1596
Anal. Chem. 1983, 5 5 , 1596-1599
Constant Current Linearization for Determination of Electron Capture Mechanisms Albert Zlatkls," C. K. Lee,' and W. E. Wentworth Department of Chemistty, Unlversify of Houston, Houston, Texas 77004
E. C. M. Chen School of Science and Technology, University of Houston, Clear Lake Clty, Houston, Texas 77058
The electron capture detector response for compounds that are known to respond by different mechanlsms have been measured as a function of temperature by using the constant current mode. The electron aff initles and actlvatlon energies obtalned from thls study agree wlth previous values supportlng the use of the constant current mode for determlning electron capture mechanisms. The electron aff inity of m -nltrotoluene was determined to be 0.8 f 0.1 eV whlie the activation energles for dissoclatlve capture were determined to be € * = 0.22 f 0.05 eV for 1,1,2,2-tetrachloroethane and E " = 0.1 f 0.05 eV for m-dibromobenzene. The minimum detectable quantity and the linear dynamlc range were determined for 1,3,5-trlchlorobenzene and benzaldehyde. Both quantlties depend on the capture mechanism and hence the temperature of the detector.
In spite of its limited linear dynamic range, the electron capture detector (ECD) has achieved a prominent position in analytical chromatography because of its unique sensitivity and selectivity. The original detector (1) used a constant dc voltage to collect the electrons. This gave not only a nonlinear response but also erratic responses. The use of a pulsed voltage for electron collection (2) eliminated the latter and led to the development of a kinetic model for the reactions in the ECD. By use of this model at a constant pulse interval (t) of about 1000 bs, the decrease in current can be related to the fundamental rate constants for the reactions according to the equation (3)
b - {e-) -- K'a (e-) where b is the electron concentration in the absence of the capturing species, (e-) the concentration of electrons in the presence of the capturing species, and a is the concentration of the capturing species. The electron capture coefficient, K') is related to the fundamental rate constants for the individual reaction steps in the capturing process. This equation is valid for both dissociative and nondissociative capture. A commercial analog computer has been described which uses this relationship to linearize the ECD response (4). A second method of linearization was described by Maggs et al. (5). This is based on the assumption that the electron concentration is related to the frequency; f = l / t which is required to obtain a fixed electron current by {e-I =
l/f
which leads to the relationship
f---fo - K ' a
(3)
To
where fo is the frequency in the absence of a capturing species. Patterson (6) optimized the design of an electron capture detector and developed a commercial linearization device using this relationship. Because it is easier to implement "constant current" linearization, this technique is currently being used in the majority of the commercial detectors. However, there has been no systematic investigation of the constant current mode for compounds which respond by different electron capture mechanisms. The only compound for which the capture mechanism has been clearly established using the constant current mode is dieldrin. Both Maggs (5)and Patterson (6) determined the temperature dependence of this compound and established a dissociative capture mechanism. Grimsrud and co-workers (7) have used the constant current mode in oxygen enhancement studies. The purpose of this research is to investigate the use of the constant current mode in establishing the electron capture mechanism. In order to accomplish this, the response for compounds that are known to respond by different mechanisms will be measured as a function of temperature using the constant current mode. A t least one example of each of the four observed mechanisms will be studied. These compounds also illustrate generally accepted trends for the effect of substituents on the electron capture response, e.g., the halogenated benzenes. These results will be applicable only to the detector used in these studies but favorable results will support the validity of eq 2 and will hence be generally applicable to all constant current linearizations.
