Post Column Derivatization Using Reaction Flow High Performance Liquid Chromatography Columns

Podsumowanie

A protocol for the use of reaction flow high performance liquid chromatography columns for methods employing post column derivatization (PCD) is presented.

Streszczenie

A protocol for the use of reaction flow high performance liquid chromatography columns for methods employing post column derivatization (PCD) is presented. A major difficulty in adapting PCD to modern HPLC systems and columns is the need for large volume reaction coils that enable reagent mixing and then the derivatization reaction to take place. This large post column dead volume leads to band broadening, which results in a loss of observed separation efficiency and indeed detection in sensitivity. In reaction flow post column derivatization (RF-PCD) the derivatization reagent(s) are pumped against the flow of mobile phase into either one or two of the outer ports of the reaction flow column where it is mixed with column effluent inside a frit housed within the column end fitting. This technique allows for more efficient mixing of the column effluent and derivatization reagent(s) meaning that the volume of the reaction loops can be minimized or even eliminated altogether. It has been found that RF-PCD methods perform better than conventional PCD methods in terms of observed separation efficiency and signal to noise ratio. A further advantage of RF-PCD techniques is the ability to monitor effluent coming from the central port in its underivatized state. RF-PCD has currently been trialed on a relatively small range of post column reactions, however, there is currently no reason to suggest that RF-PCD could not be adapted to any existing one or two component (as long as both reagents are added at the same time) post column derivatization reaction.

Wprowadzenie

High performance liquid chromatography (HPLC) coupled with post column derivatization (PCD) is a powerful tool that is useful in solving a number of issues in the analytical laboratory. It can be used to detect compounds that are otherwise undetectable with the suite of detectors available^1,2, increase the signal of the target analyte, which allows lower limits of detection and quantitation^3-5 or selectively derivatize a target analyte in order to avoid matrix effects⁶. Commonly used PCD reactions include the reaction of amines, such as amino acids, with ortho-phthaladehyde^7-9, ninhydrin^9,10 or fluorescamine^11,12, the derivatization of reactive oxygen species (ROS) with the 2,2-diphenyl-1-picrylhydrazil radical (DPPH^•)^13,14 or 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS)^15,16, and the use of the iodide-azide reagent to derivatize sulfur containing compounds^17,18.

There are, however, numerous drawbacks to the use of PCD reactions with HPLC systems⁶. Principally among these is the use of reaction coils between the point of addition of the derivatization reagent(s) and the detector, which allow time for mixing and the reaction to occur⁸. These reaction loops often have volumes of 500 µl or more, which is significant compared to the volume of the rest of the HPLC system¹⁹. The use of these high volume reaction loops results in increased peak broadening compared to what would be observed without the presence of the reaction loop. This results in shorter, wider peaks that have higher limits of quantitation and detection and negatively effects chromatographic resolution. Figures 1 and 2 highlight the deterioration of peak shape that result from the addition of various post column reaction loop volumes. This analysis was performed with a mobile phase composition of 94% methanol and 6% Milli-Q water. The flow rate of the mobile phase was 1 ml/min, the injection volume was 20 µl and the analysis wavelength was 265 nm. Coils of varying dead volumes from 20 µl to 1,000 µl were inserted between the column and the detector to simulate the effects of reaction loop dead volume in PCD methods. These loops were prepared from stainless steel tubing of 0.5 mm internal diameter. The experiment was performed on a HPLC system consisting of a controller (SCL-10AVP), a Low Pressure Gradient Valve (FCL-10ALVP), a Pump (LC-20AD), an Injector (SIL-10ADVP), and a pda detector (SPD-M10ADVP). The mobile phase was pumped through a degasser prior to introduction into the HPLC system. The separation was performed using a 250 mm x 4.6 mm i.d. 5 µm column. Experimental conditions were chosen to be typical of PCD reactions that have recently been published in the literature.

