Abstract:
A linear equation which infers elution time in a gas chromatograph as a function of temperature is assigned to a chemical compound. The linear equation is useful in constructing a computer data base which contains a long list of chemical compounds, a list of chromatographs each having a separation column containing a different adsorbent material, and a long list of linear equations associating each chemical compound with each chromatograph. The data base, which is arranged for computer searching, aids researchers in gas chromatography by automating a systematic search for optimum combination of separation columns and operating parameters.

Description:
This invention relates to gas chromatography. In one aspect, it relates to a method for finding the optimum choice of separating column and operating parameters to achieve a separation by gas chromatography. In another aspect it relates to a method for the characterization of chemical compounds. In yet another aspect it relates to a method for the identification of chemical compounds. 
     BACKGROUND OF THE INVENTION 
     The gas chromatograph is perhaps the most useful analytical tool available today to the chemist. The gas chromatograph takes a fixed volume of sample gas, or liquid which can be vaporized, and introduces the fixed volume of sample into a separating column which contains a stationary phase of adsorbent material. The sample is transported through the separating column using a mobile phase carrier, and individual molecules of the sample gas are adsorbed and then released at different times from the adsorbent stationary phase material in the column. 
     When the adsorbent material in the separating column and the operating parameters are properly selected, the separated components of interest elute or emerge from the column completely separated from each other and from any other component that may be present. This eluting stream is passed through a detector and the relative response of the detector is sensed by an electronic unit and recorded as a peak on a chart. This chart is referred to as a chromatogram. 
     Experience has shown that depending on the chemical components to be separated, a specific type of separating column, column temperature, flow rate, film thickness and other operating parameters will produce a more satisfactory result than other columns and operating parameters. As used herein the type of column refers to the particular adsorbent material contained in the column in addition to the manner in which the column is operated i.e. a capillary column or a packed column. 
     While one can conceivably find an optimum combination of column type and operating parameters for a nonroutine separation by making trial and error injection, or by searching literature for various retention data tables, this is not particularly satisfactory because the time required to establish an optimum combination for the particular separation is often excessive. In most cases, these latter approaches are only starting points for a nonroutine separation in which the chromatographer must still adjust several parameters to make the desired separation. In some cases, such adjustments can require several days of laboratory work. 
     Over the years there has developed the need for a low cost technique that would simply permit the selection of operating parameters and column type that will simulate a gas chromatograph within a few minutes. 
     Accordingly, it is an object of the invention to provide a method for automatically predicting the characteristics of a gas chromatographic separation for a given type of column with given operating parameters. 
     It is another object of this invention to provide a method for the characterization of a chemical compound which will predict its chromatographic elution time. 
     It is yet another object of this invention to provide a method for the identification of a chemical compound from its experimentally determined chromatographic elution time. 
     It is a further object of this invention to store elution data for a substantial number of chemical compounds in a memory space compatible with a personal computer. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided a method for constructing a simplified computer data file and retrieval system to aid researchers in gas chromatography by automating a systematic search for optimum combinations of operating parameters and separating columns over a given temperature range. A typical data file contains elution time data for about 2,000 organic compounds for use with 5 preferred stationary phases of adsorbent material. This substantial volume of data, which is stored in a memory size compatible with most personal computers, is input into the computer in a format which permits searching and retrieval of the data. 
     The data storage in a personal computer is based on the discovery that a large number of chemical compounds can be assigned a unique linear characteristic equation that infers its elution time in a gas chromatograph for a given stationary phase over a given temperature range. Thus, the only data necessary for characterizing compounds are the constants required in the characteristic equation. The required constants are stored in the computer memory and are retrieved to calculate retention time for a desired compound. The unique characteristic equation is of the form: 
     
         Log (S)=a.sub.1 T.sub.1 +b.sub.1                           ( 1A) 
    
     where: 
     a 1  and b 1  are experimentally determined constants, 
     T 1  is the separation column temperature in °C., and 
     S is a defined parameter dependent on the carbon number C and the distribution coefficient K n  given by the following equation: 
     
         S=Log (K.sub.n)/C                                          (2A) 
    
     If desired the step of dividing the log (K n ) by the carbon number C to define the parameter S in equation (2A) may be omitted, and a parameter M may be defined according to equation (2B): 
     
