Selecting a Type of Syringe

While selecting the correct type of syringe pump for your application is important, users should also take care in selecting the appropriate syringe. Below are some common features of a syringe like its size, plungers, and tubing that users should consider when selecting a syringe type for an application with a syringe pump.

parts of a syringe

Barrel

There are three features of a syringe barrel that should be evaluated: the outer diameter, the inner diameter, and the volume.

Outer diameter

The outer diameter represents the thickness of the syringe barrel. This value of syringe size ultimately determines whether the syringe will physically fit on the syringe pump. Typically, most syringe pump manufacturers indicate the allowable syringe sizes for a syringe pump by the volume of the syringe. While this approach often works because most syringe sizes of a certain volume have a similar outer diameter, the outer diameter of some specialty syringes (gastight, steel, etc.) is drastically different from their more common counterparts. Users should be aware of the actual outer diameter of their syringes’ size as well as the maximum outer diameter allowed by the syringe pump.

Users should also be aware that some syringes may have uneven or tapered barrels that may cause the syringe to not lie horizontally in the syringe block holder. This can cause issues because the plunger is not horizontal and the plunger cap is not flush against the pusher block resulting in the pusher block is pushing horizontally against a plunger that is not horizontal. This can cause inaccurate flow rates and potential damage to any syringe type.

Inner diameter

The inner diameter represents the thickness of the cavity inside the barrel where the fluid is stored. This value of syringe size is extremely important because it needs to be known to be able to set accurate flow rates on the syringe pump. The flow rate of a syringe pump is based on the linear distance the syringe plunger needs to travel to transfer a specific volume in a set amount of time. Because the inner cavity of the syringe barrel is a cylinder, the linear distance the plunger needs to travel can be determined based on the volume equation for a cylinder. The following represents the cylinder volume equation that has been rearranged to calculate the linear distance the plunger travels to transfer a specific volume.

l = V / πd/202

where l is the linear distance the plunger travels in cm, V is the volume in mL, and d is the diameter in mm.

Luckily, on a Chemyx syringe pump, this type of calculation is done for the user. Just enter the inner diameter of the syringe into the syringe pump interface or load a syringe from our built-in syringe library, and the pump determines how far and fast to push the syringe plunger.

The flow rate limits for a Chemyx syringe pump are determined based on the linear distance limits. The maximum flow rate is determined by the fastest linear speed the pusher block can travel. The minimum flow rate is dependent upon the step resolution of the stepper motor in the syringe pump. Both of these limits are primarily dependent on the type of stepper motor in the syringe pump. Generally, users should choose a syringe with a volume/inner diameter that puts the linear movement near the middle of these two limits.

Volume

The syringe volume is important primarily so that it has enough volume to satisfy the user’s application. However, the user should be careful to select a syringe that does not excessively exceed their volume requirements.

Typically, syringes have approximately a 1% error in volume for glass syringes (about 5% error for plastic syringes). This means that users should select the smallest syringe possible for their application that still allows. For example, a 50-mL syringe would have about a 0.5 mL error in volume, which would be problematic for any user intending to transfer only 1 mL of fluid because the actual volume dispensed could range from 0.5 to 1.5 mL.

Plunger

The size of the cap of the syringe plunger must be considered for applications that require syringe withdrawal. During withdrawal, the syringe cap must be secured by a bracket on the pusher block of the syringe pump. The brackets on Chemyx syringe pumps try to accommodate a wide variety of syringe sizes with different plunger cap sizes. In some rare cases, very small syringes or very large syringes may not have plunger caps that will fit within the bracket.

Flange

The size of the flange is important for the stability of the syringe on the syringe pump. The flange should be large enough that when it is pressed against the syringe holder block, it will help prevent the syringe from moving forward when the plunger is being pushed. On syringe pumps that can withdraw, the flange should be large enough that the flange bracket can clamp on to it.

Needle/Tubing

The needle or tubing system connected to the tip/termination of the syringe can have a significant effect on the use of syringe pump. The needle or tubing can often restrict the flow of the fluid which may cause enough back pressure to cause the syringe pump to stall. In general, to minimize the amount of back pressure, a larger inner diameter and/or shorter length tubing should be utilized when possible.

Syringe Material

The type of material used for the syringe can have an effect on the user’s application as well as the use of the syringe pump.

Plastic

Plastic syringes are an inexpensive, disposable option that typically comes sterile out of the box. Typically, these syringes come with either a full plastic plunger tip or a rubber plunger tip. The rubber plunger syringe tip type is often a better option to minimize fluids leaking past the plunger.

However, plastic syringes do have a couple of significant drawbacks. Plastic syringe plungers and barrels have some flex under pressure which leads to volume inaccuracies as high as 5%. Additionally, rubber-tipped plungers tend to absorb some organic solvents such as DMSO.

Glass

Glass syringes are the most common type of syringe used with a syringe pump. Glass syringes are typically compatible with most applications, reusable for multiple applications, and more accurate than most syringe types. The plunger of a glass syringe can be made of only ground glass, glass with a Teflon syringe tip type, metal-only, or metal with a Teflon tip.

A ground glass plunger is typically the cheapest option, but it can suffer from leakage of fluid past the plunger which may lead to volume inaccuracies. Additionally, some chemicals may cause the ground glass to fuse rendering the syringe useless.

The other types of syringe plungers tend to lead to syringes that are the most accurate with minimal leakage (some are even air-tight). The decision to use one type over the other usually depends on the compatibility of the fluid with the syringe plunger type. Usually, the biggest downside for these syringes is the cost, which is significantly greater than most syringes.

Stainless Steel

Stainless steel syringes are the most durable syringe available. These syringes are primarily used for high-pressure dosing applications that would cause a glass or plastic syringe to burst. While having an accuracy similar to glass syringes, they do not typically come in sizes much smaller than 5 mL, which minimizes their application for smaller scale use cases. Additionally, steel syringes are typically expensive and not as easy to use as other syringes because they are not transparent, making loading and removing air bubbles a little more challenging.

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