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Anal. Chem. 1980. 52. 2440-2442
information on related compounds. Dimerization by morphine and not by codeine implicates the 3-OH group in the association mechanism. T h e supply of 3-MAM was too limited to make a comparison study. Intermolecular hydrogen bonding involving the OH group and the dihydrofuran ring oxygen is a likely structural model for the association. T h e interesting result is that the molal ellipticity of the dimer is greater in magnitude and opposite in sign to that for the monomer. This result might be rationalized in terms of a sector rule theorem ( B ) , although such a model has not been prepared for solutes in structured solvents. Structural determinations by X-ray (16, 17) diffraction have detected distortion of the plane of the aromatic ring in the monomer. How hydrogen bonding might affect planarity is beyond speculation a t this time. In summary it has been successfully demonstrated that cholesteric liquid crystal solvents interact very specifically with dissolved solutes to the extent that compounds of similar molecular structure can be distinguished by their LCCD spectra. The technique of differential LCCD is fairly routine with reproducible spectra readily obtainable for chiral as well as for achiral molecules. Cholesteric solvents offer a new concept in the study of solutesolvent interactions in that each layer in the helical stack approximates to a surface, and their unique orientation allows one the use of electronic absorption spectroscopy as a probe for the interactions.
ACKNOWLEDGMENT We are indebted to Mallinckrodt, Inc., and Research Triangle Institute and the National Institute for Drug Abuse for their assistance in obtaining samples. LITERATURE CITED (1) Mason, S. F. Q . Rev., Chem. SOC. 1961, 75. 287. (2) Jaffe, H. H.; Orchin, M. "Theory and Applications of Ultraviolet Spectroscopy"; Wiley: New York, 1962;Chapter 20. (3) Lambert, J. 8.; Shurvell, H. F.; Verbit, L.; Cooks, R. G.; Stout, G. H. "Organic Structural Analysis"; Macmillan: New York, 1976. (4) Cahill, J. E. Am. Lab. 1979, 77 (ll),79. (5) Cahill, J. E.; Padera. F. G. Am. Lab. 1980, 72(4),101. (6) Bowen, J. M.; Purdie, N. Anal. Chem. 1980, 5 2 , 573. (7) Weiss, U.; Rull, T. Bull. SOC. Chim. Fr. 1965,3707. (8) DeAngelis, G. G.: Wildman, W. C. Tetrahedron 1969,25, 5099. (9) Sackmann, E.; Krebs, P.; Rega, H. U.; Voss, J.; Mohwald, H. Mol. Cryst. L i q . Cryst. 1973,24, 283. (10) Sackmann, E.; Mohwald, H. J. Chem. f h y s . 1973, 58,5407. (11) Saeva, F. D.; Sharpe, P. E.; Olin, G. R . J . A m . Chem. SOC.1973, 95, 7660. (12) Saeva, F. D.; Olin, G. R. J . Am. Chem. SOC. 1976, 98,2709. (13) Pirkle, W. H.; Rinaldi, P. L. J. Org. Chem. 1980, 45, 1379. (14) Gottarelli, G.; Samori, 8.; Folli, U.; Torre, G. J. Phys. Colloq. (Orsay, Fr.) 1979,C3-25. (15) Siek, T. J. J . Forensic Sci. 1974, 75, 193. (16) Gylbert, L. Acta Crystallogr., Sect. B 1973, 29, 1630. (17) Bye, E. Acta Chem. Scand., Ser. B 1976, 830, 549.
RECEIVED for review June 9,1980. Accepted August 25,1980. We wish to thank the National Science Foundation for the support of this work under Grant No. NSF CHE 76-19819.