         M=Log (K.sub.n)                                            (2B) 
    
     where K n  is defined above. 
     Using the M parameter, an alternative equation that infers the elution time in a manner similar to Equation (1A) is of the form: 
     
         Log (M)=a.sub.2 T.sub.1 +B.sub.2                           ( 1B) 
    
     where: a 2  and b 2  are experimentally determined constants, and T 1  is defined above. 
     Using either Equation (1A) or (1B) a large number of organic and inorganic compounds are characterized and the appropriate values for the constants a 1  and b 1 , or a 2  and b 2 , are stored in the computer data file. By searching the data file and then making appropriate calculations, the user can automatically obtain a simulated chromatogram showing the elution of components of interest from a specified type of column, or can automatically obtain an optimum combination of separating column and operating parameters for a specified separation, or alternately can obtain a list of compounds having a specified chromatographic retention time for a specified column type. 
     Other objects and advantages of the invention will be apparent from the foregoing brief description of the invention and the claims as well as the detailed description of the drawings which are briefly described as follows: 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified illustration of a basic chromatograph system. 
     FIG. 2A is a plot of Log (K n ) as a function of temperature for a series of n-alkanes. 
     FIG. 2B is a plot of the defined parameter M as a function of temperature for a series of n-alkanes. 
     FIG. 2C is a plot of the defined parameter S as a function of temperature for a series of n-alkanes. 
     FIG. 3A is a computer flow diagram which illustrates the log (S) method of retrieving elution time from the data base. 
     FIG. 3B is a computer flow diagram which illustrates the log (M) method of retrieving elution time data from the data base. 
     FIG. 4A is a computer flow diagram which illustrates a method of retrieving column information from the data base. 
     FIG. 4B is a computer flow diagram which represents a method of retrieving compound identification information from the data base. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a better understanding of the invention the derivation of the characteristic equations (1A) and (1B), along with the necessary conditions for obtaining data illustrated in the plot in FIG. 2B or 2C, is present below. 
     The elution time of a nonretained chromatographic peak is defined as t o , which is simply the time it takes a nonretained peak, normally air, to travel from the injector to the detector. The time t o  is equal to the distance of the path length of the column system divided by the average linear velocity of the carrier gas. In other words, a component with a retention time of t o  simply spends all of its time between the injection and the detector in the mobile phase. 
     Most components when injected do spend a finite amount of time in the stationary phase of the column as they move through the column. The time a component (n) spends in the stationary phase is called the adjusted retention time and is equal to the elution time t n  of component (n) minus the time of the nonretained peak t o . All components spend the same amount of time in the mobile phase equal to t o  but for a separation to occur, a different amount of time must be spend in the stationary phase. Therefore, one of the basic concepts of gas chromatograph is a ratio of the amount of time a component spends in the stationary phase to the amount of time the component spends in the mobile phase, or: 
     
         (t.sub.n -t.sub.o)/t.sub.o =k.sub.n                        (3) 
    
     In Equation (3) k n  is this ratio, often called the capacity factor or partition ratio for component (n). The International Union of Pure and Applied Chemistry recommends the term &#34;mass distribution ratio&#34; in preference to either term. More importantly, k n  can be related to the more fundamental thermodynamic distribution coefficient K n . The significance of K n  will become apparent hereinbelow. A rearrangement of Equation (3) into Equation (4) shows that the adjusted retention time is equal to the product of k n  and t o . 
     
         t.sub.n -t.sub.o =t.sub.o k.sub.n                          (4) 
    
     Thus, any change in the adjusted retention time of component (n) must be the result of a change in this product. The partition ratio is temperature dependent and will remain constant as long as the column temperature remains constant. Changes in t o  are brought about by any change in the ratio of the path length of the column system (L) to average linear velocity (μ), since: 
     
         t.sub.o =L/μ 
    
     The resulting ratio and not necessarily the individual values for (L) or (μ) is significant, although certainly both (L) and (μ) must be operated within practical limits. Likewise, the average linear velocity (μ) is dependent upon other factors such as the compressibility factor, however, since the retention time depends directly upon the values for t o  and k n , the individual values of (L) and (μ) must be within certain ratio limits to provide a t o  that is compatible with the efficiency and analysis time requirements. 
     For wide-bore capillary columns, one guideline for determining (μ) is that the flow rate be around 2.5 mL/min. for an average (μ) of ca 0.2 meters/sec., for highest column efficiency; 10 mL/min. average (μ) or ca 0.7 meters/sec. for best general operation of the column, or about 30 mL/min. average (μ) ca 2 meters/sec. for fastest analysis time. 
     A wide-bore column, when operated to provide the fastest analysis, will also have about an order of magnitude fewer number of theoretical plates than the same column when operated for highest efficiency (the latter has approximately 2,100 theoretical plates/meter). However, it should be noted that even when operated for fastest analysis, a 50 meter wide-bore capillary column typically provides more than twice the number of theoretical plates than found with the standard 6 ft.×2 mm i.d. packed column. 
     In addition, wide-bore capillary columns generally have a sufficient number of theoretical plates to permit the chromatographer to essentially &#34;tune in&#34; the efficiency requirements for a particular separation. In a sense, any extra efficiency should be &#34;traded-in&#34; by operating the column at a higher carrier flow rate which provides a shorter analysis time. The significant point is that for a defined length of time, a (t o ) can be approximated with a (μ) that reflects the general requirements necessary to make the desired separation. 
     In most cases, as long as the value of t o  provides the approximate desired efficiency, the adjusted retention time should first be varied by a change in k n  rather than a change in t o . Thus, in order to simulate a chromatogram, an initial t o  can be defined as an input statement to a computer program which will reflect the approximate efficiency and time requirements of the separation. Once this t o  is defined, then a realistic simulation can be generated that is based on k n  and temperature. In turn, if the initial simulation does not entirely meet the chromatographers needs, then additional simulations can be easily generated by first changing k n  and then if necessary modifying t o . 
     It was previously mentioned that k n  was related to the more fundamental distribution coefficient or constant K n . The relationship is shown in equation (6) in which (β) is related to the openness of the column and the percent loading of the stationary phase. 
     
         k.sub.n =K.sub.n /β                                   (6) 
    
     β is called the phase ratio since it is a ratio of the volume occupied by the gas phase to that volume occupied by the liquid stationary phase. The significance of K n  is that it is a true equilibrium constant and is only governed by the compound (n), the stationary phase, and the temperature. 
     As shown in equation (6), the value for k n  depends upon the ratio K n  to β, in which β typically has values between 5 and 35 for packed columns; however, packed column β values are generally not readily available for use in equation (6). In contrast, the determination of a β value for capillary columns can be calculated from equation (7A); or in many cases, where d&gt;&gt;r, equation (7B). 
     
         β=(r=2d)/2(d)                                         (7A) 
    
     
         β=(r)/2(d)                                            (7B) 
    
     where (r) is the inner radius of the column and (d) is the film thickness of the stationary phase. Phase ratio values for capillary columns typically range from about 50 to 1,500, with a much smaller number of preferred ratios for β. 
     If a void volume time t o  and the phase ratio β are defined as an input parameter to a computer system, then the simulation will depend only on the value of K n  for each compound at the desired column temperature, and retention data can be obtained for each of five groups of preferred stationary phases stored in the computer. The classical equation tht commonly relates the K n  value to temperature is shown in equation (8). 
     
         Log (K.sub.n)=a/T.sub.1 +b                                 (8) 
    
     where (a) and (b) are considered constants and T 1  is the temperature in degrees Kelvin (°K.). 
     It is noted that equation (8) can be used to simulate the chromatograms, however, an alternative approach is preferred in the present invention. 
     FIG. 2A shows a graph of log (K n ) as a function of temperature in degrees centigrade for a series of n-alkanes. As illustrated the resulting curves are nonlinear and can be represented by a general expression of the form: 
     
         Log (K.sub.n)=b.sub.o (10)a.sub.o T                        (9) 
    
     where a o  and b o  are constants and (T) is the column temperature in degrees Centigrade. 
     As shown in equation (10) and FIG. 2B a series of &#34;linear plots&#34; are obtained if the Log of (Log K n ) is plotted versus temperature. These &#34;linear plots&#34; are really logarithmic values that are mapped in a linear domain and have a general expression: 
     
         Log [Log (K.sub.n)]=a.sub.o T+b.sub.1                      (10) 
    
     or since log (K n )=M 
     
         Log (M)=a.sub.2 T.sub.1 +b.sub.2                           (1B) 
    
     Equation (1B) is also a basic relationship which could be used to characterize the compounds and simulate a chromatogram, however, if desired one additional step can be included. The carbon number is included as part of the basic expression for a parameter S defined as: 
     
         S=Log (K.sub.n)/C                                          (2A) 
    
     where C is the carbon number of the compound of interest. In equation (2A) dividing log (K n ) by the compounds carbon number only results in the expression having a different set of values for the constants a 2  and b 2  compared to a 1  and b 1  in Equation (1A). In turn, if equation (9 ) is converted with the appropriate constants to its logarithmic form and expressed in terms of equation (2A), then the following expression is obtained: 
     
         Log (S)=a.sub.1 T.sub.1 +b.sub.1                           (1A) 
    
     where a 1  and b 1  are constants for the particular compound and stationary phase, and T 1  is the column temperature in degrees Centigrade °C. 
     One interesting aspect of using this approach to simulate a chromatogram is that the carbon number is a known parameter that can be used as part of the input statement to the data file. In this manner, the response time of the computer system is faster since only those compounds with the correct carbon number need to be searched in the data file system. 
     Using the carbon number in the basic expression also has the effect of normalizing the various individual expressions into a small range of values as illustrated in FIG. 2C, which in some cases may be useful for predicting compounds that are not in the computer file. For the temperatures ranges used, the S values were generally between 0.2-0.6. Consequently, Log (S) is a negative number whose absolute magnitude increases as S decreases. 
     EXAMPLES 
     The plots presented in FIG. 2C, for the chromatographic characterization of n-alkane is on 0V-101 stationary phase of poly(dimethylsiloxane), illustrate the relationship between the previously defined parameter S and temperature for a series of n-alkanes. These plots were obtained using the equipment illustrated in FIG. 1. 
     Equipment Used: 
     
         ______________________________________Chromatograph System           Model 2100, OPTICHROM ® gasincluding programmer 24,           Chromatograph System fromoven 12, and    Applied Automation, Inc.,detector 16.    Bartlesville, Oklahoma.Column 10       Capillary type, 50 meters X           .53 mm i.d. from Quardex Corp.,           New Haven, Connecticut.Column stationary phase           OV-101, poly(dimethylsiloxane)Column film thickness           1 × 10.sup.-6 meters.Carrier gas 22  HeDetector 16     Flame ionization type.Injection 20    0.5 mL MAT injection valve from           Mess and Apparatetechnik,           Mumr, West Germany.Recorder 18     Model 5601-S from Easterline           Angus, Indianapolis, Indiana.______________________________________ 
    
     Referring now to FIG. 1 with the column 10, located in oven 12, stabilized at the desired temperature, chromatograms 14 for the components of interest and for a nonretained component were obtained. 
     From the chromatogram 14, the retention time of each component t n  and the retention time for a nonretained component were recorded for a number of operating temperatures. Normally two separate injections at different carrier flow rates were recorded for each temperature point selected over the operating temperature range. It is noted that a change in the carrier flow rate verified that the resulting k n  values were essentially independent of the carrier velocity. 
     In turn, an average k n  value based on at least two injections for each of the components was calculated from equation (3), and the distribution coefficient K n  is calculated from equation (6). Once the distribution coefficients K n  for a component was determined, equation (2A) was used to relate the log of K n  to its S value. 
     At this point, S was plotted versus temperature on a semi-log scale as illustrated in FIG. 2C. The constant a 1  for the slope of equation (1A) and the constant b 1  for the intercept are determined from a least squares fit of a straight line equation to the data plotted in FIG. 2C. The procedure is essentially the same for determining the constants a 2  and b 2  for an M plot illustrated in FIG. 2B. 
     The above-described equipment and method for calculating the S parameter has been applied to an assortment of organic compounds as listed in Table 1. Table 1 includes the constants for the linear equation for log (S) as a function of temperature for the assortment of compounds listed in column 2. Column 4 lists the experimental temperature range over which the data was collected. 
     
                       TABLE 1______________________________________The Log (S) Values for Assorted Compoundson an OV101 Liquid Stationary Phase                              Tem-Carbon Compound                    peratureNumber Name          Log(S) =      Range (°C.)______________________________________5      n-Pentane     -0.00320T-0.3391                              42.2-60.55      t-Pentene 2   -0.00334T-0.3260                              40.0-50.05      c-Pentene 2   -0.00323T-0.3223                              40.0-60.05      l-Pentene     -0.00300T-0.3585                              26.7-140.05      Cyclopentane  -0.00296T-0.2857                              40.0-60.05      2Methylbutane -0.00283T-0.3828                              40.6-127.86      n-Hexane      -0.00291T-0.3380                              40.0-160.06      l-Hexene      -0.00293T-0.3465                              26.7-140.06      Methylcyclopentane                -0.00285T-0.3167                              60.0-80.06      Cyclohexane   -0.00277T-0.2952                              60.0-80.06      Benzene       -0.00258T-0.3163                              60.0-148.96      2Methylpentane                -0.00282T-0.3691                              40.6-127.86      3Methylpentane                -0.00286T-0.3541                              40.6-127.87      n-Heptane     -0.00285T-0.3330                              60.5-160.07      l-Heptene     -0.00291T-0.3372                              26.7-140.07      2Methylhexane -0.00282T-0.3561                              40.6-127.87      3Methylhexane -0.00285T-0.3484                              40.6-127.87      2,3Dimethylpentane                -0.00289T-0.3493                              60.0-80.07      Methylcyclohexane                -0.00271T- 0.3227                              60.0-90.07      Toluene       -0.00259T-0.3128                              60.0-148.98      n-Octane      -0.00276T-0.3330                              60.0-160.08      l-Octene      -0.00282T-0.3343                              26.7-140.08      Ethylbenzene  -0.00254T-0.3195                              60.0-148.99      n-Nonane      -0.00274T-0.3285                              60.0-160.09      Propylbenzene -0.00254T-0.3226                              60.0-148.910     n-Decane      -0.00271T-0.3259                              60.0-160.010     Butylbenzene  -0.00254T-0.3216                              60.0-148.911     n-Unicane     -0.00269T-0.3240                              60.0-160.012     n-Dodecane    -0.00267T-0.3236                              60.0-160.013     n-Tridecane   -0.00265T-0.3234                              60.0-160.014     n-Tetradecane -0.00264T-0.3230                              60.0-160.0______________________________________ 
    
     Tables 2 and 3 show the log (S) results of a series of n-alkanes separated on a Carbowax 20M column, and on an 0V17 liquid stationary phase respectively. The experimental data obtained for constructing Tables 2 and 3 and the format of the data is the same as for Table 1. Table 2 indicates that components with wide ranges in polarity do not affect the log (S) expression to simulate their retention characteristics. 
     Table 4 shows the log (M) results of a series of n-alkanes on an 0V101 liquid stationary phase. 
     
                       TABLE 2______________________________________The Log(S) Values for Assorted n-Alkaneson a Carbowax 20 M Liquid Stationary PhaseCarbon Compound                   TemperatureNumber Name        Log(S) =       Range (°C.)______________________________________8      n-Octane    -0.00287T-0.5020                             62.2-138.39      n-Nonane    -0.00293T-0.4734                             &#34;10     n-Decane    -0.00285T-0.4749                             &#34;11     n-Undecane  -0.00280T-0.4567                             &#34;12     n-Dodecane  -0.00279T-0.4470                             &#34;13     n-Tridecane -0.00276T-0.4405                             &#34;14     n-Tetradecane              -0.00273T-0.4345                             &#34;______________________________________ Column: 50 meters × 0.53 mm i.d. fused silica capillary column containing a bonded 1 μm film of Carbowax 20 M, a poly (alkyleneoxide) 
    
     
                       TABLE 3______________________________________The Log(S) Values for Assorted n-Alkanes and n-Alcoholson an OV17 Liquid Stationary PhaseCarbon Compound                   TemperatureNumber Name        Log(S) =       Range (°C.)______________________________________8      n-Octane    -0.00308T-0.3777                             62.2-168.39      n-Nonane    -0.00300T-0.3692                             &#34;10     n-Decane    -0.00296T-0.3616                             &#34;11     n-Undecane  -0.00290T-0.3573                             &#34;12     n-Dodecane  -0.00285T-0.3554                             &#34;13     n-Tridecane -0.00281T-0.3539                             &#34;14     n-Tetradecane              -0.00278T-0.3521                             &#34;6      l-Hexanol   -0.00296T-0.1643                             79.4-168.37      l-Heptanol  -0.00287T-0.1846                             &#34;8      l-Octanol   -0.00282T-0.2004                             &#34;9      l-Nonanol   -0.00279T-0.2109                             &#34;10     l-Decanol   -0.00277T-0.2195                             &#34;______________________________________ Column: 50 meters × 0.53 mm i.d. fused silica capillary column containing a 1 μm film of OV17, a poly(50methyl/50 phenylsiloxane). 
    
