Laser ablation of solids for elemental analysis by inductively coupled

Anal. Chem. 1907, 59, 1437-1444

1437

Laser Ablation of Solids for Elemental Analysis by Inductively Coupled Plasma Mass Spectrometry Peter Arrowsmith’

SCIEX, 55 Glen Cameron Road, Thornhill, Ontario, Canada L3T 1P2

A technique for direct elemental analysis of solids is described. A focused NdYAG laser efflcientiy ablates many materials, including ceramks and polymers, that are difficult to prepare for solution analysis. Ablated particulate material is transported by gas flow into an Inductively coupled plasma (ICP) and the resulting ions are detected by ma88 spectrometry. The laser may be used either in singispulse mode to give a transient signal or at 10 Hr repetition rate, resuiting in a continuous signal. The continuous signal may be maintained constant over long periods by translation of the sample, resuiting in improved precision and duty cycle for data acqulilnlon. Quantitative analysis is obtained by internal standardization on either a known analyte or the sample matrix signal. Analytical curves obtained for NBS microprobe steel standards are finear over 4 orders of magnitude and estimated detection limits are 0.2-2 pg g-‘ in the solid. Precision and accuracy are approximately f5 %.

Interaction of laser radiation with a solid may cause ablation, vaporization, and excitation by processes that depend upon both the characteristics of the laser beam and the physical properties of the solid. The various species produced, including particulates, ground-state atoms, excited atoms, and ions, have all been utilized for elemental analysis via atomic absorption (AA), atomic emission spectroscopy (AES), and mass spectrometry (MS). A description of the laser-solid interaction and a comprehensive review of analytical techniques has recently been published (1). The advantages of using lasers for analysis of solids include little or no sample preparation, resulting in high sample throughput, application to almost all materials, and high spatial resolution, allowing analysis of small selected areas. Techniques that separate sample introduction (accomplished by laser ablation or vaporization) from sample atomization, excitation, or ionization (performed in a separate furnace, discharge, or plasma) are of particular interest since each process may be independently optimized. Laser ablation has been combined with AA ( 2 , 3 ) , microwave induced plasma (M1P)-AES (4, 5), inductively coupled plasma (1CP)-AES (6-9),and ICP-MS (IO). In these studies, low-repetition-rate (80% have been measured for a similar housing and flow system developed for electrothermal sample introduction into an ICP-MS (24,25),however, it is likely 0 1987 American Chemlcal Society

1498

F&e

ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987

1. Schematic ot

laser ablatbn houshg fw 45' laser beam

angle of incidence.

Table I. Operating Conditions coolant flow rate auxiliary flow rate aerosol (transportgas) flow rate transfer tube inside diameter forward power sampler orifice diameter skimmer orifice diameter standard torch sampling position extended torch sampling position

12 L mi+ argon 2.0 L mid' argon 1.6 L m i d argon 0.25 in. (6.4mm) 1250 W 0.045 in. (1.1mm) 0.035 in. (0.9 mm) 15 mm from load coil 20 mm from load coil

that laser ablation produees a different particle size distrihution with consequent change in the transport efficiency.) Operating conditions are shown in Table I. The transport-gas flow rate was measured by a mass flow controller. The optimum flow, corresponding to maximum ion signal, was higher than other values reported for laser ablation (8)because the relatively large diameter transfer tube used in this work reduced the flow velocity. A two-way tap loeated in the transfer tube allowed the system to be purged for a short time hefore directing the flow to the ICP torch. As shown in Figure 1,the laser beam was incident on the sample at 45' to reduce absorption of the beam by the plume of ablated material, which was generally ejected normal to the surface. This geometry has the disadvantage that translation in the vertical direction causes the laser beam to move across the sample surface and so a housing for normal beam incidence was also developed. Both the housing and sample are moved relative to the fixed laser beam by means of a zero-hacklash, three-axis translation

