Anal. Chem. 1809, 65, 2596-2600
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Helium Microwave Induced Plasma Atomic Emission Detection for Liquid Chromatography Utilizing a Moving Band Interfacet Peter B. Mason, Liming Zhang, and Jon W. Carnahan. Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115
Randall E. Winand Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439
A moving band interface is used to separate HPLC solvent from analyte before introduction into a microwave-induced plasma atomic emission detector. Spectral scans indicate that all detectable solvent is removed prior to analyte introduction. Analyte memory effects are not detectable. Chlorine element selective detection limits are 140,410, 220, and 770 pg/s for 9-chlorofluorene,pchlorobiphenyl, 4-chlorobenzophenone,and a,a'-dichloro-o-xylene,respectively. If the vaporization region is heated significantly, chlorine selective response is dependent upon the boiling point of the compound due to analyte volatilization before it reaches the detector. INTRODUCTION The need for a sensitive and selective detector for liquid chromatography(LC) is intrinsic to the method itself. Many commonly utilized LC detectors exploit eluent bulk physical or chemical properties, such as refractive index or W-visible absorption. These types of detectors lack the combination of high sensitivity and selectivity. Due to the finite resolving capabilitiesof today's chromatographiccolumns, the problem of analyte coelution must be addressed. On a fundamental basis, the technique of atomic emission spectrometry used for LC detection has the potential to provide both enhanced sensitivity and selectivity. With the low detection limits afforded by typical atomic emission techniques and the inherent Selectivity of elemental line emission, it would seem that an atomic emission detector would be ideally suited for use in chromatography. One chromatographic technique which has successfully employed helium microwave-induced plasma (MIP) atomic emission detection is gas chromatography (GCl.1-7 Several operational aspects of GC make it a suitable partner for MIP coupling. In particular, helium gas may be used as both the GC mobile phase and the plasma support gas, flow rates of the mobile phase and the plasma gas are similar, and analyte is present in the vapor phase; thus lower power plasmas ( 100
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t Presented in part at the 19th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Cleveland, OH, 1990; Paper 44. (1)Estea, S.A.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1981,53,1829. (2) Van Dalen, J. P. J.; de Lezenne C o h d e r , P. A.; de Galen, L. Anal. Chim. Acta 1977, 94, 1. (3) Bradley, C.; Carnahan, J. W. Anal. Chem. 1988,60, 858. (4)Slatkavitz, K. J.: Uden, P. C.; Barnes, R. M. J.Chromatom. 1986, 355, 117. (5) Wu, M.; Lee, M.; Farnsworth, P. B. J. Anal. At. Spectrom. 1992, " I, IYI. (6) Webster, G. H.;Doggett, W. 0.; Boss, C. B. Anal. Chim. Acta 1992,
.,."
257,309. (7) Quimby, B. D.; Sullivan, J. J. Anal. Chem. 1990,62,1027.
0003-2700/83/0385-2596$04.00/0
W) are capable of efficient atomization and excitation. Unfortunately, GC represents a unique case. The presence of the mobile phases of other chromatographic techniques may alter selected plasma characteristics, making mating to the MIP more problematic. Most areas of LC plasma research focus on direct sample introduction. Several interfacing methods have been investigated, including the introduction of effluent vapor: nebulization,8Joand thermospray." While some success has been achieved, a fundamental limitation of this approach is that the bulk of the introduced material is the chromatographic mobile phase. Due to molecular emission (Cz, CH, CO, CN, Nz, etc.) caused by organic solvents, the mobile phase often contributes to increased noise due to increased spectral background. Additionally, lower power plasmas are generally incapable of desolvation, atomization, and excitation. The presence of even small amounts of mobile phase may cause analyte signal reduction or even extinguish the plasma. It has been suggested by Jansen, Huff, and DeJongl2 that a means of achieving the desired sensitivity and low detection limits with liquid chromatography-plasma atomic emission interfaces requires the separation of the bulk solvent (i.e., mobile phase) from the analyte prior to introduction into the plasma. The use of a solvent evaporation-analyte transporttype interface between HPLC and MIP represents a potential means of solving this problem. Zhang and co-workers18 exploited the large differences in volatility between mobile phases and inorganic LC eluates. They examined the use of a moving wheel interface to couple HPLC with the He-MIP. The column effluent was nebulized and depositedon a rotating stainlees steel wheel. ABthe wheel rotated, the effluent passed under a flow of hot nitrogen which served to vaporize and carry away the mobile phase. The dry analyte on the wheel then rotated into the plasma where it was vaporized, atomized, and excited. While temperature control was sufficient to separate solvent from inorganic solutes, the high heat capacity of the thick steel wheel coupled with direct plasma contact reduced the feasibility of the use of this system with organic analytes. Finer temperature control and more rapid temperature changes are required. Additionally, detection limits were limited by flicker noise induced by plasma-rotating wheel contact. An ideal transport-type interface should possess the followingcharacteristics: the samplaloading capacity should (8)Billiet, H.A. H.; Van Dalen, J. P. J.;Schoenmakers, P. J.;De Galen, L.Anal. Chem. 1983,55,847.
