column 2. Atmospheric pressure a t the time of analysis is shown in column 3, and the volume of mercury, in column 4 . The volume of oil. column 5 . added to the volume of mercury gives the volume of fluorine, column 6, in the sample. Volume % Fz is given in column 7 . Results of analysis of a second mixture of 02 and F2 are given in Table I, Tests 4 and 5. A precision of *0.3% is indicated by data-in Table I.
Table I.
Temp., Test
O
c.
Volume
70FQin Mixture of Fz and 02
Atm. press., mm. Hg
1-01. Hg, ml
1
27.0
681
25
2 3 4 5
28 0 26 0
68 1
25 25 25 25
24.7 21 2
681 679 678
I-01.
Yol.
oil, nil
FP,ml
66 66 67
91 91 92
90
115
90
115
Fz, R 27 2 27 2 27 5 34 4 34 4
A Chromatographic Method for Determination of Trace Impurities in Grade-A Helium AI Purer and C. A. Seitz, Helium Research Center, Bureau of Mines, Amarillo, Texas
method is defor the determination of neon, hydrogen, oxygen, nitrogen, and met,hane in helium with a sensitivity of 1.2, 28.0, 1.2, 1.0, and 1.6, p.p.m. per division, respectively, using a 1-mv. recorder which has a 100-division full scale. This method is suited for the analysis of helium a t the point of use, giving one analysis every 5 minutes and using approximat,ely 200 cc. of sample. Two systems have been used previously for the analysis of Grade-l helium. The first method, involving concentration of trace impurities in G r a d e d helium by freezing them at liquid helium temperature ( 2 ) followed by mass spectrometer analysis, though good for analyzing standards, requires equipment not normally available. Thermal conductivity methods have also been used to analyze Grade-A helium, but the indicated impurity is CHROMATOGRAPHIC
A' scribed
V
expressed only in terms of total parts per million nitrogen equivalent. The two original 5-02. sample loops of the Beckman GC-2Ai chromatograph were changed to 8 and 16 cc. to give the needed range of sensitivity. Letting one sample have about twice the volume of the other permitted a high and low sensitivity. EXPERIMENTAL
h chromatographic column 13 feet long by 3/,6-1nch i d . , filled with 30- to 60-mesh 13X Linde Molecular Sieve maintained a t 40" C., was used. These conditions gave good resolution for neon, hydrogen, oxygen, nitrogen, and methane with retention times of 41, 46,
70, 97, and 130 seconds, respectively. A 22-foot column !\a> usrd for better reqolution of neon and hydrogen in ipecial case>. I t i+ deyirable to uqe a constant IToltage tianbformer to regulate the 110 volt, a c . to the chromatograph and recorder. Before purifying the helium carrier (already below 25 1i.p.m. total impurity) negative peaks were often obtained bec$se the-carrier gas contained approximately the same type and concentration of impurities as thP sample. The purifier used was a liquid nitrogen-cooled charcoal trap built by -1.D. Little, Inc., for purification of helium ahead of the -1. D. Little-Collins helium cryostat. Subsequrntly, a somewhat simpler but effective purification system
I
5. C.
Fltxibla tubb ' Contomr of W r t U 4
eorritr lnlaf
u7
T I M E , minutes
Figure 1. tograph
1694
Sample and carrier-gas flow to chroma-
ANALYTICAL CHEMISTRY
Figure helium
2.
Chromatographic
scans
of
Grade-A
was constructed, consisting of a 22-foot length of 1/2-inch i.d. copper tubing wound into a 4-inch diameter helix and filled with 10- to 20-mesh coconut charcoal, as shown in Figure 1. The purifier is conveni2ntly operated a t a pressure of about 40 p s i g., which permits throttling and control of the flow of the purified helium into the carrier-gas systzm a t the chromatograph. Operation above at,mospheric pressure also increases slightly the adsorption capacity of the charcoal for neon and hydrogen impurities in the carrier helium ( I ) . When a conventional pressure regulator attached to the sample cylinder was used to regulate the pressure of the sample, difficulty was encountered in obtaining reproducible data when the impurity content was less than 50 p.p.m. Apparent,Iy the arrangement of the internal volumes of the regulator was such that a few parts per million impurities were picked u p by the sample stream. Since the regulator could not easily be purged in less t'han 25 minutes the sampling system shown in Figure 1 was adopted. .A cylinder valve connector having a small needle control valve, 2, is attached to the cylinder, I , containing the gas to be analyzed. A flexible 0.0625-inch 0.d. by 0.04inch i.d. stainless steel capillary tube, 3, is used to deliver the sample to the gas inlet of the chromatograph. A tee-connection, 4 , is placed a t the end of the t,ube just ahead of the gas inlet of the chromatograph. From the side connect,ion of this tee, a length of plastic tubing, 6, carries excess sample flow to a glass tube, the open end of which is submerged in mercury to provide a seal. X short length of plastic tubing is used to connect the gas sample inlet' of the chromatograph to the other outlet of the tee. When the sample flow is dead-ended a t the chromatograph for a second or so when the sample loop is switched to t,he carrier-gas stream, the sample gas escapes through the mercury leg. The system is ready for operation when the bridge current of the Beckman GC-PA chromatograph is set at
400 ma. The carrier-gas pressure gauge on the chromatograph is set a t 20 p.s.ig. giving a flowrate of 80 cc. per minute; there is liquid nitrogen around the purifier; the recorder is t,urned on and properly attenuated; and sample is flowing through the sample loop.
Table 1. Chromatograph Concentration Comparison with Analyses in P.P.M. by Volume
Sample Impurity 1
s2 0 2
Ne
RESULTS A N D DISCUSSION
2 The first peak that appears is due to a pressure surge and serves as a convenient starting mark. The recorder pen returns quickly to the base line, and about 30 seconds later the neon peak appears with a sharp, well defined height. hbout 30 seconds after the neon peak, the oxygen peak appears and the nitrog-n peak appears about 20 to 25 seconds later. Typical scans are shown in Figure 2. Hydrogen, if present, appears between neon and oxygen. Oxygen and argon normally appear as a single peak. This, however, can be resolved by operating the column a t reduced temperatures (SI Analyses of the helium samples are obtained by comparing the peak heights registered for the various impurity components with their calibration curves which were obtained by comparing the chromatograms of synthetically prepared mixtures previously analyzed by another method ( 2 ) . The two different sample volumes gave the same peak height for neon at low parts-permillion concentrations; the peak area, however, was doubled for the large sample. This does not affect the accuracy of the neon analysis from 0 to above 150 p.p.m. Table I gives the results of comparative analyses of 10 samples of heliini obtained with the chromatograph method and with the impurity concentration method ( 2 ) commonly used. The analyses for the concantration method are the average of dual analyses of the same samples a n d demonstrated
s2
Ne N2
310
Ne N2 0 2
4
0 2
Te
5
N2 0 2
Ye 6
NZ 02
Te
7
9
200 1 8 310 1 8
0 2
3
Chromatograph 5 1
E2 0 2
hTe
Tracea 8 48 16 9 11 2
9 3 1
9 8 s* 28 4 02 54 Ne 9 ?;2 370 Trace5 0 2 8 Ne 630 10 NO 180 0 2 9 Ne a Below 0.5 p.p.m.
Alternate method (2)
6 1 12 210 1 10 305 I
8 310 1 6 50 15 12 9 2 9
5 1 12
28 5 50 370
Tracea 8
620
170 8
a deviation of 10% as compared to 5% for the chromatograph when 10 analyses of the same sample are obtained. LITERATURE CITED
( 1 ) Czaplinski, A., Zrelinski, E., Prremysl Chem. 38,87-8 (19%). (2) Kirkland, C. G., Brandt, L. W., Deaton, W. M., Bureau of Mines Report of Investigations ?io. 5644, 12 pp.. (1960). ( 3 ) Mosen, A . W.,Buzzelli, G., ANAL.
CHEM.32, 141-2 (1960).
New Design of an Internal Proportional Flow Counter for Radio-Gas Chromatography F. Schmidt-Bleek and
F. S.
Rowland, Chemistry Department, University of Kansas, Lawrence, Kan.
separation and G immediate . chromatographic assay of separated radioAS
active compounds have been frequently applied over the last 8 years ( I , 4, 5, 8, 9 ) . ITse of internal proportional flow counters for the high efficiency, practical assay of low energy beta-emitting radioisotopes has also been discussed in detail (6-9). Since the widely used glass counters with silver cathodes are susceptible to chemical attack by some gaseous compounds--e.g., amines-lead-
ing to the eventual destruction of the cathode, other counter designs have been sought which are more inert toward possible components of the flow stream (8). Figures 1 and 2 show the design of a brass-Teflon counter proved to have the following qualities after a variety of testing and after analytical use over a 2-year period: I t can be used a t elevated temperatures (tested up to 160" (3.); background is low even without further
shielding; by simply unscrewing, the counter can be taken apart and cleaned, if necessary; counting volume can readily be adjusted by variation of the cathode length (we have a set of different cathode party); and the counter is virtually indestructible. Table I gives indication of the properties of the metal flow counter. The counter ha? been constructed from inespensive materials with simple machinis$'s tools a t a cost of les- than VOL. 36, NO. 8, JULY 1964
1695
