Inductively coupled plasma-mass spectrometry: Capabilities and applications

MICROCHEMICAL

Inductively

JOURNAL

46,

Coupled

302-312 (1992)

Plasma-Mass Spectrometry: and Applications’

Capabilities

M. A. MARABINI,~ B. PASSARIELLO, AND M. BARBARO Istituto per il Trattamento dei Minerali, CNR, Via Bolognola 7, 0013&Rome, Italy Received August 13, 1991; accepted February 19, 1992 Researchers in the field of trace elements analysis are continuously in search of new instrumental solutions for obtaining better results in terms of analysis speed, precision, accuracy, detection power, and applicability to a wider range of analytical problems. One of the more recent innovations in this field is the inductively coupled plasma (ICP) source coupled with a mass spectrometry (MS). An ICP-MS system consists of an ICP torch which ionizes the species present and a mass spectrometer for the separation under vacuum of the different species. The main advantages of this technique with respect to graphite furnace atomic absorption spectrometry (GFAAS) and to ICP atomic emission spectrometry (ICPAES) are: (a) detection limits better than those obtained with graphite furnace, i.e., down to the ng g-’ level, due to the high sensitivity of the channel electron multiplier, which transforms the mass of each ion into an electric signal; (b) the possibility of detecting refractory elements, lanthanides, and all the other elements including halogens, C, and S; (c) high analysis speed (up to 90 elements in 5 min) due to the velocity of the quadrupole mass spectrometer in selecting different masses with respect to the speed necessary to scan different wavelengths; (d) spectral simplicity, because spectra have peaks only at the mass of each isotope and all elements have at least one isotope free from spectral overlap of other analytes; (e) capability of determining individual isotopes of each element. The instrument, therefore, allows not only quantitative elemental analyses to be carried out, but also semiquantitative assays of all the elements present and isotopic ratio analyses to determine quantitatively two or more isotopes of the same element. The most interesting application fields of this technique are in environmental chemistry, geochemistry, oil chemistry, technology of semiconductors, and biochemistry. o 1~2 Academic press, Inc

INTRODUCTION

Inductively coupled plasma-mass spectrometry (ICP-MS) is a new, highly sensitive, accurate method for elemental and isotopic analyses. It permits new instrumental solutions for obtaining better results in terms of analytical speed, precision, accuracy, detection limits, and applicability to a higher number of analytical problems, especially for the determination of trace elements. Its high sensitivity, which allows determinations at the rig/kg level, stems from the specimen excitation source which reaches temperatures of around WOO-10,000K, thus permitting elimination of chemical interferences. Accuracy, instead, depends essentially on the possibility of eliminating spectral interferences and matrix effects, thanks to the characteristics of the ICP-MS spectrum of an element: in fact, each i Submitted in conjunction with the Fifth Italo-Hungarian Symposium on Spectrochemistry: Quality Control and Assurance in Life Sciences, Pisa, Italy, September 9-13, 1991. ’ To whom correspondence should be addressed. 302 0026-265X/92 $4.00 Copyright All rights

0 1992 by Academic Press, Inc. of reproduction in any form reserved.

CAPABILITIES

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303

element has peaks only at the mass of each isotope, and at least one isotope is free from spectral overlaps of the other analyte elements. The first papers on this technique were published in 1980(1-8). In 1983, SCIEX, a Canadian company manufacturing mass spectrometers, built the first complete instrument which used this technique. The apparatus, known as ELAN, is produced by Perkin-Elmer and SCIEX. ICP-MS, as the term implies, is a synergic combination of an inductively coupled plasma with a mass spectrometer. It is designed for the rapid multielemental determination of elemental species at low concentrations, and it is complementary to both ICP emission spectrometry and graphite furnace atomic absorption spectrometry (GFAAS), thus combining the beneficial characteristics of both techniques. It consists of an ICP torch which ionizes the species present, and a mass spectrometer which allows the separation under vacuum of the different species, dispersing the ions according to their mass and quantitating the number of ions present. MATERIALS

AND METHODS

The schematic diagram of the apparatus is illustrated in Fig. 1. It consists of an ion source, an interface, and a mass spectrometer as detector. The source is plasma, namely Ar in a highly ionized state. Operation is as follows: The Ar which flows through the torch is first subjected to a high-intensity electric discharge which causes ionization of some Ar atoms and hence the release of electrons. The electrons liberated start their circular path around the torch on a plane perpendicular to the latter. The circular movement of these electrons is hindered by the ‘magnetic field existing around the torch; this causes the development of heat which provokes the ionization of other Ar atoms and hence the production of other electrons. The initial electrical discharge caused by a spark generator thus serves to trigger the production of electrons, while the magnetic field generated by a radiofrequency generator serves to maintain the excited plasma in existence. The name inductively coupled plasma indicates the formation of plasma induced by a coupling with a magnetic field. The source attains a temperature of 8000 to 10,000 K. The sample is drawn through the torch via a pneumatic pump INTERFACE

ICP ION SOURCE

VACUUM

MASS SPECTROMETER

SYSTEM

FIG. 1. Scheme of an ICP-MS apparatus.

