Problems

in

obtaining

precise

and

accurate

Sr

isotope

analysis

from

geological

materials

using

laser

ablation

MC-ICPMS

This

paper

reviews

the

problems

encountered

in

eleven

studies

of

Sr

isotope

analysis

using

laser

ablation

multicollector

inductively

coupled

plasma

mass

spectrometry

-LRB-

LA-MC-ICPMS

-RRB-

in

the

period

This

technique

has

been

shown

to

have

great

potential

but

the

accuracy

and

precision

are

limited

by

-LRB-

-RRB-

large

instrumental

mass

discrimination

-LRB-

-RRB-

laser-induced

isotopic

and

elemental

fractionations

and

-LRB-

-RRB-

molecular

interferences

The

most

important

isobaric

interferences

are

Kr

and

Rb

whereas

Ca

dimer/argides

and

doubly

charged

rare

earth

elements

-LRB-

REE

-RRB-

are

limited

to

sample

materials

which

contain

substantial

amounts

of

these

elements

With

modern

laser

-LRB-

nm

-RRB-

and

MC-ICPMS

equipment

minerals

with

ppm

Sr

content

can

be

analysed

with

a

precision

of

better

than

ppm

and

a

spatial

resolution

-LRB-

spot

size

-RRB-

of

approximately

μm

The

LA

MC-ICPMS

analysis

of

of

both

carbonate

material

and

plagioclase

is

successful

in

all

reported

studies

although

the

higher

ratios

do

suggest

in

some

cases

an

influence

of

Ca

dimer

and/or

argides

High

Rb/Sr

-LRB-

-RRB-

materials

have

been

successfully

analysed

by

carefully

measuring

the

in

standard

material

and

by

applying

the

standard-sample

bracketing

method

for

accurate

Rb

corrections

However

published

LA-MC-ICPMS

data

on

clinopyroxene

apatite

and

sphene

records

differences

when

compared

with

measured

by

thermal

ionisation

mass

spectrometry

-LRB-

TIMS

-RRB-

and

solution

MC-ICPMS

This

suggests

that

further

studies

are

required

to

ensure

that

the

most

optimal

correction

methods

are

applied

for

all

isobaric

interferences

Introduction

The

radioactive

beta

-LRB-

β

-RRB-

decay

of

to

is

an

important

isotope

system

that

has

been

widely

applied

for

geochronological

purposes

More

importantly

the

isotopic

system

is

extensively

used

to

constrain

the

rates

and

fluxes

involved

in

a

wide

range

of

geological

processes

operating

from

within

the

hydrosphere

of

the

Earth

to

the

deep

mantle

In

addition

recently

Sr

isotopes

have

become

widely

applied

as

provenance

tracers

in

many

different

scientific

disciplines

-LRB-

e.g.

biology

nutrition

medical

forensic

and

art

history

-RRB-

-LSB-

-RSB-

Sr

isotope

ratios

have

always

been

difficult

to

analyse

to

high

precision

-LRB-

better

than

ppm

-RRB-

because

there

are

large

differences

in

the

abundances

of

the

isotopes

and

In

many

materials

the

low

abundance

of

Rb

and

their

relatively

young

age

leads

to

small

isotopic

variations

-LRB-

e.g.

in

mantle

rocks

and

biogenic

carbonates

-RRB-

This

range

is

considerably

smaller

than

the

combined

U-Th-Pb

isotope

system

for

example

Recent

technical

improvements

in

thermal

ionisation

mass

spectrometry

-LRB-

TIMS

-RRB-

design

most

notably

in

terms

of

Faraday

collectors

and

amplifier

electronics

now

allow

analytical

precisions

better

than

-LRB-

ppm

SE

-RRB-

Despite

this

significant

improvement

in

precision

the

classical

TIMS

technique

requires

time-consuming

liquid

chromatographic

techniques

to

remove

matrix

and

interfering

elements

-LRB-

-RRB-

inhibiting

any

possibility

for

an

online

in

situ

analytical

technique

The

arrival

of

the

first

commercial

multicollector

inductively

coupled

plasma

mass

spectrometry

-LRB-

MC-ICPMS

-RRB-

instruments

in

the

first

half

of

the

coupled

with

laser

ablation

-LRB-

LA

-RRB-

facilities

promised

an

enormous

new

potential

for

in

situ

analytical

techniques

in

petrology

marine

sciences

and

many

other

applications

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

The

study

of

small-scale

variations

of

Sr

isotopes

in

geological

materials

has

provided

very

important

constraints

on

the

rates

of

volcanic

processes

and

the

fluxes

that

operate

in

diverse

geological

environments

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

However

these

studies

are

extremely

time

consuming

typically

averaging

more

than

hours

per

sample

because

they

require

careful

microdrilling

techniques

low-blank

liquid

chromatographic

separation

techniques

and

TIMS

analyses

The

MC-ICPMS

coupled

with

a

laser

ablation

system

should

be

able

to

perform

a

Sr

isotope

analyses

within

minutes

while

importantly

maintaining

the

spatial

resolution

and

avoiding

the

extensive

wet

chemistry

and

warm

up

times

on

the

TIMS

instrument

Despite

the

great

potential

of

LA-MC-ICPMS

analyses

for

Sr

isotopes

and

its

availability

for

more

than

a

decade

the

technique

has

not

become

routine

Eight

out

of

eleven

papers

published

in

the

period

that

reported

in

situ

analyses

of

Sr

isotopes

by

LA-MC-ICPMS

are

mainly

focussed

on

technique

development

-LSB-

-RSB-

There

is

clearly

a

huge

potential

and

a

demand

for

the

application

of

in

situ

Sr

isotope

analysis

Therefore

the

goals

of

this

paper

are

to

-LRB-

-RRB-

describe

the

different

approaches

that

have

been

followed

to

date

-LRB-

-RRB-

summarize

the

problems

encountered

-LRB-

-RRB-

discuss

the

solutions

attempted

to

solve

the

encountered

problems

and

-LRB-

-RRB-

discuss

studies

which

are

needed

in

the

near

future

to

improve

the

LA-MC-ICPMS

technique

so

that

routine

Sr

isotope

analysis

becomes

possible

Description

of

instrument

setups

All

LA-MC-ICPMS

instrument

setups

have

the

same

basic

geometry

of

LA

system

pulsed

laser

focussed

onto

ablation

chamber

sample

transfer-inlet

system

differential

vacuum

pumping

system

mass-energy

filter

and

signal

detection

-LRB-

Fig.

-RRB-

It

is

beyond

the

scope

of

this

review

to

discuss

the

actual

performance

of

the

different

mass

spectrometers

so

we

concentrate

upon

the

sample

handling

and

data

interpretation

aspects

of

the

LA-MC-ICPMS

technique

Fig.

LA-MC-ICPMS

setup

with

He

as

carrier

gas

ESA

electrostatic

filter

analyser

Laser

ablation

systems

Laser

ablation

systems

have

been

tremendously

improved

in

the

last

decade

-LRB-

see

-LSB-

-RSB-

for

a

review

-RRB-

This

development

has

mainly

focussed

on

how

to

obtain

maximum

sensitivity

in

both

sample

ablation

and

transfer

of

the

ablated

sample

to

the

ion

optics

of

the

ICP-MS

and

a

minimized

fractionation

of

elemental

ratios

for

example

Th/U

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

The

early

studies

of

Christensen

et

al.

-LSB-

-RSB-

and

Davidson

et

al.

-LSB-

-RSB-

used

a

pulsed

quadrupled

Nd

YAG

laser

whereas

more

recent

studies

have

tended

to

use

shorter

wavelengths

e.g.

pulsed

and

Excimer

lasers

-LRB-

Table

-RRB-

Owing

to

the

more

efficient

absorption

of

shorter

wavelength

light

by

most

materials

the

reported

interelement

fractionation

is

less

with

lower

wavelength

lasers

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Consequently

the

application

of

lower

wavelength

lasers

tends

to

produce

more

accurate

Sr

isotope

results

However

it

is

difficult

to

judge

from

the

published

papers

which

laser

system

has

the

highest

sensitivity

as

this

is

highly

dependent

on

the

repetition

rate

of

the

laser

the

pit

size

or

raster

technique

used

and

the

energy

density

of

the

laser

system

Moreover

ICP-MS

inlet

design

and

instrument

sensitivity

have

improved

by

several

orders

of

magnitude

in

the

last

decade

so

direct

comparison

between

the

sensitivity

of

systems

of

different

age

is

perhaps

pointless

Table

setups

and

materials

analysed

in

eleven

publications

on

Sr

isotope

analyses

by

LA-MC-ICPMSReferenceLaser

typePit

sizes

-LRB-

μm

-RRB-

Materials

-LRB-

Sr

concentration

-RRB-

InstrumentSensitivity/blank

levels

-LSB-

-RSB-

Nd

YAG

nm

Hz

mJ

pulse

carrier

gas

ArSpot

and

-LRB-

ppm

-RRB-

VG

ppm

sample

material

gives

V

total

SrFeldspar

-LRB-

ppm

-RRB-

No

blank

reported

-LSB-

-RSB-

Nd

YAG

nm

Hz

mJ

carrier

gas

not

specifiedSpot

rasterPlagioclase

-LRB-

ppm

-RRB-

IsoProbeNo

blank

reported

-LSB-

-RSB-

Nd

YAG

nm

Hz

mJ

carrier

gas

not

specifiedSpot

rasterCarbonate

fresh

water

otolith

-LRB-

ca.

ppm

-RRB-

VG

blank

reported

-LSB-

-RSB-

Nd

YAG

nm

and

nm

Hz

mJ

carrier

gas

ArSpot

apatite

sphene

clinopyroxene

otolithVG

Axiomca

V

ppm

-LRB-

solution

work

-RRB-

blank

mV

and

-LSB-

-RSB-

Excimer

nm

Hz

J

cm

carrier

gas

HeSpot

-LRB-

ppm

-RRB-

IsoProbeNot

reportedCarbonate

-LRB-

ppm

-RRB-

-LSB-

-RSB-

Excimer

nm

-LRB-

see

-LSB-

-RSB-

-RRB-

Spot

-LRB-

ppm

-RRB-

plagioclase

carbonateIsoProbeNot

reported

-LSB-

-RSB-

Nd

YAG

nm

Hz

J

cm

carrier

gas

HeSpot

raster

-LRB-

ppm

-RRB-

ThermoFinnigan

blank

mVPlagioclase

-LRB-

ppm

-RRB-

Clinopyroxene

-LRB-

ppm

-RRB-

Basaltic

groundmass

-LRB-

ppm

-RRB-

-LSB-

-RSB-

Excimer

nm

Hz

J

cm

carrier

gas

HeSpot

rasterCarbonate

otolithNu

PlasmaNot

reported

-LSB-

-RSB-

Nd

YAG

nmRaster

deepCarbonate

fresh

water

otolith

-LRB-

ca.

ppm

-RRB-

ThermoFinnigan

NeptuneNot

reported

-LSB-

-RSB-

Excimer

nm

Hz

mJ

carrier

gas

HeRaster

wideCarbonate

otolithsThermoFinnigan

NeptuneNot

reported

-LSB-

-RSB-

Nd

YAG

nm

HzSpot

rasterMelt

inclusionsThermoFinnigan

Neptuneca

V

ppm

-LRB-

solution

work

-RRB-

Blank

not

reported

He

-LRB-

L

min

-RRB-

is

used

as

the

transport

gas

through

the

ablation

cell

in

nearly

all

studies

-LRB-

see

Table

-RRB-

following

the

publication

of

-LSB-

-RSB-

which

demonstrated

that

He

gives

a

higher

sensitivity

and

less

inter-element

fractionation

-LRB-

Rb/Sr

-RRB-

compared

with

other

potential

carrier

gases

-LRB-

e.g.

Ar

-RRB-

The

He

flow

is

mixed

with

Ar

flow

before

it

enters

the

plasma

in

a

ratio

between

and

-LRB-

see

Fig.

-RRB-

With

laser

ablation

analyses

there

are

essentially

two

possible

ablation

methods

spot

analysis

and

rastering

A

spot

analysis

leaves

the

laser

beam

in

the

same

position

where

the

laser

beam

progressively

ablates

material

from

deeper

levels

of

the

ablation

pit

There

are

two

advantages

of

spot

analysis

first

the

optimal

spatial

resolution

is

obtained

typically

between

μm

for

Sr

isotopes

second

this

approach

limits

the

influence

of

surface

contamination

A

significant

disadvantage

of

the

single

spot

analysis

is

that

on

many

laser

ablation

systems

the

laser

will

go

out

of

focus

if

the

depth

to

diameter

ratio

of

the

ablation

pits

is

larger

than

one

-LSB-

-RSB-

This

will

result

in

lower

beam

intensities

due

to

lower

ablation

rates

coupled

with

less

efficient

sample

fragmentation

that

also

reduces

ionisation

efficiency

in

the

plasma

Furthermore

if

the

depth

to

diameter

ratio

becomes

larger

than

significant

fractionation

occurs

between

elements

-LSB-

-RSB-

which

could

make

the

correction

on

less

accurate

For

a

spot

analyses

the

maximum

ablation

time

is

min

-LRB-

assuming

μm

s

ablation

rate

see

Fig.

