Exploration Geochemistry

How to Select Analytical Procedures

Detection Limits

In making analytical selections, clients should carefully consider the detection limits achievable by the various procedures. Every measurement over the entire analytical range has some degree of uncertainty associated with it. A detection limit stated by ALS has an uncertainty of +/- 100%; in other words, a detection limit of 1 ppm implies an uncertainty of 1 ppm +/- 1 ppm. Our procedures have been developed to provide the best precision across the widest concentration ranges.

If a client requires very precise data in the region of 1-5 times the detection limit of a particular method, then we recommend that an alternative procedure be selected that offers a lower detection limit.

For example, if an explorer requires very precise data in the range of 1-5 ppm for base metals, then we would recommend ICP-MS procedures that offer superior detection limits and hence better precision in the required range.

Technical Limitations

All methods have limitations to some degree. For example, an aqua regia digestion for gold is suitable for soils and sediments because the gold is expected to be largely available for chemical attack. The same cannot be said for an aqua regia digestion of rocks and drill cuttings where the gold may be retained in an insoluble quartz matrix or otherwise encapsulated. For rocks and drill cuttings, a fire assay fusion is clearly the superior method of gold analysis.

Analysis of elements other than gold also requires attention be paid to the digestion procedures and their applicability to the geology and sample matrix. Sample Decomposition and Pre-Treatment has information on a range of dissolution methods.

The various ICP-MS packages offer an attractive mix of elements and detection limits but these packages are applicable only to trace level materials. Mineralized material is not suitably analysed by this technique. We would recommend that samples with higher metal content be analyzed by other methods such as OG46, OG62 or ICP81.

Contamination Control

Everyone recognizes the need to control potential contamination during laboratory analysis and this is best accomplished by a cooperative effort between clients and laboratories. Good communication between the laboratory and the client ensures that the best analytical procedures are selected. Within the laboratory one of the best means of contamination control is to classify all samples according to their expected metal concentrations and to route them through separate analytical batch streams. ALS has always been a proponent of this system and since our inception we have encouraged clients to identify their samples as trace level (i.e. geochemical) or ore grade (i.e. assay), based on the likely upper concentration levels expected.

In order for us to produce accurate data, it is important for clients to assist us by screening out samples having elevated concentrations (i.e. >1% of an individual metal, or >3% total base metals).

Sample Decomposition and Pre-Treatment

When batches of samples have been prepared for analysis and routine quality control testing has confirmed that the samples have met or exceeded specifications, the analytical process is ready to begin. One common process is to dissolve the geological samples in hot concentrated acid(s) in order to solubilise the element(s) of interest. This process readily dissolves most base elements but in some cases, alternative methods are preferred. A chemical fusion (that is, attacking the geological matrix with a high temperature molten flux) can frequently be more effective in dissolving resistant minerals and liberating constituents. The classical fire assay for gold and precious metals analysis is an example of a chemical fusion. Pelletisation is an example of a process that does not require chemical treatment at all.

Acid dissolution

The use of acids to attack geological samples can be accomplished in many different ways. A weak acid attack is generally referred to as a "leach" and while it may result in a quantitative extraction of the element(s) of interest, most of the sample will remain undissolved. A series of leaches utilizing different acids or other chemical reagents is sometimes referred to as a "sequential extraction".

Strong acid attacks are generally referred to as "digestions" and these are more powerful than leaches. At ALS, we classify digestions as " near total" or "partial" depending whether they are capable or not of fully dissolving the element(s) of interest. Some of our more common digestions are as follows:

Aqua Regia Digestion

The standard aqua regia digestion consists of treating a geological sample with a 3:1 mixture of hydrochloric and nitric acids. Nitric acid destroys organic matter and oxidizes sulphide material. It reacts with concentrated hydrochloric acid to generate aqua regia:

3 HCl + HNO3 = 2 H2O + NOCl + Cl2

Aqua regia is an effective solvent for most base metal sulphates, sulphides, oxides and carbonates but only provides a partial digestion for most rock forming elements and elements of a refractory nature..

