The effects of a torch shield on performance of the vacuum interface ofan inductively coupled plasma mass spectrometer

Brett S. Duersch, James E. Patterson and Paul B. Farnsworth*

Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, USA

Received 19th November 1998, Accepted 3rd February 1999

The effects of a torch shield on the performance of a vacuum interface for an inductively coupled plasma massspectrometer (ICP-MS) have been studied by laser excited atomic and ionic fluorescence. A metallic shield wasinserted between the torch and load coil of the ICP, which was alternately grounded and floated. Relative ion andatom densities upstream from the sampling cone and downstream from the tip of the skimmer cone of the interfacewere determined by laser excited fluorescence. Doppler shifts in the excitation spectra were used to determine ionand atom velocities inside the skimmer cone. Pb atoms, Ba and Sc ions, and Ar metastable atoms were used as theprobe species. The shield had no measurable effect on the densities of the probe species upstream from the samplingcone. Inside the skimmer cone, the densities of all probe species dropped significantly when the shield was floated.No changes were observed in the velocities of the probe species as the shield was floated and grounded.

solutions were introduced into the plasma with an ultrasonicIntroductionnebulizer (U 5000, Cetac, Omaha, NE, USA). The aerosol

When ICP sources that had been developed for optical spec- was desolvated by a heater and condenser that were integraltroscopy were adapted for use as ion sources for plasma mass parts of the nebulizer. Pertinent operating parameters arespectrometry, many of them produced secondary discharges listed in Table 1. The three-turn load coil was grounded by abetween the plasma and the grounded first stage of the vacuum piece of copper braid that was soldered at one end to theinterface. Such discharges caused numerous deleterious effects, downstream turn of the coil and bolted at the other end toincluding rapid deterioration of the sampling and skimming the first stage sampling plate. During all experiments a com-cones, contamination of the plasma with sampler and skimmer mercial torch shield (Hewlett-Packard, Palo Alto, CA, USA)material, high photon noise and high ion energies with large was inserted between the load coil and the torch. A high-energy spreads.1–4 Secondary discharges arose because the voltage switch connected the shield to ground. The plasmagrounding configurations used for emission source ICPs give was ignited with the shield retracted toward the base of therise to a net positive potential in the plasma with respect to torch. With the plasma running, the shield was pushed intothe grounded sampling interface.4,5 The positive potentials are place, and if desired, grounded by closure of the high-voltagethe combined result of capacitive coupling between the load switch.coil and the plasma and different mobilities of the positive The decision to float the shield rather than retract it wasand negative charge carriers. based on early observations that there was little difference in

A number of different load coil configurations have been instrument performance between the two modes of disablingused to minimize the plasma potential and to eliminate the the shield. Because our home-built instrument provided nosecondary discharge. They include ‘reversed’ load coils,5,6 automatic mechanism for shield retraction, the option ofcenter tapped load coils,4 interleaved load coils,7 balanced breaking the ground connection was chosen for convenienceload coils and shielded load coils.8 The performance character- and safety.istics of the various load coil geometries have been widely A viewport in the first vacuum stage allowed visual obser-discussed, both in the scientific literature and in promotional vation of the region between the sampling and skimmingliterature by instrument manufacturers. The scientific charac- cones. Under the conditions given in Table 1, there was noterizations have focused on the ion kinetic energy distributions visible change in the first vacuum stage as the shield groundingproduced by different geometries or on the effect of geometry switch was opened and closed. An obvious secondary dischargeand plasma potential on overall instrument perform-ance.5,6,8–11 The development of optical probes that can meas- Table 1 Instrument operating conditionsure ion and atom densities and velocities in the second vacuum

Incident power 1.25 kWstage of an ICP-MS12–14 has given us a unique tool with whichOuter gas flow 14 l min−1to study the effect of load coil configuration on the perform-Intermediate gas flow 0.4 l min−1ance of a critical component of an ICP-MS, namely, theNebulizer gas flow 1.2 l min−1 avacuum interface. In this paper we report the characterization Sampling depth 10 mm

of a single load coil geometry, a reversed load coil, combined Sampler orifice diameter 1.0 mmwith a torch shield that was either grounded or allowed to float. Skimmer orifice diameter 0.70 mm

Sampler–skimmer separation 10 mmFirst stage pressure 1.1 TorrSecond stage pressure 13×10−3 TorrExperimentalaThe central gas flow was turned off for the argon fluorescenceThe ICP-MS that was used in these experiments has been measurements.

described previously,15 as have the optical probes.12,13 Sample

J. Anal. At. Spectrom., 1999, 14, 615–619 615

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

ded

on 2

2/10

/201

4 17

:40:

09.

