Anal. Chem. 1995,67,131-138
Semiquantitative Analysis with Laser Ablation Inductively Coupled Plasma Mass Spectrometry Evan F. Gromwell* ISM Storage Systems Division, 5600 Cottle Road, San Jose, Califomia 95193
Peter Atvowsmith Celestica Inc., EM4 Don Mills Road, Toronto, Ontario M3C 1V7, Canada
Techniques for semiquantitative analysis of solid materials by laser ablation sample introduction for inductively coupled plasma mass spectrometry (ICPMS) are investigated. Under some ablation conditions, nonrepresentative subsampling or fractionation is a major source of analytical error, particularly for samples that have relatively low melting or boiling point components. Use of high laser fluence and reduced spatial overlap of laser pulses on the sample surface was found to reduce fractional ablation. Improved analytical results were obtained by calibration with aqueous solution standards using two-channelsample introduction to give equivalent ICP conditions for ablated material and a solution standard. This method has a further advantage of not requiring solid reference standards. Semiquantitative analysis results for steel and glass SRMs obtained under matched plasma conditions are discussed. Inductively coupled plasma mass spectrometry (ICPMS) has gained wide acceptance over the last 10 years as a technique for quantitative analysis and trace element detection.'I2 ICPMS is most commonly used for the analysis of solution phase samples because accurate, multielement calibration standards are readily available and matrix-matching of the sample and standard is generally straightforward. Efficient methods of solution sample introduction, such as ultrasonic nebulization and direct injection nebulization,3 and improvements in the design of ion transfer optics have further decreased detection limits to the femtogram per gram (parts per quadrillion) range for solution analysis: The use of pulsed laser ablation (LA) sample introduction for ICPMS is a more recent development that is of particular interest because it enables analysis of solid materials to be performed directly, without sample di~solution.~-~~ (1)Houk, R S.Anal. Chem. 1986,58,97A (2) Holland, G.,Eaton, A N., Eds.; Applications of Plasma Source Mass Specfrometm Thomas Graham House: Science Park, Cambridge, U.K, 1991. (3)Wiederin, D.R;Houk, R S. Appl. Spectrosc. 1991,45,1408. (4)Jarvis, K E.; Gray, A L.; Houk, R S. Handbook ofhductively Coupled Plasma Mass Spectromety; Blackie: Glasgow, Scotland, 1991. (5) Arrowsmith, P. Anal. Chem. 1987,59, 1437. (6)Pearce, N. J. G.; Perkins, W. T.; Fuge, RI. Anal. Atom. Spectrom. 1992,7, 595. (7) van de Weijen, P.;Baiten, W. L. M.; Bekkers, M. H. J.; Vullings, P. J. M. G. J. Anal. Atom. Spectrom. 1992, 7, 599. (8) Crain, J. S.; Gallimore, D. L. J. Anal. Atom. Spectrom. 1992,7, 605. (9)Chenery, S.:Hunt, A; Thompson, M. I. Anal. Atom. Spectrom. 1992, 7, 647. (10)Denoyer, E. R /. Anal. Atom. Spectrom. 1992, 7, 1187. 0003-2700/95/0367-0131$9.00/0 0 1994 American Chemical Society
There are many instances when laser ablation sample introduction is advantageous. Perhaps the most common is when a sample, such as a ceramic, glass, or acid resistant alloy, is difficult or impossible to prepare for solution sampling. Laser ablation allows analysis of such materials with minimal sample preparation. Ablation is particularly useful when spatial information is required and with carefully designed optics lateral and depth resolutions of 1pm can be achieved. Common examples of spatially resolved analysis include grains and inclusions in minerals, depth analysis of thin h s , small features in electronic devices, and contaminants and particulates on surfaces. With laser sampling, it is possible to analyze these featureswithout the introduction of large amounts of background or substrate material. Although the relative sensitivity of LA-ICPMS is poorer compared to solution ICPMS, because much less material is injected into the ICP, trace detection with submicrogram per gram (ppm) sensitivity in the bulk is routinely achieved. Laser ablation also has poorer precision, partly because of fluctuations in the amval time and size of individual ablated particles transported to the ICP. There are a number of concerns and problems that have prevented LA-ICPMS from becoming a general technique for analysis of materials. In part this is because high-energy lasermaterial interactions are complex and not completely understood.12 Practical