DETERMINATION OF MERCURY IN THE RANGE OF ng/l USING CV-AAS

Transcription

1 MR. TELLIARD: Our first speaker this afternoon is Ruth Wolf. Ruth is with Perkin-Elmer, and she is going to be talking about determination of mercury in the range of 1 to 100 ng using cold vapor AAS. (The author s verbatim transcript follows.) DETERMINATION OF MERCURY IN THE RANGE OF ng/l USING CV-AAS MS. WOLF: All right. I am going to be talking about determination of mercury for low levels, 1 to 100 ppt using cold vapor AAS, and before I begin, I would like to give due credit to my colleagues in Germany, Manfred Leyrer and Gerhard Schlemmer, who did all of the work that you are going to see presented today. And I think they promised to turn the heat up a little bit right after I asked for long underwear and down booties. So, hopefully, we can get it not so cold. There are, of course, many reasons to measure mercury at low levels in the environment. Mercury is toxic at extremely low levels. It also bioaccumulates in the body which leads to other problems. Mercury has also shown up on the list of 50 identified endocrine disruptors which could have other implications here in the near future, and there has been a lot of interest lately in the speciation of mercury or the determination of inorganic forms of mercury versus organic forms of mercury. This is a summary of the current mercury regulations in both the U.S. and in Europe. You see the drinking water limits here for the U.S., 2 ppb. Wastewater, universal treatment standards, TCLP extract in soils, all in the low part per billion range for those kinds of samples. Then, some of the new things that I think you heard about this morning in the last talk before lunch, some new criteria that are coming out for ambient water which require detection at much lower levels, ambient water criteria of 12 ppt and 1.8 ppt in the Great Lakes region. So, obviously, we are going to have to look at some different ways to detect mercury at those low levels. This slide here shows a comparison of different cold vapor atomic spectroscopy methods for determination of mercury. These were all done using the flow injection system that Perkin-Elmer supplies for doing mercury, and we have just a list of the method, whether pre-concentration was used, the detection system, and a typical detection limit. These first couple here are using the flow injection unit to put the sample into a system and then using an AA spectrometer to do the actual detection. The middle two here using flow injection with and without pre-concentration in a system that we developed specifically for mercury. It is called the FIMS or the flow injection mercury system. Without pre-concentration, we get detection limits around 4 ppt, and with pre-concentration which is going to be the data I am going to show you today, we are getting detection limits around 0.2 ppt. We have also done some work using flow injection and graphite tube pre-concentration and also the amalgamation pre-concentration with AAS and then using ICP/MS as the determination step and also getting much lower detection limits with those techniques. June

2 So, as you can see by the last two slides, most of the regulatory levels can be met with the existing levels if you are doing drinking waters, waste waters, soils, sludges. Most of those kinds of things we have methods that can reach the limits required, but when we start getting down into the ambient level analysis for mercury, we need to look at methods for doing ultratrace analysis. I think you saw some of this this morning in the last talk before lunch. This is a comparison of Method with Method 1631, just the sample digestion. Method uses the permanganate digestion; 1631 uses the bromate digestion. Method does not use pre-concentration; 1631 does. And then, a different determinative step, atomic absorption versus atomic fluorescence. Detection limit under Method and this was actually done, I think, with Method 245.1A which was the automated version of that method that we helped get through the approval process...of 4 ppt using the FIMS, and then the reported detection limit in the new Method 1631 of 0.2. Then, a little bit of difference here between the two methods was sample volume. With the automated FIMS system, we can use a sample volume as low as 0.5 ml which was a big advantage to some folks, because they could actually digest 100 ml of sample and do several measurements on it instead of using it all up with a non-manual system. Then, the draft Method 1631, they reference total sample volumes between 100 and 2000 ml. So, one of the things that we noticed when we compared the methods was that it may be possible to expand the detection capability of the cold vapor AAS system by using pre-concentration, and we started a project in Germany to explore that. What we wanted to do was just see what kind of detection levels we could get with the cold vapor flow injection system by using pre-concentration and the new bromate digestion system. We wanted to take a look at, you know, specifics with the digestion step, take a look at the preconcentration requirements, and also the analysis conditions for doing low-level mercury. The system that was used was the Perkin-Elmer FIMS 400 with an automated amalgamation system. This system has been available, I want to say, at least five years, because I remember having one when I first joined Perkin-Elmer, to play with, and that was five years ago. The data that you are going to see today was not done in a clean hood or a clean room. It was done in a normal laboratory in our facility in Germany. I do admit, though, that my colleagues in Germany are very meticulous. One of the things we would like to do in the future is put this system in a Class 100 clean sampling hood and just see if we can make any improvement with detection limits. We did try and get a hold of the highest quality reagents we could, especially for contamination with mercury. We wanted to make sure that the reagents were not contaminated with mercury. This is just a list of the reagents that we used and some of the vendors that we used to supply those reagents. The stannous chloride came from Merck... and some of the part numbers for those vendors June 1999