mount (Micro Controle, Model MR 50). The vertical (Z)axis is manually adjustable over 8 mm of travel to enable the optimum focus position to be located. The X and Y axes (16 mm travel) are each driven by a stepper motor and separately controlled hy a microcomputer-based stepper-controller. The distance moved per step is 2.5 r m and up to 6400 steps can he moved in each direction a t rates from single step to 200 Hz. Programs stored in memory enable the sample to he moved to any position, repeatedly stepped back and forth along a single axis, or moved in a raster pattern. In addition, the stepper-controllercan trigger the laser. Thus a sequence in which, for example, the sample is stepped into position and a selected number of laser pulses fired after some delay, can he repeated any number of times. Some possihle combinations of sample translation and laser pulse sequence, together with the resulting ablation patterns, are shown in the first three columns of Table 11. Parallel (p) polarization of a laser beam at a surface gives maximum coupling of the beam energy into the substrate (26). Hence, for the horizontally polarized beam used in this work.. the .nreferred direction of ablation (Le. the long axis of a raster scan) corresponds to motion of the sample along the iY axis of Figure 1. ICP-MSInstrumentation. A standard SCJEX ELAN Model 250 inductively coupled plasma mass spectrometer with improved ion optics (271 was used without further modification. Sample introduction could he rapidly changed from laser ablation to pneumatic nebulization for solutions hy disconnectingthe transfer tube from the sample inlet of the ICP turch and reconnecting a nehulirer and spray chamber. The resolution of the instrument was set to approximately unit width at 1090 of the wak maximum over the m / z 0-250 range. The ELAN can be operated in either a scan mode (i.e. signal as a function of m , z) or a multiple ion monitoring mode in which the quadrupole mass spectrometer is switched hetween selected masses (or 'peak-hopped") and the signal is ohtained m a function of time. Spectra comprising a single mass scan were recorded with 20 sample points per dalton at relatively long measurement times of 50 ms per point for improved precision. In the peakhopping mode a single point corresponding to the peak maximum was sampled for 5 or 100 ms at each mass (a maximum of 36 masses can he requested). Peak-hopping is particularly useful for acquiring data over short time interval9 since both the sample raw per mas and the duty cycle are relatively high. Also, a TI'L trigger with variable delay (or advance) relative to the start of the data acquisition period is available from the ELAN computer. This enabled the laser w be triggered directly and the resulting transient sigma1 captured within a narrow window. rhus avoiding unnecessary sampling of the background. It was occasionally necessary (every 1-2 weeks) to check the optimlation of the electrcetaticlenses for maximum ion multiplier count rate. This was accomplished with the laser on, by peak.

Table 11. Sample Translation and Analysis Modes translation (A) none

(B) none (C) single axis or raster

laser pulse sequence*

ablation pattern

single pulse, triggered by MS 10 Hz,no trigger single pulse, triggered by

0

ICP-MS form of signal1 peak hop transient

application single area analysis

0 peak hop long transient single area depth profile OcOcO peak hop multiple low resolution spatial distribution transients

s-c

(D) single axis or raster

(E) single axis repeat or

10 Hz bursts,

@+CZ+@

triggered by s-c 10 Hz, no trigger

peak hop multiple long transients

Ommmmo peak hop

continuous and steady

spatial and depth profile analysis signal maximization, high resolution spatial and quantitative analysis

raster

(F) single axis

10 Hz,no trigger mass scan continuous qualitative and semiquantitative analysis repeat or mass raster spectrum 'Triggered by MS and 8-c refers to external triggering of the laser by the mass spectrometer computer and by the stepper-controller. respectively.

ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987 hopping between two or three analyte elements evenly distributed over the m/z range. Optimum lens settings were similar to those reported for solution nebulization (27). Samples. Samples up to 15 mm thick and 35 mm wide can be accommodated in the ablation housing. Thin samples were mounted on blocks to raise them to within range of the vertical travel of the translator. Sample preparation is required for powders, either by pressing into disks with a binder (200 mesh powder with 5% poly(viny1 alcohol)) or fusion into a lithium tetraborate (LiPB407)matrix. For these samples it is important to achieve good homogeneity and to ensure the binder or fusion flux is free from any contamination, which could give high blank levels. Samples used without any preparation included homogeneous microprobe steels and glasses in the form of rods (NBS, standard reference materials (SRMs)), high-purity copper standards in the form of disks, and various ceramic materials. Samples were held in position with double-sided adhesive tape. Surface quality is not critical because it is modified by the first few laser pulses (in the case of ablation at the same area) and no surface preparation was employed. Samples that are translated should maintain a constant distance from the laser focus lens along one or both horizontal axes. In practice, the signal is not strongly dependent upon focus position for the Q-switched laser because the irradiance is sufficient for ablation over a waist length of approximately 500 pm (for focal lengths 150 mm). Also, some ablation may occur via secondary heating by the laser-induced plasma formed above the sample surface (28). Hence, samples do not have to be flat or parallel to better than *250 pm.

RESULTS AND DISCUSSION The various modes of using the laser, sample translator, and mass spectrometer, shown in Table 11, will be discussed in terms of the results obtained with different samples. The focused Nd:YAG laser beam was found to be capable of ablating a range of materials including reflective metals, polymers, glasses, and refractory ceramics. Both free-running and Q-switched operation were used, although the results differed. Of the materials tested, only high-purity quartz could not be ablated effectively, probably because of its high transmission at 1.06-km wavelength. Background Mass Spectra. Significant intensities of background species such as C+, N+, O+,H20+, NO+, ArN+, ArO+, and Arz+were observed with the dry ICP. However, the levels of H- and 0-containing species were reduced by an order of magnitude compared to those obtained by nebulization of aqueous solutions (20). Possible sources of background species include impurities in the Ar gas, desorption from the walls of the sample housing and transfer tube, loss of volatile additives from the transfer tube and entrainment of air in the 10-mm gap between the end of the ICP torch and the tip of the sampler orifice. In later experiments, entrainment of air was eliminated by replacing the standard length ICP torch with an extended torch (see Table I), which differed only in the length of the outer tube and allowed the sampler tip to be positioned inside the end of the torch. This gave a 1-2 order of magnitude decrease in the intensities of N- and 0-containing species but no observable change in analyte ion signals. The laser-off background was constant a t approximately 10 counts sT1 throughout the mass range apart from species that occur at mlz 580 and impurities from the Ar gas (Kr over the region mlz 80-86 and Xe over mlz 128-136). For determination of analyte elements that occur at these mlz values, it was usually possible to find a minor isotope free from interference a t the cost of reduced sensitivity. Single Laser Pulse Transient Signals (Modes A and C in Table 11). A laser pulse fired at the sample produces a plume of ablated material that is entrained in the transport-gas flow as solid particles. A study of particle formation in laser ablation of metals has been recently published (29). Mixing and spreading occur in the ablation cell, transfer tube, and ICP torch, and the ion signal is observed as a transient of approximately 500-ms duration (fullwidth at half-maximum

700000

1439

7

..oo~o~

140

Nd 142

YI

sooooo/ 400000

8

i7

200000

,-

C.0 168 NdO 161

0 0.0

0.5

- I .o

1.5 TIME

2.0

2.6

3.0

(8)

Figure 2. Single laser pulse ablation of a sample containing rare-earth elements. The indicated m l r values were sampled with a measurement time of 5 ms.

