Determination of trace elements in shellfish tissue samples by inductively coupled plasma mass spectrometry

Analytica Chimica Acta 382 (1999) 215±223

Determination of trace elements in shell®sh tissue samples by inductively coupled plasma mass spectrometry Shoji Sakao, Hiroshi Uchida* Kanagawa Industrial Research Institute, 705-1, Shimoimaizumi, Ebina, Kanagawa 243-0435, Japan Received 4 June 1998; received in revised form 6 November 1998; accepted 8 November 1998

Abstract The determination of trace elements in shell®sh tissue samples by inductively coupled plasma mass spectrometry (ICP-MS) has been investigated. After determination of major and minor inorganic elements by ICP atomic emission spectrometry (AES), the internal standardization was discussed as a correction method for matrix effects in ICP-MS. The results obtained by quadrupole (Q)-ICP-MS after acid digestion almost agreed well with reported values. The high resolution (HR)-ICP-MS technique was applied to the determinations of V, Cr, Co, Ni and As in order to remove the mass spectral overlapping caused by polyatomic ions. A sealed bomb decomposition was also discussed for the determinations of As, Se and Hg. The lanthanide patterns obtained by Leedy chondrite normalization showed smooth curves. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma mass spectrometry; Internal standardization; Trace element determination; Acid digestion; Sealed bomb decomposition; Lanthanide pattern

1. Introduction Because of the high sensitivity, wide dynamic range and relative freedom from interferences, inductively coupled plasma mass spectrometry (ICP-MS) has been developed for the determination of trace and ultra-trace levels of elements in various ®elds [1]. Especially, a lot of analytical values for many kinds of trace elements have been reported in biological samples [2±9]. Although there are many advantages in ICP-MS, it suffers from both spectroscopic and nonspectroscopic interferences. However, mass spectral overlapping with polyatomic ions, has been recently overcome by the use of high resolution (HR)-ICP-MS [10]. The most severe non-spectroscopic interference *Corresponding author. Fax: +81-458128314.

is the space charge effect, where the ion beam is disturbed in the path through the ion optics and mass spectrometer [11]. Non-spectroscopic interferences involving chemical and ionization in the ICP have been commonly corrected by internal standardization [12,13]. Shell®sh tissue and sea weed samples are wellknown to concentrate metals, and have been used as the monitor of the sea water pollution. Several standard reference materials (SRMs) have been prepared for the trace element determination in marine products. The National Institute for Environmental Studies (NIES, Japan) has prepared the certi®ed reference materials (CRMs) of marine product samples of No. 6 Mussel, No. 9 Sargasso, No. 11 Fish Tissue, No. 14 Brown Alga (Hijiki) and No. 15 Scallop. The certi®cation for the Brown Alga and Scallop, prepared for

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00797-1

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arsenic speciation [14], is still in progress. In our previous report, the determination of trace elements in CRMs of Sargasso and Brown Alga, and also in edible hijiki samples was investigated by the use of ICP atomic emission and mass spectrometry [15]. In this report, the determination of trace elements in shell®sh tissue samples has been investigated mainly by the use of the ICP-MS technique. In addition to the NIST (USA) standard reference materials (SRM) Oyster tissue (1566), the NIES No. 6 Mussel and No. 15 Scallop, and the remainder of the scallop except for the adductor are examined. Internal standardization is discussed in order to correct for the interference of coexisting elements. The high HRICP-MS technique is applied to remove the mass spectral overlapping of polyatomic ions, and a sealed bomb decomposition is examined for the determination of As, Se and Hg. The lanthanide patterns in shell®sh tissue samples are also discussed. 2. Experimental 2.1. Instrumentation and operating parameters The ICP quadrupole (Q) mass spectrometer of the SPQ8000 (Seiko Instruments, Japan) was mainly used for the determination of trace elements, and the HRICP mass spectrometer of the JMS-Plasmax-2 (Japan Electronic Optical Lab., Japan) was applied to avoid the mass spectral interferences. The SPS1200VR ICP atomic emission spectrometer (Seiko Instruments, Japan) was also used for the determination of major and minor inorganic elements. Details of the operating conditions are summarized in Table 1, and the analytical wavelength used for AES and the atomic mass unit (amu) values for MS are listed in Table 3.

A sealed decomposition bomb used consisted of the inner PTFE vessel (25 ml) and outer stainless steel guide (San-ai Kagaku, Japan) [16]. 2.2. Analytical samples Analytical samples were the SRM of the Oyster tissue (NIST No. 1566), the CRMs of No. 6 Mussel and No. 15 Scallop (NIES), and the remainder of the scallop. The CRM Scallop was prepared only from the adductor part [14]. On the other hand, 20 kg of the remainder of the scallop except for the adductor was freeze-dried and crushed. This remainder of the scallop was used for the cooperative determinations of Cd in the analytical laboratories belonging to local governments in Japan [17]. 2.3. Other chemicals Working solutions for calibration were prepared from commercially available 1000 mg lÿ1 solutions for atomic absorption spectrometry (Kanto Chemicals, Japan). Ultrapure nitric, hydro¯uoric and perchloric acids of Tamapure 100 (Tamakagaku Kogyo, Japan) were used for the digestion of analytical samples and for the preparation of working solutions. The ultrapure water used was obtained from a Milli-Q system (Millipore, Japan). The concentrations of the working solutions for calibration are listed in Table 2. 2.4. Sample preparation The sample of 0.5 g was weighed into a PTFE beaker after drying at 858C for 3 h, it was heated on the hot plate with a PTFE cover at 408C for 16 h after the addition of 10 ml of nitric acid, and heated again at 1208C for 4 h. Then, 1 ml of hydro¯uoric and 1 ml of perchloric acids were added after removal of

Table 1 Operating condions of Q-ICP-MS, HR-ICP-MS and ICP-ES

Radio frequency (MHz) Radio frequency power (kW) Outer Ar gas (l minÿ1) Intermediate Ar gas (l minÿ1) Carrier Ar gas (l minÿ1) Sampling point (mm above work coil)

Q-ICP-MS

HR-ICP-MS

ICP-ES

27.12 1.0 16 0.8 0.75 12

40.68 1.2 16 0.2 0.9 10

27.12 1.2 14 0.8 0.65 14

S. Sakao, H. Uchida / Analytica Chimica Acta 382 (1999) 215±223

217

Table 2 Composition of mixed solutions for calibration Element ÿ1

Major and minor inorganic elements(mg kg )

Trace elements (mg kgÿ1)

Lanthanides (mg kgÿ1) Internal standard (mg kgÿ1)

K Na Mg Al Ti

Concentration

Ca Fe Mn

Sr Zn

As Zn Cu Rb V Mo

Cr Cd

Ni Pb

LaLu Co

Y

Bi

the cover, and heated to nearly dryness at 1808C with the addition of an appropriate amount of nitric acid. The residue was dissolved in 5 ml of nitric acid and diluted to 50 g with ultrapure water. The prepared sample stock solution was commonly diluted 10 times with the addition of 10 mg kgÿ1 Co, Y, and Bi as the internal standard. All procedures were carried out in a clean room with an actual cleanliness class of 1000 (Federal Standard). A sealed bomb decomposition was carried out as follows. The sample of 0.1 g was taken to the PTFE vessel and 5 ml of nitric acid was added. After sealing of the stainless guide, the bomb was left in the oven at 1308C for 16 h. The content was diluted to 100 g by ultrapure water with the addition of 5 ml nitric acid and 10 mg kgÿ1 Co, Y and Bi. 3. Results and discussion 3.1. Determination of major and minor inorganic elements Determinations of major and minor inorganic elements in shell®sh tissue samples by ICP-AES were carried out for the discussion on the matrix effect. Analytical results were summarized in Table 3 together with the analytical wavelengths used. The analytical values did agree well with certi®ed values for Oyster tissue [18] and Mussel [19]. The standard

Ba Zr U

0 0 0 0 0

10.0 2.0 0.5 0.25 0.1

20.0 4.0 1.0 0.5 0.2

40.0 6.0 2.0 1.0 0.4

0 0 0 0 0

10.0 5.0 2.5 1.0 0.5

20.0 10.0 5.0 2.0 1.0

40.0 20.0 10.0 4.0 2.0

0 10

0.5

1.0

deviations for the remainder of the scallop are almost at the same level of the other SRMs, which indicates that the homogeneity of the sample is good enough for the elemental analyses. The concentrations of inorganic elements in the adductor of the scallop are lower than those in the remainder, except for K. 3.2. Internal standardization The ion signal suppression caused by the co-existing elements in the Oyster tissue sample is shown in Fig. 1. One of the reasons for this suppression is the decrease of the sample introduction rate to the plasma, which is caused by the decrease in the aspiration rate and nebulization ef®ciency. Furthermore the space charge effect is more severe in ICP-MS. Internal standardization is one of the useful techniques for the correction of these interferences. The internal standards of 59 Co‡ , 89 Y‡ and 209 Bi‡ were examined, because the internal standard with a mass close to that of the analyte is preferred to correct for the space charge effect [12]. Five or ten mg kgÿ1 of Co, Y and Bi were added to each diluted sample or calibration solution. In addition to the mass number, the internal standard with the ®rst ionization potential (IP) close to that of the analyte has to be recommended for the biological samples [15]. Although there is a little difference between the IP value of analytical and internal standard elements, an accuracy of 955% is expected for the digested solution with a dilution

