Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry

SPECTROCHIMICA ACTA PART B

Spectrochimica Acta Part B 52 (1997) 633-641

ELSEVIER

Study of a pulsed glow discharge ion source for time-of-flight mass spectrometry Yongxuan Su, Zhen Zhou, Pengyuan Yang*, Xiaoru Wang, Benli Huang Department of Chemistry, Laborator3' of Analytical Science, Xiamen Universi~, Xiamen 361005, PR China

Received 8 July 1996; accepted 24 October 1996

Abstract This paper describes a new type of glow discharge (GD) ion source coupled to a time-of-flight mass spectrometer (TOFMS). The GD is operated in the microsecond pulse (/zs-pulse) mode. The operational parameters of the/zs-pulse GD were optimized against the ion signals, giving 180 Pa for the discharge pressure, 3 A for the transient discharge current, 1.75 kHz for the discharge frequency and 2/~s for the discharge pulse duration. Experimental results show that the discharge current in the/~spulse mode can be one order of magnitude higher than that obtained in the d.c. mode. The structure of the interface between the /~s-pulse GD and the mass spectrometer was found to be critical, and a Macor disc must be applied in front of the sampling orifice in order to shield the sampling plate from the anode of the GD to achieve both a good vacuum and the best sputtering. A transient sputtering rate of 24.4 ~g s -~ mm 2 was reached in the/zs-pulse mode and was significantly higher than that for the d.c.-GD. Typical mass spectra of brass and nickel samples were studied and are discussed. © 1997 Elsevier Science B.V. Keywords: Glow discharge; Ion source; Microsecond-pulse glow discharge; Surface analysis; Time-of-flight mass spectrometry

1. Introduction Recently, the glow discharge (GD) ion source has been attracting more and more attention in regard to coupling to a time-of-flight mass spectrometer (TOFMS) [1,2], an ion trap mass spectrometer and a Fourier transform-ion cyclotron resonance analyzer, as well as a quadrupole mass spectrometer [3-5]. Because of its characteristics of stable, low ion energy distribution, little interference and high sensitivity, the GD is now considered to be one of the best devices for solid sample analysis [3]. Conventional GD designs have undergone a number of modifications to fit the requirements of solid sample analysis. The earlier modified GD sources are * Corresponding author. Fax.: +86 592 218 6401.

likely to be Grimm types [6,7], utilized in optical emission spectrometry (OES). However, features of the Grimm-type GD device are the special shape of its anode and the critical distance between anode and cathode, and thus it is relatively complicated to design and machine. Later modifications to the GD removed the above design restrictions. Winchester et al. constructed an r.f. discharge source with an external sample mount geometry [8]. This design requires only that the sample is sufficiently flat to ensure a vacuum when the sample is pressed against an O-ring. Myers et al. have designed a GD interface to a T O F M S [2]. This interface has a sliding Teflon seal which allows rapid changing of the sample without breaking the vacuum of the mass spectrometer. Ren et al. have reported a new type of GD device having a major modification in the cathode and anode structures [9]. In their GD

0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0584-8547(96)01621-7

Y. S u e t al./Spectrochimica Acta Part B 52 (1997) 633-641

634

design, the sample is used directly as the cathode, no separate cathode being required. The discharge can be confined by a quartz ring between the anode and the cathode. In addition, the anode structure is simplified for ease of manufacture. In the present study, we have modified the GD designed by Ren et al. [9], and have developed a new ion source coupled to a linear TOF analyzer. The new GD is powered in the microsecond-pulse and high-current mode, and is used to replace the previous pin-type GD [1]. The structure and characteristics of the ion source are studied. The operating parameters are optimized and discussed. Typical spectra of real samples are given.

