Ask the Expert Question:
Can you explain the difference between total metals versus dissolved metals analysis and what form of the metal is being analyzed?
Expert Repsonse:
Total metals analysis for water samples include the metals content both dissolved in the water and present in the particulates in the water. Typically a dissolved metals analysis of a water sample is performed by removing the particulates with a filter, then analyzing the filtered water for metals. The most common filters used for this purpose have a 0.45 um pore size.
Total metals analysis results should always be greater than or equal to dissolved metals analysis results, because dissolved metals is a subset of total metals. Dissolved metals are generally considered more mobile and biologically available. Thus, the dissolved metals results are useful for risk assessment and fate & transport studies.
The specific metal species (or form) present in the dissolved fraction is highly dependent on the metal of interest. Most of the dissolved species are solvated metal cations such as Na(I) or Cr(VI). Some metals species are present as oxy anions such as arsenite.
Water solubility of the elemental metals (i.e. neutral valence state) is generally quite low. For example this website states elemental mercury [Hg(0)] has a water solubility limit of 56 ug/L at 25C (http://www.inchem.org/documents/cicads/cicads/cicad50.htm#2.1).
Most regulations use total metals results because it is often considered more conservative and protective, however, depending on the purpose of the regulation it might be based on dissolved or total metal concentrations.
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If a metals sample is not digested, and analyzed unfiltered, can it still be classified as a dissolved because it has not been digested? More specifically this is for Sulfur which is not digested. So is the overall difference between Dissolved and Totals is if it is filtered or not?
ReplyDeleteThank you for the question!
ReplyDeleteHistorically the primary distinction between dissolved and total metals content in a water sample is based on filtration through 0.45 um filter. The filtration is done either in the field or soon after receipt at the laboratory. The filtration step must be done prior to preserving the water sample with the common acid preservatives. Also, the filtration should occur soon after collection before the water sample constituents start to precipitate out due to exposure to air during the collection process.
Normally filtered water samples are digested even though the metals are already in solution. The digestion process produces a water digestate that closely matches the matrix and metals species used in the instrument calibration standards to improve accuracy. The final step of the digestion process is a secondary filtration to remove any residual particles that would damage the analytical instrument.
If the digestion step was skipped on an unfiltered water sample, in theory the only metals (or sulfur) measured would be those already in solution (e.g. dissolved). The secondary filtration step would have to be performed within the unpreserved holding time. Some accuracy might be sacrificed without the matrix matching with the calibration standards that normally occurs during the digestion step.
In short, field filtration is generally the best way to distinguish dissolved from total metals. Laboratory filtration upon sample receipt is a close second. Skipping the digestion step would produce results that resemble those of a traditional filtered/dissolved process. However, the “no digestion” process is not recommended.
If the samples were not field filtered and the laboratory did not filter/preserve within 24 hours of sample collection, what is the potential bias for the results?
DeleteI have some test results for Copper and Zinc (totals and dissolved) and the dissolved levels are higher than the totals
ReplyDeletefor zinc; total is 0.119, dissolved is 0.143
for copper; total is 0.002, dissolved is 0.0023
(all in g/m³)
how is this possible?
In addition to TestAmerica's response below, the filtration process itself may have contributed to some of the additional concentrations detected. That is, zinc and copper are common in the laboratory environment and may have possibly been in the filters and its apparatus used during the sample filtration/preparation step prior to analysis. In your situation, you should check for detections of copper and zinc in the dissolved sample's associated dissolved field blank and/or dissolved preparation (method) blank, which are QC indicators of possible contamination to the samples. Hopefully, the laboratory is not shy in informing you that there may have been a slight contamination issue either from the sampling activities or within the laboratory itself.
DeleteI am not with TeatAmerica, but have worked at several respectible environmental laboratories and am currently working within a large environmental consulting firm, ARCADIS. My opinions are those of mine, from decades of experience evaluating and validating laboratory data. -sn
When analyzing a sample for total and dissolved metals, it is normally expected that the dissolved metal concentrations would be less than the total metals. However, the accuracy of the analytical results are dependent on a combination of random and systematic bias due to sub-sampling, sample preparation, and analytical operations. When samples are processed under sufficiently uniform conditions, bias is controlled through the use of quality control indicators, such as: method blanks, laboratory control samples (LCS), calibration checks, sample duplicates, matrix spikes and internal standards.
ReplyDeleteA sample analyzed for total and dissolved metals, where the metals concentrations are indistinguishable different (i.e., metals present in the sample are 100% dissolved), may have slightly different concentrations due to normal variations in the analytical process. Slightly higher concentrations present in dissolved versus total analyses can occur even though all quality controls are acceptable.