(9) Michlewicz, K. G.; Carnahan, J. W. Anal. Chem. 1986,58,3122. (10) La Freniere, K. E.; Faeeel, V. A.; Ekkels, D. E. Anal. Chem. 1987. 59, 879. (11)Roychowdhury, S. B.; Koropchak, J. A. Anal. Chem. 1990, 62, 484.
(12) Janeen, G. W.; Huf, F. A.; De Jong, H. J. Spectrochim. Acta,Part B 1986,40, 307. (13) Zhang, L.; Carnahan,J. W.; Winans, R. E.; Neill, P. H.Anal.
Chem. 1989,61, 895.
0 1993 A W n C4-temlcal Sockty
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Figure 1. Overview dlagram of the LCMIP system with the movhg band interface. Detab appear in the text.
be high, the solvent removal process should be efficient, the analyte transport efficiency to the detector should approach loo%,sample memory effects should be nonexistent, and it should be possible to quickly alter the temperature of the sample transport surface to provide low-temperature sample deposition and high-temperature sample vaporization. Designed for HPLC mass spectrometry, the moving band interface manufactured by Finnigan MAT (San Jose, CAP4 possesses many of these characteristics. Applications of this interface to maas spectrometry were reported by several researchers.1617 This moving band interface maintains high sampling efficiency while removing enough solvent so that the remaining dry analyte can be introduced into the detector. The interface provides rapid temperature changes on the moving band so as to minimize sample loss due to decomposition and volatilization prior to entering the vaporization chamber. Presented in this paper are initialpromising results based on HPLC-MIP coupling with the moving band interface. EXPERIMENTAL SECTION A block diagram of the HPLC-MIP system is presented in Figure 1. The individualcomponentsof this system are described below. High-Performance Liquid Chromatography System. The LC pump was a miniPumpfrom Laboratory Data Controlcapable of flow rates from 0.48 to 4.8 mL/min. A Rheodyne (Cotati, CA) Model 7125 injector was utilized with a 20-pL injection loop. Sample injection was performed manually. Separation was accomplished using a 25 X 0.46 cm Chromanetics Spherieorb 5-pm particle diameter ODS (3-18 column. The 20% deionized water in methanol mobile phase was degassed ultrasonicallyfor 10 min. Microwave-Induced Plasma System. The plasma torches used throughout this research were capillary quartz tubes, nominally 2 mm i.d. X 7 mm 0.d. The torch was inserted into aT&loreeonantcavity (91.5-mminnerdiameter,12.2-mmdepth) of the same basic design as that described by Beenakker." Microwave power was provided by a 125-W, 2450-MHz Microtherm Model CMD, generator from Raytheon Manufacturing Co. (Waltham, MA) with a Times Fiber Corp. No. 07145RG214/U (Wallingford, CT) coaxial cable. Impedance matching was performed using a Model 1878B 3-stub tuner from Maury Microwave (Cucamonga, CA). The plasma was maintained at 60 W with a helium flow of 20 mL/min. Spectrometer System. Plasma emission was focused onto the entrance slit of the polychromator of a microwave plasma ~~
(14) Finnigan MAT HPLC-MS Interface Operator's Manual, Manual No. 92000-9091, &vision B, February 1982. (15) McFadden, W. H.;Schwartz, H.L.; Evans, S. J. Chromtogr.