     
                       TABLE 4______________________________________The Log(M) Values for Assorted n-Alkaneson an OV101 Liquid Stationary PhaseCompound                   TemperatureName          Log(M) =     Range______________________________________n-Pentane     0.3565-0.003153T                      30-100n-Hexane      0.4465-0.003018T                      30-100n-Heptane     0.5200-0.002937T                      30-100n-Octane      0.5860-0.003006T                      30-100n-Nonane      0.6251-0.002691T                      30-200n-Decane      0.6704-0.002635T                      40-200n-Undecane    0.7034-0.002547T                      100-200n-Dodecane    0.7409-0.002519T                      100-200n-Tridecane   0.7772-0.002506T                      100-200n-Tetradecane 0.8107-0.002500T                      100-200n-Pentadecane 0.8386-0.002473T                      100-200______________________________________ Column: 50 meters × 0.53 mm i.d. fused silica capillary column containing a l μm film of OV101, a poly(dimethylsiloxane). 
    
     As a practical matter for building a data file that is compatible with most personal computers, about 2,000 compounds can be characterized using equations (1A) or (2A). These compounds can be characterized for 5 different preferred stationary phases as listed in Table 5 below. Several authors have indicated that this group of 5 preferred stationary phases will accommodate more than 80% of the required separations in gas chromatography. 
     
                       TABLE 5______________________________________Preferred Liquid Stationary Phases______________________________________(1) A poly(dimethylsiloxane), such as SP2100, OV-1, OV-101 or    SE 30 GC grade.(2) A poly(50%-methyl/50%-phenylsiloxane), such as OV-17 or    SP-2250.(3) A poly(alkylene oxide), such as Carbowax 20 M.(4) A poly(50% methyl/50% -3,3,3-trifluoropropylsiloxane), such    as OV-210, OV-202 or SP-2401.(5) A poly(cyanoalkylsiloxane), such as AN600, OV225, SP2300,    or Silar 5CP.______________________________________ 
    
     Next the data for characterizing the compounds, and if desired any other data relating to the compound which can form a collection of logically related files, was stored in a computer data base. Preferably the computer is a small desk top or personal computer which is coupled to an input/output apparatus and includes real time operating system software. The input/output apparatus includes at least a keyboard, a CRT screen and a printer, and the computer system software includes a set of programs which can be used to define, create, access and maintain a data base. A suitable computer system can include an IBM Model 50 (System-2) personal computer. 
     Referring now to FIG. 3A, there is illustrated the computer logic to retrieve the elution time of a specified compound through a specified column using the Log (S) characterization of the compound. Essentially a user can request, by inputting required information through a keyboard, the predicted elution time of a compound for a specific column. The computer first fetches the appropriate constants for the named compound that have been prestored in the data base and calculates the log (S) according to equation (1A). In the next step S is calculated by taking the antilog of log (S). Following this step the log (K n ) is calculated by multiplying S by the carbon numbers C corresponding to the named compound. Then the antilog of log (K n ) yields a value for the distribution coefficient K n . Next the β value for the column is calculated according to the equation (7A) or (7B) from the column dimensions, and then the capacity factor k n  for the named compound is calculated according to equation (6). The elution time for the named compound is then calculated according to equation (4). The thus calculated t n  is reported to the user on one of the output devices associated with the computer and can appear as a printed decimal number on the CRT screen or alternately can appear as a tick mark on an appropriately timed graph thus simulating a chromatogram. 
     FIG. 3B illustrates the computer logic to retrieve the elution time of a specified compound through a specified column using the Log (M) characterization of the compound. FIG. 3B is similar to FIG. 3A, except that the carbon number is not used in the input statement and the a 2 , b 2  constants are retrieved in place of the a 1 , b 1  constants. 
     Referring now to FIG. 4A, there is illustrated computer logic to retrieve column-type information from the data base, Essentially an operator can request through a keyboard, the optimum column type for a specific separation of two named compounds. The computer calculates the elution time for each named compound according to one of the methods illustrated in FIG. 3A or 3B, for each stationary phase for which the data base contains characteristic information for that compound. The computer then compares the elution times for each compound to be separated and reports the column type which yields the maximum difference in elution time for the compounds. 
     FIG. 4B, which is similar to FIG. 4A, illustrates computer logic to retrieve compound identification information from the data base. 
     The identification of an unknown compound from the data base generally requires the use of the log (M) expression since the carbon number for an unknown compound is also an unknown parameter. Therefore, although the log (S) expression may have some advantages, the log (M) form may be the more useful general expression for both the identification of compounds and the prediction of gas chromatographic retention times. Generally, both the log (S) and the log (M) parameters will be stored in the data base file for user convenience. 
     The invention has been described in terms of the presently preferred embodiment using capillary type columns at a constant temperature as illustrated in FIGS. 1 through 4. However, the log (S) and the log (M) expressions are valid for temperature programmed gas chromatography, where the analyst defines an initial temperature T o  and a temperature rate R 1  as illustrated in equation (11) below. 
     
