Portable XRF analysis of occupational air filter samples from different

workplaces using different samplers: final results, summary and

conclusionsw

w For the first three parts of the study see ref. 4, 5 and 6.

Martin a Harper, Bruce a Pacolay,

aExposure Assessment Branch, Health Effects Laboratory Division,

National Institute for Occupational Safety and Health, 1095

Willowdale Rd., MS 3030, Morgantown, WV 26505, USA

b Patrick Hintz,

b Spokane Research Laboratory, National Institute for Occupational

Safety and Health, 315 E. Montgomery Ave., Spokane, WA 99207,

USA

c David L. Bartley,

c

Guest Researcher, National Institute for Occupational Safety and

Health, 4676 Columbia Parkway, Cincinnati, OH 45226, USA

James d

 d E. Slaven and Michael E. Andrew

dBiostatistics and Epidemiology Branch, Health Effects Laboratory

Division, National Institute for Occupational Safety and Health,

1095 Willowdale Rd., MS 4020, Morgantown, WV 26505, USA

This paper concludes a five year program on research into the use of a portable X ray

fluorescence (XRF) analyzer for analyzing lead in air sampling filters from different

industrial environments, including mining, manufacturing and recycling. The results from four of

these environments have already been reported. The results from two additional metal processes

are presented here. At both of these sites, lead was a minor component of the total airborne

metals and interferences from other elements were minimal. Nevertheless, only results from the

three sites where lead was the most abundant metal were used in the overall calculation of

method accuracy. The XRF analyzer was used to interrogate the filters, which were then

subjected to acid digestion and analysis by inductively coupled plasma optical emission

spectroscopy (ICP OES). The filter samples were collected using different filter holders or

‘‘samplers’’ where the size (diameter), depth and homogeneity of aerosol deposit varied from

sampler to sampler. The aerosol collection efficiencies of the samplers were expected to differ,

especially for larger particles. The distribution of particles once having entered the sampler was

also expected to differ between samplers. Samplers were paired to allow the between sampler

variability to be addressed, and, in some cases, internal sampler wall deposits were evaluated and

compared to the filter catch. It was found, rather surprisingly, that analysis of the filter deposits

(by ICP OES) of all the samplers gave equivalent results. It was also found that deposits on some

of the sampler walls, which in some protocols are considered part of the sample, could be

significant in comparison to the filter deposit. If it is concluded that wall deposits should be

analyzed, then XRF analysis of the filter can only give a minimum estimate of the concentration.

Techniques for the statistical analysis of field data were also developed as part of this program

and have been reported elsewhere. The results, based on data from the three workplaces where

lead was the major element present in the samples, are summarized here. A limit of detection and

a limit of quantitation are provided. Analysis of some samples using a second analyzer with a

different X ray source technology indicated reasonable agreement for some metals (but this was

not evaluated for lead). Provided it is only necessary to analyze the filters, most personal samplers

will provide acceptable results when used with portable XRF analysis for lead around applicable

limit values.

Introduction

A useful goal of research into novel exposure measurement

techniques is to develop methods that provide instantaneous

feed back of results. This allows a clearer link to be made

between changes in work process or practice, as well as faster

interventions when exposures are deemed unacceptable.

Where such direct reading instruments are not available, a

secondary goal is to turn around the analytical results as

quickly as possible. Portable X ray fluorescence (XRF) tech

nology promises the possibility of analytical results within

minutes of the end of sample collection. Bench top XRF

analyzers have been used for determining lead on air filters 1 for many years in the UK. However, although there has been

some effort in evaluating portable XRF methodologies from
the US National Institute for Occupational Safety and Health

(NIOSH) and Occupational Safety and Health Administration

(OSHA), these have not generally been considered sufficiently

accurate by those agencies for use as anything other than

screening tools, and there has been some question as to their

ability to be used to support citations based on compliance

with legally enforceable limit values.2,3

The objective of this study was to evaluate a portable XRF

analyzer for airborne lead, and the study concentrated on one

particular analyzer as representative. A secondary objective

was to determine the appropriate sampler to use with portable

XRF analysis. Airborne lead is collected on filters housed in

filter holders or ‘‘samplers’’. The filter is presented to the XRF

analyzer. Different samplers produce different patterns of dust

deposit on the filter. Two issues are of importance for surface

analysis: spatial homogeneity and thickness. The optimum

sample would be homogenous across the filter, with a precisely

known outer boundary, and minimal thickness. When the

sample is thick, the excitation X rays can be absorbed before

reaching the element of concern, or the generated fluorescence

X rays can be absorbed before reaching the detector. Given

the lower energy of the fluorescence X rays, the latter is the

greater concern. The most serious absorption, however, will

occur when the fluorescence X rays can themselves excite

other elements in the sample, and this is the case for the lead

Lb radiation, which is of high enough energy to excite other

elements. The different samplers, therefore, were used to

collect samples in different environments, which presented

different mixtures of metals. Lead may be used in industrial

processes or encountered as a by product. At a lead mine

concentrator mill,4 lead is found as the sulfide, in association

with lesser amounts of iron and zinc. At a lead acid battery 4 recycler, lead was predominant, with much less antimony and

 5 tin. At a solder manufacturer, only tin approached the air

borne lead levels. At a bronze foundry,6 copper, iron and zinc

were present at concentrations approximately equal to, or

greater than, the lead, and at a nickel chromium recycler

plant (reported here), many other metals were present at levels

equal to or greater than lead. Finally, at a primary copper

smelter (also reported here), lead was only a minor component

of the aerosol, which was dominated by copper and iron. To

account for possible elemental interferences, factory calibra

tion of the XRF analyzer uses a multi element inverse matrix.

Since evaluation of lead analysis by portable XRF was the

primary aim of this study, the analysis of other metals was

secondary insofar as it might explain anomalies in the lead

results. Throughout the study, XRF results were compared to

inductively coupled plasma optical emission spectroscopy

(ICP OES) measurements from the same samples after diges

tion of the filters.

The nickel chromium recycling facility for which data are

reported in this manuscript operates a secondary submerged

arc smelting furnace dedicated to recovery of nickel, chro

mium, and iron. The process starts by feeding waste solids, via

a screw conveyor, to a pelletizing disk using either water or

liquid wastes. Pellets are transferred to a rotary hearth furnace

operating at 1260 1C (2300 1F) for the reduction of oxidized

metal species to their metallic form. Reduced pellets are fed to

the main furnace where they are smelted for extraction of

metal components. The molten metal is cast into approxi

mately 24 pound (10 kg) ‘‘pigs’’ which are used for the

manufacture of stainless steel. The process is recognized by

the US Environmental Protection Agency as a best demon

strated available technology (BDAT) for metals reclamation

of stainless steel waste and nickel cadmium batteries (cad

mium is recovered in a separate distillation process). The

original scope of the waste stream entering the facility has

grown over the past 20 years and now includes, in addition to

the above, metal bearing waste from nickel and chrome plat

ing, nickel metal hydride batteries, waste magnesium, and

spent nickel and chrome catalysts. About 60 000 tons of

material annually are recovered that would otherwise be

disposed of in a landfill. Areas sampled included around the

thermal oxidizer, the blending conveyor belt, the pelletizing

disk, the electric arc and rotary hearth furnaces (particularly

around the feed areas), and in casting. These areas were

selected to give a range of lead concentrations and do not

represent personnel exposures. No personal samples were

taken in this facility. By virtue of its volatility, zinc is one of

the most abundant airborne metals, with levels approaching

those of iron, the metal found in the highest concentrations.

After these two metals likely comes chromium, but due to

matrix dissolution problems, as described below, ICP OES

analysis may have underestimated the total. Magnesium is

next highest, along with likely similar concentrations of nickel

and manganese, but again the ICP OES analysis of the latter

metals is problematic. Cadmium and lead are present at

roughly similar levels, about one sixth the concentrations of

zinc and iron and about one half the likely concentrations of

nickel and manganese. The sources of lead include the term

inals of batteries and solder on printed circuit boards.

Data are also presented for samples from a copper smelter.

The primary products are 99% pure copper cast anodes, which

are eventually sold to a refinery for further processing, and

sulfur dioxide gas, which is captured and processed at the

company’s acid plant. The smelting process begins by loading

a stock material containing a mixture of recycled, copper

containing material from various sources together with mine

ore concentrates into an Isa vessel at a rate of 100 110 tons per

hour using a conveyor belt. The Isa vessel is a vertical heating

chamber containing a gas lance which burns a mixture of

natural gas, oxygen, and blast air to melt the stock material at

1200 1C (2200 1F). The molten stock material is then tapped

from the bottom of the Isa vessel and poured into the main

electric furnace. Here the stock material continues to be heated

between 1200 1C and 1260 1C (2300 1F) until a separation of

slag and matte occurs. The slag is tapped and discarded while

the remaining matte is tapped and collected into large ladles

which feed converters. Silica is added as a fluxing agent, and

arsenic is added for refining purposes. Oxygen is injected from

the bottom and the ensuing reaction further separates the

molten charge material to purify the copper. The slag material

from the converter contains between 5 50% copper, so it is

returned to the main furnace to re enter the process. The

smelter contains four converters, of which two to three are

always operating while one is maintained on a regular rotating

cycle. Air samples contain mostly copper with some iron and

arsenic. Lead, zinc and molybdenum are present in the ore and
form minor components of the air samples. Again, no personal

samples were taken in this facility. Samplers were located at

various locations along the process, focusing on decks above

the converters and the main furnace from which metal fume

rises. Samplers were also placed in the matte tapping room

which taps matte from the main furnace into the converter

ladles and the upper deck where stock material is loaded into

the Isa vessel.

The US OSHA enforces an airborne permissible exposure

limit (PEL) of 50 mg m-3 for personal samples and also an

action level of 30 mg m-3

, which requires employers to institute

aspects of a lead control program. The US Mine Safety and

Health Administration (MSHA) enforces an airborne PEL of

150 mg m-3 for personal samples. Appropriate personal pro

tective equipment may be used where concentrations are above

these levels. When all six industrial locations are combined, we

have concentration values extending from near the limit of

detection to around 5000 mg m-3.

Experimental

Samples were taken for approximately 4 6 hours. The sam

plers used in this study have been described extensively and

include: the 37 mm styrene/acrylonitrile filter cassette (Omega

Specialty Instruments, MA, (now SKC South, TX), USA),

operated in the ‘‘closed face’’ mode and referred to as the

CFC, the Institute of Occupational Medicine (IOM; SKC Ltd,

UK) sampler, the German GSP (Gesamtstaubprobenahme;

GSM NEUSS GmbH, Germany) or ‘‘cone’’ sampler, the

Button sampler from the University of Cincinnati (SKC,

Inc., PA, USA), and a 25 mm styrene/acrylonitrile filter

cassette (Omega Specialty Instruments, MA, USA), operated

in the ‘‘open face’’ mode. Samples collected from the nick

el chromium recycler were: 44 GSP; 53 CFC (24 analyzed by

XRF without their support pads as had been done in the past,

and 29 analyzed with the support pad under the filter); 46

IOM; 43 Button; and 49 25 mm open face cassettes, for a total

of 249. Samples collected from the copper smelter were 18

CFC; and 18 IOM. Mixed cellulose ester filters were used for

all samples. After returning samples to the laboratory, wipe

samples were taken from the interiors of the CFC and 25 mm

open faced samplers from the nickel chromium recycler and

from the interiors of the IOM and CFC samplers from the

copper smelter. These wipes were submitted for analysis by

ICP OES. A radioactive source XRF unit (

109Cd excitation

source, main emission at 22.1 keV, Si PIN detector, Model

XL701, THERMO NITON Corporation, Billerica, MA,

USA) was used to analyze all samples as previously de 4–6 scribed. The analytical window is a 2 cm x 1 cm rectangle.

This instrument has been evaluated for precision of replicate

analyses, and coefficients of variation are low and similar to

those of OES analysis.7 the reference ICP A second portable

XRF instrument, which uses a different X ray generation

technology was used to analyze samples from the nickel

chromium recycler. The model XT 240 (Innov X Systems,

Inc. Woburn, MA, USA) is a tube type energy dispersive

instrument with a silver anode that can generate X rays in

the energy range 10 to 35 keV. It is cradled in a bench top

docking station. The 1.4 cm diameter circular probe window

(1.54 cm2 area) is flush with the upper stand in the docking

station and also houses a Si PIN detector.

The analytical window for these analyzers is less than the

deposit area for the filters in the samplers under test. In bench

top instruments analytical windows may be large enough to

cover the entire filter area, and samples may also be able to be

rotated to account for non uniform deposition, but this

feature is not available with the portable units. Two different

algorithms have been used for combining three readings from

different parts of the filters from CFC samplers.2,3 These have

been reviewed previously.

The OSHA method ascribes the difference to the fact that

their protocol calls for the support pad to be included under

the filter in the analysis. Although no proof was offered, the

supposition could be reasonable as this would place the filter

slightly closer to the analyzer. However, the thickness of the

cardboard filter holders (see below) is not optimal for enclos

ing both the filter and support pad. In order to determine

whether the presence of the back up pad really did affect the

analysis of the CFC, two sets of CFC samples were taken at

the nickel chromium recycler; one set was analyzed with the

support pad in place and one set without.