THEORY The kinetic model for electron capture has been described earlier but will be summarized in the following (3,8)where AB represents any polyatomic molecule capable of attaching an electron:
P
+ Ar/lO% e-
CH4
+@ e-
-+ kd2P
k'D
e-
R
neutral species
+ AB -% A + Be- + AB AB-
-+ kl
AB-LAB+eAB-
+ P*+
Present address: Department of Chemistry, National ChungHsing University, Taichung, Taiwan 400, Republic of China. 0003-2700/83/0355-1596$01.50/00 1983 American Chemical Soclety
ka
A
B-
@
(4) (5) (6)
(7)
ANALYTICAL CHEMISTRY, VOL. 55,NO. 9, AUGUST 1983
where @*is used to represent a @-particlewith reduced energy as a result of thermal electron production. The rate constant, k&R,, represents the overall rate of thermal electron production. A few characteristicia generally associated with the model are as follows: (1)The current meamred with a pulsed ECD is due to the collection of all of the electrons at the anode in the cell by the application of short pulses. Negative ions are relatively immobile and contribute little to the measured current. The diffusion loss of electrons can be neglected. (2) The positive ion concentration decreases at the same time that the electron concentration decreases. The positive ion concentration must be considered as a variable (8). (3) A steady state is reached by the intermediate negative species during the time when the pulse is off. (4)The rate of production of thermal electrons is constant and is not affected by the presence of the added capturing species. The kinetic model can then be described by a system of differential equations which have been solved numerically with the above assumptions. The solution leads to the relationship
b - e-
Four electron capturle mechanisms have been observed: mechanism I, nondissolciative capture, k12and k2 = 0; mechanism 11,unimolecular tlissociation, kl and kl = 0; mechanism 111, dissociation by means of the crossing of two negative ion states; mechanism IV, dissociation by thermal activation of a single negative ion intermediate. From eq 13, three kinetic regions based on relative values of the rate constants, k-1, k2, and k’&) can be defined. These lead to the following mathematical relationships for the response. a: kl > k’,,(eJ > k2 mechanism I at high T , mechanisms I11 and IV at intermediate T
k h1 2k D
+
A + ERT
(15)
p: k’,,(@J > k1> h2 mechanisms I, 11, and IV a t low T , mechanism I1
y:
1
Flgure 1. Constant current electron capture coefficientsvs. reciprocal temperature for nondissociatlve compounds: (A) benzonitrile; (B) naphthalene; (C) benzaldehyde; (D) m-nitrotoluene; (E) nitrobenzene; (F) p-chloronltrobenzene.
the exothermicity of the reaction to determine the mechanism.
EXPERIMENTAL SECTION
lim -= K’u
In K’?“3/’2 = 7 In A
1
1597
k-, > k2 > k’Nl(@) mechanisms I11 and IV at high T
The exact definitions of high, intermediate, and low temperature depend on the compound. The potential energy curves for the four meclhanisms and the expected temperature dependence are shown in ref 9. In every case, a negative slope on a In K10/2vs. 1 / T plot corresponds to a dissociative process. However, it is not possible to differentiate between a @ and a y region. A positive slope with an intercept in the region of 12 to 18 indicates the formation of a stable negative ion. If the positive slope is small and the response is high, then it is difficult to differentiate between dissociative and nondissociative capture. In this case, one must resort to auxiliary information such as th.e electron affinity of the molecule or
The gas chromatograph used in this study was a Varian Model 3700 GC equipped with a constant-current pulse modulated ECD like the system described by Patterson (5). The cell contains a 8 mCi 63Nifoil and is polarized with a negative voltage of 50 V and 0.64 ps width. A mixture of 90% argon and 10% methane (Linde, Union Carbide) was used as the carrier gas and was passed through molecular sieves, type 5A-1/16in. pellets (Linde Division, Union Carbide) and an oxygen trap (Alltech) to remove traces of moisture and oxygen. All compounds were analyzed with a flame ionization detector to identify the major peaks. Since there was only a single major peak and the ECD data were obtained on a single peak, further purification was not necessary. The samples were injected with a Hamilton 1 0 - ~ Lsyringe. The solvents used were all low capture compounds such as benzene and hexane and were nanograde or pesticide grade. The capture coefficients of the compounds at different temperatures were determined from the chromatograms of the solutions of known concentrations of a test compound and standard compounds (1,3,5-trichlorobenzene or o-dichlorobenzene). The internal standard eliminated the variations due to instrumental parameters such as the flow rate and the recorder parameters. The experimental parameters which are determined are the response (area) for the compound x, the response for the standard, the relative concentrations of the species, and the absolute response of the standard as a function of temperature. The capture coefficient is given by area, n, K , = K , -area, n, The peak areas were integrated by an Autolab System computing integrator (Spectra Physics). All response factors were determined at low fractional capture. The temperature was measured with a thermocouple in the detector. The temperature range was chosen to clearly define the mechanism within the upper temperature limitation of the detector and the lower temperature limitation of the volatility of the compound.
