Determination of Silicate in Seawater by Inductively Coupled Plasma Atomic Emission Spectrometry

Journal of Oceanography Vol. 48, pp. 283 to 292. 1992 Determination of Silicate in Seawater by Inductively Coupled Plasma Atomic Emission Spectrometry KAZUO ABE and YASUNORI WATANABE Seikai National Fisheries Research Institute, 49 Kokubu-cho, Nagasaki 850, Japan (Received 4 January 1992; in revised form 23 March 1992; accepted 31 March 1992) Inductively Coupled Plasma Atomic Emission Spectrometry (ICP) was adopted for direct analysis of silicate in seawater, eliminating necessity for pre-treatment. Via this method, the determination of silicate is rapid and easy compared with conventional methods of colorimetry. In seawater analysis, a decrease of sensitivity of about 24% was observed due to interference by coexisting elements, mainly Na. Examination of the analytical conditions revealed a detection limit of 0.3 µm Si, and precisions of approximately 3.2, 2.0 and 1.3% for Si levels of 4, 15 and 94 µm, respectively. Using this method, silicate determination in the East China Sea was attempted. 1. Introduction Silicate is one of the important parameters which control marine productivity. Many reports have been published on its behavior (e.g. Banahan and Goering, 1986; Demaster et al., 1983; Nelson and Gordon, 1982). Silicate levels in seawater range from 0 to maximum concentrations of about 240 µm in the Bering Sea (Tsunogai et al., 1979). Determinations are ordinarily conducted by colorimetric methods. Atkins (1923, 1926) applied the yellow silicomolybdic acid method to the analysis of seawater and Robinson and Thompson (1948) described the method with minor modifications. Briefly, this method involves the reaction of molybdate with silicate in seawater, resulting in the formation of silicomolybdate complexes having a yellow color. Mullin and Riley (1955) employed the addition of a reducing solution to the silicomolybdate complex to induce a blue color, hereby enhancing sensitivity. Generally, the formation of the silicomolybdate complex requires several hours. AAS (Atomic Absorption Spectrometry) can be adopted for direct analysis of silicate, but seawater samples are not suitable for analysis by the equipment, due to interference by alkali and alkaline earth salts. Separation of a specific element from these salts should be conducted prior to analysis. In the case of Si determination, the colorimetric methods noted above require the addition of chemical reagents to sample solutions for the formation of complexes. It is sometimes very difficult to obtain silicate-free water or to remove reagent contamination. Consequently, an accurate blank value can not be determined. Separation of a specific element from alkali and alkali earth salts usually requires some kind of treatment, including the addition of chemical reagents. Thus, the risk of reagent contamination should be considerable. The Inductively Coupled Plasma Atomic Emission Spectrometry (ICP) method has been used since 1964 (Greenfield et al., 1964). This is a very sensitive method for multi-element determination, and can be applied to seawater analysis. Akagi et al. (1985) have studied the determination of some trace metals in coastal seawater using this method via coprecipitation with gallium hydroxide. This method has an advantage in that seawater and can be introduced to the

