JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1993, VOL. 8 42 7

Low Pressure Inductively Coupled Plasma Source for Mass Spectrometry*

E. Hywel Evans? and Joseph A. Caruso University of Cincinnati, Department of Chemistry, Cincinnati, OH 45221, USA

The generation of a low pressure Ar inductively coupled plasma has been successfully performed without any modification to the torch box of a commercial inductively coupled plasma mass spectrometer. A water-cooled, low pressure torch and interface, designed to allow sampling for mass spectrometry, are described. Using gas chromatography sample introduction, it was possible to measure the analytical signal for a sample of 1 -bromononane. The background spectrum contained many of the polyatomic ions seen for atomospheric plasmas, thought to be due to small leaks in the vacuum seals. Keywords: Low-pressure plasma; inductively coupled plasma mass spectrometry; gas chromatography

The pioneering work on inductively coupled plasma mass spectrometry (ICP-MS) was performed by Houk et a1.l and Date and G r a ~ . ~ - ~ Later, Douglas and c o - ~ o r k e r s ~ - ~ investi- gated microwave induced plasma mass spectrometry (MIP- MS). Subsequently the MIP source has received some attention in conjunction with MS8 and has been used for gas chromatography (GC) d e t e c t i ~ n , ~ - l ~ pneumatic nebulisa- tion13-19 and electrothermal vaporization (ETV)20-22 sample introduction. The advantage of the MIP is the ease with which plasmas can be formed in a number of gases, particularly He, resulting in a mass spectrum which is less prone to Ar-containing polyatomic ions.

An alternative to the generation of plasmas at atmo- spheric pressure is to generate them at low pressure. In this way air can be excluded, the gas flows much reduced, considerably lower power used to sustain the plasma, and ICPs can be generated using He, 02, N2 and other gases with much greater ease. Low pressure ICPs have been investi- gated for emission s p e c t r ~ m e t r y , ~ ~ J ~ and low pressure MIPS for MS.25-28 The application of a low pressure plasma in MS is particularly attractive since the exclusion of air and consequent reduction in interferences on P and S, would make it a highly sensitive and selective GC detector for these elements, and indeed this has been investigated using a low pressure IVIIP.~~

This paper describes preliminary results obtained using a low pressure Ar ICP, which has been designed so that very little modification to an existing ICP-MS instrument is required for operation.

Experimen tall Mass Spectrometer All experiments were performed using an ICP-mass spectrs- meter (VG PlasmaQuad, VG Elemental, Winsford, Chesh- ire UK). The pumping capacity of the expansion stage was increased from approximately 400 to 1900 1 min-', by addition of a second port, on the opposite side of the interface to the existing expansion port, and linked to a 1500 1 min-l rotary pump (Edwards E1M-80, Edwards High Vacuum, Crawley, Sussex, UK). The extraction lens voltage supply was replaced with an alternate supply (Keithley 247 High Voltage Supply, Keithley Instruments, Taunton, MA, USA), variable between + 999 and - 999 V. The ion lenses were tuned by monitoring A r k + at mlz 80, and optimizing them for maximum signal. No problems

*Presented at the Eighteenth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Anaheim, CA, USA, October 6-1 I , 1991.

?Present address: University of Plymouth, Department of Environmental Sciences, Drake Circus, Plymouth, UK PL4 8AA.

were encountered when tuning the lenses and voltages were similar to values found for atmospheric pressure plasmas, with the exception of the extraction lens. The extraction lens was an exception because no distinct optimum could be found for the dry, atmospheric pressure plasma, with a more negative voltage resulting in greater signal, but also greater continuum background. A compromise setting of - 190 V was used. This phenomenon was typical for the instrument used in this study.

For the low pressure Ar plasma, an optimum could be typically found at - 150 V and for the low pressure He plasma, at between -80 and -95 V. No continuum background was observed with these plasmas.

Data acquisition parameters for the full mass range scan were: mass range, 4-90 mlz; dwell time, 160 ps; sweeps, 100; and channels, 4096. Data acquisition parameters for the chromatographic temporal scan were: mass, 79 mlz; and dwell time, 500000 ps .

Low Pressure ICP Interface A low pressure ICP interface was constructed as shown in Fig. 1. The interface was similar to that described by Creed and ~ o - w o r k e r s . ~ ~ ~ ~ ~ who studied low pressure microwave plasma mass spectrometry, except that in this work the sampler was designed to accept a modified ICP torch. The sampling cone was fabricated from aluminium (University of Cincinnati, Department of Chemistry Machine Shop) such that a 0.75 in ultra-Torr fitting was incorporated into it (Fig. 1).