Variation for duplicate samples for metals analysis is controlled using relative percent difference (RPD), at 20%. The RPD between total and dissolved results can be used to evaluate if the concentrations are statistically indistinguishable. The RPDs for total and dissolved Zinc and Copper in the examples below are 18.3% and 14.0%, respectively. Since the metal concentrations present are indistinguishable between totals and dissolved, it can be concluded that 100% of Zinc and Copper are in a dissolved state.
Zinc, Total = 0.119 mg/L
Zinc, Dissolved = 0.143 mg/L
Zinc RPD = 18.3%
Copper, Total = 0.002 mg/L
Copper, Dissolved = 0.0023 mg/L
Copper RPD = 14.0%
RPD Calculation:
RPD = |Total – Dissolved|
(Total+Dissolved)/2
mg/L = g/m3
Actually, RPD = (|Total-Dissolved|/(Total+Dissolved)/2)x100
Deletebut the values calculated were correct. -sn
i want to know the objective from analyzing dissolved heavy metals in wastewater,if the dissolved heavy metals is very low can we use wastewater in agriculture or we must analyze free and complexed and oxidn states .i work in my master in analyzing total,dissolved heavy metals using atomic absorption but the voltammetry technique is not available for knowing free ,complexed and oxidn states,what is the solution ,please reply me
ReplyDeletethanks
Dissolved metals tend to be more biologically available and thus have greater impact on plant and animal life. Knowing that the dissolved heavy metal content is low might not be sufficient to provide assurance that there would be no negative impacts from particulate heavy metals in that water. Knowing that metal species (e.g. oxidation state, organic groups bound to the metal or inorganic minerals) can be useful to predict bioavailability and bioaccessibility. The metal fractionation (e.g. free, complexed, particle bound) can also be useful in this effort. The usage conditions of the water should also be considered as this can cause shift in speciation and fractionation. The specific plant or animal species should also be considered since biological uptake can be dependent on the specific organisms.
ReplyDeleteBecause of the complexity of predicting whether heavy metals will have negative impacts on plants grown with water containing the pollutants, pilot scale tests in restricted areas are the most common way to assess impacts. In some cases heavy metals are taken up by the plants with minimal impact on plant growth. If these plants will be used as a feedstock, then gastro-intestinal uptake by the target animals should also be considered. Literature searches related to bioavailability and bioaccessibility should find many plant and animal studies for potential heavy metal exposure.
I cannot filter in the field, but I want dissolved metals results. How quickly do I need to get the samples to the lab for lab filtering? Is there a protocol or guidance?
ReplyDeleteThe US EPA recommends field filtration in both SW-846 and the most recent Method Update Rule. There is no specific EPA guidance on laboratory filtration for dissolved metals content, because this is not an EPA recommended procedure. When the field sample collection crew cannot filter water samples in the field, the laboratory can filter at the lab using standard 0.45 um filters. There is no official EPA holding time on this process, but the laboratory default is 24 hours between sample collection and filtration.
ReplyDeleteThe potential for changes in dissolved content due to air exposure of the water sample or temperature change during collection and transport to the lab is significant. That is why the EPA recommends field filtration. QED Environmental Systems (http://www.qedenv.com) has water sample collection equipment and experts advice that can facilitate filtration under most field circumstances.
For a syringe filter, what material membrane should the 0.45 micron filter be made of? PTFE, PES, nylon, etc.? And then for being analyzed on an ICPMS. Thanks.
Deletehow can i make use of results of total heavy metals and dissolved metals (i mean how can discuss the results of total &dissolved metals of wastewater plant and make a relation between them ,please help me because i want a clear objective for that in my thesis
ReplyDeletethanks
for dissolved metals ,what is the maximum time for preservation of sample after digestion.i mean if i filtered the water sample for dissolved heavy metals ,then adding HNO3 and made digestion for two hrs. and cool it then preserve it in the refrigrator in a plastic container for 6 months,is it correct because i have not possibility to analyze it dirctly on AAS.
ReplyDeletethanks
Using dissolved and total metals data for water samples representing aquatic systems usually involves the realm of toxicology and in particular bioavailability. The dissolved metals content is usually the most bioavailable and the most likely to cause a biological effect. The non-dissolved or particulate metals content (e.g. total – dissolved) can in some instances function as source for producing more dissolved metals if the equilibrium is changed. In other instances, the particulate metals content is sufficiently stable, that is not considered available. This area of metals bioavailability is an area of active research at many universities and government sponsored laboratories around the world.