The simplest, most common post column reactor setup is termed a non-segmented tubular reactor which is effectively a long, thin tube through which the liquid can flow and the reaction can take place. In this system peak broadening is dependent on not just the dead volume added to the system, but also the internal diameter of the tube as highlighted by Iijima et al.⁸. Furthermore, coil geometry plays a part in the observed brand broadening. Stewart ²⁰ stated that coiling of the reactor changes the secondary flow profiles, resulting in better mixing, meaning that the dead volume may be minimized. It has been stated that peak broadening is not significant when using an open tubular knitted coil ²¹ . When the peak broadening is excessively large, other types of reactors may also be considered ^20,22. These may include bed reactors or segmented flow reactors. These reactors are particularly useful for slow reactions that would otherwise require large reaction loops. As non-segmented tubular reactors are the most common types of reactors used in PCD applications, the rest of this article deals with this type of reactor setup.

The design of the reaction flow (RF) column incorporates a multi-port end fitting that allows mobile phase to exit (or enter) the column through either a single port located at the radial central region of the column or three ports located at the outer wall region of the column (see Figure 3). These two streams are separated using an end fitting containing a central porous frit that is surrounded by an impermeable ring that is in turn surrounded by an outer porous frit that extends out to the column wall. Due to the central impermeable ring cross flow is not possible between the two porous regions.

During reaction flow chromatography, the derivatization reagent(s) are pumped against the direction of mobile phase flow into one or two of the outer ports of the reaction flow column. The column eluent is mixed with the derivatization reagent(s) in the outer frit and passed to the detector through a free outer port. Reaction flow can be used for either a single reagent derivatization (1 port for the derivatization reagent, 1 port to pass the column eluent to the detector and 1 port blocked) or a dual reagent system (2 ports for the derivatization reagents and 1 port to pass the column eluent to the detector). The flow from the central stream can either be used to detect the underivatized column eluent, effectively multiplexing detection²³, or passed to waste.

One major tuning technique that is available when running RF-PCD chromatography is the ratio of the central and peripheral flows. The optimum ratio for each derivatization depends on a number of factors such as whether the central flow will be detected or passed to waste. Therefore once the optimum ratio has been determined, it should be ensured that the correct flow ratio is achieved prior to each run being performed.

It has been found that the use of a frit to mix the column eluent stream and the derivatization reagent in RF-PCD results in more efficient mixing compared with traditional mixing techniques that typically employ a zero dead volume T-piece or low dead volume W-piece to mix the two streams. This has allowed for the use of relatively small reaction loops, or even the elimination of the reaction loop altogether. The reduction of the reaction loop size results in sharper peaks compared to traditional post column derivatization methods. This means that, despite the fact that not all of the column eluent is derivatized, greater signal to noise ratios are observed and therefore lower limits of detection and quantitation can be achieved.

Reaction flow chromatography has been developed to overcome difficulties with the adaption of PCD reactions to modern HPLC columns and systems, particularly the loss in efficiency caused by band broadening due to large post column dead volumes caused by the need to employ large volume reaction loops. The more efficient mixing processes in RF-PCD compared to conventional PCD mean that smaller reaction loop volumes may be employed leading to an increase in observed separation efficiency. Furthermore RF-PCD chromatography shows both increased signal and decreased noise compared to conventional PCD techniques resulting in lower limits of detection and quantitation compared to conventional PCD methods. An additional advantage of RF-PCD compared to conventional PCD methods is the ability to monitor the underivatized stream that elutes from the central port of the RF column as well as the derivatized stream that elutes from the peripheral region of the column. RF-PCD is a relatively new but promising technique that displays many advantages over traditional PCD methods.

Connection of the RF column is achieved in almost the same way as a conventional HPLC column with the major difference being the number of end fittings on a RF column. Fittings used to connect a standard HPLC column to the HPLC system are able to be used to connect a RF column to the HPLC system.

