Dunn Colin E. Biogeochemistry in Mineral Exploration

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Table 7-III shows the typical detection levels available by ICP-MS from a com-

mercial laboratory, and the mean and standard de viation of the data from 513

determinations of V6 (dry) interspersed among exploration-oriented biogeochemical

samples over a two-year period.

The most severe lim itations of ICP-MS are polyat omic interferences on the el-

emental signals, originating from argon and/or the sample matrix. W ith these lim-

itations in mind, instrument manufacturers such as Thermo Electron Corporation

and VG Elemental, UK, have addressed this problem and now offer analysis by

magnetic-sector high-resolution ICP-MS (HR-ICP-MS) and multi-collector ICP-MS

(MC ICP-MS). Although such sophistication is not a requirement for the exploration

biogeochemists, this separation of analyte ions from spectral interferences is a pre-

requisite for those who require and can afford this extra level of accurate and precise

elemental analysis. Furthermore, the HR-IC P-MS provides lower detection levels for

a number of elements in dry plant tissues that typically are close to or below the levels

of detection by quadrupole ICP-MS – notably Be, Bi, In, Re, Sb, Se, Te, Tl, U, V, W,

some of the REEs and all of the PGEs.

Thermo Electron Corporation manufactures the Finnigan ELEMENT2

HR-

ICP-MS and its recent successor the ELEMENT XR

for even greater sensitivity.

To quote from the manufacturer’s website www.thermo.com (3 March 2006), fea-

tures of this instrumentation are as follows.



Multi-element analysis across the periodic table covering a milligram per litre to

sub-picogram per litre concentration range – compatible with inorganic and or-

ganic solution matrices and solids.



High mass resolution to access spectrally interfered isotopes – produces unam-

biguous elemental spectra.



A multi-elemental detector for transient signals – for example, CE (capi llary elect-

rophoresis), HPLC (high performance liquid chromatography), GC (gas chroma-

tography), FFF (ﬁeld ﬂow fractionation) and laser ablation.



High precision isotope ratios – on non-interfered or interfered isotopes.



Fully automated tuning and an alysis – in conjunction with a comprehensive, cus-

tomizable quality-control system.



Reliability and robustness to serve as a 24/7-production control tool – highest

sample throughput.



Highest ﬂexibility and accessibility to serve as an advanced research tool.

Through the combination of a single Faraday collector with the SEM, the linear

dynamic range of the Finnigan ELEMENT XR can be increased by an additional

three orders of magnitude, when compared to the Finnigan ELEMENT2, to over

Multi-collector ICP-MS instrumentation is not routinely applied to plant anal-

ysis, and falls into the realm of the advanced research instrumentation that was

mentioned in the opening paragraph to this chapter. It is, therefore, not yet a viable

option for biogeochemical exploration.

202

Plant Analysis

TABLE 7-III

Multiple analyses of control V6 (dry) by ICP-MS after a nitric acid/aqua regia digestion

Detection limit

n ¼ 513

Mean

Standard

deviation

Ag (ppb)

2.6

Al (%)

0.01

0.05

0.01

As (ppm)

0.1

0.6

0.2

Au (ppb)

0.2

0.7

0.4

B (ppm)

Ba (ppm)

0.1

0.8

Bi (ppm)

0.02

0.01

Ca (%)

0.01

0.75

0.04

Cd (ppm)

0.01

0.24

0.02

Co (ppm)

0.01

0.41

0.04

Cr (ppm)

0.05

4.4

0.3

Cs (ppm)

0.005

0.03

0.003

Cu (ppm)

0.01

8.3

1.2

Fe (%)

0.001

0.08

0.005

Ga (ppm)

0.1

0.15

0.05

Hg (ppb)

3.8

K (%)

0.01

0.09

0.007

La (ppm)

0.01

0.9

0.08

Mg (%)

0.001

0.13

0.01

Mn (ppm)

2.6

Mo (ppm)

0.01

0.3

0.02

0.001

0.01

0.002

Ni (ppm)

0.1

3.5

3.4

0.001

0.05

0.003

Pb (ppm)

0.01

0.9

0.01

0.06

0.019

Sb (ppm)

0.02

0.06

0.01

Sc (ppm)

0.1

0.3

0.08

Se (ppm)

0.1

0.2

0.06

Sr (ppm)

0.5

2.1

Te (ppm)

0.02

0.02

Th (ppm)

0.01

0.1

0.02

Ti (ppm)

1.9

Tl (ppm)

0.02

0.02

U (ppm)

0.01

0.06

0.01

V (ppm)

1.3

1.8

W (ppm)

0.1

0.10

Zn (ppm)

0.1

2.7

203

Biogeochemistry in Mineral Exploration

Instrumental neutron activation analysis (INAA)

During the 1980s and 1990s, prior to the widespread availability of ICP-MS,

INAA was perhaps the best technique for the biogeochemist. Among its numerous

advantages are



multi-element capability,



high selectivity and sensitivity,



relative freedom from interferences, and



the ability to analyse samples of varying weights directly without the need of

digestion procedures.

