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Optical Emission Spectrometry with an Inductively Coupled Plasma Operated in Argon-Nitrogen Atmosphere Akbar Montaser" and J. Mortazavi Department of Chemistry, Arya Mehr University of Technology, Tehran, Iran
A 27.12-MHq 2.5-kW generator using a crystal-controlled oscillator was employed to generate an annular-shaped inductively coupled plasma (ICP) operating in a low flow argon-nitrogen atmosphere. The plasma gas flow may contain from 0 l o 100% N,. The minimum RF power irequired l o sustain the Ar-N2 plasma was about 1000 W. Compared to the Ar ICP, the Ar-N, plasmas were physically smaller in size, their spectra exhibited greater intensity, and the optimum observation heights were located lower in the plasma. Comparison of the detection h i t s of Ag, AI, As, B, Bi, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mo, Ni, Se, and Zn in the Ar and Ar-N, ICPs operated at identical power and flow rates indicated that the Ar plasma yielded generally superior results.
The Ar inductively coupled plasmas (ICPs) are remarkable vaporization-atomization-excitation sources which can provide high powers of detection, large dynamic ranges, and relative freedom from interferences for almost all elements. The analytical utility of ICPs operated in the Ar atmosphere used in conjuction with optical emission spectrometry (OES) has been documented (1-4). Furthermore, the Ar ICI? has been evaluated as an atomization source for atomic absorption ( 5 ) and atomic fluorescence spectrometry (6). Despite the increasing acceptance of the Ar ICl?s, the use of an Ar ICP has prevented its acceptance for spectrochemical analysis in those countries that Ar is either expensive or not produced locally. It would, therefore, be advisable to evaluate the use of economical diatomic gases, such as N2 as the discharge medium. Historically, Greenfield and co-workers (3, 7-10) were the first to introduce mixtures of argon and diatomic gases into the plasma gas or the nebulizer carrier gas flows to examine the effect of NP, Oz, He, and air on spectrochemical data. Later, Capitelli et al. (12) found that the extent of aluminum oxide particles decomposition in their plasma was greater in the Ar-N2 admixture than in the Ar atmosphere. Although Truitt and Robinson (12,13)were also able to introduce nitrogen into the plasma, the emission intensity was not significantly affected. This observation, however, might be peculiar to the plasma configuration, ( 5 , 14-18) the operating conditions, and the mode of plasma generation (1-4, 16, 17) utilized. Recent theoretical and experimental investigations of ICPs operating with diatomic gases have failed to provide stable low power plasinas which can operate a t low gas flow rates (19-23). In the present study, a 2.5-kW, 27.12-MHz crystal.controlled generator is employed to generate a low flow annular-shaped ICP operating in an argon-nitrogen atmosphere. The nitrogen gas is introduced only in the plasma gas flow of the ICP. The nebulizer carrier gas is still argon as used in the conventional ICP. In the sections that follow, the experimental facility and the procedure for obtaining a stable argon-nitrogen plasma are described. Our experimental facility differs from those of the earlier reports on Ar-N2 ICP (7-13), but it is similar to almost all of the commercial Ar ICPs using a low flow torch (17). The influence of 70N2 in the plasma gas flow on plasma
Table I. Experimental System component
description and supplier
scanning monochromator
Model EUE-700 (GCAI McPherson Instrument, Acton, Mass.) a 0.35-m single-pass Czerny-Turner monochromator with a 1200 linesimm plane grating replica blazed for 250 nm; reciprocal dispersion of 2 nmimm and aperture ratio of f i 6 . 8 . Model EU-701-30 photomultiplier module (GCAi McPherson Instrument, Acton, Mass.). R372 photomultiplier tube with S-5 spectral response. Model EU-700-32 controller (GCAIMcPherson Instrument, Acton, Mass.). Consisted of Model HFP-2500 D RF generator with a rated power output of 2500 W, and a crystal-controlled frequency of 27.12 MHz, a Model APCS-1 auto power control, a Model AMN-2500 E auto matching network, a Model PT-2500 plasma torch stand, a water-cooled load coil made from 3 turns of lIa-in.silverplated copper tubing, and a Model T1.5 torch assembly. The onepiece torch is constructed of precision bored and ground quartz tubing with an overall length of 1 2 5 mm, an i.d. of 18 mm, and 0.d. of 20 mm (Plasma Therm, Inc., Kresson, N.J.). Model TN-1 right-angle pneumatic type nebulizer and Model SC-2 dual tube aerosol chamber (Plasma Therm, Inc., Kresson N.J.). The aerosol was fed directly to the plasma and no external desolvation apparatus was used.
