Vortr ge
Instrumental analytical techniques in geochemistry: Requirements and applications J. P. Willis
Department of Geochemistry, University of Cape Town, Rondebosch 7700, South Africa
Instrumentelle analytische Verfahren in dcr Geochemie: Erfordernisse und Anwendungen Zusammenfassung. Die instrumentellen Techniken werden
betrachtet, die notwendig sind, um Proben aus einem aul3erordentlich grogen Bereich von irdischen und planetarischen Materialien zu analysieren. Die wichtigsten analytischen Techniken, die vom Geochemiker heutzutage benutzt werden, sind: AAS, ICP-OES, INAA, MSID und XRFS sowie die Elektronenstrahl-Mikrosonde ftir in situ-Analysen yon Mineralien. Einige Anwendungen dieser Techniken fiir die L6sung schwieriger Probleme in der Geochemie werden diskutiert. Die Bedeutung von zertifizierten Referenzmaterialien und von geochemischen Daten hoher Qualit/it wird betont. Es wird festgestellt, dab sich die allgemeine Qualit/it der Daten von Spurenelementen in den letzten 25 Jahren verbessert hat; ein direktes Ergebnis der Anwendung moderher instrumenteller Techniken. Uberraschenderweise hat sich in diesem Zeitraum die Qualit/it der Daten von bestimmten Hauptelementen verschlechtert, wenn man mit Daten vergleicht, die mit klassischen chemischen Methoden erreichbar sind. Es werden Voraussagen in Bezug auf den instrumentellen Bedarf der n/ichsten Generation von Geochemikern gemacht. Summary. Geochemists must analyse an extremely wide
range of terrestrial and planetary materials. The instrumental techniques necessary to cope with this difficult task are considered. The most important analytical techniques in use by the geochemist today are AAS, ICP-OES, INAA, MSID and XRFS, and the electron microscope for in situ mineral analysis. Some applications of these techniques to solving major problems in geochemistry are discussed. The importance of certified reference materials and of high quality geochemical data are emphasized. It is concluded that the general quality of trace element data has improved over the past 25 years, as a direct result of the application of modern instrumental techniques. Surprisingly, the quality of data reported for certain major elements has deteriorated over that time, when compared with data obtainable by classical chemical methods. Predictions are made concerning the instrumentation needs of the next generation of geochemists.
parts. In principle, geochemistry is as broad as all of chemistry and all of the earth sciences, but is both more restricted and also more extensive in scope than geology. Geochemistry deals with the distribution and migration of the chemical elements within the earth in space and in time. In order to properly understand the chemistry of the earth it is essential to know as much as possible about the chemistry and history of the sun, planets, stars and interstellar and interplanetary space [21]. Geochemistry is therefore closely inter-related with cosmochemistry. The main tasks Of geochemistry are: - the determination of the relative and absolute abundances of the elements and isotopes in the earth; - the study of the distribution and migration of the individual elements in the various parts of the earth (core, mantle, crust, hydrosphere and atmosphere) and in minerals and rocks, with the object of discovering the principles governing their distribution and migration. The methods and techniques of geochemistry are being increasingly applied in earth sciences, and it is becoming difficult to distinguish geochemistry in its traditional sense from chemical geology. Modern geological laboratories are beginning to look more and more like chemical laboratories, and the earth scientist is forced to have a much greater knowledge of chemistry, especially analytical chemistry, than was the case in the past [3]. The broad array of scientific disciplines with which geochemistry is associated are shown in Fig. 1.
CHEMISTRY
Mat~er COSMOCHBMISTRY
Energy
ATMOSPHERIC SCIENCES
485
GEOCHEMISTRY Distribution and
--
Migration of Elements
GEOCHRONOLOGY
BIOLOGY Living matter
OCEANOGRAPHY
Dating of Minerals & racks
Study of the oceans
GEOLOGY
Introduction
Geochemistry may be defined as the science concerned with the chemistry of the earth as a whole and of its component
PHYSICS
Study of the earth
Fig. 1. The association between geochemistry and related scientific
disciplines Fresenius Z Anal Chem (1986) 324:855-864 9 Springer-Verlag 1986
Geochemists are called upon to analyse a great variety of materials, including gases, dust particles, marine and fresh waters, soils, sediments, rocks, meteorites, diamonds, precious metals, all types of minerals, solid and liquid fuels, plants and biological samples. Many instrumental techniques are available, and necessary, to assist in this difficult task. Applications of instrumental techniques include largescale geochemical prospecting and mapping programmes, fundamental research on the composition and evolution of rocks and minerals, age dating and isotopic studies, materials transfer, pollution studies and speciation, and the relationship between human and animal diseases and the composition of waters, soils, plants and foods. The size and type of material to be analysed ranges from huge pieces of whole rock to the smallest single mineral grain, from tons to micrograms, for the whole periodic table of elements, and for major, minor and trace elements ranging in concentration from parts per trillion to 100%. There is no single instrumental technique that fulfils all the analytical requirements of the geochemist. The choice of an instrumental method depends on the material to be analysed, the elements to be determined and the type of analysis required. Obviously some instrumental techniques are more frequently used than others, and in this paper the advantages and limitations of only the most important techniques will be considered, together with examples of applications to current problems in geochemistry. Predictions are made as to the analytical requirements of the coming generation of geochemists. Gathering geochemical data
The geochemist requires a comprehensive collection of analytical data on the different types of material described above to achieve his goals. Many analytical techniques, both classical and modern instrumental methods, are used to obtain the necessary data. The geochemist, as an analyst, should always use the most appropriate technique for a particular analytical problem, although for any number of reasons this may not always be possible and compromise will be necessary. The great variety of materials and elements to be analysed has resulted in most analytical methods having been applied to geochemical samples at one time or another. Many modern instrumental techniques Qre restricted in their application for a number of reasons. They may: - have a limited element coverage; - be applicable over only a limited concentration range; - be subject to serious analytical difficulties, e.g. matrix effects or spectral interference; - be very expensive to operate; - have a limited availability, e.g. synchrotrons, nuclear reactors. The choice of analytical method for a particular analytical task will depend on the following criteria: - the type of research: pure or applied; - the type of sample to be analysed (solid, fluid or gaseous); - whether elemental or isotopic analyses are required; - whether bulk samples (rocks, soils, water), individual grains (minerals) or small particles (air particulates) are to be analysed; - the accuracy required. 856
The type of research, pure or applied, will determine the degree of accuracy required. For much applied research a high degree of accuracy is not essential, only relative variations in concentration being sufficient, e.g. geochemical prospecting. On the other hand, for certain types of 'pure' research only the highest possible degree of accuracy may be acceptable, e.g. age dating, where accuracies for isotopic ratios of 5 parts in 100,000 are essential.
