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Related Information: Chemical Sampling - Formaldehyde


   
Date: June 2004 Mary Eide
   
Methods Development Team
Industrial Hygiene Chemistry Division
OSHA Salt Lake Technical Center
Sandy UT 84070-6406

 

Introduction

The purpose of this study was to determine the sampling rate variation (SRV) for the Assay Technology ChemDisk 571 Aldehyde Monitor. These samplers are intended by the manufacturer for use to measure the amount of formaldehyde, and possibly other aldehydes, present in workplace air. The sampler uses 2,4-dinitrophenylhydrazine (DNPH) chemistry to produce a stable aldehyde derivative.

SRV has been established by OSHA as a measure of sampling rate error for diffusive samplers.1 SRV is the diffusive sampler equivalent of the often cited 5% sampling pump error used for active samplers. It is a unique number that is experimentally determined for each individual design of diffusive sampler because SRV is presumed to be a function of sampler design. SRV provides the sampling error component for Sampling and Analytical Error (SAE) calculations.2

SRV has been defined as the pooled relative standard deviation of sampling rates obtained in a modified version of the 16-run factor test described in the NIOSH testing protocol for diffusive samplers.3 This test is based on determination of diffusive sampling rates of test atmospheres containing five different aldehydes. The test requires sample collection from 16 different combinations of high and low analyte concentrations, short and long sampling times, high and low face velocities, high and low relative humidities, and parallel and perpendicular sampler orientations to air flow direction in a sampling chamber.

The formaldehyde atmosphere was generated from a solution of formaldehyde in water freshly prepared from paraformaldehyde. Most commercial preparations of formaldehyde are stabilized with methyl alcohol, but the methyl alcohol reacts with the formaldehyde forming methoxymethanol and dimethoxymethane (non-formaldehyde species).4 These non-formaldehyde species are unstable and readily decompose back to formaldehyde and methanol.5 These nonformaldehyde species readily react with the derivatizing agent, whether DNPH or 2-(hydroxymethyl)piperidine, to form the formaldehyde derivative. The paraformaldehyde test atmospheres, including formaldehyde water solution, contain mostly formaldehyde, while the atmospheres produced from formaldehyde that has been stabilized with methyl alcohol contain formaldehyde and non-formaldehyde species. The non-formaldehyde species are of higher molecular weight than formaldehyde and, therefore, they have different sampling rates. This difference in sampling rates causes lower loadings of formaldehyde derivative on the passive samplers when compared to the active samplers taken from the same test atmosphere. This difference could result in formaldehyde results for the diffusive sampler which are as much as 35% lower than the active samplers.6 Diffusive samplers, with sampling rates determined using test atmospheres prepared from paraformaldehyde, give analytical results similar to active samplers, when sampling atmospheres that do not contain the non-formaldehyde species. This study employed formaldehyde test atmospheres generated with formaldehyde solution prepared from paraformaldehyde to establish sampling rates for ChemDisk-Al. Therefore, sampling rates shown here are for formaldehyde alone. Formaldehyde atmospheres in the workplace could result from formaldehyde stabilized with methyl alcohol, and would contain the non-formaldehyde species, causing diffusive sampler formaldehyde results that would be lower than active sampler results, if the samples were taken side-by-side.

Reagents

Acetaldehyde, Aldrich Chemical Company, 99.5+%, lot CO 02962AO
Benzaldehyde, Aldrich Chemical Company, 99.5+%, lot 00208TI
Butyraldehyde, Aldrich Chemical Company, 99.5+%, lot BO 03519DI
Glutaraldehyde, Aldrich Chemical Company, 50% in water, lot 01907 CI
Paraformaldehyde, Aldrich Chemical Company, 95+%, lot 08710 AA
Acetonitrile, Fisher Chemical Company, 99.9%, lot 031027
Phosphoric acid, JT Baker, Baker-analyzed, 85.9%, lot D25821
2,4-Dinitrophenylhydrazine (DNPH), Aldrich Chemical Company, lot 7627JK (DNPH is light sensitive, so all solutions and samples should be protected from the light in light-impervious containers.)
Toluene, Alfa-Aesar, 99.8%, lot K06M13
N,N-Dimethylformamide (DMF), Aldrich Chemical Company, 99.8%, lot 04643LA

A freshly prepared solution of formaldehyde in water was prepared by heating paraformaldehyde at 80 C, and bubbling the vapor through deionized water. These mixtures were quantitated by titration7 before use in the vapor generation system.

