Characteristics of an Inductively Coupled Argon Plasma Operating with Organic Aerosols Part 3.* Radial Spatial Profiles of Solvent and Analyte Species D. G. WEIR? AND M. W. BLADES1 Department of Chemistry University of British Columbia Vancouver BC Canada V6T 121 The effect of organic solvent load on the radial emission profiles for Mg I Mg 11 C I C and CN was measured. The three dimensional information reveals the observation zones where the emission intensity of these species is proportional to solvent load and plasma excitation conditions. Keywords Inductively coupled argon plasma; organic solvent; solvent plasma load; radial spatial projiles This paper is the third part in a series on the physical characteristics of an inductively coupled argon plasma (ICAP) operating with organic aerosols.The first part described the experimental system for making spectral spatial and temporal measurements and examined the effect of solvent and solvent load on the background spectra and visual features of an ICAP in general.' The second documented the effect of chloro- form load on the axial emission spatial profiles for C (I) C (diatomic carbon) and CN (background species) Mg I Mg I1 (analyte species) and the ratio of Mg I1 to Mg 1.' This paper presents the effects of solvent and solvent load on the radial spatial emission profiles of Mg I Mg 11 C (I) C and CN and the emission intensity ratio of Mg I1 to Mg. Several physical phenomena are evident in the radial data notably air entrainment into the argon jet the result of vortex shedding entrainment of solvent material into the outer argon stream by a recirculation eddy at the base of the discharge and shrinking of the induction region by a thermal pinch effect.The radial spatial structure of analyte emission in the ICAP has been studied by several research group^.^-'^ In order to facilitate spatial studies many ICAP investigators have taken advantage of imaging monochromators equipped with one or two dimensional array detectors mounted vertically in the exit focal plane. This configuration allowed them to record rapidly either vertical or lateral profiles of ICAP emission for a single wavelength channel. For example Koirtyohann et ~ 1 . ' ~ recorded both laterally and axially resolved profiles of calcium emission from the ICAP and studied the effects of concomitant potassium.Blades and Horlick using this methodology were able to classify emission lines according to the behaviour of their axial profile^'^ and to study the radial spatial effects of concomitant easily ionizable elements on analyte emi~sion.~ This general approach has been used for the collection of spatial emission profiles for this paper. The purpose of this paper is to present the effects of solvent load power and nebulizer gas flow rate (or more correctly central gas flow rate) on the radial spatial distribution of emission from Mg 11 Mg I the ratio of Mg I1 to Mg I and * For Parts 1 and 2 of this series see references 1 and 2. t Present address 10228 109 Street Fort Saskatchewan Alberta 1 To whom correspondence should be addressed. Canada.Journal of Analytical Atomic Spectrometry species formed from solvent atomization uiz. C I C2 and CN. The detailed preamble has been presented in the previous publications. A subsequent publication will correlate these observations with more fundamental ICAP properties electron number densities excitation temperatures and excited state population densities. EXPERIMENTAL The instrumentation and procedure for igniting the plasma generating the solvent aerosol preparing solutions controlling the operational parameters and measuring spectroscopic quantities were presented in the previous paper^.'-^,'^*^^ Only the new procedures specific to surveying the spatially resolved parametric response of the solvent loaded ICAP are presented here. Monochromatic images of either the tail cone or induction region of the discharge were recorded with a bandpass of approximately 0.4 nm a lateral resolution of 0.09 mm and a vertical resolution of 0.6mm.Moreover images of the fore- ground background and dark signal were recorded for the purpose of background subtraction. In order to sample the emission profile adequately and to meet the requirements of the numerical Abel inversion procedure,17 up to 200 intensity samples were recorded along the lateral coordinate of the discharge at each observation height. The discharge was trans- lated across the axis of the light collection optics so that the image of the discharge was translated laterally across the entrance slit of the monochromator. The monochromator was equipped with a one-dimensional photodiode array detector mounted with its long axis mounted vertically,' such that several lateral profiles each at different observation heights could be collected simultaneously. As a result an entire monochromatic image of the discharge could be recorded in a single lateral scan.The bandpass of the monochromatic image was determined by the width of the array detector and the reciprocal linear dispersion of the monochromator. For example the width of the detector of 0.506 mm multiplied by the reciprocal linear dispersion of the monochromator at 516 nm (0.76 nm mm-') gave a spectral bandpass of 0.38 nm. This bandpass could easily be reduced by placing a mask over the detector but this was not found to be necessary. The sampling interval along the detector corresponded to 0.6 mm intervals along the axis of the discharge.The total axial range was 25 mm. This range extended either from 5 to 30 mm above the top of the load coil (in the tail cone) or from -15 to 10 mm (the induction region). In the lateral direction the sampling interval was variable but was generally set at 0.045 mm or 0.09 mm in all cases 200 lateral positions were sampled giving a lateral range of 4.5 or & 9.0 mm depending on the region being sampled. In order to generate radially resolved contour plots from Journal of Analytical Atomic Spectrometry January 1996 Vol. 11 (43-52) 43line of sight images the light collection envelope for each point