ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1 9 7 9
Table 11. Catechol and Hydroquinone Found in CSC from 1R1 Cigarettes
a
run 1 132 run 2 140 Guerin methoda 130 Value determined in our laboratory.
(12) Mold, J. D.: Peyton, M. P.; Means, R. E., Walker, T. B. Ana/yst(London), 1966. 189-194 .., 91. . .. . .. (13) Guerin, M: R.; Olerich, G. Tob. Sci. 1976, 20, 19-21. (14) Ishiguro, S.;Sato, S.; Sugawara, S.;Kaburaki, U. A @ Biol. Chem. 1976. 40. 977-982. (15) Sakuma, H.; Kusama, M.; Sato, S.; Sugawara, S. Agric. Biol. Cbem. 1976, 4 0 , 2013-2020. (16) Brunnemann, K. D.; Lee, H.; Hoffmann, D. Anal. Left. 1976, 9 , 939-955. (17) Snook, M. E.;Scholtzhauer, W. S.; Chamberlain, W. J. Tob. Sci. 1978, 22, 106-108. (18) Schlotzhauer, W. S.; Walters, D. B.; Snook, M. E.; Higman, H. C. J. Aric. Food Cbem. 1978, 2 6 , 1277-1281. (19) Ishiguro, S.;Sugawara, S. Beitr. Tabakforscb. 1978, 9 , 218-221. (20) Dietz. F.; Traud, J.; Koppe, P.; Rubelt, C. Cbromatographia 1976, 9. 380-396. (21) Sattar, M. A.; Paasivirta, J.; Vesterinen, R.; Knuutinen, \ I . J . Cbromatogr. 1977, 735, 395-400. (22) Walters, D. B. J . Anal. Toxicoi. 1977, 1 , 218-220. (23) Lee, K. S.; Lee, D. W. Chung, Y . Anal. Cbem. 1973, 45, 396-399. (24) Beg. M. M.; Vsmani, Q. S.;Shukla, I.C. Analyst(London) 1977, 702, 306-307. (25) Sanke, G. H.; Akheel, A. S. Indian J. Cbem. Sect. A 1977, 75, 907-911. (26) Rao, N. V.: Prasad, K. M. M. Curr. Sci. 1978, 47, 50-51. (27) Barek, J.; Berka, A.; Koreckova, J. Microcbem. J . 1977, 2 2 , 484-488. (28) Weissmann, G. J . Cbromatogr. 1976, 129, 431-435. (29) Drawert, F.; Leupold, G. Cbromatographia 1976, 9 ,605-610. (30) Roe, F. J. C.; Sahman, M. H.; Cohen, J. Br. J . Cancer1959, 73, 623-633. (31) Wynder, E. L.; Hoffmann, D. Cancer 1961, 74, 1306-1315. (32) Bock, F. G., Swain, A. P., Stedman. R. L. J . Natl. Cancer Inst. 1971, 47, 429-436. (33) Bock, F. G. J . Natl. Cancer Inst. 1972, 4 8 , 1849-1853. (34) Walters, D. B.; Chamberlain, W. J.; Akin, F. J.; Snook, M. E.; Chortyk, 0. T. Anal. Cbim. Acta 1978, 99, 143-150. (35) Hecht, S. S.; Thorne, R. L.; Maronpot, R. R ; Hoffmann, C). J , Mat/. Cancer Inst. 1975, 55, 1329-1336. (36) Klimisch, H. J. Fresenius' 2 . Anal. Cbem. 1973, 264, 275-278. (37) Pillsbury, H. C.; Bright, C. C.; O'Corron, K. J.; Irish, F. W. J . Assoc. OM. Anal. Cbem. 1969, 52, 458. (38) Rothwell, K., Ed. "Standard Methods fw the Analysis of Tobacco Smoke," Research Paper 11; Tobacco Research Council, London, 1972. (39) Higman, E. 5.;Severson, R. F.; Arrendale, R. F.; Chortyk, 0. T. J . Agric. Food Chem. 1977, 25, 1201. ~
p gicigaret te
catechol
1819
hydroquinone 170 158 148
day. Therefore, losses of catechol should be negligible. Table I1 gives some results of our quantitation studies. The numbers listed for the Guerin method (13) are our results. These values agree favorably with those reported by other workers (12, 13, 16). In summary, we have developed a one-step gel chromatographic method that isolates catechols and hydroquinones from CSC in relatively pure form. The method reduces to a minimum the number of steps needed to isolate these labile compounds. The isolated compounds are readily quantitated by GC. T h e method should have wide application to the isolation, identification, and quantitation of these important compounds whose environmental sources are numerous.
