Report
Double-Focusing Mass Spectrometers
in ICPMS
I
nductively coupled plasma MS (ICPMS) has matured into one of the most successful methods in atomic spectrometry because of its sensitivity and ability to make multielement measurements. Nevertheless, since the very first days of ICPMS, the Achilles' heel has been the number of spectroscopic and nonspectroscopic interferences that have limited the analytical figures of merit. Many techniques have been considered to reduce interferences, but none 01 these can cope with the problem in general (i). All are limited to some specinc mterferences or are applicable for some selected elements only. The only general method to overcome limitations from spectroscopic interferences is high mass resolution, which necessitates using double-focusing instruments that combine a magnetic and
MS (ICPHRMS) applications The only general inhigh-resolution the analytical community, as reflected in increasing number of publications. (In method for overcom- the this Report, ICPMS will refer to general asof the method and ICPQMS will refer ing spectroscopic topects instrumentation with a low-resolution interference requires quadrupole mass analyzer.) interferences double-focusing Spectroscopic Since it was introduced for elemental analysis, the ICP has developed into one of the instrumentation. most successful sources. Initially, it had
an electric sector field analyzer. This is in contrast with the low-resolution instruments that manage with simpler and cheaper quadrupole analyzers. Although double-focusing ICPMS instruments have been on the market since 1988, they were not widely accepted because of their high cost. Prices have reLuc Moens cently dropped with the appearance of Ghent University (Belgium) second-generation instrumentation. The Norbert Jakubowski better value of these instruments gave Institut für Spektrochemie und angewandte Spektroskopie (Germany) strong impetus to the development of ICP
been used as an excitation source, primarily in combination with emission spectroscopy. During the past 15 years, it has been widely used as an ion source for MS. Certain technological problems related to the sampling of ions had to be solved for a successful combination of ICP and MS into ICPMS. In the ICP, ions are generated at atmospheric pressure; whereas for operating a mass spectrometer, a pressure of less than 10"5 mbar is a prerequisite. The bottleneck in between is the interface, which is used for ion extraction and pressure reduction.
Analytical Chemistry News & Features, April 1, 1998 251 A
Report
T a b l e 1 . M a s s resolution n e c e s s a r y t o s e p a r a t e t y p i c a l interferences. Nuclide
Resolution
16/~\ +
32g+
35C|160+
51V+
40Ar14N+
54ra+
40A_160+
56pe+
1801 2572 2088 2502 1916 2213 28033
Interfering ion
R < 3000 Molecular
Isobaric
40Ar1601I_J+
57pe+
40Ar-18Q+
58pe+ 58N|+
58pe+
R = 3000-7500 32g160+ 32s2+
48Tj+ 64Znt-
2519 4261
R= 7500-10000 40Ar35pi+
75 A s +
40Ar 2 +
80Srt+
In the early days of ICPMS, this was realized simply by a single, water-cooled, nozzle-like orifice with a diameter of only 50-70 um. The problem with this arrangement was that, in a cool boundary layer in front of the cone, many different molecular ions were generated. This problem was overcome by increasing the diameter of the entrance orifice to about 1 mm, so that the boundary layer is punctured and ions are directly sampled from the "undisturbed" plasma. This technique became known as continuum sampling and therefore the became known as the "sampler"
7775 9688
and decreased spectral interferences by orders of magnitude. Nevertheless, spectral interferences are still one of the main limitations (2,3). Spectroscopic interferences are caused by atomic or molecular ions having the same nominal mass as the analyte isotope of interest The resulting signal may disturb, or even obscure, the true analytical signal; so the accuracy of the determination as well as
the detection limits may be considerably deteriorated. The sources from which the interfering species may arise are many; so far, no generally accepted model exists to explain all of the contributing factors, but it is now well accepted that the interface still plays an important role in the appearance of molecular species (4). Spectroscopic interferences may be subdivided into isobaric atomic ions, multiply charged ions, intense adjacent signals, and polyatomic ions of various origin. Isobaric overlap exists when isotopes of different elements coincide at the same nominal mass. For each element, with the exception of indium, at least one isotope can be found that is free from isobaric overlap, but in many cases this will not be the most abundant isotope. Multiply charged ions wiil be found in the mass spectrum at a position m/z. Mainly doubly charged ions of the major matrix components and multiply charged ions of die discharge contribute to the mass spectrum The signals of neighboring ions with a very high intensity such as those coming from a matrix element may contribute to the sitmal r»f an arliarpnt i^nrnnp hv tailinc if me abundance is not sufficient Polyatomic ions may consist of atoms of the discharge gas and its contami
Because the gas flow through this sampler is much larger than before, the pressure must be reduced by differential pumping in two or more steps. This is why a aecond dozzle is placed downstream of the sampler and the space in between is evacuated by a forepump with a high pumping rate. Because of the high difference in pressure between the ICP and the first pumping stage, the ions are sucked into the interface and accelerated to supersonic velocities. To avoid turbulence at the second cone, it was machined with sharp edges to skim the ions from the supersonic beam, and therefore the name "skimmer" became widely used. The arrangement consisting of a sampler and a skimmer cone with diameters of about 1 mm became known as the "interface" (Figure 1). This was the breakthrough in ICPMS that made ion extraction more effective, and thus improved intensity, 252 A
Figure 1 . Sketch of an ICPHRMS. 1, ICP ion source; 2, interface, including sampler and skimmer cones; 3, transfer and focusing optics; 4, acceleration and beam focusing; 5, entrance slit; 6, electromagnet; 7, electric sensor; 8, exit slit; 9, conversion dynode; 10, electron multiplier.