(fwhm)) by peak-hopping the mass spectrometer. (Spreading in the transfer tube and ICP torch was found to be relatively unimportant since injection of SiF4gas into the tube just above the ablation cell gave a fast rise and decay in the observed Si+ signal (241.1 Figure 2 shows transient signals obtained with the externally triggered Q-switched laser for a Li2B407fusion sample doped with approximately 1% rare-earth elements. The levels of oxide species are of particular interest and from the integrated signals on both Ce and Nd the ratios MO+/M+ are less than 0.1 %. This compares to refractory oxide levels of 1-5% obtained by solution nebulization and indicates the dry ICP contains little oxygen. No attempt was made to extend the duration of the transient with consequent reduction in the peak signallbackground ratio by changing the optimum transport-gas flow rate. For these short duration transients the number of elements that can be determined quantitatively is fewer than 10 since the precision is limited by the number of data points acquired over the transient for each element. (For a peak-hopping quadrupole mass spectrometer the sample rate per mass is inversely proportional to the number of masses or elements requested due to the single-channel nature of the instrument.) In addition, optimization of transport-gas flow and laser focus position is tedious for transient signals. For these reasons the single laser pulse transient signal mode of operation was not normally used for quantitative analysis. Applications of single laser pulse ablation include single area analysis for determination of inclusions and investigation of sample heterogeneity. Multiple Laser Pulses (Modes B and D in Table 11). In the absence of sample translation, multiple laser pulses produce “hole-drilling” by repeated ablation of sample material. The amount removed by each pulse depends on the physical properties of the sample and (for a given wavelength and energy) the pulse duration, i.e. whether the laser is free running or Q switched (28). Experimentally, it was observed that only two or three pulses from the free-running laser were required to produce a cavity of depth approximately equal to diameter in alumina ceramics. Thus, a layer approximately 50 pm thick, corresponding to 1 pg of material, is removed by each laser pulse. Scanning electron microscope (SEM) photomicrographs of the cavities showed a tendency for material to be deposited around the crater rim. This is consistent with the steady-state melting-flushing mcdhanisrn proposed for ablation by a relatively long duration free-running pulse comprising many short laser spikes (28). The smallest craters were observed to have diameters of 80 pm. This is consistent with the calculated spot diameter of 50 pm since heat conduction into the bulk tends to produce craters larger than the

1440


-11

Ti 48

TIME ( m n )

Flgure 3. Continuous signals obtained by ablation of an NBS microprobe glass (SRM K963) wlth the 10 Hr Q-switched laser. Signals were observed at four mlz values with a measurement time of 100 ms. The sample was translated back and forth 40 times along a single axis. Isotopic compositkns are '%a 0.95%. '%u 0.50%, and 48Ti 0.24% by weight.

beam diameter. In contrast, a Q-switched pulse was estimated to remove only 10 ng of ceramic, corresponding to a crater depth of approximately 1 pm. The significantly smaller amount ablated is probably due to absorption of the Qswitched pulse by the laser-induced plasma formed above the sample surface at irradiances greater than lo9 W cm-2 (28). Secondary ablation of the ceramic surface by this plasma may produce craters much larger than the diameter of the laser beam. Under the SEM, craters produced with the Q-switched laser appeared relatively clean with little or no deposition around the rim. With the 10 Hz repetition rate laser and peak-hopping mass spectrometer, it is possible, in principle, to perform bulk depth profiling for selected elements. Samples such as steels, which ablated slowly with the Q-switched laser, gave a signal that appeared continuous and was constant for several minutes. However, the signal decayed rapidly for samples that ablated at a high rate, since the laser beam was defocused as the cavity deepened. For this reason the signal is termed a "long transient" in Table 11. The duration of the signal could be improved, at some loss of spatial resolution, by using a longer focal length lens to increase the depth of field of the focused laser beam. Signals with a similar time dependence have been observed by ICP-AES (11,13). Continuous Signal with Peak Hopping (Mode E of Table 11). The 10-20 Hz repetition rate laser produces successive (transient) plumes of ablated material, which appeared as a steady stream of particles at the inlet to the ICP torch. (Also, ablation of a sample containing rare-earth elements was observed to give steady emission in the central channel of the ICP.) In this mode the sample was constantly translated to prevent defocusing of the laser beam by holedrilling. The mass spectrometer was peak-hopped (measurement time 100 ms) to give a continuous signal that could be held constant for long periods, depending on the sample translation speed and the ablation rate. Figure 3 shows continuous signals obtained from an NBS microprobe glass (SRM K963) over a period of 20 min. A slow decline in the signal is apparent since the sample was in the form of a 2 mm wide rod and translation was back and forth along a single axis, resulting in gradual defocusing. A relatively rapid decline in signal of approximately 50% over 10 min. has been reported for ablation of pressed powders in DCP-AES (16). This mode was found to be ideal for both optimization of operating conditions and data acquisition. The optimum focus position and transport-gas flow rate could be quickly obtained

