Instrumentation & Methods: ICP/MS, Uranium

Transcription

1 Instrumentation & Methods: ICP/MS, Uranium Jeff Brenner Minnesota Department of Health Overview and Fundamentals of ICP-MS Determination of Metals Using Inductively Coupled Plasma Mass Spectrometry Overview & Fundamentals of ICP-MS What we will cover Overview and Fundamentals ICP-MS Theory Interferences Reports 1

2 EPA ICP-MS Definition An analytical technique to determine Elements using Mass Spectrometry from Ions generated by an Inductively Coupled Plasma. Mass Spectroscopy Separation and measurement of the mass of individual atoms making up a given material EPA Analytical Benefits of ICP-MS Rapid multi-element quantitative analysis Very low detection limits Rapid semi-quantitative analysis Wide dynamic range Isotopic analysis Spectral simplicity Speciation (with HPLC) EPA Isotopes and Mass Spectra Isotopes of an element differ in the number of neutrons in the nucleus U Atomic Number U has 142 neutrons 235 U has 143 neutrons 238 U has 146 neutrons 2

3 U Isotope Abundance Isotope Half Life Natural Specific Years Abundance Activity (pci/ug) 234 U 246, % U 700 million 0.72 % U 4.47 billion % Isotopes and Mass Spectra The Isotopic abundance of most elements is constant Pb may differ slightly based on the source of the Pb Pb is analyzed as the sum 206 Pb 207 Pb 208 Pb Ions and Mass Spectra Positive ions are produced by the energy in the plasma In order to utilize a mass spectrometer an ion is necessary ICP-MS analyze isotopic ions The ions are steered throughout the ion path of the spectrometer. 3

4 ICP-MS Spectrum A series of peaks that correspond to mass to charge ratio (m/z) Peaks could be the sum of different isotopes of different elements Doubly charged ions will appear ½ its mass 138 Ba double charges will appear at 138/2 = 69 Isobaric Spectral Overlaps Signal at given amu is the summation of all the isotopes at that amu It is best to avoid potential overlaps by monitoring a clean mass Overlaps are correctable in software 4

5 Isobaric Spectral Overlaps Several factors must be considered when selection an isotope: Concentration of analyte Concentration of interferences Abundances of isotopes at the given mass Molecular Overlaps Polyatomic or molecular ions will occur Common ones are Ar, O, and H based Be aware of molecular overlaps that are formed: Plasma (Ar) Solvents (O, H, Cl, N) Samples (C, Cl, S) 5

6 Molecular Overlaps Elements in the ICP do not fully break apart and recombination of highly concentrated elements will occur Example 56 Fe and 40 Ar+ 16 O Background spectral features have been well characterized Factors Affecting Ion Intensities Isotopic Abundance Intensity Intensity of an isotope is proportional to its natural abundance The sum of the signals from all isotopes of an element are compared to the signal from a mono-isotopic element, the signals ideally should be equal Example: Element Percent Relative Isotope Abundance Intensity 55 Mn U U U Factors Affecting Ion Intensities Percent Ionization Element % Ionized Na 100 As 50 Se 34 F Most elements are ionized greater than 90%. 6

7 ICP-MS System Courtesy: Perkin Elmer Spray Chamber and Nebulizer ICP-MS Ion Source Region Plasma creates ions from the components in the sample. Heat from 6,000K-10,000K dries, aerosol, then atomize, and ionize components of the sample. 7

8 ICP-MS Ion Source Region (Plasma) Plasma is formed by a stream of argon gas flowing between to quartz tubes. Radio frequency (RF) power is applied through the coil, and an oscillating magnetic field is formed. An electrical discharge creates seed electrons and ions. ICP-MS Ion Source Region (Plasma) Inside the induced magnetic field, the charged particles are forced to flow in a closed annular path. As they meet resistance, heating takes place and additional ionization occurs. Reaction Cell Pressurized with a reactive gas Convert isobar to a different ion which does not interfere Convert analyte to polyatomic ion which is not interfered The specific chemistry is dependent on: Nature and density of the reactive gas Electrical fields within the cell 8

9 ICP-MS Ion Source Region (Lens) Before sampler cone 760 torr Before skimmer cone 3 torr After skimmer cone 1e-3 torr ICP-MS Ion Source Region (Lens) Material extracted from the plasma are composed of a mixture of the following: Neutral atoms (Ar) Molecules (O 2 ) Positively charged atomic and molecular ions (Ar+, O 2 +) Reactive metastable atoms and ions Negatively charged atomic and molecular ions Photons Electrons ICP-MS Ion Source Region (Lens) The lens captures and guides the positively charged ions to the quadrupole. By applying a positive potential to the lens, the ions will be focused to the center of the lens. Small ions are optimized at lower voltages. As the voltage is increased, higher mass ions are better focused. If the voltage is to high the ions are repelled. 9

