Comparison study of two commercial spectrometers for heavy metal analysis of laser induced breakdown spectroscopy (LIBS)

Abstract

The purpose of this paper is to compare the performance of two spectrometers that are manufactured from the same company. In this work, heavy metals like lead Pb and copper Cu in the KBr matrix were analyzed using the laser induced breakdown spectroscopic technique. A Q-switched Nd:YAG laser with 90 mJ per pulse operating at the fundamental wavelength of 1064 nm and pulse duration of 10 ns was used to generate plasma at the focal region. The important experimental parameters such as the laser energy, integration time, distance between the lens and sample, distance and angle of the optical fiber from the target were optimized. Two spectrometers manufactured by Ocean Optics namely as Maya2000Pro and USB 4000 were employed for anlyzing the spectral lines. The experimental setup and conditions were remained the same for both experiments. The production of spectral lines from each of the interested elements was analyzed and compared with the NIST (National Institute of Standards and Technology) database. The sensitivity, repeatability and limit of detection for each of the systems are discussed in detail.

PHOTONIC SENSORS / Vol. 4, No. 1, 2014: 6369

Comparison Study of Two Commercial Spectrometers for Heavy Metal Analysis of Laser Induced

Breakdown Spectroscopy (LIBS)

M. ARAB, N. BIDIN*, Z. H. RIZVI, S. SAFIE, and M. A. ALSAEDI

Advance Photonics Science Institute, Faculty of Science, Universiti Teknologi Malaysia, 81310, Johor Bahru, Johor, Malaysia

*Corresponding author: N. BIDIN E-mail: mailto:[email protected]

Web End [email protected]

Abstract: The purpose of this paper is to compare the performance of two spectrometers that are manufactured from the same company. In this work, heavy metals like lead Pb and copper Cu in the KBr matrix were analyzed using the laser induced breakdown spectroscopic technique. A Q-switched Nd:YAG laser with 90 mJ per pulse operating at the fundamental wavelength of 1064 nm and pulse duration of 10 ns was used to generate plasma at the focal region. The important experimental parameters such as the laser energy, integration time, distance between the lens and sample, distance and angle of the optical fiber from the target were optimized. Two spectrometers manufactured by Ocean Optics namely as Maya2000Pro and USB 4000 were employed for anlyzing the spectral lines. The experimental setup and conditions were remained the same for both experiments. The production of spectral lines from each of the interested elements was analyzed and compared with the NIST (National Institute of Standards and Technology) database. The sensitivity, repeatability and limit of detection for each of the systems are discussed in detail.

Keywords: Heavy metal, spectrometer, LIBS, LOD

Citation: M. ARAB, N. BIDIN, Z. H. RIZVI, S. SAFIE, and M. A. ALSAEDI, Comparison Study of Two Commercial Spectrometers for Heavy Metal Analysis of Laser Induced Breakdown Spectroscopy (LIBS), Photonic Sensors, 2014, 4(1): 6369.

1. Introduction

Laser induced breakdown spectroscopy (LIBS) is a prominent technique for detection and analysis of chemical, biological, explosive, and hazardous materials. The LIBS involves interaction of a target with an intense laser pulse which generates plasma. The spectral emission from the plasma contains the specific signature of atoms of the material [1, 2]. The LIBS is, in principle, a straightforward and simple analytical technique that can be employed even by non specialist users. It is a quick and portable measurement technique providing results

practically instantaneously after analysis. Also, it is applicable in situ that is, on the object itself and under certain conditions are nearly nondestructive [3].

The basic principle behind the LIBS is as

follows. The output of a pulsed laser is focused onto the target material so that a luminous micro plasma or spark is produced on the surface, the optical emission of which is characterized of the target material. A fraction of the micro plasma light is collected and analyzed by an optical spectrometer, and the results of the measurement are displayed on

Received: 10 September 2013 / Revised version: 22 November 2013

The Author(s) 2013. This article is published with open access at Springerlink.com

DOI: 10.1007/s13320-013-0144-1

Article type: Regular

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the computer screen as signals within a few seconds [4]. The LIBS offers several advantages compared to other spectroscopic techniques including (1) short time for sample preparation, (2) simultaneous multiple element analysis for almost all the elements in nature, (3) real-time response, (4) applicability to all states of the sample i.e., solid, liquid or gas, (5) only need for little amount of the sample, and (6) high sensitivity [5].

The LIBS technique has now been widely used

to study various materials like metals, alloys, biological samples, polymers, soil, environmental pollutants, land mines, and explosive materials [6, 7]. For elemental analysis by the LIBS, the appropriate choice for the experimentalist is based on the type of the spectrometer, which requires a balance among the wavelength coverage, spectral resolution, read time, dynamic range, and detection limit.

The type of the spectrometer is an important factor to be considered in optical emission spectroscopy (OES) for any plasma characterization or analytical spectrochemistry experiment [8, 9]. The ideal spectrometer depends on several parameters to get a good spectrum leading to the elements in the sample like: (a) a high resolution to resolve more lines of interest and avoid overlapping, (b) wide wavelength coverage, typically from 200 nm to 800 nm to be able to detect simultaneously several elements, and (c) high sensitivity and a linear response to radiation. Furthermore, for rapid analysis, the readout and data acquisition time should be shorter [10]. Currently, these factors can be obtained by using different types of spectrometers. In this work, a comparison study between two commercial spectrometers, USB4000-UV-VIS with Toshiba TCD1304AP detector and Maya2000 Pro with Hamamatsu S10420 detector, was performed. The lead (Pb) and copper (Cu) in the KBr mixture were analyzed at the atmospheric pressure. Both spectrometers were conducted in the range of UV-visible. The KBr powder with the high purity (99%) was chosen as a

matrix for Pb and Cu powders because it can produce strong signals and clear spectrum lines. The aim of this research was to select the suitable spectrometer to give us the best limit of detection for heavy metal analysis and determine the sensitivity of our LIBS system.

2. Experimental setup

In this work, a Q-switched Nd:YAG laser (AL-14 FP6000) with the wavelength of 1064 nm and pulse duration of 10 ns was employed. The laser was operating in the repetition mode with the rate of

Hz, and the energy remained constant as 90 mJ per pulse. The laser pulse was focused by a lens with the focal length of 8 cm. The beam diameter was 2 mm on the sample, and the distance between the laser aperture and sample was fixed at 10 cm. Hence, the energy density delivered on the target was estimatedto be as 2.3 J/cm2. Potassium bromide (KBr), lead (Pb) powder and copper (Cu) with the purity of more than 99.5% were used as the target sample. KBr was used as a reference sample, and the following with the mixture of KBr with Pb and Cu of different concentrations varied from 0.01 g to

1

g.

Two commercial spectrometers namely

USB4000 and Maya2000 Pro manufactured from Ocean Optics were utilized as the spectral analyzer. The specifications for both spectrometers are summarized in Table 1. In order to investigate the performance for both systems in the LIBS analysis, the experimental conditions and setup were remained the same. Initially, all the powders were heated in an electrical furnace at 60 for 2 hours. The powders were then weighed accurately by a digital balance (Precisa-XT 220 A) and mixed in an appropriate ratio to make samples of desired concentration prior to be palletized by an electrical pelletizing press (Herzog Germany) with a pressure 50 kN for 5 minutes. Each of prepared pellets of KBr+Pb and KBr+Cu had the same dimension of 5 mm in thickness, 40 mm in diameter, and 10 g of the weight. Finally, all the pellets were

M. ARAB et al.: Comparison Study of Two Commercial Spectrometers for Heavy Metal Analysis of Laser Induced Breakdown Spectroscopy (LIBS)

65

heated at 60 for 2 hours before executing the LIBS analysis. The plasma radiation was collected via a collimating lens prior to passing through a 2-m long optical fiber of the 600-m (pure silica) core diameter coated with dopped-flourine silica. The fiber optic cable passed the collected radiation to the spectrograph through an entrance slit of the 5-m

width. The spectrograph dispersed the radiation into its constituent wavelengths, and a charge coupled device (CCD) detector recorded them as a spectrum graph during an integration time of 100 ms. Figure 1 shows the experimental setup of the LIBS system. Both spectrometers were used in the same experimental setup.

Table 1 Specifications of USB4000 and Maya2000Pro spectrometers.

Specification USB4000 spectrometer Maya2000 Pro spectrometer

Manufacturer Ocean Optics Ocean optics

Type of detector Toshiba TCD1304AP linear CCD array Hamamatsu S10420 detector

Spectral range (nm) 200 1100 165 1100

Optical resolution 0.1 10.0 nm FWHM (grating dependent) 0.03510 depending on grating groove density and slit size Entrance aperture Slit-5 (5-m width 1-mm height) Slit-5 (5-m width 1-mm height)

Signal to noise ratio At full signal about 300:1 At full signal about 450:1

Integration time 3.8 10 seconds 6.0 5 seconds

Lens Rotational stage

Plasma Collecting lens

Spectrometer with CCD

Computer

Power supply

NdYag ;aser

Wavelength separator

Fiber optic mounted on stand

Intensity (a.u.)

Fig.1 Experimental setup of the LIBS system.

3. Results and discussion

Typical results of the spectral analysis of the KBr pellet are shown in Figs. 2(a) and 2(b), which illustrate the detailed LIBS spectra obtained from USB4000 and Maya2000Pro spectrometers, respectively. In both cases, the spectral range of study is 200 nm 700 nm.

In Fig. 2(a), no lines of K are observed, except at 581.21 nm. That is overlapping with neighboring lines, while Fig. 2(b) shows clear lines of K and Br in the spectrum obtained from the same KBr pellet.

The spectral analysis of the (KBr + Pb) pellet is shown in Fig. 3. Two lines of lead appeared from

USB4000 spectrometer at 368.93 nm and 406.21 nm, respectively. The Maya spectrometer has displayed many spectral lines of lead which were obtained from the same pellet in the spectral range of 200 nm 700 nm. The difference between the spectral lines produced from different spectrometers is shown in Figs. 3(a) and 3(b).

35 000 30 000 25 000 20 000 15 000 10 000

5000

0

200 300 400 500 600 700

Wavelength (nm)

(a)

(b)

Fig. 2 LIBS spectra of the KBr pellet sample with (a) USB4000 spectrometer and (b) Maya2000 Pro spectrometer.

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(a)

(b)

Fig. 3 LIBS spectra of the KBr + Pb pellet sample with (a) USB4000 spectrometer and (b) Maya 2000Pro spectrometer.

The spectral analysis of the (KBr + Cu) pellet is shown in Figs. 4(a) and 4(b). Two lines of copper were observed in the spectrum using the USB4000 spectrometer at 324.75 nm and 374.47 nm. Similarly, the Maya spectrometer has shown many spectral lines of copper produced from the same pellet in the same spectral range of 200 nm 700 nm.

LIBS spectra by the USB4000 spectrometer for the pellets were recorded, and the data of K line appeared only at 581.21 nm, but the spectral line at 581.21 nm was overlapped with a nearby Fe I line. Pb lines were observed only at wavelengths of 368.93 nm and 406.21 nm, for Cu lines only at wavelengths of 324.75 nm and 374.47 nm. With the second spectrometer Maya2000Pro for the pellets in the same spectral range 200 nm 700 nm, the lines of K appeared at wavelengths of 330.16 nm, 344.63

nm, 393.43 nm, 396.67 nm, 404.41 nm, and 422.56

nm, and Pb lines were observed at 239.37 nm, 257.72 nm, 283.3 nm, 357.27 nm, 363.95 nm, 368.34

nm, 373.99 nm, 405.78 nm, 438.64 nm, and 560.88

nm, for Cu at wavelengths of 203.58 nm, 222.77 nm, 236.81 nm, 279.17 nm, 324.75 nm, 390.31 nm, 406.81

nm, 435.52 nm, and 453.07 nm.

We also observed some clear lines representing

emission from gases of the atmosphere such as Ar, Ne, and Xe. The wavelength data in spectra indicated the peaks exclusive for Pb and Cu spectral lines, and we noticed the difference with the number of those lines appeared from two spectrometers. All the spectral lines were compared with the NIST (National Institute of Standards and Technology) database [11].

(a)

(b)

Fig. 4 LIBS spectra of the KBr + Cu pellet sample with (a) USB4000 spectrometer and (b) Maya2000 Pro spectrometer.

The accuracy of calibration curves between the two spectrometers strongest lines of Pb was selected at 406.21 nm for the USB4000 spectrometer and 373.99 nm for the Maya2000Pro spectrometer. The Pb content in the KBr samples varied from 0.01 g to 1 g. As shown in the Fig. 5, the two curves were reasonably linear and comparable in terms of the sensitivity (slope) and repeatability. In the first calibration curve for this system PbI 406.21 nm emission line was employed, and the correlation coefficient (R2 = 0.956) was obtained, while in the second calibration curve at PbI 373.99 nm, we acquired the correlation coefficient (R2 = 0.987).

The strongest lines of Cu element were selected at 374.47 nm for the USB4000 spectrometer and 236.81 nm for the Maya2000Pro spectrometer. In

M. ARAB et al.: Comparison Study of Two Commercial Spectrometers for Heavy Metal Analysis of Laser Induced Breakdown Spectroscopy (LIBS)

67

this study, the Cu content in the KBr samples varied from 0.01 g to 1 g. The two curves were reasonably linear and comparable in terms of the sensitivity (slope) and the repeatability. In the first calibration curve in this system, Cu 374.47 nm emission line was employed, and the correlation coefficient was obtained as R2 = 0.9.

