Organic solvents used in flash chromatography and their vapors can be flammable at the right temperatures and in the right concentrations, given an ignition source.
Flammability and explosion risk is reduced by controlling environmental temperatures, solvent vapor concentrations, and by controlling or keeping sources of ignition away from these environments. Temperature and vapor control recommendations below are adapted from the
Flammability Guide, section 1.3.
Control flammability risk by controlling the temperature of both the solvent and the surrounding air:
A solvent's flash point and autoignition temperature will be indicated in the Safety Data Sheet (SDS) provided by your supplier.
Controlling vapor levels
Ensure that ventilation is adequate:
- “Vapors need to be diluted to well below the LEL to ensure safe operation. A level of < 25% is often specified."[10] Why “well below?" First, the occupational exposure limit could be even lower than the LEL. Also, the LEL is a
theoretical limit—the actual limit might be lower. Consequently, a margin of safety below the LEL is needed.
- To ensure adequate ventilation, a mechanical ventilation system may be necessary. It should provide six air changes per hour.
As with its flash point and autoignition temperature, a solvent's lower and upper explosion limits will be indicated in the SDS provided by your supplier.CombiFlash flash chromatography systems have internal vapor sensors to monitor the organic vapor levels inside the instrument to help detect internal leaks and prevent hazardous conditions by keeping levels well below that lower explosion limit. This vapor sensor doesn't replace the need to place the system in an adequately ventilated environment, as it is not intended to detect leaks or solvent vapor sources external to the system. The sensor works by detecting vapors present inside the system. When vapor levels are excessive, the system will stop the pumps.
|
Sensitivity Setting |
Percentage (relative to the LEL of hexane) |
|
Low |
45% |
|
Medium |
15% |
|
High |
5% |
Currently, all Teledyne LABS CombiFlash chromatography systems have this vapor limit feature.
Controlling ignition risks
The
Flammability Guide (section 2.5) lists several ignition sources that could be found in a laboratory, including open flames and smoldering (e.g., from matches or gas burners), hot surfaces (e.g., ovens, furnaces, lasers), sparks (electrical or mechanically produced), and electrical discharges. Perhaps easily overlooked are solvents in rags, filters, etc. These can self-heat from chemical reactions and create an ignition source.
Electrical discharges include those from motors, switches, fuses, extension cords and—less obviously—from calculators, cell phones, and flashlights. Inadvertent ground loops can be a source of discharge.
But static electricity is the most important and common accidental ignition source.[11] Static charge can accumulate in the solvent itself and create conditions for a potential spark. This process, and the means of controlling it, will be explained in detail below.
Electrostatic Hazards and Flash Chromatography
As noted earlier, static electricity is the most common accidental solvent ignition source. Static electricity is an imbalance between positive and negative charge within a material. You can generate it during ordinary activities including walking on carpet in your fleece pajamas with covered feet, rubbing a balloon on your head, and removing cling wrap from a package or container. You probably wouldn't do any of these in your laboratory, right?
Or would you? In the lab, you might walk across a carpet with rubber-soled shoes, idly comb your hair, or remove plastic wrap from new equipment.
Furthermore, static electricity can arise from solvent handling, liquid flow, stirring, agitation, or mixing. The main concern for static electricity generation in flash chromatography is charging by liquid flow.[12] Refer to the
Flammability Guide, section 3.2, from which the following discussion is adapted.
How solvents become charged
Charge accumulation and dissipation
Solvent pumping and pipe flow—especially turbulent flow at high velocities—are important sources of electrostatic charging during flash chromatography. This can be further aggravated if the liquid passes through a fine filter or contains suspended particles or other immiscible matter.
Charge separation in pipe flow (the opposite polarity also occurs). Adapted from
Flammability Guide - Best Practice Guidelines for Safe Handling of Flammable Solvents; European Solvents Industry Group (ESIG): Brussels, June 2013, p. 19. Copyright 2023 European Solvents Industry Group.
The amount of charge generated increases proportionately to the linear velocity of the solvent. This makes the choice of system so important for your process. Systems should be designed to use an appropriately sized tubing diameter to ensure that linear velocity doesn't increase drastically as flow rates increase. The CombiFlash Torrent, for example, uses much wider tubing than the CombiFlash NextGen, allowing it to safely go up to 1 L/min.
However, while static builds up within a liquid, it also dissipates—continually leaks—to the walls of its container (or tubing). Fast dissipation is desirable because it prevents hazardous voltage differences between the solvent and the walls of its container. How quickly the solvent charge can dissipate to ground depends on the material of the container or tubing (stainless steel or plastics) and the conductivity of the solvent.
Tubing materials
Most non-metallic tubing is non-conductive; it doesn't allow the efficient transfer of charge from the motion of the fluid to dissipate to the tubing and ground to the atmosphere. This results in the accumulation of static charge and presents a risk.
The use of special static dissipative tubing on Teledyne CombiFlash systems allows the charge generated by the fluid to be transferred to the tubing surface, dissipates it to ground through the instrument or through moisture in the air, and prevents it from accumulating a large charge potential.
