Effects of Sampling Artifacts on

Occupational Samples of Diesel

Particulate Matter

JAMES NOLL* ,† AND M. EILEEN BIRCH ‡ 

Pittsburgh Research Laboratory, U.S. Department of Health

and Human Services, Public Health Service, Centers for

Disease Control and Prevention, National Institute for

Occupational Safety and Health, 626 Cochrans Mill Road,

Pittsburgh, Pennsylvania 15236, and Division of Applied

Research and Technology, U.S. Department of Health and

Human Services, Public Health Service, Centers for Disease

Control and Prevention, National Institute for Occupational

Safety and Health, 4676 Columbia Parkway,

Cincinnati, Ohio 45226

* Corresponding author phone: 412-386-6828; fax: 412-386-4917; 

e-mail: JIN1@cdc.gov. † Pittsburgh Research Laboratory. 

‡ Division of Applied Research and Technology. 

Total carbon (TC)is sometimes used tomeasure or characterize

diesel particulate matter (DPM) in occupational settings

such as underground mines. DPM samples are collected on

quartz fiber filters. When using quartz fiber filters, adsorption of

gas phase organic carbon (OC) has been reported, causing

a positive bias in the particulate TC results (adsorption artifact).

Most of the data on the sampling artifacts and corrections

apply toenvironmentalair sampling,wheresamplesarecollected

at a much higher filter face velocity and the OC concentrations

are generally much lower relative to occupational sampling.

In this study, we investigated the effects of adsorption artifact

on samples from occupational settings. Samples were

collected with and without denuders to determine the amount

of gas phase OC collected and the accuracy of certain

corrections. In underground stone mines, the adsorption artifact

was found to positively bias the particulate TC by greater

than 20% for filter loadings below 25 μg/cm2 TC (8-h time

weighted average ) 262 μg/m3

). The tandem filter correction

reduced the effect of the artifact, as high as 60% of the TC value,

to less than 11% for laboratory data. It also significantly

reduced the effect of the artifact obtained for field samples.

Introduction

Long-term exposure to diesel exhaust is a public health 

concern because diesel particulate matter (DPM) has been 

reported as a potential (1) or probable (2) human carcinogen. 

While environmental exposure is a concern, occupational 

exposure is a greater one because workplace concentrations 

of DPM are generally much higher than environmental (3–8). 

Therefore, it is important to monitor and control workplace 

exposure to DPM, especially for underground miners, which 

can be the highest exposed workforce (4–8). 

Measuring DPM mass is prone to interferences in many 

workplaces, and gravimetric methods are not sensitive 

enough for low-level determinations. Therefore, a surrogate 

is needed to determine exposure to DPM. In many occupational settings, elemental carbon (EC) is used as a 

surrogate because there are no other substantial sources of 

EC besides DPM (5, 8, 9). In some circumstances, when 

interferences to total carbon (TC) measurement are minimal, 

TC is used because it accounts for over 80% of the DPM 

(10, 11). The Mine Safety and Health Administration (MSHA) 

uses both EC and TC for compliance sampling of DPM in 

underground metal/nonmetal mines (8). TC is both measured 

directly and calculated based on EC results and a predetermined EC fraction. If both the calculated and measured 

TC are above the interim limit (350 μg/m3 TC), the sample 

is out of compliance. TC is commonly determined by 

collecting air samples onto quartz filters and analyzing them 

for organic carbon (OC) and EC (TC ) OC + EC) using NIOSH 

Method 5040 (12). Quartz filters are used because of their 

high collection efficiency for particulate matter and resistance 

to the heat generated during the analysis. 

Researchers have reported two anomalies, commonly 

called sampling artifacts, when collecting particulate OC on 

quartz fiber filters. These artifacts do not affect the EC results 

(or the calculated TC from EC results used by MSHA to 

determine compliance measurements), but they cause a 

positive or negative bias in the measurement of particulate 

OC (and TC). Eatough et al. (13, 14) observed loss of 

semivolatile OC from particles during sampling, referred to 

as the “negative” or evaporation artifact. This artifact causes 

an artificially low result for particulate OC (and TC) because 

OC initially present in air as condensed matter is lost through 

evaporation from the filter during sampling. Several studies 

have also demonstrated an artifact that results in overestimation of the true airborne particulate OC due to filter 

adsorption of gas phase OC (15–23). This is considered the 

“positive” or adsorption artifact. OC in the vapor or gas phase 

is present in the air from multiple sources. The quartz fiber 

filter primarily collects airborne particulate matter and allows 

gases and vapors to pass through, but adsorption of some 

gas (vapor) phase OC does occur. Thus, this positive artifact 

causes an artificially high result for particulate OC (and TC) 

because both particulate and gaseous OC are quantified. 

