Page 34 - Tandem Techniques

Page 16

It consists of the normal septum injection device, but of somewhat smaller proportions, and the exit
carrier gas, containing the sample as vapor, splits into two streams; one stream passes to the column and
the other through an adjustable restriction to waste. By adjusting the restriction, the proportion of the
sample that passes onto the column can be selected. This would appear to be a satisfactory solution to
the problem of injecting samples onto a capillary column. Unfortunately, however, the split proportion
varies with the diffusivity of the solute and consequently its molecular weight. Thus the sample placed
onto the column may not be truly representative of the original sample injected, and the precision and
accuracy of the quantitative aspect of the analysis can be impaired. Nevertheless, the reduced
quantitative accuracy is sometimes tolerated for the sake of improved resolution and reduced analysis
time.

In an attempt to effect a compromise between the packed and capillary columns, the large bore capillary
column was introduced (0.053 in. I.D.). The large-bore column carries a much larger load of stationary
phase and is wide enough to allow a hypodermic needle to enter and allow on-column injection.
Unfortunately, the wider bore significantly reduces the resolution obtained from a given length of
column and also introduces other injection problems. On injection into an open tube, a sample is often
split into two zones, which results in single solutes providing double peaks on elution from the column.
Solutions to this problem have been suggested that tend to complicate the normally simple injection
procedure. These include the 'retention gap' method [10], where the first few centimeters of column are
stripped of stationary phase so that the sample is injected into an empty tube. This method can reduce
the peak splitting but a more successful, though more complicated procedure is to employ the solute
focusing method [11]. This procedure requires two ovens that can be heated and cooled independently.
The sample is injected onto the first part of the column, which is hot, and the sample vapor
subsequently condensed onto a second cooled portion of the main column. As a consequence, all the
solutes are focused at a point on the front portion of the cold column. The column is subsequently
heated to develop the separation in the normal way. This procedure reduces the chance of peak splitting,
but is more involved and demands more complicated and expensive equipment. A