LITERATURE CITED Van Duuren, B. L.; Katz, C.; Goldschmidt, B. M. J . Natl. Cancer Inst. 1973, 57,703-705. Van Duuren, B. L.; Goldschmidt, B. M. J . Nati. Cancer Insf. 1976, 56, 1237-1242. Cooper, R. L.; Wheatstone, K. C. Water Res. 1973, 7 , 1375-1384. Jacquemain, R.; Remy, F.; Guinchard, C. J. Fr. Hydro/. 1975, 16, 25-32. Jahangir, L. M.; Samuelson, 0 . Anal. Cbim. Acta 1976, 85, 103-115. Tattar, T. A.; Rich, A. E. Pbflopathology 1973, 63, 167-169. Rahn, W.; Koenig. W. A. J . High Resolut. Cbromtogr. 1976, 7 , 69-71. Lepri. L.; Desideri, P. G.; Coas, V. Ann. Chim. (Rome) 1976, 66, 451-457. Jones, L. A.; Foote, R. S. J . Agric. FocdCbem. 1975, 23, 1129-1131. Maskarinec, M. P.; Alexander, G.; Novotny, M. J . Cbromatogr. 1976, 726, 559-588. W a k , P.; Hausermnn, M.; Krull, A. 5ei& Tabakfwscb. 1965, 3 , 263-277.
RECEIVED for review April 20, 1979. Accepted June 15, 1979. Presented in part at the 32nd Tabacco Chemists' Research Conference, Montreal, Quebec, Canada, October 1978. Reference to a company or product name does not imply approval or recommendation by the USDA.
Enhancement of Electron Capture Detector Sensitivity to Nonelectron Attaching Compounds by Addition of Nitrous Oxide to the Carrier Gas M. P. Phillips and R. E. Sievers D e p a r t m e n t of Chemistry, University of Colorado, Boulder, Colorado 80309
P. D. Goldan, W. C. Kuster, and F. C. Fehsenfeld" A e r o n o m y Laboratory, N O A A Environmental R e s e a r c h Laboratories, U.S. D e p a r t m e n t of Commerce, Boulder, Colorado 80303
The sensltivity of a variety of compounds in an electron capture (EC) detector is enhanced by the addition of N,O to the carrier gas. The mechanism for this enhancement derives from the low pressure steady-state negative ion chemistry of N20 in a N, carrier gas,
-
+ N 2 0 0- + N2 0- + N20 NO-+ NO e
(1) (2)
0003-2700/79/0351-1819$01.OO/O
NO-
+ N2-
NO
+ ?J2 + e
(3)
in which electrons are attached, Reaction 1, and released, Reaction 3. A reaction that converts 0- to nonreacting negative ions interrupts the sequence given by Reactions 1-3 and results in electron loss from the EC detector plasma and a corresponding selective sensitization of the detector to compounds that do not normally produce a strong response. This effect is demonstrated for CO,, H2, and CH,. 1979 American Chemicai Society
1820
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11. SEPTEMBER 1979
Table[. Gases Used in These Measurements gas supplier grade N, N,O
CH, H, CO,
NBS Matheson Matheson Matheson Matheson
Ultra-high Purity Ultra-high Purity Coleman Instrument Purity
purity 98.0 99.97 99.999 99.99
It has long been recognized that impurities in the carrier gas can strongly influence the response of the electron capture (EC) detector. In certain instances the presence of these impurities can enhance the sensitivity of the EC detector t o specific compounds. An increased sensitivity for butylbromide was noted by Van de Wiel and Tommassen in a fixed frequency, pulsed EC detector when O2 was added to the nitrogen carrier gas ( I ) . Recently, Grimsrud and Miller ( 2 ) and Grimsrud and Stebbins (3) demonstrated that O2 purposely added t o the nitrogen carrier gas could be used to increase the sensitivity of the E C detector to certain halogenated organic compounds. In separate experiments Simmonds ( 4 ) also has observed that O2 added to the nitrogen carrier gas enhances the sensitivity of the EC detector to CO,. In general, the mechanism responsible for EC detector response t o a compound is direct electron attachment to the compound of interest leading to formation of a stable negative ion. However, in References 2 and 3 it was noted that a t elevated temperatures the collisional detachment of electrons from Oz- competes with the attachment process so that 0 2 and electrons are in thermodynamic equilibrium. In this circumstance a reaction which converts 02-to a stable negative ion produces electron removal from the EC detector plasma and, therefore, generates an EC detector sensitivity to certain compounds even in the absence of a direct electron attachment mechanism. In the present series of measurements an alternative strategy t o the equilibrium 02-chemistry is presented. This alternative relies on the reactive steady-state chemistry of N 2 0 (51, i.e.,
+ N,O 0- + N,O NO- + N2 e
--
0- + N2
(1)
+ NO NO + N 2 + e NO-
(3) so that electrons, 0-,and NO- have concentrations in the EC detector that depend on the rate constants for Reactions 1-3, the concentration of N 2 0 in the N2 carrier gas, and the operating temperature of the detector. Accordingly, for this system, E C detector response is produced by compounds (analyte, A) that react with 0-. 0- A stable negative ion (4) +
+
-
This is particularly significant because 0- reacts with many compounds that do not directly capture electrons. The present measurements demonstrate this process for the EC detector response t o C02, HZ,and CH4.