As shown in Fig. 1, the experimental set-up consists of three parts: the t~s-pulse GD ion source, the ion optical system, and a linear TOF mass analyzer. Ions generated in the ion source are focused into a repulsion zone by the ion optical system, and then are extracted orthogonally into a flight tube and recorded by a microchannel plate detector (MCP; 36 mm in diameter; Electronics Institute of the Chinese Academy of Science, PR China). Signals are recorded with a 100 M Flash ADC (Scientific and ~=~I-~L1

L2 1.,3 Qx Slit

IIIt=l S,;¢-~.~L.~_.. 11. .d='/--

~-7~11~ Fir;tsS~/sg:pump

2.1. Ion source

2.1.1. Structure of the GD ion source

2. Experimental

n II

Technical University, PR China) and an IBM compatible DX33 personal computer. In addition to the present instrumental configuration, we have also modified the vacuum system and ion optics described previously [ 1]. An ion extracting cone (S 1) was added and a new ion lens system was installed (see Fig. 1). Due to the improvement of the vacuum system and the ion optics, we have found that ion collisions are reduced during the transmission process and the ion throughput is greatly improved. The operating conditions of the #s-pulse GD-TOFMS are listed in Table 1.

QY Se¢°4d~ tL/g;ecPu m l~

~

__R

I

C

---~"/-~-[~ Y

~

The GD ion source, as shown in Fig. 2, is made of stainless steel. The discharge chamber is 6 mm thick Table 1 Typical working conditions for the #s-pulse G D - T O F M S Parameter or component ~s-Pulse GD Discharge frequency Discharge pulse width Discharge current Discharge voltage W o r k i n g gas flow rate Gas pressure

1.75 k H z 2 #s 3 A - 750 V 4 0 Pa I -~ s 180 Pa

TOFMS SI LI L2 L3 Q, Q, Slit

Repeller bias Repeller pulse Repeller pulse width

Fig. 1. Schematic diagram of the #s-pulse GD-TOFMS. The ions are extracted in a direction perpendicular to the supersonic beam by a repelling pulse synchronized with the discharge pulse of the G D . S 1, extracting cone; L I , L 2 and L3, Einzel lenses; Q~, x-dimension poles; Q,, y-dimension poles; R, repeller; C, Faraday cup; G1 and G 2 , T O F M S entry grids; X, steering plate; Y, deflection plate; A, preamplifier.

Value

G1 G 2 , flight tube, Y bias X A r deflecting pulse A r deflecting pulse width

First stage pressure Second stage pressure Third stage pressure MCP Front

Centre Back

- 600 V - 120 V - 60 V - 250 V 15V -0.5 V - 375 V - 7 V 155 V at 1.75 k H z 2/~s

Ground - 2000 V - 1920 V 140 V at 1.75 k H z 3 0 0 ns 10 Pa 6.5 x 10 2 Pa 9 x 10 4 Pa - 2445 V - 1615 V - 764 V

Y. Suet al./SpectrochhnicaActa Part B 52 (1997) 633-641 To vacuum detector

?

'~-thode mounting lartz restrictor ring

/

Macor di

~ulator To TOFM~ q~30

tmple /'

Sample

,r..g

k"

qb20

Cid.

".

'[ 0.3

635

enhancement of the emission intensities was observed. A thermocouple gauge (ZDO-54; Chengdu Instrument Factory, PR China) is coupled close to the discharge cell to monitor the vacuum. The sample is processed as a disc about 35 mm in diameter and 5 mm thick. The surface of the sample is polished with metallographic abrasive paper, then washed with pure alcohol and dried immediately before use.

-; 2.1.2. Power supply f o r the #s-pulse GD

Argon ill

Quartz restrictor ring

Fig. 2. Structure of the glow discharge ion source. The restrictor quartz ring separates the anode and sample cathode, and confines the stable discharge. The Macor disc isolates the mass spectrometer sampler from the GD anode to ensure a stable discharge.

and 10 mm in diameter. The thickness of the discharge chamber is purposely selected to ensure that the sampiing plate is close to the boundary region between the cathode dark space and the negative glow [10], where the most highly ionized species are believed to be sampled into the TOFMS. We have manufactured a series of discharge cells with different diameters of the sampler orifice. The results show that an orifice diameter of 2 mm gives good performance. The critical factor for the discharge structure is the addition of a restricting quartz ring to separate the cathode and anode, and to restrict the sputtered area. Several quartz rings were made, of thicknesses 2.5 mm and diameters 30 mm, and with centre hole diameters of 2, 3, 4, 5, 6, and 7 mm, respectively (see Fig. 2). One side of the quartz ring (facing the sample) has a disclike groove 0.3 mm deep for obtaining better sputtering results [9] (see Fig. 2). The gap between the restricting quartz ring and the cylindrical anode is only 0.3 mm wide (see Fig. 2) and serves as an inlet for the support gas. Therefore, a relatively high gas velocity can be obtained when the argon gas passes through the inlet. As a consequence, more sputtered atoms can be carried into the negative glow region. On the other hand, deposition of the sample atoms can also be reduced and short-circuiting between the anode and the cathode can be eliminated. Banks and Blades [11] have investigated a jet-assisted GD source and found that application of the jets resulted in an increase in sample atomization, but no uniform

The /zs-pulse GD power supply is a laboratorybuilt set-up with adjustable frequency (100 H z - 5 kHz), pulse duration (200 n s - 5 #s) and discharge current (30 ~ A - 3 A). A d.c. voltage ( - 750 V) is used to pre-burn the sample and sustain the plasma when the discharge pulse is switched off. Compared with the d.c. power supply, the /zs-pulse operational mode can provide a transient power of more than 2 kW. 2.2. Ion optics and vacuum system

As shown in Fig. 1, the ion optical system is composed of a skimmer, an extracting cone (S 1), a cylindrical lens (L1, L2 and L3), a d.c. quadrupole ( Q , Q,), and a slit. The d.c. quadrupole and slit are applied to compress the ion beam in the repeller (R) zone. The equipment is installed in a three-stage vacuum system. The discharge cell is pumped by the first vacuum stage through a 2 mm orifice in the sampler. The pressure in the discharge cell is adjustable between 0.1 and 350 Pa by means of an argon gas flow. The first stage lies between the sampler and the skimmer, and is evacuated using a 15 1 s ] rotary mechanical pump (2X-I 5A; Shanghai Vacuum Pump Works, PR China), where the pressure is maintained at 10 Pa. The second stage, between the skimmer and the extraction cone, is pumped by a 450 1 s -~ turbomolecular pump (FI-450A; Zhejiang University, PR China) to a pressure of 6.5 × 10 2 Pa. The second stage has a 1 mm orifice on the skimmer cone, and plays a key part in ensuring the success of the detection system which follows. The third stage, including the ion optics and the flight tube, is evacuated by means of a second 450 1 s -1 turbomolecular pump to a pressure of 9 x 10 -4 Pa.

636

V. Suet al./Spectrochimica Acta Part B 52 (I 997) 633-641

2.3. TOF analyzer and signal detection system The ratio of the length of the extraction step (the distance between the repeller and G l; see Fig. 1) to that of the acceleration step (the distance between G I and G2) crucially affects the resolving power [12]. Based on such a ratio calculation, the length of the accelerating field has been increased to 38 mm from the previous 16 mm [1]. With this modification, less noise and better resolution have been obtained in the experiments. A high-voltage power supply (PS325; Stanford Research System, Inc.) is utilized to provide a potential between G1 and G2 for the minimal shift of the accelerating voltage. The potential applied to G2 is also applied to the flight tube which is insulated from the vacuum chamber walls. A Steering plate (X) is used to correct the velocity component of the ion beam in the x-direction. A narrow pulse is applied to a deflection plate (Y) at an adjustable delay time after the repeller pulse. This delay and the pulse width are adjusted for minimal Ar + and ArH + signals at the detector. The deflection plate Y is perpendicular to the steering plate X. The signal detection system is identical to that reported previously [1 ], and includes two cascade microchannel plates and a wide-band preamplifier.

3. Results and discussion 3.1. The #s-pulse GD regime 3.1.1. Unique characteristics of the tzs-pulse GD as an ion source for the TOFMS A relatively high ion transmission efficiency has been achieved by coupling the ~s-pulse GD to a TOFMS. In the t~s-pulse GD, a 2-tzs-duration discharge can produce an approximately 4 #s (fullwidth-at- half-maximum) ion packet in the repeller region [ 1]. If we expel properly the focused ion packet at a pulse width of 2/zs, a high sampling efficiency can be obtained with fairly good mass resolution. When a plasma is operated in the conventional d.c. mode, ions are produced and enter the ion optical system consecutively. The ion beam must be modulated into small ion packets in order to enable flight time analysis to be performed. For example, ions from an atmospheric inductively coupled plasma ion source have been

0.2--10 ms

h

Discharge pulse

Next pulse

Repeller delay Repeller pulse Ar deflecting delay Ar deflecting pulse Signal triggering delay

