Anal. Chem. 1989, 6 1 , 895-897
895
Moving Wheel Liquid Chromatography-Helium Microwave-Induced Plasma Interface Liming Zhang and Jon W. Camahan*
Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115 Randall E. Winans* and Paul H. Neil1
Chemistry Division, Argonne National Laboratory, 9700 South Cuss Avenue, Argonne, Illinois 60439
An Interface between a liquid chromatograph and an atmospheric pressure helium microwave-Induced plasma for halogen-selective detection Is presented. Halides and oxohalogens are separated by anion-exchange hlgh-performance ilqukl chromatography and directed, as a mist, to a continuously moving wheel interface. The aqueous solvent Is evaporated with a flow of hot nitrogen, leaving the anaiyte In residue form. The moving wheel then carries the dry anaiyte Into the plasma where it Is volatilized, atomized, and excited. The plasma Is a small-volume helium microwave-induced plasma operated at 100 W with a helium support gas flow of 3.1 L/min. Selectivity is observed when the elemental signals of coelutlng peaks are monitored. Detection limits are In the range of 0.4-20 pg of halogen. When this system Is used wlthout the chromatographic column for bulk solution analysis, halogen detection iimHs range from 0.2 to 20 pg/mL.
INTRODUCTION Helium microwave-induced plasma (He-MIP) atomic emission spectroscopy is a rapidly developing technology, with the inherent capability to produce sensitive and selective analytical response for nearly all elements in the periodic table (1-5). This technology has opened new doors in the field of atomic emission spectroscopy. One of the greatest advantages of HeMIP is the ability to detect the nonmetal elements (6-9), which are difficult to determine in trace amounts by conventional atomic spectrometric methods. High-performance liquid chromatography (HPLC) is a primary method for the separation of multicomponent liquid mixtures. However, usual HPLC detection systems based on spectrophotometry, refractive index, etc., are often nonselective, and simultaneously eluting species often produce analytical interferences. For chemical systems in which the complete resolution of species may be hindered by the similar retention times, such nonselective detectors yield serious analytical difficulties. By utilization of a highly selective detection mode such as plasma emission, the analytical utility of such systems can be significantly improved (10-13). Because atoms produce line emission a t specific wavelengths that are characteristic for each element, chromatographic peak overlap problems may be overcome by monitoring the emission wavelength of an atom contained only in the molecule of interest. Most HPLC-plasma spectroscopy techniques developed thus far have been limited to the detection of metals (14-21). For the determination of nonmetals with the He-MIP, direct nebulization of LC effluent directly into the plasma has been successful (22). This study was performed with a 500-W He-MIP. An inherent disadvantage of an interface of this nature is that the solvent from HPLC is also introduced into the plasma, which can cause both spectral and excitation
interferences. These spectral problems are even more pronounced when one is working with organic mobile phases. Lastly, this HPLC-He-MIP system required a relatively high flow rate of helium plasma gas (17 L/min) to maintain optimal performance. In order to combine the separation characteristics of HPLC with the nonmetal-selective detection capability of the HeMIP with the consideration of cost and the solvent interference, an interface device between HPLC and a He-MIP has been designed and is presented in this article. The effluent from the HPLC column is introduced onto a moving stainless steel wheel with a conventional pneumatic nebulizer. The wheel carrying the effluent is then passed through a hot region produced by heated nitrogen gas. The comparably low boiling point solvent is evaporated in this region, leaving only the analytes in residue form on the wheel. Finally, the residue is moved into the plasma and vaporized, atomized, and excited. In this way, the solvent interference is significantly reduced. Preliminary experiments have been performed, and the results are quite promising.
EXPERIMENTAL SECTION A block diagram of the system is presented in Figure 1. The ion-exchange separation was accomplished with a Hamilton PRP-X100 (polystyrene-divinylbenzene trimethylammonium) anion-exchange column (150 mm X 4.1 mm) utilizing 1 mL/min of a 4 mM p-hydroxybenzoic acid buffer in water (pH 9.5, adjusted with KOH) as the mobile phase. The LC pump was a Waters and Associates, Inc. (Milford, MA) Model No. M-6000A. The interface device consisted of a stainless steel wheel of 77.7-mm outer diameter, 62.9-mm inner diameter, and 1.5-mm thickness. The wheel was friction-drivenby a high-torque motor system. The driving motor was obtained from Talboys Engineering, Inc. (Emerson, NJ) and was Model No. 138. Several auxiliary parts, such as a heating coil of copper tubing wrapped with electric heating tape and packed with molecular sieves, were attached to the interface. The modified Beenakker TMolocavity was connected at a right angle with the interface in such a way that the edge of the wheel was inside the cavity resonator structure. The plasma was produced by a low-power microwave generator manufactured by Kiva Instruments (Rockville, MD), Model No. MP6-4M-229, and maintained with 100 W of forward power and a helium flow rate of 3.1 L/min. The plasma torch was constructed from fused silica (id. = 2.5 mm; 0.d. = 7.0 mm). The head of the torch w a directed ~ toward the wheel in such a position that the plasma contacted the edge of the wheel. The detailed structure of the interface is shown in Figure 2. Chromatographic effluent was transferred onto the edge of the moving wheel interface by directing a mist from a concentric glass nebuIizer (constructed at Northern Illinois University and modeled after the Meinhard Concentric tube nebulizer) operated at 10 psi with nitrogen nebulization gas. The axial image of the plasma was observed through a fused silica plate located at the back of the torch and was focused with a magnification of 1.7 onto a 0.5-m Ebert monochromator (Jarrell-Ash Co., Waltham, MA, Model No. 82-516) equipped with a 100-fimentrance slit and a 30-fim exit slit with a 2.5 cm diameter (15-cm focal length) fused silica lens. The output of the photomultiplier tube (R212, Hamamatsu,
0003-2700/89/038 1 -0895$0 1.50/0 0 1989 American Chemical Society
886
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989
-r 1 NEBULIZER t
i 1 4
I
0 5 METER: SPECT?OMETER;
r->*A=+zz rebulirer
I!
