4 Methods for chemical analyses
4.4 Digestion and quantification of silver and silver nanoparticles in aqueous and sediment samples (OECD 219, test with chironomids) and sediment samples (OECD 219, test with chironomids)
4.4.1 Procedure
After thoroughly shaking the samples (vortex) 1 mL of the aqueous mixture was transferred into quartz digestion vessels and 2 mL of conc. nitric acid as well as 4 mL of Ultra-Pure water were added. The subsequent digestion was performed using an Ultra Clave II microwave (MLS GmbH, Leutkirch im Allgäu, Germany).
Aqueous samples
The following microwave program was applied:
Step 1: 25 min heating up to 220 °C Step 2: 30 min at 220 °C
Thereafter, the digested samples were poured into volumetric flasks and filled up with ultrapure water to an exact volume of 15 mL. This final solution was analysed by ICP-OES for its amount of silver.
The digestion procedure was performed according to DIN ISO 11466 and DIN EN 13346/DEV S7a. Therefore, prior to digestion the sediment was dried at 105 °C until constant weight for at least 12 h. Then approximately 3 g of the homogenised material was weighed and 28 g of aqua regia were added. After 16 h without agitation at room temperature this mixture was heated un-der reflux for two hours. To avoid over-boiling glass chips were added and foaming was avoided by adding a few drops of 1-octanole. The mixture was cooled to room temperature and then carefully brought to an exact volume of 100 mL. This aqua regia extract was filtered (0.45 µm, Syringe Filter, Supor membrane, Pall Corporation, New York) and the silver concentration was determined by ICP-OES with a matrix-adjusted calibration.
Sediment samples
4.4.2 Analytical measurement
Nitric acid was of “Suprapur®” (supplied by Carl Roth, Karlsruhe) and hydrochloric acid of “in-stra-analysed” quality (supplied by Mallinckrodt Baker, Griesheim, Germany). The water used was purified using a Pure Lab Ultra water purification system (purified water
resistiv-ity >18 MΩ∙cm).
Reagents for silver analysis
A commercially available silver ICP-standard containing 1000 mg/L Ag in nitric acid 2-3% (lot no.
HC936000, CertiPUR®, Merck, Darmstadt, Germany) was used to prepare appropriate stock solutions and respective calibration solutions. All prepared standard solutions had a final HNO3 concentration of 3%.
The analysed certified aqueous reference material was purchased from Environment Canada (TMDA-70, lot 310). Unfortunately, the certified value is 10.9 µg Ag/L. For higher calibration ranges this certified value is above the limit of quantification because the LOD (limit of detection) and LOQ (limit of quantification) are strongly dependent on calibration. Therefore the measured silver concentrations in TMDA-70 could not be quantified reliably. Furthermore, a multi element Merck IV Standard (appropriately diluted to fit in the range of samples, lot HC957274, purchased from Merck, Darmstadt, Germany) was analysed along with the samples to verify the measured results. To verify the microwave procedure, Merck IV solution was also digested along with the aqueous test samples.
Certified reference materials (chapter 21.1.6) and verifying the method
For sediment samples the certified reference material CRM026-050 Sandy Loam (RT Corpora-tion, Laramie, USA, reference value for Ag is 0.57 mg/kg) was digested along with the sediment samples to additionally verify the microwave procedure.
The silver concentrations in digested and non-digested reagent blanks were always below the limit of quantification.
All materials used for sample treatment were suitable for the analysis of silver at trace levels.
The glassware (beakers and volumetric flasks) was cleaned using a Miele washer “Automatic Disinfector” combined with a water de-ioniser “Aquapurificator”, steamed out with HNO3, rinsed with ultrapure water and dried at approximately 60°C. The pipettes used were adjustable to vari-able volumes (50 - 250 µL, 200 - 1000 µL, 1000 - 5000 µL) and were purchased from Gilson (Abimed, Langenfeld, Germany) and Eppendorf (Wesseling, Germany).