"ll Signal triggering pulse

Fig. 3, Time sequence of the ion packet modulation. The delay logic drives the repeller pulse when the ion packet has completed the extraction zone. Other delay logics are set according to this repeller pulse for deflecting the Ar + and ArH + ions and for triggering the Flash ADC.

sampled into a TOFMS [12], and the sampled ions were modulated and pulsed into the TOF analyzer at a high frequency (5-10 kHz). However, the sampling efficiency in such a system is relatively low, usually below 5% [12]. In contrast, a sampling efficiency of more than 25% [1] can be obtained in the /zs-pulse GD-TOFMS. In Fig. 3 is shown a delay logic diagram designed according to the ion flight time. The discharge power supply operates in the/zs-pulse mode with a frequency of 100 H z - 5 kHz. A cycle takes about 0.2-10 ms, which is long enough for the heaviest mass to fly from the repulsion zone to the detector before the next ion packet is produced. A high-voltage pulse is applied to the repeller and the ions are extracted orthogonally into the flight tube. A narrow pulse is utilized on the deflection plate to expel Ar + and ArH + ions when they pass by. The typical delay time between the repeller pulse and the discharge pulse is 15 #s, and the delay time between the argon deflecting pulse and the repeller pulse is 3.8 /zs. A synchronized pulse is used to trigger the Flash ADC when recording the spectrum. The spectrum of interest can be recorded by adjusting the delay time between the trigger pulse and the repeller pulse. For a flight length of 1.25 m, an accelerating voltage of 2 kV and a mass of 209 a.m.u. (Bi), the flight time is typically 29/~s. Thus, the/zs-pulse GD can generate ion packets periodically in a very short duration, and the TOF analyzer can record the signal

Y. Su et al./Spectrochimica Acta Part B 52 (1997) 633-641

S00~

.

.

.

. ~-

4501 I

.

.

.

.

plateau would become higher if the maximum voltage of the power supply could be increased to more than -750 V.

+----+-m-+ , , ,~

+,-

4004

350-

637

3.2. Sputtering results

/

d

=. 300i o

/

250

.¢

200 i +

150i 100

l

0

o15

~

1.s 2 Current, A

21s

;

3'.s

Fig. 4. Voltage versus current curve for the GD operated in the #spulse mode. (Argon pressure, 180 Pa; discharge frequency, 1.75 kHz; discharge duration, 2.0 #s.)

quasi-simultaneously. Therefore, the coupling of a/zspulse GD with a TOFMS provides a good technique for analyzing transient processes occurring in an ion source.

3.1.2. The relationship between the discharge voltage and current Fig. 4 depicts the voltage and current curve (V-A) for the /zs-pulse GD. The characteristics of the V-A curve are quite different from those for the d.c.-GD. Two oscilloscopes (40 M; BS-5504; Samtron Corp, Korea) were utilized to monitor the peak current and the peak voltage simultaneously. The transient current of the/~s-pulse GD can be above 3 A, while the maximum current of the d.c.-GD is usually below 100 mA. When the current is below a certain value (about 1.6 A in our experiment), the voltage drop between anode and cathode increases quickly with the discharge current. The voltage drop tends to saturation and becomes a constant (about 450 V) when the current is higher than 1.6 A. A plateau appears in the V-A curve with a turning point (1.6 A, 420 V) and the power increase occurs mainly up to the current increase after the turning point. When the /zs-pulse GD operates in the plateau region, the voltage peak widens as the discharge current increases in height (observed from the oscilloscope). This peak widening occurs possibly because of the finite voltage output of the /zs-pulse GD power supply used in this experiment. We speculate that the turning point on the

