Argon versus Nitrogen

While most anoxia treatments are being carried out with nitrogen, the use of argon deserves consideration. Although helium is also effective, this very light gas has a relatively high diffusion rate through most plastic films, creating containment problems; it is also expensive. Argon is attractive because its cost in

many locations may be comparable to nitrogen. Additionally, studies by Valentín showed that the time necessary for 100% assured kill with argon was 65-70% of the time with nitrogen (Fig. 1.3). The species compared at 40% RH, 300 ppm oxygen in argon or nitrogen, and at 20, 30, and 40 °C included the old house

borer and the cigarette, furniture, deathwatch, powderpost, drugstore, book, and black carpet beetles. The study showed, for example, that at 20 °C, it took

7 days to kill all life stages of the furniture beetle with nitrogen but only 5 days with argon. Argon has been the anoxant of choice for Valentín for pest control in humid regions where the cost of nitrogen and argon is generally comparable.

She found that when using a 6.2 m 3 commercial bubble, based on a film made with poly(vinyl chloride) and reinforced with polyester, the oxygen leak rate

after 11 days was 2.3% with nitrogen and 1.6% with argon. It is not clear why this should be the case, but the oxygen leak rate was also higher with nitrogen than with argon for poly(vinylidene chloride) bags—0.6% versus 0.5% over one year. The slightly slower influx of oxygen when argon is used may help explain the higher rates of insect kill with this gas.

Koestler and Mathews (1994) conducted a pilot program in 1992 in which they used argon to treat ancient documents, infested with book lice, from the library of the Great Lavra Monastery on Mount Athos in Greece. Seventy manuscripts were rendered free of lice by sealing them inside plastic bags based on nylon film as the oxygen barrier. The bags were flushed with humidified argon to bring the oxygen content to 700-1000 ppm before heat-sealing. While Koestler and Mathews claimed that 20 days of exposure would have been sufficient in a museum that maintains a constant room temperature, the variable and some- times quite low temperatures at Mount Athos led to a minimum of 30 days of treatment. Unfortunately, the pilot program was not expanded to treat the entire, very large manuscript collection, although Rentokil has since done selec- tive fumigations at the monastery with carbon dioxide (Smith 1996).

Whether the cost of argon is comparable to nitrogen or substantially higher may depend on the location and the availability of suppliers other than prime dis- tributors of research-grade gases. The cost of the two highest purity grades of nitrogen, based on the February 1996 listed prices of prime suppliers, ran between $4.08 and $5.10 per cubic meter in Los Angeles, California, while the corresponding grades of argon ranged from $5.10 to $6.80 per cubic meter (Table 3.4). This is based on prices ranging from $37.00 to $65.00 for a T-cylinder of gas. An industrial grade of nitrogen, assayed at 20 ppm oxygen, can often be obtained from secondary suppliers for a fraction of the price of the research grade. The savings on industrial argon are less but still significant.

Proponents of argon cite advantages other than speed of kill, but the practical importance of these has yet to be demonstrated. For example, argon is some- what heavier than oxygen and less dense than carbon dioxide (Table 4.1). This might lead to stratification that would create lower oxygen concentrations around insect-infested objects placed on the floor of a containment unit. However, the magnitude of the density difference is not likely to create dramatic stratification effects. Additionally, if there are diurnal temperature swings in the facilities where the bags are stored, then thermal convection currents within the bags are likely to eliminate any such stratification. Also, there is a perception

that microbial or fungal growth on museum objects might feed on nitrogen, which would not occur with argon. However, this is not likely to happen on

O perational Problems and Practices

Table 4.1

Density of common gases at 1.0 atmosphere

Gas

Kg m -3

and 0°C.

Carbon dioxide

wooden pieces in the absence of liquid water (Blanchette 1995) and does not pose a genuine threat to the use of nitrogen for anoxia.

Argon versus Nitrogen