Nuclear Forensics Summer School 2010

Laboratory Experiment #4

Determination of Uranium Concentrations and Uranium Isotope Ratios in Unknown Samples

Introduction

ICPMS Background

General

Inductively coupled plasma mass spectroscopy (ICPMS), developed in the late 1980's combines, an easy and quick sample introduction of ICP technology with the accurate and low detection limits of a mass spectrometer. The resulting instrument is capable of simultaneous multielement analysis, often at the part per trillion level. ICPMS has been used widely over the years, finding applications in a number of different fields including drinking water, wastewater, natural water systems/hydrogeology, geology and soil science, mining/metallurgy, food sciences, and medicine.

Operation

Samples are decomposed to neutral elements in a high temperature argon plasma and analyzed based on their mass to charge ratios. An ICPMS can be thought of as four main processes, including sample introduction and aerosol generation, ionization by an argon plasma source, mass discrimination, and the detection system. Figure 1 illustrates this sequence of processes.

Sample Introduction

Aqueous samples are introduced into the ICPMS by a nebulizer which aspirates the sample with high velocity argon, forming a fine mist. The aerosol then passes into a spray chamber where larger droplets are removed and sent to waste. Typically, only 1-2% of the original mist passes through the spray chamber and enters the torch region.

Ionization by Argon Plasma

Once the sample passes through the nebulizer and is partially desolvated, the aerosol moves into the torch body and is mixed with more argon gas. A coupling coil is used to transmit radio frequency to the heated argon gas, producing an argon plasma located in the torch region. The hot plasma removes any remaining solvent and causes sample atomization followed by ionization.

Because atomization/ionization occurs at atmospheric pressure, the interface between the ICP and MS components becomes crucial in creating a vacuum environment for the MS system. The entire mass spectrometer must be kept in a vacuum so that the ions are free to move without collisions with air

molecules. Since the ICP is maintained at atmospheric pressure, a pumping system is needed to continuously pull a vacuum inside the spectrometer. In order to most efficiently reduce the pressure, a roughing pump and a turbo pump are typically used to gradually reduce pressure to 10 -6 torr before the ion stream reaches the quadrupole.

Figure 1: ICPMS Operation

Mass Spectrometer

Ions formed in the plasma flow through the small orifice (~ 1mm diameter) of the sampler and skimmer cones and are pumped into the mass spectrometer. An ion beam is produced and focused further into the actual unit. There are several different types of mass analyzers which can be employed to separate isotopes based on their mass to charge ratio. Quadrupole analyzers are compact and easy to use but offer lower resolution when dealing with ions of the same mass to charge (m/z) ratio. Double focusing sector analyzers offer better resolution but are larger and are more expensive.

The quadrupole mass filter is made up of four metal rods aligned in a parallel diamond pattern. A combined DC and AC electrical potential is applied to the rods with opposite rods having a net negative or positive potential. Ions enter into the path between all of the rods. When the DC and AC voltages are set to certain values only one particular ion is able to continue on a path between the rods and the others are forced out of this path. This ion will have a specific m/z ratio. Many combinations of voltages are chosen which allows an array of different m/z ratio ions to be detected.

Sample Introduction and Aerosol Generation

Ionization by Argon Plasma

Mass Discriminator and Detector

Data Analysis

Detector

The most common type of ion detector found in an ICPMS system is the channeltron electron multiplier. This cone or horn shaped tube has a high voltage applied to it opposite in charge to that of the ions being detected. Ions leaving the quadrupole are attracted to the interior cone surface. When they strike the surface additional secondary electrons are emitted which move farther into the tube emitting additional secondary electrons. As the process continues even more electrons are formed, resulting in as many as 108 electrons at the other end of the tube after one ion strikes at the entrance of the cone. The detector measures the number of ions counted per second.

ICPMS Advantages

Low detection limits Rapid analysis for multiple analytes (high sample throughput) Ability to obtain isotopic ratios Ability to couple with various sample introduction systems for oxidation state

measurements and solid analyses

ICPMS Disadvantages/Limitations

Polyatomic and isobaric interferences Limited matrices Limited resolution for quadrupole systems Cost

Applications- Qualitative Analysis

The ICPMS can be used to scan an unknown solution or solid material to determine what masses are present in the sample. The entire mass range of the periodic table can be scanned with a few exceptions. These masses correspond to a list of possible analytes present in the unknown, which can later be quantified.

