Frequently Asked Questions

Q1: How do you assure the quality of your products?

Quality is assured by adhering very strictly to specified, documented systems covering all aspects of membrane manufacture. Our quality management systems are ISO 9001: 2008 and ISO 13485: 2005 certifed.

Q2: In case of problem what support can I expect from mdi?

mdi will respond very quickly by assisting you in analyzing the problem and isolating the source. All shipments have retained material. This retained material is crucial in identifying if the problem is coming from the membrane or not. Although very very rare, corrective action will follow including full replacement of the lot on priority if the problem is confirmed from the membrane.

Q3: What are your production capacities?

mdi has been in business for about 35 years. Thousands of sq .ft. of membrane is produced everyday. Capacities are being further expanded constantly.

Q4: How should I order? Who should I contact and how long does it take for material to arrive?

Full details are given in adjoining column.

Q5: I expect products from India to be cheaper. Are your membranes cheaper?

Ask for a quote. You may be pleasantly surprised.

Q1: What is different about mdi NC membrane compared to other brands?

Superior consistency and stability. Due to optimized surface chemistry and unique processing method mdi membranes are highly consistent, point to point as well as roll to roll.

Q2: For lateral flow membranes, what is the significance of pore size?

Pore size is only a name designation. One manufacturer’s pore size designation can be quite different from another’s. Bigger pore size is designated for membranes with faster wicking rates.

Q3: If pore size is only a name, how do you differentiate between different membranes?

The wicking time is an important parameter for lateral flow membranes. mdi membranes of designated pore size will exhibit highly repeatable wicking time.

Q4: What is the difference between CNPF, CNPC, and CNPH membranes?

All the membrane are directly cast on a polyester backing. CNPF exhibit lower protein binding, CNPC exhibit higher protein binding and CNPH exhibit highest protein binding.

Q5: Should I block the mdi membrane?

All mdi membranes exhibit stable wicking rates without blocking. It is possible to make many good assays without blocking. However, should there be a need to wet block the membrane for stabilizing the Ab, reducing nonspecific reaction, or improving the migration of conjugate particles, all mdi membranes can be blocked.

Q6: Do you have detergents/wetting agents in the membrane?

NC is a hydrophobic polymer. To be able to use the NC membrane successfully it is necessary to add a small amount of detergent or a combination of detergents. The amount is so small that it does not affect the binding properties of the membrane.

Q7: The gold particles move very slowly and they do not clear after halfway up the membrane. Is this due to membrane?

The problem is in the chemistry of the conjugate, conjugate release pad, and the sample pad. Adding buffers to maintain pH at 7.4 and adding blocking agents such as PVP will help.

Q8: What is the shelf life of the membrane as received from mdi? Is there any precaution I should take in storing the membrane?

Properly stored, mdi NC membranes are stable for at least one year. Membrane should be kept away from heat at 40-70% R. H. Whether in roll form or in laminate format, membranes must not be allowed to lie in the open for long periods.

Q9: I got a very faint shadow of false positive. I suspect the membrane. Please comment.

False positive is primarily due to nonspecific binding of the conjugate particles. Using bigger pore size membrane can help reduce this, usually at the cost of sensitivity. Addition of detergents in the sample pad can also help minimize it.

Q10: I get broken lines while printing the reagent line and zones where membrane does not absorb the liquid. How can the problem be resolved?

If the NC membranes are allowed to stay in the open for a few days they tend to attract hydrophobic dust particles or other moieties which can cause above problem. Addition of 2-3% Isopropanol in the reagent formulation will help minimize above problem.

Q11: Will serum migrate on mdi membranes?

Yes, serum and plasma migrate well on mdi membranes.

Q12: How much time should I wait for the Antigen/Antibody to bind on the membrane before an assay is run?

The binding of the protein to the NC membrane grows stronger as it is dehydrated more and more. Usually, at room temp, (20-30° C), 50-60% RH the binding will reach a maximum in 3-5 days. For development, reasonable results can be expected with overnight drying at room temperature.

Q13: My tests are not stable with time. Is the membrane to be suspected?

Membrane does not affect the stability of the assay. It is related to the chemistry of the assay, and many a times it improves by adding sucrose (0.5-1.0%) to the capture Antibody solutions.

Q14: How should I select a sample pad?

The sample pad should not bind proteins nonspecifically, should exhibit good flow characteristics, and have adequate volume capacity. Its thickness should be such that it fits in the test device.

Q15: Are any additives added in the sample pad and why?

Sample pads may have many additives which a user can add to achieve desired results from the assay. The additives may be blocking agents for the membrane for rapid and uniform movement of conjugate particles, buffers to control the pH and improve specificity, detergents for improved flow and reduction of false positives.

Q16: Why is a whole blood separator required?

Whole blood separator removes Red Blood Cells (RBC) to give free plasma so that analytes of interest may be detected directly from the blood sample without the need of centrifuge or clotting to get serum.

Q17: What does a whole blood separator do in a lateral flow test?

Removal of RBC from blood is necessary for use of whole blood in lateral flow assays as RBC not only clog the membrane but can also hemolyse causing abortion of the test. mdi blood separators effectively stop RBC while allowing free plasma to migrate across the membrane with or without the driving buffer.

Q18: How is blood separator used in a lateral flow test?

There are two ways in which Whole Blood test is accomplished in a lateral flow system. - By using a driving buffer supplied with the kit. - No buffer is used and plasma separated from whole blood itself migrates across the membrane to complete the test. Different blood separators are used for each case. The separators replace the normal sample pads in a lateral flow device.

Q19: How much blood is required to be added in the whole blood test?

For assays with driving buffer, tests may be made using as little as 5 micro liter, whereas for tests without any buffer,2-4 drops of blood is required.

Q20: How much time does it take to complete the whole blood test?

A typical whole blood assay may take up to 10 minutes to complete.

Q21: RBC tend to leak past most available separators. How is mdi separator system different from other brands?

Most blood separation systems rely on differential migration speeds of RBC and plasma in a matrix and when blood with high hematocrit is applied, the RBC leak past the separator onto the membrane. mdi blood separators virtually immobilize RBC where blood is applied and RBC do not go past the separator even with excess applied blood with high hematocrit.

Q1: What is a maximum vessel size for which the ITF filters can be used as vents?

ITF filters can be used for venting of up to 5 liter vessels. For large vessels PTFE capsule filters type DTL are recommended.

Q2: I have been using 50 mm ITF filters for venting 5 liter media bottle. Sometimes the flow of air across the filter is not enough, these bottles break due to the pressure build up during autoclaving.

This can happen if the filter after autoclaving and before being installed on the bottle has not been dried to remove condensed water vapour inside its pores. This can be done by blowing filtered air through the ITF filter.

Q3: Capsule filters are relatively more expensive and not very large area filtration devices. Why not use a more cost effective membrane disc filters or if required a cartridge filter for batch filtration?

mdi capsule filters have very low hold up volume (< 5 ml for 1" capsule) therefore maximize product recovery. This is of critical importance in case of high value products such as hormone vaccines or oncology drugs. Whereas, disc filters or cartridge filter systems have a large hold up volumes (>150ml).

Q4: What is the maximum volume these systems can be used for?

Capsule filtration systems for up to 20 liters are available. However, large systems can be custom designed to suit specific needs.

Q5: We are using a 5 liter mdi capsule filtration system in our lab with a 1" DPL 0.2µm capsule filter. The flow rate is very slow. Please explain?

It is very important to completely remove entrapped air from the capsule filter by crack opening the vent on the inlet side. This will help maximize flows.

Q6: The peristaltic pump tubing is slit open after 2-3 filtrations resulting in loss of valuable fluid. Sometimes these are hazardous materials and we are very concerned about user safety?

This can sometime happen and in order to avoid this, it is recommended to keep changing the tubing position inside the pump head.

Q7: We have a 10 liter CFSP-B, the capsule filtration system with pressure vessel. Can I use it to filter 500 ml culture media?

Yes, the pressure vessel is designed to push out even very small volumes and can be used to filter even 500 ml.

Q8: What is the total volume of fluid the cup of the bottle top filter can hold?

Up to 400ml

Q9: How much volume of fluid the Vacufil can be used to filter?