in the image had to approximate to a line integral. The optical train was designed to meet this requirement as follows a planoconvex lens' was fitted with one of three apertures (5 mm diameter 10mm diameter or 5 mm widex30mm high) and was used to form a 0.5 x image of the discharge onto the entrance slit of a 1 m focal length Czerny-Turner monochroma- tor.At 516nm the object distance was 450mm (from the discharge axis to the front principal point of the lens) while the image distance was 225 mm (from the back principal point of the lens to the entrance slit). The lens maker's equation for thick lenses was used to calculate image and object distances at other wavelengths. The planar surface of the lens faced towards the entrance slit in order to minimize spherical aberration. Finally the entrance slit was lined up with the cylindrical axis of the discharge.The spatial response of the monochromator along the entrance slit was determined exper- imentally and was found to be slight over the 25 mm imaging range (k 6.25 mm along the entrance slit) and was not corrected for. Contour plots of the absolute light collection efficiency of the optical system were calculated using an exact ray tracing algorithm developed by Farnsworth et aL1* Beyond providing the light collecting efficiency of the optical train these contour plots confirmed that the light collection optics met the require- ments for Abel inversion. The spectral response function of the detector was also determined using an irradiance standard lamp. The monochromatic images were corrected for background or dark signal contributions by subtracting the background or dark signal images.Next the images were smoothed in both the axial and lateral directions with a digital Because the axis of the discharge reproducibly veered to one side with increasing height the veering was removed by shifting the lateral profile at each observation height by a predetermined number of sampling intervals (< 6 pixels). Once background corrected smoothed and straightened the images were radially inverted. After the profiles had been inverted they were condensed from 200 x 40 intensities to 50 x 40 intensities by taking the average of 4 points in the radial direction. This rendered the images much more manageable for later analysis with little or no significant loss of information. The corrected smoothed radially inverted and condensed images were then converted to topographical contour plots.Radially resolved contour plots were generated for 516.56 nm (C2) 388.34 nm (CN) 247.86nm [C (I)] 257.61 nm (Mn 11) 279.55 nm (Mg 11) 285.21 nm (Mg I) several Fe I and I1 lines Ar I (687.13 nm) and H I (486.16 nm). Some of these were measured for three solvents methanol water and chloroform over a range of inner argon flow rates and forward powers. Spatially resolved intensity contour plots can be regarded as emission from the planar slice through the discharge. Most slices are bounded by z = 5 mm to z =30 mm and r = -7 mm to r = + 7 mm; however a few are bounded by z = - 15 mm to z= + 10 mm and r = -7 mm to r = +7 mm. Obvious shadows of the induction coil reveal which contour plots extend into the load coils (that is those bounded by z= - 15 mm to z= +10mm).Intensity from the planar slice is represented by topographical isocontours as shown in Fig. 1. The isocontours always depict the normalized intensity ranging from 0.1 to 1.0 in increments of 0.1. These isocontours were either normalized to the maximum intensity for individual contour plots or the over-all maximum for a set of contour plots. RESULTS AND DISCUSSION The diagrams in Fig. 1 are a summary of the emission intensity contour plots of all the analyte and background species examined for this paper. These contour plots were measured for an ICAP operated at 1.25 kW with an inner argon flow rate of 0.81 1 min-l and a chloroform load of 4.5 mg s-'. From left to right the first contour plot depicts emission from the CN bandhead at 388.34 nm and second contour plot depicts the Cz bandhead at 516.56nm.These features are concentrated at the boundary regions of the plasma. The CN contour plot is located at the downstream boundary region where air is entrained into the plasma jet. In effect the CN contour plot indicates the interface between the hot atomic plasma and the cold room air. Within the induction coil the C2 contour plot partially outlines the upstream boundary region of the plasma aerosol channel. This region surrounds the base of the plasma and lines the inside of the aerosol channel. The C2 emission may be regarded as a dissociation front between the hot atomic plasma and the relatively cold mixture of argon and undissociated solvent vapour. The next three contour plots in Fig.1 are emission from atomic carbon and argon. The C (I) and Ar (I) emission contour plots essentially define the volume of the atomic plasma (where atomic emission predominates) and reveal how it 'nests' within the molecular boundary region (where molecu- lar emission predominates). Two views of C (I) are provided one within the induction coil and one above the torch rim. Within the induction coil the C (I) isocontour overlaps the toroidal induction region of the plasma. Above the torch rim both Ar I and C (I) contour plots reveal that the toroidal induction region coalesces into a tail cone. As mentioned earlier the isocontours of atomic emission overlap with the boundary layer emission to a greater extent downstream from the torch rim than within the induction coil.It is possible that the temperature gradient downstream is more gradual but more likely that the downstream boundary is time averaged. The next three contour plots on the right side of Fig. 1 are the intensity distribution of analyte emission. The first two plots are the plumes of emission typically displayed by hard lines (atomic ion lines or atom lines with excitation potentials > 6 eV). The intensity distributions of Mn I1 (257.6 nm) and Mg I1 (279.5 nm) are shown. The upstream base of the hard line plume begins at approximately 5-10 mm above the top of the induction coil. From this location the plume extends 15-20 mm downstream up to 25 mm above the induction coil. Typically the hard line plume varied in length width and intensity when one varied the operating parameters.In spite of this the plume retained its distinctive oval shape with no obvious tapering or constrictions upstream or downstream. The oval plume always displayed an unambiguous axial maxi- mum residing well downstream at 10 to 15 mm above the induction coil. The plume extended radially to approxi- mately 3 mm. In general this maximum radius was found at approximately 15 mm above the induction coil. In contrast the third contour plot is the structure typically displayed by soft lines (atom lines with excitation potentials < 6 eV). This plot depicts the spatial distribution of emission from the Mg I (285.2 nm) line an atom line with an excitation energy of 4.35 eV. The most conspicuous differences between this soft line structure and the hard line plumes to its left are the relative positions of their maxima and their characteristic shapes.The maximum for the soft line structure lies signifi- cantly further upstream or at a lower height above the induction coil than the hard line maxima which reside above 10mm above the induction coil. The bases of the hard line plumes also lie well downstream so that the plumes display a characteristic oval shape. In contrast the base of the soft line plume appears to stretch out into a narrow shaft which extends into the torch giving it the characteristic club shape for soft line emission. The last rightmost frame of Fig. 1 is a contour plot of the ratio of Mg I1 (279.55 nm) and Mg I (285.21 nm) line emission. In the previous paper we discussed how this 44 Journal of Analytical Atomic Spectrometry January 1996 Vol.11CN CI Mg Ii MnII MgII Mgl Mg I -8 -4 0 4 8 -0.4 0.0 4.0 -1 5 -a -4 o 4 a Radial distancehrn Fig. 1 Overview of spatially resolved maps of the analyte and background emission from the solvent loaded ICAP. See text for description ratio can be used to diagnose thermal conditions in the plasma and how the ratio is an indicator of robustness of the plasma.' Note that there are two distinctive features of CN emission the central plume and the conical outer mantle. The central plume is absent under conditions of high power and low Emission from CN-The Downstream Boundary solvent load (in the lower left hand plots) but increases in height and extends upwards to meet the outer mantle with The eighteen spatially resolved contour plots of emission from the CN bandhead (388.34 nm) in Fig.2 indicate how CN emission responds to chloroform load and forward power. incieasing load and decreasing power (in the upper right hand plots). On the other hand the outer mantle (which caps the atomic plasma or resides at the boundary between the plasma 1 .OO kW 1.25 kW 1.50 kW 3.4 mg s-l 4.2 mg s-' 6.2 rng s-l 7.4 mg s-' 8.6 mg s-l 10.0 rng s-l 30.0 25.0 20.0 15.0 10.0 Fin -80.0 -40.0 0.0 4.0 8.0-'- Radial distancehm Fig. 2 Isocontour maps of CN (388.34 nm) emission intensity for a chloroform loaded ICAP. Inner argon flow 0.81 1 min-'. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in an absolute sense by counting contours.The power is indicated on the left side of the diagram the solvent load is indicated across the top and the axial and radial scale is provided on the lower right hand map Journal of Analytical Atomic Spectrometry January 1996 Vol. 11 45and the surrounding air) appears to collapse inward and downward with decreasing power and increasing solvent load. Also note the intensity response of the CN emission from left to right or with increasing chloroform load the CN intensity of the mantle increases almost linearly with chloroform load at all locations. The response of CN intensity to power is more ambiguous (power increases from the top row plots to the bottom row plots).Although the CN intensity increases with forward power at specific locations (for example z = 10 mm and r = 6 mm) a linear increase at all locations is not evident. Indeed at some locations the CN intensity appears to decrease with forward power. In an effort to understand the response of CN intensity to chloroform loading and forward power one may first remove the spatial aspects by integrating the intensity over all space in a manner analagous to that of Pan et Alternatively one may choose a spatial location where the structure of the plasma remains fairly constant even when the forward power and chloroform load are varied. On the axis and beyond the tip of the plasma (z=30.0 mm r=0.0 mm) the intensity response appears to be largely determined by how far the atomic plasma extends downstream.Here the effects of power and solvent load are particularly evident. Similarly at a position on the axis and at an intermediate observation height (z = 15.0 mm r = 0.0 mm) the intensity is also determined larg- ely by how far the inner plume extends downstream and whether or not the plume extends past z = 15.0 mm. In contrast the structure of CN emission near z=15.0mm and r = - 5.2 mm appears to be relatively independent of power and solvent load. Because the structure here appears to be constant this is a good location to examine the response of CN intensity to forward power and solvent plasma load. At 1.5 kW the intensity increases almost linearly with solvent load indicating that excitation conditions in the CN mantle are constant and that the amount of solvent material determines the CN inten- sity.At lower powers the CN response departs from direct proportionality and displays a maximum at intermediate load- ing. Evidently high levels of solvent plasma load sap enough energy from the discharge to lower the CN emission intensity. Moreover it is likely that vortex shedding' entrains air into the argon stream effectively mixing N2 with the argon and solvent material so that the solvent carbon combines effectively with the atmospheric nitrogen and the thermal conditions in the boundary region are intermediate between the plasma and the air according to