ANALYST, MARCH 1986, VOL. 111 265 Determination of Bromide Using A Helium Microwave Induced Plasma with Bromine Generation and Electrothermal Vaporisation for Sample Introduction Mohamed M. Abdillahi and Richard D. Snook* Department of Chemistry, Imperial College, London SW7 2AY, UK A helium microwave induced plasma is utilised for the determination of bromide. The bromide sample solution is either introduced into an oxidation - generation mixture of potassium dichromate - sulphuric acid, or is vaporised electrothermally and then swept into the helium plasma. The microwave induced plasma (MIP) in a TMoto cavity excites bromine and the emission measurements are taken at both the Br II 470.5-nm and 478.6-nm lines. The calibration graphs were linear from 5 ng to 50 pg of bromide using the chemical generation technique at both analytical lines, whereas the calibration data from the graphite rod vaporisation sample introduction technique showed a linearity from 5 ng to 10 pg of bromide.Detection limits for both techniques were 1 ng. Fluorine, chlorine and iodine as well as other common anions and cations do not significantly interfere with the measurements, below 100 times the amount determined. Keywords: Bromide determination; helium microwave induced plasma; bromine generation; electrothermal vaporisation The determination of bromine by an argon microwave induced plasma (MIP) has been recently reported.1 A continuation of the work revealed that a helium plasma sustained in a TMolo cavity at atmospheric pressure provides a more sensitive method for the determination of bromide.The helium MIP has sufficient energy to excite bromine ion emission because of its high ionisation energy of 24.59 eV.2 This is illustrated in this work, by the fact that the most intense bromine lines in the visible spectrum are the ionic lines at 470.5 and 478.6 nm. Argon plasmas, on the other hand, are unable to excite these bromine ion lines in either the MIP or inductively coupled plasma (ICP) used in our laboratory. Beenakker2 has used the TMolo cavity for both argon and helium plasmas and concluded that halogen atomic and ionic emission is more easily induced in a helium microwave plasma than in an argon microwave plasma; he reported better detection limits for the halogens in the helium plasma. Carnahan and Carus03 reported a detection limit of 8 ng of bromine by measuring the emission at the 478.6-nm ion line in a helium MIP by analysing high relative molecular mass halogenated organic compounds.Van Dalen et a1.4 stated that the detection limits for bromine are similar in low and atmospheric pressure microwave induced plasmas and repor- ted detection limits of 0.24 and 0.56 yg ml-l using the 470.5- nm Br I1 line. This paper presents a method with better detection limits for determining bromine. Bromine is generated from a potassium dichromate - sulphuric acid mixture , after micro- litre aliquots of sample solution are added to the generation apparatus. After sufficient time has elapsed to generate bromine, it is then introduced into the plasma. Alternatively, bromide aliquots can be desolvated and vaporised into the microwave induced plasma using an electrothermal vaporisa- tion device.In both sample introduction techniques, the emission intensities of the Br I1 470.5- and 478.6-nm lines were measured. Experimental Reagents and Instrumentation The reagents used for the stock solution and the oxidation - generation mixture were of AnalaR grade (BDH Chemicals Ltd., Poole, Dorset). The oxidation mixture was prepared by dissolving 0.05 g of potassium dichromate in 10 ml of concentrated sulphuric acid. Doubly distilled , de-ionised water (Milli-Q water) was used throughout the experiments for preparing both the stock and working solutions. The helium plasma gas was of high-purity research grade (BOC Ltd., Wembley). The instrumental set-up is shown in Fig.1 and is similar to that used previously' except that the microwave cavity (EMS, Wantage, Berkshire) is a TMolo quarter wave modification of the Beenakker cavity. The helium plasma is supported in a quartz tube (6.4 mm 0.d.; 1.7 mm i.d.) in the microwave cavity, which is viewed axially by the monochromator (Optica Model CF 2768). Sample