EXPERIMENTAL The experimental setup used in these measurements is shown in Figure 1. The sensitivity and linearity of the EC gas chromatograph was measured by introducing aliquots of a sample containing known concentrations of the test compound in N2 into the chromatograph. The samples containing traces of each gas were prepared by diluting the test compound with N2 in a dynamic flow system with six stages of turbulent mixing. This technique allowed preparation of samples with mixing ratios of 1 ppm or greater of the test compound under study. The gases from which the test samples were blended are listed in Table I. A carrier gas mixture containing 1000 ppm of N 2 0 in N2 was made by a single static dilution. Prior to the introduction into the gas chromatograph this carrier gas mixture is passed through
comments major impurities are major impurities are major impurities are major impurities are
N,, 0,, and H,O CO,, N,, C,H,, 0,, and H,O He, 0,, CO, CO,, and hydrocarbons N,, O,, CO, and H,
NpO/N2 Blended Carrier Gas Mixture
Heoter
1 7r
Molecular Sieve 13X Trap
'
1
N 2 0 Monitor
-
__-__A-
Packord-Becker
I
Line
Primary Analysis Perkin-Elmer Model 3920 Gas Chromatograph
Vacuum
NZ Carrier Gas
GC Line
Sampling Interface
,Nz ~
Containing lpprn-1000pprn H z , CH, or COz
4
Mixing Bridge
Y,
Hp,CHqorCO2
Figure 1. Schematic diagram of the experimental setup
a stainless steel trap containing Molecular Sieve 13X (Sorbent A) maintained at 25 OC that is activated by overnight heating to 300 "C while being purged with a 1 STP atm cm3 s-l flow of N2. This trap effectively removes all detectable impurities from the carrier gas with the exception of O2and CF2C12.In addition, the N 2 0 initially is partially removed by the trap. Thus, the concentration of N 2 0 in the carrier reaches a level equal to the concentration of N 2 0 in the original mixture after several days to several weeks of operation depending on the operating temperature of the trap. This slow increase in N 2 0 concentration with time allowed the measurements to be carried out with N 2 0 concentrations varying from 10 ppb to 1000 ppm. After passing through the trap, the carrier gas mixture passes into the gas chromatograph. The N 2 0 concentration in the carrier gas is monitored at two points. The carrier gas mixture is sampled after it exits the trap but before it enters the GC. This is accomplished by bleeding off a small quantity of the carrier gas mixture through a stainless steel bellows valve into the sample loop of a second, monitoring gas chromatograph (Packard-Becker, Model 417). The N20 monitoring GC is equipped with a 1/8-inch 0.d. 8-ft long stainless steel column packed with Porapak Q operating at 80 "C with a B3NiEC detector maintained at 350 "C. A pure nitrogen carrier gas was used in the N 2 0 monitoring GC. In a similar fashion the effluent from the EC detector of the primary GC is diverted and analyzed for N20. Figure 2 shows an example of the chromatograms recorded by the monitor GC-EC detector of samples of the carrier gas mixture exiting from the primary GC-EC detector. In all cases the concentration of N 2 0 at the entrance and exit of the primary gas chromatograph was equal within experimental error. Aliquots of the test gases in N2were introduced into the sample loop of the primary gas chromatograph (Perkin-Elmer, Model 3920). This chromatograph is equipped with a 63Ni constant current, frequency-modulated, pulsed, temperature variable EC detector. In this scheme the frequency of pulses (pulse height = -55 V, pulse width = 250 ns) required to maintain a fixed standing current is inversely proportional to the average electron density in the cell. The standing current was adjustable from 0.5 to 3.5 nA with a current of 1nA typically. The pulse frequency
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
1821
Table 11. Response Factors, F,a as a Function of N,O Concentration and Detector Temperature, T d F (CO,) Column Porasil C at 50 "C
TD,"C
200
noN,O 4.83 X 3.02 X 1.92 x 1 0 - 6 1.62 x 10-5
TD,"C 350 300 250 200
noN,O 1.33 X 9 . 8 3 X lo-, 3.28 X l o - ' 1.92 x 10-5
T D ," c
no N,O 1.29 X lo-' 2.04 X 1.53 X 8.11 x 10-5
350 300 250
350 300 250 200
0.066 4.27 X 3.34 X 2.38 x 2.76 x
ppm
0.436 ppm 1.19 ppm 2.32 ppm 3.60 X lo-' 1.55 X lo-' 4.77 X 10.' 3.12 X 1.32 X l o - ' 3.95 X 2.62 x 1.02 x 10-3 2.22 x 10-3 2.02 x 6.43 x i o - , 1.70 x 10-3 F (CH,) Column Mol. Sieve 5A at 150 " C 0.90 ppm 6.61 ppm 1 2 PPm 24 ppm 1.60 X 7.91 X 9.49 X 1.00 X 10.' 7.48 X lo-, 4.01 X 5.39 X 6.26 X 1.79 X lo-' 2.27 X 1.99 X 2.58 X 10.' 6.71 x 10-5 7.39 x io-, 7.26 x 10-1 1.20 x 10-3 F (H,) Column Mol. Sieve 5A at 150 "C 10.' 10.' 10'~
0.90 ppm 3.81 X 10.' 2.49 X 10'' 8.17 X 10.' 1.15 x 10-5
6.61 ppm 2.08 X 8.93 X 10'' 2.60
10.' -2.21 x 10-5 X
1 2 PPm 3.71
X X X
10.'
