Many metals, when dissolved in waters can be toxic to plants, animals and people.   Some metal compounds can be acutely toxic at very low levels. Others can accumulate in plants or animals, building up to higher concentrations.  This `bioaccumulation` can results in toxic levels of some metals (mercury compounds are well known for this effect).

For these reasons, many governments have strict regulations regarding the amount of metals that can be discharged into the environment.   These `regulatory drivers` are the primary targets for our testing protocols.  In addition to Regulatory Guidelines and Limits, Agencies are requiring that industry not impair the aqueous environment in any way and as a result laboratories have been required to report metals to lower and lower levels.

Fortunately, instrumentation has improved greatly over the past few years to the point where ultratrace metals analysis is not limited by the instrument but rather the cleanliness of the sampling and sample handling techniques.  Our laboratories employ clean rooms to minimize laboratory sources of contamination and prove clean our containers and preservatives but what about the field environment?

One limitation of the instrumentation is that the ultratrace reporting limits cannot be achieved on samples with high dissolved solids (> 1%) and the method is not appropriate for such samples.

1. What are important considerations when collecting samples for low level metals analyses?

When collecting samples for ultratrace metals analyses, it is important to ensure that the samples that are ultimately analysed by the laboratory are representative of the environmental conditions from which the samples were collected.  Therefore, the samples must be collected in a manner and in containers that maintain the integrity of the sample and the metals being measured from the time the sample is collected to the time it is analyzed by the laboratory.

It is also important to minimize exposure of the sample to the ambient environment.  Extraneous contamination by metals in the sampling environment is a risk that increases significantly as the requirement for increased analytical sensitivity increases.  Metal contamination can occur almost anywhere from many different sources including: dust; automobile exhaust; cigarette smoke; some personal care products; etc.   It is important, therefore, to keep sample containers capped until the sample is collected.  Then cap the sample container as soon as possible after collection, keeping them sealed until they are opened by the laboratory.

2. What are appropriate sample containers for the determination of low level metals?

Sample containers for ultratrace metals analyses should be inert to the metals of interest.  Metals should not leach from the container, nor should any metals “stick” to the container walls.  Sample containers are “proofed” to ensure that they are not a source of metals themselves.

For ultra-trace testing of waters, containers and preservatives are proven free of metals to the part per trillion range.   Glass is not used for water samples because it contains significant levels of many metals.  Similarly, acid preservatives can contain significant levels of metals. High Density Polyethylene (HDPE) is a common choice for sampling waters for trace metals. It is light, impermeable to water and resistant to breaking.  Different brands can contain some metals, however.  Bureau Veritas has exhaustively researched to find the cleanest HDPE containers and preservatives available.

3. Do I need to filter my water samples in the field?

In general, surface waters are analyzed for total metals and groundwaters for dissolved metals.  Drinking waters must be analyzed for total metals.  In some situations, e.g. environmental modelling, background studies, it may be beneficial to analyze for both total and dissolved metals.

When testing for dissolved metals at routine Regulatory levels, it is recommended to field filter the sample through a proven clean 0.45 μm filter1 prior to preservation with acid. If the sample is not filtered, then some particulate matter may be dissolved by the acid preservative.  This would result in high bias for dissolved metals.  Unfiltered, preserved samples are unsuitable for filtration at the laboratory.

For ultratrace analysis, our experience has shown that it is extremely difficult to field filter without introducing contamination.  For that reason, we recommend either the use of a specially designed Bureau Veritas field filtration kit, or filtration in the lab under clean room conditions.  Although it is possible that some analytes may dissolve or precipitate between sampling and lab filtration / preservation, these changes are small compared to the issues associated with field filtration.  Transport the sample to the lab asap to minimize any changes.

4. Is it necessary to preserve water samples for metals analyses?

When collecting samples for environmental analyses, it is important that the sample remains in the same “state” as it was when it was sampled.   Sample preservation for trace metals helps to ensure that the metals in the sample bottle stay at the same concentration as when they were sampled.   For most metals, adding nitric acid (HNO3) to lower the sample pH to < 2 will ensure that the metals of interest remain in solution.  For mercury analyses, hydrochloric acid (HCl) is typically used as a preservative.