Laboratory equipment
Silver concentrations of aqueous samples were measured using an IRIS Intrepid II ICP-OES (Thermo Electron, Dreieich, Germany). Silver was detected at the wavelengths 328.068 nm, and 338.289 nm. Calibrations were performed before each measurement. Depending on the concen-tration range in the samples the following calibration solutions were used: blank, 2.5 µg/L, 5.0 10, 25 50, 100, 250, 500, 1000, and 2000 µg/L. The calibration formula was calculated using the linear regression algorithm of the ICP-OES instrument software. Due to spectral interferences at the wavelength of 338.289 nm, especially in the sediment samples the obtained data from 328.068 were used for calculating concentrations. Correlation coefficients (r) were at least ICP-OES (raw data example: chapter 21.1.2)
0.9999. For each sample, at least three internal measurements were performed and the mean was calculated and printed by the instrument software.
The applied LOD/LOQ calculations are:
LOD: 3 * method standard deviation from calibration line LOQ: 10 * method standard deviation from calibration line.
The information about the LOD/LOQ and correlation coefficient is compiled in Table 6.
A representative calibration line is shown in the raw data chapter.
Coefficients of determination (r) for respective calibration functions were taken from ICP-OES instrument outputs.
Table 6: Silver in aqueous and sediment samples: LODs/LOQs, correlation.
Measurement date, description LOD [µg/L]
digested aqueous samples 7d, 14d and 28d
1.9 6.3 1 0.9999 March 02, 2011,
digested aqueous samples 0d, 1d 3.6 12 1 1.0000 March 02, 2011,
digested sediment samples 3.9 13 1 1.0000
1 Internal LOQ calculation was performed with more digits
Instrumental and analytical set-up of the ICP-OES -Thermo IRIS Intrepid II
-Thermo Electron Corporation, Germany -Analytical conditions
-Nebuliser: Concentric glass nebuliser, Thermo Electron Corporation, Dreieich, Germany -Spray chamber: Glass cyclonic spray chamber, Thermo Electron Corporation, Dreieich, Ger-many
-Nebuliser gas flow: 0.68 L/min -Make-up gas flow: 0.5 L/min -RF power: 1150 W
-Wavelengths: 328.068 nm, 338.289 nm (not evaluated due to spectral interferences)
The certified reference material TMDA-70 (certified with 10.9 µg/L Ag) was analysed as quality assurance sample with solution samples from the test. According to the quality assurance re-quirement, the silver recovery was in the range of ± 15% of the certified value. However, regard-ing Ag concentrations measured by ICP-OES, the mean recovery (accuracy) and precision of the non-digested CRM TMDA-70 measurements were 104 ± 5.5% (n = 6).
Quality assurance measurements
The recovery for digested Merck IV standard solution samples containing 2.5 mg/L (to verify the digestion procedure for aqueous samples) was 105 ± 1.4% (n = 12). For non-digested Merck IV
samples the accuracy was determined to 106 ± 0.4% (n =2) for 500 µg/L and 101 ± 0.8% (n = 2) for 250 µg/L.
Analysis of acidified ultrapure water as reagent blanks as well as digestion and analysis of SiO2 as blanks for sediment samples revealed silver concentrations which were always at least below the limit of quantification of the respective measurement series.
For further quality assurance, recalibration samples were analysed along with the samples and the mean accuracy was determined to 98.4 ± 1.2% (n = 3) for an Ag concentration of 100 µg/L and 99.2 ± 0.6% (n = 2) for 200 µg/L.
For collecting validation information on the digestion procedure of sediment samples as well as the analytical method, the mean recovery of silver in the certified reference material “026-050 Sandy loam” was determined to 114 ± 3.4% (n =3). Regarding the quality requirements the re-coveries of metals in soils and sediments must be in the range of 100 ± 25%.
The validation information is summarised in Table 7.
Table 7: Silver in aqueous and sediment samples: method validation information.