Nitric-Perchloric-Hydrofluoric Acid Digestion

Reverse Aqua Regia Digestion

In this digestion system, nitric acid and hydrochloric acid are combined in a 3:1 proportion, exactly the reverse of the standard aqua regia digestion. The high nitric acid content is efficient in destroying sulphides and the acid combination is still sufficiently strong to solubilize easily soluble metals. The reverse aqua regia digestion is used for the analysis of copper concentrates for example.

Hydrochloric Acid-Potassium Chlorate Digestion

The reaction of chloride and chlorate generates chlorine at a relatively low temperature, thus producing a highly oxidizing environment capable of dissolving many elements of interest and without the loss of potentially volatile elements such as arsenic and antimony.

Hydrobromic Acid-Bromine Digestion

Elemental bromine can also act as an effective oxidant for elements such as tellurium and some precious metals. An excess of bromide helps to keep the elements soluble through the formation of bromide complexes.

Selective Leaches

Selective leaches, as the name implies, are designed to extract specific elements while leaving the bulk of the geological sample intact. There are many different variants that have been developed by researchers over the years. We offer a variety of non-proprietary methods, all involving the use of ICPMS technology. Although none have become the universal standard, several of the following leaches continue to offer great appeal:

• Cold hydroxylamine: This leach dissolves manganese oxides which are extremely powerful scavengers of mobile metal ions. Iron oxides remain substantially undissolved.
• Hot Hydroxylamine This leach dissolves amorphous iron oxides but leaves crystalline iron oxides substantially undissolved.
•  Sodium pyrophosphate: This leach liberates organically-bound heavy metals. It does not attack sulfide minerals nor does it dissolve crystalline iron oxides.
• Ionic Leach: This new and innovative leach technique is designed for near surface soil samples to improve geochemical mapping and enhance the potential to detect and resolve subtle geochemical anomalies over ?blind? mineralisation. It is suitable for gold, silver, PGM, uranium and base metal exploration. This procedure employs a heavily buffered alkaline cyanide solution in conjunction with other complexing agents to selectively dissolve or solubilise metal ions that have been leached from the primary source, migrated and then redeposited near the surface.

Alkaline digestion procedures

Alkaline digestion procedures are used rarely at ALS because of the efficiency and ease of acid digestions. However one very important alkaline digestion is the cyanide leach for extractable gold. Not only is cyanide very efficient in extracting gold in an alkaline environment but it would be lethally dangerous in an acid environment due to the formation of deadly hydrogen cyanide.

Fusions

Fire assay fusion is discussed in the Precious metals section.

Sodium Peroxide Fusion

Sodium peroxide (Na2O2) is a very powerful and aggressive flux ideally suited to the attack and dissolution of high grade sulphide minerals and refractory or resistant minerals. Because sodium peroxide is such an aggressive flux, sample dissolution is complete and this results in a very high salt content in the analyte solution. Typically this solution must be significantly diluted prior to analysis and this can result in elevated detection limits for trace elements. However the procedure is ideal for major and minor elements and for the determination of certain base metals in high sulphide ores.

Lithium Borate Fusions

A lithium borate fusion is the preferred fusion for whole rock analysis (WRA) in which rock characterisation can be made through an analysis of major and minor elements. Tantalum also responds well to this fusion with follow up by ICPMS . The fusion melts froma lithium metaborate sample dissolution can be poured into disks in preparation for X-ray fluorescence (XRF) analysis or they can be dissolved in acid for subsequent ICPMS analysis.

Other Fusion Techniques

A number of individual elements require specific fusion techniques. For example, samples for fluoride analysis are fused with sodium carbonate and potassium nitrate to solubilise fluoride in an alkaline environment in order to prevent its loss by volatilisation. Samples for geochemical tungsten analysis are fused with potassium persulfate to solubilise tungsten minerals such as wolframite.

Pelletisation

Many samples to be analysed by X-Ray Fluorescence (XRF) do not require any chemical manipulation prior to analysis. Pulverised material is simply weighed into an aluminium cap with a bonding agent and pelletised with high pressure to ensure sample integrity under the vacuum and a consistent surface to receive the x-rays. Samples are then ready for irradiation.

Separation of the Element(s) of Interest

When samples have been successfully dissolved, many of the elements can then be determined directly using one or more of the spectroscopic techniques available in our laboratories. However, in some cases, it is necessary to do more chemical manipulation in order to separate the element(s) of interest. This is done for a number of reasons:

•  Chiefly to lower the detection limit of the element to a level that is useful for the Explorer.
• It may also be done in order to remove an element from a potential interference or to convert the element into a more easily measured form or oxidation state.

