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Isotope dilution mass spectrometry
International Journal o f Mass Spectrometry and Ion Processes, 118/119 (1992) 575-592
575
Elsevier S o e n c e Publishers B.V., A m s t e r d a m
Isotope dilution mass spectrometry* Klaus G. Heumann Instttut J~r Anorgamsche Chemw der Umversttat Regensburg, Umversttdtsstrasse 31, W-8400 Regensburg (Germany) (Recewed 26 August 1991)
ABSTRACT In the past isotope dduUon mass spectrometry (IDMS) has usually been apphed using the formation of posltwe thermal ions of metals. Especially m cahbratmg other analytical methods and for the certification of standard reference materials this type of IDMS became a routine method. Today, the progress m this field hes m the determination of ultra trace amounts of elements, e.g of heavy metals m Antarctic ~ce and m aerosols m remote areas down to the sub-pg g-~ and sub-pg m -3 levels respectively, m the analysis of uramum and thormm at concentrations of a few pg g-t m sputter targets for the production of mtcroelectromc devices or m the determination of sub-plcogram amounts of 23°Th m corals for geochemical age determmattons and of 226Ra m rock samples. During the last few years negatwe thermal iomzation IDMS has become a frequently used method The determination of very small amounts of selenmm and technetmm as well as of other transmon metals such as vanadmm, chrommm, molybdenum and tungsten are important examples m this field Also the measurement of sdlcon m connectmn wsth a re-determmatmn of Avogadro's number and osmium analyses for geological age determmatmns by the Re/Os method are of specml mterest Inductwely-coupled plasma mass spectrometry ts increasingly being used for multi-element analyses by the ~sotope dtluUon techmque Determmatmns of heavy metals in samples of marine origin are representatwe examples for this type of mulU-element analysis by IDMS Gas chromatography-mass spectrometry systems have also been successfully apphed after chelation of metals (for example Pt determmatlon in chmcal samples) or for the determination of volatile element speoes in the environment, e.g &methyl sulfide However, IDMS---especlally at low concentration levels m the enwronment--seemshkely to be one of the most powerful analytical methods for specnatlon in the future Th~s has been shown, up to now, for species of Iodine, selenmm and some heavy metals m aquatic systems.
INTRODUCTION
Accurate analytical results are becoming more and more an unalterable precondition for further progress in high technology and in many fields of natural sciences. The knowledge of accurate trace amounts of elements and elemental compounds is also an important topic in environmental analytical chemistry. However, even if one obtains reproducible results in trace analysis * Paper presented at the 12th International Mass Spectrometry Conference, A m s t e r d a m , The Netherlands, 26-30 August 1991. 0168-1176/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
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K G Heumann/Int. J Mass Spectrom. Ion Processes 118/119 (1992) 575-592
they are not necessarily accurate as well. Therefore, reliable methods are required, which can be used for the determination of elements even at very low concentration levels and which can also be used for calibrating other analytical methods. Isotope dilution mass spectrometry (IDMS) is a method of proven high accuracy, which means that it is a technique for which the sources of systematic error are understood and controlled. There are a number of recent papers which deal with the contribution of IDMS to accuracy in trace analysis and with its application for reference measurements [1,2]. Today, the isotope dilution technique is applied in the determination of element traces and organic substances as well. The determination of organic compounds in biomedical and clinical chemistry by IDMS is an especially established method for diagnostic purposes. This part of IDMS is reviewed in the same issue of this journal by De Leenheer [3]. This article, however, revmws selected important topics in IDMS for element traces and their species which have been published during the last 3-4 years. For a more detailed description of the fundamental principles o f l D M S of elements and their applications in the past see the reviews in refs. 4 and 5. ISOTOPE DILUTION TECHNIQUE
The principles of IDMS are illustrated by the schematic mass spectrum of an element with two stable isotopes at masses m~ and m 2. An exactly known quantity of a spike isotope, normally in solution form, which is enriched in the isotope with minor natural abundance, is added to the sample. Afterwards, the sample and the spike isotopes must be equilibrated, which means that decomposition of solid samples must be carried out. After the sample and spike isotopes have been completely mixed, loss of substance during the following isolation procedures normally has no effect on the analytical result. This non-quantitative isolation is one of the main advantages of IDMS, especially for trace analyses at very low concentration levels. Another positive feature of this method is the use of a stable (or long-lived radioactive) isotope as a spike, which is the most ideal form of an internal standardization in analytical chemistry. The element to be determined must have at least two stable or long-lived radioactive isotopes. This is valid for all elements except the monoisotopic elements F, Na, P, Sc, As, Y, Rh, Pr, Tb, Ho, Tm, Au and some of the radioactive elements. In Fig. 1 the measured isotope ratio R of the isotope-diluted sample is given by the following equation: R = / z = N s x h ~ + N s p ×h~p /1 N s x h~+Nsp × h~p
(1)
K G. Heumann/lnt. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
•
577
SampLe
~] Splke
I
Nsp" hsp #-
c
/ / / / l J- J-t
2
Nsp" hsp
_ __
1
Ns • h s
/ / / / -_/_/j
Ns" hs2
/fV', i
m2
ml
m/z
=
F i g 1. T h e p r i n c i p l e o f I D M S i l l u s t r a t e d b y a s c h e m a t i c m a s s s p e c t r u m o f a n e l e m e n t wnth t w o i s o t o p e s o f m a s s e s m I a n d m2.
where N is the number of atoms, h is the isotope abundance, S is the sample and Sp the spike. Nsp, hsp, and hs are normally known so that after transformation of eqn. 1 the content Gs of the element to be determined in the sample is obtained by Gs = 1.66 x 10 -'8 x Wss x Nsp
x h ~ - ~ss// [pgg-']
(2)
where M is the atomic weight of the element and Ws the sample weight in grams. IONIZATION
METHODS
USED FOR IDMS
The ionization methods used for element analyses in IDMS can be divided into two groups: mono- and oligo-element methods on the one hand, and multi-element methods on the other. Thermal ionization mass spectrometry in the positive (PTI-MS) as well as in the negative ion mode (NTI-MS) has been the most frequently applied method, up to now, for element trace analyses with the isotope dilution technique. Due to the ionization process, metals can be analysed with the PTI mode, whereas many non-metals, semi-metals and transition metals or their oxides are able to form negative thermal ions. Electron impact mass spectrometry (El-MS) is still the preferred method for IDMS of noble gases and other volatile compounds. This is the reason why gas chromatography-mass spectrometry (GC-MS) systems are often combined with the isotope dilution technique. Spark source mass spectrometry (SSMS) is not an expanding analytical method at the moment. However, excellent investigations with SSMS using the isotope dilution technique have been carried out in the past by Jochum
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K G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
TABLE 1 Recent applications m the determination of metal traces at extremely low concentration levels w~th PTI-IDMS Topic
Analysed element
Ref.