View Article Online / Journal Homepage / Table of Contents for this issue

was present only when the switch was opened at extremely cence signals from the two atom reservoirs were recordedsimultaneously on two boxcar averagers. The hollow cathodehigh central channel flow rates. The evidence for a secondary

discharge was a marked brightening between the sampler and lamps were operated at their maximum rated dc current level,then switched off for 100 ms 20–30 ms before the laser wasskimmer cones. There were no obvious changes in the plasma

outside the vacuum system. It is also important to note that fired. When the lamps were switched off the backgroundemission dropped much more rapidly than the atom density,in the vast majority of the experiments there were no measur-

able changes in the pressures of the first and second vacuum and clean excitation spectra were obtained for all threeelements.stages coincident with switching of the ground connection. On

two occasions, a slight increase (0.02 Torr) was noted in the The apparatus was modified for the measurement of Armetastable atom fluorescence, as shown in Fig. 2. In this casefirst stage pressure as the grounding switch was opened, but

in both of these cases, irregularities in other parts of the the diode laser was CW instead of pulsed. The boxcar averagerswere replaced by lock-in amplifiers, and an optogalvanicapparatus were noted and the data were not used.

Sc, Ba and Pb were used as test analytes at concentrations (OGE) signal from the argon-filled hollow cathode lampserved as a stationary reference.of 200, 50 and 100 mg l−1 , respectively. Pb and Ba solutions

were prepared from the nitrate salts, and the Sc solution from Reference density measurements made in the ICP upstreamfrom the sampling orifice were also essential to confirm thatSc2O3 dissolved in acid. Sc and Ba were detected as singly

charged ions, lead as a neutral atom. Argon metastable atoms changes measured in the second vacuum stage did not simplyreflect changes in the plasma itself. Such reference measure-(4s[3/2] J=2) were detected as representative of the support

gas. All four species were chosen because they have efficient ments were made with an external fluorescence probe that hasbeen described in ref. 14. It recorded relative densities in thenon-resonant fluorescence schemes with large differences

between the excitation and fluorescence wavelengths. The plasma from a volume of approximately 1 mm3 located justupstream of the sampling orifice.excitation and detection wavelengths are listed in Table 2.

It was essential in these experiments that we had the abilityto record both relative density and velocity data for the probe Results and discussionspecies. Velocity information was extracted from Dopplershifts in the excitation wavelengths listed in Table 2. To With the plasma operating conditions set as indicated in

Table 1, we recorded fluorescence intensities upstream fromeliminate the effects of drift in the wavelength readouts of thelasers used in the experiments, we simultaneously recorded the the tip of the sampling cone and inside the tip of the skimmer

cone with the shield switch open and closed. The results ofexcitation spectra of the selected species in a stationary atomreservoir (a hollow cathode lamp) and in the 2 mm downstream these experiments are summarized in Table 3. The ratios in

Table 3 were recorded at the peak excitation wavelengths forfrom the tip of the skimmer cone, a region defined by theoverlap of the excitation and emission optical paths.14 Two each probe species, which were different inside and outside the

vacuum chamber. The drops in the intensities of the ionicdifferent experimental configurations were used: one for theanalytes and the other for the argon metastable atoms. The fluorescence inside the vacuum chamber were expected. Based

on reported differences in ion energies between shielded andconfiguration used for the analytes is shown in Fig. 1. Thepulsed dye laser was scanned in 1 pm increments and fluores- unshielded coils, an increase in ion velocities in the ungrounded

configuration that would shift the absorption resonance out-

Table 2 Laser fluorescence wavelengths

Excitation FluorescenceSpecies wavelength/nm wavelength/nm

Sc II 358.094 432.501Ba II 455.404 614.172Pb I 283.306 405.783Ar I 801.479 842.465

Fig. 2 Experimental block diagram for measurements of Ar metastablefluorescence.

Table 3 Ratio of fluorescence intensities (shield floating/shieldgrounded). The uncertainties are short-term standard deviations ofthe ratios

In plasma, Downstreamupstream from tip from tip of

Species of sampling cone skimmer cone

Sc II 0.99±0.09 0.53±0.03Ba II 1.0±0.1 0.45±0.03Pb I 1.0±0.1 0.76±0.05

Fig. 1 Experimental block diagram for measurements of Sc, Ba, and Ar I 0.480±0.003Pb fluorescence.

616 J. Anal. At. Spectrom., 1999, 14, 615–619

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

ded

on 2

2/10

/201

4 17

:40:

09.