Applications- Quantitative Analysis

The ICPMS can be calibrated for a known analyte of a specific mass, generating a linear calibration curve of intensity (ion counts per second) vs. concentration over several orders of magnitude. The analysis of unknown solutions will generate intensities at a specified mass corresponding to the analyte of interest. The intensities can be interpolated using the calibration curve to determine the concentration of the analyte in the unknown.

Applications- Isotopic Analysis

ICPMS can be used to determine the isotopic ratio(s) of various analytes in unknown samples. Intensities of the masses of interest can be collected, then by dividing the intensity values the isotopic ratios can be

obtained. Isotopic ratios can be useful in many fields such as the environmental, forensics, geological, and radiological.

Quality Assurance and Quality Control

Quality Assurance is all planned and systematic actions necessary to provide adequate confidence that a product or service will satisfy given requirements. Usually, the requirements are set by the client or the funding agency. A quality assurance program is a set of procedures that assist laboratory personnel meet requirements. These procedures include checks and balances, paperwork, and documentation. A quality assurance program is typically more expensive and takes more time compared to not having a quality assurance program. However, if the researcher fails to document necessary information or incorrectly performs an experiment, the cost and time to repeat the work may be more expensive and time consuming than having a quality assurance program in the first place.

Quality Control are a laboratory processes that is used to ensure that collected data meets accuracy and precision requirements and that the data generated is reportable to the client or funding agency. In order to ensure that the researcher is measuring what he/she is supposed to be measuring (i.e. hitting the bull’s eye) an assessment of accuracy is conducted. In order to ensure that the researcher is generating the same measurement for the same sample (i.e. hitting the same spot on the bull’s eye every time) then a determination of precision is performed. The amount of quality control samples that are analyzed with each analytical batch can vary depending on the client or funding organization requirements, the instrumentation or analytical technique, and the sample composition.

Some Types of Quality Control Samples

Calibration Blank: The matrix that was used to prepare the calibration standards is analyzed to ensure that there are no interferences that will impact the measurement of the desired analyte.

Initial Calibration Check (ICC): This QC sample is used to ensure that the calibration curve is accurate, or is the calibration curve measuring what it is supposed to be measuring? The ICC is a standard that is prepared by a different person than the person who prepared the calibration standards. The same matrix water is used and if possible a different stock solution is used. The concentration of the standard must be within the calibration curve. Calculating percent recovery (Equation 1) of the prepared ICC concentration to the measured ICC concentration allows the analyst to evaluate the accuracy of the calibration curve. Usually a percent recovery of 90-110% is acceptable. Only one ICC is required per analytical batch.

Percent Recovery = Measured Concentration (Equation 1) Actual Concentration

Duplicates: Sample duplicates are used to measure analytical precision, or is the instrument measuring the same values for the same sample over time? A sample duplicate is an unknown sample in an analytical batch that is analyzed twice. If there are many samples in a batch, 1 duplicate is usually analyzed for every 15-20 samples. Calculating the percent difference (Equation 2) of the sample

X 100%

concentration and the duplicate concentration will determine instrument precision. Usually a relative percent difference of ± 15 % is acceptable.

Percent Difference = (Concentration Sample-Concentration Duplicate) (Equation 2)Average Concentration Sample and Duplicate

Matrix Spikes: Often it is not possible to prepare calibration standard in the same matrix as the unknown samples. For example, the unknown samples may be from a spring and the calibration standards are prepared in 1% nitric acid. Although the samples and the standards may look similar, the spring water has a different composition than the 1% nitric acid and this difference may result in a change to the analyte signal (either by enhancing it or suppressing it). To test the compatibility of the calibration standard matrix to the unknown sample matrix, a matrix spike is prepared and analyzed.

One of the unknown samples is analyzed as collected or processed to determine the concentration of the analyte of interest. An aliquot of this same sample is then spiked with a known concentration of the desired analyte and the spiked sample is analyzed. The analyst should observe that the concentration of the analyte in the spiked sample is the concentration in the unspiked sample plus the concentration that was added to the spiked sample. A calculation of spike recovery (Equation 3) will determine whether or not there is a matrix effect. If the spike recovery is within 75-125%, then there is not a matrix effect and the calibration standards may be used to determine the concentration of the analyte in the unknown samples. Only one matrix spike is required for each different matrix in an analytical batch.

