Tip of the Month
Direct Injection in GC Systems with Electronic Pressure Control (continued)
To prevent pressure differentials during direct injections in EPC systems, various manufacturers have designed inlet liners with a small hole in the liner wall (Figure 3). The hole allows a portion of the incoming gas to escape from the liner and equalize the pressure at the downstream sensor, thereby eliminating pressure malfunctions. Use of these liners requires no hardware modifications, software modifications, or flow adjustments. Results for any direct injection analysis will be best with small samples, 1 µL or less, and a high carrier gas flow rate through the column. When a sample in 1 µL methylene chloride is injected into a heated injection port, the liquid sample can expand into a gas volume of more than 400 µL (Table II), which potentially can backflash out of the inlet liner \'d1 liners with 1 mm, 2 mm, or 4 mm i.d. have available internal volumes of 30 µL, 118 µL, or 471 µL, respectively. Furthermore, at a carrier gas flow rate of 1 cc/min, helium through a 30 m x 0.32 mm i.d. column (20 cm/s), it will take almost half a minute to transfer the vaporized sample from the inlet liner to the column inlet. If the carrier gas flow is increased to 7 cc/min (80 cm/s), 400 µL of vaporized sample will be transferred from the inlet liner to the column inlet in less than four seconds. This rapid transfer of the sample cloud will ensure narrow initial sample bandwidths. Similarly, smaller sample volumes will reduce the transfer time and will minimize the potential for sample backflashing. Also, if possible, choose a sample solvent with a small expansion volume. References (1) Grob, K., Split and Splitless Injection for Quantitative Gas Chromatography (Wiley-VCH Verlag GmbH).♦