Analytical Chemistry News & Features, April 1, 1998
nants, plus components of the solvent and matrix. Of all these different groups of spectro scopic interferences, polyatomic ions cause the most severe problems. Polyatomic ion interferences may be introduced by the analytical sample itself. For example, ox ides can survive passage through the hot zone of the plasma because of their higher bond strength. They may also arise as con taminants from either the chemical pretreatment stage, or from the discharge gas and from air trapped in the plasma. Spec troscopic interferences of this kind can, in principle be separated from the affected analyte isotope by high resolution. Mass resolution Whether an interference will be separated from an analytical signal depends on their mass difference and the instrument resolu tion. Mass resolution R is generally defined as m/Δm, in which Am is the mass diifer ence necessary to achieve a valley of 10% between two neighboring peaks of identical intensity at a mass m and mass m+Am. Because the intensities of neighboring peaks are rarely identical, however, an alter native definition is much more useful. In this definition, Am is derived from the peak width at the points in the profile that corre spond to 5% of the height. Thii spppoach wili lead to the same value as in the 10% defini tion mentioned before, if the neighboring peaks are equally high. It should be pointed out that, in general, the theoretical value is only a lower estimation for the resolution required because most often the signal in tensity of the interfering species exceeds the analyte intensity by orders of magnitude Typical examples of spectroscopic inter ferences are shown in Table 1. One of the most often discussed examples of a spec tral interference is 56Fe and 40Ar160+. The latter is a product created from the dis charge gas argon and from oxygen con tained in the solvent used. In this example, the isotopes ^Fe, 57Fe, and 58Fe can be used alternatively for analysis, but the 58Fe is isobarically interfered with by the iso tope 58Ni. Whereas, the others are inter fered with, to a certain extent, by 40Ar14N+ or 40Ar1601H+ the best choice overall is to use 57Fe However as its natural abun dance is only 2 2% the detection limit for
Figure 2. Spectrum at mass 28 of fivefold diluted human serum, a 2 0 ng/L silicon standard solution, and a sample preparation blank. Mass resolution is 3000. (Adapted with permission from Ref. 11.)
this element is extremely poor if low-. resolution instruments are used. Neverthe less, a resolution of less than 2500 is suffi cient to separate the spectral interference from the interfered analyte isotope at an m/z of 56. A more problematic example is 75As, if chloride is present in the analyte sample. In the case of a monoisotopic element, no alternative isotope can be chosen and the required resolution must be increased to about 7800, which is at the upper end of the required resolution scale shown in this Table. However, a resolution of 3000 will be sufficient to eliminate more than 90% of the interferences caused by polyatomic ions. Commercial high-resolution instru ments have a maximum resolution some where between 7500 and 12 000 so that most of the interferences in Table clearly be Nevertheless, high mass resolution is not a panacea to all types of spectroscopic interferences. Most isoboric interferences cannot be resolved by using commercial instruments. For example, 58Fe, 58Ni, and even some polyatomic argides, hydrides, and oxides need a resolution that comes close to the upper end of the achievable range; others require an even higher reso lution, which cannot be obtained. Double-focusing instruments High mass resolution is usually achieved with a double-focusing instrument on the
basis of combining magnetic and electric sector fields. These instruments have an even longer tradition in MS than do quadrupoles, but they are technically more sophisticated and therefore more expen sive. The heart of a double-focusing instru ment is a magnetic sector field. If ions that have uniform energy but differ in mass are injected perpendicular to a magnetic sector field, they pass thefieldon a circular trajec tory because of the Lorentz force. The ra dius of the trajectory depends on the mass of the ion leading to a mass dispersion. If the ion beam diverges from a slit with a certain angle, then the beam comes to a focus (directional focusing) behind the magnetic sector. Mass separation can now be realized if a slit is positioned behind the sectorfieldjust at this focus point, result ing in a well-defined radius and selection of a specific mass. Decreasing the slit width can be used to increase the mass resolution but only if the ions are mono-energetic, because any spread in energy will deterio rate the resolution. From this point of view the ICP is not an ideal ion source The endistribution of ions is far too broad to be accepted by a magnetic sector device operated in high mass-resolution mode Therefore, the energy dispersion of an electric sectorfieldis used to exactly com pensate for the energy dispersion of the magnet so that, in the whole device, only mass dispersion is left. Both magnetic and electric sector instruments have angular
