Quantitative Analysis of Aluminum Alloys by Laser-Induced Breakdown Spectroscopy and Plasma Characterization

M O H A M A D S A B S A B I a n d P A O L O C I E L O *

National Research Council Canada, Industrial Materials Institute, 75 De Mortagne Blvd., Boucherville, QuEbec J4B 6Y4, Canada

Laser-induced breakdown spectroscopy has been applied to perform el- emental analysis of aluminum alloy targets. The plasma is generated by focusing a pulsed Nd:YAG laser on the target in air at atmospheric pressure. Such a plasma was characterized in terms of its appearance, emission spectrum, space-integrated excitation temperature, and electron density. The electron density is inferred from the Stark broadening of the profiles of ionized aluminum lines. The temperature is obtained by using Boltzmann plots of the neutral iron lines. Calibration curves for magnesium, manganese, copper, and silicon were produced. The detec- tion limits are element.dependent but are on the order of 10 ppm. Index Headings: LIBS; Emission spectroscopy; Aluminum alloy; Time resolution; Plasma temperature; Electron density.

INTRODUCTION

Elemental analysis based on the emission from a plas- ma generated by focusing a powerful laser beam on a solid sample surface is known as laser-induced break- down spectroscopy (LIBS). For fluxes exceeding the breakdown threshold of the material present in the focal volume, a plasma with high temperature and electron density is formed. It is characterized by a visible flash of light and an audible popping sound. The matter present in this region is vaporized and dissociated into atoms, singly and multiply charged ions in excited states, and free electrons. The plasma characteristics depend on the laser intensity, wavelength, pulse duration, and target ma- terial and also on the surrounding atmosphere. The ob- served flash of light results from several mechanisms: brernsstrahlung (free-free), electron attachment and ra- diative recombination (free-bound), and radiative decay of excited atoms, ions, or molecules (bound-bound). The spectroscopic analysis of the light emission can be used to determine the composition of the sample. 1.2 The use of the LIBS technique has well-known advantages over more conventional methods of atomic emission spec- troscopy. The most relevant are the following:

1. It can be applied to both conducting and noncon- ducting samples.

2. Sample preparation is not necessary. 3. A small mass of sample is required (submicrogram). 4. There is the possibility of performing on-line analysis

at a distance in hostile environments. 5. Sample vaporization and excitation is possible in a

single step; i.e., there is no need for particles to be transferred to an external source.

Received 7 September 1994; accepted 16 December 1994. * Author to whom correspondence should be sent.

The basic features of this technique are described in sev- eral review papers, t-4 while a variety of applications have been reported. 5-17

In the present work, the optimum conditions for quan- titative analysis of aluminum alloys by LIBS have been studied. The laser-induced plasmas have been character- ized by emission spectroscopy in terms of their spectra, electron density, and excitation temperature. The tem- perature was evaluated from the emission intensities of Fe(I) lines observed in the plasma of aluminum alloys containing small amounts of iron. The electron density was determined from the Stark broadening of ionized aluminum lines.

We produced calibration curves for silicon, magnesi- um, copper, and manganese, over the concentration rang- es typically found in aluminum alloys.

EXPERIMENTAL

Our experimental arrangement is shown in Fig. 1. A Q-switched Nd:YAG laser (Surelite ! 10) was used for ablation of solid samples. Plasma is generated by focusing the YAG laser on the sample surface by a first lens ( f = 25 cm). The Nd:YAG laser is operated at the fundamental wavelength of 1.064 #m. The beam diameter before and after focusing was nearly 6 mm and 0.6 mm, respectively. The pulse energy could be varied from the threshold up to 500 mJ. The laser, Q-switched through a Pockels cell, emitted a pulse of about 8-ns duration. The maximum repetition rate is 10 shots per second, but for most mea- surements a rate of only one shot per second was used. The relative standard deviation of the pulse energy mea- sured with a calibrated calorimeter was typically 1%.

The target samples could be positioned and moved in all three directions by micrometer screws with respect to the laser beam. The target could be rotated around an axis normal to the optical plan formed by the intersection between the laser beam and the optical axis of the spec- trometer, thus allowing the laser to bombard the sample surface at variable angles. This configuration gave us some information about the spatial distribution of the plasma spark and allowed us to simulate configurations similar to an industrial setup.

