Chemistry Reference
In-Depth Information
uid mobile phase is continuously pumped through
the column. The stationary phase is usually a chemi-
cally modifi ed silica or polymer. The analytes inter-
fere to a different extent with the stationary phase
and the mobile phase. This determines the length of
time each analyte resides in the column. Usually the
LC system is coupled to a specifi c detector. Such a
setup is perhaps the most common in elemental spe-
ciation analysis. High-performance liquid chroma-
tography columns (HPLC) form a widely used subset
of LC, with small diameter particles (3-5
9.2 Gas Chromatography
Only volatile and thermally stable species qualify
for separation by gas chromatography. Very few com-
pounds fulfi ll these requirements. Fortunately, the
analyst can resort to chemical reactions that transform
nonvolatile compounds into volatile, thermally stable
ones. This process is referred to as “derivatization”
(García Alonso and Encinar, 2003). Liu and Lee (1999)
have written a comprehensive review on chemical
modifi cation or derivatization of analytes in speciation
analysis.
Naturally occurring volatile species are dimeth-
ylmercury (Me
2
Hg), dimethylselenium (Me
2
Se),
tetramethyltin (Me
4
Sn), trimethylantimony (Me
3
Sb),
trimethylbismuth (Me
3
Bi), methylated arsines, tetra-
alkylated lead compounds in sewage sludge, and
many more gases from municipal waste disposal sites.
This list is not exhaustive. Feldman (1997) has done
interesting research on these compounds, describing
innovative ways to convert nonvolatile into volatile
species by derivatization techniques. Various separa-
tion schemes have been developed. Most common is
the cryogenic trapping and sequential thermal desorp-
tion from the packed columns. This method is not very
selective, but unstable compounds can be preserved
for a long time before analysis. Next comes gas chro-
matography on packed columns, offering an improved
separation of the analytes through interaction with
the column, combined with separation on the basis of
their volatility (Szpunar
et al.
, 1996). Gas chromatogra-
phy with capillary columns offers a much-improved
resolution. Their very small loading capacity forms the
limiting factor for their exploitation.
The most common detector for this type of specia-
tion is inductively coupled plasma mass spectrometry
(ICP-MS), additionally inductively coupled plasma
atomic emission spectrometry (ICP-AES). It is also
possible to do isotope dilution measurements and iso-
topic ratio patterns when a high-resolution ICP-MS is
available as the elemental detector.
m) as the
stationary phase, the mobile phase being pumped
under increased pressure. A good overview of the
role in elemental speciation can be found in Ackley
and Caruso (2003). The most common types of LC are
size exclusion chromatography (SEC), ion-exchange
chromatography (IC), and affi nity chromatography
(AC). Today, it is also possible to couple an LC setup
to a soft ionization system to obtain structural infor-
mation. Examples of a soft ionization technique are
electrospray mass spectrometry (ES-MS) (Chassaigne,
2003) and tunable plasma (Leach
et al.
, 2003). Figure
1 shows an example of the separation of Ni specia-
tion analysis in the hyperaccumulating plant
Sebertia
acuminata
(Schaumlöffel
et al.
, 2003). The authors fi rst
applied size exclusion chromatography on the plant
extract and measured Ni with ICP-MS, followed by
chromatography of the m/z fraction containing Ni
on Superdex peptide HR (30 cm × 10 mm) column
also coupled to ICP-MS. Next, electrospray ionization
mass spectrometry (ESI-MS) was done of the frac-
tions corresponding to the major peak. This allowed
the identifi cation of the compound as a complex of Ni
with nicotinamide.
Multiple procedures are described in the litera-
ture for the separation of specifi c elemental species or
groups of species. As an example, approximately 100
chromatographic conditions have been listed in the
literature for the separation of organotin compounds
(Harrington
et al.
, 1996), selenium species (Montes-
Bayón
et al.
, 2000), arsenic species (Ali and Jain, 2004;
B'Hymer and Caruso, 2004), mercury species (Har-
rington, 2000), elemental species bound to proteins
(Templeton, 2005), elemental species bound to humic
acids (Heumann, 2005), etc. Variations in the choice of
the column and the elution conditions are legendary.
Enrichment and derivatization of the species have
been comprehensively outlined by Bouyssiere
et al.
(2003). Liquid chromatography will surely remain
the major separation technique in the decades ahead.
The electronic databanks prove to be very helpful in
putting together a procedure that is ideally suited for
the combination of matrix, analyte, and the available
infrastructure of the laboratory.
µ
9.3. Capillary Electrophoresis
The principle of separation by capillary electro-
phoresis (CE) is based on differences in the electrically
driven mobility of charged analytes, similar to conven-
tional electrophoresis. A high voltage electrical fi eld
(typically 20-30 kV) is applied along an open tube col-
umn with small internal diameter (Michalke, 2003).
This technique can be used as a primary or as a sec-
ondary separation technique (e.g., after HPLC) when
it is referred to as a two-dimensional technique. Taking
into account the very small loading capacity of CE, the