Spike Recovery = (Concentration Spiked Sample-Concentration Unspiked Sample) (Equation 3)Concentration of Spike Added

Continuing Calibration Check (CCC): This QC sample is used to ensure that the analytical instrument is continuing to measure what it is supposed to be measuring throughout the analytical batch. One of the mid-range calibration standards is analyzed at a minimum at the end of the analytical batch. If the batch contains many unknown samples, then a CCC is analyzed every 15-20 samples. Calculating percent recovery (Equation 1) of the CCC standard concentration to the measured CCC standard concentration allows the analyst to determine whether the instrument is still in calibration. Usually a percent recovery of 90-110% is acceptable.

X 100%

X 100%

Experimental Procedure

NOTE: Ensure that PPE (at least lab coats, gloves, and safety glasses) are applied before starting this procedure.

A. Determination of Uranium Concentrations in Unknown Samples- Quantitative Analysis

You will be supplied with two unknown solutions containing various concentrations of uranium that are within your calibration curve. Quality control samples: ICC, Duplicates, Matrix Spike, and CCC are required.

Refer to the Elan DRC II User Guide for this part of the procedure.

Part 1: Preparation of Calibration Standards (Student 1) & QC Samples (Student 2)

Materials:Uranium stock solution 100 ppbVolumetric FlasksTransfer PipettesPipettors and Pipette Tips1% Nitric Acid50 mL Centrifuge Tubes15 mL Centrifuge Tubes

Student 1: Calibration Standard Preparation1. Calculate the amount of 100 ppb uranium that needs to be added to each 50 mL flask to make the following concentrations of uranium: 0.1 ppb, 0.5 ppb, 1 ppb, 5 ppb, 10 ppb.

2. Rinse each volumetric flask three (3) times with 1% nitric acid and then partially fill each volumetric flask with 1% nitric acid. Use a transfer pipette if necessary.

3. Label each volumetric flask with the concentrations listed in step 1, your initials, and the date.

4. Pour an aliquot of the 100 ppb uranium stock solution into the supplied container.

5. Pipette the calculated volume of 100 ppb uranium into each flask using the appropriate pipettor.

6. Fill each volumetric flask to the mark with 1% nitric acid, cap (or parafilm) and shake.

7. Label six (6) 50 mL centrifuge tubes with Blank (1% HNO3), 0.1 ppb U, 0.5 ppb U, 1 ppb U, 5 ppb U, and 10 ppb U and your initials and date on all of them.

8. Pour some of the 1% nitric acid into the Blank (1% HNO3) tube then pour each of the standards from the volumetric flasks into the appropriate centrifuge tubes.

Student 2: ICC and Matrix Spike Preparation1. Calculate the amount of 100 ppb uranium that needs to be added to the 50 mL flask to make a concentration of 1 ppb uranium.

2. Rinse the volumetric flask three (3) times with 1% nitric acid and then partially fill the volumetric flask with 1% nitric acid. Use a transfer pipette if necessary.

3. Label the volumetric flask with the prepared concentration, the letters ICC, your initials, and the date.

4. Pour an aliquot of the 100 ppb uranium stock solution into the supplied container.

5. Pipette the calculated volume of 100 ppb uranium into the flask using the appropriate pipettor.

6. Fill the volumetric flask to the mark with 1% nitric acid, cap (or parafilm) and shake.

7. Label a centrifuge tube with the letters ICC, the prepared uranium concentration, your initials, and the date.

8. Pour the contents of the 50 mL volumetric flask into the centrifuge tube from step 7.

9. Select one of the unknown samples as the matrix spike and decide on the concentration that will be spiked (1-5 ppb).

10. Calculate the amount of 100 ppb uranium that needs to be added to a 10 mL volumetric flask to have the spike concentration selected in step 8.

11. Rinse the volumetric flask three (3) times with a small volume of sample and partially fill the flask with sample. Use a transfer pipette if necessary.

12. Label the volumetric flask with the unique sample identifier, spike, the spike concentration, your initials, and the date.

13. Using the 100 ppb uranium stock solution aliquot from step 4, pipette the calculated volume of the 100 ppb uranium stock solution into the volumetric flask using the appropriate pipettor.

14. Fill the volumetric flask to the mark with the unknown sample, cap (or parafilm), and shake.

15. Label a 15 mL centrifuge tube with the unique sample identification number, “Spike” designation, spiked concentration, your initials, and the date.

Part 2: ICPMS Start UpRefer to the attached Elan DRC II User Guide.

1. Follow the steps in the Elan DRC II User Guide for “ICP-MS Start-up” (pages 1-5).

2. Save the hard copy of the Daily Performance Report to include in the final laboratory report.

Part 3: Sample AnalysisRefer to the attached Elan DRC II User Guide.