Water-cooled Torch for Inductively Coupled Plasma Emission Spectrometry Hiroshi Kawaguchi,
Tetsumasa Ito, Shue Rubi, and Atsushi Mizuike
Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464, Japan
One of the disadvantages in the use of the inductively coupled plasma (ICP) is the large consumption of argon gas. Typically, 10-20 L/min of argon flows are required for the total of plasma, auxiliary, and carrier gases. Among several proposals to decrease the argon consumption (1-3), a water-cooled torch described by Kornblum et al. (3) permitted the most drastic reduction, down to 2 L/min. However, their torch had serious drawbacks resulting from the complexity of the design, the difficulty involved in introducing sufficient amounts of samples and hence the high detection limits, and the relatively large phosphate interference on the calcium emission. Water-cooled torches have also been described by Zil'bershtein ( 4 ) and Britske et al. ( 5 ) for a high-power ICP (40 MHz, 4 kW). The latter authors operated it at low total gas consumption (3.5-4 L/min) and obtained good analytical results in the determination of rare earth and some other refractory elements. T h e torch described by Kornblum et al. (3)had the inlet and outlet of the cooling water a t the bottom of the water jacket, and the structure became complicated in trying to prevent air or steam bubbles from lingering. These may destroy the torch by local overheating. Though they excluded designs in which the water supply was mounted on top of the load coil, such designs are possible if the torch is positioned into the coil from the top end of i t before attaching the spray chamber to the torch. The most promising torch we constructed requires 4 L/min of the plasma gas and 0.8 L/min of the carrier gas. Though it requires more argon than those described by Kornblum et al., it has definite advantages in that i t can be used with a conventional nebulizer and that the detection limits are superior and interference effects are comparable to those of conventional ICP torches.
EXPERIMENTAL SECTION Instruments. Detection limits and interference effects were measured by using an 0.5-m grating monochromator (Nippon Jarrell Ash, JE-50E, grating 1200 grooves/mm, slit width 10 gm) with a photomultiplier (HTV, R-106) and an electrometer (Shimadzu Seisakusho, SE-41). A conventional plasma power supply (Nippon Koshuha, 27 MHz, crystal controlled) with a coupling unit (Shimadzu Seisakusho) and a concentric glass neubulizer with a dual-barrel spray chamber were used without modification. Axial emission profiles from the plasma were measured by using a photodiode array spectrometric system described previously (6). Torch Design. Three torches we constructed are shown in Figure 1. They are made of silica glass except for a Teflon sleeve and rubber O-rings to position a central sample tube which carries sample aerosols. A water flow of 2.5 L/min was sufficient to run the plasma at the radio frequency (rf) power from 1 to 1.8 kW with any designs shown in Figure 1. The temperature increase of the cooling water was 2-5 "C depending on the power. With the design in Figure la, the plasma was easily initiated at 4 L/min of the plasma gas when the carrier gas was not introduced. When the latter flow was slowly increased, the plasma gas flow had t o be decreased at the same time to prevent the plasma from extinguishing. A hole is generated in the plasma at about 0.3 L/min of the carrier gas and 1 L/min of the plasma gas. However, when the carrier gas was further increased and the sample uptake became appreciable, the plasma suddenly shrank and became like a glowing "O-ring", and only a tail flame of the plasma could be seen at the top end of the torch from a horizontal direction. This change of the plasma shape was caused by the excessive introduction of molecular substances such as water and air, because the latter introduction into the discharge zone from the top end of the torch increased as the plasma gas was decreased. To prevent the air from entering the discharge zone, we attached a silica collar tube on top of the water jacket as shown in Figure
0003-2700/80/0352-2440$01.00/00 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980 b
2441
C
0
L94
Flgure 1. Three designs of the water-cooled torch for the low-flow
ICP. Some relevant dimensions are given in millimeters.
5
10
15
20
25
30
35
40
Height above load coil ( n m )
Figure 3. Effect of the carrier gas flow on axial profiles of the Ca I1 393.4-nm line. The carrier gas flows are given in L/min. The dotted line is used only to avoid confusion for two of the curves. Plasma gas rate was 5 L/min; rf power was 1.1 kW.
