Atomic Spectroscopy for Metal Analysis - Fundamentals of Environmental Sampling and Analysis - page 224

N
N ¼

6
2 e 3:3710 19 =ð1:3810 23 2600Þ ¼ 2:5010 4

ðat 2600KÞ

For the N * / N ratio at 2610 K, we replace 2600 K with 2610 K in the above equation, which
yields N =N ¼ 2:5910 4 , a 3.7% increase in the number of excited sodium atoms due to
the temperature increase from 2600 to 2610 K.

The above example has several important implications: (a) The ratio of excited vs.
ground state is very low (e.g., 2

5010 4 at 2500 K), hence most of the atom at such
temperature is in the ground state (e.g., Ca 0 ). (b) From earlier discussions, we know
that the atomic absorption measures the absorption of atom in the ground state. This
means that the atomic absorption is less dependent on the temperature variation. On
the contrary, atomic emission relies on the number of the excited atoms. An increase
in 10 K (2600 to 2610 K) results in a 3.7% increase in excited atoms (Ca 0* ). This
means that temperature variation needs to be better controlled in atomic emission to
assure reproducible data. (c) Since atomic absorption methods are based on a much
larger population of ground state atoms, they are expected to be more sensitive than
the flame emission methods.

:

Understanding ''Nebulization'' and ''Atomization'' Process: Why
Higher Sensitivity is Achieved in Flameless GFAA Than Flame FAA

In flame absorption spectrometry, a liquid sample (normally several milliliters) is
pumped into a mixing chamber where a process called ''nebulization'' is taking
place. Nebulization is to aspirate liquid sample into small liquid particles (aerosols).
It is estimated that only 10% of the fine aerosols will reach the burner, and all the
remaining larger droplets of the samples will condense and will be drained out of
the chamber. This results in lower efficiency in flame FAA as compared with
graphite furnace GFAA, because the entire sample is atomized for the latter.
It is critical not to be confused between ''nebulization'' and ''atomization''
which are taking place in flame or furnace. Atomization is a process that converts
sample elements into atomic vapor (gaseous atoms such as Ca 0 ). In flame atomi-
zation (FAA), the overall efficiency of atomic conversion and measurement of ions
present in aspirated solutions has been estimated to be as little as 0.1%. This is in
significant contrast with almost 100% in electrothermal atomization in graphite
furnace (GFAA). This difference simply implies that, in theory, the detection limits
in GFAA will be 100-1000 times improved over the FAA.

Quantitation and Qualification of Atomic Spectroscopy

Performing the qualitative and quantitative measurement is generally straightfor-
ward in atomic spectroscopy. Qualitative information, such as which elements are
present in the sample, is normally obtained for atomic emission spectroscopy rather
than atomic absorption. This is because in emission, a series of spectral lines
characteristic of the element of interest is identified. For quantitation, the concen-
tration of an element present in the sample is described by Beer's law, which