Page 103 - Geochemistry of Oil Field Waters

MASS SPECTROMETRIC METHODS 91

described above for barium, boron, iron, manganese, and strontium; the
aluminum emission lines at 3082.5 a can be used. However, if the alumi-
num concentration is less than 5 mg/l, the aluminum should be separated
and concentrated from the aqueous phase. This can be done by adjusting the
pH of a sample containing up to 100 pg of aluminum to pH 0.4 with
hydrochloric acid, adding 10 ml of a 6% aqueous solution of cupferron,
adjusting the pH to 4.8 with sodium acetate, and extracting the aluminum
complex into chloroform. The chloroform phase then is aspirated into the
plasma arc using the same conditions and internal standard line that is
described above for beryllium.

MASS SPECTROMETRIC METHODS FOR STABLE ISOTOPES
The ratios of the stable isotopes of deuterium and hydrogen and of
oxygen-18 and oxygen-16 differ in water taken from various sources. These
differences are useful in studying the origin of a water, and of studying
paleoenvironments if the water is geologically old. The isotopic ratios are
measured on a mass spectrometer and are always compared to the ratios
found in a standard material because such a comparison proyides greater
precision than direct analysis of absolute ratios.

Deuterium

Friedman and Woodcock (1957) developed a method whereby deuterium
is converted to hydrogen gas by reacting a 0.01-ml sample with hot uranium
metal. A mass spectrometer (Friedman, 1953) is used to compare the
deuterium/hydrogen ratio in the emitted gas to the ratio in a standard gas.
Replicates agreeing within k0.176 usually are considered satisfactory. The
results usually are expressed as deuterium enrichments (+6 values) or deple-
tion (-6 values) relative to SMOW (standard mean ocean water, with a D/H
ratio of 158 x (Craig, 1961b). The standard deviation is about 0.2%,
and a sample with a 6 value of -5 has 5% less deuterium than SMOW.
Oxygen-18

Epstein and Mayeda (1953) developed a method to analyze water samples
for l80. A 10-ml sample of water is equilibrated with carbon dioxide at
25OC and an aliquot of the COz is analyzed using a mass spectrometer for
l80. The isotope ratios in the sample are compared to those in a standard
material, using the mass spectrometer, which gives a greater precision than
direct analysis of the absolute ratios. The standard generally used in SMOW
(standard mean ocean water) which is distributed by the National Bureau of
Standards (Craig, 1961a). Delta units express the isotopic data as: