CHAPTER 4: ANALYTICAL INSTRUMENTATION

4.1 INTRODUCTION

In this section, a review of the analytical instrumentation used during sample

preparation and analysis is presented which includes an overview of the

instruments, principles and techniques used.

4.2 INSTRUMENTS USED DURING SAMPLE PREPARATION

4.2.1 Industrial microwave systems

The first commercial laboratory microwave unit with pressure feedback control in

1989, and the first with temperature feedback control in 1992 has allowed for

more rigorous design and control of microwave sample preparation procedures [60, 61]. These developments, coupled with the evolution of the microwave vessel to

point where it is capable of monitoring reaction conditions throughout digestion,

has allowed researchers to systematically study the decomposition mechanism of

a variety of matrices.

Principle of operation

Microwave chemistry is the science of applying microwave irradiation to

chemical reactions. Microwaves act as high frequency electric fields and will

generally heat any material containing mobile electric charges, such as polar

molecules in a solvent or conducting ions in a solid. Polar solvents are heated as

their component molecules are forced to rotate with the field and lose energy

through collisions. Semiconductive and conductive samples heat up when ions or

electrons within them form an electric current and energy is lost due to the

electrical resistance of the material.

Advantages

Conventional wet-sample preparation methods for the decomposition of solid

samples generally involve heating the samples over long periods of time using

either a hot plate, heating mantle, or oven. The process is stopped when the

analyst considers that the decomposition of the sample is sufficiently complete.

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This type of open-vessel digestion has many limitations including the use of large

volumes of reagents, a real potential for sample contamination by external

materials and the laboratory environment, and the potential exposure of the

analyst and the laboratory to corrosive fumes.

Closed-vessel microwave decomposition uses significantly different technology to

achieve sample decomposition. Decomposition of most solid samples can be

achieved using only 10 mL reagent, and can be completed in a short period of

time. The higher temperatures achieved in the closed system give microwave

digestion a kinetic advantage over hot plate digestion, as described by the

Arrhenius Equation:

(d ln k)/(dT) = (Ea)/(RT2) (4.1)

Integration of this equation gives:

(k2/ lnk1) = (Ea)/(2.303R)[(1/T1) – (1/T2)] (4.2)

In this expression k1 and k2 are rate constants for the reaction of interest at T1 and

T2 respectively, Ea is the activation energy, and R is the ideal gas constant. These

equations show that the reaction rate increases exponentially with increasing

temperature. This translates into approximately a 100-fold decrease in the time

required to carry out a digestion at 175 °C when compared to a digestion at 95 °C.

In addition, because the mineral acid converts microwave energy into heat almost

instantaneously, rapid heating of the sample is achieved, further decreasing the

reaction time. Digestions are also more complete because many acids (e.g. nitric

acid) show improved oxidation potential at elevated temperatures.

A typical closed microwave system with build-in pressure and temperature control

can be seen in Figure 4.1 and Figure 4.2 which also show the microwave

consumables used during routine operation.

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Figure 4.1 CEM Mars 5 microwave systems in operation,

(Photographed by, Impala laboratory management, 2006)

Figure 4.2 CEM Mars microwave 5 systems with high pressure and temperature

consumables, (Photographed by Impala laboratory management, 2006)

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4.3 INSTRUMENTS USED FOR SAMPLE MEASUREMENT

4.3.1 Inductively coupled plasma optical emission spectrometry (ICP-OES)

ICP-OES instruments are used for a number of different applications in a variety

of industries. In the mining industry, ICP-OES is used in conjunction with x-ray

fluorescence spectroscopy for the analysis of major elements.

Principle of operation

The first investigations of alkali and alkali earth elements with the aid of a

spectroscope were reported by Bunsen and Kirchhoff in 1860. Spectrochemical

analysis was further developed in the 20th century when flame, arc and spark

technology were introduced [66]. An illustration of an ICP-OES as it appears today

is shown in Figure 4.3.

 

Figure 4.3 An illustration of a SPECTRO GENESIS ICP-OES as

provided by SPECTRO, S.A.

The principle of operation involves a sample introduction system which

supplies sample at a constant rate to the ICP emission source (plasma),

where desolvation, atomization, excitation and light emission occur. During

the excitation process, molecular, atomic and ionic species in various energy

stages are produced. Energy is released in the form of electromagnetic radiation

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and, as a result, a wavelength is formed, which is characteristic of the emitting

specie. An expression for the absolute intensity (Iqp) of a spontaneous emission

line arriving from an electronic transition from a higher state q to lower state p

can be shown to be: [64,65]

Iqp = d/4π Aqp hvqp Nq (4.3)

Where d is the depth of source, vqp is the transition frequency, Aqp is the line

transition probability, h is Planck’s constant and Nq is the number of excited level

species. The intensity of an elemental atomic and ion line is used as the analytical

signal in quantitative atomic emission spectroscopy.

Components of the ICP-OES instrument include the peristaltic pump, the

nebuliser, the spray chamber, the rf generator, the torch, the optical systems, the

detectors and the data processing system. The ICP-OES components are displayed

in figure 4.4 and are discussed briefly thereafter.

Figure 4.4 The SPECTRO CIROS ICP-OES schematic diagram as provided by

SPECTRO, S.A.

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Nebulizer and spray chamber

The nebulizer works in conjunction with the spray chamber. The major function

of the spray chamber is to act as a droplet size discriminator, passing only

droplets below a certain size. The size of droplets passed by the spray

chamber will largely depend upon the geometry of the spray chamber, but

to a lesser extent will depend upon the gas flow rate. A further function of the

spray chamber is to dampen pulses originating from the peristaltic pump.

Torch, rf generator and coil

Plasma RF power primarily affects the plasma temperature i.e. the greater the

power intake, the higher the plasma temperature. The net effect of power on

analyte sensitivity depends on the ratio of analyte signal to background

noise. Most plasmas are operated between 0.8 – 1.1 KW, with the exact

power chosen in accordance with the most crucial elements to be evaluated.

The plasma is sustained by energy from the RF Generator and coil. The

ICP – OES torch is centered within the induction coil, and the center tube

is 1-2 mm below the bottom of the induction coil. The three concentric

quartz tubes of the torch serve to define three separate gas flow paths i.e.

the coolant, auxiliary and carrier gas paths.

Plasma discharge

Radio frequency energy from the induction coil causes charged particles to

accelerate in a circular pathway in the same plane as the coil windings.

These charged particles collide with Argon gas atoms, causing ionization and

thereby forming a plasma. High in the plasma is the radiation zone, where

excitation, ionization, and emission take place. This region (approximately 6000

to 7000 oK) is normally utilised for analytical measurement.

Optics

The spectrometer isolates analytical wavelengths from the emitted light plasma.

The majority of wavelengths lie within the region 160 to 860 nm .

Separation of light into its component wavelengths is normally achieved

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using a diffraction grating. There are three types of diffraction grating: ruled,

holographic and echelle grating. Many spectrometers are either flushed with

nitrogen or argon or are maintained under vacuum to remove any oxygen.

Detectors

New generation detectors have recently been introduced. These are solid-state

detectors, but are also referred to as charged - transfer devices. There are two

sub-classifications: charge – coupled devices (CCDs) and charge-injection

devices (CIDs). A CID consists of a two dimensional array of detector

elements which, when coupled to an Echelle spectrometer, is capable of

simultaneous line analysis over the range 170 -800 nm.

Readout devices and data processing

The detector produces an electrical signal which is processed by an

electronic circuit before being measured by a read-out device.

In modern spectrometers the computer controls the operating parameters of

the plasma as well as performing the task of sample logging, operation of

the auto-samplers, construction of calibration curves and facilitates the rapid

and efficient handling of data.

4.3.2 Inductively coupled plasma mass spectroscopy (ICP-MS)

Since the commercialization of ICP-MS in 1983 [64], it has undoubtedly been the

fastest growing trace element technique for a series of applications. The platinum

industry however, only truly started employing this technique for the analysis of

exploration samples, Final Tailings streams and Mill Circuit Product streams for

trace levels of precious metals about six years ago. And as yet ICP-MS is still not

perceived as an instrument for routine evaluation analysis unlike FAAS and ICP-

OES.

Principle of operation

The principle of operation for ICP-MS is similar to that of ICP-OES and

includes a sample introduction system which supplies sample at a constant

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rate to the ICP emission source (plasma). It is important to differentiate the

function of the plasma in ICP-MS compared to that in an ICP-OES. In ICP-OES

the plasma is used to generate photons of light, by the excitation of electrons of a

ground-state atom to higher energy level. Wavelength specific photons are

emitted, which are characteristic of the element of interest. A schematic setup of

an ICP-MS is shown in Figure 4.5.

 

Figure 4.5 A Schematic setup of the SPECTRO MASS 2000 ICP-MS provided

by SPECTRO, S.A

In ICP-MS the plasma is used to generate positively charged ions and not photons.

Once the ions are present in the plasma, they are projected via a low vacuum

interface into the mass spectrometer chamber and focused via an ion lens system

onto a quadrupole mass filter, Figure 4.5. The interface region consists of two

metallic cones (usually nickel), called the sampler and skimmer cones which

allow the ions to pass through to the ion optics, Figure 4.6. The ions which reach

the quadropole are separated based on their mass- to- charge ratio prior to the

detector and to filter out all the nonanalyte, interfering and matrix ions. The final

step is to convert the ions into an electrical signal with a dynode detector.

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Figure 4.6 The interface region and ion optics of an ICP-MS system as provided

by SPECTRO, S.A

4.3.3 Graphite atomic absorption spectroscopy (GFAAS)

The use of furnaces as atomizers for atomic absorption spectroscopy was a major

breakthrough as it allowed the measurement of very low concentrations when

compared to that achievable by flame atomic absorption spectroscopy.

Principle of operation

There are many different high temperature furnace designs. The objective of

GFAAS is to generate free atoms such that atomic absorption can be measured.

The principle of operation involves three stages: [67,69]

• A drying stage during which, the solvent is removed and dried to a salt

deposit.

• An ashing stage which removes organic or inorganic material.

• An atomization stage in which free atoms are formed within a small zone.

The absorption signal produced in the atomization stage is a sharp peak, the height

of which can be related to the amount of analyte element present. The GFAAS

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can achieve low limits of detection as the sample is completely atomized and

vaporized and the atoms are kept in the atomic reservoir for an extensive period.

4.4 COMPARING GFAAS, ICP-OES and ICP-MS TECHNIQUES

Atomic spectroscopic methods have many advantages which include: linear

dynamic ranges, low detection limits, easy and rapid qualitative analysis,

simultaneous multi-element analysis, good precision and high sensitivity. Specific

criteria are used to choose appropriate atomic spectroscopic methods for different

applications. A series of criteria shall be utilised to highlight the respective

advantages of the GFAAS, ICP-OES and ICP-MS techniques.

Detection limits

Typical detection limit ranges for major atomic spectroscopy techniques are

shown in Table 4.1.

Table 4.1 Detection limit ranges [70-72]

Atomic spectroscopy techniques Detection limit range (μg/L)

FAAS 1 – 1000

ICP-OES – Radial view 1 – 100

ICP-OES – Axial view 0.1 – 10

Hydride generation FAAS 0.01 – 1

GFAAS 0.01 – 0.1

ICP-MS 0.001 – 0.01

It is clear from the data in Table 4.1. , that ICP-MS offers the lowest detection

limit followed by GFAAS and axial ICP-OES. Radial ICP-OES and FAAS show

similar detection limits, but ICP-OES can atomize refractory and rare earth

elements more effectively due to the higher temperature that can be achieved by a

plasma as when compared to a flame. Hydride generation FAAS offers

exceptional detection limits for mercury (Hg), arsenic (As), bismuth (Bi),

antimony (Sb), selenium (Se) and tellurium (Te). ICP-MS in conjunction with

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collision / reaction cells or magnetic sector technology can achieve detection

limits as low as parts-per-quadrillion (ppq) for many elements [63]. It is important

to emphasize that these detection limits are only achievable in simple matrixes.

Linear dynamic range (LDR)

ICP-MS is considered to be an ultra-trace element technique which has an LDR in

excess of 105. GFAAS has a limited LDR of 102 – 103, but can be used for higher

concentrations when a less sensitive analytical line is used. ICP-OES is used for

trace and major element analysis with a LDR of 105.

Precision

Short and long term precision are a good indication of how stable an instrument

is. Precision is usually expressed in percent relative standard deviation (%RSD).

The short term precision for ICP-MS is between 1 to 3 %RSD and the long term

precision is less than 5 %RSD. The short term precision for ICP-OES is between

0.3 – 2 %RSD and the long term precision depending on the nebulizer should be

not more than 3 %RSD. Precision for both ICP-OES and ICP-MS may be

improved by using internal standardisation. GFAAS has a short term precision of

between 0.5 – 5 % RSD, but long term precision is more a function of the number

of graphite tube firings than time [63].

Interferences

Although appropriate techniques are chosen for each application by experienced

staff, interferences still need to be addressed during method development. Specific

interference common to various techniques are shown in Table 4.2.

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Table 4.2 Summary of instrumental interferences [73,74]

Technique Type of interference Method of compensation

FAAS Ionisation Chemical Physical

Ionisation buffers Release agents or nitrous oxide-acetylene flame Dilution, matrix matching or standard addition

GFAAS Chemical, physical

Molecular absorption

Spectral

Standard temperature platform furnace conditions, matrix modifiers, standard addition

Zeeman or continuum source background correction

Zeeman background correction

ICP-OES Spectral Matrix effects Ionization

Background correction Inter elemental corrections Alternate analytical line Internal standardization Ionization buffers

ICP-MS Spectral Matrix acids Doubly charged ions Matrix effects Ionization Space charged effects Isobaric effects

Background correction Inter elemental correction Alternate analytical lines Higher resolution systems to resolve masses less than 1 amu apart Internal standard Matrix matching, sample dilutions, standard addition, isotope dilutions Matrix matching, internal standard

Analysis time and sample throughput

Analysis time and sample throughput rates are influenced by the accuracy and

precision required plus the type of instrument employed. FAAS is a simple and

fast technique with a measurement time of less than 30 s per replicate of 3

integrations. ICP-OES and ICP-MS have measurements times of about 3 min per

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replicate of 3 integrations, while GFAAS is a very tedious technique with a

measurement time of up to 5 min per replicate of 3 integrations.