for a new sample by maximizing the count rate on an analyte or a matrix element. Since count rates >IO6 s-l cause gain depression of the ion multiplier detector and matrix elements gave (extrapolated) intensities up to lo8 counts s-l, it was necessary to use either a minor isotope or attenuate the signal (by temporarily detuning one of the instrument electrostatic lenses) for optimization on a matrix element. Since the precision and duty cycle were substantially improved compared to the single laser pulse transient signal mode of operation, the continuous signal mode was preferred for quantitative analysis of homogeneous samples. For inhomogeneous samples, it is possible to map the spatial distribution of selected elements along a single axis or over an area of the sample by selecting the appropriate stepper-controller program. The present work was performed with homogeneous samples. It is interesting to note that high rates of ablation (1pg per pulse) with the 10 Hz repetition rate laser are comparable to liquid nebulization with a solution of approximately 5% total dissolved solids (see below). At these levels, deposition on the sampler inlet orifice is significant for refractory samples and partial blockage of the sampler did occur during ablation of a ceramic with the free-running laser. The problem could be alleviated by either attenuating the laser beam energy, reducing the focused spot diameter, or operating the laser Q switched. Since the free-running laser ablates at relatively low temperature, with possible preferential loss of volatile elements, it was desirable (and convenient) to use the Qswitched laser for analytical work. Continuous Signal with Mass Scan (Mode F in Table 11). The continuous-signal mode of operation (Le. constant sample translation and 10-20 Hz repetition rate laser ablation) enabled complete mass spectra to be acquired over long scan times with good precision. Because these spectra comprise a single sequential mass scan they are subject to systematic variation in the signal due to, for example, defocusing of the laser beam. Hence, mass spectra were not used for quantitative analysis, although they could yield semiquantitative information in conjunction with element response factors (see below). Figure 4 shows a mass spectrum of a microprobe glass (NBS SRM K963) and demonstrates the advantage of laser ablation for rapid analysis of nonconductors. Quantitative Analysis. Several factors may cause variation in the amount of sample ablated and the observed signal. They include pulse-to-pulse variation in laser energy, defocusing of the beam during ablation or upon changing the sample, and differences in physical properties between samples. The first factor is not important due to the high reproducibility of the Nd:YAG laser and the short-term averaging responsible for the steady signal obtained at 10 Hz repetition rate. An internal standardization technique was chosen to correct for systematic and sample-to-sample variations in the signal. A primary advantage of this approach is that knowledge of the absolute amount of sample ablated is not required. Internal standardization has been widely used to improve the precision of ICP-AES (30) and only aspects relevant to laser ablation-ICP-MSwill be discussed. Assuming each analyte element E is ablated from a sample S and transported into the ICP with equal efficiency, the observed intensity may be written as I(E,S) = K.C(E,S).R(E).A(S) where K is a constant which relates concentration (pg g-l) to intensity (counts s-l), C(E,S) is the concentration of the element or isotope in the solid, R(E) is the element dependent instrument response and A(S) is a sample dependent ablation yield. R(E) is the product of two factors, the degree of ionization of element E in the ICP and the transmission through the ion optical chain and mass spectrometer. In the internal


1441

81130.2)n

M I 2

Flgure 4. Mass spectrum of NBS microprobe glass (SRM K963) obtained with the 10 Hz Q-switched laser; total scan time approximately 4 min

with repeated single-axis translation. Note the logarithmic intensity axis. The numbers in parentheses refer to elemental compositions in weight percent. standardization method the analyte signal is ratioed to that of a reference (or internal standard) element IS present in the same sample, allowing cancellation of the sample dependent terms in eq 1 to give