Solvent Extraction

Solvent extraction is the process whereby an element is extracted from its analyte solution through the use of an organic solvent, often in conjunction with a chelating agent. In this way, an element may be concentrated from a large volume into a smaller volume, thus making it easier to measure and lowering its detection limit. If the analytical technique to be used is atomic absorption spectroscopy, then there is usually an additional benefit of increased metal sensitivity due to alteration of flame characteristics caused by the presence of the organic solvent. The most common organic solvent that we use at ALS is Diisobutyl ketone (DIBK), while some of the common chelating agents that we use are:

• Trioctylphosphine oxide (TOPO)
• Aliquat-336

Several methods for gold analysis require solvent extraction.

• Aqua Regia Digestions down to 1ppb detection limit gold.
• Cyanide Leach Procedures down to 0.05ppb detection limit gold.
• Metallurgical Samples.

Volatilisation

Volatilisation can be successfully used to separate some low boiling metals or metal compounds. In the case of mercury analysis for instance, mercury can be reduced to its elemental state by reaction with stannous chloride. This elemental mercury can then be volatilised by purging with air or an inert gas such as nitrogen and swept into an absorption cell. The net effect of this separation is to produce a mercury analysis with the extremely low detection limit of 10 ppb.

Precipitation

A classical chemical procedure is the precipitation of an element of interest. This procedure can be used for elements such as barium that typically occurs in significant concentrations. It can also be used to separate picogram amounts of radioactive species such as the daughters of uranium and thorium which can be separated by precipitation with lanthanum fluoride.

Ignition

Ignition of geological samples can be used to successfully separate elements such as carbon and sulfur by converting them to their gaseous oxide forms. These oxides can then be trapped and measured by various means.

Hydride Generation

A number of elements such as arsenic, antimony, bismuth and tellurium can be reduced and separated as their volatile hydrides. It can be tricky however to determine all members of this group simultaneously by hydride generation as it is necessary to reconcile some fundamental differences within the complex chemistry of the group

For example, complete hydride reduction may not occur if an element (such as arsenic) is present in two different oxidation states. The net result is that it is frequently easier to determine these elements individually rather than as a group.

The hydride separation scheme is susceptible to some significant interferences (such as elevated levels of copper) that can prevent quantitative hydride generation. The useful working range of hydride generation is quite small and limited to low concentrations. At higher concentrations, it is usually necessary to revert to other methods of measurement.

Analytical Methodologies

At ALS we operate several full service analytical laboratories with a wide range of analytical techniques. These techniques can be divided fundamentally into spectroscopic techniques and non-spectroscopic techniques.

All elements absorb and emit radiation at specific and characteristic wavelengths. Spectroscopic techniques are used for measuring the absorption and emission of this characteristic radiation. In this way an element may be identified by its characteristic radiation and the spectroscopic technique may be used for its quantitative measurement.

Non-spectroscopic techniques utilize other measurement methods to carry out quantitative analysis; for example, an element may be isolated through basic chemical procedures and then quantified by volumetric or gravimetric methods.

Spectroscopic techniques

Atomic Absorption Spectroscopy (AAS)

In atomic absorption spectroscopy, an element in its atomic form is introduced into a light beam of appropriate wavelength causing the atom to absorb light (atomic absorption) and enter an excited state. At the same time there is a reduction in the intensity of the light beam which can be measured and directly correlated with the concentration of the elemental atomic species. This is carried out by comparing the light absorbance of the unknown sample with the light absorbance of known calibration standards.

A typical atomic absorption spectrometer consists of an appropriate light source (usually a hollow cathode lamp containing the element to be measured), an absorption path (usually a flame but occasionally an absorption cell), a monochromator (to isolate the light of appropriate wavelength) and a detector.

The most common form of atomic absorption spectroscopy is called flame atomic absorption. In this technique, a solution of the element of interest is drawn through a flame in order to generate the element in its atomic form. At the same time, light from a hollow cathode lamp is passed through the flame and atomic absorption occurs. The flame temperature can be varied by using different fuel and oxidant combinations; for example, a hotter flame is required for those elements which resist atomisation by tending to form refractory oxides.

