Determination of Cl/C and Br/C ratios in pure organic solids using laser-induced plasma spectroscopy in near vacuum ultraviolet{

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1 Determination of Cl/C and Br/C ratios in pure organic solids using laser-induced plasma spectroscopy in near vacuum ultraviolet{ Saara Kaski,* Heikki Häkkänen and Jouko Korppi-Tommola Department of Chemistry, University of Jyväskylä, P.O. BOX 35, FIN-40014, Finland. Received 27th November 2003, Accepted 3rd March 2004 First published as an Advance Article on the web 18th March 2004 Several solid organic compounds containing bromine and chlorine were analyzed with laser-induced plasma spectroscopy. Emission lines were detected in the near vacuum ultraviolet spectral region by using a gas-purged spectrograph and an intensified charge-coupled device detector. The performance of this setup in the determination of the halides in the organic samples was evaluated. Carbon emission lines in the near vacuum ultraviolet were used as internal standards for the measurement of chlorine and bromine. Linear correlation was found between the carbon and halogen emission signal ratio and the corresponding atomic ratio of the compound. DOI: /b315410f Introduction Laser-induced plasma spectroscopy (LIPS), which is also known as laser-induced breakdown spectroscopy (LIBS), is a method to analyze rapidly materials in solid, gaseous or liquid form. The basic principle of LIPS is similar to other forms of plasma optical emission spectroscopy, i.e., analysis is based on spectroscopic information from light emitted by sample atoms and ions in the plasma. The emission spectrum is characteristic of each emitting species and allows identification of the elements in the sample. However, in the LIPS method the plasma is formed by focusing a high energy laser pulse at the surface of the sample material and sample preparation is not normally needed. Another feature is that the emission is analyzed directly from this laser-induced plasma at the sample. This simplifies the experimental setup significantly when compared to, for example, inductively coupled plasma optical emission spectroscopy (ICP- OES), where the plasma generation is continuous and the sample is transported in to the plasma. On the other hand, direct analysis from the laser-induced plasma creates more demands, for example, on repeatability and reproducibility of the measurements. The inaccuracy in the measurements can be caused by shot to shot laser fluctuations or by the matrix, i.e., the mechanical, chemical and physical properties of the sample. The reliability of LIPS measurements may be improved by collecting several laser pulses or using an internal standardization against the major element in the sample. Also characteristic of LIPS is the time-gated detection. At the beginning of the plasma generation the emission from plasma is nearly a featureless continuum and at later times the emission lines can be distinguished from the background. In quantitative LIPS analysis concentrations are determined by using for calibration a set of samples that have a similar matrix to the analyzed sample. This may turn out to be difficult to accomplish, especially for inhomogeneous solid samples. Nevertheless, the analytical potential of LIPS has been shown with a variety of different materials and is summarized in several review articles and books. 1 4 The LIPS method is seen to be especially promising in applications to on-line analysis and it has been used in the analysis of molten steel, 5,6 iron ore slurry, 7 recycled thermoplasts, 8,9 furnace exhaust fumes, 10,11 etc. { Presented at the Second Euro-Mediterranean Symposium on Laser Induced Breakdown Spectroscopy, Hersonissos, Crete, Greece, September 30th October 3rd, Most often the LIPS measurements are carried out in the visible and ultraviolet regions, where the majority of the elements have strong emission lines for analysis. Important elements like sulfur, phosphorus, halogens, carbon, nitrogen, and arsenic, have their strongest lines in the near vacuum ultraviolet (VUV) spectral region ( nm). However, analysis of these elements in the near VUV by LIPS method has so far gained only little attention. One reason is that the detection of atomic emission in the near VUV spectral region becomes difficult with ordinary LIPS equipment because of atmospheric absorption below 180 nm. Atmospheric absorption can be removed with