Previous installments in this series covered the basics of valving and its applications in gas chromatography (GC). This month, we will discuss the use of fluidic switching devices for open-tubular column manipulations, such as heartcutting and comprehensive multidimensional GC×GC.

Open-tubular (capillary) gas chromatography (GC) columns challenge mechanical flow switching valves with requirements for higher speeds and lower volumes. Soon after the initial development of capillary columns, researchers recognized that fluidic flow switching devices were well suited for multidimensional capillary GC applications. David R. Deans (1) was one of the pioneers who published papers and patents on the subject in the late 1960s; the term Deans switch is synonymous with such applications. But practical multidimensional capillary column implementations depend on precise pressure settings and timing intervals, and the mechanical pressure regulators and control systems found in earlier gas chromatographs were not up to the challenges of widespread application. Broader adoption of capillary column multidimensional applications was delayed until the onset of modern solid-state electronics, microprocessor control, and the silicon microelectromechanical systems (MEMS) devices that constitute the pressure and flow components of electronic pressure control systems (EPC) found in modern GC instruments.

Fluidic Switching

Rotary valves directly connect to sample loops, columns, and detectors for detector selection, column-to-column manipulations, and backflushing, to name a few applications. Earlier installments in this series (2,3) described how chromatographers configure rotary valves to redirect column output to different detectors or other columns. Implementations with rotary valves are robust and generally simpler to set up and maintain than fluidic switches. However, mechanical actuators have limited switching speeds and are subject to mechanical stress and wear over the long run. Valve rotors have temperature limits to be considered as well. Diaphragm valves, which use pressure to actuate and relieve polymeric seals across the valve ports instead of a rotating valve core, switch faster and have extended lifetimes. Mechanical valves are ideal for packed, micropacked, and some wider bore capillary columns, and fluidic switches perform best with narrower capillaries and at higher switching speeds.

The Deans switch originally employed a series of interconnected tees and crosses with control flows driven by electrical solenoids. Modern implementations use more integrated designs that incorporate the minimum number of external unions. Precisely formed internal passageways are allocated to the necessary cross-connections; these also form internal restrictions where desired. These designs have minimum dead volumes and their construction materials present highly inert surfaces to passing solutes. The control pressures are established and modulated over time with electronic pressure control (EPC).

Figure 1

In Figure 1a, the control pressure applied at S1 is higher than that applied at S2. As a result, flow inside the device at the input stream's central point of entrance runs from left to right, and the input stream is directed to outlet 2. When the pressures at the control points are reversed, as in Figure 1b, then the flow proceeds from right to left, and the input stream is directed to outlet 1.

The input stream, which is most often the carrier gas from a first column, is diluted with a small amount of control gas on its way through the switching device. Careful choice of pressures relative to the columns and restrictions minimizes the amount of dilution and normally does not create any problems with sensitivity or peak resolution — in fact, resolution is improved as the result of better peak shapes. A minimum amount of flow at all times from both of the pressure control inputs prevents back flow and flushes the inner passageways. In contrast, the mechanical switching valve arrangements discussed in earlier installments (2,3) do not contribute any additional gases to the main carrier streams.

The associated columns and restrictors are not shown in this diagram. The inlet point and each outlet point of the fluidic switch would connect in an application-dependent fashion to a column or restrictor, whose pressure and flow characteristics determine the passage of carrier and control gases through the switching system in concert with the incoming pressure settings.