Analytical performance of a low-gas-flow torch optimized for

low-flow torch as compared with the conventional ICP. A number of promising plasma sources are currently being used in analytical atomic spectrometry...
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Anal. Chem. 1984, 5 6 , 283-288

(12) Langer, S.H.; Sheehan, R. J.; Huang, J.-C. J. Phys. Chem. 1882, 86, 4605-4618. (13) Karger, B. L.; Snyder, L. R.; Eon, C. J. Chromatogr. 1978, 125, 71-a~. (14) Liptay, W. "Excited States"; Lim, E. C., Ed.; Academic Press: New York, 1974;Vol. 1, pp 129-229.

(2) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. J. Am. Chem. SOC. 1877, 99,8027-6038. (3) Brady, J. E.; Carr, P. W. J. Phys. Chem. 1982, 86, 3053-3057. (4) Onsager, L. J. A m . Chem. SOC.1938, 58, 1486-1493. (5) Block, H.; Walker, S. M. Chem. Phys. Lett. 1873, 19, 363-364. (6) Reitz, J. R.; Milford, F. J.; Christy, R. W. "Foundatlons of Electromagnetic Theory", 3rd ed.; Addison-Wesley: Reading, MA, 1979; pp

86-87. (7) Box, G. E. P.; Hunter, W. G.; Hunter, J. S."Statistics for Experiments"; Why-Interscience: New York, 1978;Chapters 6-7. (6) Kamlet, M. J.; Kayser, E. G.; Jones, M. E.; Abboud, J.-L. M.; Eastes, J. W.; Taft, R. W. J. Phys. Chem. 1978, 82, 2477-2483. (9) Nitsche, K A . ; Suppan, P. Chimia 1882, 36, 346-348. (10) Parcher, J. F.; Hansbrough, J. R.; Koury, A. M. J. Chromatogr. Sci. 1870, 16, 183-189. (11) Conder, J. R.; Young, C. L. "Physlcochemical Measurement by Gas Chromatography"; Wiley: New York, 1979;Chapter 5.

283

RECEIVED for review June 13,1983. Accepted October 3,1983.

J. E. Brady was supported by an A.C.S. Analytical Divisional Full Year Fellowship sponsored by the Upjohn Company. This work was supported in part by grants from the National Science Foundation (CHE-8205187)and the 3M Co. (St.Paul, MN).

Analytical Performance of a Low-Gas-Flow Torch Optimized for Inductively Coupled Plasma Atomic Emission Spectrometry Akbar Montaser,* G . R. Huse, R. A. Wax, and Shi-Kit Chan

Department of Chemistry, George Washington University, Washington, D.C. 20052 D. W. Golightly, J. S. Kane, and A. F. Dorrzapf, Jr.

US.Geological Survey, 957 National Center, Reston, Virginia 22092

An Inductively coupled Ar plasma (ICP), generated in a lowflow torch, was lnvestlgated by the simplex optimization technique for simultaneous, multieiement, atomlc emission spectrometry (AES). The variables studied included forward power, observation height, gas flow (outer, intermedlate, and nebulizer carrler) and sample uptake rate. When the ICP was operated at 720-W forward power wllh a total gas flow of 5 L/min, the signal-to-background ratios ( S I B ) of spectral llnes from 20 elements were either comparable or inferior, by a factor ranglng from 1.5 to 2, to the results obtained from a conventional Ar ICP. Matrlx effect studles on the Ca-PO, system revealed that the plasma generated in the low-flow torch was as free of vaporizatlon-atomization interferences as the conventional ICP, but easily Ionizable elements produced a greater level of suppresslon or enhancement effects which could be reduced at higher forward powers. Electron number densltles, as determlned via the series limit line merglng technlque, were lower In the plasma sustained In the low-flow torch as compared with the conventional ICP.

A number of promising plasma sources are currently being used in analytical atomic spectrometry. They are inductively coupled plasmas (ICP) ( I ) , direct current plasmas (DCP) (1, 2), and microwave-induced plasmas (MIP) (I,3-5). Among these sources, argon-supported ICPs are excellent vaporization-atomization-excitation-ionization sources which are commonly employed for analytical atomic emission spectrometry (AES). Although the Ar ICP-AES method exhibits superior analytical performance for elemental analysis of a variety of materials ( I ) , it has disadvantages of requiring a relatively high rf power, a lot of laboratory space, and high argon gas flows. With reference to the rate of gas consumption, it is important to note that the conventional ICP torch requires 15-22 L/min of argon. If the ICP is run for 40 h per week, the estimated gas cost approaches $10 000 to $12 000

per year. The relatively high operating costs and the initial cost of an ICP instrument thus can be considered as one of the impediments to the acceptance of ICP-based methods. Furthermore, because of the cited limitations, the commercially available ICP-AES instruments presently are not applicable to analysis which has to be conducted in a mobile laboratory or a ship. T o reduce the argon gas flow and the input power requirements of the ICP, a number of investigators have explored the use of water-cooled torches (6-9) or torches cooled externally by compressed air (9,10) or have reduced the diameter of the gas introduction nozzle, the annular spacing between the intermediate and the outer tube, or the actual torch size (11-1 7). Although the resulting plasmas apeared stable, they suffered from the disadvantages of exhibiting inferior detecting powers and enhanced interferences, or they required a modification of the load coil and the impedance matching network (7-1 7). Evidently, what would be desirable is an ICP torch of the size (18 mm i.d.) commonly utilized in analytical laboratories, but with internal dimensions designed to allow operation at reduced input power and gas flow levels, while a t the same time preserving the excellent analytical performance of the conventional ICP torches. Recently, i t has been possible (18-20) to operate an 18 mm i.d. torch at a total gas flow and a forward power of 5 L/min and 450 W, respectively. However, the analytical performance of this torch for the elemental analysis of a variety of samples has not yet been critically documented. In the present study, the simplex technique (21-32) is utilized to optimize the plasma sustained in a low-flow torch. By use of a suitable objective function (32) optimization is conducted simultaneously for many elements while the gas flow levels (outer, intermediate, and nebulizer aerosol carrier gas flows), forward power, observation height, and sample uptake rate are changed simultaneously. After the optimum conditions are established, the analytical capabilities of the low-flow torch are compared to those of a conventional torch for atomic emission spectrometry. This evaluation includes

0003-2700/84/0356-0283$01.50/00 1984 American Chemical Society

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Table I. Experimental Facilities and Operating Conditions for the Spectrometers system I

system I1

direct readeriscanning monochromat or

Model 1160 (Jarrell-Ash Co, Waltham, MA); 0.75 m focal length, 2400 grooves/mm grating blazed at 270 nm, reciprocal linear dispersion of 0.53 nm/mm in the first order; 25 pm entrance slit; 50 pm exit slit; 3 mm slit height

wavelength for line (nm) and background measurementa

Sn I1 189.9 (-32), As 1 1 9 3 . 6 (-19), Mo I1 202.0 (t 23), Pb I1 202.3 (--32), Cr I1 267.7 (-32), B 1249.7 (-32), Ge 1 2 6 5 . 1 (-29), Mn I1 257.6 (-13), CO I1 228.6 (-29), Cd I1 214.4 (-23), Si I 251.6 (-17), Fe I1 259.9 (t 31), P I 214.9 (-32), Ba I1 455.4 (t 28), Ni I1 231.6 (t 28), Ti I1 334.9 (-32), Be I1 313.0 (-27), Th I1 283.7 ( + 16), V I1 292.4 (-32), and Ca I1

Model 2061 monochromator and Model 787 microdrive (GCAiMcPherson Instrument, Acton, MA); 1 m focal length; 1200 grooves/mm holographic grating with an efficiency of 35% at 300 nm; reciprocal linear dispersion of 0.833 nm/mm the scanning monochromator is used for matrix effect studies and electron number density measurements

imaging optics

off-axisconcave front surface mirror, magnification of 3.4X (Jarrell-Ash Co.)