Aerosols in remote areas Antarctic ice
Pb, Rb, Ba Heavy metals Heavy metals
Pure chemicals Microelectromcs Geological samples
Heavy metals U, Th, heavy metals Ra
Rosman et al. [15] V61kenmg and Heumann [16] Boutron and G6rlach [17] V61kenmg and Heumann [18] Nakamura et al [19] Herzner and Heumann [20] Volpe et al. [21]
et al. [6] and Paulsen et al. [7]. Inductively-coupled plasma mass spectrometry (ICP-MS) is an ideal method for applying IDMS because normally a solution is introduced into the plasma torch, which enables a simple equilibration with the spike solution used. However, multi-element analyses can also be carried out with this technique, which is a good precondition for making ICP-MS one of the dominant methods for IDMS in the future. DETERMINATION OF METAL TRACES AT EXTREMELY LOW CONCENTRATION LEVELS WITH PTI-IDMS
IDMS has been routinely used for many years in geochronology for age determinations [8] and in nuclear technology for accurate determinations of uranium and transuranium elements [9]. Recently, IDMS has also played an important role in the certification of standard reference materials [10-14]. However, an important trend using PTI-IDMS is in the determination of metal traces at extremely low concentration levels. A selection of different applications in this field is listed in Table 1. It was possible using PTI-IDMS to determine, for example, Pb, Ni, Cu, Cd, and TI in aerosols down to the lowest pg m -3 range in the remote areas of the South Atlantic [16] and to determine Pb, Rb, and Ba in the atmosphere over the Pacific Ocean [15]. The concentrations analysed for heavy metals in Antarctic ice normally lie below 10 pg g-~ and very often the concentration of elements such as T1, Cd, and Pb is < 1 pgg-t [17,18]. If contamination is under control, it is possible to obtain relatively accurate results by IDMS even at this extremely low concentration level. The heavy metal concentration of highly purified chemicals (HC1, HNO 3, etc.) is in the same concentration range [19]. The use of highly purified chemicals is very important in the production processes of microelectronic devices. Refractory metals of high purity (mainly molybdenum and tungsten) as well as their silicides have been increasingly
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
579
L
L5 O'
0
IDMS
SIMS
SSMS
GDMS
GDMS
(MR)
(LR)
Fig. 2. Comparison of results (/~gg- t) obtained by different mass spectrometricmethods for a tungsten sample [23]. used for gate electrodes, interconnections and diffusion barriers in integrated circuits as a result of recent developments in higher density very large scale integrated (VLSI) systems. Besides the radioactive elements U and Th other heavy metals affect the reliability of integrated circuits. It is assumed that the concentration in materials for the VLSI systems should not exceed 1 ng g- ~for U and Th and 10 ng g-~ for the heavy metals [22]. For the ultra large scale integrated (ULSI) systems (64 Mbit chip), which are now under development, even better purities must be reached. This means that reliable analytical methods, which are able to exactly characterize the trace impurities in the primary materials such as refractory metals, aluminium, quartz, etc., must also be available. Figure 2 shows a comparison of results obtained by different mass spectrometric methods for a tungsten sample [23]. The results for U, Th, and Fe differ widely. Also, analyses with glow discharge mass spectrometers (GDMS) in the high (HR) and low resolution (LR) mode do not agree very well for U and Th. Therefore, a procedure for the determination of U, Th and other heavy metals with PTI-IDMS was developed for a more accurate characterization of trace impurities in these primary materials for chip production. The results are given in the first column of Fig. 2. The relative precision of PTI-IDMS for U and Th was about 2%, which was much better than for the other mass spectrometric methods. The detection limits for U, Th and a number of other elements in refractory metals and their silicides with PTI-IDMS are listed in Table 2. All detection limits are below the suggested limiting values for VLSI systems and should also fit the needs of ULSI systems, especially for the critical elements U and Th. The higher detection limits for Fe and Ca are due
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TABLE 2 Detection limits of trace elements m refractory metals and their slllcldes with P T I - I D M S [20,24] Element Detection hmit (ng g - l )
U 0.001
Th 0.008
Cd 0.08
Cr 0.5
N1 0.6
Cu 0.8
Fe 5
Ca 10
to the fact that the blank is the limiting factor in the analysis of these ubiquitous elements. For elements, where contamination during the analytical process is not a general problem, concentrations down to the fg g-1 level (10-15 g g - l ) can be determined with PTI-IDMS owing to the extremely high sensitivity of the thermal ionization technique. An excellent example of such an analysis is the determination of 226Ra traces in different rock samples carried out by Volpe et al. [21]. In this case an enriched 228Ra spike (half-life only 5.75 years) was used for the isotope dilution procedure. 226Ra in rocks is formed by the radioactive decay of natural uranium. The results for three different rocks are presented in Table 3. The analytical uncertainty associated with the isotope ratio measurement was better than 1.5%. Also, the two independent analyses of each sample agreed very well even at the low fg g-l level. A P P L I C A T I O N O F N T I - I D M S IN E L E M E N T T R A C E ANALYSIS
NTI-IDMS has been used more and more during the last few years for element trace analyses. Singly-charged atomic ions M - or element oxide ions MO~- are usually formed by many elements, especially the non-metals, the semi-metals and the transition metals. Owing to the physical background of this ionization process the electron affinity of the atom or compound to be ionized should be high (at least 1.8 eV) and the electron work function of the filament material should be low ( < 4 eV) [4,5]. The electron work function of the material can be lowered by chemical additions to the filament surface (e.g. BaO, La203) or by using substances such as LaB 6 for ionization [4,11]. TABLE 3 Determination of femtogram amounts of 226Ra in geological samples with PTI-IDMS [21] Sample
Analysis no.
Concentration of 226Ra (fg g ~)
Volcamc rock
1 2 1 2 1 2
(1.063 + (1.068 + 69 1 + 67 8 + 14.4 + 13.9 +
Mldocean basalt 1 Mldocean basalt 2
0 010) 0 O11) 08 0.9 02 0.2
x
10 3
× 10 3
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
ki
Be
B
C
N
({¢{i
No Mg
AI
Co Sc
Ti
V
Cr
Mn
Cu
Zn
Tc IRu ~Rh ~Pd Ag
Cd
' l / I ,
Rb Sr
Y
Fe
Co
,11~
•I ~:]
Ne
Si
P
S
Cl
Ar
Se
Br
Kr
Te
I
Xe
IIIII
Ga Ge As
r I///~
Zr INb I Mo
Cs Ba La Hf Ta
Ni
F
(.H
l l l l l ,
K
0
581
In
Sn
Sb
t(¢{l¢
W [Re lOs
Ir
Pt Au Hg TI Pb Bi
Po At Rn
Procedure for isotope ratio determmotlon devetoped
Negative thermal ions
detected
Fig 3. Elements that can be measured as M - or MO)- ions by NTI-MS (elements with a developed procedure for isotope ratio determinations are apphcable for IDMS).
All elements for which IDMS procedures with the NTI technique have so far been developed are marked in Fig. 3. The recently developed techniques in this field are for Si [25], S [26], V [27], Cr [28], Mo [29], Sb [30], W [31], Os [32,33], Ir [34] and Pt [35]. Pt- ions are used for measurements of platinum, MO2 ions for Si, Sb, and Ir and MO3 ions for V, Cr, Mo, W, and Os. The main advantages of NTI-MS are the selective ionization method with only a few interferences and the relatively high ion currents obtained. A selection of recent applications with NTI-IDMS is listed in Table 4. The determination of femtogram amounts of technetium by Rokop et al. [37] shows that extremely low concentrations can also be determined with NTI-IDMS if there is no blank problem. A 97Tc spike solution was analysed using a 99Tc standard reference solution. Concentrations of I fg or less of the TABLE 4 Recent applications of NTI-IDMS Topic
Analysed element
Ref
Biological samples Meteorites Enwronmental samples
I I, Br, CI Mo W Cr B Se S~ Tc
Gramhch and Murphy [11] Heumann et al. [36] K6ppe and Heumann [29] Heumann et al. [26] Rottmann and Heumann [28] Lamberty et al. [10] Heumann and co-workers [13,14] Renner et al. [25] Rokop et ai. [37]
Rye grass Food, sediments Spike reference matenal Standard solution
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long-lived isotopes 97Tc, 98Tc, and 99Tc can be analysed using NTI-IDMS. This is possible because high ion currents of TcO4 are produced with the NTI technique from pertechnetate salts, which was first shown by Heumann and co-workers [4,38,39]. The advantage of NTI-MS compared with PTI-MS is that the most abundant technetium ion is TcO4, whereas molybdenum, which is always present in filament materials as an impurity, forms only MoO3 ions. Owing to both the interference in PTI-MS between molybdenum and technetium isotopes, and the high ionization potential of technetium, the detection limit with PTI-IDMS is about 1 pgTc [40]. Therefore, the application of NTI-IDMS instead of the positive thermal ionization mode is a great step forward, which also makes it possible to detect 99Tc contaminants in the environment and to measure the flux of high-energy solar neutrinos over the past several million years in technetium samples isolated from molybdenite ores [41]. NEW CLOCKS IN MASS SPECTROMETRY FOR GEOLOGICAL DATING
Sensitive thermal ionization techniques have enabled the application of two new age determination methods for geological samples with mass spectrometry. 1. The uranium series disequilibrium method, which is based on the radioactive decay of z38U: 2 3 8 U - ~ . . . . . 234U--~ 23°Th--L-~ 226Ra ~ , . . . . .
2.
(3)
The Re/Os method with the following decay of the J87Re isotope with a natural abundance of 62.6%:
187Re 'b'7~ 187Os (stable)
(4)
In the case of the first mentioned method a member of the series given in eqn. 3 must have been separated by a geological or geochemical process. Afterwards, the decay of this nuclide or the production of the daughter nuclide can be followed. Age determinations include isotope ratio measurements and quantitative determinations of the corresponding nuclides by means of IDMS [8]. Calcium carbonate deposited in oceans contains appreciable concentrations of uranium but is virtually free of thorium. The increase of the thorium isotope ionium (230Th) in such a calcium carbonate formation or even in corals is a function of time and, therefore, can be used as a geological clock. Previous datings with the uranium series disequilibrium methods were carried out by 0c-counting of the corresponding nuclides. Isotope ratio measurements of very small amounts of 23°Th ( > 108 atoms of 23°Th are measurable) by PTI-MS and the use of 229Th and 233U spike solutions for isotope dilution makes this dating much more precise than the 0~-counting method. The
583
K G. Heumann/lnt..I. Mass Spectrom. Ion Processes 118/119 (1992) 575-592 varloble Foroday
\
~ ~ - ~ "
.........
