846
Anal. Chem. 1985, 57,846-851
Moderate-Power Helium Plasma as an Element-Selective Detector for Gas Chromatography of Dioxins and Other Halogenated Compounds David L. Haas’ a n d J o s e p h A. Caruso* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221
A moderate-power helium mlcrowave dlscharge Is characterized as an element-selective detector for the gas chromatography of halogenated compounds. Plasma power and gas flow are optimized utlllrlng a mixture of chlorinated dloxlns and pesticides. Not surprisingly, the lowest flows possible with the tangentlal flow torch produced the best sensltlvlty. Unllke earller solution nebulization work, the hlgher plasma powers utlllzed led to poorer S / N . Multlelement chromatograms are determined at several wavelengths corresponding to the elemental emlsslon available from specific compounds. Detection limits and linear ranges are presented. I n addition to the quantitative Importance of this technique, elemental ratlos (leading to emplrlcal formulas) are determined with high accuracy by using an off-line background correction scheme. On-llne background correction schemes were also useful although Inferior to the off-line method. When no background correction Is employed, the accuracy of the elemental ratio Is considerably degraded.
During the last 20 years, the microwave-induced plasma (MIP) has become popular as an element-selective detector for gas chromatography (1, 2). Selectivity in the detection of chromatographic eluents is gained since the He-MIP responds primarily to the elemental constituenb of a compound, rather than to the entire compound. The element-selective capability of the microwave discharge allows more flexibility in the determination of gas chromatographic eluents, vs. electron capture and flame ionization detectors which respond nonselectivity to all compounds or to a given class of compounds. Multielement determinations are facilitated by the utilization of a polychromator for the spectrometric determination of many compounds. The grating of the polychromator disperses radiation from the plasma along a focal curve, which contains exit slits and photomultiplier detectors for emitted radiation of the elements of interest. Ideally, all elements in the periodic table could be determined simultaneously provided data acquisition could be performed at a rapid rate, and a sufficient number of detectors could be positioned within the instrument. Commercial Ar-ICP/polychromator systems claim the capability of determining many elements simultaneously. Also, the time required for multielement determination of a sample is greatly decreased vs. single channel or slew-scanning monochromator systems. Gas chromatographic (GC) separations have been reported utilizing both an Ar-ICP (3) and He-MIP (4-8) in conjunction with polychromatic detection. In addition, the simultaneous multielement capability of the He-MIP/polychromator has allowed determination of elemental ratios of compounds eluting from a gas chromatograph (4-7). ‘Present address: Union Carbide Corp., P.O. Box 8361, South Charleston, WV 25303.
Table I. Components, Models, and Manufacturers for a Moderate-Power MIP/GC System component gas chromatograph 3-way valve transfer block power supply polychromator
model or type
manufacturer
570 2033 120 V, 85 W
F&M Scientific Carle Instruments Chromalox Jarrell-Ash
stepper motor photomultiplier
66-000 1.5 m, 1180 G/mm M061-F608 1P28 R426
amplifier computer
i to e
resonant cavity generator tangential flow torch helium
Slo-Syn (McGraw Edison) RCA Hamamatsu laboratory built laboratory built
Intel-8080 64K micro TMo,ointernally laboratory built tuned 420-1, 600 W Micro-Now 816 o.d.1i.d. laboratory built quartz 99.0% Wright Brothers
The elution of C-containing compounds presents a potential source of error in multielement determination, due to background shifts at the elemental line of interest caused by emission from molecular species such as CN, Cz+,and CO. Relatively poor selectivities for C vs. such elements as Cl and Br have been attributed to this continuum shift (9). As a C-containing compound enters the plasma detector, the background continuum increases producing intensity at all spectral lines. Since this intensity is not due to the element of interest, “false peaks” in the chromatograms and nodinear response are common problems with determinations at elemental lines other than carbon. To avert this problem, either on-line (10) or off-line (8) background correction schemes may be employed. On-line background correction involves monitoring the background intensity shift caused by C a t all spectroscopic lines when the element of interest is not present. This background response due to C is then subtracted from the signal intensity when the element of interest is present, to yield the net signal intensity (in area or peak height) for the element. Off-line background correction can be performed utilizing an oscillating refractor plate which shifts the spectra on and off the emission lines of interest. Data collection, both on-line and off-line, thereby allows calculation of net signal intensity. Many workers have employed the low-power He-MIP for the determination of halogenated compounds. With the availability of moderate-power microwave generators for analytical work (11, 12), a moderate-power He-MIP is potentially an attractive alternative as an excitation source for GC detection. The object of this work is to characterize a moderate-power He dischasge as an element-selectivedetector for the gas chromatography of a variety of halogenated compounds, including the environmentally important dioxin group.
