Spore traps (C) Daniel FriedmanPopular Air Sampling Cassette Comparison by Practical Field Tests
     


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Spore trap reliability for indoor air sampling: how accurate & consistent are air sampling cassetts for airborne mold, dust or other particle testing?

Air sampling cassette tests for mold: accuracy and consistency comparison: this practical air sampling field study performed side-by-side comparisons of two popular air sampling cassettes in order to report on similarities and differences between the two products and their field or laboratory performance characteristics.

Also see ACCURACY vs PRECISION of MEASUREMENTS.

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A Field Comparison of Air-O-Cell and Allergenco-D Air Sampling Cassettes

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel FriedmanThis article is the first of two discussions on variability in air sampling cassette test results and particle trace patterns. This article describes and present photographs of measurements of particle traces produced by the Air-O-Cell® and Allergenco-D® sampling cassettes when air samples using the two cassettes were collected simultaneously.

A separate quantitative analysis reporting and comparing actual particle counts forms part two of this topic and remains to be completed. Those results will also be provided here.

This study was conducted and financed by InspectAPedia.com. No financial support was provided by any third party, but an air sampling pump and a supply of Allergenco-D air sampling cassettes were provided by Environmental Monitoring Systems who have reviewed this article and who comment that since the original study of their A1-D cassettes described here, product design and quality control improvement have been continuing.

More information about product changes appear in notes included in the article below. Comments from Air-O-Cell or Zefon Corp. have not been received. [Note: peer reviews of articles found here may have provided technical corrections or suggestions but do not indicate reviewers' agreement or disagreement with the article or its conclusions.] Comments or critique are invited - CONTACT us.

Study Objectives

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel FriedmanThe purpose of this field study was to observe similarities and differences between the field performance of two popular air sampling cassettes, the Air-O-Cell® cassette distributed by Zefon International, and the Allergenco-D® Cassette distributed by Environmental .

Sampling cassettes were mounted to face in the same location, oriented in the same direction and same floor height, and they collected the same volume of air simultaneously, using identical and calibrated air sampling pumps operating at the same airflow rate.

The cassettes were separated horizontally to avoid possible airflow interference between the two devices.

Examples of our field testing apparatus and controls are shown below.

 

Air Sampling Cassettes

The Allergenco-D® Sampling Cassette is described by EMS-Sales as:

The AllergencoD™ sampling cassette is an inertial impactor utilizing kinetic energy to impinge airborne particulate into an adhesive media. This type of impactor employs the use of a nozzle (or venturi) to accelerate air onto an adhesive coated glass substrate. The glass substrate is subsequently removed and the impaction zone (or trace) is then studied for quantitative analysis. The AllergencoD™ contains a laminar flow venturi (US patent#7,051,605) which directs airborne particles perpendicular to the plane of impaction. This creates a more well defined impaction trace resulting in more efficient collection and quantitative analysis.

The Air-O-Cell Sampling Cassette is described by Zefon International as:

Zefon cassetteThe Air-O-Cell® is a unique air sampling cassette specifically designed for the rapid collection of a wide range of airborne aerosols including mold spores, pollen, insect parts, skin cell fragments, fibers (e.g. asbestos, fiberglass, cellulose, clothing fibers, etc.) and inorganic particulate e.g. ceramic, fly ash, copy toner, etc.). The Air-O-Cell® collects both viable and non-viable sample specimens, providing a much broader overview of potential allergens contaminants than conventional sampling techniques.

The Air-O-Cell® operates upon the principle of inertial impaction. Particulate laden air is accelerated as it is drawn through the cassettes tapered inlet slit and directed towards a small slide containing the collection media, where the particles become impacted, and the air flow continues out the exit orfice. The adhesive nature of the collection media prevents the collected particulate from blurring or being washed off during the laboratory staining process, and eliminates sample loss from vibration during handling and shipment.

Air Sampling Cassette Field Testing Apparatus

Our parallel air sample collection apparatus set-up is shown below-left in use in the Watergate building in Washington, D.C. in November 2005. Other air sampling cassette tests were conducted in New Jersey and New York in 2005 and 2006. At below right is a similar test conducted in New York.

The air sampling pumps included a built-in flow-meter that was first calibrated in our laboratory using a standardized bubble chamber. In the field each pump was adjusted to the same air flow rate - 15 lpm.