There are alternative ways of generating the atomic species of an element which do not require the use of a flame. These "flameless" methods generally offer a superior detection limit. One of the more common flameless methods involves vapour generation of the element of interest. As described in volatilisation, mercury can be easily reduced to its elemental form and then swept into an absorption cell through which a light beam is passed. Similarly, a number of elements may be chemically converted to their volatile hydride forms and swept into an absorption cell. See Hydride Generation. A second common flameless method involves the use of a graphite furnace to electrically heat and volatilise an element of interest into an absorption cell.

Advantages of Atomic Absorption Spectroscopy

The main advantages of atomic absorption spectroscopy are as follows:

• The principles of measurement are straightforward and well understood.
• The technology is relatively inexpensive and the equipment is relatively easy to use.
• The technique is well-suited to the measurement of gold, gold pathfinders and base metals
• There are relatively few matrix and other interference effects
• Sample throughput is high as each measurement can take only seconds when the instrument is calibrated.
•  The technique is applicable over a wide range of concentrations for most elements.

Limitations of Atomic Absorption Spectroscopy

• All measurements are made following chemical dissolution of the element of interest. Therefore the measurement can only be as good as the quality of the sample digestion.
• AAS is a sequential (that is, one element at a time) analytical technique. It is better suited to the measurement of small suites of elements as larger suites become progressively uneconomic.
• Occasionally interferences from other elements or chemical species can reduce atomisation and depress absorbance, thereby reducing sensitivity.
• Some elements such as Li, Na, K, Rb and Cs ionise rather easily, again reducing atomisation and complicating the measurement technique

Inductively Coupled Plasma Emission Spectroscopy (ICP-AES)

In plasma emission spectroscopy, a sample solution is introduced into the core of an inductively coupled argon plasma (ICP) at a temperature of approximately 8000 C. At this temperature all elements become thermally excited and emit light at their characteristic wavelengths. This light is collected by the spectrometer and passes through a diffraction grating that serves to resolve the light into a spectrum of its constituent wavelengths. Within the spectrometer, this diffracted light is then collected by wavelength and amplified to yield an intensity measurement that can be converted to an elemental concentration by comparison with calibration standards. This measurement process is a form of atomic emission spectroscopy (AES).

Advantages of ICP-AES Spectroscopy

• Many elements (up to 70 in theory) can be determined simultaneously in a single sample analysis; the largest ICP only package offered by ALS includes 34 elements.
• Instrumentation is readily amenable to automation, thus enhancing accuracy, precision and throughput.
• High instrumental productivity permits very competitive pricing of analytical packages, thus giving the explorer a significant return on a relatively small expenditure.
• Electronic data capture and transfer to the LIMS ensures that no manual data transcription errors occur.
•  ICP-AES offers a useful working range over several orders of magnitude.

Limitations of ICP-AES Spectroscopy

• Complex instrumentation requires highly skilled staff both for routine operations and for repairs and maintenance.
• The emission spectra are complex and inter-element interferences are possible if the wavelength of the element of interest is very close to that of another element; for example, one of the phosphorus wavelengths suffers from both copper and aluminum interference.
• As with atomic absorption spectroscopy, the sample to be analysed must be digested prior to analysis in order to dissolve the element(s) of interest. In certain ICP packages (e.g., the ME-ICP41), a significant number of elements are only partially digested. • Rigid temperature and humidity control is required for best stability of the spectrometer.

Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)

In plasma mass spectroscopy, the inductively coupled argon plasma (ICP) is once again used as an excitation source for the elements of interest. However in contrast to plasma emission spectroscopy, the plasma in ICP-MS is used to generate ions that are then introduced to the mass spectrometer. These ions are then separated and collected according to their mass to charge ratios. The constituents of an unknown sample can then be identified and measured. ICP-MS offers extremely high sensitivity to a wide range of elements.

