MZTLTI-ELEMENTAL TRACE ANALYSIS OF SOILS BY INDUCTIVELY ...

MZTLTI-ELEMENTAL TRACE ANALYSIS OF SOILS .
BY INDUCTIVELYCOWLED PLASMA MASS SPECTROMETRY

USING SLURRY NEBULIZATION, FLOW INJECTION
AND MIJUCID-GAS PLASMA

A thesis submitted to the Department of Chemistry
in Conformitywith the requirements for
the degree of Master of Science

Queen's University
Kingston, Ontario, Canada

December 1999
Copyright 0 Angela L. Misseri 1999

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Dedicoted ToMy Parents, Rose and Sam Miseri

for their lave, samrice and ertcounLgeltient

Q

The National ~esearchkouncilof Canada, Dr. Heather Jarnieson and
Queen's University provided the instrumentation, soii samples and fiinding necessary to
completethis project.

Speciai thanks to Dr. Diane Beauchemin for her valuable teachings and input
regardingthis projecf as well as her husband, Daniel, for his time and effort spent towards
repainng the ELAN 500 ICP-MS instrument.

Co-Authorship

Al1work compiled within this thesis was carried out at the Department of
Chemistry at Queen's University, Kingston, Ontario, by the auîhor, under the direction of
Dr, Diane Beauchemin.

In this thesis, a follow up on the work of persaudl1was conducted to find conditions
dowing trace elemental analysis of soi1using as simple a method as possible. Slumïes
were prepared using the "Boîtle and Bad" approach and analyzed with flow injection and
extemal calibration A variety of methods were trïed with a 2-mm injector torch,
including varying nebulizer gas flow rates and a 5% nitrogen-argon mixed-gas plasma A
1-mm injector torch was also utilized at 1.4 mm and 1.7 mm distances f?om the sampler to
the top of the load c d , to determine ifresidence t h e andlor injector width was the
determining fàctor for successiid trace elemental analysis. Through these detenninations,
it has been realized that residence time and injector width are the two major parameters to
be optimized for quantitativeanalysis of soils. Incorporation of a 5% mixed-gas plasma
with a 2-mm injector torch at optimal nebulizer gas flow rates is an accurate method for
trace analysis of vanadium. A less optimum nebulizer gas flow rate at -0.08 L/min with a
2-mm injector torch without mixed-gas plasma yielded comparable results for NIST 2710
with suggested values, However, the fact that NIST 2711,in general, could not be
consistently determined suggests that the atomization/ionizationconditions are still not
optimal.

Aclmowledgements
CO-Authors*
Abstract
Table of Contents
List of Tables
List of Figures

1.1 Fundamentals of Inductively Coupled Plasma Mass Spectrometry
1.1.1 Histoncal Sisnificance
1.1.2 Generai Description o f QuadrupoleBased ICP-MS
1.1.3 Nebulization System
1.1-3.1 Nebulizers
1.1.3-2 Spray Chamber
i -1.4 Torch and Plasma
1.1.5 Sampling Interfàce
1.1.5.1 Sampler
1.1.5.2 Skimmer
1.1-6 QuadrupoleMass Spectrometer

1-2 Advantages of ICP-MS

1 3 Limitations of ICP-MS
1-3.1 Non-Spectroscopic Interferences
1-3-2 Isobaric Overlap
1.3-3 Polyatomic Species
1-3-4 Oxide Interferences

1.4 Methods to CompensateFor Limitations
1-4.1 Calibration Strategies
1-4-1.1 Interna1 Standardization
1-4.1-2 External Calibration
1-4-1.3 Standard Addition

1.4-1-4 Isotopic Dilution
1-4.2 Slurry Nebulization

1.4.2.1 Advantages
1.4-2.2Particle Size
1.4.2-3 SlurryPreparation
1.4-2-4Analysis Conditions
1-4-3 Flow Injection

1-4.4 Mixed-Gas Plasma

1.5 Thesis Objectives

2.1 SlurryPreparation
2.1.1 Reagents
2.1-2 StandardReferenceMaterials
2.1-3 Soil Slurry PreparationProcedure

2.2 Equipment
2.2.1 Fiow Injection Set-up

2.4 Mixed-gas Plasma

2.5 ExperimentalDesign

2.6 Data Acquisition and Processing

Chapter 3: ResuCts and Discussion

3.1 Optimal Sampling Conditions

3-2 Peak Area and PeakHeight Interpretation
3.3 Vanadium and Chromium
3-4 Laothanum and Molybdenum

3-6Zinc and Lead

Chapter 4: Conclusions and Future Work

4-1 Conclusions
4.2 Future Work

Appendijlt 1: Abbreviatiom
References
CumBculumVttae

List of Tables

28 Concentrationof standard solutions fiom
E (lowest concentration) to A (highest
concentration),

Elan 5000 software aqwsition measuring
parameters.

Optimal plasma operating conditions.

Optimal flow injection and mass
spectrometer conditions for ICP-MS.

Quantitative results for each element in

NIST 2710 according to each operating

condition.

Quantitativeresults for elements in NIST
2710 acçording to each operating condition.

Certified and suggested values for elements

inNIST 2710 and 2711.

Relative standard deviation (Yof)or NLST

2710 for each element under different
operating conditions.

Relative standard deviation (%) for NIST
2711for each element under dBerent
operating conditions.

Log of sensitivity for elements quantifïed for
the 1-mm injector and 2-mrn injector at the
optimum and -0.05 L/min fiom the optimum

NGFR experiments.

Log of sensitivityfor elements quantified for
the 2-mm injector at -0.08L/mh and

0.20L/& cornthe optimum NGFR and 5%

mixed gas plasma.

List of Figures

1 3 Elan mode1 ICP-MS schematic,

2 5 Schematic diagram of conespray nebulizer.

3 6 Scott double pass spray chamber.

4 7 Schematic of lCP torch.

Plot of a typical pattern of analyte response
versus mass of rnatrix element at a
concentration of 1000pg IL-'.

Spectnim of Cr, Ni and Co at 100 ng rnC1
showing major polyatomic ions.

Loading position for FI set-up.

Injection position of FI set-up.

DSerence between peak area and peak
height for V, Cr, La and Mo in NIST 27 IO.

Concentration of V, Cr and La for Soil2710
according to each experimental method.

Concentration of V, Cr and La for Soit 2711
according to each experimental method.

Relative standard deviation (%) of V, Cr and
La for soil2710 accordingto each
experimental method.

Relative standad deviation (%) of V, Cr and
La for soii 2711accordingto each
experimental method.

FI for Cr fkom two replicates ofNIST 2711
using the 1-mm injectorat x = 1.4 mm
samphg.

FI peak for Cr fiom two replicates of NIST

2711 using the 2-mm injector at optimal
NGFK

Concentration of Mo for soi1 2710 and 2711
according to each experimentai method.

Relative standard deviation of Mo for soils
2710 and 2711 accordingto each
experimental method,

Concentration of Pb for soils 2710 and 2711
according to each experimental method.

Chapter 1: Introduction

1.1 Fundarnentnls ofhductively CoupledPlasma Mass Spectrometry (ICP-MS)

1.1-1 Wtsfon'calSignzficance
According to definition, an inductively coupled plasma (ICP), is an electrodeless

dischargein a gas, typicaiiy argon, at atmospheric pressure that is maintained by energy
coupled to it nom a radiofrequencygeneratorl. The concept of an electrodeless plasma
generated fiom a discharge originaily dates to W. H- Hittorfin 1884~.During that time,
electrical discharges in gases were a subject of considerable interest.