the respective gas temperatures and heat capacities. It should be noted that one can optimize the CN signal as a working diagnostic by selecting the right viewing location; z = 15.0 mm and Y = - 5.2 mm appears to be the best place to monitor solvent plasma during routine analysis.0.6 I min-' 0.7 I min-' m e ' 8 0 lw v- Emission from C,-The Upstream Boundary In contrast to CN the C emission contour plots reveal primarily the upstream boundary of the discharge. This bound- ary is characterized by solvent pyrolysis and a steady recircu- lation eddy rather than air entrainment and vortex shedding. Both processes are evident in the C emission contour plots presented in Fig. 3. This figure depicts C emission within the torch for an ICAP loaded with meta-xylene. For these contour plots the meta-xylene load was 0.2 mg s-' the forward power was set at 1.25 kW and the inner argon flow rate was varied from 0.6 to 1.1 lmin-' in 0.1 1 min-' increments.The inner argon flow rate increases from left to right. At low inner argon flow rates the C2 emission wraps around the base of the plasma while the central plume of C2 emission only extends a short distance along the axis. It is unlikely that diffusion could account for the C emission around the base of the discharge. On the contrary a recirculation eddy near the base of the discharge predicted by computer sir nu la ti on^,^^^^^ could account for convective mixing of solvent material with the outer argon stream. Evidently such a recirculation eddy entrains solvent material quite effectively into the outer argon stream at low inner argon flow rates thus reducing the load in the axial channel. However when the inner argon flow rate is increased the eddy is less effective at entraining solvent material.Interestingly computer simulations predict that at extremely high inner argon flow rates the inner argon stream may actually sweep the recirculation eddy away. Consequently the load on the axial channel increases and the central plume extends farther downstream while the outer C2 emission intensity decreases. Moreover the entire profile settles down into the torch. This indicates that the plasma translates axially when the outer argon flow is loaded with solvent material. C (I) emission contour plots provide further insight into this apparent translation. Contour plots of C emission from an ICAP loaded with other solvents at different rates of solvent plasma load are all consistent with the behaviour depicted in Fig.3. In response to methanol loading the peripheral component of C emission was more intense than the central plume and the central plume extended a shorter distance downstream. Evidently the inner argon stream laden with methanol vapour could not penetrate the recirculation eddy as effectively as an inner stream laden with heavier solvent molecules because the methanol laden stream would have had less momentum. Indeed chloroform loading displayed the opposite response. In summary two features of the flowfield are evident in the parametric response of C2 and CN emission contour plots vortex shedding associated with air entrainment and a recircu- 0.8 I min-' 0.9 I min-' 1 .O I min-' 1 .I I min-' ,-8O.O -40.0 0.0 4.0 8.0 Radial distance/mm Fig.3 Isocontour maps of C (3516.16 nm) emission intensity for a rneta-xylene loaded ICAP. The solvent plasma load was 0.2 mg s - l the forward power was set at 1.25 kW and the inner argon flow rate was varied from 0.6 to 1.1 1 min-' in 0.1 1 min-' increments. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in an absolute sense by counting contours. The axial and radial scale is provided on the right hand map 46 Journal of Analytical Atomic Spectrometry January 1996 Vol. 11lation eddy at the base of the discharge which entrains solvent material into the outer argon stream. The extent to which solvent material is entrained depends on the flow properties of the inner stream.Emission from C (I )-The Atomic Plasma Contrasting quite sharply with the CN results is the response of spatially resolved emission from atomic carbon [from the C (I) line at 247.9 nm]. The fifteen contour plots shown in Fig. 4 indicate how C (I) emission responds. Once again the top row corresponds to the lowest power of 1.00kW the middle row to the intermediate power of 1.25 kW and the bottom row to the highest power investigated of 1.50 kW. From left to right the chloroform load increases from 3.4 to 8.6 mg SKI. The spatial distribution of C (I) is characteristic of the plasma region where atomic line emission predominates over molecular emission indicating that the C (I) emission emanates from hot atomic plasma and not from the molecular flame like conditions of the boundary regions. Nevertheless it is important to note that the spatial distribution of C (I) emission overlaps with the spatial distribution of CN emission.Fig. 5 is a contour plot of C (I) emission from the induction region of the ICAP. The four contour plots on the left reveal the response of C (I) emission to inner argon flow rate and methanol load while the four on the right reveal the response to inner argon flow rate and chloroform load. In each set of four contour plots the inner argon flow rate increases from 1 .OO kW 1.25 kW 1.50 kW 3.4 mg s-’ top to bottom while the solvent plasma load increases from left to right and the isocontours are normalized to the over- all maximum intensity.The contour plots show that the volume containing the C (I) emission shrinks in the axial direction in response to an increase in solvent load or decrease in inner argon flow rate. In fact the volume shrinks in the axial direction by 1Omm when the methanol load is increased and the inner argon flow rate is decreased. In a previous paper it was shown that an enormous increase in electron number density accompanies the axial shrinking.15 For methanol load- ing the electron density between the top and middle turns of the induction coil increases from 8.0 x 1015 cm-3 to 1.3 x cm-j. Evidently the contour plots in Fig. 5 reveal a thermal pinch effect.’ The effect is less conspicuous for chloro- form loading because chloroform is less effectively entrained into the outer argon stream for reasons discussed earlier.Alternatively the bond dissociation enthalpy of the C- 0 bond in methanol may contribute much more significantly to the thermal pinch effect than C-C1 or C-H bonds. Emission from Mg-The Aerosol Channel In general two distinctive responses were observed for contour plots of analyte emission. One response was observed for hard lines or atom lines with excitation potentials > 6 eV and all atomic ion lines and the other response was observed for soft lines or atom lines with excitation potentials <6 eV. Fig. 6 is a plot of hard line emission responding spatially to solvent load (Mg I1 279.55 nm). The outermost contour in 4.2 mg s-’ 6.2 mg s-l 7.4 mg s-’ 8.6 mg s-’ 1 -80.0 -40.0 0.0 4.0 8.0-.- Radial distance/mrn Fig.4 Isocontour maps of C ( I ) (247.61 nm) emission intensity for a chloroform loaded ICAP. The inner argon flow was 0.81 1 min-’. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in an absolute sense by counting contours. The power is indicated on the left side of the diagram the solvent load across the top and the axial and radial scale is provided on the lower right hand map Journal of Analytical Atomic Spectrometry January 1996 Vol. 11 470.61 I m-’ 0.3 mg s-’ methanol 1 .O mg s-’ methanol 3.2 rng s-’ CHCI 10.0 mg s-’ CHCI 10.0 5.0 E E - .- m E -1 0.0 -15 0 . -80.0 -40.0 0.0 4.0 8.0 Radial distancelmm Fig.5 Isocontour maps of C ( I ) (247.61 nm) emission intensity in the induction region for a chloroform or methanol loaded ICAP. The left hand group of four maps are for methanol loading and the right hand group of four for chloroform loading. The inner argon flows were 0.61 (top row) and 1.1 (bottom row) 1 min-’. The rf power was 1.25 kW. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in a absolute sense by counting contours 6.2 mg s-’ CHCI 8.t 4.2 rng s-’ CHCI ,7.\ mg s-’ ..!I3 I mg s-l CHC13 -7 30.0 - 25.0 E E - 20.0 $ .- E 10.0 5.0 -8.0 0.0 8.0 Radial distancelmm Fig. 6 Isocontour maps of Mg I1 (279.55 nm) emission intensity for a chloroform loaded ICAP.The inner argon flow was 0.81 1 min-’ the rf power was 1.25 kW and the solvent load is indicated across the top. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in a absolute sense by counting contours each frame represents the lowest relative emission intensity of 1. The contour interval is 1 arbitrary unit and the frames can be compared with one another in an absolute sense. At the lowest attainable solvent load depicted in the left hand frame the hard line plume displays the lowest intensity presumably because the low condenser temperature required to trap the solvent vapour has also lead to significant sample loss.The entire plume and its intensity maximum also ‘sit’ the furthest upstream at the lowest solvent load. With an increase in solvent load shown in the second frame the over-all intensity increases significantly presumably because the condenser no longer traps a significant amount of analyte. The intensity maximum also moves marginally upstream whereas the plume lengthens significantly so that its tip resides at 25 mm above the top of the induction coil. A further increase in solvent load results in the plume shown in the third frame. With this increase in solvent load the top of the plume extends no further than previously and the intensity maximum only moves marginally upstream by perhaps 1.5 mm.On the other hand the base of the plume moves downstream by 3 mm and clear of the torch while the over-all intensity of the plume decreases significantly. In addition to these major changes the boundary traced out by the outer isocontour has become narrower possibly indicating that the analyte is confined closer to the axis. Further increasing the solvent load to the maximum tolerable load (for chloroform in this case) results in the plume shown in the right hand frame. With this increase in solvent load the plume continues to decrease in intensity and become narrower while the downstream tip of the plume once again extends no further than 25 mm above the top of the induction coil. It is interesting to note that this downstream limit for the tip of the plume overlaps the downstream limit for the atomic plasma as revealed by the C (I) contour plots and by the lowest intensity isocontour for the CN plots.In order to interpret the response of the hard line emission plume to solvent load several physical processes must be taken into consideration. These physical processes determine the local concentration of atomized or ionized analyte and the energy available to excite the analyte so that it emits. Relatively far upstream from the plume the analyte is essentially confined to the aerosol stream because undesolvated particles and droplets must follow the gas stream owing to their minute inertial moments compared with their high viscous drag. 48 Journal of Analytical Atomic Spectrometry January 1996 Vol. 11However once the analyte begins to desolvate and vaporize it becomes free to diffuse across the stream lines of the plasma flow and can disperse radially as it flows across the boundary region of the aerosol channel and into the plasma.Once in the plasma the analyte continues to diffuse across the stream- lines. As the analyte is transported upward and out from the axis by convection and diffusion energy is transported upwards and in towards the axis from the toroidal energy loading region by convection radiation and heat conduction across enormous thermal gradients. As a result the time averaged density of analyte and the time averaged density of energy available to excite the analyte vary enormously throughout the plasma. As a first approximation only two properties determine the local emission intensity of hard line species.These are the time averaged density of analyte and the time averaged density of energy available to excite the analyte. Consequently the maxi- mum in hard line emission resides where the maximum amount of energy is available to excite