hydrogen capacitively coupled microwave plasma

Specrrochimica Acta! Vol. 48B. No. 4, pp. 521-529. Printed in Great Bntain.

Analytical characteristics

05&t-8547/93 $6.00 + 80 iQ 1993 Pergamon Press Ltd

1993

of a helium/hydrogen microwave plasma*

capacitively

coupled

W. R. L. MAsAMBAtand J. D. WINEFORDNER~ Department

of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A. (Received

11

September

1992;

accepted

21 November 1992)

Abstract-Excitation temperatures and rotational temperatures have been determined in a helium/hydrogen plasma as a function of the concentration of hydrogen in the plasma gas. The helium/hydrogen plasma was then evaluated in terms of matrix effects of Na+ and PO:- on Ca atomic emission signals. Little interference by Na+ on Ca atomic emission signals was observed while there was no suppression of Ca atomic signals by IQat PO:- concentrations below 100 ppm. Above this concentration, some suppression of the Ca signal was observed. Limits of detection determined in this work were comparable to the limits of detection by other workers by capacitively coupled plasmas using molecular gases as the plasma gas.

1. INTRODUCTION COMPARED to

other plasma systems used in optical emission spectroscopy (OES), such as inductively coupled plasma optical emission spectroscopy (ICP-OES), microwave plasma optical emission spectroscopy has the advantage of lower initial and operational cost. The popular use of microwave ovens has resulted in the ready availability of low cost magnetrons capable of delivering medium to high power microwave energy levels. Also, most of the microwave plasmas operate at gas flow rates below those employed in ICP-OES. Most of the early microwave systems were operated at low (cl50 W) or medium (150-400 W) levels resulting in low tolerance for molecular species introduced into the plasmas and severe matrix effects. Microwave induced plasmas (MIPS) show better stability and lower background noise for OES compared to capacitively coupled plasmas (CMPs) operated at comparable power levels and do not suffer from possible contamination from electrode material. In spite of these drawbacks, CMPs have distinct advantages over MIPS. CMPs can be formed over a wide power range. In MIPS, the coaxial cable, feeding power to the cavity, heats up and radiates microwaves at power levels greater than 150 W [l] so that most MIPS employing coaxial cables have to be operated at rather low power levels, with concomitant problems with introduction of aerosols and molecular species, such as solvent vapor, into the plasma. The MIP was therefore initially used as a detector for gases [2], for gas chromatography [3, 41, and for simple easily-fragmented volatile compounds [5]. High power levels may be used in CMPs, which usually employ waveguides rather than coaxial cables as a means of energy transfer from the magnetron to the plasma gas. Operation of CMPs at medium to high powers results in more plasma tolerance to the introduction of aerosols and molecular species. Since the early application of CMPs to the analysis of real samples [6, 71, other applications of CMPs have been reported. GOVINDARAJU et al. [8] reported a method for the analysis of silicate rocks and minerals, after buffering with Sr and Li to eliminate matrix interferences, using a CMP operated at 600 W. BURMAN and BOSTRON [9] compared the performance of CMP and ICP systems for the analysis of geological materials and concluded that CMPs suffered severely from matrix effects except in the presence of ionization buffers. No such buffers were needed for the analysis using * Research supported by NIH-5-ROl-GM38434-05. t On study leave from the University of Malawi, P.O. Box 280, Zomba, Malawi. $ Author to whom &respondence should be addressed. 521