30 000

25 000

20 000

15 000

10 000

y=14 254x+9184

R2=0.956

y=12 114x+4259

R2=0.987

5000

0

Intensity

USB4000

Maya2000Pro

0.2

0 0.4 0.6 0.8 1.0 1.2

Pb concentration (g)

Fig. 5 Calibration curve for the concentration of Pb in the KBr matrix by using USB4000 at 406.21 nm and Maya 2000Pro at 373.99 nm.

However, in the second calibration curve, the Cu at 236.81nm had the correlation coefficient of R2 =

0.983 as shown in Fig. 6.

25 000

20 000

15 000

10 000

LOD (ppm) USB4000 Maya2000Pro PbI 406.21 7293 116

373.99 3040 67 CuI 374.47 2981 63

236.81 2166 37

y=17 171x+1294

R2=0.983

y=3591x+1179

R2=0.900

Intensity

USB4000

Maya2000Pr

o

5000

0

It should be highlighted that the sensitivity and the limit of detection of a system depend on the type of the spectrograph as well as its detector. The capability of a spectroscopic detection system is deduced from the combination of the components making up the system, specifically dispersing element (i.e., grating or prism) and the detector. In this case, the dispersing element was grating with 600 grooves/mm in both spectrometers. Also, the entrance slit had the same dimensions i.e., 5 m

0 0.4 0.6 0.8 1.0 1.2

Pb concentration (g)

0.2

Fig. 6 Calibration curve for the concentration of Cu in the KBr matrix by using USB4000 at 374.47 nm and Maya 2000Pro at 236.81 nm.

The limit of detection (LOD) calculation is based on the 3 IUPAC (International Union of Pure and Applied Chemistry) definition [12, 13]:

3

LOD S

mm. Therefore, there was a higher possibility that the differences resulted in our spectra were the consequence of different detector types used by these spectrometers. A detector may even respond differently in different spectral regions due to its variable quantum efficiency. In addition, the pixel size of the detector does matter in resolving power

= (1)

where is the standard deviation of the background, and S is the slope of the calibration curve.

The limit of detection of each element in the

KBr matrix calculated by using (1), the standard deviation (SD), and LOD for both spectrometers are listed in Table 2. The LOD of the two elements Pband Cu in the KBr matrix by using th USB4000 spectrometer is not as good as their LOD values when using the Maya spectrometer. Poor LOD i.e.116 ppm was obtained with PbI 406.21 nm line recoded by the USB4000 spectrometer while thebest LOD i.e., 67 ppm was obtained with PbI 373.99 nm line recorded by the Maya Pro spectrometer. Copper lines produced by USB4000have the LOD of 63 ppm with emission line CuI 374.47 nm which from Maya was about 37 ppm atthe wavelength of 236.81 nm. Thus, the LOD of the studied elements in the same matrix mainly depended on the type of the deployed spectrometer.

Table 2 Limit of detection for various elements in the same KBr matrix obtained by two spectrometers.

Element Wavelength

(nm)

Standard deviation

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of the spectrum. Better resolution is obtained with smaller pixels. It is clear from Figs. 2 4 that the detector with bigger pixels (8 m 200 m) i.e., Toshiba TCD1304AP detector (in USB4000) generates low resolution spectra as compared to Hamamatsu S9840 detector (in Maya pro) that has significantly smaller pixels (i.e., 14 m 14 m). Therefore, the LIBS detection system that can produce rich and well resolved spectra, is said to be a good detection system and can provide a good amount of information. Such a system would be more appropriate for the application. Other parameters to be considered are the spectral range and resolution power, which present some limits related to the well of the CCD detector of the spectrometer.

4. Conclusions

A comparative study has been carried out between two commercial spectrometers using the LIBS technique. Spectral lines of Pb and Cu in the KBr matrix were analyzed. The two spectrometers were running under the same experimental conditions and observed within the same range of 200 nm 700 nm. Some differences in the production of spectral lines attributed via Pb and Cu were realized. The Maya system has shown better performance since it has lower LOD and higher repeatability R2 in the calibration curves for both heavy metals Pb and Cu as compared to the USB4000 system. In the future studies, the delay time parameter in the LIBS setup will be considered for achieving a suitable strategy in identification of heavy elements.

Acknowledgment

The authors would like to express their thanks to government of Malaysia through MOHE, UTM, GUP grant vote No. 00G79 for the financial support in this project. Thanks also due to UTM through RMC for financial management and monitoring the progress of the project.

Open Access This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.

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