ACKNOWLEDGMENT The authors wish to thank Ralph N , Adams for many helpful discussions and suggestions during the course of this work.
Received for review June 8, 1973. Accepted March 5, 1974. Acknowledgment is made to the University of Kansas’ General Research Fund for partial support of this research.
Absolute Determination of Atmospheric Halocarbons by Gas Phase Coulometry Daniel Lillian and Hanwant Bir Singh Department of Environmental Sciences, Rutgers University-The
State University of New Jersey, New Bruns wick, N.J. 08903
Gas phase coulometry was evaluated and used for the measurement of ambient halocarbons since the method permits operation at maximum sensitivity and is absolute. Ionization efficiencies were determined for 14 compounds to range from less than 0 to 90%. Reciprocal efficiencies were verified to increase linearly with flow rate as predicted by a stirred tank reactor model. At intermediate efficiencies (40 to 60%), coulometric measurements of CC13F and CC14 exhibited a constant error of about 25% over a representative concentration range. At efficiencies greater than 90%, the per cent error in coulometric analysis was less than 5% compared to primary standards. Compounds identified and measured coulometrically in the New Brunswick, N.J., area were CCI3F, CHd, CH3-CC13, CC14, CHCI=CCIz, and CCIZ=CCIZ.
Under optimum conditions of near 100% ionization in an electron capture (EC) detector, strongly electron-absorbing compounds produce a response of such great sensitivity that femtogram gram) analysis has been suggested ( I ) . In an ongoing study of the formation and decay of atmospheric halocarbons, present in concentrations less than (v/v), a coulometric gas chromatographic method of analysis based on gas phase electron absorption has been evaluated and used for measuring the ambient concentrations of several such compounds. Developed by Lovelock ( 2 ) , this method is based on a 1:l equivalency a t 100% ionization between the number of solute molecules in a carrier stream to the number of electrons absorbed by them in the EC detector. Accordingly, the solute concentration can be calculated directly from the number of electrons absorbed. At ionization efficiency of less than 10070, the use of two identical detectors in series enables one to determine the fractional ionizations in the EC detector and thereby maintain the absolute nature of analysis by correcting for the unionized molecules. When operated coulometrically at 100% ionization efficiency, an EC detector is a t its maximum sensitivity since all solute molecules are “counted.” Since the method is absolute, sources of mixing and contamination errors inherent in the preparation of extremely dilute calibration mixtures are precluded. An additional advantage of this mode of operation, particularly in air chemistry studies, is the ability to determine the cbncentration of an unknown compound and possibly deduce its identity from spatial (1) J. E. Lovelock, in “Gas Chrornatatography 1968.” C. L. A. Harbourn, Ed., Elsevier, Amsterdam, 1969. (2) J . E. Lovelock, R . J . Maggs, and E. R . Adlard, Ana/. Chem., 43, 1962 (1971).
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
and temporal distributions of concentrations. Furthermore, rate of thermal electron attachment is an excellent aid for confirmation of identity when used in conjunction with retention data. The compounds selected initially for an evaluation of coulometric application were: CC14, SFs, CC13F, CBrFzCBrF2, C C I F C C ~ ~ , CHJI, CC12F2, CHC13, CH3-CC13, CC12F-CClF2, CHCl=CC12, trans-CHCl=CHCl, CH2= CC12, and CHzC12. THEORY Consider two identical EC detectors in series and let their signals be X I and X2 in coulombs due to an injection of a plug of W moles of compound AB. If p is the fractional ionization of AB, at high ionization efficiencies and low solute concentrations, one can write:
Xi
Xj
= PQ
= P(Q
(1)
- PQ)
p = 1 - - x2
(2)
(3)
XI
where, from Faraday’s law, CJ is equal to 96,500 W . The gram moles of solute W in the EC detector are, therefore, from Equations 1 and 3
W =
XI 96,500(1 -
2)
(4)
For an injection of V ml of sample a t t “C, the volumetric mixing ration CAB is: CAB
=
8.5 X
X
v(1-
(273
+ t)(X,)
(5)
$2)
With a recorder, the signals and x2are simply the respective chromatogram areas in coulombs. Ionization efficiencies can theoretically be predicted by considering the EC detector as a stirred tank reactor in which molecules undergo pseudo-first-order reactions with electrons present in large excess. For a carrier gas passing through the detector at constant flow rate Fo simple mass balance gives:
where K is the first-order rate constant and V d is the detector volume. Verification of the model offers the possib i l i t ~of determining optimum parameters for coulometric analysis.
--
5.0
9.0
81)
4.0
60 7.0
5.0
-
4.0
-
-P
c
I
/
/
3.0
2.0
" 0
IO
20
30
40 M 60 70 80 90 100 110 120 FLOW R A T E I M L I M I N I
Figure 2. Effect of flow rate on ionization efficiencies CONCENTRATION ( P P B X S A M P L E VOLUME
I
Figure 1. Peak height vs. concentration for CCl4 and CC13F. FSD for CC14, A; FSD for CC13F, 3 X A
Table I. Ionization Efficienciesa 25
Ionization efficiency
Compound
0.90 0 . 85b 0.84 0.70 0.69 0.63 0.33 0.20 0.10 0.06 0 .oo