R e q u i r e m e n t s o f t e c h n i q u e s for g e o c h e m i c a l a n a l y s i s
In an ideal, general sense, a routine analytical technique for geochemical analysis should: - have wide element coverage; - have a wide concentration range (analytical or dynamic range) [5]; - be accurate and precise; - have high sensitivity; - be capable of analysing a range of sample sizes, from single grains to grams; - be capable of analysing 4 to 6 samples per day for up to 40 elements; - be a simultaneous or near-simultaneous technique, if the method is destructive; - have trace element capability in the part per b i l l i o n (ppb) range; - preferably be capable of analysing solid samples, because many important minerals are notoriously difficult to dissolve; - be cost-effective; - be safe and easy to use by relatively unskilled personnel; - be as free as possible from matrix effects. One day we may be fortunate enough to have such a technique at our disposal, but meanwhile, with all their limitations, we must use those techniques presently available.
I n s t r u m e n t a l t e c h n i q u e s u s e d in g e o c h e m i s t r y t o d a y
The selection of the most important instrumental techniques used by geochemists today is a difficult task, and the choice must be very subjective. Techniques used by geochemists studying organic matter will not be considered. Most of this research is carried out by scientists employed by petroleum companies and is of a very applied nature. The number of geochemists engaged in research on organic as opposed to inorganic materials is small, although the importance of this field of research is growing steadily. The relatively small number of scientists involved in organic geochemistry is probably due both to the complexity and difficulties of the subject, and to the fact that most earth scientists never manage to get to grips with organic chemistry! Many different analytical techniques are used by geochemists today. Some of the instrumental techniques commonly applied in geochemistry are listed in Table 1. In my opinion the six most important instrumental methods employed by geochemists today are Atomic Absorption Spectrophotometry (AAS), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), X-ray Fluorescence Spectrometry (XRFS), Instrumental Neutron Activation Analysis (INAA), thermal source Mass 486
Vor r ge 1. Some instrumental techniques applied to geochemical samples
Table
Table
2. Comparison of instrumental analytical techniques Qualitative analysis
-
--
-
-
-
---
-
-
-
Atomic absorption spectrophotometry: flame and electrothermal Atomic fluorescence spectrometry Charged particle excitation techniques Chromatography: gas, liquid and ion Electron microprobe Fluorometric methods Gas and thermal source mass spectrometry Inductively coupled plasma mass spectrometry Inductively coupled plasma optical emission spectrometry Infrared absorption techniques Ion microprobe Laser ionisation mass spectrometry Mass spectrometric chromatography Mass spectrometric isotope dilution analysis Neutron activation analysis: instrumental and radiochemical Nuclear microprobe Potentiometric techniques Spark source mass spectrometry Spectrophotometric methods Surface analysis techniques: Auger spectrometry, X-ray photoelectron spectroscopy, secondary ion mass spectrometry (SIMS) Voltametrie/Polarographic techniques X-ray diffraction X-ray fluorescence spectrometry: conventional and particle induced
The application of many of these techniques often requires some degree ofpreconcentration or separation of selected elements, either to bring the required element into a measurable concentration range or to remove interfering elements
Spectrometric Isotope Dilution analysis (MSID), and Electron Microprobe Analysis (EMPA). AAS and ICP-OES are discussed by a number of other authors in this volume and comments in this paper will be confined mainly to the other four techniques.
Development
of
techniques
for
geochemical
Table
Speed
Sensitivity Selectivity
wide wide limited wide limited wide wide wide wide
v. poor high v. high moderate high v. high high moderate high
v. high v. high v. high high high moderate high v. high moderate
moderate moderate moderate high high moderate high v. high high
3. Comparison of instrumental analytical techniques Quantitative analysis
AAS SEQ. ICP-OES SIM. ICP-OES SEQ. XRFS SIM. XRFS EDXRFS INAA MSID EMPA
Dynamic range
Few samples
Many samples
Precision
poor good good good good good good good moderate
fast fast fast moderate fast fast moderate moderate moderate
slow moderate fast moderate fast fast moderate slow moderate
high moderate moderate high high low high v. high high
tion, of elements and isotopes in terrestrial geochemical processes. The increasing availability of sophisticated, high quality, automatic, computer- or microprocessor-controlled instrumentation was fundamental to this expansion [14]. The characteristics of some important techniques are listed in Tables 2 - 5.
