Dependence of Laser-induced Breakdown Spectroscopy Results on Pulse Energies and Timing Parameters Using Soil Simulants

Summary

LIBS detection capabilities on soil simulants were tested using a range of pulse energies and timing parameters. Calibration curves were used to determine detection limits and sensitivities for different parameters. Generally, the results showed that there was not a significant reduction in detection capabilities using lower pulse energies and non-gated detection.

Abstract

The dependence of some LIBS detection capabilities on lower pulse energies (<100 mJ) and timing parameters were examined using synthetic silicate samples. These samples were used as simulants for soil and contained minor and trace elements commonly found in soil at a wide range of concentrations. For this study, over 100 calibration curves were prepared using different pulse energies and timing parameters; detection limits and sensitivities were determined from the calibration curves. Plasma temperatures were also measured using Boltzmann plots for the various energies and the timing parameters tested. The electron density of the plasma was calculated using the full-width half maximum (FWHM) of the hydrogen line at 656.5 nm over the energies tested. Overall, the results indicate that the use of lower pulse energies and non-gated detection do not seriously compromise the analytical results. These results are very relevant to the design of field- and person-portable LIBS instruments.

Introduction

Laser-induced breakdown spectroscopy (LIBS) is a simple method of elemental analysis that uses a laser-generated spark as the excitation source. The laser pulse is focused onto a surface that heats, ablates, atomizes and ionizes the surface material resulting in the formation of plasma. The plasma light is spectrally resolved and detected and elements are identified by their spectral signatures. If properly calibrated, LIBS can provide quantitative results. LIBS can analyze solids, gases, and liquids with little or no sample preparation.¹ These characteristics make it ideal for analyses that cannot be carried out in the laboratory.

Currently, LIBS is being studied for many different applications especially those that require field-based measurements for quantification.^1-8 This requires the development of LIBS instrumentation using rugged and compact components suitable for a field-based system. In most cases, these components will not have the full capabilities of laboratory-based instrumentation, thereby compromising the analysis performance. LIBS results are dependent on laser pulse parameters and other measurement conditions that include sampling geometry, surrounding atmosphere, and the use of gated or non-gated detection.^9-12 For field-based LIBS instrumentation, two important factors to consider are the pulse energy and the use of gated versus non-gated detection. These two factors determine to a large extent the cost, size, and complexity of the LIBS instrument. Small, ruggedly built lasers that can generate pulses from 10-50 mJ at repetition rates of 0.3-10 Hz are commercially available and would be highly advantageous to use. Therefore, it is important to know what, if any, loss in detection capabilities will result from the use of these lasers. The pulse energy is a key parameter for LIBS as it determines the amount of material ablated and vaporized and the excitation characteristics of the plasma. In addition, the use of gated detection can increase the cost of the LIBS system; as a result, it is imperative to determine the differences between spectra and detection capabilities using gated and non-gated detection.

Recently, a study was performed comparing gated detection to non-gated detection for minor elements found in steel. The results showed that the detection limits were comparable if not better for non-gated detection.¹² One important characteristic of LIBS is that the technique experiences physical and chemical matrix effects. An example of the former is that the laser pulse couples more efficiently with conducting/metal surfaces than non-conducting surfaces.¹³ For this study, we wanted to determine the effects of pulse energy and timing parameters for non-conducting materials like soil simulants.

Although, field portable LIBS instruments have been developed and used for some applications, a comprehensive study on the detection capabilities has not been performed comparing higher energy and gated systems to lower energy and non-gated systems using soil simulants. This study focuses on laser pulse energy and timing parameters for determination of trace elements in complex matrices. The laser pulse energy ranged from 10 to 100 mJ to obtain a comparison between lower and higher energies. A comparison of the use of gated versus non-gated detection was also performed over the same energy range.

Protocol

1. Laser System

1. Use laser pulses produced by a Q-switched Nd:YAG laser operating at 1,064 nm and at 10 Hz.
2. Focus the laser pulses onto the sample with a 75 mm focal length lens.
3. Collect the plasma light with an optical fiber pointed at and placed near the plasma formed on the sample.
4. Use an Echelle spectrograph/ICCD to spectrally resolve and record the LIBS spectrum.
5. Operate the ICCD in both non-gated and gated modes using a gain of 125.
6. Use a 0 μsec time delay (t_d) in non-gated mode and a 1 μsec t_d in gated mode.
7. For both modes, use a gate width (t_b) of 20 μsec with a 3 sec exposure (integrating the plasma light on the ICCD camera chip); this will result in 30 individual laser shots being added to produce each spectrum.
8. Record a total of 5 such spectra for each sample analyzed.
9. Use a digital delay generator to control the timing between the laser and the ICCD gate pulse. The experimental set up is shown in Figure 1.
10. Verify the timing with an oscilloscope.
11. Operate the laser at pulse energies of 10, 25, 50, and 100 mJ using both non-gated and gated detection.
12. Continually monitor the laser energy and adjust to correct for drift, if necessary.
13. Safety Consideration: The Nd:YAG laser is a Class IV laser; wear appropriate laser safety goggles at all times when operating the laser and establish room interlocks in conjunction with the room door and laser.¹⁴

