial seawater (RGHS) were 0.53%, 0.61%, 0.52%, 0.89%, 3.06%, 0.32%, and 1.81%, respectively. The RSDs of K, Ca, Na, Sr, and Mg were less than 1%, which showed well accuracy. However, for the RSDs of B and S, the RSDs were larger due to their low content and low sensitivity of the test spectrum lines. Nevertheless, the accuracy requirements of major elements in seawater can still be met.
Regarding metal concentrations, the contents of nearshore seawater (JAHS) and artificial seawater (RGHS) are similar, with K content at 400 mg mg/L, Ca content at 403 mg/L, Na content at about 11,200 mg/L, Mg content at about 1283 mg/L, and S, Sr, and B content at 1125 mg/L, 8496 µg /L, and 4870 µg /L, respectively. The test precision for these elements ranged from 0.30–1.74%. And the recoveries of K, Ca, Na, Mg, S, and Sr are between 97–107%. In offshore seawater (JAHS), the recovery rates of K and Mg are 106–107%, whereas the recovery rates of B in both types of seawater are poor, ranging from 110–112%, which may be related to the amount of standard solution used.