By 1895, Thomsonhad measured ionic velocities and mas-to-charge ratios, while
Rutherford focused on their mobilities2. Identification of ions by use of mas-to-charge
ratios followed in 1910by mass spectrometricprinciples, through Thomson's
identification of two of the isotopes of N&.

Researchers such as Aston, Dempster, Spenser Smith, Lunt and eregg2 were all
major contributors during the next two decades for investiga~gthe applicability of mass
spectrometry (MS) as an instrument for chemical analysis with flames, chemical reactors
and electrical discharges.

Deckers, Van Tiggellan and calcote2 extracted ions fiom low pressure flames for
mass analysis, while Knewstubb and Sugden reported ion extraction fiom Iow pressure
flames buming fkeely at atmospheric pressure2. This work may be considered the f i t
published work on flame ionizationusing a high temperature ion source for analysis of
aqueous solutionusing mass spectrometrictechniques2. Knewstubb and Hayhurst in 1971
declareci the idea of an electrical flame, as an ion source, was scientificaiiycredible for
solution analysis based upon the advantageous higher plasma temperature of 5000 K
compared to the chemical flame of 3000IC2. Knewstubb and Hayhurst reported the
higher plasma temperature yielded simpler spectra containing fewer molecular species
because of the more inert atmosphere.

-2-

Knewstubb and Hayhurst's work was a prelude to Alkemade's dernonstrationto
prove the feasibilityof couphg an AA flame, as an ion source, to a quadrupole mass
spectrorneter2. This findamental coupling of an atoniic source with a mass spectrometer
accordingly to Alkemade's priociple, provided the stimulusin all forthcomingresearch
groups for broadening and improvingthe Limitations and applicability of ICP-MS as it may
be considered today.

1.1.2 GeneralDescription of QuadhpooleBased ICP-MS
An ICP mass spectrometer is a precision instrument designed to perfom elemental

and isotopic analysis3. Such analysis is possible through the combination of ICP and MS
instnunentation. For example, Figure 1shows a schematic of an Elan-500Mode1ICP-MS

instrument. Most of the ions generated by an ICP source are atomic, singly charged and

detectablewith a quadrupole mass spectromete$. An argon plasma is ignited by a Telsa
discharge in argon flowing continuously at atmospheric pressure in a senes of concentric
glas tubes known as a torch3r4. A maximm of 2.5 kW of radio fiequency power
supplied by an RF generator can be applied to a load coil surrounding the outlet end of the
plasma torch3. This applied power helps to sustain the plasma so that sample solutions
aspirated by means of a nebulizer may be introduced into the plasma where vapourization
and atomization occurs3.

Ions fomed withui the hot plasma are extracted into a vacuum chamberwhich
houses the quadrupole mass spectromete$. An ion lem systemtransmits formed ions to a
quadrupole mass filter for separation accordingto their mass-to-charge ratios (m/z). The
separated ions are detected by a channeltron electron multiplier which produces an
electncal pulse accordingto the impact responses of each individual ion. These impact
signals are converteci and transmitted to a cornputer for analysis.

Figure 1: Elan mode1 ICP-MS schematic3
Except for orientation and cos grounding arrangement,the inductively coupied
plasmas used in MS and atomic emission spectroscopy (AES) are virtualiy the sarne1. The
mass analyzer, ion detectors and data collection systems used in ICP-MS are simiiar to
those developed for quadrupole GC-MS systems!

1.1-3.1 Nebuli'ers
Sample introduction and nebdization is the moa criticai step during analysis in

ICP-MS. Nebufizersare devices used for converthg solutions into an aerosol spray
wmposed of varying droplet sizes4 for injection into a plasma5.

Generdy, there are three types of nebuhers: 1) pneumatic nebulizers,
2) Babington style nebuiizers and 3) ultrasonic nebulizers (US&),

sharp6y7 first constructecithe cortespray nebulizerya Babington style nebulizer,
and described its robust purpose in terms of fluid mechanics. ~ h a r p r~ep3o~rted the
conespray nebulizer as having an excellent pefiormance rating when compareci to the
Meinhard concentric pneumatic nebulizer with respect to nebulization of high dissolved
solids and blockage prevention.

Later, Ivaldi et aL6adapted Sharp's construction to nebulize solutionswith high
levels of dissolved solids. According to Ivaldi et aL6,this reconstnicted nebulizer may be
considered a type of Babington nebulizer because liquid does not pass through a narrow
orifice, hstead, it flows fieely over a surface with a gas orifice (figure 2). The absence of
the narrow tip found in pneumatic nebulizers, reduces biockage for solutions with high . .
dissolved solids or a suspended solid matrix6.

The adaptation of Sharp's original construction by the Ivaldi g ~ o q fwas tested
ushg a 10 m@ solution of Mn in 10% NaCl aspirated at a rate of 3 mL/rnin. The signal
was stable for 10to 120 min, having a minimum to m a u m peak spread of 3.5% with an
average short term RSD of 0.5% and a range of 0.2 to 1.2%~.M e r a period of 120
minutes, Ivaldi et aL6reported a sharp signal decline that indicated clogging of the torch
injector, not the nebulizer, as the gas flow rate through the onfice was constant
throughout all the injections. Removal of the torch revealed salt deposits on the injector
tip6. To reduce this problem, Ivaldi's group6 increased the uiside diameter of the injector
to pennit longer hours of operation.

Accordhg to Jarvis and Wiiams8, most published work on ICP-MS incorporaîed
pneumatic nebulization of solutionsfor sample introduction into an ICP. However,
different instrumental operating conditions are requïred for slurry nebulizaîion than for

solution analysis8. In general, Babington V-groove style nebulizers are more appropnate

to allow conthuous nebulization of slumes without nebulizer clogging.

Figure 2: Schematic diagram of conespray nebulizer (Ivaldi et al6)
Ambrose and coworkersgalso recognized the need for a suitable nebulizer with slurry
nebulization and therefore used an Ebdon nebulizer,

Replacement of a pneumatic nebulizer with a Conespray nebulizer during analysis
aiiowed slurry nebulization without blockage or degradation of the precision of results?
Persaud et aL never experienced blockage of the Ivaldi adapted Conespray nebulizer,
and reported improved detection iimits and less observable noise in cornparison with a
pneumatic nebulizer.

1.1.3.2 Sprqy C h b e r
M e r passage of sample solution through the nebulizer, aerosol droplets of varying

sizes reach the spray chamber. The primary fiincîion of a spray charnber is to remove
large droplets greater than 10prn in diameter f?om the gaseous stream174. The presence
of large droplets in the gas Stream may indeed extinguishthe plas& These large
droplets typically consist of98% of a sample solution4.

The gas flow carrying the aerosol droplets enters the spray chamber and undergoes
sharp directional changes that are not easily undertaken by the large dropletsl. As a
result, the large droplets strike the sides of the spray charnber, condense and subquently
exit throughthe drain4¶12.

An ideal spray chamber is represented by one that possesses flow reversal cycionic
action and impact beadsl. Droplets which Leave the ideal spray chamber are less than 10

pm in diameterly 4. Additionai spray chamber ideals include high analyte mass fluxes,
transport efficiency, a low wash-out time and good pressure-temperature aabilityl.

Moa of the criteria of an ideal spray chamber are met by the double pass spray
chamber developed by Scott et aL1*(Figure 3), which consists of a chamber with an inner

barrel12.

Figure 3: Scott double pass spray chamber (Jarvis et al.l)
Spray chambers may be constmcted of Rytoq glas or polyethylene. Ryton is
corrosion resistant and does not experience memory effects suffered by glas4. Ryton also
has a greater wetability, dowing easier passage of solution compared to
Ebdon et al.12J9 used a Scott double pass spray chamber constnicted of Ryton which
yielded good recoveries for rock and soi1 siunies.
Cooled spray chambers oEer the additional advantage of improving recoveries47 12.
Ebdon et d l 2examineci the effeçts of a water-cooled Scott double-pass spray chamber
for the analysis of mal by ICP-MS. The cooling action improved precision while
decreasing noise by actingto lower vapour ioading in the plasma4. 12.

The nebulizer and spray chamber yield an aerosol in the plasma region which d

undergo four sequential steps4that are:

1) desolvation
2) vapourization
3) atomization
4) ionization

The energy generated fiom the radiofiequency generator that creates the

electrodeless discharge in the ICP is achieved by a coupling coii. The coupling coi1has

two main hctions, firstly as a radio fiequency transformer and secondly, as a source of

the discharge12.

-A commonlyused plasma gas is argon, which becomes partialiy ionized4accord'mgto equation 1.

Ar ~ r ++ e- (1)

Generation of the plasma occurs inside and at the open end of an assembly of

quartz tubes known as a torch4.5: l2 (figure 4).

a

-suarna.~oruurii

Figure 4: Schematic of ICP torch (Jarvis et al.l)

The outer tube containsthe main plasma support gas which acts to sustain the
plasma and protect the walls of the torch fiom the plasma itsep. An auxiliary gas flow is
introduced intofie interrnediafembe to ensure the hot plasma is kept well away ffom the
tip of the central capiiiaryinjector tube to prevent melting12. The cenbal tube is the
injectortbrough which the aerosol sample is inaoduced into the center of the plasma4. 12.

A load coi1of 2-4 turns of copper wire cooled by water flow is locatedjust below
the mouth of the torch12. An RF current is supplied by a generator and a spark is created
using a Telsa coi14. Acceleration of newly fomed electrons occurs around the magnetic
field in circuiar orbits, while the electncal energy supplied to the coi1 is converted into
kinetic energy of electrons4~12. Upon collision with Ar atoms, ohmic heating occurs
resulting in a bright discharge12.

The cool injector gas flow punches a channel through the center of the plasma,
allowing aerosol passage12. Aerosol droplets containingthe analyte are desolvated and
vapourkation of the analyte salts and oxides occurs5. Atornization of the analyte occurs
at 5000 K in the vicinity of the copper c d . The region of atomization is termed the
initia2 radiation zone (EU) and typicaily occurs 1-2 mm above the load coil. This region
may take on, for example, an amber appearance due the emission by Ca0 molecules when
agricultural sampies are nebulized5. Similady, emission by calcium atoms or ions, m e r
downstream, then altersthe amber colour to deep blue or purple. This region of
ionization is temed the n o r d andyîicd zone (NAZ)(seen in figure 4) and is typically
held at a temperature of 7000 lK4. 5.

1.1.5 Sampling Iiakrjiace
The sampling and skimmer cones are criticalwmponents of the interface and are

often thought of as the heart of an ICP-MS instrument. Their main h c t i o n is to extract
plasma gas containing the analyte of interest.

1.1.5.1 SampIer

A sampler is in direct contact with the plasma4. A majority of the argon gas that

enters the sampler &mes fkom the centre of the plasma. Materialsused to constnict a

sampler cone Vary f?om dumïnum, copper, nickel and platinum', although, nickel samplers

are commonlyused due to an equal compromise between durabilay, cost effectiveness,

high thermal and electricai conductivityl. Sampler cone blockage may at times becorne a

problem due to high dissolved or suspended solids. However, the problem can be

aüeviatbil by removd of the cone and thorough cleaningl. According to Gray and

~ i l l i a m ss~ol~ut,ions con-g up to 0.2% dissolved solids can be tolerated by the

sampler cone for long periods with usage of a wide bore injecter.

1.1.5-2 Skimmer

A nickel-based skirnntet is mounted directly behind the sampler cone at distances
between 6-7 mm1.4. Sensitivity of the ICP-MS is directly related to the physid
condition of the skimmerl.

The gas which flows throughthe sampling orifice expands into a superso~ijcet,
because the pressure changes fiom 1bar to lmbarl. m e in this condition, the plasma
becornes fiozen and little reactions occur in the h e l y expanding region, often termed the
zone of silence? To avoid loss of ions and scattering, the zone of silence passes directly
through the skimmer'.

Ll.6 QuodirpoleMars Speetromeîer
In its simptest fonn, the mass spectrometer performs two essential fbncti011~~~

which are:
1) The accelerated ions are separated according to their mass-to-

charge ratios (&) in an electrÏcfield.

2) The ions with a particular mas-to-charge ratio are detected.
Accordingto Jarvis et al.l, the ion lens provides little or no separation of
mass-to-charge ( d z ) species in the extracted ion beam In quadrupole-based ICP-MS
instruments, this fùnction is achieved using a mussjZter. The quadrupole mass filter
consists of four straight metal rods, situated pardel to and equidistantfiom the axïsls 16.

A RF voltage, as well as a DC voltage, are applied to opposite pairs of rodsl~'~T. he
mass filter is scamed by ramping both voltages1?16. AppropnateRF and DC voltages
allow the ions to oscillate so that only ions of a given m/z will have a stable path and wiil

emergefiom the opposite end of the quadmpole17-

Peak shapes vary with resolution. At low resolution, peaks are slightîycurved at
the maximum,but not flat-toppedl. With increasing resolution, peaks become sharper and

peak sides become closer together? Continuedincreases in resolution, diminishes the
peak height, demonstraîing a compromise between resolution and sensitivityl.

1.2 Advantages of ICP-MS

According to ~ r a ayn IC~ P-MS instniment in the hands of a capable analyst has

the capability of analyticaiperformance unmatched by any other rapid instsumental

laboratoq technique. ICP-MS is a method with high sample throughput, a wide linear

dynamic range14 and low detection limits. Detection limits are one to three orders of

magnitude lower by ICP-MS than by ICP-A&. T h e added benefits of high precision

make ICP-MS a superior instrument1'.

Entwistle and ~brahams'd~isaissed the abiiity ofICP-MS to detennineprecious

m),metals and rare earth elements sp&g 14 elements f?om 13?La to 175~u,as

being a tremendous benefit to geological sciences.

Duane et 24 evaluated the perfo-ce of ICP-MS with other instrumental

techniques such as ICP-AES, ETAAS and XRFS by c o m p a ~ rge d t s obtained with
samples from an inoperable miningsite. For screeningheavily contaminated soils, XRFS

was most suitab~e2~4.~.However, for mediumto low concentration,
ICP-MS was regarded as superior, due to its wide iïnear dynamic range, even though
digestion of samples was required prior to instrumental analY~is2234.~

Jarvis and ~illiams*sîudied the suitabiIif~of analysis for geological materials
ushg ICP-MS. Jarvis and wfiams8found that for most elements, precision was better

than 5% MD. Precision was particularly good for volatile elements such as As and sn8.

Accuracy varied for all of the samples determined8. Overd the authors stated that "the

accuracy of the technique was not easy to as ses^..."^.

Acquisition of mass spectra is rapid and simple compareci to that of emission
spectra fiom ICP-AES~. The spectra observed for the entire penodic table in ICP-MS,
contains only a few hundred peaks according to the number of isotopes present, while
spectra for ICP-AES may have thousands of emision lines for a single element4.

Easy analysis of aqueous solutions is another benefit and there is relative fieedom
nom chernical interferencesdue to the high plasma temperatures4. FinaJly, in two
minutes, less than 2 mL of solution can be analyzed containingup to 70 elements,
including ail their isotopes30.