Method Set-up1. Follow the steps in the Elan DRC II User Guide for “Setting-Up a New Method” (p. 6-9) to measure the concentration of uranium in the unknown samples.

2. In the “Timing” Tab, right click in the analyte column and select U (238) from the periodic table and click “OK”.

3. In the “Timing” Tab, enter 15 sweeps/reading, 1 reading/replicate, and 3 replicates.

4. In the “Calibration” Tab, right click on the Sample Units column and select “ppb” from the list. Right click on the Standard Units column and select “ppb” from the list.

5. In the “Calibration” Tab, enter the standard concentrations prepared in “Part I, Student 1” above from lowest concentration to highest concentration.

6. In the “Sampling” Tab, select the appropriate autosampler and tray as described in the Elan DRC II User Guide.

7. In the “Sampling” Tab, enter the standard concentrations for each of the Blank and Standards 1-5 in the Solution ID column.

8. Place the Blank and the calibration standards on the autosampler tray. Enter the positions of the Blank and calibration standards in the A/S Loc. Column in the “Sampling” Tab.

9. Follow the Elan DRC II User Guide to enter the appropriate flush, read delay, and wash pump speeds and times.

10. Follow the Elan DRC II User Guide to enter the appropriate information for the “Report” tab and to save the method file. “Your Folder” will be called Radchem SS.

Sample Batch Set-up1. Follow the steps in the Elan DRC II User Guide for “Creating a Sample Batch” (p. 10-11) to set up an analytical batch for the unknowns and QC samples.

2. The analytical batch should look something like Figure 2.

Dataset Set-up

1. Open the “Dataset” window using the short-cut key:

2. To open the “Radchem SS” database, go to File -> Open -> Radchem SS, then click OK.

Sample Analysis

1. Open the “Sample” window and your analytical batch will appear.

2. Highlight all of the samples on the list to be analyzed from the “Batch Index” column. In Figure 2, Batch Index #1-8 would be highlighted.

3. Click on “Analyze Batch”.

4. If you are asked to “Clear/Remove Blank and Calibration Data”, select “Yes”.

5. If you are asked to save any of the files you have created, save them if necessary.

Part 4: Post Sample Analysis

1. Open the “CalibView” window using the short-cut key: to view the uranium calibration curve in intensity (cps) vs. concentration (ppb).

2. Click on the “Stats” button to view the correlation coefficient value (Figure 3). If the correlation coefficient is 0.995, then the calibration curve is acceptable. If not, remove any outliers from the calibration curve or re-prepare the calibration standards. Save and print the calibration curve.

Figure 2: Sample Analytical Batch

3. Obtain the sample data from the printer and visually inspect the data to ensure that the ICC, Duplicate(s), Spike, and CCC meet the requirements outlined in the “Introduction”. If all QC samples pass, proceed to step 4. If one or more of the QC samples fails, re-analyze the QC sample(s) one time. If the QC sample(s) do not pass the second analysis, then re-prepare them and re-run the entire analytical batch.

4. Transfer electronic data from the computer hard drive to your memory sticks. Go to the “Desktop” -> Shortcut to Report Output -> Radchem SS ->”Your file specified in the method”. This file can be opened with Excel using comma and “ delimiters. The electronic file will look similar to the hard copies.

5. Open the “RptOption” window using the short-cut key:

6. Go to “File” -> “Open” -> “Quantitative Calibration.rop” to open the report format for the calibration curve.

Figure 3: Calibration Curve and Correlation Coefficient

7. Open the “RptView” window using the short-cut key:

8. Print two copies of the calibration data for the uranium standards.

9. Open the “RptOption” window using the short-cut key:

10. Go to “File” -> “Open” -> “Sample Batch Tracers.rop” to open the report format for the analytical batch.

11. Open the “RptView” window using the short-cut key:

12. Print two copies of the sample batch information.

Results- Quantitative Analysis

The data package will include the following:

Calibration curve and correlation coefficient data

Analytical batch summary

Method summary (analyte and mass measured, sweeps per reading, readings per replicate, number of replicates, calibration standard concentrations)

Reduced data containing calculations for Percent Recovery (Equation 1), Percent Difference (Equation 2), Spike Recovery (Equation 3)

Final sample data report including sample unique identification numbers and concentration of uranium in each sample (for the sample(s) that were analyzed in duplicate, be sure to report the average of the sample concentration and the duplicate in the final report) B. Determination of Uranium Isotope Ratios in Unknown Samples- Isotopic Analysis

You will be supplied with four unknown solutions that have various isotopic ratios of U-238 and U-235. You will analyze each of the samples and determine the isotopic composition of each of the unknowns. A reference solution will also be analyzed that contains a U-235:U:238 ratio of 0.0072526. One sample will be analyzed in duplicate to measure analytical precision.

