Greater exposed surface produces a greater uptake of the toxic vapor and a more accurate measurement. Additionally, the activated granular carbon in the sampler is the most adsorbent form of carbon and we use a large quantity of this superior adsorbent.
Passive samplers have been used since the early 1900s, but only with the development by Ed Palmes at New York University in the 1960 of a sampler with an internal air gap was the technology put on a sound foundation. His initial sampler – the so called “Palmes tube” – could measure nitrogen dioxide and sulfur dioxide. We offer an improved version that can measure almost an unlimited number of vapors.
Active sampling uses a pump to draw air into a sampling chamber; passive sampling relies on molecular diffusion to accomplish the same. Pumps always need recalibration; molecular diffusion, on the other hand, under the same conditions, always remains the same.
The only limitation is that the compound must be stable on the adsorbent, or if it isn’t, be converted chemically to something that is. For example, formaldehyde is not stable when adsorbed on charcoal, but by using as the adsorbent, a glass fiber filter impregnated with dinitrophenylhydrazine, the sampled formaldehyde is rapidly converted into a stable compound that can be quantified later using liquid chromatography.
Diffusive samplers are used anywhere a Time Weighted Average (TWA) concentration or Short-Term Exposure Limit (STEL) is desired.
With passive dosimeters one gets a time weighted average (which is what the regulatory agencies usually want), and because other compounds are also adsorbed, we can determine if there are other toxic vapors present and at what concentration. With a MIRAN, if one does not look for a compound at the time the samplings are made, those compounds will remain undetected.
One of two ways. For some compounds we add a chemical to the adsorbent to bind it. For the others, no binding chemical is needed because the attractive forces on the surface of the charcoal retain the chemical. This binding force is commonly called the van der Waals force, and is the same attractive force that causes water molecules to come together and form a liquid. Charcoal has a huge surface area where this attractive interaction can take place – often a surface area of over 1,500 square meters per gram!
Once the badge is returned, it is opened and the charcoal is placed in a vial. At that time a fixed amount of carbon disulfide is added. The carbon disulfide also contains trace amounts of other chemicals that aid the desorption process. This solvent mixture dissolves the adsorbed toxic chemical from the charcoal.
We measure the performance of the badge at the critical concentration (either the TLV or PEL), then at both half and at twice the critical concentration. We make certain that the sampling error at the 0.05 probability level is less that ±35% at the lowest concentration and within ±25% at the other two concentrations. This is the same as the OSHA requirement for stain length detector tubes.
By chromatography. There are two common ways to do this. The first, gas chromatography involves a sample being vaporized and injected onto the head of the chromatographic column. The sample is transported through the column by the flow of inert, gaseous mobile phase. The column itself contains a liquid stationary phase that is adsorbed onto the surface of an inert solid. The heavier components are retained, because of their being more strongly absorbed, in the column for a longer period of time. Each component emerges from the column separately as a peak, with, as you would expect, the lighter compounds coming out first and the heavier components last. The time that a peak takes to pass through the chromatograph tells us what the compound is. The area under the peak tells us how much of that compound was present.
In principle only in that the carrier is a liquid, not a gas. However to make it work we also have to use different types of columns and detectors. We use gas liquid chromatography when the compound to be analyzed cannot be vaporized and thus cannot be analyzed by gas chromatography.