Capillary Column Efficiencies in Gas ChromatographyMass Spectral Analyses ROY TERANISHI, RON G. BUTTERY, W. H. McFADDEN, T. R. MON, and JAN WASSERMAN Western Regional Research Laboratory, U. S. Department of Agriculture, Albany, Calif.
b Column efficiencies have been calculated with n-hexanol, n-amyl acetate, n-octanal, and limonene, and results show no significant loss in efficiency by operating one end of the capillary column under mass spectrometer vacuum. Optimum efficiency for the compounds examined and for the columns used is achieved at a linear velocity of 15 cm. per second under vacuum or at atmospheric pressure. Operating temperature range and extent of stationary liquid phase bleed from several liquids coated on stainless steel capillary columns have been examined with hydrogen flame ionization detector and with a fast-scan mass spectrometer. Limitations of the various stationary phases for combined mass spectral-GLC analysis at 200" C. are discussed.
K
Im,ia,
Bate, Costa, and Forman
( 6 ) , Keller and Stewart ( 7 ) , and Preston (13) have recently discussed
limitations and changes occurring in stationary liquid phases used in gas liquid partition chromatography with packed columns. Programmed temperature control of capillary columns permits analysis of very comples mistures of wide boiling ranges (16, 16). A fast-scan mass spectrometer can be used to monitor directly the material separated by such columns ( I , 4,6,9-11). Such experimentation imposes another limitation on the operating temperature range of stationary liquids: column bleeding must not obscure the mass spectral patterns of the material being separated and analyzed. Also, if the column is operated with a vacuum a t the esit, the question arises whether the column is operating as efficiently as with the conventional atmospheric pressure at the esit. We wish to describe the stationary liquids we have used with stainless steel capillary columns, the extent of column bleed as detected with the hydrogen flame ionization detector and with the mass spectrometer, and the comparison of capillary column efficiencies with the detector end of the calillary column at atmospheric pressure to the hydrogen flame detector with those a t vacuum to the mass wectrometer.
EXPERIMENTAL
Apparatus and Procedure. T h e mass spectral d a t a were obtained with t h e 13endix time-of-flight mass spectrometer, Model 12, a n d r c m r d e d on a Minneapolis-Honeywell Visacorder. T h e combination of fast-scan mass spectrometry with capillary column gas chromatogra1)hy and sointx rt,s:!lts have been described ( 2 , $1-11, 15). The mass spectral chromatogram? for the theoretical illate calculations were obtained by recording the ionization due to mass 43 for n-hexanol, n-amyl acetate, and n-octanal and that due to mass 41 for limonene. The gas chromatography equipment used with the mass spectrometer was built, in our laboratory (14, 16). The hydrogen flame ionization detectors and electrometers were patterned after those described by McWilliam and Dewar ( l a ) , and the signals from the electrometers were recorded on Varian G-14 recorders. The background currents were measured with a Keithley electrometer, Model 600. The stainless steel capillary columns, 0.01-inch i d . , 0,022-inch o.d., 200 feet long, were purchased from Superior Tube Co., Norrist,own, Pa,, and from J. Bishop and Co., Malvern, Pa. These columns were wound on aluminum spools 3 inches in diameter and 3 inches tall. The column spool is heated with a 400-watt disk heater fastened to the bottom (16). The injector and streamsplit,ter unit, mounted on top of the column spool, is heated with two 50-watt cartridge heaters inserted into an aluminum heater block. Both of these heater systems are connected to variable transformers for manual heat control. This column-injector unit is moved easily from the hydrogen flame ionization detector to the mass spectromet'er. Two methods of coating capillary columns have been summarized by Littlewood (8). More vigorous conditions were used in coating and conditioning t,he columns used in these experiments than previously described. Three portions of 100 to 200 p l . of 10 to 2070 (w./w.) solutionsof stationary liquid in chloroform were forced through the columns at room temperature with nitrogen at several hundred pounds per square inch pressure, flow from injector t,o detector end. The solvent, and excess stationary liquid were allowed to blow out for several hours between each portion. Then all columns were condit,ioned for at least several days at
