THE ROLE OF THE

Traditional Role of the Chemist in the Automobile Plant Environment 21
The chemist in the automobile industry may be called upon not only for the manufacture
of these compounds but also for the analysis in order to maintain the proper
concentration of the compound. The detection in an HPLC system is most widely
accomplished by UV-visible detectors. Most compounds used in production chemicals
have chromophores that are detectable in the UV region. Table 2.6 shows molar
absorption values for common functional groups [5]. Care must be taken that the solvent
used in the analysis is invisible to the analysis. As can be seen from Table 2.6,
typical detection range is 180–210 nm. A chromophore is the region between two
different molecular orbitals and will absorb UV light due to the conjugated system
of electrons or to a metal complex bound to a transition metal to a ligand.
2.9 some Problems wItH HPlc as a lab tool
In HPLC, an analyte is forced through a column of a stationary phase utilizing high
pressure. A sample of the chemical being analyzed is then introduced into the mobile
phase. Within the stationary phase, the sample will interact with the column material
and be separated within the column by chemical and physical interactions. The
variables of the sample itself, the column, and the mobile phase all contribute to
the retention time on the column. As the separation of the sample occurs within the
column, the pressure of the HPLC will increase the speed and reduce the diffusion
seen in a normal chromatogram. Typically, methanol and acetonitrile are used as the
solvent in the mobile phase for separations dealing with cutting fluids and hydraulic
additives. Most often a gradient will be applied to vary the concentration of methanol/acetonitrile
to enhance the separation.
Reverse phase HPLC is also used due to the nature of the polar constituents that
are examined. In reverse phase, the stationary phase is nonpolar and the mobile
phase will be a mixture of polar solvents and aqueous media.
Silica columns are generally used with long chain alkyl groups connected to
R(CH 3) 2SiCl. The polar fats, amines, and other polar functional groups in the additive
packages of concern will then come through the column more readily than the
longer chain nonpolar components in the base oils. From the discussion on lubricity
additives, one can see that reverse phase chromatography is a good choice for separation
of these compounds.
2.10 Plate tHeory and rate tHeory
Martin and Synge first described plate theory in chromatography in the early 1940s
[6]. Their thought was that a column can be considered as a long continuum of narrow
horizontal layers called theoretical plates. These theoretical plates are discrete
but thought of as being contiguous. Between the plates, equilibrium with the solute
and the two phases takes place. Thinking of a separation this way envisions a stepwise
process between each theoretical plate. A useful term, “height equivalent to
theoretical plate, H,” is related to the efficiency of a column by the equation: