in K are tested for by comparing adjacent K values. X change in K by more than 1 part in 300 between adjacent mass values is assumed to represent an abnormal change in K-e.g., discontinuous momentary shift in K-and the immediately preceding K value is used for the following mass value determination-i.e., the abnormal value of K is rejected. The first computed K factor for each sample is used as a starting K-factor for analysis of the next sample, and the overall process is continued until all samples are analyzed. All of the arithmetic operations are carried out by an IBM 1620 computer program in this laboratory.
specific mass numbers starting a t mass 43 and ending a t mass 170. The Kfactors computed during a typical day's production of these analyses vary between two extremes--e.g., 401,000 to 403,000. If either value had been used to determine a mass number where the other K-factor was correct, the mass unit would be incorrectly identified, with a resultant error in the final analysis of the sample. This illustrates the potential errors that can arise by the use of average X-factors.
At the time of this writing we have analyzed hundreds of samples, both gas and gasoline, by means of the described editing routine without a single failure. There are several innovations in the procedure, but the key to its success is the repetitive calculation of new Kfactors each time a mass number is identified. The outstanding advantage of this procedure, as compared to that of Thomason ( I ) , is the certainty of mass number identification (with this burden carried entirely by a computer) while requiring only normal MS operator effort. The gasoline hydrocarbon type method used in this laboratory requires
LITERATURE CITED
(1) Thomason, E. XI., ANAL. CHEM.35, 2165 (1965).
High Sensitivity Oxygen Analysis of Metallic Samples with Fast Neutrons Richard W. Benjamin, Kenneth R. Blake, and Ira L. Morgan, Texas Nuclear Corp., Austin, Texas
for oxygen in metallic T samples has been performed for some time by using fast neutron activaRACE AKALYSIS
tion analysis techniques in conjunction with a pneumatic rapid transfer system ( I , 3, 4). h system has now been designed which is capable of measuring oxygen in concentrations below 10 p.p.m. in cylindrical metallic samples 3//16 t o 0.480 inch in diameter and '/*to 1 inch long. Oxygen activation analysis with fast neutrons may be done accurately and rapidly with the O16(n,p)N16 reaction. Because of the short half life of the 6 " activity, it is advantageous to use some sort of rapid transfer system, such as has been described in some detail by hnders and Rriden ( I ) . h 150-k.e.v. Texas Xuclear Corp. Cockcroft-Walton positive ion accelerator provides an abundant source of 14-m.e.v. neutrons through the H3(d,n)He4reaction. The target used on the accelerator consists of 3 to 5 curies of tritium adsorbed onto a titanium layer which has been vacuum-deposited on a copper backing. The target assembly appears in Figure 1. The deuteron beam impinges on the tritium target (A) which is held against an O-ring by the retaining ring ( B ) ,thus sealing the accelerator vacuum system. The target is cooled by water which flow? directly over the copper backing. This allows higher beam currents and a longer target life. The thin stainless-steel sample tube (C) is fitted flush against the retaining ring, ensuring that the sample is exposed to the highest p o s i j l e neutron flux. The sample is centrally and reproducibly constrained in the tube by a polyethylene finger (D), the inside diameter of
PLEXIGLAS TUBING \
w
I
\ L J yA
" B" RETAINING RING
3
-
"A" TRITIUM TARGET
,,,,>
''
8
8
"C" STAINLESS STEEL SAMPLE TUBE
,
I
SILVER SOLDER
PRESSURE FITTING FOR NITROGEN GAS
POLYETHYLENE FINGER. WATER FITTING FOR TARGET COOLING \
b Figure 1.
Target assembly VOL. 38, NO. 7, JUNE 1966
947
QUICK
DISCONN FLANGE
SAMPLE RECWING TUBE
W
Figure 2.
Sample receiving tube assembly
which is a maximum of 0.002 inch greater than the outside diameter of the sample. Although the sample is very close to the target and there is considerable difference in neutron flux from front to back, the sample was not rotated during irradiation as done by
Table I.