Some approaches that have been used to correct for the 

positive adsorption artifact are the use of tandem quartz 

filter samplers (incorporating two quartz filters in series) and 

the use of an additional sampler containing a quartz filter 

behind a Teflon filter (15–23). When sampling with tandem 

quartz filters, particulate matter is collected only by the first 

filter, while both the first and second filters are exposed to 

and adsorb gas phase OC. For the correction to be effective, 

both filters must be in equilibrium with the sampled airstream, adsorb the same amount of gas phase OC, and not 

have a significant amount of OC loss through evaporation. 

The OC on the second filter can then be subtracted from the 

OC on the first filter to account for the adsorbed gas OC. This 

correction has not always been reported as effective. Several 

studies have reported the tandem filter correction to 

underestimate the adsorption artifact (15–18). However, other 

studies have shown effective correction when using the 

tandem filter method under the right sampling conditions 

(19–22). The efficacy of the correction depended upon 

sampling conditions such as filter face velocity, sampling 

time, loading, and filter media in these studies (19–22). 

If a sampler containing a Teflon and quartz filter is used 

for correction, the Teflon filter collects particulate matter 

but OC gas adsorption is negligible, so only the quartz filter 

beneath it adsorbs gas phase OC. For this correction to be 

effective, the Teflon filter cannot adsorb any gas phase OC 
and the Teflon filter cannot lose particulate OC. Several 

studies have shown the quartz beneath Teflon to have a 

greater OC value than the quartz beneath quartz (tandem 

filter corrections) (15, 18). Some researchers have attributed 

this to the quartz beneath quartz not reaching equilibrium 

with the sampling stream thereby underestimating the 

adsorption artifact (15, 18). Others have attributed this 

phenomenon to the evaporation artifact being more prevalent 

when using a Teflon filter instead of a quartz filter and 

reported the quartz behind Teflon to overestimate the 

adsorption artifact (21). Several studies have shown no 

difference when using either type of correction (20, 22). Use 

of two quartz filters is preferable to Teflon and quartz because 

both the collection and blank filters are in the same sampler. 

Currently, there is very little information on sampling 

artifacts in occupational settings, and it is not well understood 

how these artifacts will affect the TC results for workplace 

DPM samples (e.g., from mines). Most of the existing data 

on sampling artifacts and their corrections apply to environmental air sampling. Results for occupational samples 

may differ from environmental samples because sampling 

conditions are much different, and the effects of artifacts 

and corrections are known to depend upon conditions such 

as face velocity, composition of the sampling air stream, 

sampling time, and filter media (15–23). Environmental air 

samples are usually collected at much higher face velocities 

(20-80 cm/s as opposed to 3-4 cm/s for occupational 

samples). In addition, the concentrations of carbon are much 

lower in environmental air than in most occupational settings 

(3–8), and the types of aerosols sampled are different (e.g., 

aged aerosol from multiple environmental sources, as 

opposed to aerosols close to source). Olson and Norris (18) 

reported limited data on sampling artifacts for indoor 

sampling at flow rates similar to occupational sampling (2 

Lpm), but the collected aerosols and their concentrations 

were not similar to most occupational settings where DPM 

is monitored. 

The effect of the positive sampling artifact on occupational 

samples of DPM was the focus of this study. Several studies 

have indicated that adsorption (rather than evaporation) is 

the dominant artifact, especially at lower face velocities 

(15, 16, 23). Therefore, given the much lower filter face 

velocities typical of occupational sampling, positive artifact 

was expected over evaporation. The objective of the study 

was to determine the contribution of the adsorption artifact 

to occupational DPM samples and the accuracy of the 

corrections for these types of samples. This was accomplished 

by comparing samples with and without a denuder and using 

both the tandem quartz filter and quartz behind Teflon 

corrections for DPM samples collected in the laboratory, 

underground mines, and a loading dock area. 