analysis
The application of instrumental techniques to geochemical analysis became widespread after the second world war. The lunar exploration programme in the 1970s had a profound effect on the development of instrumental analytical techniques applicable to earth science problems in general, but especially on techniques applicable to geochemical analysis. The total mass of material brought back to earth was relatively small, and the samples released to scientists for analysis were only a small fraction of the material available. Competition a m o n g researchers for access to the lunar samples was intense, and only those laboratories producing the highest quality data obtained material for analysis. Special laboratories were established in the U S A for lunar sample analysis and techniques were developed to levels never before achieved in terms of accuracy, precision, lower limits of detection and the small aliquots of sample required for analysis. During the late 1970s and early 1980s scientists began applying techniques initially set up or developed for lunar sample analysis to terrestrial materials. The result was a rapid expansion in the production o f high quality data in all fields of geochemical research, with a corresponding increase in our knowledge o f the distribution, and especially migra-
487
AAS SEQ. ICP-OES SIM. ICP-OES SEQ. XRFS SIM. XRFS EDXRFS INAA MSID EMPA
Range
Quality
of
data
The gathering of geochemical data is generally timeconsuming and expensive, and it is essential that the data quality be good enough that analytical error does not obscure the information required. In a few applications, such as geochemical prospecting o r reconnaissance work, low quality data may be acceptable, although it is important even in these applications to know the actual levels of accuracy and precision. However, geochemists are coming to realise that for "pure" as opposed to "applied" research only data o f the highest quality are acceptable. Indeed, many geochemists consider it a waste of valuable time and money to carry out a research project unless good quality data are available or can be provided. For a geochemist, or any scientist, to do his work well, it is necessary that he should have faith in the quality of his data. For this reason many geochemists prefer to carry out their own analytical work whenever possible. Only then can they have a full knowledge of analytical difficulties that might affect the data quality. These difficulties can be taken into account when interpreting the data. Unfortunately, this approach is not always possible, or successful. G o o d quality analyses take time and require con857
Table
4. Comparison of instrumental analytical techniques Sample type
AAS SEQ. ICP-OES SIM. ICP-OES SEQ. XRFS SIM. XRFS EDXRFS INAA MSID EMPA
Table
Liquid
Solid
yes yes yes yes yes? yes yes yes no
no no no yes yes yes yes yes yes
Destructive
Method setup
Matrix effects
Spectral interference
yes yes yes no no no no? yes no
moderate difficult difficult moderate moderate moderate moderate moderate moderate
moderate 10w low high high high none low high
low high high low low high high low low
5. Cost of instrumental analytical techniques Price - $1,000 AAS SEQ, ICP-OES SIM. ICP-OES SEQ. XRFS SIM. XRFS EDXRFS INAA MSID EMPA
1 5 - 65 60-- 90 80--250 100-150 2 0 0 - 350 40-- 90 2,000 500 300
1 O0
z
m,'<.
s
;~'-~ 1.0
m m m
ta 0.1
........ 0.01
siderable care, effort and attention to detail. A geochemist m a y not have the necessary time to carry o u t his own analyses, and, even worse, m a y n o t be a g o o d analyst. As a result m u c h o f the geochemical d a t a r e p o r t e d in the literature is o f very doubtful quality. One o f the essential attributes o f a g o o d geochemist is the ability to recognize p o o r quality data.
Assessing the quality of data There are a n u m b e r o f ways in which a geochemist can check the quality o f his d a t a : analyse certified reference materials ( C R M s ) as unknowns and c o m p a r e the results with certified values; analyse for the same element by m o r e than one technique; analyse the same samples by the same technique b u t in different laboratories; take p a r t in r o u n d - r o b i n tests to c o m p a r e with other laboratories: this is n o t always successful because other laboratories m a y be even worse than y o u r own[ know w h a t concentration levels to expect in different types o f material. A knowledge o f expected concentrations comes only with considerable experience and reference to the literature. A n y a n o m a l o u s Concentrations should be regarded as being due to analytical error until proved to be correct - either by reanalysis but preferably by using a different technique.
Certified
reference
materials
(CRMs)
M o s t m o d e r n instrumental techniques (with the exception o f M S I D which is an absolute method) must be calibrated 858
,"~'I~=,~ m 9
I 0.1 wr.
'
' ; .....
I
. . . . . . . .
1.0 ~g
i
10
. . . . . . . .
lO0
CONSTITUENT
Fig. 2. The relationship between concentration and spread of analytical data on two sets of certified reference materials. A marked improvement is shown in the quality of minor element ( < 1%) determinations from the 1950s to the 1970s, but a degradation for some of the major elements. * Allende meteorite, 1970s; 9 G-l/ W-l, 1950s
using k n o w n standards. In addition, most instrumental methods are subject to analytical bias to a greater or lesser degree, and d a t a must be checked for accuracy at all times. The difficulty o f so doing is reflected in the rapidly mcreasing p r o d u c t i o n throughout the world of s t a n d a r d or certified reference materials. W i t h o u t this plentiful supply o f C R M s geochemical analysts would be in very serious difficulties. However, a w o r d o f warning must be sounded. If geochemists are not careful they m a y spend much o f their working careers doing nothing else but analysing C R M s l Also, there are now so m a n y reference materials available that the quality o f the d a t a for a number o f elements is simply not g o o d enough for geochemical purposes. The first two reference rocks, G - I and W - l , were prep a r e d in the late 1940s for the express p u r p o s e o f calibrating spectrographic techniques [2]. Since then an ever increasing number o f geochemical C R M s o f different types have been and are being prepared [1, 8]. Since the editorial policy o f most journals now prevents the publication of d a t a on C R M s , the G e o s t a n d a r d s Newsletter has become an extremely i m p o r t a n t f o r u m for the publication of such data. The magnificent efforts o f Dr. G o v i n d a r a j u and his colleagues in this regard cannot be too highly commended. The first results for G - I and W - I d e m o n s t r a t e d only too clearly (Fig. 2) the generally p o o r quality o f analytical d a t a being p r o d u c e d by the geological fraternity at that time [7].
488
Vortv ige The well-known inverse correlation between analytical error and concentration is clearly demonstrated. Following the publication of the G - I / W - I results the general quality of geochemical data did improve, especially for trace elements. Twenty years later, during 1969, material from the Allende meteorite was prepared as a CRM, both as a meteorite standard and as a ~eference material for lunar samples [11]. Data obtained on this C R M from a number of selected laboratories using different analytical techniques provided a second opportunity of testing the quality of geochemical data. The results from the Allende investigation are plotted in Fig. 2 for comparison with G - I / W - I . The figure clearly shows that the data quality had improved for lower concentrations, but had deteriorated for higher concentrations. Most of the data on Allende were obtained by instrumental techniques. The participation of many of the world's top geochemical analytical laboratories in the Apollo lunar programme provided a further opportunity of assessing the general standard of geochemical data [13]. Distressingly, the quality of the data at concentrations greater than 1% was generally even worse than for Allende (Figs. 2 - 4). However, for the common major elements at lower concentrations there was again a significant improvement compared to G-1 / W-l, with a maximum deviation (RSD) of about 10% at concentrations > 0.1%. For most trace elements (Fig. 3) the RSD varied from 6 to 40% with no correlation between the magnitude of the error and the concentration. Many laboratories received only very small quantities of the uncrushed lunar soil, samples which may not have been representative. It must be stressed that a substantial proportion of the total analytical error is probably due to sample inhomogeneity and poor sampling technique. Further, the major element data were obtained from a number of different analytical techniques, not atl of which were suited to the analysis of many of the elemems reported. Similar data from o n e laboratory for a single technique (XRFS), which is well suited to this type of analysis [15], are plotted m Fig. 4. The average relative deviation from the true values (as percentage of the given concentration) for twenty CRMs are plotted against the mean concentration for each element. Analytical data from the single technique are distinctly better than those for either G - 1 W-l, the Allende meteorite or the lunar soil. The information reported above emphasizes one of the first and most important lessons a geochemical analyst must learn m order to obtain good quality data it is essential to use an analytical technique that is suitable for the analyses required A N D that sampling errors must be eliminated as far as is possible. The second lesson, unpalatable though it may be, is to recognize that certain laboratories produce better quality data than others.
Building quality checks into analytical methods All modern instruments now produce data output in printed form. and it is unfortunate that most people accept printed data at face value without criticism. It is therefore essential that quality checks be built into software converting measured intensities to concentrations. Any experienced analyst with an intimate knowledge of a technique should be able to devise suitable quality checks. All analysts writing their own software must include as many error checks as possible. It is even more important for instrument manufacturers to do so, because their instruments are often used by in489
i00 Sulphur o
7-
0
,<
9
w o
10 84
<. 1.0
. . . . . . . .
i
0.1
. . . . . . . .
1.0 WT.
~
OR
t
. . . . . . . .
i
. . . . . . .
10 1O0 ppm CONSTITUENT
1000
Fig. 3. The precision of data obtained for major and minor elements in a single lunar soil by a number of mainly instrumental techniques [13] 9 major elements Wt. %; 9 trace elements ppm
u~ 100
1-
10
"
V:z:Z:2~,,z:~ .............
9 :-b-,..
z
1.0 a
0.1
0~01
0.1 WT.
9
1.0 CONSTITUENT
10
llXl
Fig; 4. The average relative error obtained fourmajor and minor ekemeats by XRFS analysis of 20 certifie~t reference materials (rocks). -.... Lunar soil, t970s; Allende meteorite, t97Os; - - G-t/W-I. chemical, 1950s; 9 20 Rock Standards, XRFS, 1980s
experienced analysts without the necessary expertise to recognize bad data. One example is given for XRFS. The general equation relating net peak intensity and concentration is C = k . (Ip -
Ib)'F(mac)
where C = concentration; k = s l o p e of calibration line; I p = g r o s s peak intensity; /b--background intensity; F(mac) = some function of the mass absorption coefficient. It is known that background intensity is inversely proportional to the mass absorption coefficient. Therefore, any deviation from linearity in a plot of Ib vs. F(mac) indicates an error in either one or both values and warns the analyst to take appropriate action. Plots similar to that in Fig. 5 have proved extremely valuable in our laboratory in recognizing, tracking down and preventing further analytical errors.
Some current problems in geochemistry Some of the problems presently challenging geochemists are: the compositions of stars, meteorites and interstellar dust to provide clues to the origin, formation and history of the universe and our solar system; 859
600 Interference on b a c k g r o u n d or m.a.c, t o o high
"- 500
r
20
~1500
c-
,/
q)
-
t- 400
:3
.