vacuum or chambers filled with suitable buffer gases. Also, the transmittance of the optical materials and limited sensitivity of the detectors in the VUV region put constraints on the use of the spectral region. Only a few LIPS results have been reported in the near VUV: the analysis of steel samples, 6,12 16 rubber mixtures 17 and sulfuric acid aerosols 18 have been published. Although detection with a charge coupled device (CCD) allows low readout noise, a wide spectral range and a large dynamic range, of these studies only Khater et al. 15,16 have applied such a detector. They had previously compared the intensified photodiode array with the CCD detector and reported that CCD offers better spectral resolution and intensity in the VUV region than a photodiode array. 19 Also in ICP-OES analysis the potential of CCD detection in the near VUV region has only recently been investigated Intensified charge coupled device (ICCD) detectors offer, for example, better signal to noise ratio and temporal detection when compared with traditional CCD. 23 At present the spectral range can also be extended to VUV with ICCD detectors. We have recently presented an alternative setup for laser-induced plasma measurements in the near VUV using ICCD detection. An ICCD with quartz input optics and a special gas purge arrangement allowed measurements down to 165 nm, although the spectral range was specified only to 180 nm. By using an ICCD equipped with magnesium fluoride input optics spectral lines down to 130 nm were observed. 24 Our gas-purged setup with ICCD detection has been applied to identify and classify sulfide minerals at a wavelength range from 170 to 210 nm. 25 The aim of this study was to determine the analytical limits of our gas-purged VUV setup in near VUV detection. Suitable emission lines of chlorine and bromine were analyzed from pure organic solids. Near infrared and visible spectral regions are traditionally used in the analysis of chlorine and bromine J. Anal. At. Spectrom., 2004, 19, This journal is ß The Royal Society of Chemistry 2004

2 The emission line intensities, however, may be relatively low in those regions, because of the high energies needed to make the transitions to the excited states corresponding to these lines. Therefore, near VUV offers a more suitable spectral range for detection of chlorine and bromine elements. In this work the approach of Tran et al. 30 was chosen, i.e., limits of detection were studied in terms of halogen and carbon atomic ratio in the compound, but now in near VUV. Organic chlorine and bromine compounds were also chosen as samples, because they tend to be persistent and accumulative in biological tissues and their analysis is of great interest for example in environmental sciences. Such compounds are used in industry in the production of polymers, synthetic solvents, pesticides, pharmaceuticals, dyes, etc. The use of the halogenated organic compounds is not likely to decrease in the near future, therefore it is important to develop methods for their rapid analysis. Table 1 (a) Chlorine Compound Solid organic compounds used in this study Cl/C ratio 1-Chloro-9,10-anthraquinone 1/14 C 14 H 7 ClO 2 M w ~ g mol 21 4-Chlorobenzoic acid 1/7 C 7 H 5 ClO 2 M w ~ g mol 21 Structure Experimental The solid organic compounds listed in Table 1 were studied. Compounds were obtained from several chemical manufacturers (Aldrich, Kodak, Fluka, and Acros) and they were 96 99% pure. Sample powders were pressed into pellets. The experimental setup is presented in Fig. 1. Atmospheric absorption inside the 150 mm Czerny Turner spectrograph (Acton Research Corporation, SP-150) was eliminated with nitrogen gas flow. In our previous setup 24 samples were kept in ambient air near to the slit of the spectrograph and the gas flow through the slit was sufficient to displace air between the plasma and the slit. In the present experiments the sample was placed in a small sample chamber (50 mm 6 50 mm 6 50 mm) made of steel, equipped with two windows. A quartz window was used for laser input and a magnesium fluoride window for the detection of emission. The chamber was placed near the entrance slit of the spectrograph and nitrogen flow through the spectrograph cleared the optical path between the chamber and the spectrograph. The first measurements were carried out by using only a nitrogen purge through the spectrograph, i.e., without the sample chamber. The