Two different neat aldehyde mixtures were prepared. The first mixture was 1:1 (by volume) ratio of butyraldehyde:benzaldehyde, and the second mixture was 1:1:0.1 (by volume) of the formaldehyde solution:acetaldehyde:glutaraldehyde. These mixtures were used to generate test atmospheres and to prepare standards.

DNPH extracting solution for extracting ChemDisk-Al and DNPH coated glass fiber filters. The solution was composed of 1-g DNPH and 5-mL phosphoric acid in 1-L acetonitrile. The same solution was used to prepare analytical standards. The DNPH was purified by recrystalation with hot acetonitrile.

DNPH glass fiber filter coating solution. The solution was composed of 4-g DNPH and 20-mL phosphoric acid in 1-L acetonitrile. The DNPH used was recrystalized from hot acetonitrile.

Adsorbent tube extracting solution. The solution was composed of 0.2 µL/mL DMF in toluene.

Sampling Media

Assay Technology ChemDisk 571 Aldehyde Monitor, lot 571AT1D03, containing a glass fiber filter coated with DNPH and phosphoric acid.

SKC 226-117 and 226-54 sampling tubes, lot 2952, containing XAD-2 coated with 10% (w/w) 2-(hydroxymethyl)piperidine (HMP XAD-2). These sampling tubes were packed with the same adsorbent, but contained differing amounts of the coated resin in the tubes, and thus have different amounts of formaldehyde background. The 226-54 has two sections containing 45- and 23-mg coated resin, and is used for short-term sampling. The 226-117 has two sections containing 150- and 75-mg coated resin, and is used for long-term samples. These sampling tubes were used to establish the concentrations of acetaldehyde, butyraldehyde, and formaldehyde in test atmospheres.

Glass fiber filters coated with 2-mg DNPH and 10-µL phosphoric acid (DNPH GFF). The cassette was loaded with three coated filters, with a spacer between each filter, and an extra spacer on the top to emulate open face sampling. These filters were used to establish the benzaldehyde and glutaraldehyde concentrations in the test atmospheres. The DNPH used was recrystalized from hot acetonitrile. The DNPH coated GFF were prepared by placing glass fiber filters on a clean glass plate and pipetting 0.5 mL of a solution of 4-g/L DNPH and 20-mL/L phosphoric acid in acetonitrile.8 The filters were allowed to dry 20 minutes in a hood, then they were placed in a light impervious container (brown glass jar) loosely sealed with a lid, allowed to dry completely overnight in a drawer, and then the lid was tightly sealed and the jar was placed into a freezer for storage.

Apparatus

Shaker. An Eberbach shaker was used to extract the adsorbent tubes.

Rotator. A Fisher Roto Rack rotator was used to extract the ChemDisk-Al and DNPH GFF samples.

Gas chromatograph (GC) with a nitrogen-phosphorus detector. An Agilent 6890 gas chromatograph with a 7683 injector, and 3396 Series II integrator was used for analysis of HMP XAD-2 samplers. Separations were performed using a Restek Stabilwax DB capillary column (60-meter x 0.32-mm x 1-µm df).

An electronic integrator or some other suitable means of measuring peak areas. A Waters Millennium32 Data System was used in this evaluation.

A liquid chromatograph equipped with a UV detector. A Waters 600 Controller and pump, a Waters 2487 Dual wavelength absorbance Detector, and a Waters 717 plus Autosampler was used for analysis of ChemDisk-Al and DNPH GFF samples. A 4.6- x 250-mm column packed with 5-µm Pinnacle TO-11 (Bellefonte, PA) was used in this evaluation.