1976,122,389. (18) McFadden, W.H.; Bradford, D. C.; Games, D. E.;Cover, J. L. Am. Lab. 1977, 10, 55. (17) Hayea,M. J.;Lankmayer,E. P.; Vouroa, P.; Karger, B. L.; McGuire, J. M. A w l . Chem. 1983,55, 1745.
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\ VacuumArea Belt m r o 2. Moving band Interface with the entke houslng chamber. For inltlai spectral scans,the front portron of the housing surrounding the vapwizatlonchamber was removed and heilum was Introduceddhctly into a part on the vaporization chamber.
Vaporizatloi Chamber
Outer Sleeve
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Plasmat Torch
VaporizaG Chamber
Flgwe 3. Exploded view of the front portion of the housing system to envelope the vaporization chamber. Details are &cussed In the
text.
detector (MPD) 860 organic analyzer from Applied Chromatography Systems LM. (Bedfordshire,England). This 0.76-m focal-lengthspectrometeremploysthe Rowland circle design with a 960 groovefmm grating and multiple exit slits to achieve simultaneousdetection. For this study, the 479.54nm chlorine ion line was monitored using an RCA 1P28 PMT (Lancaster, PA). The PMT was biased at -900 V. The PMT output was converted to a voltage and amplified. The amplified signal was monitored using a Houston Instruments (Austin, TX)singlechannelstrip-chartrecorderModelB5117-2. Initialspectralecane were obtained with a 0.5-m focal-length Model 82-000 JarrellAsh (Waltham, MA) Ebert spectrometer. Reagents. Reagentsemployed in this research were analytical grade and used as received. The following chemicalswere used. 4-chlorobenzophenone from Matheson Co., Inc. (Joliet, E); p-chlorobiphenylfrom Aldrich Chemicals (Milwaukee,WI);a,ddicbloro-o-xylenefrom Eastman Organic Chemicals (Rochester, NY);9-chlorofluorenefrom Aldrich Chemicals (Milwaukee, WI) and methanol. It was found using HPLC with UV-vieible detection that at least one impurity was present in 9-chlorofluorene. A second sample of 9-chlorofluorenefrom Aldrich was obtained and yieldedthe same impurity. Pure helium (99.995 % ) was purchased from Great Lakes Airgas, Inc. (West Chicago,
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RESULTS AND DISCUSSION Design and Operating Principles of the Moving Band Interface. The modified moving band interface ia diagrammed in Figures 2 and 3. Operation of the interface occure in five steps. In the first step, the HPLC eluent is direded onto the moving band as shown in Figure 2. The band ie a polyimide film 3.2 mm wide and 0.05 mm thick moving at a speed of 2.5 cm/s. The column eluent was directed onto the band through a '/le-in.-o.d., 0.01-in.4.d. flexible PEEK tube (Upchurch Scientific,Inc., Oak Harbor, WA) held 1mm above the band with a 6-mm-o.d., 2-mm4.d. glass sleeve which was bent 30° and mounted on the interface body with a Swagelok fitting. It may be calculated that a flow rate of 1 mL/min deposita a0.2-mm-thick eluent layer on the band. The second step involves initial solvent evaporation as the band passes beneath an infrared heater. Third, the effluent passes into a vacuum region served by a roughing pump. In this region residual solvent vapors are removed. The dry analyte ie then
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1 2. 3.