Advantages of ICP-MS Spectroscopy

• ICP-MS is a multielement analytical technique capable of determining an extremely wide range of elements to very low detection limits (typically sub ppb), better than those of graphite furnace atomic absorption spectroscopy.
• The technique is ideally suited for ultratrace geochemical methods such as sequential extractions and selective leaches.
•  Analytical sensitivity is sufficiently good to allow for the determination of isotopes.
• ICP-MS offers a large linear working range of several orders of magnitude.
• The technique is a useful alternative measurement method for those elements not easily measured by emission or absorption spectroscopy.

Limitations of ICP-MS Spectroscopy

• The total dissolved salt content of the analyte solution must be kept low or else instrument performance is adversely affected; this dilution can result in less attractive detection limits for some elements.
• Common matrix elements and other molecular species can interfere with the determination of some base metals; for example, chloride will interfere with a number of elements and ArCl has the same mass as As.
• Some doubly charged ionic species create difficulties.
•  Ultrapure acids are required for leaches and digestions and this will increase the cost of measurements.

X-ray Fluorescence Spectroscopy (XRF)

In X-ray fluorescence spectroscopy, a beam of electrons strikes a target (such as Mo or Au) causing the target to release a primary source of X-rays. These primary X-rays are then used to irradiate a secondary target (the sample), causing the sample to produce fluorescent (secondary) X-rays. These fluorescent X-rays are emitted with characteristic energies that can be used to identify the nucleus (i.e. element) from which they arise. The number of X-rays measured at each characteristic energy can therefore in principle be used to measure the concentration of the element from which it arises.

The fluorescent X-rays are then dispersed and sorted by wavelength using a selection of different diffraction crystals, hence the term wavelength-dispersive X-ray fluorescence. The dispersed X-rays are then detected with a thallium-doped sodium iodide detector or a flow proportional counter. Each X-ray striking the detector causes a small electrical impulse which can be amplified and measured using a computer-controlled multichannel analyzer. Samples of unknown concentration are compared with well-known international standard reference materials in order to define precise concentration levels of the unknown sample.

• A lithium borate fusion or simple pelletisation can be used to prepare both samples and calibration standards prior to measurement.

Advantages of XRF Spectroscopy

The technique is ideal for the measurement of major and minor elements and is thus a preferred technique for Iron Ore, Bauxite, and Whole Rock characterization.

•  The fusion technique minimises particle size effects that could otherwise cause problems with the measurement process.
•  Numerous trace elements can also be determined from the same fused disk, e.g. Y, Nb, Zr. The disks themselves can be stored indefinitely.

Limitations of XRF Spectroscopy

• Fluorescent X-rays can be easily absorbed by the sample itself (self-absorption). It is therefore important that the sample matrix match as closely as possible to that of the calibration standards. If this is not possible, then empirical correction factors must be applied.
• Lighter elements are not easily determined by XRF as they have inherently less sensitivity. The lower energy XRF emission from these elements means that they have less penetrating power and hence less sensitivity. ICPAES spectroscopy is the preferred technique for these lighter elements.
• Sample fusion enhances the XRF measurement technique by minimizing particle size effects but sometimes refractory minerals dissolve slowly and do not give satisfactory fusions.
• Samples high in sulphide minerals do not fuse well with lithium borate and are best analyzed using an AAS or ICP package or alternative fusion prior to the XRF procedure. See Ores and High Grade Material

Classical and Other

Many traditional chemistry techniques are non-spectroscopic in nature. These techniques rely on chemical separation of the element of interest followed by a quantification of that element by a non-spectroscopic method. These methods include volumetric or titrimetric methods, gravimetric methods and conductivity methods.

Gravimetric Methods

Gravimetric methods involve the use of balances to weigh the element of interest, either in its pure elemental form or as a chemical compound. One of the most common gravimetric determinations is that of gold and silver following a Fire Assay Fusion and cupellation. The precious metal bead that remains following cupellation is an alloy of silver and gold. Weighing this bead will give the total weight of silver and gold. If the bead is then treated with dilute nitric acid, it is possible to remove the silver quantitatively. The residual mass consists of pure gold which can then be weighed separately, thus allowing the silver to be determined by difference. The balances used for this purpose are microbalances capable of weighing to the nearest microgram (one millionth of a gram). Analysis of Bullion for gold, silver and base metal content is another common procedure.

Another common gravimetric method is the determination of barium by the chemical precipitation and weighing of barium sulfate, a highly insoluble compound. Copper in concentrate can be measured by electroplating and weighing the copper deposit.

