4 Methods for chemical analyses
4.7 Dissolution and quantification of nano-Au in aqueous and sediment sam- sam-ples sam-ples
In order to dissolve gold nanoparticles within aqueous samples Aqua regia was applied. There-fore 1 mL of Aqua regia was added to 1 mL of the aqueous test sample. The mixture was care-fully vortexed or shaken (for 5 min) and remained for at least for additional 24 h without agitation at room temperature prior to analytical determination.
Dissolution of gold nanoparticles in aqueous samples
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. Thereafter, approx. 3 g of the homogenised material were weighed and 28 g of Aqua regia were added. After 16 h at room temperature - without agitation - this mixture was heated under reflux for two hours. To avoid foaming and overboiling a few drops of 1-octanole and glass chips were added. 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, VWR, Darmstadt) and the gold concentration was determined by ICP-OES with a matrix-adjusted calibration.
Dissolution of gold nanoparticles in sediment samples
4.7.2 Analytical measurement
All materials used for sample treatment were suitable for working with gold at trace levels. The applied glassware (beakers and volumetric flasks) was cleaned with a Miele washer “Automatic Disinfector” combined with a water de-ioniser “Aquapurificator”, afterwards washed three times with aqua regia (mixture of conc. hydchloric and conc. nitric acid in a ratio of 3 : 1) and rinsed three times with ultrapure water. The glassware was dried at approx. 60 °C.
Laboratory equipment and chemicals
The pipettes used in variable volumes (50 – 250 µL, 200 – 1000 µL, 1000 – 5000 µL) were pur-chased from Gilson (Abimed, Langenfeld, Germany) and Eppendorf (Wesseling, Germany).
The water used for the analytical investigation was purified with a Pure Lab Ultra water purifica-tion system from ELGA LabWater, Celle, Germany. The purified water has a resistivity greater than 18 MΩ∙cm.
The applied acids were:
-Hydrochloric acid – “Instra-Analyzed®” (supplied by Mallinckrodt Baker, Griesheim, Germany) -Aqua regia is not commercially available and was freshly prepared prior to usage. Therefore, concentrated hydrochloric and concentrated nitric acid were mixed in a ratio of 3:1.
For ICP-OES measurements a commercially available gold standard containing 1000 mg/L Au in tetrachloroaurat acid in 7% hydrochloric acid (CertiPUR®, Merck, Darmstadt, Germany) was applied. With this standard solution appropriate stock solutions and subsequently calibration solutions were prepared.
To determine the accuracy of the applied analytical method the NIST reference material 8011 (gold nanoparticles, nominal diameter 10 nm) was analysed along with aqueous samples of the test and the recovery was determined. For this material only an informative value of 51.56 ± 0.23 µg nano-gold /g was given. However, to verify the dissolution of nano-gold and the subse-quent analytical investigation this reference material turned out to be very feasible. The amount of nano-gold corresponded to 51.56 mg/L and was therefore appropriately diluted with ultrapure water to fit in the concentration range of test samples.
(Certified) Reference materials (chapter 21.1.7)
In order to further verify the analytical method, recalibration samples were analysed along with test item samples and recoveries were determined.
Gold concentrations of aqueous samples were measured using an IRIS Intrepid II ICP-OES (Thermo Electron, Dreieich, Germany). Gold was detected at wavelengths of 197.819, 208.209, and 242.795 nm. Calibrations were performed before each measurement. Depending on the concentration range in samples the following calibration solutions (matrix adjusted) were used:
blank, 1.0, 2.5, 5.0, 10, 20L, 25, 50L, 100, and 250 µg/L. The calibration formula was calculated using the linear regression algorithm of the ICP-OES instrument software. The wavelength hav-ing the best correlation and recoveries for reference materials and recalibration samples were used for evaluation (197.819 nm). Correlation coefficients (r) were at least 0.9992. For each sample, at least three internal measurements were performed and the mean was calculated and printed by the instrument software.
ICP-OES (raw data example: chapter 21.1.4)
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 5.
A representative calibration line is shown in the raw data chapter 21.1.
Coefficients of determination (r) for respective calibration functions were taken from ICP-OES instrument outputs.
The resulting values are reported in Table 11.
Table 11: Au in daphnids and chironomids: LODs/LOQs.
Measurement date, description LOD [µg/L]
sediment samples from chironomids test
2.7 9.0 0.999997
January 04, 2012
aqueous samples from chironomids test after 28 days
2.6 8.7 0.999168
December 19, 2012
aqueous samples from Daphnia test after 0 and 48 h and aqueous samples from chironomids test after 7 days
1.4 4.3 0.999922
November 30, 2011
aqueous samples from chironomids test after 0 and 1 days
1.6 5.4 0.999983
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: 197.819 nm (used for evaluation), 208.209 nm and 242.795 nm.
The NIST reference material 8011 (gold nanoparticles, nominal diameter 10 nm, informative value of 51.56 mg/L, appropriately diluted to fit in concentration range of samples) was digested and analysed as quality assurance sample with samples from the test. According to the quality assurance requirement, the gold recoveries (accuracy and precision) were in the range of ± 15%
of the certified value. However, regarding Au concentrations measured by ICP-OES, the mean recoveries (accuracy and precision) of NIST 8011 measurements were 97.4 ± 12.9% (n = 2, dilution factor 3750), 98.1 ± 2.6% (n = 2, dilution factor 750) and 97.8 ± 1.4 (n = 12).
Quality assurance measurements
To further verify the analytical determination the recoveries of recalibration standards were de-termined to 99.0 ± < 0.1% (n = 2) for 250 µg/L, 97.0 ± 2.6% (n = 6) for 50 µg/L and 107% (n = 1) for 12.5 µg/L.