Ultra-Torr fitting Coolant out 4 Outer

Heated transfer

gas Low Diessure I t !

I Make-up gas

I 1 Coolant in sampler

Liquid cooled 7r- ICP torch

E?%Q Fig. 1 and the gas chromatograph coupled to the low pressure interface

Schematic diagram of the low pressure sampler and torch

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428 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1993, VOL. 8

Table 1 Dimensions of the low pressure sampler and ICP torch

Torch- Water jacket 0.d. (mm) 19 Water jacket i.d. (mm) 16.5 Intermediate tube o.d./mm 16 Spacing between intermediate

and outer tubeslmm x0.5 Injector tip i.d./mm 1 .o

Sa mpier- Ultra-Torr i.d./in Sampling orifice i.d./mm Sampling orifice depthlmm

3 4 1 .O 2.0

Table 2 Operating conditions for GC

Injector temperaturePC 250 Start temperature/"C 110 Final temperature/"C 200 Heating ratePC min-l 32 Transfer line temperature/"C 270 He column gas flow/ml min-' 3.0 Solvent venting time/s 75

A water-cooled ICP torch was fabricated (Precision Glassblowing of Colorado, Englewood, CO, USA), such that it could be fitted into the low pressure sampler, and a vacuum seal made using the ultra-Torr fitting (Fig. 1). The external diameter of the torch was similar to that of a standard ICP torch, so that the existing load coil and torch box on the ICP-MS instrument could be used. However, it was necessary to adjust the distance between the load coil and the sampler to 30 mm to accommodate the ultra-Torr fitting. The dimensions of the low pressure sampler and torch are listed in Table 1. The cooling fluid around the torch was maintained at 10 "C.

Gas Chromatograph The gas chromatograph consisted of an HP 5700 oven with a UNI K- 10 direct splitless injector. The column was a DB- 5 (40 mx0.25 mm i.d.) with a film thickness of 25 pm (J&W Scientific, Austin, TX, USA). A six-way valve (Valco, Houston, TX, USA) was incorporated to allow venting of the solvent. The transfer line between the gas chromato- graph and the ICP torch consisted of 1 m x & in i.d. stainless-steel tube wrapped with heating tape, insulated with fibre-glass tape and maintained at a temperature of approximately 270 "C. The transfer line was connected to the rear of the torch by means of an ultra-Torr fitting (Fig. 1) and a stainless-steel T connector. Make-up gas was introduced through the stainless-steel T connector. Operat- ing conditions for GC are shown in Table 2.

Reagents A stock solution of 1-bromononane, 1035 pg g-l (Halogen Chemical, Columbia, SC, USA) was prepared in hexane and subsequently diluted in methanol (HPLC grade, Fisher Scientific, Cincinnati, OH, USA). Liquid Ar (Wright Broth- ers, Cincinnati, OH, USA) and 99.999% purity He (American Air Liquide, Countryside, IL, USA) were used throughout.

Results and Discussion Optimization For the low-pressure Ar plasma there was no necessity to use the auxiliary gas flow and a stable plasma could be maintained using a coolant gas flow of between 0.3 and 0.8

12

10

a

6

4

2 2 3

10 -

5 -

200 300 400 500 600 100 inner gas flow/mi min-'

Fig. 2 Effect of: (a) outer gas flow rate; and (b) inner gas flow rate at forward powers of A and C, 350 and B, 450 W, using an Ar low pressure plasma, and expansion stage pumping rates of A and B, 1900 and C, 400 1 min-* for 25 ng of Br injected as 1.-bromononane. Signal monitored at mlz 79

1500

rn

3 0

>

C Q)

+

P *z 1000 c

w .- - (D

0, iTj 500

0 50 100 150

Time/s

Fig. 3 Signal for 25 ng of Br as 1-brornononane, monitored at m/z 79

1 min-', and an inner gas flow of 0.15 1 min-l at a power of CIS0 W. Under these conditions the plasma still resembled am atmospheric pressure plasma, with a clearly discernable torus-like structure ( i . ~ . , the inner gas flow 'punched' the plasma).

An optimization was performed by taking 2pl injections of 12.5 pug g-l of Br (25 ng of Br absolute) as a solution of 1 -bromononane in methanol, monitoring mlz 79 and integrating the resultant peak.