Experimental Section

Laboratory Experiments. To determine the effects of the 

adsorption artifact and the accuracy of the tandem filter 

correction, samples of DPM were collected in an environmental chamber (24) at flow rates and loadings similar to 

those in underground mines. For sampling, two quartz fiber 

filters (QQ1 and QQ2) were placed in tandem in a 37 mm 

three-piece plastic cassette (SureSeal, 225-2LF) along with a 

stainless steel support screen. Prior to use, screens were 

washed and dried and filters were precleaned in an oven at 

800 °C for at least one hour to remove possible OC 

contamination. To avoid potential variability due to lot-tolot differences, filters from the same lot were used to collect 

a given sample set. Once loaded, the tandem filter cassettes 

were compression sealed. 

Three tandem quartz filter samplers were used to collect 

DPM, hereafter referred to as “inlet samples”. These samples 

were subject to OC adsorption artifact and possible loss of 

semivolatiles. Additionally, three carbon plate denuder 

(Sunset Laboratory, Tigard, OR; similar to the ones used by 

Eatough et al. 25, 26) sampling trains were assembled. The 

denuders are designed to remove gas phase OC, but not 

particulate OC. Two sampling cassettes (each with two quartz 

filters in tandem) were placed downstream of each denuder. 

The second cassette was used to determine whether any 

semivolatiles evaporated from the first one. 

Presuming that the denuders efficiently remove gas phase 

OC and loss of semivolatiles is minimal, the difference 

between the results for the inlet samples and the denuder 

samples provides an estimate of the amount of gas phase OC 

adsorbed by the quartz fiber filter since both were exposed 

to particulate matter but only the inlet samples were exposed 

to gas phase OC. The concentration determined using the 

denuder samples (QQ1) should represent the particulate OC 

only. The accuracy of the tandem filter correction could then 

be evaluated by comparing the corrected value (QQ1 - QQ2) 

for the inlet (no denuder) samples to the OC concentrations 

(particulate only) determined using QQ1 results for the 

denuder samples. 

After assembly, the inlet samplers and denuder setups 

were placed in the environmental chamber. Exhaust from a 

Kubota genset diesel engine (V1200-B engine, 4 cylinders), 

operating under 80% load, was plumbed into the chamber. 

A tapered element oscillating microbalance (TEOM) (Thermo 

Electron Corp., East Greenbush, NY), designed to measure 

the ambient particulate mass in real time by using a vibrating 

microbalance to measure the mass of the particles collected 

on a filter, was used to monitor the mass concentration of 

diesel exhaust. Vacuum pumps and critical orifices were used 

to draw the exhaust through the samplers at 1.7 Lpm or 5.1 

Lpm. (5.1 Lpm was used for some samples to determine 

whether this higher flow rate would affect the denuder 

characteristics. No difference was seen between the denuder 

results obtained at the two different flow rates.) 

After sampling, the pumps and the engine were turned 

off and residual DPM was ventilated from the chamber. When 

all the DPM was exhausted, the cassettes were removed, 

sealed, and preserved for later carbon analysis. Samples (both 

top and bottom filters) were analyzed for elemental, organic, 

and total carbon with a Sunset Laboratory carbon analyzer 

(Sunset Laboratory, Tigard, OR) according to NIOSH Method 

5040 (thermal-optical method). The NIOSH 5040 procedure 

is described in detail elsewhere (12). Eleven different experiments were performed by operating the engine at different 

loads (idle, 5%, 10%, 25%, and 80% loads). The purpose was 

to achieve a variety of OC/EC ratios, various DPM concentrations and filter loadings, and different gas phase OC 

concentrations. 

Field Samples. In addition to laboratory tests, field 

samples were collected in underground mines and two 

outdoor locations to further investigate the potential impact 

of the adsorption artifact on actual occupational samples. 

As with laboratory sampling, two precleaned quartz fiber 

filters from the same lot were placed in a three-piece plastic 

cassette with a stainless steel support screen. As previously 

discussed, most of the aerosol carbon is collected by the top 

quartz filter (QQ1), while the bottom filter (QQ2) is primarily 

exposed to gas phase OC. Thus, the TC contributed by the 

adsorption artifact might be estimated by comparing the OC 

on QQ2 to the TC on QQ1. 