the hard line species and where the hard line species has not dispersed appreciably by any mass transport process. By similar reasoning the limits of the hard line plume reside where the energy available to excite the hard line is cut off or where the local concentration of hard line species is very low. Accordingly the upper boundary of the hard line plume coincides with the plasma boundary the radial limits of the plume are determined by the radial trans- port of analyte and the base of the plume is determined by vaporization atomization and ionization processes that convert the analyte into hard line species.The contrasting response of a relatively soft line Mg I (285.21 nm) is depicted in Fig. 7. Once again the outermost contour in each frame represents the lowest relative emission intensity of 1. The contour interval is 1 arbitrary unit and the frames can be compared with one another in an absolute sense but the scale is different from that for the Mg 11. In contrast to Mg 11 the soft line plumes depicted here do not extend past the plasma boundary. On the contrary they appear to be nested within the hard line plumes. In fact it appears as though the soft line emission occupies a cooler temperature band in the plasma than the hard line plumes a band that surrounds the hollow inner boundary.This suggests that the geometry of soft line plumes can be rationalized in terms of 4.2 mg s-' CHCI 6.2 mg s-' CHCI 8.6 mg s-l CHCI3 7.4 mg s-l CHC13 I 30.0 25.0 E E 20.0 5 t c .- 15.0 2 m .- 2 10.0 5.0 -8.0 0.0 8.0 Radial distance/mm Fig. 7 Tsocontour maps Mg I (285.21 nm) emission intensity for a chloroform loaded ICAP. The inner argon flow was 0.81 1 min-I the rf power was 1.25 kW and the solvent load is indicated across the top. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in an absolute sense by counting contours norm temperatures as Blades and Horlick have pointed out previo~sly.'~ The norm temperature for an optical transition is simply the temperature at which the emission intensity for that transition displays a maximum.In a thermal plasma one generally encounters a single maximum for line intensity with increasing plasma temperature because of two competing processes. First with the increasing temperature electron collisions increasingly populate an excited bound state for a particular line according to the Boltzmann function. As a result the emission intensity for that line increases with temperature. Second at sufficiently high temperatures the atomic species begins to ionize into the next ionization stage thus depopulating the excited state. Alternatively emitting molecules dissociate with increasing temperature thus depopu- lating molecular excited states.Overall atomic ionic and diatomic emission from the ICAP can be roughly characterized by a norm temperature (if one ignores dynamic processes). As it happens the electron kinetic temperature in the plasma the temperature which largely governs electronic excitation of plasma bound states ranges from approximately 6000 to 9000 K. In general the norm temperatures for molecular species fall below this range by approximately 1000 K whilst the norm temperatures for soft lines fall within this range and the norm temperatures for hard lines exceed this range by approximately 1000 K (see for example references 6 9 and 1 1 ). Consequently molecular emission generally occupies the plasma boundary soft line emission occupies diffuse tempera- ture bands within the plasma (time averaged bands of course) and the most intense hard line emission may generally be found where the most plasma energy is available for electronic excitation. Contour plots of the ratio of ion line to atom line intensity (Mg I1 279.55 nm and Mg I 285.21 nm) are presented in Fig.8. These are consistent with the axial profiles presented in the previous paper but in this case additionally show the off-axis behaviour. It is apparent in these contour plots that the cool regions lie close to the inner boundary and that the excitation environment grows hotter towards the downstream limit where the contour plots become discontinuous. Discussion We have previously discussed the occurrence of a vortex shedding mechanism that is responsible for air entrainment in the tail flame' and as is evident from the isocontour plots presented in this paper these mechanisms are primarily respon- sible for the determination of the location of the downstream boundary of the plasma.It cuts the plasma off by folding cold room air into the argon stream and thus extinguishing the plasma. As a result the tail cone of the plasma cannot be regarded as a region where the plasma decays gradually and steadily owing to microscopic processes such as three body recombination or radiative loss. On the contrary one must consider the possibility of an abrupt fluctuating discontinuous limit at the downstream boundary of the plasma more akin t o the bounds of the potential core in a round jet. The recirculation eddy also has an effect on the radial spatial structure of the plasma discharge.It influences how the solvent load is distributed over the argon stream and hence determines whether the axial channel or the induction region will be heavily loaded with solvent material. The balance between these two extremes determines the temperature and density profile of the plasma gas downstream from the torch. Hence it determines how energy is transported to the analyte. Beyond mass transport by convection and diffusion the effects of heat conduction and heat capacity are evident in the contour plots of C (I) emission from the induction region. Journal of Analytical Atomic Spectrometry January 1996 Vol. 11 494.2 mg s-l CHC13 6.2 mg s-’ CHCI 7.4 mg s-’ CHCI 8.6 mg s-’ CHC13 -8.0 0.0 8.0 Radial distance/mm Fig.8 Isocontour maps of the ratio Mg I1 to Mg I emission intensity (ratio Figs. 6 and 7) for a chloroform loaded ICAP. The inner argon flow was 0.81 1 min-’ the rf power was 1.25 kW and the solvent load is indicated across the top. The outermost contour in each frame represents a relative emission