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1993, VOL. 8 429

lx108

iX1o7

1x106

iX1o5

1x10~

1x10~

1x102

E 10

Y Y > 1

3 0

- .- 0

; 1x108

.S iX107

.- - (II

v)

1x106

1x10~

iX1o4

iX1o3

1x102

10

1

40Ar+ (skipped)

/

I

( b ) 14 16 + "Ar' r 30;i+ O (Skipped)

0 5 10 15 20 25 30 35 40 m/z

Fig. 4 Background spectra between mlz 4 and 40 for: ( a ) Ar atmospheric pressure plasma; and (6) Ar low pressure plasma

The results of the optimization for the outer gas are shown in Fig. 2(a). As can be seen the outer gas flow had a considerable effect on peak area signal, the best signal being achieved at the lowest gas flow of 0.3 1 min-l, at both 350 and 450 W power. Similarly, a lower inner gas flow rate was

preferable [Fig. 2(b)], though in this case an optimum was observed at 0.25 1 min-', for both 350 and 450 W forward power. Presumably the higher gas flows caused a decrease in the plasma temperature, though this was not offset by increasing the power as would be expected. A much greater

Table 3 Operating conditions for atmospheric and low pressure plasmas

Parameter Ar atmospheric Ar low pressure He low pressure Plasma-

Forward power/W Reflected powerlW Inner gad1 min-l Intermediate gad1 min-' Outer gad1 min-I Sampling depth*/mm

1350 3 50 100 < 10 30 50

0.65 0.15 0.063 1.0 0 0

14 0.35 0 10 30 30

Mass spectrometer- Sampler material Ni A1 A1 Skimmer material Ni Ni Ni Sampler-skimmer spacinglmm 5.5 7.5 7.5 Expansion pump11 min-' 400 1900 1900 Expansion pressure/mbar 2x 100 2 x 10-1 2x 10-2 Intermediate pressurelmbar t i x 10-4 ti x 1 0 - 4 ti x 10-4 Analyser pressure/mbar 6 x 7x lo-' tl x 1 0 - 8

*Sampling depth is defined as the distance between the outside surface of the sampling orifice and the foremost turn of the load coil.

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430 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1993, VOL. 8

28 29

4 0 ~ ~ 1 6 0 1 H+ Si Si+

1x106

lx1o5

1 ~ 1 0 ~

1x10~

1x102

g 10

.- 5 1

3 0 Y m

l X l O 6 1 40Ar40Art

1x10~

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1

(skipped} I

,

40 45 50 55 60 65 70 75 80 85 90 mlr

Fig. 5 Background spectra between rnlz 40 and 80 for: (a) Ar atmospheric pressure plasma; and (b) Ar low pressure plasma

effect on signal was caused by adding the 1500 1 min-l pump to the expansion stage, to result in a potential pumping capacity of 1900 1 min-l [Fig. 2(a) and (b)]. At a pump rate of 400 1 min-l the signal was much reduced compared with a pump rate of 1900 1 m1n-l.

Observation of the tip of the skimmer was possible by means of a quartz window set into the side of the expansion chamber. When a small amount of air was allowed to leak into the low-pressure system, the characteristic red Ha emission at 656.3 nm, from moisture in the air, was clearly visible. However, no definite 'barrel shock' region was visible.

An example of a chromatogram obtained for 1 -bromono- nane, using the operating conditions listed in Table 3, is shown in Fig. 3. For an injection of 25 ng of Br as l-bromo- nonane a peak area of 225000 counts was observed which was reproducible from day to day, though no attempt was made to optimize the chromatographic conditions. Peak tailing could have been caused when the column effluent was switched to the low-pressure environment, causing a temporary increase in the column flow rate. The advantage of the low pressure plasma is that all of the GC effluent, and hence all the analyte, must pass through the sampling orifice into the expansion stage. If the skimming conditions are suitably optimized it might then be possible to achieve lower detection limits.

Background Spectra An example of the background spectrum obtained for the low pressure plasma with Ar as the support gas i s shown in Figs. 4(b) and 5(b) and compared with the spectrum obtained for Ar at atmospheric pressure [Figs. 4(a) and 5(a)]. Spectra were obtained using the operating conditions shown in Table 3.

The spectra between rnlz 4 and 40 for the plasmas studied are shown in Fig. 4(a) and (b). The spectrum for Ar at atmospheric pressure [Fig. 4(a)] was obtained with a dry plasma and contained many of the so-called gas peaks due to Q2, Nz and moisture entrainment from the air, and as contamination in the liquid Ar supply. Lithium, Na, Mg and K peaks were due to contamination on the torch from previous experiments. The A1 peak was due to A1 in the cleaning agent used to polish the Ni cones.