I
aperture
counter dece[erotlon
lenses
SEM /
-
center ~
quodrupole mass h t t e r
/~magnet m, lm'
cups
sht
I I I I ~.~
ion source
Fig. 4. Schematicdiagram of a thermal iomzation mass spectrometerwith enhanced abundance sensitiv]ty [44]. relative precision is in the range of about 1%, which is a factor of 5-10 better than by o-counting. The time range for this dating method is from a few years up to about one million years. For example, Edwards e t a | . [42] were able to determine the precise timing of the last interglacial period by PTI-MS measurements of 23°Th in corals to be (122-130) x 103 years. Recent applications of geological dating with the U/Th disequilibrium method require the measurement of 23oTh/232Th ratios in the range 10- s_ 10- 6 [43]. For such an extreme isotope ratio the abundance sensitivity of a normal thermal ionization mass spectrometer is not adequate. Therefore, new types of tandem mass analyser systems have to be developed for enhanced abundance sensitivity. One instrument of this type has been designed by Laue and Habfast [44]. The schematic diagram of this mass spectrometer is shown in Fig. 4. An extended geometry 90 ° magnetic sector field instrument (MAT 262, Finnigan) was coupled with a quadrupole mass filter. The interface between the two mass analysers is a decelerating immersion lens and a beam-shaping static quadrupole lens. The system is used together with a conventional multicollector array and with an ion counter system. The abundance sensitivity of this equipment is at least 10 -g, which allows satisfactory measurements even for extreme 23°Th/232Th ratios. Recent experiments have shown that the system presented in Fig. 4 could also be run without any quadrupole filter using only the deceleration lenses. The Re/Os method is especially suitable for the dating of sulfide minerals and iron meteorites, because other geochronometers are not applicable to these materials. Moreover, the Re/Os couple can be used to study the differentiation of the Earth's mantle and the growth of the continental crust [8]. The
584
1
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
lf*
tt~
12 03
p.O
10
08"
f
Re/Osage=1415±0 161x109o
•('STos/'ar0s ), t~
6
8
1'0
187Re/la60s Fig. 5. Re/Os dating of two iron meteorites (Canyon Diablo and Tocopilla) with SIMS and RIMS [45,46]. application of the Re/Os method has been retarded by analytical difficulties which have arisen from the low osmium concentrations in silicate minerals and the lack of sensitive and precise isotope ratio measurements. However, osmium rich materials, e.g. iron meteorites, could be dated in the past by SIMS [45] and also by resonance ionization mass spectrometry (RIMS) [46]. The results of the analyses of two different iron meteorites (Canyon Diablo and Tocopilla) by SIMS and RIMS are shown in Fig. 5. Using a half-life of 4.23 × 109 years for IS7Re the Re/Os age was determined to be (4.15 + 0.16) x 109 years with an initial ratio of (87Os/186Os), = 0.8119 _ 0.0094. In 1989 Wachsmann and Heumann [47] showed for the first time that osmium isotope ratios can be measured by NTI-MS. This technique was later improved by Vrlkening et al. [32] and by Creaser et al. [34]. Recently, the most precise and most sensitive NTI technique for osmium isotope ratio determinations has been developed at the University of Regensburg by the introduction of oxygen or freon into the ion source during the measuring procedure [33]. In Table 5 the isotope ratios with the corresponding relative TABLE 5 Progress in osmium isotope ratio deterrninatlons by NTI-MS Technique RIMS NTI-MS
Isotope ratio
RSD (%)
Ref.
187OS/186Os
190Os/ 192Os
32 1.3
Walker and Fassett [48]
'STOs/192Os 188Os/192Os 189Os/192Os
0.07 0.01 0.004
Walczyk et al. [33]
K.G Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
585
TABLE 6 I C P - I D M S of trace elements m a lobster standard reference material (LUTS-1) [54] Element
NI Cu Zn Sr Cd Hg Pb
Concentration (pg g- 1) ICP-IDMS
Certified value
0 235 15.9 12,4 2.24 2.18 0.017 0.010
0 200 15.9 12.4 2.46 2 12 0.0167 0 010
_ 0.011 + 02 _+ 0.2 + 0.04 + 0.04 + 0 002 + 0.001
___0.034 ___ 1.2 + 0.8 __+0.28 + 0.15 + 0 0022 + 0 002
standard deviations (RSD) obtained by RIMS [48] and NTI-MS [33] are listed. This comparison shows the progress in osmium isotope ratio measurements between the RIMS method for Re/Os dating and NTI-MS. By the improvement of NTI-MS it should be possible to widely expand applications of the Re/Os method. M U L T I - E L E M E N T A N A L Y S E S BY I C P - I D M S
Even if a few publications also describe the determination of a single element by ICP-IDMS, e.g. of antimony in high purity copper [49] or of boron in saline waters [50], the main advantage of this method lies in the multielement analysis. This was formerly suggested in 1983 by Douglas et al. [51] because spiking of solutions, which is the usual form of samples introduced into the ICP-MS, is easy. Thus, the time-consuming chemical separation of elements, which is necessary for thermal ionization IDMS determinations, can usually be avoided. While the precision of isotope ratios measured by the thermal ionization technique is better than ICP-MS data by one order of magnitude or more, the precision of isotope ratios determined by ICP-MS is usually adequate for trace element determinations. In cases where the amount of element added by the spike drastically exceeds the amount of this element in the sample owing to low sample concentrations, the precision of the isotope ratio measurement can be the limiting factor in IDMS. Under these conditions the precision of the ICP-MS method is often inadequate. With respect to the simple sample introduction for liquids it is of consequence that one of the first analyses by ICP-IDMS was carried out with water samples [52]. However, the isotope dilution technique has also been applied to multi-element analyses in solid samples, e.g. from marine origin, by McLaren and co-workers [53,54]. Table 6 summarizes the results of seven trace elements in a lobster hepatopancreas sample. A comparison of the
586
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ICP-IDMS results with the certified values of this standard reference material (LUTS-1) shows excellent agreement. The precision of the ICP-IDMS data is always better than the uncertainty of the certified values. The main advantages of the discussed application of ICP-IDMS in a biological material were the following. 1. Only microwave digestion with a mixture of HNO3/H202 and no other sample preparation was necessary. 2. A simultaneous determination of elements with a concentration difference of more than three orders of magnitude (Cu and Pb) was possible using an instrumental feature which permits a reduction of the sensitivity for selected elements while retaining full sensitivity for the other elements. 3. The simultaneous determination of mercury and other elements which is often a great analytical problem because of partial loss of mercury during the digestion steps for other elements was possible. In the case of this analysis the isotopic equilibration between spike and sample was achieved before possible mercury losses. 4. The total time for the ICP-MS measurement of each solution is only about 5 min. 5. Precise and accurate data can be attained for elements, which are not influenced by isobaric interferences. As a result of these advantages, ICP-MS will become one of the dominant methods for IDMS in the future. However, there are also disadvantages in ICP-IDMS, for example the fact that the samples should be dissolved for the isotope dilution step whereas electrothermal evaporation or laser systems allow the direct introduction of solid samples into the plasma torch. Owing to interferences and cross-contamination problems the number of spike solutions is certainly limited. The use of a multi-element spike is limited by the different concentration levels needed for various samples. Mostly, the accuracy for elements affected by interference is relatively poor and in the case of extremely low concentrations the precision of the isotope ratio measurement for ICP-MS (relative standard deviations are typically above 0.1%) can be the limiting factor of the analysis. CHELATION OF ELEMENTS COMBINED WITH GC-IDMS A special application of IDMS is the chelation of the element to be determined followed by a gas chromatographic separation and the measurement of isotopic peaks of the chelate in a normal "organic" mass spectrometer using electron impact for ionization. One important precondition for this technique is the formation of volatile chelates, which must be thermally stable. This type of IDMS analysis may be of special interest to clinical laboratories because there is a growing accessibility of GC-MS systems in this area.
K.G. Heumann/lnt. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592 CF3-CH2
.S
S.
~N--C// "'"Pt / CF3 -CH 2/
\s /
587
/ CH2- CF3
\C--N
",,"S~
\ CH2-CF 3
Fig. 6. Platinum complex of the chelating agent bls(tnfluoroethyl)dithiocarbamate used for GC-IDMS [59].
