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Lecture 15
ANALYTICAL METHODOLOGY
SAMPLE PREPARATION
Isotopic analysis can be performed on minerals, rocks, and solutions. Analysis of minerals requires that the first be separated from the rocks that contain them. This begins with crushing, usually fol- lowed by some form of magnetic or density separation. The latter may involve devices such as shaking table or heavy liquids, the former usually involves a device called a “Franz”. The final step is often hand-picking under a microscope. The final step of preparation is usually to grind the sample to a powder, which greatly facilitates dissolution. Care must be excercised not contaminate the sample in this step, something even apparently “inert” grinding materials such as alumina oxide, titanium car- bide, and opal can do. In some cases, such as analysis of Pb isotope ratios in many basalts, it is better to avoid powdering and use crushed fragments (~10 mg each) instead. Water samples, of course, do not require this preparation, but they should be collected in carefully cleaned containers. In most cases, it is advisable to acidify the sample immediately following collection to avoid absorption of particle- reactive elements, such as Pb, Hf, and the REE, on container walls.
PREPARATIVE CHEMISTRY
The techniques described below generally require that we first purify the element to be analyzed. Thus some form of preparative chemistry is usually required. The first step for solid samples is disso- lution. For silicate rocks, this requires hydrofluoric acid (HF). Many fluorides, particularly CaF 2 and MgF 2 are highly insoluble, however. To insure a soluble sample once digestion is complete, a small amount of a high-boiling point acid such as perchloric acid (HClO 4 ) is usually to the HF. Basaltic rocks and some minerals can be digested in teflon beakers on hot plates, generally overnight, but rocks with more resistant minerals, such as zircons, as well as those minerals, must be digested in pressure vessels (“bombs”) in ovens at 200˚ C or more. Once digested, the sample is evaporated to dryness. The sample is again taken up in acid solution and the element of interest isolated by ion exchange. Pb and Sr can be isolated with a single step ion exchange process, while Hf and Nd require 2 or 3 separate ion exchange steps. With water samples, digestion is, of course skipped, and the procedure begins with ion ex- change. The separation of Os involves entirely different procedures, due to the volatility of OsO 4. There are several techniques, one of which is “fire assay” in which the rock powder is mixed with a flux such as nickel sulfide, heated and fused. The platinum group metals will concentrated in the nickel sulfide, making their ultimate purification easier. As second technique is Carius tube digestion. In this tech- nique, sample powder is heated with aqua regia (HCl and HNO 3 ) in sealed glass tubes (Carius tubes). In both approaches, Os is ultimately purified by distillation of OsO 4 from nitric acid solution. The ultimate product of these techniques is a small amount, picograms to micrograms, of a salt of the element of interest.
THE MASS SPECTROMETER
In most cases, isotopic abundances are measured by mass spectrometry. The exceptions are, as we have seen, short-lived radioactive isotopes, the abundances of which are determined by measuring their decay rate, and in fission track dating, where the abundance of 238 U is measured, in effect, by in- ducing fission. (Another exception is spectroscopic measurement of isotope ratios in stars. Frequencies of electromagnetic emissions of the lightest elements are sufficiently dependent on nuclear mass that emissions from different isotopes can be resolved. We will discuss this when we consider stable iso- topes.) A mass spectrometer is simply a device that can separate atoms or molecules according to their mass. There are a number of different kinds of mass spectrometers operating on different principles. Undoubtedly the vast majority of mass spectrometers are used by chemists for qualitative or quantita-
Lecture 15
tive analysis of organic compounds. We will focus exclusively, however, on mass spectrometers used for isotope ratio determination. Most isotope ratio mass spectrometers are of a similar design, the magnetic-sector, or Nier mass spectrometer*, a schematic of which is shown in Figure 15.1. It consists of three essential parts: an ion source, a mass analyzer and a detector. There are, however, several variations on the design of the Nier mass spectrometer. Some of these modifications relate to the spe- cific task of the instrument; others are evolutionary improvements. We will first consider the Nier mass spectrometer, and then briefly consider a few other kinds of mass spectrometers.
The Ion source
As its name implies, the job of the ion source is to provide a stream of energetic ions to the mass ana- lyzer. Ions are most often produced by either thermal ionization , for solid-source mass spectrometers, electron bombardment , for gas-source mass spectrometers, or by inductively exciting a carrier gas into a plasma state in the case of inductively coupled plasma–mass spectrometers (ICP-MS). In thermal ionization, a solution containing the element(s) of interest is dried or electroplated onto a ribbon of high-temperature metal, generally Re (rhenium), Ta (tantalum), or W (tungsten), welded to two supports. The ribbon is typically 0.010" thick, 0.030" wide and 0.3" long. In the simplest situation, the ribbon is placed in the instrument and heated by passing an electric current of several amperes through it. At temperatures between about 1100° C and 1800° C the sample evaporates in the low pres- sure environment of the mass spectrometer. Depending on the element and its first ionization potential (i.e., the energy required to remove one electron from the atom), some or all the atoms will also ionize. The efficiency with which the sample ionizes determines the amount of sample needed. The alkali metals ionize quite easily; the ionization efficiency for Cs, for example, approaches 100%. For some other elements, it is as low as 0.1% or lower. On top of this, modern mass spectrometers have transmis- sion efficiencies of only 50%, which is to say only 50% of the ions produced reach the detector. In some cases, the rare earth elements for example, there is a tendency for the element to evaporate as a molecule, most typically an oxide, rather than as a metal atom. This problem can be overcome by us- ing two or three filaments. In this case, the sample is loaded on one or two filaments, from which it is evaporated at relatively low temperatures. The neutral atoms or molecules are then decomposed and ionized by another filament kept at much higher temperature (~1900-2000°C). In general, a double or
- (^) It was developed by Alfred Nier of the University of Minnesota in the 1930's. Nier used his instrument to deter- mine the isotopic abundances of many of the elements. In the course of doing so, however, he observed variations in the ratios of isotopes of a number of stable isotopes as well as Pb isotopes and hence was partly responsible for the fields of stable and radiogenic isotope geochemistry. He also was the first to use a mass spectrometer for geochro- nology, providing the first radiometric age of the solar system. In the 1980's he was still designing mass spectrome- ters, this time miniature ones which could fly on spacecraft on interplanetary voyages. These instruments provided measurements of the isotopic composition of atmospheric gases of Venus and Mars. Nier died in 1994. 60° Ion Source Collector Array Figure 15.1. The magnetic sector or Nier mass spectrometer. This in- strument uses a 60° magnetic sector, but 90° magnetic sectors are also sometimes used.