10 Reaction Cell or Collision Cell A reaction gas is introduced into the cell. The reaction of the gas with the interfering species is set up to remove these interferences from the path. Quadrupole Courtesy: Perkin Elmer Mass Filtering System Separates on type of element (ion) from another with an electromagnetic field. Only one mass (m/z) will make it through at a time. Many masses enter, only one makes it out. Perkin Elmer Optimization After initiating the plasma, allow the instrument to warm up while aspirating a blank solution for at least 15 minutes. Mass Calibration Tune DRC II Tuning Solution (1 ppb Mg, In, Ce,Ba,Pb, U) and check for responses and RSDs. Generate and evaluate a tune report. 10

11 Perkin Elmer DRC II Optimization Suggestions Suggested guidelines for an acceptable tune for method Sensitivity: Mg > 8,000 cts/0.1 sec/10 ppb In >40,000 cts/0.1 sec/10 ppb U >30,000 cts/0.1 sec/10 ppb Precision: Mg < 5 % RSD (0.1 sec integration time) In < 5 % RSD ( ) U < 5 % RSD ( ) Oxides: < 3.0% Ba++/Ba+ < 3.0% Background: Mass 220 < 2 cps Mass Accuracy: +/ AMU Daily Performance Check Sensitivity Nebulizer Autolens x-y adjustment Detector Optimization Oxides to High: Reduce nebulizer flow (plasma temperature increases) Dirt cones Reduce peristaltic pump speed Increase RF power Double Charged ions too high: Decreased RF power Increase nebulizer flow Check skimmer 0-ring Poor precision Check entire sample introduction system Check the nebulizer Check that the correct method is used Perform a visual check of the plasma! Is it stable? Isobaric Correction Counts at mass 114 = 114 Cd Sn 114 Cd = mass Sn We cannot measure the counts of Sn at mass 114 directly since 114 Cd can also be present. However, we can measure another isotope of Sn (118) that is free from overlap by Cd. Therefore: 114 Cd = mass 114 (a 114 Sn/a 118 Sn)*( 118 Sn) 11

12 Isobaric Correction The abundance ratio (a 114 Sn/a 118 Sn) of these two isotopes is (0.65%/24.23%) and is reasonably constant. Therefore: 114 Cd = mass 114 (0.65%/24.23%)*( 118 Sn) Correction = -(0.0268)*( 118 Sn) Polyatomic Correction Interference of Chloride on Arsenic High concentrations of chloride react with argon in the plasma to form the following: 40 Ar 35 Cl interfering on 75 As 40 Ar 37 Cl interfering on 77 Se As has only one isotope at mass Ar 35 Cl can cause isobaric overlap & Erroneously high results Must measure 40 Ar 35 Cl contribution and subtract it from the total counts at mass 75 Total counts mass 75 = counts from 75 As plus counts from 40 Ar 35 Cl 75 As = mass Ar 35 Cl Polyatomic Correction We cannot measure the ArCl contribution at mass 75, however, we can measure the ArCl contribution from 40 Ar 37 Cl at mass 77 The equation then becomes: 75 As = mass 75- (a 40 Ar 35 Cl/a 40 Ar 37 cl)*( 40 Ar 37 Cl) The relative intensities of 40 Ar 35 Cl and 40 Ar 37 Cl are determined by the isotopic ratio of 35 Cl to 37 Cl %/24.23%= As = mass *( 40 Ar 37 Cl) Correction = * 77 Se 12

13 Polyatomic Correction If Se is present in the sample, the correction becomes more complicated. 77 Se will contribute intensity counts to mass 77. Therefore, measure Se at mass 82 and multiply the result by the ratio of 77 Se to 82 Se. 75 As = mass *(mass77-77 Se) 75 As = mass *[(mass77-(a 77 Se/a 82 Se)* 82 Se] 75 As = mass *[(mass * 82 Se] Correction * 77 Se+2.733* 82 Se Types of Methods Measuring Uranium Total concentration method Uranium analysis by ICP-MS Results reported as ug/l Not very labor intensive Limitations Can not detect 234 U and 235 U isotope Conversion is accurate if isotopes are present in natural abundance Bias radioactivity concentration low Uranium Calculation Uranium radioactivity A (pci/l) = U (ug/l) * 0.67 (pci/ug) Where: A = activity of uranium U = uranium concentration 0.67 = conversion factor 40 CFR part Analytical methods for radioactivity. Footnote 12 13

14 Types of Methods Measuring Uranium Total activity method Uranium chemically separated Analyzed on alpha-beta proportional counter Total activity of all three uranium isotopes Reported as pci/l Limitations Can not distinguish isotope Conversion is accurate if isotopes are present in natural abundance Bias mass concentration high Labor intensive Types of Methods Measuring Uranium Isotopic activity method Uranium chemically separated Similar to total activity Alpha spectrometer Able to distinguish uranium isotope Results can be reported as pci/l or ug/l Limitations Labor intensive U Isotope Abundance Isotope 234 U 235 U 238 U Half Life (years) 246, million 4.47 billion Natural Abundance % 0.72 % % Specific Activity (pci/ug) 6, Relative Intensity