Instrumental improvement of the XYZ-translation system for a Perkin-Elmer Sciex Elan 500 ICP-MS

Spectroch~m~caActa, Vol 48B. No 3, pp 475483. Rmted

1993 0

m Great Bntam

TECHNICAL

Instrumental

improvement

0584-8547/93 s6 @.I + 00 1993 Pergamon Press Ltd

NOTE

of the XYZ-translation system Elan 500 ICP-MS

for a Perkin-Elmer

Sciex

(Received 24 June 1992; accepted 29 September 1992)

1. INTRODUCTION IN ORDER to optimize instrument performance, most commercial inductively coupled plasma mass spectrometry (ICP-MS) instruments are equipped with a system to position the torch relative to the sampler/skimmer configuration. In a Perkm-Elmer Sciex Elan 500, this system is called the XYZ-translation system. By using this system, the central channel of the plasma is positioned relative to the sampler/skimmer configuration in such a way that the sensitivity for singly charged ions is maximized while the response for both the doubly charged ions and the oxides is minimized. The ions usually used for optimization are Rh+, Ba+ and Ba*+, and Ce+ and CeO+ [l]. The optimization in the X- and Y-direction is always very straightforward. The three types of ions mentioned above exhibtt practically the same optimum in both the Xand Y-direction. However, in the Z-duection, optimization is somewhat more complicated. In general, the sensitivity for the three types of ions peaks when the torch is close to the When the distance between torch and sampler/skimmer sampler/skimmer configuration. configuration is increased, the sensitivities are decreased. The optimum Z-value is reached when the ratios Ba*+/Ba+ and CeO+/Ce’ are minimized. At this position, sufficient singly charged ion sensitivity is still guaranteed. The Perkm-Elmer Sciex Elan 500 is equipped with an XYZ-translation system by which the X- and Y-position are changed by manually tummg two knobs that are coupled to two screws that move the complete matching network box, on top of which the torch is mounted. In the Z-direction, the matching network is displaced along a gutdance system and is held in position by clockwise turning a knob connected to a locking-mechanism. Because of the absence of a display with which the position of the torch can be monitored, it was felt that optimization was difficult to perform. It was decided to modify the system by changing the manual controls using an electronic control device. At that time, Spectrotec GmbH (Trebur, F.R.G.) offered an electronic system for the X- and Y-direction of an Elan 500 ICP-MS, but unfortunately the Z-direction was excluded. JAMESW. MCLARENof the National Research Council of Canada has also installed a completely different mechanical torch alignment system in his Perkin-Elmer Sciex Elan 250 [2].

2. GENERAL DESCRIFTION A block diagram of the lay-out of the XYZ-translation system is presented in Fig. 1. For each direction, a small d.c. servo-motor is used to displace the torch box. The X- and Y-axis servo-motors were coupled to the original flexible axis and screws. The guidance system of the Z-direction was also maintained, but the Z-servo-motor was coupled to a newly installed ballbearing spindle. On every d.c. servo-motor the displacement is measured by means of a ten turns potmeter, coupled to the axis of the motor. The displacement is transmitted by means of two home-made gearwheels. The ten turns potmeter is integrated into a Wheatstone bridge that drives--by means of an amplifier-the d.c. servo-motor until the bridge is in equilibrium. The potmeter read-out is displayed on a digital voltmeter. By means of an adjustable potmeter mounted on the front panel of the control module and also integrated into the Wheatstone bridge, the equilibrium of the bridge can be disturbed. Closing the switch between the amplifier and the servo-motor will bring the bridge back into equilibrium again. In this way, the system can be operated in two different ways. By keeping this “normal open” switch closed, the servomotor immediately reacts on the disturbance of the equilibrium of the Wheatstone bridge. In this way the torch box can be displaced continuously. With the switch opened, first the required 475

476

Technical note

0 DVM.

*wvo

motor

v

FIN 1 Block diagram of the lay-out of the XYZ-translation system. value can be adjusted on the potmeter of the control module. Closing the contact between amplifier and servo-motor ~111 bring the bridge into equilibrium again. In this way the torch box can be displaced stepwise. An overview of the complete system is presented in Fig. 2.

3. ELECTRONIC DESCRIPTION The electronic diagram of the power supply of the XYZ-translation system is presented in Fig. 3. Two separate BY164 rectifiers are used, the first to provide the unstable 18 V used as power for the d.c. servo-motors, the second to provide-by means of voltage regulators-the stabilized 15 V used as power for the Wheatstone bridge and the operational amplifiers. The 5 V and 1 A power for the digital voltmeter IS provided by a separate power supply. In Fig. 4, the electronic diagram of only one axis is given. The diagrams for the X-, Y- and Z-axis only differ in the resistance values of RI-Rd. Both the servo potmeter and the front pane1

2 Rg. 2. An overvlew of the system components of the XYZ-translation system.

477

Technical note

Fig. 3. Electromc diagram of the power supply of the XYZ-translation system

+15v I

RI = 12K

+lBV unstab -ISV

RI -12K

Fig. 4. Electronic diagram of the X-, Y- or Z-axis.

478

Technical note

Fig. 5. Overview of the electronic drive of the Z-axis.

potmeter have 1 kR resistance and are ten turn potmeters. The front panel potmeters are provided with a built-in mechanical display. The total distance for every axis to be displaced was mainly adjusted by means of an appropriate choice of the RI-R4 resistance values. By means of the 2K2 extension, the total distance can be fine-tuned. The mechanical zero of every axis is fixed by means of a 20 K adjustable resistance. By means of a second 20 K adjustable resistance, the electronic zero of the digital voltmeter was fixed. Finally, a 20 K span is used to match the digital voltmeter display with the arbitrary value displayed on the front panel potmeter. The axis to be displayed on the digital voltmeter is selected by means of a threeposition switch. The d.c. servo-motors used are supplied by RS Components Ltd (Corby, Northants, U.K.). For the X- and Y-axis, a d.c. servo-motor of 40 rpm was used (stock no. 330799). To ensure a rather rapid displacement of the torch box in case of maintenance on sampler and skimmer, a 130 rpm motor was used for the Z-axis (stock no. 330-777). To avoid any high frequency interference, a Ferrite Bead (stock no. 238-283) was mounted in the transfer line between knitter and servo-motor.

4. MECHANICAL DESCRIFT~ON The total distance the torch box can be displaced along the X- and Y-axa equals 12 mm and one digit on the digital voltmeter equals a displacement of 0.0127 mm. For the Z-direction, these values equal, respectively, 35 and 0.0389 mm. As mentioned before, the displacement of the d.c. servo-motor is transmitted to the servo potmeter by means of two home-made gearwheels (see Fig. 2). The gearwheel mounted on the servo-motor axis is made of brass and has 12 teeth. The gearwheel of the potmeter is made of NOVOTEXR (Poelman, Hutzen, The Netherlands) and has 36 teeth. Both gearwheels have so-called involute teeth (module l), which is especially used when a relatively large weight has to be displaced. The combination of NOVOTEXR and brass is chosen to ensure a silent and a vibrational insensitive mechanical transmission. As can be seen in Fig. 5, a ball-bearing spindle was installed underneath the matching network box. The spindle used has an outer diameter of 10 mm, a pitch of 2 mm and was supplied by SKF (SKF-Multitec B.V., Amsterdam, The Netherlands). Finally, the clearance of the translation system in the X-, Y- and Z-direction equals, respectrvely, 0.06, 0.12 and 0.20 mm and is overridden by always operating every axis from the same side. NOVOTEX is a registered trademark of AEG Isolier und Kunststoffe GmbH, Kassel, F.R.G.

479

Technlcal note

1

x-axis

(4 06

n

06

“JLU

Dmplacement, m mm 1

-r

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-1

0

1

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3

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y-axis

04 06

0.6

04

02

0

7

-3

-2

Displacement, m mm

Fig. 6. Contmued.

5. ANALYTICAL PERFORMANCE The analytical performance of the XYZ-translation system is illustrated in Figs 6 and 7, where cross-sections of the plasma are displayed. In Fig. 6(a)-(c), the analytical response for ‘Li, 45Sc, lo3Rh, 175Lu and =*U is displayed in the X-, Y- and Z-direction, respectively. As mentioned before, optimization in the X- and Y-direction is very straightforward to perform. According to Fig. 6(c), in the first instance this also holds for the Z-direction. A more comprehensive example of Z-axis optimization is given in Fig. 7. A solution containing 100 ppb of Ce and Ba was continuously aspirated while the torch was displaced in the Z-direction starting at the largest (analytical) distance between torch and sampler. The intensities for ‘We+, 13*Ba+, “‘TeO+ and 13*Ba2+ (at mass 69) were continuously monitored, while WJeO+ was expressed in

Techmcal note

0.0

06

04

0.2

0 12

10

8

6

4

2

0

Ftg. 6 Cross-sections of the argon plasma wtth respect to the ion count rate of ‘LI, ?jc, ro3Rh, 175Lu and *W m the (a) X-direction, (b) Y-directton and (c) Z-dtrectton. In (a) and (b), the zero corresponds to the centre of the axial channel. In (c), the zero corresponds to the position at which the sampler tip barely mtsses the top of the torch as the mass spectrometer interface IS moved down mto the sampling posttton.

