4 Methods for chemical analyses
4.5 Quantification of silver in the extracts of DGTs, diffusive gradients in thin films films
4.5.1 DGTs in general
DGT means diffusive gradients in thin films. DGT devices can collect the dissolved ions. The concentrations can then be determined by instrumental analysis.
“The simple device uses a layer of Chelex resin impregnated in a hydrogel to accumulate the metals. The resin-layer is overlain by a diffusive layer of hydrogel and a filter. Ions have to dif-fuse through the filter and diffusive layer to reach the resin layer. It is the establishment of a constant concentration gradient in the diffusive layer that forms the basis for measuring metal concentrations in solution quantitatively without the need for separate calibration” [Technical documentation], http://www.dgtresearch.com].
For detailed information please see the above mentioned technical documentation and refer-ences cited within.
4.5.2 Procedure
For preparation, the DGT devices were placed into a 0.01 mol/L NaCl solution. Argon was intro-duced into the solution for one hour. Afterwards the vessel containing the DGTs and the NaCl solution was tightly closed and remained for 24 h under inert gas. Thereafter the DGT devices were carefully introduced into the test setup with chironomids by pushing them on the sediment.
Preparation and application of DGTs
The devices remained in the test vessels for 48 h. After their removal they were thoroughly rinsed with ultrapure water and wrapped into polyethylene bags for storage at 4 °C for no longer than 4 days. The DGTs were broken up and the resin layer was extracted and directly trans-ferred into 1.5 mL 1 mol/L nitric acid for elution of silver ions for at least 24 h. An exact volume of 1 mol/L nitric acid was added and the solution was sampled and analysed for its amount of silver
4.5.3 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 multi element ICP-standard containing 1000 mg/L Ag in nitric acid 2-3% (lot no. HC957274, ICP Multi Element Standard Solution IV, CertiPUR®, Merck, Darmstadt, Germany; chapter 21.1.6) was used to prepare appropriate stock solutions and respective cali-bration solutions. All prepared standard solutions had a final HNO3 concentration of 3%.
The analysed certified aqueous reference materials – appropriately diluted to fit in the concen-tration range of samples - were purchased from Environment Canada (TMDA-70, lot 310, certi-fied with 10.9 µg/L Ag and TMDWS2, certicerti-fied with 9.97 µg/L; chapter
Certified reference materials (chapter 21.1.6) and verifying the method
21.1.6).
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 approx. 60 °C. The pipettes used were adjustable to variable volumes (50 - 250 µL, 200 - 1000 µL, 1000 - 5000 µL) and were purchased from Gilson (Abi-med, Langenfeld, Germany) and Eppendorf (Wesseling, Germany).
Laboratory equipment
Silver concentrations of aqueous samples were measured using an Agilent 7500ce ICP-MS in-strument (Agilent Technologies, Waldbronn, Germany). Silver was detected at the isotope 109 in the no-gas mode of the machine. Calibrations were performed prior to the measurement se-ries. Depending on the concentration range in samples the following calibration solutions were used: 0.25, 0.50, 1.0, 2.5, 5.0, 10, and 25 µg/L. The calibration formula was calculated using the linear regression algorithm of the ICP-MS instrument software. Correlation coefficient (r) is 0.9999. For each sample, at least three internal measurements were performed and the mean was calculated and printed by the instrument software.
ICP-MS (raw data example: chapter 21.1.3)
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 8.
A representative calibration line is shown in the raw data chapter 21.1.2.
Coefficient of determination (r) for respective calibration function was taken from ICP-MS in-strument outputs.
Table 8: Determination of silver ions: LODs/LOQs, correlation.
Measurement date, descrip-tion
LOD [µg/L]
LOQ [µg/L]
Correlation coefficient r
March 4, 2011 0.0013 0.0039 0.9999
Instrumental and analytical set-up of the ICP-MS -Agilent 7500i (Agilent Technologies, Germany) -Analytical conditions
-Nebuliser: Micro Mist, Agilent Technologies, Germany -Spray chamber: Scott Type, Agilent Technologies, Germany -Nebuliser gas flow: 0.95 L/min
-Make-up gas flow: 0.12L/min -RF power: 1500 W
-No-gas mode I-sotope: 109Ag
The certified reference materials TMDA-70 (certified with 10.9 µg/L Ag) and TMDWS2 were ana-lysed as quality assurance sample with solution samples from the test. According to the quality assurance requirement, the silver recovery was in the range of ± 15% of the certified value.
However, regarding Ag concentrations measured by ICP-MS, the mean recovery (accuracy) and precision of CRM TMDA-70 (dilution factor 10) measurements were 95.8 ± 4.0% (n = 5) and 97.5 ± 5.5% (n = 6) for CRM TMDWS2 (dilution factor 5).
Quality assurance measurements (certificate of reference material chapter 21.1.6)
The silver concentration in reagent blanks analysed along with the actual samples were mostly at least below the limit of quantification (LOQ = 0.039 µg/L, n = 10). Because of the low LOQ the measured values of three additional reagent blanks exhibited higher concentrations (0.004 µg/L, 0.020 µg/L and 0.005 µg/L). However, the latter concentrations were far below the measured amounts in actual samples and therefore did not influence the analytical measurement series.
The validation information is summarised in Table 9.
Table 9: Determination of silver ions: information on method validation.
Validation parameter Results Comment
Selectivity Isotope 109Ag for ICP-MS interferences can be excluded for
109Ag
Linearity applied calibration functions were linear see Table 6 correlation coeffi-cient (r) at least 0.9999
Limit of detection (LOD) 0.013µg/L see Table 6
Limit of quantification (LOQ) 0.039 µg/L see Table 6
Reagent blanks < LOQ = < 0.039 µg/L (n = 10);
due to low LOQ three reagent blanks > LOQ:
0.004 µg/L, 0.020 µg/L, 0.005 µg/L (n = 3)
no influence on the analytical results
Accuracy and precision mean recovery for TMDA-70:
95.8 ± 4.0% (n = 5)
diluted to 1.09 µg/L (factor 10) for low concentration range
Accuracy and precision mean recovery for TMDWS2:
97.5 ± 5.5% (n = 6)
diluted to 1.99 µg/L (factor 5) for low concentration
Reproducibility mean recovery for TMDA-70:
95.8 ± 4.0% (n = 5)
diluted to 1.09 µg/L (factor 10) for low concentration range
Reproducibility mean recovery for TMDWS2:
97.5 ± 5.5% (n = 6)
diluted to 1.99 µg/L (factor 5) for low concentration
The amount of silver in dispersion in NM300K provided by the producer ´Rent a Scientist´ is determined by UV-VIS measurements without digestion. Because a certified standard solution containing nano-Ag is not available yet, the calibration used for this method was performed with a silver standard. Therefore, the analytical result that is provided by the producer was measured without matrix-adjusted calibration and may differ from the real value.
