“Krypton Triode” as Selective Detector for Gas Chromatography SIR: The argon triode is well known as a sensitive detector in gas chromatography. It will not detect the permanent gases or methane because their ionization potentials are greater than the excitation potential of argon. Some workers have attempted to make a universal detector by using helium as the carrier gas because of its higher (19.8 ev us. 11.6 ev for argon) excitation potential (1-4). These attempts were successful although highly purified helium was found necessary in several cases. We have taken the opposite course by using krypton (excitation potential 10 ev) as a carrier gas in a standard argon triode; the resulting “krypton triode” shows potential uses as a selective detector for gas chromatography. The basic operation of the argon triode has already been well described (5,6)and only an outline is given here. The operation of the detection process should be the same for all the rare gases. In the detection cell, accelerated beta particles excite the rate gas atoms into neutral and relatively long lived metastable states. When an excited rare gas atom strikes a sample molecule it can ionize that molecule. Presence of a number of positive ions then causes the cell to conduct, allowing an external current to flow. This external current is amplified and applied to a strip chart recorder output. EXPERIMENTAL
Materials and Equipment. A Microtek 2500R chromatograph equipped with a standard Microtek argon triode detector was used. The column oven was modified to allow injection directly onto the head of the column. The column was a 4-foot by ‘/r-inch coiled glass tubing packed with 2% SE 30 silicone rubber on 70/80Chromosorb-G AW-DMCS. Ultrahigh purity argon and research grade krypton were supplied by Matheson Co. Compounds analyzed were API standards or Phillips Petroleum Co. research or pure grade hydrocarbons. Methyl ethyl sulfide was obtained from Eastman Kodak Chemicals, 1,s-hexadiene from Columbia Chemicals, and 1,3-cyclohexadiene from Aldrich Chemical c o. Column head injection, glass columns, and siliconized supports were all used to reduce losses of the subnanogram quantities of compounds which were required. Operating Conditions. The column was operated at the optimum temperature for each component to emerge rapidly after injection and to be sharp and well defined for good measurement. The outlet block and flame detector were operated at 170’ C, and the carrier gas flow was approximately 40 cc per min. Polarizing voltage was set at 1000 volts d.c.
(1) R. Berry, Nature, 188,579 (1960). (2) A. Karmen, L. Guiffride, and R. L. Bowman, Ibid.,191, 906 (1961). (3) W. A. Wiseman, Ibid.,190, 1187 (1961). (4) Ibid.,192,906 (1961). ( 5 ) A. B. Littlewood, “Gas Chromatography,” 1st ed., p. 267, Academic Press, New York, 1962. (6) J. E. Lovelock, ANAL.CHEM.,33, 162 (1961). 640
ANALYTICAL CHEMISTRY
Procedure. Solutions containing about 80 mg per liter were prepared for each compound. Hydrocarbon solvents were chosen to be higher boiling than the compound so that they would emerge last. Hence, the compound was not
Table I. Argon and Krypton Sensitivities Signal-to-noise ratio at mass Sensitivity flow rate of ratio, 50 mol/sa argon to Ionization Compound Argon Krypton krypton potential“ n-Pentane 73 1.3 56 10.6 n-Hexane 45 1.6 28 10.4 n-Heptane 66 2.4 28 10.4 22 10.2 n-Octane 57 2.6 61 2.9 21 10.2 n-Nonane n-Decane 5s 2.6 21 10.2 Benzene 33 1.6 9.4 53 Toluene 6.5 37 1.8 9.2 73 34 2.1 8.9 pXylene 1,3-Pentadiene 63 45 1.4 9.7 1,5-Hexadiene 59 36 1.6 9.5 1,3-Cyclohexadiene ... 80 49 1.6 n-Butyl mercaptan 49 10 4.9 ... Methyl ethyl sulfide 54 29 1.9 ... Diethyl disulfide 61 33 1.8 ... Naphthalene 83 44 1.9 8.3 a From “Ion Production by Electron Impact,” R. I. Reed, Academic Press, New York, 1962.
Table II. Argon and Krypton Signal-to-noise ratio at mass flow rate of 50 mol/sec Argon Krypton Compound 1.2 118 Cyclopentane 1.8 97 n-Pentane 2.7 89 Methylcyclopentane 4.1 137 n-Octane 7.5 128 Cyclohexane 132 11.0 Methylcyclohexane trans-1,ZDimethyl93 12 cyclohexane 116 45 Benzene 154 44 To1uen e 44 137 p-Xylene 11s 37 Ethylbenzene 63 13 Cyclopentene 149 21 Cyclohexene 104 20 I-Octene 88 12 trans-4-Octene 2,2,4Trimethyl130 11 pentane
Sensitivities Sensitivity ratio, argon to Ionization krypton potential0 98 11.1 54 33 33
17 12 7.8 2.5 3.5
3.1 3.1 4.8 7.1 5.2 7.3 12
10.6
...
10.2 10.3
...
...
9.4 9.2 8.9 9.1 9.3 9.2 9.5
... ...
0 From “Ion Production by Electron Impact,” R. I. Reed, Academic Press, New York, 1962.
detected immediately after the equilibrium of the cell had been upset by the mass of solvent vapor. All the preliminary experiments to determine the operating conditions were carried out with argon carrier gas. These conditions included : operating voltage, sample size for linear response, solvent for each compound, and optimum column temperature. Two series of runs were made, each consuming a 50-liter cylinder of krypton. ‘The first included a variety of hydrocarbons and three sulfur compounds. The second included a wider range of hydrocarbon types. A run was made with the same compounds using a’rgon as the carrier gas. RESULTS AND DISCUSSION
The results for each series of runs are shown in Tables I and 11. The first two columns of data are the sensitivities for each compound with argon md krypton. They are a measure of the signal-to-noise ratio obtained for each compound at a fixed mass flow rate of 50 mol/sec. The third column is the argon/ krypton sensitivity ratio. Results in Table I show a distinctly poorer sensitivity for the n-paraffins in krypton compared with the other compounds. It appears fro n these data that a distinct break point occurs between compounds with ionization potentials above and below 10.0. Those below 10.0 ev are detected at the same magnitude as in argon. Those above 10.0ev show much less sensitivity in krypton compared to argon. The second series of results (Table 11) confirms this same break point at an ionization potential of about 10.0 ev. The data for methyl cyclohexane, trans-l,2-dimethylcyclohexane,
and 2,2,4-trimethylpentane indicate that the change in sensitivities is not abrupt since these compounds, which would be expected to have ionization potentials close to 10.0 ev. show intermediate sensitivities. Krypton in an argon triode gives a detector for gas chromatography that is selective for compounds having ionization potentials below about 10.0 ev. The krypton detector responds readily to simple aromatics, olefins, diolefins, mercaptans, and sulfides and is therefore a valuable tool for detecting and measuring small amounts of such materials in otherwise saturated samples. Examples are: odorants in natural gas and liquefied petroleum gas (LPG), aromatics in paraffinic solvents, and olefins and aromatics in waxes. It can also be used as a dual detector system for characterizing individual components. CHARLES D. PEARSON ROBERT S. SILAS Phillips Petroleum Co Research & Development Department Research Division Analysis Branch Bartlesville, Okla. RECEIVED for review October 27, 1966. Accepted January 30, 1967. Paper presented at the 1966 Pittsburgh Conference on Analytical Chemistry and Spectroscopy (paper No. 94), February 20-26,1966.
Re-Evaluationof ChronopotentiometricData for Adsorption of Riboflavin SIR: The calculations of Tatwawadi and Bard ( I ) for the extent of electroactive adso ption of riboflavin onto mercury electrodes are generally incorrect. The unweighted least-squares analysis employed by these authors is based on the fundamental assumptions that the values of the ordinate are free from error and that the values of the abscissa are associated with equal absolute errors. These conditions are not satisfied in the present case because bot i the ordinate and abscissa are functions of the same measured variables. It is a quite general conclusion that the parameters estimated according to a correctly rormulated least-squares analysis are insensitive to mathematical reformulations of the model (2) because mathematical reformulation alters not only the quantities plotted as ordinatc: and abscissa but it also alters the function which weights 1he data in just such a fashion that there is no net effect. Thus, the observation of Tatwawadi and Bard that mathematizal reformulation of their models led to apparently different values for the parameters D and nFr is a clear indication that their data analyses are in error. We can illustrate this point by considering the analysis according to the mathematically equivalent models 3 and 3*.
(1) S . V. Tatwawadi and A J. Bard, ANAL.CHEM., 36, 2 (1964). (2) W. E. Deming, “The Statistical Adjustment of Data,” Wiley, New York, 1943; RepLblished by Dover, New York, 1964, p. 156.
Model 3:
ir Model 3*: jr1/2
- nFr
- br1I2= 0
- nFr/r1/2 - b
= 0
n F r represents the extent of adsorption expressed as charge per cm2 and the symbol b denotes the chronopotentiometric constant, n F d s C / 2 . The other symbols have their usual electrochemical significance ( I ) . n F r and b may be estimated from the chronopotentiometric data by solving the following matrix equation (3-5). Model 3:
Model 3*:
(3) Zbid.,Chap. X . (4) W. C. Hamilton, “Statistics in Physical Science,” Chap. IV, Ronald Press, New York, 1964. ( 5 ) P. J. Lingane, ANAL.CHEM.,39, 485 (1967). VOL. 39, NO. 4, APRIL 1967
541