General

The most commonly employed instrumental techniques in the platinum industry

are that of FAAS, GFAAS, X-ray fluorescence spectroscopy, ICP-OES and their

capabilities and analytical limitations are well known. ICP-OES would appear to

have become the most popular routine technique for inorganic multi-elemental

analysis of dissolved samples, despite requiring significant capital investment and

having running expenses which are much higher than those for FAAS.

4.5 INSTRUMENTATION FOR THE IDENTIFICATION OF

MINERALS.

4.5.1 Scanning electron microscope (SEM)

SEM uses a focused beam of high-energy electrons that generate signals which

reveal information about the sample such as external morphology, chemical

composition, crystalline structure and the orientation of the sample composition.

Principle of operation

Accelerating electrons in a SEM carrying significant kinetic energy generate

radiation signals when the electrons are decelerated in a solid sample as a result of

electron interactions. These signals include secondary electrons (which produce

SEM images), backscattered electrons (BSE), diffracted backscattered electrons

(EBSD) (which are used to determine crystal structures and the orientation of

minerals), photons (characteristic X-rays that are used for elemental analysis and

continuum X-rays), visible light and heat [76].

Many SEM installations have an energy dispersive X-ray detector system (EDS),

which allows for spectral analysis of X-rays generated from the sample directly

under the electron beam. X-ray generation is produced by inelastic collisions of

incident electrons with electrons in discrete atomic orbitals. As the excited

electrons return to lower energy states, they yield X-rays of a fixed wavelength.

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Characteristic X-rays are produced for each element in a mineral which is

"excited" by the electron beam.

Advantages and disadvantages

Some advantages of this technique include are as follows:

• Excellent characterization of solid samples.

• The electron beam can be scanned over a very small area of the sample.

• SEM in conjunction with an EDS detector allows morpholograpic,

topographic, crystallographic and compositional information to be

obtained rapidly and simultaneously from the same area.

• SEM analysis is considered to be "non-destructive", meaning there is no

volume loss of the sample, and as such it is possible to analyze the same

materials repeatedly.

Unfortunately, SEM cannot detect very light elements (H, He and Li) or

elements with an atomic number less than 11. Although SEM with an EDS

detector is fast and easy to use, it suffers from poor energy resolution and poor

sensitivity towards elements present in low abundance compared to

wavelength dispersive X-ray detectors (WDS) or electron probe micro

analyzers (EPMA).

4.5.2 Electron probe micro-analyzer (EPMA)

The EPMA is a micro-beam instrument used for the in situ non-destructive

chemical analysis of minute solid inclusions [75].

Principle of operation

An electron microprobe operates under the principle that if a solid material is

bombarded by an accelerated and focused electron beam, the incident electron

beam has sufficient energy to dislodge inner-shell electrons of the constituent

atoms in the sample to be analyzed. Outer-shell electrons fill these inner-shell

vacancies, losing energy by the emission of characteristic X-rays as in X-ray

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fluorescence. These electrons can be focused to a very fine beam through a set of

electromagnetic lenses between the electron source and the sample to be analyzed.

The X-rays generated are analysed by a crystal spectrometer with wavelength

dispersive X-ray detector. Of most common interest in the analysis of geological

materials are secondary and back-scattered electrons, which are useful for surface

imaging or obtaining an average composition of the material. The electron optical

path and X-ray spectrometer of an electron microprobe are illustrated in Figure

4.7.

 

Figure 4.7 The electron optical path and x-ray spectrometer of an electron

microprobe [68].

Advantages and disadvantages

Some advantages of this technique are as follows:

• Although similar to SEM it is equipped with a range of crystal

spectrometers which enable quantitative chemical analysis at high

sensitivity.

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• The EPMA is capable of analysing sample areas as small as 1-2 micron

diameter and even minute single phases in a material.

• Chemical analyses can be obtained in situ, which allows the detection of

small compositional variations within chemically zoned material.

• It is considered to be a “non-destructive” technique.

EPMA is also unable to detect the very light elements (H, He and Li) and as a

result cannot detect “water” in hydrous minerals. It is also known that some

elements generate x-rays with overlapping peak positions which need to be

separated. The absolute detection limit for most elements is not as good as that

achievable by x-ray fluorescence because of the presence of a continuum

spectrum.

Nevertheless, the ability of an EPMA to obtain quantitative chemical analysis on a

minute volume of sample or mineral grain ensures its continued popularity in the

study of minerals.