Anal. Chem. 1993, 65,1998-2002
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On-Column Bromine- and Chlorine-Selective Detection for Capillary Gas Chromatography Using a Radio Frequency Plasma Stig Pedersen-Bjergaard' and Tyge Greibrokk Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway
A 350-kHz rf plasma sustained inside the end of a 0.25- or 0.32-mm4.d.fused-silica GC column was evaluated for element-selective detection of brominated and chlorinatedcompounds separated by gas chromatography. Due to the small volume of the plasma cell, dilution of the GC effluent with make-up gas was unnecessary and a stable 25-W plasma was maintained in only 1.5-5 mL/min GC carrier gas. With the plasma sustained in pure helium, carbon formed from eluting material was deposited in the plasma region causing serious peak tailing and loss of sample. This was avoided by dopingthe GC carriergas with traces of oxygen. With the 350-kHz rf plasma sustained on-column, detection limits for bromine and chlorine were improved by a factor of 50 to 0.9 and 1.1 pg/s, respectively. For both bromine and chlorine, element-to-carbonselectivities approaching 1Os:1 and detector linearity over at least 3 orders of magnitude were obtained. The on-column detection system has been applied to a screening for halogenated compounds in snow from Antarctica and in drinking water produced offshore.
tigations, combined use of these hyphenated techniques results in a strong tool for identification purposes.iZJ8 For most atomic emission detectors coupled with capillary GC, the plasma is sustained inside a discharge tube of 1-4mm i.d. In order to stabilize the plasma within this volume, typically 50-150 mL[min make-up gas (helium) is added to the effluent prior to detection. Unfortunately, the introduction of make-up gas causes a reduction in the detectability because the column effluent is diluted and because the residence time of emitting species in the plasma is reduced. In this work, detector sensitivity was dramatically improved by eliminating the introduction of make-up gas. This was accomplishedfor the F i t time by sustainingthe plasma inside the end of a fused-silica GC capillary column (on-column detection). Due to the low volume of the detedor cell, a stable plasma was maintained in only 1.5-5 mL/min helium used as the GC carrier gas. A 350-kHz rf plasma was used as an efficient on-column excitation source. This plasma had previously been used for the purpose of GC-AES by Rice et al.14 and by Skeleton et aI.1"" Compared with the 350-kHz rf plasma operated in a 1-mm-i.d. discharge tube with 60 mWmin make-up gas, detection limits for brominated and chlorinated compounds were improved by a factor of 50 using on-columndetection. Technical considerations, optimization, and applications of this system are presented.
INTRODUCTION
EXPERIMENTAL SECTION
Over the last two decades, atomic emission spectroscopy (AES)has been widely explored as one of the most selective and sensitive detection systems for gas chromatography (GC).ia A distinctive advantage of atomic emission detectors in GC is that they provide highly selective information based on the elemental content of the eluting compounds. Direct quantitative determination of elements in the effluent allows empirical formulas to be calculated,- as well as universal calibration.lOJi The information on elemental content provided by GC-AES is complementary in nature to the data from GC/MS-FTIR, and especially for environmental inves-
Apparatus. The gas chromatograph used was a Model 4200 GC equipped with a split/splitless injection port (Carlo Erba, Milan, Italy). For the optimization and performance studies and for the analysis of the snow sample, the GC was supplied with a 25 m X 0.32 mm i.d. fused-silica GC column coated with 0.17-rm HP-1methyl silicone (Hewlett-Packard,Avondale, PA). For the drinking water analysis, the GC was supplied with a 30 m X 0.25 mm i.d. fused-silicacolumn coated with 0.25-rm DB-5 methyl phenyl silicone (J&W Scientific,Folsom, CA). In addition, a 10 m X 0.10 mm i.d. fused-silica GC column coated with 0.17pm HP-5 methyl phenyl silicone (Hewlett-Packard)was tested for on-column detection. The on-column detector cell which was placed in the FID block on the top of the GC is illustrated in Figure 1. A 5-cm long piece of polyimide coating and stationary p b e was carefully burned off at the end of the fused-silica GC column. The last 2 cm of this uncoated GC capillary served as the plasma tube and was placed inside a 2-cm piece of protecting silica tube (VB-in. o.d., 0.5-mm i.d.). The protecting tube was fitted with a graphitized
* Author to whom correspondence should be addressed.
(1)Rieby, T. H.; Talmi, Y. CRC Crit. Reu. Anal. Chem. 1983,14(3), 231-265. (2)Ebdon, L.;Hill, S.; Ward, R. W. Analyst 1986,111,1113-1138. (3)Uden, P. C. Trends Anal. Chem. 1987,6(91,238-246. (4)McLean, W. R.;Stanton, D. L.; Penketh, G. E. Analyst 1973,98, 432-442. (5)Dingian, H.A,;De Jong, H. J. Spectrochim. Acta 1983,36B,777781. (6)Uden, P. C.; Slatkavitz, K. J.; Barnes, R. M. Anal. Chim. Acta 1986,180,401-416. (7)Wylie, P. L.; Sullivan, J. J.; Quimby, B. D. J. High Resolut. Chromatogr. ISSO,13,49946. (8)Yie-ru, H.; Quing-yu,0.;Wei-le, Y. J. Anal. At. Spectrosc. 1990, 5,116120. (9)Pedersen-Bjergaard, S.;Asp, T. N.; Vedde, J.; Greibrokk, T. J. Microcolumn Sep. 1992,4,163-170. (10)Qwyu 0,.; Duo-chuen, W.; Ke-wei, Z.; Wei-le,Y. Spectrochim. Acta 1983,38B,419-425. (11) Pederwn-Bjergaard, S.; Asp, T. N.; Greibrokk, T. Anal. Chim. Acta 1992,266,87-92. 0003-2700/93/0365-1998$04.00/0
(12)David, F.;Sandra, P. J. High. Resolut. Chromatogr. 1991,14, 554-557. (13)Pedersen-Bjergaard, 5.;Asp, T. N.; Vedde, J.; Carlberg, G. E.; Greibrokk, T.Chromatographia 1993,35,193-198. (14)Rice, G.W.; D'Silva, A. P.; Faseel, V. A. Spectrochim. Acta 1986, 40B,1573-1584. (15)Skelton, R.J.; Farnsworth, P. B.; Markidea, K. E.; Lee, M. L. J. High Resolut. Chromatogr. 1988,11,75-81. (16)Skelton, R. J.; Chang, H . 4 . K.; Fameworth, P. B.; Markides, K. E.;Lee, M. L. Anal. Chem. 1989,61,2292-2298. (17)Skelton, R. J.; Markides, K. E.; Lee, M. L.; Farmworth, P. B. Appl. Spectrosc. 1990,44,853-857. 0 1993 Amerlcan Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
RESULTS A N D DISCUSSION
top electrode
I protecting silica tube
1888
plasma region (GC column) _ _ j light
collection
GC-column Figure 1. Detector cell.
Vespel ferrule into the top of a 111~118-in.reducing union (Swagelok,Solon,OH). The fused-silica GC column was extended through this reducing union and a 4.5-cm piece of l/1&. steel tubing (transfer line) to the GC oven. The reducing union and the transfer line were mounted in the heated FID block in order to prevent condensationof high-boiling compounds. A steel wire (1-mm diameter) placed at the column outlet served as the top electrode. The plasma was generated inside the end of the uncoated fused-silicaGC column between this top electrode and the grounded reducing union by a HPG-2 radio frequency power supply (350 kHz; EN1 Power Systems, Rochester, NY). The plasma was sustained in the GC carrier gas with no addition of make-up gas. The optical part of the system was similar to the setup used by Skeleton et. al.1617and by Pedersen-Bjergaard and GreibrokkJ8 The plasma was viewed side on through the wall of the fusedsilica column and the protecting silica tube. Atomic emission was measured in the near-infrared portion of the spectrum by a Model H-20 IR monochromator (Instruments SA, Metuchen, NJ), providing a 0.4-nm resolution with 50-pm slits. A long-pass filter with a 595-nm cutoff (Melles Griot, Irvine, CA) was used to reject second- and third-order radiation, and a pair of achromaticlenses cf = 58 mm, 12.7" diameter;Newport Corp., Fountain Valley, CA) were used to focus the emission light in front of the monochromator. The photomultiplier tube used was a R2658 and was operated with a Model C665 dc power supply (both Hamamatsu, Shizuoka-ken,Japan). Signals from the photomultiplier tube were collected by a Model 428 current amplifier (Keithly Instruments, Cleveland, OH) and recorded on a SR6335 strip-chart recorder (Graphtec Corp, Yokohama, Japan). Ultrahigh-purity helium (99.9999096 ; Hydro, Oslo, Norway) was used as the GC carrier gas. The GC carrier gas was doped with traces of oxygen (99.998%;AGA, Oslo, Norway). Oxygen was introduced through a restrictor connected to a tee union in the GC carrier line between the helium tank and the GC. The helium was passed through a Supelco OMI-l indicating purifier (Supelco, Bellefonte, PA) prior to the addition of oxygen. Sample Preparation. To 1 L of a snow sample collected near a researchcamp in Antarctica was added 2 mL of acetonitrile and the pH was adjusted to 3 with HNOa. The sample was then eluted through a C18 Empore extraction disk (47 mm; Analytichem International, Harbor City, CA). The extraction disk was extracted by acetonitrile for 1 h and subsequently with ethyl acetate for 1 h. The final GC analysis was carried out on the combined extract. A 50-mL drinking water sample produced off-shorewas salted out with NaCl and extracted for 30 min with pentane according to the method of Peters.18 (18) Pedersen-Bjergaard, S.;Greibrokk, T. J. High. Resolut. Chromatogr. 1992,15,677-681. (19) Peters, R.J. B. Ph.D Thesis, Technische Universitet Delft, 1991.