522

W. R. L. MASAMBA and J. D. WINEFORDNER

the ICP-OES. LARSONand FASSEL[lo] and BOUMANSet al. [ll] also indicated superior performance of the ICP compared to the CMP for OES, especially with respect to matrix interferences. In both cases, the CMP was operated at 600 W, whereas the ICP was operated at power levels greater than 1 kW. NAKASHIMAet al. [12] and SUZUKI [13-151 reported CMP-OES analysis of steel. HUANG and BLADES [16] and UCHIDAet al. [17] used CMP-OES systems as GC detectors for organotin compounds. ALI et al. [ 181 used direct solid sampling CMP-OES. ZHANG et al. [19] determined Na, K, and Ca in oyster tissue and low boron glass using CMP-OES. Other workers have determined limits of detection of various elements [19-211. In spite of the potential for analytical use, relatively few studies on CMP-OES have been reported. Helium plasmas have been employed for CMP-OES in order to take advantage of the high ionization energy of helium that should result in efficient excitation of elements introduced into the plasma, especially elements of high excitation energies such as As, halogens, NZ, 02, Fe and Sn [22]. However, limits of detection of chlorine and bromine were lower in a He ICP-OES compared to Ar ICP-OES [22]. The electrical resistivity, heat capacity and thermal conductivity of He are greater than for Ar, leading to more rapid heat dissipation to the torch walls in the case of Ar. This results in difficulties in torch design (231 since the He plasma tends to adhere to the torch walls. Research on He CMPs has therefore developed concurrently with special torch designs [23-291. With Ar CMPs, it was necessary to wet the plasma gas with water for stability [30, 311, whereas with He CMPs, this was not necessary. Addition of Hz to the He plasma gas eliminated the problem of the plasma adhering to the torch walls. CHAN and MONTASER[22] mentioned that He ICPs usually required a mixture of other gases such as Ar and air for stability. The CMP formed when H2 was added to the He plasma gas is centered in the torch and became thinner and taller as the proportion of H2 was increased. The reduction of plasma size with the addition of molecular species has been observed in ICPs [32-351 and has been attributed to the smaller electrical diameter of these plasmas compared to Ar plasmas, to absorption of energy required for dissociation of the molecular species, which led to a cooler plasma surface and to reduced thermal conductivity resulting in a thermal pinch [32, 331. This work investigated the properties of a helium/hydrogen capacitively coupled microwave plasma with respect to the effect of added hydrogen on excitation temperature (T,,,), rotational temperature (T,,,), and emission signals of Zn and Cr introduced by solution nebulization. Also the effect of microwave power on Cr, Mn, and Al signals, and the matrix effects of PO:- and Na+ on Ca emission signals, as well as limits of detection of several elements introduced by solution nebulization into the He/H2 CMP-OES-system were determined.

2. EXPERIMENTAL The experimental system described elsewhere [36] was used. The plasma was formed at the tip of a graphite electrode. T,,, and T,,, were determined at 800 Wand at different concentrations of hydrogen in the plasma gas. Fe and OH were used as the spectrochemical species as described previously [36]; the Fe lines used for T,,, were 372.0, 374.7, 374.8, 374.9, 375.8, and 376.4 nm and the Q,2, Q,4, Q,5, Q,6, and Q,9 rotational lines of OH in the 307-310 nm range were used for T,,,. All of the Boltzmann plots were linear. The He plasma gas flow rate was kept at 6.5 I/min and the hydrogen (which was subsequently mixed with the He) flow rate was varied so that its concentration varied to about 14% (v/v). Emission signals and signal-to-noise ratios (S/N) were determined at 800 W for Zn (10 ppm at 481.1 nm), Cr (5 ppm at 425.4 nm), and Al (10 ppm at 396.2 nm) using pneumatic nebulization without desolvation and were found to give maximal values around 5 mm for these metals as well as most others. Therefore a compromise observation height of 5 mm above the CMP-electrode was used for all subsequent studies. He was used as the carrier gas. The effect of Na’ and PO:- on Ca emission signals was investigated by measuring the emission signals of 20 ppm Ca on Na+ or POj- concentrations of up to 1000 ppm. All solutions

523

Analyticalcharacteristics of a He/H, CMP 5000

4000 G ? 3 FJ G E”

3000

2000

: 1000

0 0

I

1

I

I

3

6

9

12

Hydrogen

content

in plasma

15

gas (%v/v)

Fig. 1. Effect of hydrogenon (a) excitationtemperatureand (b) rotationaltemperatureat 700W. were prepared from commercial atomic absorption standards (Fisher Scientific Co., Fair Lawn, N.J. or Inorganic Ventures Inc., Brick, N.J.).