1.42 1.68 -1.93 x 1 0 - 4
24 ppm 5.28 X 2.10 X 10.' no peak -4.22 x 1 0 - 4
5.89 ppm 1.23 X 8.21 X lo-' 4.78 x 10-3 2.81 x 10-3
11.3 ppm :!.58 X 1.60 X 9.37 x 1 0 - 3 3.62 x 1 0 - 3
33 ppm N,O 1.84 X 10.'
33 ppni N,O 7.99 X
The response factor quoted in units of pVs/nTorr cm3 (= 22.99 pV,s/femtomol). N20
t
sampling interface was free of detectable contamination and exhibited no test compound retention.
RESULTS
v) W
t 0
a v)
B L
0 +
u
e,
c
W
a
L
100 2 0 0 s Figure 2. Example of analysis made of the carrier gas containing N,O which passed through the gas chromatograph used for the primary analysis. This example indicates that 1.73 ppm N,O was present in the effluent gas sample along with an O2which was an impurity
0
is converted to voltage (10 MV= 1 count s-l) and plotted. Three columns, each packed in '/,-inch 0.d. stainless steel tubing, are employed in this study, (1)a 396-cm column packed with 100-150 mesh Porasil C operated at 100 "C for general survey work and for some measurements of C 0 2 when operated at 50 "C, (2) a 366-cm column packed with 80-100 mesh Porapak Q operated a t 50 "C for the remaining C 0 2 measurements, and (3) a 280-cm column packed with 100-120 mesh Molecular Sieve 5A operated at 150 "C for the H2 and CHI measurements. The samples were introduced into the gas chromatograph via a 10-port sampling valve equipped with a 5-mL sample loop and a 19-mL sample loop. This sample injection interface was maintained at 100 "C (373 K). Samples at subambient pressure are accommodated by an evacuated inlet system, with sample loop pressure measurement made using a stainless steel bonded strain gauge pressure transducer. The vacuum line of the sample injection interface is protected with a Molecular Sieve 13X trap and a liquid nitrogen trap to prevent pump vapor backstreaming. To check for possible contamination of the sampling interface, routine checks are made by injecting a pure N2 blank or by injecting the carrier gas through an evacuated sample loop. The chromatographic traces of these blank samples indicated the
The addition of N 2 0 to the N2 carrier gas produces a large increase in the sensitivity of the EC detector to H,, CHI, and C02. A sample of the response enhancement for COz is shown in Figure 3. The lower trace indicates the COS response observed in the absence of NzO with a sample containing 1200 ppm of C 0 2 in N2. T h e sample size was 312 Torr cm3. T h e attenuation factor on the detector signal for this chromatogram is 128, and the small peak a t about 50 s corresponds to CO,. T h e top trace gives the CO, response with 20 ppm of N 2 0 added to the carrier gas. The large peak at 51 s arises from the CO, in a sample containing 17.5 ppm C 0 2 in Nz (sample size, 250 Torr cm3). The attenuation of the detector signal is the same as the previous chromatogram. In addition to a peak arising from CO,, an oxygen impurity peak is observed at 25 s, and a small negative response is recorded at 63 s which corresponds to the elution time for N 2 0 from the column. This negative peak indicates the lower concentration of N 2 0 in the sample relative to that in the carrier gas. This negative N 2 0 peak is sufficiently separated from that of the C 0 2 so that it does not influence the measurement of C07. As expected, the enhanced sensitivity observed for the test compounds is associated with a n increased peak height with no detectable change in peak width, peak shape, or retention time. Table I1 shows numerically the enhanced sensitivity of the EC detector as a function of N 2 0 addition a t various E C detector temperatures. The sensitivity is quoted as a response factor, F ,
pv.s
nTorr cm3
(5)
where u: is the full width of the peak a t half maximum in seconds (s), and H is the sample size normalized peak height expresed in units of microvolts (pV) divided hy the volume (cm3)and partial pressure (nTorr = 1OP Torr) of' the gas under study contained in the sample loop which is maintained at 100 "C. The sample size factor, nTorr cm3,is simply a measure of the number of sample molecules injected onto the GC column; Le., 1 nTorr cm3 a t 373 K = 2.62 X lo7 molecules = 4.35 x IO-'' mol. H,, CH4, and C 0 2 do not directly attach electrons in the EC detector. However, all three compounds are observed to
1822
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
co2 I
\ \
1
\\
\
i
0,
Z LL
(b)
i
N20
:
COZ I
-
(a)
t-
L--'
0
100
1
I
