Steam Carrier Gas-Solid Chromatography of Organic Samples with Thermal Conductivity Detector with High Temperature Precolumn Akira Nonaka Institute for Optical Research, Kyoiku University, 22- 17, Hyakunincho-3, Shinjuku-ku, Tokyo, Japan
Steam carrier gas-solid chromatography (SSC) can be carried out wlth a thermal conductivity detector (TCD) for organic samples. The sensitivity of the TCD increases n-fold ( n I 70) times by using a hlgh temperature precolumn (HTPC) attached to the entrance of the TCD. The HTPC causes the reaction of the organic samples with the carrier steam and generates hydrogen and light gases which are more sensitive to the TCD wlth steam. An open tubular quartz column suffices for the HTPC in most cases. Quantitative analysis for organic samples In this system is llmited to lo-' g samples, although the detection limit Is reduced to lo-' g by mixing 1 YO ammonia in the steam carrier gas.
Steam carrier gas-solid chromatography (SSC) has been reported to be useful in analyzing polar and high-boiling samples, especially in detecting dilute aqueous samples (1-4). In the SSC, which has been practiced only with the hydrogen flame ionization detector, the thermal conductivity detector (TCD) can also be used effectively. Steam carrier gas is like other heavy carrier gases (5-7) in that the outputs from the TCD are not always regular. The signals out of the TCD are sometimes positive as usual, but there are also many cases of negative signals. Moreover, the response of the TCD is not only very small but also sometimes anomalous when the samples have a thermal conductivity similar to that of water vapor and/or the sample size is comparatively small. In such cases, precise gas-chromatographic analyses become difficult. An attempt has been made to overcome the shortcoming in the use of TCD with steam carrier gas by attaching a high temperature precolumn (HTPC) between the TCD and the analytical column. The H T P C makes the signals from the TCD significantly large and the polarity of the signals constant (always minus) for ordinary organic substances. A few examples in which a reaction, or pyrolysis precolumn was employed with the TCD have been reported (8-10). In this study, however, it is shown t h a t not a packed column but an open column is preferred as the precolumn in many cases and that the increase in TCD-response is principally due to the formation of hydrogen in the reaction of organic samples with the steam carrier gas. T h e use of the HTPC not only makes the quantitative analyses of most organic materials very easy in SSC with TCD, but also serves to protect the TCD from being contaminated by sorption of, or corrosion by, the organic samples.
EXPERIMENTAL Apparatus. An Ohkura Gas Chromatograph Model 701 was reconstructed to meet the steam carrier gas chromatograph, in which the sensing cell of the TCD was connected with reference cell in series via a buffer column (Teflon tube, 1 m x 1 mm i.d., l00/150 mesh glass beads), a sample inlet, an analytical column, and the high temperature precolumn. The analytical column was packed with noncoated adsorbents such as activated alumina, diatoma-
ceous firebrick, phosphoric acid-modified firebrick, etc., according t o the kind of the samples, that is, hydrocarbons, alcohols, carboxylic acids, etc., respectively (1-4). Another analytical column (stainless steel tube 5 m X 2 mm i.d., 40/60 mesh active carbon) was prepared for the component analysis of the decomposition gases in the HTPC. Various kinds of HTPC, open quartz tube, platinum tube with catalysts, etc., were prepared. The electronic furnace for the HTPC was able to heat up to 1000 'C. The steam carrier gas was generated by introducing a constant flow rate (10 to 20 mg/min) of water into the head of another buffer column (stainless steel tube, 1 m X 2 mm i.d., 100 mesh firebrick powder 170 "C) with an Ohkura SSC-1 liquid pump. The buffer column was connected with the reference cell of the TCD. Both tungsten and platinum-rhodium filaments (60Q) were examined with steam carrier gas in the TCD. The former were two times more sensitive than the latter and in practice, could withstand the steam carrier gas for a fairly long time when the cell wall temperature was 170 O C and the dc supply was 150 mA.
RESULTS AND DISCUSSION Sensitivity of TCD with Steam Carrier Gas. T h e TCD used with steam carrier gas was less sensitive than t h a t with helium or hydrogen carrier gas for ordinary organic materials. The outputs from the TCD took both polarities, although it was negative for most low molecular weight organic substances such as low-boiling hydrocarbons (up to about 200 "C of b.p.) other than organic halides. Table I shows the relative weight response found in the TCD used in this study for various organic materials with steam carrier gas. In the cases where the thermal conductivity of steam was nearly equal to that of the sample and/or the quantity of the sample was comparatively small, the signals from the TCD sometimes had a "W" shape which were just like the ones found in the cases when the hydrogen was detected by a TCD with helium carrier gas (11) or methane with nitrogen carrier gas ( 5 ) . Use of High Temperature Precolumn. The HTPC was placed in front of the sensing side of the TCD and maintained a t 800 to 1000 "C so that most organic materials would decompose (or react with water vapor) to form a mixture of lower molecular weight organic or inorganic substances which might be more easily sensed by the TCD with steam carrier gas. When a quartz open tube 5 to 10 cm long and 2 to 3 mm i.d. was used as the HTPC, for almost all organic samples the outputs from the TCD became n-fold (n I70) times larger in peak height and showed a constant polarity (usually negative). Table I1 presents the approximate magnification in the peak height by the use of H T P C for various organic compounds (column 2) and the peak height magnitude relative to the case where helium carrier gas was used without the H T P C (column 3). Figure 1 shows a typical chromatogram obtained in the SSC-TCD-HTPC system, in which formic and other low molecular weight fatty acids in a dilute aqueous solution are easily eluted having a fairly large peak height without any influence of the sample water both on detection and full separation. ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
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mln
4
2
0
Figure 1. Chromatogram of an aqueous solution of formic acid ( I ) , acetic acid (2),and propionic acid (3) Column, Chromosorb P-AW, 30160 mesh, baked after addition of 3% H3P04. noncoated. 2.4 m X 2 mm i.d., 130 OC; carrier gas, steam, 13 mg/min; high temperature precolumn. open quartz tube, 70 mm X 2.5 mm i d., 900 OC; detector, TCD: sample size, 1 pl of 0.5% aqueous solution
T h e increase in the TCD-response was principally due t o the formation of hydrogen in the HTPC. This was ascertained by the gas chromatographic analysis (active carbon column and the TCD with steam carrier gas) of the mixture gas produced in the H T P C which had been attached directly to the sample inlet. Figure 2 shows a typical chromatogram in the case of cyclohexane, where the hydrogen peak is larger than the peaks of carbon monoxide, methane, carbon dioxide, etc., although the peak areas are not always in proportion t o the volumes of the products in the HTPC. Incidentally, the chromatogram is an interesting example of SSC showing that light gases, as well as high-boiling materials, existing in water or water vapor can be analyzed with a stationary solid column. Construction of H T P C . A number of conventional catalysts such as platinum asbestos, silica-alumina, etc., which could hasten the decomposing process were tried as the packing of the HTPC, but none of them were significantly more effective than the open tubular quartz column. It was found, moreover, that the response of the T C D t o ordinary organics decreased abruptly a t a small sample size when any catalytic materials were used in the construction of the HTPC. This is due to the reaction with or adsorption of the hydrogen formed in the H T P C by such materials. Figure 3 is a graph of peak area of T C D with the H T P C s vs. amount of hydrogen. In the case of the H T P C with platinum asbestos (curves I11 and IV), especially a t a high temperature, the response to hydrogen was significantly reduced and failed a t a fairly large amount of hydrogen. Even in the use of a n open quartz tube, the reduction of TCDresponse was experienced a t a high temperature of 1000 "C, although a t a low temperature such irregularity was not found. Catalytic materials, which might adsorb or consume hydrogen, should not be used as packing or tubing for the HTPC. Limit of Detection. Although the signals from the T C D increased n-fold ( n = 16-70) times by using the H T P C for ordinary organic materials, the detection limit for these samples was about g, since even in the use of a n open quartz tube as the HTPC, there is a nonsensitive region for less than about g hydrogen, as seen in Figure 3 (curve 11). The limit for the quantitative analysis, however, was improved to about g by mixing about 1%ammonia in the steam carrier gas. This seems to be the result of the formation of hydrogen by partial decomposition of ammonia in the HTPC. Effect of the H T P C on Separation. T h e use of the 384
min 6 4
2 0
Figure 2. Chromatogram of a gas mixture produced in the high temperature precolumn with steam carrier gas Sample, cyclohexane; high temperature precoiumn, open quartz tube, 70 mm X 2.5 mm i.d. 1000 OC; analytical column, activated carbon, 40/60 mesh, 5 m X 2 mm i.d., 160 O C : carrier gas, steam, 9.4 mg/min: detector, TCD
Table I. Relative Weight Response of the TCD Used with Steam Carrier Gas0 R e la t ive response
Compound
Cyclohexane -100 n-Nonane -4 0 Benzene -4 2 Carbon tetrachloride +loo Dichlorobenzene +43 Methyl alcohol -40 Ethyl alcohol -58 n-Butyl alcohol -3 3 n-Hexyl alcohol -1 3 n-Propylamine -61 Tri-n-butylamine -1 2 n-Octylamine -0 Formic acid -30 Propionic acid -60 Acetone -1 0 a TCD thermostat temperature, 170°C; TCD filament, tungsten ( 6 0 n ) ; TCD filament current, 150 mA.
Table 11. Peak Height Magnification by Using the HTPC'
ANALYTICAL CHEMISTRY, VOL. 48, NO. 2, FEBRUARY 1976
Magnification b y HTPC
Sample
a
with steam carrier gas
n-Hexane Benzene m-Xylene Methyl alcohol Ethyl alcohol n-Butyl alcohol Acetone n-Propylamine Formic acid Propionic acid Open tubular quartz column,
14
16 40 50
40 37
Ratio t o the case of He carrier gas without HTPC
6 2.6
... 5 10
70
7 6
16 33
3 5
14 ... 7 cm x 2 mm i.d., 1000°C.
