A Simple Method for the Calibration of Sensitive Gas Chroma tog ra phic Detectors SIR: The absolute calibration of very sensitive gas chromatographic detectors makes it necessary to prepare accurately measured samples having masses below 1 yg. In the case of volatile compounds this problem has been solved in the past by a variety of methods all aimed a t diluting the sample with large volumes of gas (3-5, 7 ) . If, however, the diluent gas is saturated with the vapor of a very dilute solution, microgram size samples of the solute may be obtained a t once in a convenient volume of gas. Previously this method has been used to obtain an unvarying sample (6), but not as a means of absolute calibration. At the very low concentrations involved it is theoretically probable that the partial pressure ( p J of the solute above the solution will obey Henry's law, p , = kc, where k is the Henry's law constant for the particular solvent and solute under consideration, and c is the concentration of the solute, (grams/1000 grams of solution). The absence of explicit data on the values of k has apparently prevented the use of this means of absolute calibration in the past. In fact, the data on the vapor pressure of solutions existing in the literature permits the calculation of k for many systems, particularly those involving water as a solvent. In this work the method has been tested by the calibration of an Argon Ionization Detector for ethanol and acetone. The partial vapor pressure-concentration data for the solute in aqueous ethanol (2) and acetone (1) systems were plotted and were of relatively slight curvature below 20% solute, so that k could be found to within *3% in both cases,
calibration was checked by the conventional procedure of applying a liquid ethanol-acetone mixture of known composition to the chromatograph using a capillary micropipet. Solutions of 1 to 2y0 of the mixture were prepared to obtain the necessary small sample in a total volume of about 0.1 11. %-Heptane was used as the solvent as it contained no impurities likely to obscure the ethanol or acetone chromatogram peaks. The calibration results can also be checked roughly by using them to calculate the volume of the sample delivered by the micropipet. This was estimated independently by weighing the pipet filled and empty on an Oertling microbalance. Successive weighings of the micropipet used (constructed from a 3.5-mm. length of steel hypodermic needle tubing) showed that a sample of n-heptane evaporated from it a t about 1% per second a t 22' C. Therefore, using a standard procedure taking 10 to 15 seconds, about 10 to 15% of the initial sample is lost. Thus the sample volume calculated from a chromatogram, the effective sample volume, is about 0.875 of the volume of the micropipet. However, the ratio of ethanol to acetone as measured from the chromatogram remained well within 10% of the w e i g h t of 0.5 I
calculated value, even when as much as 30% of the sample had evaporated, so that the relative calibration described earlier is still valid. The loss by evaporation and the consequent possible error can be reduced markedly by using a longer capillary of finer bore-e.g. a glass capillary of 0.11 pl. volume, 9.0 mm. long, lost only O.3y0of an nheptane sample per second a t 22' C. RESULTS
Figure 1 presents graphically the results of both absolute and relative calibrations. In the former, the systematic error in the calculated sample weights due to errors in k, the Henry's law constant, and the standard error of the mean peak areas are both =t3Y0. The slight nonlinearity is to be expected a t the high detector voltage used to obtain the desired sensitivity (1250 volts applied through a 2000 megohms linearizing resistance to a 20-mc. Srw detector). In the latter confirmatory experiments, the quantity of acetone present in a sample was calculated from the chromatogram by the calibration and this, combined with the known mixture composition, gave the weight of ethanol equivalent to the ethanol peak.
ethanol I .o I
(pg.) I. 5
-
I
Ethanol/water k = 0.0839 mm. Hg gram-' kg., temp. = 25" C. Acetone/water k = 0.455 mm. Hg gram-' kg., temp. = 25" C. EXPERIMENTAL
Calibration, Argon was saturated with the vapor above aqueous solutions of ethanol (concentrations up to 1.7 grams per kg.) and acetone (concentrations up to 0.07 gram per kg.) held a t 25.0' f 0.1' C. Samples, of 4.96 f 0.02 ml. volume, were injected using a gas-tight syringe. It was necessary to use a diglycerol precolumn (8) to remove the water from the calibrating sample because of its marked auenching effect on the sensitivitv of the argon de&ctor (4). Confirmation of Calibration. The relative response of the detector to ethanol and acetone indicated by the 1 162
ANALYTICAL CHEMISTRY
Weight
Figure 1. 0
0 5
of a c e t o n e (pg.)
+
Calibrations for ethanol and acetone
Calibration points for ethanol Calibration points for acetone Relative colibration points for ethanol
The relative calibration points found in this way agree reasonably well with the absolute values. The mean of 18 estimates of the effective sample volume calculated from chromatograms was 0.095 & 0.005 pl. which agreed well with the value found by weighing, 0.100 =!= 0.005 pl., and thus confirmed the accuracy of the absolute calibrations. It is therefore considered practicable t o calibrate accurately the response of sensitive gas chromatographic systems by a combination of the absolute method described here and the relative method
already widely applied and used as a check in this work.
Thompson, R. J., Proc. of Bnd Sqinp. on Gas Chromatography, Amsterdam (1958) p. 165, Butt,erworths, London, 195s.