reproducibility between those measurements was significantly poorer than in the chamber experiments, because nitrogen flow at the sample tends to be unstable during the measurements. Also, especially in the samples with high carbon content, the behavior of halogen/carbon signal ratio versus the corresponding atomic ratio was scattered. This was probably caused by the formation of the molecular species with nitrogen in the plasma. Therefore, use of a sample chamber turned out to be necessary for quantitative work. The sample chamber technique allows the use of various buffer gases and reduced pressure The sample pellet was placed in the chamber filled with argon at a pressure of about 50 torr. The laser pulses were focused on the surface of the pellet with a 40 mm focal length lens made of fused silica. Each measurement consisted of five sampling points for each sample with an accumulation of 10 laser shots at each location. This allowed estimation of the standard deviation of the measured intensities. A motorized translation stage (Standa) was used to move a distance of 300 mm between the sampling locations. The number of laser shots per location was kept moderate because of the small height of the plasma in side-view detection at the energies used. The pulse energy of the KrF excimer laser (Lambda Physik, Optex) at the sample was, on average, about 5 mj and the pulse duration was 12 ns. The power density at the sample was y2 GWcm 22 in these measurements. The optimal delay in these samples was found to be 100 ns and the gate time was 500 ns. Plasma lifetimes are significantly shorter when created with the low energy excimer lasers used in this study than those observed with a Nd:YAG laser generated 2,3-Dichloroquinoxaline 1/4 C 8 H 4 Cl 2 N 2 M w ~ g mol 21 2,6-Dichloro-1,4-benzoquinone 1/3 C 6 H 2 Cl 2 O 2 M w ~ g mol 21 (b) Bromine Compound Tetradecyltrimethylammonium bromide C 17 H 38 NBr M w ~ g mol 21 Br/C ratio 1/17 1-Bromoadamantane 1/10 C 10 H 15 Br M w ~ g mol 21 4-Bromobenzoic acid 1/7 C 7 H 5 BrO 2 M w ~ g mol 21 1,3-Bis(bromomethyl)benzene 1/4 C 8 H 8 Br 2 M w ~ g mol 21 Structure plasma. In addition to the lesser influence of plasma shielding with UV laser wavelengths, this allowed the use of higher repetition rates. An ICCD detector (Andor DH520 with pixel imaging area) equipped with magnesium fluoride optics was used to detect emission from the plasma. In order to minimize the background of the detector the readout time per pixel was chosen to be 16 ms, which limited the repetition rate to 15 Hz in these measurements. No collection optics were used. MgF 2 lenses could have been used to enhance the collection efficiency, but then the purge system at the optical path between the sample chamber and the slit of the spectrograph should have been redesigned. The absence of the collecting lens was compensated by minimizing the distance between the slit of the spectrograph and the plasma J. Anal. At. Spectrom., 2004, 19,

3 Fig. 1 Schematic diagram of the experimental LIPS setup for near VUV measurements. inside the sample chamber (v2 cm). The spectrograph had a holographic grating with 2400 grooves mm 21 and the resolution of the spectrograph was about 0.5 nm in the measurement region. Laser pulse energies were measured with an energy meter (Ophir Optronics Ltd.). Results and discussion In the analysis of solid, halogenated compounds the best results have been obtained in a helium atmosphere, because emission lines of argon in the near IR have caused interference to the analyzed lines and have increased the background. 30,32 This is not the case in the near vacuum ultraviolet region and therefore argon was used as a buffer gas. A typical bromine emission spectrum in near VUV measured in argon is shown in Fig. 2. The most intensive bromine line at 154 nm was used for quantitative analysis. The carbon emission line at y156 nm, consisting of five unresolved lines, was used as an internal standard to correct for, for example, matrix effects and laser pulse energy fluctuations. This approach was adapted from the previous studies of elements in carbon-rich materials. 