-RRB-

Fig.

intensity

obtained

by

ablating

different

spot

sizes

-LRB-

and

μm

-RRB-

and

rastering

-LRB-

spot

moving

at

μm

s

-RRB-

as

a

function

of

Sr

abundance

in

a

mineral

-LRB-

modified

from

Fig.

in

-LSB-

-RSB-

-RRB-

The

ablation

rate

is

μm

s

and

the

typical

efficiency

is

-LRB-

e.g.

in

ions

that

get

ablated

are

counted

by

the

detector

-RRB-

This

diagram

illustrates

that

with

a

spot

size

only

the

minerals

with

ppm

can

be

analysed

with

sufficient

precision

Rastering

results

in

slightly

better

signals

but

its

greatest

advantage

is

that

the

beam

does

not

run

out

of

focus

during

the

analysis

and

the

ion

current

does

not

decay

as

with

a

spot

analysis

The

rastering

technique

involves

the

movement

of

the

sample

with

a

low

speed

-LRB-

typically

μm

s

e.g.

see

-LSB-

-RSB-

-RRB-

The

advantage

is

that

the

ion

beam

size

is

more

stable

over

time

and

the

signal

is

larger

-LRB-

see

Fig.

-RRB-

Disadvantages

are

that

the

surface

contamination

of

the

sample

could

be

a

problem

and

the

advantage

of

high

spatial

resolution

of

LA

is

diminished

The

surface

contamination

problem

can

be

overcome

by

pre-cleaning

the

ablating

area

with

the

same

raster

area

but

with

a

lower

ablation

rate

MC-ICPMS

instruments

Five

different

MC-ICPMS

instruments

have

been

used

to

obtain

Sr

isotopes

LA-MC-ICPMS

-LRB-

see

Table

-RRB-

Although

different

in

configuration

the

type

of

MC-ICPMS

instrument

does

not

seem

to

be

very

important

All

MC-ICPMS

instruments

can

collect

the

Sr

isotope

masses

and

the

additional

masses

required

to

perform

the

isobaric

corrections

on

the

Sr

isotope

masses

There

are

probably

differences

in

the

sensitivity

whereby

the

older

generation

instruments

-LRB-

Isoprobe

and

Axiom

-RRB-

have

a

lower

sensitivity

than

more

recently

designed

instruments

-LRB-

ThermoFinnigan

Neptune

and

Nu

Plasma

instruments

-RRB-

The

collision

cell

arrangement

is

not

beneficial

for

Sr

isotope

analysis

-LRB-

maybe

for

Fe

interferences

see

below

-RRB-

nor

is

the

high-resolution

capability

of

the

Nu

Plasma

HR

Nu

Plasma

and

the

ThermoFinnigan

Neptune

instruments

because

isobaric

interferences

such

as

on

can

only

be

resolved

with

a

mass

resolution

of

significantly

beyond

the

capabilities

of

all

MC-ICP-MS

instruments

Materials

ablated

Table

lists

the

different

types

of

geologically

relevant

materials

which

have

been

analysed

Except

for

the

melt

inclusion

work

reported

by

Jackson

and

Hart

-LSB-

-RSB-

most

of

the

tested

materials

are

characterized

by

low

Rb/Sr

-LRB-

-RRB-

ratios

The

majority

of

studied

materials

are

marine

carbonate

and

magmatic

plagioclase

Most

studies

have

used

present-day

marine

carbonate

as

an

in-house

standard

assuming

that

it

has

the

composition

of

present-day

seawater

-LSB-

-RSB-

Marine

carbonate

has

in

the

order

of

ppm

Sr.

Plagioclase

is

another

suitable

material

since

it

has

also

low

Rb/Sr

ratios

and

high

Sr

contents

-LRB-

ppm

-RRB-

Although

these

minerals

have

low

Rb/Sr

ratios

the

Ca/Sr

ratios

are

high

which

could

result

in

potential

interferences

from

Ca

-LRB-

see

below

-RRB-

Other

materials

that

have

been

ablated

and

analysed

successfully

are

clinopyroxene

magmatic

carbonates

-LRB-

Sr-rich

ppm

and

Rb-poor

ppm

-LSB-

-RSB-

-RRB-

apatite

and

melt

inclusions

from

relatively

Sr-rich

alkaline

melts

These

materials

generate

several

types

of

isobaric

interferences

which

will

be

discussed

below

The

highest

Rb/Sr

ratios

materials

were

analysed

by

Jackson

and

Hart

-LSB-

-RSB-

In

their

study

they

analysed

basaltic

melt

inclusions

with

Rb/Sr

ratios

of

up

to

although

the

reported

external

reproducibility

of

the

is

about

times

worse

than

that

of

a

melt

inclusion

with

a

Rb/Sr

of

Factors

influencing

the

data

quality

of

Sr

isotope

analysis

by

LA-MC-ICPMS

The

following

factors

influence

the

quality

of

data

which

can

be

obtained

by

LA-MC-ICPMS

-LRB-

-RRB-

counting

statistics

-LRB-

-RRB-

blank

levels

-LRB-

-RRB-

instrumental

mass

discrimination

and

laser-induced

elemental

and

isotopic

fractionation

and

-LRB-

-RRB-

molecular

interferences

Counting

statistics

The

precision

which

can

be

obtained

by

Sr

isotope

analyses

by

LA-MC-ICPMS

depends

upon

the

number

of

ions

counted

The

number

of

ions

that

can

be

collected

by

the

Faraday

cups

depends

on

-LRB-

-RRB-

the

amount

of

Sr

in

the

sample

-LRB-

see

Fig.

-RRB-

-LRB-

-RRB-

ablation

spot

size

ablation

rate

-LRB-

energy

density

and

pulse

rate

-RRB-

-LRB-

-RRB-

the

laser

ablation

efficiency

-LRB-

e.g.

the

particle

size

distribution

of

ablated

material

-RRB-

-LRB-

-RRB-

transport

from

sample

chamber

to

plasma

and

-LRB-

-RRB-

tuning

conditions

of

the

plasma

and

mass

spectrometer

The

amount

of

Sr

brought

to

the

plasma

is

unclear

in

most

publications

Davidson

-LSB-

-RSB-

reported

that

approximately

ng

of

Sr

is

needed

for

a

beam

of

The

theoretical

best

possible

precision

for

such

beam

intensity

is

then

ppm

Background

-LRB-

blank

levels

-RRB-

The

instrumental

background

levels

for

and

that

have

been

reported

are

mV

-LRB-

-LSB-

-RSB-

see

Table

-RRB-

Since

running

samples

spiked

with

and

resulted

in

non-natural

background

isotopic

composition

of

these

isotopes

Waight

et

al.

