Exploring Various Membrane Filters Biology Essay

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Membrane filters or "membranes" are polymer films with specific pore ratings. Membranes retain particles and microorganisms that exceed their pore ratings by acting as a physical barrier and capturing such particles on the surface of the membrane (Lake et al., 2002). The two main types of filter glass or quartz membranes and polymeric membrane including polyvinyl chloride (PVC), mixed cellulose ester polymers (MCE), polytetrafluoreothylene (PTFE) and polycarbonate (sometimes referred to as TMTP). Cassinelli et al.,(1994) reported tests performed on products available at the time showed that many filters to be 95% efficient at capturing particles greater than 0.1um aerodynamic diameter, even though the pore-size is as large as 8um.

The major prerequisite in selecting a sampling system is to determine what size range of particles is to be monitored and the method of analysis. The analytical method selection is very pivotal, because test will dictate the type of sampling media compactable with the sampling system. Several air sampling filters are available, and the specific filter used depends upon the desired physical and chemical characteristics of the filter and analytical methods used. Particle sampling filters consist of a tightly woven fiber mat or plastic membrane penetrated by microscopic pores. Several characteristics are important in selecting a filter media; these characteristics include (Cassinelli et al, 1994):

Particle sampling efficiency

Mechanical stability

Chemical stability

Temperature stability

Blank concentration

Flow resistance and loading capacity; and

Cost and availability

A comparison of several air sampling filters types with their chemical and physical characteristics and the corresponding chemical analytical methods that can be used for analysing the sample is presented in Table 1 (Cassinelli et al, 1994).

Temperature and humidity are important parameters that can affect the mass of sampling filter in analytical laboratory inn order to achieve accurate results. Aerosol sampling filter have the ability to absorb moisture from the atmosphere; the higher the humidity the more moisture is absorbed hence the mass of the filter will be affected. It is pivotal that relative humidity in the weighing room is keep at a contact range of 45% to 55% and temperature should be approximately 22 °C to 25 °C (MHSC, 2007).

The three recently developed or modified analytical and sampling methods for accessing various sampling methods are OSHA Method ID-215, NIOSH 7703 and NIOSH Method 7605 (Cassinelli et al, 1994). The availability of these methods presents an opportunity to compare and evaluate their performance in determining concentrations of aerosols in the workplace atmosphere. The NIOSH method 7703 was developed primarily for assessing aerosol exposures in occupational environment where sampling results are needed within a short period (i.e., less than two hours of sampling). This method has shown to be the simple, fast, practical and economical alternative to the laboratory methods, with an analytical detection limit slightly higher than the fixed-site laboratory methods. The NOISH 7703 procedure allows the use of various sampling medias (USDL, 1998).

Table . Example of representative filter media for particle sampling (Cassinelli et al., 1994).

Filter type

Filter size (mm)

Characteristics

Analytical methods

Chemical

Physical

Ringed Teflon Membrane

25

37

47

Low blank levels

Low blank weight

No carbon analysis

Low hydroscopic tendency

Inert to gas adsorption

Thin membrane

White, nearly transparent surface

High particle collection efficiency

High flow resistance

Melts at 60 degrees

Cannot be accurately sectioned

Multiple pore size available

Minimal diffusion of transmitted light

GRAV, XRD, OA, PIXE, INAA, AAS, ICP/AES, ICP/MS, IC, AC

Teflon membrane polypropylene backed

47

Low blank level

No carbon analysis

High blank weight

Inert to gas adsorption

High background levels for PIXE and XRF

Low hydroscopic tendency

Thin membrane

White opaque surface

High particle collection efficiency

Melts at 60 degrees

High flow resistance

Diffuses transmitted light

GRAV, PIXE, XRF, INNA, ASS, ICP/AES, ICP/MSIC, AC

Nylon membrane

25

37

47

Low blank weight

Low hydroscopic tendency

High HNO3 collection efficiency

Passive adsorption of low levels of NO, NO2, PAN, and SO2

Thin pure nylon membrane

Diffuses transmitted light

High flow

Resistance

Melts at 60 degrees

1um pore size

IC. AC

Silver membrane

25

37

Resistant to chemical attack

Passive adsorption of organic vapours

High blank weight

Low hydroscopic tendency

Thin Membrane

Gray-white surface

Diffuses transmitted light

High flow resistance

Melts at 350 degrees

GRAV, XRD

Cellulose ester membrane (cellulose nitrate mixed ester and cellulose acetate)

37

47

Dissolve by several organic solvents

Negligible ash content

Hydroscopic

Low blank

weight

Thin membrane

White opaque

Surface

Multiple pore size available

Surface diffuse transmitted light

High flow

Resistance

Melts at 70 degrees

GRAV, OM, TEM,SEM, XRD,

Biomedical application

Polycarbonate membrane

47

No carbon analysis

Low blank levels

Low blank weight

Low hydroscopic tendency

Smooth, thin surface with straight capillary holes

Light gray, nearly transparent surface

Minimal diffusion of transmitted light

Multiple pore size available

Use of particle size classification

Low particle collection efficiency for some pore sizes

Retain static charge

Moderate flow resistance

Melts at 60 degrees

GRAV, OA, OM, SEM, XRF, PIXE

Pure quartz filter

25

37

47

203x254

Contains large and variable quantities of A1 and Si

Low blanks level for ions

Passive adsorption of organic vapours

Little adsorption of HNO3, NO2and SO2

Low hydroscopic tendency

White opaque surface

Diffuses transmitted light

Edges of filter flake in holder

High particle collection efficiency

Moderate flow resistance

Melts at less than 900 degrees

ICP/AES, ICP/MS, IC, AC, OA, T, TOR, TMO, TOT

Mixed quartz fiber (quartz filter with 5 % borosilicate content)

203x254

Contains large and variable quantities of Na, Al, and Si plus variable levels of other metals

Passive adsorption of organic vapours

Little adsorption of HNO3, NO2and SO2

High blank weight

Low hydroscopic tendency

White opaque surface

Diffuses transmitted light

High particle collection efficiency

Becomes brittle on heating

Can melt at 500 degrees

GRAV, XRF, PIXE, AA, ICP/AES, ICP/MS, IC, AC, T, TOR, TMO, TOT

Glass fiber (borosilicate glass fiber)

203x254

High blanks level

High blank weight

Adsorption of HNO3, NO2and SO2

High hydroscopic tendency

White opaque surface

Diffuses transmitted light

High particle collection efficiency

Melt at 500 degrees

Low flow rate resistance

GRAV, XRF, PIXE, AAS, ICP/AES, IC, AC,

Cellulose fiber ("paper" filter)

25

37

47

No carbon analysis

Low blank levels: high purity

Most useful for adsorption of gases e.g. HNO3, NO2and SO2, NH3 after impregnating with reactive chemicals

High blank weight

Hydroscopic

Adsorbs gases particularly water vapour

White opaque surface

Diffuses transmitted light

Low particle collection efficiency

High mechanical strength

Variable flow resistance

Burns at 500 degrees

GRAV, XRF, PIXE, AAS, ICP/AES, ICP/MS, INAA, IC, AC,

1.1 Mixed Cellulose Ester (MCE)

The standard filter for most aerosol sampling in industrial hygiene applications, mixed cellulose ester (MCE or MEC) membrane filters are comprised of pure, biologically inert mixtures of cellulose acetate and cellulose nitrate. MCE membrane filter is characterized by a smoother and more uniform surface than pure nitrocellulose filter. MCE membranes are naturally low in metal background and are compatible with dilute bases and acids and aromatic and aliphatic hydrocarbons. MCE filters are hydrophilic and auto-cleavable (Noble et al., 2003 and Cassinelli et al, 1994). In gravimetric analysis using ashing techniques, (MCE) Nitrocellulose filters yield a residue of less than 0.045% of their initial weight. MCE filters have a non-cytotoxic wetting agent extractable level of less than 4% of their weight.

MCE filters are commonly used for metal analysis as they dissolve easily by oxidising acid as nitric or perchloric. The standard pore size is 0.8um, but this is not compatible with high flow rate sampling. Larger pore-sizes are available but the efficiency for collection of small particle can be compromised if the pores become too large. A large pore-size of 1.2 um is available and may allow high flow-rates with low particle losses (Lake et al., 2002 and Noble et al., 2003). The filters are hydrophilic making them a poor choice for gravimetric analysis, but the attraction of being able to measure total particulate mass collected as well as to perform chemical analysis causes them to be used this way in some European countries such as Sweden. MCE filters are easily digested by dilute nitric acid which makes it ideal for atomic absorption spectroscopic analysis and their pore structure collapses when exposed to acetone vapour and offers minimal interferences in fibre counting. This makes it highly suitable for the collection and analysis of asbestos, silica dust and other fibers (Lake et al., 2002).

SKC MCE membranes filters meet NIOSH specifications for the collection and analysis of airborne metals. They are hydrophilic, economical membrane for air monitoring applications. They dissolve completely using standard digestion procedures and suitable for use in most all NIOSH analytical methods which require MCE filters, including asbestos fibers monitoring (Lee et al., 2002). The MCE membrane thickness can be controlled to plus or minus 10 microns to accommodate various applications. Clears completely, possesses low artefacts, and offers minimal interference in fiber counting (Noble et al., 2003). MCE filters are affected greatly by temperature changes.

 

MCE filters features are:

Hydrophilic

Economical membrane for air monitoring applications

Suitable for air monitoring applications.

Dissolves completely using standard digestion procedures.

Suitable for use in most all NIOSH analytical methods which require MCE filters, including asbestos fibers monitoring.

Clears completely, possesses low artefacts, and offers minimal interference in fiber counting.

Table . Examples of various MCE filter applications (Cassinelli et al, 1994).

Application

Colour

Pore Size(µm)

Micro dialysis of DNA and proteins

White

0.1

Sterilizing filtration, bioassays

White

0.22

Sterilizing filtration, air monitoring, particle monitoring, particle removal, bioassays

White

0.3

Clarification of aqueous solutions, particle removal and analysis, microbiology analysis

White

0.45

Fluorescent bacteriological assays, particle monitoring, bioassays

Black

0.45

Particle monitoring, particle removal, dairy microbiology, retention of yeasts, molds and algae

White

0.65

Air monitoring, particle monitoring, particle removal, bioassays

White

0.8

Fluorescent assays, particle monitoring, air monitoring

Black

0.8

Clarification of aqueous solutions

White

1

QC of fluid holding tanks, fluid monitoring, air monitoring, particle collection and analysis

White

3

QC of fluid holding tanks, fluid monitoring, particle collection and analysis

White

5

QC of fluid holding tanks, fluid monitoring, air monitoring, particle collection and analysis

White

8

1.2 Polyvinyl chloride (PVC) Filters

These are the filters most commonly used filters for gravimetric analysis in the United States because of the exceptional weight stability, even though their price is significantly higher than the glass fiber filters. It is possible to perform additional chemical analyses on particles collected by PVC filters, although PVC filters are difficult to digest (Cassinelli et al, 1994). Methods for the analysis of silica calls for the PVC to be digested in organic solvent tetrahydrofuran, or destroyed by plasma ashing. Strong acid digestion generally leads to very viscous matrix. Particles may be efficiently removed from PVC filters by ultrasonic agitation and then digested, but this procedure has not been extensively evaluated. Filters that are mixed polymer of PVC and polyvinyl acetate are also available (Lake et al., 2002).

PVC filters features are:

Filters composed of pure medical-grade PVC

Use for air monitoring applications to analyze silica, carbon black or quartz particulates

Available in 5.0 µm filters

Filters composed of vinyl/acrylic polymer

Used for air monitoring applications to analyze silica, carbon black or quartz particulates

Available in 0.8 µm filters

1.3 Glass and Quartz Fiber Filters

Glass-fiber filters without binders are recommended for analytical and gravimetric determinations. Glass-fiber filters are manufactured from 100% borosilicate glass. These depth filters combine fast flow rate with high loading capacity and retention of fine particulates. The small diameter fibers give micro glass media superior efficiency and dirt holding as compared to cellulose and synthetic media (Cassinelli et al, 1994). They are the least expensive and have been in use longest. They are commonly used in gravimetric analysis in Europe, but not in the USA. Their weight stability is not quite good as the PVC, but it is considered adequate in countries where used (Lake et al., 2002). They can also be used in methods requiring digestion of particulate as long as they are bind-free and there are no analytical interferences.

Quartz fiber filters are more expensive compared than glass-fiber filters. They are mostly reserved for special applications, for example in sampling diesel exhaust, where organic contamination must first be removed from the filter by baking at high temperature. Glass and Quartz filters offers little resistance to air flow and thus are the most common filters for high flow rate application (Lake et al., 2002).

Glass fiber filters features are:

Made of borosilicate glass fiber without binders or with binder.

Stability at high temperatures: It keeps its properties up to 500 °C and 180 °C for Grade GF10.

Usable as Pre-filter for membranes to prevent the membranes from silting up.

Large surface area provides an outstanding retention capacity.

High flow speed and high permeability to air.

Reduce filtration costs and premature clogging when filtering difficult-to-filter or highly contaminated solutions.

Excellent wet strength for easy handling and filter integrity.

1.4 Polytetrafluorothylene (PTFE) filters

PTFE membrane media for filtration is made of PTFE (polytetrafluorothylene). The PTFE membrane was laminated with great variety of fabric and paper. They are new filter media. Applied to extensive industries, including biochemistry, microelectronic, lab material and etc. Directly and indirectly related to pharmacy brewing, manufacture of pure water and special need water, beverage and dairy, chemical regent, biochemical regent, air filtration of fermentation tank in microelectronic, purification and filtration in microelectronic plants, filtration and separation of antibacterial fluid, production of medicine, air conditioning of hospitals and commercial buildings(Cassinelli et al, 1994). These filters are the most expensive, but also the most inert, hydrophobic and free from interferences. They are often used for sensitive chemical analyses of reactive compounds such as polycyclic aromatic hydrocarbons. Although they are prone to static problems, with careful handling they can be used for gravimetric analysis to a sensitivity of 1 to 2μg per sample (Kennedy et al., 1994).

PTFE filters features are:

PTFE membrane with supporting layer polyester or polypropylene

The PTFE membrane can effectively filtrate microorganism and other particulates

Wide chemical compatibility

High temperature resistance

Low starting resistance

PTFE filters applications are as follows:

Filtration of strong acids and aggressive solutions

Venting applications

Phase separations

Aerosol sampling

1.5 Polycarbonate Filters

These filters have the smoothest surface most suitable for microscopic analyses. They are used mostly for optical microscopy, frequently for bio aerosol and transmission electron microscopy such as asbestos.

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