"Y
H T P C never affected the chromatographic separation, if the HTPC was an open tubular quartz column 7 cm long and 2 mm in i d . , was maintained a t about 1000 "C, and the sample size was less than about g for ordinary organics. For a sample size of more than 2 X g, however, the eluted peaks a t the same HTPC condition were distorted and widened a little, and the separation was somewhat reduced
Y ..2
B
0 x
0
'OI 20
t
/
r 0
a
100
LITERATURE CITED (1) (2) (3) (4) (5)
50'
(6) (7) 0
200
400
Hz,
600
(8) (9) (10)
800
p t ( 2s.c)
Figure 3. Plots of peak area of the TCD with high temperature precolumn vs. amount of hydrogen
(11)
Curves I and 11, open quartz tube as the high temperature precolumn at 190 "C and 1000 "C, respectively; curves Ill and IV, quartz tube packed with platinum asbestos at 190 "C and 1000 OC, respectively
A. Nonaka, Anal. Chem., 44, 271 (1972). A. Nonaka, Ana/. Chem., 45, 483 (1973). A. Nonaka, BunsekiKiki. 11, 236 (1973). A. Nonaka, Adv. Chrornatogr., 12, 223 (1975). J. G. Keppler, G. Dijkstra, and J. A. Schols, in "Vapour Phase Chromatography", Proceedings of the Second Symposium, London, May 1956, D. H. Desty. Ed.. Academic Press, New York, N.Y., p 222. L. J. Schmauch and R. A. Dinerstein, Anal. Chem., 32, 343 (1960). A. B. Littlewood, "Gas Chromatography", 2nd ed.. Academic Press, New York, N.Y., 1970, p 356. W. R. Moor and H. R. Ward, J. Am. Chem. Soc., 80, 2909 (1958). A. E. Martin and J. Smart, Nature (London), 175, 422 (1955). I. R. Hunter, V. H. Ortegren, and J. W. Pence, Anal. Chem., 32, 682 (1960). J. J. Madison, Anal. Chem., 30, 1859 (1958).
RECEIVEDfor review May 5 , 1975. Accepted October 20, 1975.
Gas Chromatographic Determination of Free Parent Acids in Cyclic Anhydrides L. W. Haas* and R. J. DuBois Hercules Incorporated, Bacchus Works, Magna, Utah 84044
A gas chromatographic method was developed for measuring the free parent acid in the presence of the cyclic carboxylic anhydride. Prior to chromatographic analysis, the anhydride in the sample was pre-reacted with morpholine. The free acid was then converted to a silyl derivative with BSA. The samples were analyzed on a 6-ft X %-inch SS column packed with 5 % SE52 on Gas Chrom Q 60180 mesh temperature programmed from 125-250 OC at 10 OC1min. A constant helium flow of 60 cm3/min. and a glass lined inlet maintained at 250 OC were also used. Interferenc.es from organic or mineral acids or basic catalysts were avoided. The procedure was demonstrated quantitatively for four anhydrides, although applicable to many others. Relative standard deviations of 1 YO and 3 YO were found at the 20 % and 1 % free acid levels, respectively.
Cyclic anhydrides are frequently employed as curing agents for epoxy resins. The level of free acid present can have an important effect on final properties of the cured resin. Thus, an accurate and sensitive quantitative measure of the free acid is necessary for acceptance testing of the anhydride, for establishing shelf-life of the curative, and especially for optimizing new epoxy resin formulations. Comprehensive reviews of the various methods for determining the parent acid in the presence of the correspond-
ing carboxylic acid anhydride are available ( I , 2). The most frequently used procedures all employ acid-base titrations in one form or another. Indirect methods are available (3, 4 ) based on measurements of the difference between the anhydride content and the total acid after hydrolysis. These methods suffer from a lack of precision particularly a t low acid concentrations. Lucchesi ( 5 ) reported a means for obtaining both the anhydride and the acid content from a single titration and a simple calculation. However, this approach assumed no other components in the system. Direct methods which do not require subtracting the anhydride contribution are much more suitable. A direct potentiometric titration using Ai-ethylpiperidine or tri-n -propylamine in dry acetone was developed by Siggia ana Floram0 (6). With this procedure, the free acid is titrated directly without interferences from the anhydride. Accurate results down to the 0.1% acid level have been obtained for maleic and phthalic acid in the presence of the anhydride. However, this method can only be used for acids having a pK of 3 or less, which limits its use to only a relatively few acid-anhydride systems. T o extend the usefulness of the method to weaker acids, an acidity enhancement technique was developed by Greenhow and Jones ( 7 ) . A series of acids with a pK > 3 were directly titrated in the presence of their anhydride in acrylonitrile solvent, barium perchlorate being used to en-
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