150000

I

100000

The internal standard may be either an analyte of known concentration, a matrix element, or an added element in the case of pressed powders and fusions. Results will be presented for two of these cases; analysis of steel and copper standards with analyte and matrix internal standards, respectively. For the steel standards (NBS microprobe SRMs 661-665) the minor isotope 60Niwas chosen as the internal standard since preliminary mass scans gave "Ni signals of 103-105 counts s-l. Also, @Ni is well separated from the intense (saturated) Fe isotope peaks a t m / z 54-58 and there are no significant interferences from either background or analyte species at m / z 60. Continuous mode ablation with the &switched laser (mode E of Table 11)w a ~used and signals were recorded for a period of 4 min, Corresponding to approximately 80 intensity measurements for each selected m / z value. Typical signals for several isotopes are shown in Figure 5, the rapid decay of the signal when the laser was turned off indicates a short cleanout time for the ablation cell and transfer tube. The noise in these signals masks any obvious correlation between them and indicates that @Ni may not be the best choice for an internal standard. In the data analysis procedure, each of the n intensities for an analyte element was divided by the corresponding intensity for the internal standard to give ratios Ii(E)/Ii(@Ni),i = 1, n. (Intensities were initially blank subtracted with a laser-off background of 20 counts s-l at all m/z values.) In addition, the variance of each ratio was calculated from the variances of the analyte and internal standard intensities and the correlation coefficient between them. The individual ratios and variances for each analyte were summed to give a (weighted) mean intensity ratio I(E)/I(@Ni)and standard deviation of the mean (31). These ratios, which varied in magnitude from 10 (RSD = 1 % ) to 0.001 (RSD = 20%) were multiplied by the certified concentration of Ni in the corresponding standard to give a quantity proportional to the analyte concentration C(E) (see eq 2). The final results are shown in Figure 6 in the form of log-log analytical curves calculated by least-squares fit for seven analytes. The mean deviation of the data points from the lines is f5% except for points at low concentration (C1 Kg g-l), which have deviations of approximately f20%. This

-

1 1 i

I

I

I

o

25000

4

. . . . ~ . . . . ,. . . I . . . . ,

0

0.0

0.5

1.0

1.5

20

. . . , . . . . , . . . . , . . . 2.5

3.0

3.5

1

I

TIME (mi")

Figure 5. Signals observed for the determination of Cu, As, Sb, and Ni in NBS steel (SRM 664). The 10 Hz Q-switched laser was turned on at 0.7 min and off at 2.9 min. The measurement time was 100 ms.

3

Flgure 6. Analytical curves obtained for the NBS microprobe steels (SRMs 661-665) wlth internal standardization on 80Ni. Preclslon of the data points was within the symbols except for those marked by error bars (& 1a).

large scatter probably arises from the uncertainty in the concentrations of Nb, Zr, and Sb (which are not certified for

1442


Table 111. Detection Limits from Analysis of NBS Steel SRMs %

detectn lim

isotope

abund

(3u), l g g-'

re1 response R(E)/R(mNi)