However, accurate analytical results for toxic and essential trace elements are increasingly important for diagnostic purposes. Reamer and Veillon [55] were among the first to use the GC-IDMS technique for the determination of selenium traces in food samples. With respect to the above mentioned interest in medicine, Aggarwal and co-workers [56-59] have developed GC-IDMS methods for the determination of the heavy metals Ni, Cr, Cu, and Pt in urine using lithium bis(trifluoroethyl)dithiocarbamate as the chelating agent. Figure 6 shows the platinum complex of this chelating agent. When the NIST freeze-dried urine reference material SRM 2670 was analysed with GC-IDMS, a platinum concentration of (125 + 6)ngm1-1 was found [59]. This agreed well with the recommended value of 120 ng Pt ml- i. SPECIES ANALYSES
In the introductory part of this review it was pointed out that the species analysis of the element carbon, which more generally means the analysis of organic compounds, by IDMS will be discussed separately [3]. A review, which deals with IDMS of other element species, has recently been published [60]. It is not possible to directly determine element species with the ionization methods normally used in inorganic mass spectrometry, that is with thermal ionization, SSMS, and ICP-MS. These ionization methods preferably produce atomic or oxide ions independent of the chemical form of the sample compound. For example, iodide and iodate samples form the same I- ions in NTI-MS. Prior to the mass spectrometric measurement a complete separation of the various species must, therefore, be carried out. Additionally, the following preconditions must be fulfilled. 1. An isotopically enriched spike in the same chemical form as the species to be determined must be available. Otherwise the species must be converted into the chemical form of the spike after its quantitative separation. 2. No isotopic exchange between the different element species is allowed until the species have been completely separated from each other. It is, however, not necessary to isolate the particular species quantitatively after the isotope dilution process has taken place. It has been possible to analyse the following species, so far, with NTI-IDMS: nitrite and nitrate in food samples [61], selenite, selenate and organoselenium
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(including trimethyl selenonium ions) [62,63], iodide, iodate and organoiodine compounds in aquatic systems [64]. With PTI-MS G6tz and Heumann [65] have been able, for the first time, to differentiate between chromium complexes with humic substances, which are kinetically inert for isotopic exchange, and those where the spike and sample compounds are in equilibrium in an aqueous system. Volatile compounds can also be determined by the isotope dilution technique with a GC-MS system if the species to be determined is available in form of a labeled spike compound, which is stable enough for the gas chromatographic separation. A recently published example of this type of analysis is the determination of ppt levels of dimethyl sulfide (DMS) in the atmosphere using deuterated d6-DMS as spike [66]. The importance attached to the analysis of low DMS levels is due to the fact that this compound has become a key component in the transfer of sulfur from biogenic sources in the ocean to the atmosphere, where it can affect climatic processes through its oxidation to SO2. Using anion chromatographic separation methods it is possible to differentiate four iodine species in natural aquatic systems: iodide, iodate and two organoiodine compounds, one of them elutable with a sodium nitrate solution from a column filled with an anion exchange resin (elution of this anionic organoiodine compound between iodate and iodide) and another organoiodine compound with high molecular weight non-elutable under the conditions described. Figure 7 represents the very different distribution pattern of these iodine species in a moorland lake sample compared with a river water sample analysed with NTI-IDMS [64]. Whereas in the moorland lake water only the organoiodine species could be detected during two different samplings, in the river water all four iodine species were analysed with comparable concentrations. It must be pointed out that this iodine speciation was carried out with NTI-IDMS at concentration levels of mostly less than 1/tgl -~. Obtaining accurate iodine determinations at this low level is a well-known problem when applying other analytical methods, even if only the total content has to be analysed. This example evidently shows the high sensitivity and accuracy of NTI-IDMS. ICP-MS coupled with chromatographic methods is increasingly applied for speciation [67,68] because of the great advantage of its use as an on-line system. Beauchemin et al. [69] applied ICP-IDMS to determine methyl mercury in a marine standard reference material. Because of the relatively simple on-line coupling for most of the chromatographic methods, the fast analysis times and the accurate results by the isotope dilution technique, ICP-IDMS will certainly become one of the most powerful methods for speciation in the future.
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
589
7
1
Moorland lake
/ . 12187
_J ..,,,
4/88 o
¢b 0 u
River w a t e r "10 0
2-
Samphng date
~ iodide
iodate
12
onmonlc non-elutable orgnno-I orgono- I
Fig. 7. Distribution pattern of iodine speciesm a moorland lake and a river water sample [64]
FUTURE TRENDS
The necessity of accurate analytical results even at extremely low concentration levels becomes more and more obvious in the different fields of high technology, in natural sciences and for environmental protection. IDMS will, therefore, increasingly be used at least as a calibration and control method to guarantee reliable analytical results. With respect to this the following topics will play an important role in the future when using IDMS for elements. 1. Calibration of other analytical methods and the certification of standard reference materials. 2. Trace element determinations at extremely low concentration levels, including radionuclides with half-lifes down to a few years. 3. Accurate multi-element analyses using ICP-MS. 4. Accurate routine analyses in medicine for diagnostic purposes. 5. Application of the Re/Os dating method using NTI-MS. 6. Species analyses by on-line coupling of ICP-MS with different chromatographic methods.
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REFERENCES l P De Bi6vre, Fresenius' J. Anal. Chem., 337 (1990) 766 2 P. De Bi6vre, J. Savory, A. Lamberty and G. Savory, Fresenius' J. Anal. Chem., 332 (1988) 718. 3 A. De Leenheer, Int. J. Mass Spectrom. Ion Processes, 113/114 (1992) in press 4 K.G. Heumann, in F. Adams, R. Gijbels and R. van Grieken (Eds.), Inorganic Mass Spectrometry, Wiley, New York, 1988, p. 301. 5 K G. Heumann, in H. Gfinzler, R. Borsdorf, W. Fresemus, W. Huber, H. Kelker, I L/iderwald, G. T61g and H. Wisser (Eds.), Analytiker-Taschenbuch, Vol. 9, SpringerVerlag, Heidelberg, 1990, p. 191. 6 K P. Jochum, H.M Seufert, S. Medinet-Best, E. Rettmann, U. Schbnberger and M. Zlmmer, Fresenlus' Z Anal. Chem., 331 (1988) 104. 7 P.J. Paulsen, R Alvarez and C.W. Mueller, Anal. Chem., 42 (1970) 673. 8 G Faure, Principles of Isotope Geology, 2nd edn., Wiley, New York, 1986. 9 W. Beyrich, W. Golly, G. Spannagel, P. De Bi6vre and W. Wolters, The IDA-80 Measurement Evaluation Programme on Mass Spectrometric Isotope Dilution Analysis of Uramum and Plutonium, Vol. l: Design and Results, Report KfK 3760, EUR 7990e, Kernforschungszentrum Karlsruhe, 1984. 10 A. Lamberty, V. Holland, A. Verbruggen, F. Hendrickx and P. De Bi6vre, Fresemus' Z. Anal. Chem., 332 (1988) 645. I l J.W. Gramhch and T.J. Murphy, J. Res. Natl. Inst. Stand. Technol., 94 (1989) 215. 12 J. Volkenlng and K.G. Heumann, Fresemus' Z. Anal. Chem., 335 (1989) 478. 13 K.G. Heumann and N. R/idlein, Fresenius' Z. Anal. Chem., 335 (1989) 751. 14 K.G. Heumann and M Wachsmann, Fresenius' J. Anal Chem., 335 (1991) 755. 15 K.J.R. Rosman, C.C. Patterson and D.M. Settle, J. Geophys. Res., 95D (1990) 3687. 16 J V61kenmg and K.G. Heumann, J. Geophys. Res., 95D (1990) 20623. 17 C.F. Boutron and U. G6rlach, in J.A.C. Broekaert, S. Gficer and F. Adams (Eds.), Metal Speoation m the Environment, NATO ASI Series, Vol. 23, Springer-Verlag, Heidelberg, 1990, p. 137. 18 J. V61kenlng and K.G. Heumann, Fresenius' Z. Anal. Chem., 331 (1988) 174. 19 S. Nakamura, K. Chaki and M. Murozumi, Nippon Kagaku Kaishi (1988) 735. 20 P. Herzner and K.G. Heumann, Mikrochim. Acta, in press 21 A.M. Volpe, J.A. Olivares and M.T. Murrell, Anal. Chem., 63 (1991) 913. 22 H.M. Ortner, W. B1fdorn, G. Fnedbacher, M. Grasserbauer, V. Krivan, A. Virag, P. Wilhartitz and G. Wfinsch, Mikrochim. Acta, Part I, (1987) 233. 23 K.G. Heumann, P. Herzner and H.-E. G~ibler, in H. Blldsteln and H.M. Ortner (Eds.), Proc. 12th Int. Plansee Seminar 89, Vol 4, Verlagsanstalt Tyroha, Innsbruck, 1990, p. 191. 24 P. Herzner and K.G. Heumann, in preparation. 25 M. Renner, A. Lamberty and P. De Bi6vre, Analusis, submitted 26 K.G Heumann, M. K6ppe and M. Wachsmann, Proc. 37th ASMS Conf. Mass Spectrometry and Alhed Topics, Miami Beach, FL, 1989, p. 414. 27 M. Wachsmann, L Rottmann and K.G. Heumann, Abstracts 12th International Mass Spectrometry Conference, Amsterdam, 1991, p. 304. 28 L. Rottmann and K.G. Heumann, Abstracts 12th International Mass Spectrometry Conference, Amsterdam, 1991, p. 304. 29 M. Koppe and K.G Heumann, Fresenius' Z. Anal. Chem, 331 (1988) 118. 30 M Wachsmann and K.G. Heumann, Int. J. Mass Spectrom. Ion Processes, 108 (1991) 75.