9~r 1200"
'lO C
3 12
900"
300
~8 o
200
M.a.c.
0
error
m
in
too
low o r background
,
i
300~ E
1O0
/
L=.
/
/
a. '
.02
i
'
.04
=
.
.06
i
.08
.1
a.
.12
Reciprocal Moss Absorpfion Coefficient
9
0
.
.
.
300
.
i
.
.
,
9
900
.
i
.
,
1500
ppm Sr rock slabs
0
~ Wt.
4
8
12
16
20
Fe203 rock slobs
Fig. 5. Correlation between background intensity and reciprocal mass absorption coefficient in the XRFS analysis or rock samples. Deviations from linearity indicate errors in either one or both measurements
Fig. 6. Comparison of data for the trace element Sr and major oxide Fe203 determined on powder briquettes and solid disks of the same rock. The ease and accuracy with which an unknown sample could be classified into one of the two groups using the disk samples is obvious
studies of the composition of planetary atmospheres and the surfaces of the moon and planets; - the age, formation, composition and chemical evolution of all parts of the earth, especially the lower and upper mantle and the lower and upper crust; the formation and movement of "hotspots", and inferences from compositional differences between "hotspot" lavas and mid-ocean ridge basalts as to the structure of the mantle; inferences on the structure and evolution of the mantle from the composition of minerals enclosed in diamonds and from kimberlitic nodules; - the study of the isotopes of rare gases to provide constraints on theories of the evolution of the earth's mantle; - sea floor spreading and the subduction of oceanic plates below the continents; the causes of mass extinctions of flora and fauna as evidenced by the fossil record; -environmental and pollution studies, including ozone, acid rain and the effects of increasing CO2 production on global temperature, climate and sea level; pathways of the elements through the weathering, transport, depositional and diagenetic cycles; the uptake and dispersion of metals by organisms; - disposal of radioactive waste; - development of new methods for geochemical exploration, particularly for ore bodies deeply buried in the crust or below sand cover.
simply corrected for or eliminated. Recent developments allow analyses of reasonable accuracy ( ~ 5 - 1 0 % ) to be carried out without standards, which is a major breakthrough. The accuracy of the 'no standards' calculations should improve even further over the next few years. In addition to being the general workhorse of the research geochemist, the technique is widely applied in geochemical prospecting and reconnaissance programmes. An unusual application, and one difficult to achieve with any other technique, is the analysis of slabs of whole rock to provide rapid, reasonably accurate major and trace element data (Fig. 6) with a minimum of sample preparation and at very low cost [6]. The results are good enough to allow the selection of samples for further more accurate analysis, either by XRFS or by another method. The data also provide a very rapid method for correctly classifying fine-grained volcanic or sedimentary rocks, an otherwise difficult and time-consuming task. XRFS is also important as a portable analytical technique, and, among many other applications, has been used for the in situ determination of gold concentrations in Witwatersrand gold mines [19].
Application
of instrumental
methods
to current
problems
in geochemistry
X-ray fluorescence spectrometry ( XRFS) XRFS is the best general purpose instrumental technique available at present for the analysis of solid geological samples. It can determine most (about 40) of the geochemically important elements at concentrations down to a few ppm, and below the ppm level in low absorption samples such as coal. Properly applied, the technique can provide both precise and accurate data relatively quickly. Although subject to matrix effects these can be readily and 860
Instrumental neutron activation analysis (INAA ) I N A A has played an extremely important role in geochemistry, specially in the analysis of meteorites and lunar samples. The main advantages of the technique are the ability to analyse very small samples for many elements at very low concentrations (ppb range for some elements), and nondestructively, if one can wait for the radioactivity to decay away! The REEs, Hf, Ta, Cs, W, Th, U and some of the platinum group metals (PGMs), which are difficult to determine by other techniques, are commonly determined by INAA. I N A A and XRFS are essentially complementary techniques, in that the weaknesses of the one technique are the strengths of the other. Used together, as they often are by many geochemists, they form a very powerful combination with which most of the required elements can be determined. A recent application of I N A A has been in the determination of Ir in sediments from the Cretaceous-Tertiary bound-
490
Ye r ge ary, in which Ir is enriched to about 30 times its normal concentration in rocks [4]. Rocks at the Cretaceous-Tertiary boundary correspond to one of the many periods in the geological record in which there was world-wide mass extinction of plant and/or animal life. The cause of the mass extinction, and of others at approximately equal intervals throughout the last 250 million years, is the subject of hot debate in the geological literature [14]. Geochemical analysis of sedimentary rocks formed during periods of extinction will assist in finding an answer to the problem.
Mass spectrometric isotope dilution (MSID) MSID is one of the most important analytical techniques applied to geochemical analysis. The main applications of the technique are for radiogenic isotope determinations and age dating. It is also used for very accurate and precise determinations and has very high sensitivity (ppb). Its main advantages are freedom from systematic errors and interference problems, and the capability of analysing extremely small quantities of sample (rag). MSID is inherently the most accurate and precise of all the techniques applied to geochemical analysis, and, being an absolute technique, does not require calibration with standards. An important application published last year was the use of MSID to date minute garnet grains encapsulated in diamonds from relatively young (90 Myr) kimberlites [18]. 2.7 to 13 mg aliquots of garnets were separated from between 300 and 600 kimberlitic diamonds and analysed for Rb-Sr and Sm-Nd concentrations and isotopic ratios. The quantities of garnet Sr and Nd determined were 2 - 3 5 ng and 1 0 - 4 5 ng respectively. In-run precisions (2 s.d.) of 0.007% for 87Sr/S6Sr and 0.005% for 143Nd/~44Nd were obtained for as little as 10 ng of each element, illustrating the extreme capabilities of the technique. The garnets were shown to have ages of 3,2003,300 Myr, suggesting that the diamonds formed very early in the earth's history and that their formation was unrelated to that of the host kimberlites. The work indicated that by 3,300 Myr the earth had already developed a solid mantle/ crust to a depth of approximately 150 km with temperatures between 9 0 0 - 1 , 1 0 0 ~ at that depth, the conditions necessary for diamond stability. These conditions are in conflict with deductions made from other work on ancient igneous crustal rocks, and have caused geochemists to reconsider theories relating to the formation of the earth's mantle and crust.