LITERATURE CITED
(1) Beare, W. G., McVicar, G. A., Ferguson, J. B., J . Phys. Chem. 34, 1310 (1930). (2) Butler, J. A. V., Thompson, D. W., Maclennan. W. H., J . Chem. SOC.1933, 674. (3) Desty, D. H., Geach, C. J., Goldup, A., Proc. of 3rd Symp. on Gas Chromatography, Edinburgh (1960), p. 46. Butterworths, London, 1960. (4) Loyelock,. J. E., Zbid., p. 16. (5) Primavesl, G. R., Oldham, G. F.,
( 6 ) Purnell, J. H., Ilept. of Physical
Chemistry, Cambridge, England, private communication, 1962. ( 7 ) Scott, R. R., Stannard, B. W., Chem. Zntl. (London) 1960, 1259. (8) . . Swoboda. P. A. T.. Zbid., P . 1262. and Proc. of 3rd Sump: on G& tography, Edinburgh (1960) p. 354,
Butterworths, London, 1960. M. G. BURNETT P. A. T. SWOBODA Low Temperature Research Station Cambridge, England
Titration of Aromatic N-Oxides in Acetic Anhydride with Perchloric Acid SIR: Wimer has reported that both amides (9) and sulfoxides (10) when dissolved in acetic anhydride can be titrated with perchloric acid in acetic acid. Following the completion of the present work i t was learned that Wimer had also titrated certain amine oxides (11) dissolved in acetic anhydride with perchloric acid in dioxane. Mackenzie and Winter (6-8) and Burton and Praill (1, 8) have presented considerable evidence that. a solution made from perchloric acid, acetic anhydride, and aceti: acid has CH~COZH‘, (CH3CO)zOH-’, and CH3CO+as the acidic species in equilibrium and that their acidities increase in the order listed. In this work eight aromatic N-oxides dissolved in a mixture of acetic anhydride and acetic acid have been titrated with perchloric acid in acetic acid. The results of the titration studies are summarized in Table I.
N-acetoxy tertiary amine perchlorates were isolated from many of the titrated solutions. These have been identified by elemental analysis and by conversion to the parent N-oxide when treated with sodium hydroxide. For example, N - acetoxy - 2 - methylquinolinium perchlorate, m.p. 152-3’ C., (Anal. Calcd. for C12H12N02: C, 47.77; H, 4.01; N, 4.64. Found: C, 47.76; H, 4.23; N, 4.52) yields 2-methylquinoline N-oxide dihydrate when reacted with sodium hydroxide. Based on the isolation of the Nacetoxy tertiary amine perchlorates the success of the titrations could be due to the reaction of either CH&O+ or (CH3C0)20H+ or both with an Noxide. It is postulated that the Noxides in the present study reacted predominately with protonated acetic anhydride as shown by the following equation.
Table 1. Titration of Aromatic N-oxides in Acetic Anhydride with Perchloric Acid
A’-oxide of Pyridine 2-Methvlpyridine 3-Methylpyridine 4-Methylpyridine 2,BDimethylDvridine &&Loline Iaoquboline 2-Methylquinoline dihydrate 2-Carboxypyridine 4-Nitro-2methylpyridine
Meall Calcd. Observed Neut. Neut. VariEquiv. Equiv. ance 95.1 95.5 0.1 109.1
109.6
0.1
109.1
109.5
0.2
109.1
109.2
0.2
123.1 145.2 145.2
123.3
146.3
145.3
0.3 1.3 0.2
195.2
195.7
0.5
139.1 Unsucc. 154.1 Unsucc.
d r-coca, Jaffe (5) has found that pyridine Noxides substituted with electron-withdrawing groups are weaker bases than are pyridine N-oxide and its homologs. Wimer (11) reports that N-oxides of 4 nitro- and 4-cyano-pyridine can be titrated in a system in which CHaCO+ is likely to be present, whereas in the present work N-oxides of 2-methyl-4 nitropyridine and 2-carboxypyridine could not be titrated. If CHIGO+ is a
,oo/ 700
200
y.
I
0
2
4
6
0
10 12
14
16 18
2 0 22 24 26
ML. O F O.SN ncio4
Figure 1 . Tiiration of 3-picoline-Noxide ( 1 ) and isoquinoline-N-oxide (2)
stronger acid than (CH3COz)zOH+, then i t is reasonable that the difference between Wimer’s results and our results is due to a predominance of CH3CO+ as the acidic species in his titrations, whereas (CH8C0)20H+predominated as the acidic species in our titrations. Also, it is possible for some aromatic Noxides to react with xetic anhydride without the presercc I acid (4). Additional subs4 uted pyridine N oxides and somc aliphatic N-oxides will be studied L~ an effort to learn more about ’ le generality of the titration method. EXPERIMENTAL
The N-oxides listed in Table I were distilled or recrystallized prior to dissolving them in 10 to 20 ml. of acetic acid. The samples of N-oxide usually weighed between 0.2 and 1.5 grams. Generally 10 to 25 ml. of acetic anhydride was added as well as 2 to 3 drops of methyl violet solution, and the resulting solution was titrated immediately with 0.1 to 0.5M perchloric acid in acetic acid (3). A Beckman Model H2 glass electrode pH meter with a glass electrode and a calomel electrode was used. For convenience the titrant VOL. 34, NO. 9, AUGUST 1962
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