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system and the elevated temperature of the torch provokes high ionization of the elements as well as complete destruction of the matrix. Ionization is never complete because of the presence of a great number of electrons (9, 20). Figure 1 iliustrates a schematic diagram of an ICP source. Most elements of the periodic table are efficiently ionized under typical plasma conditions. The most critical part of the whole apparatus is certainly the interface (II). This consists of two metal cones (Ni is used since it stands up well to the high temperatures) with the point cut to provide a hole about 1 mm in diameter (Fig. 1, Item 1). A vacuum of ca. 100 Pa is created between these two cones to permit aspiration of the ions produced by the torch; these are sampled through another orifice, the skimmer (Fig. 1, Item 2), to form a narrow ion beam. The ions then pass into the ion optics (Item 3), whose functions are: (a) to separate the ions from the neutral species still present, (b) to focus all the ions present in a beam so as to permit their entry into the mass spectrometer, (c) to block the nonionized species and photons. The ions enter the quadrupole mass spectrometer (Fig. 1, Item 4) of the ELAN, which separates them according to the ratio of their mass and charge (12, 13). A quadrupole mass filter consists of four parallel rods with opposite pairs of rods. Direct current (DC) voltage and radiofrequencies (RF) are applied to opposite pairs of rods. The DC accelerates the ions and the RF provokes a magnetic field that endows the ions with a particular trajectory which depends on the mass/charge ratio. As the trajectory is a constant of the instrument, every individual isotope can be examined by using a particular RF and DC value. However, if the amplitude of the RF and the DC are varied continuously, all the elements present are determined sequentially. A detector known as the channel electron multiplier (CEM; Fig. 1, Item 5) is the modern version of the photomultiplier. Thus functions as an ion counter, determining the ions of the individual elements quantitatively and those of the sequentially transmitted elements as a whole, semiquantitatively. The instrument is, of course, controlled by an internal microcomputer.

CONCENTRATION

FIG. 2. Example of a calibration

PPM

curve for ICP-MS.

CAPABILITIES

Performance

OF ICP-MS

305

of the ELAN

The ICP-MS can be utilized in two operating modes for: -quantitative analysis of the individual elements and qualitative and semiquantitative analysis of all the elements present; -analysis of the isotopic ratio and analysis by isotope dilution. Elemental and multielemental analysis. Elemental analysis is employed to ascertain the concentration of a given element which is known to be present in solution. This is generally done by measuring the quantity of one of its isotopes after calibrating the instrument on solutions of known concentration. The qualitative-semiquantitative mode is very useful for discovering what elements are present in an unknown solution and roughly ascertaining the amount thereof. Theoretically up to 90 elements can be identified in a single sample in about 5 min. Not more than three elements are used for calibration so as to cover the entire range of mass; the semiquantitative evaluation is obtained with an accuracy of ca. 10 to 30%. Table 1 reports the detection limits of elements detectable with the ICP-MS technique. As can be seen, values are even better than those obtainable by means of GFAAS. Furthermore, the detection limits of refractory elements, lanthanides, and rare earths are at the rig/ml level, which cannot be achieved with the best plasma spectrometers. In most cases the spectrum configuration is very simple, thus facilitating interpretation. In fact, all elements except In have at least one isotope which is free from spectral overlaps from other analyte elements. Isotope measurement and isotope dilution. The isotope ratio technique is used when one or more isotopes of the same element have to be quantitatively deter-

TABLE 1 Typical Detection Limits (rig/ml, ppb) Afforded by ICP-MS

1 Fr IRal Ac] *

EL gt i

Pm t% E

m

z

Np Pu Am Cm Bk Cf Es Fm

Negative Ion Measurement

Pa F

&

T”;, zy H”b E’r

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mined (1, 8, 14-16). The result is expressed in terms of intensity ratio between a specific isotope and the reference isotope. No standardization is needed and the precision is between 0. I and 3.0%. The concentration of an element can also be determined using the isotope dilution technique. This is based on measurement of the ratio of the isotopic abundance of two isotopes of the element contained in the sample to which a known quantity of isotope is added. The advantage of this method is the high accuracy obtainable. In practice it can be considered as an internal standardization which also compensates for any possible losses of element during sample preparation. Fields of Application ICP-MS can be employed in all analytical areas in which ICP-AES and GFAAS are commonly used. However, it is very useful for determinations that are not easily performed or not possible with other techniques. The sectors most interested in ICP-MS are thus likely to be as follows: Ecology - All polluting elements in drinking water and waste water can be determined with one sample. -All movements of large masses of air can be followed, determining the trace elements and their isotopic configuration. Geochemistry -Precious-metal exploration. -Mineral dating; following the isotope ratio, for instance, of Pb in a rock. -Distribution of rare earths in an ore can provide indications as to the geological history of the sample. Petrochemistry -Analysis of V, Ni, and Fe present naturally in crude oils to avoid catalysts being poisoned during the refining processes. -The isotope ratio technique, using several elements occurring in the crude, can be adopted to ascertain the geological history of the field, as well as data on age and composition. Semiconductors -Control of substances used for manufacturing microcircuits; determination of impurities at ultratrace levels in very pure water, solvents, acids, oxidants, substrates, doping agents, etc. Biochemistry -Study of the dynamics of the isotope ratio of toxic or nutritional elements in physiological liquids, tissues, etc. -Study of the metabolism of Pt employed as an anti-tumor agent. -Approach to the preparation of a periodic table of elements characterizing diseases.

CAPABILITIES

307

OF ICP-MS

TABLE 2 Instrumental Parameters for Quantitative Analysis Plasma RF power Plasma flow Auxiliary flow Nebulizer gas flow Sample uptake flow Sample delay

1400w 12 liters/mitt 1.6 liters/min 0.82 liter/min 0.2 ml/min 120 s

Ion lens settings Barrel Plate Einzel-1 stop-2 Resolution Measurement/peak Scanning mode Measurement mode Measurement time Repeats Dwell time

33 21 91 39 Low (1.1 amu, 10% means) 1 Elemental Multichannel IS

3 50 ms

With the isotope dilution technique ICP-MS can generally be used advantageously in this sector to replace radioactive substances. Metallurgy

-Trace-element

analysis.

7.6 7.0

6.0

Pd

-i ATOMIC

MASS

FIG. 3. Results of the semiquantitative analysis of sample 89.

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TABLE 3 Quantitative Analysis: Sample 89

Element

Net concentration (mgkter)

Yttrium Cerium Lanthanum (Rhodium)

0.03908 0.05293 0.02220 (4.613e4)

-Simplification of certain difficult determinations, such as Ce in steel, Hf in Zr alloys, and rare earths in steels and alloys. Examples of Application of ICP-AES to Rare Earth Determinations A method has been developed for the use of ICP-MS for semiquantitative and quantitative detection of trace and ultratrace rare-earth elements in ores. TABLE 4 Quantitative Analysis: Sample 157

Element Lithium Beryllium Scandium Molybdenum Tin Cesium Praseodymium Neodymium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Thallium Thorium Uranium Yttrium Lanthanum Cerium Niobium (Rhodium)

Net concentration (mg/liter) 0.07401 0.01084 9.21le-4 4.879e-4 0.04184 0.3466 3.888e-5 3.866e-4 I .786e-4 3.025e-5 3.618e-5 -

1.467a-5 9.152e-5 2.173e-3 0.3016 4.830e-4 1.355e-3 2.223e-3 7.356e-4 0.01051 (8.014e4)

CAPABILITIES

OF ICP-MS

TABLE 5 Quantitative Analysis: Sample 158

Element Lithium Beryllium Scandium Molybdenum Tin Cesium Praseodymium Neodymium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Thallium Thorium Uranium Yttrium Lanthanum Cerium Niobium (Rhodium)

Net concentration (mglliter) 0.05184 0.06890 7.857e-3 5.057e-4 1.016 0.5664 3..584e-4 1.695e-3 3.788e-4 7.192e-5 1.671e-4 2.292e-4 8.775e-5 1.688e-4 4.62Oe-4 0.1695 1.282 0.01076 2.19Oe-3 2.924e-3 3.520e-3 0.4893 (8.014e4)

FIG. 4. Results of the quantitative analysis of sample 89.

309

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200-

AND

BARBARO

Th

lOCo

Hf

U

Ta

Y

La

Ce

N,b

$Rh

FIG. 5. Results of the quantitative analysis of sample 157.

