(4) Y. Hayashi, S. Matsushita. T. Kumamaru. and Y. Yamomoto, Talanta, 20, 414 (1973). (5) F. D. Pierce, M. J. Gortatowski, H. D. Mecham. and R. S. Fraser, Anal. Chem., 47, 1132 (1975). (6) American Public Health Association, American Water Works Association. Water Pollution Control Federation. "Standard Methods for the Examination of Water and Wastewater," M. J. Taras, A. E. Greenburg, R . D. Hoak, and M. C. Rand, Ed.. American Public Health Association, Washington, D.C., 1971, 13th ed.. Parts 107A and 107C, pp 69 and 73.
(7) F. D. Pierce, H. R . Brown, and R. S. Fraser, Appl. Spectrosc., 29, 489 (1975). (8) C. R. Parker, "Water Analysis by Atomic Absorption Spectroscopy," Varian Techtron Ry. Ltd., Springrale, Australia, 1972, p 35.
for review October ber 22,1975.
239
Accepted Decem-
Direct Injection of Gaseous Samples into a Microwave Induced Discharge F. A. Serravallo and T. H. Risby" Department of Chemistry, The Pennsylvania State University, University Park, Pa. 76802
Vinyl chloride has been determined in air by direct injection into a microwave-induced reduced pressure (4.7 Torr) helium plasma using the CI(II) line at 4794.5 d. Matrix effects due to the quenching properties of oxygen and competing reactions of the nitrogen and oxygen necessitate the use of a number of gas standards. The emission spectrum of vinyl chloride in a helium plasma, together with calibration curves using mixtures of vinyl chloride in helium and vinyl chloride in air are presented. The reduced pressure discharge can accept sample sizes at least as large as 100 pl without affecting stability. A detection limit of 390 ppm of vinyl chloride in air with a selectivity ratio of 300 as compared to methane has been obtained. This poor detection limit was due to the presence of air which quenches the CI( II) emission.
Microwave discharges have been used to fragment solute vapors as they are eluted from gas chromatographic columns and to excite atomic and molecular species (1-21). Deactivation of these species is viewed spectroscopically to give chromatographic peaks with the result t h a t emission from excited heteroatoms leads to a high degree of selectivity. These large selectivities normally observed result in the possibility of injecting gaseous samples directly into the plasma, thereby obviating the need for a gas chromatograph. Taylor et al. (22) have used microwave-induced discharges for the determination of sulfur dioxide by direct injection. Our study was concerned with the effect of air on the signals obtained when gaseous pollutants were injected into a microwave-induced plasma. This work reports on the matrix effects observed and how they limit this application as compared to prior gas chromatographic separation.
EXPERIMENTAL Helium (99.99+% pure) was obtained from the cryogenic laboratory of The Pennsylvania State University. Vinyl chloride (99.9% pure) was obtained from Matheson. Oxygen (99.5% pure) was obtained from Phillip Wolf and Sons (Lewistown, Pa.). Methane (99.95%pure) was obtained from Linde. A p p a r a t u s a n d P r o c e d u r e . The apparatus was the same as that described previously with the exception that the sample was introduced through an injection port (23). Injections were made through a rubber septum located two feet from the sidearm on the discharge tube (quartz, 1.5 mm i.d., 6 mm 0.d.). The injection port was connected to the sidearm by means of Teflon tubing (%6 inch i.d.). The greatest flow of helium (95% of the total) was through Gases.
the top of the discharge tube, with only a minimal flow rate through the injection port to sweep the samples into the discharge region. In this way, the plasma could accept injections at least as large as 100 p1 without overloading. If injections were made into the main stream of helium, the largest sample size which could be admitted without affecting discharge stability was 15 pl. Injections were made with a 100-p1 gas-tight syringe (Hamilton, No. 1710). Gas mixtures used as standards were prepared by adding a known pressure of vinyl chloride or methane to an evatuated glass bulb and then bringing the pressure of the flask up to atmospheric with either helium or ambient air.