In one of the mines (a stone mine), a denuder setup similar 

to that used for laboratory work was used, but an impactor 

was added upstream of the denuder to exclude larger dust 

particles. Again, tandem filter samplers without denuders 

were used to permit a direct comparison between adsorbed 

gas phase and particulate OC, and to determine how well the 

tandem filter correction might work under actual mining 

conditions. 
TABLE 1. Comparison of TC With and Without a Denudera

TC(QQ1) after % deviation between

difference between subtraction with and without

TWA (μg/m3

) TC(QQ1) and TCpb of TC(QQ2) vs TCpb denuder

sampling flow rate engine TC deviation deviation

time (min) (Lpm) load sample (QQ1) TCpb TCcorr (%)b (%)c EC

33 5.1 0% laboratory 1 171.52 122.21 135.55 40 11 6

54 1.7 0% laboratory 2 162.72 101.74 100.90 60 -1 1

71 1.7 0% laboratory 3 265.93 189.02 179.70 41 -5 4
 

184 1.7 5% laboratory 4 388.73 291.92 291.39 33 0 11
 

62 1.7 10% laboratory 5 315.07 246.34 238.69 28 -3 5

80 1.7 10% laboratory 6 269.07 213.44 200.13 26 -6 4

73 5.1 10% laboratory 7 248.75 206.97 197.96 20 -4 4
 

100 1.7 10% laboratory 8 465.01 329.43 334.25 41 1 3
 

53 5.1 25% laboratory 9 167.23 143.16 139.15 17 -3 3

98 1.7 80% laboratory 10 427.50 344.83 404.55 24 17 15
 

286 1.7 field 1 149.29 132.57 122.68 13 -7 -5
 

419 1.7 field 2 203.65 140.90 119.98 45 -15 17
 

411 1.7 field 3 168.00 118.47 140.60 42 19 12
 

403 1.7 field 4 179.48 141.27 150.38 27 6 15
 

a TWA (time weighted average): μg/(flow rate (Lpm) × 480 minutes) × 1000. TCpb is total particulate carbon on QQ1 after

denuder. TC(QQ1) is uncorrected TC on QQ1 without denuder. TCcorr is TC(QQ1) minus TC on QQ2 without denuder. % b deviation EC ) (EC[without denuder] - EC[with denuder])/(EC[without denuder]) × 100. % deviation ) (TC(QQ1) - TCpb)/ c TCpb × 100. % deviation ) (TCcorr - TCpb)/TCpb × 100.

Some researchers have reported underestimation of the 

adsorption artifact with tandem quartz filters relative to a 

quartz filter under a Teflon filter, while other investigations 

have reported no difference between the two types of 

corrections (15–23). To investigate the potential effects of 

this phenomenon on occupational samples, samplers loaded 

with Teflon and quartz filters (i.e., upper Teflon filter and 

two quartz filters beneath) as well as the tandem quartz filter 

setup were used to collect air samples in a loading dock area 

and a smokers’ shelter nearby. Details regarding the sample 

collection methods and different sampling locations are 

provided in following sections. 

Mine Surveys. Submicrometer particulate matter was 

collected using SKC DPM Cassettes (part 225-317; SKC, Inc., 

Eighty Four, PA) with an MSA Elf personal pump operated 

at 1.7 Lpm. Area and personal breathing zone samples were 

collected during actual production in two stone mines and 

one metal mine for 2.5-8 h. The DPM Cassettes contained 

an impactor with a 0.8 μm cutpoint at 1.7 Lpm and two quartz 

fiber filters in tandem. Each quartz filter was analyzed for 

OC, EC, and TC using NIOSH Method 5040. On some days, 

a denuder setup that included an impactor (SKC DPM 

Cassette) without filters was used in exhaust areas in one of 

the stone mines to exclude the mineral dust from the DPM 

sample. 

Loading Dock/Smokers’ Shelter Samples. Eight 37-mm 

cassettes were each loaded with two precleaned quartz-fiber 

filters. In four of the eight cassettes, a Teflon filter was placed 

on top of the two quartz filters. Filters in four additional 

cassettes served as passive field blanks, meaning no air was 

pulled through them. The bottom quartz filters in all eight 

cassettes, as well as the four directly under the Teflon filters, 

provided measures of adsorbed OC. The second (bottom) 

quartz filters in the cassettes with the Teflon filters were 

compared to the bottom quartz filters in the cassettes 

containing only a quartz pair to see if there was any difference 

in the two types of corrections. 