Determination of these elements by conventional methods has been plagued by at least one of the following problems: lack of specificity, low sensitivity and/or detection limits, poor precision and/or accuracy, cost and/or long analysis time. For example, both gravimetric determination coupled with fractional crystallization, and the calorimetric techniques suffer from all these problems except expense. In contrast, precise, accurate analysis can be achieved by the isotope dilution MS technique and NAA. However, both of these are expensive, timeconsuming, and available to only a limited number of geoscientists. The ICP-AES technique is generally more readily available than NAA but it requires the preconcentration and separation of rare earth elements before instrumental detection. Detection limits range from 1 to 60 Czg/liter. ICP-MS provides enormous potential for rare earth analysis since it is more capable of discriminating elemental species: in fact, the rare earths are not easy to separate from one another because of their similar chemical properties, while they are difficult to isolate in pure form owing to their reactivity. Both characteristics are, of course, attributable to their electronic structure. 600400 200-

- _Li

808 800 400

Sfl cs Be

sc

kdo

n

l-l

Pf

Nd

Sm

I

FIG. 6. Results of the quantitative analysis of sample 158.

CAPABILITIES

OF ICP-MS

311

The analytical results obtained on various samples containing rare-earth elements are given below. They are discussed in terms of background, standardization, and detection limits. Optimum instrumental conditions are indicated. Semiquantitative and quantitative analyses were performed on three samples containing rare-earth elements, namely samples 89, 157, and 158, which are micas and feldspars from the Adolo Mine in Sidamo Province, Ethiopia. The samples (0.50 g) were dissolved in a microwave furnace using acid digestion (1 part HCl, 3 parts HNO,, and 6 parts HF) under pressure. They were then diluted to 100 ml and analyzed in a Perkin-Elmer ICP-MS spectrometer, Sciex Model Elan 500. Twenty-two rare-earth elements were determined, a calibration graph being constructed for each, using three standard solutions up to 0.1 mg/liter. Figure 2 provides an example of a calibration graph. Optimum parameters for quantitative analysis are shown in Table 2, especially as regards RF intensity, flow of gas and sample, setting of ion separation devices, resolution, measurement time, and number of measurements for each element. Figure 3 illustrates the histogram of the semiquantitative analyses of all the elements present in Sample 89, while the quantitative results obtained on the three samples are reported in Tables 3, 4, and 5 and and Figs. 4, 5, and 6. It can be seen from the results of quantitative analysis that minimum concentrations-less than around 5 x 10K5 mg/liter-can be detected by optimizing instrumental parameters. In terms of percentage on the solid sample, it is thus possible to determine with the same precision rare-earth grades down to limits of less than around 1 x 10W7%,namely &liter. CONCLUSIONS

ICP-MS is a relatively new analytical technique which uses a plasma torch coupled with a mass spectrometer. The main advantages of this technique compared to other instrumental techniques such as GFAAS and ICP-AES are: -detection limits better than those obtained with GFAAS and ICP-AES; -the possibility of detecting refractory elements; -high analysis speed (up to 90 elements in 5 min); -spectral simplicity; -the capability of determining individual isotopes of each element. ACKNOWLEDGMENT Special thanks are extended to Mr. A. Casciello for help in experimental work.

REFERENCES 1. Houk, R. S.; Fassel, V. A.; Flesch, G. D.; Svec, H. J.; Gray, A. L.; Taylor, E. Anal. Chem. 1980, 52, 2283. 2. Gray, A. L.; Date, R. Dyn. Mass Spectrom., 1981, 6, 252. 3. Date, A. R.; Gray, A. L. Analyst, 1981, 106, 1255. 4. Eur. Spectrosc. News, 1982, 43, 13. 5. Houk, R. S.; Fassel, V. A.; Svec, J. Dyn. Mass Spectrom. 1981, 6, 234. 6. Houk, R. S.; Svec, H. J.; Fassel, V. A. Appl. Spectrosc. 1981, 35, 380. 7. Houk, R. S.; Fassel, V. A.; Svec, H. J. Org. Mass Spectrom. 1982, 17, 1. 8. Houk, R. S.; Thompson, J. J. Mineralogist, 1982, 67, 238.

312 9. 10. 11. 12. 13. 14. IS. 16.

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Fassel, V. A. Science, 1978, 202, 183. Barnes, R. M. CRC Crit. Rev. Anal. Chem., 1978, I, 203. Douglas, D. J.; Houk, R. S. Prog. Anal. Atom. Spectrosc., 1985, 8, 1. Roboz, J. Introduction to Mass Spectrometry Instrumentation and Techniques, Wiley, New York, 1968. Dawson, P. Quadruple Mass Spectrometry and Its Applications, Elsevier, Amsterdam, 1976. Date, A. R.; Gray, A. L. Spectrochim. Acta, 1983, 388, 29. Date, A. R.; Gray, A. L. Int. .I. Mass Spectrom. Ion Phys., 1983, 48, 357. Smith, R. G.; Brooker, E. J.; Douglas, D. J.; Quan, E. S. K.; Rosenblatt, G. Geochem. Explor., 1984, 21, 385.