2112
(19)
(20) 1211 (22) (23) (24)
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 Symposium, National Bureau of Standards, Gaithersburg, Md., April 10, 1978. R. C. Zepp, G. F. Baughman, N. L. Wolfe. and D. M. Cline, Environ. Left 6, 117 (1974). P. D. LaFleur and W. P. Reed, Report of Investigation- Research Material 50-Albacore Tuna. Y. Dokiva. M. Taouchi. s. Toda. and K. Fuwa. Anal. Chem.. 50. 533 (1978) T C Rains, Report of Analysis G Westoo, Acta Chem Scand , 20 213 (1966) G Westoo, Scence, 181, 567 (1973)
(25) C. A. Bache, W. H. Gutenmann, and D. J. Lisk, Science, 172, 951 (1971).
.I
RECEIVED for review July 28, 1978. Accepted September 19, Protection 1978. The authors thank the ~ ~ ~ , i ~ Agency for Partial support of this work through t h e Interagency Energy/Environment Program (EPA-IGA-D5-E684). The specification of commercial products does not imply endorsement by the National Bureau of Standards.
Atmospheric Pressure Helium Microwave Detection System for Gas Chromatography Bruce D. Quimby, Peter C. Uden," and Ramon M. Barnes Department of Chemistry, GRC Tower I, University of Massachusetts, Amherst, Massachusetts 0 1003
A microwave emission detection system for gas chromatography has been devised which utilizes the TMoraresonant cavity to sustain a plasma in helium at atmospheric pressure. The effluent from the gas chromatograph is split between a flame ionization detector and a heated transfer line directing it to a small auxiliary oven containing a high temperature valve. The valve allows the effluent to be directed either to a vent or to the plasma. Atomic emission from the constituent elements of species entering the discharge is observed axially with an echelle grating spectrometer. The system allows for highly selective and sensitive detection of elements of interest by monitoring an appropriate wavelength corresponding to that element. Performance of the system is described in terms of linearity, selectivity ratios, and detection limits for the halogens, silicon, phosphorus, sulfur, lead, mercury, and manganese.
Element selective detectors have been an important means of simplifying difficult separation problems in gas chromatography for many years. When the compound or compounds of interest contain a particular element not present in the other components of t h e sample matrix, the utilization of a detector which responds solely to that element generally leads to a significant reduction in the chromatographic requirements of the analysis, since only those compounds containing the element of interest need be separated from each other, and not necessarily from other components in the sample. The desirable characteristics of an element specific GC detector are that it should be capable of monitoring any of a large number of elements with a high degree of selectivity and sensitivity, simple to operate and maintain, and compatible with the wide range of gas chromatographic techniques currently being employed. A microwave emission detector (MED), first described by McCormack, Tong, and Cooke ( I ) , appears to be well suited for these purposes. With this detector, compounds eluting from the gas chromatograph are directed into a microwave discharge which is sustained in either argon or helium. Observation of the optical emission spectrum resulting from the fragmentation and excitation of compounds entering the plasma affords sensitive, element selective detection. The MED has been demonstrated to be useful for the selective 0003-2700/78/0350-2112$01.00/0
detection of several metallic elements including Hg ( 2 , 3 ) ,Cr ( 4 , 9), A1 (8, 9), Cu, Ga, Fe, Sc, V (81,and Be (9) in addition to many nonmetallic elements such as Se ( 5 ) ,As (6, 7 ) ,Sb (a, Si (IO),P , S, Br, C1, I(11, 12), C, H, D, N, and 0 (12). With the types of resonant cavities employed in the above studies, discharges can be sustained in argon at either atmospheric or reduced pressure (usually 5-50 Torr) and in helium a t reduced pressure only. Line emission is observed for all elements in the helium plasma, while in the argon plasma F, C1, Br, N, and 0 exhibit only diatomic molecular spectra. Since the measurement of band spectra has presented problems when applied for selective detection ( I , 13), the helium plasma, although somewhat less convenient to operate, is preferred because of its wider range of applicability. T o overcome the necessity of operating the helium plasma at reduced pressure, Beenakker (14-16) introduced the TMol0 cylindrical resonant cavity. Owing to its increased efficiency for transfer of microwave power to the discharge, an atmospheric pressure helium (or argon) plasma can be sustained a t the same low power levels as used with previous cavities. A further advantage offered by this design is the ability to view light emitted from the plasma axially. With earlier cavities in which the helium plasma is operated a t reduced pressure, the plasma is usually viewed transversely through the walls of the quartz discharge tube. Deposition of materials on the discharge tube walls and devitrification of the quartz when using the helium plasma result in gradual attenuation of sample response with time. The addition of small amounts of oxygen or nitrogen to the helium to act as a scavenger gas (12)reduces carbonaceous deposits, but deposition of metals and devitrification still present limitations. Since the plasma can be viewed directly when the TMolocavity is employed, situations of this type are avoided. Using an exponential dilution flask, Beenakker ( 2 5 ) assessed the analytical performance of the atmospheric pressure helium plasma for the selective detection of C, H, C1, Br, I, and S, and found the detection limits to be considerably lower than those attained with earlier cavity designs. The construction of a GC-MED system incorporates three fundamental units, the source, the monochromator-readout system, and the interface to the gas chromatograph. T h e atmospheric pressure helium plasma was chosen for reasons discussed above. To accommodate the wide range of elements to be monitored, the monochromator operates over a waveC 1978 American Chemical Society