intensity of 1. The contour interval is 1 relative intensity unit for each frame and the frames can be compared with one another in a absolute sense by counting contours Inspection of these contour plots reveals that the plasma volume shrinks in the axial direction in response to solvent loading. This indicates that the discharge responds to solvent loading with a thermal pinch in the axial direction rather than a simple translation downstream.Although such an effect may be obscured in the C (I) contour plots by the mass transport of carbon in the argon stream the thermal pinch effect is corroborated by observations and comparison with similar effects reported in the literature. Phenomena related to heat conduction are also evident beyond the torch rim. For example contour plots of the Mg I1 to Mg I intensity ratio show that solvent dissociation suppresses the transport of energy from the outer regions into the axial channel. Only once all the solvent material has dissociated does an appreciable amount of energy flow from the outer region into the axial channel. However it is not clear how the energy is transported from the outer regions into the channel. Even so the contour plots provide no reason to invoke anything beyond thermal transport or heat conduction.Any further argument about radiation trapping the transport of metastable species or other non-thermal channels of energy transport must be regarded as speculative; after all the results presented here are obscured by temporal averaging. Air Entrainment Vortex shedding has been observed for flame-jets as well as for round jets of cold gas and is most readily observed when the jets are excited acoustically. Becker and Massaro have reviewed the literature on acoustically excited jets up to 1967.24 Their review references work from 1858 when L e ~ o n t e ~ ~ reported that a coal gas flame-jet jumped in response to certain notes from a violoncello and suggested that ‘We must look upon all jets as musically inclined’.The jumping flame was just a manifestation of vortex shedding. Soon afterwards it was found that combustion was not essential for acoustic activity in a gas jet-round jets of cold gas made visible by smoke particles behaved in a similar manner. Later on Lord Rayleigh analysed the instability problem and employed stroboscopic illumination to study it. He found that both varicose and sinuous instabilities could be acoustically excited in round jets. The varicose instabilities took the form of symmetrical swelling and constriction of the jet’s diameter synchronous with the exciting tone while sinuous instabilities took the form of rhythmic undulation or twisting of the jet (vortex evolution is associated with the varicose instability).Varicose instabilities predominate in jets with a flat or top hat velocity profile with a thin boundary region. Becker and Massaro’s literature review also reveals that vortex shedding and the acoustic sensitivity of jets has been extensively studied in more recent times and that the associated theory has also been extensively developed. Becker and Massaro themselves have presented photo- graphic records and detailed observations of vortex evolution in a round jet. Their study focused on an axisymmetric jet of cold gas from a nozzle with a flat flow velocity profile at the nozzle mouth except for a thin boundary layer of laminar flow near the nozzle wall. Their study was very informative because it spanned a wide range of‘ Reynolds numbers for the axisym- metric jet including the range typically encountered for ICAPs.They divided the complete range of Re = 600-20 000 into eight flow regimes. [Incidentally the ICAP (Re = 100-600) resides in the first flow regime of 600<Re< 1450.26-29] They found that successive vortices shed off the jet according to a general 50 Journal of Anahtical Atomic SDectrometrv Januarv 1996. Vol. 11frequency law (or wave velocity law). In general J A - 2 constant UO (5.1) where f is the vortex shedding frequency A is the wavelength of the varicose disturbance and Uo may be regarded as the centre line velocity of the jet. The constant is approximately 0.5 so vortices shed off the jet at approximately half the velocity of the jet stream. Vortex shedding followed this frequency law both in the presence and absence of acoustic excitation.Interestingly they found that when the vortex shedding was excited by pure acoustic tones discrete frequency jumps were observed which turned out to be related to the resonant properties of the nozzle tube. Among other things they found that varicose instabilities were prevalent for thin boundary layers whereas sinuous instabilities were prevalent for fully developed laminar flows. One further point worth mentioning is that they observed the transition from vortex shedding to turbulent flow (vortices need not be turbulent). In certain flow regimes successive vortices would collide then break up into turbulent eddies. In other flow regimes the onset of turbulent flow resided upstream from the tip of the potential core.In which flow regime the ICAP is found if it indeed displays similar phenomena is not known. The ICAP displays many of the characteristics of an axisym- metric round jet in which one would expect vortex shedding. Before the hot argon jet flows out of the confinement tube into the relatively stagnant air of the torch box its flow is essentially laminar rather than turbulent. Its flow profile also appears to have a thin boundary layer and a top hat velocity profile except for the axial channel.23 Moreover the argon exists as unionized gas as a coolant stream close to the torch wall. So at least the outer flow bears similarities to a round jet prone to varicose instabilities. On the other hand it may be dangerous to assume that the flow dynamics of hot plasma bear any similarity to those of cold argon.It should also be noted that the ICAP has both axial and tangential velocity components of flow. It is not understood how tangential components affect varicose instabilities. If the ICAP behaves as a round jet the