Page 95 - Geochemistry of Oil Field Waters

EMISSION SPECTROMETRY a3

EMISSION SPECTROSCOPY

The basic requirements for all spectroscopic measurements are a source, a
dispersion element, and a detector. The source may be an emitter whose
emission is to be measured, or it may be a continuum that emits all wave-
lengths, within a certain range, so that absorption by material in the light
path may be measured.
In general, emission spectra are concerned with transitions from upper
state to lower state electronic levels in atoms and in simple molecular
species. Some flames are hot enough to excite upper electronic levels in
neutral atoms (un-ionized) and in molecules. Electric discharges produce
more vigorous excitation, and a high-voltage spark tends to increase the
ionization of the emitters.
In spectrographic analysis the light source first vaporizes and dissociates
the sample and second excites the atoms causing them to radiate charac-
teristic spectra. The intensities of the spectral lines of elements excited in a
light source are proportional to the concentration of the elements in the
sample, thus providing a basis for quantitative analysis. Excitation is mainly
thermal in the sources, flames, arcs, and sparks.
Temperature is very important in spectrographic analysis because some
elements are not easily excited in a thermal source while others are. The
ionization potential of the element determines the ease of exciting its
spectra. The alkali elements with ionization potentials of 4-5 V are excited
in low energy sources while the rare gases with ionization potentials up to 25
V require high temperatures to be excited. A Bunsen flame gives a tempera-
ture of about 1,700'C; an oxyacetylene flame, about 2,700'C; an electric
arc, 3,700'C-6,700'C; and an electric spark, about 9,700"C. In the follow-
ing procedures a plasma arc source was used, capable of temperatures up to
7,700' C .
The plasma arc was adapted to analytical spectrography by Scribner and
Margoshes (1961). The temperature of a direct current arc is increased by
thermal-pinch effect.
The internal standard method is used in the following procedures, and
with this method the intensity of a line of the element present in unknown
concentration is measured relative to that of an invariant line of a reference
element. With this method the intensity ratio must be highly reproducible.

Barium, boron, iron, manganese, and strontium

The emission characteristics of barium, boron, iron, manganese, stron-
tium, and lanthanum in 10 solvent systems have been studied (Collins,
1967). The greatest emission enhancement was found in a mixture consisting
of 30 ml of water plus 20 ml of 35% n-amyl-alcohol and 65% acetone, as
illustrated by Fig. 3.7. Because n-propanol is easier to work with, it was used
in the following procedure; however, if additional sensitivity is needed, the
n-amyl-alcohol-acetone mixture can be used.