Chemistry Reference
In-Depth Information
Separation Techniques
Complex mixtures, such as those that occur in ancient human remains or
as residues in ancient vessels and tools, may need to be separated into their
components before they can be analyzed.
Separation techniques
are ana-
lytical techniques used for separating and sometimes even identifying the
components of chemical mixtures (Setford 1994). Two widely used separa-
tion techniques are
chromatography
and
electrophoresis
.
Chromatography
is based on allowing a gaseous or liquid mixture or solution, usually known
as the
analyte
, to seep through a
stationary
, usually
solid medium
that sep-
arates between the components; the separation depends on the rates at
which the different components of the analyte move through or along the
stationary medium. If the moving mixture consists of gases, the technique
is known as
gas chromatography
(GC); if it consists of liquids or is a liquid
solution, it is known as
liquid chromatography
(LC) or
high-performance
liquid chromatography
(HPLC) (Setford 1994; Miller 1988).
Hyphenated Analytical Techniques
Two or more combined analytical techniques, generally called
hyphenated
or
tandem techniques
, provide more information than that obtained from
single traditional physical techniques, thus yielding more reliable results
(Rouessac and Rouessac 2000). Many challenging analytical problems, for
example, involve more or less complex mixtures that require, as an impor-
tant first step, separation of their components. Some hyphenated
techniques, therefore, couple a separation technique with an analyzing
technique so as to separate and analyze complex mixtures. Such is the case
with gas chromatography (GC), which is coupled with another separation
and analyzing technique, mass spectroscopy (MS), to yield the
gas chro-
matography
-
mass spectrometry
(GC-MS) analytical technique. The com-
bination of the two techniques allows a much finer degree of substance
identification than does either technique used separately (Kitson et al.
1996; Scott 1997).
In many instances when analyzing archaeological objects, removing a
sample may be aesthetically deleterious to the object, therefore rendering the
removal of samples totally inadequate, or allowing the removal of extremely
small samples so as not to affect the appearance of the objects. In such cases
it is often necessary to turn to specialized techniques, such as nondestruc-
tive techniques, which do not require the removal of samples altogether, or
to micro analytical techniques, for which extremely small samples are
needed (see Textbox 11).