1.3 Limitations of ICP-MS
13eauchemin4stated four factors which affect the detectionlimits of ICP-MS,

they are:
1) degree of ionizationof the analyte

2) natural abundanceof the isotopeused for the determination
of a particular element

3) non-spectroscopie interferences
4) mass discrimination

Like any instrumental technique, ICP-MS has its difnculties due to interferences.

These problematic issues are addressed below.

t3.1 Non-Speettoscopic Interfierences

With ICP-AES, one of the possible forms of matrix suppression observed is a shifk

in the ionization equilibrium induced by the addition of easily ionizable elements such as

Na, y caZ2. Elements which are easily ionized produce a sigNscaflt increase in electron

population in the central channel after i ~ n i z a t i o nC~o~n.centrations greater than 0.1% w/v

may be s d c i e n t enough to produce a suppression in ICP-MS. However, selection of

compromise conditions may reduce this effect for concentrationsless than 1%22.

According to Date and similar or greater mat& effects have been observed in

ICP-MS th= ICP-AES. depending on the m a 6 Figure 5).
1.25

mars of suppressani

Figure 5: Plot of a typicai pattern of analyte response versus mass ot matrix element at a
concentration of 1000pg m d (Date and ~ r a y ~ ~ )

eauc chemin^ stated suppressionand recovery are worse for iighter elements while heavier

eiements recover faster. Suppression is ais0 worse for analytes with a higher ionkation

1.3.2 Isoburk OverIq
Isobaric overlap may oc- between coincident isotopes of neighbouringelements,

causing seriousinterferences4p Most elements in the periodic table have one, two or
three isotopes fke nom isobaric interferences, for example Co, Sm and Sn respectivelyl.
A number of overlaps occur with argon, the plasma gas. As a d e , isotopes with odd

masses are fiee fiom isobaric overlap, while several even masses are not4. 22. The severity
of interferenceis dependent upon the samplemathv and the relative proportions of

occurring elements.
1.3.3 PolyafomèSpecies

The plasma contains neutral and ion populations of plasma gas and rnatrix elements
at levels of 10' times greaterthan a trace analyte of 1pg rnZI122- Reactions between
such populations are inevitable, resulting in additional interferences. The most abundant

species are Ar, O and H, which yield species of OH3, Ar0 and ArH, showing a full range

of isotopic combinationsl. 47 22. The extent of polyatomic interferenceis dependent upon
several factors including operating conditions, extraction geometry, the plasma, sample
ma& nebulVRr systems and the nature of the acid used in sample preparationi.

Figure 6 shows a mass s p e c t m ftom a 100ng mLel solution of Cr, Ni and Co in
1%NaCl that displays the interferhg polyatomic ions at d z 51, 53,54, 56, 62 and 63 for
35~1160+3,7~1160+A, rN+, A*+, N%O+ and NaAr+ respectively.

Interferences are also expected fiom the acids used in sample preparation. For
instance polyatomic ions containingN, Cl, P and S, will give lise to increasingiy serious
interferencess2. As a result, great care should be taken when selecting acids for sample
preparation and it is for this reasonthat nitric acid is most commonlyused for an acidic
medium.

mass

Figure 6: S p e c t m ofCr,Ni and Co at 100 ng m ~ - slhowing major polyatomic ions

(Date and

Oxide species occur either as a result of incomplete dissociation of the sample

matrk or recombinationin the plasma taill?12- Resulting interferences are from ions of

~ ~ ~ 0 2M'~o+,
or ~~~0~m~ass units. The oxides expected are prediaable

according to monoxide bond strengthof the element concernedl. Elements, such as Ce,

with the greatest oxide bond strengthyield the highest MO+ ions. Table 1lists expected

oxides for some elements,

Table 1: Element-for~ningoxides (~eauche~nin~)

Elemeat Masr Unit ofOnde (m/z)

Ca 56,58-60,62,64

MO I08,I10-114,116

W 196,198-200,202

Ti 6246

Sc 5 1

Y 105

Zr 106. 108-110.112

Horlick et and Jarviset al. explain that the level of MO+ ions generated is
dependent on the RF forward power and the nebulizer gas flow. Water as a vapour in the
plasma introduces oxygenl. The water reduces plasma temperature, resulting in a
dramatic alteration in plasma equilibrium because of the energy required to dissociate the
water moleculesl.

The highest MO+& ratios are observed at highest nebulizer gas flow
According to McLaren et al." the amount of ondes detected is proportional to the
-ber of undissociated oxide ions extracted in the expansion stage.

The detennioation of REEs in soi1by Zhang et al.l7showed interference due to
the oxide ions 147~m1601, 50~m16a0n,d 15g~b16in0 determinations of 163Dy, 1 6 6 ~arnd
175~uH. owever, interferencesfrom 137~a1601, 41~r1601, 43~d16a0nd l%di60 and
156~d16w0 d d not be neglected for accurate analYsisl7.

1.4 Metbods To Compensate For Limitations

There are several methods to compensate for inteflerences due to matruc species,
isobaric overlap, polyatomic and oxide species. Correction for isobaric overlap involves
measurement of another isotope of the i n t e r f e ~ gelement, then multiplication by an
abundance factor1. 4. Background interferences are successfùily corrected with, for
example, a reagent blank for compensation of 4 0 ~ 1an6d ~1%160.Other more
elaborate methods of compensation are descn'bed on the following pages.

Intemal standardization, also known as surrogate addition, enables a user to
correct for signal instability, atomizationinefficiency, sample handling and in moa cases,
matrix effects2? According to ~ r o w mnat~rix~effects will not be corrected by intemal
standardizationifthere is s e l d v e chemistry for the analyte of interest rather than the
surrogateitselP8. In such instances,an additional surrogate should be considered
whereby one surrogate may correct for ma& effects and the additional internai standard
may compensate for W i t y effects2?

Results fiom Zhang et al?' for the detenninaticnofREEs in soil by ICP-MS
found the precision of analysis improved fiom 28.2 to 37.4% without intemal
standardkation to less than 9.8% wjth an intemal standard, despite the fact that the signal
intensity was significandy suppressed up to 5 0 % ~ ~ .

Intemal standardkations involve addition of a fixed amount of an element which is
not present in the sample42~8. The chosen intemal standard must be distinct nom the
aualyte, yet behave as similarly as possible to the analyte4*28. To nilfill this Cnteria,
Beauchemin explained the surrogate must be simiiar in mass ( d z ) ,have comparable
ionizationpotentials and undergo identical chemistry to the d y t e under instrumental
conditions.
1.4.1.2 EdemZ CaZibration

For quantitativeanalysis, a çalibration cuve provides good accuracy and
precision. ~ r o w n e~x*plained there are two major assumptions involved when using
extenial calibration, they are:

1) The signal is stable over time
2) na* effects are negiigible

However, this is not usually the case for elemental analysis and correction for drift using
an intemal standard or fiequent caiibration is advisable4.

Amther consideration when using external calrirationis rnatchingthe rnatrix of
the standard solution to that of the sample. Of course, at least one isotope must be fkee of
spectroscopie interferences4.

1.4.1.3 &kmdbrdArjlrlrtion

According to eauc chemin^, if matrix matching is complex or impossible, the

method of standard addition is an alternative. Standard addition corrects for matrix effects
and yields good accuracy and precision4.

The method of standard additioncannot correct for drift or instability and

**.therefore may require Eurrogate addition4. The type of internai standard selected is

dependent upon the type of drift present. If drift is present during sampleuptake or
nebulization, or ifthere is discrimination of aerosol droplets in the spray chamber, then
only one surrogate is needed for all elements4. If drift occurs in the plasma and some
elements are desolvated quicker than others, several surrogatesmay be required4.