Humid air generator. A Miller-Nelson Model HCS-401 Flow-Temperature-Humidity Control System was used to generate humid for use with controlled test atmospheres. This instrument was equipped with a 500 L/min mass flow controller.

Relative humidity and temperature tester. An Omega Digital Thermo-hygrometer Model RH411 was used to determine the relative humidity and temperature of the test atmospheres within the exposure chamber. The probe was calibrated by the manufacturer.

Gas test meter. An Equimeter no. 750 gas meter was used to measure dilution flow rates. This meter had been checked at several flows against a Singer DTM 115 gas meter (that had been tested by the local natural gas distributor and found to be accurate).

  Figure 1. This is a diagram of the test atmosphere generation and sampling apparatus. The air stream of a known flow and humidity is introduced into the apparatus from the Miller Nelson Flow-Temperature-Humidity Control System. The aldehyde mixtures come from the ISCO syringe pumps and are teed into the air stream. The stream is heated to vaporize the aldehydes. The air and aldehydes flow into a glass mixing chamber to form a homogeneous test atmosphere. This test atmosphere then flows to the exposure chamber. The exposure chamber is large enough for the diffusive samplers to fit inside, and has side ports from which active samplers can be taken. The test atmosphere then flows out of the exposure chamber into the exhaust.

Figure 1. This is a diagram of the test atmosphere generation and sampling apparatus. The air stream of a known flow and humidity is introduced into the apparatus from the Miller Nelson Flow-Temperature-Humidity Control System. The aldehyde mixtures come from the ISCO syringe pumps and are teed into the air stream. The stream is heated to vaporize the aldehydes. The air and aldehydes flow into a glass mixing chamber to form a homogeneous test atmosphere. This test atmosphere then flows to the exposure chamber. The exposure chamber is large enough for the diffusive samplers to fit inside, and has side ports from which active samplers can be taken. The test atmosphere then flows out of the exposure chamber into the exhaust.

Syringe pumps. The two aldehyde mixtures were metered into the system using two Isco 100DM syringe pumps equipped with a cooling/heating jacket and an insulating cover. Both pumps were operated in the constant flow mode. The temperature of water in the cooling jacket was maintained at 19 C with a Forma Scientific Model RH411 Bath and Circulator.

The chemical vapors were generated by pumping the two aldehyde mixtures through a short length of 0.53-mm uncoated fused silica capillary tubing into a vapor generator where they were heated and evaporated into the dilution air stream (Figure 1). The entire apparatus was placed in a walk-in hood. The glass vapor generator consisted of a 15-cm length of 5-cm diameter glass tubing with a side port for introduction of the capillary tubing. The glass tube of the vapor generator was wrapped with heating tape to evaporate the chemicals in the mixture. A Miller Nelson Flow-Temperature-Humidity controller was used to regulate the humidity, temperature, and volume of the dilution stream of air. The test atmosphere passed into a glass mixing chamber (76-cm x 30-cm) from the vapor generator, and then into a glass exposure chamber (76-cm x 20-cm). The humidity and temperature were measured at the exit of the exposure chamber with an Omega Digital Thermo-hygrometer. Face velocities of the test atmospheres were calculated by dividing the volumetric flow of each atmosphere by the cross-sectional area available for the air flow in the exposure chamber. The cross-sectional area available for the air flow was the cross-sectional area of the exposure chamber reduced by the cross-sectional areas of the samplers.

Experimental

Sample Analysis

The HMP coated XAD-2 adsorbent tubes were opened, each section was placed into a separate 2-mL vial, and 1-mL of toluene with 0.2 µL/mL of DMF as the internal standard was pipetted into the vial. The vials were sealed and were placed on a shaker for 1 hour. Standards were prepared by injecting microliter amounts of aldehyde spiking solutions, prepared by diluting both aldehyde mixture solutions, into vials containing the 150 mg of the HMP XAD-2 resin for high standards, and 45 mg for low standards. Standards were prepared with the same amount of coated resin as was contained in the samples. Standard blanks of each amount of resin were also prepared, due to the background amount of formaldehyde present. The standards were allowed to react overnight. The standards were blank corrected before plotting the calibration curve. The standards were extracted in the same manner as the samples.

Adsorbent tubes were analyzed by gas chromatography (GC) using a nitrogen-phosphorus detector. Separations were performed using a Restek Stabilwax DB capillary column (60-meter x 0.32-mm x 1-µm df). The injection volume was 1 µL with a 1:10 split. The GC temperature program was 60 C for 4 min then 7 C/min to 220 C and hold for 2 min. The hydrogen carrier gas was 2.5 mL/min, hydrogen detector gas was 2 mL/min, the nitrogen auxillary gas was 10 mL/min, and the detector air was 60 mL/min. The injector temperature was 220 C and the detector temperature was 260 C.

ChemDisk-Al were opened, the coated glass fiber filter removed and placed into a 4-mL light impervious (amerber) vial, 2-mL of a solution of the DNPH extracting solution was added, and they were capped. They were extracted for 1/2 hour on a Fisher Roto Rack.

DNPH GFF cassettes were opened, each filter placed into separate 4-mL amber vials, and then 2 mL of a solution of the DNPH extraction solution was added, and they were capped. They were extracted for 1/2 hour on a Fisher Roto Rack.

DNPH standards were prepared by injecting microliter amounts of spiking solutions, prepared by diluting both aldehyde mixture solutions, into 4-mL amber vials containing 2 mL of DNPH extracting solution. Standards were allowed to react for 1 hour before analysis.

ChemDisk-Al and DNPH GFF samples were analyzed with a liquid chromatograph equipped with a UV detector. A 4.6- x 250-mm column packed with 5-µm Pinnacle TO-11 was used in the evaluation. The injection volume was 10 µL. The mobile phase was 65:35:0.02 (v:v:v) acetonitrile:water:phosphoric acid pumped at 1 mL/min. The wavelength was 365 nm.

Extraction efficiency

It was not necessary to perform an extraction efficiency study of the aldehydes from HMP XAD-2, as the same medium was spiked with the aldehydes in the mixture to make the analytical standards as was used for spiking.9

The extraction efficiency study of the DNPH GFF was performed by spiking the five aldehydes onto the DNPH GFF, in amber vials, and allowing them to react overnight in a drawer. Six filters at each of six levels were spiked. The loadings studied were: acetaldehyde 1 to 86.4 µg/sample; benzaldehyde 1 to 208.4 µg/sample; butyraldehyde 1 to 141.6 µg/sample; formaldehyde 0.3 to 22 µg/sample; and glutaraldehyde 0.2 to 9.8 µg/sample.

The extraction efficiency study of the ChemDisk-Al was performed by opening the samplers, removing the coated glass fiber filter, placing the coated glass fiber filters in amber vials, spiking the five aldehydes onto the coated glass fiber filter, and allowing them to react overnight in a drawer. Six samplers at each of six levels were spiked. The loadings studied were: acetaldehyde 1 to 43.2 µg/sample; benzaldehyde 1 to 104.2 µg/sample; butyraldehyde 1 to 70.8 µg/sample; formaldehyde 0.3 to 11 µg/sample; and glutaraldehyde 0.2 to 4.9 µg/sample.

Sampling rate and capacity

The sampling rate and capacity of ChemDisk-Al for each of the aldehydes was determined by exposing sets of three passive samplers to an aldehyde test atmosphere for increasing time periods. The test atmosphere contained a target concentration of acetaldehyde (2 ppm or 3.6 mg/m3), benzaldehyde (2 ppm or 8.68 mg/m3), butyraldehyde (2 ppm or 5.9 mg/m3), formaldehyde (0.75 ppm or 0.92 mg/m3) and glutaralehyde (0.2 ppm or 0.8 mg/m3). This test atmosphere level will be referred to as the 1 x level, and the 1/10 of this level as 0.1x level in this study. Unless otherwise noted, the sampler orientation was parallel to the flow direction of the test atmosphere. The average relative humidity, temperature, and face velocity of the test atmospheres, except in the factor tests, were 77%, 30 C, and 0.4 m/s, respectively. Six active samplers were collected with each set of three diffusive samplers. The six active samplers consisted of three cassettes containing DNPH GFF and three HMP XAD-2 tubes. The sampling rates were 100 mL/min for DNPH GFF, and 50 mL/min for HMP XAD-2 tubes.

Reverse diffusion tests were performed by sampling the 1 x concentration for one-half the total sampling time, and then sampling clean humid air for the remainder of the sampling time. Eight passive samplers were exposed for 2 hours, four were removed and analyzed, and then the other four were exposed for 2 hours to clean, humid air and then analyzed. The relative humidity, temperature, and face velocity were 76%, 29 C and 0.4 m/s respectively.

Factor tests

A 16-run factor test was performed using a modified version10 of the NIOSH Factor Test.11 NIOSH has identified six factors that can affect the performance of diffusive samplers: analyte concentration, face velocity, relative humidity, exposure time, interferant, and sampler orientation. Sixty-four experimental runs (26) would be required to fully evaluate combinations of each factor at two levels. NIOSH recognized that this would be an excessive number of tests, and has devised a 16-run fraction of the full factorial that is capable of revealing any of these factors having a significant effect, free of two-factor interactions, on sampler performance. Some two and three-factor interactions can also be screened by this design. The test is based on comparison of each factor effect to experimental error so that the significance of that effect can be determined. Experimental conditions are shown in Table 1. Interferant was provided by the components of the aldehyde mixture. For example if formaldehyde was examined, then acetaldehyde, benzaldehyde, butyraldehyde, and glutaraldehyde were the interferants, and the levels were either high (1x) or low (0.1x).

Table 1
Experimental Design of the Factor Test

run
no.
analyte
concn
RH
(%, C)
inter
level
time
(min)
face vel
(m/s)
sampler
orien

1 0.1x 21,30 low 120 1.9 perp
2 1x 19,29 low 30 0.2 perp
3 0.1x 80,30 low 30 2.0 paral
4 1x 80,30 low 120 0.2 paral
5 0.1x 21,30 high 120 0.2 paral
6 1x 20,30 high 30 1.8 paral
7 0.1x 79,30 high 30 0.2 perp
8 1x 79,30 high 120 1.8 perp
9 1x 80,29 high 30 0.2 paral
10 0.1x 77,30 high 120 1.8 paral
11 1x 20,30 high 120 0.2 perp
12 0.1x 21,31 high 30 1.9 perp
13 1x 77,30 low 30 1.8 perp
14 0.1x 78,30 low 120 0.2 perp
15 1x 21,29 low 120 1.8 paral
16 0.1x 20,29 low 30 0.2 paral

concn = concentration; inter = interfence; face vel = face velocity; orien = orientation; perp = perpendicular; paral = parallel

Results and Discussion

  Table 2
Extraction Efficiency (%)
 
  medium acet benz buty form glut
 
  DNPG GFF 100.0 100.1 100.0 100.0 100.0
  ChemDisk-Al 100.2 100.1 100.0 100.1 100.0
 

where: acet = acetaldehyde; benz = benzaldehyde; buty = butyraldehyde; form = formaldehyde; and glut = glutaraldehyde


Extraction efficiency

A summary of the average extraction efficiencies is in Table 2. The extraction efficiencies were high and constant over the ranges studied.