Table 1. Analytical figures of ment correspondmg to the optimum Zaxrs value for different nebulier argon flow rates Nebulizer argon flow rate (l/mm) Optunum Z-axis value* Sensitivtty for r3sBa+ (cps+pm) Sensitivity for 140Ce+ (cpdppm) Ratio Ba2+IBa+ (%) Ratio CeO’iCe’ (%)

1.5 3.0 mm

16 44mm

1.7 6.6 mm

2 00 x 106

3.03 x 106

3.64 x 106

2 79 x 1P 2.2 2.1

3.93 x 106 19 22

4 62 x lo6 1.9 2.5

*The Z-axts value is defined as the distance m mm between the sampler ttp and the top of the outer tube of the torch. The 0 mm value corresponds to the position at which the sampler tip barely misses the top of the torch as the mass spectrometer interface IS moved down into the samplmg positton.

to “‘OCe”, and 138Ba2+ in proportion to 13sBa+. As can be seen in Fig. 7, the maximum M+ sensitivity does not correspond to the minimum levels of M2’ and MO+. The optimum Z-value is chosen with respect to the minims levels of both w’ and MO+ while maintaining sufficient M+ sensitivity. For instance, in the case of 1.6 l/mm nebulizer flow rate, the compromise Z-value is 4.4 mm involving a loss in sensitivity of 41% for Ce’ and 45% for Ba+. Figure 7 also shows that the optimum Z-value strongly depends on the nebulizer argon flow rate used. In Table 1, the analytical figures of merit corresponding to the optimum Zvalue for every nebuhzer argon flow rate are presented. It is clear that in order to keep the levels of oxides and doubly charged species as low as possible, and maintaining sufficient single charged sensitivity, it is very important to have a reproducible Z-axis positioning, especially when the nebulizer argon flow rate is changed. proportion

481

Technical note

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100

I

1.5 Vmin.

(4

60

06

\ c

12

10

6

6

4

2

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0

Dasplacement, I" mm

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@I

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1.6 Vmin. 60

60

06

I

04

02

0

12

10

6 Dwplacemmt,

6

4

2

0

mmm

Fig. 7. Contmued.

6. BUILDING-IN THE ICP-MS

INSTRUMENT

During approximately six months, the XYZ-translation system has been used with the control module placed on top of the instrument. When the Elan 500 was upgraded by Perkin-Elmer with the new computer system, the space that became available after the old hard disk and computer were removed was used to build in the XYZ control module. The Bayard Alpert gauge control module and the Edwards type 1500 pressure display [3] were also incorporated into the front panel (see Fig. 8).

Technical note

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(c)

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1.7 Vmin.

100

I

ceovce 80

60

aa+*I aa+ 12

10

8 Dwplacement,

6

4

2

0

I” mm

Fig. 7. CrosssectIons of the argon plasma with respect to the Ion count rate of 14”Ce+ and 138Ba+ (left ordmates) and the ion count rate ratios of 156Ce0+/i40Ce+ and 138Ba*+/*38Ba+ (right ordmates) m the Z-dIrectIon, for dlfferent nebuhzer gas flows of respectively (a) 1.5 l/mm, (b) 1.6 l/mm and (c) 1 7 Umin

Fig. 8. Overvlew of the newly constructed Elan 500 front panel.

7. CONCLUSION Modification

of the XYZ-translation

system

for the Perkin-Elmer

Sciex Elan 500 ICP-MS

is

easily done, providing a more convenient and reproducible way to position the torch box and thus to choose optimum analysis conditions in terms of maximizmg the ion count rate and minimizing the influence of oxide- and doubly charged ions.

483

Technical note 8. SUMMARY

A Perk&Elmer Sciex Elan 500 ICP-MS is equipped with a translation system with which the torch box can be positioned manually by means of three knobs. In the course of our work, it was felt that this positioning was difficult to perform and hardly reproducible. It was decided to modify the system by changing the manual controls using an electronic control device. Three d.c. servo-motors were built into the Elan 500, driving the torch box in the X-, Yand Z-directions. For each axis, the displacement is measured and displayed on a digital voltmeter. The analytical performance of the XYZ-translation system is illustrated by means of maximizing ion count rate while minimizing the influence of oxide- and doubly charged ions. Finally, the XYZ-translation system was completely incorporated into the Elan 500. HUUB KLINKENBERG

DSM Research Department FA-AE P.O. Box 18 NL-6160 MD Geleen The Netherlands

TONBEEREN VAN BORM

WERNER

BERTMEVISSEN CEESVANDONGEN

DSM Research Department SE-TO P.O. Box 18 NL-6160 MD Geleen The Netherlands

Acknowledgements-The authors are grateful to HARRY CAUBO, Jo HENDRIXand KAREL FLACH for their assistance m building the XYZ-translation system and modifying the Elan 500 ICP-MS.

REFERENCES [l] Perkin-Elmer, Elan 500 ICP-MS course, Uberlingen, F.R.G. (1988). [2] D. Beauchemin, J. W. McLaren and S. S. Berman, J. Anal. At. Spectrom. 3, 775 (1988). [3] H. Khnkenberg, Zicheng Peng, T. Beeren and K. Flach, Spectrochim. Acta 47B, 585 (1992).