introduction into the helium MIP was achieved by using the chemical generation cell as described previously,' or a graphite rod electrothermal vaporisation unit .5 Procedure A 10-pl bromide sample solution was added to the generating mixture through the suba-seal septum.' The bromine gener- ated, during 1 min, was swept into the helium plasma and the emission measurements were taken at the 470.5- and 478.6-nm lines.Alternatively, the emission intensity measurements were taken at those wavelengths, after a 10-pl bromide solution was desolvated, and vaporised electrothermally and then swept directly into the microwave induced plasma. Signal regis- tration was achieved using an EM1 9601B nine-stage photo- multiplier tube, the output of which was connected across a 22 kohm load to the input of a JJ Lloyd CR450 chart recorder. TMOIO cavity Monochromator Sample introduction Lens recorder Reflected power meter Flow meter Power supply Microwave power generator Helium d * Present address: Chelsea Instruments Ltd., 5 Epirus Road, London SW6 7UR. Fig. 1. Schematic diagram of the MIP system. A, Signal; B, background266 . _ rn e .- c 60- 50- g 4C- >. w .- ANALYST, MARCH 1986, VOL.111 : 1.0 Results and Discussion c .- C .: 80 .- cn E Lu 40 Wavelength Selection It was found that the Br I1 470.5-nm line is twice as intense as the next brightest Br I1 478.6-nm line, in agreement with other workers.6 - Background - A A . a A A r ' - - Optimisation of the Operating Parameters The microwave forward power, helium flow-rates, entrance and exit slit widths, photomultiplier voltage, applied voltage to the graphite rod atomiser and the generating solution were optimised using the univariate search method. Using the TMolo cavity, the effect of microwave forward power on the emission of Br I1 lines showed an optimum value of 130 W (Fig. 2). In a previous paper we showed that the argon MIP used for determining bromine1 required an optimum forward power of 30 W.This fundamental power difference is inherent in the design of the different cavities employed7 and the nature of the plasma gas used. The physical properties of argon and helium are given in reference 8 (argon: thermal conductivity at 25 "C = 39 x cal s-1 cm-l O C - 1 , specific heat at 25 "C = 0.13 cal 8-1; helium: thermal conductivity at 25 "C = 340 x cal s-l cm-1 OC-1, specific heat at 25 "C = 1.25 cal g-1). The higher thermal conductivity and specific heat mean that helium dissipates more energy and therefore requires higher power to sustain than an argon plasma as was found in the 70 Y At?--.----- 60 70 80 90 100 I I I I I 110 120 130 140 150 Microwave forward powerNV Fig. 2. Effect of microwave forward power on the emission of bromine in a helium MIP.A, Signal; and B, background 1 I I I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Helium flow-rate/l min-1 Fig. 3. Effect of helium flow-rates on the emission of bromine in a TMolo cavity MIP. A, Signal; and B, background TMolo cavity. Varying the helium flow-rates independently of other variables yielded a maximum signal-to-background ratio at 0.43 1 min-1 (Fig. 3). When the helium plasma gas flow-rate is higher than 0.5 1 min-1, the signal to background ratios were gradually reduced and may be attributed to the decrease in residence time of bromine in the plasma, as the intensity of the signal lowers. It was also observed that at lower helium flow-rates, the analyte was not sufficiently swept into the plasma and the peaks tended to be smaller and broader.We have reported1 that the optimum slit widths for the determination of bromine in an argon plasma using a 314 wave Broida cavity was 100 pm. The wide slit widths were used to isolate the molecular (Br2) band emission, which is much wider than the ionic Br I1 lines. Optimising the slit widths, an optimum of 10 pm was found for both the entrance and exit slit widths using the Br I1 470.5- and 478.6-nm lines. Altering the photomultiplier voltage showed that the best signal-to- background and the highest signal-to-noise ratios were at 1000 V. This differs from the voltage employed in reference 1, because the efficiency of the PMT is worse in the 470-478 nm region than at 291 nm. Optimisation of the generation mixture volume and concentration remain the same as used previously.1 The bromide sample is being oxidised to bromine and then introduced into the plasma where it is subsequently atomised and excited.The voltage applied to the graphite rod was varied to determine the optimum peak height for a 10-yl bromide sample solution. Applied voltages of 1 V (60 s), 3 V (5 s) and 8 V (3 s) were found to be the optimum for evaporation, ashing and vaporisation, respectively, of the bromide sample solution before introduction into the helium microwave induced plasma. These are equivalent to temperatures of 105,300 and 1900 "C, respectively. Table 1 summarises the optimum conditions. Table 1. Optimum conditions for the determination of bromide in a helium MIP Microwave foward power . . . . . . . . Helium flow-rate (generation) .. . . . . Refelected power . . . . . . . . . . Helium flow-rate (ETV) . . . . . . . . Entrance and exit slit widths . . . . . . Photomultiplier (EHT) . . . . . . . . Sample solution volume . . . . . . . . Graphite rod temperatures: Wavelengths selected (Br 11) . . . . . . Ashing . . . . . . . . . . . . . . Drying . . . . . . . . . . . . . . Vaporisation . . . . . . . . . . . . 130 W 0.4 W 0.43 1 min-* 0.8 1 min-1 10 pm 1000 v 10 1.11 470.5 and 478.6 nm 105 "C 300 "C 1900 "C 103 rn C 3 c .- > 102 F 9 s .: 10 c .- C t t c .- 15 Fig. 4. Calibration graphs of Br I1 emission at A, 470.5 nm and B, 478.6 nmANALYST, MARCH 1986, VOL. 111 267 I I I I 102 103 104 0.1 ' 10 Amount of bromidehg Fig. 5. Response data for Br I1 emission at A, 470.5 nm and B, 478.6 nm in an atmospheric helium MIP using ETV Calibration Graphs, Detection Limits and Interferences Double logarithmic calibration graphs (Figs.4 and 5 ) were obtained with respect to bromide in the solution. Fig. 4 illustrates the calibration graphs for Br I1 emission in the helium MIP at A, 470.5 nm and B, 478.6 nm when the analyte is generated chemically as bromine from the potassium dichromate - sulphuric acid oxidation mixture. The detection limit, defined as the sample concentration that produced a signal-to-noise ratio of two, at both the emission lines was 1 ng of bromide and the log - log calibration graphs were linear in the range from 5 ng to 50 yg with a slope of 0.8. Fig. 5 shows the calibration data for the same emission wavelengths when electrothermal vaporisation was used for the sample intro- duction technique.The detection limit was 1 ng and the linear dynamic range was from 5 ng to 10 yg of bromide. Although the 470.5-nm line emission is twice as intense as the 478.6-nm line, the linearity of both lines breaks below 5 ng, hence showing no difference in the detection limits or the linearity of the calibration graphs (Figs. 4 and 5 ) . Possible interference effects of fluorine, chlorine, iodine, sulphate, nitrate, sodium, potassium, iron and mercury were studied. A 1000-fold excess of Na, K, Fe, Hg, SO4 and NO3 did not affect the 10 p.p.m. bromine emission signal. However, it seems that the other halogens have slight interference effects if their concentrations are more than 100-fold greater than the 10 p.p.m. bromide solution. Conclusion These methods present two sample introduction techniques for the helium microwave induced plasma (MIP) and provide a sensitive, rapid and simple way of determining bromide. The two most intense emission lines for bromine in a helium MIP (470.5 and 478.6 nm) have been evaluated and it was found that both lines exhibit similar behaviour, in terms of detection limits and linear dynamic ranges. 1. 2. 3. 4. 5. 6. 7. 8. References Abdillahi, M. M., Tschanen, W., and Snook, R. D., Anal. Chim. Acta, 1985, 172, 139. Bennakker, C. I. M., Spectrochim. Acta, Part B, 1982,32,173. Carnahan, J. W., and Caruso, J. A., Anal. Chim. Acta, 1982, 136,261. Van Dalen, H. P. J., Kuwee, B. G., and De Galan, L., Anal. Chim. Acta, 1982, 142, 159. Long, S. E., Snook, R. D., and Browner, R. F., Spectrochim. Acta, Part B, 1984, 40, 553. Tanabe, K., Haraguchi, H. , and Fuwa, K., Spectrochim. Acta, Part B, 1981, 36, 119. Mulligan, K. J., Hahn, M. H., and Caruso, J. A., Anal. Chem., 1979, 51, 1935. Robin, J. P., Prog. Anal. At. Spectrosc., 1982, 5 , 79. Paper A5J19.5 Received April 31st, 1985 Accepted September 2nd, 1985