Instrumental Considerations. With the plasma generator operated at 350 kHz, a stable rf plasma was easily ignited and maintained inside a 0.32-mm4.d. fused-silica GC column. This was also the case when the internal diameter of the GC column was reduced to 0.25 mm. With a 0.1-mm i.d. GC column, however, it appeared difficult to maintain a stable on-columnplasma. Consequently,on-columndetection was carried out with either a 0.32-mm4.d. column (optimization, performance, and snow sample studies) or a 0.25-mmi.d. column (drinking water analysis). Due to the small volume of the detector cell, dilution of the column effluent with makeup gas was unnecessary, and the rf plasma was sustained in only 2-5 mL/min helium used as the GC carrier gas. Besides a considerable improvement of the detector sensitivity (discussed below), the elimination of make-up gas caused a 20-50 times reduction of the helium consumption. In order to be an efficient power supply, the electrical impedance of the plasma generator must be equal to the impedance of the plasma discharge. Since the rf generator used in this work is designed to match a very wide range of impedances, the rf energy was easily and efficiently coupled with the simple electrode system used. With optimization of the frequency and impedance matching of the rf generator, the reflected power was less than 5 % . In spite of the small plasma cell dimensions, the wall of the fused-silica GC column was only slightly affected by the rf plasma a t power levels below 30 W. Even after 1 week of continuous operation, no peak tailing and no loss of sensitivity were observed due to interactions between the plasma and the capillary. Thus, plasma capillaries could be used for several days before replacement. Replacement was easily performed within 5 min by cutting off the used uncoated end of the GC column followed by removal of a new 5-cm length of polyimide coating. Above 30 W, the plasma became unstable, and at power levels higher than 35-40 W, the capillary was damaged. The interelectrode distance (and the size of the plasma) was determined by the length of the silica tube protecting the uncoated end of the GC column. The signal-to-noise ratios were not significantly affected by the length of the plasma. A 2-cm protection tube resulting in a plasma region of 3.5-4 cm was found to be appropriate. For atomic emission detectors, signals are sensitive to positional changes of the plasma. Positional changes may arise from mechanical fluctuations of the discharge tube or from plasma fluctuations inside the tube. In this work, the position of the capillary containing the plasma was fixed relative to the optical system by a protecting silica tube. With on-column detection, positional fluctuations inside the discharge tube were suppressed by the small volume of the detector cell. In addition, since the monochromator was equipped with 500-pmslits, homogeneous illumination of the entrance slit was achieved. In a previous work, side-on and end-on collection of light from a 350-kHz rf plasma were compared.18 Signal-to-noise ratios obtained with the two different configurations were almost the same (difference less than lo%),and good longterm stability was obtained with both geometries. In this work, collection of light through the fused-silica GC column and the protecting silica tube (side-on) was preferred because it simplified the construction of the detector cell. With the side-on configuration, however, detector sensitivity and longterm stability are affected by the optical quality of the plasma cell. High optical transparency was achieved by carefully burning off 5 cm of the polyimide coating prior to installation of the GC column. During operation, no deposita were observed in the uncoated part of the fused-silica GC column
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 15, AUGUST 1, 1993
On-column detection Chlorine 837.6 nm
I/
Detection with make-up gas
A
3
Chlorine 837.6 nm
signal
Bromine 889.8 nm 889.8 nm
A
0
10
20
30
applied power Flguro 3. Effect of applied power on signals and signal-to-noiseratios. Trlchlorobenzene was used as model compound.
--c-----ct-
6.65 6.70 6.75
min
7 . 4 0 7.45 7 . 5 0 min
--
6.65 6.70 6.15
7.40 7.45 7.50
min
min
Table I. Effect of Compound Structure on Bromine Response compound relative Br response dibromobenzene tribromobenzene tetrabromomethane tetrabromoethane
Fbure 2. Oncolumn detection and detection in a 1-mm-1.d. discharge tube with 60 mL/min He: elution profiles of trichioro- and tribro-
mobentene. if traces of oxygen were added to the GC carrier gas (discussed below). Thus, the high optical quality of the light path was maintained during operation, and a acceptable long-term stability was achieved; the intensity of helium-, bromine-, and chlorine-background emission driftedless than 10%over a 18-h period. Effect of GC Carrier Gas Doping. Initial experiments with on-columndetection revealed a very low tolerance toward eluting material when a pure helium plasma was used. Significant peak tailing and loss of sample (poor repeatability) were observed from replicate injections even at the lownanogram level. This was due to carbon formed and subsequently deposited in the plasma region. In order to keep the carbon volatile, traces of oxygen were added to the GC carrier gas. The oxygen level in the plasma was controlled by the intensity ratio between the 777.2-nm oxygen line and the 706.5-nm helium line. This was preferable because the small flow rates of oxygen required were difficult to measure directly. At oxygen levels corresponding to IOIIH~> 5-7, carbon deposits were avoided, even with the plasma turned on during solvent elution. Thus, doping of the GC carrier gas with traces of oxygen was inevitable in order to enhance both the solvent tolerance and the repeatability (