photomultiplier tube and power supply
data acquisition system ICP automatic system
nebulization system
stability, physical size, and plasma spectra is presented and compared to results obtained from an Ar ICP. Detection limits of 16 elements are also reported. The influence of A1 on Ca emission a t the concentration ratio of 1000 are compared for Ar ICP and the Ar-N2 plasmas.
EXPERIMENTAL Instrumentation. The components of the ICP-OES system are described in Table I. A 1:l image of the plasma was formed on the entrance slit of the monochromator with a spherical plano-convex lens made of Suprasil having a diameter of 5 cm. An
0003-2700/80/0352-0255$01.00/0 0 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
y)o FORWARD POWER, W
Figure 2. Influence of N, flow rate in the plasma gas flow and the forward power on the tuning capacitance. The number on each curve refers to the N, flow rate. The total plasma gas flow rate is 15 L/min FORwMO p o w ,w
Figure 1. Influence of N, flow rate in the plasma gas flow and the
forward power on the reflected power. The number on each curve refers to the N, flow rate. The total plasma gas flow rate is 15 L h i n adjustable diaphragm placed on the lens allowed matching of the lens and monochromator aperature. The three-dimensional positioning of the entire plasma torch system was controlled by a mechanism similar to that described previously (6). Argon and nitrogen mixed in a standard T-connector were introduced into the plasma gas flow input of the torch assembly. Procedure for Obtaining a Stable Argon-Nitrogen Plasma. The plasma should first be generated in an all Ar atmosphere by the procedure commonly used ( 2 7 ) . The forward power is increased to about 1000 W and the nebulizer carrier gas flow is initiated. In order to introduce N2 into the plasma gas flow, an Ar auxiliary plasma gas flow rate of about 1.5 L/min was maintained. The Ar in the plasma gas flow is then decreased toward zero, while the N2 flow is increased to about 15 L/min. Since the automatic servo-driven matching network should follow the impedance alteration of the plasma, the change from 100% Ar to 100% N2 in the plasma gas flow should be relatively slow and span over a time interval of about 2 min. Prepration of Solutions. Stock solutions of 1000 pg/mL of all elements were prepared from their reagent grade salts. All solutions were stored in polyethylene bottles and prepared within hours of use to minimize ana& absorption. All analyte and blank solutions contained I % nitric acid.
RESULTS AND DISCUSSION Effect of % N2i n P l a s m a Gas Flow on P l a s m a Stability. The power requirements of the plasmas, determined when a wet aerosol was introduced into the ICP, were found to be related to the plasma gas type. The minimum forward power required for sustaining the Ar and the Ar-N, ICPs were about 750 and 1000 W, respectively. The stability of the optimized ICP systems was similar for the same analyte concentration. For example, the relative standard deviation of 10 consecutive determinations of a 0.1 pg/mL Cd solution was about 1-29’0 for both plasmas at a forward power of 1200 W. The influence of the 70N2 on the reflected power is shown in Figure 1. The reflected power increases with 9’ N2 in the plasma gas flow. For a particular Ar-N2 composition, the reflected power decreases as the forward power is increased. The tuning and the fine tuning capacitors also change with YO N2 and the forward power. This is illustrated in Figure 2. For an Ar plasma operated at 1200 W, the tuning and fine tuning capacitances are about 100 and 30 pF, respectively. When the N2 flow rate is increased to 1 L/min (Ar = 1 4 L/min), the tuning capacitance decreased automatically to about 94 pF. In order to reduce the reflected power to about
‘“.----1-----;7 Qa
-.
