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Properly setting up your HPLC-CAD system is crucial for maximizing the overall detector performance. This page presents best lab practices for installing your HPLC-CAD system, but you should always refer to our operating manuals as the primary source of information!

The top factors to consider before installing your CAD include what types of mobile phases were previously used with your LC system, gas supply, exhaust and lab conditions.

LC instrumentBuffers from previous runs can interfere with the CAD response so avoid using mobile phases with nonvolatile chemicals, even when your CAD is disconnected from the instrument
ColumnsHave dedicated columns that are not used for other analysis with a high concentration of non-volatile buffers. Flush all new columns to waste for several column volumes before connecting to your CAD
Nitrogen supplyA gas generator to supply stable ultra-pure nitrogen flow is highly recommended over tanks and cylinders
ExhaustCentralized exhaust systems have less pulsation and are the better choice over individual motors
Lab conditionsAvoid placing your HPLC-CAD system in areas with heavy airflow and temperature fluctuations

 

CAD setup

 

 

HPLC instrument

The best-case scenario is for you to have a dedicated LC-CAD system free of use with non-volatile additives, like phosphate buffers or non-volatile ion pairing reagents. But, if you do plan to use your LC instrument for multiple methods, you should avoid using non-volatile mobile phase additives, even when your CAD is disconnected.

Of course, this situation may not be ideal for your budget or lab space, so the next best practice is to perform extensive flushing and exchanging all wetted parts of your LC system that can retain non-volatiles as they may have a “memory effect.”

Memory effect refers to the slow and persistent release of semi- and non-volatile compounds used during previous runs, which can result in baseline disturbances, noise, and higher background currents. Typically, this exchange includes your solvent lines, inlet frits and columns.

Flushing out inorganic salts from columns that were used with non-volatile buffers can take an extremely long time, so consider dedicating a column to use with CAD-friendly mobile phases.

In cases of severe contamination, you may want to swap other components such as the degasser cartridges.

Multi-detector setups

You may want to couple your CAD to an optical detector like a variable wavelength detector (VWD) or diode array detector (DAD). Because the CAD is a destructive detector, you must use the CAD as the last detector in the flow path, downstream from an optical detector.

Flow splitting

  Complementary detection techniques can give much deeper insights about a sample, so you can use the CAD with one or more optical detectors, and a mass spectrometer (MS) as well.

But, keep in mind that the CAD and MS are both destructive, so you’ll need to split the mobile phase flow between the two detectors. For setups using both CAD and MS, you can add an additional optical detector after the column, prior to the flow splitter.

 

 

Nitrogen gas supply

The CAD requires a constant gas supply pressure of 4.8 bar (70 psi) with a maximum supply pressure of 6.2 bar (80 psi), and typically uses 4 L/min of nitrogen gas.

You should ensure the gas source is ≥ 95% purity and free from water vapor, hydrocarbons/oil vapor, and particulates (> 0.1 μm). If necessary, use a sub-micron particle or carbon filter combined with a water condensation trap close to the gas source.

You can use pressurized air with the CAD if the solvents are not highly combustible — we highly recommended nitrogen gas. For solvents like THF that form combustible or explosive mixtures, you MUST use nitrogen gas of 99% purity.

There are three common approaches you can use for supplying nitrogen gas to your CAD: Generators, cylinders, and liquid nitrogen tanks. Of all these options, we recommended you use a gas generator to help provide an on-demand, uninterrupted supply of dry nitrogen from a compressed air source.

Generator

Many laboratories use generators that produce high-purity nitrogen gas by passing pressurized air through a bundle of fibrous semipermeable membranes.

Nitrogen generators provide a stable, safe, and easy-to-use source of high-quality nitrogen. While the initial cost is high compared to other solutions, the running costs are very low, and interruptions usually only occur due to scheduled annual maintenance.

Nitrogen generators are available in various sizes. Some supply whole buildings while others can supply a single instrument. Similarly, the pressurized air can come from a centralized in-house source, or a local compressor dedicated to a single nitrogen generator.

If you use one source to supply nitrogen to multiple instruments, make sure the recommended pressure and flow ratings for your CAD are met even during times of maximum nitrogen consumption.

When   splitting flow from a large format generator to a CAD, you may need to install an upstream gas regulator to ensure compatibility with other instruments. A suitable modular stand-alone combination tuned to the needs of the CAD is available.

Cylinder/tank

Nitrogen gas cylinders* are maintenance-free, and convenient to use. Nitrogen is pressurized at around 200 bar (2900 psi) in a steel cylinder, so you must use an output gas regulator to reduce the incoming gas pressure to meet the requirements of the CAD.

The filling volume of gas cylinders commonly used in the laboratory is around 9.5 m3, which corresponds to approximately 40 hours of continuous CAD operation. This approach suffers from frequent operational interruptions as the output pressure must be regularly adjusted as the cylinder empties, and cylinders must be replaced when emptied.

This option can be expensive in the long run, and you must remember to open the valve before connecting the CAD to remove any debris in the tubing.

*Gas cylinders pose a laboratory hazard if mishandled; take appropriate safety precautions.

Liquid nitrogen

Liquid nitrogen* in an insulated Dewar container uses ambient energy to evaporate part of the liquid nitrogen and generate pressurized gaseous nitrogen.

These tanks typically require adjustment of the pressure-building valve to achieve elevated and/or maintain a specified output pressure. A 50 L dewar can deliver nitrogen for up to approximately 140 hours of operation, requiring less frequent interruptions and is typically more cost-effective than gas cylinders.

*Liquid nitrogen, even more so when pressurized, poses a laboratory hazard so take appropriate safety precautions.