150" to 175" C. for Tween-20 and 225" to 250" C. for the silicone oils, until no more stationary liquid emerged from the column. The flow of nitrogen during the conditioning was from the detector to the injector end a t 50 to 100 11.s.i. With such conditioning, the columns could be used immediately after installation in the inst,ruments. If a column had a low theoretical plate value, it was washed with the series of solvents, chloroform, ether, acetone, water, and then back to chloroform again and then it was recoated. Carr was taken to use small portions of solvents, or the small diameter column5 would be permanently plugged, especially if a n apl~reciable amount of stationary liquid was in the column. Because of the method of coating, no attempt was made to determine the amount of stationary liquid left in these small diameter columns. However, the amounts left in larger tubing were determined. These results showed that very little is accomplished by using more than three portions. The stationary liquids used w r e : silicone oil SF 96-50 (General Electric), silicone oil DC 710 (Dow Corning), silicone oil Versilube F50 (General Electric), polyethylene glycol Carbonax 1540 (Union Carbide), and polyethylene sorbitan monolaurate Tween-20 (.\tlas Powder). The conditions used for obtaining the theoretical plate d a t a were: detector temperature, 115" f 5" C . ; injector temperature, 225" f 10" C.; column temperature, 125" i 2 " C . ; helium carrier gas average linear velocit'y, 15 f l cm. per second, unless varied for H E T P studies. The sample loads were 0.2 p1. split 1/200 with t,he hydrogen flame detector and 1.0 pl, split ljl00 with the mass spectrometer. The average linear velocities werr calculated from the time required for met'hane to arrive a t the hydrogen flame detector, and argon a t the mass spectrometer. The compounds limonene, n-amyl acetate, n-octanal, and n-hexanol were chosen because these are represent,ative of compounds encountered in our research. These are commercially available. Purification was not necessary since the capillary columns separated the impurities from the main material, and the mass spectral d a t a verified thc chemical structure of t,he material used. Theoretical plate values were calculattd on the peaks from the compounds 1 ) ~ .th(, methods recommendrd (S). VOL. 36, NO. 8, JULY 1964
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0.2 I I 1 I I 20 30 40 AVERAGE LINEAR VELOCITY (cm./sec.) Figures 1 to 4. HETP curves, 200 foot, 0.01 -inch i.d.
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Figure 1 . Figure 3.
SF 9 6 - 5 0 , hydrogen flame C a r b o w o x 1540, hydrogen flame
RESULTS A N D DISCUSSION
of Column. Table I summarizes t h e comparison of the HETP Efficiency
d a t a obtained with various stationary liquids: column temperature, 125" C . , average linear velocity of helium carrier gas, 15 em. per second. These d a t a show t h a t , within esperimental error, the column efficiencies are not significantly reduced in changing the capillary columns from the hydrogen flame detector to t'he mass spectrometer conditions. Figure 1 shows the H E T P relationship with the change of linear velocity of helium carrier gas with silicon oil SF 96-50 stationary liquid, hydrogen flame detector, and observed with n-hesanol, n-octanal, n-amyl acetate, and limonene. Figure 2 shows a similar relationship with the column attached to the mass spectrometer. With this htationary liquid, there is some loss in column efficiency calculated with nhesanol in changing from the hydrogen 1510
ANALYTICAL CHEMISTRY
0
I IO
I 1 20 30 AVERAGE LINEAR VELOCITY (cm./sec.) ,
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n-hexanol, A n-amyl acetate, 0 n-octonal, -f limonene
Figure 2. Figure 4.
SF 9 6 - 5 0 , mass spectrometer C a r b o w a x 1 5 4 0 , mars spectrometer
flame detector to the mass spectrometer, but there is none with the other compounds. Figures 3 and 4 show similar curves with Carbowas 1540 stationary liquid, Table I.
Stationary liquid
SF 96-50
IIC 710
Versilube F50 Carbowax 1540 Tween-20
40
and in this case, there is little loss in efficiency with n-hesanol when the mash spectrometer is used. With limonene, there seems to be an increase in efficiency, whereas, with n-amyl acetate,
HETP Comparison, 125" C., 15 cm. per second
Compound n-Hexanol n-Amyl acetate n-Octanal Limonene n-Hexanol n-Amyl acetate Limonene n-Hesanol n-Amyl acetate Limonene n-Hesanol %-Amylaretate n-Ortanal Limonene n-Hexanol n-Amyl acetate n-Octanal Limonene
Hydrogen flame, cm. 0.17 0.10 0.12 0.07
(tailing) 0.13 0.12 0.12 0.19 0.06 0 20 0.12 0 21 0.28
0.20
0.05 0 13 0.06
Mass
spectrometer, cm. 0 35
0.12 0.12 0 09
(tailing) 0 08
0.09 0 22 0.14
0.09
0 22 0.15 0 '10
0.2'7 0 12
0.12 0 12
Table II.
Increase in Hydrogen Flame Background Current
D o w Corning 710 200 C
Current increase Liquid SF 96-50
DC 710 1-ersilube F5O Carbonax 1540 Tween-20
(1 x lo-"
Temperature range, "C.
amp.)