Validation pa-rameter
Results Comment
Selectivity two different wavelengths for ICP-OES method interferences only observed at 338.289 nm Linearity applied calibration functions were linear see Table 6 correlation coefficient (r) at
least 0.9999 sample from measurement series of March 02, 2011 < LOQ = < 11.9 µg/L
-
Accuracy and precision
mean recovery for TMDA-70:
104 ± 5.5% (n = 6)
for low concentration range of samples (10.9 µg/L)
Accuracy and precision
mean recovery for non digested Merck IV (500 µg/L): 106 ± 0.4% (n = 2)
corresponds to concentration range of samples (500 µg/L)
Accuracy and precision
mean recovery for non digested Merck IV (250 µg/L): 101 ± 0.8% (n = 2)
corresponds to concentration range of samples (250 µg/L)
Accuracy and precision
mean recovery for recalibration (100 µg/L):
97.2± 1.1% (n = 2)
corresponds to concentration range of samples (100 µg/L)
Accuracy and precision
mean recovery for recalibration (200 µg/L):
98.2 ± 1.0% (n = 2)
corresponds to concentration range of samples (200 µg/L)
accuracy and precision
mean recovery for digested Merck IV (2.5 mg/L):
105 ± 1.4% (n = 12)
corresponds to concentrations of applied loadings
Accuracy and precision
mean recovery for recalibration samples of 100 µg/L: 98.4 ± 1.2 (n = 3)
corresponds to concentration range of samples (100 µg/L)
Accuracy and precision
mean recovery for recalibration samples of 200 µg/L: 99.2 ± 0.6 (n = 2)
corresponds to concentration range of samples (200 µg/L)
Accuracy and precision
mean recovery for silver in digested CRM026-050:
114 ± 3.4 (n = 3)
Certified with 0.570 mg/kg
Reproducibility mean recovery for TMDA-70:
104 ± 5.5% (n = 6)
for low concentration range of samples (10.9 µg/L)
Reproducibility mean recovery for non digested Merck IV (500 µg/L): 106 ± 0.4% (n = 2)
corresponds to concentration range of samples (500 µg/L)
Reproducibility mean recovery for non digested Merck IV (250 µg/L): 101 ± 0.8% (n = 2)
corresponds to concentration range of samples (250 µg/L)
Reproducibility mean recovery for recalibration (100 µg/L):
97.2± 1.1% (n = 2)
corresponds to concentration range of samples (100 µg/L)
Reproducibility mean recovery for recalibration (200 µg/L):
98.2 ± 1.0% (n = 2)
corresponds to concentration range of samples (200 µg/L)
Reproducibility mean recovery for digested Merck IV (2.5 mg/L):
105 ± 1.4% (n = 12)
corresponds to concentrations of applied loadings
Reproducibility mean recovery for recalibration samples of 100 µg/L: 98.4 ± 1.2 (n = 3)
corresponds to concentration range of samples (100 µg/L)
Reproducibility mean recovery for recalibration samples of 200 µg/L: 99.2 ± 0.6 (n = 2)
corresponds to concentration range of samples (200 µg/L)
Reproducibility mean recovery for silver in digested CRM026-050:
114 ± 3.4 (n = 3)
certified with 0.570 mg/kg
One millilitre of the samples were digested and to filled to 15 mL with ultrapure water (dilution factor 15). Prior to measurements the stock solution samples were additionally diluted 1:10 be-cause of their higher concentration (dilution factor for stock solution samples is 150). When the measured value by ICP-OES without dilution factor was below the limit of detection (LOD) or quantification (LOQ) this is denoted in the tables. To obtain the measured silver concentrations of the instrument the values have to be divided by 15, and the data for the stock solutions by 150.
Presentation of the results
The amount of silver in dispersion was determined by UV-VIS measurements without preceding digestion. As a certified standard solution containing nano-Ag is not yet available, the calibration used for this method is performed with a silver standard. The quantification of total silver carried out after total digestion differs from the amount reported by the producer as the measurement performed by the producer was performed without matrix-adjusted calibration.