detection system

Type R 300, R 427, R 955, and R 889 side window photomultiplier tubes (Hamamatsu Corp., Middlesex, NJ) were used to cornert light intensities to electrical signals; the JarrellAsh readout system consisted of a PDP 1 1 / 3 4 minicomputer (Digital Equipment Corp.) which controlled individual analog integrators and the multiplexed A/D converter

recorder

Omni Scribe Recorder (Houston Instrument, Austin, TX) 2.5 kW, 27.12 MHz rf generator (Model 2000, Jarrell-Ash Co.) using the automatic forward power control unit with no automatic impedance matching network. The water cooled, 27 mm i.d. load coil had 4 turns of in. copper tubing; the distance between the intermediate tube and the bottom of the lower coil was normally 2 to 3 mm; a fixed cross flow nebulizer and a single-tube spray chamber was used; a peristaltic pump (Minipuls 2, Gilson Medical Electronics, Inc., Middleton, WI) was used to deliver sample to the nebulizer a mass flow controller (Model 8240, Matheson Gas Co., East Rutherford, NJ) was used for the nebulizer carrier gas line; other components were similar to those described elsewhere ( 3 3 )

317.9 ( + 29)

inductively coupled plasma system

gas handling system

spherical planoconvex quartz lens, 5 cm diameter, 20 cm focal length; a 1:l image of the plasma was formed on the entrance slit of the monochromator Type 9558QB (EM1 Electronics Ltd., Middlesex England) 11-stage photomultiplier tube with a spectral response of 160-850 nm; the cooled tube was operated a t -800 V and its current output was amplified by a linear current-tovoltage converter (Model 427) or an 8decade logarithmic current-to-voltage converter (Model 26220, Keithley Instrument Inc., Cleveland, OH) Model 164 (Houston Instrument, Austin, TX) 5 kW, 27.12 MHz rf generator (Model HFD5000D, Plasma Therm, Inc., Kresson, N J , with associated electronics such as automatching network and automatic forward power control unit; all other details were similar to system I except that a 3-turn load coil was employed and concentric and cross-flow nebulizers, using a dualtube spray chamber, were used

same as system I

a The numbers in parentheses refer to the position of an oscillating refractor plate (located behind the entrance slit) which is used to shift the spectral line profiles for peak and background measurements. The background for each line is selected at an x unit (each unit is approximately 0.032 nm) to the low (-) or the high ( + ) wavelength side of each spectral line peak.

a comparison of signal-to-background ratios, relative freedom from interference effects, and the performance of the torches for the elemental analysis of a reference geological material. EXPERIMENTAL SECTION A. ICP-AES System. Two ICP-AES systems, described in Table I, are used in this study. Details on the design and construction of the low-flow torch are given elsewhere (18,19). The procedure for plasma generation and stabilization is similar to that of a conventional torch. B. Selection of Spectral Lines, Response Function, and Optimum Conditions. So that the data may be generally applicable to analyte lines commonly observed with an Ar ICP, the behavior of 20 different ion and neutral atom lines in the wavelength region of 189 to 455 nm was measured. The selection of spectral lines, listed in Table I, was influenced by the availability of suitable lines on the polychromator as well. Most of these lines (33-36) have exhibited excellent sign&-to-background (SIB)ratios in the conventional Ar ICP. The figure of merit used for the calculation of response function (32)was the weighted S I B value summed over all elements. Because of its greater statistical

variation (33),the signal-to-noise ( S I N ) ratio was not employed for response function calculation. An integration time of 1 s was used for both peak and background measurements. As shown in Table I, the background for each line is measured at an x unit (each unit is approximately 0.032 nm) to the low (-) or the high (+) wavelength side of each spectral line peak. Simplex optimization was initially conducted for the first ten elements listed in Table I simultaneously. The six parameters used in optimization studies were observation height (distance above the load coil), forward power, outer gas flow, intermediate gas flow, nebulizer carrier gas flow, and sample uptake rate. In accordance with the recommendation of Yarbro and Deming (37), a large initial simplex was employed. Optimum conditions were reached in 30 to 40 simplices. Responses were taken as optimal when the parameters had converged to within the precision with which they could be adjusted. When the simplex optimization was repeated for other elements, similar optimum conditions were obtained. Thus, one set of operating conditions may be used to determine all elements simultaneously. C. Preparation of Solutions. For optimization studies and S I B measurement, two sets of multielement solutions were made

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Table 11. Ranges of Variables Studied and Optimum Conditions Determined by the Simplex Method for the Low-Flow Torch optimum conditions commonly for the low-flow torch employed conditions for the six parameter four parameter boundary limits conventional torcha optimization optimization variable 14 11 1.observation height,b mm 8-29 15-18 920 7 20 650-1400 1100 2. forward power,c W 11 4 3. outer gas flow: L/min 4-1 5 18-22 0.16 0.5 4. intermediate gas flow, 0.0-2.0 0.0 L/min 5 . nebulizer carrier gas flow,b 0.1-0.66 0.66-1.0 0.56 0.5 L/min 0.66 0.5 0.2-1.0 1.0 6. sample uptake rate,b mL/min Experimental constraints prevent measurements a Commonly employed conditions in this and other laboratories ( 3 4 ) . outside boundary limits. The upper boundary limit for forward power should be reduced to 950 W when the torch is operated at an outer gas flow rate of 4 L/min. up from stock solutions of individual elements prepared from their reagent grade salts. One solution contained Sn, As, Mo, Pb, Cr, B, Ge, Mn, Co, and Cd while the other solution was prepared from Si, Fe, P, Ba, Ni, Ti, and Be. To prevent spectral interferences (35,36),single-element solutions were prepared for Th, V, and Ca. For matrix effect studies, phosphorus was added as reagent grade H3P04(85%) in the Ca-P04 system. For the study of the Ca-Na system, the Na concomitant solutions were prepared from NaCl. All solutions contained 10% nitric acid so that any acid effect would not bias the results. For electron number density measurements, the aluminum concentration was 2500 Kg/mL and the reference blank was distilled water. The geological reference material was digested (38) and diluted by a factor of 500.

RESULTS AND DISCUSSION A. Simplex Optimization of the Low-Flow Torch. The simplex optimization process was initially started with six variables which were thought to have an important influence on the analytical capabilities of the ICP torch. The ranges of the variables studied and the optimum conditions identified are summarized in Table 11. Because of plasma instability, the possibility of torch destruction, or other experimental constraints, parameters outside boundary limits could not be selected. For example, a t an outer gas flow rate of 4 L/min, torch overheating was observed if the forward power was increased beyond 950 W. Our initial optimization studies provided two conclusions. First, with the exception of intermediate gas flow, the optimum values of all parameters were generally less than those commonly used for the conventional Ar ICP (34). Second, while the optimum Ar gas flow in the low-flow torch, when six parameters were optimized simultaneously, was about 50% less than that of a conventional torch, further reduction in gas consumption was desirable. Since plasma stability deteriorated at an outer gas flow of less than 3.5 L/min, the outer gas flow was fixed to 4 L/min and a second simplex optimization was initiated with a new set of boundary conditions for the forward power. To protect the tip of sample aerosol tube at higher powers, the intermediate gas flow was also fixed a t 0.5 L/min. Results of this optimization study, presented in column five of Table 11, indicate that at the outer gas flow rate of 4 L/min, the low-flow torch should be observed at lower observation height and that it required (a) lower forward power, (b) lower aerosol carrier gas flow, and (c) less sample compared to either the conventional torch or the same low flow torch optimized for six parameters. It is important, however, to compare the analytical performance of the low flow and the conventional torch under these conditions. B. Comparison of S / B Ratios for the Conventional Torch and the Low-Flow Torch. A comparison of S I B ratios obtained for aqueous samples injected into the con-

