Yes. Filters will perform very differently in a gas/air stream than in a liquid stream. These differences can be observed by comparing flow rates and retention ratings between gas /air and liquid for the same medium. In liquid, particles generally follow the flow of the solution as it travels through the filter, and it is primarily particles that are directly intercepted by the filter matrix which will be retained. In gas/air filtration, direct interception also applies, but there are other additional mechanisms that can cause particles to be captured by the filter. These mechanisms include inertial impaction, gravitational settling, electrostatic interaction, and Brownian Motion.
Ash is residue remaining after ignition of cellulose filters at 900°C in air. It is a useful measure of the general purity of the filter, with lower ash content representing greater purity. It is essential to know the ash content when performing gravimetric analyses. Ash level can generally distinguish bleached wood cellulose grades from high alpha cotton cellulose grades. The ash levels for wood pulp grades will be ≥ 0.2%, whereas high alpha cotton cellulose will have a typical ash level of 0.06% or below.
Qualitative papers are generally used for recovering retained materials and identifying the nature of the filtrate, not for quantitation. Quantitative paper is used to measure the quantity of material remaining on the filter. Quantitative filter papers are often used in gravimetric analysis, where constituents of the material to be filtered are separated and their quantities determined by weighing. Many standard inorganic analyses are based on this analytical process.
The ash content (inorganic material) is monitored as part of the production process, however extractables are not typically measured. Extractable levels can be influenced by the liquid being used, contact time, temperature, and other factors. Extractables are specific to individual applications and methods. However, if high purity is required then generally it would be best to use a quantitative paper which has low ash content.
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When filtering under vacuum, often the highest vacuum levels tend to be used to try to achieve the maximum flow rate. However, in practice, the flow rate through cellulose and glass fiber filters only tends to increase with increasing differential pressure up to about 5 cm Hg. After this point, the flow rate only increases slowly with further increases in differential pressure. However, high differential pressures can affect the retention characteristics of depth filter media adversely and so in practice it is recommended to use as little vacuum pressure as possible.
Pure cellulose fiber filter papers have been successfully autoclaved. It is generally recommended to protect the papers from becoming saturated with excessive moisture during the autoclave process.
Qualitative filter papers are not specifically acid treated to remove any inherent elements present in the natural high alpha-cellulose cotton linters, resulting in an typical ash content of 0.06%. When strength is needed, a small amount of wet strengthener is added, as in grades 113 and 114. Within the quantitative grades, the 40 series is treated with acid to remove some of the elements, resulting in lower ash content (i.e., 0.007%) than in the qualitative range. The 50 and 540 series are treated slightly differently in order to impart wet strength and, in the case of the 540 series, to minimize the ash level. These papers will withstand handlinge when wet handling and precipitate removal by scraping from the filter paper.