Rapid Mass Spectrornetric-Gas Chromatographic Analysis of Nonolefinic Naphthas V. A. Cirillo, D. J. Skahan, Betty Hollis, and Harry Morgan, The Atlantic Refining Co., Philadelphia, Pa. analysis of nonolefinic naphthas, using low temperature fractional distillation to remove C6 and lighter components in combination with a mass spectrometer hydrocarbon-type analysis @,I),has been standard practice in the petroleum industry for a number of years. Since nonolefinic naphthas in this laboratory represent a major part of the analytical work load, a method was sought to minimize the time reGas quired for their analysia. chromatographic backflushing p ~ o cedures to recover CS and heavier material proved unreliable. The use of a rapid one-plate distillation for the removal of the Cs and lighter hydrocarbons was also unsatisfactory. The method described here ia similar in some respects to that of Howard and Ferguson (3). They used a mercury orifice and constant-volume pipets to introduce total naphtha into the mass spectrometer and determined the Cs and lighter material by gas chromatography. In the present method no mercury orifice is necessary, constantvolume pipets are not used, and the computational procedure is different. THE
The C6 and lighter material is determined by gas chromatography, using a 15-foot column of 20% by weight Dow Corning silicone grease on a (2-22 acidwashed firebrick, 28- to 42-mesh. The column temperature is 110" C., with a helium flow of 50 cc. per minute. The total time required for determining C2 to Ca is 3 minutes. If a sample contains more than 15% of the C5 and lighter components, cyclopentane is included in the gas chromatographic analysis, by means of a 25foot column using isoquinoline as the substrate. This increases the gas chromatographic analysis time from 3 to 15 minutes. To introduce total naphtha into the mass spectrometer, remove the sample from the refrigerator ( ~ 3 8 "F.). Insert a probe into the sample and introduce it through a silicone stopper (Chase Walton Elastomers, Inc.). Raise the temperature to 450" F. for 2 minutes. Obtain the spectrum. Figure 1 illustrates in detail the introduction of total naphtha into the mass spectrometer. Total naphtha contains Cc to Clz nonolefinic hydrocarbons with up to 30% of CSand lighter material. Samples seldom contain more than 30% of the lighter hydrocarbons. The introductory probe is a No. 20 hypodermic needle notched on one side, with the portion above the notch sealed with silver solder. The system is completely sealed during the time of
Silver Solder N
450OF.
Figure 1. Diagram of sample introduction port and sample injector
sample injection, as the capillary containing the sample is shorter than the rubber stopper, as shown. To prevent cross contamination and because small memory effects are associated with the silicone rubber, a new rubber stopper is used for each sample. No time is lost here, because during the 12 minutes
Table 1.
Methane Ethane Propene Propane Isobutane n-Butane Cyclopentane Isopentane n-Pentane Paraffin CS C7
c; c 9
CIO 9 1 1
G Z Cycloparaffin Dicycloparaffin Tricyclopnrafin Alkylbcnzene CS
c7 CS
c 9
c*o c 1 1 c 1 2
Indan Naphthalene Total
+
Gravity, Cs btms. Measured Calculated a
11.3 7.8 6.4 5.9 1.2
... ...
0.5 1.6
...
, . .
4.8 4.5 11.7 7.3 6.3 5.3 1. o
12.3 8.1 6.7 5.3 0.8
12.4 7.9 6.8 5.0 0.7
...
...
...
...
Sample 3 Pod4 G.C.b
...
... 0.4 1.4 ... 4.4 4.3
...
... . I .
... ... ... ...
...
0.4
...
11.41
4.6 6.9
13.1 13.5 13.8 9.8 3.5
12.6 13.G 13.2 9.4 3.8
... ...
... ...
3.7 0.3 0.3
3.7 0.3 0.3
24.6 1.1 ...
24.7
4.0 15.7 17.7 12.4 2.5 0.2
3.9 15.7 17.7 12.2 2.3
4.1 15.8 17.8 12.0 2.4 0.2
1 .o 2.4 3.2 1.9 0.2
... 0.1 -
0.1
1.1 2.6 3.4 1.9 0.2 ... 0.1 0.1 0.5
3.7 0.3 0.3
3.8 15.2 17.5 12.2 2.5 0.2 0.1 100.0
Sample 2 Poda G.C.b
0.3 1.6 0.2 4.7 3.8
3.8 0.3 0.4
... ...
Although this technique gave excellent results, it was preferable not to expose the entire inlet manifold to the atmosphere, so the valve shown in Figure 2 was installed. With this valve, the inlet volume exposed to the atmosphere, during the time required to change the rubber stopper, is considerably less than the volume previously exposed-5 cc. us. 200 cc. At first, leaks were encountered a t the O-ring, but a larger O-ring rectified this. The possibility of cross contamination from trapped gas was investigated, using Cg and heavier hydrocarbons. There was no cross contamination. None was detected in degassing tests. The quality of the results demonstrates that, if present, the effect of hydro-
Naphtha Analyses
Sample 1 Podo . G.C.b ... ... ... ... ... ... 0.1 ... 0.4 1.6 0.2 5.2 4.0
required for running a sample, the operator has ample time to remove the used stopper, insert a new one, and pump the inlet manifold in readiness for the next sample.