Trace elements by ICP-MS (mg kgÿ1) amu Al V 51 HRg Cr 52 HR Mn 55 Ni 60 HR Co 59 Cu 63 Zn 66 As 75 HR Sealed dec. Se 82 Sealed dec. Rb 85 Y 89 Sr 90 Zr 91 Mo 95 Cd 114 Sn 120 Ba 138 Hg 202 245 22.4 1.72 1.03 0.817 16.8 1.23 0.966 (0.316)h 57.9 790 16.2 13.4 ± 2.22 ± 4.51 (0.404) 9.45 0.304 0.232 3.24 1.54 5.06 ND 5.18 0.0560.004

0.14040 3.430.16

10.10.7

4.50.5

2.080.20

0.370.04 632 85424 13.01.2

17.01.2 1.010.09

0.6500.080

25523 2.70.2

0.4950.022 0.1330.010 0.02550.0023 0.930.07 0.1400.012 0.001700.00012 0.01950.0011 0.08540.0024

2247 15.42.9 1.07 1.070.15 0.949 14.90.1 0.8980.07 0.872 (0.344) 4.920.23 1071 12.90.5 9.50 8.94 1.420.10 1.90 2.800.13 (0.111) 16.60.4 0.2190.016 0.8560.013 0.8060.016 0.9030.083 0.9100.050 ND

1.000.02 0.2100.002 0.02020007 0.5260.038 0.1370.002 0.001500.00016 0.01670.0003 0.01070.0002

Founde

Foundc

Reportedd

Mussles

Oyster tissue

Major and minor inorganic elements by ICP-AES (%) Wavelength (nm) Na 589.59 0.504 Mg 280.72 0.130 Al 396.15 0.0243 K 766.49 0.919 Ca 317.93 0.138 Mn 257.61 0.00178 Fe 238.20 0.0204 Zn 213.86 0.0880

Element

Table 3 Analytical results of major and trace elements in shellfish mustle samples

0.05

0.820.03

17

1.5

0.37 4.90.3 1066 9.20.5

16.31.2 0.930.06

0.630.07

220 0.450.31

1.000.03 0.210.01 0.0220 0.540.02 0.130.01 0.001630.00012 0.01580.0008 0.01060.0006

Reportedf

7.220.80 8.350.21 0.672 0.3620.014 0.430 1.090.14 0.4780.054 0.523 (0.0132) 0.8190.046 58.20.5 6.670.63 3.70 3.34 0.7860.05 0.974 8.120.07 (0.0033) 4.470.05 0.0210.002 0.0480.007 0.1620.004 0.1310.008 0.0410.003 ND

0.5820.012 0.1870.003 ND 1.740.02 0.03090.00055 0.002990.00007 0.001050.00008 0.005870.0001

Found

Scallop(adductor)a

20.10.5 2.51 6.640.37 3.79 27.30.2 9.010.46 4.06 (0.706) 34.60.5 1571 13.40.3 11.8 11.2 7.810.42 10.6 3.670.04 (0.372) 5.142.3 1.690.06 1.450.03 1101i 0.3650.031 12.30.2 ND

1.870.03 0.3650.009 0.1330.004 0.7670.024 1.100.06 0.003060.00014 0.1670.006 0.01660.0002

Found

Scallop(remainder)b

218 S. Sakao, H. Uchida / Analytica Chimica Acta 382 (1999) 215±223

208 209 232 238

309 439 71.2 283 64.6 14.1 67.9 10.1 58.0 11.6 33.4 4.64 31.9 4.85

± 0.478 (0.0055) 0.0532 0.123

b

NIES CRM [14]. Prepared for the cooperative determination [17]. c Obtained as the average from two samples. d NIST reported values [18]. e Obtained from five samples. f NIES certified and reference values [19]. g Determined by HR-ICP-MS. h Determined using 115 In as the internal standard. i Average value:11618 mg kgÿ1. j Reported value by INNA [21].