3 One of the difficulties that I have found in doing trace work is finding good sources of clean reagents, and that is probably going to be one of the difficulties in any ultratrace method, is to identify a supplier of clean reagents. So, that is why we have tried to include this information here. For those of you who are not familiar with automated flow injection, this is a block diagram of the flow injection system, and I will just kind of briefly go through this. I don't know if anybody can read the labels on the components, but if you can't figure out what they say, they are in German, so if you are not familiar with the word and you are not a German speaker, that could be the reason. The flow injection system consists of a flow injection valve and two pumps connected through a computer. Everything is controlled through our software. The first pump here...we will call it pump 2...controls the speed and flow of the hydrochloric acid carrier stream and also the reductant, in this case, stannous chloride. This pump is turning and provides a constant stream of the hydrochloric acid carrier into this device here which is our...it is called a mixing block...where the sample eventually will be mixed with the stannous chloride. The sample then comes out of the mixing block and goes into the gas-liquid separator, and the purpose of this device is to separate the liquid from the mercury vapor, and then the mercury vapor is taken into the desired detection system. How the FIAS valve functions, the second pump will control the sample flow coming from the autosampler here through the valve. The valve is shown here in the fill step which is shown in the little insert down here. In the fill step, the sample is pulled from the autosampler tray into the valve, fills the sample loop if one is being used, with any excess going out to the drain. The reason this is controlled with a separate pump is once you have the loop filled, you want to turn the pump off so that you don't waste all of your sample. Then, the sample is switched into the inject position, and that pushes the sample plug that is in this loop into the carrier stream in a discrete plug if you are using it in the discrete sampling mode. That sample then goes in the mixing block and mixes with the stannous chloride to form the mercury vapor and goes into the gas-liquid separator. Now, we have a variety of options at this point. We have taken the mercury vapor onto our amalgamation system. We have also taken this mercury vapor into a graphite tube, and we can also take it into the plasma of an ICP optical or a mass spec system, or we can just take it into the quartz cell and determine the mercury directly that way. These are some of the parameters used by the FIMS. The detection system on the FIMS uses a fixed wavelength light source at nm. We did all our determinations with peak height with a read time of 20 seconds. For the mercury amalgamation system, we did not use a sample loop. We used continuous flow for two fill steps of 10 ml each. So, we used a total of 20 ml of sample for each replicate and then argon carrier flow. June

