Anal. Chem. 1981, 53, 1829-1837
1829
Microwave-Excited Atmospheric Pressure Helium Plasma Emission Detection Characteristics in Fused Silica Capillary Gas Chromatography Scott A.
Estes, Peter C. Uden," and Ramon M. Barnes
Department of Chemistty, GRC Tower
I, University of Massachusetts,
Interfacing of fused silica gas chromatographlc caplllary columns to a mlcrowave-exclted atmospheric pressure hellum plasma (MED) is Investigated. The system for glass and fused slllca caplllary operatlonr incorporates an Interface where excess solvent can be vented by a chemically deactivated, fluidic-logic gas swltchlng system. A TMola resonant cavity allows axial viewing of plasma emission. A quartz refractor plate background corrector improves selectivity ratios for elements whose emlsslon occurs In the high-carbon (cyanogen) background reglon. Background emission characterlstlcs of the helium plasma under varlous condltlons are established from 200 to 500 nm. Calibration curves, selectlvlty ratlos, and detection Ilmits are establlshed for the elements vanadium, nloblum, chromium, molybdenum, tungsten, manganese, iron, ruthenium, osmlum, cobalt, nickel, mercury, boron, aluminum, carbon, slllcon, germanium, tin, lead, phosphorus, arsenlc, sulfur, selenlum, fluorlne, chlorlne, bromine, iodine, hydrogen, and deuterium.
Chemically inert fused silica wall coated open tubular capillary columns have made more feasible high-resolution gas chromatography of chemically active and thermally sensitive volatile compounds (1,23. Such highly efficient separation necessitates consideration of the detection mode. Although standard detectors show the number of compounds in a sample, there is often a need to distinguish their elemental composition. This becomes critical when chromatographically interfering substances overlap compounds of elemental significance. Among the element-specificgas chromatographicdetectors, only plasma emission detectors are multielement responsive. These include the inductively coupled argon plasma (GC-ICP) ( 3 , 4 ) ,the direct current argon plasma (GC-DCP) ( 5 , 6 )and the microwave-induced and sustained inert gas plasma (GCMED). The first microwave induced and sustained inert gas plasma emission detection system (GC-MED) was described by McCormack et al. (7)arid utilized both argon and helium low-pressure plasmas. Other investigators have used GC-MED systems based on various low-pressure plasma cavity designs (8, 9) for detection of metallic and nonmetallic elements, including Hg, Cr, Al, Cu, Ga, Fe, Sc, V, Be, Se, As, Sb, Si, P, S, Br, C1, I, C, M, D, El, and 0 (10-12). There are two drawbacks to low-pressure plasma cavities. Since reduced pressure (5-50 torr) must be maintained to sustain the helium plasma, emitted light containing analytical information must be viewed transversely through the walls of the quartz discharge tube. These continually undergo changes from carbonaceous and metallic oxide deposition and devitrification of the quartz tube walls. The continual change of transparency of the optical emission window creates a reproducibility problem. The microwave-induced discharge is also disrupted by solvent quantities greater than 1pg, since operation of the 0003-2700/8 110353-1829$0 1.25/0
Amherst, Massachusetts 101003
plasma at low pressure makes the use of solvent venting devices complicated. Repetitive sample analysis involves continual plasma disruption and reignition (10, 11). The development of the TWlocylindrical resonance cavity of Beenakker (13-15),which transfers microwave power to the helium more efficiently, allows operation of an atmospheric pressure plasma at the low power levels (40-100 W) as in reduced pressure cavities. This permits axial viewing of the plasma, which negates the problems of viewing transversely through the quartz tube. Beenakker (14) showed the analytical superiority of the atmospheric pressure helium plasma discharge. We have (12) interfaced the Beenakker cavity with a gas chromatograph using an auxiliary oven containing a higlhtemperature stainless steel valve to elute compounds to the plasma and vent solvent. This system, incorporating a high-resolution echelle monochromator, was used to determine Pb, Mn, Hg organometallics, organic compounds containing Si, P, S, and the halogens, haloforms in drinking water (169, and chlorination products of humic acids (17). A similar