RESULTS AND DISCUSSION The constant current ECD data are plotted as In KP12vs. 1/T. The compounds responding by mechanism I are shown in Figure 1, those responding by mechanisms I1 and IV are shown in Figure 2, and those responding by mechanism I11 are shown in Figure 3. The a regions are obvious for benzonitrile, naphthalene, benzaldehyde, nitrobenzene, and benzylacetate. The @ regions can be clearly seen for nitrobenzene, nitrotoluene, 1,2-dichloropropane, and 1,1,2,2tetrachloroethane. Benzyl acetate and the halogenated benzene compounds show y regions. The chloronitrobenzene
1598
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
Table I. Electron Affinities and Activation Energies
a
compound
EA(CC), eV
EA(CF),a eV
benzonitrile naphthalene benzaldehyde benzyl acetate 3-nitrotoluene
0.26 0.14 0.39 0.1 5 0.8
0.24 0.13 0.43 0.17
compound
E*(CC), eV
1,3-dichloropropane 2-chlorotoluene chlorobenzene bromobenzene 1,2-dichlorobenzene iodobenzene benzyl acetate m-dibromobenzene 1,1,2,2-tetrachloroethane
0.43 0.38 0.39 0.28 0.38 0.06 0.39 0.1 0.22
E*(CF)," eV 0.33 0.44 9.40 0.26 0.30
0.07 0.41
From ref 9.
Table 11. Detector Performance at Different Temperaturesa detector temp, K
slope of log-log plot
6 50 541 429
0.9613 0.9592 0.9525
428 530 630
0.9707 0.9667
sensitivity, mL/(g s)
minimum detection, g
linear range
corr coeff
1,3,5-Trichlorobenzene 5.49 x 1013 5.09 X lo-'' 1.45 x 1013 2.37 X lo-" 2.37 X lo1' 1.32 X lo-'''
2.59 X lo2 2.63 X l o 2 2.58 X 10'
0.9995 0.9995 0.9998
8.51 X 10' 5.13 X 1 0
0.9993 0.9982
Benzaldehyde 9.66 X 10" 4.80 X lo-''' 8.13 x 10-9 5.50 X 10" 1.23 X 10"
a Column, 8 ft x 2 mm i.d. nickel-10% OV-225 on Chromosorb Q (100-120 mesh); carrier gas 10% methane in argon; benzaldehyde flow rate 54 mL/min, column temperature, 413 K; 1,3,5-trichlorobenzene flow rate 38 mL/min, column temperature 403 K.
I
ci
\
T--1
2 T'
f03K-']
Figure 2. Constant current electron capture coefficients vs. reclprocal temperature for dissociative capture, mechanisms I 1 and I V : (G) benzyl acetate; (H) 1,3dichloropropane;(I) 1,1,2,2-tetrachloroethane.
could be either an a or a ,8 region. The electron affinities and activation energies determined in this study (CC) are compared with the corresponding quantities obtained by using the constant frequency (CF) (9) mode in Table I. The values are in agreement within the experimental error. The average deviation is 0.02 eV for the electron affinities and 0.04 eV for the activation energies. The data for m-nitrotoluene, 1,1,2,2-tetrachloroethane, and dibromobenzene are new and illustrate the use of the constant current response in determining mechanisms. The combination of an a and a ,8 region indicates the formation of a stable negative ion and supports the nondissociative capture for nitrobenzene and perhaps chloronitrobenzene. From the data, the electron affinity of nitrotoluene is 0.8 f 0.1 eV. The activation energy for tetrachloroethane is 0.22 f 0.05 eV while the activation energy for m-dibromobenzene is 0.1 f 0.05 eV.
1
2 T-' bo3 K-l]
Figure 3. Constant current electron capture coefficients vs. reciprocal temperature for dlssoclative capture, mechanlsm I 1I: (J) o-chlorotoluene; (K) chlorobenzene; (L) bromobenzene; (M) odichlorobenzene; (N) mdlbromobenzene; (0)iodobenzene.
By analogy to alkyl halides and mono- and dibromobenzenes, it is expected that these two compounds should undergo dissociative capture. It is clear that the constant current mode can be used to establish the mechanism of electron capture. Furthermore the relative responses obtained from the constant current data agree with the relative responses obtained in the constant frequency mode so that the large body of this latter data can be used as a guideline for the constant current data. This is illustrated by the fact that the response of these compounds covers 5 orders of magnitude. In addition, the response for the halobenzenes is in the order I > Br > C1 and for multiple substitution the order is tri > di > mono. On the basis of these results, it was decided to experimentally measure the linear dynamic range and the minimum
1599
Anal. Chem. 1983, 55, 1599-1603
6
composition of the sample in the detector or parts leading to the detector. The minimum detectable quantity is obviously related to the electron capture coefficient. Consequently, it will also be a function of the capture mechanism and the temperature. For 1,3,5-trichlorobenzene, and the highest temperature, picogram quantities can be detected. Registry No. 1,3,5-Trichlorobenzene,108-70-3;benzonitrile, 100-47-0;naphthalene, 91-20-3; benzaldehyde, 100-52-7;benzyl acetate, 140-11-4; 3-nitrotoluene, 99-08-1; 1,3-dichloropropane, 142-28-9; 2-chlorotoluene, 95-49-8; chlorobenzene, 108-90-7; bromobenzene, 108-86-1; 1,2-dichlorobenzene, 95-50-1; iodobenzene, 591-50-4; m-dibromobenzene, 108-36-1; 1,1,2,2-tetrachloroethane, 79-34-5; nitrotoluene, 1321-12-6; nitrobenzene, 98-95-3; p-chloronitrobenzene, 100-00-5.