4 So, all of these parameters are controlled through the software on the instrument. Now, when we are using the gold amalgamation system on the FIAS, we do have a second FIAS valve that we use to control the flow of the mercury vapor in and out of the amalgamation system, and this is just a close-up of the valve. The mercury vapor comes out of the gas-liquid separator, and when the valve is in the fill or inject position, the mercury vapor goes through here, into the mercury amalgamation system which uses a gold-coated gauze to amalgamate the mercury vapor. Once the sample has gone through here, we switch the valve position, closing off the stream from the gas-liquid separator, and then the amalgamation system is heated up to release the mercury vapor into the quartz cell where it is detected. This slide just shows the amalgamation parameters. This is actually a screened down boff of some of the software. The first two steps here are actually just to flush the sample line and get all the lines filled with your sample. This second step here with a pump speed of 120 rpm for 60 seconds loads approximately 10 ml of sample through the mixing block to react with stannous chloride, and then that is preconcentrated onto the gold gauze. We repeat this step again so that we use 20 ml of sample total. Once that is done, we switch the position of the valve to the fill step again, so we close off the sample flow into the mixing block, and then the amalgamation cell is heated to release the mercury vapor into the detection cell, and after that is done, there is also an automated cool-down step for the amalgamation system. So, this is all controlled. You don't have to think about it. The next few slides are going to compare mercury detection without the amalgamation system with the FIMS and with the amalgamation system. Now, this slide here is a picture of the signal that you get from the FIMS system without the amalgamation for a 10 ppt standard using a 500 ul sample loop. What I would like you to notice is this is the absorbent scale on this side. You get an absorbance of roughly units for a 10 ppt without amalgamation. With this system, we did the EPA Appendix B detection limits over three days, and we do get a detection limit without amalgamation of 4 ppt. These are some signals that we get off the system with the amalgamation unit in use. Preconcentration time 120 seconds. Total sample volume for one replicate would be 20 ml, and you can see here the absorbency scale goes from to 0.006, and if we look at these traces here, this is the blank signal here, the yellow line. 1 ppt is this red line. The green line is 2 ppt, 5 ppt, and 10 ppt. So, probably the most astounding thing is that for a 10 ppt now, we are getting an absorbance of about which is about 25 times the signal that we were getting without the amalgamation system. The next experiment that they did was to look at some blank values. Obviously, we had a little bit of a blank peak there in that last slide, so now we are working on ways to reduce that blank. They took a look at the stannous chloride solution not purified, and the apparent mercury concentration in that blank was about 16 ppt June 1999

5 Once that stannous chloride was purified by purging it for 1 hour with an argon flow, that level was reduced down to less than 1 ppt. So, we found that the significant source of contamination in this system was actually the stannous chloride solution. So, a clean-up step will have to be included in the method that is eventually written for this. And then the levels in the 0.5 percent nitric acid and 0.5 nitric acid with permanganate in it, less than 2 ppb. This is a plot of a typical calibration curve for the amalgamation system from 1 to 10, showing standards at 1, 5, and 10 ppt. So, very good linearity here, and I don't have the data...it didn't make it to us Monday, because Germany was on holiday last week... but I do have the detection limit data with me if anybody is interested in it, but we do...i get a calculated detection limit for a 3 sigma 10 replicates of the blank of 0.2 ppt. We also ran some water samples through this, some lake water samples, that were collected on three different days. The lake water came from Lake Constance which is right next to our facility in Uberling in Germany, and I think this was an excuse to go wind surfing or something, but the water authority that controls and monitors Lake Constance has reported that that lake has an ambient level that ranges from 0.8 to around 1.0 ppt pretty consistently. So, they collected three samples. This is the level that was measured in those three samples. We did a 10 ppt spike on that sample and got excellent spike recovery. Did the same thing with a local tap water. Ambient level about...average there is probably about 1.5, and then, again, excellent spike recovery at 10 ppb. So, to compare the results that we got with the FIMS system without amalgamation and with amalgamation, the working range without amalgamation would be from 20 ppt to about 20 ppb. This would be a suitable working range for most of the other regulatory analyses that you would do on drinking water and wastewater and TCLPs and soil digest. With amalgamation, we get a working range from about 1 ppt up to about 200 ppt. Detection limits without amalgamation, 4 ppt, and with amalgamation, 0.2 ppt. So, equivalent to what we have been reporting with Method Pre-concentration time on the amalgamation system is 120 seconds. Total sample time for three replicates without amalgamation would be 2 minutes and with amalgamation is a little bit longer, 18 minutes. Sample consumption without the amalgamation...and, again, this was with a 500 ul sample injection loop...the total sample used here would be 2.5 ml. The total sample with amalgamation with three replicates would be 60 ml, and in both cases, everything was totally automated which would have some advantages. We will get to those in a minute. So, just kind of a slide to summarize what you may give up a little bit if you are doing ultratrace mercury determinations. Direct determination without amalgamation, very good speed. Less than 2 minute a sample is pretty good. Very low sample consumption, and because the FIAS system is completely contained, you have a very limited amount of contamination that can take place. June

