A N e w Detector for Gaseous Components Using Semiconductive Thin Films SIR: The authors have developed a new type detector for gaseous components, which is based on the fact that the adsorption and desorption of gases cause the change in electrical conductivity of semiconductors. Although this phenomenon is known in some degree, the authors found that a t high temperatures (near 400" C.) the adsorption and successive desorption processes on the surface of semiconductors take place very rapidly and may indicate a marked change in electrical conductivity by the use of thin film semiconductors. This property of thin films is applicable to the detection of gaseous componenbs. Therefore, the authors attempted t o use thin films as a gas chromatographic detector instead of the ordinary thermal conductivity cell. The experimental arrangement is simple (Figure 1). The current, depending upon the resistance of film, S,is converted to voltage by variable reCARRIER GAS OUTLET ZINC
CAQRIER
GAS
INLET
OXIDE F I L M
n
i-d
BOROSILICATE GLASS TUBE
ZINC
T-ERMOCOUPLE
OXIDE FILM
I
e RECORDER
Figure 1.
Experimental arrangement
sistance, R,, which is recorded by an electronic recorder without amplification. The input voltage of the recorder is adjustable a t desired sensitivity by the variable resistance, R,. The use of zinc oxide films is shown as an example. The thin films of zinc oxide TI ere prepared by vacuum evaporation of metallic zinc onto the borosilicate glass plate and by subsequent oxidation in air a t G O " C. for 10 hours. Their thicknesses are of the order from 20 to 1000 A. The borosilicate glass plates as support have a dimension of 20 x 4 sq. mm., and the electrical contacts are made by platinizing pretreatment of both ends. The thin film is set in a borosilicate glass tube, of 8-mm. i d . , which is connected with the outlet of the gas chromatograph. When the carrier gas (nitrogen) is passed through the borosilicate glass tube, the conductivity of thin film keeps a fixed value in the steady gas flow. However, addition of a small amount of other gas to the carrier gas causes a change in conductivity of the film. The increases are brought about by the adsorption of carbon dioxide, benzene, ethyl alcohol, etc., which behave as an electron donor to the zinc oxide of a n-type semiconductor. On the other hand, the adsorption of oxygen decreases the conductivity and behaves as an electron acceptor. With succesive desorption of gases, the conductivity is restored t o its original steady values. At room temperature or 100" C., the change in conductivity is too small t o detect, and the remarkable, peakshaped change may appear a t higher temperature than about 200" C.
The results of detection with the present method for several common compounds are shown in Table I. In this case, the zinc oxide film used has the resistance of 1 megohm at room temperature, and the flow rate of carrier gas, nitrogen, ranges from 60 to 80 ml./min. And these measurements were carried out without packing the 1-meter gas chromatographic column. The results of a comparison between this method and that using a thermal conductivity cell are shown in Figures 2 and 3. The curves in these figures were obtained on a 4-meter column, 4 mm. i.d., packed with Celite 545 impregnated with 30 grams of dioctyl phthalate per 70 grams of the Celite. The column temperatures in Figures
A
0
.5 1.0
-.Table I.
Species Toluene Toluene Benzene Ethyl ether Ethyl ether Ethyl alcohol Propane Carbon dioxide Carbon dioxide 1502
0,001 0.001 0,001 0.001 0.002 0.001 1.o
1.0 5.0
ANALYTICAL CHEMISTRY
E
Results of the Present Method
Variable Temp. of Quantity, resistance, detector, ml. R,, ohm C. 100 100 100 100 100 10 500 100
100
Peakheight, mv .
411
0.98
403 411 406 432 380
1.65 1.6 3.6 2.97 1.25 1.08 5.0
404
432 429
I .o
-
Peakwidth,
Peakarea, ml. X mv. per
30
mg. ( 1 ) 270 287 189 177
7
286 480
min. 30 7.5 3 3
10 1.5
5
203
17 27
0.5
I 1
3
2
4
6
8
1
0
T i m e , minutes Figure 2. Results of commercial p r o pane gas using thermal conductivity cell (A) and zinc oxide film (6).
1
R. were 485" C. and 10 ohms in B of Figure 2, and 490' C. and 30 ohms in B of Figure 3, respectively. The results of Table I and Figures A
HZ
I
2
3
5
4
Time, minutes
Figure 3. Results of a gas 50% mixture (5% HP
1.5
=
C3HS
+
45%
+
n-Ch)
using thermal conductivity cell ( A ) and zinc oxide film (6)
in
1.0
>
..E
2.5
2 and 3 show that the peak widths of the present method are generally large in comparison with those using the thermal conductivity cell. This is because the adsorption and desorption of gases require a relatively long time, depending upon the species of gases, the temperature, and thickness of the zinc oxide film. Selection of proper conditions would improve the results. The above results, however, show that the reproducibility of measurement is sufficient. The experimental and theoretical investigation of the relation between the height or area of peak and the sample quantity is being continued. The sensitivity for the above detection is 50 1 p.p.m. and about a hundred times that of the thermal conductivity method. The sensitivity will be increased still higher by amplification.
-
LITERATURE CITED
c:
(1) Dimbat, M., Porter, P. E., Stross, F. H., ANAL.CHEM.28,290 (1956).
, 1
2
3
I
4
5
6
Time, minutes
2 and 3 were 41' C. and 40' C., respectively. The quantity of sample used was 1 milliliter in each case. The flow rates of the carrier gas, nitrogen, were 17 ml./min. in A and B of Figure 2 and 26 ml./min. in A
and 20 ml./min. in B of Figure 3. The cell currents of the thermal conductivity cell were 64 ma. in A of Figure 2 and 82 ma. in A of Figure 3. The temperature of zinc oxide film and the value of variable resistance
TETSURO SEIYAMA Am0 KATO KIYOSHI FUJIISHI MASANORI NAGATANI Department of Applied Chemistry Faculty of Engineering Kyushu University Hakozaki-machi Fukuoka-shi, Japan
RECEIVEDfor review May 3, Accepted August 10, 1962.
1962.
Determination of Benzoic Acid in Phthalic Anhydride by Gas Liquid Chromatography Sir: R e have developed a method of analysis for benzoic acid in phthalic anhydride. This undesirable by-product has been noted in varying amounts in crude phthalic anhydride (PAA) produced both by fluid- and fixed-bed catalysis. It represents B loss of yield, and in refined PAA, when present in significant amounts, it exerts a "chain stopping action" on the esterification process employed in alkyd resin manufacture. This action results in lower molecular weight polymers and in a sweetish, objectionable odor. The determination of small amounts of benzoic acid in the anhydride by titrimetric methods is complicated by the presence of maleic and phthalic acids. Quantitative chloroform extrac-
tions, and subsequent titration with standard alkali are successful only in amounts greater than 2% (3). Spectrophotometric techniques are also successful but possess 'inherent difficulties
(4)*
Gas liquid chromatography (GLC) does not have these limitations and furnishes a quick, convenient method for carrying out the determination of benzoic acid in crude and refined phthalic anhydride in the presence of other impurities. Because the retention times differ significantly (Figures 1 and 2), this technique permits the simultaneous estimation of other impurities such as toluic acid, l,Cnaphthoquinone, phthalide, and unoxidized naphthalene (or o-xylene) . Hence the method can be used for
checking plant reactor effluent streams for benzoic acid, and for following its concentration during condensing, treating, and refining cycles, as well as for analyzing the finished PAA product. The method is rapid and well suited for a control procedure. Principle of the Method. T h e sample is dissolved in benzene, and the free acids are esterified by diazomethane t o yield the methyl esters and nitrogen. EXPERIMENTAL
Apparatus. Gas chromatograph, Wheelco Model 10, Rockford, Ill. Carrier gas, argon, 7.5 p.s.i. inlet pressure. Detector, @-ionization (Rs 226 source). Column: 8-foot, 6-mm. VOL. 34, NO. 1 1 , OCTOBER 1962
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