Presentation on theme: "Instrumentation & Methods: ICP/MS, Uranium"— Presentation transcript:
1 Instrumentation & Methods: ICP/MS, Uranium Jeff BrennerMinnesota Department of Health
2 EPA Method 200.8 Overview and Fundamentals of ICP-MS Determination of Metals Using Inductively Coupled Plasma Mass Spectrometry
3 Overview & Fundamentals of ICP-MS What we will cover Overview and FundamentalsICP-MS TheoryInterferencesReports
4 EPA ICP-MS DefinitionAn analytical technique to determine Elements using Mass Spectrometry from Ions generated by an Inductively Coupled Plasma.Mass SpectroscopySeparation and measurement of the mass of individual atoms making up a given material
6 EPA 200.8 Isotopes and Mass Spectra Isotopes of an element differ in the number of neutrons in the nucleusU Atomic Number 92234U has 142 neutrons235U has 143 neutrons238U has 146 neutrons
7 EPA Method 200.8 U Isotope Abundance Isotope Half Life Natural SpecificYears Abundance Activity (pCi/ug)234U 246, %235U 700 million %238U billion %
8 EPA Method 200.8 Isotopes and Mass Spectra The Isotopic abundance of most elements is constantPb may differ slightly based on the source of the PbPb is analyzed as the sum206 Pb207 Pb208 Pb
9 EPA Method 200.8 Ions and Mass Spectra Positive ions are produced by the energy in the plasmaIn order to utilize a mass spectrometer an ion is necessaryICP-MS analyze isotopic ionsThe ions are “steered” throughout the ion path of the spectrometer.
10 EPA Method 200.8 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 elementsDoubly charged ions will appear ½ its mass138Ba double charges will appear at 138/2 = 69
11 EPA Method 200.8 Isobaric Spectral Overlaps Signal at given amu is the summation of all the isotopes at that amuIt is best to avoid potential overlaps by monitoring a “clean” massOverlaps are correctable in software
14 EPA Method 200.8 Isobaric Spectral Overlaps Several factors must be considered when selection an isotope:Concentration of analyteConcentration of interferencesAbundances of isotopes at the given mass
15 EPA Method 200.8 Molecular Overlaps Polyatomic or molecular ions will occurCommon ones are Ar, O, and H basedBe aware of molecular overlaps that are formed:Plasma (Ar)Solvents (O, H, Cl, N)Samples (C, Cl, S)
16 EPA Method 200.8 Molecular Overlaps Elements in the ICP do not fully break apart and recombination of highly concentrated elements will occurExample56Fe and 40Ar+16OBackground spectral features have been well characterized
17 EPA Method 200.8 Factors Affecting Ion Intensities Isotopic Abundance IntensityIntensity of an isotope is proportional to its natural abundanceThe 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 equalExample: Element Percent RelativeIsotope Abundance Intensity55Mn234U235U238U
18 EPA Method 200.8 Factors Affecting Ion Intensities Percent IonizationElement % IonizedNaAsSeFMost elements are ionized greater than 90%.
19 EPA Method 200.8 ICP-MS System Courtesy: Perkin Elmer
21 EPA Method 200.8 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.
22 EPA Method 200.8 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.
23 EPA Method 200.8 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.
24 EPA Method 200.8 Reaction Cell Pressurized with a reactive gasConvert isobar to a different ion which does not interfereConvert analyte to polyatomic ion which is not interferedThe specific chemistry is dependent on:Nature and density of the reactive gasElectrical fields within the cell
25 EPA Method 200.8 ICP-MS Ion Source Region (Lens) Before sampler cone 760 torrBefore skimmer cone 3 torrAfter skimmer cone 1e-3 torr
26 EPA Method 200.8 ICP-MS Ion Source Region (Lens) Material extracted from the plasma are composed of a mixture of the following:Neutral atoms (Ar) Molecules (O2)Positively charged atomic and molecular ions (Ar+, O2+)Reactive metastable atoms and ionsNegatively charged atomic and molecular ionsPhotonsElectrons
27 EPA Method 200.8 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.
28 EPA Method 200.8 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.
29 EPA Method 200.8 Quadrupole Mass Filtering System Courtesy: Perkin ElmerMass Filtering SystemSeparates 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.