The Vacufil has a large filtration area, as it has a 75mm diameter high flow rate, high throughput membrane, it can be used to filter up to 1 liter of solution.

Q10: What is the protein loss with mdi PES syringe filters?

mdi PES syringe filters offer protein recovery of > 99.9%.

Q11: My sample volume for growth regulators is not more than 500µl? These are very expensive materials and I do not want to loose any of it?

mdi 4mm PES Syringe filters type SY4PL-S have hold up volume of < 5µl and will help maximize sample recovery.

Q12: For serum samples of up to 50ml, I have to use multiple syringe filters and also the hand pressure required is too much.

mdi SY25KG-S syringe filters are specially designed to incorporate a pre-filter layer of microglassfiber that protects the downstream membrane layer and enhances throughputs at much reduced pressure.

Q13: Can purified DNA be sterilized using any of these filters?

Yes, because of very small volumes SY4PL-S with a hold up volume of <5µl should be used.

Q14: Can I use the SY25KG-S for 1 liter media sterilization using a syringe?

Normally for volumes more than 200ml, it is recommended to use 50mm IKG-S with a peristaltic pump, as it would be a little inconvenient to filter large volumes with a syringe.

Q15: Are mdi filters suitable for HPLC sample filtration?

Yes, mdi filters are HPLC certified to assure that these do not add artifacts to the sample.

Q16: My samples are very turbid even after filtration through a 0.2µm syringe filter the downstream is somewhat hazy.

Use SY25GN of SY25TG syringe filters. These are special filters with a microglassfiber pre-filters that retains turbidity causing very fine colloidal particles. This pre-filter also protects the downstream membrane thereby enhancing throughputs.

Q17: There is a loss in area under the peak for the drug compound in my sample after filtration. How do I minimize it?

This normally happens due to adsorption of the drug compound on the filter membrane. mdi PTFE syringe filters are inert materials and do not adsorb or leach out.

Q18: My sample volume is only 50µl and I am afraid to loose most of it during filtration. Which filter should I use?

Use 4mm mdi syringe filters as these have a hold up volume of <5µl.

Q19: What is the difference between SCN and SCNJ membranes if both are Nitrocellulose?

SCN is pure Nitrocellulose where as SCNJ is internally supported Nitrocellulose for superior handlability.

Q20: Can I use the SCNJ membrane from any side or is there a specific side which will give better results?

Unlike other supported Nitrocellulose membranes available in the market, mdi SCNJ membrane offers uniform surface on both sides and give identical results.

Q21: What advantages do I get for switching over from Nitrocellulose membrane type SCN to PVDF membrane type SVF?

The SVF membrane has higher protein binding capacities in comparison to Nitrocellulose membranes and also offer higher strength than SCN membranes.

Q22: I have recently started using the SVF membrane for western blots, but did not get any bands. On the other hand the SCN membrane was working fine.

The SVF is a hydrophobic PVDF membrane and needs to be pre-wetted with methanol before it is used. Also ensure that the membrane is soaked in water for 2 minutes after wetting and then equilibrated in transfer buffer for a minimum of 5 minutes.

Q23: I am not getting good results with small molecular wt. proteins (<20kd) by using Nitrocellulose membranes. Please suggest a solution?

Try the 0.2µm SCNJ membrane with superior binding capacities for small protein molecules.

Q24: Are cartridge filters suitable for sterile gas applications?

mdi PTFE cartridge filters are validated for liquid-based bacterial retention challenge with Brevundimonas diminuta as per ASTM F-838-05 and are suitable for critical applications where sterility of the filtered gas is of utmost importance.

Q25: I use 0.2µm PTFE cartridge filters for distilled water tank venting. The filter did not allow intake of air while drawing water from the tank and the tank collapsed due to the resulting vacuum.

Distilled water tanks are maintained at 80ºC. The water vapour coming out of the tank through the vent filter condenses inside the pore of the cartridge filter due to much lower outside temperature. This results in water logging the filter which in turn does not allow the passage of air. This result is build up of vacuum.

Q26: What is 'Blow Down Time'?

Even the most hydrophobic of filters may end up liquid logged under some set of circumstances, particularly during steaming and integrity testing. This will invariably affect the flow characteristics in an adverse manner. The time required to blow dry the filter enough to restore a usable or minimum required flow rate is referred to as the blow-down time.

Q27: Can the mdi PTFE cartridge filters be steam sterilized?

Yes, the mdi PTFE cartridge filters type CPTF can be steam sterilized for 100 cycles at 121°C for 30 minutes. High teperature resistant CPTH cartrage filters can be steam sterilized for 80 cycles at 135°C for 30 minutes each.

Q28: Are capsule filters suitable for sterile gas applications?

mdi PTFE capsule filters are validated for liquid-based bacterial retention challenge with Brevundimonas diminuta as per ASTM F-838-05 and suitable for critical applications where sterility of the filtered gas is of utmost importance.

Q29: Can the mdi PTFE capsule filters be Inline steam sterilized?

No, the mdi PTFE capsule filters type DTL cannot be Inline steam sterilized. These can be autoclaved at 121°C for 30 minutes.

Q30: Are cartridge filters validated for microbial retention?

All mdi sterilizing grade filters are validated for microbial retention as per ASTM F-838-05. These filters are also validated for other performance parameters such as filter fluid interaction, heat stability etc. as per PDA Technical Report #26.

Q31: How many times can I autoclave the CPPKS cartridge filters?

30 times at 121°C for 30 minutes.

Q32: Are capsule filters validated for microbial retention?

Q33: How many times can I autoclave the PES capsule filters?

25 times at 121°C for 30 minutes.

Q34: We are using a 5 ltr mdi capsule filtration system in our lab with a 1" DKS 0.2µm capsule filter. The flow rate is very slow. Please explain?

It is very important to completely remove entrapped air from the capsule filter by crack opening the vent on the inlet side. This will help maximize flows.

Q35: We use the 0.2µm DKS-S 1" capsule filter for media preparation. Can I reuse these pre-sterilized capsule filters?

We do not recommend reuse, as any trace amount of biological fluids inside the filter will facilitate microbial growth thereby contaminating subsequent batches.

Q36: Are these capsule filters certified for sterility?

A certificate of Quality stating the same accompanies each Filter.

Q37: What should be the cartridge filter size for a flow rate of 3m³/hour?

As a thumb rule a well designed filtration system should have one 10" cartridge filter for every 1m³/hour of flow rate. So a 30" cartridge filter is recommended.

Q38: We are producing recombinant vaccines and I want to sterilize protein solutions. Please let me know which membrane type should I use?

mdi CPPKS cartridge filters or DKS capsule filters offer very high protein recoveries as these incorporate very low protein binding polyethersulfone membrane.

Q39: We are producing high value hormones for human therapeutic use and are using 0.2µm, 5" hydrophilic cartridge filters. Please let us know if we can reduce our filtration losses (we loose about 100ml as hold up volume in a 70 liter batch).

You can use mdi 0.2µm DKS, 5" capsule filter with a hold up volume of 30ml to maximize recoveries. These capsule filters also offer very high flow rates and throughputs.

Q40: We have a 1000 liters distilled water tank and are using 10" 0.2µm PTFE cartridge filters as sterile vent filters. These filters are choking very fast and it is too uneconomical to change so frequently. Please help?

Distilled water tanks are maintained at 80°C to prevent microbial growth, water vapors, formed at this temperature escape out of the vent filters and condense as soon as these come in contact with the cool filter housing, thus water logging the filters. We recommend use of steam jacketed vent filters housing to prevent water logging.

Q41: We have used mdi 0.2µm/10" CPTF cartridge filters for the recommended 100 cycles for sterile venting of our distilled water tank. Can I continue to use these?

mdi PTFE cartridge filter type CPTF are tested to withstand up to 150 autoclaving/steam sterilization cycles and the recommendation of 100 cycles is to build a safety margin. However, we recommend integrity testing of the filter before and after every batch to ensure sterility.

Q42: We are facing heavy rejections due to black particles. Please help?

Black particles are normally formed from bungs on inline gasket seals in the wash water lines. Please check wash lines for any disfigured seals. Also check the quality of rubber bungs. Poor quality bungs can release rubber particles to cause heavy rejections.