inary preparation of the sample that is necessary. A specific detector also makes it possible to analyze for a group of compounds having a common chemical functional group. For example, it is possible to detect alcohols by preparing labeled acetates with carbon-14-labeled acetic anhydride, analyzing the mixture by gas chromatography, and detecting the acetates with a radioactivity detector (6). I t is also possible to prepare chloroacetates of the alcohols and detect these using a detector sensitive to halogens ( 7 ) . Both of these procedures thus facilitate detecting compounds with a reactive functional group in the presence of large amounts of other compounds that do not have this group.
ACKNOWLEDGMENT
The author expresses his appreciation to Maurice Lofters for expert assistance in performing this work.
LITERATURE CITED
(1) Dean, J . A , , “Flame Photometry,”
( 7 ) Landowne, R., Lipsky, S.R., ANAL. CHEM.35, 532 (1963). (8) Lovelock, J. E., Ibid., 33, 162 (1961). (9) Pvlc\Villiam, I., Dewar, R. A,, ,Yature 181, 760 (1958). (IO) Monkman, J. L., DuBois, L., “Gas Chromatography,” Noebels, Wall, Brenner, eds., p. 333, Academic Press, Xew York. 1961. (11) Padley, P. J., Page, F. M., Sugden, T. XI., Trans. Faraday SOC.57, 1552 (1961).
p. 43, XcGraw-Hill, Kew York, 1960.
12) Goulden, R.. Goodwin. E. S..’ Davies. L., Analyst 88, 951 (1963). ( 3 ) Gunther, F. A , , Blinn, R. C., Ott, ~
D. E., ASAL. CHEM.34, 303 (1962). (4) Honma, XI., AKAL.CHEM.27, 1656 (19j5). (5) harmen, A,, Giuffrida, L., Sature 201, 1204 (1964). (6) Karmen, A., PIlcCaffrey, I., Kliman, B., Anal. Biochem. 6, 31 (1963).
RECEIVED for review February 14, 1964. Accepted May 4, 1964. 2nd International Symposium on A4dvances in Gas Chromatography, University of Houston, Houston, Texas, March 23-26, 1964. This study was supported in part by United States Public Health Service-National Institutes of Health Grants GM-11535-01 and 1-S.O. 1-FR-OJ145-01.
Performance and Characteristics of an Ultrasonic Gas Chromatograph Effluent Detector F. W. NOBLE, KENNETH ABEL, and P. W. COOK laboratory o f Technical Development, National Heart Institute, National Institutes of Health, Bethesda, Md. The theory and instrumentation for a detector based on the measurement of the velocity of ultrasound in column effluents are discussed. The characteristics of binary gas mixtures and the propagation of sound through these mixtures allow the quantitative prediction of response when hydrogen and, to a lesser extent, helium are used as carrier gases. The response ( a t constant mole fraction) i s directly proportional to molecular weight up to a molecular weight of about 400 for 4-mc. operation and i s linear from a mole fraction of about 1% to the minimum detectable sample which presently i s of the order of 1 O-I4 mole for molecular weight 100. The detector cells, with internal volumes of 5 to 50 PI., can b e used with packed or capillary columns, corrosive samples, and at tempera ures to 270” C. Factors affecting the attainment of high sensitivity levels are discussed and experimental verification of theory i s presented.
