RECENT IMPROVEMENTS IN SPME-GC/MS DETECTION OF ACETIC AND FORMIC ACID IN AIR

Jens Glastrup and Morten Ryhl-Svendsen

The National Museum of Denmark

ABSTRACT

1): Formic acid sampling:

Following what we presented last year in Oxford, about detection of acetic acid in air by SPME-GC/MS, we have now investigated the performance of SPME as a sampling media for formic acid also. The method and experimental setup are identical to the description in last years Oxford paper [1].

Fig. 1: Chromatogram of acetic and formic acid in air, sampled on SPME (PA 85µ)

By exposing the SPME fibre (PA 85µm) to formic acid standards in the concentrations 10, 110, 210, 310, 410, and 510 µg/m³ (in triplicates), we have found a linear response between the standard concentrations and the GC signal with a correlation coefficient better than r=0.98. The detection limit of the method was 28.9 µg/m³.

Fig. 2: Calibration curve, formic acid

By the same method we have previously found a detection limit for acetic acid of 5.3 µg/m³ (linear response between 50 - 650 µg/m, r=0.975)

We also tested new SPME fibers, with different phases: polydimethylsiloxane/divinylbenzene (blue), carbowax/divinylbenzene (orange), Carboxen/polydimethylsiloxane (black), and the polyacrylate (white) we have been using until now.
All fibers were exposed to mixed standards of 200 µg/m³ of formic acid and 200 µg/m³ of acetic acid, followed by analysis by GC/MS. The result was promising, new fiber types like the polydimethylsiloxane/divinylbenzene has a much increased performance when collecting acetic acid, however, the Carboxen/polydimethylsiloxan fiber has far the best performance for both acetic and formic acid (fig. 3). The sensitivity is around 40 times better for acetic acid and 5 times better for formic acid than the old polyacrylate fibre.

Fig. 3: Comparison between different phases: polydimethylsiloxane/divinylbenzene (blue), polyacrylate (white), carbowax/divinylbenzene (orange), and carboxen/polydimethylsiloxane (black).

2): Practical use of SPME sampling:

Being a fast and easy-to-use technique, we have found SPME sampling useful for acid emission detection from wood.

In recent experiments, we have been looking into the effect of reactive materials in showcases on the concentration of air pollutants. Our hypothesis is that the presence of museum objects reactive to air pollutants (e.g. lead objects reactive to acetic and formic acid) in a confined system like a showcase, may act as a scavenger, thus actually cleaning the air.

While the flux of pollutants emitted out of construction materials and onto the surface of the reactive museum objects may be more or less unchanged, the concentration of the pollutants in the case air may decrease to very low or not detectable. This may especially be the case where the reactive surface area of objects is large compared to the case volume and area of emitting construction material (many objects in a showcase) thus there's constantly a fast removal of pollutants from the air onto the object surface.

If only the concentration of pollutants in the showcase air is used as a measure on the showcase air quality, one could be lead to a false sense of being secure, when measuring a low or no concentration of pollutants, e.g. with passive samplers. We propose, that determination of the area-specific emission rate of construction materials used in showcases is a better measure of the "corrosion potential" of that case, than the concentration of pollutants in it.

We have demonstrated this by the following experiment:

In a steel emission test chamber of 0.227m³, a more than 10 years old plank of oak was enclosed. The air exchange rate of the system was 1.5/24h, illustrating a semi-airtight showcase. Within the chamber the air was constantly mixed by a fan. The air used for ventilation was purified by activated charcoal and conditioned to 23°C and 45%RH. The plank had a surface area of 0.433 m², which gives a volume/area loading in the chamber of 1.9.

During the experiment the concentration of acetic and formic acid was monitored in the exhaust air from the chamber. During the first 21 days the concentration just increased constantly.

At the 21st day, a lead foil with a polished and clean surface was inserted into the chamber with the oak plank. This foil had a surface area of 0.143m², which is approximately 1/3 of the oak plank area.

Right after this the chamber concentration of acids off course decreased dramatic, as the chamber system had been disturbed by opening the chamber door to the ambient lab air. But the concentration of both acetic and formic acid stayed low, and actually continued to decrease for the next 60 days until the experiment was terminated (fig 4).

Fig. 4: Decay curves for acetic acid (top) and formic acid (bottom) concentrations over 80 days in the chamber set-up. The lead foil was inserted on day 21.

If we assume that the emission rate of acids from the wood was unchanged* it is obvious that the sudden fall in chamber air concentration means that almost all the emitted mass of acids is being used by reaction on the lead surface. This means that a high amount of pollutants take part of deteriorating reactions within the chamber (showcase) despite the low concentration of acids in the chamber air which we measured.

*) Actually there is a fair chance that the emission rate from the wood would even increase during the decribed conditions. As the emission rate of a pollutant from a material is dependent on the difference in concentration of the pollutants over and below the material surface. When the surrounding air concentration drops, this difference increases, thus increasing the rate of emission.

Reference:
[1] Ryhl-Svendsen & Glastrup: "Direct measurements of acetic acid by SPME-GC/MS, and calculation of emission rates from emission chamber tests". Third Indoor Air Quality Meeting, Oxford Brookes University, 10th-12th July 2000, On-line: http://iaq.dk/iap/iaq2000/2000_14.htm

Jens Glastrup and Morten Ryhl-Svendsen
The National Museum of Denmark
Department of Conservation
P.O.Box 260, Brede, DK-2800 Lyngby, Denmark
E-mail: jens.glastrup@natmus.dk
E-mail: morten.ryhl-svendsen@natmus.dk