Commercial processing of samples is simple and highly automated. The usual

procedure is to activate samples by irradiation with neutrons in a reactor to produce

radioactive isotopes by neutron capture-type reactions. In the case of dry vegetation,

dry milled material is compressed into a pellet using a conventional XRF briquetting

press. The amount of material used is commonly 15 g, but both 8 g and 30 g bri-

quettes are sometimes selected. Although the data obtained on each size are quite

comparable, obviously the 30 g briquette is more representative of the original sam-

ple (basically the equivalent of one assay tonne), but the 8 g sample, which is slightly

cheaper to analyse, provides data that are perfectly adequate for exploration pur-

poses. Even a small vial containing only 2 g of tissue provides comparable and

consistent data, but with slightly poorer precision.

In the case of plant tissue that has been reduced to ash, the ash is packed into a

small polyethylene vial, and then capped and heat-sealed. An optimal amount of ash

has proved to be 0.25 g, although similar results are obtained with 0.5 g samples.

Under normal instrumental settings, analytical precision decreases quite substantially

when samples smaller than 0.25 g are irradiated. Up to 30 of the briquettes or 120

small vials at a time can be stacked and irradiated.

The instrumentation required to measure a gamma spectrum comprises three

parts:



The detector, invariably a Ge (Li) crystal which becomes ionized by the gamma

photons thereby generating an electronic signal;



electronic ampliﬁcation; and



a multi-channel analyser to sort and store detected pulses.

After irradiation, samples are left for a 5–7-day decay period before the simul-

taneous determination of up to 36 elements (Table 7-IV). This decay, or ‘cooling’

period as it is sometimes called, is required to allow reduction in unwanted sh ort-

lived radioisotope activity (e.g., Na). Speciﬁc isotopes are identiﬁed by their char-

acteristic gamma-ray energy or energies, normally in the range 60–1600 keV and they

are quantiﬁe d by measurement of the peak area. Calibration is carried out using one

or more SRMs that is irradiated and counted under identical conditions.

204

Plant Analysis

TABLE 7-IV

Detection limits in humus, dry vegetation and vegetation ash analysed by INA (from Hoff-

man, 1992)

Humus Dry Ash

Au (ppb) 1 0.1 5

Ag (ppm) 2 0.2 2

As (ppm) 1 0.01 0.5

Ba (ppm) 100 5 10

Br (ppm) 1 0.01 1

Ca (%) 0.01 0.01 0.2

Co (ppm) 1 0.1 1

Cr (ppm) 1 0.3 1

Cs (ppm) 0.5 0.05 0.5

Fe (%) 0.05 0.005 0.05

Hf (ppm) 0.5 0.05 0.5

Hg (ppm) 0.5 0.05 1

Ir (ppb) 5 0.1 2

K (%) 0.01 0.05

Mo (ppm) 0.5 0.05 2

Na (ppm) 100 1 10

Ni (ppm) 10 2 50

Rb (ppm) 20 1 5

Sb (ppm) 0.1 0.005 0.1

Sc (ppm) 0.1 0.01 0.1

Se (ppm) 2 0.1 2

Sr (ppm) 100 10 300

Ta (ppm) 0.5 0.05 0.5

Th (ppm) 0.5 0.1 0.1

U (ppm) 0.1 0.01 0.1

W (ppm) 1 0.05 1

Zn (ppm) 20 2 20

REEs

La (ppm) 0.1 0.01 0.1

Ce (ppm) 1 0.1 3

Nd (ppm) 3 0.3 5

Sm (ppm) 0.1 0.001 0.1

Eu (ppm) 0.2 0.05 0.01

Tb (ppm) 0.2 0.01 0.5

Yb (ppm) 0.1 0.005 0.05

Lu (ppm) 0.1 0.001 0.05

205Biogeochemistry in Mineral Exploration

A separate suite of short-lived isotopes can be measured using epithermal neu-

trons. These include Al, Cl, Cu, I, In, Mg, Mn, Ti and V. Hoffman (1992) and Hall

(1995) both provide detailed accounts of INAA in geo-analysis addressing biogeo-

chemical applications using dry tissue and ash. Table 7-IV shows the various de-

tection levels typically obtained for humus, dry vegetation and vegetation that has

been reduced to ash by controlled ignition.