Volumetric or Titrimetric Methods

In volumetric methods of analysis, the analyte is determined titrimetrically through a chemical reaction with a reacting species of known concentration. A knowledge of the volume and concentration of the reacting species allows the analyst to determine the total amount of analyte present. A chemical indicator is frequently used to indicate the end of the reaction by signalling that the analyte has been fully consumed. The major elements of base metal concentrates such as lead, zinc and copper can be measured in this way.

Specific Ion Electrode Methods

Specific ion electrode methods are based on the principle of measuring the potential difference that exists between a standard ion electrode and a solution of the same ion.

• The technique is most frequently used for the determination of fluoride or chloride in solution following a chemical fusion.

Induction Furnace Methods

High temperature induction furnaces can be used to rapidly pyrolyze a sample and thereby convert some common chemical species to volatile forms in order to separate and measure them. This process is useful for elements such as carbon and sulfur which can be found in a number of forms (for example, carbon can be present as carbonate, graphite or organic carbon;; sulfur can be present as sulphate, sulphide or sulphur). Induction furnace pyrolysis converts both elements quantitatively to their oxides which can then be measured by other standard volumetric or titrimetric methods.

Ultra-Trace Geochemical Analysis

Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) is an excellent technique for the measurement of many trace elements to extremely low detection limits. By combining ICP-MS with conventional atomic emission plasma spectroscopy (ICP-AES), we have designed a number of interesting analytical packages, which are particularly attractive for ultra-trace metal exploration and reconnaissance programs.

We offer three such extended sensitivity packages, denoted ME-MS41, ME-MS61 and ME-MS81. Samples submitted for any of these packages will initially be analyzed by ICP-AES to pre-screen samples to make sure that no elevated metal concentrations are present. Elevated metal concentrations (defined as >1% of an individual base metal, or >3% cumulative) cannot be introduced to a plasma mass spectrometer without causing cross contamination. Samples showing elevated metal concentrations will not be analyzed by ICP-MS but will only have ICP-AES data reported. For higher grade samples where the accompanying pathfinder elements are of critical interest, a diluted sample can be introduced to the ICP-MS machine with an accompanying loss of sensitivity.

ME-MS41 Ultra-Trace Package

In this package, elements are reported following an aqua regia digestion. As for any aqua regia digestion, many minerals will be incompletely dissolved. For a description of elements determined and their

ME-MS61 Ultra-Trace Package

Elements are reported following HF-HNO3-HClO4 digestion, HCl leach. Only the most resistive elements, such as Zircons, are incompletely dissolved using this procedure.

Trace Level Methods by ICP Analysis

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) is a popular analytical technique owing to the amount of information generated for a relatively small cost to the explorer. This section details information on our new range of packages.

ME-ICP41: Aqua Regia and ICP-AES.

Data for elements are reported giving the explorer the widest possible range of information. Even though the leach is considered "partial", in most cases it is still sufficiently strong to dissolve over half of the elements in a quantitative manner. The remaining elements are dissolved in a manner that is usually incomplete. These elements are outlined both in our Service Schedule and on our Certificates of Analysis. In addition to offering the widest range of information about elemental concentrations, the ME-ICP41 package is also the most economical of the multi-element packages, thus providing extremely good value.

This package has been designed for soils, silts, lake and stream sediment analysis. Rock characterization is better accomplished using the ME-ICP61 package outlined below.

Notes: Detection Limits

Detection limits vary from element to element. Several different factors such as analytical sensitivity of an elemental spectral line and interelement interferences can have a major effect on the detection limit offered.

Interelement Effects

The concentration values of some elements in the ME-ICP41 package are routinely corrected for interelement effects caused by spectral line overlap. Great care and attention is taken to ensure that these corrections are properly made. High Concentrations of certain major elements, such as Aluminum and Iron can interfere upon trace elements (e.g. Beryllium), depending on the analytical wavelength that has been selected. Although these interferences can usually be compensated for, in extreme cases the effect may be sufficiently great as to prevent the measurement of a small number of elements.

Evaluation of data for incompletely dissolved elements

Silicates, clays and resistant minerals are incompletely dissolved in aqua regia digestions. Elements such as Aluminum, Barium, Titanium, Sodium and Potassium will rarely be fully dissolved. Hence, data for these elements will typically not match those generated by more aggressive dissolution techniques such as total digestions or whole rock fusions.