The light emitted from the plasma was imaged 1:1 by a second quartz lens ( f = 10 cm), into the entrance, typ- ically a 20-~m-wide slit, of a 2 /3 -m Czerny-Tulrner spec- trometer. The spectrometer has a grating with 2400 grooves per mm blazed at 300 nm, and reciprocal linear dispersion of 0.63 nm/mm. The He-Ne laser is used as a pointer. The spectra were monitored by an optical mul-

Volume 49, Number 4, 1995 0003-702s/95/4904.049952.00/0 APPLIED SPECTROSCOPY 4 9 9 © 1995 Society for Applied Spectroscopy

Trigger

f

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I

Target

Schematic diagram of the experimental setup.

YAG laser

[

tichannel analyzer (OMA) (Princeton Instrument IRY- 700 S) with 700 sensitized channels. The detector of this measuring system was a gateable intensified photodiode array with gate width range from 40 ns to 80 ms. The width of the spectral window covered by the detector and monitored on a screen was 10 nm at 300 nm. The spectral window could be shifted from 185 to 650 nm by rotation of the grating through a stepping motor. The high-voltage gateable intensifier in front of the detector made it pos- sible to perform measurements of weak transient signals.

The temporal history of the plasma was obtained by recording the emission features at predetermined delay times for the gateable intensifier using a variable delay generator which was triggered by the laser pulse. The intensities of the lines were obtained by integration of the peak area with automatic spectral baseline correction. The data were sent to a PC through a GPIB card for treatment. The wavelength calibration was carried out through a mercury spectral lamp as well as by identifying the iron lines obtained in the spectrum after a delay of 30 us. The energy was adjusted by using attenuators in the path of the beam and not by adjusting the supply voltage to prevent changes of the energy distribution in the laser beam.

PLASMA CHARACTERIZATION

The emitted spectral line intensity ~mn is a measure of the population of the corresponding energy level of this element in the plasma. If the plasma is in local thermo- dynamic equilibrium (LTE), the population of an excited level can be related to the total density N(T) of neutral atom or ion of this element by Boltzmann's law:

¢,nn 4"trhmn U(T) gmAmneXp - (1)

where Xm,, Am,, and g,, are, respectively, the wavelength, the transition probability, and the statistical weight for the upper level; E,, is the excited level energy; T is the

temperature; and k and h are the Boltzmann and Planck constants, respectively. U(T) is the partition function.

There are thus two main factors influencing the emitted line intensity. The first is the number density of the atoms, and the second is the temperature of the plasma. The number density of free atoms is dependent on the mass ablated by the laser shot and on the fraction of material (particles, droplets, etc.) evaporated in the plasma. The vaporized amount, in turn, depends on the plasma shield- ing and the absorption of the laser radiation which is related to the electron density of the plasma. Knowledge of the temperature and electron density is vital to the understanding of the dissociation, atomization, ioniza- tion, and excitation processes occurring in the plasma.

For this reason we carried out an analysis of these pa- rameters to determine the best experimental conditions for the measurements in order to obtain the most favor- able analytical precision.

Temporal Characteristics of the Plasma Emission. Emission spectra of aluminum alloy plasmas in air at atmospheric pressure were recorded at different delay times td in the wavelength region 180 nm -< ~ - 650 nm. Because of the transient nature of the laser-induced plas- mas, the atomic and ionic populations present in the plume rapidly evolved with time and position. As a con- sequence, the spectra emitted by the laser-induced plas- mas vary significantly, depending upon the observation time after the impact of the laser pulse. At the first mo- ments, the plasma emission consists of an intense con- tinuum that decreases with time. This emission was at- tributed to both collisions of electrons with ions and at- oms (free-free emission) and recombinations of electrons with ions (free-bound emission)? s At later times, emis- sion lines due to the radiative decay of excited species dominate the radiation process as the plasma cools and expands. Figure 2 illustrates the temporal evolution of the plasma in the spectral region centered at 284 nm. Figures 2A, 2B, and 2C show the behavior of the plasma at several delay times. The early traces (td -< 500 ns) in this figure were dominated by singly ionized lines of Al(II) and Mg(II). These lines were strongly broadened, shifted, and superimposed on the intense continuum. They grad- ually disappear and are replaced by emission from excited neutral Mn, Si, and Mg atoms at longer delay times from the laser pulse impact.