284 K. Abe and Y. Watanabe inside of the plasma torch without separating alkali and alkaline earth salts. In this paper, a rapid and simple method for the determination of silicate in seawater by ICP was examined. Subsequently, this method was adapted through water samples from the East China Sea. 2. Experiment 2.1 Apparatus and reagents Analysis of silicate were conducted using the Jarrell-Ash ICAP (Inductively Coupled Argon Plasma Atomic Emission Spectrophotometer)-575 MARK II. Specifications of this equipment are shown in Table 1. After setting analytical conditions, and making background corrections for wave length spectra in accordance with the standard solution profile, sample or test solutions were introduced via the nebulizer inside the plasma torch. Intensity was measured at 17 mm above work coil and determination of silicate in seawater was made by standard additional methods. All reagents used in this study were of special grade, and a Si standard solution was prepared from a 1000 ppm standard solution (Na 2 SiO 3 in 0.2 M Na 2 CO 3 ) for AAS analysis (Wako Pure Chemical Industries, Ltd.) by diluting with distilled water, prior to each analysis. 2.2 Analytical conditions Analytical conditions were examined using a test solution (about 10 µm Si). The possible effects of gas flow rate (coolant, auxiliary, carrier) and spectral interferences from major elements present in seawater (Na, Mg, K and Ca) were examined in order to determine suitable conditions. Furthermore, parameters such as voltage applied to work coil, integration time for calculation, and output power were checked. Values of silicate determined under these analytical conditions were also confirmed by colorimetric methods. 2.3 Field observation Surface water samples were collected via a clean, plastic bucket at the stations (St. 1 St. 6) shown in Fig. 1 on October 24 25, 1991. In addition, vertical sampling was carried out with Go- Flo sampling bottles at St. 7 (see Fig. 1) on 25 October 1991. After collection, all samples were transferred to carefully acid-washed plastic centrifuge tubes and frozen immediately to circumvent adsorption to tube walls and/or chemical change until laboratory analysis. Table 1. Operational parameters. Spectrometer Jarrell-Ash ICAP-575 MARK II frequency 27.12 MHz output power (max) 2.5 kw Nebulizer Cross-flow type Monochrometer 0.75 m Zerny-Terner type grating 1800 grooves/mm reciprocal linear dispersion 0.79 nm/mm entrance slit width 25 µm exit slit width 25 µm Wave length 251.61 nm Torch Fassel type, Quartz

Determination of Silicate in Seawater 285 Fig. 1. Sampling stations in the East China Sea. 3. Results and Discussion The profile for the Si standard solution analyzed by ICP in this laboratory is shown in Fig. 2. The detected signal (intensity) shown in this figure is composed not only of emission spectra line originating from the specific element, but also from other components that are not proportional to the concentration of an objective element. If the intensity (concentration) of a certain element is sufficiently high in comparison to the intensity of these other components, this phenomenon can be ignored. A background correction will effectively compensate for this interference. In Fig. 2 the symbol, *, corresponds to the intensity from interferential components. Thus, subtracting signals (intensity) from these components gives a true intensity which depends only on the specific element. L and H show the low and high points of the wave length, respectively, for the background correction sphere. Figure 3 shows the effect of flow rate of coolant gas for Si. Open and closed circles correspond to distilled water and seawater, respectively. Coolant gas is necessary for cooling the quartz torch and preventing air contamination of the argon plasma. When the flow rate is low, emission of NO, NH, or N 2 may be generated due to air contamination. This can cause spectrum interference, so background correction can not be made easily, and a melting of the quartz torch may possibly occur. As shown in Fig. 3, only a small change of intensity is observed with variations of flow rate of the coolant gas. For the two reasons noted above, flow rates above 12 l/min should be applied. The auxiliary gas is necessary for the argon plasma to float above the torch and prevents from sticking salts or carbon inside the torch. Therefore, when analysis of a solution containing highly concentrated salts (e.g. seawater) is carried out, the flow rate should be kept high. Although decreasing intensity is observed with increasing flow rate during Si analysis (Fig. 4), flow rates of more than 0.5 l/min are necessitated.

286 K. Abe and Y. Watanabe Fig. 2. Profile of Si standard solution. The symbol * indicates background intensity. L and H correspond to the low and high points of background correction. Fig. 3. Flow rate of coolant gas and the Si intensity. Open and closed circles correspond to distilled and seawater, respectively. Carrier gas is used to introduce an aerosol (sample) from the nebulizer into the plasma. This flow rate is a main indicator of sensitivity and precision. Variation in intensity as a function of flow rate is shown in Fig. 5. Open and closed circles indicate distilled and seawater, respectively. The highest sensitivity for Si determination was 0.40 l/min. Figure 6 shows the effects of coexistence elements on the determination of silicate. In this study the effect of Na, Mg, Ca and K were examined because of their abundance in seawater. The Y and X axes indicate relative intensity compared with that of distilled water without any kind