Table 7-V provides an example of the long-term reproducibility of the data ob-

tained by INAA. The data represent results compiled over several years for 272

portions of control V6 (ash), and the average and standard deviation are calculated.

Also, data for 19 samples of dry V6 (15 g pellets) were compiled and the same

parameters calculated.

INAA is particularly sensitive for As, Co, Cr, Cs, Hf, Ir, Sb, Sc, Ta, Th, U and

most of the REEs. It is especially suitable for vegetation, as dry tissue is highly

concentrated in such elements as C, N, H and O, which create a very low-background

spectrum, and hence cause few interfer ences.

Whereas there are many advantages to using INAA for determining the trace

element content of vegetation samples, the method does have some limitations. For

example, when samples have a high U content the correction becomes sufﬁciently

large that data for some elements either cannot be reported (e.g., Mo) or have very

high-detection limits. Other potential problems include improper ﬂux monitoring

and maintaining consistent sample-to-detector geometry. These last items are tech-

nical problems that the analyst must address. However, it is as well for geochemists

to be aware of these problems, because it assists in carefully evaluating the data

quality. On occasion, data have been released that appear to be incorrect by a factor

of ﬁve. On questioning the laboratory that provided the data it was found from their

investigations that the detector had been set at an incorrect distance from the sample

that was being measured – i.e., too far away, so fewer gamma rays were detected. Of

course, by inserting appropriate standard samples in the sequence, problems such as

this can be identiﬁed and readily resolved. The issue of inserting ‘blind’ (i.e., un-

identiﬁed) SRMs throughout a sequence of samples cannot be too highly stressed. As

will be mentioned time and again in this book, the need to accurately assess data

quality is of paramount importance when conducting a geochemical survey –

whether using rocks, soils, sediments, waters, or vegetation.

From the exploration biogeochemist’s point of view, the drawback of INAA is

that not all of the elements of potential interest can be obtained by this method,

because they are either impossible or difﬁcult to determine.



If an exploration programme is oriented toward base metals, several key elements

are missing from the suite available from INAA – notably Cd, Cu and Pb.



Nickel detection limits are high (see Table 7-IV).



The only platinum group element determined by direct irradiation is iridium. It is

highly unusual for Ir to be present in dry tissue above the detection limit of 0.1 ppb

Ir. Commonly, values reported at or above the detection limit have proved to be

206

Plant Analysis

TABLE 7-V

Analysis of control V6 by INAA using 0.5 g of ash, and 15 g pellets of milled dry tissue

Ash (n ¼ 272) Dry (n ¼ 19) Ash (n ¼ 272) Dry (n ¼ 19)

Mean Standard deviation Mean Standard deviation Mean Standard deviation Mean Standard deviation

Au (ppb) 17 7 0.83 0.4 Sb (ppm) 1.2 0.11 0.07 0.01

Ag (ppm) o2– o0.2 – Sc (ppm) 4.5 0.31 0.28 0.02

As (ppm) 7.6 0.8 0.51 0.09 Se (ppm) 2 – o0.1 –

Ba (ppm) 404 37 25 4.4 Sr (ppm) 948 198 31 13

Br (ppm) 12 3.0 2.6 0.1 Ta (ppm) o0.5 – o0.05 –

Ca (%) 15.83 1.21 0.94 0.08 Th (ppm) 3 0.34 0.23 0.05

Co (ppm) 8.7 1.0 0.6 0.05 U (ppm) 1.3 0.26 0.06 0.02

Cr (ppm) 68 5.3 4.6 0.5 W (ppm) o1– o0.05 –

Cs (ppm) 1.2 0.6 0.05 0.05 Zn (ppm) 773 67 37 3.7

Fe (%) 1.76 0.13 0.11 0.008 REEs

Hf (ppm) 5.2 0.5 0.3 0.03 La (ppm) 21 1.4 1.1 0.05

Hg (ppm) o1– o0.05 – Ce (ppm) 42 3.4 2.2 0.16

Ir (ppb) o2– o0.1 – Nd (ppm) 22 2.6 0.8 0.35

K (%) 3.65 0.6 0.23 0.04 Sm (ppm) 3.1 0.2 0.15 0.009

Mo (ppm) 4.9 1.6 0.3 0.13 Eu (ppm) 0.8 0.10 0.03 0.006

Na (ppm) 11305 662 542 19 Tb (ppm) o0.5 – o0.01 –

Ni (ppm) o50 – o2 – Yb (ppm) 1.7 0.18 0.10 0.012

Rb (ppm) 45 7 2 1.8 Lu (ppm) 0.3 0.06 0.01 0.002

207Biogeochemistry in Mineral Exploration

unrepeatable, or are a result of a missed correction. INAA is a wonderful tech-

nique for PGEs that have ﬁrst been separated from plant tissue by either chemical

procedures or NiS ﬁre assay, but the former is time-consuming and the latter

requires a lot of material and is a costly enterprise.