3.2.1. Effect of sampler orifice on sputtering The diameter of the sampler orifice has an obvious effect on the sputtering results. Normally, the smaller the diameter of the sampler orifice, the better will be the vacuum of the TOF analyzer. However, experimental results show that the small orifice results in poor sputtering and therefore a weak ion signal. The centre of the bottom of the crater turned a faint yellow when the small orifice was used, and the glow discharge was transferred to an arc-like plasma. In contrast, the sputtering results seemed to be satisfactory when a hollow sampler was used, but the vacuum of the third stage was so poor (having a pressure of more than 9 × 10 3 Pa) that the working requirement of the MCP could not be met. From the above experimental results, we speculate that the anode should be hollowtube-like for better sputtering, but the sampler orifice should be small in diameter to give a a better vacuum in the TOFMS. To satisfy these conflicting requirements for both better sputtering and a better vacuum, the anode and the sampler should be separate with respect to their geometry. The sampler in the earlier design is used not only as an anode in the GD but also as a sampler in the mass spectrometer. The anode geometry has a considerable influence on the sputtering results, which can be explained by the variation in the electric potential [13]. We believe that a conducting sampler plate affects the stability of the plasma in its violation of the requirement of a hollow anode. Therefore, we have shielded the sampler plate with a Macor disc having a 2 mm orifice in the centre. This Macor disc isolates the sampler plate electrically from the hollow anode. Experiments show that the sputtering results improved to become as good as those for a hollow anode, while the vacuum was also guaranteed for the TOFMS. 3.2.2. Sputtering rate In the /xs-pulse GD mode, the transient sputtering rate (Rt) is fairly high but the average sputtering rate (R,) is relatively low. The sputtering rates Rt and R~

E Suet a/./Spectrochimica Acta Part B 52 (1997) 633 641

638

can be represented as Rt-

W

TxFxDxS

and R,~-

W

TxS

where R~ and R~ are in milligrams per second per square millimeter, W is the amount lost (mg), T is the sputtering time (s), S is the sputtered area of sample (mm2), D is the pulse duration (s) and F is the pulse frequency (Hz). For a GD operation, the sputtering rate is an important parameter for judging the sampling efficiency. We determined an amount lost of 12.8 mg for a brass sample after the sample was sputtered for 2 h. The devotion of d.c.-GD discharge to the sputtering rate is negligible because the d.c. discharge current is only about 30 ~A, and the sample is very little sputtered when the discharge pulse is off. The transient sputtering rate is about 24.4 ~g s ~ mm 2 and the average sputtering rate is only 0.128 /~g s ~ mm 2 for a duty cycle of 1:190 (with a discharge frequency of 1.75 kHz and a pulse width of 3/~s) and a sputtered area of 13.85 mm-. Compared with the sputtering rates for the d.c.-GD and the r.f.-GD [14,15], the transient sputtering rate of the /.¢s-pulse GD is significantly large, although its average sputtering rate is relatively low. As a result, the ~s-pulse GD can produce many more atoms and ions in every pulse shot, and can consequently offer good depth profile resolution in surface analysis.

observation under an SEM (S-520; Hitachi Corp., Japan) shows that many small cones emerge on the sputtered surface when the copper sample is sputtered in either the d.c. mode or the ~s-pulse mode, we notice with the SEM that the cones are smaller when obtained in the/xs-pulse mode than in the d.c. mode. The density of the small cones is about 80 mm -2 when the copper sample is sputtered. The population of small cones is about nine times greater than for the d.c. mode. Ren et al. [18] reported that the formation of the cones was due to the masking of the matrix by sputtering second-phase precipitates or oxide particles, as well as an electrostatic effect. Thus, /xspulse GD is a potential method for obtaining the best layer profile. This capability is demonstrated because a finer sputtered surface will provide a higher resolution of layer analysis in the ~s-pulse mode.

3.3. Effect of discharge parameters on the spectral intensity 3.3. l. Discharge current of the/xs-pulse GD power supply In Fig. 5 is shown the dependence of the ion signal (63Cu +) on the discharge current. As can be seen, the intensity of the 63Cu+ ions increases quickly when the current is below 1.6 A, and then tends to a plateau which is believed to be related to the unique characteristics of the V-A curve in the /xs-pulse mode. To observe visually the actual/~s-pulse GD, we have also used a GD-OES device constructed previously with a

3.2.3. Observations of the sputtered surface The surfaces of the same types of copper samples sputtered in the/xs-pulse and d.c. modes were studied. The GD sputtering topography of brass and alloys has been studied using a scanning electron microscope (SEM) and a scanning Auger spectrometer [16,17]. Ren et al. have also reported and explained some characteristics of the sputtered surface [ 18]. However, their experiments were all performed in the d.c. mode. We have observed with our naked eyes that the sputtered surface obtained using the/xs-pulse GD is finer than that obtained using the d.c.-GD. When the sputtered sample is being polished, we find that the convex shape of the crater close to the anode wall almost disappears at the same rate as the central part of the sputtered surface. We have reported the topography of copper samples studied with an SEM [19]. Although

~,;~

-

1.4 1.2 > ._o= C •-~ (D

i"

A-

1 a, 0.8

0.6 0.4 i/ 0"21

1~2 1~4 1.6

118 =2

2.2

2.4

2.6

2.8

3

Discharge current, A Fig. 5. Effect of the discharge current on the 6~Cu+ signal. The operational parameters were the same as those given in Fig. 4. Each point is an average value based on 1000 pulses.