2. Samples and Sample Preparation

1. Use synthetic silicate certified reference materials with known element concentrations as samples; these mimic common soil samples with minor and trace amounts of selected elements spanning a range of concentrations.
2. Concentrations of the trace elements ranged from a few ppm to 10,000 ppm. Table 1 lists the elements monitored here including their line types and wavelengths used for analysis. The line types labeled as I and II signify neutral atoms or a singly ionized atoms, respectively. The common base composition of each silicate sample is SiO₂ (72%), Al₂O₃ (15%), Fe₂O₃ (4%), CaMg(CO₃)₂ (4%), Na₂SO₄ (2.5%), and K₂SO₄ (2.5%).
3. Press the samples into 31 mm diameter pellets using a hydraulic press to create a smooth surface for LIBS analysis. The smooth surface helps to create consistency with the LIBS results.
4. Analyze a new sample spot for each spectrum recorded.
5. Safety consideration: The synthetic silicate samples contain a wide variety of elements at various concentrations; wear gloves during handling.

3. Preparing Calibration Curves

1. Prepare calibration curves for the various elements in both gated and non-gated detection over the range of laser energies tested.
2. Make these curves by plotting the peak area or the ratioed peak area (y-axis) against element concentration (x-axis).
3. Use a linear trend line to fit the calibration curve. [screen shot 1]
4. Calculate detection limits using 3σ detection as defined by IUPAC.¹⁵ [calculation 1]

4. Plasma Temperature Determination

1. Measure plasma temperatures from Boltzmann plots.
2. Use a set of iron lines [Fe(I)] between wavelengths of 371-408 nm to create Boltzmann plots using: ln(Iλ/gA) = -E_u/kT - ln(4ρZ/hcN₀) (Eq. 1) where I is the intensity of the transition as determined from the peak area, λ is the wavelength, A is the transition probability, g is the degeneracy of the transition, E_u is the upper state for emission, k is the Boltzmann constant, T is the temperature, Z is the partition function, h is Planck's constant, c is the speed of light, N₀ is the total species population.
3. Chose Fe lines that have known E_u, g, and A values.

• The Fe(I) lines used here are 371.99, 374.56, 382.04, 404.58, 406.36 nm.
• The E_u, g, and A values can be found on this website (http://physics.nist.gov/PhysRefData/ASD/lines_form.html)
• Make sure to select the option to show the "g" under additional criteria labeled as level information.
• Use the E_k and g_k values.

4. To determine temperature, plot ln(Iλ/gA) against E_u and fit the data with a linear trend line; the slope is equals to -1/kT.^16,17 [screen shot 2]

5. Electron Density Determination

1. To measure the electron density, use the full width at half maximum (FWHM) of the hydrogen line at 656.5 nm.
2. Take this data using t_d=0.5 μsec and t_b= 4.5 μsec on the ICCD.
3. Measure the FWHM of the hydrogen line. [screen shot 3]
4. Calculate the electron density using: N_e = 8.02 x 10¹²[Δλ_1/2/α_1/2]^3/2 (Eq. 2) where N_e is the electron density, Δλ_1/2 is the measured FWHM of the hydrogen line, and α_1/2 is the reduced wavelength which is a function of the temperature and the electron density. The values for the reduced wavelengths are provided in Griem's Appendix IIIa.^16-18
5. Calculate the electron density using a temperature of 10,000 K (this was the close to the average temperature of the plasma). [screen shot 4]

6. Work up All Data Using a Program that Can Determine the Peak Areas and/or Microsoft Excel

Results

Effect of laser pulse energy and detection modes on detection capabilities. LIBS spectra of the synthetic silicate samples were recorded using gated and non-gated detection over the range of laser pulse energies tested. Over 100 calibration curves were constructed from these data to evaluate the effect of the laser pulse energy. Calibration curves were prepared by (1) using the area under the analyte peak and (2) by ratioing the area of the analyte peak to the area of the iron peak at 405.58 nm. The iron...

Discussion

When comparing non-gated and gated detection modes, the detection limit data show that the gated detection mode allowed for detection of all of the elements including those that were not seen using higher laser energies in non-gated detection mode. Using gated detection, the initial high background from the formation of the plasma is not observed and the background is decreased showing the elemental emission better resolved. Furthermore, the detection limits were slightly lower using gated detection.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Equipment
Nd:YAG laser	Continuum	Surelite II
Echelle spectrograh/ICCD	Catalina/Andor	SE200/iStar
Digital delay generator	BNC	Model 575-4C
Hydraulic Press	Carver	Model-C
31-mm pellet die	Carver	3902
Power meter indictor model	Scientech, Inc.	Model number: AI310D
Power meter detector model	Scientech, Inc.	Model number: AC2501S
Oscilloscope	Tektronix	MSO 4054
Optical fiber	Ocean Optics	QP1000-2-UV-VIS
Lens kit (this kit contains the 75 mm f.l. lens)	CVI Optics	LK-24-C-1064
Reagent/Material list
Synthetic silicate sample	Brammer Standard Company	GBW 07704
Synthetic silicate sample	Brammer Standard Company	GBW 07705
Synthetic silicate sample	Brammer Standard Company	GBW 07706
Synthetic silicate sample	Brammer Standard Company	GBW 07708
Synthetic silicate sample	Brammer Standard Company	GBW 07709
Aluminum caps (for pressing synthetic silicate samples)	SCP Science	040-080-001