Axial Viewing and Modified Cup Design for Direct Sample Insertion Inductively Coupled Plasma Atomic Emission Spectrometry CAMERON D. SKINNER AND ERIC D. SALIN* Department of Chemistry, McGill University,Montreal, Quebec, Canada H3A 2K6 Axial viewing generally provides an advantage over lateral optical design since the plasma tail is directed at the optical viewing of the plasma for direct sample insertion. The signal system. Typically, some form of sampling cone or sheathing change is element-specific and has produced detection limit gas is used to protect the optic that is exposed to the plasma.5 improvements as high as a factor of 10 in the set studied.In To enhance the optical design of commercial instruments some the axial viewing mode the incandescent cup does not manufacturers are abandoning the traditional mirror-based contribute to an increased background; however, the optical systems in favor of lens systems.6 Since the central background and subsequent background noise levels are channel of the plasma is cylindrical, a cylindrical achromatic increased for most elements. Carrier gases were introduced lens may be used to focus the circular region in the central into the cup via a hollow stem resulting in a reduction in the channel into a line image on the entrance slit to maximize the background signal, notably in the longer wavelength region.light throughput. As a result of the higher throughput, smaller When 1000 ppm Freon-12 in argon is used as a carrier slit-widths may be used to increase the resolution and improve through the center of the cup, the normally problematic the signal-to-background ratio. The combination of these refractory elements are vaporized easily.Modifications and modifications in turn reduces the detection limits and improvements to the design of the direct sample insertion minimizes interferences. device are also discussed. The second liability of ICP-AES is the poor sample introduction eciency of pneumatic nebulizers.ETAAS overcomes this Keywords: Axial viewing; direct sample insertion; inductively disadvantage by producing a transient signal that has a high coupled plasma; Freon; atomic emission spectrometry; sample analyte concentration in the sample cell. This high analyte introduction concentration gives rise to increased sensitivity and lower detection limits. A similar increase in sensitivity can also be Inductively coupled plasma atomic emission spectrometry observed in ICP-AES when sample transport eciency is (ICP-AES) has been the workhorse of modern analytical increased with the use of devices such as ultrasonic7 and frit8 elemental determinations for many years because of its multi- nebulizers.In addition to these more traditional liquid sample element capability, large linear dynamic range, relative freedom introduction techniques, alternative sample introduction from non-spectral interferences and low detection limits.methods such as electrothermal vaporization (ETV)9 and direct However, for samples that require great sensitivity and lower sample insertion (DSI)10 may be used. With ETV, the sample detection limits, electrothermal atomic absorption spec- vapor is swept out of the furnace and introduced into a trometry (ETAAS) and more recently ICP mass spectrometry standard ICP. In the DSI technique, a sample is introduced (MS) have been the techniques of choice.ETAAS is primarily into the plasma on a sample carrying probe. Our laboratory a single element technique and suers from interferences and has primarily focused on the use of graphite cups as probes a shorter linear dynamic range than ICP-AES. These draw- for DSI experiments but metal probes have also been used.11 backs are unfortunate because of the low detection limits and With DSI the sample is vaporized within the plasma, and the relatively low cost of the technique.On the other hand, ICP-MS technique can be considered to be a 100% ecient sample has very low detection limits and is capable of determining introduction method. multiple elements by rapidly scanning through the masses of Axial viewing in combination with a sample introduction the elements of interest; however, the instrumentation is expens- technique that is 100% ecient would appear to have the ive to purchase and operate, and is more prone to interferences potential to reduce detection limits of ICP-AES significantly.than ICP-AES.1 The ideal solution would seem to be an To this end we have attached a DSI device to an ICP extension of the working range of ICP-AES to levels that are configured for axial viewing. We also speculated that DSI may competitive with those of ETAAS and ICP-MS. gain an additional advantage in that the background intensity In an attempt to augment the capabilities of ICP-AES, many might be considerably lower with axial viewing as the DSI instrument manufacturers are switching from lateral to axial cup would appear to be black.Given the significantly higher viewing of the plasma. This method oers several potential light levels anticipated from axial viewing, one could expect a advantages of which the most important are increased optical dramatic improvement in signal-to-background ratio and throughput and higher line-to-background ratios.2 The consequently detection limits.5,6 resulting configuration provides lower detection limits that Even though DSI-ICP-AES with a thin-walled