The recommendations offered here embody, with some modification, those made a t the symposium held in London in 1956 (8). I n accordance with those recommendations it may be emphasized that the use of the standard solutes and stationary phases (which are now becoming available commercially) mentioned therein should contribute to the achievement of uniformity of results. Values for the tables and figures have been taken from the paper by Littlewood, Phillips, and Price (8) except for those for hydrocarbons in diisodecyl phthalate in Figure 3 which were taken from the paper by Porter, Deal, and Stross (9). ACKNOWLEDGMENT
This paper was written a t the request of the committee of the Gas Chromatog-
raphy Discussion Group, organized under the auspices of the Hydrocarbon Research Group, Institute of Petroleum. The authors form a subcommittee charged with formulating proposals for the collection of data and their publication in a readily usable form. The contribution of one of the authors (D.A.) to this paper forms part of the program of the Chemical Research Laboratory and publication is made by permission of the Director. The authors are indebted t o R. P. mi. Scott for advice and criticism. LITERATURE CITED
(1) Desty, D. H., ed., “Vapour Phase Chromatography,” p. 5, Academic Press,
Nev- York,. 1957. (2) Ibid., p. xi. (3) Herington, E. F. G., Analyst 81, 53 (1956). (4) Hoare, ill. R., Purnell, J. H., Trans. Faraday SOC.52, 222 (1956).
(5) James, A. T., Biochem. J . 52, 242 (1952). (6) James, A. T., Martin, 4. J. P., Ibid., 50, 679 (1952). eulemans, A. I. M., Kwantes, A,, (7!nK”Vapour Phase Chromatography,” ed. by D. H. Desty, p. 15, Academic Press, New York, 1957. (8) Littlewood, A. B., Phillips, C. S. G., Price, D. T., J . Chem. SOC. 1955 1480. (9) Porter, P. E., Deal, C. H., Stross, F. H., J . Am. Chem. SOC.78, 2999 (1956). (10) Purnell, J. H., in “Vapour Phase Chromatography,” ed. by D. H. Desty, p. 52, Academic Press, New York, 1957. (11) Thomson, G. W., Chent. Revs. 38, 1 (1946).
RECEIVED for review November 29, 1957. Accepted April 24, 1958. Division of Analytical Chemistry, Symposium on Advances in Gas Chromatography, 132nd Meeting, .4CS, New York, N. Y., September 1957.
Terms and Units in Gas Chromatography H. W. JOHNSON and F. H. STROSS, Shell Development Co.,Emeryville, Calif. ,This i s a report on terms and units in gas chromatography, sponsored b y a Study Group of Section L on Gas Chromatography of Research Division IV, American Society for Testing Materials Committee D-2 on Petroleum Products and Lubricants. This group consists o f R. 0. Clark, Gulf Research a n d Development Co., and F. H. Stross, Shell Development Co. The object of the report i s to make definitions of terms and units reflect the widest cross section of opinion practical.
T
significance of the confusion in the definition of terms and units in gas chromatography n-as recognized early in the work of one of the subordinate groups (Research Dirision IV) of ASTRI Committee D-2 on Petroleum Products and Lubricants. The scope or objective of this committee is “the promotion of knowledge of petroleum, of petroleum products (including products derived from petroleum), and of lubricants; and the recommendation of standards pertinent to these materials.” Because gas chromatographic methods are now being developed for specification purposes, clarification of gas chromatographic nomenclature by such a standardizing body appears to be a logical development. ASTM Committee D-2 is composed of producing, consuming, and general interest members embraring a wide cross section of many industries other than petroleum. A small [Torking group was HE
1586
ANALYTICAL CHEMISTRY
created to prepare definitions of terms and units peculiar to gas chromatography which represent the majority of expression of o j n i o n in this country. The following ‘ tabulation and discussion is the first report resulting from this effort. This compilation has been approved and adopted by its sponsoring group, Research Division IV. Recognizing that other individuals are also concerned with this general problem, it was thought advisable to give as complete coverage of the chemical profession as is practical. ANALYTICAL CHEhfISTRP mas considered most appropriate for this purpose, and the cooperation of its editor is gratefully acknowledged. It is hoped that publication of the report will achieve the objective which is intended: to make these definitions of terms and units reflect the widest cross section of opinion as is practical. Comments and recommendations are solicited and may be sent to F. H. Stross. TECHNIQUE
Gas Chromatography. All chromatographic methods in which the moving phase is a gas. Gas-Liquid Chromatography (GLC) , or more fully, gas-liquid partition chromatography. All gas chromatographic methods in which the fixed phase is a liquid, distributed on a solid support.
