Instrumentation & Methods: ICP/MS, Uranium

Jeff Brenner

Minnesota Department of Health

EPA Method 200.8Overview 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

EPA 200.8ICP-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 200.8Analytical 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 200.8Isotopes and Mass Spectra

Isotopes of an element differ in the number of neutrons in the nucleus

U Atomic Number 92 234U has 142 neutrons 235U has 143 neutrons 238U has 146 neutrons

EPA Method 200.8U Isotope Abundance

Isotope Half Life Natural SpecificYears Abundance Activity

(pCi/ug)

234U 246,000 0.0055 % 6208.2235U 700 million 0.72 % 2.17238U 4.47 billion 99.27 % 0.336

EPA Method 200.8 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 Pb207 Pb

208 Pb

EPA Method 200.8 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.

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 elements

Doubly charged ions will appear ½ its mass

138Ba double charges will appear at 138/2 = 69

EPA Method 200.8 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

EPA Method 200.8 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

EPA Method 200.8 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)

EPA Method 200.8 Molecular Overlaps

Elements in the ICP do not fully break apart and recombination of highly concentrated elements will occur

Example 56Fe and 40Ar+16O

Background spectral features have been well characterized

EPA Method 200.8 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

Intensity55Mn 100.0 100.0234U 0.0055 0.0055235U 0.7200 0.7200238U 99.2745 99.7245

EPA Method 200.8 Factors Affecting Ion Intensities

Percent IonizationElement % IonizedNa 100As 50Se 34F 0.001

Most elements are ionized greater than 90%.

EPA Method 200.8 ICP-MS System

Courtesy: Perkin Elmer

EPA Method 200.8 Spray Chamber and Nebulizer

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.

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.

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.

EPA Method 200.8 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

EPA Method 200.8 ICP-MS Ion Source Region (Lens)

Before sampler cone 760 torr Before skimmer cone 3 torr After skimmer cone 1e-3 torr

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 ions Negatively charged atomic and molecular ions Photons Electrons

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.

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.

EPA Method 200.8 Quadrupole

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.

Courtesy: Perkin Elmer

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 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.

Perkin Elmer DRC II Optimization Suggestions

Suggested guidelines for an acceptable tune for method 200.8

Sensitivity:Mg > 8,000 cts/0.1 sec/10 ppb

In >40,000 cts/0.1 sec/10 ppbU >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: +/- 0.05 AMU

EPA Method 200.8 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?

EPA Method 200.8 Isobaric Correction

Counts at mass 114 = 114Cd + 114Sn114Cd = mass 114 - 114Sn

We 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)

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)

EPA Method 200.8 Polyatomic Correction

Interference of Chloride on Arsenic High concentrations of chloride react with argon

in the plasma to form the following: 40Ar35Cl interfering on 75As 40Ar37Cl interfering on 77Se

As has only one isotope at mass 75 40Ar35Cl can cause isobaric overlap &

Erroneously high results Must measure 40Ar35Cl contribution and subtract

it from the total counts at mass 75 Total counts mass 75 = counts from 75As

plus counts from 40Ar35Cl75As = mass 75- 40Ar35Cl

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 77

The 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%=3.127 75As = mass 75-3.217*(40Ar37Cl)

Correction = -3.127* 77Se

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 75-3.127*(mass77-77Se) 75As = mass 75-3.127*[(mass77-(a77Se/a82Se)*82Se] 75As = mass 75-3.127*[(mass77-0.874*82Se]

Correction -3.127*77Se+2.733* 82Se

EPA Method 200.8 Types of Methods Measuring Uranium

Total concentration method 200.8 Uranium analysis by ICP-MS Results reported as ug/L Not very labor intensive

Limitations Can not detect 234U and 235U isotope Conversion is accurate if isotopes are present

in natural abundance Bias radioactivity concentration low

EPA Method 200.8 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 141.25 Analytical methods for radioactivity. Footnote 12

EPA Method 200.8 Types of Methods Measuring Uranium

Total activity method 908.0 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

EPA Method 200.8 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

EPA Method 200.8U Isotope Abundance

Isotope 234U 235U 238U Half Life (years) 246,000 700 million 4.47 billion

Natural Abundance 0.0055 % 0.72 % 99.27 %

Specific Activity (pCi/ug) 6,208 2.17 0.336

Relative Intensity 0.0055 0.72 99.27