Protokół

Caution: Please refer to material safety data sheets (MSDS) for all materials and reagents before use (i.e., MSDS for methanol). Ensure the use of all appropriate safety practices when handling solvents and High Performance Liquid Chromatography (HPLC) eluent. Ensure appropriate use of engineering controls of HPLC, analytical balance and detector instrumentation, and ensure the use of personal protective equipment (safety glasses, gloves, lab coat, full length pants, and closed-toe shoes).

Note: This protocol describes 3 methods of reaction flow post-column derivatization (RF-PCD) techniques, each with a different reagent specific to the nature of a chemical compound of interest. For the analysis of ROS go to section "1. Detection of ROS using DPPH^•", for the analysis of primary amines see section "2. Detection of primary amines using fluorescamine", and for the analysis of phenolic compounds go to section "3. Detection of phenols using 4-aminoantipyrene and potassium ferricyanide". Use ultra-pure water (e.g., Milli-Q water) throughout.

Note: Connection of the RF column is achieved in almost the same way as a conventional HPLC column with the major difference being the number of end fittings on a RF column. Fittings used to connect a standard HPLC column to the HPLC system are able to be used to connect a RF column to the HPLC system.

1. Detection of ROS Using DPPH^•

1. Set up of HPLC instrument
   1. Prepare the HPLC instrument with 100% water on line A and 100% methanol on line B as the mobile phase. Purge the pumps as per manufacturer's requirements.
   2. Set up the HPLC instrumental components and the additional derivatization pump as illustrated in Figure 4A.
   3. Set the UV-Vis detector to analyze at a wavelength of 520 nm.
2. Set up of RF column
   1. Connect the inlet of the RF column to the HPLC instrument.
   2. Connect a 15 cm length of 0.13 mm i.d. tubing to outlet central port of the RF column.
   3. Connect an outlet peripheral port to the UV-Vis detector using a 15 cm length of 0.13 mm i.d. tubing.
   4. Connect the DPPH^• pump line to a peripheral port on the outlet of the RF column.
   5. Block the unused peripheral port on the outlet of the RF column using a column stopper.
   6. Bring the flow rate of the HPLC pump to 1 ml min^-1 at 100% line B - 100% Methanol.
   7. Equilibrate the column with the 100 % methanol mobile phase for 10 min for a 4.6 mm i.d. x 100 mm length column. This time should be scaled according to the dimensions of other columns the user may employ.
3. Preparation of DPPH^• reagent
   1. Prepare a 0.1 mg/ml solution of DPPH^• in methanol.
   2. Sonicate the flask containing the DPPH^• reagent for 10 min.
   3. Cover the flask in foil to prevent exposure to light.
   4. Purge the DPPH^• pump with the prepared DPPH^• reagent as per the manufacturer's requirements.
4. Tuning the RF column outlet
   1. Accurately weigh two clean and dry vessels. Label one vessel as central and the other as peripheral.
   2. Collect the effluent exiting the central port into the vessel labeled central for 1.0 min.
   3. Re-weigh the central port vessel and calculate the weight of the flow from the central port as follows:
      Weight of Central Port (g) = Final Weight of Central Port Vessel (g) - Initial Weight of Central Port Vessel (g)
   4. Repeat steps 1.4.2 and 1.4.3 for the effluent exiting the UV-Vis that is attached to the peripheral port of the RF column and calculate weight for the peripheral port vessel as follows:
      Weight of Peripheral Port (g) = Final Weight of Peripheral Port Vessel (g) - Initial Weight of Peripheral Port Vessel (g)
   5. Calculate the percentage of the flow coming from the central and peripheral ports as follows:
      % Central Port = Weight of Central Port (g) / (Weight of Central Port (g) + Weight of Peripheral Port (g)) x 100
      % Peripheral Port = Weight of Peripheral Port (g) / (Weight of Central Port (g) + Weight of Peripheral Port (g)) x 100
   6. Ensure the segmentation ratio between the central flow and the peripheral flow is 30:70 (central:peripheral). If the central flow is above 30%, decrease the amount of flow exiting by adding a length of 0.13 mm i.d. tubing to the outlet of the central port. If the central flow is below 30% decrease the length of 0.13 mm tubing from the central port.
   7. Repeat step 1.4.1 to 1.4.6 until a segmentation ratio of 30:70 (central:peripheral) is achieved.
   8. Set the flow rate of the DPPH^• reagent pump to 0.5 ml min^-1.
      Note: The RF column set up with DPPH^• reagent is now ready for analysis. Samples may now be injected.
5. Post run shutdown conditions
   1. Once all of the samples have been injected, indicating that the run has finished, stop the derivatization reagent pump flow.
   2. Remove the DPPH^• reagent pump line from the peripheral port and stopper the port.
   3. Equilibrate the column with the mobile phase in which it is to be stored by allowing the mobile phase to pass through the column at 1 ml min^-1 for 10 min.
   4. Stop the flow of the mobile phase pump on the HPLC instrument.
   5. Replace the DPPH^• reagent with methanol and purge the additional pump.
      Note: The HPLC system can now be shutdown.