cup is rela- may allow ICP-AES to replace ETAAS for a variety of tively free from memory eects, refractory compounds and applications.However, axial viewing of the plasma is not elements can be problematic because of their low volatility. In without its disadvantages. There is an increased risk of inter- some cases, incomplete vaporization may result in tailing that ferences by viewing through the plasma tail.3 To help counter does not return to the baseline during a 10–15 s insertion.this problem the torch can be extended, which some manufac- Carbide-forming elements are especially troublesome since turers claim maintains the traditional freedom of the ICP from most carbides vaporize at temperatures greater than those that chemical interferences.4 Viewing the plasma axially presents some diculties in the can be obtained in a conventional DSI cup in an argon plasma.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (725–732) 725Vaporization of these compounds can be improved by increas- deep into the base of the electrode for an adapter that mates the hollow glass shaft to the cup. Next, a 5.16 mm (13/64 in) ing the temperature using oxygen in the plasma (outer) gas12 or by the introduction of halogenating agents.13 Increasing the hole 9.7 mm deep is bored into the face of the electrode to form the cup.The wall of the cup is produced by removing forward power that is applied to the plasma eectively increases the temperature; however, it is often not sucient to obtain carbon from the electrode with the cutting tool. The wall requires several light passes. If too much carbon is removed complete vaporization. The elements that form refractory oxides and carbides all have halide forms that vaporize at in one pass, the cup tends to deform and crack. When the underside of the cup is being cut, the tool must significantly lower temperature.This property has been exploited by using solid halide salts.14 We have modified the be oriented such that only the point of the tool is in contact with the base of the cup (see Fig. 1). If the heel of the tool is DSI so that gaseous halogenating agents (usually Freon-12) may be introduced directly into the cup during the insertion. allowed to rub the base of the cup, the friction and the resulting torque inevitably breaks the stem.The base of the cup and the Note that others have directly introduced Freon into the plasma gases.15 Freon breaks down to produce fluorine and stem are all cut in one pass since any pressure on the thin stem usually results in a broken cup. Deformation and cracking chlorine radicals which react rapidly to form volatile halides.16 Gases injected through the cup establish a central channel are not a problem when machining the base of the cup because of the high structural strength of the base.The dimensions of that reduces the background and also increases the analyte signal intensity by entraining the analyte vapors up through the cup are given in Fig. 2. After the cup has been machined it must be cleaned prior the viewing zone. The addition of low volumes of halogenating gases to the plasma gases does not establish a central channel to use. The cup is cleaned by inserting it into the plasma so that the top of the cup is approximately 5–10 mm above the but does enhance the volatility.The purpose of this study was the evaluation of axial viewing top of the load coil (ATOLC). An insertion to this depth ensures that contaminants in the base and stem do not vaporize for DSI-ICP-AES and a study of direct injection of gases through DSI cups. during routine use. Typical analytical insertion depths are to 0 mm ATOLC. The primary contaminants that have been observed in the plasma during the cleaning step are sodium EXPERIMENTAL (from handling) and iron from the cutting tool. The 0.7 mm thick stem is not the thinnest stem that has The DSI cups fabricated for this investigation are made from been produced.Stems as fine as 0.5 mm can be easily produced standard carbon electrodes, Table 1. The cups are an improvebut have been found to be too delicate for practical purposes. ment over those that are traditionally used because the walls Cups with such fine stems tend to break on handling and do and the base of the cup are especially thin to minimize the not survive the vibrations of the DSI device during insertions mass introduced into the plasma.This allows the plasma to and retractions. The survival rate of the 0.5 mm stem cups heat the cup and sample rapidly. We have found from experidepends on the individual cup. Some cups have survived ence that one of the most important parameters in cup design hundreds of insertions whereas some break on the first run.is the thickness of the cup base.17 If the base or wall is too This large inter-cup variability is not found with the thicker thick then the analytes are slow to vaporize and the signal tends to tail.17–19 Memory eects are only observed on carbideforming and highly refractory elements. In the past, the cups were machined on a lathe (Table 1) that rotated at 1180 rev min-1; however, we have recently found that operating the lathe at its maximum speed of 2880 rev min-1, allows a cup of finer dimensions to be produced.The cutting tool is a simple high speed steel cutter with a slightly rounded triangular point. The sharpness of the cutting edge is critical to the success of cutting the cup. The cutter must be resharpened after every 3–5 cups are machined due to the abrasive nature of the graphite. Fig. 1 Cutting of DSI cup stem. The first step in cup production is the drilling out of the base. A 3.18 mm (1/8 in) twist drill is used to bore a hole 9 mm Table 1 Equipment supplies and manufacturers Carbon cup blanks Bay Carbon Electrode: #S-8 high density Bay City, MI, USA Boiler caps: #BC-1 Spectrometer modifications TruLogic, and upgrade Mississauga, Ontario, Canada Plasma Unit, Model 2500 Plasma Therm, Kresson, NJ, USA Ball valve F.C.ProValve, Pointe Claire, Quebec, Canada 1000 ppm Freon-12 in argon Matheson Ville St. Laurent, Quebec, Canada Lathe Norvik Moore, Emco Compact 5 Ville St. Laurent, Quebec, Canada Fig. 2 DSI cup dimensions. 