...
100.0
... ... ...
1oo.o
...
... ... ...
loo.0
136-T
... 0.1
0.806 0.797
0.5
0.809 0.799
1 .o
...
1oo.o 0.740 0.729
Low temperature fractional distillation, sintered disk.
* Gas chromatography, rubber disk.
VOL 34, NO. 10, SEPTEMBER 1962
1353
Teflon Valve
ONE RUBBER
is then calculated (1). Using the average carbon numbers of the paraffins and the alkylbenzenes, the approximate density of the CN portion of the sample can be calculated. Density,
+ +
= 0.66 0.02 (carbon number, 6) Density,,, = 0.75 0.01 (carbon number, - 6)
Density,b
-
=
0.876
where p indicates paraffins, cyp indicates cycloparaffins, and ab indicates alkylbenzenes. First approximate density of Cw
=
% p X density,
With the present method, no loss in accuracy or reproducibility was experienced, and a considerable amount of costly distillation time was eliminated.
+ % cyp X density,,, + % ab x density,t, x%
M S BOTTLE
Figure valve
To demonstrate the validity of the method, samples were analyzed by low temperature fractional distillation and mass spectrometry (Table I). The C6+ portions were introduced into the mass spectrometer through a heated indium-covered sintered disk and the results were compared with those obtained using the gas chromatographmass spectrometer method.
2. Improved
inlet
carbon absorption on the valve itself must be negligible. The arithmetic procedures for correcting the mass spectrum of total naphtha for the contributions of the Cs and lighter components obtained by gas chromatography, are shown below. After correction for light ends, the hydrocarbon-type analysis is computed in the usual manner ( I ) . The mass spectrum of thc total naphtha is obtained, and the pressure is recorded. The pressure is corrected for any air that may be present. Ail approximate hydrocarbon-type analysis
From the carbon numbers of the Daraffins and alkvlbenzenes and the approximate type inalysis, the molecular weight Of the ‘st can be computed. Approximate MW of Cs+ =
yo p jS6
P, CYP, ab ACKNOWLEDGMENT
The authors express appreciation to J. Colucci for assistance in obtaining the mass spectra.
+ 14 (carbonLT,number, - 611 + 70 cyp [84 + 14 (carbon number, ab 178 + 14 (carbon number.h - 6)1 2
% P, CYP,ab
The volume percentages of light comI)OllentS determined by gas Chromatography are multiplied by appropriate volume-mole factors and normalized to the total microns of air-free sample. The molar contribution of each light component is calculated a t masses 41, 43, 55, 57, 67, 69, 7’. hydrocarbon-type analysis is then recalculated, using 2 values COrreCted for the light components.
Gas Chromatography of Food Volatiles-An
- 6)l +
LITERATURE CITED
(1) Am. SOC. Testing Materials, Philadelphia, Pa., “ASTM Standards on Petroleum Products and Lubricants.” 38th ed., Vol. 1, Appendix VII, pp. 1120-7, 1961. R. ANAL. 239 430 (3) Howard, H. E,, Fprguson, W. (>,,
(2{l:&’yn’
Zbid., 31, 1048 (1959). (4) Lumpkin, H. E., Thomas, B, W., Elliot, Annelle, Zbzd., 24, 1389 (1952).
Improved Collection System
Irwin Hornstein and Patrick F. Crowe, Meat Laboratory, Eastern Utilization Research and Development Division, Agricultural Research Service, United States Department of Agriculture, Beltsville, Md.
AS CHROMATOGRAPHIC ANALYSIS Of
G trace volatiles that contribute flavor
to foods requires preliminary concentration. Collection and concentration are effected by passing a stream of air or nitrogen over the samples and condensing the volatile compounds in a packed or unpacked refrigerated trap (2, 5, 6). Sawar and Fagerson (4) in an effort to improve the efficiency of the freeze-out process have recycled air or nitrogen in a closed system that incorporates a refrigerated trap. The collected volatiles are then heated to facilitate transfer and introduced onto the column through a series of valves or stopcocks. These techniques are cumbersome and during transfer material may be lost by condensation and adsorption on connecting surfaces; in 1354
ANALYTICAL CHEMISTRY
addition, the subsequent decontamination of traps and valves represents a formidable problem. It has also been suggested that, for quality control purposes, vapors above heated vegetables and food products can be sampled with a syringe and injected directly into the gas liquid chromatography (GLC) unit (1 3). Here, too, particularly at the high sensitivities required, contamination from the syringe and the septum in the heated injection port can lead t o extraneous peaks in subsequent analyses unless additional bake-out or cleanup procedures are used. The technique described here utilizes a refrigerated, stainless steel or copper coil filled with column material for the collection trap. Swagelok
fittings are used t o make this coil an integral part of the column. The chromatogram is obtained without any gas transfer systems and without the use of heated injection ports, and the collection coil itself can be used repeatedly without further cleanup. EXPERIMENTAL
Apparatus. The gas chromatograph is a n F&M Model 609 equipped with a flame ionization detector and incorporating linear temperature programming. The insulated column heating chamber is cylindrical, 7 inches i.d., and 7 inches in height. The columns described are fitted t o these dimensions. Columns are 7.5 feet long, packed with
