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Two PMDs were evaluated for pneumatically controlled GC injection, specifically the Agilent Capillary Flow Technology™ (CFT) and SGE SilFlow™ Deans’ switch microfluidic wafers. PMDs are constructed from a series of thin SS plates that are laser etched and carefully aligned to obtain the required flow path architecture. Diffusion bonding was used to form a single cohesive wafer with a complex internal architecture. Column

unions are then welded to the wafer inputs and outputs to allow connection to capillary columns, and a control stream of auxiliary carrier gas. The internal surfaces of the PMDs were also chemically deactivated to prevent possible analyte retention or degradation from occurring during separation.

GC convection ovens are constructed from SS sheets that are insulated with a glass fibre insulation material to minimise heat loss during temperature programming. The thermal mass of most GC ovens is greater than 2 kg, all of which must be accurately heated during temperature-programmed analysis, which limits the maximum temperature-programming rate of these GC instruments. While PMDs represent a relatively small thermal mass compared to a GC convection oven, their ability to

accurately track oven temperatures has not been previously established in the academic literature. Insufficient temperature tracking leads to the development of cold spots in the carrier gas flow path and causes extra-column peak broadening. The surface area, mass and surface area to mass ratio of the Agilent and SGE PMD Deans’ switch are shown in Table 5.

Table 5 Physical properties of Agilent and SGE Deans’ switch microfluidic wafers

Agilent SGE

Wafer mass (g) 32.3 15.5

Wafer surface area (cm2_{) 46.5 17.8} Surface area to mass ratio (cm2 _g-1_{) 1.4 1.1}

To evaluate the ability of each PMD to track GC oven temperatures during

temperature-programmed analysis, a κ-type thermocouple was attached to each PMD. The relative temperature of the PMD was plotted against time during a series of

different temperature programs to determine the ability of the wafers to track the oven temperature set point during a typical GC run (Figure 12 and Figure 13).

Figure 12 Temperature measurements of the Agilent Deans’ switch PMD, relative to the programmed oven temperature for five linear temperature

programs. Temperature programming rates (°C min-1_{) are indicated in the figure}

legend. Initial oven temperature 40 °C with a 30 s hold, followed by temperature

programming to a final temperature of 300 °C with a 60 s hold.

Figure 12 revealed some large temperature deficits between the PMD temperature and the oven temperature. Ideally, all of the plots should show a relative difference of 0 °C between the PMD and the oven temperature set point. However following the initiation of the temperature ramp, the PMD was unable to track the oven temperature accurately, which caused the wafer to trail the temperature set point of the oven by 6 to 22 °C, depending on the speed of the temperature ramp selected.

Figure 13 Comparing the temperature of the SGE and Agilent Deans ’switch wafers relative to the oven cavity during a fast temperature ramp. Initial

temperature 40 °C for 60 s, then the oven was temperature programmed at a rate

of 30 °C min-1 _{to 300 °C, with a final hold time of 60 s afterwards.}

Figure 13 shows an overlay of the Agilent and SGE PMDs temperature during a 30 °C min-1 _{programming rate, relative to the oven temperature. Both microfluidic wafers} exhibited thermal hysteresis after the temperature programming was initiated at 60 s, with an average temperature deficit of -22 °C. A difference of this magnitude would cause significant amounts of extra-column peak broadening. Initially it was expected that the SGE wafer would have superior temperature tracking capabilities compared to the Agilent wafer, since it was smaller and had a much lower mass (44 % less). This proved to not be the case, and both wafers had similar temperature tracking profiles as shown in Figure 13. To reconcile this observation, the surface area of each wafer was calculated and compared to each wafers’ mass (Table 5) and the Agilent wafer was found to have a much larger surface area to mass ratio compared to the SGE wafer.

After 550 s, there is a steep return towards 0 °C relative temperature difference between the PMD and the oven temperature (Figure 13) that corresponds with reaching the final oven programmed temperature. A 60 s hold at 300 °C is then initiated. Both wafers equilibrate towards the 300 °C set point at a rate of approximately 12 °C min-1_, suggesting that 12 °C min-1 _{is the maximum heating rate that either PMD can be heated} within the 6850 GC oven cavity, during temperature programmed GC. Further

experiments showed that 2 min was sufficient to ensure that PMDs were equilibrated at the final temperature of 300 °C. The Agilent 6850 GC is specified to be capable of

temperature ramps at rates between 1 and 70 °C min-1_{, however this specification} appears to only be accurate when a capillary column is the only item being heated. Larger items, such as PMDs or rotary valves are large thermal masses that limit the upper programming limit of fast convection oven heating. To overcome this limitation, auxiliary heating is necessary to ensure cold spots do not develop in the flow path within GC convection ovens, alternatively a high temperature compartment, such as an isothermal oven or heating block can be used to ensure that PMDs are maintained at a sufficiently high temperature compared to the oven.