argon flow remains laminar just beyond the exit of the torch but varicose instabilit- ies arise in the cylindrical surface of discontinuity in the flowfield. As a result the surface of discontinuity eventually folds into a toroidal vortex much like a breaking wave. As this vortex moves downstream it entrains air and radically convolutes the cylindrical interface between the air and the argon. As it continues on its course downstream it grows in thickness folds in on itself and continues to entrain more air into the argon jet at its interior.As the toroidal vortex moves upwards its inner edge makes contact with the hot plasma and probably folds cool material in with the plasma gas quenching the plasma. In other words the plasma is probably confined to the potential core of the jet where the potential core is simply the region of the flowfield unperturbed by air entrainment. In short the plasma boundary is probably defined by a modulated limit or cut-off determined by vortex entrainment of cold air rather than by thermal or radiative dissipation of energy. The time averaged picture of this modulated plasma bound- ary is revealed by the isocontours of CN intensity in Fig. 2. In the outer boundary region the CN emission contour plots and the tail flame observed above the plasma are probably time averaged pictures of the vortex shedding. Within this region the interface between air and plasma over which mass trans- port by diffusion takes place becomes radically convoluted even though the flow may still be laminar (experiments with cold gas jets and diffusion flames reveal that the flow eventually becomes turbulent as the vortices collide with each other and disintegrate at downstream distances of more than two jet diameters from the nozzle).The net result of the vortex shedding is a complex pulsating mixing mechanism with a frequency corresponding to the shedding frequency of the vortex rings which modulates the outer diameter of the plasma. It seems reasonable to conclude then that the overlap of the contours for CN and C (I) contour plots above the torch rim is a consequence of temporal averaging while the minimal overlap between C (I) and C2 isocontours within the torch is characteristic of the steady plasma boundary there.To our knowledge detailed pictures of this vortex shedding process are only available for cold jets and diffusion flames and not for plasma jets. But reliable experimental evidence shows that the ICAP also displays vortex shedding. This evidence includes high speed movie frames in which vortex structures are plainly visible and noise power spectra in which bands corresponding to vortex shedding frequencies are ~nmistakable.~~ Analytical Consequences The spatial relationships between the analyte plumes and background emission are of particular relevance because the ratio between analyte signal and background emission is something that the analyst would like to maximize.Also relevant to the analytical performance is the influence of vortex shedding and air entrainment. Air entrainment influences the analytical signal in several ways. It introduces flame-like conditions to the periphery of the analyte plumes rendering the analyte signal susceptible to all of the matrix interference effects normally encountered with flames. It also introduces noise by modulating the analyte plume. It may even corrupt the analytical blank by entraining dust or other pollutants. Finally vortex shedding and air entrainment are sensitive to acoustic excitation and changes in the flow dynamics of the room air. Consequently environmental sound and changes in the flowfield of the room air may corrupt the analytical signal.Apparently it would be beneficial to eliminate air entrain- ment altogether or to reduce the effect during measurement. End-on viewing is probably one means of achieving this end. The diagnostic usefulness of radially resolved monochro- matic images of emission intensity contour plots for both physical diagnostics and control diagnostics is evident in this paper. The contour plots reveal that emission from C2 CN and C (I) may all be used as control diagnostics for solvent plasma load provided that they are viewed at the appropriate location. For example the C2 intensity is proportional to solvent load when viewed down the axis while CN and C (I) intensity are proportional to solvent load when viewed off- axis.On the other hand for physical diagnostic work the C (I) contour plots within the induction region reveal that the plasma shrinks both axially and radially in response to solvent load. Cz emission within this region indicates how solvent material is distributed over the argon flowstream. From a physical perspective a downstream conical limit is evident in all contour plots of emission intensity at locations above the torch rim. This limit and the overlap between isocontours of different emitting species may be attributed to time averaged varicose instabilities in the plasma jet. In contrast the flowstream within the induction region is appar- ently steady and unperturbed by fluctuations. In contour plots of this region it is evident that the distribution of solvent over the argon stream depends on the inner argon flow rate and the properties of the solvent.Briefly less solvent material is distributed over the outer argon stream when the momentum (flow rate x density) of the inner stream increases. This response may be attributed to a recirculation eddy at the base of the discharge. It is also evident in these contour plots that the Journal of Analytical Atomic Spectrometry January 1996 Vol. 11 51plasma responds to solvent plasma load by shrinking both axially and radially. From an analytical perspective the emission contour plots reveal the analytical usefulness of emission from argon and solvent pyrolysis products. In particular the three dimensional information reveals the observation zones where the emission intensity of these species is proportional to solvent load and plasma excitation conditions. 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