 

 

 

 

Exhaust setup

The gas exhaust from your CAD may contain flammable, poisonous, or corrosive components, so you must connect the detector to an exhaust system to safely remove exhaust from your lab.

Avoid connecting the gas exhaust to strong negative or pulsating pressure ventilation systems since this may result in unstable pressure within the CAD, which can lead to reduced performance. Do not attach an active exhaust pump to your CAD exhaust.

You’ll also want to prevent disruptions in the exhaust pressure equilibrium while your CAD is in operation. Opening and closing fume hoods using the same exhaust vent may cause a change in the baseline CAD signal. 

Basic exhaust guidelines include:

Ventilation sourcesUse a fume hood or other industrial vents, avoid a gas-tightened connection to the exhaust of the detector and prevent siphons from the exhaust outlet to the external vent. You can use an optional 90° elbow for easy installation
Ventilation rateExhaust ventilation rate > 4 L/min (per detector) with ventilation at atmospheric or at slightly negative pressure with no positive pressure applied
Tubing lengthExhaust tubing length should be 2.5 m, with 1/2 inch inner diameter. You can shorten the tubing to any length needed
Tube fittingUse an exhaust tube fitting with 3/8 inch outer diameter

 

 

Lab environment

A stable laboratory environment can help reduce baseline artifacts, noise and drift, and improve your overall detector performance.  The best way to prevent disruptions from your lab settings are:

1. Avoid temperature fluctuations

  • Don’t place the CAD in direct sunlight, near heating or cooling sources, or under an air duct
  • Avoid locations with significant changes in temperature and strong air drafts
  • Consistent AC/heat setting
  • No uneven airflow

2. Provide good nitrogen and exhaust conditions

  • Stable and steady gas supply
  • No pulsating exhausts

Besides adhering to the standard best practices and considerations for HPLC, we suggest the following guidelines when preparing your CAD for operation:

  1. Verify that the correct nebulizer gas pressure setting, as stated in the nebulizer certificate, is set on the device or in the software depending on the model.

  2. Always turn on nitrogen gas flow prior to turning on the mobile phase flow. Wait at least 5 minutes after turning on the gas flow before turning on the liquid flow. Failure to do so can flood the detector.

  3. Ensure you set an upper flow rate limit in the control software and do not exceed the flow rate rating of the CAD.

  4. Like temperature equilibration for VWD or DAD, you should allow the CAD to equilibrate before starting an analysis. Equilibrate using the initial analysis conditions like evaporation temperature (EvapT), mobile phase composition and flow rate, and let the baseline and noise levels stabilize. 

Following these procedures is necessary to remove hazardous materials, and ensure no residue is present anywhere in the CAD when mobile phase solvents evaporate. Otherwise, remaining static liquids, especially acidic modifiers, may corrode some components inside your CAD.

For short stand-by periods (e.g., overnight)

  1. Maintain nitrogen gas flow to the CAD to prevent any buildup or residue.

  2. Reduce the mobile phase flow rate to 50 μL/min, until normal operation resumes.

  3. Set the injection valve in the autosampler to the inject position.

  4. Make sure the column temperature does not exceed 40 °C.

For prolonged *shutdown periods (e.g., one week or longer)

  1. Flush your HPLC-CAD system with an appropriate, pure solvent (minimum LC/MS-grade). Make sure that residual sample components, impurities from the column, or buffers are completely removed from the detector.

    • If no buffer is used, flush the system, for example with methanol.

    • If a buffer is used, flush the system with several volumes (for example, 1.0 mL/min for 10 minutes with the standard system) of methanol and water (50:50) to prevent salt buildup in the fluidics. If the solvents in the detector are not miscible with water, use an appropriate intermediate solvent.
       
  2. Remove the column from the flow path and then clean the system and detector with a mixture of high purity water and methanol 50/50 (v/v) for at least 1 hour

  3. If you see a high background current, pronounced drift, or a high noise level, extend this flushing period until good working conditions are restored.

  4. Next, flush the system with pure LC-MS grade isopropanol.

  5. Stop the mobile phase but maintain gas flow to the CAD for 5 minutes to let the detector dry out.

  6. Turn off the gas flow and then wait until the system pressure reaches zero.

  7. Turn off the gas supply to the detector.

*Refer to our operating manuals for detailed information on how to transport and startup your CAD detector after prolonged shutdown periods.

  1. If your CAD starts to consistently show high noise, you first should rule out if the issue is from the LC system or mobile phase.

    • Clean the flow path by flushing an appropriate pure solvent, such as water (without a column) and verify the CAD signal is low, around 1 pA.
       
  2. If you determine the LC system and mobile phase are not the problem then the likely root cause is the contamination of the nebulizer, spray chamber or evaporation tube. A more aggressive flushing procedure can help to remove contamination and restore CAD performance.

    • Clean the system by flushing (without a column) with a suitable cleaning solvent mixture — like the Thermo Scientific ChromaCare LC-MS Instrument Flush Solution: a quaternary mixture of LC/MS grade acetonitrile, methanol, water, and 2-propanol, or a 50/50 (v/v) mixture of ultrapure water/methanol.
       
  3. Monitor the background current while flushing the CAD system. In most cases, the background current will decrease over time and after several hours you will see a normal background current.

    • Periods of increased EvapT (20 minutes at 35 °C, then 20 minutes at 70 °C, 20 minutes back at 35 °C) can help to re-establish good operating conditions faster.

    • If the background current does not show a significant improvement after 24 hours of flushing, we recommend full preventive maintenance from a service technician, which includes a cleaning of the nebulizer, spray chamber, and evaporation tube.

 

 

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