After assembly, the samplers were positioned in a portable 

dust chamber (27) designed for dust sampling in mines. 

Twelve cassettes were mounted symmetrically inside the 

chamber, which allows for simultaneous collection of up to 

eighteen samples. A homogeneous dust distribution was 

presented to the samplers inside the chamber. Air sampling 

pumps connected to ports on the outside of the chamber 

were programmed to run at 2 Lpm over an 8 h period. In one 

case, the pump was stopped after 23 min. 

Samples were collected on six different days: five days in 

the loading dock area and one day near a smokers’ shelter 

in the same vicinity. A diesel truck was operating in the 

loading dock area, over different periods, on three of the five 

sampling days. On the other two days, a truck entered the 

area but was not left in idle. 

Results and Discussion

DenuderEffectiveness.As previously discussed, carbon plate 

denuders (Sunset Laboratory) were used to prevent gas phase 

OC from collecting on the quartz filter. This denuder is 

designed to adsorb gas phase OC at the optimum flow rate 

of about 8 Lpm; however, in this study, we mostly used a 

flow rate of 1.7 Lpm, which is typical for samples collected 

in underground mines (5.1 Lpm was also used for some 

samples). To ensure that no particulate carbon loss occurs 

in the denuder at these lower flow rates, we compared EC 

results for samples collected with and without the denuder. 

As EC exists only in the particulate form, results for the two 

samples should be comparable if no particle loss occurs. If 

the deviation (%), calculated as 

% deviation )

(EC without denuder - EC with denuder) 

× 100 (1) (EC without denuder) 

was above 20%, we did not include the data point in our 

results as this would indicate a potential sample collection 

problem. As seen in Table 1, the deviations for the laboratory 

data were at or below 11% for all samples except one, which 

had a deviation of about 15%. 

In the field test, the deviation for one of the samples was 

above 20%. The other results showed deviations below 18%. 

The deviations in the EC values obtained with and without 

the denuder were higher for the field data than the laboratory 

data. In the field, more EC may have been trapped by the 

denuder setup because the size-selective sampling device 

used upstream of the denuder may have disrupted the flow 

pattern in the denuder. The higher deviation in the field 

samples could also have occurred due to spatial variation, 

which has been reported to cause a deviation of as high as 

20% in other field samples (28, 29). 
When using denuders, other concerns are gas phase 

breakthrough and the loss of semivolatiles in the particulate 

phase through evaporation. The efficiency of the denuders 

used in this study has not been reported but these denuders 

are similar to the ones used by Eatough et al. (25, 26). The 

efficiency of the denuders used by Eatough et al. has been 

reported to be around 87% at 35-40 Lpm (26) and increases 

with decreasing flow rate (30). Therefore, we would expect 

an efficiency between 90 and 100% for the denuders at the 

flow rates used. However, it is still important to determine 

the significance of any breakthrough of gas phase OC under 

the specific conditions used especially since it has been 

reported that the efficiency of other denuders could have 

been affected by the composition of the sampling stream 

(18). It is also important to determine the effect of the 

evaporation artifact on the results when using a denuder 

especially since it has been shown that the evaporation artifact 

is more prevalent when using a denuder than without one 

(21). As with previous studies (20), a set of quartz filters behind the collection quartz filter was used to indicate whether 

breakthrough or evaporation occurred. If a significant 

contribution by breakthrough or evaporation was indicated, 

the sample was not included in the data set. 

With a few of the laboratory samples, the bottom filter 

(QQ2) in the denuder sampler (i.e., first after the denuder) 

contained a significant amount of OC (e.g., more than 0.9 

μg/cm2, or over two times higher than typical media blanks), 

which would indicate that some gaseous/semivolatile OC 

passed through the denuder or there was a loss of semivolatiles from the particulate. These laboratory samples were 

not included in the data analysis because they would not 

provide reliable quantification of the results. However, most 

of the OC results for QQ2 and the other two filters in the 

second cassette after the denuder were between 0.3 and 0.9 

μg/cm2. Some sets of field data also indicated gaseous OC 

passage (or OC evaporation) and also were not included in 

the analysis, but most results for OC on QQ2 (after denuder) 

and the two quartz filters in the second sampler after the 

denuder were at or below 0.8 μg/cm2. 

