2995
Anal. Chem. 1985, 57,2995-2996
Mass Spectrometric Characterization of High-Temperature Outgassing of Anisotropic Pyrolytic Boron Nitride Paul L. Fortucci, Vincent D. Meyer, and Edward K. Pang* GTE Lighting Products, Technical Assistance Laboratory, Danvers, Massachusetts 01923 Mass spectrometry is a useful tool for industrial quality control. An important application is the measurement of the outgassing rate of materials used in a high-temperature and high-vacuum environment such as the fabrication of semiconductors (1). For example, anisotropic pyrolytic boron nitride (APBN) increasingly finds applications in areas such as material evaporation (molecular beam epitaxy), crystal growing, and zone refining. Its outgassing characteristics are important for such applications. Hu (2) recently reported a pyrolytic mass spectrometric method for rapid characterization of polymeric materials, but the all-glass pyrolysis unit described is not suitable for high-temperature applications. This paper describes a system for fast mass spectrometric characterization of high-temperature outgassing measurements and measuring the total quantity of gas evolved. The major outgassing unit is a quartz sample tube (25-mm 0.d. X 250-mm length) with graded seal (25-mm 0.d. X 64-mm length) which is connected to an all-glass inlet system (Figure 1). The inlet system consists of the gas-handling and rough-pumping systems, a Perkin-Elmer 5 L/s getter ion pump, and an oil diffusion pump with a liquid nitrogen trap. A calibrated Du Pont gold molecular leak (leak rate 0.188 cm3/s) was placed between the inlet system and a UTI Model lOOC quadrupole mass spectrometer tuned to the following settings: optimum emission setting, 2.3 mA; optimum focus setting, -30 V; basis sensitivity relative to Nz(Faraday cup), 2.6 X A/torr; electron multiplier noise at gain of 1 X lo5, 1X A; btalpressure emission setting, 0.5 mA; ion energy, 15 V; electron energy, 70 V. Calibrations have been performed for the species reported. Partial pressures have been corrected with the appropriate sensitivity factors. Pressure rise during the outgassing period was measured by a MKS Baratron capacitance manometer (10 mmHg full-scale with ranges X 1,0.1, 0.01). The temperature was measured using a chromel-alumel thermocouple which was held against the quartz sample tube. The sample tube was heated by an electric furnace (Lindberg Co., Inc., Chicago, IL) connected to an adjustable voltage (0-140 V). Before measurements were made, the quartz sample tube was baked (>lo50 “C) for 14 h under a pressure of 1 X torr or less. Typical spectra taken on the inlet showed the major residual gas to be hydrogen. The base pressure in the mass spectrometer section was 6 X torr. The APBN sample (1.06 g) was placed in the side arm (the “cold zone”) during the bakeout. The temperature of the “cold zonen during bakeout was 30 OC. Special precautions were taken to maintain the temperature of the “cold zone“ at 30 “C during the outgassing period. A ferromagnetic rod placed behind the sample made it possible to move the sample while maintaining vacuum integrity. With the stopcocks closed and the “hot zone” empty, gases evolved from the inlet were collected and analyzed. The total quantity was 4.2 X lo+ torr L/g. The major constituents were hydrogen and carbon dioxide. The quartz sample tube volume was valved off from the pumping and inlet systems. The sample was then transferred from the “cold zone” to the “hot zone” held at 1000 OC. With the stopcocks closed, the gases evolved were collected in the volume (0.41 L)enclosed by the stopcocks. Partial pressure measurements of noncondensable gases were made periodically by opening an all-metal leak valve maintaining a cons*t inlet pressure for analysis. 0003-2700/85/0357-2995$0 1.50/0
TO DlFfWlON P U M P AND G E T T E R I O N P U M P c
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nion
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M LEAK
O
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Flgure 1. Design of outgassing system.
Table I. Results for the Composition (in % and torr L/g) of Gas at 1000 “C for 10 min
species detected
%
torr L/g
HZ CHI CzHz NdCO CZH6 C&
91.1 2.07 1.42 4.90 0.29 0.22
0.143 0.0032 0.0022 0.0077 0.0005 0.0003
Table 11. Results for the Composition (in % and torr L/g) of Subsequent Gas at 1000 “C for 10 min
species detected H Z
CH4 CzHz NZ/CO CZH6 CRHR
%
torr L/g
71.8 1.54 0.55 25.1 0.65 0.45
0.00287 0.00006 0.00002 0.00100 0.00003 0.00002
For outgassing measurements at extremely high temperatures, an alumina sample tube (18-mm 0.d. X 250-mm length) may be connected to the glass inlet system by applying a low vapor pressure resin (Torr Seal, made by Varian Associates, Palo Alto, CA), but the resin has to be maintained at temperatures below 100 “C. The furnace used was microprocessor-based and programmable with a maximum temperature of 1650 “C (Applied Test Systems, Inc., Butler, PA). Table I shows the composition of gases from an APBN sample (no. 680) heated at 1000 OC for 10 min. The total quantity is 0.157 torr L/g. The analyses showed an extremely high percentage of hydrogen and hydrocarbons which might present a problem in the application of APBN in some processes. The outgassing patterns of many materials often depend on pretreatment. Prevacuum degassing of such materials can often eliminate the outgassing problem. After the first outgassing measurement, the sample remained in the system under a pressure of about lo4 torr. The quartz sample tube was allowed to cool to room temperature. The sample was outgassed again at 1000 “C for 10 min to evaluate the effect of the first outgassing on subsequent vacuum degassing. Table I1 shows the results of the second outgassing measurement. The total gas collected was 0.004 torr L/g. The 0 1985 American Chemical Society
2996
Anal. Chern. 1985, 57,2996-2997
total quantity was considerably larger than that of gas evolved from the inlet. The percentage of hydrogen was relatively lower than the first measurement, and there was a corresponding increase in the percentage of m / z 28 component; however, the quantity of gases was significantly less than in the first analysis. This method offers rapid characterization of high-ternperature outgassing from many materials used in high-vacuum furnaces, and furthermore, it can be used to evaluate different
treatments of such materials so as to enhance their quality. Registry No. Hz, 1333-74-0;CH4,74-82-8;CzHz,74-86-2; Nz, 7727-37-9; CO, 630-08-0; C2H6, 74-84-0; C,H,, 74-98-6; BN, 10043-11-5.
