Ask the Expert: Best Practices for Aseptic Sampling from Stainless-Steel Equipment

BPI Contributor

March 3, 2020

4 Min Read

Turn-key single-use aseptic sampling devices (ASDs) have diminished bioprocess contamination risks significantly. But depending on testing, facility, and storage needs, some ASD container types are more effective than others are. Bobbi Allen (technology expert at Sartorius Stedim Biotech North America, SSB) focused her 8 January 2020 “Ask the Expert” presentation on “what, why, when, and where” operators must sample aseptically from stainless-steel tanks. Using data from in-house testing of aseptic sampling containers, Allen offered key considerations for sterility, process monitoring, and endotoxin sampling.

Allen’s Presentation
Regulatory agencies require process sampling at the beginning or completion of significant manufacturing steps, and operators must withdraw samples without tainting them or their sources. Compared with needles and septa, valve-and-bottle assemblies, and steam-in-place (SIP) valves, turn-key ASDs have enhanced sampling significantly. ASDs are self-contained, and because they are presterilized single-use systems, only their product- contact surfaces require SIP. Moreover, multiple independent sampling and removal pathways eliminate the need for SIP between collections.

The sheer number of required assays can obfuscate their sterility requirements. Operators can classify aseptic sampling events into two categories: materials for process monitoring (PM) and microbiological (MB) testing. PM assays test for pH, osmolality, dissolved oxygen, metabolites, and cell viability and density. MB assays test for bioburden, sterility, non-host organisms, and endotoxins.

All samples must be drawn without contaminating a vessel. But MB testing also requires sample protection. Thus, sterilized bags, bottles and tubes, or syringes can work for PM sampling; MB samples must be collected in closed systems — meaning bags or bottles and tubes. Container selection also should account for technicians’ handling preferences, space limitations, storage conditions, and sample-volume control.

Syringes are ideal for pH and gas analyses. Syringes are inexpensive and can be manifolded for greater savings. They work with lower activated valves, which can expand a line’s sampling capacity. And syringes save time: Bags introduce logistical steps that can delay sample analysis and skew gas readings, whereas syringe-drawn samples can go directly to an analyzer, ensuring minimal change in pH, O2, and CO2.
All plastics are permeable to gases, but syringes are preferable to bags in this respect, too. The thick, rigid plastics used in syringes limit gas permeability. Moreover, sample gas exchange occurs where fluids interface with air and container surfaces, and syringes have lower, more stable surface area/volume ratios than bags do. Thus, even though bags are popular, syringes are best for pH and gas assays.

Material compatibility is critical in endotoxin assays. Endotoxins can adsorb to container surfaces, which can hinder detection and quantitation efforts. To identify optimal containers, SSB compared the affinity of control standard endotoxin (CSE) for common containers: 15-mL polystyrene (PS) centrifuge tubes, 60-mL polycarbonate (PC) bottles, 50-mL polypropylene (PP) centrifuge tubes, 60-mL polyethylene terephthalate glycol (PETG) bottles, and bags composed of polyethylene and ethylene-vinyl acetate. Researchers spiked three units (representing four lots) of each container type with 0.5 EU/mL of CSE solution and then measured solution recovery at 1, 6, 12, and 24 hours after sample extraction.

Both bags showed high adsorption and were excluded from further study. PC and PS containers exhibited the best recovery at the one-hour mark, but PS and PETG materials yielded the most accurate results after six hours. Moreover, PP and PC unit performance dropped over time. Accounting for the standard deviation of average recovery over time revealed that PS containers earned overall higher scores than those of their PC and PETG counterparts.

When aseptic sampling for sterility and bioburden, any closed container will do. But endotoxin samples should be assayed within an hour of extraction using polystyrene containers with high recovery rates and stability across lots and over time.

Questions and Answers
What tank interface options are available? Triclamp (TC) options can range from 0.75 to 2.00 inches. Some tanks are equipped with 25-mm “in-gold fittings” or “safety ports.”

What materials interact with bioprocess fluid during ASD sampling? Only tank connections and face plates contact fluids. Connectors usually consist of platinum-cured silicone or 3/16-inch stainless steel. Face plates are often polyesterimide-based.

How many samples can a single ASD take? The largest ASDs come with two-inch TC fittings and with up to nine sampling pathways. Bags, bottles, and tubes can be manifolded for up to five collections, yielding 45 samples.

More Online
The full presentation of this webcast can be found on the BioProcess International website at the link below.

Watch the full webcast now.

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