Impact oftheAdsorptionArtifact on Occupational DPM

Results. When collecting samples in an atmosphere of just 

diesel emissions at flow rates and loadings typical of a mining 

environment, the adsorption of gas phase OC sometimes 

caused a significant bias in TC results. Table 1 lists TC results 

for DPM samples collected in a laboratory environmental 

chamber at flow rates and loadings typical in underground 

mines. As mentioned earlier, one set of samplers (inlet 

samples) had no sampling inlet preselector, and one set used 

a denuder to scrub out the gas phase OC. The denuder 

samples collected only particulate carbon, and the inlet 

samples collected both particulate and gas phase carbon. As 

indicated by the percent deviation (Table 1, column six) 

between the inlet and denuder samples, the TC sampled in 

the presence of gas phase OC was over 30% higher than the 

TC for samples not exposed to gas phase OC for 50% of the 

samples. The TC sampled in the presence of gas phase OC 

was also always larger than the TC for samples not exposed 

to gas phase OC, indicating an OC adsorption on all samples, 

and that the adsorption artifact had a greater contribution 

than the evaporation artifact. Sampling times did not appear 

to affect the contribution of the adsorption artifact to these 

samples. 

Table 1 also shows TC results for denuder samples 

collected in an underground stone mine (field samples). 

Again, samples with and without a denuder were collected. 

As shown in column six, the TC samples exposed to gas phase 

OC were over 40% higher for 50% of the samples and were 

always higher in TC concentration. Thus, the adsorption 

artifact also had a major contribution to the TC in actual 

mining samples. This was further seen when taking air 

samples using an impactor during production in several 

underground metal/nonmetal mines (Figure 1). As discussed, 

QQ2 is not exposed to particulate carbon, so the majority of 

TC on QQ2 is adsorbed gas phase OC. Some of the OC could 

be semivolatiles that evaporated from the first filter, but most 

is likely due to gas adsorption (15, 16, 23), given the much 

lower (relative to environmental sampling) face velocities 

used in our study. In our study, we found higher TC for 

samples exposed to gas phase OC and affected by sampling 

artifacts relative to denuder samplers unaffected by gas 

adsorption, demonstrating positive sampling artifact. 

As shown in Figure 1a, in underground mines the 

adsorption artifact can contribute between 0.3 and 5.8 μg/ 

cm2 on the filter. The average gas phase OC adsorbed by the 

quartz filters (i.e., OC on QQ2) was 3 μg/cm2, with a 95% 

confidence range of 0.3-5.8 μg/cm2. This range is fairly 

independent of the DPM concentration; therefore, the gas 

phase OC would be a larger portion of the collected TC at 

the lower TC concentrations. As shown in Figure 1b, the 

ratio of gas phase OC to TC (TC[QQ2]/TC[QQ1]) for many 

of the samples was over 20% when the TC collected was <25 

μg/cm2. The effect of the gas phase diminished as the 

concentration of DPM increased. As with the denuder data, 

the sampling time did not appear to affect the contribution 

of the adsorption artifact for these samples. 

The adsorbed gas phase OC (QQ2) also was a large portion 

of the OC collected on QQ1 for most of the loading dock area 

samples (Table 2). The gas phase OC to TC ratio (QQ2/QQ1) 

was over 30% for 4 of 6 samples. Results for adsorbed OC on 

passive (i.e., no air drawn through filter) filter blanks are 

usually substantially lower than those obtained for bottom 

filters after tandem filter sampling and will underestimate 

the correction. 

Accuracy oftheTandemFilterCorrection.As mentioned 

earlier, researchers have applied the tandem filter correction 

to correct for the adsorption of gas phase OC. This correction 

relies on the second filter adsorbing the same amount of gas 

phase OC as the primary filter, which is not always the case 

with environmental sampling (15–23). We therefore tested 

the accuracy of this correction when sampling DPM in 

occupational settings. 

As shown in Table 1, the tandem filter correction worked 

well when applied to laboratory data that simulated the DPM 

sampling procedure in underground mines. TC on the first 

filter went from being over 30% higher than the particulate 

TC for 50% of the samples to being within 11% of the 

particulate TC when the tandem filter correction was applied 

for all but one sample. For one sample, the deviation (%) 

only decreased from 24 to 17%, but this sample also had a 

higher deviation between the EC values than the other 

samples (15% compared to under 11% as described earlier), 

which suggests that sample collection had a higher contribution to the deviation for this sample. 