-LSB-

-RSB-

concluded

that

the

Sr

and

Rb

background

is

an

accumulation

of

material

from

samples

within

the

introduction

system

It

is

important

to

note

that

these

backgrounds

are

significantly

higher

than

those

reported

for

Pb

and

Hf

isotopes

and

that

careful

monitoring

of

the

background

is

clearly

vital

to

obtain

accurate

results

In

nearly

all

studies

the

blank

is

measured

by

collecting

data

with

conditions

as

during

the

analysis

except

for

not

firing

the

laser

Only

in

two

studies

-LSB-

-RSB-

are

baselines

measured

off-peak

as

with

a

standard

TIMS

analysis

Instrumental

mass

discrimination

and

laser-induced

elemental

and

isotopic

fractionation

Both

the

laser

system

and

the

MC-ICPMS

induce

isotopic

fractionations

The

isotope

fractionation

in

the

MC-ICPMS

is

generally

referred

to

as

instrument

mass

discrimination

In

this

case

the

measured

elemental

or

isotope

ratio

are

different

from

the

true

ratio

due

to

differences

in

ionisation

potentials

space

charge

effects

effects

of

the

matrix

of

the

sample

preferential

transmission

of

one

type

of

ion

and

reactions

in

the

ICP

-LRB-

e.g.

oxide

formation

-RRB-

The

instrument

mass

discrimination

is

generally

corrected

during

solution

MC-ICPMS

analyses

by

using

a

stable

isotope

pair

which

can

be

corrected

by

an

exponentional

law

-LRB-

similar

to

TIMS

mass

fractionation

correction

-RRB-

or

when

there

is

no

stable

isotope

pair

available

-LRB-

e.g.

Pb

isotopes

-RRB-

corrected

using

a

standard-sample

bracketing

method

-LSB-

-RSB-

In

addition

to

the

instrument

mass

discrimination

it

is

now

well

established

that

during

the

ablation

process

of

a

sample

both

isotopic

and

elemental

fractionations

are

induced

The

best

studied

elemental

fractionations

have

been

reported

for

U/Pb

for

which

an

increase

of

the

U/Pb

ratio

by

a

factor

during

a

single

ablation

has

been

reported

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Laser-induced

isotopic

fractionation

has

also

been

reported

e.g.

for

Fe

isotopes

-LSB-

-RSB-

Both

the

laser-induced

isotopic

fractionation

and

the

instrument

mass

discrimination

can

be

corrected

for

during

Sr

isotope

analyses

by

using

the

stable

ratio

and

an

exponential

correction

-LSB-

-RSB-

All

LA

ablation

studies

have

done

so

except

one

-LSB-

-RSB-

in

which

no

mass

bias

correction

was

applied

to

the

data

Elemental

fractionation

induced

by

the

laser

is

another

important

factor

The

Sr

isotope

ratios

can

be

internally

normalized

to

but

correcting

for

the

interference

on

the

-LRB-

see

below

-RRB-

is

complicated

by

the

fractionation

of

the

Rb/Sr

ratio

In

seven

of

the

published

studies

it

is

assumed

that

the

laser-induced

isotope

fractionation

and

instrument

mass

discrimination

are

the

same

for

both

Sr

and

Rb

This

is

possibly

not

the

case

as

suggested

by

the

study

of

Jackson

and

Hart

-LSB-

-RSB-

who

employed

a

standard-sample

bracketing

method

to

establish

the

total

Rb

laser-induced

fractionation

and

instrument

mass

discrimination

in

order

to

cope

with

large

Rb

corrections

-LRB-

see

below

-RRB-

Similarly

most

authors

make

the

Kr

corrections

on

the

Sr

masses

by

assuming

that

the

laser-induced

isotope

fractionation

and

instrument

discrimination

are

the

same

for

Sr

and

Kr

For

a

good

Kr

correction

the

mass

bias

has

to

be

established

in

an

independent

way

We

will

use

the

term

mass

bias

to

describe

both

the

laser-induced

isotope

fractionation

and

the

instrument

mass

discrimination

Molecular

interferences

Measurement

and

correction

of

the

interferences

of

elements

and

molecules

on

the

Sr

masses

-LRB-

Table

-RRB-

is

perhaps

the

most

challenging

aspect

of

Sr

isotope

analyses

by

MC-ICPMS

This

is

especially

true

when

comparing

LA

analysis

with

solution

work

where

some

of

the

interfering

elements

-LRB-

e.g.

Ca

Rb

and

the

REE

-RRB-

can

be

removed

by

chromatographic

purification

of

the

sample

or

burnt

off

by

heating

at

temperatures

below

that

required

for

Sr

ionization

on

a

TIMS

instrument

Unfortunately

for

LA-MC-ICPMS

Sr

isotope

analysis

large

corrections

for

interfering

elements

need

to

be

made

Since

these

interferences

can

be

more

than

half

of

the

signal

intensity

on

the

Sr

isotope

mass

these

corrections

are

large

and

the

correction

procedures

need

to

be

extremely

rigorous

if

accurate

and

precise

Sr

isotope

ratios

are

to

be

obtained

Table

isotope

masses

and

possible

interferences

in

the

mass

region

of

Fe/Zn/Ga

There

are

many

potential

interfering

elements

and

molecules

on

the

Sr

isotope

spectrum

most

of

which

have

been

extensively

discussed

by

Ramos

et

al.

-LSB-

-RSB-

This

study

concluded

that

careful

monitoring

and

correcting

for

the

interferences

can

produce

Sr

isotope

data

with

a

precision

suitable

for

use

in

most

geological

applications

In

the

next

section

we

discuss

the

main

interferences

and

how

they

can

compromise

Sr

isotope

data

leading

to

errors

in

excess

of

those

that

can

be

tolerated

for

everyday

applications

Rubidium

Rb

is

a

well-known

interference

on

during

TIMS

analyses

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

The

presence

of

a

significant

Rb

signal

can

hamper

TIMS

and

solution

and

LA-MC-ICPMS

analyses

of

Sr

isotopes

Whereas

with

TIMS

and

solution

MC-ICPMS

work

the

Rb

can

be

separated

from

the

Sr

by

chromatographic

techniques

this

is

obviously

not

possible

during

LA

analysis

All

LA-MC-ICPMS

studies

-LRB-

see

Table

-RRB-

use

the

peak

stripping

method

for

the

Rb

correction

by

measuring

the

signal

on

mass

and

subtracting

the

signal

from

using

the

value

of

-LSB-

-RSB-

An

additional

problem

with

LA-MC-ICPMS

analysis

is

the

unknown

mass

bias

of

Rb

during

the

analysis

Most

studies

-LRB-

see

Table

-RRB-

assume

that

the

mass

bias

of

Rb

and

Sr

-LRB-

derived

from

the

ratio

-RRB-

is

constant

and

correct

the

Rb

interference

using

the

Sr

mass

bias

However

Jackson

and

Hart

-LSB-

-RSB-

deployed

a

different

strategy

to

correct

for

high

Rb/Sr

alkaline

melt

inclusions

The

mass

bias

of

Rb

was

measured

by

sample-standard

bracketing

natural

basalt

glasses

with

sample

material

The

measured

was

than

used

to

correct

the

mass

bias

of

the

melt

inclusions

measured

between

successive

glass

standards

By

determining

a

more

accurate

Rb

mass

bias

they

were

probably

in

a

better

position

to

measure

high

Rb/Sr

material

although

the

reproducibility

of

their

isotopic

data

deteriorated

with

increasing

Rb/Sr

A

similar

approach

was

followed

by

McCulloch

et

al.

-LSB-

-RSB-

Unfortunately

Jackson

and

Hart

-LSB-

-RSB-

did

not

report

measurement

of

standards

of

known

Rb

and

Sr

isotopic

composition

so

it

is

difficult

to

fully

assess

the

accuracy

of

their

method

There

is

no

consensus

as

to

how

high

the

Rb/Sr

ratio

of

materials

can

be

analysed

to

still

produce

a

reliable

LA

analysis

Ramos

et

al.

-LSB-

-RSB-

proposed

that

the

materials

should

have

Rb/Sr

whereas

Jackson

and

Hart

-LSB-

-RSB-

and

Davidson

et

al.

-LSB-

-RSB-

report

successful

corrections

of

materials

with

Rb/Sr

of

and

respectively

Krypton

The

noble

gas

krypton

interferes

with

masses

and

and

is

therefore

an

important

interference

that

needs

to

be

corrected

The

source

of

the

krypton

is

the

argon

-LRB-

and

helium

-RRB-

gas

used

to

transport

the

sample

into

the

plasma

and

to

generate

the

argon

plasma

-LRB-

see

Fig.

-RRB-

The

amount

of

krypton

in

the

argon

gas

is

supplier

dependent

Studies

that

report

the

magnitude

of

the

Kr

interference

indicates

a

total

Kr

contribution

between

mV

-LSB-

-RSB-

and

mV

-LRB-

-LSB-

-RSB-

-RRB-

Woodhead

et

al.

-LSB-

-RSB-

observed

that

Ar

derived

from

compressed

gas

cylinders

gives

a

more

stable

Kr

signal

than

those

from

liquid

Ar

supplies

They

further

noted

that

the

Kr

abundances

in

the

Ar

supply

vary

largely

between

different

batches

It

is

important

to

note

that

Jackson

and

Hart

-LSB-

-RSB-

suggest

that

there

are

also

isobaric

interferences

of

unknown

origin

on

the

peak

making

a

good

Kr

correction

extremely

difficult

-LRB-

see

Table

-RRB-

In

addition

as

with

Rb

it

is

impossible

to

monitor

directly

the

mass

bias

of

the

Kr

isotope

ratios

during

an

analysis

if

there

are

isobaric

interferences

on

or

so

sample-standard

bracketing

may

be

required

Four

different

correction

methods

have

been

deployed

to

correct

for

the

Kr

interference

Gas

blank

or

on

peak

zero

-LSB-

-RSB-

This

method

measures

the

Kr

levels

as

a

blank

measurement

with

the

He

flow

going

through

the

sample

chamber

but

without

ablating

the

sample

-LRB-

laser

shutter

closed

or

no

laser

firing

-RRB-

The

advantage

of

this

method

is

that

there

is

no

mass

bias

correction

involved

The

gas

blank

also

corrects

for

minor

amounts

of

Sr

and

Rb

on

masses

and

which

are

thought

to

be

derived

from

the

sample

introduction

system

-LRB-

see

above

-RRB-

Peak

stripping

In

this

case

the

-LRB-

or

-RRB-

signal

is

used

to

calculate

the

and

corrections

on

mass

and

respectively

Christensen

et

al.

-LSB-

-RSB-

used

this

method

to

correct

for

the

and

ratios

However

they

do

not

report

how

they

performed

the

mass

bias

correction

on

this

ratio

but

instead

used

a

ratio

of

which

is

significantly

higher

than

the

natural

ratio

of

-LSB-

-RSB-

To

date

there

is

no

report

in

the

literature

of

a

method

to

calculate

the

mass

bias

of

Kr

from

the

ratio

which

should

be

possible

provided

there

are

no

isobaric

interferences

on

or

Waight

et

al.

-LSB-

-RSB-

used

the

on-peak-zeros

to

correct

for

the

Kr

interference

and

subsequently

correct

any

remaining

Kr

interference

by

the

peak

stripping

technique

by

monitoring

the

signal.Jackson

and

Hart

-LSB-

-RSB-

monitored

both

the

and

the

and

observed

deviations

of

up

to

a

factor

of

in

the

-LRB-

ca.

-RRB-

Therefore

these

authors

deployed

another

technique

to

correct

for

Kr

on

the

Sr

masses

which

uses

the

most

abundant

Kr

isotope

at

mass

-LRB-

abundance

-RRB-

instead

of

Kr

This

mass

has

a

major

interference

that

can

reach

of

the

signal

Correction

is

made

by

subtracting

from

the

signal

until

the

internationally

accepted

ratio

of

is

obtained

for

the

ratio

-LSB-

-RSB-

Iterations

are

necessary

for

the

mass

bias

correction

-LRB-

e.g.

substitute

the

Kr

number

in

the

ratio

and

repeat

the

calculations

until

there

is

no

change

in

the

calculated

isotope

ratios

-RRB-

This

technique

will

only

work

if

there

are

no

other

interferences

involved

on

masses

and

A

significant

disadvantage

of

this

Kr

correction

technique

is

that

the

ratio

can

not

be

used

to

check

if

the

interference

corrections

on

the

ratio

were

successful

by

providing

an

independent

check

that

the

obtained

from

the

analysis

is

the

same

as

the

internationally

accepted

value

of

In

summary

there

are

several

ways

to

correct

for

Kr

on

masses

and

The

most

widely

applied

technique

is

the

on

peak

zero

which

seems

to

be

successful

for

low

Rb/Sr

-LRB-

-RRB-

ratios

The

method

published

by

Jackson

and

Hart

-LSB-

-RSB-

is

the

only

one

reporting

Sr

isotope

analyses

on

high

Rb/Sr

-LRB-

-RRB-

samples

Calcium

dimers

and

argides

When

ablating

sample

material

with

high

Ca/Sr

ratios

-LRB-

e.g.

marine

carbonate

ca.

and

plagioclase

ca.

-RRB-

calcium

dimer

and

calcium

argides

can

be

formed

Ca

dimers

have

been

reported

to

be

interfering

molecules

during

SIMS

analyses

-LSB-

-RSB-

Waight

et

al.

-LSB-

-RSB-

and

Bizarro

et

al.

-LSB-

-RSB-

were

the

first

to

suggest

that

Ca

argides

-LRB-

e.g.

-RRB-

could

interfere

with

the

Sr

isotope

masses

for

materials

with

high

Ca/Sr

ratios

The

published

studies

on

Sr

isotopes

are

confusing

regarding

Ca

dimer

and

argides

Ramos

et

al.