~
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
length range of 190-800 nm. Generally, selectivity can be increased by using a dynamic spectral background correction technique (17, 18), or by increasing the spectral resolution of t h e monochromator ( I ) . For this work a n echelle grating spectrometer was employed because of its high spectral resolution and because the entrance aperture is physically well matched to the size of the source when viewed axially. With respect to interfacing the gas chromatograph to the source, a major consideration is t h e limitation on the amount of organic material which can be introduced into the plasma per unit time. In most cases the injected solvent will extinguish the plasma and/or result in carbon deposits on the discharge tube walls. In previous studies, this was usually avoided by allowing the solvent peak to pass through the discharge tube before initiating the plasma. With some elements, however, t h e sample response has been found to be a function of the time elapsed between initiation of the plasma and elution of the analyte (10). To avoid these conditions, the interface in t h e system described here incorporates a high temperature valve which permits venting of the solvent or other large peaks away from the plasma, thus eliminating t h e need for repeatedly initiating the plasma after each injection.
Figure 1.
2113
Block diagram of GC-MED system
EXPERIMENTAL Spectrometer. A prototype version of the Spectraspan I11 echelle grating monochromator (Spectrametrics, Inc., Andover, Mass.) with a reciprocal linear dispersion of 0.122 nm/mm at 400 nm was used. The entrance slit width was 50 pm; slit height. 500 wm; exit slit width, 100 pm; and exit slit height, 200 pm throughout the experiment. The image of the plasma (viewed axially) was focused onto the entrance slit with the 10-cm f.l. quartz lens supplied with the monochromator. The photomultiplier tube (Hamamatsu R446) was maintained at 700 V with a high voltage power supply (Northeast Scientific RE1603); the photocurrent was monitored with a picoammeter (Keithley 410). A variable low pass filter was inserted between the picoammeter and the dual pen stripchart recorder (Omniscribe, Houston Instruments) and a 1-s time constant was used throughout. Gas Chromatograph. A Varian 2440 gas chromatograph equipped with a flame ionization detector (FID) was employed. Stainless steel chromatographic columns were used. These were a 3 ft X in. o.d. column of 5% OV-17 on 100,'120 mesh Chromosorb 750 (adapted for on-column injection) which was used for the mercury evaluation, a 3 ft X in. o.d. column of 3% QF-1 on 100/120 mesh Varaport 30 (adapted for on-column injection) for the phosphorus evaluation, a 6 ft X in. o.d. column of 6% Carbowax 20M on 100/120 mesh Chromosorb P for the silicon determination, and a 6 ft X 1/8 in. o.d. column of 2.5% Dexsil 300 on 100/120 mesh Chromosorb 750 for the remainder of the elements investigated. Helium carrier gas flow rates were 70 mL/min for diphenylmercury and 50 mL/min for all other investigations. The injector, detector, transfer line, and interface oven were maintained at 170 "C for the manganese and lead investigations, and a t 200 "C for the remainder of the work. The column temperatures used are indicated on the chromatograms. Microwave Equipment. The microwave generator (Scintillonics HV15A) employed was capable of producing 0-100 watts of microwave power at 2.45 GHz. .4n adjustable line with a perpendicularly positioned trombine line (General Radio) was inserted in the 50-R coaxial line (Hewlett-Packard) to allow generation of an argon or helium atmospheric pressure plasma without changing the discharge tube position within the cavity (16). All experiments described here were carried out with the atmospheric pressure helium plasma. The microwave power level was maintained at 75-80 W throughout for reasons discussed by Beenakker (15). The TMoiocavity (RKB Products, Inc., Lexington, Mass.) was constructed from copper after Beenakker's description (14) except that the inner diameter was 92.7 mm and the UG-58 connector was mounted on the cavity without further modification (14). Although the reflected power could be tuned to a minimum using the copper tuning screws in the cavity, its value when operating the helium plasma remained high (ca. 35%), thus necessitating the use of the adjustable lines. With this type of tuning, the reflected power could be reduced to less than 1%
-5" *
I
Construction of heated transfer line connecting splitter and interface oven Figure 2.
of the forward power, and the plasma remained stable down to 20 K forward power, below which level it was extinguished. Interface. A block diagram of the entire GC-MED system is presented in Figure 1. The system was designed to allow detection with both the FID and the MED simultaneously. The effluent from the column was split using a low dead volume I / 16-in. tee located inside the column oven. From the splitter, one third of the effluent was directed to the FID, and the balance to the MED. The split ratio was adjusted by crimping the transfer line to either the FID or the MED, as described by McLean et al. (12). All transfer lines were 1/16 in. o.d. X 0.01 in. i.d. stainless steel. The portion of the column effluent directed to the MED passed through a heated transfer line which extended from inside the column oven to the interface oven. This transfer line consisted of 1/16 in. o.d. X 0.01 in. i.d. stainless steel tubing inside 'is in. o.d. X 0.085 in. i.d. aluminum tubing. This was wrapped with a double layer of glass electrical tape (3M), then with glass insulated nichrome wire (The Berquist Co., Minneapolis, Minn.) with the coils spaced about in. apart, wrapped with another double layer of glass tape, and this entire assembly covered with 1'/2 in. o.d. X i / 4 in. i.d. blown silicone thermal insulation tubing (Ja-Bar Silicone Corp., Andover, N.J.). The temperature of the transfer line was adjusted with a Variac attached to the nichrome wire and was measured by a type K thermocouple inserted between the last layer of glass tape and the silicone insulation. The '/8-in. aluminum sleeve was used to ensure even distribution of heat throughout the 1/16-in.sample line to eliminate hot or cold spots (19). The construction of the transfer line is indicated in Figure 2. The interface oven is illustrated in Figure 3. This design was chosen for two reasons: first, the high temperature valve (CV4-HTa, Valco Instruments, Houston Texas) must be operated in the range of 156300 "C and the use of an auxiliary oven allowed the column temperature to be independent of the interface temperature; secondly, the oven was constructed so as to afford easy access to the discharge tube assembly for replacement and adjustment of the discharge tube. The interface oven consisted of an aluminum box 13 in. X 9 in. X 5 in., the inside walls of which were insulated with Fiberglas. Heat was supplied by two 500-W duct type air heater elements (SEF-120, Chromalux) each powered by a Variac. The heater elements and the aluminum mounting bracket of the valve were affixed to the oven wall by ceramic electrical insulators which were tapped to accept screws on both ends. The construction reduced conduction of heat to the outside wall of the oven and thus maintained the valve at the same
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
Figure 4. Response to chlorine at CI(I1) 479.5 nm as a function of total helium flow rate Figure 3. Interface oven: (1) inlet from transfer line, (2) high temperature valve, (3) 1 / 4 in. X 1/,6 in. S.S. union, (4) discharge tube, (5) compression fitting, (6) cavity, (7) optical mount of spectrometer, (8)
"plasma He" inlet, (9) O2 inlet, (10) '/18-in. tee, (11) aluminum valve bracket, (12) "carrier replacement He" inlet, (13) vent, (14) heater elements, (15) connections to Variacs, (16, 17) ceramic electrical insulators, (18) thermocouple, (19) Fiberglas insulation, (20) asbestos card, (21) cylindrical fan temperature as the rest of the oven. To provide for even distribution of heat throughout the oven, a small cylindrical fan was located between the heater elements directly under the valve. After passing through the valve, the sample was directed to the plasma tube through another short length of transfer line. The transfer line was connected to the 6 mm o.d. X 1 mm i.d. quartz discharge tube (Quartz Scientific) with in. X in. stainless steel reducing union. .41/16-in.hole was drilled into the side of this fitting into which a line from a brass 1/16-in,tee was soldered. Lines carrying oxygen and helium were attached to the tee. Both lines had coils containing about 10 in. of tubing inside the oven to preheat the gases before they contacted the sample to preclude its condensation. The discharge tube was held in the reducing union with a graphite ferrule. The discharge tube was affixed to the cavity by a compression fitting containing a high temperature O-ring, which was located on the cavity. The cavity was attached to the optical mount of the spectrometer which contained provisions for adjusting the position of the cavity (and discharge tube) with respect to the lens and entrance slit. Since the cavity was not attached to the oven but to the spectrometer, any small movement of the oven from turning the valve or other manipulations did not disturb the position of the plasma with respect to the optical system. Materials. Methylcyclopentadienylmanganesetricarbonyl (MMT) was purchased from Strem Chemicals, Danvers, Mass.) (20), tetraethyllead and tetravinylsilane from Ventron Corp. (Alpha Division, Danvers, Mass.), 2,5-dimethylthiophene from Pfaltz and Bauer, Inc., (Stamford, Conn.), 2-bromofluorobenzene from Aldrich Chemical Co., (Milwaukee, Wis.) and the remaining chemicals from Eastman Kodak Co. (Rochester, N.Y.). Commercial grade helium was used throughout and was passed through a molecular sieve 3A trap before entering the GC-MED system. Operating Procedure. The total helium flow rate through the discharge tube (carrier replacement helium plus plasma helium) was adjusted to the desired level in the range of ca. 45-450 mL/min. The microwave input power was set to 75-80 UTforward power, and the system tuned to minimum reflected power. The plasma was initiated by momentarily inserting the tip of a small diameter piece of wire, which was attached t o ground, a few millimeters into the end of the discharge tube. After an initial warm-up time of about 30 min, the system was again tuned to minimum reflected power. The wavelength setting of the monochromator was optimized for mercury, manganese, and lead using hollow cathode lamps and a small mirror placed between the lens and the cavity. The wavelength setting for silicon was adjusted by the use of the background emission from the discharge tube itself. For all other elements investigated, the wavelength setting was optimized by introducing small amounts of vapor containing the element of interest into the plasma by placing
solvents in a bottle connected via a tee (see Figure 1)to the carrier replacement helium line. After wavelength adjustment, the carrier replacement helium flow rate was set to a value approximately equal to that of the carrier gas flow rate. When a sample was injected into the gas chromatograph, the valve was turned to the "valve closed" (v.c.) position, venting the portion of the effluent split to the MED to the atmosphere. After the bulk of the solvent had passed through (as indicated by the FID), the valve was turned to the "valve open" (v.0.) position directing the sample components into the plasma. Any peaks other than the solvent which resulted in more than ca. 1 pg/s of organic material entering the plasma had also to be vented, to preclude carbonization of the discharge tube and possible extinguishing of the plasma. If the plasma tube was inadvertantly carbonized but the plasma not completely extinguished, the immediate introduction of ca. 0.5-2 mL/min of oxygen into the plasma helium line (see Figure 1)would usually clean the discharge tube within a few minutes. If the plasma was extinguished and could not be re-ignited, the plasma tube had to be removed and cleaned or replaced. For all investigations presented here. oxygen was used only for the above mentioned purpose and was not doped into the plasma support gas on a continuous basis.
RESULTS AND DISCUSSION Optimization. Effect of Flow Rate. The effect of the total flow rate of helium through the discharge tube on sample response was determined for all elements investigated. This was done by repeatedly injecting a standard solution while varying the "plasma helium" flow (see Figure l), with the carrier gas flow rate and column temperature maintained constant. Beenakker (15) observed that the response to carbon a t 193.1 nm is almost independent of flow rate at flows greater than 50 mL/min when the carbon is present a t a constant concentration in helium. Similar results were obtained by van Dalen e t al. (21) under similar conditions for a low pressure helium microwave plasma; but for a constant rate of introduction of carbon, response decreased with increasing flow rate. The response for all elements investigated in our study was found to be significantly affected by the total flow rate of helium through the discharge tube. Figure 4 illustrates the dependence of chlorine response at 479.5 nm on flow rate. The response remains constant over the range of 42-50 mL/min and then decreases sharply with increasing flow rate; the response decreased by 70% a t 120 mL/min. Flow-response curves for chlorine at 481.0 nm, bromine a t 470.5 nm, and iodine at 206.2 nm are virtually identical to that in Figure 4. Mercury, phosphorus, sulfur, and fluorine all exhibited the same general behavior (Le., their response decreases above ca. 50 mL/min) when monitored a t the wavelengths indicated on the chromatograms in Figure 8. Although the response is relatively constant in the 42-50 mL/min range, the flow rate was maintained a t approximately 50 mL/min for all subsequent experiments with these elements, because the plasma becomes increasingly unstable if flow rates are decreased below this value.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2115
Table I. Detection Limits and Selectivities
element
wavelength, nm
F
this work det. lim.,
685.6 481.0 470.5 206.2 545.4 253.6 251.6 253.7 257.6 283.3
c1 Br I S
P Si Hg Mn Pb
sel.
pg/s 8.5 16
Ref. 1 5 det. lim., pg/s sel.
3500 2400 1400
10
1100
31 63 2.1 29 1.0 0.25 0.49
250 2.6 x 1 0 4 3900 9.1 x 1 0 4 1.9 x 106 1.3 X l o 6
---
---
7 5 3 25
200 220 130 200
-----------
---------
---
Ref. l Z a det. lim., Pg/s sel. 30 30 45 25 45
-----------
3100 1300 7300 3500 87 0
-------
-----
Ref. l l b det. lim., Pg/s sel.
-
---
---
40 13 33 33 6
120 240 380 55 2400
---------
---------
The authors in Ref. 1 2 did not indicate whether or not the detection limits reported were corrected for the split ratio The between the FID and the MED. We assumed that they did not, thus their detection limits have been divided by two. detection limits reported in Ref. 11 were taken at S/N = 3 instead of S / N = 2, and thus have been multiplied by ’/, for comparison purposes. 1^P
--1
/I
. _G-
’CC
’52 CP F L O A
25c
2,: RATE
332
rrI/rrir
Flgure 5. Response to manganese at Mn(I1) 257.6 nm as a function of total helium flow rate
The flow-response curve for manganese at 257.6 nm is given in Figure 5 . In this case, the response increases with increasing flow rate, reaching a maximum a t 220 mL/min, decreasing sharply thereafter. A similarly shaped curve is obtained for lead a t 283.3 nm, except that the maximum is a t 100 mL/min. Because it is a constituent of the discharge tube, silicon exhibits unique behavior with respect to flow rate. Not only does its response continually increase with increasing flow rate up to the value a t which the plasma becomes unstable (ca. 475 mL/min), but the selectivity (see below) increases in a similar fashion. A chromatogram of tetravinyl silane injected in diethylether solution at a flow rate of 60 mL/min is given in Figure 6b. The time a t which the valve was opened to direct that portion of the column effluent split to the MED into the plasma is denoted by “V.O.”. The first three peaks after the solvent are nonsilicon-containing impurities in the ether, and the fourth peak contains 31 ng of Si entering the plasma as tetravinylsilane. Figure 6a corresponds to the injection of a somewhat larger volume of ether solution and only 3.1 ng of silicon entering the plasma, with the flow rate a t 450 mL/min. As can be seen from comparing the FID and MED responses in Figure 6b, the selectivity is only slightly greater than unity, and the sensitivity is rather poor. In Figure 6a, the sensitivity has increased by about one order of magnitude, and the impurities are barely detectable. The background level of silicon emission at 251.6 nm is about 2 orders of magnitude greater a t 60 mL/min than at 450 mL/min, indicating t h a t much higher levels of silicon are liberated from the walls of the discharge tube at the lower flow rate. Spatial Dependence of Response. The image presented by the helium plasma when viewed axially consists of a small
Figure 6. Effect of total flow rate of helium on response to silicon. Column temperature, 60 O C . V.O. = valve open
zone within the plasma which is somewhat brighter in appearance than the rest of the discharge and is located near the wall of the discharge tube, as illustrated in Figure 7 . Optimal results for silicon and manganese were obtained by monitoring the region opposite the bright zone between the center of the discharge and the wall. The response to iodine was greatest a t the edge of the bright zone near the center of the plasma and, for the remainder of elements studied, monitoring the region within the bright zone produced the best results. Detector P e r f o r m a n c e . Sensitiuity. Chromatograms demonstrating the sensitivity of the detector for the elements evaluated are presented in Figure 8. The peak which appears a t this time in many of the chromatograms derives from response to the solvent tail which had not completely eluted when the valve was opened. In some of the chromatograms (Le., Figures 8c, f, g, and h), the base-line level when the valve was closed was higher or lower than the base-line level when the valve was open. This was found to be due to a mismatch in the carrier and carrier replacement helium flow rates, and although the effect could be eliminated by careful adjustment of the carrier replacement flow, mismatch of these flow rates had no observable effect on sample response. T h e quantities of element or compound indicated above each peak in Figure 8 refer to the amount of material entering the plasma (Le., amount injected corrected for the split ratio). The detection limits, defined as the mass flow rate of element
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
0
?
2mrr
Figure 7. Axial view of discharge tube
entering the plasma required to produce a signal-to-noise ratio of two, are listed in Table I, together with the detection limits and selectivities obtained previously with the atmospheric
a)
Cl,I!:48'Onrr
pressure (15) and reduced pressure (11, 12) microwave plasmas. The detection limits presented in Ref. 11 and 12 have been recalculated according to the above definition. The detection limits obtained here compared with those of previous workers, with the exception of iodine, are higher than those from Ref. 15 by a factor of about 2.5, and, with the exception of iodine and sulfur, are typically lower by a similar factor than those obtained with reduced pressure systems in Ref. 11 and 12. Selectiuity and Linearity. The selectivity for each element a t the wavelength of analysis is defined here as the ratio of the peak response per gram-atom of element to the peak response per gram-atom of carbon. n-Dodecane was used as the source of carbon for this work. To assist comparison, the selectivities from Ref. 11 and 12 have been recalculated using this definition.
3) Z',;''
685 E r r ,
2 -3-c.ro' JcrcDerzere - Chiorc - 2 l c d c benzene ~
2 2ngZ I
: 30 "C
s:
d)
I(I)
4 5 "C
SC'C
165-C
H s ! I ; 233,'rn
2C6.2nm
1
~
C hloro 2 - o d o b e n z e r e ~
Figure 8. Element selective gas chromatograms obtained with GC-MED system. Column temperatures are indicated in each chromatogram
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
As can be seen in Table I, selectivities obtained for bromine, iodine, and sulfur in the present study were one fifth to one third of those noted by McLean et al. (12),but higher values were obtained for fluorine and chlorine. Considerably higher selectivities were reported here than were shown by Bache and Lisk ( 1 1 ) and by Beenakker (15). Higher resolution of the monochromators employed here and by McLean et al. almost certainly gave rise to the superior selectivities. The very large selectivity ratios observed for lead, manganese, and mercury result from a combination of two factors; (i) the high sensitivities observed for these elements, and (ii) the favorable wavelength region employed with respect both to optical resolution of the monochromator and the minimal interference by molecular band emission from hydrocarbons. Thus far, the linear dynamic range for chlorine, bromine, iodine, and lead has been investigated for the present system. For the halogens, the linear range is about lo4, showing agreement with results reported previously. The linear range extends from the detection limit up to a concentration at which the deposition of carbon and/or quenching significantly alter the plasma characteristics (ca. 1 pg/s of organic material entering the plasma). In the case of lead, however, the linear range is only lo3,and the upper limit at element sample levels 2-3 orders of magnitude below the upper limits for the halogens. This behavior is due to the deposition of metallic lead or lead compounds onto the walls of the discharge tube when large amounts of lead-containing species enter the plasma. Similar results have been reported for other metal-containing compounds (7,8). After injection of an amount of lead corresponding to ten times the upper limit of linearity, the background level of lead emission a t the analytical wavelength is increased, and the selectivity vs. hydrocarbons decreases significantly. To determine whether the increased response to hydrocarbon under these conditions was caused by increased molecular band emission from the hydrocarbon or was due to emission from lead, the experiment was repeated monitoring wavelengths on both sides of the lead line (283.3 nm). Since the increased response to hydrocarbons was not observed a t these wavelengths, but only a t the lead line, it appears that the hydrocarbon causes release of lead deposited on the discharge tube walls into the plasma. A similar effect with respect to selectivity is observed in the case of silicon as mentioned above, except that the element is present in the discharge tube wall and does not necessarily derive from a sample overload. The analogous experiment of injecting hydrocarbon and monitoring wavelengths in the region of the silicon 251.6-nm line under the conditions in Figure 6b (helium flow rate = 60 mL/min) yielded results similar to those in the previous experiment, Le., the response to hydrocarbons derived from increased emission from silicon. In addition, the injection of a hydrocarbon when the background level of oxygen emission a t 777.2 nm was monitored produced a negative response. This behavior may arise from reduction by carbon of oxides of lead and silicon, which are released from the inner wall of the discharge tube by the action of the plasma, in a reaction analogous to that discussed previously for several metal oxides in a fuel-rich oxygen-acetylene flame (22). This suggestion is consistent with the observation that the injection of oxygen produces a negative response when monitoring the silicon 251.6-nm line. The mechanism also suggests that under conditions for which the amount of silicon and oxygen released into the plasma from the discharge tube wall can be reduced, the response to hydrocarbons a t the silicon wavelength should also be reduced. As illustrated in Figure 6, the background emission from silicon and oxygen is greatly decreased at higher helium flow rates, presumably due to increased cooling of the discharge tube, and the response to hydrocarbon is corre-
2117
spondingly much smaller. Further investigation into the cause of this behavior is warranted owing to its pronounced effect on the analytical performance of the detector with respect to silicon, lead, and possibly other elements. Reproducibility. The short-term reproducibility of the system was assessed when several solutions containing different chlorinated hydrocarbons and a chlorinated internal reference standard were injected five times each while the C1 481.0-nm wavelength was monitored. The relative standard deviation of the peak height ratios ranged from 0.5-2.0%. If care is taken in adjusting the total helium flow rate, the wavelength setting of the monochromator, and the spatial position monitored, the day-to-day response of the MED is reproducible to within 570.This is illustrated in Figure 8c, in which the last two injections of bromobenzene were obtained on the next working day after the first two. The MED system was shut down completely overnight. To maintain the ambient temperature of the laboratory at a relatively constant level during operation of the detector is necessary, as large changes in temperature cause the wavelength setting (and thus detector response) to drift, necessitating readjustment of the wavelength every 2-3 hours.
CONCLUSIONS The system described here provides a versatile element selective detector for gas chromatography which is convenient to operate and maintain.. Limitations encountered with previous microwave plasma emission systems are circumvented through the use of the axially viewed atmospheric pressure helium plasma in conjunction with a high temperature diversion valve in the interface. The enhanced sensitivity of the atmospheric pressure helium plasma and the high optical resolution of the echelle grating monochromator are combined to afford low detection limits and high selectivities for the elements investigated. For some elements such as sulfur, bromine, and iodine, higher selectivities than those obtained here may be desirable, especially in complex mixtures containing high concentrations of compounds not containing the element of interest. In this situation, the use of a system with dynamic spectral background correction may be preferred to the approach taken here.
ACKNOWLEDGMENT The authors thank Richard K. Brehm of RKB Products, Inc., for the use of the microwave cavity, and William G. Elliott for making available the echelle spectrometer and providing helpful suggestions with respect to its use. Helpful discussions with Charly D. Allemand are greatly appreciated.
LITERATURE CITED (1) A. J. McCorrnack, S. C. Tong, and W. D. Cooke, Anal. Cbem., 37, 1470 (1965). (2) Y . Talmi, Anal. Cbim. Acta, 74, 107 (1975). (3) C. A. Bache and D. J. Lisk, Anal. Cbem., 43, 1950 (1971). (4) F. A. Serravallo and T. H. Risby, J . Chromatogr. Sci., 12, 585 (1974). (5) Y. Talrni and A. W. Andren, Anal. Cbem.. 46, 2122 (1974). (6) Y . Talrni and D. T. Bostick, Anal. Cbem., 47, 2145 (1975). (7) Y. Talrni and V. E. Norvell, Anal. Cbem., 47, 1510 (1975). (8) R. M. Dagnall, T. S.West, and P. Whitehead, Analyst(London),98, 647 (1973). (9) H. Kawaguchi, T. Sakamoto. and A . Mizuike, Talanta, 20, 321 (1973). (10) D. T. Bostick and Y. Talmi, J . Chromatogr. Sci., 15, 164 (1977). (11) C. A. Bache and D. J. Lisk, Anal. Cbem., 39, 786 (1967). (12) W. R. McLean, D. L. Stanton, and G. E. Penketh, Analyst. (London), 98, 432 (1973). (13) R. M. Dagnall, T. S. West, and P. Whitehead, Anal. Chem., 44, 2074 (1972). (14) C. I. M. Beenakker, Spectrocbim. Acta, Part 8 , 31, 483 (1976). (15) C. I. M. Beenakker, Spectrocbim. Acta, Part E , 32, 173 (1977). (16) C. I. M. Beenakker and P. W. J. M. Boumans, Spectrocbim. Acfa, Part E , 33, 53 (1978). (17) P. M. Houpt, Anal. Cbim. Acta, 66, 129 (1976). (18) C. Feldman and D. A. Batistoni, Anal. Cbem.. 49, 2215 (1977).
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
(19) R. J. Lloyd, R. M. Barnes, W. G. Elliott, and P. C. Uden, 28th Pittsburgh Conference on Analytical Chemistry and Applied Spect~oscopy,Cievehnd, Ohio, March, 1977, Abstract 161. (20) P. C. Uden, R. M.Barnes, and F. P. DiSanzo. Anal. Chem., 50, 852 (1978). (21) J. P. J. van Dalen, P. A. de Lezenne Couiander, and L. de Galen, Ana/, Chim. Acta, 94, 1 (1977).
(22) T. G. Crowiey, V. A. Fassei, and R. N. Kniseley, Spectrochim. Acta, Part B , 2 3 , 771 (1968).
RECEIVED
for Rview August 7, 1978. Accepted September 27,
1978.