200
300
I
I
4OC S
Figure 3. Chromatograms demonstrating NO , induced selective electron capture sensitization (SECS) for CO,. Lower chromatogram was made using a N, carrier gas, and a 312 Torr cm3 sample containing 1200 ppm of CO, was injected. The upper chromatogram was made using a N2 carrier as containing 20 ppm of N20. I n this chromatogram a 250 Torr cm sample containing 17.5 ppm COPwas injected. In both chromatograms the Porapak Q column at 50 OC was used, the EC detector was maintained at 350 OC, and the signal was attenuated by a factor of 128
Y
2.0
1.0
0
3.0
IOOO/T ( K - ' ) Figure 5. Plot of Fw for CO,, CH, and H, as a function of temperature. The experimental data for CO, are plotted as the open circles (0),for CH, as open triangles (A), and for H, as inverted triangles ( 0 ) .The concentration of N,O in the N, was 12 ppm (VIV) for these measurements
i 1
I ' 01
1 1
IO
R Figure 4. Plot of FNIOfor CO,, CH, and H, as a function of the N20 volume mixing ratio, RN,o. The experimental data for CO, are plotted as the open circles (0),for CH, as open triangles (A),and for H, as inverted triangles (V).The solid curves are smooth fits to illustrate the trends in the data
induce a small response in the EC detector in a purified N2 carrier gas. The measured response factors for the analytes with no NzO in the carrier gas are referred to as Fo in the following text. The increase in the response factor attributable to the presence of NzO is plotted graphically in Figure 4. In this plot FNlo = F - Fo is shown as a function of N 2 0 addition where the NzO volume mixing ratio is designated as RN@. The response factors for the three compounds with N 2 0 sensitization exhibit a similar behavior. At 1 ppm of N 2 0 in the
I02
X IO6
Figure 6. Plot of average peak to peak noise vs. N,O volume mixing ratio, RN,O, in N, carrier gas
carrier gas, the response factors are ca. 2 X 10-4, 5 X and 11 X for Hz, CH4, and COS, respectively. A t concentrations in excess of 10 ppm in the carrier gas, the response factor increase levels off, indicating some detector saturation. T h e dependence of FN20 for H2,CH4, and C 0 2 as a function of EC detector temperature is shown in Figure 5. For each compound the response factor is observed to increase rapidly with temperature. As the NzO concentration in the carrier gas is increased, the electron density decreases and, therefore, the base-line operating frequency of the EC detector increases. This is accompanied by an increase in the background noise. The
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
i
t
Negative
1823
I
Figure 8. Illustration of the steady-state negative ion chemistry used
to explain the N,O induced selective enhancement of capture sensitivity Rk20 X IO6 Figure 7. Detection limit for COP as a function of RNgO.The detection limit is defined as the number of molecules injected onto the Porasil
C column maintained at 50 "C needed to produce a peak response twice the average peak to peak base-line noise. For CO,, 10" molecules have a mass of 7 3 pg and = 1.66 X lo-'' mol
effect is discussed in References 2 and 3 and a typical plot of the average peak to peak noise level as a function of N 2 0 concentration at 350 "C is shown in Figure 6. However, these values should be considered as only representative. The N 2 0 response of this instrument, and therefore the base-line frequency and noise, depends on the presence of certain impurities in the carrier gas flowing through the EC detector and varies accordingly. The minimum detectable amount of CO, observed with the present instrumentation as a function of the volume mixing ratio of NzO in the carrier gas is shown in Figure 7. In this figure the minimum detectable signal is the absolute amount of sample injected onto the GC column required to produce a peak response twice the base-line noise level. For C 0 2 these figures apply to studies made with the 13-ft Porasil C column a t 50 "C. The present results indicate that the minimum detectable levels of H, and CHI are comparable to that for
co,.
Finally, one of the important features of the constant current ECD is the wide linear dynamic range. In limited tests performed in those experiments, the sensitization produced by N 2 0 addition to the carrier gas appears to be linear. For example, with 20 ppm of N 2 0 in the N2 carrier gas the response was linear for a factor of 19.4 increase in the CO, sample.
DISCUSSION The preceding section presents results which demonstrate that N 2 0 added to a nitrogen carrier gas enhances the response of the EC detector to non-electron capturing compounds such as CO,, H2,and CH4. This effect is attributed to the reaction of these compounds with 0- to form stable negative ions, thereby, interrupting the steady-state reaction sequence. The scheme is illustrated in Figure 8. The net result of converting 0- by reaction with an appropriate analyte is to decrease the free electron density and thus selectively induce electron capture detector sensitization. Thus, this technique is termed "Selective Electron Capture Sensitization", SECS. T h e binary reactions t h a t comprise this chemistry, Reactions 1-3, are relatively well understood (5-10). Three-body reactions which serve to convert electrons to 0- (9, IO) and 0- to NO- (9) can be neglected compared to Reactions 1 and
2 a t the EC detector temperatures used in this study. The exprate constants for Reactions 1-3 are h , = 7.2;X [-4800/T] cm3 SC' (6, 7), h , = 2.2 X .LO-'' cm3 Si-' (5, 91, and k 3 = 10-l' exp[-iOO/T] cm3 s-' (8). Since KO- is destroyed by reaction with the N2 carrier gas a t a much faster rate than it is produced by the reactions of 0- with NzO, the concentration of NO- expected is much smaller than that of electrons or 0- and, therefore, NO- chemistry will be omitted from further discussion. The time required to establish a reactive steady state depends on the time constant for the slowest reaction. For any reasonable combinations of N 2 0 concentrations and detector temperatures below 400 "C, the constraint, on the approach to steady state will be imposed by Reaction 1. For a cell pressure of 760 Torr that lifetime will be T~ = 2.2 X lo-' RKZo-'(s) a t 200 "C decreasing to r l = 2.6 X l W 9 R ~ ~ 0(s) -l T,will be short at 350 "C. For N 2 0concentrations, RN, L compared to the passage time of the gas mixture through the detector. Alone, the steady-state chemistry of Figure 8 does not provide for electron removal from the EC detector plasma and fails to account for the observed EC detector sensitivity to N20when using N2 as a carrier gas. The present understanding of N 2 0 response in the detector indicates that this removal must be explained either by higher order reactions of 0- with N 2 0 (9-11) which have not been studied under similar experimental conditions or, alternatively, by reactions of 0- with impurities in the carrier gas. Impurity levels of compounds such as O2 and H,O of the order of 1 ppm in the gas in the detector would be sufficient to account for the observed N 2 0 response. As the N 2 0 concentration in the carrier gas increases, the rate of formation of 0- through Reaction 1 increases. T h e 0- production through Reaction 1 is balanced by the destruction through Reaction 2 and conversion to stable ions through Reaction 4. These two processes dictate that a steady-state concentration of 0- relative to electrons is achieved which, for k 2 [ N 2 0 ]>> h,[A], is given by,
At this point, the detector response enhancement associated with 0- secondary reactions becomes saturated. The approach to this saturation in FNzo is indicated for HP, CH,, and C 0 2 in Figure 4 as a flattening of the curves a t concentrations of N 2 0 above ca. 10 ppm NzO. The N 2 0 concentrat ion a t which saturation occurs depends on the details of the high pressure
1824
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
secondary reactions of 0- with N 2 0 , and the concentration of impurities that react with 0-in the EC detector. The SECS response factor, FNzO,a t saturation depends on the rate a t which 0- is destroyed by the test compounds, and the ratio of 0- to electrons as deduced from Equation 6 which will in turn depend strongly on temperature as indicated in Figure
02
5. The ion chemistry of CH4, C02, and H 2 with 0- is well known. The CHI reaction is
0-
+ CH4
-
OH-
+ CH3
(7)
with k , = 8 X lo-" cm3 s-l (12). The reaction shows little energy (temperature) dependence. As a consequence, the temperature dependence of F N 2 0 for CH4 almost entirely reflects the temperature dependence of Reaction 1 as indicated in Equation 6. The slope of F N 2 0 for CHI in Figure 5 implies an activation energy for hl of 0.44 eV in excellent agreement with the value of 0.46 eV measured by Warman et al. (6) and 0.45 eV measured by Wentworth et al. ( 7 ) . The reaction of C 0 2 with 0- is a three-body process,
0- + COS + N2 --* CO3-
+ NZ
(8)
The rate constant for this reaction is 3.1 X cm6 s-l (13) a t room temperature with 0, as a third body. In the ECD at atmospheric pressure and 350 "C, N2 = 1.2 X 1019molecule ~ m - COz ~ , more rapidly reacts with 0- than does CHI and, consequently, F ~ ~ o ( c 0 2>) FN,o(CH,). The temperature dependence of FNzo(COz)does not yield a straight line in the Arrhenius plot of Figure 5 as does CHI. The slight upward curvature observed for FN20(C02)is explained by the negative temperature dependence exhibited by Reaction 8. The reaction of Hz with 0- has two channels at low energy ilOH- + H Pa) 0 - - H, L H,O + e (9b) The rate constant for this reaction is k9 = 6.4 X cm3 s-l (14). However, at low temperature the associative detachment channel is dominant, while the production of OH- increases rapidly with temperature. The competition of these two processes