We did not detect any doubly ionized lines for AI(III), such as the strong lines at 360.2 and 361.2 nm. This result is in agreement with that of Colon et al) 9 As discussed by several authors, 5,t5,2° the surrounding atmosphere af- fects the emission spectra, the crater size, and the amount of sample vaporized by the laser pulse. In our case, the plasma was formed in air, so that lines of C(I) (193.1 and 247.8 nm) from atmospheric carbon dioxide dissociated in our plasma; traces of N(I) and a molecular band of A10 were also detected.

In order to obtain a good compromise between the signal-to-background ratio and the emission intensity, a proper choice of the time delay is required for the mea- surement of emission lines for analytical purpos- es. 6'10'12'13'15'20-26 Delay periods from 150 ns to 2 us have been chosen by other investigators. 2°,24,27-29 In this work we have chosen for the quantitative analysis a value of 10 us for the delay time and 10 us for the collection (gate)

500 Volume 49, Number 4, 1995

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286 288 290 292

Wavelength (nm)

Fro. 2. Spectra of an aluminum alloy sample (AL6010) taken at dif- ferent delay times after the impact of the YAG laser pulse. Energy density was 21 J/cm 2.

TABLE I. Spectroscopic data of neutral iron lines.

A,.,.. 10 8 X,.. (nm) E,. (cm-')" E,,. (cm-') ° g,." (s ')" &A%

370.55 415.9 27,395 7 0.032 10% 371.99 0 26,875 11 0.163 10% 372.25 704 27,560 5 0.049 10% 373.48 6928 33,695 i 1 0.902 10% 373.713 415.9 27,167 9 0.142 10% 374.56 704 27,395 7 0.115 10% 375.82 7728 34,329 7 0.634 10% 376.38 7986 34,547 5 0.540 10%

"Reference 30.

time, in order to discriminate against the continuum and to restrict the observed species to neutral atoms. We recall that the optimal time delay is related to the energy of the laser, its wavelength, the target characteristics, and the surrounding atmosphere.

Determination of the Plasma Excitation Temperature. Relative emissivities of lines from a given state of exci- tation can be used to evaluate the plasma temperature. The lines must be well resolved for accurately evaluating their emissivities (~m,), and the transition probabilities (Amn) must be known. Since the populations of the excited states are given by the Boltzmann distribution, Eq. 2 describes their relative emissivity:

~kmnlf.mn , [N(T) '~ E m lngmAm------~-ln~u~] k T (2)

where the symbols were defined above. A plot of the expression on the left-hand side vs. Em has a slope of - 1/ kT. Therefore, the plasma temperature can be obtained without knowing N(T), the total number density, or U(T), the partition function.

The temperatures were determined from the emission intensity of eight Fe(I) lines observed in the laser-induced plasma of an aluminum alloy (AL3003) containing 0.7% Fe. The degeneracies and transition probabilities for these lines were taken from Fuhr et al. 3° and are given in Table I. A typical spectrum of our plasma showing those lines is presented in Fig. 3. They are very close but not blended in the spectrum, making corrections for the spectral re- sponse of the system unnecessary. A typical plot of Eq. 2 is shown in Fig. 4 (td = 3 ~S, E = 60 m J); the data were fitted with least-squares approximation. The slope of this line yields a temperature of 6730 K.

The temperature was measured for three laser inten- sities (three laser energies for the same spot diameter of 0.6 mm on the target). The temporal variation of the obtained temperatures is shown in Fig. 5. The gate width was l ps for t < 5 ps and 5 ps for longer delay times. Taking into account the uncertainty of transition prob- abilities, the multiple measurements have uncertainties of about 6%. This incertitude would be higher for the low emission intensities obtained at much longer delay times or for shorter delay times where the lines are broadened and interfere with other lines.

It can be seen that the temperature drops very fast in the first few microseconds. Conversely, only small tem- perature changes can be observed on a microsecond time scale at later times. The strong continuum background and broadening of the emission profiles of iron lines pre-

APPLIED SPECTROSCOPY 501

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CO

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CO wi t O

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FIG. 3. Emission spectra o f LIBS on an a luminum alloy sample (AL3003) containing 0.7% Fe. Time delay and gate width were both 5 its. The observed emission lines were mostly assigned to neutral a tomic iron.

vented determinations of temperature before time delays of 0.5 #s in the case of lower emission intensities and 1 tts for higher intensities (Fig. 5).