Analytical Chemistry News & Features, April 1, 1998 253 A
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Table 2. Selected applications. Matrix Biological Human serum Biological standard reference materials Human urine Environmental Sediment and airborne particulates Water River and lake water Geological samples Grass Materials, nuclear, f o o d High-purity Y 2 0 3 and G d 2 0 3 Al203 Radioactive waste Wine
Elements
Reference
V, Fe, Cu, Zn, Ag, Al, Si, P, S, Ti, Cr, Mn, Cd, Sn, U 1, As, Se, rare earth elements
11,13
Pt, Rh, Pd, Ag, Pt
17, 18
Pb
19
Si, As, Cd, Cr, Co, Ni, Cu, Pb, Zn Cu, Ni, Mo, Pb Tc Pt
20,21 22 23 24
Rare earth elements V, Cr, Mn, Fe, Ga, C, Ni, Cu, Zn, Ce Tc, Th, U, Np Rare earth elements
25 26
focusing properties and the combined system focuses by angle and energy. This is why these instruments are called double focusing. Different geometries for combining a magnetic and an electric sector are possible, but double-focusing conditions can be realized with a well-defined combination of electric and magnetic sector angles only. The position sequence of the two analyzer components is optional. Traditionally, the electric sector is placed before the magnetic sector field. A 90° electrostatic sector combined with a 60° magnetic sector became widely known as Nier-Johnson geometry. Nowadays, the so-called "reverse geometry" with the electric sector behind the magnetic sector is usually considered advantageous because the high ion currents from the source are first reduced by analysis and only ions of the selected mass are subjected to the subseouenteneryanalysis This confighetos imorove abundance sensitivity In normal sector field arrangements, double-focusing conditions are obtainable at only one point, where the exit slit is located. Some very special arrangements have been developed, however, which guarantee double focusing in a whole plane. Such arrangements have been used for simultaneous detection by photoplates or by multicoliector Faraday cup equip254 A
14-16
27 28
ment. The latter is advantageous, especially for high-precision (relative standard deviation) isotope ratio measurements, because all isotopes of an element can be measured simultaneously. Therefore, precision is not limited by time-dependent fluctuations of the source. Although often not designed for high mass-resolution, precise isotope ratio measurements with doublefocusing instruments are an important application, for example in dating geological samples or taking measurements at nuclear plants. A schematic of an ICP double-focusing MS instrument is shown in Figure 1. An ion source, a sampling interface, and a subsequent lens system are necessary, as they
are in standard, low-resolution quadrupolebased instruments. A major difference is the need for an accelerating voltage of up to 8000 V. A special lens system is normalll used, shaping beams and focusing ions into the mass analyzer. The resulting bent geometry of doublefocusing instruments provides certain advantages when comparedwiththelinear geometry of quadrupoles, because it keeps the noise level low and guarantees a high transmission. No ion losses occur at otherwise offset lens systems or photon stop arrangements as is the case with quadrupole instrumentation. For these reasons, sensitivities of up to 109 cps per ug/mL for the low--esolution mode and noise levels of less than 0.1 CDS have been reported (5). Reducing noise and improving sensitivity can improve detection limits by orders of magnitude even when the instruments are operated in lowresolution mode The first ICPHRMS instrument with Nier-Johnson geometry on the market was the Plasmatrace I, introduced in 1988 by VG Elemental. Design of the ion source and the interface was based on quadrupole ICPMS systems by the same manufacturer. Nowadays, double-focusing instruments with high-resolution capabilities are available from different manufacturers (6). Some peculiarities of double-focusing instruments should be mentioned. At first, die peak form looks different. Quadrupoles are operated with constant peak width and therefore linearly increasing resolution. However, double-focusing instruments are operated for a fixed slit width with constant resolution, and therefore the absolute peak
T a b l e 3. S e l e c t e d s p e c t r a l i n t e r f e r e n c e s in a h u m a n s e r u m r e f e r e n c e m a t e r i a l {11,
23).