explains the sharp downward curvature of FNzO (H,) in Figure 5. In fact, the response of H, in the EC detector becomes negative at the lower temperatures as shown in Table 11. When the associative detachment process, Reaction 9b, dominates, electrons are produced from 0-accounting for the negative response. In this study of NzO sensitivity enhancement, the minimum detectable limits of Hz, CH4, and COz for a signal to noise ratio of two is approximately lo'* molecules. This sensitivity is comparable to the detection limits typically quoted for flame ionization detection of CHI (15). The present sensitivity allows measurement of H2, CH4, and COz in atmospheric samples. This is illustrated for H, and CH4 in the chromatogram shown in Figure 9 of a 250 Torr cm3 sample of ambient air containing approximately 1.0 ppm of H 2 and 1.6 ppm of CHI. The peak at 39 s corresponds to H2,while the peak at 97 s is due to CH4 These two peaks are separated by the large peak corresponding to 02.This chromatogram was made using a Molecular Sieve 5A column operated a t 150 "C. Although an impressive enhancement of sensitivity for H2, CHI, and C 0 2 has been obtained, the EC sensitivity for these compounds is still far below the optimum detection limits for compounds which rapidly attach electrons. For example, in this same instrument, the detection limit for CFC13 is approximately los molecules. The present limits on the sensitivity of the N 2 0 enhancement technique are imposed by a number of factors. In steady state, the concentration of 0relative to free electrons does not depend on However, it does depend critically on temperature. This is shown in
u
0
100
200
s
Figure 9. Chromatographic trace of a 250 Torr cm3 sample of ambient air, demonstrating sensitization for H, and CH,. The chromatogram was made using the Molecular Sieve 5 A column at 150 "C. In this analysis the N2 carrier gas contained 12 ppm of N,O the results for J"@ in Figure 5. This trend of increasing Fxz0 with increasing temperature will continue until [O-]/[e-] is greater than unity. According to Equation 6, at 350 "C [O-]/[e-] = 0.15. At all temperatures, the largest sensitivities are obtained a t the cost of greatly increased noise. This is associated with the attachment of free electrons in the presence of N 2 0 . However, this attachment cannot be explained with our present understanding of the ion chemistry of N 2 0 and may involve impurities in the EC detector. As suggested above, the most likely impurities in the present experiments are 0, and HzO introduced into the carrier gas stream through small leaks. Both 02 and HzO react rapidly with 0- by three-body processes, such as Reaction 8, to form 03- and OH-aOH (13). This speculation is supported by work presently being carried out in our laboratory using a gas chromatograph in which the valves, columns, and EC detector are housed inside a box sealed in such a way that the composition of the atmosphere surrounding these elements can be controlled. The detection limit on this instrument for NzO is increased by a factor of 8.8 when pure nitrogen replaces air as the purge gas. The enhancement in detector sensitivity toward H2, CHI, and COz can be expected for a wide variety of other compounds including a wide spectrum of hydrocarbons (10, 16). Note Added in Proof. We have recently determined experimentally that picogram amounts of hydrocarbons such as hexane, pentane, butane, propane, ethane, and other organic compounds can be detected by SECS techniques ( J . Chrornatogr., in press). The response will depend on the nature of the 0- interaction. For example, by virtue of the associative detachment reaction, CO is observed to give a negative response at all detector temperatures as Hz did at lower detector temperatures. With N 2 0 induced SECS, an enhanced sensitivity is observed only for analytes that react more rapidly with 0-than directly with electrons. For this reason, the detection limits of the EC detector to compounds such as CFCl, or CC14 which rapidly convert electrons to stable negative ions is raised on addition of N 2 0 to the carrier gas simply by virtue of the increased base-line noise. Practical advantage may be taken of this equalizing phenomenon in the frequently encountered instances in which two or more analytes of interest have markedly different response factors necessitating repeated independent analyses. Moreover, the EC detector is usually considered a selective detector. It appears that by the simple expedient of switching the carrier gas from pure nitrogen to
ANALYTICAL CHEMISTRY, VOL. 51, NO. 11, SEPTEMBER 1979
a carrier gas containing the sensitization agent, NzO, the same detector becomes much less compound specific. In addition, the technique holds the promise that relative response factors characteristic of various classes of compounds in EC compared with SECS, coupled with retention indices measurements, can be used to aid in the confirmation of identification of unknown peaks. Finally, it is interesting to note t h a t the steady-state ion chemistry outlined in Figure 8 represents electron catalyzed conversion of N 2 0 to N2 and NO,
2 N 2 0 --% Nz+ 2NO
(10)
This cycle suggests alternative strategies for the detection of compounds which interrupt the cycle of ion chemistry either by attaching electrons or by reaction with 0- or NO-.
