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Element Compatibility and Stability for ICP Standards: EPA 200.7
Brian Alexander
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Topics
• Overview of EPA Method 200.7
• the effect of acid matrix
• proper container materials
• potential concentration limits
• other factors affecting stability
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Method 200.7
• Approved revisions: 4.4 and 5.0
• Scope: Trace element determinations using ICP-AES (OES)
– water, wastewater, solid wastes
• Consolidated existing methods including SW-846 (wastes)
• Applicable to 32 analytes (31 elements + SiO2)
• Detailed information on how to prepare analytical standards
– Starting materials
– Digestion/dissolution
– Matrix (both HNO3 and HCl)
– Potential issues
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200.7 Element Overview
• Alkali, alkaline earth, first row transition, others
• Mix of chemistries
• elements requiring HCl or HF (Ti, Sn, Si)
• elements that dislike Cl- (Ag, Pb, Tl)
• elements that dislike HF (alkaline earth, REE)
• Hg
• Method specifies HNO3 / HCl mixture for multi-element standards
• notes that concentrations can change upon aging
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Different Container Materials
• 200.7 recommends FEP for solution storage
(FEP = fluorinated ethylene propylene)
• Other common container materials include:
• LDPE (low density polyethylene)
• HDPE (high density polyethylene)
• PP (polypropylene)
• PMP (polymethylpentene)
• Borosilicate glass
• All are compatible with mineral acids except glass/HF
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Container Material Considerations• Plastic containers
• All 200.7 elements stable except,• Mercury in HNO3
• suffers from adsorption to plastic surfaces
• negligible effect at higher (>100 ppm) concentrations
• Lower concentrations recommend glass containers
• BUT: Hg in HNO3 in PE is stabilized by AuCl3 (EPA)
• Borosilicate glass
• Suitable for Hg in HNO3 at all concentrations
• Contamination risk for Na, Al, B, Ba, Ca, K and lower levels of Fe, Ni, Zn, Sr
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Plastic Container Contaminants
Leaching study
• 1% v/v HNO3
• 60 hours @60 °C
• LDPE cleanest
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Concentration Limits: Multi-element standards
Mixing “rules”
• lower is better
• what doesn’t work at 100 ppm can work at 10 ppm
• less acid required
• ~65 elements together at ppb levels (1% HNO3, tr. HCl, tr. HF)
• more elements can actually help
• order of addition can be critical
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Concentration Limits: Multi-element standards
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200.7 Compatibility Concerns
Problem elements
• Sn – either HCl (10%) or HF with HNO3
• Mixing Sn(HF) with REE (Ce, Y) can be tough
• Hg:
• HNO3 + LDPE = ~1 ppm adsorption to container walls
• 5% HNO3 in LDPE at high concentrations (>100 ppm) – can be stabilized at low concentrations with Au
• in 5% HNO3 in glass at all concentrations
• in 10% HCl at lower concentrations (<100 ppm) in LDPE
• Sb – if antimony stabilized as tartrate, do not mix!
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Stability of Multi-element standards (ppb)
Study – ppb stability 65 elements: 1% (v/v) HNO3, tr. HCl, tr. HF (LDPE)
• 2, 10, 100 ppb mixtures
• Tested at 1, 3, 25, 75, 137, 300, and 375 days
Results• Hg was not stable long enough to measure (minutes)
• Au unstable at all concentrations after 3 days
• Ag unstable at 10 and 100 ppb after 137 days.
• Mo, Sn unstable only at the 2 ppb level at 375 days.
• All other 200.7 elements stable at 2-100 ppb for 375 days
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Factors Affecting Stability
• Chemical stability
– Well designed multi-element standards stable indefinitely
• Human factor (ooops!)
– e.g., cross contamination, mistakes
• Transpiration
– Loss of water vapor = systematic error
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Transpiration: Effect of Temperature
Higher temperatures
= faster transpiration
0
0.2
0.4
0.6
0.8
0 150 300 450 600 750
Perc
ent C
hang
e
Days
125 mL Bottles 30 °C
20 °C
≤5 °C
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0
0.5
1
1.5
0 1 2 3 4 5
Perc
ent C
hang
e
Years
125mL
500mL
Transpiration: Effect of Bottle Size
Smaller bottles transpire faster
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Transpiration: Container Material and Fill Level
Rate of transpiration:
- LDPE > HDPE > glass
- Partially full > full
0
0.5
1
1.5
2
0 25 50 75 100
Tran
spira
tion
Rat
e / Y
ear
Bottle Fill %
LDPEHDPEGlass
250 mL bottles
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Summary - Element Compatibility and Stability
Can we mix everything together? Yes, but…
• lower is better
• acid matrix – what can you use in your lab?
• Container materials can be critical
• be aware of starting materials (e.g., thallium)
• element matrix chemistry (Sb-tartrate + Hg)
• For EPA methods, Sn and Hg are likely limiting factors
• Transpiration (systematic error) can be significant
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