Recent increased use of Personal (diffusive) Monitors by industrial hygienists is due to their efficiency versus other methods. The need for efficiency has been driven by a growing realization of the need to acquire adequate statistical data demonstrating compliance with PELs and STELs.
Personal Monitoring devices for chemicals can be divided into two groups:
(a) "Reactive" Monitors which form a non-volatile derivative from the collected chemical, and
(b) "Sorbent" Monitors which "sorb" the analyte but do not modify it chemically.
"Reactive" Personal Monitors which sample by diffusion and "trap" the analyte chemically work well in a variety of situations and are widely accepted because they avoid reverse diffusion (analogous to "breakthrough" in tube sampling) by forming a non-volatile derivative in the Monitor. Aldehydes, being reactive, can be collected on a variety of media which has been impregnated with a reagent system.
DESIGN CONSIDERATIONS FOR AN ALDEHYDE MONITOR
Below are listed the Design Goals for the Aldehyde Monitor and how each has been addressed in the development of the ChemDiskÔ Aldehyde Monitor.
Sampling Grid provides fixed Diffusion Layer to provide reliable Sampling Rate... The Sampling Grid previously designed for the Organic Vapor Monitor (and known to provide unit-to-unit variation of better than 3%) has been employed. This Grid, of injection-molded polypropylene, has been shown to be dimensionally reproducible, non-contaminating, inert, and non-sorbing with respect to a variety of organics.
Reagent System reacts completely with Sample to form a single Aldehyde Derivative... For a century, the reagent 2,4-dinitrophenylhydrazine (DNP) has been used for definitive characterization of aldehydes through the melting points and infra-red spectra of their crystalline 2,4-DNP derivatives. Electrochemical, spectrophotometric, and HPLC methods for analysis of aldehyde-DNP derivatives have also been described.
In 1979, Jan-Olof Levin, of the National Board of Occupational Safety and Health (Umea, Sweden), described an aldehyde air sampling method utilizing DNP-treated XAD-2 media. An improved version was published, followed by a definitive method utilizing Glass Fiber Filters treated with DNP usable as collection media for either active or passive sampling of aldehydes. A fourth peer-reviewed article by Levin described application of this sampling and analytical method to glutaraldehyde.
In 1987, EPA issued Method TO-11, utilizing a DNP-treated silica gel cartridge with an analytical procedure (HPLC) similar to Levin's. In the same year OSHA issued its Method 64, which described sampling of glutaraldehyde using DNP-treated Glass Fiber Filters, a method directly based on Levin's work.
Experience with DNP-treated media (included herein) has been characterized by blank values typically at 0.1 ug or less coupled with rapid reaction times which permit the use of a wide variety of passive and active samplers operating over a wide range of sampling rates with lower detection limits and less likelihood of breakthrough.
PRIOR USE DNP REAGENT
in VALIDATED ALDEHYDE SAMPLING METHODS
|J.-O. Levin et al.
Chemosphere 1979, 8, 823-827
|DNP on XAD-2||Formaldehyde||HPLC of DNP Derivs|
|J.-O. Levin et al., Scan. J. Work Environ. Health 1981, 53, 168-171||DNP on XAD-2||Forrmaldehyde||HPLC of DNP Derivs|
|J. -O., Levin et al., Anal. Chem. 1985, 57, 1032-1035.||DNP on GFF||Aldehydes||HPLC of DNP Derivs|
|J. -O. Levin et al., Chemosphere 1981, 10, 275-280.||DNP on GFF||Glutaraldehyde||HPLC of DNP Derivs|
|EPA TO-11||Sulfuric Acid||15 Aldehydes||HPLC of DNP Derivs|
|OSHA Method 64||DNP on GFF||Glutaraldehyde||HPLC of DNP Dervis|
Low Background Blank for System and Reagent to enable low detection limits... Ultimate sensitivity in an analytical system is a product of the quantity of sample collected and the inherent sensitivity of the analytical method used. The high sensitivity of the HPLC analysis with UV-visible detector in the vicinity of 355 nm balances the moderate sampling rates available with diffusive samplers and, overall, provides a method capable of detecting less than 0.01 ppm of most aldehyds for an 8-hr sample.
Special Process Controls to Obtain Low Background Blank... Since formaldehyde is ubiquitous in the environment, every laboratory has experienced problems with high formaldehyde blanks arising from the presence of formaldehyde in air, water, building materials, paper, plastics shampoo, etc.
Accordingly, each material in contact with product inside the hermetically-sealed pouch (including the pouch itself) has been tested for its contribution to the background blank. In addition, key components are heat-treated during manufacture to further minimize any accumulation of aldehydes.
The reagent-treated wafer on which formaldehyde is collected has been prepared from an inert fiberglass shown to be superior to paper with respect to both its inertness and its lower formaldehyde content.
To minimize formaldehyde and enhance shelf life, the process of treating and packaging the wafers has been designed to be carried out entirely in an environment of nitrogen which serves to minimize pickup of ambient formaldehyde and to preserve the reagent system from oxidation.
SAMPLE CAPACITY OF CHEM DISKÔ MONITOR
Sample capacity is determined largely by the quantity of DNPH Reagent System present in the reagent-treated Wafer which collects the aldehyde sample. A quantity of DNPH Reagent per Monitor is present which is capable of reacting with more than 3 umoles of aldehyde (e.g. equivalent to more than 100 ug of formaldehyde). Taking the Sampling Rate for formaldehyde into account, the Monitor has the capacity to sample a formaldehyde exposure in excess of 80 ppm-hour (equivalent to 10 ppm for an 8-hr sampling time). A Linear Relationship between Formaldehyde Exposure Level (ppm) and formaldehyde (ug) collected has been shown for formaldehyde levels in the range of 0.1-30 ppm-hr.