0003-2700/85/0357-0846$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
847
torch ooian quartz
-
threaded
1/16”
discharge
insert
a i6 T n i R T
2 , 7 - d i CDD
1 , 2 , 4 - t r i CDD
Methoxychlor
Citex BN-21
1 , 2 , 3 , 4 , - t e t r a CDD
tubing
r Tt
----transfer block ranganrial
Citex BC-26
Flgure 2. Chlorinated dioxins and pesticides.
f l o w gas
Figure 1. Modified tangential flow torch/GC interface. A heated transfer block prevents condensation of analyte in route to the plasma.
EXPERIMENTAL SECTION Instrumentation. The system schematic diagram is similar to that reported earlier (8). Experimental components and model numbers are listed in Table I. The internally tuned resonant cavity (11)was mounted on a gas chromatographic oven (F & M Scientific, Hewlett-Packard). To provide a means of venting a solvent peak, a three-way valve (Carle Instruments, Model 2033) was placed after the chromatographic column (lI4 in. X 3 ft quartz packed with 2% OV-101 on a silica support). The solvent peak must be vented with a low-power He-MIP to avoid extinguishing the plasma. At moderate power levels, the plasma was stable with the introduction of relatively large amounts (ca. 5 pL) of organic solvent. As might be expected, however, C background increased dramatically and thus solvent venting was employed throughout the investigation. During solvent venting, the plasma was maintained with a tangential flow gas as discussed below. Provisions were made to heat the tangential gas by passing it through 30 ft of 1/8 in. stainless tubing before entry to the plasma. To minimize peak broadening, all existing union and “T” connectors were replaced with low dead volume fittings (Swagelok fittings, Cincinnati Valve and Fitting, Cincinnati, OH). Torch Design and Chromatographic Interface. To avoid decomposition of the containment tube, it is critical that the moderate-power plasma remain centered. To satisfy this requirement, a torch was designed similar to the modified tangential torch discussed recently (12, 13). The torch design and chromatographic interface are given in Figure 1. A ‘/I6 in. X 0.010 in. stainless steel tube from the chromatographic column passed in. “T”connector and into a threaded, through one end of a stainless steel insert placed within the discharge tube. This threaded insert is also threaded onto the low volume lIl6in. tubing from the column, both securing the insert in position and allowing its easy replacement. The stainless steel insert was quadrathreaded (four threads, 90° apart), with a thread depth of 0.020 in. and a pitch of 3-114 turnslin. The threaded insert fits snugly into a quartz discharge tube (8 rnm o.d., 6 mm i.d.) which is secured in the other end of the “T” connector. To dissipate additional heat, the discharge tube was cooled by flowing Nz gas through an outer cooling tube. To prevent condensation of the analyte as it is transported from the column to the plasma, the quartz discharge tube was passed through an aluminum insert (lI2in. o.d., 8 mm i.d. x 3 in.) which was secured in a l/z in. hole of an aluminum heating block. Power for the heating blocks (Chromlox, 120 V, 85 W) and thermocouples were provided with the controls of the gas chromatograph. Emission Lines. Four emission lines (channels)were available for spectral observation. Namely C I at 247.9 nm; Br I1 at 470.5 nm; C1 I1 at 479.5 nm; and H I at 656.3 nm. The H I line was monitored with a red-sensitive photomultiplier (Hamamatsu R-446). The others were as described previously (8). Data Acquisition. The plasma image was focused through a quartz lens (4 cm, 15 cm focal length, Ealing Optical) onto the entrance slit of the polychromator. Emission from the plasma passed through an oscillating quartz refractor plate and onto a
concave grating where the radiation was dispersed along the Rowland circle of the polychromator. A stepper motor controller (8)oscillates the refractor plate to allow dynamic off-line background correction. The current of each photomultiplier is converted to voltage and sent to the PMT data preprocessor which “latches” the data before it is sent to the ADC board of the 8080 microcomputer for digitization. Sample Preparation and Procedures. 2,7-Dichlorodioxin (2,7-di CDD), 1,2,4-trichlorodioxin (1,2,4-tri CDD), 1,2,3,4tetrachlorodioxin (1,2,3,4-tetraCDD, also given as TCDD in some of the figures), Citex BC-26, Citex BN-21, Methoxychlor, and hexachlorocyclohexane (BHC) solutions all were prepared by weighing on a microbalance the amount of compound needed to prepare a 10000 ppm stock solution, followed by dilution with benzene or isooctane in a 10.0-mLvolumetric flask. These compounds were provided by the U.S. FDA. The dioxins also were obtained from Chem Service, Inc. BHC was obtained from the US. EPA. Diethyl phthalate (DEP) and dibutyl phthalate (DBP) are liquids and were obtained through the U.S. EPA. They were prepared by removal of the volume needed t o produce a 10000 ppm stock solution, followed by dilution with benzene in a 10.0-mL volumetric flask. Serial dilutions were then performed to obtain desired solution concentrations. Five microliters of the appropriate mixture was injected onto the chromatographic column. SOFTWARE Data manipulation was performed utilizing appropriate software as discussed in an earlier work (8)but modified to provide additional features to store the chromatographic data on disk, recall of a chromatographic data set from the disk, calculation of detection limits and elemental ratios, and output of all calculations on the printer. R E S U L T S AND DISCUSSION (1) Plasma Power. A study was performed to determine the effect of applied power to the helium discharge on the C1 signal intensity and signal-to-noise ratio of C and C1 in several halogenated compounds. A test mixture containing 2,7-di CDD, 1,2,4-tri CDD, 1,2,3,4-tetraCDD, Methoxychlor, Citex BN-21, and Citex BC-26 was selected due to the environmental significance of these compounds. Their structures are presented in Figure 2. All compounds except BN-21 contain chlorine. Citex BC-26 contains both chlorine and bromine. The mixture, consisting of 1000 ng (6.94 p L of 144 ppm) of each compound was injected. Off-line background correction was used throughout the experiment. The resulting separation is shown in Figure 3. Chromatographic operating conditions for this separation are listed in Table 11. The effect of power on the signal intensity and signal-tonoise ratio for C and C1 emission illustrates a similar trend, as is illustrated in Figure 4 for C with a decrease followed by a slower increase in intensity and signal-to-noise ratio for both C and C1, as power to the discharge is increased from 145 to 425 W (as measured a t the generator). The noise on both channels (elements) steadily increases with power, and thus the optimal power level is the lowest (ca. 145 W) that could be obtained with this generatorlcavity configuration. Un-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
848
CHROMATOGRAPHIC
POWER STUDY
PARAMETERS
ov-101
2%
Carbon Channel
7 5 0 mL/mln He Carbon
20R
I
I
0
YethOXyChlor
I
1
e
;
A
BC-26
I n t
0100
-
0
; 10-
U
:
SIN
0
I
n
I
I
Signal
s &
i I
Chlorine
Y
1-
0
L ;
I
1 .o
4.0
7.0
I
I
125
225
'
I
1
325
425
Power (W) Flgure 4. Power study on the C channel. Increased C signal-to-noise ratio in the He plasma discharge was obtained at lowest powers. The
chlorine channel shows similar trends.
