Determination of Heavy Metals in Whole Blood Using Inductively-Coupled Plasma- Mass Spectrometry: A Comparison of Microwave and Dilution Techniques

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1 Determination of Heavy Metals in Whole Blood Using Inductively-Coupled Plasma- Mass Spectrometry: A Comparison of Microwave and Dilution Techniques By: Laura Schweitzer, Advisor-Dr. Charles Cornett University of Wisconsin Platteville; Department of Chemistry and Physics The determination of heavy metals in forensic toxicology investigations is often limited by the use of relatively insensitive wet chemical techniques or single-element methods employing atomic absorption or graphite furnace methods. This project addresses the need for the development of rapid, sensitive, multi-elemental methods for the analysis of metal toxins in whole blood, which would expand inorganic analysis in toxicology investigations. The project utilizes the capabilities of inductively coupled plasma mass spectrometry (ICP-MS) to develop and validate a quantitative method for the determination of toxic and acute levels of arsenic, selenium, cadmium, mercury, thallium, and lead in blood. The project also compares the sample preparation method of microwave digestion to that of simple dilution using standard reference materials and quality control standards. Microwave digestion proved to produce more reliable data, in terms of accuracy and reproducibility. Analysis of the standard reference materials and quality control standard results will be discussed. INTRODUCTION: Years of warnings about consuming too much fish and numerous toy recalls in the recent years invading the headlines for their use of lead are finally making people take notice of the dangers of heavy metals. Unfortunately, little is still known about the remaining dangers involving heavy metals such as arsenic, cadmium, selenium, and thallium. Arsenic, which is often found in pesticides, can make its way into ground water, fish, and other foodstuffs resulting in unaware ingestion. Arsenic ingestion can lead to problems with gastrointestinal, cardiovascular, and the central nervous systems, and eventually death. Chronic exposure to arsenic in drinking water can also be the cause of lung, bladder and kidney cancer. Cadmium is another toxic heavy metal that can lead to death, which is usually a result of kidney failure. Long-term exposure has also been linked with skeletal damage in rare cases. The major sources of cadmium exposure are through cigarettes and nickel cadmium batteries. 1 Thallium, which is used in rat poison, has been the cause of accidental as well as intentional poisoning and is more 1 Järup, Lars. Hazards of Heavy Metal Contamination British Medical Bulletin 68, 2003, pg

2 toxic then cadmium, lead, and mercury. 2 Even though the problem with heavy metals has escaped the public eye for years the toxicology community has always been monitory it. The field of forensic toxicology is largely focused on the determination of drugs in biological matrices, but can also include acute intoxication, poisoning, and death investigations relating to the ingestion of toxic heavy metals. These cases can involve both intentional poisonings as well as accidental ingestion. According to the American Association of Poison Control Centers Toxic Exposure Surveillance System roughly 13,000 heavy metal intoxication cases occurred in the United States in This statistic proves that heavy metal poisoning cases may not be the largest part of a toxicologist s workload, but are prevalent enough to deserve a good look at the possible improvements for their methods of analysis. Surprising over half, roughly 7,000, of those 13,000 heavy metal cases involved either arsenic, selenium, cadmium, mercury, thallium, or lead justifying an appropriate method in which to quantify these analytes. 1 The fact that these cases would be few and far between calls for a quick and easy method that can quantify both acute as well as toxic levels of all six of these analytes. Inductively coupled plasma mass spectrometry (ICP-MS) is an instrumental method that would have the ability to perform the quantification of all these analytes in acute or toxic levels in a single analysis. The ability of ICP-MS to perform multielemental analyses is just one of the advantages it has over other instrumentation that may be used in toxicology investigations. ICP-MS s ability to test for numerous elements simultaneously can reduce total analysis time as well as cost. The use of ICP-MS provides the potential to screen for up to 70 elements in a single analysis; in 2 Nriagu, Jerome O. Thallium. Chemical & Engineering News. 81(36) 2003 pg Watson, William; et all; 2004 Annual Report of the American Association of Poison Control Centers Toxic Exposure Surveillance System < 2