View Article Online

side the laser bandwidth was expected. The drops in the argonand lead fluorescence intensities were surprising. Neutralspecies should be unaffected by plasma potential.

Two additional sets of experiments were conducted in aneffort to understand the nature of the changes induced bygrounding or floating the torch shield. In the first set, fluores-cence excitation scans of ions inside the tip of the skimmercone and in hollow cathode lamps were recorded simul-taneously. The results of these experiments are presented inFig. 3–6. Perhaps the most important feature of these fourfigures is that grounding or floating the torch shield onlyaffects the magnitude of the fluorescence signals. The nor-malized wavelength profiles produced by the excitation scans

Fig. 5 Excitation scans for Pb atom. Top, raw data; bottom, nor-malized scans. &, Hollow cathode lamp; +, shield grounded;*, shield floating.

Fig. 3 Excitation scans for Sc ion. Top, raw data; bottom, normalizedscans. &, Hollow cathode lamp; +, shield grounded; *,shield floating.

Fig. 6 Excitation scans for Ar metastable atoms. Top, raw data;bottom, normalized scans. ··-··, Hollow cathode OGE signal; ——,shield grounded; - - - -, shield floating.

under the two conditions are identical within experimentalerror.

A second set of experiments was performed to ensure thatthe results of the excitation scans were not unique to a singleset of plasma conditions. For the three probe metals, the shieldwas alternately grounded and ungrounded as the central flowrate was raised in small increments. The results are shown inFig. 7. Clearly, the behavior observed at a central flow rate of1.2 l min−1 is not unique. Consistent with reports by Rosset al.,6 the optimum flow rates for the grounded andungrounded conditions are different. However, the sensitivitiesmeasured by fluorescence are worse under all flow conditionsfor the ungrounded shield than they are for the groundedFig. 4 Excitation scans for Ba ion. Top, raw data; bottom, normalizedshield. The differences between the Pb atom traces and thosescans. &, Hollow cathode lamp; +, shield grounded; *,

shield floating. for the Sc and Ba ions are expected. The ion curves are typical

J. Anal. At. Spectrom., 1999, 14, 615–619 617

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

ded

on 2

2/10

/201

4 17

:40:

09.

View Article Online

excitation spectrum is almost certainly produced by atomsthat are thermalized in a shock structure or other flowdisturbance at the tip of the skimmer cone. A detailed studyof the argon spectra is reported in ref. 13.

The optically-measured velocities reported here are consist-ent with those reported by Fulford and Douglas,10 but difficultto reconcile with those measured by other workers.8,9,11 Themost direct comparison can be made with the study by Gray,8in which shielded and unshielded coils were compared.Although in our study the comparison is between a groundedshield and a floating shield, the qualitative differences weobserved between the two conditions should be similar tothose reported by Gray. The floating shield changes thecapacitance between the load coil and the plasma but doesnot eliminate it.

In Gray’s study, significant decreases in the energy of cobaltion were reported as an unshielded coil was replaced by ashielded coil. The changes were measured over a broad rangeof central channel flow rates. We see no such changes. Theinterpretation of Doppler shifts in a fluorescence excitationspectrum is very straightforward. This suggests that the dis-crepancies between the two studies must be due to subtledifferences between the interfaces, residual shielding by thefloating shield, changes in velocities arising downstream fromthe skimmer cone, or artifacts in the ion stoppingFig. 7 Effect of torch shield as a function of central gas flow rate. +,measurements.Shield grounded; *, shield floating. Top graph, Sc ion; middle

Interpretation of the decreases in the magnitudes of thegraph, Ba ion; bottom graph, Pb atom.fluorescence signals is not straightforward. If the fluorescencesignals were representative of the total flux of material through

of those recorded for mass spectrometric detection.16 The the interface, then the opening of the grounding switch wouldatom curves are typical of a species favored by low tempera- of necessity be accompanied by a large drop in vacuumtures. At intermediate flow rates the lead ion dominates and pressure in the first and second vacuum stages. Clearly, thethe atom signal is low. At high flow rates, cooler gas is pushed drops in fluorescence signal do not signify a proportional droptoward the sampling cone, and the fraction of the lead that is in total flux through the interface. Two alternative explanationsionized decreases. At low flow rates, some lead atoms appear could account for the lack of a pressure change accompanyingas a result of ion–electron recombination. the large drops in fluorescence intensity. One is that the species