Whereas INAA is perhaps the best technique for Au in vegetation, some of its

pathﬁnder elements are not determined – e.g., Bi, Te, Tl – and detection limits for

others (e.g., Ag, Hg and Se) are a little too high.



Rhenium and Li each have their place in biogeochemical exploration and data for

these elements are not provided by INAA.

No analytical technique is perfect for all elements and so compromises need to be

made. A significant advantage of INAA is that it is a non-destructive technique. As a

result, if a doubtful or particularly interesting analysis is received, the same sample

can be re-analysed to ensure that it is not an analytical problem. Analytical costs are

low, and the precision (Table 7-V) and accuracy are excellent for most elements that

can be readily determined.

X-ray ﬂuorescence (XRF)

The Swedish Geological Survey has used XRF extensively for the analysis of

their ‘biogeochemical samples’ (stream bank organic material comprised largely of

roots). However, nowadays with other techniques available, although it remains a

viable approach for some elements, it is not the optimum method for vegetation

analysis. Advantages are that analysis is non-destructive, rapid, only a weak X-ray

source is required and there are few spectral line interferences. Wilson (1998) indi-

cates that results from the XRF analysis of plant tissue compare favourably with

data obtained by AAS and ICP-ES. However, without special concentration pro-

cedures, the sensitivity for most elemen ts is inferior to that obtained by INAA or

ICP-MS. XRF is of particular use for its very precise measurement of the major

elements; for Sr, Rb; and for high ﬁeld strength elements (HFSE) – Hf, Nb, P, Ti, Y

and Zr. It is not practical to determine element concentrations in ash, because 5 g of

ash is a usual requirement, and to obtain that amount of ash requires a large original

sample – about 100 g of dry foliage, 200 g of twigs or almost 2 kg of conifer trunk

wood.

Very much smaller samples are required for Total Reﬂection XRF (TXRF). In

this technique X-rays are guided to impinge on the surface of a sample at a glancing

angle (only 41) such that total reﬂection occurs. The X-rays excite atoms in the top

layers of the material and the ﬂuorescence is detected by a Si (Li) detector placed

above the sample. The sampl e is applied as a thin layer of a dried solution or a slurry

and sensitivity is vastly improved with detection limits of a few ppb, but it requires

very ﬂat samples. The X-ray source can be an X-ray tube or a synchrotron. An early

study of this technique claimed success in determining element distribution patterns

in wood, bark, needles and ﬁne roots of healthy and diseased spruce (Berneike et al.,

208

Plant Analysis

1987). The consumption of only mL amounts of solution (digestate) by TXRF was an

important advantage in the analysis of single ﬁne roots. Results by both ICP-ES and

TXRF compared extremely well for Ca, K, Fe, Mn, Sr, Ba, S, Zn and Cu. Alu-

minium, Mg, Na and P required detection by ICP-ES, while Cd and Pb demanded

the low-level detection capability of TXRF.

The new generation of high-resolution XRF instruments (e.g., the Axios [PAN-

analytical B.V.]) promises to determine sub-ppm levels of trace elements in vege-

tation. During 2004, PANanalytical introduced a new range of Wavelength

Dispersive XRF (WDXRF) spectrometers that included the Axios-Advanced – a

system with an enhanced capability for measuring the range of elements commonly

analysed in soils. In order to overcome speciﬁc problems associated with determining

low elemental concentrations, the Axios system is compatible with PANalytical’s

Pro-Trace module that permits quantiﬁcation down to sub-ppm levels.

SUMMARY

There are many excellent texts that provide summary tables showing the advan-

tages and disadvantages of the principal analytical techniques used in the analysis of

plants, and all provide comments on which elements are best determined by a par-

ticular method (e.g., Markert, 1994; Hall, 1992, 1995; Ayrault, 2005). This is valuable

information for those seeking to selec t the optimal method for determining a par-

ticular element in plant tissue.