         T.sub.e =R.sub.1 t.sub.n +T.sub.o                          (11) 
    
     where Te is the final or elution temperature. 
     The reciprocal of Equation (3) solved for t n  is shown in Equation (12) below: ##EQU1## 
     Equation (12) represents the fraction of the total retention time on the column corresponding to the distance traveled by a compound at a specific temperature or K n  value. Therefore starting with an initial temperature, T o , and a fixed nonretained peak time, t o  a first value for K n  is calculated according to FIG. 3A or 3B and the corresponding fraction of the total time is then calculated according to Equation (12). This calculation is periodically repeated for example at tenth of a minute intervals and the fractions calculated according to Equation (12) are summed in Equation (13) below. ##EQU2## where T o  and T e  are the initial column temperature and the final column temperature illustrated in Equation (11). 
     When the various fractional time period summed in Equation (13) total 1, component (n) has traveled the length of the column with the last temperature parameter equal to the elution temperature T e . The factor of 10 appearing in the denominator of Equation (13) converts minutes to tenths of minutes for correspondence with the measurement increments of tenths of a minute utilized for generating new K n  values. Once the T e  value is known, the retention time (t n ) is readily obtained from Equation (11). 
     Table 6 compares the final temperature T e  and the partition ratio k n  predicted according to the method of the present invention with actual temperature program chromatogram for a series of n-alkanes. All of this temperature programmed data was obtained in a 50 meter×0.53 mm i.d. 0V101 column with a 1×10 -6  film thickness. The initial temperature was 50° C. with a temperature rate of 5° C./min. The comparative data illustrated in Table 5 indicates excellent agreement between predicted and actual results in temperature programmed chromatography. 
     
                       TABLE 6______________________________________Comparison of Elution Characteristicfor n-alkanes at R = 5° C./Min.  Retent.Compd. Time     Observed Observed                           Predicted                                   PredictedName   (Min.)   T.sub.e in °C.                    k.sub.n                           T.sub.e in °C.                                   k.sub.n______________________________________Air    1.47     --       --     --      --n-C6   2.36     61.8     0.605  61.75   0.599n-C7   3.34     66.7     1.27   66.75   1.28n-C8   5.09     75.4     2.46   74.75   2.37n-C9   7.54     87.7     4.13   86.75   4.00n-C10  10.40    102.0    6.07   101.25  5.97n-C11  13.38    116.5    8.10   116.25  8.01n-C12  16.17    130.8    10.0   130.25  9.92n-C13  18.91    144.6    11.86  144.25  11.82n-C14  21.52    157.6    13.64  156.75  13.52______________________________________ 
    
     Reasonable variations and modifications of the present invention possible by those skilled in the art are within the scope of the described invention and the appended claims. Since the distribution coefficient, K n , is a true equilibrium constant dependent only on the compound, a modification such as using a packed column is within the scope of this invention. Further, a method for predicting rentention times for use with multicolumn systems such as employing two or more columns in series is within the scope of this invention. Still further, a method for predicting retention times in liquid chromatography wherein the temperature parameter in Equations 1A, 1B, and 8-10 is replaced with a solvent composition factor at a constant temperature is within the scope of this invention.