a

Lanthanides by ICP-MS (ug kgÿ1) amu La 139 Ce 140 Pr 141 Nd 145 Sm 147 Eu 151 Gd 157 Tb 159 Dy 163 Ho 165 Er 166 Tm 169 Yb 171 Lu 175

Sealed dec. Pb Bi Th U

<500 <60

15 <200 <200

69.5 163

370 420

0.052 0.1210.008

0.4800.030

2122 3174 39.30.7 1523 31.11.0 6.380.10 30.61.0 4.130.14 20.60.6 3.890.11 10.20.5 1.320.03 8.230.26 1.150.04

0.0376 0.8680.050 (0.0093) 0.03700.014 0.08450.0010

<23j <2.0j

2.70.5j <200i

17612j 3604j <7300j <350j 452j 7.31.4j

0.041j 0.077j

0.910.04

3.50.7 5.21.0 0.60.1 2.80.6 0.70.2

0.0716 0.0410.003 ND 0.001710.00019 0.004350.00031

3484 75318 87.21.2 3485 81.12.0 19.90.4 85.31.4 13.20.4 76.01.8 15.30.3 44.40.7 6.780.31 44.50.8 6.740.14

0.152 5.110.16 (0.116) 0.1140.005 0.3150.006

S. Sakao, H. Uchida / Analytica Chimica Acta 382 (1999) 215±223 219

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using 115 In as the standard. The original concentration of In should be negligible, compared with 10 mg kgÿ1 added as the internal standard. The results obtained as the average from two samples are listed in Table 3 in parenthesis. The total concentration of the original and added was used subsequently for the internal standardization. 3.3. Determination of trace elements by Q-ICP-MS after acid digestion

Fig. 1. Mass spectra for 59 Co‡ obtained by HR-ICP-MS solid line, Oyster tissue digested solution after 5 mg kgÿ1 Co addition as internal standard; dashed line, 5 mg kgÿ1 Co solution.

factor of more than 500, which corresponds to 0.5 g sample prepared in 2500 ml solution [15]. There are signi®cant spectroscopical interferences in ICP-MS. For the selection of internal standard elements, spectral overlapping with polyatomic ions should be examined ®rst. Although the ion signal of 59 Co‡ is sometimes used as the internal standard, overlapping of mass spectra is observed, as shown in Fig. 1. The spectrum was measured with a resolution of 3000 (M/dM). The small signal observed in the higher mass range might be caused by 43 Ca16 O‡ or 42 Ca16 O1 H‡ , because this peak was clearly observed in the Oyster tissue and the remainder of the scallop, where the concentration of Ca is relatively high. The effect of this overlapping on the actual determination using Q-ICP-MS might not be too serious, because the formation of polyatomic ion is less easily observed in 27.12 MHz Q-ICP-MS than in 40.68 MHz HR-ICPMS [20]. The added concentration of Co as the internal standard should be higher, because then the effect of polyatomic ion signals should be more reduced. For Y‡ and Bi‡ of other internal standard elements, mass spectrum overlapping was not observed for the investigated shell®sh tissue samples even with a resolution of 8000 (M/dM) in the same HR-MS system. Furthermore the concentration of Co, Y and Bi in original samples might not be negligible. The original concentrations of these elements were determined before the addition of Co, Y and Bi by HR-ICP-MS

The results obtained of trace elements by Q-ICPMS, after acid digestion and 10 times dilution with the addition of the internal standard elements, are shown in Table 3 with amu values used. An internal standard was used with a mass selected close to each ion for 59 Co‡ , 89 Y‡ and 209 Bi‡ . Found values of Al, Mn, Se, Rb, Sr, Cd, Ba, Pb and U did agree well with reported values for the Oyster tissue SRM [18]. However, values of V, Cr, Ni and As are found to be larger than those reported, which is caused by the mass spectral overlapping with polyatomic ions. For the Mussel CRM, Al, Mn, Cu, Se, Sr, Cd, and Pb also agree well with certi®ed and reference values [19]. The spectral overlapping seems to be also existent. Some results for the elements obtained in both SRM and CRM are in good agreement with reported values by neutron activation analysis (NAA) [18,21]. The content level of the trace elements for the adductor of the scallop is found to be signi®cantly lower than that of the remainder. The remainder part seems to be effective in concentrating metals, as shown for Cd (more than 100 mg kgÿ1). 3.4. Removal of mass spectral interference by HRICP-MS As overlapping of molecular ion mass spectra was observed for some elements, the signal measurements of 51 V‡ , 52 Cr‡ and 60 Ni‡ (3000 M/dM) and 75 As‡ (8000 M/dM) were carried out using the HR-ICP mass spectrometer. The analytical results are listed in Table 3. The overlapping of 51 V‡ with 35 Cl16 O‡ was indicated for the digested Sargasso solution [15], which might be caused by the decomposition of the sample by addition of HClO4. It is very dif®cult to remove the amount of perchloric compounds in digested samples.