There must be at least one isotope fiee of spectroscopie interferencesfor each
analyte4. Disadvantages of standard addition include requirement of a preliminary analysis
to find the approxknateconcentration- The spiking required also makes it a time
consuming method.
1.4.1.4 IsofopicDilution

Isotopic dilution is a powerfù2 absolute method, achievablethrough mass
spectrometrictechniques- The rapid scanning of the MS detector easily separates naturd
isotopes fiom spiked isotopes28.

Caldation of the two isotope ratios of a padcular element is achievable by
Klinkhammer' s e ~ p a t i o n ~ ~ ( e ~ u a3t).i o n

=total concentration of al1 isotopes

leiMJ = concentration of enriched isotope of M in the spiked solution

VI=volume of spike
V, =volume of sample
R, =qeasged ratio for sample and spike (''w~M)
R,= (niMl%d) for pure spike

&= ratio for natural abundanceM

ni =natural isotope
ei =e ~ c h e idsotope

A, =natural abundance of "M

Isotope difution has three main disadvantageswhich are: 1) it is a tirne c o d g

method, 2) at least two stable isotopes fkee of spectroscopie interferences are required for

each element determined and 3) a prelimioary anaiysis is required.

hductively coupled plasma mass spectrometry is a usenil analyticai technique for

chernical analysis of soils and ceramic materials. The analysis of a majority of elements in

the periodic table is possible using ICP-MS, includingthe capability of isotope ratio

measurementss. Originally, ICP-MS was designed for solution analysisl', however, other

matrices such as soils are possible as long as the matrix canbe introduced into the gas

flowK

Sluny analysis is sirnilar enough to solution analysis that conventional

instrumentation can be used without labonous modificationll. 30. Depending on the

homogeneity ofthe sample, calibrations may be easily accomplished using aqueous

standards as the behavior of a slurry may be similar to that of a solutionwithin the spray

chamber, torch and 30.

1.4.2.1 Advrmtages

The method of sample presentation into plasmas is a crucial step in any analytical

procedu&. Traditionally, samples may be prepared using digestion processes such as

acid dissolutionor fusion21. Although digestion procedures facilitate homogenization

and caùaration, they also contributesto contamination, evaporative losses of volatile
elements and incomplete dissolution, especiallywhen using siliceousminerais, refiactory
compounds and ceramics, in particular oxides of vanadium and chromium, which are
partiailady resistmt to acid attack1l. 21* 30. Furthemore, dissolution methods are often
tedious, costly and time consuming, involving hazardous usage of concentrated acids such

a~HF a d ~ ~ 1 0 ~ ~ 7 ' .
Entwistle and ~brahamls4 studied the applicability of ICP-MS for rapid site

investigation of historical Scottish sites but reverted to using a HN03 - HC104 acid

digestion technique, despite forelcnowledgethat incompletedissolution was Likely.
Furthemore, volatilization of seleniun, arsenic, iodine, antirnony and mermry occurred
from the high boiling point of ~ ~ 1 0En~twi'stl~e an.d ~ b r a h a m s ldi~scovered the
influence of incomplete sample digestion huidered the accuracy of result and data
interpretation, in particular, values for zirconium,bromine and chromium were less than
50% when compared with standard reference materials (SRMs). A more aggressive,

tedious and hazardous HF - HN03 - HC104 acid digestion was required to improve

accuracy, but only succeeded to irnprove analytical recovery for zirconiumup to 59%
comparedto the SRM

A study of Jarvis and wiuiams8used slurry nebulization to determineits suitability
as a soil preparation method, in cornparisonto acid dissolution techniquesfor d y s i s by
ICP-MS. Slurry nebulizattion was shown to be a viable and usefùl method of analysis with

a relative standard deviation (RSD) between +/- (5 - IO)% for the elements studied8.

As a resultydirect introduction of soi1slurries into the plasma prevents these
diflicultie~~O~t.her notable benefïts of slurry preparation include use of few chemicals,
thereby reducing costs, no analyte loss, no use ofhazardous chemicals and a simpEed

solutionpreparation procedurell. S l m y nebulhtion also reduces polyatomic

interferences due to the absence of acid rnatrix elements as in dissolutionmethods,

Slurry nebulization has been incorporated into a variety of applications, most

commonly ICP-AES~~A.ccord'mg to ~ b d o net aL2Is,lurry nebulization with ICP-MS

has increased in popularity recently. Yet, published works in this area are difiîicuit to

retrieve, as it is a younger spectroswpic method of analysis. Nonetheless, the approach

has distinct advantages, for example, low cost and easy implementation, over direct

methods of solid analysis such as laser ablation and direct sample insertion into the

plasma2l.

1.4.2.2 Parfide Size

The particle size distributionof a slurry is the critical factorcontrolling analytical

recovesrllp 21. In an ideal slurry, ali particles should be of similar small size to ensure

both suspensionin a Liquid medium during analysis and atomization efficiency of the

particle in the 21y 30.

A study by Ebdon et al.l2found that magnesïum recovery, used as a measure of

slurry transport efficiency relative to the solution, is dependent upon particle size. A

particle diameter of 10prn had a transport efficiency, d e h e d as the percentage of sample

entering the torch injecter, of 3-4%, while particles 5 Pm in diameter had irnproved

transport efficienciesnom 14-15%12.

Vien and F ~21 us~ing d~irect *current plasma (DCP) AES to analyze plant tissue

samples and to study the effect oftransport efficiencies, discovered particles larger than 23

prn are not delivered to the plasma. Instead, they settIe out of the aerosol strearn, exiting

the spray chamber through the drain13. According to Goodall et al2'$37 the maximum

allowable particle size is one in which each aerosol droplet is occupiedby one solid

particle.

As shown, the solid particle size of a slurry is the iimiting factor for efficient slurry

nebubation into the ICP. To avoid transport inefficienciesthe maximum recommended

particle size is 5pm by grinding 30.

1.4.2.3 S I q Prepmm-on

To achieve the desired particle size distributionthere is a range of grinding

techniques available. These include: micronizing d l ; mixingmill; vibration pot d l ; the

bottle and bead method and the puck type grinder.

To demonstrate the usage of slurry nebukation for GFAAS and ICP-AES, Fuller

and c o ~ o r k e r s4~0 ~gr.ound samples for 30 minutes in a micronising d e r under acetone,

redting in a mean particle size less than 6 40

The choice of grinding agent is important and may affect the analytical recovery16~

24. According to Ebdon et aL21, the grinding matenal should be harder than the sample

material. Nso, the grinding or milhgagent should not contain elements that wodd

interferewith a n a ~ ~ s iAs n~a~ly.sis of grinding blanks should therefore be conducted to

ensurethat the grhdingor milling agent has not contributed to additional contamination.

Due to the nature and varying deosity of solid particlesll, a suitable grinding

rnethod is selected upon consideration of sample type and its mechanical strength30.

Brittle materials are successfùliyground wMe coarse grained rocks may pose a difliculty

due to the grain size, friability and hardness30.

The least expensive grinding of partidar interest here, is the bottle and bead

method introduced by Williams et al.33- It involves weighing 0.1-1.Og of sample into a

30 mL polypropylene bottle with 10g of zirconia beads. Deionized distillecl water

(DDW) is added to completely submersethe beads. The bottie is shaken on a wrist action

shakerfor a suitable amount of tune so as to achieve a homogeneous sluny with desired

diameterll. The resultant slurry is decanted to separate thezirconia beads nom the soil
slurry and the slurry is diluted to volume with dispersant. If an acid is used as a
dispersant, then it also serves as a leaching agentIl.

S1urry concentration is another important factor, Slurrïes can be diluted, but
within a smali range, without affectingprecision21. Very dilute soil slurries result in poor
precision due to the srnaUer number of particles remauiùig in solution after dilution, while

wncentrated slurriesresult in sampler cone blockage if nebulized contuiuouslyll. 21-

Ebdon and ~ i l l r i n s o ndi~sc~ussed that signal to background increased linearly with
increasing slurry concentration up to 200/0 (mhi). Continuous nebulization o f 0.05% (m/v)
soil slunies, with diameters less than 3 pm, occurred without sampier cone blockage.

Analysis of 1% (mh) slurries were made possible without blockage by
ïmplementation of flow injectiong. Blockage would normaiiy occur approxixnately 30
minutes after wntinuous nebulization in the absence of flow injection17.

Lobinski et anaLyzed zirconium powder using ICP-AES to investigate slurry
preparation parmeters, operating variables and their effect on slurry stability- Lobinski's
group f o d that slmies were stablefor pH ranges between 1-2 and 10-1143. The use
of organic solvents for slurry preparation was not advised due to possible contamination
and plasma instabw3.

Persaud in 19941° based a master's thesis on developing slurry nebulization of
mils for ICP-MS. In terms of slurry preparation, ~ersaudlOexaminecithe sample size, for
example, a 0.1 g sample and a 0.25 g soil sample. Similar relative standard deviations
were obtained for slunïes prepared nom the two different masses. However, a sample
size much less thau 0.1 g would increase the associated relative standard deviation
WD)Io.

Grindingt h e of the soil slurrieswas also tested by backscattered electron
imaging. 1twas observed by persaudlo that der 24 hours of grinding, no particles larger
than 1p m in diameter could be detected-

Slurry stab'ity was a subject of concem by ~ e r s a u d lA~s. a result, slunies were
analyzed one weeiq then three months &er the initial preparation. The data showed
similarresults, indicating slurry stabüity over a long period of tirnelO.

In terms of dispersing agents, 1%(vh)and 1MHN03 solution were compared
and the latîer was found to be a better medium for s1un-y analysis. Triton-100 used as a
dispersal agent additive to improve wettability of the tubing and spray chamber was also
compared to 1~ HN0310. persaudl0 explaiaedthat a 1MHN03sluny yielded optimal
r e d t s in terms of detection limits, precision and sensitivity-

Ebdon and collier21741 examined the effect of various torch injector tubes (2.2
mm, 3 mm and 4 nmiin diameter) and their effect on analyte sample transport7~41. Ebdon

and collier2l*41 indicated that the 4 mm injector tube was unsuitable for analysis, while
the 3 mm injector was most suited for slurry nebulization. The authors found the interna1
diameter ofthe injector iimited the mean particle droplet aerosol size reachingthe

plasma219 41.
JaMs and wrlliams87 21used a 3-mm plasma torch injector for s l u q nebulization

into an ICP. Both authors emphasized the importance of the wider bore injector which
inmeases the cross sectional am, yet decreases samplevelocity through the injector
aperture and therefore creates a longer residence time in the plasma for improved
ionUation/atomization of refiactory particles8> l. Using the 3-mm injector, Jarvis and
~illiarns*721 found "withinsampleYpyrecision was better than 5% RSD for most elements
and "outer sample" precision was typically 10%RSD.

Totland et aL7.42 used aqueous d i r a t i o n standards in the analysis of geological

samples by ICP-MS without matrix matchingg Fagioli et a1.1°7 44 used 0.25 g - 0.50g of

organic SRMs which were with sulphuricacid in 50 mL digestion tubes and heated

for approximately one hour at 350%. Fagioli et al.Io* used aqueous standards for

calibrationwhich yielded comparableresults for ICP-AES as with flarne atomic absorption

spectrometry (FAAS). Precision was less than 5% indicating slurry~ n i f o r m i t y44~- ~ ~

Wrlliams and 30 determined the differences between continuous analysis of

slurries using ICP-AES and ICP-MS and found for analysis by ICP-MS, it was necessary

to Iunit the levels of total dissolveci soiid to less than 2000 pg/mL to prwent blockage of

the sampler cone orifice.

1.4.3 Flow Injection

Flow injection Owas first used by Ruzicka and m e n 3 in 1975. FI is a useful

technique where a known sample volume is injected into a continuousmobile a m e r
strearn16.

In its simplest form, the apparatus for flow injection only requires a peristaltic
pump, micro-bore tubing and a sample injection valve1. The total volume injected into the
system is smalland therefore small volumes of reagents and samples can be used, reducing
andysis and sample preparation costsl. According to Gray and I3atS2, flowinjection (Fl)
systems offer numerous advantages for analysis of samples with a high mtrix
concentration. Aggarwal et stated that the throughput of small volumes using flow
injection provides sensitivity, minimalsample consumption, and speed when using

ICP-MSsystems.

Ambrose et al9.21 reported the use of flow injection for the dysis of soil
slurries using ICP-AES. They stated that flow injection systerns are as applicable to soil

slmies as they are to solutions and provide stability ofresuits, as well as introductionof a

concentrated slurry into the plasma by a carnier strearn9y21. FI allowed the analysis of 1%

w h slurries, which would have resultedin blockage after about 30 of continuous

nebulizationlL7l3-

Minimal samplewosumption in ultratrace analysis of REEs is advantageous. A

studyby Aggarwal et al.32demonstrated detection Limits for REEs between 0.05 to 0.5

ppt for samples injected with a 460 pL FI loop.

FI is partidarly usefiii for samples with high dissolved solidsbecause there is a

reduction in nebulizer and tubing blockage when using a liquid carrierL0.persaudl0

reported that constant flow of DDW through Teflon tubing reduced intersample

contamination, as well as sampler cone and nebulizer blockage.

Persaud et al.l1used flow injection to minimize transport effects of acid leached

slumes. Sensitivitywas found to be 85% of that achieved by continuous nebukation

using a 0.25 rnL loop and a DDW carrier".

Persaud et al." anaiysed SRMs using slurry nebukation, FI, 1-mm injector

plasma torch and mixed-gas plasmas. Compromise conditions were selected for

multielernental analysis of nhe slumesusing aqueous standard solutionsfor extemal

calibrationl! No accurate determination of Co, Ni or Cu was possible due to polyatomic

ions arising eom the matricesH. However, accurate determinationsofLa and Pb were

achieved, while a positive bias for Zn was observed as a result of spectroscopic

M e r e n c e s eom the matrices1! Results for Mo were better in contrat to Zn, however,

the concentration ofMo was too low in the sample for accurate quantitative analysisll.

Very low results were obtained for V and Cr in cornparisonto certinedvalues1! Persaud

et al.l1mncluded that the systematidy low results for these elementsindicated that the

selected operating conditions did not d o w complete ionization/atomization of the soi1

particles. According to Persaud et al.11, a wider plasma torch injector may increase the
residence time in the plasma and the mean particle aerosol droplet size reaching the
plasma,

Introduction of a nitrogen-argon muid-gas via a gas proportioner was studied by
Beauchemin and ~ r a i g D~u~e t-o problems with plasma extinction, nitrogen was added
slowlyto prevent plasma shrinkage4%Using the gas proportioner, sipal to noise ratios
were improved, thereby improving detection limits, however sensitivitywas
~orn~romised~~.

~ersaudl*studied the effecîs of slurry nebulization and an argon-nitrogen mixed
gas plasma. Addition of nitrogen to the plasma significafltlyincreased temperature and
proportionallythe risk of melting the injector torch. Therefore, I?ersaudlo used a higher
coolant gas flow rate. An increase in outer gas coolant fiom 12Llmin to 15 Wmin
showed no merence in sensitivity, detection limt and precision for sluny analysido.

