This article is based on a poster originally authored by Rui Santos, Oliver Buettel, and Siqi Sun.
EN 16174:2012 analyzes multi-elements in the dissolution of treated biowaste, sludge, and sediment in aqua regia. However, the dissolution of environmental specimens using aqua regia may not completely disintegrate the element, resulting in the extraction from the specimen not reflecting the total concentration levels of the target analytes.
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To achieve the total disintegration of elements, various acid mixtures were measured against the aqua regia results. To get a quicker procedure for multi-elemental analyses of soil specimens with inductively coupled plasma-mass spectrometry (ICP-MS), one-step microwave-facilitated dissolutions were explored.
Certified reference material stream soil (NCS DC 73325), supplied by the China National Analysis Center for Iron and Steel, was utilized. The extraction capabilities investigated were for the following reagent mixtures: HCl/HNO3 (3:1 v/v, i.e., aqua regia) following the EN 16174 method and a mixture of HNO3/HCl/HF (8:1:1 v/v/v) for a full dissolution.
The chosen elements were Al, As, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Se, Sn, Sr, Ti, Tl, U, V, W, Zn, and Zr.
The majority were analyzed using aqua regia to dissolve the sediment specimens. Aqua regia was easy to use and produced adequate availability for environmental screening for Li, Be, Ca, Ti, V, Cr, Co, Ni, Cu, Zn, Se, Zr, Mo, Cd, Sn, W, Hg, Tl, Pb, and U. When HNO3/HCl/HF (v/v/v 8:1:1) was used, the level of recovery for these elements improved.
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Instrumentation
The PlasmaQuant MS, featuring integrated collision/reaction cell (iCRC) technology for removing polyatomic species in plasma and improving the accuracy and precision of analysis, was used.
The ICP-MS system was integrated with a CETAC ASX-560 autosampler, an ASXPress Plus injection valve, a Scott-type spray chamber with a Peltier chiller, and a MicroMistTM nebulizer 0.1 mL/min to quantify 31 elements in Certified Reference Material sediment (NCS DC 73325) supplied by the China National Analysis Center for Iron and Steel, Beijing, China.
A closed container, microwave-facilitated speed wave XPERT system dissolved the sediment specimens. PlasmaQuant MS determined the mix of the chosen metals and metalloids in the formulated solutions. The testing was completed in a standard analytical laboratory. A ‘clean room’ was not utilized.
Table 1 summarizes the instrument operating parameters, including iCRC modes that used helium and hydrogen gases to eliminate spectroscopic interferences on the first-row transition metals.
Table 1. PlasmaQuant MS Elite operating conditions. Source: Analytik Jena US
| Parameter |
Specification |
| Plasma Gas Flow |
10.5 L/min |
| Auxiliary Gas Flow |
1.50 L/min |
| Sheath Gas Flow |
0.00 L/min |
| Nebulizer Gas Flow |
1.06 L/min |
| Sampling depth |
5.0 mm |
| Plasma RF Power |
1.40 kW |
| Ramp Rate |
20 rpm – black/black PVC pump tubing (<1 mL/min) |
| Stabilization delay |
10 s |
| iCRC Gas Setting |
He 120 mL/min: 44Ca, 52Cr, 56Fe, 95Mo, 112Cd, and 200Hg |
| No Gas: 7Li, 9Be, 11B, 23Na, 24Mg, 27Al, 39K, 49Ti, 55Mn, 59Co, 60Ni, 65Cu, 66Zn, 86Sr, 90Zr, 118Sn, 137Ba, 182W, 205Tl, 206+207+208Pb, 209Bi and 238U |
| H2 120 mL/min: 51V, 75As and 78Se |
| Dwell Time |
10 ms (No Gas) and 20 ms (iCRC) |
| Scan per Replicate |
25 (peak hopping, 1 pt/peak) |
| No. of Replicates |
6 |
| Sample uptake time |
0 s – OneFAST Sample Introduction system used |
| Internal Standards |
159Tb and 175Lu at 5 μg/L, interpolate correction |
Sample Preparation and Reagents
Certified reference material sediment (NCS DC 73325), China National Analysis Center for Iron and Steel, Beijing, China, was utilized for verification. Every solid specimen was placed in the pre-cleaned speed wave XPERT dissolution system container, mixed with the reagent (s). 0.1 g of test sample was dried, sieved, and weighed before transferring to the decomposition container.