2. Detection of Primary Amines Using Fluorescamine

1. Preparation of mobile phase
   1. Prepare 1 L of a of 10 mM ammonium acetate solution, adjusting the pH of the solution to 9.0 with 5 M ammonium hydroxide prior to dilution to volume.
   2. Add 52.6 ml of acetonitrile (ACN) to the ammonium acetate buffer to attain a premixed mobile phase of 95:05 (buffer:ACN).
2. Set up of HPLC instrument
   1. Prepare the HPLC instrument with the pre-mixed prepared buffer on line A as the mobile phase. Purge the pump as per manufacturer's requirements.
   2. Set up the HPLC instrumental components and the additional derivatization pump as illustrated in Figure 4A.
   3. Attach a pulse dampener coil to the derivatization pump.
   4. Set-up the fluorescence detector (FLD) with an excitation wavelength of 390 nm and emission wavelength of 475 nm.
3. Set up of RF column
   1. Connect the inlet of the RF column to the HPLC instrument.
   2. Connect a 15 cm length of 0.13 mm i.d. tubing to outlet central port of the RF column.
   3. Connect an outlet peripheral port of the RF column to the FLD detector using a 15 cm length of 0.13 mm i.d. tubing.
   4. Connect the derivatization pump line to a peripheral port on the outlet of the RF column.
   5. Block the unused peripheral port on the outlet of the RF column using a column stopper.
   6. Bring the flow rate of the HPLC pump to 1 ml min^-1 at 100% line A - 10 mM Ammonium Acetate buffer pH 9, premixed with 5% ACN.
   7. Equilibrate the column with the 100 % line A mobile phase for 10 min for a 4.6 mm i.d. x 100 mm length column. This time may be scaled according to the dimensions of other columns the user may employ.
4. Preparation of fluorescamine reagent
   1. Make 100 ml of a 0.1 mg ml^-1 fluorescamine reagent.
   2. Sonicate for 1 min.
   3. Cover with foil to prevent exposure to light.
   4. Purge the reagent pump with the prepared fluorescamine reagent as per the manufacturer's requirements.
5. Tuning the RF column outlet
   1. Accurately weigh two clean and dry vessels. Label one vessel as central and the other as peripheral.
   2. Collect the effluent exiting the central port into the vessel labeled central for 1.0 min.
   3. Re-weigh the central vessel and calculate the weight of the flow from the central port as follows in step 1.4.3.
   4. Repeat steps 2.5.2 to 2.5.3 for the effluent exiting the FLD that is attached to the peripheral port of the RF column and calculate weight for peripheral as follows in step 1.4.4.
   5. Calculate the percentage of the flow coming from the central and peripheral ports as follows in step 1.4.5.
   6. Ensure the segmentation ratio between the central flow and the peripheral flow is 43:57 (central:peripheral). If the central flow is above 43%, decrease the amount of flow exiting by adding a length of 0.13 mm i.d. tubing to the outlet of the central port. If the central flow is below 43%, decrease the length of 0.13 mm tubing from the central port.
   7. Repeat step 2.5.1 to 2.5.6 until a segmentation ratio of 43:57 (central:peripheral) is achieved.
   8. Set the flow rate of the mobile phase pump to 0.7 ml min^-1.
   9. Set the derivatization pump to flow at 0.1 ml min^-1.
      Note: The RF column set up with fluorescamine reagent is now ready for analysis. Samples may now be injected.
6. Post run shutdown conditions
   1. Once all of the samples have been injected, indicating that the run has finished, stop the derivatization pump.
   2. Remove the derivatization pump line from the peripheral port and stopper.
   3. Equilibrate the column with the mobile phase in which it is to be stored by allowing the mobile phase to pass through the column at 1 ml min^-1 for 10 min.
   4. Stop the flow of the mobile phase pump.
   5. Replace the fluorescamine reagent with acetonitrile and purge the derivatization pump.
      Note: The HPLC system can now be shutdown.