High and low conductivity solvents
The rate at which dissipation occurs in a solvent depends in great part on its conductivity. Charge dissipates quickly from a solvent with higher electrical conductivity, while a low-conductivity (i.e., ≤ 50 pS/m) liquid may take several seconds to lose most of its charge. Generally, “hydrocarbon solvents usually have low conductivity whilst most polar solvents (e.g. many oxygenated solvents such as alcohols) have high conductivity."[13] The
Flammability Guide, p. 21, has a useful table of common solvents and their conductivities; this list is similar:
|
Substance |
Conductivity pS/m |
Dielectric constant |
Relaxation times s |
|
Acetone |
6 x 106 |
21 |
3.1 x 10-5 |
|
Isopropanol |
2 x 106 |
18 |
8.1 x 10-5 |
|
Ethyl Acetate |
3 2 x 106 |
6.2 |
2.8 x 10-5 |
|
Ethanol |
4 x 106 |
25 |
5.6 x 10-5 |
|
Methanol |
7 x 106 |
33 |
4.2 x 10-5 |
|
n-Heptane |
4 |
1.9 |
4.3 |
|
n-Hexane |
24 |
1.9 |
0.71 |
|
n-Octane |
9 |
1.9 |
1.9 |
|
Toluene |
5 |
2.4 |
4.3 |
|
n-Pentane |
24 |
1.8 |
0.68 |
|
m-Xylene |
9 |
2.4 |
2.4 |
Adapted from Flammability Guide - Best Practice Guidelines for Safe Handling of Flammable Solvents; European Solvents Industry Group (ESIG): Brussels, June 2013, p. 21. Copyright 2023 European Solvents Industry Group.
But don't let the choice of a higher conductivity solvent make you complacent: electrostatic charge can accumulate in medium- or even high-conductivity solvents during certain operations.[14] This is why good instrument design is critical for safe operation for all solvents used in flash chromatography.
Furthermore, even some high-conductivity materials have an elevated risk of ignition: “Ethyl acetate has been associated with a number of electrostatic ignition incidents and even though it has a high conductivity, it should be treated as a low conductivity material. It would be prudent to treat other light esters similarly."[15]
Effect of humidity levels on static electricity
Although humidity isn't discussed in the
Flammability Guide, the lack of it can create hazardous conditions because dry air facilitates the transfer and accumulation of static electricity instead of allowing it to dissipate. Dissipation from solvents is aided by humidity in the air because the particulate matter in the water vapor gives surface charge someplace else to go and allows electrostatic charge to dissipate from surfaces. Dryer air doesn't do this as well. If the air is especially dry, even walking across a carpet can generate an enormous static charge.[16] Increasing the humidity in the air can prevent this.
You may notice the laboratory humidity can vary greatly; the success of a moisture sensitive reaction could depend on the season. For example, humid Nebraska summers aren't ideal for Grignard reactions, but during the cold dry winters they work splendidly. Your laboratory environment may vary depending on its location and on the time of year. A laboratory in a tropical region might be expected to have more humid air than one in a colder region. However, a heavily air-conditioned laboratory might have dry air year-round in either location if it is not humidified.
Ultimately, a 5% swing in humidity could be the difference in generating an amount of static electric charge on ungrounded, non-conductive tubing to create a potential hazard situation. Proper system design, grounding, and the use of static dissipative tubing take the guesswork out of varying laboratory humidity levels.
Controlling static electricity
Refer to the
Flammability Guide, section 3.3, for static control measures for solvents. Measures from this document are summarized below.
Grounding
Adequately ground equipment.
- Ground all conductive system components to prevent sparking. The path to earth should have a resistance of < 10 Ω.
- Regularly verify that equipment complies with earth resistance requirements and that conductive or dissipative hoses meet end-to-end resistance requirements. CombiFlash systems are designed with a static-dissipative black tubing. This tubing is unique to Teledyne chromatography systems.
- Ground your solvent and waste bottles during operations when pumping from or into a container, when filling, or when emptying a container. Also, be sure to ground yourself during any of these operations.
Personal protective equipment
Electrostatic dissipative gloves and footwear should be worn; flooring should be dissipative as well. If dissipative gloves do not offer sufficient hand protection for an operation in other respects, that operation should be re-evaluated to avoid material handling or to ensure proper grounding of items carried by the operator.
Operating procedures
Design operating procedures to reduce turbulence, velocity, and splashing of solvents.
-
Keep conductive materials out of tanks with properly designed operating procedures and clothing.
Be patient when filling, dipping, or otherwise handling solvents. (These are discussed in the
Flammability Guide, section 1.3)
-
Wait. Give charge time to dissipate.
-
Avoid splash filling. Dip pipes or bottom filling should be used for flammable, low-conductivity solvents.
Know your solvent containment system.
-
Avoid non-conductive or non-dissipative plastic containers. “Do not fill solvents into any insulating plastic containers or tanks [greater than 5 L] unless the [flash point] of all solvents handled in the vicinity is > 60 °C."[17]