_ _
Figure 1. LC-MIP system schematic. ~ o view p
11
cI o x I
c11 :n-
Br (11) 0
4
8
Br-
BrOj
206.24 nm a
ii
i1 1,l
uacuun- bot N2
Side View
IOi
0
4
8
1-
103/1-
Flgure 3. Retention time characterizatlon: top, separation of chlorate and chloride viewed at the 479.45-nm CI I1 line; center, separation of bromate and bromide viewed at the 478.55-nm Br I1 line; bottom, separation of iodate and icdiie viewed at the 206.24-nm I I line. Each chromatogram corresponds to a single injection monitored at the listed
wavelength. Arrows correspond to sample Injection.
8
b
Flgure 2. LC-MIP interface: (a) top view; (b) side view. Key: 1, friction wheel drive: 2, guide wheel gearing; 3, interface chamber housing: 4, TMolo resonator; 5, separation plate; 6, stainless steel wheel; 7, fused silica plasma torch (2.5-mm i.d., 7.0-mm 0.d.) with fused silica face plate; 8, driver wheel shaft; 9, N connector; 10, coupling loop; 11, plasma region.
Toyko) was converted to a voltage and amplified with an operational amplifier based current-to-voltage converter/amplifier. The voltage output was monitored on a standard strip chart recorder.
RESULTS AND DISCUSSION Optimization of Experimental Conditions. The analytical sensitivity is largely dependent upon the plasma gas flow rate, nebulization gas flow rate, rotation speed of the moving wheel, and the relative position of the torch to the wheel. Among all these factors, the rotation speed and the position of the wheel are of the most importance. A very uniform rotation rate is required. Care had to be taken to align the wheel so that contact with any part of the housing was avoided. Any obstruction causing the wheel to rotate at a speed that is not constant causes different sections of the wheel to have different contact times with the plasma, contributing significantly to plasma flicker noise. These problems will largely increase the background noise, causing an increase in the detection limit. It was found that a wheel rotation rate of 7.1 cm/s provided the maximum signal. The second important factor is the relative position of the wheel with respect to the plasma torch. If the distance between the two is too large, the sample is not exposed to the highest temperature region, causing the efficiency of excitation to be low. However,
if the distance is too short, a depression of atomic line emission is observed, probably from plasma perturbation. Therefore, a compromise distance is required. In this work, the distance from the head of torch to the edge of wheel is about 1.5 mm. Detector Selectivity. In order to initially characterize the HPLC-He-MIP system, model samples were chosen in three sets, which were a chlorine set (as NaCl and NaClOJ, a bromine set (as NaBr and NaBr03), and an iodine set (as NaI and NaI03). Initial retention characterizations were determined by injecting 96-pL sample sets containing 50-pg amounts of halides, as the halide and the oxohalogen, and observing the emission line of the corresponding halogen. The results are shown in Figure 3. Fairly good separation was obtained for the compounds within each set. From the chromatograms obtained, it can be seen that the difference in the retention characteristics of the two components within the same set is large enough to be detected. But the retention differences of the components in different sets, such as C1- and Br03-, are small. Such a small difference in retention would make both quantitative and qualitative determination difficult with nonselective detectors. However, when the respective element emission wavelength is set, only the emission signal of the corresponding element will appear, even if another species is coeluted. Mixtures containing each of the six components were injected, with the resulting chromatograms identical with those in the right column of Figure 3. A response was not observed to any significant degree from the anions other than those containing the halogen being monitored. Calibration Plots. The calibration plots were constructed on the basis of peak area (intensity) versus the amount of halogen. The plots were divided into two categories; the first set was performed by direct solution nebulization. These results are shown in Figure 4. Generally, calibration plots
ANALYTICAL CHEMISTRY, VOL. 61, NO. 8, APRIL 15, 1989
1.o
signal is defined as the peak height in the case of chromatography and as the signal above the base line for direct solution nebulization. These limits of detection are listed in Table I and range from 0.2 to 20 pg/mL for direct solution nebulization and from 4 to 300 ng/s for HPLC. While these detection limits do not approach those of ion chromatography using standard detectors, the combined selectivity and preliminary detection limits of the system are promising. It is clear that the major source of background noise is the slightly variable rotation rate of the moving wheel.