-LSB-

-RSB-

conducted

solution

work

using

NIST

doped

with

Ca

in

such

a

way

that

the

Ca/Sr

ratios

varied

between

and

They

did

not

observe

a

change

of

the

outside

the

reported

error

In

addition

Jackson

and

Hart

-LSB-

-RSB-

did

not

find

a

change

in

the

ratio

outside

analytical

error

in

alkali

basalts

with

Ca/Sr

ratios

of

ca.

In

contrast

Woodhead

et

al.

-LSB-

-RSB-

reported

Ca

argide

and

Ca

dimer

signals

of

approximately

mV

for

all

Sr

masses

during

the

ablation

of

carbonates

This

study

also

reports

that

the

influence

on

the

accuracy

of

the

ratio

is

beyond

the

levels

of

within-run

precision

In

addition

Woodhead

et

al.

-LSB-

-RSB-

also

reported

that

the

ratio

can

be

significantly

modified

by

Ca

dimer

and

argides

when

ablating

carbonates

increasing

the

ratio

from

to

They

successfully

corrected

the

interferences

on

mass

and

by

monitoring

the

peak

on

mass

Peak

stripping

resulted

in

the

correct

ratio

because

the

relative

isotopic

abundances

of

these

argide

and

dimer

molecules

are

very

similar

Therefore

it

is

not

necessary

to

know

the

correct

argide/dimer

ratio

It

is

important

to

note

that

Jackson

and

Hart

-LSB-

-RSB-

reported

significant

variations

in

the

which

they

attributed

to

interferences

Ca

argides

and

dimer

appear

a

probable

source

of

such

interferences

REE

The

rare

earth

elements

-LRB-

REE

-RRB-

can

interfere

on

the

Sr

isotope

masses

and

on

interfering

element

masses

as

doubly

charged

ions

-LRB-

see

Table

-RRB-

The

formation

of

doubly

charged

REE

in

an

Ar

plasma

depends

on

the

operating

conditions

-LRB-

e.g.

RF

power

gas

flows

-RRB-

but

is

generally

in

the

order

of

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

The

REE

interferences

of

course

depend

on

the

material

analysed

For

recent

marine

carbonates

with

low

absolute

REE

abundances

-LRB-

ppm

total

REE

-RRB-

the

effect

can

be

neglected

but

for

clinopyroxene

with

significant

REE

abundances

-LRB-

Dy

ca.

ppm

Yb

ca.

ppm

-RRB-

it

could

be

a

potential

problem

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Waight

et

al.

-LSB-

-RSB-

and

Ramos

et

al.

-LSB-

-RSB-

were

the

first

to

study

the

interferences

of

Er

and

Yb

in

detail

For

Sr

isotope

analyses

and

are

relevant

because

these

ions

interfere

with

and

respectively

-LRB-

Table

-RRB-

With

an

increasing

Er

interference

on

mass

the

ratio

will

decrease

due

to

an

incorrect

Rb

correction

-LSB-

-RSB-

The

net

effect

is

that

the

amount

of

on

mass

is

overestimated

and

too

much

will

be

subtracted

from

the

ion

current

resulting

in

a

low

ratio

In

a

similar

way

an

increasing

interference

of

Er

will

result

in

a

higher

ratio

because

the

Kr

correction

on

the

mass

is

not

correct

The

signal

on

mass

is

assumed

to

be

and

therefore

the

measured

ratios

will

be

too

high

due

to

the

presence

of

Another

potential

interfering

REE

on

the

Sr

isotope

masses

is

Yb

-LRB-

see

Table

-RRB-

Yb

has

five

isotopes

which

appear

on

masses

and

as

doubly

charged

ions

The

and

ratios

will

increase

with

increasing

Yb

content

of

the

ablated

material

whereas

the

ratio

will

decrease

-LSB-

-RSB-

The

correction

for

these

interfering

doubly

charged

REE

can

be

done

by

monitoring

half

masses

Erbium

can

be

accounted

for

by

measuring

at

mass

during

an

analysis

Peak

stripping

with

mass

-LRB-

mass

-RRB-

can

then

be

done

using

a

natural

Er

solution

prior

to

the

Sr

LA

measurement

to

establish

the

Er

mass

bias

The

mass

bias

can

also

be

established

by

measuring

the

ratio

on

masses

and

and

assume

that

the

mass

bias

for

Yb

is

equal

to

that

of

Er

-LSB-

-RSB-

The

Yb

corrections

can

be

deployed

by

using

the

abundance

and

mass

bias

from

the

ratio

Ramos

et

al.

-LSB-

-RSB-

also

examined

if

Hf

-LRB-

e.g.

-RRB-

interferes

with

the

Sr

isotope

masses

but

did

not

find

a

significant

contribution

Zn

Ga

and

Fe

Schmidberger

et

al.

-LSB-

-RSB-

did

not

observe

correlations

between

the

FeO

content

and

Sr

isotope

ratios

of

ablated

clinopyroxenes

Iron

can

potentially

generate

two

isobaric

interferences

-LRB-

-RRB-

which

are

according

to

Schmidberger

et

al.

-LSB-

-RSB-

eliminated

by

the

collision

cell

in

the

Isoprobe

instrument

Ramos

et

al.

-LSB-

-RSB-

only

reported

a

limited

effect

of

Fe

oxide

on

the

Sr

isotope

ratios

and

did

not

correct

for

Fe

oxide

interferences

Ga

and

Zn

oxides

-LRB-

e.g.

and

-RRB-

could

also

interfere

with

the

Sr

isotope

masses

However

Ramos

et

al.

-LSB-

-RSB-

did

not

find

significant

deviations

in

solution

work

with

Sr/Ga

-LRB-

-RRB-

and

Sr/Zn

-LRB-

-RRB-

ratios

typical

of

geological

materials

and

decided

not

to

correct

for

these

potential

interferences

during

laser

ablation

work

Calcium

phosphates

Due

to

the

large

interference

of

on

during

LA-MC-ICPMS

-LRB-

see

Table

-RRB-

it

has

become

apparent

that

in

situ

analysis

of

biogenic

phosphates

and

apatite

is

extremely

challenging

and

currently

does

not

yield

accurate

results

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Proposed

solutions

to

correct

for

isobaric

interferences

From

the

above

discussions

of

the

interferences

on

the

strontium

masses

-LRB-

see

Table

-RRB-

it

is

clear

that

the

order

in

which

the

interference

corrections

are

applied

is

very

important

Nearly

all

interference

corrections

interact

with

other

corrections

For

example

the

Rb

correction

is

important

for

the

but

is

also

influenced

by

the

Yb

correction

In

some

cases

iterations

of

calculations

are

necessary

to

minimize

the

errors

on

the

calculations

In

Fig.

the

order

of

corrections

is

given

for

the

eleven

published

Sr

LA-MC-ICPMS

procedures

The

correction

routines

can

be

divided

in

two

groups

-LRB-

I

-RRB-

Kr

-LRB-

gas

blank

-RRB-

Rb

Sr

mass

bias

correction

and

-LRB-

II

-RRB-

the

Jackson

and

Hart

-LSB-

-RSB-

method

with

Kr

correction

using

and

Rb

mass

bias

correction

by

sample

standard

bracketing

Fig.

of

interference

corrections

in

the

eleven

publications

concerning

Sr

isotope

analysis

by

LA-MC-ICPMS

Not

mentioned

in

publication

but

inferred

from

published

isotope

ratios

See

text

for

discussion

The

group

I

correction

method

in

its

most

simple

form

has

been

deployed

by

Christensen

et

al.