The specimens were removed using two different methods:
- Following EN 16174: two drops of water, 6 mL of HCl, 2 mL of HNO3 were consecutively added and mixed well;
- Full dissolution: two drops of water, 1 mL of HCl, 8 mL of HNO3, and 1 mL HF were consecutively added and mixed well.
The decomposition containers were closed when there were no violent reactions in either method. The extraction mixture temperature was elevated at 10 ºC/min to 175 +/- 5 ºC and held for 10 minutes (EN 16174). For the full dissolution, a temperature of 230 +/- 5 ºC was obtained and held for 20 minutes. The containers were cooled to room temperature for 30 minutes before being opened.
Before the ICP-MS analysis, the extracts were diluted 10-fold with 1% (v/v) HNO3.
Results and Discussion
Table 2 reveals that using the aqua regia as a reagent for wet digestion achieves a reasonable rate of extraction (i.e., higher than 70%) for Be, V, Cr, Mn, Co, Cu, As, Mo, Cd, Ba, Hg, Pb and Bi.
An extraction rate above 50% was achieved for Li, Al, Fe, Ni, Zn, Sr, Sn, Tl, and U. Aqua regia removed 22 of 31 elements with higher than 50% recoveries. The recoveries were larger for some metals (e.g., Ti, Ni, Cu, Zn, Zr, Mo, Sn, and Tl).
The major exclusions to these results were Mg, Al, Ca, Sr, and Ba in solutions containing HF. Al was found in lower concentration levels due to the precipitation of aluminum fluoride. The other alkaline earth elements (Mg, Ca, Sr, and Ba) had exceptionally lower 1-9% recoveries.
This is explained by the nature of metal-soil binding and low solubility of MgF2 (Ksp = 5.16 x 10-11), CaF2 (Ksp = 3.45 x 10-11), SrF2 (Ksp = 4.33 x 10-9) and BaF2 (Ksp = 1.84 x 10-7). The other earth-alkaline element, Be, increased in recovery, with HF becoming the water-soluble BeF2. Mixing Ti in an acidic solution like HNO3 and HCl creates insoluble oxo-complexes. However, Ti reacts to HF and yields a soluble fluoro complex [TiF6].
The results for Tl can be explained by Tl chemistry. In the presence of HCl, Tl forms the barely soluble TlCl (Ksp = 1.86 x 10-4). The reaction of Tl and HF forms TlF, which is soluble, resulting in an elevated recovery rate of Tl. The results for Sn indicate an elevated recovery when a stronger reagent for dissolution is used (e.g., HF).
When Na and K, two significant elements, were analyzed, the relative standard deviation (RSD) values decreased when a stronger dissolution method was utilized. The recoveries of Na and K did not improve significantly with HF. The small volume of HF utilized (only 1 mL) to process more than 32% of SiO2 present could be a reason. RSD was present at approximately 5% or less for most elements.
