Anal. Chem. 1995, 67,2479-2485
Determination of Elemental Content of Rocks by Laser Ablation Inductively Coupled Plasma Mass Spectrometry Frederick E. LicMe US. Geological Survey, P.O. Box 25046, WS 973,Denver Federal Center, Denver, Colorado 80225
A new method of analysis for rocks and soils is presented using laser ablation inductively coupled plasma mass spectrometry. It is based on a lithium borate fusion and the free-running mode of a NWAG laser. An Ar/N2 sample gas improves sensitivity 7 x for most elements. Sixty-threeelements are characterizedfor the fusion, and 49 elements can be quantified. Internal standards and isotopic spikes ensure accurate results. Limits of detection are 0.01 pglg for many trace elements. Accuracy approaches 5%for all elements. A new quality assurance procedure is presented that uses fundamental parameters to test relative response factors for the calibration. Geological samples are often cited as among the most difticult materials to analyze because of their wide composition range and complex mineralogy. These minerals vary in nearly all physical and chemical parameters including size, hardness, composition, and solubility. Trace elements may reside within various minerals, may be a major element of a trace mineral, or may exist as a combination of both. An elemental analysis of rocks is important to classify the type of rock and to determine its origin and economic value. Historically, the determination of trace and minor elements in granitetype rocks has depended on a host of analytical techniques including the following: instrumental neutron activation INAA,'s2 energy-dispersive and wavelength-dispersiveX-ray fluorescence spectrometry3-6 inductively coupled plasma atomic emission and mass spectrometry OCP-AES, ICPMS) ,7,8 and atomic absorption spectrometry.gJ0 As a consequence, many elemental ratios used for petrogenetic modeling are calculated using data from different analytical techniques and subsamples. Most ICPMS methods involve dissolving the sample either with acids or fusion prior to instrumental analysis. Lithium borate fusions are popular because all major elements and many trace elements can be determined following dissolution of the fusion (1) Gordon, G. E.; Randle, IC;Goles, G. G.; Corliss, J. B.; Beeson, M. H.; Oxley, S. S. Geochim. Cosmochim. Acta 1968,32, 369-396. (2) Baedecker, P. A; Rowe, J. J.; Steinnes, E. /. Radioanal. Chem. 1977,40, 115- 146. (3) Bertin, E. P. Principles and Practice ofX-ray Spectrometric Analysis; Plenum Press: New York, 1979. (4) Giaque, R. D.; Garrett, R B.; Goda, L. Y. Anal. Chem. 1977,49, 62-67. (5) Adler, I. X-ray Emission Spectrography in Geology; Elsevier Publishing Co.: New York, 1966. (6) Fabbi. B. P. Am. Mineral. 1972,57, 237-245. (7) Crock, J. G.; Lichte, F. E. Anal. Chem. 1982,54, 1329-1333. (8)Lichte, F. E.; Meier, A. L.; Crock, J. G. Anal. Chem. 1987,59, 1150-1157. (9) Baedecker, P. A ; McKown, D. M. Methods for Geochemical Analysis. US. Geol. Sum. Bull. 1987,1770, Hl-Hl4. (10) Viets, J. G.; O'Leary, R. M. /. Geochem. Erplor. 1992,44, 107-138. This article not subject to U.S. Copyright. Published 1995 Am. Chem. Soc.
