JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991 VOL. 6 21 1 Direct Solid Sampling in Capacitively Coupled Microwave Plasma Atomic Emission Spectrometry Abdalla H. Ali Kin C. Ng* and James D. Winefordnert Department of Chemistry University of Florida Gainesville FL 3261 1 USA A capacitively coupled microwave plasma operating in the range between 500 and 700 W is used as an excitation source for the analysis of solid samples. National Institute of Standards and Technology (NIST) Standard Refe- rence Materials (SRMs) Tomato Leaves (SRM 1573a) and Coal Fly Ash (SRM 1633a) are used for the evaluation of the technique. The system contains a graphite electrod-up in which the solid sample is deposited. Heating of the electrode-cup vaporizes the analyte into the plasma for atomic emission spectrometry.The detection limits (defined as 30 of the background) for Mn Ca Mg Zn Cu As Rb and Pb in Coal Fly Ash and Cd Fe Cu Zn Zn Sr Rb Mg and Pb in Tomato Leaves were determined. The plasma gas used in this study was 20% nitrogen and 80% helium. Keywords Capacitively coupled micro wave plasma; atomic emission spectrometry; analysis of solids; Tomato Leaves; Coal Fly Ash In atomic spectrometry direct analysis of solids is important for several reasons. Dissolution of solids requires the use of hazardous chemicals which can be very time consuming and common methods of liquid sample introduction particularly by pneumatic nebulization are known to be inefficient (~10% sample throughput). Furthermore contamination and losses may occur in the process of dissolution.Deterioration of sensi- tivity and detection power occurs owing to the dilution and the degradation of the plasma as the atomization and excitation source by the solvent that is introduced (with the analyte) into the plasma. The resultant solution after dissolution can contain a high salt content with a potential for clogging the nebulizer. These problems are not encountered in a direct solid sample introduction approach. Also it is beneficial to have a technique that rapidly quantifies the elemental content of a sample before it is subjected to a time consuming dissolution procedure sometimes requiring the use of expensive and/or hazardous chemicals for more precise analysis. Currently arc and spark emission techniques work well for this preliminary quantification. However in the spark technique the sample must be electrically conducting or if it is non-conducting the sample must be mixed with a conducting powder.Several methods have been developed for solid sample in- troduction into inductively coupled plasmas (ICPs) which have been reviewed recently.'-* They include laser ablation electrothermal vaporization arcs sparks direct solid sample insertion powder injection and sluny nebulization. Microwave-induced plasmas which are usually operated at low powers ( ~ 2 0 0 W) have been mainly successful for gaseous samples particularly gas chromatographic eluate^.^.^ Capacitively coupled microwave plasmas (CMPs) which can provide high power levels have the capability of efficiently vaporizing liquid aerosols and atomizing and exciting analyte atoms.Hence most previous research has been focused on the analysis of liquid ~ampIes.~-~ For both types of microwave plasma very few studies have involved solid sample introduc- tion methods.x-'3 In most instances electrothermal vaporiza- tion devices were used to facilitate the introduction of solid samples into the but no work has been reported on direct solid sampling methods without analyte vapour trans- port for microwave plasmas. In this paper a new method for the rapid screening of solid samples is reported. This technique exploits the requirement for an electrode for plasma generation in the CMP and the sub- * On leave from the Department of Chemistry California State Univer- t To whom correspondence should be addressed.sity at Fresno Fresno CA 93740-0070 USA. sequent heating of the electrode this heating effect being used to advantage in the sampling procedure. A graphite electrode with a cup end was constructed into which solid powder was placed. Heating the electrode effected vaporization of the sample into the plasma enabling emission measurements to be made. The rapid heating of the electrode caused rapid vapori- zation of the sample thus producing a high transient concen- tration of the analyte. Coal Fly Ash [National Institute of Standards and Technology (NIST) Standard Reference Ma- terial (SRM) 1633al and Tomato Leaves (NIST SRM 1573a) were the materials chosen for evaluation of the technique. Experimental Instruments The experimental set-up is shown in Fig. 1 and the compo- nents are listed in Table 1.The electrode+xp system is shown in Fig. 2. The torch employed is similar to a conventional ICP torch except that the central tube is larger in diameter in order to accommodate the graphite electrode. The cavity for the gen- eration of the microwave plasma has been described else- where.6.10.1 I Sample Preparation For the determination of most elements the sample was used without further treatment. In Coal Fly Ash the concentrations of Mg Ca and Zn were so high that they were outside the linear response range of the procedure therefore dilution of array Electrodecup Diode array \ Fig. 1 atomic emission spectrometry Experimental set-up for capacitvely coupled microwave plasma212 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY APRIL 1991.VOL. 6 .o Graphite Table 1 Instrumentation for capacitively coupled microwave plasma atomic emission spectrometry 450 Instrumentlcomponents Manufacturer -Graphite electrode Diode array OSMA Model 1R4- 1024 Princeton Instruments Princeton NJ USA Spectrometer Princeton Instruments Software (I- 120) Princeton Instruments Jobin-Yvon HR1000 1 m 2400 grooves mm-I linear dispersion 0.5 nm mm-’ OSMA detector controller PC computer High voltage d.c. power supply Model 805- I A (maximum power output 5 kW) Princeton Instruments IBM Hipotronics Brewster NY. USA Magnetron Model 2M131 (frequency 2.45 GHz maximum power output 1.6 kW) Hitachi Des Plaines IL. USA Electrode-cup system Torch (Spex graphite rod; grade HPND) Three concentric quartz tubes Laboratory made Laboratory made Fig.2 mm Graphite electrode<up for the solid sample insertion. Units are in the sample was necessary in order to establish the detection limits. For this purpose the Coal Fly Ash was ground further using a procedure described previ~usly’~ and eventually passed through a 200-mesh nylon sieve. A 100 mg portion of this fine particle coal was diluted in spectroscopic grade graph- ite powder (Union Carbide New York NY USA) so as to obtain a 1% m/m Coal Fly Ash mixture. Even though grinding may increase the chances of contamination it is known to improve the homogeneity and hence the precision of the ana- lytical signals.I5 Procedure Tomato Leaves (5-10 mg) and the original or the 1 % Coal Fly Ash were deposited in the cup. The cup was placed on the top of the electrode and inserted into the central tube of the torch which held the electrode in position.There was a tight fit between the cup and the electrode in order to ensure good thermal and electrical contact. The mixed-gas (intermediate) plasma flow-rates (1 1 min-’ for N and 4 1 min-’ for He) were then adjusted. The outer He gas flow-rate was 6 1 min-I. There was no injection gas. The plasma was initially ignited at a low power (about 100 W) in order to ash the sample. When the ashing step was omitted the sample popped out of the cup causing flares in the plasma during the atomization step leading to plasma instability. Ashing in situ was first used in a direct sample insertion (DSI) ICP by AbduIlah et a[.’(‘ After about 15 s the power was raised to a pre-selected value of 400 W for Tomato Leaves and 700 W for Coal Fly Ash.Lower powers were used for liquid samples which were em- ployed solely for the identification of analyte emission lines. The observation height was 2mm above the cup and the emitted radiation was focused on the entrance slit of the spec- trometer by a system consisting of two matched quartz lenses (focal length = 4 in). The emission was monitored with a pho- todiode array. For better resolution second to fifth orders were used with a single scan (33 ms) spectrum in order to determine the detection limits. Results and Discussion Before the solid samples were introduced the graphite cup the electrode and the graphite powder were checked for impurities of the elements of interest by monitoring their emission lines.No measurable signals were observed for any of the elements. Moreover blanks were run before every measurement to ensure the absence of memories from previous runs and atmo- spheric contaminants. The limits of detection (LODs) based on 30 of some elements in Tomato Leaves and Coal Fly Ash are listed in Table 2. The background was measured at 0.1 nm off-peak. For the determination of Mg Zn and Ca in Coal Fly Ash a blank (consisting of graphite powder) was used. The low LODs obtained for the elements arose largely from an in- creased vaporization rate of analyte when the sample was diluted with graphite powder. Dilution of the sample reduced the matrix background and improved vaporization and atomi- zation efficiencies resulting in higher signal to background ratios.Similar increases in sensitivity were observed by Brenner et al.” who used an ICP for the determination of Cu and Zn in silicate materials when diluted in graphite. The addi- tion of graphite resulted in a higher rate of analyte evolution a stronger reducing environment and complete consumption of the solid sample.I8 A positive influence on the vaporization has also been achieved by adding organic halides to the plasma gas and/or chemical modifiers.19.”’ The precision of the meas- urement of the analytical signals was between 12 and 18% which considering the small amounts of samples being ana- lysed is acceptable. The presence of the majority of the elements were confirmed at multiple wavelengths and the lines were identified by using hollow cathode lamps (HCLs).Solution nebulization and aqueous solution deposition in the cup were also used when HCLs were not available or when the intensity was too low for definitive identification. When using an HCL nearby neon emission lines made identification of the elementsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. APRIL 1991 VOL. 6 213 Table 2 LODs (based on 30) of some elements in Coal Fly Ash (NIST SRM 1633a) and Tomato Leaves (NIST SRM 1573a). Element Vnm LOD/ng Tomato Lealyes- Cd cu Fe Mn Pb Rb Sr Zn Ca Mg Mn Rb Zn cu As Pb Mg Coal Fl-v Ash- 228.8 324.8 258.6 403.1 285.2 283.3 780.0 460.7 2 13.9 422.7 279.5 403. I 780.0 2 13.9 324.8 193.7 283.3 0.3 8 134 69 1 2 4 14 30 5 3 46 80 0.1 22 51 I33 of interest difficult and the method of deposition of liquid solu- tions required a cleaning step as there were analyte residuals present. Therefore solution nebulization was most frequently used for line identification. For the elements investigated a higher power was needed for Coal Fly Ash than for Tomato Leaves in order to observe sufficient signal to background ratios indicating that the former is more resistant to thermal decomposition than the latter.(The cup temperature increased with the power.) The duration of the emission signals depend- ed on the power the analyte and the matrix. Manganese which is present in similar concentrations in the two samples gave an emission signal in Coal Fly Ash which lasted twice as long as that in Tomato Leaves for the same amount of sample under the same operating conditions.This may have been a result of the chemical form in which the analyte exists in the sample. The system described here is simpler than the DSI-ICP system in terns of cost and operation. Unlike the DSI-ICP the electrode in the CMP is held in a fixed position by the central tube of the torch and can only be changed by moving the torch. Therefore it is not prone to poor reproducibility of posi- tioning of the cup. In DSI-ICP studies of analytical signals as a function of cup position showed that a variation of 1 mm produced signal intensity changes as large as 10%.'h.21 In DSI- ICP drying is generally carried out externally by use of aux- iliary heating devices.?' In the CMP described here both drying and ashing can be performed in situ. 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