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31 J. V61kening, M. K6ppe and K.G. Heumann, Int. J. Mass Spectrom Ion Processes, 107 (1991) 361. 32 J. V61kening, T. Walczyk and K.G. Heumann, Int. J. Mass Spectrom. Ion Processes, 105 (1991) 147. 33 T. Walczyk, E.H. Hebeda and K.G. Heumann, Fresenius' J. Anal. Chem., in press. 34 R,A. Creaser, D.A. Papanastassiou and G.J. Wasserburg, Geochlm Cosmochim Acta, 55 (1991) 397. 35 M. Wachsmann, Thesis, Umversity of Regensburg, 1991. 36 K.G. Heumann, N. Neubauer and W. Reifenh/iuser, Geochim. Cosmochlm Acta, 54 (1990) 2503. 37 D.J. Rokop, N.C. Schroeder and K. Woifsberg, Anal. Chem., 62 (1990) 1271. 38 P. Kastenmayer, Thesis, University of Regensburg, 1984. 39 K.G. Heumann, P. Kastenmayer, W. Schmdlmeler, M. Unger and H. Zemmger, Proc. 31st ASMS Conf. Mass Spectrometry and Alhed Topics, Boston, MA, 1983, p 581. 40 R.L Walker, in W.S. Lyons (Ed.), Radioelement Analys~s, Progress and Problems, Ann Arbor Science, Ann Arbor, MI, 1980, p. 377. 41 G.A. Cowan and W.C Haxton, Science, 216 (1982) 51. 42 R.L. Edwards, J.H. Chen, T.-L. Ku and G.J. Wasserburg, Science, 236 (1987) 1547. 43 S.J. Goldsteln, M.T. Murrell and D.R. Janecky, Earth Planet. Sci Lett., 96 (1989) 134 44 H.-J. Laue and K. Habfast, Proc. 37th ASMS Conf. Mass Spectrometry and Allied Topics, Miami Beach, FL, 1989, p. 1033. 45 J.-M. Luck and C.J. All6gre, Nature, 302 (1983) 130. 46 R.J. Walker and J.W. Morgan, Science, 243 (1989) 519. 47 M. Wachsmann and K.G. Heumann, unpublished results (1989). 48 R.J. Walker and J.D. Fassett, Anal Chem., 58 (1986) 2923. 49 H. Umeda, I. Inamoto and K. Chiba, Bunsekl Kagaku, 40 (1991) 109. 50 D.C. Gregoire, J. Anal. At. Spectrom., 5 (1990) 623. 51 D.J. Douglas, E.S.K Quan and R.G. Smith, Spectrochlm. Acta, Part B, 38 (1983) 39. 52 J.R. Garbarino and H.E. Taylor, Anal. Chem., 59 (1987) 1568. 53 J.W. McLaren, D. Beauchemin and S.S. Berman, Anal. Chem., 59 (1987) 610. 54 J.W. McLaren, K.W.M. Siu, J.W. Lam, S.N. Wilhe, P.S. Maxwell, A. Palepu, M. Koether and S.S. Berman, Fresenius' J. Anal. Chem., 337 (1990) 721. 55 D.C. Reamer and C. Veillon, Anal Chem., 53 (1981) 2166. 56 S.K. Aggarwal, M. Kmter, M.R. Wdls, J. Savory and D.A. Herold, Anal. Chem., 61 (1989) 1099. 57 S.K. Aggarwal, M. Kinter, M.R. Wills, J. Savory and D.A. Herold, Anal. Chem., 62 (1991) 111. 58 S.K. Aggarwal, M. Kmter and D.A. Herold, Anal. Blochem., 194 (1991) 140. 59 S.K. Aggarwal, M. Kinter and D.A. Herold, J. Am. Soc. Mass Spectrom., 2 (1991) 85. 60 K.G. Heumann, m J.A.C. Broekaert, S. Giicer and F. Adams (Eds.), Metal Speciation m the Environment, NATO ASI Series, Vol. 23, Springer-Verlag, Heidelberg, 1990, p. 153. 61 M. Unger and K.G. Heumann, Fresemus' Z. Anal. Chem., 320 (1985) 525. 62 K.G. Heumann and R. Groger, Fresenius' Z. Anal. Chem., 332 (1989) 880. 63 D. Tanzer and K.G. Heumann, Anal. Chem., 63 (1991) 1984. 64 C. Relfenh~iuser and K.G. Heumann, Fresemus' J. Anal. Chem., 336 (1990) 559. 65 A. G6tz and K.G. Heumann, Fresenius' J. Anal. Chem., 331 (1988) 123. 66 D.C. Thornton, A.R. Bandy, R C Ridgeway, A.R Driedger II1 and M. Lalevic, J. Atmos. Chem., 11 (1990) 299.
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67 D. Beauchemin, K.W.M. Siu, J.W. McLaren and S.S. Berman, J. Anal. At. Spectrom., 4 (1989) 285. 68 B. Gercken and R.M. Barnes, Anal. Chem., 63 (1991) 283. 69 D. Beauchemin, K.W.M. Siu and S.S. Berman, Anal. Chem., 60 (1988) 2587.
575
Elsevier S o e n c e Publishers B.V., A m s t e r d a m
Isotope dilution mass spectrometry* Klaus G. Heumann Instttut J~r Anorgamsche Chemw der Umversttat Regensburg, Umversttdtsstrasse 31, W-8400 Regensburg (Germany) (Recewed 26 August 1991)
ABSTRACT In the past isotope dduUon mass spectrometry (IDMS) has usually been apphed using the formation of posltwe thermal ions of metals. Especially m cahbratmg other analytical methods and for the certification of standard reference materials this type of IDMS became a routine method. Today, the progress m this field hes m the determination of ultra trace amounts of elements, e.g of heavy metals m Antarctic ~ce and m aerosols m remote areas down to the sub-pg g-~ and sub-pg m -3 levels respectively, m the analysis of uramum and thormm at concentrations of a few pg g-t m sputter targets for the production of mtcroelectromc devices or m the determination of sub-plcogram amounts of 23°Th m corals for geochemical age determmattons and of 226Ra m rock samples. During the last few years negatwe thermal iomzation IDMS has become a frequently used method The determination of very small amounts of selenmm and technetmm as well as of other transmon metals such as vanadmm, chrommm, molybdenum and tungsten are important examples m this field Also the measurement of sdlcon m connectmn wsth a re-determmatmn of Avogadro's number and osmium analyses for geological age determmatmns by the Re/Os method are of specml mterest Inductwely-coupled plasma mass spectrometry ts increasingly being used for multi-element analyses by the ~sotope dtluUon techmque Determmatmns of heavy metals in samples of marine origin are representatwe examples for this type of mulU-element analysis by IDMS Gas chromatography-mass spectrometry systems have also been successfully apphed after chelation of metals (for example Pt determmatlon in chmcal samples) or for the determination of volatile element speoes in the environment, e.g &methyl sulfide However, IDMS---especlally at low concentration levels m the enwronment--seemshkely to be one of the most powerful analytical methods for specnatlon in the future Th~s has been shown, up to now, for species of Iodine, selenmm and some heavy metals m aquatic systems.
INTRODUCTION
Accurate analytical results are becoming more and more an unalterable precondition for further progress in high technology and in many fields of natural sciences. The knowledge of accurate trace amounts of elements and elemental compounds is also an important topic in environmental analytical chemistry. However, even if one obtains reproducible results in trace analysis * Paper presented at the 12th International Mass Spectrometry Conference, A m s t e r d a m , The Netherlands, 26-30 August 1991. 0168-1176/92/$05.00
© 1992 Elsevier Science Publishers B.V. All rights reserved.
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K G Heumann/Int. J Mass Spectrom. Ion Processes 118/119 (1992) 575-592
they are not necessarily accurate as well. Therefore, reliable methods are required, which can be used for the determination of elements even at very low concentration levels and which can also be used for calibrating other analytical methods. Isotope dilution mass spectrometry (IDMS) is a method of proven high accuracy, which means that it is a technique for which the sources of systematic error are understood and controlled. There are a number of recent papers which deal with the contribution of IDMS to accuracy in trace analysis and with its application for reference measurements [1,2]. Today, the isotope dilution technique is applied in the determination of element traces and organic substances as well. The determination of organic compounds in biomedical and clinical chemistry by IDMS is an especially established method for diagnostic purposes. This part of IDMS is reviewed in the same issue of this journal by De Leenheer [3]. This article, however, revmws selected important topics in IDMS for element traces and their species which have been published during the last 3-4 years. For a more detailed description of the fundamental principles o f l D M S of elements and their applications in the past see the reviews in refs. 4 and 5. ISOTOPE DILUTION TECHNIQUE
The principles of IDMS are illustrated by the schematic mass spectrum of an element with two stable isotopes at masses m~ and m 2. An exactly known quantity of a spike isotope, normally in solution form, which is enriched in the isotope with minor natural abundance, is added to the sample. Afterwards, the sample and the spike isotopes must be equilibrated, which means that decomposition of solid samples must be carried out. After the sample and spike isotopes have been completely mixed, loss of substance during the following isolation procedures normally has no effect on the analytical result. This non-quantitative isolation is one of the main advantages of IDMS, especially for trace analyses at very low concentration levels. Another positive feature of this method is the use of a stable (or long-lived radioactive) isotope as a spike, which is the most ideal form of an internal standardization in analytical chemistry. The element to be determined must have at least two stable or long-lived radioactive isotopes. This is valid for all elements except the monoisotopic elements F, Na, P, Sc, As, Y, Rh, Pr, Tb, Ho, Tm, Au and some of the radioactive elements. In Fig. 1 the measured isotope ratio R of the isotope-diluted sample is given by the following equation: R = / z = N s x h ~ + N s p ×h~p /1 N s x h~+Nsp × h~p
(1)
K G. Heumann/lnt. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
•
577
SampLe
~] Splke
I
Nsp" hsp #-
c
/ / / / l J- J-t
2
Nsp" hsp
_ __
1
Ns • h s
/ / / / -_/_/j
Ns" hs2
/fV', i
m2
ml
m/z
=
F i g 1. T h e p r i n c i p l e o f I D M S i l l u s t r a t e d b y a s c h e m a t i c m a s s s p e c t r u m o f a n e l e m e n t wnth t w o i s o t o p e s o f m a s s e s m I a n d m2.