bead. The fusion/solution method requires relatively high flux to sample ratio to maintain stable solutions. High flux to sample ratios increase salt deposits on the sampling orifices of the mass spectrometer and the blank levels. Metal oxide ion levels are high because of the water and cause significant isobaric interferences for many trace elemenk8 Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) is a new analytical technique that seems destined to play a major role in the analysis of geologic materials because of its wide dynamic range and elemental sensitivity. This technique uses laser energy to directly sample solid material and present it as a fine aerosol to the plasma. The fundamental processes that occur during the laser/sample interactions are quite complex and have been studied in detail.11-14 Briefly, laser sampling occurs when the fluence from the laser is high enough to cause the sample to boil. One method used to adjust the fluence is through the electronic discharge of the laser. For the Nd/YAG laser, the Q-switched mode results in a pulse width lo0 ,us. This large change in discharge mode greatly alters the characteristics of the laser. Several methods of analysis have been devised for the LAICPMS, and it is rapidly gaining acceptance as a microprobe technique's that can be applied to bulk analysis. Methods based on pressed pellets have offered semiquantitative re~ults.'~J~ Perkins et al.'* recently published a method for quantitative analysis of rocks based on a lithium borate fusion using a Q-switched laser. They noted the color of the glass disk varies with the composition of the rock which affected the sampling efficiency of the laser. This study develops a new method of analysis based on a lithium borate fusion and laser ablation ICPMS using the freerunning mode of a Nd/YAG laser. Iron oxide is added to produce black disks that are more uniformly sampled with the laser. The method offers a low flux to sample ratio and has no significant isobaric interferences from metal oxide ions. Internal standards (11) Thompson, M.; Chenery, S.; Brett, L. /. Anal. At. Spectrom. 1990,5, 49-
55. (12) Allmen, M. Laser Ablation of Electronic Materials: Basic Mechanisms and Application, Proceedings of the 1991 Summer School on Laser Ablation of Electronic Materials, Gironde, France, September 16-19, 1991; Elsevier Science Publishers BV.: Amsterdam, 1991. (13) Dittmar, IC;Wennrich, K Prog. Anal. At. Spectrosc. 1984,7, 551. (14) Darke, S. A; Tyson, J. F. 1.Anal. At. Spectrosc. 1993,8, 145-209. (15) Jackson S. E.; Fryer B. J.; Grosse W.; Hearley D. C.; Longerich H. P.; Strong D. F. Chem. Geol. 1990,83, 119. (16) Gray, A. L. Analyst 1985,110, 551. (17) Broadhead, M.; Broadhead, R; Hager, J. W. At. Spectrosc. 1990,11, 205. (18) Perkins, W. T.; Pearce, N. J. G.; Jeffries, T. T. Geochim. Cosmochim. Acta 1993,57,475-482.
Analytical Chemistty, Vol. 67, No. 14, July 15, 1995 2479
wl-I
F] --1-
MASS SPECTROMETRY
1
LASER MASS
SPECTROMETER
CELL
Figure 1. Block diagram of instrument. Table 1. Instrumental Conditions
Laser type Nd/YAG wavelength (nm) 1064 discharge free-running power (d) 450 repetition rate (pulses/s) 8 laser pulses/spot 4 no. of spots/sample 6x7matrix raster repeats 2 Mass Spectrometer Fisions Ins. PlasmaQuadII+ mass range (amu) 22-240 channeldamu 17 channels integrated 15 dwell time per channel (us) 40 integration time (s) 150 Inductively Coupled Plasma power (W) 1250 gas flow rates (Wmin) coolant 17 auxiliary 0.4 sample argon 0.50 nitrogen 0.080 20 sampling distance load coil (mm)
and isotopic spikes ensure accurate results. A single method and a single sample preparation are applicable for the quantjfication of 49 geochemically important elements in rocks, soils, and sediments. EXPERIMENTAL SECTION Apparatus. Figure 1gives a block diagram of the instrument. A Fisions Instruments PlasmaQuad 11+ ICPMS equipped with a Spectron Inc. Nd/YAG laser is used for all studies. The sample stage is under computer control with stepper motors providing x-y movement in 0.05 pm steps. All instrumental conditions are listed in Table 1. Sample Preparation. A flux mixture is prepared from 5.4 parts LizB407,0.6 parts Li2C03,3.0 parts LiOH6Hz0, and 1.5parts FezO3. Preparation method: weigh 0.15 g of sample and 0.20 g of flux into a disposable 10 mL polycarbonate titration cup. Spread the powders over the bottom of the cup, and add 75 pL of the internal standard solution at the concentration levels listed in Table 2. Molybdenum and tungsten isotopes are prepared separately in 1%LiOH and are added as a 20 pL solution. Mix the liquid and powders with a Teflon stirring rod and transfer the wet mixture to a 15 mL flat-bottom graphite crucible. Place the crucible into a 1040 "C muffle furnace for 25 min, remove, and allow to cool. 2480 Analytical Chemisrry, Vol. 67,No. 74, July 75, 7995