3. RESULTSAND DISCUSSION 3.1.

Effect of H2 on T,,, and T,, The excitation temperature (T,,,) decreased gradually as the H2 concentration in the plasma gas was increased (Fig. l(a)). Reduction in T,,, with increase in Hz content is likely to be a result of thermalization of the plasma reducing the overall excitation energy of the plasma [37]. Since molecular species can absorb microwave energy, thereby exciting vibrational modes, the absorbed energy is not available for excitation. ABDALLA and MERMET[38] found no difference in TexC for pure Ar and Ar-N2 (7.5%) MIPS. BLADES and CAUGHLIN[39] observed lower T,., when xylene was aspirated into an Ar plasma compared to the temperature obtained when aqueous solutions were introduced into the Ar ICP at the same power level. A decrease in T,,, with increased concentration of N2 up to 6% has been reported for the Ar-N2 ICP [40]. The rotational temperature (T,,,) increased slightly with an increase in Hz concentration (Fig. l(b)). T,,, is usually taken to reflect the gas temperature. These results are therefore in agreement with the early observations of COBINEand WILBUR [41] where the addition of molecular species to inert gases resulted in higher temperatures enabling the CMP to be used as a welding torch when it was not possible to ignite a piece of paper in the pure inert gas plasma. The increase in the rotational temperature may be the result of heat of association of atoms dissociated in the plasma by the microwaves. ABDALLA and MERMET1231 observed a slight increase of T,,, in an Ar-N2 (10%) plasma (T,,, of 2900 K) compared to a pure Ar plasma (T,,, of 2570 K). A less pronounced effect was observed using a He (T,,, of 2030 K) and a He-N2 (7.5%) plasma (T,,, of 2050 K). The results of this work are in agreement with those obtained for electron temperature and gas temperature measurements [42], where the presence of water vapor in an Ar ICP thermalized the plasma. 3.2. Effect of Hz on emission signals Figures 2 and 3 show the signals and S/N (signal-to-background noise) ratios for Zn and Cr, respectively, introduced into the plasma by solution nebulization. The S/N ratios for Zn and Cr (4.5% Hz gave a slightly larger SIN) were almost independent

524

W. R. L. +

MASAMBA and J. D. WINEFORDNER

0

Signal

SIN

11000 x C : al F .E .:

10000

‘5

6000

150

120

9000 8000

\

90

7000

+

2 2

0

\

60

F .;ii a, I:

O1:‘;::-:

5000 4000

30

3000 2000

0

0

3

% hydrogen

9

6

in plasma

12

gas (v/v)

Fig. 2. Effect of hydrogen on Zn emission signal and signal-to-noise

+

0

Signal

ratio at 800 W.

S/N

18000 C>

16000

z al ;: .-

14000

.-: .-:

12000

: .-,” ;;j a,

10000

;

20

16

12 2 3i ”

0

8

0

8000 I-

6000 4000 0

3 % hydrogen

9

6 in plasma

12

gas (v/v)

Fig. 3. Effect of hydrogen on Cr emission signal and signal-to-noise

of the Hz content in the He plasma gas. Therefore, case of nebulization was sample introduction noise.

ratio at 800 W.

the major source of noise in the

3.3. Effect of solution flow rate on Cr emission signals For these studies, the hydrogen concentration in the plasma was held constant at 4.5% and the carrier gas flow rate was found to be optimum for Cr at 6.5 Ymin; these parameters gave maximal signal-to-noise ratios for Cr. The optimum (maximal SIN) solution flow rate for Cr was found to be 1.4 ml/min (Fig. 4). 3.4. Effect of Power The effect of power on the signal and S/N ratios of Cr, Mn, and Al is shown in Figs 5-7. In all cases, a sharp increase of signals with power is observed. The S/N

Analytical characteristics

of a He/H, CMP

525

0

1

0.30

0.70

1.10 Solution

1.50

1.90

flow rate (ml/min)

Fig. 4. Effect of solution uptake rate on Cr emission (+) and signal-to-noise 800 W.

450

550

650

2.30

750 Power

850

950

ratio (0) at

1050

(W)

Fig. 5. Effect of microwave power on Cr emission (+) and signal-to-noise

ratio (0).

ratios also increased continuously over the power levels investigated. This indicates that power levels greater than 1000 W would most likely improve sensitivities and limits of detection. Unfortunately, the electrodes required replacing more frequently at power levels greater than 800 W. A power level of 800 W was therefore chosen for all subsequent measurements. 3.5. Matrix interferences Figure 8 shows the effect of POQ- on a 20 ppm Ca atomic emission signal. The Ca signal decreased by about 20% as the phosphate increased from 10 to 1000 ppm. Figure 9 shows the effect of Na’ on a 20 ppm Ca atomic emission; the Ca signal increased by less than 10% as the phosphate concentration increased from 10 to 1000 ppm. The influence of phosphate on the atomic signal from Ca is a classic example of

526

W. R. L. MASAMBAand J. D. WINEFORDNER 4000

80 3000

60 2000

1000

0 400

500 600 700 800 Power

900 10001100

(W)

Fig. 6. Effect of microwave power on Mn emission (+) and signal-to-noise ratio

8000

(0).

400

7000 320 6000 z .-& fn S ‘iij .? k= .-k? z 5L z

5000

240 ? v)

4000 160

3000

2000 80 1000

0 500

600

700

800

Power

900

1000 1100

(W)

Fig. 7. Effect of microwave power on Al emission (+) and signal-to-noise

ratio (0).

