elasticity to establish the connections. [For details of handling glass capillary columns, see (7).] It is desirable, furthermore, that the same type of gas chromatograph be used for GC/MS as for pure GC. This allows a given sample to be run first on GC with FID. When the optimal analysis conditions have been found,
(7) K. Grob and H. J. Jaeggi, Chrornatographia, 5, 382 (1972).
the column is moved, without any modifications of the glass parts, to the MS. After identification of interesting substances, it is often moved back again to GC for quantitative estimation. Received for review February 26, 1973. Accepted March 23, 1973. All above-mentioned work has been made possible through the generosity of F. J. Burrus & Cie, Boncourt, Switzerland.
Coupling of Permeation and Exponential Dilution Methods for Use in Gas Chromatographic Trace Analysis
Fabrizio Bruner, Claudio Canulli, and Massimiliano Possanzini Laboratorio lnquinamento Atmosferico del C. N.R., c / o lstituto di Chimica Analitica, Universita 00785 Roma, ltaly
Trace analysis by gas chromatography is becoming more and more important since air pollution and similar topics became the objects of extensive research and of routine determination as well. In this connection a precise knowledge of the reliability of chromatographic columns is very important in quantitative determinations and calibration curves must be carried out rather often. Thus, a standard device to make calibration curves is desirable as a necessary accessory for any gas chromatograph used for trace analysis. In a recent paper, the use of the exponential dilution method in the analysis of sulfur compounds in air has been described ( I ) ; the results showed that by making very careful measurements, a relative standard deviation of about 5% for the SO2 curve slope could be achieved. However, syringe injection of the sample into the exponential dilution flask can often be a definite source of error, in some cases affecting severely the accuracy of measurements, and the curve obtained is completely out of the 5% indicated above. Advantages in the determination of the initial concentration Co are obtained when permeation tubes are used. On the other hand, the necessity of mixing the effluent of the permeation tube with additional gas a t different flow rates in order to vary the sample concentration according to the need of making a calibration curve, can be the source of considerable errors. The combination of a permeation tube (P.T.),which ensures a high degree of accuracy in obtaining the value of initial concentration, and of the exponential dilution flask (EDF), to obtain precise concentrations between C, and the detection limit is a technique which grants the advantages of both methods. In this paper, an apparatus serving for this aim is described. The SO2 permeation tube was prepared in this laboratory according to the usual procedure (2); the exponential dilution flask, constructed entirely in Teflon (Du Pont) was the same as described in Ref. (1). The total system is described (1) F. Bruner, A. Liberti, M. Possanzini, and I. Ailegrini, Anal. Chem., 44, 2070 (1972). (2) F. P . Scaringelli, A. E. O'Keeffe, E. Rosenberg, and J. P . Bell, Ana/. Chern.. 42, 871 (1970).
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in Figure 1. The gas line, including EDF and sampler, is saturated with the mixture coming from the permeation tube. The diluting gas is kept a t a constant flow with the system described in Ref. (1). In our experimental conditions, the SO2 concentration in the gas entering the EDF was 29.30 ppm. Once the EDF and the entire line are saturated with the SOz-containing mixture, flowing at a rate of 60 ml/min, the 4-way and 8-way valves are rotated at the same time and exponential dilution takes place. Area us. concentration points are taken in the same way as in Ref. (1).
The flow rate of the nitrogen diluter is kept at the same value (60ml/min). The calibration curve is made according to the procedure of Ref. ( I ) . Particular care must be taken to be sure that the initial concentration of SO2 is reached in the EDF. This was tested by connecting the EDF directly to the Flame Photometric Detector (Tracor, Inc., Austin, Texas) and by recording continuously the signal obtained. In this way, the first part of the graph of Figure 2 is obtained. When the SO2 level reaches a constant value, the 4-way valve is rotated and the dilution starts. The second part of the graph of Figure 2 is obtained in such a way. One hour or less is sufficient to obtain a constant signal (EDF saturation). By running the calibration curve for SO2 according to the procedure described, we could note the following improvements: The relative standard deviation on all the points obtained by plotting the calibration curve using all 20 dilutions was 1%.The slope of the calibration curve (log peak area us. log concentration) was 1.85 [the column used was the same described in Ref. ( I ) ] . The accuracy of measurements does not depend on the operator, and this is important if many apparatus of this type should be used for monitoring purposes. The error connected with the fact that by using the conventional operation of EDF, the initial concentration of the sample is not preserved in the sampler (3) is practically eliminated. In our system the saturation involves the (3) G. Greco, Jr., F. Gioia, and F. Alfani, Chim. Ind. (Milanj, 53, 1133 (1971).
A N A L Y T I C A L C H E M I S T R Y , VOL. 45, NO. 9, AUGUST 1973
SAT U RAT ION
Needle
Colu
Detector
DILUTION
-.Ic---o
Figure 1. Scheme of
-
the coupling device
mV
Saturation
/ /
Dilution
I 1
Figure 2. Typical detector signal during saturation of EDF and dilution
sampler also, so that the signal obtained with the first injection corresponds exactly to the C, in the EDF. Finally, analogous improvements were found for other samdes. such as CH&H and n-butane (relative standard deviations of calibration curve slope were 4% and 2%, respectively), showing that this system can be successfully used for any gas for which a permeation tube is feasible. -
ACKNOWLEDGMENT The authors thank Andrew E. O’Keeffe for helpful discussions and Vincenzo Di Palo for technical assistance.
I
Received for review January 8, 1973. Accepted March 12, 1973.
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 9 , A U G U S T 1973
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