i i , , ! - 3 - 2 - 1
0
1
2
3
4
Log w
[.gl Figure 4. Constant currisnt response vs. amount injected at various temperatures (K)for benzaldehyde (A)and 1,3,54richIorobenzene (0).
detectable quantity for a dissociative compound, 1,3,5-trichlorobenzene, and a nondissociative compound, benzaldehyde. The responses are plotted as relative area vs. the amount injected on a log-log plot at different temperatures in Figure 4. The analytical parameters obtainled from the data are tabulated in Table 11. The maximum linear dynamic range is 3 orders of magnitude. For benzaldehyde, there is no linear range at the highest temperature. This could be due to de-
LITERATURE CITED Lovelock, J. E.; Llpsky, S. R. J . Am. Chem. SOC. 1980, 82, 431-433. Lovelock, J. E. Anal. Chem. 1981, 33, 162-177. Wentworth, W. E.; Chen, E. C. M.; Lovelock, J. E. J . Phys. Chem. 1988, 70, 445-458. Fenimore, D. C.; Davis, C. M. J . Chromatogr. Scl. 1970, 8 , 519-523. Maggs, R. J.; Joynes, P. L.; Davles, A. J.; Lovelock, J. E. Anal. Chem. 1971, 43, 1966-1971. Patterson, P. L. J . Chrofytogr. 1977, 134, 25-37. Grimsrud, E. P. I n Electron Capture-Theory and Practice Chromatography”; Zlatkis, A., Pooie, C. F., Eds.; Elsevier: New York, 1981; pp 91-117. Wentworth, W. E.; Chen, E. C. M. J . Chromatogr. 1979, 786, 92-1 18. Wentworth, W. E.; Chen, E. C. M. I n “Electron Capture-Theory and Practice in Chromatography”; Zlatkis, A., Poole, C. F., Eds.; Elsevier: New York, 1981; pp 27-68.
RECEIVED for review February 16,1983. Accepted May 2,1983.
Quantitative Analysis without Analyte Identification by Refractive Index Detection Robert E. Synovec and Edward S. Yeung” Department of Chemisfty and Ames Laboratory, Iowa State University, Ames, Iowa 500 1 I
Contrary to the accepted notion that qualitative analysis must precede quantitative (analysis,we show that one can determlne the volume fraction of an arbitrary anaiyte In a flowing s identity. The physlcai and chemsystem without knowing H ical properties of the anaiyte are thus unknown, and no working curve can be established. The scheme Is based on the refractive index detector, whlch produces two distinct responses for the same analyte when two separate eluents are used. The informiation obtained can be used to predlct the volume fractlon of the analyte, as well as Its refractive index. Experimental verlflcation of thlts scheme Is reported.
Chemical analysis deals with the solutions to scientific questions through the identification ,and the quantitative determination of the composition of matter. It is generally accepted that the former precedes the latter. That is, one must identify or specify the species of interest before one can determine its concentration. This is because all analytical methods are based on some particular physical and/or chemical property of the species. The experimental observable must be calibrated against this particular property so that a concentration can be deduced. Analytical working curves thus
serve to provide the needed calibration, but they can only be constructed if the identity of the species is not in doubt. There are certain situations where it is desirable to know the concentrations of the components before any attempts at identification. One example is the assay of supposedly pure material. There, one tries to determine the type and the amount of each impurity present, the latter being of primary concern. Since quantitative methods are generally species specific, it is difficult to be sure that all possible impurities have been searched for. If however one can first ascertain the amount of all impurities present, the scope of the problem becomes much more tractable. Another example is the control of pollution emission and waste discharge. Knowing the concentrations of any foreign matter released without first requiring speciation is advantageous. A third example is forensic chemistry, where finding out whether any contaminant exists at all is an important fist step. A fourth example is in organic synthesis, where it may be desirable to know the yields of the various reaction products on the microscale, even when these products cannot be identified. And, when the products are identified, it may not be possible to isolate sufficient quantities of each to use traditional analytical calibration curves. It is therefore appropriate to pose the question whether qualitative analysis is a prerequisite for quantitative analysis.
0003-%700/83/0355-1599$0’1.50/0 0 1983 American Chemical Society
