Trace Analysis Guide - Container Transpiration
Overview
A problem that has gone relatively unnoticed when achieving reliable measurements is that of transpiration. Transpiration is defined as the passage of vapor from within a container to the outside. This loss of vapor can occur through the container walls or between the cap and threads, resulting in an increase in concentration. Transpiration becomes a problem when samples are stored for a long period of time. It is a greater problem for standard solutions, since they typically remain in use or storage for periods of up to one year.
Inorganic Ventures has been studying the transpiration loss of containers for several years. The data presented in this section will be for LDPE. This is one of the best container materials because it's the cleanest of the plastics and it's inexpensive. It has been gaining considerable popularity for use in the handling of both samples and aqueous standards.
Initial Transpiration Study
Figure 6.1 shows the rate of transpiration for 125 mL and 500 mL LDPE bottles with conventional PP caps. This study, which was conducted over a five year period, showed a significant transpiration rate difference between the two bottles. Initially, it was thought that transpiration was occurring through the container walls. However, no correlation could be found that would explain the difference of the slopes. Surprisingly, it was discovered that there was a direct correlation between the transpiration rate and the circumference of the opening.
Figure 6.1: Transpiration - 125 and 500 mL LDPE Bottles
The transpiration rate for the study shown in Figure 6.1 was found to be directly proportional to the ratio of the circumference of the bottle opening to the surface area of the solution. No correlation of the transpiration rate to the bottle surface area could be found. These findings suggest that transpiration is occurring around the cap/bottle opening and not through the container walls. This discovery was unexpected. Therefore, additional transpiration studies were conducted in an attempt to better define the transpiration mechanism.
Additional Transpiration Studies
The next study was conducted for LDPE bottles of 30, 125, 250, and 500 mL capacity. The purpose was to determine if the correlation of the transpiration rate to the bottle opening held up for a variety of bottle sizes. In addition, other variables were added as illustrated in Figures 6.2 and 6.3.
Figure 6.2: Transpiration - 125 ml Bottles
| No tape, Never opened | ||
| Tape sealed, Never opened | ||
| No tape, Opened periodically (30 times) | ||
| Tape sealed, Opened periodically (30 times) | ||
| No tape, Half-full, Opened periodically (30 times) | ||
| Inorganic Ventures' Transpiration Controlled Technology™ (TCT)1 |
Figure 6.3: Transpiration - 30 ml Bottles
| No tape, Never opened | ||
| Tape sealed, Never opened | ||
| No tape, Opened periodically (30 times) | ||
| Tape sealed, Opened periodically (30 times) | ||
| No tape, Half-full, Opened periodically (30 times) | ||
| Inorganic Ventures' Transpiration Controlled Technology™ (TCT)1 |
The results for the 250 ml and 500 ml bottles are congruent with the data shown above, with the exception that the transpiration rates are proportionately lower. So the question then becomes, What factor is proportionately effecting the rate of transpiration?
Figure 6.4 shows a comparison of the transpiration rates for all of the bottles capacities as compared to each bottle size surface area. No correlation could be found, indicating that the mechanism for transpiration is not through the bottle walls.
Figure 6.4: Transpiration Rates vs Bottle Surface Area
Figure 6.5 shows a comparison of the transpiration rates for all of the bottles as compared to the ratio of the bottle opening circumference to the solution surface area -- i.e. Transpiration Rate vs. [π * dc] ÷ [π * (Rb)2] where:
π = 3.14
dc = cap diameter in centimetres
Rb = radius of bottle (main body)
Figure 6.5: Transpiration Rates vs Bottle Opening Geometry
This study revealed a correlation that fits to within 6 % relative, indicating that transpiration around the cap is the correct mechanism and not through the bottle as previously assumed.
As additional support for this argument, Figure 6.6 shows a dramatic illustration of the fact that there is a near perfect correlation of transpiration rate to bottle opening geometry and no correlation to the bottle surface area.
Figure 6.6: Bottle Opening vs Area Geometries
Summary of Findings
Through this study, the following conclusions were drawn:
- Transpiration occurs around the bottle opening and not through the bottle as previously thought.
- Taping the bottle has no influence upon the transpiration rate.
- Bottles that are half full and opened periodically have the highest transpiration rate.
- Opening the bottle periodically greatly increases the transpiration rate.
- Smaller bottles transpire faster.
- 30 ml bottles can not have a 1 year expiration date if certified to an uncertainty of ≤ ±0.6 % relative.
- Transpiration Controlled Technology™ (TCT)1 totally eliminates transpiration.
Studies involving different bottle construction materials and 'bottle vapor space' have also been conducted and will be published at a later date.
1. Transpiration Controlled Technology (TCT) - a patented technology that virtually eliminates transpiration loss, greatly increasing a standard's shelf life. Developed by Inorganic Ventures for the optimal storage of standards after packaging.