3 addition, multiple elements can be simultaneously quantified. 4 Methods currently employed in toxicology analyses such as atomic absorption can detect only 45 elements, cannot detect some metalloids or nonmetals, and often do not have the ability to do so simultaneously. 5 In fact, none of the currently employed techniques have the ability to simultaneously quantify elements as ICP-MS does. This means that in the time it takes to test for one element with the traditional method the ICP-MS will have tested for and perhaps even quantified all the elements of interest. Sensitivity is another major aspect in the attraction to ICP-MS analysis versus the usual atomic absorption spectroscopy. Atomic absorption methods generally are only sensitive enough to measure in the parts per million. On the other hand ICP-MS is sensitive enough to measure well into the parts per billion and even parts per trillion for certain elements. The instrument also has detection limits in the same range. The major heavy metals that can cause harm to the human body are arsenic, selenium, cadmium, mercury, thallium, and lead. Atomic absorption is known to have detection limits of only 0.1, 0.13, 0.002, 0.28, 0.02 and 0.03 ppm for these elements respectively. 6 These same elements have been quantified using ICP-MS in blood at levels of 0.11, 1.6, 0.04, 0.26, 0.005, and 0.07 ppb respectively. 7 Lastly ICP-MS has a large linear dynamic range and can accurately measure from percents down to parts per trillion. This broad dynamic range is well suited for toxicology analysis because of the need to screen for acute levels as well as low-level, chronic toxicity. ICP-MS is not only unique in its advantages but also in it operation. The chemistry happens within the plasma flame, which can reach degrees of 15,000 to 20,000 K. A carrier gas 4 Thomas, Robert. Practical Guide to ICP-MS; Marcel Dekker Inc.: New York, NY Robinson, James. W Atomic Absorption Spectroscopy; Marcel Dekker Inc.: New York, NY Varma, Asha. Handbook of Atomic Absorbance Analysis; CRC Press: Boca Raton, Fl Goulle J-P, Mahieu L, Castermant J, et al; Metal and metalloid multi-elementary ICP-MS validation in whole blood, plasma, urine and hair: reference values; For. Sci. Int. 153, 2005,

4 of argon in sent through the instrument, which generates a reduced temperature gap in the plasma. The sample is then aspirated into the instrument and the intense energy source of the plasma. Inside the plasma atoms become charge though collisions with argon atoms and other species within the plasma. These charged species or ions can then be analyzed by the mass spectrometer to achieve intensities for number of isotopes. Literature has provided to major sample preparation methods for the analysis of blood by ICP-MS. These methods, including simple dilution with a diluent and microwave digestion, both have analytical benefits. The dilution and digestion procedures can minimize effects from dissolved solids in the blood samples. Additionally digestion can dissolve the carbon rich components often found in biological samples, eliminate a number of matrix effects, and limit contamination. 8 MATERIALS AND METHODS: Chemicals and Reagents: Standard reference materials (SRMs) were obtained from Toxicology Centre and the Nation institute of Public Health in Quebec Canada and the National Institute of Standards and Technology (NIST). The sample diluent consisted of 0.5% nitric acid, 0.01% triton X-100, and 0.5% butanol. The internal standards stock consisted of 250 ppb germanium, rhodium, and iridium in a 2% nitric solution. The diluent stock was a 0.5% nitric acid, 0.01% triton X-100, and 5% butanol solution containing 50 ppb germanium, rhodium, and iridium. Calibrators were prepared from Cerilliant stock solution and contained the elements of interest at concentrations demonstrated in table 1. Quality control (QC) spiking solutions were prepared from Spex stock solution and contained the elements of interest at concentrations demonstrated in table 2. 8 Carrilho, Elma Neide, et al; Microwave-Assisted Acid Decomposition of Animal- and Plant-Derived Samples for Element Analysis J. Agric. Food Chem., 50 (15), 2002,

5 As Se Cd Hg Tl Pb Low Medium High Table 2: Concentration in ppb of each element in the quality control spiking solutions As Se Cd Hg Tl Pb Cal Cal Cal Cal Table 1: Concentration in ppb of each element in the 4 calibrator spiking solutions Simple Dilution: Samples for the simple dilution procedure were prepared in 15 ml disposable centrifuge tubes. To the centrifuge tube 0.6 ml of blood, either SRM or pigs, and 0.12 ml of the internal standards stock solution was added. In addition to the SRMs 0.6 ml of 2% nitric acid and to the pigs blood 0.6 ml of a stock calibrator solution were added. The samples were diluted to 6 ml with a sample diluent. QC samples were prepared in an identical fashion to the standards using the QC spike solutions. The samples were centrifuged at 4000 rpm for 10 minutes to separate the solids. Microwave Digestion: The microwave digestion samples were prepared for analysis by adding 0.75 ml aliquots of blood sample and 1.5 ml of concentrated Optima grade nitric to a quartz insert. Next, teflon digestion vessels are filled with 2 ml of concentrated hydrogen peroxide and 8 ml of 18 MΩ or deionized water. The peroxide permitted higher digestion temperatures by reducing the nitric vapors. The quartz inserts were then lowered into the teflon vessels, capped and digested in an Ethos advanced microwave workstation from Milestone Step Time Power Temp min 1000W 85 C min 1000W 135 C min 1000W 230 C min 1000W 230 C Table 3: Microwave digestion parameters for blood digestion 5