where N is the number of atoms, h is the isotope abundance, S is the sample and Sp the spike. Nsp, hsp, and hs are normally known so that after transformation of eqn. 1 the content Gs of the element to be determined in the sample is obtained by Gs = 1.66 x 10 -'8 x Wss x Nsp
x h ~ - ~ss// [pgg-']
(2)
where M is the atomic weight of the element and Ws the sample weight in grams. IONIZATION
METHODS
USED FOR IDMS
The ionization methods used for element analyses in IDMS can be divided into two groups: mono- and oligo-element methods on the one hand, and multi-element methods on the other. Thermal ionization mass spectrometry in the positive (PTI-MS) as well as in the negative ion mode (NTI-MS) has been the most frequently applied method, up to now, for element trace analyses with the isotope dilution technique. Due to the ionization process, metals can be analysed with the PTI mode, whereas many non-metals, semi-metals and transition metals or their oxides are able to form negative thermal ions. Electron impact mass spectrometry (El-MS) is still the preferred method for IDMS of noble gases and other volatile compounds. This is the reason why gas chromatography-mass spectrometry (GC-MS) systems are often combined with the isotope dilution technique. Spark source mass spectrometry (SSMS) is not an expanding analytical method at the moment. However, excellent investigations with SSMS using the isotope dilution technique have been carried out in the past by Jochum
578
K G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
TABLE 1 Recent applications m the determination of metal traces at extremely low concentration levels w~th PTI-IDMS Topic
Analysed element
Ref.
Aerosols in remote areas Antarctic ice
Pb, Rb, Ba Heavy metals Heavy metals
Pure chemicals Microelectromcs Geological samples
Heavy metals U, Th, heavy metals Ra
Rosman et al. [15] V61kenmg and Heumann [16] Boutron and G6rlach [17] V61kenmg and Heumann [18] Nakamura et al [19] Herzner and Heumann [20] Volpe et al. [21]
et al. [6] and Paulsen et al. [7]. Inductively-coupled plasma mass spectrometry (ICP-MS) is an ideal method for applying IDMS because normally a solution is introduced into the plasma torch, which enables a simple equilibration with the spike solution used. However, multi-element analyses can also be carried out with this technique, which is a good precondition for making ICP-MS one of the dominant methods for IDMS in the future. DETERMINATION OF METAL TRACES AT EXTREMELY LOW CONCENTRATION LEVELS WITH PTI-IDMS
IDMS has been routinely used for many years in geochronology for age determinations [8] and in nuclear technology for accurate determinations of uranium and transuranium elements [9]. Recently, IDMS has also played an important role in the certification of standard reference materials [10-14]. However, an important trend using PTI-IDMS is in the determination of metal traces at extremely low concentration levels. A selection of different applications in this field is listed in Table 1. It was possible using PTI-IDMS to determine, for example, Pb, Ni, Cu, Cd, and TI in aerosols down to the lowest pg m -3 range in the remote areas of the South Atlantic [16] and to determine Pb, Rb, and Ba in the atmosphere over the Pacific Ocean [15]. The concentrations analysed for heavy metals in Antarctic ice normally lie below 10 pg g-~ and very often the concentration of elements such as T1, Cd, and Pb is < 1 pgg-t [17,18]. If contamination is under control, it is possible to obtain relatively accurate results by IDMS even at this extremely low concentration level. The heavy metal concentration of highly purified chemicals (HC1, HNO 3, etc.) is in the same concentration range [19]. The use of highly purified chemicals is very important in the production processes of microelectronic devices. Refractory metals of high purity (mainly molybdenum and tungsten) as well as their silicides have been increasingly
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579
L
L5 O'
0
IDMS
SIMS
SSMS
GDMS
GDMS
(MR)
(LR)
Fig. 2. Comparison of results (/~gg- t) obtained by different mass spectrometricmethods for a tungsten sample [23]. used for gate electrodes, interconnections and diffusion barriers in integrated circuits as a result of recent developments in higher density very large scale integrated (VLSI) systems. Besides the radioactive elements U and Th other heavy metals affect the reliability of integrated circuits. It is assumed that the concentration in materials for the VLSI systems should not exceed 1 ng g- ~for U and Th and 10 ng g-~ for the heavy metals [22]. For the ultra large scale integrated (ULSI) systems (64 Mbit chip), which are now under development, even better purities must be reached. This means that reliable analytical methods, which are able to exactly characterize the trace impurities in the primary materials such as refractory metals, aluminium, quartz, etc., must also be available. Figure 2 shows a comparison of results obtained by different mass spectrometric methods for a tungsten sample [23]. The results for U, Th, and Fe differ widely. Also, analyses with glow discharge mass spectrometers (GDMS) in the high (HR) and low resolution (LR) mode do not agree very well for U and Th. Therefore, a procedure for the determination of U, Th and other heavy metals with PTI-IDMS was developed for a more accurate characterization of trace impurities in these primary materials for chip production. The results are given in the first column of Fig. 2. The relative precision of PTI-IDMS for U and Th was about 2%, which was much better than for the other mass spectrometric methods. The detection limits for U, Th and a number of other elements in refractory metals and their silicides with PTI-IDMS are listed in Table 2. All detection limits are below the suggested limiting values for VLSI systems and should also fit the needs of ULSI systems, especially for the critical elements U and Th. The higher detection limits for Fe and Ca are due
580
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TABLE 2 Detection limits of trace elements m refractory metals and their slllcldes with P T I - I D M S [20,24] Element Detection hmit (ng g - l )
U 0.001
Th 0.008
Cd 0.08
Cr 0.5
N1 0.6
Cu 0.8
Fe 5
Ca 10
to the fact that the blank is the limiting factor in the analysis of these ubiquitous elements. For elements, where contamination during the analytical process is not a general problem, concentrations down to the fg g-1 level (10-15 g g - l ) can be determined with PTI-IDMS owing to the extremely high sensitivity of the thermal ionization technique. An excellent example of such an analysis is the determination of 226Ra traces in different rock samples carried out by Volpe et al. [21]. In this case an enriched 228Ra spike (half-life only 5.75 years) was used for the isotope dilution procedure. 226Ra in rocks is formed by the radioactive decay of natural uranium. The results for three different rocks are presented in Table 3. The analytical uncertainty associated with the isotope ratio measurement was better than 1.5%. Also, the two independent analyses of each sample agreed very well even at the low fg g-l level. A P P L I C A T I O N O F N T I - I D M S IN E L E M E N T T R A C E ANALYSIS
NTI-IDMS has been used more and more during the last few years for element trace analyses. Singly-charged atomic ions M - or element oxide ions MO~- are usually formed by many elements, especially the non-metals, the semi-metals and the transition metals. Owing to the physical background of this ionization process the electron affinity of the atom or compound to be ionized should be high (at least 1.8 eV) and the electron work function of the filament material should be low ( < 4 eV) [4,5]. The electron work function of the material can be lowered by chemical additions to the filament surface (e.g. BaO, La203) or by using substances such as LaB 6 for ionization [4,11]. TABLE 3 Determination of femtogram amounts of 226Ra in geological samples with PTI-IDMS [21] Sample
Analysis no.
Concentration of 226Ra (fg g ~)
Volcamc rock
1 2 1 2 1 2
(1.063 + (1.068 + 69 1 + 67 8 + 14.4 + 13.9 +
Mldocean basalt 1 Mldocean basalt 2
0 010) 0 O11) 08 0.9 02 0.2
x
10 3
× 10 3
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
ki
Be
B
C
N
({¢{i
No Mg
AI
Co Sc
Ti
V
Cr
Mn
Cu
Zn
Tc IRu ~Rh ~Pd Ag
Cd
' l / I ,
Rb Sr
Y
Fe
Co
,11~
•I ~:]
Ne
Si
P
S
Cl
Ar
Se
Br
Kr
Te
I
Xe
IIIII
Ga Ge As
r I///~
Zr INb I Mo
Cs Ba La Hf Ta
Ni
F
(.H
l l l l l ,
K
0
581
In
Sn
Sb
t(¢{l¢
W [Re lOs
Ir
Pt Au Hg TI Pb Bi
Po At Rn
Procedure for isotope ratio determmotlon devetoped
Negative thermal ions
detected
Fig 3. Elements that can be measured as M - or MO)- ions by NTI-MS (elements with a developed procedure for isotope ratio determinations are apphcable for IDMS).