6 The amalgamation system, you get very good detection limits. And there is a typo here. That was a Tuesday morning oops. But you do get good detection limits, and with the amalgamation system, you get the specificity for mercury, but you do give up a lot with speed, and you are using quite a bit more sample. So, there is...you can't have everything. There is a little bit of a tradeoff there. So, some conclusions that we made from this preliminary work. Obviously, mercury is going to be an element of interest for a long time. We do need to come up with some lower detection limit methods for specialized applications, especially since we are seeing more and more interest in ultra-low level determinations and speciation of mercury. The cold vapor generation system, coupled with pre-concentration and the sensitive detection system used on the FIMS system...and one of the reasons that that detection system is so sensitive is the long path length of that quartz cell used. It is a 25 cm long cell. So, it is quite a long cell that gives us good detection limits...does allow for ultratrace detection limits using the atomic absorption detection system. Of course, contamination is going to be an issue, and you have to take extreme care to control your contamination at every stage, including sample collection and analysis. Detection limits can depend on your reagent purity and contamination introduced during processing of the sample. Again, there is going to have to be, probably in most routine labs, if they are going to attempt ultratrace analysis, some training of people on clean sample handling techniques. Since I have been in the ICP/MS field for the last six years, that is always an issue, training people how to deal with ultratrace analysis. There are some special techniques you have to take care to use. The FIMS amalgamation system is automated, and it does reduce exposure to contamination because it is a closed system. You are not manually manipulating the sample a lot, so you do reduce some of the risk for contamination. The bromate digestion is much faster. It uses cleaner reagents than we found with the old permanganate digestion, and it does also reduce exposure to contaminants, because you don't have that long digestion period. Some of the additional work that we are going to be following up with the rest of the summer include some additional method validation work and development of a standardized procedure for doing cold vapor analysis of ultratrace mercury using the amalgamation system. And one final thing that we would like to do is put the entire system inside our clean hood and see if we get any improvements with our detection limits. With that, I would like to close and entertain any questions. QUESTION AND ANSWER SESSION MR. TELLIARD: Questions? MS. NEWMAN: Just two questions. Debby Newman, City of Cincinnati June 1999

7 The MDL was performed in reagent water? MS. WOLF: Yes, I...no, they were not. They were done on a low-level spike. I think they were done on a 1 ppt spike. MS. NEWMAN: Of water? MS. WOLF: Yes. MS. NEWMAN: Reagent water? MS. WOLF: Yes. MS. NEWMAN: You didn't do...okay, and have you used this on any industrial effluents? MS. WOLF: I don't know if they have yet or not. That is some of the continuing work we need to do. The preliminary work was just to see, you know, how low could this system potentially go, and, you know, we have shown that in a clean sample, we can get down to the 0.2, so the rest of the work will be to, you know, run real samples and real effluents through it and see what will happen. The FIMS is in use without the amalgamation system in quite a few labs, running all kinds of effluents, and I don't think we have run into many problems with it. So, I don't anticipate anything. MS. NEWMAN: Right, but, basically, you are using this in ambient water quality... MS. WOLF: Yes. MS. NEWMAN:...conditions. MS. WOLF: Yes. MS. NEWMAN: Okay, thank you. MR. JAGESSAR: My name is Patrick Jagessar, New York City DEP. Is the bromate good enough to take care of organic mercury? Potassium permanganate is probably the ultimate oxidizing agent. MS. WOLF: I don't really know the answer to that. I don't think...nick does, since Nick helped develop the method. MR. BLOOM: I know the answer to that question. Actually, free halogens are a much better oxidizing agent for mercury organic compounds than potassium permanganate is, and potassium permanganate actually will not break the methylmercury bond without the presence of chloride in the solution. MR. JAGESSAR: Thank you. One other question. Is there any studies been done on effect of barometric pressure on mercury determination? I had some personal experience whereby while doing mercury analysis, using known standard, for example, sometimes you see, for a known standard, say 1 June