726 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12stemmed cups and so the 0.7 mm thick stem has been used mirror can sit on top of the torch box. The chimney is also removed from the torch box cover and a 90° folding mirror is throughout this work. There was no appreciable dierence in the performance of the two cup types. The lifetime of the cup placed directly above the plasma. With this arrangement the magnification of the system remains constant, see Fig. 3. with the 0.7 mm stem has been found to be in excess of 200 insertions, and probe demise is usually due to a handling The folding mirror directly above the plasma is protected from the plasma tail by a gas cut-o that is directed across accident. The cups that are designed to introduce gases into the the tail of the plasma. The gas cut-o is created by passing a 14 l min-1 nitrogen gas stream through a 2 in Perkin-Elmer plasma through their bases are machined similarly; however, after the cup has been drilled out with the 5.16 mm drill bit, flame atomic absorption slot burner head.The DSI device (DSID) that was used for this work has the center of the electrode is drilled out with a 1.58 mm (1/16 in) or 0.88 mm (0.035 in) drill bit. The full depth of the undergone extensive modifications from the original design of Sing and Salin.21 The modifications were necessary in order bit is used to drill through the entire probe in one pass.The method of cutting the stem is the same except that the diameter to allow greater ease in set-up and alignment as well as the introduction of gases into the plasma through the cup. An is increased to 2.28 mm to accommodate the hollow stem (Fig. 2). There was no significant dierence in analytical per- explanation of the new design follows as well as a schematic diagram (Fig. 4). formance between the two cups, so the cup with the 1/16 in hollow diameter shaft was used throughout.The shaft on which the DSI cup is placed is 1/4 in medium wall glass tubing. The shaft is mounted near the center of a The adapter plug that mates the DSI cup to the glass shaft is cut from graphite and is designed to fit into the hole in the brass block by a 1/4 in to NPT PTFE Swagelok fitting. The shaft extends to about 5 cm below the block. The Swagelok base of the cup 3.18 mm (1/8 in) and into the glass shaft (approximately 2.5 mm) (Fig. 2). A second adapter with a hole fitting was drilled out to allow the shaft to pass through the fitting.This extension allows a gas fitting to be attached bored out to 1.58 mm was made so that gases could be introduced into the DSI cup. directly to the end of the glass shaft which can then be used to introduce gases into the cup via the hollow stem cup and A boiler cap is a commercially available lid that fits snugly over the graphite electrodes that the cups are made from. The shaft adapter (Fig. 5).The brass drive block has a 12.7 mm (1/2 in) hole drilled DSI cup that was used for the boiler cap experiment had a thicker stem of 1.4 mm and slightly thicker walls of 0.4 mm through it to accept a 1/2 in od aluminium guide shaft that extends the length of the drive section of the DSID. On the with an exterior diameter of 6.17 mm (0.243 in) so that the boiler caps would fit the cup. Three boiler caps were drilled opposite face of the drive block, a 2 mm thick plate of aluminium is mounted with six Allen screws.The two ends of the taut out to orifice diameters of 0.40 mm (1/64 in), 3.18 mm (1/8 in) and 8.73 mm (11/32 in). Two of the three caps were made from drive belt are held in position by tightening the plate down over the belt with the screws (Fig. 5). After these experiments standard Bay Carbon boiler caps which have an orifice of 0.97 mm (0.038 in); the third was made from carbon rod had been performed an additional guide shaft was installed on the system so that the DSI shaft is bracketed by the guide stock, Table 1.The spectrometer used for this series of experiments was a shafts. This addition provides further insertion precision and prevents the guide block from rotating while in motion. Jarrell-Ash 750 direct reader originally designed for spark emission. The spectrometer has been modified for rapid back- At the bottom of the DSID, the drive belt passes over a sprocket that is attached to the stepper motor.An idler ground correction with the installation of a galvanically driven quartz refractor plate and high speed electronics (Table 1).20 sprocket is located at the upper end of the DSID. An optical interrupter switch is mounted on the body of the DSID and Throughout this work data