3
0
>
I
YI & C
-.C
0
5
10
15 20 25 30 35 H e l g h t obove l o a d c o i l (
40
mm)
Figure 2. Effect of the plasma gas flow on axial profiles of the Ca 11
393.4-nrn line. The plasma gas flows are given in liters per minute. Carrier gas rate was 0.8 L/min; rf power was 1.1 kW. lb. Though this torch could be operated more stably than design (a),it was not possible to increase the carrier gas flow rate to more than 0.5 L/min preserving the normal shape of the plasma. These torches consist of only two tubes instead of the conventional three-tube arrangement. This may explain some of the instabilities. Finally, a central tube of a “tulip” shape was used as shown in Figure IC. With this torch, the plasma was ignited at about 5 L/min of the plasma gas. After the flow was increased to 10 L/min, the carrier gas could be easily introduced piercing a hole through the plasma as with conventional torches and then the plasma gas flow could be decreased down to 2 L/min. In both designs (a) and (b),the cooling water was observed to boil close to the plasma region and cracks were sometimes generated at the inside wall of the water jacket when the plasma was turned off after prolonged operation. In design (c), however, boiling of the cooling water is not observed and the cracking can be completely avoided by increasing the plasma gas flow to about 10 L/min just before turning off the plasma. Since the plasma is observed through the silica tube just above the water jacket, strict concentricity is required in the positioning of the central tube to prevent the silica wall from devitrifying. The torch without a collar tube and with a tulip-shaped central tube was also tested, but the plasma became unstable when the plasma gas was decreased to less than 5 L/min. The following experimental data were obtained with the torch shown in Figure IC. RESULTS AND DISCUSSION Axial Profiles of Calcium Emission. By use of a photodiode array spectrometric system described previously (6), axial profiles of calcium emission (background signal was subtracted) were measured under various operating conditions (Figures 2-4). A calcium chloride solution (200 Fg Ca/mL) was introduced into the plasma by a conventional concentric
0
5
10
15
20
25
30
35
40
Height a b o v e l o a d c o i l ( m m )
Flgure 4. Effect of the rf power on axial profiles of the Ca I1 393.4-nm line. The rf powers are given in kilowatts. Plasma as rate was 5 L/min and carrier gas, 0.8 L/min.
nebulizer with a spray chamber. The uptake rate of the solution was 1.5 mL/min at 0.8 L/min of the carrier gas. The effect of the plasma gas flow on axial profiles of the Ca I1 393.4-nm line is shown in Figure 2. Relatively abrupt and small deviations on the curves in this figure and others (Figures 3 and 4) are caused by both array and silica window imperfections and do not represent spatial structure. The highest intensity of the emission is observed just above the water jacket, 10-12 mm above the load coil. The plasma gas flow should be greater than 4 L/min to observe the most useful part of the plasma from above the water jacket. The peak intensity, however, did not increase appreciably when the plasma gas flow was increased to more than 4 L/min. Figure 3 shows the effect of the carrier gas flow on axial profiles of calcium emission. The peak intensity increases with increasing carrier gas up to 0.7 L/min and then decreases. The upward shift of the peak position with the increase of the carrier gas is also observed. These behaviors are similar to those for conventional torches (6), presumably owing to the same mechanisms. The effect of the rf power on axial profiles of calcium emission is shown in Figure 4. The peak intensity increases and the peak position slightly shifts downward with increasing rf power. The signal-to-background ratio, however, does not increase appreciably because the background intensity also increases with increasing rf power. Detection Limits. Although the optimum operating conditions were not extensively examined, the detection limits of 14 elements were measured and compared with those of
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table I. Detection Limits
element AI I BI Ba I1 Ca I1
co I1
Cr I1 La I1 Mg I1 Pvln I1 Ni I1 Sm I1 Ti I1 Zn I Zr I1