Many of the polyatomic peaks are difficult to assign, but from a comparison of the atmospheric pressure plasma [Fig. 4(a)] with the low pressure plasma [Fig. 4(b)] the gas peaks are generally of the same order of magnitude in intensity for both plasmas. The only substantial differences are those for OH+* at mlz 18 and 20, which were less intense in the low pressure plasma, and WC2 and 320+2, which were more intense, though by less than an order of magnitude. Also, the Ar+ signal was much smaller in the

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1993, VOL. 8 43 1

low pressure plasma, presumably because of the reduced power and gas flows. No peak due to *'Al+ was observed in the low pressure spectrum [Fig. 4(b)] despite the fact that the sampler was made of Al. Erosion of the sampler is unlikely at the low power studied, but the absence of 27Al+ also indicated that there was no intense secondary discharge in the expansion stage.

The most likely explanation for the persistence of the gas peaks for the low pressure plasma is that a small leak in the interface between the gas chromatograph and the torch, or between the torch and the sampler, allowed minute amounts of atmospheric air into the system, all of which were consequently sampled. The most likely point of entry would be the interface between the hot transfer line from the gas chromatograph and the torch, where the ultra-Torr seal could have suffered from thermal degradation.

A comparison of the spectra at rnlz 40-80 shows that the atmospheric pressure plasma [Fig. 5(a)] suffered from interferences due to Cr+, Ni+ and Cu+, which may have been caused by a secondary discharge in the expansion chamber, whereas the low pressure plasma [Fig. 5(b)] had greater interferences due to 40Ar14N+ and ArAr+. A signifi- cant difference between the atmospheric and low pressure plasmas is the absence of the continuum background of approximately 10 counts in the latter case. The continuum background was a characteristic of the instrument used in this study when operating a dry atmospheric plasma.

The deconvolution of many of the polyatomic interfer- ences is difficult, especially since the low pressure plasma has not yet been fully optimized with respect to sampling depth, sampler-skimmer spacing and sampling orifice size. It is possible, for instance, that the formation of polyatomic ions was due to skimming downstream of the mach disc.

Helium Plasma One advantage of the low pressure plasma is the ability to form He and molecular gas plasmas easily, and it was indeed possible to do this using the conditions given in Table 3. However, it was not possible to increase the forward power above 100 W owing to the extremely high reflected power of 50 W, but higher powers should be practical if a suitable matching network is used. The main characteristic of the low pressure He plasma was the much increased intensities of polyatomic species between mlz 4 and 40 compared with the atmospheric Ar plasma. This was presumably due to a number of factors such as: the extremely low power, which probably allowed greater formation of polyatomic species; the more efficient ioniza- tion of O2 [ionization potential (IP)= 13.62 V], N2 (IP= 14.53 V) and H2 (IP= 13.6 V); and the unoptimized nature of the interface, which meant that the expansion gases could have been sampled from a region where recombination of the extracted gases was prevalent. Simi- larly, the spectrum between rnlz 40 and 80 contained much larger peaks due to SiO+ compared with the atmospheric pressure Ar plasma. One advantage of the He low pressure plasma may be for coupling with laser ablation, ETV, hydride generation, or other methods of gaseous sample introduction, since the attenuation of 40Ar+, 40Ar40Ar+ and other Ar-containing polyatomic ions could allow the deter- mination of 39K+, 40Ca+, and isotopes of Se.

Conclusions A low pressure Ar plasma was successfully generated and used with mass spectrometry. Using GC, a sample of 1 -bromononane was analysed. Plasmas were easily gener-

ated and sustained at low pressure in both Ar and He, but the total exclusion of atmospheric air was difficult and depended on the effectiveness of several vacuum seals. Future work in this area will include the simplification of the torch design and GC interface to reduce the number of vacuum seals necessary; generation of higher power He plasmas by improving the tuning of the RF circuit; and generation of plasmas in other gases such as 02, N2 and air.

The authors are grateful to the National Institute of Environmental Health Sciences for providing support through grants numbered ES03221 and ES04908. We are also thankful to the National Institute of Health Shared Instrument Grant Program for providing the VG Plasma- Quad.

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Paper 2/04 I2 7H Received July 31, I992

Accepted Novem ber 2 7, I 992

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