Time (min) Flgure 3. Gas chromatographic separation of chlorinated dioxins and pesticides, 190 W He plasma.
STUDY
Table 11. Moderate-Power He-MIP/GC Operating Conditions Utilized in Optimizing Plasma Conditions and Calculations of C / C l Ratios plasma optimization
calculation of C/C1 ratios
Plasma Conditions forward power reflected power He flow (tangential) He flow (column)
190 w
190 w
ow
750 mL/min
25 mL/min
ow
I
750 mL/min 25 mL/min
I
\
400
1200
GC Conditions column size column packing column flow temp (injection port) temp (transfer block) temp gradient
in. x 3 ft
in. x 3 ft
2% ov-101
2% ov-101 25 mL/min 300 "C 300 "C
I/,
25 mL/min 300 "C 300 "C time, min 0 2
injection volume
temp, time, "C min
teomp, C
190
0
100
190
2.3 7
240 240
3 240 7 24 0 6.9 WL
5.0 ALL
fortunately, the plasma would not center at 145 W, preferring to cling to the wall of the discharge tube. To avoid etching the containment vessel, 190 W was chosen for the remainder of the studies as the plasma was easily centered at this power level. (2) Plasma Gas Flow. The identical separation discussed above was utilized to determine the flow rate which yielded the greatest increase in C and/or C1 signal intensity and signal-to-noise ratio in a 190-W He discharge. Again, relative intensities for C and C1 responses were averaged over all compounds and are depicted in Figure 5. Both C and C1 responded similarly and showed increased intensity and SIN (results not shown) with decreased flow. This phenomenon is common with plasma sources and indicated a response dependent upon analyte residence time in the discharge. A tangential flow of 750 mL of Helmin was needed to center
2000
mL/min He Figure 5. Tangential flow study with C and CI. Low flows provided increased intensity and S I N on the C and CI channels, 190 W He
plasma. the discharge and thus to avoid degradation of the containment vessel. The remainder of the studies were run a t this flow. (3) Chromatography. T o investigate on-line vs. off-line background correction, and background interference due to C on the C1 channel, a test mixture was produced which contained three chlorinated compounds (Lindane (BHC), 1,2,3,4-tetra CDD, and Methoxychlor) and two non-chlorinated compounds (DEP and DBP). Chromatographic conditions are listed in Table 11. The chromatographic separation, monitoring the C, C1, and H channels, is presented in Figure 6. (4) Detection Limits and Precision. Detection limits and percent relative standard deviations for C and C1 were calculated from averages of five replicates of the mixture described below. A 5.0-pL injection, containing 500 ng of DEP, 500 ng of BHC, 500 ng of DBP, 360 ng of 1,2,3,4-tetra CDD, and 440 ng of Methoxychlor, was used for all detection limit and precision studies. The detection limits and precision for
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985 20000 r
2000000-
2000'
:
on Carbon C h a n n e l
Response
DEP
I
n'
A
DBP
0
DEP
849
rn
Methoxychlor
n
TCDD
0
BHC
1000000-
0
I
1
ng
7
1
time ( m i d
Figure 6. Separation of chlorinated and nonchlorinated priority pollutants. Responses on the C, CI, and H channels are indicated. Relative intensities are given on the vertical axis.
Table 111. Comparison of Detection Limits and Precision of the Moderate vs. Low Power He-MIP/GC element (channels)
mod power (single)=
mod power (multiple)b
low power (8) (multiple)b
Detection Limits (pg/s) C
c1
76
100
120
250 50
H
NDc
C
3.0 6.7
24 76
ND
Precision ( % RSD)
c1
Single element monitoring.
