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Pharmaceutical Article

Using a 1mm Inlet Liner for Headspace Analysis Residual Solvents Class 2 Mix A on Rtx®-1301 (G43)

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by Rick Lake, Pharmaceutical Innovations Chemist

The function of an injection port in headspace analysis is very different than in conventional direct liquid injection. In a direct flash injection, the injection port vaporizes a liquid sample for transfer into the capillary column. When the sample vaporizes, it rapidly expands in volume, filling the liner. A 4mm liner is commonly used for flash injections as its volume is large enough to handle the expansion of most solvents when injecting small volumes (1-2µL).

In contrast, in headspace analysis, the sample is vaporized inside the headspace vial and the gaseous sample is transferred into the injection port either by a transfer line or by a syringe injection. The injection port therefore is used for sample transfer and a large volume liner can cause deleterious effects such as band broadening, denoted by a broad peak width or a loss of peak efficiency. More volatile, early eluting compounds are more prone to band broadening because they cannot be properly focused on the analytical column. For example in USP<467> choosing a 4mm liner affects peak efficiency, causing resolution between acetonitrile and dichloromethane to fail system suitability criteria (Figure 1).

For headspace applications involving a transfer line, a smaller bore liner, preferably 1 mm, is recommended. This lower liner volume decreases band broadening and allows quicker sample transfer by increasing the linear velocity through the inlet.


Figure 1  Using a 1mm Inlet Liner for Headspace Analysis Residual Solvents Class 2 Mix A on Rtx®-1301 (G43)

  1. acetonitrile
  2. dichloromethane

Column:
Rtx®-624 30m, 0.32 ID, 1.8µm (cat. # 10970)
Sample:
USP <467> Class 2 Mixture A Standard Solution (cat#. 36271) in 20mL headspace vial
Inj.:
Headspace injection (split ratio 1:5), 1mm split liner Siltek® deactivated and 4mm split IP deactivated liners
Inj. temp.:
140°C
Carrier gas:
helium, constant flow
Flow rate:
2.16 mL/min., 35.3 cm/sec.
Oven temp.:
40°C for 20 minutes, to 240°C @ 10°C/minute, hold for 20 minutes
Det.:
FID @ 240°C
Headspace Conditions
Instrument:
Tekmar HT3
Transfer Line Temp.:
105°C
Valve Oven Temp.:
105°C
Sample Temp.:
80°C
Sample Equil. Time:
45min.
Vial Pressure:
10psi
Pressurize Time:
0.5min.
Loop Fill Pressure:
5psi
Loop Fill Time:
2.00min.
Inject Time:
1.00min.

GC_PH00912