2284
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
Direct Sample Insertion Device for Inductively Coupled Plasma Emission Spectrometry Sir: The inductively coupled plasma (ICP) is currently the most effective source for simultaneous trace multielement analysis of solution samples by atomic emission spectrometry (1-5). However, more efficient solution sample introduction systems are required for the ICP and, as well, the analytical capability of the ICP would be greatly extended if other sample forms such as powders and solids could be directly introduced into the plasma and effectively analyzed. Although several workers have developed and investigated various approaches for introduction of such sample forms into the ICP (3, 6-12), no one system has yet been widely accepted or utilized. In this correspondence, a sample introduction system for the ICP is described that allows the direct insertion of small amounts of sample (powders, solids, or desolvated liquids) into the central core of the ICP. The sample container is a conventional undercut cup graphite electrode such as those used in dc arc emission spectrometry. The conventional ICP torch has been modified so that the electrode is held in a simple mechanical insertion system that replaces the normal central aerosol tube of the torch. Solid and powder samples ranging from 20 mg to less than 1 mg and liquid samples as small as 5 pL can be analyzed with this system. A schematic drawing of the direct sample insertion system is shown in Figure 1. A torch essentially identical to the conventional design has been utilized except that it is constructed without the central aerosol tube and is mounted in a Teflon block. The graphite electrode sits on top of a solid quartz rod that is placed inside a central quartz tube. The rod extends completely through the Teflon block to a sliding platform. Several types of electrodes have been used including Ultra Carbon #781, Spex 4033, and Spex 4004 undercut graphite cup electrodes. When igniting the plasma, the top of the electrode is set a t a height that is identical to that of the top of a conventional aerosol injector tube. After the plasma is lit and the desired flow rates and power level are set, the electrode containing the sample is inserted into the plasma simply by raising the lower platform by hand. The electrode is inserted to a position just above the top of the load coil and the whole process takes less than 1 s. No problems have been encountered in maintaining the plasma either during electrode insertion or while the electrode is in the plasma. Little if any adjustment of the automatic matching network occurs when the electrode is inserted. The electrode appears to be almost white hot in about 2 s after insertion. Essentially no consumption of the electrode occurs, a consequence of the inert Ar atmosphere of the plasma. Solid and powder samples, once placed in the electrode, can be inserted into the plasma with no further treatment; however, it is necessary to desolvate liquid (aqueous) samples before insertion. Liquid samples were placed in the cup using a 5-pL Eppendorf pipet. Desolvation was accomplished by inductively heating the electrode before igniting the plasma. The electrode containing the 5-pL aqueous sample is inserted to the center of the coil region and very low forward power is applied (-50 W). With a little practice, the sample can be quickly and effectively desolvated in about 1 min in this manner. Although not tested, inductive heating may also be useful for ashing samples directly in the cup. If the normal ignition level of forward power is applied to the system while the electrode is inserted in the coil region, it is quickly heated to a bright orange color and in 2 to 3 s the plasma will ignite. In fact, it was by inserting a graphite rod, that Reid ignited
his plasma systems back in the early 60’s. The normal procedure is, however, to lower the electrode containing the desolvated sample back to the starting position before starting the plasma and then insert the electrode as described previously. A commercially available radiofrequency inductively coupled plasma source was used in this investigation (Plasma-Therm Inc., Kresson, N.J.). The source consisted of a Model HFP-2500D 2.5-kW RF generator (27.12 MHz), a Model ADC5-3 automatic power control, a Model AMN-2500E automatic matching network, and a Model PT-2500 plasma torch assembly. The source was operated at a forward power of 2 kW and a plasma gas (coolant) flow rate of 17 L/min. No auxiliary or central (“aerosol”) gas flows were used. Several measurement systems were used including a computer-coupled photodiode array spectrometer (13),a single channel scanning monochromator (Heath EU-700)-photomultiplier tube system, and a Bausch & Lomb 2-meter dual grating spectrograph. The spectrum resulting from a 5-pL aqueous sample (1ppm Zn and Cd) as measured with the photodiode array spectrometer is shown in Figure 2. The sample was inductively desolvated before insertion and the spectral signal was integrated for 12.5 s after electrode insertion. The ICP spectral background was not subtracted for this spectrum; as a result, the cadmium and zinc lines are seen superimposed on the NO bands and continuum that occur in this spectral region (14). The total spectral coverage of the photodiode array spectrometer is about 50 nm. The 12.5-s integration of the spectral signal was achieved by the summation of 5 consecutive 2.5-s integration time cycles of the photodiode array measurement system. It was clear while observing the successive spectra on the photodiode array oscilloscope readout that Zn and Cd were not vaporizing a t the same time. T o determine the emission time behavior of these two species, the photodiode array integration time was set at 0.16 s and the intensities of Cd(I1) 226.5 nm and Zn(I1) 202.5 nm were monitored for 18 consecutive integration cycles (2.88 s) after electrode insertion. Plots of the intensities of these lines as a function of time are shown in Figure 3. From these results, it is clear that this sampling system will require extensive characterization in the time domain. An analytical curve was measured for Zn concentrations from 1 to 100 ppm. The sample was added as a solution (5 pL) and inductively desolvated before electrode insertion. The curve was linear with a correlation coefficient of 0.98. The relative standard deviation for samples of the same concentration was 15%. The most likely sources of imprecision include reproducibly introducing a 5-pL sample to the electrode cup and manual control of the desolvation and electrode insertion steps. The spectrum of a powder sample is shown in Figure 4. The sample was a commercial powder standard available from Spex Industries and normally used in dc arc analysis. This sample was one of the G-7 series standards and contained 0.0001% of 49 elements in a graphite matrix along with 0.1 70 indium for use as an internal standard where required. About 5 mg of this standard were added to the cup of the electrode. The spectrum was obtained about 1 s after insertion of the electrode and the spectral signal was integrated for 0.64 s. The photodiode array spectrometer was set for the Zn-Cd region. At the particular point in time at which this spectrum was acquired, most of the Cd had already vaporized, although the Zn lines are still quite prominent. With a 5-mg sample,
0003-2700/79/0351-2284$01.00/00 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
[
2285
GRAPHITE ELECTRODE
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-
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PLASMA GAS (COOLANT) AUXILIARY GAS
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il
TEFLON BASE CENTRAL GAS
h
Z n I 2138
ZnI12061Z n I I 2025
SOLID QUARTZ ROD
WAVELENGTH ( n m ) Flgure 4. Spectrum of 0.0001% G7 Spex Industries graphite matrix powder standard
ADJUSTABLE STOP BLOCK PLAT FORM SCREW
Flgure 1.
Schematic drawing of the direct sample insertion torch
CdI 2288 CdiI 2265
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\ Zn
Zn 202 5
-9
Spectrum of a 5-pL aqueous sample containing 1 ppm of
Cd and Zn
202.5n m
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TIME (sec) Figure 3.
,
,
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tog Mass Zn (gm)
WAVELENGTH (nm) Figure 2.
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,
Time study plots for Cd and Zn
only about 5 ng of each element is present in this sample. T o test the quantitative capability of this sample introduction system for powdered samples, a set of 5 powdered samples containing Ag, Pb, B, and Zn were prepared in a graphite powder matrix. The samples were run in Spex #4004 electrodes complete with boiler caps. The use of boiler cap electrodes provided more reproducible time behavior. The sample size was about 15 mg and was weighed in all cases. The single channel monochromator measurement system was used and Zn was determined a t the 213.8-nm line. The amount of Zn in the samples ranged from 1.8 ng to 12 pg. As can be seen in Figure 5, the analytical curve is linear up to about 1.3 pg. The slope of the line is 0.98 and the correlation coefficient
Figure 5.
matrix
Analytical curve for Zn in powder samples with a graphite
is 0.99. The detection limit (S/N = 2) is estimated to be about 0.1 ng. Finally it should be noted that this analytical curve was obtained without the use of an internal standard. Because of the nature of the sample cup and the capability of analyzing powders directly, comparison of this system to a dc arc is inevitable. Thus a direct semiquantitative comparison was carried out using a Bausch & Lomb 2-meter dual grating spectrograph. The Spex G standards containing 49 elements but without the In internal standard (0.1, 0.01,0.001, and 0.0001%) were run on this system using both a dc arc source and the direct sample insertion device (DS1D)-ICP source. In both cases the samples were run in 781 Ultra Carbon electrodes. SA-1 plates were used and only the wavelength region from 240 to 340 nm was recorded. The results are summarized in Table I. Essentially equivalent detection limits were obtained for Ag, Be, Cd, Cr, Fe, Mg, Mn, Sb, and Sn. The DSID-ICP was better for As, Bi, Cu, Ga, In, Na, Pb, T1, and Zn while the dc arc was better for B, Ca, Ge, Mo, Ti, and V. Certain elements were seen by only one or the other system. Co, Nb, Ni, and W were seen only with the dc arc and Hg, P, and Sr only with the DSID-ICP. This brief study is by no means meant to be a definitive comparison, but was simply carried out to obtain a feeling for the capability of these two systems under reasonably identical conditions. In addition, it should be kept in mind that only the wavelength range from 240 to 340 nm was observed. For both techniques, more sensitive lines for several elements occur outside this region, particularly for the ICP ( 1 5 ) . The main conclusion to be drawn both from the results shown in Table I and Figure 5 is not which technique is best but the fact that the ICP when coupled to the direct sample insertion system can be utilized directly for quantitative, semiquantitative, and
2288
ANALYTICAL CHEMISTRY, VOL. 51, NO. 13, NOVEMBER 1979
Table I. Semiquantitative Comparison of dc Arc and DSID-ICP Using the Spex G Standards (240-340)nm) dc arc element % concn. line Ag
As
B Be Bi Ca Cd
co Cr cu
Fe Ga Ge Hg In Mg Mn Mo Na Nb
Ni P Pb Sb Sn Sr Ti
T1 V W
Zn
0.0001 0.1 0.001 0.0001 0.1 0.001 0.1 0.1 0.001 0.01 0.001 0.01 0.001
DSID-ICP % concn. line 0.0001 0.01 0.01 0.0001 0.01 0.01 0.1
328.1 278.0 249.7 249.5 293.8 315.9 326.1 245.0 283.6 327.4 239.6 294.4 365.1
0.001 0.001 0.01 0.01 0.001 0.0001 0.0001
325.6 285.2 257.6 284.8 330.2 309.4 253.2
0.1 0.01 NS NS 0.1 0.01 0.01 0.01 0.01 0.1 0.01 0.01 NS 0.01
NS 0.1 0.01 0.01
283.3 326.8 303.4
NS 0.01 0.1 0.0001 0.1 0.1
328.1 278.0 249.7 313.0 306.8 315.9 326.1
NS' 0.001 0.001
NS 0.01 0.0001 0.0001 0.001 0.1 0.01 0.1
334.9 276.8 310.2 263.5 330.3
283.6 324.8 239.6 294.4 365.1 253.7 325.6 285.2 257.6 284.8 330.2 255.3 283.3 287.8 326.2 338.1 336.1 276.8 310.2 328.2
'NS = Not seen at any concentration level. Table 11. Qualitative Analysis of NBS (SRM 1632) Coal (230-340 nm) observed element cu Fe Mn
V
concn., ppm
line,
18 8700 40 35
324.8 239.5 259.3 309.3
nm
Table 111. Direct Qualitative Analysis of NBS (SRM 1571) Orchard Leaves (230-340nm)
not observed best ele- concn., line, ment ppm nm As Cr Ni
Pb Zn
6 20 15 30 37
193.7 205.6 221.6 220.3 213.9
qualitative analysis of powder samples with a capability at least equal to the dc arc. Finally, some recent experiments indicate that it is possible to directly insert samples that have a predominantly organic matrix and cany out at least a qualitative analysis. Both NBS Coal and NBS Orchard Leaves can be packed into the cup and (without ashing) be inserted directly into the plasma. The elements and limes that have been spectrographically observed in the resulting emission from the plasma are summarized in Tables I1 and I11 along with those elements listed by NBS for these samples and not detected. In most of the undetected cases, the most sensitive line of the element was outside the observed wavelength range (C240 nm) in a region difficult to reach with the SA-1emulsion. Clearly this sample introduction system has the potential of significantly expanding the overall analytical capability of the inductively coupled plasma. Sample types and small solution volumes previously difficult to introduce into the ICP can now readily be studied. This direct sample insertion
not observed
observed
line,
ele-
ment
concn
Be Ca
0.03 ppm 2% 2.6 ppm 12 PPm 300 ppm 1.5% 0.6% 91 PPm 82 PPm 0.2% 25 PPm
cr
cu Fe K
Mg
Mn Na P
Zn
nm
313.1 315.8 284.3 324.7 239.4 404.4 285.2 259.4 330.2 255.3 334.5
ele- concn, ment ppm
As B Cd
Hg Ni
Mo Pb Ru
Sb Se U
10 33 0.11 0.15 1.3 0.3 45 12 2.9 0.08 0.03
best line, nm
193.7 249.8 2.4.4 194.2 221.6 202.0 220.3 240.3 206.8 196.0 386.0
technique could supplement existing ICP sample introduction methods much as electrothermal atomizers have enhanced atomic absorption measurements. Just as clear is the need to carefully assess the analytical characteristics of each sample type, particularly in the time domain. The time behavior observed is similar to that which is observed with dc arc sources (16)and electrothermal atomizers. However, while analogies to the capabilities and characteristics of the dc arc and electrothermal atomizers are unavoidable, important differences exist. It is felt, for example, that selective volatilization and thermochemical reactions which may take place as the sample is vaporized from the electrode can be more reproducibly controlled in the ICP discharge, which is considerably more stable than the dc arc. In addition, the relative insensitivity of ICP emission characteristics to a variety of classic matrix effects is well documented. Further characterization of this ICP sample introduction system is continuing. In particular, plasma gas environment, sample matrix, electrode geometry, and plasma power are all being investigated as to their effects on analyte emission characteristics.
LITERATURE CITED Winge, R. K.; Fassel, V. A,; Knisely, R. N.; Hass, W. J., Jr. Spectrochim Acta, Pari B 1977, 31, 327. Dahiquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1. Greenfield, S.; McGeachin, H. McD.; Smith, P. B. Talanta 1976, 23, 1. Boumans, P. W. J. M. Mikrochim. Acta 1978, 1 / 3 - 4 , 383. Boumans, P. W. J. M.; Bastings, L. C.; demer, F. J.; van Kollenburg, L. W. J. Fresenius' Z.Anal. Chem. 1978, 291, 10. Scott, R. H. Spectrochim. Acta, Part B 1978, 33, 123. Human, H. G. C.; Scott, R. H.; Oakes, A. R.; West, C. D. Ana&st(London) 1978, 101, 265. Daanall. R. M.: Smith. P. J.: West, T. S.; G-eenfield, S.Anal. Chim. Acta 1971, 54, 397. Hoare, H. C.; Mostyn, R. A. Anal. Chem. 1967, 39, 1153. Kniseley, R. N.; Fassel, V. A,; Butler, C. C. Clin. Chem. 1973, 19, 807. Greenfield, S.; Smith, P. B. Anal. Chim. Acta 1972, 59, 341. Nison, P. E.; Fassel, V. A,; Knisely, R. N. Anal. Chem. 1974, 46, 210. . Soectrosc. 1976. 30. 113. Horlick. G ~ N ADD/. Horlick; GaG. Ind. Re'sJDev. 1978, 20(8),70. Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Spectrmc. 1979, 33, 206. Boumans, P. W. J. M. "Theory of Spectrochemical Excitation"; Hllger: London, 1966.
Eric D. Salin Gary Horlick* Department of Chemistry University of Alberta Edmonton, Alberta, Canada T6G 2G2
RECEIVED for review December 8,1978.Accepted August 14, 1979.