Lecture 15
ICP-MS instruments came on the market a decade after quadrupole ICP-MS instruments and are now at the point where they achieve accuracies competitive with thermal ionization instruments. Combined with their generally higher ionization efficiency and hence higher sensitivity, they produce results that are superior to thermal instruments for several elements. As they continue to develop they will likely entirely replace thermal ionization instruments. After the ions are produced, they are accelerated by an electrostatic potential, typically in the range of 5 - 20 kV for magnetic sector mass spectrometers (in thermal ionization mass spectrometers, the filament with the sample are at this potential). The ions move through a series of slits between charged plates. The charge on the plates also serves to collimate the ions into a beam. Generally the potential on the plates can be varied somewhat; in varying the potential on the plates, one attempts to maximize the beam intensity by 'steering' as many ions as possible through the slits. Thus the source produces a nar- row beam of nearly monenergetic ions.
The Mass Analyzer
The function of the mass analyzer is two-fold. The main purpose is to separated the ions according to their mass (strictly speaking, according the their mass/charge ratio). But as is apparent in Figure 15.1, the mass analyzer of a sector mass spectrometer also acts as a lens, focusing the ion beam on the detec- tor. A charged particle moving in a magnetic experiences a force
F = q v × B 15.
where B is the magnetic field strength, v is the particle velocity, and q is its charge (bold is used to de- note vector quantities). Note that force is applied perpendicular to the direction of motion (hence it is more properly termed a torque), and it is also perpendicular to the magnetic field vector. Since the force is always directed perpendicular to the direction of motion, the particle begins to move in a circu- lar path. The motion is thus much like swinging a ball at the end of a string, and we can use equation for a centripetal force:
F = m
v^2
r
This can be equated with the magnetic force:
m
v^2
r
= qv! B 15.
The velocity of the particle can be determined from its energy, which is the accelerating potential, V, times the charge:
Vq =
mv^2 15.
Solving 6.4 for v^2 , and substituting in equation 15.3 yields (in non-vector form):
V
r
2 Vq
m
B 15.
Solving 15.5 for the mass/charge ratio:
m
q
B^2 r^2
2 V
relates the mass/charge ratio, the accelerating potential, the magnetic field, and the radius of curvature of the instrument. If B is in gauss, r in cm, and V in volts, this equation becomes:
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m
q
= 4.825! 10 "^5
B^2 r^2
V
15.6a
with m in unified atomic mass units and e in units of electronic charge. For a given set of conditions, a heavier particle will move along a curve having a longer radius than a lighter one. In other words, the lighter isotopes experience greater deflections in the mass analyzer. The radius of the Cornell mass spectrometer is 27 cm; it typically operates at 8 kV. Masses are se- lected for analysis by varying the magnetic field (note that in principle we could also vary the accel- erating potential; however doing so has a second order effect on beam intensity, which is undesirable), generally in the range of a few thousand gauss. Mass spectrometers built in the last 25 years or so employ a design 'trick' that has the effect of ex- tending their effective radius, which results in higher resolution (better separation between the masses at the collector). It was shown in the 1950's that if the ions entered the magnetic field at an angle of 26.5° rather than at 90°, the effective radius of the mass analyzer doubles. Thus the Cornell instrument has an effective radius of 54 cm. An additional advantage of this ‘extended geometry’ is that the ion beam is focused in the ‘z’ direction (up-down) in addition to the x-y direction. This is an important ef- fect because it allows the entire ion beam to enter the detector, which in turn allows the use of multiple detectors. In addition, current commercial mass spectrometers have further modifications to the mag- net pole faces to produce a linear focal plane, which is helpful in the multiple collectors currently in use. Collisions of ions with ambient gas result in velocity and energy changes and cause the beam to broaden. To minimize this, the mass spectrometer is evacuated to 10-^6 to 10-^9 torr (mm Hg ~ 10-^3 atm). Where very high resolution is required, an energy filter is employed. This is simply an electrostatic field. The electric field force is not propor- tional to velocity, as it the magnetic field. Instead, ions are deflected according to their energy. The radius of curvature is given by:
R =
2 V
V 2
where V 2 is the electrostatic potential of the energy filter and V is the energy of the ions (equal to the accelerating potential). Ion sputtering produces ions of a variety of en- ergy, hence an energy filter is generally re- quired with this method of ion production. ICP sources also produce ions that are less monoenegetic that thermal ionization or electron bombardment, so energy filters are also used with high precision ICP mass spectrometers. Instruments employing both mass and energy filters are sometimes called double focusing instruments. An example is shown in Figure 15.2.
The Collector
In general, ions are 'collected', or de- tected, at the focal plane of the mass spec- X-Y lens Y and Z deflectors focus Ion Source Electrostatic Analyzer Magnetic Analyzer Beta Slit Defining Slits Collector Alpha Slit Figure 15.2. A double focusing or Nier-Johnson mass spectrometer with both magnetic and electrostatic sec- tors. After Majer (1977).