Sampling rate and capacity

Sampling rates were calculated by dividing mass collected (corrected for extraction efficiency) by sampling time multiplied by the actual concentration of the test atmosphere (sampling rate = µg/(min x µg/L)). Sampling rate, in L/min, was converted to mL/min, the same units often used for adsorbent tubes. Theoretical concentrations were calculated from the test atmosphere generator apparatus operation parameters ((syringe pump rate x concentration in solution) dilution air volume). The actual concentration of the test atmosphere was determined from the active sampler results. The actual test atmosphere concentrations were about 98% of the theoretical concentrations (Table 3)  The selection of active medium used to establish actual test atmosphere concentrations was based on the existence of validated methodology and on technical considerations. Adsorbent tube results were used for acetaldehyde, butyradehyde, and formaldehyde. DNPH GFF results were used for glutaraldehyde and benzaldehyde. The same sampling time was used for the active and passive samplers for each test run. All samples were analyzed as soon as possible after collection. Sampling rates and capacity results are shown in Table 4 and Figure 2. The sampling rates were determined at ambient temperature and pressure and converted to their equivalent at 25 C and 760 mmHg.

Table 3
Sampling Rates and Capacity Test Atmospheres (ppm)

source acet benz buty form glut

theoretical concn 2.13 2.01 2.09 0.76 0.21
DNPH GFF results 1.83 1.98 2.01 0.72 0.20
HMP XAD-2 results 2.11 1.88 2.04 0.75 na

na = not applicable



Table 4
Sampling Rate and Capacity (mL/min)

time (min) acet benz buty form glut

5 8.92 5.23 6.23 12.03 5.46
10 9.22 5.41 6.48 12.29 5.62
15 9.39 5.53 6.69 12.49 5.83
30 9.50 5.64 6.76 12.79 5.91
60 9.69 5.71 6.79 12.91 6.01
120 9.76 5.77 6.81 13.01 6.02
180 9.92 5.84 6.91 13.11 6.06
240 9.90 5.78 6.83 13.09 5.97
360 9.62 5.66 6.80 12.79 5.91
480 9.46 5.53 6.77 12.19 5.79


Figure 2. This is a plot of the data presented in Table 4.

Figure 2. This is a plot of the data presented in Table 4.


  Table 5
Average Sampling Rate (mL/min) and RSD (%)
 
    acet benz buty form glut
 
  30-240 min 9.54 5.61 6.71 12.67 5.86
  RSD 3.26 3.35 3.02 3.10 3.26
  ChemDisk-Al sampling rate 9.77 5.81 6.83 13.05 6.03
 

 

 

The sampling rates were constant from 5 to 480 minutes. The capacity of the sampler for a component is presumed to be exceeded when the apparent sampling rate for that component decreases rapidly. The capacity wasn't exceeded after 8 hours. The test atmosphere contained acetaldehyde at 2 ppm, while the PEL is 100 ppm, so the capacity results for a sampler determined at the PEL will probably be lower than determined in this study. The average sampling rates for 5 to 480 minutes are listed in Table 5, along with their RSD. The ChemDisk-Al sampling rates in Table 5 were obtained from Assay Technology.12

Reverse diffusion occurs when the compound is lost from the sampler following collection. All diffusive samplers have the potential for reverse diffusion. These DNPH aldehyde derivatives are not volatile but loss could occur through other means so this experiment was performed. The recovery was calculated by dividing the average recovery after 4 hours by the average recovery after 2 hours. The results in Table 6 show that the recoveries for the two sampler sets were similar.

  Table 6
Reverse Diffusion
 
    acet benz buty form glut
 
  recovery (%) 99.4 99.8 100.1 99.7 100.2
 

The effects of increasing face velocity, on the sampling rates, are shown in Figure 3. The most dramatic effects occur at low to medium velocities. The overall effect is similar to the ones observed for 3M 3520 OVMs13, SKC 575-002 passive samplers.14






Figure 3. This is the plot of the face velocity (m/s)versus sampling rate (mL/min) for the five aldehydes studied. The points on the graph for acetaldehyde are: 0.2 m/s, 8.58 mL/min; 0.3 m/s, 9.06 mL/min; 0.5 m/s, 9.72 mL/min; 1.0 m/s, 9.97 mL/min; and 1.8 m/s, 10.66 mL/min. The points for benzaldehyde are: 0.2 m/s, 5.14 mL/min; 0.3 m/s, 5.37 mL/min; 0.5 m/s, 5.73 mL/min; 1.0 m/s, 5.94 mL/min; and 1.8 m/s, 6.38 mL/min. The points for butyraldehyde are: 0.2 m/s, 6.18 mL/min; 0.3 m/s, 6.35 mL/min; 0.5 m/s, 6.81 mL/min; 1.0 m/s, 6.99 mL/min; and 1.8 m/s, 7.54 mL/min. The points for formaldehyde are: 0.2 m/s, 12.55 mL/min; 0.3 m/s 12.82 mL/min; 0.5 m/s, 13.15 mL/min; 1.0 m/s 13.77 mL/min; and 1.8 m/s, 14.33 mL/min. The points for glutaraldehyde are: 0.2 m/s, 5.49 mL/min; 0.3 m/s, 5.78 mL/min; 0.5 m/s, 6.06 mL/min; 1.0 
m/s, 6.31 mL/min; and 1.8 m/s, 6.62 mL/min.

Figure 3. This is the plot of the face velocity (m/s) versus sampling rate (mL/min) for the five aldehydes studied. The points on the graph for acetaldehyde are: 0.2 m/s, 8.58 mL/min; 0.3 m/s, 9.06 mL/min; 0.5 m/s, 9.72 mL/min; 1.0 m/s, 9.97 mL/min; and 1.8 m/s, 10.66 mL/min. The points for benzaldehyde are: 0.2 m/s, 5.14 mL/min; 0.3 m/s, 5.37 mL/min; 0.5 m/s, 5.73 mL/min; 1.0 m/s, 5.94 mL/min; and 1.8 m/s, 6.38 mL/min. The points for butyraldehyde are: 0.2 m/s, 6.18 mL/min; 0.3 m/s, 6.35 mL/min; 0.5 m/s, 6.81 mL/min; 1.0 m/s, 6.99 mL/min; and 1.8 m/s, 7.54 mL/min. The points for formaldehyde are: 0.2 m/s, 12.55 mL/min; 0.3 m/s 12.82 mL/min; 0.5 m/s, 13.15 mL/min; 1.0 m/s 13.77 mL/min; and 1.8 m/s, 14.33 mL/min. The points for glutaraldehyde are: 0.2 m/s, 5.49 mL/min; 0.3 m/s, 5.78 mL/min; 0.5 m/s, 6.06 mL/min; 1.0 m/s, 6.31 mL/min; and 1.8 m/s, 6.62 mL/min.

Factor Test

The results of the factor test are presented in Table 7. The sampling rates were determined at ambient temperatures, but are expressed at 25 C and 760 mm Hg.

Table 7
Factor Test Results (mL/min)

test acet benz buty form glut

1 10.78 6.39 7.49 14.98 6.85
2 9.15 5.41 6.48 12.61 5.71
3 10.87 6.42 7.62 15.08 6.83
4 9.55 5.64 6.74 13.01 5.78
5 9.32 5.43 6.59 12.90 5.75
6 10.79 6.43 7.64 14.38 6.79
7 9.19 5.58 6.82 12.69 5.94
8 10.93 6.56 7.49 15.05 6.84
9 9.88 5.81 6.81 13.35 5.97
10 10.83 6.27 7.62 14.99 6.84
11 9.42 5.58 6.85 13.13 5.91
12 10.64 6.15 7.45 13.11 6.05
13 10.91 6.54 7.77 15.01 6.83
14 9.61 5.54 6.75 13.22 5.95
15 10.77 6.41 7.68 14.93 6.96
16 9.10 5.39 6.43 13.00 5.76

  Table 8
Percent RSDs of Average Sampling Rates
 
    acet benz buty form glut
 
  ave (mL/min) 10.10 5.97 7.14 13.84 6.30
  RSD (%) 7.46 7.67 8.04 7.28 8.11
  pooled RSD (%)     7.71    
 
 
Table 9
Analysis of Factor Test Data
 
    acet benz buty form glut
 
  error (mL/min) 0.08 0.05 0.09 0.22 0.11
  MSE (mL/min) 0.18 0.11 0.20 0.49 0.24
 
 
Table 10
Analysis of Factor Data Effect Results
 
    acet benz buty form glut
 
  concn 0.72 1.38 0.60 0.38 0.42
  RH 1.22 1.33 0.88 0.85 0.62
  interfer 0.18 0.08 0.29 0.57 0.30
  time 0.46 0.10 0.37 0.75 0.52
  face vel 7.65 7.72 5.34 3.44 3.72
  orient 0.33 0.06 0.26 0.47 0.31
  interaction none none none none none
 
  none = none significant
 
 
Table 11
Precision Data (% RSD)
 
    acet benz buty form glut
 
  HMP XAD-2 2.6 4.8 2.2 2.5 na
  DNPH GFF 5.2 2.1 3.3 4.1 2.4
  ChemDisk-Al 3.2 2.8 2.6 3.0 2.7
 
  na = not applicable
 
 
Table 12
Package Integrity Test
 
    acet benz buty form glut
 
  found (µg) 0.71 <DL <DL <DL <DL
  blank (µg) <DL <DL <DL <DL <DL
  DL 0.1 0.1 0.1 0.1 0.1
 
  DL = detection limit
 

Average sampling rates and their percent RSDs are shown in Table 8. These percent RSDs were found to be homogenous by the Cochran Test.15 The pooled percent RSD, 7.71%, is the SRV for ChemDisk-Al as determined by this work.

The data in Table 7 was further analyzed to detect factor effects, following the NIOSH protocol for diffusive samplers.16 The minimum significant effect (MSE) was calculated for each aldehyde by multiplying experimental error of the factor test by the appropriate t statistic for the nine degrees of freedom (2.26 is the t statistic at the 95% confidence level for nine degrees of freedom). The MSE for each aldehyde is found in Table 9. The analysis gave a numerical factor effect result for each aldehyde component for each of the seven factors. The absolute value of the normalized ratio of effect/MSE is shown in Table 10. Any ratio above 1 is significant at the 95% confidence level, and that effect should be studied further in additional experiments. The results for interferant may be somewhat equivocal because the remainder of the mixture was considered the interference, such as acetaldehyde, benzaldehyde, butyraldehyde, and glutaraldehyde were interferants for formaldehyde. Face velocity had the most significant effect on the sampling rates.

Precision

The pooled relative standard deviations for sets of 3 sample results for each meduium were calculated for each aldehyde from the factor tests. The precision data of the diffusive and active samplers for each component were comparable. While data is presented in Table 11 for both types of active samplers, the active sampler for acetaldehyde, butyraldehyde, and formaldehyde was HMP coated XAD-2 tubes, and the active sampler for benzaldehyde and glutaraldehyde was DNPH coated glass fiber filters.

Package integrity

Formaldehyde and acetaldehyde are common components in the air, especially in urban areas, as they are natural by-products of combustion engines. The integrity of unopened ChemDisk-Al (lot 571AT1D03) was checked by placing them in the exposure chamber, for 100 hours, while the factor tests were performed. The results in Table 12 are not blank corrected.

Conclusions

The sampling rate variation for ChemDisk-Al is 7.71% as determined by this work. Sampling rate variation is a function of the design of diffusive samplers, and is not dependent on the sorbent inside, or the chemical tested. This sampling rate variation may be used for other chemicals collected using this diffusive sampler under conditions that approximate conditions of these tests. This sampling rate variation would also apply if a different sorbent was placed inside this design of diffusive sampler for use with other chemicals.

Sampling rate variation is used by OSHA as the sampling error component of the SAE (Sampling and Analytical Error).17 The analytical error component is periodically updated from the analysis of quality control samples. Each analyte will have a unique SAE.

References

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