-LSB-

-RSB-

and

involves

Kr

correction

by

peak

stripping

peak

stripping

and

subsequently

normalization

to

ratio

of

-LRB-

see

Fig.

-RRB-

The

more

elaborate

correction

methods

of

group

I

-LSB-

-RSB-

include

corrections

for

the

tail

REE

-LSB-

-RSB-

and

Ca

argide/dimers

-LSB-

-RSB-

The

group

I

data

reduction

method

does

work

for

the

relatively

simple

matrix

of

carbonate

material

and

plagioclase

The

group

II

method

from

Jackson

and

Hart

-LSB-

-RSB-

is

unique

and

has

as

the

major

advantage

that

the

Rb

correction

is

done

very

precisely

resulting

in

the

possibility

to

analyse

higher

Rb/Sr

materials

-LRB-

see

above

-RRB-

but

more

standards

should

be

analysed

to

fully

validate

this

approach

Precision

and

accuracy

of

Sr

isotopes

by

laser

ablation

From

the

above

discussion

it

is

obvious

that

LA-MC-ICPMS

will

probably

never

obtain

the

to

precision

and

accuracy

which

are

possible

with

state-of-the-art

TIMS

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Therefore

LA-MC-ICPMS

will

not

replace

microdrilling

of

sample

material

dissolution

and

chromatographic

processing

in

a

clean

laboratory

environment

and

measurement

by

TIMS

when

high-precision

analysis

are

required

in

sample

materials

with

limited

variation

in

Sr

isotopes

-LRB-

see

-LSB-

-RSB-

-RRB-

However

the

microdrilling/TIMS

technique

is

very

time

consuming

and

requires

a

significant

skill

from

the

operator

Measurement

of

Sr

isotopes

by

LA-MC-ICPMS

on

the

other

hand

has

great

potential

if

the

precision

and

accuracy

required

is

in

the

order

of

ppm

with

the

additional

advantage

of

reduced

analysis

time

-LRB-

average

of

min

including

wash

out

and

standard

analysis

compared

with

min

for

the

TIMS

analysis

-RRB-

In

Fig.

the

precision

and

accuracy

are

reported

for

the

eleven

Sr

isotope

studies

by

LA-MC-ICPMS

for

which

data

were

available

The

precision

is

controlled

by

the

counting

statistics

that

are

mainly

controlled

by

the

concentration

of

Sr

in

the

ablated

material

From

Fig.

it

is

obvious

that

due

to

their

high

Sr

contents

calcium

carbonate

and

plagioclase

provide

the

best

precision

The

precision

for

carbonate

material

is

better

than

ppm

for

six

laboratories

that

have

published

data

on

marine

carbonate

The

precision

for

other

materials

can

be

significantly

worse

for

example

for

many

clinopyroxenes

the

precision

is

between

and

ppm

Plagioclase

also

gives

good

results

with

a

precision

better

than

ppm

in

four

studies

Fig.

between

laser

ablation

and

TIMS

in

ppm

-LRB-

-RRB-

-RRB-

for

-LRB-

a

-RRB-

and

-LRB-

c

-RRB-

and

the

precision

-LRB-

-RRB-

for

laser

ablation

analysis

-LRB-

in

ppm

-RRB-

for

-LRB-

b

-RRB-

and

-LRB-

d

-RRB-

Publications

-LSB-

-RSB-

respectively

Typical

TIMS

precision

-LRB-

in

ppm

-RRB-

is

represented

by

the

grey

shaded

area

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

The

accuracy

of

the

data

is

more

important

than

the

precision

The

accuracy

expressed

in

ppm

deviations

of

from

reported

values

is

given

in

Fig.

The

accuracy

is

calculated

for

samples

where

LA

data

and

chromatographically

cleaned

samples

measured

by

TIMS

or

solution

MC-ICPMS

were

given

Some

interesting

conclusions

can

be

made

Carbonate

material

gives

the

most

accurate

results

data

are

within

ppm

of

the

solution

MC-ICPMS

and

TIMS

values

The

good

results

for

carbonate

material

are

to

be

expected

since

corrections

for

the

REE

and

Rb

are

minimal

in

this

material

The

good

accuracy

also

suggests

that

the

Ca

dimer

and

argides

are

probably

not

a

significant

problem

for

carbonate

material

since

a

correction

for

these

interferences

was

actually

performed

in

only

one

study

-LSB-

-RSB-

However

this

is

a

question

that

needs

to

be

explored

in

greater

detail

in

future

studies

Another

observation

is

that

all

the

LA-MC-ICPMS

carbonate

data

are

shifted

towards

higher

compared

with

TIMS

and

solution

MC-ICPMS

data

-LRB-

Fig.

-RRB-

Plagioclase

tends

to

result

in

ratios

that

are

higher

than

TIMS

or

solution

MC-ICP-MS

values

The

plagioclase

samples

of

Ramos

et

al.

-LSB-

-RSB-

and

Christensen

et

al.

-LSB-

-RSB-

are

closest

to

the

TIMS/solution-MC-ICPMS

value

whereas

those

of

Waight

et

al.

-LSB-

-RSB-

and

Davidson

-LSB-

-RSB-

are

ppm

too

high

The

most

likely

cause

for

this

discrepancy

is

the

significant

Rb

correction

for

this

material

-LRB-

especially

those

of

Davidson

-LSB-

-RSB-

since

these

samples

are

artificially

enriched

in

Rb

-RRB-

In

contrast

clinopyroxene

results

tend

to

be

lower

than

the

published

TIMS/solution

MC-ICPMS

results

This

is

true

in

the

work

reported

by

Waight

et

al.

-LSB-

-RSB-

and

Ramos

et

al.

-LSB-

-RSB-

but

the

clinopyroxene

accuracy

published

by

Schmidberger

et

al.

