Chemical lnstrumentution
"'a-
5. Z. LEWIN, N e w York University, Washington Square, New York 3, N. Y.
T h i s series of articles presents a surwy of the basic principles, ciiaracterislics, and limitations of those instruments mhich find important applications i n chemical work. The emphasis is on commercially available epuipmeitt, and approximate prices are quoted to show the order of magnitude of cost of thevarious types of design andconstruction.
15. Gas Chromatographs (Continued) The nature of the packing in a gas chromatograph column has important effects on thc performance of the instrument. Since the moving phase is a compressible gas, the resistance to flow offered by the solid packing croates a,pressure d m p along the length of tho column that is greater, the longer the column, and the finer the particle size of tho packing. That is, in order to obtain a given reto of gas flow out of the exit end (outlet) of the column, the gas pressure a t the inlet ( P J must be greater than that existing a t the exit (PA): t,he difference between t,hesr . twovalues is tho pressure drop. The characteristic mode of interaction with the column packing of a constituent being chromatographcd is often reported in terms of the retention time. This is the time that is observed to elapse between the instant the component is introduced onto the chromatographic column, and the moment when its concontrat,ion peak is observed in the read-out. I t is evident that the retention timo will bo longer, the stronger the interaction is betwcon tho eonstitucnt and the rolnmn; for a given column used under precisely defined conditions, the magnitude of the retention time is a physical chcmical parameter that can serve to characterize the substance. However, since the rctcntion time is sensitive to the flow rate of the carrier gas, a more general and characteristic parameter is the retention volume, which is the volume of carrier gas required to sweep the constituent in question through the entire column. This auantitv " is eenerallv expressed in tcrms o i t h e volun~cof g& corresponding to the temperature of the column and the pressure existing st the eolumn outlet (generally atmospheric pressure). The compressibility of the gas causes the linear velocity of the carrier gas to vary along the length of the column, due to the variation in pressure. I n order to determine the corrected retention volume, Vno, from the observed retention time, In, and the obaemed flow rate, F,, of the carrier gas a t column temperature and outlet pressure, the uncorrected rotention volume, Vx, must he corroded far the campressihility effect by means of a pressure gradient factor,f. ~~
va =
F,ta
~~
The most characteristic parameter that describes the interaction between a cornponent and the column is the partition coefficient, K, which is equal to the ratio of tho amount of the component in the stationary phase to its amount in the moving phase a t any and every point in the column, if equilibrium conditions obtain. The larger the partition coeficient is, the more effectively does the stationary phase interact with the component and impede its progress bhrough the column; hence, the larger is the retention volume. The quantitative relationship between these parameters is:
K
=
Vx" - V0" VL
nhorc Voo is the total gas volume of the column ( i t is moasurod experimentally as the corrected retention volume of a camponent which is not held back a t all by the column, e.g., air in the case of a partit,ion column a t room temperature and higher), and Vr.is the volume occupied by the liquid phase in the eolumn (equal to m / p , where m is the weight of liquid phase used in preparing thc column packing, and p is the density of the liquid phaso a t bhe operating temperature of the column). The principal function of tho column is to produce a separation, or resolution of bhe components present in tho original
feature samplc. One of the measures that is used for this property is the HETP, based upon the assumption that the sepr~rs.tittian produced in s. gss chromatograph column is analogous to that obtained with a distilling column or with a. multi-stage countercurrent extraction apparatus. V7it,h these techniques, the larger the number of theoretical plates there are per unit length of column, the less the amount of lateral spreading out of the concentration profile of the component that occurs as i t moves through thc apparatus. Hence, the HETP of a. gas chromatograph can he estimated from the degree of spreading-out (i.e., the width of the elution peak) associated with n given duration of contact with thc column (i.e., retention time or volume). A relationship often used is:
n
=
[ H E T P ] X L = 16 (d/w)P
where n is the number of theoretical plates in a column of length L,d is the retention time or volume, and w is tho associated pe& width (usually estimated as equal to the intercepts made a i t h the baseline of tangents d r a m through the inflection points on either side of the peak). The relationship of these quantities to the xppcnrance of the read-out of a typical gas chrolnatogram is illustrated in Fig. 15. The numbcr of theoretical plates of a packed column often is observed to go through 8 maximum (i.e., H E T P goes through a minimum) as a function of flow rate, as shown in Figure 16 (compare a i t h Figure 3). I t decreases somewhat with increme in temperature, and with increase in sample size, but increases, though not in a linear fashion, with the length of the column (see Figure 17). I n order to chromat,ogrsph large size samples, i t is common practice to increase thc column (Continued on page A 6 )
..
A Figure 15. The reod-out of on idealized gor chromatogram. I. inrtont a t which romple i s injected. A. peak due t o air which entered with the sample. M. peak due to the sample component. This record provides the following characteristic parameter.: V,!, total gas volume of column (from Id); Vb retention volume (from AM); n, total theoretical plater (from AM and DE).
Volume 39, Number
I, January 1962
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. . Figure 16. The number of theoretical plater of a pocked column decreases with increasing temperature, and goes through n maximum as 0 function of the Raw rate. (From T. Johns, Beckman Bulletin No.756.)
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Figure 17. The number of theoretical plater of o ~ a c k e dcolumn increases with column length, but decreorer with sample sire. (From T. John., (Conlinued on page A 8 ) loc. cit.1
Figure 18. The pressure drop dong o packed column increase, linearly with the reciprocal of the The HETP decrease, with decrease in pqrticle sire. In practice, o particle size of the compromise must b e sought between number of theoretical p l d e s ond resistance to gas flow. (From T. Johns, loc. =it.)
diameter in proportion to the increase in sample size; however, even for a constant ratio of column diameter to sample size,
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the number of theoretical plates is often ohs~rvedto fall off sharply for dinrnetprc (Conlinmd on page AIO)
in excess of a/4-inch. This has been attributed t o the difficulty of achieving a uniform peeking in a, thick column, as well as t o the difficulty of introducing the sample all a t once without diluting i t
column is packed have an important effect on the performance of the column. Figure 18 shows that the pressure drop along the column increases markedly as the particle size decreases, and the increase in pressure drop is associated with a decrease in HETP. Hence, the efficiency of the column is improved by using a small particle sise for the packing, hut this requires s high pressure nt the inlet to the eolumn in order t o force a reasonal~lerate of flow of gas through the packing. The best compromise in operating conditions is generally found to correspond t o 5&60 mesh particles. I t should he noted that the fundamental property that determines the efficiency of the column is not particle sise, but pore size. The latter is h chararteristic of. the microscopic structure of bhe solid; the particles of the solids commonly used in gas chromatography are porous, and the ultimate pore sire is the same, whether the particles correspond t,o big chunks or a fine powder. This iri illustrated by the data contained in Figure 19, which shows that the effect of particle size on H E T P tends to disappear as the flow rate is reduced, since a t law flow rates the gas can penetrabe into the pores, with the result t,hnt the chromatographic process is no longer l i m i t ~ dt o the outer surfaces of the particles.
/
100 C C l Y l l
I
100
200 YIO 400 a R n C L E S,ZE,UIERONS
500
Figure 19. HETP verrvr porticle sire for the elution of n-butone from o packed column using hydrogen as the carrier gar. The pocking contained diethylhexyl rebecate deposited on red Chromororb having pores 10-50 microns in diameter. (From Boker, Lee and Wall in "Gas Chromatography;' Academic Prerr, 1961, p. 271.
The suitability of a solid substrate for use as a liquid support in GLPC depends upon its possession of a number of specific properties. I t should have a, porous structure, hut should be available in the form of aggregates of moderate partirle size, so that gas can flow easily through the packed column. I t must be wetted by the liquid phase, so that a. uniform liquid layer can be deposited on the entire surface of the solid, both external and internal. I t must be able to maintain a very thin layer of liquid on its surface for long periods a t elevated temperatures, for the thicker the liquid film is, the slau.er is the equilibration between the stationary and moving (Continwd on page A121
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phases, and the lower is the efficiency of the column. Thus, the desirability of a solid phase for GLPC depends in part upon its physical structure, and in part upon the properties of the liquid that is to be employed. For most GLPC work, the various forms of diatomaceous earths have been found to be effective substrates. One of the most popular forms is a Johns-Manville crushed firebrick designated as C-22; other forms are Celite 545, Sterchamol No. 22 (available in U. S. through Alupharm Chemicals, New Orleans 12, Louisiana), Chromosorb V , silica gel, etc. For special problems, use is made of Tide, Teflon, charcoal, and many other solids. The efficiency of a column has been expressed above in terms of the H E T P or number of theoretied plates. However, the ability of a given column to resolve a pair of components depends both upon the column elliciency m d upon the existence of a difference in the interaction of these components with the stabionmy phase. The latter factor is oiten described in terms of the senamlion. factor. parameters of the two components in que~tiotion. Thus, a separation factor of, e.g., 1.10 between two substances means that on the particular column to which this factor perbains, one component h ~ as retention time (or volume, etc.) 10% greater than the other. Given this separation factor, the actual separation that can be achieved with a given column depends upon t,he efficiencyof the column.
/:?I!:\ \ i
Figure 20. lllurtrtlting the roles of reporation factors and numbers of theoretical ploter in o a m chrornotoaroohic resolution. Both dioarams refer to the same reparation factor between the two components, but in A the column hor about 7 0 0 theoretical plates, and in B there are about 2 3 0 0 ploter. If the distribution is divided into two equal parts, the amount of impurity in each cut is shown by the crorrhatched areor.
-
- .
As Figure 20 shows, a column having 700 theoretical plates gives the same separation between the peaks of the two components (assumed to be present in the sample in equal molar concentrations) as a column having 2300 plates, but if the distribution is split into two equal cuts, each cut has 10% of impurity in the first case, and only 1% in the second case. The rel* tionship between separation factor, a,
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1.M
1.025
1.015
1.01 105 I
10' i 1 2
n
103 I
I
lo2 I
1 0 - l ~ lo-9
lo-?
1 0 - ~ 1 0 - ~ 10-3
10-2
lo-'
10
m I 2 + rn12
"F. Figure 21. Glveckovf diagram showing the relationrhips between separation foctor, a, number of theoretical plotes, n, ond fractional amount o f impurity, q , ortocialed with o partially resolved component. m, and ma are the numbers of moles o f components 1 and 2 respectively, in the originol romple.
nrunber of theoretical plates, n, and frae-
(Cdinued on page A14)
Volume 39, Number 7 , January 1962
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tional band impurity, 7, is shown grrtphically by the Glueckauf nomograph reproduced in Figure 21.
The special virtue of the capillary, or Golay column, in which there is no packing, and the stationary liquid phase is
1
2'5
2b
15
I
10
5
0
Figure 22. Comparison of resolution obtainable with o A. packed column, and o 6. capillary column. The some sample was wed in both corer, and the stotionory liquid phase uor polypropylene glycol at 90°C. I.cyclopentone. 2. rnethylcyclapentme. 3. benzene. 4. cyclohexane. 5. methylcyclohexone. 6. toluene. 7. ethylbenzene. 8. p-xylene. 9. m-xylene. 10. o-xylene. Sample r i m were 20 lambda and 0.0001 lambdo respectively.
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deposited as s. thin film on the inside capillary walls, resides in the rapid equilibration that occurs between the moving and stationary phases. For equal flow rates, the capillary column is capable of yielding much sharper separations than the packed column, as is illustrated for one case in Figure 22. If time of analysis is the prime consideration, the capillary column will give resolutions equal to those obtained with a packed column in a much shorter time. However. in order to
Figure 23. The specific retention volumes of a series of d c o h o i ~on Silicone 7 0 2 fluid plotted or a function of the reciprocal temperature. IFrom Littlewood, Phillips and Price. J. Chem. Sor, 1480,19551.
(Continued on page A16)
PEAK HElGHT/1000 (1WDir.=.92mr) INTEGRATEDSIGNAL I N MILLIVOLTMINUTES~I mrmin= 12,5W=w*l
I 18
14.4
16
12.8
-
14
11.2
-
l
l
I
I
I
I
I
PEAK HEIGHT 20 p12.2-DIME'lHYL8UTANE
20
n-HEPTANE
PEAK AREA 20 n-HEPTANE PEAK AREA 20 ('1 2.2-DIMETHYL8UTANE
I
TEMPERATURE 'C The effect of column temperature on peak height and peok oreo. Beckmon Model GC-2A gar chromotogroph with thermal conductivity detector.
Figure 24.
realize the high speed or resolution inherent in the capillary column, i t is necessary t o use a much smaller sample, and an eouivalentlv more sensitive detector, t h a n those which are adequate for the packed column technique.
Temperature Control I n order for a compound t o move through a gas chromatographic column at
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Data obtained an a
r. practical rate, it is necessary that its solution in the stationary phase have a vapor pressure roughly in the range hetween 10 and 1000 mm Hg. The effective vapor pressure is a sensitive iunet,ion of the column temperature, and an essential part of the proress of finding t,ho proper conditions for a ehromatogrzphic experiment consists in the adjustment of the temperature. Although the heat choice (Continued on page A19)
of the operating temperatnre(s) is important far optimal performance, i t is by no means a critical factor. Practical information on sample composition can be obtained with a column t,he temperature of which is as mueh as 100°C above the boiling point of the mast vola.til.tile eomponent and as mueh as 100°C below that of the lesst volatile. The retention times or volumes of component. decrease wit,h increase in column t,emperattture, a8 shown in Figure 23. I n general, tho retention time decreases about 5% per 1°C increase in temperature. Since the area under a peak does not change with temperature (except to the extent that the detector output is a function of temperature), and since the peak widbh dccrcases as the retention time d e creases, i t follows that the peak height will increase correspondingly. These relxtionships are illust,rated graphically in Figure 24. In general, peak heights inereme ahnut 2% per 1% risein temperature. It will he seen from the foregoing that the degree of temperature control required in any given chromatographic exneriment will denend unon tho orecision constancy of tho column temperature to 3~1°C is adequate. Many gas chromatographs enclose the column in s. well-insulated box with a heater and blower mount,ed inside to maintain a uniform air-bath temperature. The temperature of the column is sensed with a thermocouple, and indicated on a meter, but no effort is made to control the temperature, since a thermally lagged, enclosed space will not fluctuate by more than + l ° C for considerable periods after a steady state has been reached if the line valtago supplied to the heater stays eonstmt. An example of this kind of unit is the Column Oven of Research Specialties Co., Richmond, California (Model 601-1, $200). I t e m ncemnmodate U-shaped columns up to 6 feet long, and can be heated to 300°C. A proportional temperature controller can be added, if desired, to maintain a. temperature constant to + 0 . 0 5 T (Model 607-3, $450). The Capillary Analyzer of the JarrellAsh Company, Newtonville 60, Mass* ehusetts, has an enclosed air-bath type of column oven, the special features of which are a pair of booster heaters to bring the temperature up to the working range rapidly, and a water cooling coil attached to the cast aluminum oven body to bring the temperature dawn again rapidly after a run. The heating and cooling rates achieved in this design are illustrated by the data in Figure 25. The precision of the temperature control a t the operating temperature is +0.5'C. The Perkin-Elmer Vapor Fractometer, Models 154C, D and 188 incorporate column ovens of the air-bath type with temperature control to +0.l0C. The standard heater is suitable for temper* tures up to 225°C; an auxiliary heater, designed to be mounted surrounding the column, permits operation up to 325% The Berkman Model GC4A Gas Chromatograph provides an eighkposition (Continued on page A20)
switch calibrated in terms of steady &ate column temperature which permits the operator to select the power input needed to bring the column to within 15°C of the nominal value shown on the selector switch. A proportional control system maintains thc oven temperature constant to within zt0.0l0C over the range from 40' to 220'C. In the Shandon Universal Gas Chromatograph (U.8. distributor: Consolidated Lahomtories, Chicago Heights, Illinois), the column is bathed in the vapors of an organic liquid, which is heated by two 500-vatt heaters, one of which is the intermittent regulator. By the use of suitahle bath liquids, column t,emperatures up to 300°C may be obtained. High temperature gas chromatogrwhs, capable of operating-at temperatures up to 500°C, are available from F. and M. Scientific Corn.. Avondale. Pennsvlvania: American Instrument Co., Silver Springs,
.
0
20
40
60
80
100
120
TIME. MINUTES
Figure 25. Heating ond cooling rater of the cclvmn oven of the Jarrell-Arh Capillary Anolyrer. Moximum heating rote. ---- Maximum cooling rote, with water Row.
Maryland; Consolidated Electrodynamics
Corp., Pasadena, California; Carlo E r h , Milan, Italy (U.S. distributor: Schueler and Co., New York 38, N. Y.). I n the case of the American Instrument Co. chromatograph, the oven contains t m heaters (900 and 400 watts maximum), both controlled by variable transformers. The larger heater provides a manually adjusted constant heat input; the smaller gives a supplementary adjustable heat input which is cycled by the oven temperature regulator to maintain a controlled temperature. The automatic temperature control is eflective up to 300-C; the precision is =t0.005'C. Above 300°C, the thermal insulation provides the resistance to temperature variations. Temperature Programs When a mixture of components which have widely varying volatilities is chromntographed on a column held a t a constant temperature, the law boilers come out a t short retention times, and tend to be hunched together, while the high boilers come out a t long retention times and tend to give broad, low peaks with poor detectability. This is illustrated by the chromatogram shown in Figure 26B. However, if the temperatnre of the column in low initially, and is increased as the chromatogram proceeds, then the retention time8 of the law boilers are increased, while those of the high boilers are decreased, and the components can be spread more or less evenly over the time axis, with the dramatic improvement in detectability illustrated in Figure 26A. A numher of commercial instruments is now available with provision for programming the temperature of the column during the run. The simplest type of temperature programming is the manual programmer, which consists of starting with the column a t some initial temperature, and msnually adjusting a variable transformer to a setting which gives a considerably higher power input than that which sufficed to maintain the initial temperature. If the sample is injected a t the same time, the column temperature u3l he climbing toward a new steady state during the course of the elution of the sample. The rate of temperature rise depends upon the magnitude of the transformer setting, and can he predetermined by calibration runs. This type of manual temperature programming is possible with any gas chromatograph that is supplied with a variable heater control. The temperature program has the disadvantage of being "on-linear, and the range of rates of temperature increase that can be chosen in this fashion is limited. Instruments that employ an electronic controller to raise the temperature a t a precise and predetermined rate are much more versatile, and have been widely adopted as they have hecome available commercially. An example of a linear programmed instrument of this class is the Model 500 Gas Chromatograph of F. and M. Scientific Corp. A thermocouple controller provides the temperature program. A thermocouple is employed to sense the temperature of the column and to provide a signal which deflects a moving indicator. The position of this indicator (Continued a page A88)
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relative t o a set point determines whether the column oven heater is turned on or
off; i.e., if the thermocouple indicator is below the set point, the heater is turned on
Figure 26. lllvrtrating the effect of w r y i n g the column temperotvre during o chromatographic run. Somple is a seven-component hydrocmrbon mixture, run on o 4-foot, 5-mm column pocked with 25% DC-200 silicone on 35-80 merh Chromororb; helium flow rate was 3 5 cclmin. A. Temperature B. Conrtont temperature of 16S°C. increared linearly at 6"C/minute. 1. pentane. 2. hexone. 3. heptane. 4. 1-octene. 5. decane. 6. 1-dodecane. 7. I-tetrmdecene.
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by the controller; if a t or above the set point, the heater is turned off. If the set point were t o remain a t a fixed locntian, t,he column temperature would be controlled a t a constant value. However, in the programmed instruments, the set point is caused to move upscale hy the rotation of a specially cut cam, so that the temperature a t which the system tries to regulate itself also rises. The function according to which the set point moves with time is called the program, and may he chosen to be linear, parsholic, exponential, etc., simply hy appropriate shaping of the driving esm. The F. and 11. Model 500 Gas Chromatograph employs a linear programmer, with n choice of 18 heating
Figure 27.
Column oven design d the
F.
ond
M. ScientiRc Corp. Model 5 0 0 Gar Chromotograph. Ports ore contained in the shield which can be opened to the room air t o increore the rote of cooling after ccmpletion o f a run.
(Conlinued on page A%)
rates from 0.29 to 43"C/minute. The design oi the column oven is shown in Figure 27. The linear temperature pragrammer unit is available as a separate component (Model 40, $795). An electronic temperature programmer based upon B platinum resistance thermometer as the sensor is available from Barber-Colman Company, Rockford, IUinois (Model 33, $035). Several other manufacturers now offer electronic temperature programmers, or complete temperature programmed gas chromatographs. These include: Perkin-Elmer Corp., Wilkens Instrument and Research, Loe Engineering Co., MicroTek Instruments, Inc., Research Specialties Co. Beokmm Instruments manufactures a temperature programmer with which it is particularly eavy ior the operator to select the function according to whlch the temperature will vary. A photo of this instrument, the ThermotraC, is shown in Figure 28. The operator traces the d e
Figure 28. Beckman ThermotraC temperature progrornmer.
sired program with a strip of black tape or with India ink on a Mylar sheet, which is mounted on a rotating drum. Zero and span potentiometers are set for the desired starting temperature and the span between starting temperature and maximum desired temperature. The drum rotates at a constant speed, and an optical iollower device tracks the black line and sets the control point for the thermoregulator. The unit shown contains the chromatograph column and oven in the chassis behind the rotating drum. A 1000-watt heater operated by a proportional rontroller maintains the temperature of the column within 0.1-C of the program for rates of rise as great as 30°C per minute.
Bibliography Lavelock, J . E., "A Sensitive Detector for Gas Chromatography," J. Chromat., 1,35 (1958). Lovelack, J. E., "Ionization Methods for the Analysis of Gases and Vapors," Anal. Chem., 33,162 (1961). Martin, A. J. P., and James, A. T., "GasLiquid Chromatography; the Gas Density Meter," Bioehem. J., 63, 138 (1956). Pecsok, R. L., ed., "Principles and Praetice of Gas Chromatography," J. V'iley and Sam, N. Y., 1950. N e d : Conlinuation of the survey of gas chromalographic inslrumentalion.
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