30 EPA Method 200.8 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 TuneDRC II Tuning Solution(1 ppb Mg, In, Ce,Ba,Pb, U) and check for responses and RSDs. Generate and evaluate a tune report.
32 EPA Method 200.8 Daily Performance Check SensitivityNebulizerAutolensx-y adjustmentDetector OptimizationOxides to High:Reduce nebulizer flow (plasma temperature increases)Dirt conesReduce peristaltic pump speedIncrease RF powerDouble Charged ions too high:Decreased RF powerIncrease nebulizer flowCheck skimmer 0-ringPoor precisionCheck entire sample introduction systemCheck the nebulizerCheck that the correct method is usedPerform a visual check of the plasma! Is it stable?
33 EPA Method 200.8 Isobaric Correction Counts at mass 114 = 114Cd + 114Sn114Cd = mass SnWe cannot measure the counts of Sn at mass 114 directly since 114Cd can also be present. However, we can measure another isotope of Sn (118) that is free from overlap by Cd. Therefore:114Cd = mass 114 – (a114Sn/a118Sn)*(118Sn)
34 EPA Method 200.8 Isobaric Correction The abundance ratio (a114Sn/a118Sn) of these two isotopes is (0.65%/24.23%) and is reasonably constant. Therefore:114Cd = mass 114 –(0.65%/24.23%)*(118Sn)Correction = -(0.0268)*(118Sn)114Cd = mass 114 – (a114Sn/a118Sn)*(118Sn)
35 EPA Method 200.8 Polyatomic Correction Interference of Chloride on ArsenicHigh concentrations of chloride react with argon in the plasma to form the following:40Ar35Cl interfering on 75As40Ar37Cl interfering on 77SeAs has only one isotope at mass 7540Ar35Cl can cause isobaric overlap &Erroneously high resultsMust measure 40Ar35Cl contribution and subtract it from the total counts at mass 75Total counts mass 75 = counts from 75As plus counts from 40Ar35Cl75As = mass Ar35Cl
36 EPA Method 200.8 Polyatomic Correction We cannot measure the ArCl contribution at mass 75, however, we can measure the ArCl contribution from 40Ar37Cl at mass 77The equation then becomes:75As = mass 75- (a40Ar35Cl/a40Ar37cl)*(40Ar37Cl)The relative intensities of 40Ar35Cl and 40Ar37Cl are determined by the isotopic ratio of 35Cl to 37Cl.75.77%/24.23%=3.12775As = mass *(40Ar37Cl)Correction = * 77Se
37 EPA Method 200.8 Polyatomic Correction If Se is present in the sample, the correction becomes more complicated. 77Se will contribute intensity counts to mass 77.Therefore, measure Se at mass 82 and multiply the result by the ratio of 77Se to 82Se.75As = mass *(mass77-77Se)75As = mass *[(mass77-(a77Se/a82Se)*82Se]75As = mass *[(mass *82Se]Correction *77Se+2.733* 82Se
38 EPA Method 200.8 Types of Methods Measuring Uranium Total concentration method 200.8Uranium analysis by ICP-MSResults reported as ug/LNot very labor intensiveLimitationsCan not detect 234U and 235U isotopeConversion is accurate if isotopes are present in natural abundanceBias radioactivity concentration low
39 EPA Method 200.8 Uranium Calculation Uranium radioactivityA (pCi/L) = U (ug/L) * 0.67 (pCi/ug)Where: A = activity of uraniumU = uranium concentration0.67 = conversion factor40 CFR part Analytical methods for radioactivity.Footnote 12
40 EPA Method 200.8 Types of Methods Measuring Uranium Total activity method 908.0Uranium chemically separatedAnalyzed on alpha-beta proportional counterTotal activity of all three uranium isotopesReported as pCi/LLimitationsCan not distinguish isotopeConversion is accurate if isotopes are present in natural abundanceBias mass concentration highLabor intensive
41 EPA Method 200.8 Types of Methods Measuring Uranium Isotopic activity methodUranium chemically separatedSimilar to total activityAlpha spectrometerAble to distinguish uranium isotopeResults can be reported as pCi/L or ug/LLimitationsLabor intensive
42 EPA Method 200.8 U Isotope Abundance Isotope U 235U 238UHalf Life (years) 246, million billionNatural Abundance % % %Specific Activity (pCi/ug) 6,Relative Intensity