-LSB-

-RSB-

is

highly

variable

-LRB-

accuracy

varies

between

and

ppm

-RRB-

The

large

range

in

the

accuracy

of

clinopyroxene

is

probably

partly

caused

by

the

significantly

lower

Sr

content

of

this

material

compared

with

plagioclase

and

carbonate

material

-LRB-

see

Fig.

d

-RRB-

The

variation

in

precision

is

up

to

ppm

a

range

that

is

also

observed

in

the

accuracy

-LRB-

ca.

ppm

-RRB-

In

addition

clinopyroxene

needs

significant

corrections

for

Rb

and

the

REE

hampering

both

the

precision

and

accuracy.Sr

isotopes

by

laser

ablation

analysis

of

other

geological

materials

also

looks

promising

-LRB-

e.g.

groundmass

-RRB-

whereas

apatite

and

sphene

require

such

large

corrections

that

accurate

results

will

be

difficult

to

obtain

Unfortunately

not

all

studies

published

so

far

have

reported

the

ratio

From

the

five

studies

who

did

measure

this

ratio

the

results

from

Woodhead

et

al.

-LSB-

-RSB-

are

only

presented

in

a

figure

A

summary

of

the

data

from

the

other

four

studies

is

shown

in

Fig.

and

d

As

can

be

expected

from

the

significant

Kr

corrections

there

are

large

deviations

from

the

true

value

of

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

Results

from

clinopyroxene

display

a

large

variation

of

ca.

ppm

but

the

same

is

true

for

carbonate

material

-LRB-

ca.

ppm

-RRB-

Sphene

again

has

the

largest

deviation

from

the

true

value

suggesting

problems

with

corrections

of

Ca

dimer

and/or

argides

We

recommend

that

ratios

are

routinely

reported

as

an

indicator

for

the

quality

of

the

Sr

isotope

analysis

and

that

greater

effort

is

made

to

account

for

Ca

dimer

and

argides

Future

directions

All

published

studies

so

far

have

demonstrated

the

potential

of

Sr

isotopes

by

LA-MC-ICPMS

As

discussed

above

the

precision

and

accuracy

of

Sr

by

LA-MC-ICPMS

is

clearly

limited

by

the

corrections

involved

with

this

technique

Some

of

these

are

technique

dependent

-LRB-

such

as

the

Kr

corrections

-RRB-

whereas

others

depend

on

the

sample

material

analysed

-LRB-

e.g.

Rb

REE

and

Ca

dimer

-RRB-

Some

of

the

potential

developments

for

this

technique

are

evaluated

below

Laser

systems

In

the

past

decade

there

has

been

a

tremendous

effort

to

improve

laser

design

and

shorter

wavelength

lasers

are

now

being

used

-LSB-

-RSB-

The

net

effect

of

using

shorter

wavelength

lasers

is

the

more

efficient

volatilization

of

the

sample

which

produces

smaller

aerosol

particles

in

the

plasma

that

are

more

efficiently

ionized

-LRB-

e.g.

see

-LSB-

-RSB-

-RRB-

This

development

was

partly

driven

by

the

effects

of

trace-element

fractionation

during

laser

ablation

of

materials

where

the

measurement

of

elements

with

different

volatility

was

hampered

if

large

aerosol

particles

were

produced

for

example

U/Pb

dating

of

zircons

-LSB-

-RSB-

Since

corrections

for

isobaric

interferences

on

Sr

isotope

masses

are

so

large

-LRB-

compared

with

TIMS

-RRB-

the

assumption

that

Rb

instrumental

discrimination

and

laser-induced

elemental

fractionation

are

the

same

as

for

Sr

is

probably

not

true

Lasers

with

low

wavelength

-LRB-

e.g.

nm

-RRB-

will

reduce

this

elemental

and

isotopic

fractionation

However

the

mass-bias

contributions

by

instrumental

mass

discrimination

and

laser-induced

elemental

fractionation

need

to

be

determined

independently.Careful

determination

of

the

Rb

instrumental

discrimination

by

introducing

a

Rb

standard

aerosol

through

a

desolvating

nebulizer

to

correct

for

instrumental

discrimination

may

help

However

probably

the

best

solution

would

be

that

followed

by

Jackson

and

Hart

-LSB-

-RSB-

by

measuring

Rb-bearing

standards

with

the

laser

in

a

standard-sample

bracketing

method.The

Kr

interference

can

be

reduced

by

using

clean

sources

of

-LRB-

liquid

-RRB-

Ar

It

could

be

useful

to

experiment

with

Kr

removal

techniques

in

a

similar

manner

as

suggested

by

Zuzel

et

al.

-LSB-

-RSB-

The

interferences

of

the

REE

are

probably

only

important

for

materials

that

contain

significant

amounts

of

REE

Experimenting

with

optimal

plasma

conditions

which

reduce

the

creation

of

doubly

charged

REE

is

important.The

situation

with

Ca

argides

and

dimer

is

unclear

More

experimental

work

is

needed

especially

by

ablating

Ca-rich

materials

which

do

not

contain

Sr.

Conclusions

The

eleven

publications

reporting

on

strontium

isotope

analyses

by

LA-MC-ICPMS

all

conclude

that

it

is

a

very

powerful

technique

which

is

feasible

although

it

does

have

significant

limitations

depending

mainly

on

the

material

ablated

So

far

low

Rb/Sr

ratios

have

proved

essential

for

a

good

analysis

Successful

corrections

for

Rb

have

been

reported

for

values

up

to

This

suggests

that

carbonate

and

plagioclase

represent

reliable

materials

for

analysis

but

the

analysis

of

high

Rb/Sr

minerals

such

as

mica

and

biotite

are

not

currently

possible

and

will

remain

so

unless

there

is

a

significant

improvement

in

the

way

that

the

Rb

correction

on

is

performed

Higher

Rb/Sr

materials

have

been

ablated

using

a

standard-sample

bracketing

technique

for

correct

Rb

corrections

with

to

date

limited

success

Correction

of

Kr

interferences

on

Sr

masses

was

reported

not

to

be

a

major

problem

in

the

eleven

studies

published

to

date

however

the

accuracy

and

precision

of

the

data

reported

so

far

do

not

fully

substantiate

this

conclusion

Different

strategies

are

followed

to

correct

for

Kr

interference

in

which

gas

blank

subtraction

seems

to

be

the

most

preferable

Further

work

is

required

to

determine

the

instrumental

mass

discrimination

of

Kr

to

ensure

the

correct

and

ratios

are

obtained

Doubly

charged

REE

interferences

are

only

a

problem

in

materials

where

significant

REE

contents

are

present

such

as

clinopyroxene

However

successful

correction

is

possible

by

collecting

data

at

half

masses

The

influence

of

Ca

dimer

and

Ca

argides

is

unclear

and

needs

more

investigation

It

appears

that

the

effect

is

limited

on

the

ratio

but

for

corrections

are

necessary

Given

that

monitoring

is

a

good

way

to

establish

the

veracity

of

the

data

these

corrections

should

be

performed

routinely

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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O

laser ablation
sr isotopes
in situ analysis
interferences