ventional and the low-flow ICP is shown in Table I11 for 20 elements. The commonly used experimental conditions for the conventional torch (34)and the optimal conditions for the low-flow torch operated at an outer gas flow rate of 4 L/min are also shown in Table 111. For most atomic and ionic spectral lines which were investigated, the S I B ratios obtained with the low-flow torch were either equivalent or slightly inferior, by factors ranging from 1.5 to 2.0, to those obtained by using the conventional ICP torch. Note that the same ICP direct reader facility is used to conduct this comparison. To place the general applicability of our results in perspective, column 6 in the table shows the S I B values estimated by Winge et al. (34) for each of the elements measured a t the same wavelength. It is important to emphasize that the spectral lines selected for this comparison not only possess high excitation energies but are among the most sensitive lines commonly used in ICP atomic emission spectrometry (34-36). Similar studies on neutral atom lines of medium excitation energies yielded comparable results for the two torches. C. Matrix Effect Studies. For evaluation of this aspect of analytical performance, the matrix effects previoulsy investigated (39,40)were repeated with the low-flow ICP torch. These included the effects of increasing concentration of PO4 and Na on Ca neutral atom and ion line emissions. In this discussion and in subsequent figures the net emission intensities of a given species in the absence of an interferent are normalized to 100 arbitrary units. Figure 1presents the interference response curves for the Ca-PO, system obtained with the low-flow torch. At an outer gas flow of 4 L/min, the low-flow torch was as free as the conventional torch from interference (39,40)when the P04/Ca molar ratio ranged between zero to 1000. As the outer gas flow was increased to 7 L/min in the low-flow torch, slight depressions in both Ca atom and ion line intensities were observed at higher phosphate concentration. The magnitude of this depression, which was less than 7 % , was reduced a t higher forward power, as shown in response curve b’ in Figure 1. These results clearly document the relative freedom of the low-flow torch from the solute vaporization effects. The effects of increasing concentration of Na on the relative intensities of Ca ion and atom lines are compared in Figures 2 and 3, respectively. Addition of 10000 Hg/mL of Na to a 20 gg/mL Ca solution injected into the low-flow torch, under the optimized conditions identified by the simplex method for achieving maximum S I B ratios, reduced Ca I1 and enhanced Ca I emission intensities by approximately 20 and 250%, respectively, when an outer gas flow of 4 L/min was maintained. The use of higher forward power and/or higher outer gas flow rate decreased such effects. It is worth noting

ANALYTICAL CHEMISTRY, VOL. 56, NO. 2, FEBRUARY 1984

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Table 111. Comparison of Signal-to-Background Ratios for the Low-Flow Torch and the Conventional Torch element

wavelength, nm

concn, ,ug/mL

Sn I1 As I Mo I1 Pb I1 Cr I1 B I Ge I Mn I1

189.9 193.6 202.0 202.3 267.7 249.7 265.1 257.6 228.6 214.4 251.6 259.9 214.9 455.4 231.6 334.9 313.0 283.7 292.4 317.9

50 100 10 100 10

co II

Cd I1 Si I Fe I1 PI B a I1 Ni I1 Ti I1 Be I1 Th I1 v I1 Ca I1

low flow torcha

conventional torch 74 116 82 115 60 49 116 50 66 73 55 113 138 94 143 86 42 109 67 46

60 95 66 98 52 27 88 31 60 41 44 113 52 88 132 86 28 105 80 22

10 100 2.5 10 5 20 10 100 2 25 5 0.5 100 10 15

conventional torchC 60 56 38 70 42 63 62 55 43 60 50 48 39 46 48 39 55 46 40 45

The conventional torch a Operating conditions established by simplex optimization are listed in column 5, Table 11. was operated under the following commonly employed experimental conditions used in our laboratories: forward power = 1100 W, observation height = 1 5 mm, uptake rate = 1 mL/min, nebulizer carrier gas flow rate = 0.66 L/min, outer gas flow rate = 22 L/min. Simplex optimization of the conventional torch does not alter the commonly used conditions or the SIB values more than r5%. Literature values for the conventional torch operated at the commonly employed conditions ( 3 4 ) . r

f

Ca 1422.6 nm oCaII393.3 nm

2

o

5t-

Ca

II

393.3 nm

1

z

Y

2 c

3wK

W

2

(a')

-----a

90-

90-

80

O

1

10

100

1000

10,000

5

Na CONCENTRATION, f.a/mL

Figure 2. Effect of sodium on the net emission intensity at the Ca I1 nm spectral line. The concentration of calcium is 20 fig/mL. The outer gas flow rate is 4 L/min in (a) and (a') and 7 L/min in (b) and (bf). The forward power is 700 W (a),850 W (af),700 W (b), and 1100 W (bf). The monochromator slit height and width were 1 r**m and 20 fim, respectively. Other experimental conditions are identified in column 5, Table 11. 393.3

,

0

1

1

I

I

10

100

1000

MOLAR RATIO, IP041/1Ca)

Figure 1. Effect of phosphate on the net calcium emission intensity. The concentration of calcium is 0.5 pmol/mL. The forward power and the outer gas flow rates, respectively, are 700 W and 4 L/min in (a), 700 W and 7 L/min in (b), and 1100 W and 7 L/min in (bf). The monochromator slit height and width for the ion line were 1 mm and 20 pm, respectively. For the neutral atom line, the slit height and wldth were changed to 5 mm and 40 pm, respectively. Other experimental conditions are identified in column 5, Table 11. that the magnitude of ionic signal depression observed in the low flow torch is comparable to the level reported for the conventional ICP (39,40). In the case of the Ca I line, however, the enhancement effect could be eliminated only when the low-flow torch was operated at a forward power and an outer gas flow of 1100 W and 7 L/min, respectively. Under such conditions, the SIB ratios listed in Table I11 deteriorated by a factor of about 2 compared to the S I B values obtained for the low-flow torch under the optimized conditions. The data summarized above invite speculation on the greater susceptibility of the plasma generated in the low-flow torch to ionization type interference a t lower forward powers of 700 to 850 W. Because it is likely that electron number density, ne, would play an important role in any ionization

>1 *Ca r422.6 nm

t- 120

z, p

w

Z

t-

2

/ ---.-a

100

Ib1 Ib')

W', I

10

K 100

0

100

1000

10,000

1

Na CONCENTRATION, ,UUg/rnL

Figure 3. Effect of sodium on t h e net emission intensity at the Ca I 422.6 nm spectral line. The concentration of calcium is 20 pglmL. The outer gas flow rate is 4 L/mln in (a) and (a') and 7 L/min in (b) and (bf). The forward power is 700 W (a), 850 W (af),700 W (b), and 1100 W (bf), The monochromator slit height and width were 2 mm and 20 pm, respectively. Other experimental conditions are identified in column 5, Table 11.

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Table IV. Classification and Wavelengths of Spectral Lines Used for Electron Density Calculation

A(3pZP-ndZD) no. on figures 1

2 3 4 5 6 7 8 9 10

orbital 9d 10d lld

12d 13d

11 12

14d

13 14 15 16

15d 16d

( 4 2 ) wavelength, nm

215.07 214.56 213.47 212.97 212.36 211.83 211.49 210.99 210.85 210.33 210.94 210.84 209.62 209.44 209.13 208.91

I

208 0

l

l

2 1 5 0 208.0

WAVELENGTH. nm

Figure 4. Effect of forward power and outer gas flow rate on the wavelength scan of the series limit for AI excited in the plasma generated in a low-flow torch. The lower spectrum of each pass is a reference blank solution. The slit height and width are 4 mm and 40 pm, respectively. Other experlmental conditions are listed in column 5, Table 11, and on the wavelength scans.

equilibria that do prevail in the plasma, it is worthwhile to compare ne values of plasmas generated by low-flow and conventional torches.

D. Comparison of Electron Number Densities in ICPs Generated in Low-Flow and Conventional Torches. Electron number densities, ne, were estimated via the series limit line merging technique, as described by Montaser et al. (41, 42). This technique is based on the fact that, as the principal quantum number increases, the wings of the Stark-broadened lines start to overlap each other until finally, before reaching the series limit, the lines merge completely, forming a continuum. The principal quantum number, n,, a t which merging occurs depends on the electron number density. Figure 4 shows typical spectral scans at two forward powers and two outer gas flow levels when Al-containing solutions were introduced into the low-flow torch. The scanning monochromator-photomultiplier detection system was used to observe the gross analyte spectrum and the spectrum of a reference blank solution a t each operating condition. The shift in the continuum background caused by the presence of A1 (43) and the plasma background features ( 4 2 , 4 4 , 4 5 ) clearly observed at higher forward powers have been previously discussed for the Ar ICP. As expected, the series of 2P-2D doublets, listed in Table IV, showed broadening and decreased intensity with progression to higher numbers of the series, i.e., with progression to lower wavelength. The most important feature of the spectra was the disappearance of the higher members of the series a t higher powers, thus qualitatively verifying the expected increase in ne. Examination of the 720-W scans at outer gas flows of 4 and 7 L/min revealed that the last discernible line in both plasmas occurred at 209.44 nm, corresponding to the 15d transition. Thus, the measured electron number density, calculated on the basis of the In-

glis-Teller equation (41),was approximately 1.4 X 1014~ m - ~ . When the forward power was increased to 920 and 1100 W, respectively, for plasmas with outer gas flow of 4 and 7 L/min, the estimated ne value was increased to 4 X lOI4 cm-3 in both cases. It is interesting to note that if the conventional Ar ICP is operated at the commonly used experimental conditions (observation height of 15 mm, forward power of 1100 to 1200 W, etc.), the measured electron number density determined via the series limit line merging technique is approximately 7 X 1014cm-3 (41). On the basis of these determinations it should, therefore, be evident why the low-flow torch is more susceptible to ionization type interference effects, especially when operated at lower forward powers. Although the observations on the relative electron number densities in the ICP provide some insight on the properties of these variations, we must emphasize the following two important points. First, because the measurements are based on free atoms residing primarily in the central axial channel, the ne values are a representative average of that channel and not the remainder of the plasma. Second, a definite interpretation of the magnitude and trend of the analyte line behavior plotted in Figures 2 and 3 requires a far more detailed knowledge than is now available on the spatial distribution of analyte free atoms, electron energy distributions and number density, temperatures in their various forms, the ionization equilibria prevailing, and the role played by the Ar-sustaining gas. E. Analytical Performance of the Low-Flow Torch in the Elemental Analysis of Geological Materials. To this point we have not compared the analytical performance of the plasma generated in the low-flow torch to that of a conventional ICP for the determination of a real sample. Since great interest exists in manganese nodules (38,46)as sources

Table V. Elemental Percentage Recoveriesa for USGS Reference Nodule P-1

element

P-1 composition ( 4 6 )

Fe

5.78% 29.14% 1.15% 2240 ppm 1.34% 1595 ppm

conventional torch 1100 w

700 W

low flow torchb 900 w

1100

w

98 97 97 96 100 103 101 96 102 101 99 98 co 97 100 97 98 Ni 99 97 97 98 Zn 98 104 96 101 Percent recovery is defined as ratio of the ICP result to the recommended value ( 4 6 ) times 100. The tabulated values are results of three determinations. The operating conditions for the ICPs are listed in Table 11. The outer gas flow for the low-flow torch is 4 and 7 L/min at forward power of 700-900 W and 1100 W, respectively. Mn cu

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of the technologically important metals Mn, Fe, Co, Cu, and Ni, an analysis of US.Geological Survey (USGS) reference nodule P-1 from the Pacific Ocean was undertaken. For this aspect of the comparison, the low-flow torch was operated at three power levels while the conventional torch was utilized a t the commonly employed (34)operating conditions. The percent recovery values presented in Table V for Fe, Mn, Cu, Co, Ni, and Zn clearly show that performances of the plasmas generated in the conventional and the low-flow torches are comparable. I t is worth noting however, that the relative standard deviation (RSD) of analysis was improved from 1 3 % to f 2% as the forward power in the low-flow torch was increased from 700 to 1100 W. The RSD of analysis conducted with the conventional torch ranged between 1 % and 2%.

CONCLUSION The following general conclusions may be drawn regarding the analytical utility of the low-flow ICP torch 1. For an outer gas flow of 4 L/min in the low-flow torch, the optimum forward power, observation height, intermediate gas flow, nebulizer carrier gas flow, and sample uptake rate are 720 W, 12 mm, 0.5 L/min, 0.5 L/min, and 0.5 mL/min, respectively. Except for the intermediate gas flow rate, the magnitudes of the parameters are lower than those commonly used for the conventional Ar ICP. 2. The analytical performance of the plasma sustained in the low-flow torch ( S I B values achievable, relative freedom from vaporization-atomization-ionization interferences) closely approaches that of a conventional ICP. The uniqueness of this low-flow torch comes from the fact that its analytical capabilities are superior to those of other low-flow torches (6-17) and that its physical size (18 mm i.d.) is compatible with that of conventional ICP torches commonly employed with commercial ICP facilities. 3. The low-flow torch operates with a total gas flow of 5 L/min as compared to 20 to 22 L/min for the conventional torches. Although the use of this low-flow torch reduces the gas consumption by a factor of about 4,the development of "bench-top'' ICP-based systems, to be used in a mobile laboratory or a ship, requires a torch which consumes less than 1 L/min of argon and 200 to 500 W input power. Work in this area is currently under way in our laboratories.

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RECEIVED for review August 16,1983. Accepted October 31, 1983. This research (at GWU) was supported in part by the Society for Analytical Chemists of Pittsburgh and the NIH Biomedical Research Support Grant under Contract No. 2S07 RR07019-18.