Y. Suet al./Spectrochimica Acta Part B 52 (1997) 633-641 1.6!

1.8T---1.6-

1.4, ~zr

i 1.2~

•--~

'

ffl

! ,

0.6 ~ I

0.4

o.4~/ 1

.

I

,~

0,6-

'

0.22

.

e'.

,

0.6

.

I

1.2-' >

11 0.8

/.

1.4 ~

IA-

i

-(11 --

639

i I

2

3

4

5

6

7

8

Square of discharge current, A 2

°-ii 1200

m

t

1400

1600

1800

2000

22~00 2400

2600

Frequency, Hz

Fig. 6. Relationship between the 6-~Cu+signal and the square of the discharge current. The operational parameters were the same as those given in Fig. 4.

Fig. 7. Effect of the pulse frequencyon the 63Cu+signal. (Discharge pressure, 180 Pa; discharge current, 3 A; discharge pulse duration. 2 t~s.) Each point was an average value based on 1000 pulses.

quartz observation window. This GD device has a structure similar to that used in conjunction with the TOFMS, and can be operated in the #s-pulse mode. Through the quartz window, we observed that the discharge glow for a brass sample turns green when the discharge current increases. It can be surmised that the increase of transient power strengthens the sputtering and excitation processes, and sample atoms can acquire enough energy to be ionized efficiently. Fig. 6 displays the relationship between the intensity of the 63Cu+ions and the square of the discharge current. A similar experiment has been reported [20] for a hollow cathode lamp operated in the /~s-pulse mode for ion fluorescence spectrometry. Interestingly, the experimental results showed that the intensity of ion emission is proportional to the square of the discharge current [20]. However, no such relationship can be seen in /zs-pulse GD-mass spectrometry. It seems that the amount of ions being sampled into the TOFMS does not increase with the discharge current in a square relationship.

hand, more sample atoms can be sputtered and ionized with a high frequency. As a result, the signal-to- noise ratio can be enhanced. From Fig. 7, it seems that a frequency of around 1.7-2 kHz can produce a strong ion intensity. However, we speculate that the results obtained above 2 kHz should have stronger signals. This speculation results from the limited power capacity of the present /~s-pulse GD supply. The power supply is unable to provide enough average power at a high discharge frequency. Therefore, the excition and ionization process faulters when the average power required is higher than that which can be offered by the power supply. As a result, the ion throughput of the sample becomes low above 2 kHz in this experiment. The experimental results show that the pulse duration has little effect on the peak intensity of the signals when it is longer than 1.5/xs. However, a long pulse duration will consume more discharge power. A duration of 2 kts was found to be good in the present experiments.

3.3.2. Effect of discharge frequency and pulse duration Fig. 7 shows the effect of the discharge frequency on the ion intensity. Generally, there is a trade-off between the discharge frequency and the ion sampling interval. On the one hand, increasing the discharge frequency will decrease the discharge interval time and meet the limits of minimum flight time for the heavier mass (e.g. 29 t~s for Bi, m/z 209). On the other

3.4. Effect of discharge gas on the intensity of O3Cu+ ions Fig. 8 displays the relationship between the 63Cu signal and the gas pressure. As shown in Fig. 8, the intensity of the 63Cu signal reaches its peak value when the argon pressure is about 180 Pa. A further increase in the argon pressure will cause a decrease in intensity. The plasma tends to be unstable and arcing

640

}I. Suet al./Spectrochimica Acta Part B 52 (1997) 633-641 1.8,

2

X

1.6

16

14 HaO

L ×

1.2 t,"

u~

;<

1i

p<

~

×.

12

C

._~

0.8!

Zn

1

Ar ArH

0.8

Ar A r H

0.6!

o6

0.4;

i

0.2~

I

OI 60

Cu

18

X

1.41 >

~

0.4

80

100

120 140 160 Pressure, Pa

180

200

when the argon pressure is too high. The working gas pressure is one of the most important factors in the discharge process for the t~s-pulse GD ion source. Kawaguchi and co-workers [21 ] have illustrated the effects of the discharge-gas flow rate on the relative sensitivity factors (RSFs) in GD-mass spectroscopy with a Grimm-type ion source. They found that the ion signal as well as the RSFs varied with the gas flow rate, and were element dependent and sensitive to different sample materials. Ratliff and Harrison [22] investigated the various effects of water vapour on the glow discharge processes and discovered that water vapour had detrimental effects on both atomization and ionization in the plasma. We have obtained similar results in studies of the effects of the argon flow rate and argon impurities (mainly water vapour) on the ion intensity of a brass sample. When the pressure was fixed at 180 Pa and the gas flow rate was increased from 20 to 40 Pa 1-~ s -j (by adjusting the distance between the anode tube and the quartz restrictor ring), the intensities of the 63Cu + and 68Zn+ signals increased by 15% and 9%, respectively. When 99.95% pure (instead of 99.999% pure) argon gas was used as the discharge gas, the copper and zinc signals decreased significantly, and CuH20 + and ZnH20 + signals emerged with relatively strong intensity.

' P

°~-L__,~IiL

220

Fig. 8. Effect of the argon pressure on the 63Cu+ signal. (Discharge frequency, 1.75 kHz; discharge current, 3 A; pulse width, 2 #s.)

15

Figs. 9 and 10 show typical mass spectra of brass and nickel samples, respectively. As shown in these

25

35

I

45

55

65

75

85

95

105

115

125

Mass/Charge Fig. 9. Mass spectra of brass obtained in the #s-pulse GD-TOFMS. The results were based on an average of 500 pulses. The discharge was operated under the conditions given in Table I.

figures, the t~s-pulse GD can produce strong ion signals of the sputtered sample. For the background spectra, Ar + and ArH ÷ ions were reduced efficiently by the argon deflecting pulse. Polyatomic species (such as H3 O+, Ar~, Ar2H + and CuAr ÷) can be seen in Fig. 9. The presence of such species is dependent experimentally on the discharge pressure and on the distance between the cathode and the sampler orifice. A gas purification device is currently being tested for the removal of water vapour in the argon gas. It is believed that strong ion signals can be obtained when ions are sampled from the interface region between the cathode dark space and the negative 1.1

ArH

H20

1=

seNi

0,9 S°Ni

0,8~ 0,7~

S2Ni

> OH "--'e~ 0'6 i ,

Ar S4Ni

._g o.~! ffl

0.4 J

o.3~

!

0.2,

'

iJ

'

i'

'

', ,, ~'i!

At=

".,,,. . . . . ' ¢~...__j" ~ ,

o.1 ..,--.....~ ~ i

o

3.5. Spectra of brass and nickel obtained with the i~spulse GD-TOFMS

CuAr

NH

,o

20

so

40

s-o~-~o eo ~ 1 o o l l o 1 2 o Mass/Charge

Fig. 10. Mass spectra of nickel obtained with the #s-pulse GDTOFMS. The results were based on an average of 500 pulses. The discharge was operated under the conditions similar to those given in Table 1.

Y. S u e t al./Spectrochimica Acta Part B 52 (1997) 633-641

glow [10]. This critical positioning of the plasma can be performed by adjusting the gas pressure, and by optimizing the distance between the sampler orifice and the cathode. Under the present experimental conditions, a resolving power (T/2AT) of 220 can be achieved for the linear TOFMS. This resolution is calculated from the full-width-at-half-maximum (FWHM) of the 65Cu+ peak. The experimental results show that the resolution can be further improved if the rise time of the repelling pulse can be reduced and the accelerating grids can be machined with better precision. These modifications are currently in progress.

