Dispersion Matters: Rethinking How LC Systems Should be Categorized

For more than five decades, attempts have been made to improve liquid chromatography separations performance by reducing the particle size of the stationary phase.

However, the benefits of doing so can only be realized if the LC instrument provides an appropriate amount of extra-column dispersion (band spreading) in relation to the volume of the column being used.

Dispersion plays a key role in maximizing chromatographic performance. Despite this fact, it’s often overlooked.

On band spreading

As an analyte band becomes wider, the resulting chromatographic peak width is increased. This wider analyte band results in a dilution effect that produces a decrease in peak height accompanied by a loss in sensitivity and resolution.

Conversely, when band spreading is minimized, narrower chromatographic bands are achieved, resulting in higher efficiency.

But I usually talk “pressure” with my sales rep

While conventional instrument sales materials often weigh heavily on the role that the pressure/flow rate envelope plays on separations, these specifications essentially have little bearing on actual separation power. Instead, such factors play more into what diameter of column or size of particle can be accommodated on a given LC system.

In fact, by attempting to accommodate the highly popularized broad flow rate/pressure envelope spec, tubing I.D. is increased, which actually compromises chromatographic performance because it increases extra-column dispersion.

So what gives me better performance?

True separation performance is governed by system dispersion paired with a linear velocity that yields the highest possible performance for a given analytical column.

It’s important to understand how “performance” is defined in selecting an LC system. While current perception is that “power range” is the line of demarcation within the market, “dispersion” is the specification that best represents a system’s capacity for separation performance.

How to better categorize LC systems

It is therefore appropriate to categorize LC systems into three distinct categories in order of performance maximization:

• HPLC [High Performance Liquid Chromatography] systems generally have an extra-column dispersion of 30 µL or greater, and are optimized to run columns packed in 4.6 mm I.D. with particles greater than 3µm. Typical operating pressure generally does not exceed 6,000 PSI and flow rate is often less than 3.0 mL/min.
• UHPLC [Ultra High Performance Liquid Chromatography] systems generally have extra column dispersion between 12 – 30 µL, and are optimized to run columns packed in 3.0 mm I.D. with particles no less than 2.5 µm. Typical operating pressure generally ranges between 6,000 and 15,000 PSI, and flow rate is often less than 2.0 mL/min. The dispersion, pressure and flow rate characteristics of these systems make them ideally suited for modern 2.7 µm solid-core columns.
• UPLC [UltraPerformance Liquid Chromatography] systems generally have extra column dispersion no greater than 12 µL, and are optimized to run columns packed in 2.1 mm I.D. with particles less than 2.0 µm. Typical operating pressure generally ranges between 9,000 and 15,000 PSI, and flow rate is often less than 1.0 mL/min.

Let’s take a closer look at these three categories of LC systems, and how dispersion, system scalability, and operational costs can inform your next investment in an LC instrument.

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