oflWs It-ould be correspondingly different-Le., if e were 0.25, the values on tlie W s axis would be one fourth of t,hat shown in Figure 2. Thus, each investigator may use his own counting efficiency and prepare a nomograph rvhich is applicable to his work. The time interval is included b y introducing into t'he numerator of Equation 4 the tcrni. exti, where t , is the time elapsed from irradiat,ion to detection ( I ) . The incorporation of this term into TT-.,- could cause difficulty in obtaining the normal axis for WTS and may have to remain an entity in Equation 3. This method is cumbersome in tliat calculaticns are necessary to olitsin quantitatiw results. However. this is
compensated for by the fact that, after a little experience, one can make excellent qualitative estimations without, written calculations. After obtaining TYhr, rapid approximate mental calculation.; with Equation 3 are possible. LITERATURE CITED
1 ~ 1Aalberts, ) J. H.,
Terheijke, J. H.,
d p p l . Physics Letters 1 , 19 (1962). ' , 2 ) Freiling, E. C . , ."\izccieonics 14, S o . S, G S (1956). ( 3 ) Gillespie, A. S., Hill, IT. IT., Sztcleonics 19, 170 (1Y61). ( 4 ) Jenkins, E. S . , Smalrbs, -1.A , , Quart. Rec. Chetti. Soc. London 10. 83 (19561. [S) Leddicotte, B. W.,Re?:nold~,9. _\., U. 8. Atomic Encry?- C omrn. Rep!. AECD-3489 (1953)
(6) Meinke, I\'. I!.) AXAL. CHEM. 31, 792 (1959). ( 7 ) lleinke. \V. IY.. Science 1 2 1 . 177 (1955). (8) Meinke, \V. \\. , Maddock, R . S., ASAL. CHEM.29, 1171 (1957). (9) Mrsler, R. I3 3"ucleonics 18, so.1 , 73 (1960). (10) Schindewolf, U., Angeii. Chr' l t l . 'io, 181 (1938). (11) Stelin, J. R., ('lancy, E. F., .I-cicl eonics 13. S o . 4. 27 (1955). (12) Studh-, x.' M.;Postmus, C., Jr., LIech, J., W'alters, R. R., Sloth: E. S . , Argonne Katiunnl Laboratory, ANL6577 (1962). ~
\VURK \viis done under the auspices i \ f the Unitcd St:itcss A\tomic Energy Coniniission. Contmts of this paper do not nccrsenrily r c f l w t tlic. position of t h r V.S. :\rrny.
liquid Sample Introduction System for a Mass Spectrometer Albert D. Pattillo and Howard A. Young, Mobil Oil Co.,Beaumont Refinery, Beaumont, Texas
work. Both instruments are equipped the past several year>. memwith heated inlet systems operated at of ASTAI Committee D-2, 135' C. (276' F,). 9 project was intiRD IV have cooperated in a program to ated to determine the best means of develop a method to analyze petroleum introducing a sample in the gasoline fractions in the gasoline boiling range for boiling range into the mass spectromhydrocarbon types. One of the principal drawbacks to such a method 1 1 ~ eter. ~ The first method tried consisted of been finding a suitable nipan- of wiiple using a microliter syringe to inject a introduction into the ma+ spectrometer sample through a silicone rubber plug vacuum. The device shown in Figure 1 into the ~ a c u u msystem. Further inhas been used routinely 3t 11013i1'3 vestigation revealed selective adsorpBeaumont Refinery Laboratory for tn-o tion, predominately with the aromatics. years and has eliminated the objection. -1more commonly used device was then presented by other methods. installed consiqting of a fritted disk Two CEC Model 21-103C Ala-s covered with a heated gallium-indiumSpectrometers were used to coiidwt this tin alloy. The sample way introduced through this n i t h a capillary "dipper." Reproduction was poor as a reiiilt of the elevated temperature of the alloy, some distillation occurring through the fritted disk. Since a device himilar to the one shown in Figure 1 had been used successfully in this laboratory on gas chromatographs, it was decided to build and inytall one on the mass spectrometer. URING
D bers
m
DESCRIPTION AND OPERATION
Figure 1. Cross section of liquid introduction system for mass spectrometer
@ @ @ @
'/B-inch drill rod Charging volume (approx. 0.0006 cc.). Drill hole in '/g-inch rod, solder appropriate size needle stock in hole, machine, and polish compression cap Teflon plug ('/a-inch thick) M.S. panel Connection to M.S. inlet system
1768
e
ANALYTICAL CHEMISTRY
The sample introduction -ystem IS mounted on the front panel of the control cabinet along with the conventional gas introduction system. The liquid introduction system consists of a piece of stainless steel (6, in Figure 1) drilled so that the drill rod, 1, can be moved through i t into the maSs spectrometer. The drill rod is polished n i t h crocuB cloth. The stainless steel, 6, is threaded, and a compression cap, 3, is screwed onto it. The compression cap contains a '4 inch thick Teflon plug, 4, drilled so that the drill rod fits into it snugly. The compression cap is pulled up tight enough to maintain
the mass spcctroiiieter vacuum. Tlic small hole, 2, in the drill rod ha3 :I capacity of approximately 1 11. To obtain a sinal1 hole, install a sevtioii of hypodermic needle. With the drill rod pulled out p i 1 tially, a sample is introduced into the small hole with a syringe or any other convenient means. The drill rod is then pushed in until the sample reaches the mass spectrometer vacuum. The sample is completely vaporized into the mass spectrometer. The mash spectrometer has been operated a t 204" C. (400' F.) n i t h a satisfactory sample introduction. -4 fine stream of laboratory air is directed on the drill rod while in the charging poiition to permit the reproducible introduction of samples boiling Rt 35" C. (9.5" F.). Any sample can be introduced that has a boiling point high enough to be volatilized a t the temperature of the inlet system maintained at a pressure of less than 5 X torr before sample introduction. KO difficulty has been observed with samples ha&g a 95% boiling point of 260" C. (500' F.). Many advantages result from the installation of 5uch a liquid introduction system. Since no distillation occurs, reproducibility of pressure measurements is better. Teflon is essentially inert to hydrocarbons, resulting in no selective absorption. There is little chance to lose high vacuum due to channeling or leakage such as occurs when using fritted disks and silicone rubber plugs. A relatively volatile sample can be introduced a t room temperature into the heated inlet system. One such device has been in constant use on a mass spectrometer without replacing any parts for tn-o years, while another one has been in service for a longer period of time on a gas chromatograph.