4. Conclusion A glow discharge ion source which is easy to manufacture and assemble has been designed and coupled to the TOFMS. This coupling of the/~s-pulse GD to a TOFMS offers a promising technique for observing transient processes in the ion source because the production and detection of ions are quasi-simultaneous. It is important that a Macor disc is applied to shield the sample plate from the discharge anode in order to achieve both a good vacuum and the best sputtering. The V - A curve of the gs-pulse GD shows that the working current in the p.s-pulse mode is much higher than that in the d.c. mode. The most effective factors when considered the #s-pulse GD are the argon pressure, the discharge current and the discharge frequency, having optimal parameters of 180 Pa, 3 A, and 1.75 kHz, respectively. The transient sputtering rate for the gs-pulse GD (24.4 gg s -~ mm -2) is much higher than the sputtering rates of the d.c.-GD and the r.f.-GD, though its average sputtering rate is relatively low (0.128 gg s -1 mm-2). The sputtered surfaces of the samples are fairly fine when using the gs-pulse GD mode, which ensures that the gs-pulse GD is a promising technique for surface analysis. Spectra of brass and nickel samples are obtained, and high ion signals and a fairly good resolution of 220 are achieved. However, the background and noise level as well as the signal-to-noise ratio needs to be further enhanced to improve the power of detection of the gs-pulse GD-TOFMS.

641

Acknowledgements The Laboratory of Analytical Science is run by the State Education Commission of China (SEDC). This work is supported by the National Nature Science Foundation of China under grant number CHEM29235110-II, and partially by the Outstanding Youth Fellowship of SEDC. The authors thank Professors Jianshi Ren and Gongshu Zhang, and Mr Hongbo Ma for their helpful advice in designing the GD ion source.

References [I] W. Hang, P.Y. Yang, X.R. Wang, C.L. Yang, Y.X. Su and B.L. Hunag, Rapid Commun. Mass Spectrom., 8 (1994) 590. [2] D.P. Myers, M.J. Heintz, P.P Mahoney, G. Li and G.M. Hieftje, Appl. Spectrosc., 48 (1994) 1337. [3] S.A. Mcluckey, G.L. Glish, D.C. Duckworth and R.K. Marcus, Anal. Chem., 64 (1992) 1606. [4] C.M. Barshick and J.R. Eyler, J. Am. Soc. Mass Spectrom., 4 (1993) 387. [5] D.M. Milton, R.C. Hutton, G.A. Ronan and J. Fresenius, Anal. Chem., 343 (1992) 773. [6] N.P. Ferreira, J.A. Strauss and H.G. Human, Spectrochim. Acta Part B, 38 (1983) 899. [7] R.M. Lowe, Spectrochim. Acta Part B, 31 (1976) 257. [8] M.R. Winchester, C. Lazik and R.K. Marcus, Spectrochim. Acta Part B, 46 (1991) 483. [9] J.S. Ren, H.L. Zhao and G.S. Zhang, Chin. Patent 88 217038. 4, 1988. [10] D.C. Duckworth and R.K. Marcus, J. Anal. At. Spectrosc., 7 (1992) 711. [11] P.R. Banks and M.W. Blades, Spectrochim. Acta Part B, 46 (1991) 501. [12] G.M. Hieftje, J. Anal. At. Spectrosc., 7 (1992) 783. [13] D. Dezso, S. Laszlo, A. Miklos, J. Anal. At. Spectrosc., 7 (1992) 707. [14] J.L. You, Z.S. Wang, G.S. Zhang and J.S. Ren. Anal. Lett., 26 (1993) 541. [15] C. Lazik and R.K. Marcus, Spectrochim. Acta Part B, 47 (1992) 1309. [ 16] H. Jager and F. Blum, Spectrochim. Acta Part B, 29 (1974) 73. [ 17] J.E. Greene and B.R. Natarajan, J. Appl. Phys., 49 (1978) 417. [18] J.S. Ren, H.L. Zhao and G.S. Zhang, Acta Metall. Sin., 3 (1990) 65. [19] Z. Zhen, Y.X. Su, P.Y. Yang, X.R. Wang and B.L. Huang, J. Anal. Sci., accepted for publication, 1997 (in Chinese). [20] Z.B. Gong, Ph.D. Dissertation, Xiamen University, Xiamen, 1995. [21] T. Tanaka, T. Kubota and Kawaguchi, Anal. Sci., 10 (1994) 895. [22] P.H. Ratliff and W.W. Harrison, Spectrochim. Acta Part B, 49 (1994) 1747.