8
Figure1.
Optimizing splitless hold time.
0 15 30 45
120
75 90 105
60
late-eluting compounds
hold time (sec.)
20
16
12
8
4
0
component area (thousands)
The time period that the solenoidvalve is closed is referred to as the splitless hold-time.
The hold-timemust be optimized toobtain the best performance from the analytical system.
If the solenoidvalve is opened tooquickly, some of the samplewill be lost causing reduced
response. If the solenoidvalve is open too long, the solvent peakwill tail. The splitless hold-
timewill varydependingon column flow rate, injectionport geometry, injectionport
temperature, andvolatilityof the analytes. It is impossible topredict the optimum splitless
hold-timewithout performing some experimentationunder the exact conditions of your
analysis.
Tooptimize the splitless hold time for a particular instrument, prepare a standard that contains
both an early- and a late-eluting compound (e.g., fluorophenol andbenzo(g,h,i)perylene).
Inject this standardover a range of splitless hold times from0.1 to2.0minutes andplot the
data.An example of this optimization is shown inFigure 1.
In this example the optimum splitless hold time is 60 seconds. This is the point on the graph
where the responseof the late-eluting compound levels off.Holding the solenoidvalve closed
longerwill not appreciably increase the response of this compound, butwill greatly increase
the size of the solvent peak. Because the lower boiling compoundwill transfer onto the
column faster, its responsewill level off sooner (in this example~45 seconds).Once this data
has beenplotted, it is possible toobserve the correct splitless hold-time (once again, the point
atwhich the response of the late-eluting compounds levels off). The net effect of this
optimization is tomaximize responseof late-eluting compoundswhileminimizing solvent
tailing.
In addition tooptimizing the splitless hold-time, fused-silicawool shouldbe used in the
injectionport liner to improvevaporizationof highermolecularweight compounds.While
there aredifferent theories regarding theplacement of fused-silicawool, consistency in the
amount of packing and locationof thepacking ismost important. Restek recommends placing
the plugofwool below the point that the syringe needle reaches, but above the inlet of the
column.We also recommendusing agooseneck liner tominimize contact between the
injected sample and the bottomof the injectionport. Thiswill help improve the response of
themore reactive compounds such as 2,4-dinitrophenol, PCP, and thenitroanalines. The
gooseneck liner alsomakes thegreatest improvement in response andminimizationof endrin
breakdown forUSEPAMethod525.
Another technique tominimizemolecularweight discrimination is toperform the splitless
injectionunder ahigher columnheadpressure.Ahigh inlet pressure is advantageous during
injection to control the rapidly expandingvapor cloud in the inlet. Byusing amomentary
pressure pulse for the time that the split vent line is closed, the sample vapor cloud is
controlled and sample backflash into the gas lines entering and exiting the injectionport is
minimized. The effect of the pressure pulse is to increase the amount of analyte transferred to
the column, especially the late-eluting components. This can lead to stationaryphase
overload, however, so itmaybe necessary to increase the capacityof the columnwhenusing
this technique (see the
ColumnSelection
section formore information).
early-eluting compounds
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