2.7 4.6
2.6 3.8
Multielement monitoring.
ND,
not determined.
carbon, chlorine, and hydrogen determined in the moderate power helium discharge are listed in Table 111. Data are compared for multielement acquisition using off-line correction vs. single channel determination utilizing on-line background correction. Detection limits were roughly 25 to 100% higher with off-line vs. on-line correction. This appears reasonable as the oscillating refractor plate allows less time for actual data acquisition on a given channel. The direct comparison of off-line vs. on-line correction (in the multielement mode) could not readily be accomplished with the present instrumentation. For comparison purposes, detection limits and precision values determined in a low power helium discharge on the same polychromator are also presented (8). Detection limits for C and C1 determined in the moderate power He-MIP are approximately 4 times higher than C and C1 determined in a low-power He-MIP on the same polychromator system (8). These increased levels of detection might be attributed to several factors: (a)Helium Flow. Large He flows are required to maintain a centered plasma arrangement within the discharge tube. Larger flows decrease analyte residence time in the discharge and thus produce decreased signal intensity. The total He flow is 775 mL/min (750 mL/min tangential flow 25 mL/min column flow) in
+
500
1000
compound
Flgure 7. Carbon response per nanograms of compound. Relative intensity given on vertical axis. The chlorine response is similar.
the moderate power system vs. 80 mL/min He flow (20 mL/min auxiliary flow 60 mL/min column flow) for the low power system (8). ( b )Elevated Background Excitation and Emission. It is possible that complete molecular fragmentation also occurs a t the lower power levels (10,14, 15). If this is the case, then the higher power available from the moderate power discharge may be further exciting the molecular background of the sample and thereby degrading the S I N (resulting in higher detection limits). ( c ) Optical Coupling. The optical coupling to the polychromator was performed with different lenses in the two studies (this work and ref 8). The internally tuned cavity resonator possesses two stubs which protrude from the face plate of the cavity, limiting the focal length of the lens selected to focus the plasma image a t the entrance slit. A 4.0 cm diameter, 15 cm focal length quartz lens was used vs. a 5.0 cm diameter, 10 cm focal length quartz lens in the low power system (8). The smaller lens limits the solid angle of light collected, allowing less light to the polychromator. Although it was initially anticipated that the detection limits would be lowered with the use of higher plasma powers, the data suggest that the lower power plasmas may be even better suited for gas analysis and will allow high-quality analytical results. (5) Linear Dynamic Ranges. Single channel determinations for C and C1 were performed utilizing duplicates of 5-pL injections of the test mixture described above. The refractor plate a t the entrance slit was placed in a fixed position, and thus no background correction was performed. The C (247.9 nm) channel response of DBP, BHC, DBP, 1,2,3,4-tetra CDD, and Methoxychlor in a 190-W He-MIP (750 mL/min flow) per ng of compound and ng of C is presented in Figures 7 and 8. The C1 (479.5 nm) channel response to BHC, 1,2,3,4-tetra CDD, and Methoxychlor per nanogram of compound and nanogram of C1 is similar to that given for C so the figures are not reproduced here. Both channels demonstrate linearity from 10 to 1000 ng of compound injected. Also a plot of intensity per nanogram of compound collapses to yield a constant slope when plotted as intensity per nanogram of element. This universal response, regardless of compound structure, indicates that reproducible fragmenta-
+
850
ANALYTICAL CHEMISTRY, VOL. 57, NO. 4, APRIL 1985
Response on Carbon Channel A
2000000
Response on Chlorine Channel
DBP
A
DBP
DEP
0
DEP
rn
Methoxychlor
A
TCDD
0
BHC
3000
A
1000000
1500
I
I 350
0
7 00
0
ng Carbon
ng Carbon
Figure 8. Carbon response per nanograms of carbon. Relative intensity given on vertical axis. The chlorine response is similar.