Element (isotope)
Polyatomic ion
Resolution required
Al ( 27 AI)
13f^^M+
Si ( 28 Si)
12Q160+ 14
1454 919 1557 958 968 1801 2035 1818 2715 2572
NJ
P ( 31 P) S ( 32 S) Ti ( 49 Ti)
,4ru16f-\1 | i +
,6
o2
35C|14N+ 32S"I6/~\1 l_l+ 12C37Q| +
V ( 51 V)
Analytical Chemistry News & Features, April 1, 1998
35C|160+
Concentration (standard deviation) 1.7 (0.54) ug/L 175(20) ug/L 122.7 (1.5) mg/L 1.113 (0.021) g/L 1.09 (0.17) ug/L
61 (6.6) ng/L
width increases with mass. For normalresolution settings, the peaks have a trape zoidal peak shape, which looks needle-like at low masses, and are broader at high masses. The scan speed with which a mass spectrum can be acquired is lower, even with modern laminated magnets, because double-focusing instruments require a longer settling time for achieving stable magnetic-field condi tions. This limits the number of isotopes that can be investigated when data acquisition is by special sample introduction systems such as laser ablation. Of course the most important capability of double-focusing instruments is high mass resolution. Increasing resolution re sults in decreased peak width. The interfer ing molecule can be separated from the analytical isotope but not without a reduc tion in sensitivity. It should be noted that increasing the resolution from 400 to 4000 decreases sensitivity by about 1 order of magnitude. Even in this case, the detection limits are better than for quadrupole instru ments by orders of magnitude. Applications
The availability of double-focusing ICPMS instruments has not only facilitated ongoing ICPMS-based investigations but has also triggered new applications. Elimination of several spectral interferences and improved detection limits and precision of isotope ratio measurements are important achievements. Table 2 lists a short selection of applications in biology, geology, environmental studies, ultrapure materials, and of long-lived radio isotope measurements. Isotope ratio measurements with ICPQMS typically have a precision of 0.1-1% (RSD on 10 replicate measure ments) for elements present at a suffi ciently high concentration so as to avoid imprecision because of poor counting sta tistics. With a double-focusing magnetic sector ICPMS instrument, precision can be improved to 0.05-0.2% (7,8). When a multicollector detection system is used instead of a single detector, precision can be fur ther improved to a level comparable with that of thermal ionization MS (9). The excellent performance of this type of equipment was demonstrated by the isotopic analysis of lead at a concentration of 426 ppm in the NIST 610 glass reference mate-
Figure 3. Concentrations of cadmium, lead, and uranium in an ice core sample taken approximately 1 0 m below the surface in Antarctica. For each element, the far left bar represents the concentration in picograms per gram in the innermost part of the core and the far right bar represents the concentration in the outermost part of the core. (Data from Ref. 12.)
rial, in which six independent measurements are performed over six days (10). Each mea surement is based on the average of 12 abla tions at different spots with each ablation taking 5 s, during which 40 laser pulses
created a crater 40 μm in diameter and 60-80 um deep. The results are in good agreement with data from thermal ionization MS data for the same material. Considering the simplicity of laser ablation ICPMS and the amount of sample preparationtimethat is needed for thermal ionization MS, the ad vantages of ICPMS are obvious. In the analysis of human serum, in addi tion to the polyatomic interferences men tioned earlier, many additional molecular ions occur that are based on compounds and elements abundantly present in the serum, such as carbon, sodium, sulfur, phosphorus, chlorine, and potassium. Ta ble 3 shows some of these interferences. By using appropriate sample preparation and calibration methods, many but not all of these interferences can be avoided or corrected. At a resolution of 3000 or more, how ever, most elements can be measured free from spectral overlap. Figure 2 shows the spectrum at mass 28 of fivefold diluted hu man serum reference material (11). The large CO+ peak is clearly separated from the 28Si+ analyte peak, allowing accurate determination of silicon. The application of ICPMS with high mass resolution increases the number of elements that can be reliably measured as demon strated by the data in Table 3 for the analysis of a second-generation serum standard refer ence material. Using ICPMS can help re duce the extraordinarily intense efforts re quired for certifying reference materials. Otherwise, more laborious strategies using complementing spectroscopic techniques or neutron activation analysis are the only alter native. In combination with the isotope dilu tion technique ,acuracies san be realized thatcancompete with reference materials analyzed by definitive techniques The Antarctic ice cap is considered one of the best preserved and most detailed archives of the variability in the chemical composition of the atmosphere and there fore of great value for environmental re search. However, measuring ultratrace concentrations in ice and snow tends to be extremely difficult. Thus, the analytical method to be used should offer very low detection limits without analyte enrich ment, low sample consumption, and mul tielement analysis capability.
Analytical Chemistry News & Features, April 1, 1998 255 A
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