__
,
15
20
J
__._
25
NITROGEN FLOW R A T E , L P M
Figure 3. Influence of 100% N, plasma gas flow rate and the forward power on the reflected power. The number on each curve refers to the forward power. Argon in auxiliary plasma flow is 1.5 L/min
7 W, the fine tuning capacitance is manually changed to about 104 pF. As the N2 flow rate is increased from 1to 15 L/min, the tuning capacitance decreases automatically to about 92 pF. For these variations, however, the fine tuning capacitor has no influence on the reflected power. I t should be realized that these variations may be very specific to the model of the commercial auto-matching network utilized in our studies. The Ar-N2 plasmas could be operated at lower plasma gas flow rates. A stable Ar-N2 plasma can be obtained for total plasma gas flow rate of as little as 3 L/min. Figure 3 shows the influence of the plasma gas flow rate and the forward power on the reflected power when the plasma gas is composed only of nitrogen. For a forward power of 1200 W, the reflected power increases from 75 to 105 W as the N2 plasma flow rate is changed from 5 to 20 L/min. When the forward power is increased to 2200 W, the reflected power decreases and varies from 30 to 45 W for the same flow variations. As far as the power requirement is concerned, the present results contradict the previous observations (3, 7 , 9 ) that the use of more than 20% Nzin the plasma gas required a R F generator greater than 2.5 kW. Greenfield and associates employed a 15-kW generator (3,9) and appreciably high gas flow rates in a larger torch (the plasma, the auxiliary plasma, and the nebulizer carrier gas flow rate were 2&70,10-35, and 2-3 L/min, respectively). Recently Greenfield and McGeachin reported ( I O ) that they sustained Ar-Nz plasmas using a high flow torch at generator operating frequencies of 36 and 7 MHz when the power in the plasma was 1.11 and 1.29 kW, re-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
, Figure 4. (A) Ar plasma. The plasma gas flow rate is 15 L/min. (B) Ar-N, plasma. The plasma gas contains 100% N, 15 L/min
spectively, Therefore, our present operation is novel, not because of the low power, but because a low-flow torch was operated with nitrogen. Influence of N2 in Plasma Gas Flow on Plasma Physical Size. Increases in % N2 caused the plasma size (diameter, length, volume) reduction. Figure 4 shows a comparison of a Ar plasma and an Ar-lOO% N2 plasma. This observation is in agreement with Reed's statement that plasmas in diatomic gases are electrically smaller in diameter than the equivalent plasma in argon (24). The size reduction noted also by Greenfield and associates (3,10) was explained in terms of absorption of energy upon dissociation of the nitrogen gas. Consequently, the plasma outer surface becomes cooler and less conducting and a thermal pinch results. The above phenomenon has also been described (10, 19, 20) in terms of temperature dependence of the thermal-conductivity function of nitrogen compared to argon. The thermal conductivity of argon increases monotonically with temperature, while that of N2 rises to a peak a t about 6500 K, falls to a minimum a t approximately 9000 K, and then increases again with higher temperature. As a direct consequence of these differences, the radial temperature of the Ar-N, plasma may pass through an inflection similar to the effect predicted for the N2 plasma (201, and the discharge will appear optically smaller. The electrical conductivity of nitrogen also undergoes a sharp drop a t 7000 K (19) and may influence the plasma size reduction. Influence of N, in Plasma Gas Flow on Plasma Spectra. The plasma spectra, consisting of molecular band,
257