acquisition software (SF20) written by G. Le�ge`re specifically for transient signal acquisition was is connected to the stepper motor controller. The flag which triggers the optical interrupter is mounted onto the drive block used.A commcial version of this hardware is available from TruLogic Systems (Table 1). so that the lower limit of travel can be detected and the stepper motor controller has a zero reference position. The spectrometer input optical system uses an all-mirror design to minimize chromatic aberrations (Fig. 3). A 0.75 m focal length o-axis concave mirror is used to image the plasma onto the entrance slit. A 90° folding mirror is placed between the entrance slit and the focusing mirror to facilitate experimental set-up.In the lateral viewing mode the plasma, mirrors and the spectrometer all lie on the same optical plane. In the axial viewing mode the torch box (Table 1) is lowered below the optical plane of the spectrometer so that the focusing Fig. 4 Schematic diagram of direct sample insertion device. Fig. 3 Overhead view of axial viewing configuration. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 727reverse of the normal optical path.23 The beam can be followed from the entrance slit and the mirrors adjusted as needed to ensure that the beam focuses at the appropriate position directly above the DSI cup.Sample introduction into the cup is straightforward. The plasma is extinguished and the cup is raised to approximately 5 cm ATOLC and a volume of sample is pipetted into the cup. In this series of experiments 10 ml of standard were used. The cup is then retracted to the center of the load coil and dried inductively. The cup is then withdrawn until the DSID triggers the optical interrupter.The stepper motor controller then drives the cup up to the base of the plasma. It briefly halts (2 s) at about 5 mm below the base of the plasma to allow the plasma to stabilize. As the DSI shaft is driven up towards the plasma, the plasma can be seen to jitter slightly. The cup is then inserted into the plasma. Since the sample is pre-dried there is no need for a drying step beneath the plasma.14,21,24 Transient analyte emission is observed with the spectrometer.Fig. 5 Diagram of drive block. The on-line as well as the background intensities are measured in rapid succession. After the signal has been recorded, the cup is withdrawn from the plasma and allowed to cool before the Above the drive block and in-line with the glass shaft is a Teflon plate 1 cm thick with a hole for the shaft. This plate next sample is introduced.serves two functions: it guides the shaft up the middle of the torch and it provides a gas-tight seal between the plasma and RESULTS AND DISCUSSION the external atmosphere while the ball valve is open. In the future a sample deposition box will be added to the DSID on In order to compare the relative merits of axial viewing with top of the drive section. The cup can then be withdrawn from lateral viewing for DSI, the dependence of the transient signal the plasma into the box and the sample can be sprayed into intensity on several experimental parameters was investigated. the cup and dried while the plasma is still operating.Spray Table 2 lists the experimental parameters used for this series deposition yields better detection limits and good reproduc- of measurements in both the axial and lateral viewing modes. ibility and is preferable since it is easily automated.22 During In the axial viewing mode the gas cut-o height was varied sample deposition and drying the ball valve can be closed to first.Three dierent cut-o heights were investigated. The prevent the plasma from settling on the inner torch tube. The lowest was approximately 25 mm ATOLC and visibly sheared ball valve (Table 1) can be disassembled in situ, so that the the conical top of the plasma o. The highest was approxitorch, Swagelok fitting and part of the ball valve can be mately 70 mm ATOLC so that the luminous region of the removed as a unit for cleaning or replacement without remov- plasma was unaected by the cut-o gas.The middle height ing the DSID or losing the alignment of the system. was the median of the upper and lower heights. The plasma Additionally, the torch can be safely removed so that when was only slightly aected although the top of the discharge the DSID is removed from the system there is no danger of would occasionally be disrupted by turbulence from the gas torch breakage. cut-o. Fig. 6 indicates that the position of the gas cut-o does In the past, alignment of the DSID within the torch was a not have a dramatic eect on the signal.Since the highest gas problem. In this configuration, the bottom of the torch box cut-o height visually disturbed the plasma the least it was has been cut out and replaced with a removable torch box used for subsequent experiments. This strategy may not be plate that can be changed for each type of sample introduction advisable if samples of a complex nature are analyzed since device.The problem of alignment has been overcome by recombination takes place in the tail of the plasma and may attaching the torch and the DSID to the removable torch box lead to interferences and loss of linearity.6 plate. The torch is held at the bottom by a 1/2 in PTFE The insertion depth parameter was varied next. Fig. 7 shows Swagelok fitting. This