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HPLC/UHPLC Trap Columns: Trapping - What is it? What are trap cartridges and how are they used?

Trapping: The selective retention and subsequent elution of analytes within a sample. Trapping is a chromatographic technique, but typical phase boundary effects like partitioning are not being exploited. Many trapping applications could be described as "digital", or on/off chromatography, because solvent conditions are selected to ensure that there are only two retention states - one where solvent strength is weak enough to allow the analyte to bind to the stationary phase without eluting, and one where solvent strength is sufficient to cause immediate and complete sample elution. The term trap cartridge refers to a packed bed with suitable capacity to completely retain a given amount of target analyte within a sample. Trapping is often used in conjunction with mass spectrometry - the trap cartridge serves to clean up (and/or pre-concentrate) the analyte, while the MS handles detection and identification. The MS can also make up for low separation resolution and column efficiency that would not be acceptable with other modes of detection. In return for the trade-off in resolution, trap cartridges offer greatly reduced analysis time (a benefit common to all short/fast bed formats.)

Example 1: Sample Cleanup ("trap and dump"), Desalting

The analyst has a simple mixture - a compound of interest dissolved in a medium not suitable for injection onto an MS (highly aqueous, salt-containing, etc.). For instance, protein samples often arrive for analysis in buffered solutions. This might be for stability reasons, or the salts might be present due to a preparation step such as a digest. This salt has to be removed prior to mass spectrometry analysis, as they will foul the MS interface. A trap cartridge can be placed within the loop of an injection valve to conveniently capture the analyte. Once the buffer salts and impurities have been washed away, mobile phase strength can be increased and the compound eluted onto an analytical column, or directly onto the MS.

Example 2: LC separations on the MS timescale

A big problem with LC-MS is that the LC part takes a lot longer than the MS part. With MS, it is not always necessary to achieve a high level of resolution, but a trap cartridge can be used to provide a small amount of chromatographic separation. Using switching valves, two trap cartridges could even be used in a tandem/parallel set-up, reducing the LC cycle time by 50%. These two factors can be combined to produce an automated fast LC-MS system that works at the pace of the MS, not at that of the LC.

Example 3: Separation of a mixture of proteins

Traps are often used in protein separations. You can actually use two or more cartridges in a row to assist with separation of mixtures of charged and neutral proteins. For instance, an ion exchange cartridge could be used in-line, followed by a C18 cartridge. Additional switching and injection valves would be used to offer increased solvent routing flexibility, as follows:

Gradient pump → Injection Valve → Ion Exchange Trap → Switching Valve → C18 Trap → Mass Spectrometer

The protein or peptide mixture would first be injected inline ahead of the ion exchanger. Use of a weak mobile phase solvent would cause charged proteins to be retained by the ion exchanger, while neutral proteins would pass through to the C18 cartridge and be retained there. A switching valve in between the exchanger and the C18 bed allows switching of the solvent being delivered to just the C18 trap. The neutral proteins can be eluted from this trap onto the MS by increasing solvent strength. Then, charged proteins could be selectively eluted from the exchanger onto the C18 column via salt gradient (the valve downstream from the C18 trap would switch so that salts were sent to waste, not to the MS). A second subsequent elution off the C18 trap (in conjunction with a solvent switch) sends these charged proteins onto the MS, free of salts and contaminants.

Optimize Products for Trapping

Optimize offers a wide selection of novel packed-bed products, all of which can be used effectively in a sample trapping or on-line purification application. The following products are well-suited for use as trap cartridges, but we encourage you to call us to discuss your particular requirements - we have numerous bed formats in a variety of dimensions, and many options for standard or custom-packed stationary phases that can be tailored to your application. Our trap cartridge offering is centered around four main offerings with many variations - a low-volume capillary product suitable for direct installation into injection valve ports and columns, a variable volume quick-connect product that can be connected in-line anywhere on your system, a low-volume ultra high pressure stem trap, and a variable volume ultra high pressure hand-tight cartridge trap.

OPTI-PAK® Capillary Traps

This product is perfect for direct connection to injection and switching valves, and offers a very low-volume bed format combined with auto-adjusting ZDV connection into any 10-32 port. Use for very small sample volumes and low flow rates. This product is rated to 6,000 psi.

OPTI-LYNX™ Trap Cartridges

These cartridges offer quick-connect convenience and a larger bed for greater sample capacity and higher flow rates.

EXP® Stem Traps

The slim architecture allows it to easily fit into crowded instrument compartments or to connect directly to tightly-spaced injection ports. When tightened by hand it seals to 8,700+ psi. When tightened by wrench on the incorporated wrench flats, it seals to 20,000+ psi. The low volume, low dispersion cartridges are perfect for your UHPLC volume-critical applications.

EXP® Trap Cartridges

The Hand-Tight EXP Trap Column is rated for use at 20,000+ psi and can connect to any injection valve or column with 10-32 threads or can be connected in-line with 1/16” stainless tubing. For your UHPLC applications that require a larger packed bet, these cartridges are the best choice on the market today.