by John Stone
(Reproduced from "Environmental Lab", February/March 1993)
Noncurrent editions of Standard methods require that the glass fiber filter used for solids determinations be prepared by rinsing repeatedly with distilled or deionized waters oven drying, cooling, and then weighing to the nearest 0.1 milligram. ProWeigh Filters. a new product designed to eliminate the labor and time involved with performing a TSS and preparing the filter, meet all preparation requirements found in Standard Methods for methods 2540D and 2540E.
This product is novel in that it is simply a machine-produced duplication of the tasks necessary for TSS, and not a new test method. Because the product is a replication of present laboratory efforts. only routine blank testing is required and no equivalency testing need be done. ProWeigh meets all EPA requirements for a TSS filter, thereby meeting with EPA approval.
MANUFACTURING PROCESS
ProWeighs are washed with three aliquots of deionized water while undergoing vacuum filtration. This step removes any loose fibers to provide consistent blanks. After washing the filters move through a balling procedure where they are oven dried at 105°C for 90 minutes in a horizontal flow mechanical convection oven, and then transferred IO a dessicating cabinet to cool .
All weighings are performed in a special IOA humidity 1()011] to minimize any weight variation. Balances are computer interfaced and the filter weights are automatically printed on heat-resistant Mylar® labels. The labels are automatically affixed to the aluminum planchet. As a continuing quality check, 8 percent of all filters are redried and reweighed. This assures filter weight stability and reproducibility.
USING THE FILTERS
To actually perform a TSS sample evaluation, the analyst takes the filter from its aluminum dish, vacuums the sample through it and then returns it to the oven for final drying. Upon reweighing the filter, the initial filter weight listed on the Mylar® label is then rechecked.
To address a frequently voiced concern regarding this product, the initial recorded weight attached to the planchet in which the filter is carried will not be affected by absorbed moisture. The manufacturer's initial recorded weight is noted when the filter is completely dried and dessicated, and the testing laboratory's final weighing is performed after drying and dessicating, so all interim air moisture absorbed by the filter while in transit or on the stock shelf of the laboratory is a nonfactor.
In addition, the use of different balances should not affect accuracy. Balances must be calibrated routinely to ensure correct results. The manufacturer's balances are repeatedly certified with NIST weights to meet with NIST standards, and are calibrated internally several times each working day.
The following quality control results were obtained by the City of Orlando, Fla. The data shows the results of testing one filter per day for 90 days as a quality control measure. Each filter, weighing approximately 115mg, was washed with 100ml of deionized water and baked at I W°C for three hours. The filter was then weighed, and this weight was compared to the original manufacturer's weight. The graph (Figure I) plots the differences between these measurements. All of the filters weighed within +/-0.3mg of the initial prepared weight, well within the limits set by Standard Methods #2540D & E of +/-0.5mg.
City of Orlando QC data:
Number of Filters Checked: 90
Standard Deviation: 0.119717mg
Variance: 0.014332mg
Average: 0.077778mg
Zero runs up or down the length of >l=7.
Data point #60 registers outside 3 standard deviations.
Only one run above or below the average >7, beginning with point #24.
PRODUCT ADVANTAGES
ProWeigh is a standard 47mm filter that is used with any common 47mm filter funnel. Crucible users who do not use a filter funnel for TSS have found three distinct advantages in changing to this product:
- Most crucible filters measure less than 25mm in diameter, so thick, sludgy samples filter three to four times quicker because of the vastly increased surface area.
- Solids routinely adhere to the sides of crucibles and are then baked on. Great difficulty is encountered in scouring and scrubbing afterwards. With ProWeigh, little washing is required because solids adhering to the sides of the filter flannel are washed onto the filter with a squirt bottle of lab water.
- Finally 99 percent of all labs using crucibles already possess at least one 47mm filter funnel of some design so no new equipment purchase is required to chance over to this product. It is important however for crucible users to fully understand that only the filter will be weighed when using ProWeigh as the crucible method requires that both the filter and crucible be weighed in tandem.
Current users including Florida Power & Light, the City of Orlando, BP of America. Tuscaloosa Testing Laboratories, Martel Laboratories, Amerada Hess Corp., the U.S. EPA and East Texas Testing have found that the little variation in actual cost cannot take into account improved accuracy and convenience. Private laboratories enjoy the benefit of half the turnaround time on a TSS sample. Also, because TSS analysis routinely sells for $8.00 to $16.00 and is, therefore, generally of low value to the laboratory, the technician can perform additional and more valuable tasks.
When the filter cost was added to actual labor cost, the City of Orlando found that with ProWeigh it was able to save about 10 percent in the total cost of performing a single TSS. And with over 20 TSS's performed each day in this busy municipal lab an extra 30 minutes per day is now productively redirected to the performance of other pressing requirements.
APPLICATIONS FOR FIXED OR VOLATILE SOLIDS
ProWeighs are also available for fixed or volatile solids. After rinsing, these filters are heated in a muffle furnace to 550°C to drive off any potential volatiles present on the glass fiber. Then. as with all other ProWeigh products. They are dessicated to balance temperature, weighed to the nearest 0.1mg and quality checked for accuracy.
Also available are DoubleWeigh Filters. which are 100 percent redried and reweighed to meet the performance requirements of several states and some individual laboratories.
ProWeigh is available direct from the manufacturer. Environmental Express. 443 Long Point Rd. Suite C, Mt. Pleasant. SC 29464: (800) 343-5319 or (803) 881 -6560.
Mr. Stone is Vice-President of Environmental Express, Ltd., 490Wando Park Blvd., Mt. Pleasant, SC 29464, U.S.A.; tel.: 803-881-6560;fax: 803-881-3964.