fitting is attached to a pneumatically the optimization curve with traditional lateral viewing at a driven 1/2 in poly(vinyl chloride) (PVC) ball valve.The ball viewing height of 20 mm ATOLC. Fig. 8 shows how signal valve, pneumatic drive and torch assembly are all mounted onto a plate. Between the torch/ball valve plate and the match Table 2 Instrumental operating parameters box plate is a large (3 cm diameter) O-ring. The torch/ball valve plate is attached to the torch box plate via three Allen Plasma forward power 1 kW screws. To align the torch in the center of the load coil the Reflected power #5 W three screws are adjusted.The O-ring accommodates the tilt Plasma gas flow rate 14 l min-1 of the plate while providing a gas-tight seal. Auxiliary gas flow rate 0.8 l min-1 DSI torch purge gas flow rate #0.3 l min-1 In a similar fashion, the drive section of the DSID is (only before ignition of plasma) mounted to the underside of the match box plate with three Axial cut-o gas flow rate #14 l min-1 Allen screws and a large O-ring.This allows the cup to be Sample volume for calibration 10 ml 1–100 ppb centered in the load coil when the drive shaft is extended. Sample volume for hollow stem 10 ml at 0.5 ppm At the point where the ball valve attaches to its mounting Insertion depth 0 mm ATOLC* plate a small (1/8 in Swagelok) gas fitting is used to attach the Viewing height 20 mm ATOLC Exposure time 20 ms per position gas flow that is normally used for the nebulizer to the DSID. Number of on-line and o-line This ‘nebulizer’ gas is used to purge the ball valve when the exposures per trace 200 instrument is first set up but during routine use it is usually Galvanometer settle time 10 ms turned o.Background oset #0.8 nm Alignment of the optical system is relatively simple since the Gas flow through cup 290 ml min-1 installation of a diode laser in the zero order light trap of the spectrometer. When the laser is on, the beam follows the * ATOLC: Above top of load coil. 728 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12the case similar behavior between Zn and other volatile elements would be expected. Axial viewing in combination with DSI allows a wide range of viewing heights to be used without loss of sensitivity. There are two reasons for this; first, when DSI is employed the plasma does not have widely dierent thermal zones such as those with liquid nebulization because there is no cooling/ desolvation process.26 Second, the optical arrangement tends to average the signal intensity over the length of the plasma which ‘homogenizes’ the signal.To verify this, the cup was inserted to 0 mm ATOLC and the viewing height of the system was adjusted by moving the focusing mirror slightly. The viewing height of the system was measured as the distance from the focal point of the alignment laser to the top of the load coil (ATOLC, as is the insertion depth). Fig. 9 shows that when the plasma is viewed axially the signal is nearly indepen- Fig. 6 Intensity of emission in axial viewing mode as a function of the position of the gas cut-o. (&, Sn; +, Zn; $, Pb; X, Cu). dent of the viewing height although the signal drops close to zero as the viewing height approaches the interior of the cup. This indicates that, operating under compromise conditions, viewing height should not be a problem. The signal intensities that are observed with lateral viewing DSI show a greater dependence on the viewing height but there is no strong optimum.For comparison, both systems were subsequently tested with the same viewing height and insertion depth. The method was calibrated in both the lateral and axial viewing modes using the operating conditions listed in Table 2. Those elements for which calibration graphs were acquired are listed in Table 3 with an asterisk. In general, axial viewing does provide an advantage over lateral viewing. Net emission intensities are higher; however, background noise is also increased over that of lateral viewing.The overall result is an increase in the signal-to-background noise ratio and improved Fig. 7 Lateral viewing signal intensity versus cup insertion depth. detection limits (Table 4). The detection limits for peak height (X, Sn; $, Zn; +, Pb; &, Cu). measurements were calculated based on three times the noise of the background divided by the slope of the calibration Fig. 8 Axial viewing intensity versus cup insertion depth.(X, Sn; $, Zn; +, Pb; &, Cu). Fig. 9 Peak area in the axial viewing mode as a function of the viewing height. The viewing height is the focal point of the spectrometer, the top of the DSI cup is inserted 0 mm ATOLC. (X, Cu; peak height varies with insertion depth when axial viewing is $, Zn; +, Pb; &, Cd; ,, Mg). used. Note that Fig. 7 only covers the 0–10 mm insertion depth range whereas with axial viewing (Fig. 8) the range was from -10 to 10 mm.The reason for the smaller range was an Table 3 Elemental lines studied oversight while the instrument was set up in the lateral viewing Element Wavelength/nm mode. Zinc is the most sensitive element with respect to insertion depth. At the -10 mm insertion depth the base of C I 193.1 Zn I 213.9 Calibration graph determined the cup is sitting in the edge of the plasma base. At this Pb II 220.4 Calibration graph determined position the emission intensity was sucient to saturate the Cd I 228.8 Calibration