Trace Analysis Guide
| Ion; Concentration;Duration of Experiment(h = hour) | Material | pH where Adsorption IS significant | pH where Adsorption IS NOT significant |
|---|---|---|---|
| Ag; 1ppm; 1h | PE; PTFE | pH 2-12 | "acid solution" |
| Al(III); 1-10ppm; 24h | borosilicateglass | pH 3.5-11 | pH<1.5; pH>13 |
| Ca(II); 0.5-2ppm; 24h | borosilicateglass | pH 8-12 | pH 1.5 |
| Cr(III); 1-5ppm; 24h | borosilicateglass | pH 3.5-12 | pH 1.5 |
| Pb(II); 10-13ppm; 24h | borosilicateglass | pH 3.5-12 | pH 1.5 |
Adsorption
Adsorption is divided into the following physical or chemical types:
- Physical adsorption is an attraction between the solid surface and adsorbing species consisting of van der waals interactions.
- Chemical adsorption or chemisorption is a chemical interaction which is strong enough to break or form chemical bonds.
- Types of losses such as ion-exchange, reduction, precipitation, and diffusion into a permeable solid are often also treated as adsorption.
In this guide, adsorption is taken to mean loss through the combined effects of all interactions with the walls of vessels or with filter paper.
The pH of the solution is an important consideration. Most trace analysts prefer to prepare or adjust solutions to a pH of < 2, as shown in Table 7.1 below.
Table 7.1: Effect of pH Upon the Adsorption of Inorganic Ions onto Various Materials1Attempts have been made to prevent adsorption by complex formation, as shown in Table 7.2, but the use of relatively high levels of reagents increases the risk of contamination. Furthermore, it tends to influence the stability and chemistry of other analytes of interest.
Table 7.2: Prevention of Adsorption by Complex Formation1
Attempts to remove adsorbed ions typically require extreme conditions and are only partially successful, as seen in Table 7.3.
Table 7.3: Desorption of Adsorbed Inorganic Ions1
PPB Stability Study
An adsorption test was conducted at Inorganic Ventures in an attempt to better understand the stability of mixed element solutions at the ppb concentration level in low density polyethylene (LDPE) bottles. The stability of metals at the ppb level in this container material was of significant concern.
Experimental Design
- A blend of 65 elements from Inorganic Ventures / IV Labs' CMS-SET was prepared at the 0, 2, 10, and 100 ppb concentration level in 1 % (v/v) HNO3 at the start of the study.
- The set consists of the following:
- CMS-1 - 10 µg/mL Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y in 3.5 % HNO3
- CMS-2 - 10 µg/mL Au, Ir, Pd, Pt, Re, Rh, Ru, and Te in 3.5 % HCl
- CMS-3 - 10 µg/mL Ge, Hf, Mo, Nb, Ta, Sn, Ti, W, and Zr in 3.5 % HNO3 tr. HF
- CMS-4 - 10 µg/mL Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, and V in 3.5 % HNO3
- CMS-5 - 10 µg/mL Ag, Al, Ca, Cs, Cr+3, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn in 3.5 % HNO3
- Only LDPE bottles (500 mL) were used.
- The LDPE bottles were acid leached with 1% nitric acid for 59 hours at 60 °C. New blends prepared in the same way were compared to the original preparation at 1, 3, 25, 75, 137, 300, and 375 days.
- The New blends were compared to the original blend using ICP-MS and the relative % loss was calculated.
- The ICP-MS used is in a clean room limiting environmental contamination (opened 1 % nitric acid solutions in similar LDPE bottles placed around the auto-sampler yielded no detectable environmental contamination at times of ~ 100 hours).
- Measurements of each blend were made in the same LDPE bottle i.e. the blend was not exposed to any other container during the study.
Experiment Results
- Hg was not stable long enough to measure (minutes).
- Au was the next most unstable element, showing instability at the 2, 20, and 100 ppb levels at 3 days.
- Pd showed instability only at the 2 and 10 ppb levels at 3 days.
- Pt and Ta showed instability only at the 2 and 10 ppb levels at 137 days.
- Ag showed instability only at the 10 and 100 ppb levels at 137 days.
- Mo, Sn, and Hf showed instability only at the 2 ppb level at 375 days.
- Ir showed instability only at the 2 ppb level at 300 days.
- All other elements showed no instability at 2-100 ppb for 375 days, including:
Ce, Dy, Er, Eu, Gd, Ho, La, Lu, Nd, Pr, Sm, Sc, Tb, Th, Tm, U, Yb, Y, Re, Rh, Ru, Te, Ge, Nb, Ti, W, Zr, Sb, As, Ba, Be, Bi, B, Cd, Ga, In, Pb, Se, Tl, V, Al, Ca, Cs, Cr+3, Co, Cu, Fe, Li, Mg, Mn, Ni, K, Rb, Na, Sr, and Zn.
Data - Elements with Instability:
The stability curves for the above elements listed as having some form of instability are shown below:
