Microsoft Word - OF 01-333 A Procedural Manual for Measurement of
Uranium and Thorium Isotopes Utilizing the
USGS-Stanford
Finnegan Mat 262
By Donald D. Shamp
1
Open-File Report 01-333
2001
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey
Editorial standards. Any use of trade, firm, or product names is for descriptive purposes only and does
not imply endorsement by the U.S. Government.
U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
1
U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 Table of Contents

Introduction 3
I.
Filament Loading 4
II. Uranium metalSingle Filament Method 5
III. Thorium metal.Single Filament Method 12
IV. Some Miscellaneous Tips and Tricks 19
V. Figure1 4
Figure 2 20
Figure3 21
Figure 4 21
Figure 5 22
Figure 6 22
Figure 7 23
Figure 8 23
Figure 9 24
Figure 10 24
Figure 11 25 3
A Procedural Manual for Measurement of Uranium and Thorium
Isotopes
Utilizing the USGS-Stanford Finnegan-Mat 262
Introduction
Over the past several decades investigators have extensively examined the
238
U-
234
U-
230
Th systematics of a variety of geologic materials using alpha spectroscopy.
Analytical uncertainty for
230
Th by alpha spectroscopy has been limited to about 2% (2
).
The advantage of thermal ionization mass spectroscopy (TIMS), introduced by
Edwards and co-workers in the late 1980s is the increased detectability of these
isotopes by a factor of ~200, and decreases in the uncertainty for
230
Th to about 5 (2
) error.
This report is a procedural manual for using the USGS-Stanford Finnegan-Mat 262 TIMS to collect and
isolate Uranium and Thorium isotopic ratio data. Chemical separation of Uranium and Thorium from the
sample media is accomplished using acid dissolution and then processed using anion exchange resins.
The Finnegan-Mat262 Thermal Ionization Mass Spectrometer (TIMS) utilizes a surface ionization
technique in which nitrates of Uranium and Thorium are placed on a source filament. Upon heating, positive ion emission occurs. The ions are then accelerated and focused into a beam which passes
through a curved magnetic field dispersing the ions by mass. Faraday cups and/or an ion counter
capture the ions and allow for quantitative analysis of the various isotopes.
FILAMENT LOADING PROCEDURES
URANIUM:
1. Apply a ~2.5 µl layer of colloidal graphite (aqua dag) to the center third of the filament and take to
dryness using ~1.5 amps of current.
2. Apply the sample over the dried graphite and take to dryness with ~1.8 amps of current.
THORIUM:
1. Apply a ~1.5 µl layer of colloidal graphite (aqua dag) to the center third of the filament and take to
dryness using ~1.5 amps of current.
2. Mix the sample with an equal amount of graphite and apply in ~1µl layers and take to dryness with
~1.1 amps of current.
3. Apply a separate ~1.5 µl layer of colloidal graphite (aqua dag) over the sample and take each to
dryness between applications using ~1.5 amps of current.
Uranium
Sample
Layer
Graphite
Layer
Rhenium Filament
Filament
P 5
Thorium
Figure 1 1mm

TIMS procedure for Uranium metal.Single Filament Method
1. Start up AM:
a. Add liquid nitrogen to carousel. Confirm that vacuum is at least 10
-8
mbar.
b. Turn on ion-source by pushing button (figure 2). Turn high voltage
(KV) from 4 to 10 (figure 3). Determine if multimeter display shows
9.8 10 KV(figure 4). If it doesnt, press Return to Main Keys, press
MS Control, press Manual Mode, it will then read Computer
Mixed
Sample and
Graphite
Graphite
Layer
Rhenium Filament
Filament
P t Mode, then press Computer Mode again. Verify that high voltage is
now ~9.6 10 KV..
c. Begin filament preheating when new samples have been loaded
otherwise proceed to Section 3.
2. Begin filament preheating (this ensures the samples are outgassed and
any organics and interfering elements are evaporated)
a. Turn KV selection knob to lowest setting (figure 3) and verify that the
beam valve is closed.(figure 5).
b. Crank sample #1 into A position. (figure 6)
c. Set switch on current control box to position 5. (figure 7)
d. Turn current to 3.50 amps and leave for 10 minutes.
e. Turn off current by simultaneously depressing both High Speed buttons
on the filament current box.
f. Repeat steps b-e for all samples on carousel.
3. Set Cup Configuration
a. Press key Return to Main keys.
b. Press key MS Setup.
c. Press key Cup Configuration.
d. Select Th.
e. Press key Save Conftab. 7
4. Calibration Procedure (Ensures that short term changes in the high
voltage are corrected for by adjusting the magnetic field to minimize
system drift across the mass range)
a. Insure calibration sample (ie; U-500) is in IS position #1 (figure 6).
b. Open beam valve (figure 5).
c. Turn high voltage (KV) from 4 to 10 (figure 3).
d. Press key Return Main Keys.
e. Press key MS Control.
f. Press key Input Channel.
g. Press the number 1 key (this is the ICM counter) and press enter..
h. Press key Input Mass.
i. Type in 238 and press enter.
j. Begin filament heating of U-500 sample.
k. Set switch on current control box to position 1. (figure 7)
l. Turn up current to 3.500 amps. Note the counts for 238.
m. When the count rate is several hundred, focus the beam. (see section 6
steps a-k)
n. After getting a good focus and strong count rate (several thousand cps),
press key Input Mass.
o. Type in 235 then do a peak center and note the count rate. It should be
virtually identical to the last 238 count (U-500 is a 1:1 ratio of 238/235). If it is, then the machine is counting correctly and you may
proceed to calibrating the DAC table.
p. Press key Input Mass.
q. Type in 238 and enter
r. Press key Input Channel.
s. Press key 6 (this places the 238 in the #6 Faraday cup).
t. Press key Return Main Keys
u. Press key MS Calib.
v. Press key Magnet Calib.
w. Press key Calib RefMass.
x. Type 238 and enter (Screen will then indicate the current ref mass in
faraday # 6).
y. Press the upper far left button on the Ops. Control panel (figures 3 & 8).
z. Turn on the chart recorder.
aa. Adjust the three magnet dials till the digital display reads in the 238
range and then fine tune for the strongest signal on the chart recorder
(peaking to the left).
bb. Hit the space bar. The control will now shift back to the computer and
the software will adjust the DAC tables and printout the results. All
values should be close to 0.0 and must not exceed
± 8.0 (figure 9). (If
values exceed
± 8.0 repeat steps w through bb using 235 and 238
alternately and recheck till values are satisfactory). 9
5. Begin filament heating (Ensure all samples have been pre-heated, the
first sample to be analyzed is in the IS position and the beam valve is
closedfigures 5 & 6).
a. Press key Return to Main Keys.
b. Press key Support Programs.
c. Press key Heat/Ion/Eva .
d. Press key Autoheat Eva.
e. Type 3500,200 and hit enter. Ensure current is increasing on filament
control box . This will take 17.5 minutes.
g. When auto-heating is complete