concerns,which have been discussed previously, include the difficulty of obtaining or making matrix-matched standards that contain all the elements of interest and the accuracy of the resulting calibration.l3-I6 The problem of matrix-matching arises because the ablation yield varies with material properties, such as reflectivity,thermal conductivity, and melting and boiling points. To some extent, variations in total ablation yield can be corrected by internal standardizationusing an element present in both the standard and sample. On the other hand, nonrepresentative subsampling,or fractional ablation, is generally difficult to correct since it causes enhancements or reductions in relative analyte signal intensities and sensitivities. In terms of a matrix effect, fractionation is analogous to a nonspectral analyte interference that may depend on the nature and composition of the matrix, the ablation conditions, and/or the properties of the ablated species. (11)MoenkeBlankenburg, L. Spectrochim. Acta Rev. 1993,15,1. (12) Phipps, C. R; Dreyfus, R W. In Laser Ionization Mass Analysis: Vertes, A,
Gijbels, R, Adams, F., Eds.; John Wiley: New York, 1993;Chapter 4. (13)Thompson, M.; Chenery, S.;Brett, L. J. Anal. Atom. Spectrom. 1989,4,11. (14)van Heuzen, A A; Morsink, J. B. W. Spectrochim. Acta 1991,46B, 1819. (15)Crain, J.; Hansel, J.; Troxel, J. E. Spectroscopy 1992, 7, 40. (16)Williams, J. G.; Jarvis, K E. /. Anal. Atom. Spectrom. 1993,8, 25.
Analytical Chemistry, Vol. 67, No. 1, January 1, 1995 131
In principle, fractionation can occur in the ablation process, during transport of the ablated material to the ICP, and within the ICP itself. However, fractionation during a phase change is most common, and there is considerable evidence that volatile elements and compounds are particularly prone to fractionation. Fractionation during transport will occur if one or more elements are ablated predominantly as condensable vapor that diffuses to the walls of the transfer line or if large particulates are transported less efficiently than smaller particulates of differing composition. Hence, significant differences between sample and standard in the properties and/or size of ablated particulates may result in systematic calibration errors. There have been relatively few reports of full quantitative analysis by LA-ICPMS using matched standards and multipoint calibration plots for each element of intere~t.'~-~O Several a p proaches have been used to obtain matrix-matched standards. One technique is to grind the sample to a fine powder, mix with a suitable binder, and form pressed pellets of both the sample and ~ t a n d a r d . ~ ~Although > ' ~ - ~ ~ multielement standards can be made by this method, the accuracy of the analysis is usually limited by inhomogeneous distribution of the analyte in the binder. Fusion of the powdered sample with a borate salt to form a glass tends to result in improved matrix-matching since the material is dissolved in the g l a ~ s . ' ~The J ~ reported precision and accuracy for these methods in the best cases, i.e., with matrix-matched, homogeneous materials and internal standardization, are typically f5-20%.19,20Pelletization and fusion are not the method of choice in the present work because of the inherent loss of spatial information of the sample and other problems, including analyte dilution by the matrix and possible introduction of trace contaminants during grinding. Other calibration techniques involve use of standards of a matrix similar to the sample, e.g., glass Standard Reference Materials (SRMs) for the analysis of minerals.21 Using an alternative approach, Hager found that ratios of element sensitivity factors obtained by laser ablation of metals and solution ICPMS could be fitted with exponential factors containing (atomic) enthalpies of atomization and ionization.22 This does not appear to be a general method because ablation of nonmetals is likely to produce molecular species, such as oxides, silicates, etc. Also, for both metals and nonmetals, there is considerable evidence a large weight fraction of the particulates transported to the ICP are solidified droplets that have undergone solid-liquid-solid, rather than solid-monatomic gas, phase changesz3 Because of the difticulties of obtaining matrix-matched and multielement solid standards, semiquantitative analysis is commonly performed using element relative sensitivity factors (RSFs) or single-point calibration. Depending on the degree of matrix-matching, accuracies ranging between f10% and factors of 2-5 have been reported.6J0J4J6,20 A calibration technique developed by Thompson et al.,13 involves use of two sample introduction channels to obtain matched plasma conditions for the ablated material and a nebulized aqueous solution standard. This method has been used (17) Imai, N. Anal. Chim. Acta 1990,235, 381. (18) van Heuzen, A. A. Spectrochim. Acta 1991,46B, 1803. (19) Perkins, W. T.; Fuge, R; Pearce, N. J. G. j.Anal. At. Spectrom. 1991, 6, 445. (20) Denoyer, E. R; Fredeen, IC J.; Hager, J. W. Anal. Chem. 1991, 63, 445A. (21) Jackson, S. E.; Longerich, H. P.: Dunning, G. R; Fryer, B. J. Can. Mineral. 1992,30, 1049. (22) Hager, J. W. Anal. Chem. 1989, 61, 1243. (23) Thompson, M.; Chenery, S.; Brett, L.J. Anal. Atom. Spectrom. 1 9 9 0 , 5 , 4 9 .
132 Analytical Chemistry, Vol. 67, No. 1 , January 1, 1995
for quantitative LA-ICP atomic emission spectroscopy," although to our knowledge no results have been reported for ICPMS. Use of nonmatrix-matched standards for LA-ICPMSraises the question of how similar the sample and (solid) standard need to be and how differences in their chemical and physical properties affect the accuracy of the determination. A related question is the relative importance of fractionation during ablation, transport, and excitation in the ICP and whether fractionation can be minimized by selecting particular ablation or plasma conditions. In order to investigate these questions laser ablation was used to sample various metals and nonmetals for subsequent detection and analysis by ICPMS. Strong fractional ablation effects were observed for some elements, and we discuss ablation conditions that reduce fractionation. Use of nonmatrix-matched standards was investigated by comparing RSFs under various ablation and ICP conditions. By minimizing fractionation during ablation in conjunction with simultaneous solution sample introduction, it was found that matrix effects for solid and solution samples can be significantly reduced. These techniques have the potential to make LA-ICPMS a practical technique for semiquantitative analysis of solid materials since calibration can be performed with readily available aqueous standards. EXPERIMENTALSECTION
Preparation of Standards. Solution standards were obtained as 1000 pg/mL multielement mixtures (VHG Labs) and diluted to the desired concentration levels in 2%high-purity nitric acid with distilled deionized water. The solid materials used were all NIST SRMs. They included brass SRM 1103, steel SRMs 1263A and 1264A, and fused synthetic ore SRM 1834. Prior to ablation they were polished with alumina and/or diamond grit and washed with methanol or acetone. It is known that at least one of these materials is inhomogeneous (i.e., local variation in composition exceeds the measurement error); Pb is segregated in SRM 1103. Several precautions were taken to minimize the effects of inhomogeneity. Samples are translated in a raster pattern under the laser beam, so that a large surface area is sampled compared to the laser spot size, and the acquired signal intensities are averaged. This helps to average out any local inherent inhomogeneities (e.g., inclusions) and improves the accuracy of compositional analysis. It is also possible to induce inhomogeneity for some materials (e.g., brass) since it was observed that the composition of ablated areas can differ from the bulk. Tkis is caused by compositional changes, such as element segregation, within the laser-heated zone that typically extends -1 pm below the sample surface. To avoid these problems, the SRMs were ground well below any previous laser ablation tracks before reuse. Instrumentation. All experiments reported here were performed on a commercial ICP mass spectrometer (Elan 250/500, Perkin-Elmer Sciex) with an upgraded data system (Elan 5000), quadrupole mass filter, and ion optics. This instrument is also equipped with a multichannel scaler (ACE MCS, EG&G) for rapid coaddition of mass spectra with negligible dead time. The spectrometer can be switched between normal (multielement) operation used for isotope "peak hopping" and MCS control for acquisition of full mass spectra. For the latter, the output voltage ramp of the MCS drives the quadrupole and signal pulses from the channeltron electron multiplier (CEM) detector are diverted to the input of the MCS pulse counter. All other aspects of the
Plasma Aux.
