Introduction - Determination of Anions in Natural and Treated Waters - page 49

of different polarities often provides better separation than with a capillary column. It
sometimes happens that a capillary column is used as a supplement in the packed-column
gas chromatograph. It is best, therefore, to house the capillary and packed columns in the
same column oven and use them routinely and the capillary column is used when more
detailed information is required.
Conventionally, it is necessary to use a dual-column flow line in packed-column gas
chromatography to provide sample and reference gas flows. The recently developed
electronic base-line drift compensation system allows a simple column flow line to be
used reliably.
Recent advances in capillary column technology presume stringent performance levels
for the other components of a gas chromatograph as column performance is only as good
as that of the rest of the system. One of the most important factors in capillary column
gas chromatography is that a high repeatability of retention times be ensured even under
adverse ambient conditions. These features combine to provide ±0.01min repeatability
for peaks having retention times as long as 2h (other factors being equal).
Another important factor for reliable capillary column gas chromatography is the
sample injection method. Various types of sample injection ports are available. The
split/splitless sample injection port unit series is designed so that the glass insert is easily
replaced and the septum is continuously purged during operation. This type of sample
injection unit is quite effective for the analysis of samples having high-boiling-point
compounds as the major components.
In capillary column gas chromatography, it is often required to raise and lower the
column temperature very rapidly and to raise the sample injection port temperature. In
one design of gas chromatography, the Shimadzu GC 14-A, the computer-controlled flap
operates to bring in the external air to cool the column oven rapidly—only 6min from
500°C to 100°C This computer-controlled flap also ensures highly stable column
temperature when it is set to a near-ambient point. The lowest controllable column
temperature is about 26°C when the ambient temperature is 20°C
Some suppliers of gas chromatography are listed in Table 1.9.

Instrumentation

Shimadzu gas chromatographs
This is a typical high performance gas chromatograph version (see Table 1.9 for further
details). The inner chamber of the oven has curved walls

Table 1.9 Commercial gas chromatographs

Manufacturer Model Packed
column

Capillary
column

Detectors Sample
injection
point
system

Keyboard
control

Link to
computer

Visu
disp

Shimadzu GC- Yes Yes FID 1 Split- Yes Yes No