Sample Be 7, Run 1 Be 7, Run 2 Heat KO. 3362 Heat KO. 3363 Be 6 WA-58
Ti-1 Be 6
WA-58
948
NITXGEN GAS INLET
Anders and Briden (1) and Mott and Orange (5) because the counting geometry is nearly 47r. Reasonable values for the standard deviation (c) of several measurements were obtained without sample rotation, although the lack of truly 4~ counting geometry introduces
Precision of Measurements for Several Oxygen Concentrations
Rel. oxygen content, p.p.m. 11.0 11.8 24700 14400 5356 1050 2850 5080 1050
ANALYTICAL CHEMISTRY
Rel. std. dev., 70 10.6
14.0 1.22 0.99 1.70 1.46 1.57 2.88 1.74
Total rel. std. dev., % 10.7 14.1 1.90
1.76 2.24 Std. Std. 3.28 Std.
Sample size,
inches 0.249 X 1 0.249 X 1 0 2505 X 1 0.251 X 1 0.248 X 1 0.249 X 1 0.360 X 1 0.358 X 1 0.479 X 1
Wt. oxygen, mg.
0.016
0.017
37.2 21.6 7 75 3.938 21.95 15.1 14.550
some error. Provision is made at the bottom of the assembly for the pressure fittings required by the rapid transfer system, and at the top of the assembly for the flexible sample tube. Three removable sample tube assemblies have been built, along with several fingers for each assembly, so that all cylindrical samples from 3/!16 to 0.480 inch in diameter can be handled. Flexible sample transfer tubes are made from 1/8-inch wall polyethylene tubing and from Sylobraid, a nylon reinforced polyvinylchloride plastic tubing. Polyethylene tubing is adequate, if available with requisite inside diameter and vall thickness. The saniple receiving t i b e assembly appears in Figure 2 . The sample receiving tube ( E ) ,which is fitted with a quick disconnect for rapid sample removal and r placement, protrudes into a large paraffin and lead detector shield which houses two 3 X 3-inch NaI(T!) scintillation detectors. Initially, polyethylene figures were built to position the sample in the detector assembly, but measurement's indicated that several thousandt'hs of an inch uncertainty in this positioning did not appreciably increase the uncertainty of t'he gamma ray measurements. The XaI(T1) crystals are optically coupled t'o Dumont 6363 photomultiplier tube3 whose outputs are amplified and gated so only the high energy gamma rays from X*e are accepted. This output is then fed into a decade scaler. The entire system is automatically programmed to irradiate the sample for a predetermined time, transfer the sample to the detector (transfer time is 1 to 2 seconds), count the sample for a predetermined time, and then return the $ample to the irradiation position where the cycle begins again. Ten runs per sample are made as a rule to provide good statistics. Unknown samples are compared with standards using the same geometry to determine the oxygen concentration. Seutron flux is monitored with a simplified recoil proton telescope as described by Bame et al. ( 2 ) , and gamma ray counts for each run are normalized to a standard neutron monitor count. The recoil proton telescope has the advantages of being both very directional and neutron energy dependent. To thoroughly test this high sensitivity oxygen analysis system, high purity beryllium samples covering a large range of sizes and purity mere analyzed for oxygen. Each sample and standard was irradiated for 20 seconds and counted for 20 seconds after a constant delay of about 3 seconds which provided for transfer from the irradiation area. The standards used in these measurements Tere titanium rods which were machined t o the same size as the unknown samples; the oxygen cont'ent of the
standards was determined by several methods in several different laboratories. The results of the measurements are presented in Table I ; they give a good idea of the sensitivity of the system and the reproducibility of measurements a t various levels of oxygen content. The column titled “Rel. std. dev.” represents the relative standard deviation of the normalized gamma ray counts; this quantity was calculated from 10 measurements which were made on each sample. The column titled “Total rel. std. dev.” represents the relative standard deviation of the measured relative oxygen content of each sample; this was calculated from the usual rules of error accumulation.
Some aspects of Table I merit special mention. Sample Be-7 weighed 1.44145 grams; thus, 11 p.p.m. of oxygen represents a weight of only 16 kg. of oxygen. Since beryllium is a lowdensity metal, 11 p.p.m. of oxygen in the same size sample of another metal could correspond to considerably more oxygen and a proportionally higher count rate. The standard deviation of 11 p.p.m. of oxygen would decrease then as the density of the sample increased. Conversely, considerably smaller relative oxygen contents could be measured with the same standard deviation with higher density metals.