To evaluate the plasma temperature by this method, it is important to verify that the plasma is not optically thick for the lines used. We verified the ratio of emission intensities of resonant and nonresonant lines according to a procedure described previously by Radziemski et al. 31 and Simeonsson and Miziolek. 32 The observed in- tensity ratios were consistent with those predicted by the statistical weights of the upper levels; this result indicates that the plasma was optically thin.

The temperature values reported in this work are cal- culated from spectra which were measured by integrating the intensity over the line-of-sight in the plasma perpen- dicular to the target on the center of the laser spot. The

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F ]o . 4. A n examp le o f B o l t z m a n n p lo t ob ta ined in a i r at a tmospher i c pressure. Energy density was 21 J /cm 2. The delay t ime and the gate width were 3 and 2 tts, respectively. The slope yields a temperature o f 6730 K.

12000

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• 170 mj , • 100 mj

2000 . . . . ' ' ' i , l i i , , I i , i i I i t , , I . . . .

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T i m e ( I ts )

FIG. 5. Time-resolved plasma temperatures for three different energy densities. The sample was a luminum alloy (AL3003). The gate widths were 250 ns, 1 us, and 5 us, for delay t imes td < 1 #s, 1 , s < to < 5 us, and 5 , s < to < 60/.ts, respectively.

plasma is not spatially resolved and no Abel inversion was applied, so that the resultant temperature is a pop- ulation-averaged temperature as discussed by Boumans 33 and used by several authors. 14,31,32,34

Furthermore, the laser-induced plasma depends on many parameters, such as laser intensity, wavelength, pulse duration, target material, and surrounding atmosphere. Consequently, it is difficult to compare our results with those reported in the literature with an aluminum target at atmospheric pressure. 19,35,36 However, in general terms, the temporal variation of the temperature appears to be similar to that reported by Iida. 35 His values are slightly lower than ours and decrease rapidly with time. This result is probably due to the lower ambient pressure in his case, which less efficiently confines the plasma, so that the plasma spreads more rapidly over a large volume.

Electron Densities. The profile of a line is the result of many effects, but in our conditions the main contribution to linewidths arises from the Stark effect. The contribu- tion of other mechanisms of broadening (Doppler effect, Van der Waals broadening, and resonance broadening) can be neglected, as shown under conditions similar to ours by Colon et al. 19 The electrons in the plasma can perturb the energy levels of the individual ions which broaden the emission lines originating from these excited levels. Stark broadening of well-isolated lines in the plas- ma is, thus, useful for estimating the electron number densities provided that the Stark-broadening coefficients have been measured or calculated. 18,37 Three lines o fAl(II) were identified as candidates for electron-density mea- surements: 199.0, 281.6, and 466.3 nm. Figure 6 shows the 281.6-nm line with sufficient resolution to measure the full width at half-maximum (FWHM). All three lines showed approximately Lorentzian line shapes, 19 so that the observed line shape was corrected by simply sub- tracting the contribution by instrumental line broadening with the use of

A,X~e = A,~ob~ - AXi,,, . . . . ,. (3)

5 0 2 V o l u m e 49 , N u m b e r 4, 1 9 9 5

In our case, Z~hinst . . . . t was typically 0.03 nm, as deter- mined by measuring the FWHM of the Hg lines emitted by a mercury spectral lamp at low pressure. The FWHM AX,/2 of a line is given by ~s

AXvz(in Angstroms)

( ) (~16) TM _ wt ) "

= 2w 1 - ~ + 3.5A [1 1.2ND ,/3 [ Ne \

(4) The first term in Eq. 4 gives the contribution from elec- tron broadening, and the second term is the ion broad- ening correction; w is the electron impact parameter, which can be interpolated at different temperatures, 37 and A is the ion broadening parameter. Both w and A are weak functions of temperature) 8,38 Ne is the electron density (cm-3) and No the number of particles in the Debye sphere. No is given by: 37

T(eV) 3/2 N D = 1.72. 109.N~(cm_3)t/2. (5)

The contribution from quasi-static ion broadening (the second term of expression 4) is small in our case. Its value can be evaluated from the extrapolation of the G r i e m 37 estimation for A and w (for example, for T = 8000 K and Ne ~ 10 t7 c m -3, its contribution is less than 2%). Ex- pression 4 reduces to

No (6) ~kl/2 = 2w 1016.