8 ppb, you would have no signal, but just by shaking the tube, your signal appears after reanalyzing the sample. I know there was work published on the effect of barometric pressure on mercury determination. Is there anything in the future in the method that would mention something about barometric pressure in mercury determination? MS. WOLF: I don't really know how much effect the barometric pressure would have on the FIMS system. When you look at the gas-liquid separator, the argon flow that is coming up is around a ml/min, and that tends to pretty well mix everything together, and I would think that that would negate any effects of barometric pressure, having that high flow rate through there, but I, you know...i can check and see if there have been any studies done. I am not totally familiar with all of the work that has been done in Germany on this, but they are very meticulous about such things. So, I am sure that they have probably considered it. MR. JAGESSAR: On the gold/platinum gauze, the purge and trap, Perkin-Elmer purge and trap, after a while, it gets overheated, and, ultimately, your characteristic mass increases. Consequently, your detection limit degrades. Is there any study done to determine the stability of detection limits, say... MS. WOLF: I don't... MR. JAGESSAR:... if they took the sample? MS. WOLF: Those have not been done yet. I would expect, though, that the gold/platinum gauze eventually would have to be replaced on a fairly routine basis to maintain your best possible detection limits. MR. JAGESSAR: I am talking about...say, for example, a daily run, for example, of 50 samples. From my experience, after 20 samples...this is my opinion now...the gold/platinum gauze tends to get overheated. So, your detection, your characteristic mass of a known standard increases or degrades. Ultimately, your detection limit is going to be affected. MS. WOLF: Yes, we have not seen that phenomenon. MR. JAGESSAR: Okay, thank you. MR. TELLIARD: If you have a problem disposing of those traps, I will give you an address you can send them to. Just helping out, you know. I have a quick question. On my 1968 AA, how much is it going to cost me to put this retrofit unit on? MS. WOLF: You will not get these detection limits if you try to put this on your I have tried it, and you won't do that June 1999

9 MR. TELLIARD: What do you think a front-end unit is going to cost, though? I mean just roughly. MS. WOLF: The flow injection system with the amalgamation is probably...probably around $30,000 for the complete system. MR. TELLIARD: Okay, thank you. MR. BLOOM: May I ask you a question real quick? MS. WOLF: Yes. MR. BLOOM: In this system, are the samples residing in a carousel, sample carousel? MS. WOLF: No, the autosampler is our AS90 autosampler, the standard autosampler, and I do believe that they...there is a hood that fits over that autosampler to protect things from contamination, and I know that they have that hood over the autosampler. MR. BLOOM: But the tubes are open, so the risk of gas phase contamination is... MS. WOLF: Yes, there is, yeah. MR. BLOOM: Has that been looked at at all? MS. WOLF: I don't think to a significant extent yet. MR. TELLIARD: Thank you, Ruth. June

10 Determination of Mercury in the Range of ng/l Using CVAAS Ruth Wolf, Manfred Leyrer, and Gerhard Schlemmer The Perkin-Elmer Corporation June 1999

11 Reasons to Measure Mercury at Trace Levels Bioaccumulation concerns Health effects (possible endocrine disruptor) Speciated analysis 2 June

12 Summary of Mercury Regulations Medium US Maximum Contaminant Level (ng/l) EU Regulatory Limit (ng/l) Drinking Water Wastewater (Chlor- Alkali-Mercury Cells) (new) Universal Treatment Stds TCLP Extracts 200,000 Soils Sludges Ambient water 110,000 max for one day 48,000 avg. over 30 days 150,000 (wastewater) or 25, ,000 (nonwastewater) 1-21 mg/kg cleanup goal (10, ,000 ng/l in solution, based on 1g sample) 12 (freshwater cont. criteria) 1.8 (Quality Guidance for the Great Lakes) 50,000 ng/l before it is mixed with other wastewater mg/kg ( 1mg/kg for Agricultural soil ) (5,000 10,000 ng/l in solution, based on 1g sample) mg/kg (160, ,000 ng/l in solution, based on a 1g sample) Natural waters such as Lake Constance, Germany carry around 0.8 ng/l Hg June 1999

13 Methods for the Determination of Mercury with CVAS Technique Preconcentration Detector Detection Limit (ng/l) Flow Injection None AAS 100 Flow Injection - Continuous Au/Pt Gauze AAS 10 Flow Injection None FIMS 4 Flow Injection - Continuous Au/Pt Gauze FIMS 0.2 (20 ml) Flow Injection Graphite Tube AAS 0.5 (100 ml) Flow Injection - Au/Pt Gauze ICP-MS 0.2 (25 ml) Continuous 4 June

14 Comparison of Existing Hg Methods Cold Vapor Technique Method EPA Method CVAAS Draft Method 1631 CVAF Sample Digestion Permanganate Bromate Preconcentration No preconcentration Amalgamation Analytical Step Atomic absorption Atomic Fluorescence Detection Limit 4 ng/l (FIMS) 0.2 ng/l (ML) Sample Volume ml ml June 1999

15 Objectives of this Work Explore the factors involved in implementing the determination of mercury at ultratrace levels using CVAAS Consider - Sample digestion step - Preconcentration requirements - Analysis conditions 6 June

16 Experimental Perkin-Elmer FIMS-400 with an automated amalgamation accessory was used No clean hood used, although instrument could easily fit into one High quality reagents were used, and cleaned in some cases June 1999

17 Reagents Reagent Quality Vendor Part Number SnCl. 2 2H 2 O Reagent grade Merck HCl 30% Suprapure Merck H 2 SO 4 96% Suprapure Merck HNO 3 65% Suprapure Merck KBr Merck 4905 KBrO 3 Merck 4914 KMNO 4 Merck 5084 K 2 Cr 2 O 7 Merck 4864 NH 2 OH-HCl 1.5% Perkin-Elmer B Argon High purity % 8 June

18 Cold Vapour Generation with Flow Injection AAS, OES, MS specific detector quartz cell long path cell (25 cm) Au/ Pt gauze graphite tube plasma volume selection control of liquids control of gases June 1999

19 FIMS Spectrometer Parameters Wavelength 253.7nm Signal Peak Height Peak Smoothing 9 points Read Time 20.0 sec Read Delay 0 sec BOC Time 2 sec Replicates 3 Sample loop No Loop used, continuous flow for app. 2 x 10-mL per replicate Repeats Program step 1 to 3 repeated for one more time Argon Carrier ml/min adjusted at FIMS flow meter Argon Carrier 2 80 ml/min fixed output at the Argon Carrier of the Hg Amalgamation System 10 June

20 Hg Preconcentration on a Gold/ Platinum Gauze FIAS- Valve for Control of Hg-Vapor June 1999

21 FIMS/Amalgamation Parameters Step Time Pump 1 Pump 2 Valve Read Heat Cool Argon (s) (rpm) (rpm) Prefill Fill X X Fill X X Inject X X Inject X X Fill X X Fill X X X Fill X X 12 June

22 Determination of Hg Without Amalgamation with Hg Specific Detector ( FIMS ) Hg Determination in water Concentration: 10 ng/l Volume: 0.5 ml June 1999

23 EPA IDL for AA System with Specific Hg Detector ( FIMS ) Without Amalgamation (ng/l) Sample Day 1 Day 2 Day Mean SD xSD IDL June

24 Hg with Amalgamation Concentration Range 1-10 ng/l Preconc. time: 120 s Sample volume: 20 ml 10 ng/l 5 ng/l 2 ng/l 1 ng/l Blank June 1999

25 Blank Values Peak Height Concentration (ng/l) SnCl 2, not purified SnCl 2, Purified 1 hour with Argon ASTM Type 1 water with 0.5% HNO 3 ASTM Type 1 water with 0.5% HNO 3 and KMnO 4 %RSD June

26 Typical Calibration Curve from 1-10 ng/l June 1999

27 Results for Water Samples Sample (3 replicates) Hg Conc. (ng/l) RSD (%) Spike Concentration (ng/l) Spike Lake Water-Day Lake Water-Day Lake Water-Day Tap Water Tap Water % Recovery 18 June

28 Hg - Determination in the ng/l Range Summary Without Amalgamation With Amalgamation Working range (ng/l) Detection limit (ng/l) Preconcentration time s Time/sample (n=3) 2 Min. 18 Min. Sample consumption 2.5 ml 60 ml (3 replicates and prefill) Automation Yes Yes June 1999

29 Direct Determination vs. Amalgamation Parameter Direct Amalgam Detection Limit + Specicifity + Speed + Sample Volume ++ Contamination + 20 June

30 Conclusions Mercury will remain an element of interest for some time Lower detection limits are required for specialized applications Cold vapor generation coupled with preconcentration and a sensitive detection technique allows ultratrace detection limits Contamination is an issue and must be controlled at every stage of the sample collection and analysis June 1999

31 Conclusions Detection limits depend on - Reagent purity - Contamination introduced during processing The FIMS/Amalgamation system is automated, reducing exposure to contaminants The bromate digestion is faster, uses cleaner reagents, and reduces exposure to contaminants Further work includes additional method validation and development of a standardized procedure for CVAA 22 June

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