Gas-Solid Chromatography. All gas chromatographic methods in which the fixed phase is a n active solide.g., charcoal, molecular sieves, etc. NOTE. The ambiguous term “Vapor phase chromatography” is obsolete; it was decided a t the symposia of the American Chemical Society in Dallas and of the Institute of Petroleum in London on the subject, both in 1956, to recommend against the use of this term. APPARATUS
Sample Injector. The device by which a liquid or gaseous sample is introduced into the apparatus. The sample can be introduced directly into the carrier gas stream, or into a chamber temporarily isolated from the system by valves, which can be changed so as t o make a n instantaneous snitch of the gas stream through the chamber. The latter is a bypass injector . The sampling chamber may have a certain volume that is not part of the column; this sample injector volume, B , has a n influence on the shape and position of sample peak, and therefore on the efficiency of separation. The quantitatire relations, and correction for this effect, are discussed in the literature ( 9 ) . Column. SOLIDVOLUME is the volume occupied by the solid support or the active solid in the column. LIQVIDTOLCJIE, V L , is the volume
occupied by the liquid phase in the column, V L = J~’L/PL, where W L= the weight and p~ is the density of the liquid phase. INTERSTITIAL VOLUMEis the volume of the column not occupied by the solid support and liquid phase, or the active solid. It does not include any volume external to the column such as sample injector volume or detector dead volume. Detector. A device t h a t measures the change of composition of the effluent. A detector which measures instantaneous concentration is called a differential detector. An integral detector continuously measures the sample accumulated since the beginning of the analysis. The detector dead volume, 2, is the free space of the detector and also affects the shape and position of the sample peak. Its effect can be calculated and corrected for (Y). A further discussion of factors generally affecting detector response is to be published (IO). REAGENTS FOR GAS CHROMATOGRAPHY
Carrier Gas, eluent gas. Gas t h a t is used to transport and elute the sample as it passes through the column. Liquid Phase is a relatively nonvolatile liquid a t column operating temperature, which is used as a coating for the solid support and there serves t o dissolve the sample components. Separation depends on differences of volatility of the various components in the solution. Solid Support is normally a n inert porous solid $7-hichis covered with the liquid phase. Occasionally a n active solid is used as a solid support t o achieve special separation effects (3,6). The particle size range of the support should be specified, because it affects column efficiency and the pressure differential necessary to achieve a given flow rate. The inner wall of the column may also serve as the solid support (4). Active Solid is a porous solid capable of chromatographic separations by virtue of its adsorption activity as distinguished from the partitioning effect of a liquid phase. The particle size of a n active solid must be specified for the reasons cited above. Any chemical or physical pretreatment should also be described. CONDITIONS DURING DETERMINATION
Pressure. Inlet pressure, pi, absolute pressure a t inlet of column. Outlet pressure, p,, absolute pressure a t exit end of column. Flow Rate. Gas flow rate by volume, F,; this is usually measured at some point beyond the column a t ambient temperature and pressure,
It must be converted for the subsequent computations t o column temperature and outlet pressure. Temperature. The temperature of the column should be specified as well as the maximum deviation from this value. If the detector temperature is the same, it should so be stated; if not, it should be specified. REPORTING OF RESULTS
The following definitions apply directly to the chromatograms obtained by means of differential-type detectors or by differentiating the records obtained by means of integral detectors. Chromatogram is a plot of the detector response us. time or volume of carrier gas. Base line is t h a t portion of a chromatogram recorded when only carrier gas emerges from the column. Peak is the portion of the chromatogram recording the detector response while one particular sample component emerges from the column. The peak base is constructed by connecting a straight line between the extremities of the peak a t the points where no sample is passing through the detector. The area enclosed between the peak and the peak base is called the peak area and the distance from the peak maximum to the peak base measured parallel to the axis representing detector response is the peak height. The line parallel to the peak base, bisecting the peak height, and terminating a t the sides of the peak is called the peak width a t half height. The segment of peak base intercepted by tangents to the inflection points on either side of the peak is the peak width. Retention Time (uncorrected) t,, is the time required from start of the analysis (sample injection or admission to column if this is delayed) t o the maximum of the peak of the compound under consideration. Retention Volume (uncorrected), V,, is the volume of gas, measured a t column outlet pressure and temperature, required t o sweep the compound under consideration from the sample injector t o the detector. It is related to flow rate by the equation V R = tRFr.
Retention Volume (corrected), l’RO, is V R corrected for the pressure drop in the column,-Le., the compressibility of the gas ( 5 ) . It is the limiting value of V Ras P,-+ p ,
Total Gas Volume or Gas Holdup, Voo, is the corrected retention volume of a nonabsorbed sample and represents the volume of carrier gas required t o transport such a sample
from the point of injection t o t h e point of detection a t column outlet pressure. Partition Coefficient is defined by the equation
H = concn. of sample in liquid phase wt./ml. concn. of sample in carrier gas ’ wt./ml. and is related to V,O by
Specific Retention Volume is defined by the equation (8)
v, = (concn. of sam le in liquid phase) b 7 3 ) wt./g. (concn. of sample in carrier’ wt./ml. gas), T
It is related to H by V ,
= 273 H
were T is the temperature of the column in O K. APPARATUS PERFORMANCE
Column Efficiency (theoretical plates). I n agreement with usage becoming increasingly widespread it is recommended that column efficiency be expressed in terms of theoretical plate numbers. The h-omenclature Committee of the London Chromatography Symposium of &lay 1956 also has adopted the definition of column efficiency (9). [Reference (2’1uses the term “resolution,” but the committee has since recognized that it should be replaced by “efficiency,” because only a single peak is involved.] It is recommended that theoretical plate number be calculated by the equation Tiumber of theoretical plates = l6
(
peak width
The theoretical plate number may vary with the compound as well as the column. Therefore the compound used should be specified. The retention volume and peak width used in the equation must be consistent, If the corrected retention volume is used, the observed peak width must also be corrected for pressure drop in the column. SENSITIVITY
Detector sensitivity is a function of the voltage sensitivity of the recorder, the retention volume of the sample, and column conditions during the experinient. *4peak area term reduced to a standard basis by taking into account the factors mentioned can be made to represent detector sensitivity in a generally applicable manner as follows: VOL. 30 NO. 10, OCTOBER 1958
1587
niv . S (ml. mv. mg.-') = __-
flow rate must be constant so that the time abscissa may be converted to milliliters of carrier gas. Under these conditions
where
Peak area = (millivolts)(time) = (constant) (wt./ml.) (ml.) = (constant) (wt.)
concn. A X CI X Cz X C8 W
S A C1
= detector sensitivity
peakarea, sq. cm. recorder sensitivity, mv. per cm. of chart CZ = reciprocal chart speed, minutes per cm. CP = flow rate a t exit of column, ml. per minute corrected to column temperature and atmospheric pressure W = weight of sample introduced a t head of column, mg. = =
The detector sensitivity is a function of operating conditions. Therefore, any operatjng conditions affecting sensitivity of the specific detector, such as temperature, pressure, flow rate, and the nature of the carrier gas need to be specified. Limit of Detection can be specified for either a detector system or a complete chromatographic apparatus. It is defined as the minimum weight of a compound which can be determined with a specified statistical reliability. A limit of detection has no meaning unless the statistical criterion is specified. For the entire apparatus, several samples of the minimum weight must actually be analyzed and the standard deviation or some related quantity specified. KOTE. The detector sensitivity as calculated above does not in itself measure the limit of detection of the detector. This would require a quantitative expression of the background noise. I n the absence of a standard procedure for obtaining values of this kind, no method for specifying the limit of detection is being recommended a t this time. DISCUSSION
OF
TERMS
The following discussion applies directly to the chromatograms obtained by means of differential type detectors or by differentiating the records obtained by means of integral detectors. The chromatographic peak is the only source of quantitative analytical data. Ordinarily the chromatogram is made by a strip chart recorder which is connected to the detector. The dimensions of peak area are (millivolts) (time) and are not directly proportional to the amount of substances unless tn-o conditions are met. First, the outpuf of the detector-recorder system must be linear with concentration. The proportionality constant for a particular compound can be determined experimentally or perhaps theoretically. Secondly, the 1588
ANALYTICAL CHEMISTRY
Obviously, an error in flon- measurement will cause the same relative error in peak area. Volume of sample injector and detector, and changes in retention volume, due to a changed column efficiency, for example, do not affect peak area. Temperature does not affect peak area unless it changes the Sensitivity of the detector. Peak height can also be related to weight of a given compound for a given apparatus by calibration. Peak height is not affected by errors in flow rate measurement. However, temperature, column efficiency, amount of liquid phase, volume of detector and sample injector-in fact, any variable that affects retention volume or peak shapewill alter the peak height and must not be changed after calibration. Retention time, corrected and uncorrected retention volume, partition coefficient, and specific retention volume have all been used to describe the position of a peak. The first three of these quantities are dependent on apparatus design and experimental technique. However, theory indicates and experiments have verified (9) that there is a definite relationship between the peak positions of a given component on two different columns containing the same liquid phase a t the same temperature. This relationship exists regardless of n-ide differences in the size, efficiency, or amount of liquid substrate of the two columns, provided the solid support has no effect on the partitioning. The discussion following is applicable even when the solid support influences partitioning, as long as the ratio of liquid phase to solid support is specified. At the rate the literature of gasliquid chromatography is building up, it should soon provide a ready source of information on what peak positions will be obtained with a given component on a variety of liquid phases. This information would not only serve to identify the component, but similar information on all the expected components in a sample would show what liquid phase would give the best spacing of peak positions, and hence the best separation. Many of the published papers on gas-liquid chromatography do not contain sufficient information to ascertain the relationship between the given peak position on the author's column and that
to be expected on some other column. Because the detector in gas-liquid chromatography is ordinarily connected to a strip chart recorder with constant chart speed, the resulting chromatogram is obtained with time as the base line. From the standpoint of the author's convenience alone, it is certainly most convenient to report peak position as retention time. Unfortunately, this quantity is of limited use to any other worker. In fact, all the following additional data would be necessary before retention time could be used to predict the peak position on other columns a t the same temperature and n-ith the same carrier gas: (1) flow rate, (2) temperature, (3) pressure of the carrier gas under conditions of flow measurement, (4)ratio of absolute column inlet pressure to absolute column outlet pressure, p , / p o ; ( 5 ) the total gas volume, Voo; (6) the liquid volume V L ; and ( 7 ) sample size and manner of injection. Although it is necessary to specify these seven additional quantities in order to define a retention time completely, it is possible to choose units for the peak position which absorb all but the last of these variables and the effect of the last can be eliminated by correction (9). An obvious improvement is to multiply retention time by the rate of flow to give the uncorrected retention volume. This quantity eliminates item 1 from the above list. By establishing a standardized pressure and temperature for the flow rate measurement, items 2 and 3 could also be eliminated. James and Martin ( 5 ) originally defined flow rate to be measured at column temperature and outlet pressure. However, where authors using retention volumes fail to specify items 2 and 3, there is the chance that they may be using the flow rate measured a t the temperature and pressure of their flow rate meter. By reporting the corrected retention volume, VR', item 4 can be omitted, but this has also led to confusion. One cannot be sure whether an author's retention volume is the uncorrected V Eor the corrected VE'. Readers are further plagued by the question as to whether an author's ''corrected retention volume" might not also be corrected for Voo(item 5 ) . Many authors give peak parameters in terms of relative retention volumes. Thus all the retention volume peaks are given in terms of a "standard" substance. Of course, no universal standard substance can possibly be chosen which will be equivalent for all immobile phases and column temperatures. From Equation 2, it is apparent that the relative retention volumes for two substances on the same column at the same temperature with partition coefficients H I and H 2 will be: VRI' - - Vc" V L H ~ VR*' VC" V L H ~
+
+
This expression cannot be reduced unless V G ois assumed small enough t o be neglected, in which case VR,O/VR~' = H 1 / H 2 ,which eliminates the column parameters entirely and relates the ratio to partition coefficients exclusively. The error introduced by simply ignoring VGoin this ratio could easily be avoided, for the value of Vc0 is obtained for any column by passing through it a small sample of gas which will not be absorbed on the column liquid (such as oxygen or air) but \\-ill be detected. Since H is virtually zero, by Equation 2 the limiting retention volume (corrected for pressure drop) is V R O
(oxygen)
=
Vc"
and this quantity can be subtracted from all other VRovalues to obtain what may be termed (V,")' values. Ratios of these quantities would indeed also eliminate V L(item 6), and thus have the desired independence from column operating variables. However, all the possible random uncertainties in the measuring of retention volume will carry over t o relative retention volume, and a neiv variable is introduced-the reference compound, which is found to change from author to author and column to column. Moreover, Ambrose, Keulemans, and Purnell (I) have pointed out that relative retention volumes are not a complete measure of the factors that determine separation. Littlelvood, Phillips, and Price (8) eliminated column variables without resorting to relative values by use of a quantity for n-hich the term specific retention volume has been proposed (I). Their measurements and corrections lead to a quantity equal t o 273 (V,")'/ T , which is then divided by the weight of liquid phase ( W L ) : V u = 273 (V,)'/ W L T ,but WLIVL = p~ is the density of the liquid. Hence H = TVg pL/2i3. Porter, Deal, and Stross (9) advocated the use of H directly. Littlewood et al. ( 8 )used Vu in their study of temperature effects because the use of H would have meant dividing by a new V La t each column temperature, whereas W Lremained constant. It is not necessary to specify items 2 and 3 when partition coefficients or specific retention volumes are used, because the conditions for measuring the carrier gas volume are specified by the definitions. The partition coefficient is related t o column conditions by Equation 2, which holds only when the volume VGis measured a t column temperature and outlet pressure. This volume may also be used in computing VCafter correcting by the factor 273/T.
It is apparent that only by use of the fundamental constant, H , or a quantity such as Vu which is related to it by factors which are constants a t constant temperature, is it possible to express peak positions independent of the experimental conditions. Indeed, the distinguishing feature of a fundamental physical constant is that it may be so expressed. There has been a reluctance to use these terms because of a Tvidespread, if unfounded, belief that their use required a thoroughgoing knowledge and application of theoretical thermodynamics. T o use nongeneral properties such as uncorrected emergence times only forces the reader to do the work of converting the constant to his use; this is often made difficult or impossible by inadequate reporting of the experimental conditions. On the other hand, if constants of general significance are given, they can be applied to any system without the necessity of specifying the conditions under which they n ere derived. The only specifications required are the temperature and the identity of the partitioning liquid. If the partition coefficient or specific retention volume of another compound which can be used as a standard is known, the calculation of H or V u of a given sample is almost as simple as the comparison of emergence times. The procedure is to analyze the sample in question, and the standard sample, and also a nonadsorbed gas, such as air, under exactly the same experimental conditions. The emergence time, as read from the chart records, of the air peak is subtracted from the emergence time of each of the others; the ratio of the so corrected retention times is equal t o the ratio of the corresponding values of H or V u , one of which is known. If a compound with known H or T', value is not available, the procedure is only slightly more difficult. The retention times of a sample and of an air peak are measured under the same conditions, and their difference is obtained, as before. But novi the difference must be multiplied by the flow rate calculated a t column temperature and outlet pressure and then corrected for the p J p o ratio. T o obtain H immediately, this result is divided by the volume V L of the column liquid; to obtain V u the result is divided by the weight of the column liquid and multiplied by 273/T. All the quantities used in this computation, with the exception of V L ,are normally obtained during the course of an analysis, such
as the emergence time of the air peak, or have to be known accurately in any case to arrive a t a good analysis-i.e., the flow rate, and the inlet and outlet pressure. The weight of the column liquid is known from the preparation of the column packing; the evaluation of €3, however, involves the additional experimental effort of determining the density of the liquid a t operating temperatures. This can be done by simple methods, because the error in the density figure causes only a proportional error in the H value. The quantity V udoes not require this additional determination; the partition coefficient, H , on the other hand, offers the advantage that it can be obtained from any appropriate equilibrium measurements, not necessarily involving gas-liquid chromatography a t all. It is likely that both of them will be used in the literature, according as one or the other is more advantageous for the topic. If the authors will furnish the density figures a t column temperature in papers on the subject, the desired conversion can be made directly. ACKNOWLEDGMENT
r The authors wish to express their appreciation to the members of Section L on Gas Chromatography of Research Division IV (ASTM D-2) for their valuable criticism and suggestions, and eupecially to R. A. Dinerstein, George Natsuyama, and J. N. Thoburn. LITERATURE CITED
Ambrose, D., Keulemans, A. I. AI., l'urnell. J. H.. .kSAL. CHEM.30, 1582 (1958). (2) Desty, D. H., "Vapour Phase Chromatography," p. xi, Academic Press, Kew Tork, 1957. (3) Eggertsen, F. T., Knight, H. S., Groennings, ~ . 28, 303 - . S.,. 4 ~ 4 CIIEM. (1956 ). (4)Golay, hI. J. E., International Symposium on Gas Chromatography, Instrument Society of America, Lansing, Mich., Aug. 28-30, 1957. ( 5 ) James, ,4.T., Mart,in, h. J. P., Biochem. J . 50,679 (1952). (6) Janak, J., Erdol u. Kohle 10, 442 (1957). ( 7 ) Johnson, H. R., Stross, F. H., Sinth Pittsburgh Conference on Analytical Chemistry and Applied Spect'roscopy, March 1958. (8) Litt,lewood, 4. B., Phillips, C. S. G., Price, D. T., J . Chern. SOC.1955, 1480. (9) Porter, P. E., Deal, C. H., Stross, F. H., J . Am. Chem. SOC.78, 2999 (1956). (10) Schmauch, L. J., "Response Time and Flow Sensitivity of Detections for Gas Chromatography," submitted to ASAL. CHEM. RECEIYEDfor review April 3, 1958. Accepted June 2, 1958. ( 1)
VOL. 30, NO. 10, OCTOBER 1958
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