I P, eV

I5As w u

100 30.9 42.8 23.8 51.5 83.8 100

2 2 2 0.9 0.3 0.3 0.2

1.2 1.2 1.4 2.5 6.6 7.5 11.0

9.8 7.7 8.6 7.1 6.8 6.8 6.9

'%b

98M~ wZr 52Cr

93Nb

the relatively pure SRM 665) and the unsuitability of the constant laser-off blank correction. The curvature of the Cr line in Figure 6 is due to saturation of the detector for SRM 663 (1.3% Cr), which gave an extrapolated intensity of 1.7 X 106 counts s-l. Detection limits estimated from the fitted lines are listed in Table 111. Both the detection limits and the relative response factors R(E)/R(@"i) (which determine the displacement of the analytical lines) generally follow the first ionization potentials of the elements, suggesting that these elements are ablated with equal efficiency by the Q-switched laser. Precision similar to that of the present work was obtained for determination of Cu in ores by 20 Hz pulse rate laser ablation-DCP-AES (16),but the reported detection limit of 20 pg g-' is significantly higher. I t is important to note that the above technique is not a true internal standardization in the sense of simultaneous measurement of analyte and internal standard signals as is done in multichannel AES. Because it takes 1-2 s (Le. 10-20 elements a t 100 ms measurement time) to repeat a measurement on a given analyte in ICP-MS, short term fluctuations will not be corrected. However, rapid fluctuations, which do not arise in the ICP, are, to some extent, averaged in the sample-transfer process. The correlation coefficient is a measure of the effectiveness of internal standardization and values >0.5 indicate an improvement in precision (30). Values of 0.5-1.0 were generally obtained for the steel analysis, particularly for elements present in high concentration (Cr, Mo, W). Hence, longer term (>2 s) variations in signal should be corrected by the internal standardization technique. The internal standardization method was further tested by analysis of Cu standards (Outokumpu Oy, Copper Products Division, Finland) with the matrix element as the internal standard. This method has the advantage that prior knowledge of an analyte concentration is not required. However, ablation of a large amount of matrix causes saturation of the detector. Since Cu does not have an isotope of sufficiently low abundance to avoid saturation, the signal at "Cu was attenuated to approximately 5 x lo5 counts s-' by detuning the "B" electrostatic lens in the ELAN. Because analytes must be determined with maximum sensitivity, it was not possible to "simultaneously" measure (i.e. within 1-2 s) both analyte and attenuated internal standard signals. Hence there was poor correlation between the analyte and internal standard signals and variations in signal during the measurement period could not be corrected. The free-running laser was used to ablate the Cu standards in order to increase the amount of ablated material and improve the sensitivity. Continuous mode ablation (mode E of Table 11)was performed for three 1-min periods; for periods 1 and 3 the instrument was at maximum sensitivity and for period 2 the electrostatic lens was detuned. Intensity measurements from periods 1and 3 were summed for each analyte to give a weighted mean intensity I(E) and variance. The mean intensity Z('%u) and variance for the internal standard were obtained from period 2. (Each intensity measurement was initially blank corrected by a constant 20 counts s-l.) The standard deviation of the

.2.0y T

a

L

.1 .o

-2.0

I 1.o

0.0 LOG

2.0

(W)) ( Ng-7

Figure 7. Analytical curves obtained for the copper standards with internal standardization on the attenuated %J signal. Ablation by

free-running laser. Table IV. Detection Limits from Analysis of Comer Standards %

detectn lim

isotope

abund

(3u), ccg g-'

75As

100 57.3 34.5 51.8 100 28.9 51.7 100

6 4 0.3 0.3 0.2 0.2 0.05 0.2

lZ1Sb 130Te Io7Ag

55Mn ll*Cd 208Pb 209Bi

re1 response 1 p, R(E)/R(65Cu) eV 0.007 0.034 0.16 0.17 0.26 0.58 0.87 1.1

9.8 8.6 9.0 7.6 7.4 9.0 7.4 7.3

mean intensity ratio I(E)/I(%Cu)was estimated by combining the variances for the two intensities with a correlation coefficient of zero. Analytical curves obtained by plotting log (I(E)/I(65Cu))vs. log (C(E))are shown in Figure 7. Since C("Cu) is constant for all the samples, this term was ignored and hence the ordinate of Figure 7 has arbitrary units. The increased scatter of the data points (compared to Figure 6) could arise from several factors; the copper standards probably have poor homogeneity compared to that of the microprobe steels (although sample translation and signal averaging reduce this problem), the relatively low-temperature ablation obtained with the free-running laser may give fractional volatilization of analytes, and the absence of correlation prevents correction of signal variations. However, the lines of Figure 7 are reproducible since good agreement was obtained with analytical curves derived (from the same raw data) by "simultaneous" internal standardization on lO7Ag. Detection limits and relative response factors R(E)/R(65C~)estimated from the leastsquares-fitted lines of Figure 7 are listed in Table IV. The detection limits for Sb, Cd, and Bi take into account the roll-off a t low concentration apparent in Figure 7, which probably arises from error in the known concentrations and the unsuitability of the blank correction. The relative response factors follow the trend of ionization potential (IP) except for Cd, which has a larger value than expected for an I P of 9 eV. Hence there is evidence of preferential ablation for this volatile element. Possible sources of fractional ablation include volatilization from the surrounding bulk and revolatilization of material deposited around the crater rim by the free-running laser (see above). Time dependent variation in Mn, Cr,