Table 2. Metals and semi-metals concentration, recoveries and precision achieved in the soil CRM NCS DC 73325 after microwave-assisted digestion with various acid mixtures (n=8); Con and MDL in mg/kg. Source: Analytik Jena US
| Isotope |
Certified
value
(mg/kg) |
HCl/HNO3
(3:1 v/v) |
HNO3/HCl/HF
(8:1:1 v/v/v) |
RSD (%) |
MDL |
| Found |
REC (%) |
Found |
REC (%) |
| Li |
19.5 ± 0.9 |
11.3 |
58 |
15.6 |
80 |
2.4 |
1.70 |
| Be |
2.8 ± 0.6 |
2.0 |
73 |
2.5 |
89 |
2.4 |
0.06 |
| B |
(10) |
<LOQ |
- |
<LOQ |
- |
- |
5.22 |
| Na |
593.5 ± 148 |
158 |
27 |
165 |
28 |
4.4 |
26.5 |
| Mg |
1568 ± 181 |
685 |
44 |
23 |
1 |
5.2 |
6.25 |
| Al |
154896 ± 1800 |
5404 |
49 |
19240 |
12 |
6.4 |
27.2 |
| K |
1660 ± 166 |
357 |
21 |
522 |
31 |
4.6 |
10.6 |
| Ca |
1144 ± 143 |
338 |
30 |
107 |
9 |
8.1 |
32.4 |
| Ti |
20200 ± 500 |
4937 |
24 |
15564 |
77 |
3.8 |
10.6 |
| V |
245 ± 21 |
187 |
76 |
210 |
86 |
3.1 |
0.93 |
| Cr |
410 ± 23 |
341 |
83 |
352 |
86 |
2.7 |
0.92 |
| Mn |
1780 ± 113 |
1635 |
92 |
1176 |
66 |
2.9 |
0.96 |
| Fe |
(97) |
90332 |
65 |
85578 |
61 |
3.0 |
26.5 |
| Co |
139375 ± 6 |
87.7 |
90 |
93.5 |
96 |
1.4 |
0.73 |
| Ni |
276 ± 15 |
161 |
58 |
263 |
95 |
2.0 |
1.45 |
| Cu |
97 ± 6 |
70.1 |
72 |
92.6 |
95 |
1.2 |
0.72 |
| Zn |
142 ± 11 |
84.0 |
59 |
113 |
80 |
1.1 |
0.97 |
| As |
4.8 ± 1.3 |
4.6 |
95 |
5.1 |
105 |
3.8 |
0.34 |
| Se |
0.32 ± 0.05 |
<LOQ |
- |
0.24 |
74 |
13 |
0.39 |
| Sr |
26 ± 4 |
14.9 |
57 |
0.28 |
1 |
4.1 |
0.47 |
| Zr |
318 ± 37 |
110 |
35 |
289 |
91 |
1.9 |
1.16 |
| Mo |
2.9 ± 0.3 |
2.2 |
74 |
3.1 |
105 |
1.7 |
0.09 |
| Cd |
0.08 ± 0.02 |
0.08 |
97 |
0.08 |
101 |
13 |
0.01 |
| Sn |
3.6 ± 0.7 |
2.5 |
68 |
3.9 |
109 |
2.9 |
0.19 |
| Ba |
180 ± 27 |
125 |
70 |
16 |
9 |
2.4 |
0.66 |
| W |
1.2 ± 0.2 |
<LOQ |
- |
1.09 |
91 |
3.1 |
0.70 |
| Hg |
0.061 ± 0.006 |
0.05 |
79 |
0.06 |
107 |
8.5 |
0.12 |
| Tl |
0.21 ± 0.06 |
0.13 |
63 |
0.19 |
89 |
3.4 |
0.18 |
| Pb |
14 ± 3 |
10.9 |
78 |
12.8 |
92 |
1.4 |
1.90 |
| Bi |
0.2 ± 0.04 |
0.18 |
91 |
0.17 |
85 |
3.3 |
0.03 |
| U |
2.2 ± 0.4 |
1.16 |
53 |
1.24 |
57 |
2.8 |
0.03 |
Conclusions
It is important to remember that a low recovery rate does not determine if a method is unsuitable because recovery is affected by a minimum of four parameters. The effectiveness of the digestion process should consider the chemical properties of the analyzed element, the nature of the metal-to-matrix binding, the extraction reagent(s) utilized, and the temperature of the extraction procedure. The extraction method utilized should be founded on the investigative goals.
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This information has been sourced, reviewed and adapted from materials provided by Analytik Jena US.
For more information on this source, please visit Analytik Jena US.