The percent of nitrogen addition bdween 0% and 15% was also examineci by
~ e r s a u d l ~A. parmeter set of 15%nitrogen, 1.2 kW torch power and 800 Umui
nebulizer gas flow rate (NGFR), gave the best quantitative result for slurry analysis using
compromise conditionslO.However, cornparisonbetween caldated concentrationsand
accepteci values showed only La and Pb may be accurately determined using this
methodl09 I l.

1.5 Thesis Objectives
The goalof this thesis is to foilow up on the work of ~ e r s a u d l ~to>fi~nd~ ,

conditions aiiowing the trace elemental analysis of heterogeneous materials such as soils,
using as simple a method as possible. To this end, slurries will be prepared using the
'Bottte-and-Bead" approach, flow injection and extemal calibration- In this way, the
method will be kept inexpensive, simple and should minimize clogging indliced by soils

while providing a greater sampie throughput thanthat achieved with continuous

nebukation,
As a continuation after ~ersaudl*l,?a~wider (2-mm) bore injector, both with and

without a nitrogen-argon mixed-gas plasma, wiii be used to increase residence times, in an
attempt to improve atomizati~~onizatioenfficiency. Results wiUbe compared to those
obtained with the srnalier f1-mm) bore injector.

Slurry concentration wiU be increased from 0.1% to 1.0%to improve detection
limits for molybdenum. The continuous rinsing effect provided by FI was expected to
minimize clogging at this level.

Chapter 2: Experimental

2.1 Slurry Preparation

2.1.1 Reagents

Multielemental standard solutions were prepared using anaiytical grade solids

purchased fkom M a Aesar USA) and diiuted with 1M high purity niîric acid

(Merck, Darmstadt, Germany), using a 100 mL volurnetnc ffask Nitric acid was prepared

with deionized distillecl water @DW) @EUi-Q+, Millipore, ON, Canada).

150m .polypropylene botties and 100mL volurnetric flasks used to store and

prepare the calibration standards, were first rinsed with 1%nitric acid and then allowed to

sit oveniight, f l e d with fresh 1% nitric acid. The following day, the bottles were emptied

and Nsed thoroughly with DDW. Table 2 lias the standard solution concentrations.

Five different concentrationsof calibration standards were prepared to enable the

method of extemal dbration. Solutionconcentrations were chosen so that calibration

standards had concentrationsencompassingthose in the soi1slunies.

Table 2: Concentrationof Standard Solutions From E (Iowest concentration)
To A (highest concentration) In Units ofpg/ml

2.1.2 Standmd Refereirce Materials
Two standard reference materials were selected as soil samples based upon

availability and extent ofmetal contamination. Standard reference material NIST-2710
contains trace element concentrations. NTST-2710was obtained fiom the

National Institute of Standards& ~ e c h n o l ion G~ a~ith~er~sburg, MD, U S 4 courtesy of

Dr. Heather Jamieson of the Department of Geological Sciences, Queen's University,

Kingston, Ontario, Canada. SRM 2710 was coIlected fiom the top 10 cm of pasture land

located dong SilverBow Creek in Butte, Montana, USA The soil site is nine miles east

of the Anaconda plant and 6.5 miles south of a pond which feeds into Silver Bow Creek

Flooding of the creek occurs periodicaliy, depositing sediment high in concentrationof

copper, mannanese and zinc.

Standard reference material NIST-27 11also supplied wurtesy of Dr. Heather

Jamieson, was purchased fkom the National Institute of Standards& ~ e c h n o l o ~ ~ ~ ~ ,

Gaithersburg, MD, USA This SRM is an agricultural soii £YomMontana, USA that has

been collected in the upper 15.2 cm of a wheat field., known as the till layer. MST-2711is

considered to be a moderately elevated contaminated sojl.

2.1.3 Soi1 Slurry Preparutioli &oce&re

Polypropylenebotîles of sizes 30 mL and 500 m . ,as well as 250 mL volumettric

flash were rinsedwith 1% nitnc acid and allowed to sit overnight filled with 1%nitnc

acid. The ne- day, each of the flasks were emptied and thoroughly rinsed with DDW.

Zirconia beads, 2-mm in diameter, required as a grinding agent, were rinsed

overnight in 1Mnitric acid and washed with DDW the next day to reduce interferences

fiom contamination. Preparation of blanks followed the same procedure as for the soil

samples but without soil. This control blank was required to iden* ifany contamination

could be contributed by the zirconia grinding beads.

Soi1 sluny preparation was wnducted according to the '330ttle and Bead" method

described by Wfiams et 5 g of zirconia beads, 2.5 g of a soil sample and

10mL of DDW was placed in a clean 30 mL poiypropylenebottle and secured on a wrist

action shaker for exactly 24-hours. Exact masses for NfST 2710 and 2711 were

(2.5055 +/- 0.0001) g and (2.5061 +/- 0.0001) g respectively. M e r the 24-hour shakuig

period was complete, the polypropyiene bottles were removed from the wrist action
shaker- CarefUy decanted slurries were transferred into clean 250 m .volumetric flasks,
leaving behhd the zirconia beads in the polypropylene bottles. A majority of a soil slurry
was decanted into a volumetncflask; 1 M nihic acid was added to the polypropylene
bottle, shakenthoroughly and decanted into the volumetric flask. This procedure was
continued until no visible signs of soii remained on either the zirconia beads or the bottle.
The 250 mL volumetricflask was fDed to volume with L M nitric acid and inverteci and
shaken The slurrywas then transfèrredto a clean 250 mL polypropylenebottle. Rie
final slurry concentration was 1%(nui).

2.2 Equipment

AU data were collected with a Perkin-Elmer-Sciex ELAN 500 Inductively Coupled

Plasma Mass Spectrometer, equipped with a homemade x-y-z translational stage
undemeath the torch box to allow optimal alignment of the plasma with respect to the
mass spectrometer-

Two different studieswere conducted: one with a 1-mm bore torch injector
and one with a 2-mm bore torch injector. All rneasurements involved use of a standard

Scott double pass spray chamber and a Conespraynebulizer (Perkin - Elmer, Norwalk,

CT, USA). To regulate the aerosol camer gas flow rate, a mass flow controller was
required. (Mode1 246B,MKS Instruments, Andover, MA, USA).

A gas proportioner (mode1 7571-603-604,Matheson) with gas column numbers
603 and 604 for nitrogen and argon respectively, were used to introduce analytical grade
nitrogen at 5% to the outer (coolaut) argon gas flow.

2.2.1 FCow Injection Set-up
A manud flow injection system was used for ail sample injections wbich consistecl

of a flow injection @valveI()mode1 5020, Rheodyne Inc., Cotati, C h USA) controlled
by a switching module (Universal module, Anachem, Luton, England). The sample
injectionvalve was connecteci to the nebulizer by 1 m of 0.8 mm diameter Tenon îubing,
as in the study conducted by Persaud et al.

Position zero on the FI actuator ailowed passage of a DDW canier through a 50 ,

pL injection loop (Figure 7). This was used instead of the 250 p L loop used h

~ e r s a u d ' d ~w~ork to mhbize the risk of clogguig due to the increase in soil
concentration The sample injectionvalves were filied by suction into a syringe d i r d y
fiom a polypropylene sample bottle to minimize contaminationcontributed by the
syringe43~7. Once the FI valves were filled with sample, the loading position '0"was
manuaily switched into injection position Y", for sample passage to the nebulizer
(figure 8). To reduce thepossibility of memory effectsbetween injections, the loop was
flushed by suctionwith 1M H N 0 3 .

