From my previous post, we saw that bubbles can form from cavitation during dispensing due to excess dissolved gas, or to low pressure areas and turbulence during dispensing. Sometimes bubbles form despite the best efforts of instrument engineers to eliminate dissolved atmosphere. Unfortunately, bubbles can show up long after an instrument is designed and placed into service. Even the most well-thought-out designs can encounter strange, unexplainable, errors. The following is an example of such a situation and its solution.
Three years ago, a customer came to IDEX Health & Science with a problem: they were encountering a build-up of bubbles in a line connecting a salt water reference solution to a sensor head. The line was one meter long and had originally been made of PTFE fluoropolymer. Other fluids were connected to the sensor head through identical lengths of tubing and were kept at the same place in the instrument. Early development of the instrument had shown that unless all the fluids were degassed, they would develop bubbles in the tubes as they moved from the reservoirs to the sensor head. To ensure the fluids were properly degassed, the design engineers developed a multiple layer bag which had near-zero air permeability.
By using this bag, all the fluids could be degassed thoroughly by the instrument manufacturer, then supplied to the end-use facility where the reagents could be placed on the machine when needed. Although costly, the method of providing pre-degassed reagents was found to eliminate bubbles from all lines… except the one containing the salt water reference solution. Measurements of the amount of dissolved air remaining in the salt water reference solution showed that the salt water was becoming saturated with air overnight in the meter-long transfer line, but the formation of bubbles in the line was very puzzling. Even though the system engineers changed the transfer line from PTFE to a lower permeability fluoropolymer, FEP, they found the bubble problem still occurred at nearly the same rate.
Since the conductivity of the salt water was referenced prior to each sample in a “standard, sample, standard” protocol, a deviation in conductivity caused by an air bubble was causing the results from a test to be rejected by the software. A purge cycle was implemented in the software to assist in removing the bubbles from the reference side of the conductivity bridge, which improved results but didn’t completely eliminate them. Rejecting the results from the test due to bubbles in the reference standard was costing the end-user of the instrument considerable money—not only for the test but by requiring re-sampling of the patient in some instances. The reference solution error rate before– and after– the instrument software change to flush the bubbles is presented in figures 1 and 2.
Figure 1: Original sample data obtained without a software recognition of bubbles followed by a flush. Salt water reference solution showing noise and conductivity changes due to bubbles.
The patient samples processed through the system under the above conditions resulted in the unacceptable rate of errors as seen below.
Figure 2: Post software change which included bubble recognition and reference solution flush. Again, sample errors result from salt bridge bubbles; Original FEP tubing
After reviewing all the data, IDEX Health & Science engineers made a Transfer-Line Degasser to match the flow restriction and the format of the original line connecting the degassed reagent to the sensor head. The instrument’s on-board vacuum was used to create the vacuum inside the lumen of the Poridex™ tube, such that the entire length of the line from the connection to the bag to the sensor head was being actively degassed. Three features of the Transfer-Line Degasser were vital to the installation: Firstly, the Transfer-Line Degasser was flexible, allowing easy retrofitting to installed instruments. Secondly, the entire contents of the line could be degassed within minutes of an instrument re-start. Thirdly, the Transfer-Line Degasser took up no additional space in the instrument.
Following the initial testing of the installed Transfer-Line Degasser, the following results demonstrated the efficacy of the application.
Figure 3: Conductivity measurements of salt water reference after installation of Poridex™ Transfer Line Degasser.
As expected, the elimination of bubbles from the salt water reference standard resulted in a near-zero number of failed samples.
Figure 4: Conductivity measurements of Patient Samples following the installation of Poridex™ Transfer Line Degasser
Since the instrument had already passed FDA certification, the modification to the instrument was made to actively degas the salt bridge transfer line, then to re-certify. Although it would have been possible to degas all of the reagents instead of incurring the continuing costs of the associated degassing and packaging of each individual reagent, recertification was a barrier to this potential cost-saving effort. Where the other reagents spent only two or three hours in their respective transfer lines, the salt bridge solution could spend as much as 24 hours or more. Since air is hydrophobic, any bubble which formed in the salt solution within the lines would preferentially adhere to the fluoropolymer tubing inner wall and therefore act as a further nucleation site for increasing the bubble size. Purging the line could only remove the larger bubbles, which adhered to the inner surface. The Poridex tube that makes up the inner space of the Transfer- Line Debubbler offers a porous surface through which bubbles contacting its surface can escape into the vacuum. It also presents a vacuum to the salt solution as it rests in the space between the Poridex inner tube and the outer wall of the transfer line. The OEM customer graciously allowed us to use their figures, which accurately represent the action of the Poridex transfer line.
Active Degassing Transfer Line–Epilogue
It is possible to prevent bubble formation by degasssing the system fluid in the transfer tubing itself. The Systec® Tranfer-Line Degasser employs a unique co-axial approach to remove dissolved gases before they can form bubbles and affect critical results. Vacuum is applied to the inner tube, which pulls bubbles and dissolved gases from the solution in the outer tube.
Figure 5: Poridex™ Transfer-Line Degasser section and explanation of the process
The application we have described above is one of many different opportunities to use a transfer line to remove both bubbles and dissolved gas along its entire length. Wherever temperature, pressure or solution concentration changes may occur, so too may bubbles caused by the change in solubility of atmosphere in the fluid. Because Transfer-Line Degassers are light-weight, flexible and made using inert polymers, they are uniquely suited for removing gas and bubbles while transferring fluids long distances without additional volume inside instruments and have the added feature of being able to do so between fixed and moving stages.
The author of this post, Carl Sims, is a Senior Scientist at IDEX Health & Science. He specializes in chemical instrumentation, analytical chemistry methods, and gas-liquid transfer.