system was used by Caruso et al. for polybrominated biphenyls (18), while a simpler valveless device has been described €or vapor sampling (19). In this paper, we demonstrate the compatibility of fused silica capillary columns with the MED, utilizing an inert nonvalve interface (20),and the good selectivity obtained with wavelength modulation and a low-resolution scanning monochromator (21). The atmospheric helium plasma emission background and analytical characteristics from 200 to 500 nm are investigated under various conditions. Detection limits and selectivities of 29 elements are given and chromatograms for representative elements are shown. EXPERIMENTAL SECTION Instrumentation. The GC-MED system, described in detail elsewhere (20),includes a Varian 1200 gas chromatograph (Varian Instruments, Walnut Creek, CA), a chemically deactivated nonvalve solvent venting interface, a Beenakker type TWlocylindrical resonance cavity (J and D Instrument Co., Lexington, MA), and a low-resolution scanning monochromator having approximately 0.1 nm resolution, equipped with a quartz refractor plate background corrector. The interface allows venting of column effluent containing large quantities of solvent which would disrupt the helium discharge, while passing extremely labile species without loss. Materials. Cyclopentadienylvanadiumtetracarbonyl (CpVtetraethylgermanium (Et4Ge),tetra-n-propyltin(n-Pr4Sn), n-butylboronic acid (n-BBA),and phenylboric acid (PBA) were obtained from Alfa Products, Ventron Corp. (Danvers, MA), bis(cyclopentadieny1)nickel (Cp,Ni or nickelocene)from Arapahoe Chemicals, Inc. (Boulder, GO),2-methylthiophenefrom Pfaltz and Bauer, Inc. (Stamford,CT), and bis(cyclopentadienyl)iroin and triethyllead chloride from the Ethyl Corp. (New York, Nk', and Ferndale, MI). n-Tetradecane, n-undecane, l-bromo-lchloro-2,2,2-trifluoroethane,1,1,2,2-tetrachloroethane,bromobenzene, iodobenzene, and acetone-de were obtained fronn Eastman Kodak (Rochester, NY), methylcyclopentadienylmanganese tricarbonyl (CH3CpMn(C0)3)and diethylseleniunn (E@e) from Strem Chemic&.,Inc. (Danvers,MA), ethylene glyccll 0 1981 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
and trimethyl phosphate ((CH303P(0))from Aldrich Chemical Co., Inc. (Milwaukee,WI), and tetraethyllead (Et4Pb)and aluminum tris(trifluoroacety1acetonate)(Al(TFA),) from Research Organic/Inorganic Chemical Corp. (Sun Valley, CA). Tri-n-butylmethylsilane (n-BuSMeSi)and triethylamine (Et,As) were prepared via Grignard reactions from tetrachlorosilaneand arsenic oxide, respectively. Bis(cyclopentadieny1)ruthenium(Cp2Ruor ruthenocene),bis(cyclopentadieny1)osmium (Cp20sor osmocene), cyclopentadienylniobium tetracarbonyl (CpNb(CO)(, cyclopentadienylchromium, -molybdenum, or -tungsten nitrosyl dicarbonyl CpCr(NO)(C0)2,CpMo(NO)(CO),,or CPW(NO)(CO)~, and pentamethylcyclopentadienylcobalt dicarbonyl ((CH&CpCo(CO)z)were prepared from the appropriate metal carbonyls. Methylmercury bromide was prepared from methylmercury sulfide. The commercialgrade helium, hydrogen, and air used were passed through combination silica gel-molecular sieve 3A traps to remove most of the water and low-boiling hydrocarbons. Procedure. The total helium flow rate through the discharge tube (column effluent helium and plasma interface helium) was adjusted between 40 and 800 mL/min as each element required (Table 11). After input and reflected microwave power had been adjusted, the discharge was initiated by either (i) momentary insertion into the end of the discharge tube of the tip of a short piece of small diameter wire (ca. 1.5 cm) held in an insulator such as a piece of quartz capillary tubing or (ii) the discharge from a Tesla coil. The former procedure reduced microwave leakage from the cavity to a very low level (ca. 0.02 mW/cm2) (22). Grounded or long ignition wires can cause large microwave leakage (ca. 10-20% of input power (16)),since the wire becomes a microwave emission antenna when the cavity is shorted for ignition. After the microwave power train had reached a stable operating temperature (ca. 15-20 min), slight retuning to minimum reflected microwave power was sometimes required. Monochromator wavelength settings for V(II), Cr(II), Mo(II), W(II), Mn(II), Fe(II), CoQh Ni(II), Hg(I),BO), N I ) , SnQ),Pb(I), As(I), and Se(1) were made by using hollow cathode lamps. Wavelength settings for Si(I), C(I), and H were made by using background lines in the plasma resulting from silicon from the quartz tube walls and volatile low-level hydrocarbons in the helium carrier/plasma gas. Wavelength settings for F(I), Cl(II), Br(II), I(I), Ge(I), S(II), and P(1) were optimized by doping into the plasma, small amounts of head space vapor from a small stainless steel bottle containing volatile compounds of the element of interest, as described by Quimby (12,20). Nb(II), Ru(II), Os(II), and D(1) wavelength settings were optimized by making replicate sample injections of the appropriate mixture. Since a microbore fused silica column was used, injection volumes were kept below 0.10 pL and were split 100 to 1 at the injection port; thus a maximum of 1nL or 1pg of sample entered the column. Solvent venting was unnecessary and the end of the column could be placed within 1-5 mm of the plasma, maintaining high column efficiency (23). General GC-MED operational parameters are given in Table I. GC oven temperatures for each compound analyzed are given in Table I1 and also on the chromatograms.
RESULTS AND DISCUSSION Helium Plasma Background. The atmospheric pressure helium plasma background was determined for the wavelength region between 200 and 500 nm (Figure la) (24,25). For pure helium, only silicon from the quartz discharge tube and helium molecular or atomic emission lines would be expected; however, it contained very low levels of water, hydrocarbons, Nz, and O2not removed by the traps. Scrubbing the helium with a molecular sieve 5A trap immersed in liquid nitrogen almost completely removed the NH and OH molecular emissions; almost no effect on the N2 or N2+emissions was seen. The latter background emission might be eliminated by some other scrubbing method if moist air entrainment into the plasma via back-diffusion does not play a large role in the introduction of these impurities. The removal of OH and NH by liquid nitrogen trapping strongly suggests that moist air entrainment does not occur.
Table I. General Operating Parameters for the GC-MED System Chromatographic column packing material dimenSP-2100 wall-coated open sions tubular fused silica, Carbowax pretreated 0.3 mm 0.d. x 0.2 mm i.d. x 12.5 m (Hewlett-Packard Corp., Avondale, PA) carrier gas (helium) flow 1.0 mL/min rate injection volume 0.10 pL split 100 to 1 injector temp 190-235 "C transfer block temp 190-225 "C interface oven temp 190-225 "C compds and column temp see Table I1 Spectroscopic microwave input power 45-75 W forward, see Table I1 entrance and exit slit width 25 or 50 pL, see Table I1 height 12 mm obsd emission wavelengths see Table I1 total He plasma flow rate 40-800 mL/min, see Table I1 axial viewing positions see Figure 2a H doping flow rate 0.5-1.0 mL/min photomultipliers potential wavelength range used 200-550 nm RCA 1P28 700V RCA 4836 1100 V 550-750 nm Instrumentation Micromicroammeter Model 414 (Keithley Instruments, Cleveland, OH) 5 V full scale meter deflecoutput tion range 1 X lO-''-l X 10.' A low pass filter time con0.10 s stant strip chart recorder Omniscribe (Houston Instruments, Houston, TX) 1 V or with RPBC 0.01 V full scale input chart speed for chromatograms 1 cm/ min, for calibration curves 10 cm/min refractor plate motor drive amplitude full drive potential used frequency 148 Hz refractor plate 26mmx 1 3 m m x 3mm quartz lock-in amplifier Model 120 (Princeton Applied Research, Princeton, sensitivity reference mode phase time constant output voltage
NJ)
0.1-50 mV full scale input voltage selected external, 148 Hz + 220" 0.10 s
10 mV full scale
Hydrogen was doped continuously into the plasma discharge (ca. 0.5-1 mL/min) to determine its effect on the background, since several elements were analyzed in a mixed He-H2 atmosphere. During hydrogen doping, the plasma image focused on the entrance slit changed from its characteristic bright light yellow to the red resulting from H 656.3 nm emission. The effect of hydrogen is shown in Figure Ib. Increase in the NH molecular band emission with an accompanying decrease in the N2 and N2+band emission further verifies the presence of a significant amount of nitrogen in the discharge. If high plasma helium flow rates were used in
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
N,H tjH (OD) ( 1 1 )
18131
cHe(i)
Helium Plasma Background
1 d
i
Molecular Band
Band System
Transition
a ) N,?
Second Positive
c3a- a 3 ~ r
d ) CN e) N i
Violet Main
3 i + ~ 31: .Ground state *I---) 21T.ground state 2 ~ - + 2 ~ .ground state 2p+21: .ground state
NANOMETERS
Hydrogen Doped Helium Plasma Background
Chlorobenzene Doped H e l i u m Plasma Background /Sl(l)
I
\
200'0--;0&0
I
$0
7'0
dO-goo2
O ;
4b 3
& $0~
7'0
d0
d O . . m X
i0
4b 4&---r-r--y 60 70 80
9O50o
NANOMETERS
Flgure 1. Plasma background spectrograms: (a) helium plasma, (b) hydrogen doped helium plasma, (c) chlorobenzene-doped helium plasmal; input microwave power = 54 W, wavelength scan rate = 2 A/s, time constant = 0.1 s, micromicroammeter gain = 3 X A, and recorder chart speed
= 2.0 cm/min.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
Table 11. Optimal GC-MED Operational Parameters element and column emission compd temp, "C wavelength, nm
(CI-I,CH,),PbCI Et.Pb (C*II,O),P(0 ) Et,As S-CH,C,H,S F,CCHBrCl Cl,HCCHCl, C,H5Br C,H,Br C6H5I
110 110 24 140 160 120 160 180 130 135 170 180 150 135 140 90 140 140 160 85 140 140 120 80
110 45 23 55 80 80 100
23 (CZH, a Optimized parameter with RPBC in operation.
H 486.1 H 656.3 D I 656.1 V I1 268.8 N b I1 288.3 Cr I1 267.7 Mo I1 281.6 W I1 255.5 Mn 11 257.6
Fe I1 259.9 Ru I1 240.3 Os I1 225.6 Co I 240.7 Ni I1 231.6 Hg I 253.7 €3 I 249.8 A1 I 396.2 C 1247.9 Si I 251.6 Ge 1265.1 Sn I 284.0 Pb I 283.3 Pb 1405.8 P 1253.6 As I 228.8 s I1 545.4 F 1685.6 c1 11 479.5 Br I1 470.5 Br I1 478.6 I 1206.2 Se 1204.0
combination with hydrogen doping, the biconvex lens was fogged with a thin film, indicating increased discharge tube erosion over helium flow without hydrogen. Chlorobenzene was doped continuously into the plasma (12, 20), and the resulting background is shown in Figure IC. The plasma color changed from ita characteristic bright yellow to a very deep blue. The addition of carbon compounds to the plasma increased the molecular emission from CN, NZ3and N2+,extending the inapplicable wavelength region toward the red. For this reason, wavelength modulation for the improvement of selectivities of elements such as C1, Br, F, and S over carbon was needed, particularly with the low-resolution monochromator. Optimization. Effect of Flow Rate. The response or sensitivity of various elements in terms of chromatographic peak area per unit of analyte mass has been shown to depend on gas flow rate through the discharge (12, 16). Here, to determine optimum flow rate, repetitive injections of a solution containing a compound of the element were made at various flow rates. Two types of elemental responses were found as a function of total flow rates. One showed maximum elemental response a t a determined total flow rate, which decreased very rapidly with an increase in total flow rate and decreased very gradually with a decrease until the discharge became unstable (ca.