ACKNOWLEDGMENT
The authors are grateful to Charles D. Houston, Wright-Patterson A.F.B., Ohio, for the beryllium samples and titanium standards which were used in this work.
LITERATURE CITED
(1) Anders, 0. U., Briden, D. W., ANAL. CHEM.36, 287 (1964). (2) Bame, S. J., Haddad, Eugene, Perry, J. E., Smith, K. R., Rev. 81%.Instr. 29, 652 (1958). (3) Mott, W. E., Orange, J. M., ANAL. CHEM.37, 1338 (1965). (4) Steel, E. L., Meinke, W. W., Ihid., 34, 185 (1962).
Pulseless High-pressure Pump for liquid Chromatography R. E. Jentoft and T. H. GOUW, Chevron Research Co., Richmond, Calif.
1“
HIGH-PRESSURE liquid chromatography, the eluting solvent is generally pumped into the column by a reciprocating piston or bellows pump. Pulses generated by this pump can often be discerned by t’he detector. Besides introducing difficulties in the interpretation of the detector output, pulses also have a detrimental effect on the separating efficiency of the column. A major portion of these pulses can be eliminated by the use of surge tankse.g, , spring-loaded bellows connected to tees in the line between the pump and the column inlet. But even careful design n-ill not remove all pulses from the flow pattern and rather complicated and/or large surge tanks, with concurrent large holdup of material, are necessary to reduce the level of the flow variations to a negligible level. I n addition, damage to this pump and surge system is feasible if the outlet line is accitlentally blocked. TT’itl.1 the pump described here, high pressures up to 1000 p.s.i. can be generated at the outlet of the pump. I n comparison, many of the commercially available pumps are incapable of opei,ating above 100-1i.s.i. pressure. Delivery into the chromatographic s>.stem can be carried out without any discernible pulses for any length of time and any amount of solvent. Accidental blocking of the outlet line does not damage the pump a t all. Construction is simplei, than reciprocating plunger pumps of equivalent rating.
cury as the displacing agent prevents the dissolution of the air in the solvent. The outlet of the pump is on the top of Vessel A . For operation up to 250 p.s.i., a dual back-reference flow controller with integral micrometering valves (Millaflow No. 9986 220) is mounted between this outlet and the inlet of the chromatographic column. Because these type flow controllers are currently not available for higher pressure ratings, a fine metering valve is used for work at higher pressures. The pressure-reducing valve in the air line is adjusted to yield a pressure about one and one-half to two times the desired inlet pressure to the column, but not less than 100 p.s.i. to minimize the effect of the variations in the mercury levels. Prior to introduction into the pump, the solvent in the external reservoir is heated by a n infrared lamp. This degases the liquid sufficiently to
prevent air from coming out of solution in the column and creating artifacts in the chromatogram. At the normal mode of operation, neither of the threeway solenoid valves is energized. The air pressure, which is now connected to B , is transmitted through the mercury to the solvent in A . There is vacuum on D, and C is being refilled from the external solvent reservoir. Mercury rises in A as solvent is delivered to the chromatographic column. As soon as the mercury in Vessel A comes in contact with the upper electrode, the solenoids are energized. B is now connected to atmospheric pressure or to a regulated pressure somewhat lower than the operating air pressure in D. The latter is the desired mode of operation a t high pressures. The air pressure, which is now on D, refills A from C and, a t the same time, keeps up the uninterrupted delivery of the solvent at the External RSe22r Check Valves
T Relay
-Gravity Type Check Valves
DESCRIPTION AND OPERATION OF PUMP
Pressure Reducing
h schematic drawing of the unit is given in Figure 1. Four %inch by 8-inch stainless steel reservoii s, constructed from pipe nipples, are half-filled with m m u r y , the displacing agent. High pressure air is used as the source of operating pressure. The use of mer-
N.C.
To Atmosphere c_jt 1 3-Way Solenoid Valves
Figure 1.
4
ommon N.C.,
@ @
Pulseless high pressure pump VOL. 38, NO. 7, JUNE 1966
949