3. Detection of Phenols Using 4-Aminoantipyrene and Potassium Ferricyanide

1. Preparation of mobile phase
   1. Prepare 1 L of a of 100 mM ammonium acetate solution, adjusting the pH of the solution to 9.0 with 5 M ammonium hydroxide prior to dilution to volume.
   2. Add 52.6 ml of methanol to the ammonium acetate buffer to attain a premixed mobile phase of 95:5 (buffer:methanol).
2. Set up of HPLC instrument
   1. Prepare the HPLC instrument with the pre-mixed prepared buffer on line A as the mobile phase. Purge the pump as per manufacturer's requirements.
   2. Set up the HPLC instrumental components and the two additional reagent pumps as illustrated in Figure 4B.
   3. Attach a pulse dampener coil to each of the reagent pumps.
   4. Set the UV-Vis detector to analyze at a wavelength of 500 nm.
3. Set up of RF column
   1. Connect the inlet of the RF column to the HPLC instrument.
   2. Connect a 15 cm length of 0.13 mm i.d. tubing to outlet central port of the RF column.
   3. Connect an outlet peripheral port of the RF column to the UV-Vis detector using a 15 cm length of 0.13 mm i.d. tubing.
   4. Connect each reagent (i.e., 4-aminoantipyrene and potassium ferricyanide) pump line to a peripheral port on the outlet of the RF column.
   5. Bring the flow rate of the HPLC pump to 1 ml min^-1 at 100% line A - 100 mM Ammonium Acetate buffer pH 9, premixed with 5% methanol.
   6. Equilibrate the column with the 100 % line A mobile phase for 10 min for a 4.6 mm i.d. x 100 mm length column. This time may be scaled according to the dimensions of other columns the user may employ.
4. Preparation of 4-aminoantipyrene reagent
   1. Prepare an ammonium acetate buffer with a pH of 9 by following step 3.1.1.
   2. Weigh 150 mg 4-aminoantipyrene and dissolve in 100 ml of prepared ammonium acetate buffer (pH 9).
   3. Sonicate for 1 min.
   4. Cover with foil to prevent exposure to light.
   5. Purge the first reagent pump with the prepared 4-aminoantipyrene reagent as per the manufacturer's requirements.
5. Preparation of potassium ferricyanide reagent
   1. Weigh 150 mg of potassium ferricyanide and dissolve in 100 ml of ammonium acetate buffer (pH 9) prepared as per step 3.1.1.
   2. Sonicate for 1 min.
   3. Cover in foil to prevent exposure to light.
   4. Purge the second reagent pump with the prepared potassium ferricyanide reagent as per the manufacturer's requirements.
6. Tuning of RF column
   1. Accurately weigh two clean and dry vessels. Label one vessel as central and the other as peripheral.
   2. Collect the effluent exiting the central port into the vessel labeled central for 1.0 min.
   3. Re-weigh the central vessel and calculate the weight of the flow from the central port as follows in step 1.4.3.
   4. Repeat steps 3.6.2 to 3.6.3 for the effluent exiting the UV-Vis that is attached to the peripheral port of the RF column and calculate weight for peripheral as follows in step 1.4.4.
   5. Calculate the percentage of the flow coming from the central and peripheral ports as follows in step 1.4.5.
   6. Ensure the segmentation ratio between the central flow and the peripheral flow is 50:50 (central:peripheral). If the central flow is above 50%, decrease the amount of flow exiting by adding a length of 0.13 mm i.d. tubing to the outlet central port. If the central flow is below 50%, decrease the length of 0.13 mm tubing from the central port.
   7. Repeat step 3.6.1 to 3.6.6 until a segmentation ratio of 50:50 (central:peripheral) is achieved.
   8. Set the flow rate of the 4-aminoantipyrene pump to 0.5 ml min^-1.
   9. Set the flow rate of the potassium ferricyanide pump to 0.25 ml min^-1.
      Note: The RF column set up with the two-component reagents is now ready for analysis. Samples may now be injected.
7. Post run shutdown conditions
   1. Once all of the samples have been injected the run has finished, indicating that the run has finished, stop both of the reagent pumps.
   2. Remove the reagent pump lines from the peripheral ports and replace them with a 15 cm piece of 0.13 mm tubing.
   3. Equilibrate the column with the mobile phase in which it is to be stored by allowing the mobile phase to pass through the column at 1 ml min^-1 for 10 min.
   4. Stop the flow of the mobile phase pump on the HPLC instrument.
   5. Replace both the reagents on the reagent pumps with methanol and purge the additional pumps as per manufacturer requirement.
      Note: The HPLC system can now be shutdown.

Wyniki

The first PCD method that was adapted for use by RF-PCD was the derivatization of antioxidants using the 2,2-diphenyl-1-picrylhydrazil radical (DPPH^•)²⁴. This reaction was introduced by Koleva et al.²⁵ and has been widely used since. The detection relies on the decolorization of the DPPH^• radical in the presence of reactive oxygen species, hence the presence of antioxidants results in a drop in the observed absorbance. The DPPH^• reaction of...

Dyskusje

RF-PCD allows for the efficient mixing of the derivatization reagent with the HPLC effluent post-column without the use of reaction coils, minimizing the effects of band broadening and improving separation performance. RF-PCD methods have also shown improvements in signal response with respect to detection method. Camenzuli et al.²⁸ was the first to report the use of reaction flow columns with DPPH^• for the detection of ROS in an espresso coffee sample. Their study involved the analys...

Materiały

Name	Company	Catalog Number	Comments
HPLC instrument	Agilent	1290 Series HPLC
Additional Pump(s) for derivatization system	Shimadzu	LC-20A
RF colum			Non-commercial
PEEK tubing	Sigma Aldrich	Z227307
Column stoppers			Provided with column
PEEK tube cutter	Sigma Aldrich	Z290882
Analytical Scale Balance			4-point analytical balance
Stop watch			Non-Scientific equiptment
Eluent collection vials			Any Small vial with a flat bottom will do, e.g., HPLC vials
HPLC Vials			Will depend on instrument used
Vessels for mobile phase and derivatization solution(s)	Sigma Aldrich	Z232211
General Laboratory glassware			Volumetric Flasks, pippettes, etc. Quantity and volumes will depend on sample preparation method.
Methanol	Sigma Aldrich	34860
DPPH	Sigma Aldrich	D9132
Ammonium Acetate	Sigma Aldrich	17836
Ammonia	Sigma Aldrich	320145	Corrosive
Acetonitrile	Sigma Aldrich	34998
Fluorescamine	Sigma Aldrich	F9015
4-aminoantipyrene	Acros Organics BVBA	AC103151000
Potassium ferricyanide	AnalaR	B10204-30