All elements for which IDMS procedures with the NTI technique have so far been developed are marked in Fig. 3. The recently developed techniques in this field are for Si [25], S [26], V [27], Cr [28], Mo [29], Sb [30], W [31], Os [32,33], Ir [34] and Pt [35]. Pt- ions are used for measurements of platinum, MO2 ions for Si, Sb, and Ir and MO3 ions for V, Cr, Mo, W, and Os. The main advantages of NTI-MS are the selective ionization method with only a few interferences and the relatively high ion currents obtained. A selection of recent applications with NTI-IDMS is listed in Table 4. The determination of femtogram amounts of technetium by Rokop et al. [37] shows that extremely low concentrations can also be determined with NTI-IDMS if there is no blank problem. A 97Tc spike solution was analysed using a 99Tc standard reference solution. Concentrations of I fg or less of the TABLE 4 Recent applications of NTI-IDMS Topic
Analysed element
Ref
Biological samples Meteorites Enwronmental samples
I I, Br, CI Mo W Cr B Se S~ Tc
Gramhch and Murphy [11] Heumann et al. [36] K6ppe and Heumann [29] Heumann et al. [26] Rottmann and Heumann [28] Lamberty et al. [10] Heumann and co-workers [13,14] Renner et al. [25] Rokop et ai. [37]
Rye grass Food, sediments Spike reference matenal Standard solution
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long-lived isotopes 97Tc, 98Tc, and 99Tc can be analysed using NTI-IDMS. This is possible because high ion currents of TcO4 are produced with the NTI technique from pertechnetate salts, which was first shown by Heumann and co-workers [4,38,39]. The advantage of NTI-MS compared with PTI-MS is that the most abundant technetium ion is TcO4, whereas molybdenum, which is always present in filament materials as an impurity, forms only MoO3 ions. Owing to both the interference in PTI-MS between molybdenum and technetium isotopes, and the high ionization potential of technetium, the detection limit with PTI-IDMS is about 1 pgTc [40]. Therefore, the application of NTI-IDMS instead of the positive thermal ionization mode is a great step forward, which also makes it possible to detect 99Tc contaminants in the environment and to measure the flux of high-energy solar neutrinos over the past several million years in technetium samples isolated from molybdenite ores [41]. NEW CLOCKS IN MASS SPECTROMETRY FOR GEOLOGICAL DATING
Sensitive thermal ionization techniques have enabled the application of two new age determination methods for geological samples with mass spectrometry. 1. The uranium series disequilibrium method, which is based on the radioactive decay of z38U: 2 3 8 U - ~ . . . . . 234U--~ 23°Th--L-~ 226Ra ~ , . . . . .
2.
(3)
The Re/Os method with the following decay of the J87Re isotope with a natural abundance of 62.6%:
187Re 'b'7~ 187Os (stable)
(4)
In the case of the first mentioned method a member of the series given in eqn. 3 must have been separated by a geological or geochemical process. Afterwards, the decay of this nuclide or the production of the daughter nuclide can be followed. Age determinations include isotope ratio measurements and quantitative determinations of the corresponding nuclides by means of IDMS [8]. Calcium carbonate deposited in oceans contains appreciable concentrations of uranium but is virtually free of thorium. The increase of the thorium isotope ionium (230Th) in such a calcium carbonate formation or even in corals is a function of time and, therefore, can be used as a geological clock. Previous datings with the uranium series disequilibrium methods were carried out by 0c-counting of the corresponding nuclides. Isotope ratio measurements of very small amounts of 23°Th ( > 108 atoms of 23°Th are measurable) by PTI-MS and the use of 229Th and 233U spike solutions for isotope dilution makes this dating much more precise than the 0~-counting method. The
583
K G. Heumann/lnt..I. Mass Spectrom. Ion Processes 118/119 (1992) 575-592 varloble Foroday
\
~ ~ - ~ "
.........
I
aperture
counter dece[erotlon
lenses
SEM /
-
center ~
quodrupole mass h t t e r
/~magnet m, lm'
cups
sht
I I I I ~.~
ion source
Fig. 4. Schematicdiagram of a thermal iomzation mass spectrometerwith enhanced abundance sensitiv]ty [44]. relative precision is in the range of about 1%, which is a factor of 5-10 better than by o-counting. The time range for this dating method is from a few years up to about one million years. For example, Edwards e t a | . [42] were able to determine the precise timing of the last interglacial period by PTI-MS measurements of 23°Th in corals to be (122-130) x 103 years. Recent applications of geological dating with the U/Th disequilibrium method require the measurement of 23oTh/232Th ratios in the range 10- s_ 10- 6 [43]. For such an extreme isotope ratio the abundance sensitivity of a normal thermal ionization mass spectrometer is not adequate. Therefore, new types of tandem mass analyser systems have to be developed for enhanced abundance sensitivity. One instrument of this type has been designed by Laue and Habfast [44]. The schematic diagram of this mass spectrometer is shown in Fig. 4. An extended geometry 90 ° magnetic sector field instrument (MAT 262, Finnigan) was coupled with a quadrupole mass filter. The interface between the two mass analysers is a decelerating immersion lens and a beam-shaping static quadrupole lens. The system is used together with a conventional multicollector array and with an ion counter system. The abundance sensitivity of this equipment is at least 10 -g, which allows satisfactory measurements even for extreme 23°Th/232Th ratios. Recent experiments have shown that the system presented in Fig. 4 could also be run without any quadrupole filter using only the deceleration lenses. The Re/Os method is especially suitable for the dating of sulfide minerals and iron meteorites, because other geochronometers are not applicable to these materials. Moreover, the Re/Os couple can be used to study the differentiation of the Earth's mantle and the growth of the continental crust [8]. The
584
1
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
lf*
tt~
12 03
p.O
10
08"
f
Re/Osage=1415±0 161x109o
•('STos/'ar0s ), t~
6
8
1'0
187Re/la60s Fig. 5. Re/Os dating of two iron meteorites (Canyon Diablo and Tocopilla) with SIMS and RIMS [45,46]. application of the Re/Os method has been retarded by analytical difficulties which have arisen from the low osmium concentrations in silicate minerals and the lack of sensitive and precise isotope ratio measurements. However, osmium rich materials, e.g. iron meteorites, could be dated in the past by SIMS [45] and also by resonance ionization mass spectrometry (RIMS) [46]. The results of the analyses of two different iron meteorites (Canyon Diablo and Tocopilla) by SIMS and RIMS are shown in Fig. 5. Using a half-life of 4.23 × 109 years for IS7Re the Re/Os age was determined to be (4.15 + 0.16) x 109 years with an initial ratio of (87Os/186Os), = 0.8119 _ 0.0094. In 1989 Wachsmann and Heumann [47] showed for the first time that osmium isotope ratios can be measured by NTI-MS. This technique was later improved by Vrlkening et al. [32] and by Creaser et al. [34]. Recently, the most precise and most sensitive NTI technique for osmium isotope ratio determinations has been developed at the University of Regensburg by the introduction of oxygen or freon into the ion source during the measuring procedure [33]. In Table 5 the isotope ratios with the corresponding relative TABLE 5 Progress in osmium isotope ratio deterrninatlons by NTI-MS Technique RIMS NTI-MS
Isotope ratio
RSD (%)
Ref.