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479.5 481.0 481.9 486.1
nm CI(I1) nm CI(I1) nm CI(I1) nm H(I)
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470
I 500
Wavelength (nm) Flguro 4. Spectral scans of the hellum plasma In the 470-500-nm spectral reglon: (A) plasma of pure helium, helium Introduced through the Interface; (B) plasma with 0.5 mL/mln methanol Introduced onto the movlng band; (C) plasma with 0.5 mL/mln methanol contalnlng 1 mg/mL khlorobenrophenone deposlted onto the movlng band.
transported to the fourth functional region of the interface, the vaporization chamber. Here, flash vaporization of the analyte is achieved through radiant heating. Heating is accomplished by a nichrome heater within a quartz sleeve inside the chamber. Following samplevolatilization, the band with remaining analyte residue exits the vaporization chamber and passes under a nichrome wire cleanup heater. This heater serves to remove any remaining sample and prepare the band for the solvent deposition step. The band then cools and is ready to repeat another cycle. For initial vaporizationand spectral characterizations, the Beenakker TWlo cavity was affixed directly in front of the vaporization chamber. The quartz plasma torch was connected to the vaporization chamber with a Swagelok fitting. The configuration used for these experiments is similar to that of Figure 2 except that the forwardmost portion of the outer housing was not present and a Va-in. copper tube was soldered onto the vaporization chamber to deliver helium plasma gas through the chamber and into the torch. The plasma gas is introduced into the chamber and transports vaporized analyte into the plasma. The efficiency of solvent removal and analyte vaporizationwas examined by obtaining spectra in the region from 470 to 500 nm. These spectra are shown in Figure 4. The spectra with pure helium and with
the continuous introduction of 0.5 mL/min methanol are essentially identical. With the introduction of 0.5 mL/min methanol containing 1 ppt 4-chlorobenzophenone (bp 332 O C at 771 Torr), three intense chlorine ion emission lines were observed. Immediately upon the cessation of 4-chlorobenzophenone introduction and substitution of pure methanol, the spectra returned to that seen in Figure 4B. These results indicated that solvent analyte separation and high boiling point analyte desorption could be achieved with success. Flow injection was used to simulate transient chromatographic signals.18 When 4-chlorobenzophenone was monitored at the 479.5-nm chlorine emission line, it was found that solvent flows exceeding 0.5 mL/min suppressed the emission intensity. This behavior is probably due to incomplete removal of the greater solvent volume. While this configuration was useful for initial studies, it is not ideal in terms of maximizing analyte transport to the plasma. Of particular note are the rectangular slots where the moving band enters and exits the vaporization chamber. The entrance and exit slots have dimensions of approximately 5 mm X 0.5 mm, or a combined area of 5 mm2. It is reasonable to assume a significant loss of helium, and consequently, a lose of analyte occurs through these slots. The inner diameter of the quartz plasma torch is 2 mm, which represents an area of 3.1 mm2. On the basis of areas alone, one would expect only 38% of the analyte to exit through the plasma torch. Plasma back pressure may decrease this fraction significantly. To prevent this decreasein sample transport to the plasma, a modification to provide a helium overpressure outside of the vaporization slots was devised. Figures 2 and 3 detail this approach. A system employing inner and outer cylindrical sleeves and a 0.5 mm thick by 54 mm diameter metal face plate was machined. The inner sleeve was fitted over the vaporization chamber and secured with bolts. The outer sleeve slid over the inner sleeve and a gas-tightseal was formed by a nitrile O-ring located on the interior surface of the outer sleeve. The chamber was completely sealed by the metal face plate butting against a second nitrile O-ring located on the inner face of the outer sleeve. On Figure 2, note the placement of the thin metal spacer located between the housing and the adjacent chamber. This spacer had 12holes (3-mm diameter) to allow access to the atmosphere. Without the spacer, the vacuum was sufficient to draw atmosphere through the outlet of the torch and prevent plasma ignition. By isolating the vacuum region from the helium overpressure region with the spacer, a net positive flow of helium through the torch was maintained. With the vaporization chamber completely enclosed, helium gas was introduced. This introducedgashad two outlets: into thevaporizationchamber through the aforementioned vaporization chamber slots or out the back of the enclosed region through the moving band slots. This former flow serves as the analyte transport and plasma support gas. In this manner, the loss of analyte through the vaporization chamber slots was eliminated. To achieve a net flow of 20 mL/min through the plasma torch, approximately1.5 L/min helium was provided to the enclosed overpressure region. The bulk of introduced gas was lost through the metal spacer to the surrounding environment. Because the flash vaporization and cleanup heaters are completely enclosed in the metal sleeves, there was a 1-h period of time before the temperature of the vaporization chamber and the cleanup heater reached steady-state values. The interface was operated without introduction of mobile phase until the temperatures stabilized. The IR heater was set at 90% of maximum, yielding an approximatetemperature of 100 "C. Vaporization and cleanup heater temperatures (18)Zhang, L. Ph.D. Dissertation, Northern Illinois University, 1990.
