Page 77 - Geochemistry of Oil Field Waters

ATOMIC ABSORPTION METHODS 65

ATOMIC ABSORPTION METHODS

Atomic absorption is complementary to flame spectroscopy. The spectra
emitted are analyzed by absorption of resonance lines by free atoms of a
constituent in the vapor phase. The unexcited or ground-state atoms pro-
duced in the flame can absorb radiant energy when supplied by a suitable
external radiation source at a frequency coinciding with that of the emission
frequencies of the element atomized. The measurement of this radiation
absorbed forms the basis of absorption flame spectrophotometry - or
atomic absorption spectrophotometry, as it is usually called.
At temperatures up to 2,7OO0C, ground-state atoms usually account for
more than 90% of the atoms in the vapor phase. Hollow cathode discharge
tubes generally are used as a light source. The sensitivity of detection does
not depend upon the spectral response of the light receiver, since the absorp-
tion coefficient is a measure of the relative intensity of the light which passes
through an absorption cell versus that which does not. Additional theory can
be found in a book by Robinson (1966).
Atomic absorption is useful in water and brine analysis, and there are
several publications on the subject. Publications oriented to oilfield and sea
water analysis are Fabricand et al. (1966), and Angino and Billings (1967).
Table 3.XII illustrates the sensitivities that can be obtained using atomic
absorption to determine some metals in aqueous solutions. The sensitivities
listed are obtainable if no interferences are present. Interference usually

TABLE 3.XII
Approximate sensitivities for some metals to atomic absorption

Metal Wavelength Sensitivity Fuel and oxidant
(A) (mg/l)
- - . -
Aluminum 3093 1 .o nitrous oxide-acetylene
Barium 5536 0.2 nitrous oxide-acetylene
Beryllium 2348 0.1 nitrous oxide-acetylene
Cadmium 2288 0.04 air-acetylene
Calcium 4226 0.08 air-acetylene
Chromium 3579 0.15 air-acet ylene
Copper 3247 0.2 air-acet ylene
Iron 2483 0.3 air-acet ylene
Lead 2833 0.5 air-acet ylene
Magnesium 2852 0.02 air-acet ylene
Manganese 2794 0.15 air-acet ylene
Mercury 2536 0.01 air-acetylene
Nickel 2320 0.15 air-acet ylene
Silver 3281 0.1 air-acet ylene
Sodium 5890 0.03 air-acety lene
Zinc 21 38 0.04 air-acet ylene