DE-SORPTION EFFICIENCY & RECOVERY
Forward Method - A DNPH-impregnated wafer from a ChemDisk Monitor was spiked with a known quantity of Aldehyde using a microsyringe, then placed into a glass vial which was sealed. After equilibration, a measured volume of solvent (acetonitrile) was added. A control containing an identical quantity of Analyte in Acetonitrile was treated in a parallel manner.
Calculation of De-Sorption Efficiency - De-Sorption Efficiency (DE) was calculated as follows.
De-Sorption Efficiency (DE)
[Analyte Found (ug), DNPH Wafer]
[Analyte Found (ug), Control ]
De-sorption efficiency (i.e., % recovery) in the range 98-102% were found in all studies.
DEMONSTRATION OF SAMPLING RATE AND PERFORMANCE
Exposures were performed as described in the Exposure Challenge method included in this report. In this method, aldehyde levels were continuously generated in an inert chamber in which the environment was continuously re-circulated through a Sampling Tunnel containing Monitors to be tested. Air Samples were drawn continuously from the vicinity of the Monitors and conveyed to external twin impingers which were subsequently analyzed for formaldehyde via NIOSH 3500 (chromotropic acid method).
Similar methodology was employed by the Wisconsin Occupational Health Service in its independent study for which results are also included in this report.
Linear Regression Plots of four distinct studies performed during 1992-1993 at Assay Technology and at the Wisconsin Occupational Health Service (Madison, WI) are included in this report. A summary of these results is given below.
(1) Formaldehyde levels ranging from 0.1 to 13 ppm with exposure times ranging from 15 min to 24 hours were monitored with all values in the four studies clustering around a single regression line.
(2) The linear response of the monitor over a wide dynamic range with low background demonstrates Dosimetric Performance capable of effectively monitoring 15-minutes STELs (0.2-5ppm), 8-hr TWAs (0.01-3ppm), or 24-hr Indoor Air Quality tests (below 0.01 ppm).
(3) The Sampling Rate was determined from Least Squares Linear Regression analysis of the data shown in the included graphs. The Slope of the plot of Formaldehyde Exposure in ppm-hr (referenced to the NIOSH 3500 Method) versus Formaldehyde Found (ug) in the Monitor yielded the Sampling Rate when the Slope was multiplied by the molar volume, divided by the molecular weight, and appropriate unit conversion factors were applied.
(4) A formal Sampling Rate value was taken from data of studies performed in 1993 representing the most current manufacturing and analytical methodology. When the 1993 regression line was compared to 1992 data (including the Wisconsin Study) (see Figures), no significant differences were observed indicating that the Sampling Rate is stable with respect to manufacturing variables.
HUMIDITY CHALLENGE TEST
Diffusion rates and sampling rates of vapors in air have been shown in previous studies to be unaffected by humidity variations provided that moisture levels encountered do not create aerosols or react chemically with the analyte. Potential humidity effects were evaluated by determining if high humidity exposures led to loss of previously collected aldehyde. Humidity exposed samples showed negligible loss of aldehyde in three days.
REVERSE DIFFUSION CHALLENGE TEST
A specific test was developed by Assay Technology in 1984 to evaluate its first products, ChemChipÔ monitors to ensure that sample was not lost during or after sampling. This test has been applied to ChemDiskÔ Monitors to provide a challenge to determine whether practical sampling capacity has been exceeded in a particular application.
In this test, Monitors are exposed at a level equivalent to exposure at the PEL for eight hours, then allowed to stand under ambient conditions (as if sampling) in an environment containing no analyte.
Under such conditions, reverse diffusion experienced if practical sampling capacity is exceeded would be detected as loss of analyte when the quantity of analyte recovered by a Monitor subjected to this challenge is compared to the quantity recovered from a "control". To pass the Reverse Diffusion challenge, the quantity of Analyte recovered from the "challenge" samples exposed at the PEL must not be more than 10% less than the quantity recovered from controls when experimental error has been taken into account.
Monitors challenged with one day of post-exposure standing in open air (zero formaldehyde level) met the requirements of this test and showed negligible loss of formaldehyde. Monitors which had been re-packaged for return to the lab and challenged for one week also met the requirements of negligible loss of formaldehyde.
The ChemDiskÔ Aldehyde Monitor has been designed to provide accurate monitoring of aldehydes via a combination of a reliable design, a selective and sensitive derivative chemistry, and utmost care exercised in the design and manufacture of monitors so as to ensure (to the extent possible) a formaldehyde-free monitor with very low background.
Appropriate tests have been carried out at Assay Technology and elsewhere to ensure that the design goals have been met. A continuous program of quality control and quality improvement (including quality testing of each lot of Monitors manufactured)is maintained by Assay Technology.
The included Figures (following) show plots of Formaldehyde Exposure (ppm-hr) vs Formaldehyde Found (µg) for all studies performed separately and combined. Each Figure is labeled with the date of the study performed. In each Figure, the Least Squares Regression Line determined from the 1993 studies is compared with the data for that particular study separately and to the data for all studies taken together.