0.8
.-E v)
5
0.6
I
's 0.4
-
d
0.2 0.0
0.0
897
0.2
0.6 0.8 Amount of X (mg/mL)
0.4
1.2
1.0
Flgure 4. Calibration curve obtained by direct nebulization of sample from stock bottle: C103- (A),CI- (W), Br03- (+), Br- (O), IO3- ( O ) , I-
(A).
CONCLUSIONS A moving wheel interface between liquid chromatography and an atmospheric pressure helium microwave-induced plasma has been developed, and initial results are encouraging. Since the lower level of detection is limited by flicker noise from slight variations in the wheel rotation rate, efforts are currently being directed toward the improvement of the rotation mechanism. Therefore, a great improvement in detection limits is potentially possible. These modifications are currently under investigation in our laboratory.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
Amount of X(pg)
Flgure 5. calibration curve obtained by nebulization of LC effluent: ClO3- ( O ) , CI- (A), Br03- (W), Br- (A),IO3- (+); I-(0).
Table I. Detection Limits (DL) of Halides and Oxohalogens
species
C1 as NaC103 C1 as N a C l Br as N a B r O B Br as NaBr Ias N a I 0 3 I as N a I
wavelength, 479.45 479.45 478.55 478.55 206.24 206.24
nm
DL by HPLC, ng/s
DL by direct nebulization, PPm
30
5
54
20 9
200 300 7 4
10 0.6 0.2
were linear from 50 to 1000 pg/mL. The second set of calibrations was obtained by using the HPLC effluent and is shown in Figure 5. While some deviations from linearity were observed, calibration plots were sufficient for analytical utility. The reasons for these deviations are still unknown and are currently under investigation. Detection Limits. The detection limit is defined as the analyte concentration yielding a signal intensity twice the standard deviation of the background noise. The analytical
(8) (9) (10)
(11) (12) (13) (14) (15)
Ettre, L. S . J. Chromatogr. Sci. 1978, 16, 396. Skogerboe, R. K.; Coleman, G. N. Anal. Chem. 1976, 4 8 , 611A. Zander, A. T.; Hieftje, G. M. Appl. Spectrosc. 1981, 35, 357. Risby, T. H.; Talml. Y. CRC Crit. Rev. Anal. Chem. 1983. 14, 231. Carnahan, J. W.; Mulligan, K. J.; Caruso, J. A. Anal. Chim. Acta 1981, 130, 227. Hughes, S. K.; Brown, R. M.; Fry, R. C. Appl. Spectrosc. 1981, 35, 396. Wolnik, K. A.; Miller, D. C.; Seliskar, C. J.; Fricke, F. L. Appl. Spectrosc. 1985, 39, 930. Freeman, J. E.; Hieftje, G. M. Appl. Spectrosc. 1985, 39, 211. Nygaard, D. D.; Leighty, D. A. Appl. Spectrosc. 1985, 39, 968. Michlewicr. K. G.; Urh, J. J.; Carnahan, J. W. Spectrochim. Acta 1985, 4 0 8 , 493. Mlchlewicz, K. G.; Carnahan, J. W. Anal. Chem. 1985, 5 7 , 1092. Michlewicz, K. G.; Carnahan, J. W. Anal. Chim. Acta 1986, 186, 275. Michlewicz, K. G.; Carnahan, J. W. Anal. Chem. 1988, 5 8 , 3122. Koilotzek, D.; Oechsle, D.; Kaker. G.; Tschopel, P.; Tolg, G. Fresenius' Z . Anal. Chem. 1984. 318, 485. Nisamaneepong, W.; Ibrahim, M.; Gilbert, T. W.; Caruso, J. A. J. Chromatog. Sci. 1984, 22, 472. Windsor, D. L.; Denton, M. B. J. Chromatogr. Sci. 1979, 17. 492. Morita, M.; Uehiro, T.; Fuwa, K. Anal. Chem. 1980, 52, 351. Hausler, D. W.; Taylor, L. T. Anal. Chem. 1981, 53, 1223. Savage, R. N.; Smith, S. B. J. Ll9. Chromatogr. 1982, 5 , 463. Fraley, D. M.; Yates, D.; Manahan, S. E. Anal. Chem. 1979, 2225. Lawrence, K. E.;Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 293. Michlewicz, K. G.; Carnahan, J. W. Anal. Lett. 1987, 20, 1193.
RECEIVED for review October 17,1988. Accepted January 17, 1989. L. Zhang and J. Carnahan express their gratitude to the Division of Educational Programs of Argonne National Laboratory for support of this research project. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract no. W-31-109-ENG-38. This work was presented in part at the 1988 Winter Conference on Plasma Spectrochemistry, San Diego, CA, paper no. S14.