Calibration. A plasma blank and a single calibration standard are used to determine the sensitivity and background factors for each element. The calibration standard was prepared to have normal abundances of the major and minor elements by mixing USGS standards GSElg and AGV-1 with NIST reference material 270420 in a 1:4.5:4.5 ratio, respectively. In addition, this mixture was spiked with several metal oxides to bring all trace elements up to 50 p d g . Analysis. Place 12 samples flat side down into a Teflon carousel with two standards and a blank, and put them in the sample cell shown in Figure 1. The samples are arranged to analyze a standard at the beginning and end of each set of 12 samples. A blank is used to first condition the ion optics of the instrument and then as a wash sample to reduce carry-over between the standard and the first sample. Analyze the samples and standards using the conditions listed in Table 1. Integrated intensities are compared to a single standard and plasma background. RESULTS AND DISCUSSION Most multiple element techniques require optimization of many interrelated parameters including sample preparation and instrumental conditions. In this method, a rock sample is fused with lithium borate and then sampled using the free-running mode of a Nd/YAG laser. Laser-sampled particles and vapors are carried in an argon stream to an inductively coupled argon/nitrogen plasma where they are vaporized and ionized before being extracted into the quadrupole mass spectrometer for analysis. Each feature of the method is discussed below in detail. Sample Preparation. Lithium betaborate was studied as the fluxing agent because it dissolves most resistate minerals and forms physically stable beads that are easily handled. Unfortunately, the chemical lots of metaborate that were tested contained high blanks. Therefore, lithium hydroxide was mixed with lithium tetraborate to form the basic flux. LiCO3 was added to offer some mixing of the sample as the fusion progressed through the evolution of bubbles of COZ. Fusion disks prepared from this flux range from clear to jet black and depend on the iron content of the sample. Clear blank discs were difficult to sample using either the Q-switched or freerunning mode of the laser. Therefore, ferric oxide was added to the flux mixture at lo%,which consistently produced black fusion discs that were efficiently sampled by the laser. The flux to sample was studied to find the lowest possible amount of flux that would dissolve the rock matrix. Below 1:1, the fusion disk contained undissolved inclusions of the original rock. Between 1:l and 4:1, a consistent glass was obtained and no improvement was seen between them. Therefore, a somewhat conservative 1.3:l ratio was selected and studied in detail. This differs considerably from the 5:l ratio that is required for methods that dissolve the bead for direct nebulization and should lower blanks and reduce deposits on the sampling orifices. The last phase of the sample preparation study was to minimize the temperature and time required for the fusion. A temperature of 1040 "C was selected because it was high enough to melt all materials and produce a fluid solution. The time required for (19) Meyers, A. T.; Havens, R. G.; Connor, J. J.; Conklin, N. M.; Rose, H. J., Jr. U S . Geol. Sum. Prof: Pop. 1976,No. 1013. (20) Govindaraju, IC, Ed. Geostand. News[. 1994, 18 (Special Issue).
Table 2. Isotopes Used for Calculatlons
element Na Mg Al Si P S C1
K Ca
sc
Ti V Cr Mn Fe co Ni cu Zn Ga Ge As Se
Rb Sr Y Zr
Nb Mo
Ru Rh Pd Ag
Cd
isotope
int std
detectn limit @g/g)
element
isotope
5 0.1 10 600 1.4
In Sn
115 116-120 123 126, 128 133 134-137 139 140 141 143, 146 149, 152, 154 151,153 155-158,160 159 161-164 165 166-168 169 171-174 175 177-180 181 182,184, 184 185,187 190, 192 191, 193 194-196 197 205 206,208 209 232 238
87Rb(120 pg/mL) 23 86Sr (80 pg/mL) 25,26 27 29 31 33,34 35 87Rb 39 86Sr 42,44 45 46, 47, 49 51 52,53 55 57,58 59 'j2Ni (20 pg/mL) 62Ni 60 63 65Cu (40 pg/mL) n !% (60 pg/mL) 66,67 71 72,74 lzlSb (8 pg/mL) 75 82 a7Rb 85 88 89 90,91 95 97Mo (20 pglmL)C 95,98 99,101 103 105,106 107,109 112, 113 W d (16 pg/mL)
Sb Te
cs
Ba La Ce Pr Nd Sm
a a
100 200 0.1 0.1 0.1 0.1 0.1 b 0.03 0.2 0.1 0.1 0.03 0.03 0.03
Eu Gd
Tb DY Ho Er Tm Yb Lu Hf Ta W Re
a
0.01 0.02 0.02 0.03 0.02 0.03
Au TI
a
Pb
a
Bi Th U
os Ir
Pt
a
0.02
int std
detectn limit @g/g)
97Mo
a
97Mo IzlSb 121Sb 87Rb
0.02 0.06 a
0.01 0.02 0.02 0.02 0.02 0.05 0.03 0.02 0.03 0.01 0.03 0.01 0.03 0.01 0.03 b 0.03 0.01 0.02
Lu (80 pg/mL)
l8W(16 pg/mL)
oO.12oj v
conc, = (Int, - IntbInt(36,),/Int(36,)b)SensIntLu,,,/IntLu,a,
II
I
I
/ IA
MASS (amu)
Figure 5. Normalized sensitivity vs mass curve.