A common timer-controller (seen on the bottom of the stepladder at left, and between the two air pumps in the right hand photo) controlled the sampling interval so that we could assure that the two pumps were operated simultaneously and for an identical pumping interval.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman


Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

We also established and recorded site conditions likely to affect any air sampling measurement results, including

  • The establishment of still-air conditions no less than two hours before the samples were collected
  • Building HVAC systems were shut down before, during, and after the sample collection
  • No fans were in operation
  • Windows and doors were shut, including interior doors between rooms
  • No personnel were in the test area before or during the sample interval except for the test operator
  • Air sampling cassettes were aimed in the same direction
  • Air sampling cassettes were placed at the same floor-height
  • Air sampling cassettes were experimentally separated by distances ranging from touching-cassettes to four feet apart. We experimented with different cassette separation differences but did not observe a significant effect. As a rule we kept the cassettes 24" apart. We did not use cassette horizontal separation of more than 36" because of the importance of sampling the same indoor air and indoor air conditions. Our opinion is that if separated by 12" or more (such as the chair back in our photo at above right), there will be no air pattern interference between the sampling cassettes in a simultaneous measurement.
  • The air sampling cassettes were connected and used according to their manufacturer's protocols for equipment connections, tubing, flow rates, sampling interval, and ambient temperature.
  • On the date of use, the air sampling cassettes from each of the manufacturers were confirmed as not having reached their marked expiration date. (The airborne particle collection rate may change with out-dated air sampling cassettes if the media has become dried.)
  • In the laboratory the samples were prepared using the manufacturer's recommendations.

Some tests were conducted in duplicate, except that a second sample pair was collected after making a single "wave" of a clipboard at surfaces near the sampling equipment to stir up local, previously-settled dust.

Other Air Samplers Operated in Parallel

Some tests included operation of other air sampling devices during the test interval, for other and calibrating data comparisons, including

  • The Burkard personal air sampler (10 lpm, maximum 9 minute sample interval)
  • The Allergenco Time-Lapse impaction sampler
Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman


In our photograph of a sampling cassette test underway in Rhinebeck, NY you can see our Burkard personal air sampler at the left, and two sampling cassettes suspended over a workbench, facing, separated by 36", and placed 18" distant from moldy oriented strand board paneling in a 1930's poured concrete basement.

Indoor and outdoor temperature and relative humidity were also recorded, as well as the usual visual site observations and case history associated with building indoor air quality investigations.

 

 

Air Sampling Cassette Field Testing Procedure

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Our photograph (left) shows air sampling cassette particle trace slides that have been mounted onto microscope slides for both an Air-O-Cel air sampling cassette (top of photo) and an Allergenco-D air sampling cassette (bottom of photo).

The sampling media gel size and thickness appear similar for the two products. The square versus rounded corner sample collection media deposits is visually obvious but makes no practical difference in usage since the particle trace does not extend close to the edges of the media.

 

 

Using duplicate air sampling pumps lab and field calibrated to collect uniform simultaneous air samples, two samplers were operated in parallel, switched on and off by a common timer to sample indoor air particles in indoor locations at seven different buildings in 2005-2006.

The manufacturers' standard particle trace dimensions expected to be produced by each air sampling cassette are provided below.

Nominal air sampling cassette specifications
Bioaerosol Sampling Cassette Trace Specifications Operational Protocol Flow Rates Operational Protocol Time
Allergenco-D® Sampling Cassettes 1.1 mm x 14.4 mm
1.1 mm x 14.0 mm
5-20 lpm
15 lpm standard
1-20 minutes
Air-O-Cell® Sampling Cassettes 1.1 mm x 14.4 mm 15 lpm standard 0.5 - 10 min
  We observed in field practice that trace dimensions vary from nominal depending on dust conditions and sampling interval. See notes at right. Optimal flow rate is 15 lpm. At higher rates expect reduced capture efficiency of smaller particles - may under-represent Pen/Asp for example. Adjust sampling interval for environmental dust conditions. Do not overload slides

Variations in Air Sampling Cassette Particle Trace Dimensions

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel FriedmanThe actual particle trace in an individual air or vacuum dust sample may vary from the official trace dimensions, depending on several variables:

The airflow rate of the sampling pump and its deviation from the manufacturer's recommended flow rate, including any effects of extraneous forces such as use of a long connecting tube between the sampling pump and cassette, effect off battery condition on some battery-operated pumps, duration of the sampling interval, level of airborne dust and debris in the sample area, type and size of airborne particles in the sample area, local air currents in the sample area due to open windows, wind, operation of overhead fans, operation of HVAC equipment, and the orientation of the air sampling cassette with respect to these air currents.

To control for these variables all of our test samples were collected indoors, in still-air conditions (no fans or HVAC equipment operating for at least two hours before the sample collection), windows and doors shut, no people in the sample area other than the experimenter.