Determination of Silicate in Seawater 287 Fig. 4. Effects of auxiliary gas in Si analysis. Open circles indicate distilled water and closed circles, seawater. Fig. 5. Influence of carrier gas on Si analysis. Open and closed circles indicate distilled and seawater, respectively. of metals, and concentrations of added coexistence elements, respectively. An arrow in Fig. 6 indicates the concentration level of each element in seawater. Interference in determination of Si was observed for every coexistence element examined in this study. At seawater levels, the effect of Na interference is great (about 80%). In Fig. 7 the effects of these elements regarding sensitivity in Si determination are illustrated. In this experiment, filtered seawater (surface

288 K. Abe and Y. Watanabe Fig. 6. Interference of coexistence elements on the determination of silicate. Fig. 7. Effects of coexistence elements for sensitivity of Si determination. seawater of the Kuroshio current), distilled water, and mixed solution, i.e. seawater: distilled water = 1:1 were used. Standard additional methods were adapted, so it is not necessary that the three lines in Fig. 7 start from the origin. Sensitivities (slope) in Fig. 7 are 355, 270, and 300 intensity/1 µm Si for distilled water, seawater, and mixed solution, respectively. These results indicate a decrease in sensitivity (slope) of about 24% in seawater. This should be considered as due to interference from coexistence elements in seawater, which is consistent with the results in Fig. 6. Therefore, for determination of Si in seawater, calibration curves should be determined

Determination of Silicate in Seawater 289 Fig. 8. Relationship between applied voltage and lowest limit of detection for Si determination. Table 2. Effects of output power (kw). Output power (kw) R. Intensity C. V. (%) 0.8 0.546 1.63 1.0 0.767 1.77 1.2 0.883 2.09 1.4 0.960 2.15 1.6 0.984 2.44 1.8 1.000 2.75 Table 3. Effects of integration time. Integration time (s) C. V. (%) 1 3.57 2 3.20 3 2.82 4 2.64 5 1.88 6 2.06 7 1.39 8 1.10 using seawater (standard additional methods should be carried out). Figure 8 shows the relationship between applied voltage and the detection limit of the methodology, defined as twice the standard deviation of the blank signal. Results for voltage ranging from 450 to 850 V are depicted. Only slight variations were observed, indicating that any value of applied voltage in this range is acceptable. Though larger intensity can be obtained with increase in applied voltage, in this study, 700 V was adopted for the reasons noted above. Effects of output power (kw) and

290 K. Abe and Y. Watanabe Table 4. Precision of the methodology. Si 1. 3.58 µm 1. 15.2 µm 1. 94.4 µm 2. 3.45 2. 15.4 2. 93.9 3. 3.59 3. 15.4 3. 94.3 4. 3.73 4. 14.7 4. 96.5 5. 3.63 5. 15.0 5. 92.4 6. 3.56 6. 14.7 6. 93.9 7. 3.32 7. 14.7 7. 94.9 8. 3.64 8. 14.9 8. 94.8 9. 3.59 9. 14.6 9. 93.4 10. 3.65 10. 14.8 10. 92.6 µ = 3.57 ± 0.115 µ = 14.9 ± 0.302 µ = 94.1 ± 1.19 (3.21%) (2.03%) (1.26%) Table 5. Comparison of Si analysis with colorimetric methods. 1. 1.75 µm 2. 1.72 3. 1.75 4. 1.57 5. 1.41 6. 1.69 7. 1.78 8. 1.57 9. 1.70 1.66 µm ± 0.137 (8.23%) This study 1. 1.63 µm 2. 1.73 3. 1.73 4. 1.84 5. 1.63 6. 1.53 7. 1.63 8. 1.84 9. 1.63 1.69 µm ± 0.105 (6.21%) Colorimetry integration time are shown in Tables 2 and 3. These experiments were carried out using distilled water. Though the coefficient of variation (CV) increased with additional output power, obtained relative intensities (each intensity was normalized by the value at the output power, 1.8 kw) increased. The CV values for various integration times showed smaller fluctuations with longer integration time. In this study, 1.6 kw and 4 s were adopted for output power and integration time, respectively. In Table 4, the precision, values of about 3.2, 2.0, and 1.3% are shown as errors in a single determination at Si levels of 4, 15, and 94 µm, respectively. The analytical detection limit of our methodology, as defined in Fig. 8, was 0.30 µm Si. Table 5 shows a results comparison of Si analysis between the methods used in this study and a previous colorimetric method (Sugawara, 1969). In this experiment, filtered surface seawater from the Kuroshio current was also analyzed, and the results show good correspondence between the two methods; therefore, the applicability of our method for silicate analysis detailed in this study was confirmed. Isshiki et al. (1991) reported form of dissolved silicon in seawater, resulting in the concentrations obtained by the two methods, ICP and colorimetry were the same within experimental error. This also supports the applicability of determination of Si in seawater via ICP method. Figures 9 and 10 show concentrations of silicate as a function of longitude and water depth, respectively. Each numeral corresponds to a station in Fig. 1 (Fig. 9). All samples were filtered

