Versatile Mass Spectrometric inlet System for High Boiling Liquid Samples and for Thermoarialysis of Nonvolatile Materials Jacob Shen* and M. P. T. Bradley The Standard Oil Co. (Ohio), Warrensville Heights, Ohio 44 128
We report a versatile mass spectrometric sampling system for the introduction of both low and high boiling range liquid samples. The sampling system can also be used for mass spectrometric thermoanalysis of nonvolatile materials and for pyro1ysislMS study of polymers. A mass spectrometric sampling system for high boiling range liquid samples usually involves two vacuum stages kept at different temperatures. The viscous sample is first introduced into the first vacuum stage where air is removed by a vacuum pump. The first stage is usually kept at room temperature so that there will be no sample loss when air is pumped away. The sample is then moved into a second vacuum zone where heat is supplied for a total volatilization of the sample. Since there is a temperature difference between the two vacuum stages, the second stage is usually separated from the first one to prevent the diffusion of the sample vapor into the cold zone. The temperature maintained in the second zone has to be high enough so that adequate volatilization of the sample will take place ( I ) . Usually either Teflon seals or glass joints are used to provide vacuum seal for the high temperature zone. However, the use of Teflon seals limits the temperature in the second zone to 220 OC or less which in many cases is not satisfactory for the total vaporization of the sample ( I ) . The use of glass joints as internal seals permits higher temperature, but they are fragile, hard to provide maintenance service, and often cause considerable amounts of instrument down-time. The sampling system described in this paper uses DuPont Vespel resin to provide internal vacuum seal in the high temperature zone. The Vespel seal can stand a temperature of up to 350 "C which, in our experience, is adequate for the total volatilization of most of the high boiling range petroleum cuts. Glass lined stainless steel tubings are used for most of the transfer lines. The whole sampling system is therefore much more robust than an all-glass system. Figure 1shows the block diagram of the new liquid sampling
system used for a CEC-103C mass spectrometer. It has two parts: 1)a septum injection port for direct introduction of low boiling range samples, and 2) a two-stage sample inlet for high boiling range samples. Both ports are connected to a glass expansion volume. The septum injection port can be isolated from the expansion volume by a push button Teflon valve (Precision Sampling) outside of the heated zone. In our experience, the septum can maintain the vacuum (10-7 Torr) effectively even when the Teflon valve is open. Any liquid sample of low viscosity can be introduced through this port directly using a microsyringe. The second port used for high boiling range samples has two stages. A mechanic/diffusion pump system is used to remove air from the first stage of the sampling port which can be isolated by a toggle valve. The second stage is kept at high temperature and can be isolated from the expansion volume by a Valco high temperature switching valve. The expansion volume can be isolated from the same vacuum line by a Whitey ball valve. Figure 2 shows in more detail the inlet system for high boiling range samples which usually have high viscosity. The inlet system has two parts which can be detached from each other with ease. Part (B) is a stainless steel flash vaporizer enclosed in a metal jacket (I) using a cartridge heater (J).The flash vaporizer (B) has its shoulder in a female nut to provide a sealing surface against a Vespel insert (E) located at the end of part (A). The Vespel insert (E) functions both as an internal vacuum seal as well as an internal probe guide for the sample probe (C). A second Vespel insert (G) is also used in the front end of the flash vaporizer to guide the sample probe to reach a proper position. A ball valve (D) located outside of the heated zone is used to isolate zone (A) from zone (B). Typically the sample is introduced through the sample probe (C) having a quartz capillary tip (H) whose volume varies from 0.001 to 0.005 ml. The capillary tips of various volumes are interchangeable. The quartz capillary tip containing the sample is introduced through the sample probe first into zone (A) with the ball valve (D) closed. Air is then evacuated. The ball valve
ISOLATION
PUMP
VALVE
M
- -- - \OT
ZONE
Valve
I GLASS EXPANSION
1 B"H I
I I
I
VACUUM ZONE
YACUUM
I
ZONE
I I
1I
I I
I I I I
Valve
-.J Figure 2. Sample inlet system for high boiling range liquid and thermoanalysis of nonvolatile materials
Figure 1. Block diagram of the dual port liquid sample inlet system (A) Septum injection port for low boiling liquid samples. (B) Sample inlet for high boiling liquid samples
for
(A) First vacuum stage. (B) Flash vaporizing zone. (C) Sample probe. (D) Ball valve. (E)Vespel insert. (F) Stainless steel shoulder of B. (G) Vespel insert. (H) Quartz capillary tip. (I) Metal jacket. (J) Cartridge heater. (K) Glass liner. (L) Switching valve. (M) Insulation boards
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
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Table I. Comparison of Results for a High Boiling Range Aromatic Cut (Vol. W ) Original New sampling sampling system system Monoaromatics 24.3 6.0 12.1 0.1 Alkylbenzenes 10.2 5.9 Naphthenebenzenes 2.0 0.0 Dinaphthalenebenzenes Diaromatics 23.4 30.1 18.4 Naphthalenes 9.9 10.6 Acenaphthalenes, 13.6 dibenzofurans 1.1 Fluorenes 0.0 Triaromatics 42.3 56.8 51.1 Phenanthrenes 38.6 Naphthenephenanthrenes 3.6 5.7 Tetraromatics 0.0
0.0
Pyrenes Chrysenes Pentaaromatics
0.0
0.0 0.0
0.0
0.0
Perylenes Dibenzanthracenes Thiophenoaromatics
0.0 0.0 0.0
0.0 0.0
7.2
10.0
Benzothiophenes Dibenzothiophenes Naphthobenzothiophenes
2.4 7.6
3.4 3.8
0.0
0.0
(D) is opened and the sample probe (C) is allowed to enter into the flash vaporizing zone (B) via the internal Vespel guides. The vaporized sample then enters into the glass expansion volume which is heated by two bar heaters directly underneath the volume. A switching valve (L) is used to isolate the expansion volume from the flash vaporizer when needed. The whole heated zone is insulated by individual rigid boards which can be detached from each other. The use of rigid insulation boards forms a heated zone where temperature is kept a t 300 to 350 O C . This eliminates the use of heating tape at various parts of the sampling system. The rigid boards also provide an easy access for the whole inlet system when maintenance service is needed. Table I shows the comparison of results for a high boiling range aromatic sample using both the original as well as the new sampling system. Robinson and Cook's method ( 2 )was used for the analysis of the sample which was expected to be high in di- and triaromatics and very low in monoaromatics. However, the result obtained using the original sampling system showed a relatively significant amount (24%) of monoaromatics. The result could not be explained until the same analysis was repeated using the new sampling system. This time the result showed as expected about 6% of monoaromatics and a total of 87% of di- and triaromatics. The same sample has been sent to Gulf Research and Development Company (Pittsburgh, Pa.) for similar analysis. The results from both laboratories agreed to within 5%. The original sampling system involving the use of Teflon seals apparently has failed to provide a temperature high enough for the total volatilization of the sample (1).
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NO. 14,
The new sampling system can be generally applied to any high boiling distillates or residues with a boiling range from 600 O F to at least 1300 O F . For a liquid with an end point of less than 600 O F , it can be easily introduced through the septum injection port using a micro syringe. It should be noted that the first vacuum stage of the two-stage sampling system is not cooled. Therefore, if a mixture containing both low and high boiling compounds is to be introduced, modifications should be first made to cool the first vacuum zone to avoid the loss of any low boiling compounds. However, if the viscosity of the sample is low, it can also be introduced through the septum injection port. We have now used the new sampling system for over six months. The Vespel inserts so far still perform satisfactorily. It has been once overheated to 700 O F accidentally for a short length of time. The Vespel inserts showed some expansion and the sampling probe became hard to withdraw. After returning to a lower temperature, however, the performance of the Vespel seal was recovered and no further damage was observed. While the new sampling system is now routinely used in this laboratory for the hydrocarbon type analysis of high boiling range samples, it can also be used for mass spectrometric thermoanalysis of nonvolatile materials (3-5). By replacing the quartz capillary tip with a quartz tube of larger id., solid samples can be easily introduced into the flash vaporizing zone using the same sample probe. Thus, the evolution of volatile components from nonvolatile samples can be monitored mass spectroscopically at any temperature up to 350 "C. The technique is convenient because the off gases can be transferred in situ from the flash vaporizer to the ion source. Since the processing temperature of polymers is usually below 350 "C, the new inlet system can be easily used to study the organic volatiles evolved during the processing of polymers. Another application made possible using the new sampling system is the direct pyrolysis of polymers in the sample inlet of a mass spectrometer. Both the platinum ribbon probe and the coil probe from Chemical Data Systems (Oxford, Pa.) can be used to replace the sample probe (C). The polymer sample, either coated onto a platinum ribbon or introduced through a quartz tube placed in the middle of a coil probe, can be pyrolyzed in the flash vaporizer. Such a combined pyrolysis/MS system can be conveniently used for the characterization, the structural study, and the study of thermodegradation mechanism of polymers (6-8). The results of these applications will be the subject of separate reports.
LITERATURE CITED (1) J. E. Schiller and C. L. Knudson, Anal. Chem., 48, 453 (1976). (2) C. J. Robinson and G. L. Cook, Anal. Chem., 41, 1548 (1969). (3) R. H. Wiley and L. H. Smithson, Jr., J. Macromol. Sci., Part A, 2 (3), 589 (1968). (4) W. T. Flowers, R. N. Haszeldine, E. Henderson, A. K. Lee, and R. D. Sedgwick, J. Polym. Sci., 10, 3489 (1972). (5) G. M. Armitage and S. J. Lyle, Talanta, 20, 315 (1973). (6) H. L. C. Meuzelaar, M. A. Posthumus, P. G. Kistemaker,and J. Kistemaker, Anal. Chem., 45, 1546 (1973). (7) D.0. Hummel, H. J. Dussel, K. Rubenacker, and T. H. Schweren, Makromol. Chem., 145, 259 (1971). (8) D. 0.Hummel, H. J. Dussel, and K. Rubenacker, Makromol. Chem., 145, 267 (1971).
RECEIVEDfor review July 26, 1976. Accepted August 30, 1976.
DECEMBER 1976