RESULTS AND DISCUSSION I t has been shown previously (23)t h a t the emission characteristics of microwave discharges are strongly dependent upon the partial pressure of the plasma gas and of added gases, especially those doping gases whose ionization potential is below the energy of the excited states of the plasma gas. Since this study was concerned with analysis of vinyl chloride in air samples, the effect of adding air to a reduced pressure microwave-induced helium plasma was studied. T h e emission spectrum obtained upon the addition of ambient air to the helium discharge is shown in Figure 1, and the species observed in Table I. The intensity of emission from these species is dependent on both the partial pressure of air PA^^) and on the partial pressure of oxygen (Pop).The variation of emission intensity as a function of added air is shown in Figure 2. T h e decrease in the emission intensity of He(1) (curve 1) has been explained previously (23) on the basis of collisional deactivation a t the wall and on Penning ionization. In this case, Penning ionization of both oxygen and nitrogen can occur (26-29):
+ Nz He* +
He*
0 2
-+
+
+ N2+ + e He + 0 2 + + e He
(1) (2)
Curve 2 in Figure 2 shows the change in emission intensity of the 3755-A bandhead of the Nz second positive system as a function of added air. T h e intensity of emission increases as increases until it reaches a maximum. T h e subsequent decrease in emission intensity may be explained by collisional deactivation with the wall and by excited pair ionization as suggested by Lund and Osham (30): N2*
+ Nz*
+
NP + N2+
+e
Curve 3 in Figure 2 shows the change in emission intensiANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
673
Figure 1. Emission spectrum of air in helium plasma PH. = 0 90 Torr, Pni, = 0 15 Torr Relative intensity vs wavelength (A) \ [
3 4
3
c 7
PTo,,l Figure 2. Relative intensity of emission vs. PAir P H= ~ 0.90 Torr. (1) He(l) 3889 NO 2471 A (X5)
A,
(2) N2+ 3582
A
(X3), (3) N2 3755
Figure 3. Relative intensity of emission as a function of added 0 2
A,
(4)
+e
-
N
+N
(3)
Also, the reaction of either N2+ or N2 with oxygen to form NO can explain not only the decrease in Nz+ and N2 emissions, but also t h e fact t h a t the NO emission intensity reaches a maximum a t a higher pressure and decreases more slowly than either Nz+ or Nz emission (curve 4, Figure 2). Figure 3 shows t h e quenching effects of oxygen when added t o a plasma of constant and PA^^. Comparison of curve 1 (Figure 3) with curve 1 of Figure 2, and with the result obtained when oxygen was added to a helium plasma (23), shows t h a t the decrease in emission intensity is most pronounced where the partial pressure of oxygen is greatest. This is in agreement with the observation t h a t the rate constant for reaction 2 is greater than t h a t for reaction 1 ( 3 1 ) . It is evident from Figure 3 t h a t quenching of all species is rapid upon t h e addition of oxygen. The reduction in nitrogen emission intensities could be exploited in the analysis of air samples, provided the added oxygen did not quench the emissions from the species of interest. T h e emission spectrum of vinyl chloride in a reduced pressure (3.45 Torr) helium plasma is shown in Figure 4, and t h e species observed are given in Table 11. O(1) was observed since a very small (not measurable) amount of oxy674
PHs = 1.05 Torr, PAir= 0.10 Torr. (1) He(l)3889 A, (2) N2+ 3582 A, (X7), (3) Np 3755
ty of the Nz+ first negative system. This decrease in emission intensity can be explained by collisional deactivation. The N2+ emission intensity decreases faster than t h a t of Nz, which suggests additional loss mechanisms. Until now we have discussed only quenching of the excited states, but it should be noted t h a t loss of the species from the ground state can also be used t o explain decrease in emission intensity. For example, the following reaction can be written for N2+: N2+
( t o r r )
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
A,
(4) NO 2471 A (X7)
Table I. Species Observed in the Emission Spectrum of Air in Helium Plasma Atomic ( 2 4 )
Molecular and system ( 2 5 )
HeU) H(I)
Nz-Second positive Nz+-First negative NO-7 "-3360 8, OH-3064 8,
ow
Table 11. Species Observed in the Emission Spectrum of Vinyl Chloride in Helium Plasma Atomic ( 2 4 )
Molecular and s)stem ( 2 5 )
HeU) HU) C(1)
Cz-Swann CH-4300 8, CN-Violet CO-4th positive "-3360 8, OH-3064 %,
om
ClU) CI(I1)
gen was necessary to prevent carbonaceous deposits on the inside walls of the discharge tube. A number of Cl(1) and Cl(I1) lines were observed, but the most suitable lines for analysis, upon consideration of the intensity, the background emission, and the interference from molecular species, are the Cl(I1) lines a t 4819.5, 4810.1, and 4794.5 8,. T h e emission spectrum of vinyl chloride in a helium plasma with added air is essentially the sum of Figures 1 and 4 with no new species observed. Injections of a mixture of vinyl chloride (11 Torr) in helium were made, and t h e line a t 4794.5 8, was monitored for t h e detection of vinyl chloride. Typical instrumental responses are shown in Figure 5 . The response, which is essentially instantaneous upon injection, is a sharp spike lasting approximately 10 s, with
“CCS
Figure 4. Emission spectrum o f vinyl chloride in helium plasma Discharge pressure: 3.45 Torr; very slight bleed of wavelength (A)
O2 (not measurable) to prevent carbon deposition on inside walls of discharge tube. Relative intensity vs.