500-200:
0 7
35"-200 35 "-225
6 0
35"-200"
50"-200
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column efficiency is quite good under both conditions. Similar curves were obtained with other stationary liquids. An esamination of these H E T P curves shows that a good compromise, if time of analysis and column efficiency are considered, is a t 15 em. per second for operating most of these capillary columns with the hydrogen flame ionization detect'ors and with the mass spectrometer. Temperature Stability of Column. Table I1 summarizes the increase in t h e background current of t h e hydrogen flame ionization detector from t h e stationary liquid bleeding a t 200' C . T h e bleed from t h e Carbowax 1540 a n d from Tween-20 is less t h a n t h a t from silicone oil D C 710. Bleed from this silicone oil can probably be lessened with more vigorous conditioning. The longer life observed with Carbowax 1540 and Tween-20 with capillary columns as compared with packed columns is probably caused by the fact that our capillary columns are usually Irogrammed, and whatever oxygen is injected with the sample is eluted a t low temperatures. The increase in background current observed by the mass spectrometer a t 200' C . has not been included in Table I1 for various reasons. Because the ionization efficiencies of a hydrogen flame detector and a mass spectrometer are different, a n integrat,ed current value for the ions observed in the mass spectrometer would be misleading. Ionization due to CO+, C02+, HzO+ are significant in the mass spectrometer but are not in the flame detector. Furthermore, because the ionization in the mass spectrometer is separated into various fragment ions, the important consideration is not the amount of ionization caused by column bleed but the masses at which the ionization is observed. Figure 5 presents a mass spectral comparison of the column bleed a t 200" C. for IIC 710, Carbowax 1540, and Versilutx F5O. The l'we~n-20 pattern was very similar to that of t h r Carbowas 1540; the S F 96-50 was similar to that of I)(- 710; but in the i m l ~ r t a n trcigion ahoy(, mass 50, it is
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200' C
Figure 5. Mass spectral patterns of column bleed at w a x 1540, and Versilube F50
only about one fifth as intense and is much preferred for this type of work. I t is apparent from Figure 5 that a considerable evolution of COz (mass 44) and CO (mass 28) occurs. The S2 background (mass 28) which is present with a partial pressure of 7 to 10 X torr is about 3 times the observed 02 (mass 32) background. Xot shown in Figure 5 is the increase in background due to H20 (ionization at masses 18, 17, and 16). This is considerable for any column we have used and varies a great deal depending on the history of the column. C 0 2 and CO also seem to be dependent upon previous history, and it is concluded that these are probably slowly desorbed from the stationary phase. As a n example, one experiment with a Versilube F50 column that had been unused for several weeks gave, a t 200" C., an ion current due to H20+, O H + , and O+ of 2 X 10-12amp. (based on a n estimated electron multiplier gain of lo6). This is about 100 times higher than the normally observed HzO background. Compounds eluted a t temperatures higher than 150' C. are usually above the molecular weight 150. Mass spectral patterns for such compounds generally give characteristic ion fragments above mass 50 and only a few esceptions such as mass 31 (primary alcohols) or mass 44 (aldehydes), etc., are encountered. This is fortunate because the ionization from the various substances bled from a column a t 200" C. is generally more than 80% hcloxv inass 45. Thus, most esters, kctoncy arrtals, terpenes, aromatics,
44
200" C. for DC 710,
Carbo-
etc., that are eluted above 150" C. give their characteristic ionization above the region of serious column bleed. In our experience the ionization frequently observed a t mass 73 is the most troublesome. The scan rates required for combined mass spectral-GLC analysis (mass range 20 to 200 in 1 to 4 seconds) dictate t h e use of an amplifying system with a time constant which is not less than second, and, as a consequence, use of an electron multiplier is highly preferred. This usually means that the sensitivity of the combined method is limited by the extent of the column bleed. As would be expected, we do not consider the D C 710 to be a n acceptable column above 170" C. but the SF 96-50 column will generally be usable to 200°C. 130th the Tween-20 and the Carbowas 1540 have proved to be sufficiently stable for most purposes u p to 200' C. The Versilube F50 gives very little bleeding even at higher temperatures, and a significant part of that shown in Figure 5 above mass 44 is due to other background causes. Although it is clear that sensitivity is limited a t higher temperatures by column bleeding, it is unlikely that large increases in sensitivity can be easily gained. A tenfold increase in amplifier sensitivity would make the natural mass spectral background troublesome. Statistical limitations also exist. If a mass spectral peak is to be scanned in second, then an ion current, of amp. will correspond t o onlj. about 100 ions. In spite of this, the complcti. introVOL. 36, NO. 8 , JULY 1964
151 1
duction of the sarnple in 15 to 20 seconds leads to considerably higher sensitivities (in terms of the amount of sample required) than is obtained in conventional batch inlet mass spectrometry. Suitable mass spectral signals can be obtained with gram of a compound passing through the capillary column, and in special ca.ses, identification can be made with to IO-" gram. However, the smaller limits require that a very favorable niass hpectral pattern is formed, and in general, interpretation of a mass hpectral pattern may require as much as 30 t o 100 times the signal required for detection. The column bleed patterns shown in Figure 5 are typical and will vary a modest amount from colunin t a column and with uhe of the column. However, holm that less rigorous conditioning permits escessive bleed a t lT5" to 200" C. for most columns and frequently obscures the mass spectral pattern of the separated material.