Electron microprobe analysis ( EMPA ) EMPA is the only general purpose, readily available, nondestructive technique for major and minor element analysis of minerals in situ, and its importance and contribution to the advancement of geochemistry is inestimable. Unfortunately, lower limits of detection are 5 0 - 200 p p m for many elements, which is insufficient for the determination of most trace elements. The technique has been very widely applied to a great variety of geological materials, but particularly to the analysis of minerals from the earth's mantle. One of the major problems facing geoscientists today is how best to safely and permanently store radioactive nuclear waste products. Two new minerals in the crichtonite series, Lindsleyite and Mathiasite, which were recently discovered
491
in kimberlites from South Africa, may provide a suitable means for storing radioactive waste [10]. The composition of the minerals is very complex and was determined by EMPA. The minerals are characterized by high concentrations of Ba and K, respectively. Typical EMP analysis gave the following compositions: General formula - AM2103s Lindsleyite A = Bao.62Sro.41Cao:o9Pbo.o1Ko.ovNao.o6REEo.o3 M = Til 1.63Zro.87Alo.11Cr3.s9Fe2.87Mgl.49Nbo.08 Mathiasite A = Ko.66Cao.z2Sro.15Bao.xoNao.osREEo.os M = Tit3.18Zro.62Cr2.87Fe2.28Mgl.63Cao.22Nbo.09 The new minerals are potentially important high pressure reservoirs for refractory and large-ion-lithophile elements in the earth's upper mantle. They may be fundamental to an understanding of the dynamics of fluid movements and metasomatising compositions in the upper mantle. On a more applied and practical level, because the minerals are able to host large and small radii cations that include Ti, Zr, Cr, U, Th, the alkalies and the REEs, and are also highly refractory, they may provide a means of safely storing radioactive waste at suitable burial sites [10].
Nuclear microprobe The electron microprobe is not suited to the determination of trace elements, and the development of the nuclear microprobe shows great promise for in situ trace element analysis of mineral grains [16, 20]. There have been very few geochemical applications of the technique to date, but applications will no doubt increase rapidly as the technique becomes more readily available. In a recent application the distribution of Ni, Cr, Mn and Sr in mantle xenoliths was determined using a proton microprobe [9].
Inductively coupled plasma optical emission spectrometry (ICP-OES) When a geochemist is required to analyse solutions, either natural waters or laboratory dissolutions, one of the best techniques available today is ICP-OES. The characteristics of multi-element capability, wide dynamic range, linear calibration curves and very low limits of detection give the technique significant advantages over many other methods. For the best data it is essential to use an instrument with at least 3A/mm resolution in the first order because of the 'forest' of argon lines in the spectrum. The technique is not completely free from matrix effects, particularly when dissolutions of rock samples are to be analysed. Ca band emission can cause significant interference. The technique is particularly good for elements such as the REEs and boron. While the technique has been applied to the determination of major elements in rocks with some success, great care should be exercised when applying the technique to trace element analysis of rock samples. In many rocks, almost the total concentration of a particular element may be contained in a single mineral, e.g. Zr in zircon, Cr in chromite or Sn in cassiterite. These minerals can be extremely difficult to dissolve chemically, and may also be present as very small grains. Unless great care is taken it is possible f o r minute grains of these minerals to remain undissolved and unnoticed, leading to serious analytical error. For this reason 861
Lectures fusion with a suitable flux, followed by acid d:issolution, is preferable to acid dissolution alone. The technique is still a l o n g way from being a geJaeral purpose method for the analysis of rock samples. However, for water analysis, either for pollution studies, geochemical exploration or fundamental research, the technique is excellent.
Atomic absorption spectrophotometry (AAS) AAS has for many applications been superceded by ICPOES. However, it has a distinct cost advantage over ICPOES and in general suffers from considerably less spectral interference. For rapid analysis of a single element in solution, e.g, Cu or Zn in geochemical exploration, and for the determination of very low concentrations of elements such asAs, H g a n d Cd, the technique is an excellent choice. For the analysis of samples for which only very small volumes are available, such as interstitial waters from deep-sea sediments, AAS with electrothermal atomisation is almost unbeatable. The advantages and disadvantages of the technique have long since been thoroughly worked out, and AAS continues to hold an important place in certain fields of geochemical analysis.
Ion chromatography (IC) IC is a relatively new technique that is proving extremely useful for the determination o f ionic concentrations, both anions and cations, even in highly saline solutions [17]. The deveiopment of packed hollow-fibre membranes has simplified the system considerably. It is possible to obtain complete separations of ions with only a single eluant in a few minutes. The technique can be operated easily by inexperienced analysts and is fast, sensitive and highly reproducible. Detection limits for most ions are in the range 0.1 to 1 mg 9 1-1. Preconcentration techniques may be used for lower concentrations. The main applications are naturally in water analysis, and the technique will prove extremely valuable for speciation and pollution studies. The application of this technique to the analysis o f pore waters should revolutionise our knowledge of diagenetic reactions in sediments. Other applications will be in the rapid analysis of borehole waters for geochemical prospecting programmes, and in the analysis of seawater.