was sampled because of the improved sensitivity, and a higher dissociation temperature of the plasma (6200-6700 K) improved the vaporization of the particles. Mass Spectrometer. The ion lens settings of the mass spectrometer were adjusted to give a mass response as shown in Figure 5. This response gives low sensitivityfor low-mass major elements like sodium without compromising the sensitivity for trace elements at high masses such as uranium. The lenses are adjusted to give maximum sensitivity at the Arzt at mass 80 and then reduced a factor of 10 by increasing the collector lens setting. Beryllium at m/z 9 is the only element that cannot be determined in most geological materials because of this selection of lens settings. Calculations. All calculations are performed in a spreadsheet using linear relationships. The ablation efficiency and mass response are defined by the recovery of the internal standard lutetium (m/z 175) and the sum of the major oxides (m/z 29). Iron in the flux and water content in the samples are used in the major element summation. Mass response shift is interpolated for each mass between mass 29 and 175. Lutetium is applied directly to all elements above m/z 175. Finally, response factors for the stable isotopic spikes are applied following the ablation and mass normalization calculation. Changes in background intensity are corrected by taking the ratio of the background to 36Ar.
A generalized equation for the high masses is 2484 Analytical Chemistry, Vol. 67, No. 14, July 15, 1995
where Int is the intensity in counts per second, Sens is the calibrated sensitivity of element m, m is the metal of interest in the sample, b is the blank, and s is the sample. The interpolation between internal standards simply lengthens the equation. Quality Assurance. A quality assurance program was developed to monitor the calibration factors for each element based on theoretical response factors. This is an expanded analysis of the experiment described by Houk et aLZ3and uses the Boltzman equation to describe three temperatures: ionization, secondary ionization, and oxide dissociati~n.~~ It also adds a transport efficiency correction for elements that sublime during laser sampling. The curve as shown in Figure 5 combines these four factors with the isotopic abundance and plots the normalized response vs atomic mass. Each factor is varied to find the best curve fit. Typical temperatures of 7350, 7350, and 6750 K are calculated for the plasma, assuming 1.5 x 1015 electrons/cm3, and are used to calculate the degree of ionization, secondary ionization, and dissociation,respectively,for each element in the standard. Table 3 gives the calculated factors for each of the elements studied. The sublimation factors are given in Table 4, and are varied as a group. The temperatures and enhancement factors are excellent diagnostic tools for the instrument performance, and the individual points on the curve can be used to quickly spot individual elements that are in error by only a few percent. Finally, the coefficient of variation from the curve fit yields a quantifiable value for the calibration that can be useful for a generalized quality control program. This analysis is useful for many other reasons in addition to spotting outlying values. For instance, the data in Table 3 show that Ba is doubly ionized at 26%even though less than 1%of the Baz+is measured at mass 69, and Ce is present at > 10%as CeOt although CeO+ is measured at 0.08%at mass 156. These low interference factors are probably due to collisional deactivation of the larger or faster ions during transport through the instrument. Results. The precision of the method was determined by analyzing 10 replicate preparations of a single sample. Average (25) Gaydon, k G. Dissociation Energies and Spectra ofDiatomic Molecules, 3rd ed.; Chapman and Hail: London, 1968.
; 2.00 N
-
g 1.50
Z
0
f
1.00
C
P 0.50 0.00
I
b------I Lo Ce Pr Nd
-C
NBS 2704
Sm Eu Gd Tb Dy Ho Er Tm Yb Element NOS
+
1646a
* CSD-2
I
Figure 6. Rare earth element chondrite normalized plot.
relative standard deviation for the 48 elements reported was -3%. Data for several reference materials are given in Tables 5-7. These comparisons are based on triplicate analyses of the reference material and suggest the accuracy of the method is -5%. (26) Wilson, S. A. personal communication, 1994.
The precision and accuracy is sufficient for most geochemical studies that use major element concentrations. For the trace elements and the rare earth elements, this level of accuracy combined with the low limits of detection (see Table 2) offers perhaps the most powerful single analytical method ever devised. From just a small selection of the data, the chondrite normalized rare earth plots shown in F w r e 6 suggests the method is useful for petrogenetic studies using these elements. One technician can prepare and analyze -70 samples/day. There is no toxic waste material at the end of the analysis. Except for the cost of the instrumentation, operating expenses are quite low. Finally, depending on the nature of the sample, the sample size can be varied from a few milligrams to several grams with minimal modifcation of the general method. ACKNOWLEDGMENT Computer programs for integrating the mass spectra and transferring the data into a spreadsheet format were written by Joseph Christie. Received for review November 30, 1994. Accepted April
27,1995.@ AC9411642 Abstract published in Advance ACS Abstracts, June 1, 1995.
Analytical Chemistry, Vol. 67, No. 14, July 15, 1995
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