LITERATURE CITED (1) t i J. Van de Wiel and P. Tommassen, J , Chromatogr., 7 1 , 1 (1972). (2) E. P. Grimsrud and D. A. Miller, Anal. Chem., 5 0 , 1141 (1978). (3) E. P. Grimsrud and R. G. Stebbins, J . Chromatogr., 155, 19 (1978). (4) P. G. Simmonds, J . Chromatogr., 166, 593 (1978).
1825
( 5 ) R. Marx, G. Maucbire, F. C. Fehsenfeid, D. B. Dunkin, and E. E. Ferguson, J . Chem. Phys., 5 8 , 3267 (1973). (6) J. M. Warrnan, R . W. Fessenden, and G Bakale, J . Chem. Phys., 5 7 , 2702 (1972). (7) W. E. Wentworth, E. Chen, and R. Freeman, J. Chem. Phys., 55, 2075 (1971). (8) The rate constant quoted for reaction 3 is estimated from !he result contained in M. McFarland, D. B. Dunkin, F. C. Fehsenfeld, A. L. Schrneltekopf, and E. E. Ferguson, J . Chem. Phys., 56, 2358 (1972). (9) D. A. Parkes, J . Chem. Soc., Faraday Trans. 1 , 2103 (1972). (10) H.Shimamori and R. W. Fessenden, J . Chem. Phys., 68, 2757 (1978). (1 1) H. Shimamori and R. W. Fessenden. J . Chem. Phys.. 69, 4732 (1978). (12) W. Lindinger, D. L. Albritton, F. C. Fehsenfeld, and E. E. Ferguson, J . Chem. Phys., 63, 3238 (1975). (13) F . C. Fehsenfeld and E. E. Ferguson, J . Chem. Phys., 61, 3181 (1974). (14) M. McFarland, D. L. Albritton, F. C. Fehsenfeld, E. E. Ferguson. and A. L. Schmeltekopf, J . Chem. Phys., 59, 6629 (1973). (15) R. L. &ob, "Modem Ractice of Gas Chromatography",VViley Interscience, New York, 1977. (16) D. L. Aibritton, Atomic and Nuclear Data Tables 22. 1 (1978).
RECEIVED for review February 13, 1979. Accepted May 22, 1979. M.P.P. wishes to thank the Crraduate School of the University of&lorado for a Colorado Doctoral Fellowship, under which much of the experimental work wm conducted.
Noniterative Method for Computer Evaluation of Second-Order Kinetic Data Paul B. Kelter and James D. Carr" Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588
A noniterative method is described for evaluating the second-order rete constant and initial absorbance from absorbance vs. time data. The advantages of the method are its speed, simplicity, accuracy, and ability to deal with noisy second-order data.
Two independent equations are necessary to solve for the two unknowns, Xoand k,. This is accomplished in the following manner. For 12 points, L e t Sl =
nf2 t=l
1
-andS2 =
x,
5
1 --
t=(n/Z)+l .Yt
If 6 = a constant time interval between points, and t l = the In 1962, R. G. Cornel1 introduced a noniterative method for calculating first-order rate constants for radioactive decay processes ( I ) . The application of Cornell's method to absorbance-time data assumes that the absorbance a t any time, t , is a sum of the exponential decays of all the previous A,'s. It became necessary in our work to utilize similar procedures for second-order reactions as is described in this paper.
THEORY Consider the reaction:
X
+X
-
time of acquisition of the first data point then s 1=
2-+ k , 2x0
ni2
c (6 + t J
t=l
(3)
and
(4) To give an example of the summation, consider the times ( t J , ( t l + a), ( t l 26), and ( t l 36). Summing these four times gives:
+
Products
+
4
c = [tl + ( t l + 6) + + 26) + (t* + 36)]
or
(tl
X
+Y
t=i +
Products
if Xo = Yo and the stoichiometry of the reaction is 1:l. Under these conditions, the concentration of species X at any time t is
Xn
+ k,t
n
t=l
where X, = the initial concentration of species X. Equation 1 can be converted to the following form:
Xt
n ( n -116 c + 6) = ntl + 2 (tl
x,= 1 + h,tX, -1-_ - 1
which can be factored to give ntl + t , or in more general terms, nt, .t2;:;t . Noting that C;: t = n(n - 1)/2, it can be shown that for n data points,
(2 )
0003-2700/79/0351-1825$01.00/0
T o account for S 2 points being taken a t time (nj2)6later than S1 points, an added term is introduced into the S z expression, this term being equal to (n/2) 6(n/2)for nj2 points in the S 2 expression. Substituting Equation 5 back into Equations 4 and 3, and adding the extra term gives 0 1979 American Chemical Society