The determination of electron density by this method is independent from any assumption of LTE conditions. Figure 6 shows the 281.6-nm Al(III) line profile at dif- ferent delay times corresponding to different electron den- sities. Electron densities were determined from the Stark widths of various lines with the use of Eq. 6. The temporal behavior of the electron density obtained and averaged from different lines is shown in Fig. 7 for a laser energy density of 21 J / c m 2.

Self-absorption is another cause of broadening and may lead to erroneous results, but in our case the plasma is optically thin for the lines used. To get an order of value, the absorption coefficient for those lines can be estimated 39 as follows: By estimating the AI(II) density as approxi- mately equal to the electron density and knowing the oscillator strength and the lower energy leveP ° states, for 0.04- and 0.2-nm linewidths we can obtain absorption coefficients of 0.2 and 0.03 cm -~, respectively, for the 281.6- and 466.2-nm AI(II) lines. In this calculation, the value of the partition function was taken from Ref. 41, while the temperature and the electron density were 8000 K and 2.1017 c m -3, respectively.

Table II. Composition (%) of standard aluminum alloys.

C

0 0 LO ÷ w

AL II 281.6nm

'1t \'---"//i,,

281

" \ \

• \ \ X k \ t t n s

\ 200 \ - .

282 283 284

Wavelength (nm)

Fx~. 6. Profiles of the 281.6-nm line of AI(II) obtained at different delay times (from top to bottom respectively, 50, 200, and 500 ns; 1, 2, and 3 us), corresponding to different electron densities.

Figure 7 shows that the electron density is at a maxi- mum at the earliest times and is on the order of 10 ~s cm -3 or greater, depending on the laser energy. The density decays steadily to ~ 1017 cm -3 or less within 2 ~zs. Our values are similar to those reported by Colon et al. ~9 The electron densities obtained ranged from 1.10 's to 7.10 t5 cm-L Since for our analysis of plasma temperature we have assumed that the plasma is in LTE, the values of electron density allow us to confirm the validity of our assumption using the criterion of Ref. 42.

The lower limit for electron density for which the plas- ma will be in LTE is:

Ne(cm -3) >- 1.6. IO'2[T(K)]'/Z[AE (eV)] 3. (7)

,~E is the largest energy transition for which the condition holds. In our case AE = 3.65 eV for AI, and the lower limit given by Eq. 7 is 6.10 ~5 cm -3. For iron AE ~ 3.2 eV, and the lowest value is 4.10 ~5 cm -3. The measured densities are greater than this value throughout the plas- ma, which is consistent with the assumption of LTE pre- vailing in the plasma. In conclusion, the choice of the time delay was crucial for obtaining the best operating conditions, in particular to avoid significant line broad- ening causing spectral line overlap.

QUANTITATIVE ANALYSIS

The quantitative spectral analysis involves relating the spectral line intensity of an element in the plasma to the concentration of that element in the target. We investi- gated a set of seven samples of aluminum alloy. The

Samples Mg Si Fe Cu Mn Cr Zn Ti A1

ALl 100 0.05% 0.95 Si + Fe 0.12 0.05 0.05 0.1 0.05 Remainder AL2036 0.45% 0.5 0.5 2.6 0.25 0.1 0.25 0.15 Remainder AL3003 0.1% 0.6 0.7 0.1 1.5 0.1 0.1 0. l Remainder AL5182 4.6% 0.2 0.35 0.15 0.35 0. l 0.25 0.1 Remainder AL5754 3.2% 0.4 0.4 0. | 0.5 0.3 0.2 0.1 Remainder AL6010 0.8% 1 0.5 0.38 0.5 0.1 0.25 0.1 Remainder AL6111 0.75% 0.9 0.4 0.7 0.3 0.1 0.15 0.1 Remainder

APPLIED SPECTROSCOPY 503

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FIG. 7. Temporal behavior of the electron density of an aluminum alloy (AL6010) plasma induced by a YAG laser; energy density 21 J/cm 2.

elemental compositions of these samples are given in Ta- ble II. All values are in percentage. We analyzed the most important minor elements in our samples, which are cop- per, magnesium, manganese, and silicon. The results de- scribed above were applied to the determination of those elements in the aluminum alloy matrix. The prominent analyte and reference lines used are summarized in Table III.

Experimental Conditions for the Measurements. As mentioned above, at early times following plasma for- mation, the spectrum is dominated by an intense radia- tion continuum and ionic emissions. These lines are strongly broadened by the Stark effect, because of the high electron density that exists in the plasma initially. At later times (td > 2 #s), emissions from neutral atoms predom- inate. We have found that the optimum time td is some- what dependent upon the species. Ideally, a td analysis should be made for each element to determine the opti- mum td. In this work we used td = 10 #S uniformly for all the elements studied, with a gate window tw = 10 ~s. All these lines are from similar level energies (see Table III).

Table III. Spectroscopic data of prominent analyte and reference lines used for LIBS analysis of aluminum alloys.

Wave- Ele- length A ~,

ment (nm) E~ (cm-l)" E2 (cm-') a (10 s s ~)" g : g~"

AI +b 281.62 59,842 95,351 3.83 3 1 A1 b 256.80 0 38,929 0.23 2 4 A1 b 396.15 105 25,348 0.98 4 2 Si 251.61 223 39,955 1.21 5 5 Mg ÷ 280.27 0 35,667 2.6 2 2 Mg 285.21 0 35,051 4.95 1 3 Mg 517.27 21,864 41,197 0.346 3 3 Mg 518.36 21,905 41,197 0.575 5 3 Mn 279.83 0 35,726 3.6 6 6 Mn 280.11 0 35,690 3.7 4 6 Mn 403.08 0 24,802 0.19 6 8 Mn 403.31 0 24,788 0.18 6 6 Cu 324.75 0 30,784 1.39 2 4

"Reference 40. b Reference lines.

504 Volume 49, Number 4, 1995

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% Mg in aluminum alloys

FIG. 8. Temperature measured at 10-gs delay time and 10-gs gate window for different concentrations of magnesium in aluminum alloys.

In the experiment reported here light collection has been optimized in order to gather most of the plasma emission in the direction of observation 8,2°,2~ to avoid plasma inhomogeneity and irregular distribution of ma- terial in the plasma, which cause poor shot-to-shot re- producibility.

In order to reduce statistical errors, measurements were performed by averaging the signals of 50 shots after an initial eight shots to prepare the surface and prevent sur- face contamination to distort the results. According to s e v e r a l authors, 6,9,2°,24 a few preliminary laser shots are enough to clean the sample surface and avoid the influ- ence of the initial surface conditions.

The matrix effects are the changes in plasma compo- sition produced by the physical and chemical properties of the target. The actual significance and the extent of matrix effects in LIBS are still uncertain. 1,2 In our case, aluminum is the major component. It forms more than 90% of the sample, so that the minor components do not vary widely in concentration. Figure 8 shows the variation of temperature vs. the concentration of magnesium in aluminum alloy. It can be seen that temperature is almost constant within the limit of uncertainty, showing that there are no matrix effects in this case.

Calibration Curves. Calibration curves have been ob- tained with the use of aluminum alloy samples whose composition is shown in Table II. Figures 9A to 9D show the calibration curves. Each figure relates the emissivity of an element to its concentration in the aluminum alloy.

The calibration curves for the Mn(I) 403.3-nm, Si(I) 251.6-nm, Cu(I) 327.4-nm, and Mg(I) 285.2-nm lines are approximately straight lines up to 1%. The curves for copper and magnesium (Figs. 9A and 9C) exhibit a cur- vature for higher concentrations. The leveling of these curves indicates a decrease in the sensitivity of this tech- nique at high concentration of copper and magnesium. This observation is due to self-absorption of the Cu(I) 327.4-nm and Mg(I) 285.2-nm resonance lines. Similar behavior for calibration of aluminum in iron ores was observed by Grant et al? ° and for chromium in steel samples by Leis et al. 6

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% Mg in aluminum alloys

1 2 . . . . 1 2

% Mn in aluminum alloys

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% Si in aluminum alloys % Cu in aluminum alloys

Fie. 9. Calibration curves vs. concentration in aluminum alloys: (A) Mg(I) 285.2 nm vs. Mg concentration; (B) Mn(I) 403.1 and 403.3 nm vs. Mn concentration; (C) Cu(I) 327.4 nm vs. Cu concentration; (D) Si(I) 251.6 nm vs. Si concentration.