Refer to the Elan DRC II Isotope Ratio User Guide for this part of the procedure.

Part 1: Sample Analysis

Method Set-up1. Follow the steps in the Elan DRC II Isotope Ratio User Guide for “Setting-Up a New Method” (p. 6-9) to measure the uranium isotope ratios in the unknown samples.

2. In the “Timing” Tab, click in the analyte column and type U 235 and U 238, then follow the steps in the guide to define the group and select the reference mass.

3. In the “Timing” Tab, enter 40 sweeps/reading, 1 reading/replicate, and 3 replicates.

4. In the “Calibration” Tab, click on the U-235 standard ratio cell and type in 0.0072526.

5. In the “Sampling” Tab, select the appropriate autosampler and tray as described in the Elan DRC II Isotope Ratio User Guide.

6. In the “Sampling” Tab, enter the Blank name in the Solution ID column.

7. Place the Blank on the autosampler tray. Enter the position of the Blank in the A/S Loc. Column in the “Sampling” Tab.

8. Follow the Elan DRC II Isotope Ratio User Guide to enter the appropriate flush, read delay, and wash pump speeds and times.

9. Follow the Elan DRC II Isotope Ratio User Guide to enter the appropriate information for the “Report” tab and to save the method file. “Your Folder” will be called Radchem SS.

Sample Batch Set-up1. Follow the steps in the Elan DRC II Isotope Ratio User Guide for “Creating a Sample Batch” (p. 10-11) to set up an analytical batch for the unknowns and QC samples.

2. The analytical batch should look something like Figure 4.

Dataset Set-up

Figure 4: Example Analytical Batch for Isotope Ratio Analysis

1. Open the “Dataset” window using the short-cut key:

2. To open the “Radchem SS” database, go to File -> Open -> Radchem SS, then click OK.

Sample Analysis

1. Open the “Sample” window and your analytical batch will appear.

2. Highlight all of the samples on the list to be analyzed from the “Batch Index” column. In Figure 4, Batch Index #1-7 would be highlighted.

3. Click on “Analyze Batch”.

4. If you are asked to “Clear/Remove Blank and Calibration Data”, select “Yes”.

5. If you are asked to save any of the files you have created, save them if necessary.

Part 4: Post Sample Analysis

1. Transfer electronic data from the computer hard drive to your memory sticks. Go to the “Desktop” -> Shortcut to Report Output -> Radchem SS ->”Your file specified in the method”. This file can be opened with Excel using comma and “ delimiters. The electronic file will look similar to the hard copies.

2. Open the “RptOption” window using the short-cut key:

3. Go to “File” -> “Open” -> “Sample Batch Tracers.rop” to open the report format for the analytical batch.

4. Open the “RptView” window using the short-cut key:

5. Print two copies of the sample batch information.

ICPMS Shutdown1. Place the autosampler probe in the 1% nitric acid rinse bottle for about 2 minutes.

2. Place the autosampler probe in the DI water container for about 5 minutes.

3. Remove the probe from the DI water and place it back on the autosampler.

4. On the Elan software, open the “Devices” window using the short-cut key:

5. Click on the “Stop” button to stop the pump.

6. Open the “Instrument” window using the short-cut key:

7. Click on the “Stop” button for the PLASMA ONLY (please be careful not to stop the vacuum).

8. Loosen the pump tubing from the peristaltic pump.

Results- Isotope Ratio Analysis

The data package will include the following:

Analytical batch summary

Method summary (analyte and mass measured, sweeps per reading, readings per replicate, number of replicates)

Reduced data containing Percent Recovery (Equation 1) for Standard Reference Solution, and Percent Difference for Duplicate(s) (Equation 2)

Final sample data report including sample unique identification numbers and U-235:U-238 isotope ratio (for the sample(s) that were analyzed in duplicate, be sure to report the average of the isotope ratios for the sample and the duplicate in the final report)

Questions to be answered by students

1. How does this apply to nuclear forensics and practical applications?2. What would you change about the experiment to improve the forensic capabilities?3. What is the value of analyzing quality control samples in an analytical batch?