Table 1. LA-ICP Mass Spectrometer Operating Conditions
parameter
value
plasma (L/min) auxiliary (L/min) sample (laser ablation) (L/min) pneumatic nebulizer
12 2.0 1.8.1.4 for mixed plasma concentric (Meinhard TR-30-3A) 0.3
r -i
-
Solution
Ar gas flow
single channel flow (45 psig) &/mid mixed sample flow (30 psig) (L/min) solution uptake rate (mL/min) rf power (kw) ICP torch load coil-samplerinterface distance (mm) mass spectrometer pressures base (Torr) operating (Torr) nozzle diameters sampler (in.) skimmer (in.) ablation laser wavelength (nm) pulse duration (ns) incident pulse energy (mJ) repetition rate (Hz)
0.2
1.0 1.2-1.5 Sciex "long"type 20 5 x 10-8 (1-3) 10-5 0.042 0.038 Nd:YAG (Lumonics HY750) 266, fourth harmonic 8, Q-switched 0.5-60 1 or 10
Laser
Window Gas
II
out
~
_____________ ______________
Laser
Focus H
9 1 cm
. .. ...
8 8 8
Load Coil
.. .. ...
Figure 1. Schematic of the high fluence ablation system. The sample is fixed on a XYZtranslation stage, and the whole assembly is located in a pressurized housing (neither shown).
instrument are standard. A summary of the ICPMS instrument specifications and typical operating conditions is given in Table
1. Solid samplingwas done with a laser ablation system developed in this laboratory. The system (see Figure 1) has been described previously.24 The laser beam is focused by a singlet lens through an open-ended ablation cell, positioned with a gap of 0.5-1 mm above the sample. Typical laser spot diameters are in the range (24) Arrowsmith. P.;Hughes, S. K Appl. Spectrosc. 1988. 42. 1231.
Figure 2. Schematic of the annular adapter for mixed-sample introduction into an ICP torch. The adapter is inserted into the end of the torch and attached by a ground-glass seal with a spring clamp (clamp not shown). The various flow paths are indicated.
20-500 pm at the surface. The ablation cell and sample are enclosed by a gas-tight housing which is pressurized at 4-5 Torr above atmosphere. The flow of Ar gas through the cell entrains the plume of ablated material and carries the particulates through a 7km-long tube (internal diameter 3 mm) to the plasma torch. Although previous work from our laboratory used the Nd:YAG fundamental output (1064nm)," the fourth harmonic (266 nm) was chosen for the present work because metals have lower reflectivityand glasses and polymers higher absorption in the W. Incident laser energies were measured with a volumeabsorbing power meter (Scientech 380207). Sample translation was accomplished with an Xn axis, computercontrolled stage (model 42, Standard Stage). The sample is moved at a uniform speed in a prescribed raster pattern, relative to the fixed laser beam and ablation cell. The standard translation speed was 33 pm/s. At 1@Hz pulse repetition rate, a steady stream of ablated material enters the ICP, and since the increment between laser pulses is 3 pm, a track of partially overlappinglaser spots is deposited. Under these conditions, the laser spot samples both previously ablated and fresh sample surface and an area within a typical beam diameter of 300 pm receives -100 laser pulses. Although ablated material that remains on the surface may also be sampled by subsequent laser pulses, the contribution to the signal from redeposited blowoff material is estimated to be small (