Figure 5. Direct injections of vinyl chloride-helium mixture into helium discharge Discharge pressure: 4.70 Torr. injections: 5 pI (3), 10 pl (4). 15 pl (4), 20 pI (3). and 25 pl (3). i
0
-,
0.1
0
P
02
(torr)
0, Flgure 7. Quenching effect of oxygen on CI(II) emission Discharge pressure: 4.80 Torr: 50-fii injections of vinyl chloride-helium mixture
I00 p i
;I
2 3-1
0 t - I 0
I
I
I
I
I
size
2 0 PI
I
too
5c sample
I
3 0 1-1
(PI)
Figure 6. Calibration curves for direct injection = 11 Torr). Discharge pressure: 4.70 Torr. Curve 1: vinyl chloride in (PC~H~CI helium. Curve 2: vinyl chloride in air
a
lop1
iI
5 !J
0
1
G
40
P
C?H,CI
only a very slight tailing. As can be seen in Figure 5 , the system is both reproducible and stable. T h e calibration curve shown in Figure 6 (curve 1) was obtained with injections of 10-100 ~1 of the vinyl chloridehelium mixture. I t is evident t h a t the response is linear over this range (14-140 p p t vinyl chloride). However, if the vinyl chloride sample is brought up to atmospheric pressure with ambient air instead of helium, curve 2 of Figure 6 results. This matrix effect can be explained on the basis of the quenching effects of oxygen in microwave discharges and on the fact that, when vinyl chloride is injected with air, a number of competing reactions take place, as discussed earlier. Oxygen quenches both molecular and atomic emissions, and the effects of adding oxygen on the emission intensity of the Cl(I1) line a t 4794.5 A are shown in Figure 7 . These results were obtained by adding oxygen through the top of
E0
(torr)
Figure 8. Direct injections of vinyl chloride-air mixtures fcH3cI refers to partial pressure of vinyl chloride in gas mixtures
the discharge tube and making constant injections (50 ~ 1 ) of the vinyl chloride-helium mixture. Also, it is apparent that increasing the injection volume of the vinyl chloride-air mixture not only increases the amount of vinyl chloride, but simultaneously increases the amount of oxygen. Thus, larger amounts of oxygen counteract the effect of adding more vinyl chloride. Also, the species of oxygen or nitrogen can react with He(1) (reactions 1 and 2) and, of course, with free electrons, thus competing with the vinyl chloride. This problem can be circumvented, however, if a number of different vinyl chloride-ajr mixtures are prepared which contain different amounts of vinyl chloride. Then, by injecting the same volume of each standard, one of the variANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
675
Figure 9. Emission spectrum of sulfur dioxide in helium plasma PH, = 1 30 Torr PSO?= 0 09 Torr Relative intensity vs wavelength (A)
ables would be expected to remain essentially constant. Linear calibration curves were obtained using this technique and are presented in Figure 8. The matrix effect observed for vinyl chloride-air mixtures not only affects the linearity of the calibration curve, but also increases the limit of detection. T h e detection limit using vinyl chloride-helium mixtures was 150 ppm vinyl chloride (S/N= 2), as opposed t o 390 ppm using vinyl chloride-air mixtures. T h e selectivity ratio of the response of this monitor for vinyl chloride as compared t o methane was 300:l. I t should be mentioned t h a t attempts were made to use this system for the analysis of sulfur dioxide in air. An emission spectrum of sulfur dioxide in a helium plasma was obtained (Figure 9), and the only species observed were SO ( 5 ) and S(1) ( 4 ) . T h e emissions for S(1) were observed a t 5297, 4694, and 2169 A (2). However, no peaks were obtained upon injection of sulfur dioxide samples when monitoring any of the S(1) lines. Apparently, the emission from S(1) is quenched more rapidly than the emission from Cl(II), and is totally quenched by background air.