ReplyDeleteThe US EPA methods do not have a specific holding time for sample digestates. The stability time for water samples preserved in the field with nitric acid is six months. The heated acid digestion at the laboratory does not change this official holding time or assumed stability. Thus, in general acidic digestates are considered stable for 6 months from date of sample collection. For some samples and storage conditions, the stability might be even longer but a holding time study would be needed to demonstrate this extended stability.
ReplyDeleteMy research is about the possibility of using treated sewage wastewater in irrigation.I analyse dissolved metals and COD,BOD,TDS,TSS,TURBIDITY,CONDUCTIVITY,PH,TEMP in the treated wastewater of sewage .I want to add a geological part in my thesis.Can I add the soil properities in the surrounding area as a geological part without taking samples from soil,i mean to discuss clay type and other factors which affect sorption of dissolved metals?
ReplyDeleteand what is the difference for analyzing dissolved metals than total metals in that case?
thanks a lot
If you have sufficient historical data about the clay and soil to make your decisions you might not need to do additional soil and clay sample testing. If you do not sufficient soil characterization data to make your decisions then additional testing would be appropriate.
ReplyDeleteI don't understand the second question, "what is the difference for analyzing dissolved metals than total metals in that case?" What case are you referencing? Normally total and dissolved analyses only apply to water samples, but the preceding information in your post referred to soil and clay samples.
What if the sample was preserved digested for total metals and then about 2 weeks later, the client wants the sample filtered and analyzed for dissolved metals? The results wont be reliable, correct?
ReplyDeleteOnce a water sample has been preserved with nitric acid, it is no longer appropriate to use this same sample aliquot for dissolved metals analysis. The nitric acid preservative is designed to keep the metallic content of the water sample in solution and in some cases will actually leach metals from the small particles present in the water sample. Thus, filtering a nitric acid preserved water sample and analyzing for dissolved metallic content could produce a high biased result.
ReplyDeletei use a gridded filter(0.45micrometer) paper for filtration of dissolved metals,Is that correct or i should use ungridded filter?
ReplyDeleteand for comparison between total metals and dissolved metals i preserve total metals with nitric then i filter for dissolved and take 50 ml of each total and dissolved then add 5 ml for all for digestion is it true to increase amount of nitric for total including preservation or not?
When the laboratory recieves a sample that hasn't been preserved or filtered in the field and laboratory filtering is requested, does the holding time change? Based on my knowledge and some comments above, it seems like there would be a higher priority to get those samples filtered and preserved. How long after reciept does the laboratory filter the samples?
ReplyDelete@Anonymous (Feb 5th): Filter gridding is not needed for the preparation of dissolved metals water samples. However, the gridding probably doesn’t hinder your filtration process either. The filtration step must occur before the nitric acid preservative is added to the water sample. The filtration step should be done as soon as possible after the water sample is collected. The best scenario is to use a pump and tubing system with a capsule filter in-line with the tubing and filtration then occurs during the sample collection process.
ReplyDeleteThe US EPA 3000 series methods provide a variety of sample digestion options. They are available at http://www.epa.gov/epawaste/hazard/testmethods/sw846/online/3_series.htm
@Anonymous (Feb 8th): The US EPA Sw-846 methods don’t mention a specific holding time for laboratory filtration of a water sample intended for dissolved metals analysis, because the expectation is that the filtration will occur in the field. Many laboratories will still accept unfiltered samples and perform laboratory filtration. In those instances the filtration is usually completed within 24 or 48 hours of sample collection.
ReplyDeleteAre dissolved metals more likely to be indicative of current, active pollution sources? In the soils of urban watersheds there can be a long legacy of accumulated metals from historical pollution sources. We sometimes see high total metals in some well-forested, less urban watersheds with high gradient and erosion potential. Conversely, we tend to see higher proportions of dissolved metals in areas receiving runoff from a current pollution source such as a highway. Is that a reasonable hypothesis? It seems the longer a metal is in the environment, the more likely it is to sorb to particles.
ReplyDeleteIf i've left water samples in the fridge for 6+ months without adding nitric are these useable for total metals? And what method should i use for ICP-MS analysis
ReplyDeleteEPA Method 3005 specifies that the nitric acid preservative must be added at the time of sample collection. Other EPA methods such as 3010 and 3015 describe the use of nitric acid as a preservative but don’t specify a time frame in which the acid must be added to the sample. Occasionally limitations in the field will prevent preservation to pH < 2 and this process is completed when the samples arrive at the laboratory a day or two later. Method 200.8 allows a lag of two weeks between collection and preservation. I don’t know of any stability studies that can defend unpreserved storage for 6 months followed by nitric acid preservation.
