200.7: Element Compatibility and Stable

Standards

Brian Alexander

Topics

• Overview of EPA Method 200.7

• the effect of acid matrix

• proper container materials

• potential concentration limits

• other factors affecting stability

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

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

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

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

Plastic Container Contaminants

Leaching study

• 1% v/v HNO3

• 60 hours @60 °C

• LDPE cleanest

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

Concentration Limits: Multi-element standards

Concentration Limits: Multi-element standards

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!

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

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

Transpiration: Effect of Temperature

Higher temperatures

= faster transpiration

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0.2

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0.6

0.8

0 150 300 450 600 750

Perc

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t C

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Days

125 mL Bottles 30 °C

20 °C

≤5 °C

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1.5

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t C

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Years

125 mL

500 mL

Transpiration: Effect of Bottle Size

Smaller bottles

transpire faster

Transpiration: Container Material and Fill Level

Rate of transpiration:

- LDPE > HDPE > glass

- Partially full > full

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1.5

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0 25 50 75 100

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Bottle Fill %

LDPE

HDPE

Glass

250 mL bottles

Summary - 200.7 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

• Customers can visit our website’s

Tech Center, which includes:

– Interactive Periodic Table

– Sample Preparation Guide

– Trace Analysis Guide

– ICP Operations Guide

– Expert Advice

– And much, much more.

Technical Support – Available to EveryoneOnline Resources at inorganicventures.com