Analytical characteristics of a He/H, CMP

0.00

’ a

’

527

I

100

10 Concentratm

of PO,‘-

kmm)

Fig. 8. Effect of PO:- on the Ca emission signal.

s

3 0.30z c

0.20-

T “L

6

0

v

0

I

5 6c 0”

O.lO-

J 1000

100 Concentration

of Na’

(ppm)

Fig. 9. Effect of Na’ on the Ca emission signal.

compound formation in the condensed phase in the flame [43-451. The effect disappears in an C;H,-N,O flame or after the addition of a releasing agent [46-481. The reduced extent of this interference in the ICP has been used to demonstrate the superiority of the ICP over Aames for analytical application [49]. The effect of an easily ionizable element (EIE) in flame spectroscopy has been well-characterized. Enhancement in the emission intensity from atom lines is observed when signals with and without the EIE are compared [44, 501. The situation is not so well-defined in the ICP, partially because of the variability in plasma operation conditions between investigators and partially because the influence of the EIE in the ICP is complex. KOIRTYOHANN ef al. [Sl] observed slight enhancement of the Ca atom emission signal in the presence of an EIE up to 10 mm above the load coil, an increased enh~cement of the signal between 10 and 15 mm above the load coil, and then almost no effect above 15 mm above the load coil. LARSON et al. [52] observed enhancement of the Ca atomic emission signal, the magnitude of which increased with increase in molar ratio of Na to Ca. The enhancements also increased with increasing height above the load coil. Enhancements of atomic Ca emission signals in the OES were also observed by ABDALLA et al. [53] and KAWAGUCHI et al. [54]. DAHMEN [55] reviewed the rather significant ionization interferences for many elements (atomic lines) and supports the need for ionization buffers in many of the older CMP-OES-systems. BOUMANS et al. [ll] also indicated the severe matrix interferences, especially those owing to alkali

528

W. R. L. MASAMBAand J. D. WINEFORDNER

Table 1. Limits of detection (ppm) for several elements determined by solution nebulization into CMP and ICP

Element

Wavelength (nm)

This work* (CMP)

Al Mn cu Cr Li Sr K Na Sn Zn

396.15 403.08 324.75 425.43 670.78 460.73 766.49 589.59 303.41 481.05

0.3 0.1 0.08 0.05 0.004 0.05 0.02 0.01 0.1 0.1

Ref. [20] (CMP)

Ref. [21] (CMP)

Ref. [55] (fCP)

Ref. [56] (JCP)

0.003 0.009 0.00005 0.01 -

0.3 0.05 0.01 0.08 0.7 0.04 0.04 0.03 1.8 0.2

0.02 0.001 0.005 0.006 0.9 0.0004 0.03 0.1 0.002

0.5 0.005 0.01 0.26 0.01 5

* LODs defined as concentration in ppm resulting in a signal of three times the standard deviation of blanks. The wavelengths are those used in this work.

metals in CMP-OES and the lack of use of previous CMP-OES-systems analysis. In our studies with CMP-OES, classical matrix interferences and sodium on Ca atomic emission were similar to those in ICP-OES.

for practical of phosphate

3.6. Limits of detection and precision In Table 1, limits of detection (3~) are given for several elements including Cr and Zn introduced by solution nebulization into our CMP-OES-system. Those obtained using molecular gas CMP-OES (and ICP-OES) by other workers are included [20, 21, 55, 561 for comparison. This work gives limits of detection comparable to those obtained for previous molecular gas CMP-OES-systems [20, 551. The limits of detection in Ref. [55] however were determined at 2a while those in Ref. [20] were with aerosol desolvation. The limits of detection in this work were inferior to those by Ar ICP-OES [56] and He CMP-OES at 900 W [21]. The precision (% RSD) was approximately 2-5% for all elements in Table 1 at concentrations of 100 X the limit of detection.

4. SUMMARY

Hydrogen in the plasma gas slightly increased T,,, and reduced T,,,. Hydrogen reduced emission signals for elements nebulized into the CMP as solutions, and had little effect on the signal-to-noise ratios of these elements. Signals increased as power was increased and rather small matrix effects were observed for the atomic emission signals of Ca by Na. There was no suppression of the Ca atomic emission signal for PO:- concentrations below 100 ppm but some suppression of the Ca emission signals was observed at concentrations higher than 100 ppm. Limits of detection were comparable to those by CMPs employing molecular gases as the plasma gas but inferior to those by ICP-OES. The limits of detection could, however, be improved by the use of ultrasonic nebulization or desolvation after pneumatic nebulization using a He nebulizer. REFERENCES [l] [2] [3] [4] [5] [6] [7]

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of a He/H, CMP

529

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