Practical & Operating Details for Air-O-Cell and Allergenco-D Air Sampling Cassettes

Practical Considerations in Mounting Air Sampling Cassettes onto Microscope Slides

In each air sampling cassette, a thin glass cover slip has been coated with a patented sample collection media and affixed into position below the air venturi slot in the cassette inlet side. To prepare an air sample for microscopic examination the cassette must be opened by cutting a paper or plastic tape that seals the edges of its mating halves, and the media-coated cover slip, now containing a particle trace, is gently removed and transferred to a microscope slide for mounting, hydrating with an appropriate liquid, application of a larger cover-slip to cover the particle trace, and subsequent analysis.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Our photograph (left) shows air sampling cassette particle trace slides that have been mounted onto microscope slides for both an Air-O-Cel air sampling cassette (top of photo) and an Allergenco-D air sampling cassette (bottom of photo).

The sampling media gel size and thickness appear similar for the two products. The square versus rounded corner sample collection media deposits is visually obvious but makes no practical difference in usage since the particle trace does not extend close to the edges of the media.

(Product Update Note by EMS: Since the manufacture of these Allergenco-D cassettes in 2005, product development at EMS has continued. Nine modifications to improve collection efficiency and/or manufacturing quality control have been implemented, including the elimination of die cut adhesives. Current production uses square cut polymentic adhesive.)

Mounting the Particle Trace Micro-Slide onto a Microscope Slide

When affixing the Air-O-Cel micro-slide onto the microscope slide, while it is possible to allow the trace to "free float" on the receiving microscope slide pending addition of a hydrating solution (typically Triacetin 98%), in order to keep the trace micro-slide squarely positioned on the receiving microscope slide, some practitioners teach the practice of using a small square of clear adhesive tape at each end of the micro-slide to secure it squarely on the microscope slide. (J. Shane, previously at McCrone Research - DF). Below we demonstrate using clear adhesive tape to secure an Air-O-Cell particle trace in place onto the microscope slide.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

On removing each particle trace micro-slide from its cassette we found that a small dab of adhesive remained on the under-side of the Allergenco trace micro-slide. That material assisted in fixing the micro-slide in position onto our standard pre-cleaned microscope slide. Below we demonstrate using the cassette-mounting adhesive left on the back of the micro-slide to secure an Allergenco-D particle trace in place on the microscope slide.

This is easier and faster than the "adhesive tape" method described above. The air space between the under-side of the micro-slide and the microscope slide was not appreciably different between the two approaches, and in either approach, our triacetin fluid could easily be made to flow between the two glass planes where added optical precision was desired.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Choices of Mountant for Air Sampling Cassette Coated Glass Slides

For mounting the coated glass slide made by Allergenco, Tucker recommends (but does not require) preparing the sample with Lactophenol cotton blue stain.

(Note courtesy of EMS: Recently, ASTM Method D7391-09 Standard Test Method for Categorization and Quantification of Airborne Fungal Structures in an Inertial Impaction Sample by Optical Microscopy has been published to standardize the analysis of Inertial Impaction Samplers. This method allows for stains and non-stain optical fluids.)

OPINION: However experienced aerobiologists often prefer very light stains or no stain color at all in order to preserve the natural color of some particles that can aid in their identification. Also we have observed that slides prepared with lactophenol, despite the presence of a phenolic preservative (also a carcinogen), slides often become opaque in long term storage. We prefer using triacetin which works well with this media.

Cassette Sample Preparation Particle Float Warning

Experts also warn that improper placement of mounting fluid on the slide can "wash" away loose particles, causing a significant interference with accuracy of any particle counts attempted.

OPINION: A warning sign that this error has occurred might be the observation of particles and debris that washed to the edge of a microscope slide cover slip, or the observation of many particles floating and moving in the mounting fluid. While quantitative analysis should not be attempted when this error condition has been observed, we did not find that the particle float problem was any more frequent with one of our test sampling cassette brands than the other.

Comparison of Particle Traces on Air-O-Cell and Allergenco-D Air Sampling Cassettes

Below we show a closeup of the air sampling cassette collection media and particle trace for an Air-O-Cell cassette (below left) and an Allergenco-D cassette (below right). These were heavy traces collected for photographic clarity. For actual building air sampling these example traces may have been overloaded.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Particle Trace Length

If actual particle trace dimensions vary much from the manufacturer's standard trace dimensions the effect will be an inaccuracy in computed particle frequency calculations for laboratories that do not count 100% of the particle trace. Even when 100% of a particle trace is counted, variations in microscope and slide preparation, calibration, and human error will produce some variation in actual counts obtained.

An Air-O-Cell particle trace is shown at below-left and an Allergenco-D particle trace is shown at below-right. You can see that the Allergenco-D trace in this sample was 14mm in length and pointed in end-shape, and the Air-O-Cell trace was 13.5mm in length and more square in end-shape.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

OPINION: We observed that variations in particle trace length and width occurred but that they depended on the indoor dust conditions or airborne particle levels in the test area, combined with sampling interval and location far more significantly than any variation in particle trace dimensions between the two sampling cassette brands tested.