Determination of Silicate in Seawater 291 Fig. 9. Silicate concentration as a function of longitude. Fig. 10. Vertical distribution of silicate in the East China Sea. through 0.45 µm membrane filter prior to analysis. In Fig. 9, a decrease in concentration from the western station to the eastern station was observed. The highest value of 19.0 µm at 125.0 E and the lowest of 1.59 µm were obtained at the eastern station (surface). The vertical profile in Fig. 10 indicates a depletion of silicate in the surface water (1.56 µm) and increase with depth

292 K. Abe and Y. Watanabe to a maximum of about 75 µm near the sea floor. Input from the land or the river, horizontal and vertical convection, or biological activity can be considered to be sources that regulate these distributions. The sensitivity and dynamic range of methods of silicate analysis detailed in this report are well-suited for application to water samples from this area. In the East China Sea, 0.5 and 1.3 µm were determined via ICP in surface water from the Okinawa Trough region (Isshiki et al., 1991). Our result shows higher value, 19.0 µm at 125.0 E, 32.5 N (in surface water). Further quantitative study should be carried out. Acknowledgements The authors are indebted to the officers and crew of R/V Xiang Yang Hong 09 (People s Republic of China) for their help in the sampling. We also wish to thank the staff of Seikai National Fisheries Research Institute, Nagasaki, Japan for their useful comments. References Akagi, T., K. Fuwa and H. Haraguchi (1985): Simultaneous multi-element determination of trace metals in sea water by inductively-coupled plasma atomic emission spectrometry after coprecipitation. Anal. Chim. Acta, 177, 139 151. Atkins, W. R. G. (1923): The silica content of some natural waters and of culture media. J. Marine Biol. Assoc. U.K., 13, 151 159. Atkins, W. R. G. (1926): Seasonal changes in the silica content of natural waters in relation to the phytoplankton. J. Marine Biol. Assoc. U.K., 14, 89 99. Banahan, S. and J. J. Goering (1986): The production of biogenic silica and its accumulation on the southeastern Bering Sea shelf. Continent. Shelf Res., 5, 199 213. Demaster, D. J., G. B. Knapp and C. A. Nittrouer (1983): Biological uptake and accumulation of silica on the Amazon continental shelf. Geochim. Cosmochim. Acta, 47, 1713 1723. Greenfield, S., I. L. Jones and C. T. Berry (1964): High-pressure plasmas as spectroscopic emission sources. Analyst, 89, 713 720. Isshiki, K., Y. Sohrin and E. Nakayama (1991): Form of dissolved silicon in seawater. Mar. Chem., 32, 1 8. Mullin, J. B. and J. P. Riley (1955): The colorimetric determination of silicate with reference to sea and natural waters. Anal. Chim. Acta, 12, 162 176. Nelson, D. M. and L. I. Gordon (1982): Production and pelagic dissolution of biogenic silica in the Southern Ocean. Geochim. Cosmochim. Acta, 46, 491 501. Robinson, R. J. and T. G. Thompson (1948): The determination of silicate in sea water. J. Mar. Res., 7, 49 55. Sugawara, K. (1969): On the preparation of CSK standards for marine nutrient analysis. SCOR-UNESCO, ICES IOC, Tokyo. Tsunogai, S., M. Kusakabe, H. Iizumi, I. Koike and A. Hattori (1979): Hydrographic features of the deep water of the Bering Sea The Sea of Silica. Deep-Sea Res., 26, 641 659.