30,32,37 The internal standardization to carbon emission line significantly increased correlation to the atomic ratios, especially in chlorine samples. The relative standard deviation of the laser pulse energy measured from 500 pulses was about 2.5%. The chemical matrix could also cause intensity variations from sample to sample by differences in absorption of the excitation pulse resulting from the increased ablation rate and other factors. The emission lines of chlorine are shown in Fig. 3. Analysis of the bromine spectra revealed that the chlorine line at nm contains a contribution from singly ionized carbon emission under the measurement conditions. Although this is the strongest chlorine emission line in the region, it could not be used for quantitative analysis. The chlorine line at nm was used instead. The nearest carbon emission line at y143 nm (triplet) was selected to be used as an internal standard for chlorinated samples. For calibration the background corrected and integrated intensities of halogen lines were plotted against the halogen and carbon atomic ratio of the compound (Fig. 4). The linear correlation of the halogen and carbon line intensity ratios with the corresponding atomic ratios is reasonably good for both elements. The correlation for bromine is much better than that for chlorine. Neither of the calibration curves goes through the origin. Two reasons may exist for this. Firstly, our KrF excitation at 248 nm could increase the signal level, in particular in samples with high carbon content through further excitation of the carbon emission line at nm. Secondly, overlapping carbon emission lines interfere more with the Fig. 2 Spectrum of 4-bromobenzoic acid showing the most intensive emission lines of bromine in the near vacuum ultraviolet region. Fig. 3 Spectrum of 1-chloro-9,10-anthraquinone showing the most intensive emission lines of chlorine in the near vacuum ultraviolet region. 476 J. Anal. At. Spectrom., 2004, 19,

4 chlorine could be improved by using a spectrograph having a grating designed for VUV. Such purged spectrographs are already commercially available, which in our opinion is a sign of the growing interest in VUV detection without vacuum systems. Nevertheless, this work demonstrates that near VUV analysis can be carried out also with ordinary equipment. Acknowledgements Authors are grateful to Lab. Tech. Pia Mänttäri for sample preparations. Professor Henrik Kunttu is acknowledged for lending the ICCD detector with MgF 2 optics and MSc. Jussi Ahokas for the technical assistance with the detector. S.K. would like to thank the Magnus Ehrnrooth foundation for the travel grant, which enabled her to present this work at the EMSLIBS-II conference in Crete (October 2003). Fig. 4 (a) Bromine (154 nm) and carbon (156 nm) line intensity ratio plotted as a function of the Br/C atomic ratio in the sample. (b) Chlorine (135 nm) and carbon (143 nm) line intensity ratio plotted as a function of Cl/C atomic ratio in the sample. Limit of detection (LOD) refers to halogen/carbon atomic ratio. The standard deviations (error bars) of the intensities from sampling point to sampling point are shown. analysis of chlorine samples (cf. Fig. 4 (a) and (b)) and this might cause the slope of the calibration line to be clearly steeper for bromine than for chlorine. The limit of detection (LOD) for the bromine/carbon ratio was nearly five times better than that for chlorine. This is likely due to low transmission efficiency of the spectrograph in the emission region of chlorine, as seen in the spectra (compare the intensities of the carbon emission line at 143 nm to the line intensities of bromine and chlorine, respectively, in Fig. 4 (a) and (b)). The standard deviations of the chlorine line intensities are higher than the corresponding values for the bromine lines, which seems to be result of the significantly lower signal to noise ratio of the chlorine spectrum. The LOD values indicate that one bromine atom can be reliably determined in an organic compound containing 90 carbon atoms: for chlorine compounds the same number is 20. It can be pointed out that many of the environmentally hazardous halogenated organic compounds contain several halogen atoms. According to the results obtained, LIPS method could be used to detect halogenated organic compounds. However, if the sample contains a mixture of organic compounds, they can not be identified with the halogen/carbon ratio only. It is obvious that with more sophisticated VUV techniques than used here the analytical potential of halogen detection may be considerably improved. In particular, analysis of References 1 S. J. Hill, S. Chenery, J. B. Dawson, E. H. Evans, A. Fisher, W. 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