LITERATURE CITED (1) Colwell, 8.H. Vacuum 1970, 2 0 , 481-490. (2) Hu, J. C. A. Anal. Chem. 1981, 53, 942-943.
RECEIVED for review May 3, 1985. Accepted July 19, 1985.
Apparatus for Trace Determination of Volatile N-Nitrosamines in Small Samples Harry M. Pylypiw, Jr., Frank Zimmerman, and George W. Harrington*
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 Lucy M. Anderson
Laboratory of Comparative Carcinogenesis, National Cancer Institute, Ft. Detrick, Frederick, Maryland 21701 Because of their importance as a potential major human health hazard (1, 2), N-nitrosamines (NAs) have been the subject of many investigations. In all such studies, analytical chemistry plays a key role (3). In fact, guidelines were recently suggested to ensure accuracy in quantitation techniques (4). A significant problem that has existed in various in vivo metabolic experiments concerns difficulties associated with analyzing very small samples for very low levels of NAs. For example, the mouse, which is frequently used as the laboratory animal for metabolism studies, supplies the analyst with about 0.5-1.0 mL of blood per animal and organs that weigh between 0.3 and 1.5 g. When the levels of NAs expected are of the order of a few parts per billion, sample preparation becomes the most important step in the analysis, since the instrumental method, gas chromatography coupled with the thermal energy analyzer (GC-TEA), is well established and has a detection limit of less than 1 ppb (5). The usual method of sample preparation for volatile NAs in a wide variety of samples is the mineral oil distillation (6). While this technique is acceptable for large samples (ca., 5-20 g), it falls short of the demands imposed by small samples (ca., 0.2-1.5 9). The apparatus and procedures described here minimize sample handling and yield recoveries in the 90-100% range for small samples containing a few parts per billion volatile NAs.
EXPERIMENTAL SECTION Chemicals. All chemicals and solvents were ACS reagent grade or better. Water was distilled and purified with a Barnstead NANOpure I1 system. Morpholine (Aldrich Chemical Co. no. 13, 423-6) was double distilled,and the second distillate was collecbd and stored under nitrogen gas. The defoaming agent used was Antifoam B (Fisher ScientificCo., CS-283-4). NAs were received from the NCI Chemical Carcinogen Reference Standard Repository, a function of the Division of Cancer Etiology, NCI, NIH, Bethesda, MD. Instrumentation. A thermal energy analyzer, Model TEA-502, manufactured bv Thermo Electron Corm. interfaced with a Hewlett-Packari Model 5790A series paiked-column gas chromatograph was used for separation and detection of NAs. Specific conditions were as follows: column 6 f t X 2 mm i.d. glass, packed with 10% Carbowax 20M + 2% KOH on Chromosorb W AW, 80/100mesh; programmed from 120 to 190 "C at 5.0 OC/min; final hold time, 1.0 min; total run time, 15.2 min; carrier gas, He; flow rate, 12 mL/min; on-column injection, 250 "C; interface line, l/s in. 0.d. glass-lined stainless steel, 250 OC; TEA furnace, 525 O C ;
-8
lllr- I
.~
I
I
_____-_______
1.50-1.75mmIDcapillary
t _ _ _ _ _ _ .
Flgure 1. Distillation-extraction apparatus. All dimensions are in millimeters except where noted. All tubing is standard wall unless otherwise indicated. All tubing sizes are outer diameters except the capillary. Dimensions marked with an asterisk are critical.
TEA cold trap, -151 "C; and TEA detector pressure, 1.4 torr. All data were accumulated and calculated on a Hewlett-Packard 3390A reporting integrator. Apparatus. The apparatus is shown in Figure 1. It is a modification of a commercially available model (6826 distillation-extraction apparatus, Ace Glass, Inc., Vineland, NJ). The dimensions are given in Figure 1 and the caption. Those dimensions marked with an asterisk are critical. The heights of the flask arms are critical to ensure that the liquid-liquid interface occurs just below the capillary tip. If the interface is not properly located, a mixture of liquids will return to the lower flask. The dimension of the capillary tip is important to regulate flow rate. It was found that an optimum flow rate was required to obtain
0003-2700/85/0357-2996$01.50/00 1985
American Chemical Society