CARBON INTERFERENCE ON CHLORINE DEP
DBP
I
h
BHC
7 00
350
Figure 10. Carbon response on chlorine channel. Relative intensity
is given on vertical axis. Table IV. Elemental C/Cl Ratios and Percent Error in Determination”
Methoxvchlor Methoxychlor TCDD
compound
actual C/C1 ratio
BHC 1,2,3,4-tetra CDD Methoxychlor
1.00 3.00 5.33
single element no bkg corr
single element on-line bkg corr
multielement off-line bkg corr
0.95 (-5.0) 0.96 (-4.0) 1.00 (0) 2.92 (-2.7) 3.00 (0) 3.02 (+.67) 4.89 (-8.3) 5.12 (-3.9) 5.36 (+.56)
”Percent error in determination of actual C/CI ratio is given in parentheses. x1 chlorine x20 I
I
I
3
4
5
time (min) Figure 9, Carbon background interference on the chlorine channel. The chlorine channel was amplified 20-fold. Notice two “false peaks” appearing on the CI channel are caused by non-CI containing com-
pounds. tion of analyte occurs in the discharge. A plot of intensity vs. nanogram of element with constant slope, regardless of compound size or structure, is critical if elemental ratios are to be calculated. (6) Background Correction Schemes. A system utilizing multielement data acquisition (with off-line correction) was compared with single element data acquisition (with and without on-line correction) for the determination of C/C1 ratios. The first system utilized the multielement capabilities of the polychromator in conjunction with an oscillating refractor plate which facilitated stepping “on and off” (ca. 0.2 nm) the spectrometric line of interest. Data acquisition was synchronized so that off-line data (background) could be dynamically subtracted from on-line data (background + signal), resulting in a background corrected chromatogram. The second system involved monitoring C and C1 channels individually with the identical polychromator used in the
former study (8). As C-containing compounds eluted from the chromatograph into the plasma, the background intensity increased due to molecular species such as e N , CO, Czf, etc. The rise and fall in background emission, as the C-containing compound eluted, produced a “false p e a k on the C1 channel. This effect is illustrated in Figure 9. Both DEP and DBP (neither contain C1) appear on the C1 channel. In addition, it is reasonable to assume a slightly inflated C1 intensity for compounds containing both C and C1 due to the above cited background shift. Background correction was performed by monitoring the intensity on the C1 channel due to compounds containing no C1 (DEP and DBP). In theory, intensity on the C1 channel per nanogram of C can then be subtracted from the C1 intensity to give a net C1 intensity corrected for the background interference. A plot of intensity on the C1 channel per nanogram of C (C-containing compounds with no chlorine) is given in Figure 10. (7) Elemental Ratios. Elemental C/C1 ratios were calculated with five replicates of the mixture described above. Calculations using multiple channel acquisition were performed with the software described above. Elemental ratio calculations performed by monitoring the C and C1 channels individually must first be corrected for the background interference due to C on the C1 channel. Background correction is performed by subtracting the amount of emission on the C1 channel which corresponds to the amount (ng) of C in the chromatographic peak. Elemental C/C1 ratios as determined by multiple channel acquisition with off-line correction, and
Anal. Chem. 1985, 57, 851-857
single channel acquisition with and without on-line correction are listed in Table IV. The percent error on determination as compared with the actual C/C1 ratio is also presented. Single channel determinations of the C/Cl ratio of BHC, 1,2,3,4-tetra CDD, and Methoxychlor are all lower than the actual values for these compounds. As expected, the background interference due to C on the C1 channel does lead to significant error on the determination of the C/CI ratio of a compound. This interference inflates the value of the C1 intensity, resulting in low C/C1 ratios. Subtraction of the C1 channel interference decreases the error in the determination of these ratios. Multiple channel analysis with off-line correction leads to the determination of C/Cl ratios which are very close (