graph determined detector.It is unclear why Zn shows such a strong dependence Sn I 235.5 Calibration graph determined on insertion position. Initially we believed that it might have Fe II 259.9 been a question of hard/soft line behavior; however, both Pb Mg II 279.6 and Sn are also hard lines and yet do not display this strong Al I 309.3 Cu I 324.8 dependence on insertion position.25 In fact Cu, Pb and Sn Co I 345.3 Spark line behaved essentially in the same way even though the Cu line Mg I 383.2 Background emission only (Fig. 11) is a soft line. This anomalous behavior has also been observed Ca II 393.4 by Umemoto and Kubota, who claim that the line intensity is Pb I 405.7 Background emission only (Fig. 11) governed by the rate of vaporization;26 however, if this were Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 729Table 4 Detection limit (pg) comparison between axial and lateral viewing (10 ml sample size).To convert to the concentration detection limit divide by 10 and the units will be in ppb Lateral viewing Axial viewing Improvement Element Height Area Height Area Height Area Sn 1800 7000 390 1500 4.6 4.6 Zn 8 33 46 130 0.17 0.25 Pb 60 270 21 220 2.8 1.3 Cu 6 20 1.2 1.8 4.9 11 Fig. 11 Background emission of the plasma with a DSI inserted to 0 mm ATOLC versus wavelength and power for the two viewing modes. Data taken from o-line measurements. graph.The detection limits for the peak area were calculated by multiplying three times the noise of the background by the time that the element normally takes to vaporize divided by apparent blackbody emission curve the cup was inserted into the slope of the calibration graph. As is traditional for transient the plasma and allowed to reach its equilibrium temperature. analysis techniques, we have expressed detection limits as mass. The data acquisition system was triggered on the spectrometer In these experiments, a 10 ml sample volume was used; hence, and, after 3 s, the plasma was extinguished by stopping the for example, a detection limit of 20 pg corresponds to a plasma gas flow and tripping the rf interlock.The gas flow concentration of 2 ppb. The sample cup can actually hold over was cut o so that the cup would cool slowly and allow the 100 ml; however, one should keep in mind when predicting spectrometer to follow the decay in light emission. The data detection limits that our preferred technique for sample depos- showed that the intensity dropped close to the dark level ition is ‘aerosol deposition’, in which approximately 1 ml of within 0.5 s whereas the cup was still visibly incandescent for solution is deposited.27 at least 10 s.This indicates that the background is due to the With the range of concentrations used to prepare the plasma and not the DSI cup. The high background intensity calibration graphs (1–100 ppb) the calibration graphs were observed is similar to the increased background that has been linear in contrast to what has been observed with liquid found with liquid nebulization axial viewing.3,6 The presence nebulization and axial viewing.This is because of the limited of the DSI cup in the lower region of the plasma, where there concentration range and the minimal amount of sample that is intense continuum emission, partially masks the background is actually introduced into the plasma when compared with but the plasma ingresses into the viewing zone above the cup liquid nebulization.and produces an increased background. Calibration graphs were also determined at 1.2 and 1.5 kW. In an attempt to force the analyte plume from the cup into In all cases the slope of the calibration graph increased with the viewing zone, three dierent graphite boiler caps were power while calibration linearity was maintained. placed on a DSI cup. When the emission intensities of the Fig. 10 shows three superimposed Cu 324.8 nm traces that three caps were compared using the same analyte concentration have not been corrected for background. The plasma back- there was only a 16% increase in analyte emission intensity ground is elevated with increased power prior to insertion of (peak area) as the diameter was decreased. This minimal the cup at 1.7 s but rapidly drops to nearly the same level once improvement suggests that the analyte cloud is being rapidly the cup is inserted into the plasma.This indicates that better mixed and dispersed in the plasma as soon as it emerges from detection limits should be possible with increased power since the DSI cup. the signal-to-background ratio is increased. In the Introduction we suggested that axial viewing DSI In the lateral viewing mode, the spectrometer cannot view might provide significant improvements in the signal-to- the cup unless the cup is inserted to extreme depths (+20 mm background ratio because of the increased light levels and low ATOLC).In the axial viewing mode the DSI cup lies along background due to the presence of the cup. These experiments the optical path of the spectrometer, and the cup is clearly demonstrate that our optical arrangement does not provide incandescent while it is within the plasma. This suggested that this result. When a DSI cup is used without a central gas flow part of the increased background observed in the axial viewing the plasma ingresses into the region above the cup resulting mode may have been due to the incandescence from