CONCLUSIONS This study has shown t h a t the presence of air severely limits the application of direct injection techniques for the determination of gaseous air pollutants, since oxygen and nitrogen quench the emission from atomic species. T h e only use of such a system would be to monitor levels which are of the order of 100 ppm. T h e recently adopted OSHA temporary standard is 50 ppm with the ability to assay 5 ppm with a relative precision of f 2 0 % and, therefore, the technique described would not meet this criteria. Microwave-induced discharges could and have been used successfully for monitoring air pollutants, providing t h a t the oxygen and nitrogen were first separated from the pollut a n t by gas chromatography.
676
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
LITERATURE CITED (1)A. J. McCormack. S. C. Tong, and W. D. Cooke, Anal. Chem., 37, 1470 (1965). (2)C. A. Bache and D. J. Lisk, Anal. Chem., 37, 1477 (1965). (3)C. A. Bache and D. J. Lisk, Anal. Chem., 38, 783 (1966). (4)C.A. Bache and D. J. Lisk. Anal. Chem., 38, 1757 (1966). (5)C. A. Bache and D. J. Lisk, Anal. Chem., 39, 786 (1967). (6)H. A. Moye, Anal. Chem.. 39, 1441 (1967). (7)E. M. Bellet, W. E. Westlake, and F. A. Gunther, Bull. Environ. Contam. Toxicol., 2(5),255 (1967). (8) C. A. Bache and D. J. Lisk, J. Gas Chromatogr., 6,301 (1968). (9)R. M. Dagnall, S. J. Pratt, T. S. West, and D. R. Deans, Talanta, 16, 797 (1969). (IO)R. M. Dagnall, S. J. Pratt, T. S. West, and D. R. Deans, Talanta, 17, 1009 (1970). 1 1 ) D. A. Luippold and J. L. Beauchamp, Anal. Chem., 42, 1374 (1970). 12) C. A. Bache and D. J. Lisk, Anal. Chem., 43, 950 (1971). 13) W. Braun. N. C. Peterson, A. M. Bass, and M. J. Kurylo, J. Chromatogr., 55, 237 (1971). 14) R. M. Dagnall, T. S. West, and P. Whitehead, Anal. Chim. Acta, 60, 25 (1972). 15) R. M. Dagnall, T. S. West, and P. Whitehead, Anal. Chem., 44, 2074 (1972). 16) W. R. McLean, D. L. Stanton, and G. E. Penketh, Analyst (London),98, 432 (1973). (17)H. Kawaguchi, T. Sakamoto. and A . Mizuike. Talanta, 20, 321 (1973). (18)R. M. Dagnali, T. S. West, and P. Whitehead, Analyst(London), 98, 647 (1973). (19)Y. Talmi and A. W. Andren, Anal. Chem., 46, 2122 (1974). (20)Y. Talmi and V. E. Norveil. Anal. Chem., 47, 1510 (1975). (21)F. A. Serravallo and T. H. Risby, J. Chromatogr. Sci., 12,585 (1974). (22)H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, Anal. Chem., 42, 1569 (1970). (23)F. A. Serravalio and T. H. Risby, Anal. Chem., 47, 2141 (1975). (24)A. R. Striganov and N. S. Sventitskii, "Tables of Spectral Lines of Neutral and Ionized Atoms". Plenum, New York, 1968. (25)R. W. B. Pearse and A. G. Gaydon, "The Identification of Molecular Spectra", 3rd ed., Chapman and Hall, Ltd., London, 1965. (26)F. M. Penning, Naturwissenschaften, 15, 818 (1927). (27)F . M. Penning, Z.Physik, 46, 335 (1928). (26)F. M. Penning, Z.Physlk, 57, 723 (1929). (29)F. M. Penning, 2.Physik, 72, 338 (1931). (30)R. E. Lund and H. J. Osham, 2. Physlk, 219, 131 (1969). (31)F. C. Fehsenfeld, A. 2. Schmeltekopf, and E. E. Ferguson, Planet. Space Sci., 13, 219 (1965).
RECEIVEDfor review September 10, 1975. Accepted December 17,1975. Acknowledgment is made to the Donors of T h e Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