atom, and ion line spectra, were generally more intense in the Ar-NZ ICP as compared to the Ar plasma. The plasma spectra shown in Figure 5 were compared from 200 to 500 nm when the plasma were operated with dry argon in the nebulizer carrier gas flow. The plasma gas flow rate was 15 L/min N2 in the Ar-N2 plasma and the observation height was about 17 mm above the load coil. The molecular band spectra resulting from atmospheric and plasma gases and their reaction products included those of N2, Nzf, NH, and NO. Particular attention should be paid to the y-band system of NO (12) in the Ar-N2 plasma, which extends from 200 to 280 nm, a wavelength range in which many elements have their most useful emission lines. The NO y band was eliminated when the O2 impurity present in the local N2 tank was removed and the plasma box was flushed with pure N2. Compared to the Ar ICP, the spectrum for the Ar-N2 plasma was relatively free of bands and lines between 350 to 500 nm. Atom and ion line spectra were obtained when solutions of elements were individually introduced into the plasma. The net emission intensity of each element was evaluated by subtracting the intensity of the background emission from the intensity of the analyte plus the background emission. Both the atom and ion spectra were enhanced in the argon-nitrogen ICPs. Figure 6 shows the effect of % N2 in the plasma gas flow on the net Cd atomic emission signal, background signal, and the signal-to-background ratio. 'The plasma gas flow rate and the forward power are 15 L/min (md 1200 W, respectively. It can be seen that the Cd atomic emission is improved as the N2 flow rate is increased from 0 to about 2 L/niin. The atom line is relatively unaffected when the nitrogen flow is increased from 2 to 5 L/min, but it begins to decrease for higher N2 flow rate. When the plasma gas flow contains only nitrogen, the Cd atomic emission signal is still superior to the result obtained from an Ar-ICP at these operating conditions. Similar behavior is observed for the background signal. The signal-to-background ratio, however, irj greatest for an Ar-ICP and decreases as nitrogen replaces ilrgon in the plasma gas flow. Investigation of the effect of '70 N2 on the net analyte emission intensities indicated that the maximum net intensity occurred when the amount of N2 in the plasma flow ranged between 5 to 15% for all elements studied. This was generally observed for other observation heights. Figure 7 shows that atom line intensities, background intensities and the signal-to-background ratios are enhanced a t lower plasma gas flow rates. This is illustrated for four Ar100% N2 plasmas where Cd is used as the test element. The forward power and the observation height are about 1200 W and 17 mm above the load coil, respectively. When the forward power is changed from 1200 to 2300 W, the net emission intensity and the background intensity are increased. This
Figure 5. (A) The Ar plasma spectrum. (B) The Ar-N, plasma spectrum. The plasma gas contains 100% N,. Forward power and plasma gas flow rates are 1200 W and 15 L/min, respectively for both plasmas. The spectra are compared from 200 (left margin) to 500 nm
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980
I
8
30
6
20
4
2
3
4
8
B
10
14
I2
16
I 20
18
NITRw35N F L O W R A I E ~ L P M
Flgure 6. Influence of N2 flow rate in the plasma gas flow on the net Cd atomic emission intensity (S), background intensity (B), and the signal-to-background ratio (SIB). Forward power = 1200 W, total plasma gas flow rate = 15 LImin and h = 228.8 nm 7
I
IOt
1000
- - - - - - - - -o
NITROGEN FLOW R A T E , LPH
i
Flgure 7. Influence of 100% N, plasma gas flow rate on the net Cd atomic emission intensity (S), background intensity (B), and the signal-to-background (SIB). Forward power = 1200 W, and = 228.8 nm
auxiliary plasma gas flow rate nebulizer carrier gas flow rate sample nebulization rate
40 pm