wavelength. , nm 396.152 249.773 455.403 393.366 238.892 267.716 379.478 279.553 257.610 221.647 359.260 334.941 213.856 343.832
present study water-cooled torch conventional torch ItJb DL, pg/mL InlIb DL, pg/mL
C, 4 r n L 10
10 0.1
0.03 10
1.2 1 0.1 1
10 10
12.4 23.7 2.78 3.70 53.3 8.53 6.40 28.9 45.4 31.8
0.031 0.016
18.1
0.021 0,001 6 0.003 8 0.009 2
23.4
1 10 1
100
4.13
0.001 4 0.000 3 1 0.007 1
0.005 3 0.005 9 0.000 1 3 0.000 84 0.012
4.04 14.7 1.24 2.92 34.3 5.77 2.23 14.9 21.7 17.8 6.51 11.1
43.9 4.29
lit. (ref 8 ) conventional torch DL, pg/mL
0.074 0.020 0.002 4 0.000 3 1 0.008 7 0.006 2 0.013 0.000 20 0.001 4 0.017
0.046 0.002 7 0.006 8 0.007 0
0.028 0.004 8 0.001 3 0.000 1 9 0.006 0 0.007 1
0.010 0.000 1 5 0.001 4 0.010
0.043 0.003 8 0.001 8 0.007 1
Table 11. Operating Conditions watercooled conventiontorch al torch
R F power, kw gas flows (argon), L/min plasma auxiliary carrier
observn height (above load coil), mm observn zone, mm integration time, s
1.1
1.1
4
12
0.8
0 0.8
12
15
5 10
5 10
the conventional three-tube torch (Table I). The operating conditions (Table 11) for both torches are matched as close as possible. The detection limits (DL) were calculated from the equation where crB is the relative standard deviation of the background, C is the analyte concentration which yielded the net line intensity I,, and Ib is the background intensity obtained when distilled water was introduced. The values of gB were calculated from 18 measurements of the background intensity a t 334.94 nm and 1.9 and 1.5% were obtained for the water-cooled and conventional torches, respectively. These values were used for the calculation of the detection limit of each element because the standard deviation of the background could be assumed to be proportional to the background intensity (7, 8). T h e present uB values are somewhat higher than those reported in the literature, 0.3-1.0%, because the stabilization of the rf power generator was insufficient. In spite of the slightly larger gB for the water-cooled torch than that for the conventional torch, the detection limits for the former are generally superior to those for the latter by a factor of about 2. This is due to the lower background and hence higher signal-to-background ratios in the water-cooled torch as is shown in Table I. Further reduction of the background noise and therefore improvement of the detection limits may be
' LO
0 003 0'01 0'03 0'1 Concentration, wt'l.
0'3
1'0
Flgure 5. Effect of sodium and phosphate on the intensity of the Ca I1 393.4-nm line. Ca concentration was 10 Fg/mL.
possible by using a somewhat longer collar tube and a more stable rf power generator. Interferences. Effects of sodium and phosphate on calcium emission are shown in Figure 5. Although the depression effect of sodium is somewhat larger than in the conventional torch, the effect of phosphate is comparable to that and absent up to 0.1%. The latter effect is much smaller than the results reported by Kornblum et al. ( 3 ) . The depression of the calcium intensity in higher concentrations of phosphate is probably due to the nebulization interference (9). The collar tube is not necessarily fused to the body of the torch as shown in Figure 1 and a demountable tube may be sometimes desirable since the silica wall tends to devitrify when concentrated alkaline solutions are nebulized. Such torches are now being constructed in our laboratory. LITERATURE CITED (1) Savage, R. N.; Hieftje, G. M. Anal. Chem. 1979, 51. 408-413. (2) Allemand, C.D.; Barnes, R. M.; Wohlers, C. C. Anal. Chem. 1979, 51, 2392-2394. (3) Kornblum, G.R.; Van der Waa, W.; de Galan, L. Anal. Chem. 1979, 51, 2378-238 1. (4) Zil'bershtein, K. I. ICP Inf. News/. 1980, 5, 506. (5) Britske, M. E.; Sukach, Ju. S.; Filimonov, L. N. Zh. f r i k l . Spekfrosk. 1978, 25, 5. (6) Kawaguchi, H.; Ito, T.; Ota, K.; Mizuike, A. Specfrochim. Acta, Part 6 1980. 356. 199-206. (7) Bou&nslP. W. J. M.; DeBoer. F. J. Specfrochim. Acta, Part6 1977, 326,365-395. (8) Winge, R. K.; Peterson, V. J.; Fassel, V. A. Appl. Specfrosc. 1979, 33, 206-219. (9) Greenfield, S.;McGeachin, H. McD.; Smith, P. B. Anal. Chim. Acta 1976, 8 4 . 67-78.
RECEIVED for review June 26,1980. Accepted August 28,1980.