At higher concentration, self-absorption is no longer negligible for resonance lines. However, we can reduce

3 self-absorption by using a transition which terminates on a higher energy level. Figure 10 shows the calibration curves for Mg at 518.2 and 517.3 nm. Both magnesium lines have relatively high excitation energies (41,197 cm- t) g and, while the Mg(I) 285.2-nm line is a resonance line, 8 the 518.2- and 517.3-nm lines are due to transitions which ~ 2 end at 21,911 and 21,870 c m - ', respectively, above the ground state. The calibration curves are linear and they pass, within experimental uncertainty, through the origin.

c In conclusion, resonance lines may be used when the '~o 1 concentration of the analyte is low on the sample. This ~, observation is in agreement with the results o f Autin et a1.,24 Leis et a1.,6 and Grant et al. ,0 These lines are generally m E the strongest and would yield the lowest detection limit. If the concentration o f the analyte varies widely in the matrix, the nonresonant lines are more reliable and sen- 0: sitive for the variation and their use in LIBS is more convenient.

Preeision and Detection Limits. The lines used for an- Fro. 10. alyte elements in our analysis are free from interferences

. . . . I . . . . I . . . . I . . . . I . . . .

• 518.36 nm • 517.27 nm •

1 2 3 4 5

% Mg in aluminum alloys

Calibration curves for Mg(I) 517.2 and 518.3 n m vs. mag- n es iu m concentration in a l u m i n u m alloys.

A P P L I E D S P E C T R O S C O P Y 5 0 5

0.15

t o

0.10

.o_ 0.05

0.00 ~ ~ i ~ I i i ~ i I = i i i

0.0 0.5 1.0 1.5

% Si in aluminum alloys

F1o. 11. Calibration curve for Ratio of Si(l) line 251.6 nm to the AI(1) line 256.8 nm vs. silicon concentration on aluminum alloy.

and are well isolated. The precision of 50 consecutive measurements of Mg, Mn, Cu, and Si was 4% RSD (rel- ative standard deviation) for the strongest line at high concentration and 6% at low concentration. This value of RSD is related to the variation of laser energy (1%), the degradation of the surface from shot to shot, and the perturbation of the plasma due to the laser surface inter- action. This precision would be improved by measuring the ratio of analyte lines to the reference line with com- parable excitation energies and within the same spectral range.

The wavelength range captured simultaneously by our detector array was 10 nm. Within this range, there are three couples of lines (analyte and reference) having com- parable excitation energies (see Table III). Figure 11 shows the ratio of one of them: Si 251.6 nm to AI(I) 256.8 nm vs. concentration of Si in the sample. In such cases, the relative error of the analyte to reference concentration ratio is mainly governed by the relative error of the an- alyte concentration. Relative accuracies of 3% are achiev- able when one is measuring absolute element contents.

The detection limit is defined as the concentration that produces a net line intensity equivalent to three times the standard deviation of the background. It can be evaluated with the expression 43

CL = 3cr--'-9~ (8) S

where aB is the standard deviation of the background, and S is the sensitivity defined as the slope of the referenced atomic emission signal vs. concentration.

The detection limits for LIBS analysis of aluminum alloy calculated by Eq. 8 and the slopes of the calibration curves for each element are listed in Table IV.

It can be seen that the detection limits are a function of the element studied. This observation is due to many factors: (1) the intensity of the analytical line, which is related to the transition probability; (2) the upper energy of the emitting analytical line (in fact, with plasma sources in LTE, it is difficult to populate the higher level energy); and (3) the spectral region of the analytical line, which is

Table IV. Detection limits for various elements in aluminum alloy.

Element Concentration (ppm)

Mg 0.5 Cu 10 Si 14 Mn 2

related to the detector sensitivity. Our limits of detection are similar to those reported by Leis et al., 6 Autin et al., 24 and Carlhoff and co-workers. 2~

CONCLUSION

The LIBS method has been applied as an analytical technique for analysis of aluminum alloy samples. The limits on the accuracy of this technique are mainly due to the matrix dependence of the signal, self-absorption, and line broadening, as well as the high intensity of the continuum. These problems can be overcome or mini- mized by properly using time-resolved measurements.

The temporally resolved emission spectrum of an alu- minum laser-induced plasma has been used to evaluate the space-integrated temperature and the electron density as a function of time. The present results show that the production of a plasma in air at atmospheric pressure by means of a YAG laser is suitable for analytical applica- tions. The detection limits and precision obtained are comparable to those of conventional AES methods. To extend the dynamic range, one may select different an- alyte lines to be used alternatively within a certain con- centration range. The stronger analyte lines, even when exhibiting self-absorption at higher concentrations, could be used in the lower concentration range, while the weaker lines may be used for higher concentrations.

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APPLIED SPECTROSCOPY 507