In Fig. 3–6 the magnitudes of the Doppler shifts are prob- that accounts for the bulk of the flow through the interface,ably the most significant feature. Those shifts, with the corre- ground state neutral argon, is not affected by the shieldsponding velocities of the atoms or ions, are summarized in changes in the same manner as the minority species measuredTable 4. In the Doppler shift data there is no evidence of any by the fluorescence experiments. The second is that the changesacceleration of the ions by electric fields inside or outside the in the fluorescence signals reflect not a change in the totalinterface. Rather the data are consistent with a free expansion density of the respective species, but rather a shift to otherof the plasma with no other accelerating forces. The most states. Of the two possibilities, the former seems the mostprobable velocities of neutral lead and barium and scandium plausible. In either case, our experiments do not reveal theions are the same within experimental error, with no clear source of the changes in fluorescence intensity. We plan furtherdependence on mass. The most probable velocity for Ar, which experiments in the first vacuum stage that we expect to provideis the most precise of the measured values, corresponds to a more insight into the changes in interface performance thatgas kinetic temperature in the plasma of 7500 K.10 This is occur with the grounding and floating of the torch shield. Thewithin the range of values expected for a 27 MHz ICP at experiments reported here certainly do not answer all remain-1.25 kW.17 Because the argon measurements were made with ing questions about the effects of load coil configuration andthe central gas flow turned off, the temperature and hence the plasma potential on vacuum interface performance, but ratherterminal expansion velocity should be slightly higher than for should serve to provide impetus and direction for further study.the other probe species.

In addition to the argon population with a mean velocityAcknowledgementof 2800 m s−1, there is a significant population with a mean

velocity near zero. The low velocity peak in the argon This work was supported by the National Science Foundation,Grant no. CHE-9415384.

Table 4 Doppler shifts and ion/atom velocities. Uncertainties for Sc,Ba, and Pb are based on a wavelength uncertainty of ±0.5 steps of

Referencesthe laser wavelength drive. The uncertainty for Ar is based on anestimated uncertainty in peak location of ±5 steps in the diode

1 H. Niu and R. S. Houk, Spectrochim. Acta, Part B, 1996, 51, 779.laser scan2 R. S. Houk, V. A. Fassel, G. D. Flesh, H. J. Svec, A. L. Gray and

C. E. Taylor, Anal. Chem., 1980, 52, 2283.Stationary3 D. J. Douglas, Can. J. Spectrosc., 1988, 34, 38.Species wavelength/nm Doppler shift/pm Velocity/m s−14 D. J. Douglas and J. B. French, Spectrochim. Acta, Part B, 1986,

41, 197.Sc 358.094 3 2600±4005 A. L. Gray, R. S. Houk and J. G. Williams, J. Anal. At. Spectrom.,Ba 455.404 4 2700±300

1987, 2, 13.Pb 283.305 2 2200±5006 B. S. Ross, P. Yang, D. M. Chambers and G. M. Hieftje,Ar 801.479 7 2800±40

Spectrochim. Acta, Part B, 1991, 46, 1667.

618 J. Anal. At. Spectrom., 1999, 14, 615–619

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

ded

on 2

2/10

/201

4 17

:40:

09.

View Article Online

7 I. L. Turner, U. S. Patent 5,194,731, March 16, 1993. 14 B. S. Duersch and P. B. Farnsworth, Spectrochim. Acta, in thepress.8 A. L. Gray, J. Anal. At. Spectrom., 1986, 1, 247.

15 Y. Chen and P. B. Farnsworth, Spectrochim. Acta, Part B, 1997,9 J. A. Olivares and R. S. Houk, Appl. Spectrosc., 1985, 39, 1070.52, 231.10 J. E. Fulford and D. J. Douglas, Appl. Spectrosc., 1986, 40, 971.

16 G. Horlick, C. A. Rose, S. H. Tan and M. A. Vaughan,11 N. Jakubowski, B. J. Raeymaekers, J. A. C. Broekaert and D.Spectrochim. Acta, Part B, 1985, 40, 1555.Stuewer, Spectrochim. Acta, Part B, 1989, 44, 219.

17 M. Huang, S. A. Lehn, E. J. Andrews and G. M. Hieftje,12 B. S. Duersch, Y. Chen, A. Ciocan and P. B. Farnsworth,Spectrochim. Acta, Part B, 1997, 52, 1173.Spectrochim. Acta, Part B, 1998, 53, 569.

13 J. E. Patterson, B. S. Duersch and P. B. Farnsworth, Spectrochim.Acta, in the press. Paper 8/09063G

J. Anal. At. Spectrom., 1999, 14, 615–619 619

Publ

ishe

d on

01

Janu

ary

1999

. Dow

nloa

ded

on 2

2/10

/201

4 17

:40:

09.

View Article Online