5. What is the minimum sample size required for low level metals analyses?

For metals, typical minimum sample requirements are 20 mL for waters.  Larger volumes are recommended to allow for Quality Control samples and repeats.  A summary of sampling requirements is presented in Tables 1-2 below.

Table 1: Sample Collection – Ultratrace Metals

SAMPLING ELEMENT GROUNDWATER SURFACE WATER DRINKING WATER
Sample Container 50 mL HDPE 50 mL HDPE 50 mL HDPE
Field Filtered Yes No No
Preservative HNO3 (to pH <2) HNO3 (to pH <2) HNO3 (to pH <2)
Minimum Sample Size 20 mL 20 mL 20 mL
Recommended Sample Size 50 mL 50 mL 50 mL
Hold Time 180 days 180 days 180 days
Sample Container 50 mL HDPE 50 mL HDPE 50 mL HDPE
Dissolved Metals (filtered) Yes No No

Table 2: Sample Collection – Ultratrace Mercury

SAMPLING ELEMENT GROUNDWATER SURFACE WATER DRINKING WATER
Preservative HCl (to pH <2) HCl (to pH <2) HCl (to pH <2)
Minimum Sample Size 50 mL 50 mL 50 mL
Recommended Sample Size 100 mL 100 mL 100 mL
Hold Time 28 days 28 days 28 days

6. What are acceptable sample hold times for samples submitted for low level metals analysis in waters?

As a rule, it is ideal to analyse environmental samples as soon as possible after collection to minimize any changes to the sample or the parameters of interest over time.  Maximum hold time for preserved samples submitted for ultratrace metals analyses is 180 days and 28 days for mercury.

Table 3: Reporting Detection Limits2 – Ultratrace Metals by ICP/MS3

PARAMETER RDL ug/L
Aluminum (Al) 0.20
Antimony (Sb) 0.005
Arsenic (As) 0.02
Barium (Ba) 0.02
Beryllium (Be) 0.01
Bismuth (Bi) 0.01
Boron (B) 5
Cadmium (Cd) 0.005
Calcium (Ca) 10
Cesium (Cs) 0.05
Chromium (Cr) 0.04
Cobalt (Co) 0.005
Copper (Cu) 0.05
Iron (Fe) 0.50
Lead (Pb) 0.005
Lithium (Li) 0.10
Magnesium (Mg) 5
Manganese (Mn) 0.03
Mercury (Hg)* 0.0002
Molybdenum (Mo) 0.01
Nickel (Ni) 0.02
Phosphorus (P) 1
Potassium (K) 10
Rubidium (Rb) 0.05
Selenium (Se) 0.04
Silicon (Si) 2
Silver (Ag) 0.003
Sodium (Na) 10
Strontium (Sr) 0.05
Sulphur (S) 500
Tellurium (Te) 0.01
Thallium (Tl) 0.005
Thorium (Th) 0.001
Tin (Sn) 0.01
Titanium (Ti) 0.40
Tungsten (W) 0.04
Uranium (U) 0.002
Vanadium (V) 0.03
Zinc (Zn) 0.01
Zirconium (Zr) 0.04

*Determined using cold vapour atomic fluorescence

7. Are there special sample collection procedures for low level metals?

Yes.  With low level metals analyses, there is an increased risk of sample contamination from extraneous sources.  To address this risk, the United States Environmental Protection Agency (USEPA) published Method 1669, “Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels”4, referred to as the “Clean Hands/Dirty Hands” sampling technique.

As the nickname implies, “Clean Hands / Dirty Hands” sampling is a two-person task. One team member is identified as “Clean Hands” and the other as “Dirty Hands”, which reflects their roles and tasks associated with the sampling process. In other words the “Clean Hands” team member performs all activities requiring contact with the sample and sample container, sample collection, sample filtration and sample preservation.  The “Dirty Hands” team member is responsible for preparing any sampling devices (except the sample container itself), operating machinery, and all other activities that do not involve direct contact with the sample.


References

1 The filter media must be demonstrated to be free of metals

2 Reporting limits may vary and may not be achievable on all samples. Not all elements are routinely reported.

3 ICP/MS = Inductively Coupled Plasma/Mass Spectrometry

4 U.S. Environmental Protection Agency, Office of Water Engineering and Analysis Division, Method 1669: Sampling Ambient Water for Trace Metals at EPA Water Quality Criteria Levels, July 1996

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