Validation of a New Liquid Asymmetric-Electrode Plasma Optical Emission Spectroscopy (LAEP-OES) Method for Measurement of Total Mercury in Tuna

Abstract

Background: Mercury intake is caused by eating seafood, such as tuna and other predatory fish species. To reduce the health risks of mercury intake, it is necessary to continuously measure and monitor mercury concentrations at fish farms and markets. We have developed a compact system that can detect multiple heavy metals by liquid asymmetric-electrode plasma optical emission spectroscopy (LAEP-OES).

Objective: The validity of the LAEP-OES method for total mercury levels was evaluated using standard solutions, certified substances, and specimens of bluefin tuna and other fish species.

Methods: All specimens were dissolved in 4 M lithium hydroxide solution and then dispensed into a sample reservoir well of the single-use measurement reagent pack. Total mercury levels were automatically measured within 15 min of placement into the dedicated equipment. A total of 102 fish specimens, classified into 10 fish species, were evaluated using the new method and the results were compared to those obtained from validated analytical methods.

Results: LOD (0.02 mg/kg), LOQ (0.07 mg/kg), repeatability (4.0%), intermediate precision (9.8%), and trueness (recoveries 107%) of the proposed method were within satisfactory limits for total mercury levels in fish. Additionally, when using various fish species, the method had a strong positive correlation with the results of cold-vapor atomic absorption spectrometry (CV-AAS, the official method) with Spearman rs = 0.984.

Conclusion: The LAEP-OES method can be used for measuring total mercury levels in bluefin tuna. Total mercury measurement using this new method has the potential to be applied to other fish species.

Highlights: Total mercury levels in fish were measured using our unique analysis system. Pacific bluefin tuna, southern bluefin tuna, and Atlantic bluefin tuna distributed in the Japanese market were analyzed for total mercury in their wild and farmed fish varieties.

Figure 1.

Comparison of LAEP and LEP principles, and LAEP-OES versus CV-AAS procedures. (a) Principle of LEP (24). A prototypical LEP configuration employs platinum (Pt) electrodes positioned on either side of a narrow channel. Upon application of an electrical potential to the platinum electrodes immersed in a liquid medium, plasma generation is efficiently induced within this narrow channel. (b) Principle of LAEP. The LAEP configuration utilizes electrodes with different sizes and made of different materials (nichrome and carbon), with plasma generation occurring predominantly at the location of the smaller electrode. A notable characteristic of this approach is its capacity to enhance the concentration of metal ions at the electrode sites responsible for plasma generation prior to the actual initiation of plasma. After excitation and light emission of metal atoms, the emitted light is introduced into an optical fiber. Its spectrum is obtained using a grating mirror and then measured using a charge-coupled device (CCD) image sensor. The built-in spectroscope measures the light generated by the excited metal atoms as a luminescence spectrum. (c) (I) Flowchart for the LAEP-OES method. A solution of 4 M lithium hydroxide (4 M LiOH) was used for extraction of mercury from fish. (II) Flowchart for the CV-AAS method, which is the method for total mercury measurement based on the guidelines of the Ministry of the Environment in Japan (14). Nitric acid (HNO₃), perchloric acid (HClO₄), and sulfuric acid (H₂SO₄) were used to treat the specimens in this method.

Figure 2.

Differences in mercury concentration between wild and farmed Pacific, Southern and Atlantic bluefin tuna. Three types of bluefin tuna—Pacific bluefin tuna (T. orientalis), Southern bluefin tuna (T. maccoyii), and Atlantic bluefin tuna (T. thynnus)—are each plotted for farmed and wild-caught types. The middle line indicates the mean value, and the upper and lower lines indicate the maximum and minimum, respectively. A significant difference in mercury concentration between wild and farmed tuna was observed in T. maccoyii and T. thynnus. The Mann–Whitney test was used. *P <0.05; **P <0.01; ***P <0.001; ****P <0.0001; ns = not significant.

Figure 3.

Comparison of total mercury concentration between the two methods. The correlation analysis was performed by plotting the mercury concentration in fish determined by the LAEP-OES and CV-AAS methods on the y- and x-axis, respectively. The correlation between the total mercury value obtained by LAEP-OES and CV-AAS was significant (Spearman’s rank correlation coefficient, r_s = 0.9840, P <0.0001). A simple linear regression is shown as a dashed line.