A peristaltic pump (Rabbit, Mode193857, Raini.Instrument Co. Inc., Woburn,
MA, USA) was needed to maintain constant sample uptake during acquisitionsat a rate of
1mL/min The peristaltic pump tubing was made of Tygon and had a diameter of
0-32mm. Sampleswere injeçted in order of blank, the least concentrated calibration
solution (labeled E) to the highest concentrated standard solution (labeled A) folIowed by
a soil slurry. Five repeat injections of all standard solutions were done. Berneen three
and five replicate injections were done for soil slurries. Soil slurries were kept dispersecl
by manud agitation during flow injection, when an ultrasonic bath was unavailable.

soi1slurry sample syringe

ICP-MS peristaltic pump
DDW

Figure 7: Loading position for FI set-up

Figure 8: Injection position ofFI set-up

2.3 Initial Optimization
An argon plasma having an outer plasma gas flow rate of 12L h i n was ignited

~ h +with a forward RF power of 1.2 k W - A 100pgR. solution o f ~ i + , and ~ b i+n 1%

ultrapure HN03 was continuouslyaspirated, while the three micrometers ofthe x-y-z
translational stage beneath the plasma interface, the mass fIow controiler, also referred to
as the nebulizer gas flow rate WGFR) and the ion lens voltages (B, El, P and S2, see
table 7) were periodically altered to optimize ail three ion signals. F i optimjzation of
~ h w+as conducted by adjusting the x-y-z translational stage with respect to the sampler,
until a maximum ~ h s+ignal was achieved. Secondly, the NGFR was optunized to

maximize RhC intensity. Finaily, the ion lenses, if needed, were adjusteci to equaiized the
~ i a+nd ~ b s+ignals without muiimizingany Rh+ intensity. This optimization has resulted

in the best achievable compromise conditions for multielementl analysis, The initial
optimizationwas repeated on a daily basis because of changing instrumental conditions.

Optimizatïon of conditions for the analysis of soil slurrieswas then conducted by

alteration of the NGFR in FI mode, by increasing or decreasingthe mass flow controller,

by counts of 10L/mk to check ifthe maximum signal intensity obtained during the
onginal optimization, was the same for the slurries. This mass flow controller rate was
used for ail subsequent s 1 q acquisitions. Flow injection was used instead of continuous
nebubation for slurry optimization for fear the higher slurry concentrationwould cause
instrument clogging-

2.4 Mixed-gas Plasma
The argon plasma was initidy ignited at an RF forward power of 1.2 kW and a

12L/min argon tlow rate. Mer ignition, both parameters were increased, to 1.3 kW and
15L/min respectively. Nitrogen was cautiously added to prevent plasma shrinkage. Final

settings on the gas columns d e r addition of 5% nitrogen were 20 and 130for columns
603 and 604 respectively. The reflected torch power was minimizedby manual fine tuning
to less than 5 W.

2.5 Experimentd Design

A 1-mm injector torch was studied using an optimalNGFR at both an optimal
(1-4mm) x-direction sampling position and non-optirnal(l.7 mm) x-direction sampling
position where the plasma is moved 0.3 mm away fiom the sampler. This was done to
check ifany improvement could result by solely increasïng the residence tirne in the
plasma.

For the 2-mminjector torch, experiments were done at the optimum NGFR and
non-optimal NGFRs, narnely, -0.05 LJmia, -0.08 L/min and -0.20 L/min fiom the
optimum. Again, this was carried out to check if an increase in residence t h e would have
a beneficial effect.

A nitrogen-argonmixed-gas plasma with 5% nitrogen was also used with a 2-mm
injector torch at an optimalNGFR only, as signiscant sensifvity loss resulted fiom the
addition of nitrogen to the plasma introduced by the gas proportioner.

2.6 Data Acquisition and Processing
Version II of an Elan 5000 multielement data acquisition softwarein the Graphics

program was used in measurement mode with a 50 ms dwell time, one sweep per reading
and one measurement per peak with 250 repeats. See table 3 for details.

Table 3: Elan 5000 software acquisition measuring parameters

Measruing Parameters f Softnrare Settings
resolution(at IO% peak height) f low (1.1 u)

measiaing time 40s

measurernentlpeak 1

1

dweU time 50 ms

rePeab 250

Analyses pertaining to a given set of soil slwies were conducted on the same day

to reduce effects fkom instrumental drift and changes ffom regular instrumental

maintenance such as changing or cleaning sampler, skimrner or torch, etc. Isotopes

mo&ored were S 1 ~ ,S 2 ~ r6,8 Zn, 9 8 ~ 01,3%a a d 2 0 8 ~ b P- lease see tables 4 and 5 for

daiiy instrumental operating conditions.

Table 4: Optimal plasma operating conditions

Plasma Conditions Iostnunental Settings

RF forwardpower 1.2 W

RF forward power, mixed-gas 1 1-3 W
RF refiectedpower (with and without mixe& gas) 1 -5W
12 L/min
outer gas flow rate

outer gas flow rate with mixed-gas

Ar nebulizerflow rate: 0.800 Umin - 1.100Umin
1-mmtorch (optimizedM y ) 1-00Umin - 1.350 Umin
2-mmtorch (optimized daüy)

mixed-gas, 1-mmtorch (optimkd daüy) 1.069 Wmin

Ar auxiliarvflow rate 2Lhin

Table 5: Optimal flow injection and m a s spectrometer conditions for ICP-MS

F1and MS parameters o p t h a i settings

flow injection:

nebulizertype and DDW carrierflow rate Conespray, ImUmin

Mass spectrometer:

sampler o s c e diameter 1.14 mm

skirumer o d k e diameter - 0.89 mm
intefice pressure and MS pressure 1Torq l o J ~ o r r

MS lem settîng (optimii7ed daily):

Bessel boxbarre1 @) (3.28 to 7.63) V

Einzel lense~1& 3 (EI) - 19.85 V
Besselbox end Ienses Op) -( -6.29to 20-82) V

Photon stop (S2) 16.55 V

DOS GWBASIC programswere used for mathematical smoothing and peak area

and peak height caldation of ion count rate peaks for each individual injection.
Polynomial smoothing was achieved with a %pointmoving window.

Precision was compared ushgthe relative standard deviation (RSD) for a gïven
soi1 slurry based on three to five replicates. The relative standard deviation was celailated
by dividing the standard deviationby the mean for a given elernent in a slurry. Sensitivity
was taken as the dope of the caiibrationcurve.

Chapter 3: Resdts and Discussion

3.1 Optimal Sampting Conditions

Initia& a piece of Teflon tubing extending nom the FI vahe to the soil slmy
sampled only the top portion of the s1urry. So, reproducible peaks were difocult to obtaui
for the soil slurries. To increase reproducibility and obtain a more representative sample, a
longer 0.80 mm Tefion tubing was aîtached to the FI valve, so that it muid reach the very
bottom of each slurry sarnple. The correspondhg experirnents for all the slmies yielded

more reproduciile results and a consistent slurry - nitric acid mixture inside the sample

loop. As a result, the longer 0.80 mm Teflon tubing samplingwas used throughout the
remainder of this work.

Use of an ultrasonic bath was orighally employed, however, after brief
exambation of the bottom of each of the 250 mL polypropylenebottles during analysis, it
was noted that in each case, there were smalldeposits of soi1 particles. These s m d
deposits were not being adequately agitated by the ultrasonic bath, causing some concem
that the injections were not t d y representative of the entire sarnple. As a reçuIf all the
slunies were vigorously shakenby hand during injection, in which case, there were no
longer any soi1deposits remainùig on the bottoms of the polypropylene bottles.
Therefore, vigorous manudy agitation was employed for the remainder of the
experimentS.

3.2 PeakArea and PeakHeight Interpretation
Initially the purpose of the first experiment with the 2-mm injector torch was to

compare the merences between peak area and peak height results. From this experiment,
it was discovered that the peak area calculations are much more accurate compared to