Table V. Comparison of Solids Analysis Techniques

technique

form of signal

transport rate into ICP

detection limits, sensitivity

pg

g-'

this work

(Qswitched)

steady

0.1"

20-200b

0.2-2

100-800*

0.4-4

2-40''

0.01

not reported

0.3-2

arc nebulization

ICP-MS' laser ablation

steady

ICP-MS' (free running) laser ablation ICP-AES' (free running)

transient

lOOfJ

transient

Id

Opg s-'. bCounts s-l per pg g-'. CReference21. dFrom the quoted rate of 1 mg mi&, assuming 50% transport efficiency. eReference10. fpg per pulse. #From the quoted rate of 0.2 mg per pulse, assuming 50% transport efficiency. Counts per pg g-' per pulse. Reference 8.

and Ni signals observed by ICP-AES during free-running laser ablation of steel has also been attributed to fractional vaporization (11). Over a period of several days, the walls of the transfer tube became noticeably discolored due to deposition of particles of ablated material. This could give spurious signal (memory effects) at specific m / z values if particles were dislodged by expansion pulses caused by laser-induced heating of the gas in the sample cell. The problem was eliminated by replacing the transfer tube every few days. Alternatively, a true laser-on blank could be obtained by ablating a pure material (which should contain the internal standard element, if required). The window of the ablation cell was generally used for 1-2 weeks without cleaning. However, if the free-running laser was used to ablate some metals, sputtering of molten material produced a deposit on the window within minutes. Sputtering did not occur with the Q-switched laser. If a single standard is available then a relative response factor R(E)/R(IS) can be obtained for each analyte via eq 2. Combination of this factor with an intensity ratio I(E)/I(IS) obtained by laser ablation of the sample under similar matrix conditions yields the analyte concentration. Possible methods for semiquantitative analysis (i.e. accuracy within a factor of 2-3) by internal standardization without standard samples are under investigation. Current work is aimed a t showing whether R(E)/R(IS) factors, conveniently obtained by nebulization of a multielement solution, are applicable to laser ablation. There is considerable evidence that response factors vary with sample matrix in solution (32)and it is likely that similar effects will arise from the (relatively large) difference in ICP conditions between solution nebulization and laser ablation. (Instrument conditions such as electrostatic lens settings are held fixed for both solution nebulization and laser ablation.) If such relative response factors can be obtained, analyte concentrations may be calculated from eq 2, or alternatively, by methods based on the proportionality between the sum of the observed (matrix plus analyte) intensities and the total amount of sample ablated (33, 34). Comparison to Other Techniques. Sample introduction by pneumatic nebulization gives analyte intensities of 2 X lo5 counts s-l (pg pL-l)-I (to within a factor of 3). Assuming an uptake rate of 1 mL min-' and a nebulizer efficiency of 1%, the calculated instrument response is approximately lo-' for an analyte of atomic weight 60 g mol-', i.e. one count is detected for every lo7analyte species in the ICP. Also, a solution of 1% total dissolved solids corresponds to a transport rate into the ICP of approximately 2 pg s-l. Q-switched laser ablation of NBS steels gave a sensitivity of 100 counts s-l (pg g-l)-I (with an order of magnitude variation between elements).

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Assuming 50% transfer efficiency and an instrument response of lo-', the quantity of sample ablated is approximately 20 ng per pulse at 10 Hz repetition rate. This corresponds to a transport rate into the ICP of approximately 0.1 pg s-l, hence, laser ablation gives improved sensitivity over nebulization for samples that would require dilution to

Laser ablation of solids for elemental analysis by inductively coupled

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