Rheodyne TSG - Appendix F

Comments
Rheodyne front-loading injectors have a built-in needle port with a patented direct-connection design. It connects the tip of the syringe needle directly to the end of the sample loop. There is no connecting passage that traps sample, which can enter the loop when the next sample is loaded. Syringe accuracy is maintained and flushing after every injection is rarely necessary.

Size designations of sample loops are nominal. Actual volumes can differ from the nominal designations because of the tolerance of the tubing bore. Accuracy of large loops (2 mL) is about 5%, intermediate loops (20 µL) 10%, and small loops (5 µL) 30%.

The following Rheodyne Technical Notes contain useful information:

Technical Note 5, “Achieving Accuracy and Precision with Rheodyne Sample Injectors.”

Technical Note 7, “Pressure Drop of Valves and Tubes from 1 to 1000 mL/min.”

Technical Note 9, “How to Make Tube Connections Between Injectors, Columns and Detectors in Liquid Chromatography.”

What Happens Inside
Mobile phase in the loop is displaced when you dispense sample from the syringe during loading. The boundary between the sample and mobile phase has a parabolic profile due to laminar flow; the velocity at the center of the tubing is twice the average velocity, and at the wall it is almost zero. So, sample in the center travels 2 µL along the loop for every 1 µL dispensed.

This behavior accounts for the shape of the curve in the graph, a plot of sample volume dispensed from the syringe vs. mass of sample injected into the column. The curve has three regions.

1. When the volume dispensed is less than half the loop volume, the curve is linear. Sample has not yet reached the far end of the loop. Within this region, performance depends on syringe accuracy and the operator skill. Accuracy is typically 1%, if you use the syringe full scale volume. Precision is 0.2% to 2% RSD, depending on your skill. This is the region of the partial-filling method.

2. When the volume dispensed is between half a loop volume and about two loop volumes, the curve is nonlinear. Sample is lost from the loop, so accuracy and precision are poor. If you dispense a volume equal to the volume of the loop, you are in this region of inferior performance.

3. When the volume dispensed is several loop volumes, the mass injected is independent of the volume dispensed. The loop contains only pure sample, undiluted by residual mobile phase. Accuracy is determined by the loop (see Tips Page). Precision is 0.05% to 1%, depending on the volume loaded. Five loop volumes produce about 0.1% precision. When only a few loop volumes are loaded, in order to conserve sample, precision is improved by loading nearly the same volume each time. This is the region of the complete-filling method.

Volume of sample dispensed (in units of loop volumes) from the syringe vs. sample mass (observed peak area) injected onto the column for a Rheodyne front-loading injector.

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