ACKNOWLEDGMENT
The authors thank . J o ~ I I'cruzzi ~ for excellent cooperation in the) (Jonstruction of the instruments and I). C. Patterson for his help. LITERATURE CITED
(1) Brunnee, C., Jenckel, I,, Kronentierger, K.,10th Annual Jleeting of .4ST&I Committee E-14, S e i \ Orleans, T,a .Tune - ~~~- 1962 -( 2 )Buttery, R . G., lIcF:tdden, LV. H., Teranishi, I t , , Kealv, 31. P., l l u n , T. I Coree, J. LV., AI(.: Fadden, LV. H., Black, 1). It,, Llorgan, h. I., J . Food Sci. 28, 478 (19G3). (16) Teranishi, R., S i n i m o , C. C., Corse, J., ;\S.%I..CHEM. 32, 1384 (1960). RECEIVEDfor review Jlarch 26, 1964. 29, 1964. 2nd Interi i i r r i on Advances in Gas , University of Houston, Houston, Texas, ;\larch 2 - 2 6 , 1964. Reference to a company or product, name does not imply approval or recommendation of the product hy the C . S. Ikpartrnent of Agriculture to the esclusion of others that may he suitable.
Simulated Distillation by Gas Chromatography L. E. GREEN, L. J. SCHMAUCH, and J. C. WORMAN Research and Development Department, American Oil Co ., Whiting, Ind.
b Applications of gas chromatography to obtain distillation-type data have been reported, but efforts thus far have been aimed at reproducing low efficiency distillations. This paper describes a gas chromatographic method fo robtaining a boiling pointdistribution curve, equivalent to that obtained from a 1 00-theoreticalplate analytical distillation. An automated, temperature programmed instrument is coupled with an electronic integrator to print the per cent off on a paper tape at regular time intervals. The instrument is calibrated with a known blend of pure hydrocarbons. Only 1 pl. of sample is required. An average analysis takes one hour, which is only 1% of the time required for a distillation of comparable precision; furthermore the initial and final boiling ranges are better defined, and the method accounts for the total sample. One operator can use as many as three instruments simultaneously. The method has been applied to petroleum distillates with boiling points up to 1000" F., and to samples containing as much as 85% nonvolatiles. It has been used widely in bench-scale and pilot plant studies of hydrocarbon reactions, in process studies of crude oil distillation, in fuel blending, and in the analysis of natural gas condensates. 151 2
ANALYTICAL CHEMISTRY
S
R A L RAPIII but relatively crude distillation methods, such as ASTM 11-86 (I), are used commonly to determine t,he boiling range of ,petroleum fractions. 1Iany correlations have been made in an attempt to establish useful relationships between product properties or Ilrocess operating conditions and .\SIX boiling range. but their usefulness is restricted by the inherent error of drawing conclusions from limited data. Such one-plate distillations are also inaccurate in the initial and final boiling ranges and approach a true picture only near the midpoint. I3oiling range is determined much more accurately by a precise analytical distillation, commonly called true boiling point (1'13P) distillation. This term has many connotations, for there is no standard T13P distillation. For the purl)ose of this paper it is defined as a distillation on a column operating at an efficiency of 100 theoretical plates, x i t h data observations a t 1YG fractions. .I typical distillation requires a t least 100 hours. The data have been used estensively for 1)rocess design and pilot 1)lant studies, and they nould be extremely useful for control of manufacturing processes. However, the time required makes the method of no value for control, and the expense greatly limits its use for researcah. 1,ike XSTlI D-86, the lengthy TI3T'
distillation fails to establish the initial or final boiling l~oints accurately. Seither method completely accounts for the total sample charged, and as the amounts of sample boiling below room temperature and above 500" F. incwase, theze deficiencies become more >eriou>. I%ecauseof dihtillation column holdup. false bottom>-high boiling compounds-must be added to the sample if all of the eaml)le is to br distilled. Even so. it is difficult to determine exactly where the sample ends and the false bottoms hegin. To prevent cracking of sample components, the distillation pot temperature niust not be allowed to rise above 500" I?. K h e n that temlierature is reached, the distillation must be completed under reduced pressure. Thus, pressure variations, loss of noncondensable light ends, loss during pressure reduction and vacuum operation, and column holduIi very severely limit the confidence to be plaoed in Tf3P distillations of broad-boiling sanil~les. Gas chromatography offers a completcly neJV means of obtaining boiling ~)oint-distribution data. and several authors have re1)orted such apl)lications. Rggprtsen, Groennings, and Holst ( 3 ) dercribe a gas chromatographic method whicah yields distillation (lata rquivalrnt to that obtained from I O t o 20
