Icp- Ms Fundamentlas

8/11/2019 Icp- Ms Fundamentlas

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Instrumentation & Methods:

ICP/MS, Uranium

Jeff Brenner

Minnesota Department of Health

EPA Method 200.8

Overview and Fundamentals of ICP-MS

Determination of Metals Using InductivelyCoupled Plasma Mass Spectrometry

Overview & Fundamentals of ICP-MS

What we will cover

Overview and Fundamentals

ICP-MS Theory

Interferences Reports



EPA 200.8

ICP-MS Definition

An analytical technique to

determine Elements using MassSpectrometry from Ions generatedby an Inductively Coupled Plasma.

Mass Spectroscopy Separation and measurement of the

mass of individual atoms making up agiven material

EPA 200.8

Analytical Benefits of ICP-MS

Rapid multi-element quantitativeanalysis

Very low detection limits

Rapid semi-quantitative analysis

Wide dynamic range

Isotopic analysis

Spectral simplicity

Speciation (with HPLC)

EPA 200.8

Isotopes and Mass Spectra

Isotopes of an element differ in the

number of neutrons in the nucleus

U Atomic Number 92

234U has 142 neutrons





EPA Method 200.8

U Isotope Abundance

IsotopeHalf Life Natural Specific

Years 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 elementsis constant

Pb may differ slightly based on the sourceof the Pb

Pb is analyzed as the sum206 Pb207 Pb208 Pb

EPA Method 200.8

Ions and Mass Spectra

Positive ions are produced by theenergy in the plasma

In order to utilize a massspectrometer an ion is necessary

ICP-MS analyze isotopic ions

The ions are “steered” throughoutthe 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 ½its mass

138Ba double charges will appear at138/2 = 69

EPA Method 200.8

Isobaric Spectral Overlaps

Signal at given amu is thesummation of all the isotopes atthat amu

It is best to avoid potential overlapsby monitoring a “clean” mass

Overlaps are correctable in software



EPA Method 200.8

Isobaric Spectral Overlaps

Several factors must be consideredwhen selection an isotope:

Concentration of analyte

Concentration of interferences

Abundances of isotopes at the givenmass

EPA Method 200.8

Molecular Overlaps

Polyatomic or molecular ions willoccur

Common ones are Ar, O, and H based

Be aware of molecular overlaps thatare 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 ofhighly concentrated elements willoccur

Example56Fe and 40Ar+16O

Background spectral features havebeen well characterized

EPA Method 200.8

Factors Affecting Ion Intensities

Isotopic Abundance Intensity

Intensity of an isotope is proportional toits natural abundance

The sum of the signals from all isotopesof an element are compared to the signalfrom a mono-isotopic element, thesignals ideally should be equal

Example: Element Percent RelativeIsotope Abundance Intensity55Mn 100.0 100.0234U 0.0055 0.0055235U 0.7200 0.7200238

U 99.2745 99.7245

EPA Method 200.8

Factors Affecting Ion Intensities

Percent Ionization

Element % Ionized

Na 100

As 50Se 34

F 0.001

Most elements are ionized greater than90%.



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 thesample.

Heat from 6,000K-10,000K dries, aerosol, thenatomize, and ionize components of the sample.



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

Plasma is formed by a stream of argon gas flowingbetween to quartz tubes.

Radio frequency (RF) power is applied through thecoil, and an oscillating magnetic field is formed.

An electrical discharge creates seed electrons andions.

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

Inside the induced magnetic field,the charged particles are forced toflow in a closed annular path.

As they meet resistance, heatingtakes place and additional ionizationoccurs.

EPA Method 200.8

Reaction Cell

Pressurized with a reactive gas

Convert isobar to a different ion which doesnot interfere

Convert analyte to polyatomic ion which is notinterfered

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


Material extracted from the plasma arecomposed 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


The lens captures and guides thepositively charged ions to the quadrupole.

By applying a positive potential to the

lens, the ions will be focused to the centerof the lens.

Small ions are optimized at lowervoltages. As the voltage is increased,higher mass ions are better focused.

If the voltage is to high the ions arerepelled.



EPA Method 200.8

Reaction Cell or Collision Cell

A reaction gas is introduced into the

cell. The reaction of the gas withthe interfering species is set up toremove these interferences fromthe path.

EPA Method 200.8

Quadrupole

Mass Filtering System

Separates on type of element (ion) from another withan 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 theinstrument to warm up whileaspirating 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 andevaluate 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 ppbIn >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 bepresent. However, we can measureanother isotope of Sn (118) that is freefrom 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 subtractit from the total counts at mass 75

Total counts mass 75 = counts from 75As

plus counts from40

Ar35

Cl75As = mass 75- 40Ar35Cl

EPA Method 200.8


We cannot measure the ArCl contribution at mass75, however, we can measure the ArCl contributionfrom 40Ar37Cl at mass 77

The equation then becomes:

75As = mass 75- (a40Ar35Cl/a40Ar37cl)*(40Ar37Cl)

The relative intensities of 40Ar35Cl and 40Ar37Cl aredetermined 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


If Se is present in the sample, thecorrection becomes more complicated. 77Sewill contribute intensity counts to mass 77.

Therefore, measure Se at mass 82 andmultiply the result by the ratio of 77Se to82Se.

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 200.8Types 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 presentin 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




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 presentin natural abundance

Bias mass concentration high

Labor intensive


Isotopic activity method Uranium chemically separated

Similar to total activity

Alpha spectrometer

Able to distinguish uranium isotope

Results can be reported as pCi/L orug/L

Limitations Labor intensive

EPA Method 200.8

U 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

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