187OS/186Os
190Os/ 192Os
32 1.3
Walker and Fassett [48]
'STOs/192Os 188Os/192Os 189Os/192Os
0.07 0.01 0.004
Walczyk et al. [33]
K.G Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
585
TABLE 6 I C P - I D M S of trace elements m a lobster standard reference material (LUTS-1) [54] Element
NI Cu Zn Sr Cd Hg Pb
Concentration (pg g- 1) ICP-IDMS
Certified value
0 235 15.9 12,4 2.24 2.18 0.017 0.010
0 200 15.9 12.4 2.46 2 12 0.0167 0 010
_ 0.011 + 02 _+ 0.2 + 0.04 + 0.04 + 0 002 + 0.001
___0.034 ___ 1.2 + 0.8 __+0.28 + 0.15 + 0 0022 + 0 002
standard deviations (RSD) obtained by RIMS [48] and NTI-MS [33] are listed. This comparison shows the progress in osmium isotope ratio measurements between the RIMS method for Re/Os dating and NTI-MS. By the improvement of NTI-MS it should be possible to widely expand applications of the Re/Os method. M U L T I - E L E M E N T A N A L Y S E S BY I C P - I D M S
Even if a few publications also describe the determination of a single element by ICP-IDMS, e.g. of antimony in high purity copper [49] or of boron in saline waters [50], the main advantage of this method lies in the multielement analysis. This was formerly suggested in 1983 by Douglas et al. [51] because spiking of solutions, which is the usual form of samples introduced into the ICP-MS, is easy. Thus, the time-consuming chemical separation of elements, which is necessary for thermal ionization IDMS determinations, can usually be avoided. While the precision of isotope ratios measured by the thermal ionization technique is better than ICP-MS data by one order of magnitude or more, the precision of isotope ratios determined by ICP-MS is usually adequate for trace element determinations. In cases where the amount of element added by the spike drastically exceeds the amount of this element in the sample owing to low sample concentrations, the precision of the isotope ratio measurement can be the limiting factor in IDMS. Under these conditions the precision of the ICP-MS method is often inadequate. With respect to the simple sample introduction for liquids it is of consequence that one of the first analyses by ICP-IDMS was carried out with water samples [52]. However, the isotope dilution technique has also been applied to multi-element analyses in solid samples, e.g. from marine origin, by McLaren and co-workers [53,54]. Table 6 summarizes the results of seven trace elements in a lobster hepatopancreas sample. A comparison of the
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ICP-IDMS results with the certified values of this standard reference material (LUTS-1) shows excellent agreement. The precision of the ICP-IDMS data is always better than the uncertainty of the certified values. The main advantages of the discussed application of ICP-IDMS in a biological material were the following. 1. Only microwave digestion with a mixture of HNO3/H202 and no other sample preparation was necessary. 2. A simultaneous determination of elements with a concentration difference of more than three orders of magnitude (Cu and Pb) was possible using an instrumental feature which permits a reduction of the sensitivity for selected elements while retaining full sensitivity for the other elements. 3. The simultaneous determination of mercury and other elements which is often a great analytical problem because of partial loss of mercury during the digestion steps for other elements was possible. In the case of this analysis the isotopic equilibration between spike and sample was achieved before possible mercury losses. 4. The total time for the ICP-MS measurement of each solution is only about 5 min. 5. Precise and accurate data can be attained for elements, which are not influenced by isobaric interferences. As a result of these advantages, ICP-MS will become one of the dominant methods for IDMS in the future. However, there are also disadvantages in ICP-IDMS, for example the fact that the samples should be dissolved for the isotope dilution step whereas electrothermal evaporation or laser systems allow the direct introduction of solid samples into the plasma torch. Owing to interferences and cross-contamination problems the number of spike solutions is certainly limited. The use of a multi-element spike is limited by the different concentration levels needed for various samples. Mostly, the accuracy for elements affected by interference is relatively poor and in the case of extremely low concentrations the precision of the isotope ratio measurement for ICP-MS (relative standard deviations are typically above 0.1%) can be the limiting factor of the analysis. CHELATION OF ELEMENTS COMBINED WITH GC-IDMS A special application of IDMS is the chelation of the element to be determined followed by a gas chromatographic separation and the measurement of isotopic peaks of the chelate in a normal "organic" mass spectrometer using electron impact for ionization. One important precondition for this technique is the formation of volatile chelates, which must be thermally stable. This type of IDMS analysis may be of special interest to clinical laboratories because there is a growing accessibility of GC-MS systems in this area.
K.G. Heumann/lnt. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592 CF3-CH2
.S
S.
~N--C// "'"Pt / CF3 -CH 2/
\s /
587
/ CH2- CF3
\C--N
",,"S~
\ CH2-CF 3
Fig. 6. Platinum complex of the chelating agent bls(tnfluoroethyl)dithiocarbamate used for GC-IDMS [59].
However, accurate analytical results for toxic and essential trace elements are increasingly important for diagnostic purposes. Reamer and Veillon [55] were among the first to use the GC-IDMS technique for the determination of selenium traces in food samples. With respect to the above mentioned interest in medicine, Aggarwal and co-workers [56-59] have developed GC-IDMS methods for the determination of the heavy metals Ni, Cr, Cu, and Pt in urine using lithium bis(trifluoroethyl)dithiocarbamate as the chelating agent. Figure 6 shows the platinum complex of this chelating agent. When the NIST freeze-dried urine reference material SRM 2670 was analysed with GC-IDMS, a platinum concentration of (125 + 6)ngm1-1 was found [59]. This agreed well with the recommended value of 120 ng Pt ml- i. SPECIES ANALYSES
In the introductory part of this review it was pointed out that the species analysis of the element carbon, which more generally means the analysis of organic compounds, by IDMS will be discussed separately [3]. A review, which deals with IDMS of other element species, has recently been published [60]. It is not possible to directly determine element species with the ionization methods normally used in inorganic mass spectrometry, that is with thermal ionization, SSMS, and ICP-MS. These ionization methods preferably produce atomic or oxide ions independent of the chemical form of the sample compound. For example, iodide and iodate samples form the same I- ions in NTI-MS. Prior to the mass spectrometric measurement a complete separation of the various species must, therefore, be carried out. Additionally, the following preconditions must be fulfilled. 1. An isotopically enriched spike in the same chemical form as the species to be determined must be available. Otherwise the species must be converted into the chemical form of the spike after its quantitative separation. 2. No isotopic exchange between the different element species is allowed until the species have been completely separated from each other. It is, however, not necessary to isolate the particular species quantitatively after the isotope dilution process has taken place. It has been possible to analyse the following species, so far, with NTI-IDMS: nitrite and nitrate in food samples [61], selenite, selenate and organoselenium
588
K.G. Heumann/Int. J. Mass Spectrom. 1on Processes 118/119 (1992) 575-592
(including trimethyl selenonium ions) [62,63], iodide, iodate and organoiodine compounds in aquatic systems [64]. With PTI-MS G6tz and Heumann [65] have been able, for the first time, to differentiate between chromium complexes with humic substances, which are kinetically inert for isotopic exchange, and those where the spike and sample compounds are in equilibrium in an aqueous system. Volatile compounds can also be determined by the isotope dilution technique with a GC-MS system if the species to be determined is available in form of a labeled spike compound, which is stable enough for the gas chromatographic separation. A recently published example of this type of analysis is the determination of ppt levels of dimethyl sulfide (DMS) in the atmosphere using deuterated d6-DMS as spike [66]. The importance attached to the analysis of low DMS levels is due to the fact that this compound has become a key component in the transfer of sulfur from biogenic sources in the ocean to the atmosphere, where it can affect climatic processes through its oxidation to SO2. Using anion chromatographic separation methods it is possible to differentiate four iodine species in natural aquatic systems: iodide, iodate and two organoiodine compounds, one of them elutable with a sodium nitrate solution from a column filled with an anion exchange resin (elution of this anionic organoiodine compound between iodate and iodide) and another organoiodine compound with high molecular weight non-elutable under the conditions described. Figure 7 represents the very different distribution pattern of these iodine species in a moorland lake sample compared with a river water sample analysed with NTI-IDMS [64]. Whereas in the moorland lake water only the organoiodine species could be detected during two different samplings, in the river water all four iodine species were analysed with comparable concentrations. It must be pointed out that this iodine speciation was carried out with NTI-IDMS at concentration levels of mostly less than 1/tgl -~. Obtaining accurate iodine determinations at this low level is a well-known problem when applying other analytical methods, even if only the total content has to be analysed. This example evidently shows the high sensitivity and accuracy of NTI-IDMS. ICP-MS coupled with chromatographic methods is increasingly applied for speciation [67,68] because of the great advantage of its use as an on-line system. Beauchemin et al. [69] applied ICP-IDMS to determine methyl mercury in a marine standard reference material. Because of the relatively simple on-line coupling for most of the chromatographic methods, the fast analysis times and the accurate results by the isotope dilution technique, ICP-IDMS will certainly become one of the most powerful methods for speciation in the future.
K.G. Heumann/Int. J. Mass Spectrom. Ion Processes 118/119 (1992) 575-592
589
7
1
Moorland lake
/ . 12187
_J ..,,,
4/88 o
¢b 0 u
River w a t e r "10 0
2-
Samphng date
~ iodide
iodate
12
onmonlc non-elutable orgnno-I orgono- I
Fig. 7. Distribution pattern of iodine speciesm a moorland lake and a river water sample [64]
FUTURE TRENDS
The necessity of accurate analytical results even at extremely low concentration levels becomes more and more obvious in the different fields of high technology, in natural sciences and for environmental protection. IDMS will, therefore, increasingly be used at least as a calibration and control method to guarantee reliable analytical results. With respect to this the following topics will play an important role in the future when using IDMS for elements. 1. Calibration of other analytical methods and the certification of standard reference materials. 2. Trace element determinations at extremely low concentration levels, including radionuclides with half-lifes down to a few years. 3. Accurate multi-element analyses using ICP-MS. 4. Accurate routine analyses in medicine for diagnostic purposes. 5. Application of the Re/Os dating method using NTI-MS. 6. Species analyses by on-line coupling of ICP-MS with different chromatographic methods.
590
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67 D. Beauchemin, K.W.M. Siu, J.W. McLaren and S.S. Berman, J. Anal. At. Spectrom., 4 (1989) 285. 68 B. Gercken and R.M. Barnes, Anal. Chem., 63 (1991) 283. 69 D. Beauchemin, K.W.M. Siu and S.S. Berman, Anal. Chem., 60 (1988) 2587.