the DSI in a higher background, especially at longer wavelengths. The cup.Fig. 11 shows the o-line intensities measured at various flow-through cups were not tested in the axial viewing arrange- wavelengths and powers. The data in Fig. 11 are uncorrected ment, because significant increases in signal intensity are not for photomultiplier tube response and filter losses; however, observed unless Freon is used, and exposing the optics to the the shape of the curve is strikingly similar to that of a halogens that the Freon produces seemed imprudent.blackbody emission curve. To determine the source of this The relatively large diameter of the DSI cup, 5.16 mm (Fig. 2), gives rise to a diuse plume of analyte that is approximately twice the cup diameter at a viewing height of 20 mm ATOLC. One of the primary reasons for modifying the DSI was to allow gases to be introduced into the cup because it was believed that the carrier gas would form a central channel constraining the analyte in a narrow axial region thereby increasing the sensitivity.Fig. 12 shows that the width of the emission zone is largely independent of the presence of the carrier gas flow, suggesting that strong dispersion forces operate in the central zone of the 27 MHz plasma.We also expected that the central gas flow would reduce background intensity and this was found to be the case.Fig. 13 shows a plot of the ratio of the background intensities (measured from the o-line data) when Freon is used as a central gas to when no gas is Fig. 10 Cu 324.8 nm traces at 1.0, 1.2 and 1.5 kW obtained with axial used. A similar reduction in background intensity is also viewing and not corrected for background emission. The DSI is inserted into the plasma at 1.7 s. observed when argon is used as the carrier gas.The three 730 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12elements chosen for the plot cover the wavelength range indicated in Table 3. These data as well as the background data of the other elements show that the intensity of the background is significantly reduced in the longer wavelength regions. This is similar to the central channel that is observed with liquid nebulization. However, with the introduction of argon as the central gas, the analyte signal intensity is also reduced as a consequence of local plasma cooling.For the elements studied, the emission intensity was approximately 35–55% of the intensity observed when no argon was used in the cup. The exceptions to this are Ca, Al and Co. The Al and Co signals are comparable and the Ca signal is 2.6 times greater than those obtained when no carrier gas is used, Table 5. It appears that the signals from these three elements Fig. 12 Iron peak area as a function of radial distance and type of are enhanced because the argon sweeps the analyte out of the carrier gas introduced through the DSI cup obtained in the lateral cup as it vaporizes, reducing the probability of forming a viewing mode.Carrier gas flow rate was 290 ml min-1. X, Argon; $, argon+1000 ppm Freon-12; &, no gas. refractory compound (e.g., CaC, CoC, Al2O3). The signal does not return rapidly to the baseline with argon as the carrier gas, indicating incomplete vaporization.Calcium is by far the most refractory of the elements studied (Fig. 14), but both Al and Co show similar tailing. Despite the general loss of sensitivity in using a central injector gas, there is an advantage to using argon as a carrier gas because the noise of the background is reduced by an average of 60% of the value observed when no gas is used, Table 5. This is advantageous for the elements that are not highly volatile (Ca, Al and Co) as well as for Cu, Mg and Fe.However, for the volatile hard-line elements (Zn, Pb and Cd), a carrier gas is a disadvantage as can be seen from the reduction in the signal-to-background noise ratio (Table 5) and poorer detection limits. When Freon is used as a carrier gas, dramatic improvements Fig. 13 Ratio of the background intensities for cups with gas to cups in vaporization rate are observed for some elements, especially without gas at three dierent wavelengths. The carrier gas introduced those that are refractory.When compared with running with through the DSI cup was argon with 1000 ppm Freon-12 at argon, all of the elements displayed an increased peak height; 290 ml min-1. $, Zn 213 nm; ,, Mg 382 nm; &, Pb 405 nm. however, for the volatile elements (Pb and Cd), the increased Table 5 Figures of merit for carrier gas DSI. Determined from 25 ml of 0.5 ppm multi-element solution Carrier gas* Element sb† Peak height S/B‡ S/N§ Detection limit¶/pg Argon Zn 213.9 9.5 9880 19.7 1040 14 Freon 8.9 37300 74.7 4210 3.6 No gas 15 22500 42.2 1490 10 Argon Cd 228.8 14 13800 27.9 960 16 Freon 15 32700 65.4 2180 7 No gas 3.5 38000 75.6 10900 1.4 Argon Pb 220.4 2.3 450 0.94 199 75 Freon 3.6 800 1.65 218 69 No gas 3.4 1100 2.29 320 47 Argon Fe 259.9 16 1450 2.88 93 160 Freon 21 17700 34.1 838 18 No gas 24 2720 4.78 114 130 Argon Mg 279.6 0.36 1250 204 3420 4 Freon 0.35 2800 456 8100 2 No gas 2.1 2340 292 1110 13 Argon Al 309.3 23 870 1.30 37.5 400 Freon 21 15500 23.9 746 20 No gas 31 820 1.09 26.5 570 Argon Cu 324.8 2 6780 84.2 3420 4 Freon 3 19900 224 6560 2 No gas 3 12800 108 3880 4 Argon Co 345.3 34 4520 6.67 134 110 Freon 26 16600 22.8 630 24 No gas 45 3580 3.40 80 190 Argon Ca 393.4 2.4 1771 19.6 754 20 Freon 14 43000 1230 3030 5 No gas 2.7 690 12.4 258 58 * The carrier gas flow rate was 290 ml min-1.Freon indicates 1000 ppm Freon-12 in argon. † sb is the noise measured on the background signal while the cup is in the plasma.