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221

Fig. 2. Mass spectra for 52 Cr‡ obtained by HR-ICP-MS solid line, Oyster tissue digested solution; dashed line, 5 mg kgÿ1 Cr solution.

Fig. 4. Mass spectra for 75 As‡ obtained by HR-ICP-MS solid line, Oyster tissue digested solution; dashed line, 5 mg kgÿ1 Cr solution.

Analytical results for V became closer to the reported values for three samples, compared with those obtained by Q-ICP-MS. However, they still do not agree well with reported values. One of the reason might be the loss during the digestion. The overlapping of 52 Cr‡ with 40 Ar12 C‡ is shown in Fig. 2. The effect of 40 Ar12 C‡ could be reduced by the removal of organic compounds in acid digestion. The 60 Ni‡ determination might be interfered by 44 Ca16 O‡ in the sample, as shown in Fig. 3. This interfere is clearly found in the analytical values for the remainder of the scallop having a high concentration of Ca. Analytical

values for Cr and Ni are in fairly good agreement with reported values [18,19]. Fig. 4 also indicates the overlapping of 75 As‡ with 40 Ar35 Cl‡ . The amu of 75 is the only available for the determination of As. The overlapping did not affect the determination of As in Sargasso and Brown Alga [15], because the concentration of As is high in these CRMs. Analytical values for As determined by the HR-ICP-MS for Oyster and Mussel agreed well with reported values [18,19].

Fig. 3. Mass spectra for 60 Ni‡ obtained by HR-ICP-MS solid line, Oyster tissue digested solution; dashed line, 5 mg kgÿ1 Cr solution.

3.5. Decomposition in sealed bomb A sealed bomb has been used for the determination of biochemical samples and was able to prevent the loss of volatile elements [22]. Further, the decomposition without HClO4 should be effective for the reduction of the overlapping with polyatomic ion caused by Cl in ICP-MS. Analytical results for As, Se and Hg are also listed in Table 3. Although Hg might be lost in HNO3±HClO4 digestion, the obtained Hg value in the sealed bomb almost agrees well with the reported value, as shown in Mussel CRM [19]. The As values obtained by the sealed decomposition and Q-ICP-MS also agree well with those by HR-ICP-MS, and the value of Mussel also agrees with reported value [19]. The Cl concentration should be very low in the decomposed solution. Analytical values for Se seem to be slightly higher than those by HNO3±HClO4 digestion. However, the Se concentration in Oyster and Mussel obtained in HNO3±HClO4 digestion

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S. Sakao, H. Uchida / Analytica Chimica Acta 382 (1999) 215±223

agreed well with the reported values [18,19], as shown in Table 3. The reason for these results is not known yet. 3.6. Determination and chondrite pattern of lanthanides ICP-MS is one of the most useful methods for the determination of ultra-trace levels of all lanthanides, because of high sensitivities and low blank concentration in digested solutions. The obtained results by QICP-MS after acid digestion are listed in Table 3. The effect of BaO‡ overlapping on Eu determination was corrected by using the analytical value of Ba [15]. Analytical results for some of the lanthanides almost agree with those by NAA, but some do not [18,21]. The amount of the lanthanides for Oyster, Mussel and the remainder of scallop are almost at the same level, however, those in the adductor of scallop are fairly low. Because only a few data are available for comparison, the accuracy of the analytical results is often evaluated by inspecting lanthanide distribution patterns. The lanthanide pattern is obtained by normalizing the lanthanide abundance in the sample to that of chondrite. The results obtained by using the lanthanide abundance of Leedy chondrite [23,24] are shown in

Fig. 5. Normalized lanthanide patterns for shellfish tissue samples.

Fig. 5. The patterns indicate smooth curves with a clearly negative deviation for Eu, and also a slightly negative one for Ce in Oyster. The perfect curve was not obtained in the adductor of scallop, because of the lack in sensitivity for heavy lanthanides present at low concentrations.

Acknowledgements The authors express their gratitude to Mr. K. Ishii (JEOL) for the measurements by HR-ICP-MS and joining the discussion about the HR-ICP-MS analytical results.

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