1200 w about 3 W 1 5 L/min of N , or Ar or Ar + N, 1.5 Limin Ar for Ar-N, plasmas. 1.0 Limin Ar 2.0 mL/min
is demonstrated in Figure 8 for 100% Nz plasma gas flow rates of 5, 10, and 15 L/min. The signal-to-background ratio degrades with increased forward power, except for the plasma with plasma gas flow rate of 15 L/min where the maximum ratio occurs at 1500 W. It should be noted that under the same operating conditions, the optimum observation height for most elements excited in the h - N 2 plasma is 12 mm above the load coil which is lower than the value obtained for the Ar plasma (18 mm). Analytical Results. Following an initial orientation study described above, the ICP parameters were set a t sensibly selected levels, Table 11. For all the comparisons performed here, the forward power was about 1200 W, the auxiliary plasma gas flow rate equaled 1.5 L/min of Ar for the Ar-N2 ICP, the Ar-N2 plasma contained 10% Nz in the plasma gas flow, and the observation heights were 18 and 12 mm above
1800 2200 260( FOWARD POWER, W
Figure 8. Influence of forward power on the net Cd atomic emission intensity (S), background intensity (B), and the signal-tc-background ratio (SIB) for total N, plasma gas flow rates of 5 (a), 10 (b), and 15 (c) LImin at the analysis wavelength of 228.8 nm. Observation height = 17 mm
Table 111. Comparison of Optical Emission Detection Limits (ng/mL) of the Ar Plasma and the Ar-10% N, Plasma for a Forward Power of 1 2 0 0 W
Table 11. Experimental Parameters entrance and exit slits generator forward power generator reflected power plasma gas flow rate
1400
element Ag A1 As B Bi
Ca Cd
co
Cr cu Fe
Mg Mo Ni
Se Zn
analysis wavelength, nm 328.0 308.2 193.6 249.7 223.0 317.9 228.8 228.6 357.8 324.7 259.9 279.5 202.0 351.5 196.0 213.8
detection limits Ar ICP Ar-10% N, ICP
-
0.8
8.0 1.5
11.0
11.0
4.0
2.0 4.0 8.0 0.08 2.0
0.4 0.2 2.0 2.0 6.0 1.0 11.0 0.1
4.0 4.0 10.0 0.4 6.0 1.0 0.1
7.0 5 .O 15.0
3.0 11.0
0.3
the load coil for the Ar and the Ar-N2 plasmas, respectively. The detection limits obtained for 16 elements with an Ar plasma and an Ar-10% N2 plasma are compared in Table 111. Each solution contained one element. The detection limit was defined as the analyte concentration required t o give a n average net emission intensity equal to three times the standard deviation of the background emission. I t can be seen from the table that the detection limits yielded by the Ar plasma are generally superior t o those from the Ar-10% Nz plasma.
Anal. Chem. 1980, 52, 259-263
Detection limits were also obtained when the plasma gas flow contained 100% N2 at the same experimental conditions. Compared to the Ar-10% N2 ICP results presented in the table, all detection limits deteriorated by a factor of 2-5. It should be borne in mind that the present experimental parameters employed for the Ar plasma are generally similar to the optimum conditions used in other laboratories. However, the Ar-N2 ICP detection limits can be improved if the gas flow rates, the forward power, and the observation height are optimized. The Ar-ICP and the Ar-10% N2 ICP were also compared for the influence of 50 pmol/mL of A1 on the determination of 0.05 pmol/mL of Ca at the spectral line of 317.9 nm. The experimental parameters were similar to those for detection limit determinations, except for the observation height which was 15 mm for the two plasmas. The A1 suppressed the net Ca emission by 15 and 5% for the Ar plasma and the Ar-N2 plasma, respectively. Similar results also noted (11) by other investigators indicate that the Ar-N2 ICP is hotter than the corresponding Ar plasma.
LITERATURE CITED (1) R. M. Barnes, Crlt. Rev. Anal. Cbem., 7, 203 (1978). (2) V. A. Fassel and R. N. Kniseley, Anal. Chem., 46, I l l O A , 1155A (1974). (3) S.Greenfield, I. L. Jones, H. McGeachin, and P. B. Smith, Anal. Cbim. Acta, 74, 225 (1975).