G
that measure, directly or indirectly, the velocity of sound in gas mixtures have been utilized for a t least 70 years. A brief review of the various applications of sound velocity measurement techniques as apiilied to specific gas analysis problems up to 1948 was included in a paper by Crouthamel and Diehl ( 2 ) . A number of other applications (6, 7 , 9-11, 13, 16) and one commercial instrument (Sational Instrument Laboratory, AS ANALYSIS METHODS
Washington, D. C.) have been developed since 1948. With the exception of the ultrasonic whistle system of Testerman and JlcLeod (16) and the phase change measuring system of Noble ( I I ) , these methods have not been applied to gas chromatographic effluent detection. Robinson’s patent ( I S ) specifies application to gas chromatography, but it is not clear whether or not the method was successfully applied. The methods of Testerman and hIcLeod and of Robinson indirectly determined sound velocity by measuring frequency changes. The beat frequency occurring between a whistle operated by the column effluent and a second whistle operated by a flow of pure carrier gas was measured in the first case while Robinson used two resonant cavities (one for pure carrier and one for column effluent) and measured the difference in resonant frequency. The method described briefly by ?;oble IS an eltension of a concept used by Lawley (9) and later by Kniazuk and Prediger ( 7 ) . With this method the frequency is maintained constant and the change in wavelength accompanying the velocity change is measured by determining the change in phase of the sine wave received by a transducer a t one end of a gas-filled tube as compared with the phase of the sine wave transmitted from a second transducer a t the other end of the tube. The first part of this paper presents the theoretical considerations involved in binary gas miyture analysis utilizing phase change measurements. Partic-
ularly it is concerned with factors affecting the use of this method as applied to gas chromatographic effluent detection. The second part of the paper is concerned with the development of suitable instrumentation t o perform this type of analysis a t high sensitivity levels and with experimental verification of theory. THEORY
The time required for a sound wave of velocity, V , to travel a distance, s, is
t = -S V The phase delay in electrical degrees corresponding to t is
where f is the frequency of the wave. For pure ideal gases a t low frequency
where M is the molecular weight of the gas, y is the ratio of specific heat a t constant pressure to the specific heat a t constant volume, R is the gas constant (8314 sq. meters gram mole-] OK.-’), and T is the absolute temperature. Combining Equations 2 and 3 we h a r e
VOL. 36, NO. 8, JULY 1964
1421
We noiv consider the effect of mising mole fraction, n, of a second gas having molecular weight SI2 and specific heat ratio y2 to the first gas of molecular weight Xl and specific heat ratio 71. We assume that both gases are ideal and that the gas miyture is homogeneous so that the molecular weights and specific heats are additive. The equivalent molecular weight, J f , , of the mixture is:
equivalent specific heat ratio will be, from Equations 6 and 9
Substituting Equation 10 into Equation 4 we have 360 s j 9 =
and the square root of the equivalent molecular weight for small values of the mole fraction will be, approximately
[RTIIZ
[y+
n
{l
x
The phase change, A q , caused by the addition of ga. 2 is then A 4 = 180 sf. \-lf1/RTyi11~2 n X .
(6) The equivalent ratio of specific heats, y e , of the mi\ture is 1
[2
+n
- 11
+-
-
Y * = 71 ___
1
[2 [:: 2 ‘I
1 n - 11 - _______ l + n
(7)
where C, is the molar specific heat a t constant pressure and C, is the molar specific heat at constant volume. When n[C,,’C,, - 11 5 0.1, then
Expanding Equation 8, neglecting the term in n2, and assuming small values for the mole fraction
(1
-
$1
(9)
The square root of the equivalent molecular weight divided by the
Table I.
(12)
- l)]
The gram specific heats, e,, rather than the mole specific heats, C,, are usually tabulated in the literature. The expression, using gram specific heats, equivalent to Equation 12 is
Equation 13 is valid when the specific heats a t the frequency of the sound wave are the same as the tabulated values determined by calorimetry. ht, high frequencies these tabulated values will generally be valid only for monatomic gases. This is because a t the higher ultrasonic frequencies, the interchange of translational and vibrational or rotational energy of the niolecules cannot keep pace with the rapid oscillations of pressure. This effect is discussed in detail in most reference works on ultrasonics (1, 3, I?‘) and need not be repeated here. I t is sufficient to indicate that, a t STP, the specific heat ratio will be a maximum of 1.67 for monatomic gases and will approach a value of 1.0 as the number of atonis in the molecule increases (at f = 0). Increasing the frequency results in a shift of the specific heat ratio toward the maximum value for y of 1.67. On the assumption that the relative
He
CHa
NS
coz
A