ME-ICP41m: Elements by Aqua Regia Acid ICP-AES with Quantitative Low Detection Mercury by AAS.

The analytical sensitivity for mercury using ICP spectroscopy is adequate for some sample types, but in many cases explorers require a better sensitivity than the 1 ppm detection limit offered by conventional ICP-AES. In this package, we substitute a quantitative geochemical procedure for mercury. This procedure uses conventional cold vapour atomic absorption spectroscopy with a detection limit for Hg of 10 ppb, a one hundred fold improvement over that offered in the ME-ICP41.

ME-ICP61: Elements by HF-HNO3-HClO4 Acid Digestion, HCl Leach and ICP-AES.

This package utilises a near total digestion so that data reported for nearly all elements is considered quantitative. It is considered most appropriate for rock characterization as it includes data for all major and minor elements except silicon.

Notes: Digestion For this digestion, the acid mixture must be taken to incipient dryness. This process ensures the best possible dissolution, but also results in the loss of volatile mercury. In addition, this particular acid mixture results in the loss of silicon, an element not normally considered to be volatile.

To assist in the final dissolution of the sample residue, hydrochloric acid is added and then sample analysis is carried out in a dilute hydrochloric acid matrix.

This digestion will be "total" for most rock samples. Certain types of highly resistant minerals, for example zircons, may not be totally attacked. In these limited cases, we recommend that the Whole Rock fusion technique be used.

Re-precipitation

Certain mineral species, though capable of fully dissolving during the digestion process, are prone to re-precipitation as a result of their fundamental chemistry. Barium, even if present in relatively low concentrations, is susceptible to re-precipitation, and may co-precipitate other elements such as silver and lead. Laboratory technicians are trained to watch for this phenomenon and preventative action is taken where possible by quickly analyzing solutions following the digestion process.

ME-ICP61m: Elements by HF-HNO3-HClO4 Acid Digestion, HCL Leach and ICP-AES with Quantitative Low Detection Mercury by AAS.

As discussed in ME-ICP41m, a cold vapour mercury add-on is also available for ME-ICP61. Simply request ME-ICP61m.

FAQs

Why are my barium results by the ME-ICP61 procedure lower than those that I got by your whole rock procedure?

In ME-ICP61, samples are digested using the triple acid combination of nitric, perchloric and hydrofluoric acids with a final hydrochloric acid leach. A sample containing a significant amount of sulphides will produce sulphate ions during the digestion and this can occasionally cause re-precipitation of barium as barium sulphate, resulting in low barium recoveries. In the whole rock procedure, the samples are greatly diluted following the whole rock fusion preparation stage, and this helps prevent precipitation of barium. Similarly with the pelletisation process, barium does not precipitate, as this is a non-destructive preparation step.

Why do you have an upper limit on your ME-ICP41 package Some other labs don't have upper limits on their ICP packages?

There are a number of reasons why we adopt this approach. The main one is our insistence on contamination control by sorting samples according to expected metal concentrations and routing them through separate batch streams. In this way we can provide better service for all clients by minimizing chances of cross contamination. We prefer that samples expected to exceed our ME-ICP41 upper limits be analyzed by one of our ore and high grade assay procedures, which have been especially designed for this purpose. The digestions for these packages take place in a physically separate part of the laboratory designed for handling high grade samples. In addition, even though ICP-AES has linear calibration curves over several orders of magnitude, these curves cannot be extended indefinitely to higher concentrations. Alternatively, diluted solutions can be introduced for the ME-ICP41 procedure.

Lithogeochemistry

• Introduction
• Whole Rock Analysis by X-Ray Fluorescence
• The ME-XRF06 Basic Whole Rock Package
• Whole Rock Analysis by ICP Spectroscopy
• The ME-ICP06 Whole Rock Analysis Package by ICP Spectroscopy
• Add-on Options for Whole Rock Analysis o Basic Add-Ons
• The ME-MS81 Extended Whole Rock Add-on Package by ICP-MS
• Quality Control procedures for Whole Rock Analysis
• FAQs

Whole rock analysis (WRA) is used to identify the essential rock type of geological samples. At ALS, we offer two possible alternatives: wavelength dispersive X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). XRF has traditionally been the technique of preference for analytical chemists, although ICP-AES works equally well for the majority of samples. A description of the technique in the Analytical Methodologies section helps to explain why XRF is so well accepted for most types of geological samples.

Whole Rock Analysis by X-Ray Fluorescence

Samples for whole rock analysis by XRF are fused using a lithium borate fusion..The melt is then poured into a mould and cooled to yield a solid glass disk. The disks can then be analysed and the elements determined by comparison with standard reference materials.

The ME-XRF06 package offers thirteen major and minor elements (reported as oxides), plus loss on ignition (LOI), and the sum of the elements reported.

Whole Rock Analysis by ICP Spectroscopy

For whole rock analysis there are occasions when XRF e is inappropriate (see limitations of XRF Spectroscopy). Samples high in sulphides, or metal concentrates, are difficult to fuse with lithium borate. These samples are best fused by sodium peroxide.

The ME-ICP06 package offers the same thirteen major and minor elements, plus loss on ignition (LOI) and the sum of the elements reported, as does the ME-XRF06 package.

Add-on Options for Whole Rock Analysis

We offer basic add-on packages, which can be requested in addition to either the ME-XRF06 or ME-ICP06 packages. These packages cover: Total Carbon; Inorganic Carbon; Ferrous Iron; Moisture Water; Water of Crystallisation and Total Sulphur. The ME-MS81 Extended Whole Rock Add-on Package by ICP-MS. This package offers a comprehensive suite of 38 elements by lithium metaborate fusion.

FAQs

The sum of oxides for several of my samples is only in the range of 92-94%. Why wasn't it in the range of 100 +/- 1.5%

The whole rock analysis only sums thirteen major and minor elements plus LOI. When a sum does not reach 100 +/- 1.5%, it usually indicates that other metals are present in significant quantities; for example, base metals such as Cu, Pb, Zn; or compounds such as fluorite (CaF2). A sum value of 92-94% does not necessarily indicate that the analysis is in error.

On one of my samples, the barium by XRF was higher than by your ME-ICP61 package. Why is that?

The four acid digestion used for the ME-ICP61 package converts sulphide to sulphate as part of the digestion process. Since barium sulphate is extremely insoluble, it may have prematurely precipitated if significant amounts of either barium or sulphate were present. Alternatively, barium may have co-precipitated with another species such as lead sulphate, which is also relatively insoluble.

I asked for an XRF analysis on my copper concentrate but the Certificate of Analysis indicates it was run by ICP. Why did you switch it?

Samples such as metal concentrates and others high in sulphides do not fuse properly with lithium borates. They also cause significant damage to the platinum equipment that we use. Therefore, we switch the samples to our alternative procedure in order to give you better results.

What does the term Loss on Ignition mean How do I interpret it?

Loss on ignition refers to the weight loss experienced by a sample when it is placed in a furnace at a specific temperature and for a specific time period. It is used to give a general indication of the "volatile" species in a sample. The LOI s are carried out at a temperature of 1010 C for one hour, so the weight loss reflects all of those species that are lost at this temperature and for this time period. It would include water (both surficial moisture as well as water of crystallization) and organic carbon species. In addition, carbonates decompose to oxides with the loss of carbon dioxide and sulphates decompose (but usually only partially) to oxides with the loss of sulphur trioxide. Fluorides may also be partially lost. As a result of the complex nature of these reactions, the LOI value is best used as a general indicator of the amount of volatile species present. It cannot be used to determine the presence of individual species. Some base metals such as As will also be lost in this process.

Why do I have a negative LOI result?

For samples that are high in Iron, the LOI procedure will convert elemental Fe to Fe2O3 , thereby leading to a gain in sample mass.

Rare Earth Analysis

We have been offering trace rare earth packages for a number of years. Initially, this data was generated exclusively using neutron activation analysis (NAA). The more recent availability of inductively coupled plasma mass spectroscopy (ICP-MS) provides a better option; not only are the detection limits in general lower than those provided by NAA, but the technique itself is less prone to interference than NAA. The ME-MS81 includes all the rare earths plus uranium, thorium and yttrium.