(4)
(5) (6) (7) (8) (9)
(IO) (11)
(12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
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P. W. J. M. Bournans and F. J. DeBoer, Spectrochim. Acta, Part 8,30,
309 (1975). R. H. Wendt and V. A. Fassel, Anal. Chem., 38, 337 (1966). Akbar Montaser and V. A. Fassel, Anal. Cbem., 48, 1490 (1976). S.Greenfield, Proc. Soc. Anal. Cbem,.,2, 111 (1965). S.Greenfield and P. B. Smith, Anal. Chirn. Acfa, 57, 209 (1971). S. Greenfield and P. 8. Smith, Anal. Cbim. Acta, 59, 341 (1972). S.Greenfield and H. McGeachin, Anal. Chim. Acta. 100, 101 (1976). M. Capitelli, F. Crarnarossa, L. Triolo, and M. Molinari, Combust. Flame, 15, 23 (1970). D. Truitt and J. W. Robinson, Anal. Cbim. Acta, 49, 401 (1970). D.Truitt and J. W. Robinson, Anal. Cbim. Acta, 50, 61 (1970). S. Greenfield, I. L. Jones, and C. T. Berry, Analyst(London), 89, 713 (1964). G. W. Dickinson and V . A. Fassei. Anal. Cbem., 41, 1021 (1969). P. W. J. M. Boumans and F. J. DeBoer, Specfrochim. Acta, Part 5 , 27 391 (1972). R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon, Anal. Chem., 46, 75 (1974). C. Veillon and M. Margoshes, Spectrochim. Acta, Part 8,23, 503 (1968). R. M. Barnes and S.Nikdel, J . Appl. Pbys., 47, 3929 (1976). R . M. Barnes and S.Nikdel, Appl. Spectrosc., 30. 310 (1976). M. Thompson, 8. Pahhvanpour, and S.J. Watton, Analysf(London), 103, 568 (1978). P. B. Zeernans, S. P. Terblanche, K. Visser, and F. H. Harnm. Appl. Specfrosc., 32, 572 (1978). B. Bogdain, Kontron ICP-Berichet, Ausgabe 2, February 1978. T. 8.Reed, Adv. High Temp. Cbern., 1, 284 (1967).
RECEIVED for review December 28, 1978. Resubmitted July 25, 1979. Accepted October 30, 1979. Presented in part a t the 1978 FACCS Meeting, Boston, Mass.
Determination of the Aqueous Chlorination Products of Humic Substances by Gas Chromatography with Microwave Emission Detection Bruce D. Ouimby,’ Michael F. Delaney,’ Peter C. Uden,’ and Ramon M. Barnes Department of Chemistry, GRC Tower I, University of Massachusetts, Amherst, Massachusetts 0 1003
The aqueous chlorination products of humic and fulvlc acids have been examined by capillary column gas chromatography with an atmospheric pressure hellurn microwave emission detection system. The results indicate that in addition to trihalomethanes, significant numbers of chlorinated phenolic and/or other acidic compounds can be formed. I n addition, the presence of bromide ion in the chlorination mixture is shown to produce bromine-containing compounds. Since many of these compounds cannot be gas chromatographeddirectly, chemical derivatization with diazomethane is employed. The number of halogen-containing substances that appear in the element selective chromatograms is greatly Increased after methylatlon of the samples. Several halogenated phenols and aromatic carboxylic acids have been tentatively identified by retention times.
Halogenated organic compounds in finished drinking water have received much recent attention. In 1974 Rook ( 1 ) and Bellar, Lichtenberg, and Kroner (2) demonstrated that organic precursors in raw water are halogenated by chlorine, producing Present address: Hewlett-Packard Co., Route 41, Avondale, Pa.,
19311. * P r e s e n t address: D e p a r t m e n t of Chemistry, T u f t s University, M e d f o r d , Mass. 02154. 0003-2700/60/0352-0259$01.OO/O
trihalomethanes; a comprehensive review of the area has been published by Trussell and Umphres ( 3 ) . In general, studies indicate that the action of chlorine on humic substances in the raw water is the largest source of trihalomethanes. Very little data however regarding other possible chlorination products of humic substances are available and it is to these that the present study is addressed. Humic substances, amorphous, brown or black, hydrophilic, acidic, polydisperse substances with molecular weights ranging from several hundreds to tens of thousands ( 4 ,are classified as fulvic acid, humic acid, and hymatomelanic acid according to solubility in acid, alkali, and ethanol in the classical soil organic matter classification scheme. All three fractions are found in naturally colored waters, fulvic acid being at much higher concentration than hymatonielanic and humic acids (5, 6). It is accepted that the structures of humic materials vary with location and different conditions but consist mainly of aromatic polyhydroxy, polymethoxy, polycarboxylic acids with smaller amounts of sugars and nitrogen bases. It is not, however, clear to what extent the structures are held together by hydrogen bonds as opposed to being condensed with covalent bonds. Speculative structures were proposed by Dragunov (a,Christman and Ghassemi (8)as shown in Figure 1, and Schnitzer ( 4 ,based on various degradative or nondegradative techniques involving derivatization elemental analysis, functional group analysis, reducing properties, etc. (9). Q 1980 American Chemical Society