Mass spectrometry~gas chromatography ( M S-GC ) Most of the ore bodies exposed at the surface have already been discovered, and the search for new ore deposits must be directed to those buried below the surface. Gases emanating from buried ore deposits can be useful guides to their location. The two most widely used techniques for analysing gases in geochemical exploration are gas chromatography and mass spectrometry [12]. The gas concentrations to be measured are in the p p b t o ppm range. Helium and hydrocarbon gases are formed when organic matter is converted to coal, oil or natural gas, and gas anomalies have been found over known deposits. Sulphide ore bodies release Hg vapour, GO2 and sulphur gases but consume oxygen, and uranium ore bodies are known to release He. Gases can be collected on absorbent materials, such as activated charcoal, or special gas probes can be 862
forced into the soil. The excellent sensitivity o f ,3~S-GC allows these techniques to be used effectively for geochemical exploration.
Mtssbauer spectrometry Many elements, elg. Fe, occur in geological samples in mo~e than one oxidation state and/or in more than one mineral form. Information on the forms in which elements are present can be vital to the correct "interpretation of geochemical data. Mtssbauer spectrometry is applied, but perhaps not frequently enough, to the determination of the percentage of Fe present in different mineral phases. The technique has been of particular importance .to coal geochemistry, where it is essential to know the proportion,.of Fe present in sulphide, carbonate, oxide or clay minerals.
Inductively coupled plasma mass spectrometry ( ICP-MS) ICP-MS is still a very new, and relatively unproven, technique but has the potential for becoming one of the most widely applied instrumental techniques in geochemistry. It has almost complete elemental coverage, very high sensitivity and the mass spectrum is much simpler than that of optical emission spectrometry. Although it has been stated that spectral overlap problems are rare, this is only true when no molecules or molecular fragments are formed in the ICP source. Although this may generally be the case, much detailed investigation remains to be done: The lower limit of detection is in the p p t to ppb range in solution. Because of the necessity for dissolution and dilution of solid samples the practical lower limits of detection in the sample are of the order of ppb to ppm. The technique has the added advantagethat the isotopic composition of elements such as Pb can be determined routinely at very low concentrations, The method has a wide linear dynamic range but ionisation suppression caused by easily ionisable elements, such as Na and K, can lead to significant errors. The major application of the technique is likely to be in rapid, multi-element trace analysis and isotopic ratio determination. It will b e particularly useful for the hydride elements and the rare earth and platinum groups of elements which, because of their very low concentrations in geological materials, are difficult to determine by techniques other than INAA. Because the sample must be in solution, problems are bound to occur with incomplete dissolution, contamination and blanks. Should the introduction of solid samples (slurries) into the ICP become feasible in the future, then the technique would be extremely attractive to the majority of geochemists who analyse solid samples.
Sampling Sampling is often the greatest source of error in geochemical analyses. If the sampling error is large, e.g. 2 0 - 3 0 % , there is little point in going to considerable effort and expense to obtain datawith accuracies of 1 - 2 % . The geochemist must ensure that his sampling techniques produce a sample that is both representative and worth analysing, sometimes a very difficult task: The application of reconnaissance techniques, as described above using XRFS, is very valuable in assisting in the selection of samples for special or more accurate analysis.
492
Voltage Future analytical requirements Without doubt one of the main requirements in analytical geochemistry in the future will be ever decreasing lower limits of detection. This will challenge both the scientists developing new techniques and the instrument manufacturers who have to produce the instruments at a cost acceptable to their customers. In particular considerable emphasis in geochemical research is shifting to the use of isotopic information, which makes the use of some type of mass spectrometer essential. ICP-MS may prove to be a suitable technique for general purpose use. The extremely low limits of detection (ppt to ppb) obtainable with ICP-MS, especially when coupled with the ability to determine isotopic ratios both within and between elements, open up many new avenues for geochemical research. Considerable development of the instrumentation is still required, but we can expect more and more of these instruments to appear in geochemical laboratories. A disadvantage of the technique, as with ICP-OES, is that the sample must be in solution. Hopefully this may change, and much research is currently in progress on the introduction of powdered samples (slurries) into the plasma. Meanwhile, geochemists desperately need a technique for analysing solid samples that has capabilites similar to ICP-MS. The necessity for dissolving solid samples leads to errors due to incomplete dissolution (a serious problem for many minerals), contamination and blank problems, and the need for very expensive high purity reagents. One of the major gaps in the geochemist's analytical armoury at the present time is the lack of a suitable technique for the accurate and precise trace element analysis of small areas ( < 5 microns diameter) in situ. An instrument is needed for trace element analysis that is as easy to use as the EMPA and SEM-EDXRFS are for major element analysis and which is non-destructive, in contrast to SIMS. Nuclear microprobe techniques show some promise in this field, but not every laboratory has a reactor or accelerator available. Perhaps one valuable outcome of the so-called "star wars" research programme will be a tunable X-ray laser that can be used as the excitation device in an X-ray microprobe. Such an instrument would provide the capability of selecting the most suitable energy for exciting elements of interest, without the very high backgrounds associated with electron excitation in the electron microprobe. The technique would also be non-destructive and have lower limits of detection in the 1 - 1 0 ppm range for a wide range of elements. In the meanwhile, nuclear microprobes will find increasing applications in geochemical analysis. Many other techniques will be developed during the remainder of this century, and the next 15 years promise to be even more exciting for the analytical geochemist than those gone by. They will also pose major challenges in terms of teaching and training future geochemists. Experience has shown that the most valuable geochemical research, and often the best quality data, comes from those scientists who develop or apply new methods and/or carry out their own analytical work. There is no greater incentive to an analyst than the desire for data with which to pursue his personal scientific interests. It is in these laboratories that future geochemical analysts must be taught and trained. Geochemists must be careful not to be so blinded by the wonders of modern instrumentation as to ignore the 493
traditional tools of the earth scientist. Such low technology tools as the geological hammer, magnifying glass and optical microscope remain as important and fundamental tools of the modern geochemist as they were for the early geoscientists. Modern instrumental techniques must be used together with, not to the exclusion of, the traditional tools if geoscience is to continue to extend its recent advances.