Table 7-VI is a modiﬁcation of that prepared by Hall (1995), updated to include

trends and advances over the past decade, now that ICP-MS has become a more

accessible technique and costs are so modest. It does not include the most advanced

and expensive techniques that are at present rarely available outside of academic and

government institutions.

It is quite remarkable how much valuable and precise analytical information can

be obtained from the commercial laboratories for so little cost – typically well under

$1 per element determination by HR-ICP-MS and substantially less for the more

common quadrupole ICP-MS. Furthermore, the extreme sensitivity of the instru-

mentation has, at no a dditional cost, permitted insight into concentrations and dis-

tribution patterns of elements that were typically nearly always reported as ‘less than’

values in vegetation – notably Be, Bi, In, Re, Te and Tl, most of which can be

insightful pathﬁnder elements. This now leaves the biogeochemists of the 21st cen-

tury in the enviable situation of being able to gain new fundamental information on

the multi-element distribut ions of ultra-low concentrations in large numbers of veg-

etation samples. There is now a new frontier of insight on elements in plants and their

relationships to the underlying substrate.

Furthermore, as far as the exploration biogeochemist is concerned, the complex

world of analytical techniques and methods has been greatly simpliﬁed, now that

ICP-MS covers most of the basic requirements. Of course, many suitable controls on

209

Biogeochemistry in Mineral Exploration

TABLE 7-VI

Advantages and limitations of the principal techniques applied to the analysis of vegetation

Technique Advantages Disadvantages Best elements

ICP-MS High Sensitivity Decomposition required >50 elements from a

single digestion

Multi-element Destructive Good for many ultra-

trace elements

Low detection limits Matrix interferences

Few spectral

interferences

HR ICP-MS Extremely high

sensitivity

Decomposition required Most elements from a

single digestion

No matrix

interferences

Destructive Only effective method

for getting low level

PGEs and the halogens

(excluding F) in

vegetation

Higher cost of analysis

than ICP-MS

Limited availability

ICP-ES Multi-element Decomposition required Al, B, Ba, Ca, Cd, Co,

Cu, Fe, K, La, Mn, Mo,

Na, Ni, P, Pb, S, Sr, Ti,

V, Y, Zn

Few matrix Destructive

interferences Spectral interferences Satisfactory for Ag, Bi

and Li, but they are

rarely above dl in dry

vegetation

Low cost

AAS Few interferences Decomposition required Same as for ICP-ES, but

not B, P, S. Good for Hg

by cold-vapour, but not

superior to ICP-MS.

GFAAS greatly

enhances sensitivity, but

at extra cost

Robust and rugged Destructive

One element at a time,

therefore high cost for

many elements

Small linear dynamic

range (major and trace

Continued

210 Plant Analysis

precision and accuracy should be inserted along with the ‘ﬁeld’ samples, and intel-

ligent and informed interpretation is required. The geochemist should also have some

appreciation of the data quality that can reasonably be expected – which leads now

to Chapter 9 that deals with ‘real-world’ situations for the individual elements.

TABLE 7-VI Continued

Technique Advantages Disadvantages Best elements

elements cannot be

determined from same

solution)

INAA Direct analysis of dry

material (non-

destructive)

Nuclear reactor required Ag, Au, As, Ba, Br, Ca,

Co, Sr, Cs, Fe, Hf, K,

Mo, Na, Rb, Sb, Sc, Sr,

Ta, Th, U, W, Zn

Multi-element Decay period cannot be

rushed, so potentially

slower than by above

methods

The REEs La, Ce, Nd,

Sm, Eu, Tb, Yb, Lu

usually included with

above from a single

irradiation. High

sensitivity for Ir, but Ir

rarely above dl. in dry

tissue or ash unless near

significant PGE

mineralization

High precision and

accuracy

Separate irradiation

required for short lived

isotopes (Al, Cl, Cu, I,

In, Mg, Mn, Ti, V)

Few matrix

interferences

Poor or impossible for

the base metals Cd, Cu,

Ni, Pb

Low cost

XRF Direct analysis of dry

material (non-

destructive)

Matrix interferences Major elements and Ba,

Cl, Cr, Fe, Mn, Nb, S, P,

Rb, Sr, Th, Ti, Y and Zr

Multi-element (Na-U,

but lighter elements

with HR-XRF

[Axios])

Fusion required to avoid

matrix effects for some

major elements

High precision and

accuracy

Relatively large sample

required for pressed

pellets

High cost of fused disc

211Biogeochemistry in Mineral Exploration