The tandem filter correction seemed to also work relatively 

well for correcting the adsorption artifact in the field samples. 

As shown in Table 1, for 75% of the field samples, the 

deviations between particle TC (measured with a denuder) 

and TC without a denuder were about twice the deviation 

expected due to loss of particles through the denuder and 

analytical errors. EC is only in particulate form and is not 

affected by sampling artifacts. Therefore, the difference 

between EC results for samples with and without a denuder 

would be due to particle loss in the denuder and analytical 

and sampling errors. When the tandem filter correction was 

applied, the deviations between particle TC (measured 

through denuder) and corrected TC (using the tandem filter 

correction) were close to those expected due to sampling 

errors (deviation in EC values). That is, little effect from the 

artifact was observed when the tandem filter correction was 

made. 
FIGURE 1. (a) TC(QQ2) for DPM samples in some underground mines. (b) Ratio of TC(QQ2) to TC(QQ1) showing the contribution of the

adsorption artifact to mining samples.

TABLE 2. Total Carbon Results for Samples Collected in a Loading Dock Area and near a Smokers’ Shelter

mean TC, μg/cm2 (RSD, %) bottom quartz mean

sample seta location QQ1 TQQ1 QQ2 TQQ2 pooled RSD % QQ2/QQ1 (%) sampling time (min)

1 shelter 4.81 (9.77) 3.66 (11.75) 3.46 3.41 3.44 8.80 72 480

2 dock 3.52 (7.10) 2.82 (12.77) 2.48 2.33 2.43 9.57 70 480

3 dock 3.61 (1.39) 2.89 (1.38) 2.41 2.42 2.41 7.45 67 480

4 dock 4.59 (3.92) 1.79 (13.40) 1.41 1.36 1.38 6.46 31 23

5 dock 8.43 (3.91) 1.84 (19.02) 1.35 1.36 1.35 12.03 16 480

6 dock 22.78 (1.54) 3.62 (4.97) 3.10 3.18 3.14 4.09 14 480

a Sets of eight 37-mm cassettes: four containing two quartz filters (QQ1 upper and QQ2 lower) and four containing two

quartz filters (TQQ1 upper and TQQ2 lower) plus a top Teflon filter. TQQ1, QQ2, and TQQ2 provided measures of adsorbed

OC. See text for details.

In our studies with limited field samples, the tandem filter 

correction resulted in a good measure of the particulate TC.

Our data show that the need to correct for the adsorption 

artifact, and the number of backup filters analyzed, depends

on the sampling strategy and environment. 

Teflon/QuartzCorrections.A quartz filter behind a Teflon 

filter has been shown to collect more OC than quartz behind 

quartz (15, 18). Some researchers have reported the quartz 

behind Teflon to be a better estimate of gas phase OC, while 

other researchers have reported no difference between the 
two correction methods, or the quartz behind the Teflon 

overestimating the adsorption artifact (15–23). Therefore, as 

part of this study, we also investigated this phenomenon. As 

reported in Table 2 for samples collected in a loading dock 

area and near a smokers’ shelter, we did not obtain a 

significant underestimation of adsorbed gas with the tandem 

quartz approach. Quartz filters placed immediately below 

Teflon filters (i.e., TQQ1) had only slightly higher OC loadings 

than did the bottom quartz filters of stacked quartz pairs 

(i.e., QQ2 and TQQ2). Since the tandem quartz correction 

gave results similar to those found with a quartz filter behind 

a Teflon filter, it is unlikely that there was a significant 

volatilization artifact. Sampling time (23 min compared to 

480 min) did not appear to affect the results for this data set, 

though this is just one result and more data would be needed 

to fully understand the impact of sampling time on these 

types of samples. Our findings may differ from those reported 

in previous studies on environmental samples due to the 

lower face velocities used in our study and the air 

composition. 

Acknowledgments

We thank Shawn Vanderslice for performing some of the 

NIOSH Method 5040 analyses. The mention of any company 

or product does not constitute an endorsement by the CDC. 

The findings and conclusions in this report are those of the 

authors and do not necessarily represent the views of the 

National Institute for Occupational Safety and Health. 

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