The most popular type of gradient pumping module used to perform HPLC analysis utilizes a low pressure mixing valve in their design. These valves are electronically controlled and proportion the amount of mobile phase from one of several solvent channels into a mixer for introduction to the pump head (*the solenoid valves used for this are sometimes called gradient proportioning valves). They provide random access to multiple solvents (e.g. 4) for method development and column flushing. The mobile phase solutions are mixed at low pressure before entering the high pressure side of the pump head (where they undergo compression). This design requires only one high pressure pumping head and can allow for very high mixing accuracy (often 0.1% per channel) of the mobile phase. This allows for the formation of mobile phase gradients over time which greatly aid in resolving samples apart on the column.
The gradient proportioning valves need to be tested along with the other parts of your HPLC system on a regular basis to insure they are operating within the manufacturer's specifications. They should also be tested anytime you suspect a problem may be present. One quick way to check the operation of two of the valves is to use a tracer compound and STEP gradient to monitor their operation. You can set up a method to perform this test as suggested below.
QUICK GRADIENT COMPOSITION TEST:
Bottle A = 100% DH20;
Bottle B = 0.1 % Acetone in DH20 (*Acetone is the tracer compound);
Flow Rate = 1.000 ml/min;
Column = No column. Install a restriction capillary in place of the column to obtain a backpressure of > 60 Bars;
Detection = 265nm (10 nm bandwidth) UV;
STEP Gradient Program:
0 to 2.00 min, 0 % B
2.01 min, 20% B
4.01 min, 40% B
6.01 min, 60% B
8.01 min, 80% B
10.01 min 100 % B
12.01 min 20% B
14.00 min 20% B
Note: If the delay volume (dwell volume) of your system is large, then you may want to adjust the time values shown to LARGER values (i.e. 2 minutes delays are used in this example, but 5 or even 10 minute delays between steps may be more appropriate if your system has > 1 ml dwell volume.
Running the above method should result in a signal trace which shows a step-wise rise to 12.00 minutes (as the acetone concentration increases). The edges of the "steps" should be sharp and the risers should also be close to vertical. The final step change which starts at 10.01 minutes shows a linear gradient change back down to the 20% B level. This line should not have any bumps or dips in it and should transition smoothly back down. The height of the baseline at this point should match the height seen between 2.01 and 4.00 minutes (same 20% B). The height of the proportional steps (e.g. 20, 40, 60, 80) should also be the same. You can use your CDS to measure these height values.
Another useful aspect to view is the S/N ratio at each step. Use your CDS to establish noise windows within each range (e.g. 2.50 to 3.5 minutes). This data is useful when comparing the performance of the pump at different intervals.
If you observe deviations in the height of the proportional steps or dips in the lines, these can be caused by leaking or sticking check valves as well as leaking or sticking gradient proportioning valves. *If you have a quaternary pump, be sure and test all four of the valves used (2x per test).
Lastly, the above example is a generalized method and may or may not be applicable to your specific HPLC pump. Be sure and customize a test method which takes into account the pressure ranges, flow rates, delay volume, mixing volume, and number of low pressure channels used in your pump.