Air Sampling Cassette Actual Particle Trace Width in Field Use

Both particle traces were nominally 1mm in width, but the Allergenco-D trace (below-left) appeared to be less uniform near the trace ends and in some of our samples the trace was thinner, and more scattered near its edges than the Air-O-Cell trace in some individual samples. The two samples were collected simultaneously, in the same location, and represent an air sample of 105L obtained at 15 LPM using EMS Megalite pumps. The photograph at below-left has been enlarged more than its companion at right; both show a scale of 1mm between horizontal markings.

From this limited sample we could not conclude whether or not this difference in particle trace always occurs nor whether the determining factor was the cassette brand or other factors. But considering that the samples were collected using identical air flow rates, durations, locations, placements, it would appear that the Air-O-Cell traces were a bit more uniform in our tests of these 2005 product versions. [Results with the current products (2010) may vary from our initial tests.]

Large particle effects on trace capture: We also posed that the presence of individual large particles landing on an individual trace sample can skew the particle capture results by interfering with later arriving small particles. But notice that in both of our slide photographs (below) at least one large fiber landed in the trace. There was no visible difference in the effect of this phenomenon.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

End-Shape of Air Sampling Cassette Particle Trace Patterns

An Air-O-Cell particle trace end shape is shown at below-left and the Allergenco-D particle trace end shape is at below-right. Enlarging these trace patterns it appears that in this example the Air-O-Cell trace pattern end was a bit more square and sharply defined - a result that may vary from sample to sample.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Below are closeup photographs taken in our dissecting microscope of the particle trace ends. At left is the Allergenco-D particle trace and at right the Air-O-Cell particle trace. There is similar particle scatter at the ends of both traces. [The images are not identically magnified in these photos.]

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Air Sampling Cassette Capture Media Consistency

One of our test sample pairs found that the Allergenco-D particle capture media had included voids that caused uneven particle capture across the trace (photo below-left). The Air-O-Cell media performed more consistently (photo below-right). Because of the possibility of a handling or preparation error we did not conclude that this trace inconsistency was a recurrent problem with either device. Further testing or reports of independent study of this question are needed.

Note, courtesy of EMS: An academic study by Grinshpun et alhas been published (see our references below) and found both in laboratory and field experiments that the Allergenco-D cassette exhibited better collection on smaller spores and more consistent results than the Air-O-Cell Cassettes.

Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman Air-O-Cell and Allergenco-D cassette Trace Comparison (C) Daniel Friedman

Air Sampling Cassettes and Media Shelf Life

Spore traps (C) Daniel Friedman

Watch out: be sure to note and comply with the expiration date on these or any other biological or particle sampling supplies such as air sampling cassettes.

Because the particle-adhering media on the micro slides in air sampling cassettes may become dry with age, its ability to capture particles at its stated efficiency rate can be compromised, leading to significant errors in characterization of the level of particles in the air sample if you use the device after its expiration date.

The expiration date of the Allergenco-D sampling cassette is visible in our photograph (left)

Air-O-Cell recalled air sampling cassettes manufactured between July and December 2008 due to a change in the plastic housing, a related media dryout, and therefore an error in the anticipated expiration date of those particular products.

Concluding Comments on Air Sampling Cassette Consistency & Performance

It appears that both of these air or dust and particle sampling cassettes perform very well in the field and in the laboratory. The variations in quantitative analysis results such as spore counts between air samples appear to be dominated by factors other than cassette design or coated slide collection media.

This article is the first of two discussions on variability in air sampling cassette test results and particle trace patterns. This article describes and present photographs of measurements of particle traces produced by the Air-O-Cell® and Allergenco-D® sampling cassettes when air samples using the two cassettes were collected simultaneously. A separate quantitative analysis reporting and comparing actual particle counts forms part two of this topic and remains to be completed. Those results will also be provided here.

Watch out: OPINION: for marketing and legal reasons most aerobiology and mold test laboratories are willing to provide very precise spore-counts or particle counts obtained by partial or full particle trace analysis of air samples.

But as we explain at Causes of Variation in Airborne Particle Levels, the variability of indoor air sample particle capture rates is enormous, at least several orders of magnitude, depending on factors that are virtually never managed, nor even recorded by most field investigators. As a result, indoor airborne particle counts may be very precise (11,248 X-spores per liter of air) but very inaccurate or even dead wrong (off by three orders of magnitude from what occupants actually experience as their personal exposure). Do not confuse measurement precision with measurement accuracy.

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