6 according to the parameters demonstrated in table 3. 9 Following digestion, the vessels were cooled to the point where the inserts could be safely removed. The clear digestate was then quantitatively transferred to a disposable centrifuge tube. The pigs blood digestate was spiked with 0.75 ml of calibrator and 0.75 ml of diluent stock which contained the internal standards and brought up to 7.5 ml with 18 MΩ water. The SRM digestate was prepared in an identical fashion with the addition of 0.75 ml of 2% nitric acid in place of the calibrator stock. QC samples were prepared in an identical fashion to the standards using the QC spike solutions. ICP-MS Analysis: Following the final preparations the blood sample they were analyzed using a Perkin Elmer Elan 9000 ICP-MS. The method parameters for the blood metals analysis are provided in Table 4. Component Parameter RF Power 1200 Watts Nebulizer Flow Approx LPM Sweeps/Reading 20 Readings/Replicate 1 Replicates 3 Detector Mode Pulse Blank Subtraction After Internal Standard Measurement Units cps Process Spectral Peak Average Process Signal Profile Average Autolens On Scan Mode Peak Hopping MCA Channels 1 Dwell Time 100 ms Integration Time 2000 ms Curve Type Simple Linear Sample/Standard Units ug/l Calibration Type Standard Additions Table 4: ICP-MS blood metals method parameters 9 Milestone Microwave Laboratory Systems; Digestion Application report ; December,

7 Each day prior to the analysis of any samples the instrument was calibrated to ensure that it was operating within acceptable ranges. Once this was established the pigs blood standard curve was analyzed and linearity was evaluated. Next quality control (QC) standards and standard reference materials (SRM) were analyzed. The instrument was allowed time to rinse between each sample which ranged from 1 to 2 minutes. The diluted samples were allowed to rinse for longer periods of time to ensure that carry over was not occurring. The instrument software would supply a calibration curve and net intensities for the standards as well as concentration for elemental isotope in the samples. The results were analyzed each day to ensure the method was working efficiently. RESULTS and DICUSSION: Numerous analyses were done on the QC standards and SRMs. Table 5 displays the average of 4 digestion analyses and 3 dilution analyses for the particular SRM QMEQAS07B-06. Actual Digestion Dilution µg/l µg/l µg/l As75 14 ± ± ± 2.2 As ± 3 21 ± 5 18 ± 4 Se ± ± ± 17 Cd ± ± ± 1.4 Cd ± ± ± 1.4 Hg ± ± ± 3 Hg ± ± ± 3 Tl ± ± ± 0.6 Pb ± ± ± 50 Pb ± ± ± 50 Pb ± ± ± 50 within 95% confidence interval Table 5: Displays the excepted values as well as the average of 4 digestion analyses and 3 dilution analyses for the SRM QMEQAS07B-06 In these particular analyses one can see that the microwave digested samples produced more accurate results for all metals than the those prepared by simple dilution prep and exhibit smaller 7

8 deviations meaning better precision. All of the standard reference materials as well as the QC standards demonstrated similar results in terms of accuracy and precision. As expected from literature, the selenium results leave a lot to be desired. The chemistry of selenium in the argon plasma of the ICP-MS and its relatively high homeostatic levels in blood ( ppb) combine to make it very difficult to analyze by this method. The decision as to whether or not selenium should be included in the final method has by debated throughout the lifetime of this project. If selenium is kept the method will only have to ability to determine that toxic level are present and not an accurate quantification. The decision as to whether the digestion or dilution method should be used unfortunately depends not only on the accuracy and reliability of the method but also on the convince factor for the laboratory. Clearly the microwave digestion method produced the more accurate and reliable results but it must be noted that not only is this method more time consuming for the analyst but it requires the used of more expensive materials. The digestion method in total including time spent waiting on the vessels to cool can take anywhere from 2 to 3 hours while simple dilution preparation is roughly 45 minutes including ten minutes in the centrifuge. The ICP-MS analysis of the dilution runs do take longer as a result of increased rinse times between samples but an autosampler would most likely be employed so this would have little effect on the analyst. In a crime laboratory an analyst time is very limited and a large sample output is a huge part of an analyst job. With that said the dilution method would most likely produce a higher throughput for the laboratory with all things considered. In terms of expense other then labor, the chemicals required for the digestion procedure required a bigger budget for the laboratory. The concentrated hydrogen peroxide as well as large amounts of Optima grade nitric acid can quickly cost a lab. Optima grade nitric acid can cost roughly $1 per ml and almost $1.50 per 8

9 sample, which can add up quickly for a lab that wants to run this analysis on a majority of the toxicology samples that come through the lab such as the Wisconsin State Crime Lab-Madison would like to do. On the other hand the digestion procedure was able to remove both the complex matrix and reduce biohazard of the sample. This results in a less hazardous sample for the analysts as well as the instrumentation. As with many things the decision on which method to employ in a particular laboratory is dependent on quality versus quantity as well as budget. CONCLUSION: The result provide sufficient evidence in which to conclude that this method would be valid for use in toxicology for quantitative determination of toxic and acute levels of arsenic, selenium, cadmium, mercury, thallium, and lead in blood samples. Furthermore, in terms of sample preparation, if time and money were not an issue, the recommended method would be the microwave digestion procedure. This procedure produced the more accurate and reliable results compared with that of the simple dilution. Additionally, this procedure produced a sample that was clear compared to the dark red dilution samples. The cleaner sample and simpler matrix of the digestate demand less wear and tare on the instrumentation. Finally, the digestion lessened the biohazard of the samples making them less hazardous for the analyst. The final phases of this project are still being completed and plans are to implement this method in the Wisconsin State Crime Laboratory - Madison as soon as validation is complete. 9