Estimation of Extractable N-Nitroso Compounds at the Parts-per- BiIIion Level G. S. Drescher’ and C. W. Frank* Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Analytical methodology designed for the rapid estimation of the total N-nitroso group content In environmental samples was developed. I n a typical determination, a 100-9 sample was extracted with methylene chloride and back-extracted with distilled-deionized water to remove nitrite. The extract was dried over anhydrous sodium sulfate, concentrated, and was analyzed by acid-catalyzed denitrosation and subsequent detection of the evolved nitric oxide via its chemiluminescence reaction with ozone was performed. Detection limits for all N-nitroso compounds tested were in the nanogram range, while the only serious interferences were due to n-alkylnitrites ( 0-nitroso compounds).
Since many N-nitroso compunds are known t o be potent carcinogens in animals (I,2 ) , there has been a great deal of concern as to the extent of their occurrence in the environment (3-5). It has been estimated ( 6 ) that the “tolerable” exposure levels for the more volatile N-nitrosamines in man are as low as 5-10 pg/kg (ppb). For this reason, minimum detection limits of a t least 10 p p b are now generally accepted as an essential prerequisite for the determination of this class of compounds in environmental samples. T h e existing analytical methods, which provide sensitivity a t t h e ppb level, rely on the separation of the N-nitroso compounds by gas chromatography (7-10) or high-performance liquid chromatography (11-13) after their isolation from the original sample matrix by extraction or distillation. Because the chromatographic detectors are not specific for t h e N-nitroso functional group, these methods necessarily include tedious clean-up procedures to remove interferences from t h e chromatograms, resulting in impractically long analysis times. In addition, confirmation by mass spectrometry (14-16) is required for the unequivocal identification of the chromatographic peaks. The recent reports concerning t h e thermal energy analyzer (17-20) as a group specific detector for N-nitroso compounds has indicated that a reduction in the amount of cleanup required prior to detection has been accomplished; but this system is rather expensive and is not available in most laboratories. Since the existing analytical methodology is not wholly suitable for analyzing large numbers of environmental samples, this paper discusses t h e evaluation of a more practical Present address, Ciba-Geigy Corp., Development Department, McIntosh, Alabama. 0003-2700/78/0350-2118$01 0010
technique designed as a preliminary screening method for the estimation of N-nitroso group content in the extracts from complex environmental systems. This technique is based on the method of Downes et al. for the determination of Nnitrososarcosine (21), which relies on the acid-catalyzed denitrosation of the compound in refluxing 1,2-dichloroethane and subsequent detection of the evolved nitric oxide via its chemiluminescence reaction with ozone. A similar method has also been proposed by Gough and Woollam (22). T h e detection system described here is capable of selectively responding to most N-nitroso compounds a t the ppb level in an original sample of 100 g.
EXPERIMENTAL Apparatus. The N-nitroso compounds were denitrosated in the special apparatus shown in Figure 1, which provided for the simultaneous mixing and degassing of the sample solutions. Nitric oxide concentrations in the effluent from the denitrosation reaction were measured using a Bendix 8101-B Oxides of Nitrogen Analyzer operated in the bypass mode to detect NO only. The gas samples were introduced continuously through ‘/,-inch o.d. Teflon tubing into the ambient sampling part of the instrument using helium carrier gas at a controlled flow rate of 150-170 mL/min. Two additional traps filled with no. 10 mesh charcoal were installed between the exhaust outlet and the vacuum pump of the NO, Analyzer to prevent destruction of the pump seals by solvent vapors. The instrument output was coupled with a IO-mV strip-chart recorder. Reagents a n d Chemicals. Stock solutions of the N-nitroso compounds were made up gravimetrically in methylene chloride (Burdick and Jackson, distilled in glass) to a nominal concentration of 10-J M. Standard solutions for the calibration were obtained by suc-essive dilutions of the appropriate stock solutions. The same procedure was used for the preparation of solutions for those compounds evaluated as potential interferences with the exception of sodium nitrite and sodium nitrate, which were prepared in mithanol. The denitrosation reagent was prepared by diluting 1.7 mL of 48’70aqueous hydrobromic acid (9 M) to a final volume of 25 mL with glacial acetic acid. An additional 2 mL of acetic anhydride was added to the reagent to scavenge the excess water. The N-nitroso derivatives of proline and atrazine (2-chloro4-ethylamino-6-isopropylamino-5-triazine) were synthesized by treatment of the parent amino compounds with nitrosyl tetrafluoroborate in the presence of pyridine (23, 2 4 ) , and were characterized by melting point and mass spectrometry. Since N-nitrosoatrazine is very light-sensitive and will decompose rapidly under ordinary fluorescent lighting in the laboratory (251, all work with this compound was done in the dark using opaque glassware. All other N-nitroso compounds, solvents, and reagents were obtained commercially and used as received without further purification. .S 1978 American Chemical Society