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Figure 5. Chromatogram of 4-chlorobenrophenone,gchloroflwrene, and pchloroblphenyi. The vaporization and cleanup heaters are set at 220 and 214 O C , respectively.
were in the range of 200-250 OC. These temperatures are detailed with individual chromatograms. It should be noted that, even at the high temperatures maintained in the vaporization and cleanupregions, the polyimide band showed few signs of deterioration when operated continuously for periods of 6-7 h and intermittently over a period of 3 months. Figure 5 is a chromatogram of a 20-pL injected mixture of 8.9 pg of chlorine as 4-chlorobenzophenone,0.53 pg of chlorine as 9-chlorofluorene, and 8.9 pg chlorine as p-chlorobiphenyl in methanol. A mobile-phase flow rate of 0.43 mL/min was employed, with the vaporizationheater set at 226 OC and the cleanup heater at 214 OC. Using W-visible detection, the first and last peaks of this chromatogram were determined to be impurities in the 9-chlorofluorene. For this set of compounds, the peak areas per unit mass were similar. This observation is not surprising as the MIP is expected to produce peak areas proportional to chlorine mass. Further, because the boiling point range of these compounds is reasonably small (291-332 "C), their vaporization behaviors onthe moving band interface are expected to be similar. (It should be noted that the boiling point of 9-chlorofluorene has not been determined. However, the boiling point of fluorene is 295 "C and the chlorinated species is expected to be slightly higher.) A second set of mixtures was examined. This chromatogram was obtained using a vaporization temperature of 240 O C and cleanup heater temperature of 215 OC. The chromatogram in Figure 6 was obtained by injecting a 20-pL sample containingchlorine masses of 3.3 pg as a,a'-dichloro-0-xylene, 2.7 pg as 4-chlorobenzophenone, and 4.2 pg as p-chlorobiphenyl. Dynamic Ranges and Detection Limits. Calibration plots were obtained for the compounds separated in Figure 5 and are shown in Figure 7. Ae stated previously, the responses for these compounds are similar. Response tends to be linear up to approximately 5 pg. Masses above this level produce a "roll-over" in the calibration plot. The cause of this nonlinearity may be due to plasma "over-loading", vaporization effecta, or a combination of the two.
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Moss Chlorine [micrograms] Figure 7. Calibration plots of compounds shown in the Flgure 5 chromatogram: (0)Cchlorobenzophenone;(+)pchkmblphenyl; (0) Q-chiorofluorene.
Figure 8 is a plot of peak area versus chlorine mass for the mixture of 4-chlorobenzophenone,p-chlorobiphenyl, and a,ddichloro-o-xylene. This plot indicates a different detector response for each of the three compounds. Relative responses were 1.00 for 4-chlorobenzophenone, 0.55 for p-chlorobiphenyl, and 0.34 for a,a'-dichloro-o-xylene. Examinations of the differences in response behaviors are important in optimizing the system for analytical use. The calibration plot shown in Figure 7 (4-chlorobenzophenone, 9-chlorofluorene, p-chlorobiphenyl)was taken with a vaporization temperature of 220 OC. Fairly uniform response was seen for all compounds. Calibration data for Figure 8 were taken with a vaporization heater temperature of 240 "C. In this case, response varied from compound to compound. It is likely that the temperature of the moving band within the housing plays a major role in determining response behavior. The metal sleeves which house the vaporization and cleanup heaters are significantly heated by the vaporization and cleanup heaters. One problem which this heating poses is the possible vaporizationof analyte prior to entrance into the vaporization chamber. The boiling points and response factors for 4-chlorobenzophenone,p-chlorobiphenyl, and CY,&'dichloro-o-xylene are listed in Table I. As can be seen from
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Table 11. Detection Limits for Compounds Separated and Detected with the Moving Band HPLC-MIP Interface re1 detectn limit abs detectn limit compound (ng of Cl/s) (ng of C1)
CI
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4-chlorobenzophenone 9-chlorofluorene p-chlorobiphenyl a,a'-dichloro-o-xylene
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1 L
800
0.22 0.14 0.41 0.71
31 16 60 97
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For all compounds except 9-chlorofluorene,detection limits were taken with the conditionsof Figure 5. For 9-chlorofluorene, conditions are those of Figure 6.