ReplyDeleteThe most common ICP-MS analytical methods are 6020 and 200.8. Other methods are available from other sources as well. Each are designed to meet the objectives of different monitoring needs and programs. Read the scope and application sections of each to determine which are most applicable to your specific testing needs.
I have a HNO3, non-filtered preserved sample that has been frozen for the past month. Would it be acceptable to thaw, filter the preserved sample, then submit it for testing? From what I have read above, the crucial step in this process is to preserve and the filtering can come later; I'm wondering if there is a time limit on this "later".
ReplyDeleteHi, I would like to know for analysis of dissolved metals (Cd and Pb) by using ICP-MS, is it possible to use 0.22um pore size filter membrane instead of 0.45um pore size membrane? Will the pore size affect the dissolved metals in the aqueous solution?
ReplyDeleteThank you.
waterlab - yes it is possible to use a 0.22um filter. If you compare the results with those using a 0.45um filter you may be able to distinguish if there is a fraction of the metal associated with particles with the size between 0.45 and 0.22um, and get a better indication of what is actually dissolved metal. Bear in mind that nano particles (1-100 nm typically) which may have sorbed metals on their surfaces will still pass through a 0.22um filter size (220 nm) so you will still not be certain that you only are measuring dissolved metals.
ReplyDeleteonly if you have particulate between 0.23 and 0.44um in diameter:-)
DeleteHi,
ReplyDeleteDo you know if there's a guideline stating wether total or dissolved metals should be used in Human Health Risk Assessment? I understand that dissolved metals are more available, but considering that pH in human stomach is low, it should make metals (considering total metals) more available then the previous dissolved concentration.
Thank you!
Hi,
ReplyDeleteIs it necessary to acid digest clear laboratory made arsenic samples. What is the standard procedure for Arsenic acid digestion?
Thanks
Hi, do you know whether the EPA Action Level of 0.015 mg/L for lead (Pb) is intended to represent total or dissolved lead at the tap? Thanks!
ReplyDeleteIf you are comparing the results of total and dissolved metals in a sample that has a very high inert solids content (50-70% solids) - would you expect the dissolved metals concentration to be higher than the totals metals concentration due to the displacement of volume by the solids - essentially dilution by solids?
ReplyDeleteIs there a way to estimate the dissolved metals concentration based upon total metals data for Cd, Cr, Cu, Pb, and Zn? If so, can you direct me to that information?
ReplyDeletehow would the level of metals be affected by the increase of dissolved oxygen? would the effect be significant if the samples were analyzed within 24 hrs after collection of samples given that DO was induced during the sampling. thanks.
ReplyDeleteHi I am interested in testing the total metal content in leachate from mine wastes. What would be the better way for sample preparation?
ReplyDeleteFiltering with Whatman Grade 42 and acidifying with nitric acid (pH below 2) or acidifying first and then filtering using Grade 42?
Thanks
If you filter first & then preserve, your analysis will reflect the dissolved metals concentration of your sample. If you're interested in total metals concentration you should preserve the sample without filtration. Any filtration that occurs would be as part of the sample preparation/digestion process in the lab.
DeleteYou mentioned that a filtered water sample means that you are testing for metals that are already in solution and that by then digesting the sample would only be to matrix match. Does this mean that there can not be any lost data from any organo-metallic bonds not freed during a digestion process.
ReplyDeleteHi, I want to analyze arsenic and mercury in ground water samples. (1). which type of metals i can analyze: dissolved or total metals? (2). Should i need digestion for the same? (3). if the samples were preserved for more than 6+ months in refrigerator without filteration and acidification, can we use that samples for heavy metal analysis? (4). can we use 0.22 micron meter filter instead of 0.45 micron meter? (5). which type of filter paper is recommended for ground water filteration: membrane filter or cellulose ester filter or syringe filter?
ReplyDeleteHi. I found very strange phenomena. After minus the value of reagent blank (Acid+Ultra Pure Water), I found that the total metals of my rainwater samples is less than the dissolved metals. May I know what could be the possible reasons for this? Thank you very much.
ReplyDeleteHi I would like to estimate total concentrations of metals based on measured dissolved concentrations. I know this is a bit of a stretch and is highly dependent on many factors but is there a typical factor similar to the RPD that can be used? Thanks
ReplyDeleteHow do you preserve leachate samples for metal analysis for long term storage? If only interested in total amounts and not specific form/species - say Cu, Zn, Cd - can freezing work?
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Hi If i have filtered my water sample ( leachate ) with Grade 1 filter paper and acidified it to below 2 pH and analysed for metal concentration, will that be a total metal ?
ReplyDelete