Conclusions The geochemist is fortunate to have a considerable array of instrumental techniques available with which to analyse very varied and complex terrestrial and planetary materials. The most important techniques applied to geochemical materials analysis at present are atomic emission spectrometry, inductively coupled plasma optical emission spectrometry, neutron activation analysis, mass spectrometric isotope dilution analysis, X-ray fluorescence spectrometry and electron microprobe analysis. These techniques are applied to both bulk sample analysis and to the analysis of individual mineral grains, and a number of applications to current problems in geochemistry have been discussed. The importance of certified reference materials for the calibration of instrumental methods, and for checking and maintaining the quality of geochemical data, has been emphasized. The importance of producing only the highest quality data for "pure" geochemical research was highlighted. The quality of trace element data has improved over the last 25 years, and the improvement is a direct result of the introduction of modern instrumental techniques during the 1950s. In contrast, the precision and accuracy of the determination of certain major elements has improved little, if at all, over those levels attainable during the 1950s. The geochemist's future requirements will be for lower and lower detection limits and a technique that can analyse milligram to gram aliquots of solid sample for up to 40 elements routinely. There is also an urgent need for a simple, routine method for in situ trace element analysis in mineral grains. At present the only suitable method appears to be the nuclear microprobe, which at this stage is definitely not routine. Hopefully, the development of an X-ray laser microprobe might solve this problem. There is certainly no shortage of problems challenging both the geochemist and the analyst interested in geological samples. It will require the concerted efforts of both disciplines, acting in the closest cooperation, to develop new analytical techniques that will allow the already rapid advance of geochemical knowledge to continue into the 21st century. Acknowledgements. I wish to thank the Organizing Committee for inviting me to present this paper at the Colloquium Spectroscopicum Internationale XXIV and for financial assistance towards the cost of attending. I am also grateful to the Universtity of Cape Town for granting me leave and to the CSIR Foundation for Research Development (FRD) for generous financial assistance. Mr. R. Buchart and Mr. C. Basson assisted with the preparation of the diagrams.
References t. Abbey S (1983) Geol Surv Can Paper 83:15 2. Ahrens LH (1977) Geostandards Newslett t :157-- 161 3. Allegre CJ (1985) Chem Geol 48:1 863
4. Alvarez LW, Alvarez W, Asaro F, Michel HV (1980) Science 208:1095-1108 5. Butler LRP (1983) spectrochim Acta 38B:913-919 6. Duncan AR, Erlank AJ, Betton PJ (1984) Special Publ Geol Soc S Aft 13 : (in press) 7. Fairbairn HW, Schlecht WG, Stevens RE, Dennen WH, Ahrens LH, Chayes F (1951) U.S.G.S. Bull 980 8. Flanagan FJ (1970) Geochim Cosmochim Acta 34:121 -125 9. Fraser DG, Watt F, Grime GW, Takacs J (1984) Nature 312:352-354 10. Haggerty SE, Smyth JR, Erlank AJ, Rickard RS, Danchin RV (1983) Am Mineral 68:494- 505 11. Jarosewich E, Clarke RS, Barrows JN (1985) The Allende meteorite reference sample. Natl Mus Nat Hist, Smithsonian Inst., Washington 12. McCarthy JH (1985) Proc 2nd Int Syrup Anal Chem Expl Min Proc Mat Pretoria (in press) 13. Morrison GH (1971) Anal Chem 43 : 22 A - 31 A
864
14. Nicolaysen LO (1985) S Aft J Sci 81 : 120-- 132 15. Norrish K, Chappe11 BW (1977) In: Zussman J (ed) Physical methods in determinative mineralogy. Academic Press, New York 16. Pierce TB (1984) J Trace Microprobe Technol 2: 291 - 318 17. Pohlandt C (1984) Nucl Act Jul: 3 3 - 3 6 18. Richardson SH, Gurney JJ, Erlank AJ, Harris JW (1984) Nature 310:198--202 19. Rolle R (1985) Proc 2nd Int Syrup Anal Chem Exp Min Proc Mat Pretoria (in press) 20. Schweikert EA, Filpus-Luyckx PE, Thomas JP, Fallavier M (1984) J Trace Microprobe Technol 2:319-339 21. Winchester JM (1972) The encyclopedia of geochemistry and environmental sciences. IVA, Dowden, Hutchinson and Ross, Stroudsburg Received October 28, 1985
494