0
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Moss Chlorine (micrograms] Calibration plots of compounds shown in the Figure 6 chromatogram: (0)4chlorobenzophenone; ( *)pchlorobiphenyl; (W) a,a'dichloro-exylene. Flguro 8.
Table I. Boiling Points and Response Factors for Compounds Separated in Figure 6 and for Which Calibrations Are Shown in Figure 8 compound bp ("0 re1 response 4-chlorobenzophenone p-chlorobiphenyl a,a'-dichloro-o-xylene
332 291 239
1.00 0.55 0.34
this table, response factors increased as the analyte boiling point increased. It is likely that at the higher vaporization chamber temperature that the heating of the metal sleeve increases and this heat may be transferred to the moving band. Lower boiling point compounds may be partially vaporized before they reach the vaporization chamber and be carried out the back slots by helium in the overpressure region. Reproducibility studies were performed with each of the compounds injected and the conditions of Figures 7 and 8. The average relative standard deviations were 5.0% and the range for the compound set was 1.0-12%. Table I1 illustrates chlorine detection limits using a signal to baseline noise ratio of 3 for the compounds utilized in this study. The range of detection limits was 140 pg of Cl/s for 9-chlorofluorene to 770 pg of Cl/s for a,a'-dichloro-o-xylene. These detection limits approach those of the GC-MIPsystems. Estes et al.' listed a chlorine detection limit for 1,1,2,2tetrachloroethane of 43 pg/s. In this work, the listed detection limits average 380 pg/s. Thus, the average detection limit is only a factor of 9 higher than that obtained with GC-MIP and only a factor of 3 higher in the best case. Clearly, the detection limits for the LC-MIP system are approaching those of the GC-MIP.
The moving band interface has been demonstrated to be an efficient tool for separation of liquid chromatography mobile phase from the analyte. Using this scheme, detection limits approach those of GC/MIP systems. It appears that improvements can be made to enhance system performance even more. Accompanying the modified vaporization region was a significant amount of heating of the sleeveswhich surrounded the band. This heating may have caused significant loss of analyte due to vaporization in this region. Cooling of the sleeve by affixing a water coil to it may prevent this problem. Additionally, the capillary quartz plasma torches typically used in a low power plasma system are degraded by the high temperature plasma. Eventual large increases in plasma noise and instability necessitates changing these torches. Initial design and construction of a water-jacketed torch, with the water jacket placed immediately after the cavity has improved the lifetime of the torch approximately 2-3 times. Implementation of this torch for LC/MIP will be examined. Finally, future work should focus on the application of this system to other chromatographic separations. The He-MIP is capable of detecting a number of non-metals, including 0, N, C, H, S, P, C1, and Br. In particular, the advantage of selectivity offered by atomic emission detection should be exploited by examining separations which involve the coelution of analytes by simultaneous monitoring of several emission lines. This system shows promise and should be examined for the separation, identification and quantitation of pesticides and pharmaceuticals.
ACKNOWLEDGMENT This work was performed in part under the auspices of the Office of Basic Energy Sciences,Division of ChemicalSciences, US. Department of Energy, under Contract W-31-109-ENG38. RECENEDfor review April 12, 1993. Accepted June 23, 1993." ~~~~
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Abstract published in Aduance ACS Abstracts, August 15, 1993.
