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by Jim Bolton


Editor’s Note: From time to time, I receive questions that come in from the IUVA website. The following are some of the more interesting questions and answers.

Question: How does the ultraviolet light transmit through a quartz sleeve? And do you have any suggestion to replace a quartz sleeve with another material that is similar to a quartz sleeve?

Answer: At 254 nm, the wavelength emitted by many ultraviolet (UV) lamps, the transmittance of quartz in a UV reactor is about 95%. The only other material that has been used as a replacement for a quartz sleeve is a tube made out of polytetrafluoroethylene (Teflon), which has a transmittance at 254 nm of about 60%.

Question: I have read that “Strictly, the term quantum yield applies only for monochromatic excitation.” I 100% agree with this sentence. But I saw in some papers where they mentioned determination of quantum yields for polychromatic light. Please let me know if these quantum yields are correct?

Answer: It is true that the quantum yield is only properly defined for a specific wavelength. The reason is that the quantum yield may vary with wavelength. However, fortunately, in the vast majority of cases, the quantum yield is independent of wavelength. The reasons are as follows:

  1. For any photochemical reaction, there exists a maximum wavelength above which no photochemistry can occur. This threshold wavelength is related to the energy of the first excited state of the chromophore.
  2. If the photochemical system is exposed to wavelengths less than the threshold wavelength, the chromophore is exited to higher and higher levels of the first, second, etc. excited states.
  3. These higher excited levels decay very rapidly (in femtoseconds) to the lowest excited state from which photochemistry occurs – this is why the quantum yield would not be wavelength-dependent above the threshold wavelength.

An example of a case where the quantum yield does depend on the wavelength is the ferrioxalate photochemistry. In that case, the quantum yield is constant at 1.25 for wavelengths above 260 nm but rises to 1.48 below 240 nm (Goldstein and Rabani, 2008). The reason is probably that the second excited state has a long enough lifetime that photochemistry can occur from that state. (Reference: Goldstein, S; and Rabani, J. 2008. The ferrioxalate and iodide–iodate actinometers in the UV region, J. Photochem. Photobiol. A: Chem., 193: 50-55.)

Question: We are using UV for milk purification. What is the meaning of 100 hours of UV life? And, after 100 hours, are the germicidal properties of UV light affected greatly? Some literature says that turning off/on the UV lamp more than once in eight hours affects its lifetime. Please suggest the factors that affect the lamp life at 254 nm.

Answer: Low-pressure UV lamps have lifetimes of about 8,000 to 12,000 hours, while medium-pressure UV lamps have lifetimes of about 6,000 to 8,000 hours.

Factors that can reduce the lifetime of UV lamps include turning on and off frequently, and operating at or above the rated power.

Question: I’m running an experiment with UV-disinfection. Of course you need quartz sleeves to have a high transmittance, but my question is: How much of the UV light will go through the quartz sleeve and also through other materials (glass, etc.)?

Answer: Since you are interested in UV disinfection, I assume you are using a low-pressure UV lamp that emits at 254 nm.

The transmittance of most grades of quartz is very high at 254 nm, and it is limited only by the reflection properties. In open air where the interfaces are air/quartz/air, each air/quartz interface reflects about 4% of the UV, so the transmittance is about 92%. When a quartz sleeve is mounted in a UV reactor containing water, the interfaces are air/quartz/water. The quartz/water interface reflects very little UV, so the transmittance of UV into the water through the quartz sleeve is about 95%.

All forms of glass (e.g., window glass) block UV at 254 nm. The only other common material that transmits 254 nm UV is Teflon (polytetrafluoroethylene), where the transmittance is about 75%.

Question: I quite often use a combination of hydrogen peroxide and tetraacetyl ethylenediamine (as the catalyst) to whiten plastic – which effectively means removing the bromine from the plastic surface. It’s a simple job in the summer – place the mix outside and the sun does the hard work. A few hours later, and the yellowing/browning is gone. In the winter it doesn’t work so well in the UK. It starts to go dark at 3:30 p.m., and I have to use artificial light. I’ve never been able to find an effective UVA/UVB light to take over for the sun. What wavelength should I be looking for to move the bromine and to activate the HP?

Answer: It is probably the ultraviolet part of sunlight that is activating your photochemical process. Thus, I suggest you purchase a “black light” bulb, such as the ones that were used in discos to stimulate fluorescence in clothes. These should be available at most lighting shops. Black lights are available with several different emission wavelengths (such as 310 nm, 340 nm, 350 nm and 370 nm). I suggest you try to get one with a 310 nm emission wavelength. Make sure you purchase a bulb with a high optical power at the surface so your photochemical process will not take too long.

Question: We have installed UV glass tube lights in our food production area. We need to prevent or take preventive action to avoid glass breakage (because of food safety issues), so we need to cover these UV tube lights. This means when we use these lights during production to avoid microbial contamination, the UV light passes through the protected cover.

Answer: Unfortunately, almost any material that you use to cover the UV lamps will totally absorb the UV rays making your UV system useless. The only material that would be effective would be a thin sheet of Teflon (polytetrafluoroethylene, which transmits about 75% of the UV rays.

Question: Do you have any idea about the maintenance behavior over the life of LP UV amalgam lamps made from open quartz (with pre-coat) to emit 185 nm radiation? None of the lamp producers publish the data. Also the measurement of the 185 nm lamp output is not standardized.

Answer: I do know that the 185 nm UV create “color centers” in the quartz that eventually block the 185 nm UV. However, I do not know at what rate these color centers are formed.

I agree that the measurement of 185 nm UV is difficult. Perhaps some of the UV sensor manufacturers (International Light, sglux Solgel Technologies, UV-technik Speziallampen, etc.) might be developing sensors for the 185 nm region. The methanol actinometer has been used to measure the 185 nm output.

Question: I would like to know how to use the UV lamp for UVB irradiation of mammalian cells monolayer in culture. The lamp I am using is manufactured by UVP, Upland, California. The catalog number is 95-0341-01. It has three bulbs/tubes inside that has one each of different wavelengths:

  • Longwave, which is UVA is 315-400 nm and peaks at 365 nm. The intensity of that wavelength is 1147 microwatts per square centimeter at a 2 inch distance.
  • Midrange, which is UVB is 280-315 nm and peaks at 302 nm. The intensity of that wavelength is 778 microwatts per square centimeter at a 2 inch distance.
  • Shortwave, which is UVC is 200-280 nm and peaks at 254 nm. The intensity of that wavelength is 1,070 microwatts per square centimeter at a 2 inch distance.

I would like to irradiate cell monolayer with a midrange UVB dose of 50 J/m2. Is it possible to know for how long and at what distance I need to apply the UVB lamp in order to get that specific UVB dose?

I also assume I need to irradiate cells in phosphate buffered saline with no plastic cover. Is that right?

Answer: Since you are interested only in the UVB emission, I will focus on that. If the UVB irradiance is 778 microwatts/cm2 or 0.778 milliwatts/cm2, the time required to achieve a UV dose of 50 J/m2 = 5 mJ/cm2 is 5 / 0.778 = 6.4 seconds at a distance of 2 inches (5.08 cm). If you want to know the irradiance at other distances, you would have to use a radiometer sensitive to the UVB wavelengths.

Another option would be to use the ferrioxalate actinometer. If you wish to use this option, let me know and I’ll send you the protocol.