‡ S/B is the ratio of the peak height to the average background intensity while the cup is in the plasma. § S/N is the ratio of the peak height to the noise of the background. ¶ Detection limit determined from three times the noise of the background divided by the slope of the calibration graph. Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 731has shown that axial viewing DSI provides higher light levels than lateral viewing. In addition, axial viewing DSI usually provides better signal-to-background ratios than lateral viewing.This suggests that there are further advantages to be obtained by using axial viewing DSI with higher resolution spectrometers such as those now used commercially. The signals observed did not depend strongly on the height at which the plasma tail was cut o or the observation height but were dependent on the insertion depth, most notably with Zn. The background that was observed was not from the incandescence of the DSI cup but rather from the plasma.Constraining the analyte plume that emerges from the cup to a narrow region by using a boiler cap produces very small Fig. 14 Comparison of Ca emission with carrier gases flowing through the DSI cup. When argon is used the signal is increased but increases in intensity, suggesting that dispersion forces are vaporization is incomplete; the use of 1000 ppm Freon-12 in argon significant in the center of the plasma just above the DSI cup.saturates the system. The carrier gas flow rate through the cup was The use of hollow stem cups allows the introduction of gases 290 ml min-1. through the cup and forms a central channel. For some elements, the gas reduces analyte signal due to plasma cooling. For the refractory elements even argon increases the signal but signal is still lower than that obtained when no gas is used. tailing and incomplete vaporization remain problematic. When Fig. 15 shows that the analyte peak signal, both height and Freon-enriched argon is used, dramatic improvements in signal area, for Pb is slightly decreased when Freon-enriched argon quality are observed for refractory elements.Peak tailing is is used as the central gas. In general, lower peak heights and reduced or eliminated. Detection limits are always improved areas are observed for the volatile elements; however, Freon when compared with those obtained with argon alone, but are does significantly enhance the vaporization of refractory and not always better than those obtained when no gas is used.carbide-forming elements. The Freon-enriched argon that was used in this series of REFERENCES experiments was fixed at 1000 ppm Freon-12 in argon because it was purchased as a premixed gas. For this series of experi- 1 Shao, Y., and Horlick, G., Appl. Spectrosc., 1991, 45, 143. ments this level of Freon was sucient to promote the vaporiz- 2 Danielsson, A., ICP Inf.Newsl., 1978, 4, 147. ation. The optimum concentration of Freon was not 3 Ivaldi, J. C., and Tyson, J. F., Spectrochim. Acta, Part B, 1995, 50, 1207. determined and remains to be investigated. Concerns have 4 Fisons advertising brochure for the Maxim system. Document been raised as to the use of Freon because of its detrimental No. VGE/SM/PM/023, September 1993. eect on the atmosphere; however, in this application, the 5 Faires, L. M., Bieniewski, T. M., Apel, C. T., and Niemczyk, Freon is completely destroyed in the plasma and poses no T.M., Appl. Spectrosc., 1985, 39, 5. threat to the upper atmosphere. 6 Demers, D. R., Appl. Spectrosc., 1979, 33, 584. 7 Castillano, T. M., Vela, N. P., Caruso, J. A., and Story, W. C., J. Anal. At. Spectrom., 1992, 7, 807. CONCLUSIONS 8 Cliord, R. H., Montaser, A., Sinex, S. A., and Capar, S. G., Anal. Chem., 1989, 61, 2777. One of the primary advantages of axial viewing of the ICP is 9 Hall, G. E. M., Pelchat, J.-C., Boomer, D. W., and Powell, M., an increase in the optical throughput of the system. This allows J. Anal. At. Spectrom., 1988, 3, 791. 10 Karanassios, V., and Horlick, G., Spectrochim. Acta Rev., 1990, manufacturers to employ techniques which provide higher 13, 89. resolution (e.g., smaller slits, higher orders) without becoming 11 Karanassios, V., and Horlick, G., Spectrochim. Acta, Part B, 1989, quantum noise limited. Our work with a conventional system 44, 1361. 12 Liu, X. R., and Horlick, G., J. Anal. At. Spectrom., 1994, 9, 833. 13 Karanassios, V., Abdullah, M., and Horlick, G., Spectrochim. Acta, Part B, 1990, 45, 119. 14 Blain, L., and Salin, E. D., Spectrochim. Acta, Part B, 1992, 47, 399. 15 Fujimoto, K., Okano, T., and Matsumura, Y., Anal. Sci., 1991, 7, 549. 16 Kantor, T., Hanak-Juhai, E., and Pungor, E., Spectrochim. Acta, Part B, 1980, 35, 401. 17 Rattray, R., PhD Thesis, McGill University, 1995. 18 Fujimoto, K., Okano, T., and Matsumura, Y., Bunseki Kagaku, 1992, 41, 609. 19 Blain, L., Salin, E. D., and Boomer, D. W., J. Anal. At. Spectrom., 1989, 4, 721. 20 Le�ge`re, G., and Burgener, P., ICP Inf. Newsl., 1982, 13, 521. 21 Sing, R. L. 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Accepted April 2, 1997 732 Journal of Analytical Atomic Spectrometry, July 1997, Vol