The filters were removed from the filter holders (samplers)

and placed in a 25 mm or 37 mm filter holder, depending on

filter size. The filter holders are made of cardboard, with

Mylar film over the filter area. For the THERMO NITON

instrument, the cardboard holders were placed on the manu

facturer’s provided test stage, so that the filter holder could be

placed under the analyzers X ray beam in appropriate posi

tions. For the Innov X instrument, the samples in their Mylar

holders were placed on top of the probe window at locations

expected to duplicate the ‘‘top’’, ‘‘center’’ and ‘‘bottom’’

positions of the filter as with the THERMO NITON holder.

Data handling for the filters from the different samplers has

 4–6 been described previously.

Both XRF analyzers report readings in mg cm-2

. For the

Innov X tube type instrument, each individual reading in

volved collecting the fluorescence radiation for 240 seconds.

For the THERMO NITON source instrument the analysis

time is extended as the radiation source decays. Counts were

recorded for 240 nominal seconds, the actual time taken being

that for the number of decays equivalent to that occurring in

240 seconds when the source was initially calibrated. This

time period is considered the best trade off between accuracy

and speed of analysis. The THERMO NITON analyzer was

set up in ‘‘thin film’’ mode. The internal calibration of both

analyzers was checked with thin film standards from Micro

matter Company (Deer Harbor, WA, USA) each day before

sample filters were analyzed. Each Innov X reading was taken

in triplicate and the average was used.

After XRF analysis, the samples (filters plus Mylar film)

from the nickel chromium recycler were extracted by sonica

tion with 50% nitric acid and analyzed by NIOSH Method

7300 (metals by ICP).8 This concentration of acid normally is

sufficient to dissolve lead unless particularly difficult species

such as lead sulfate (PbSO4) or lead dioxide (PbO2) are

present, and good results were achieved in the prior studies

with this procedure. However, in this study it was noted that

50% nitric acid was not sufficient to dissolve several other
 

 

 

 

 

 

 

 

 

 

 

 

 

metals present in the nickel chromium recycler samples, and

so following this experience the samples from the copper

smelter were extracted with aqua regia on a hot block

(95 1C). Note that neither procedure actually dissolves the

Mylar film. Because of the large discrepancy noted between

XRF analysis of chromium and the ICP OES results, four

spare samples from the nickel chromium recycler, were ana

lyzed using the THERMO NITON instrument but then sub

mitted to neutron activation analysis (NAA) at a commercial

facility (General Activation Analysis, Inc., Encinitas, CA,

USA). The samples plus comparator standards were irradiated

in a TRIGA Mark I nuclear reactor at a flux of 1.8 x 1012

neutrons cm-2 s

-1

. The elements Cr, Fe, Mn and Ni produce

the isotopes 51Cr, 59Fe, 56Mn and 65Ni (

58Co) with half lives of

28 days, 45 days, 2.5 hours and 2.5 hours (71 days), respec

tively. Emissions from the irradiated samples were counted

after decay periods of one hour, one day, one week and two

weeks to obtain total mass results.

The interior surfaces of the CFC and 25 mm open face

cassette samplers from the nickel chromium recycler and the

CFC and IOM samplers from the copper smelter were wiped

using a ‘‘ghost wipe’’ as described previously,4–6 and the wipes

were analyzed by ICP OES separately using the same diges

tion procedure as the respective filters.

Statistical analysis on the THERMO NITON XRF and

ICP OES lead data for the nickel chromium recycler samples

was performed in accordance with the rationale and protocol

provided in the pilot project paper.9 The robust measure of

accuracy as described includes all data points. This method

essentially compares each XRF value directly with its corre

sponding ICP OES reference value as to whether it meets or

exceeds a pre determined measure of closeness to that value

(e.g. ± 20%, 25% or 30%). The estimate is conservative in

that the uncertainty in the reference method is not addressed.

Samplers can be ranked in accordance with the percentage of

values falling within the selected limit, and an acceptability

criterion can be used (e.g. 90 or 95%) to classify samplers as

appropriate for specific decision end points, such as compli

ance with limit values. However, alternative approaches are

possible, once a larger data set is available. Two methodolo

gies (known as the ISO GUM (Guide to Uncertainty in

Measurement) approach and the Bootstrap approach) specific

to this type of field evaluation of a non destructive method

been compared.10 have Data from some of the field studies in

this program were used to evaluate differences between the two

approaches, and the results are summarized below. However,

the data from the two studies reported here were not included

in that analysis as noted below. The Innov X instrument was

not evaluated for lead using these samples.

Results from the nickel–chromium recycler

Most of the filter analyses (except from CFC samplers) for

lead by ICP OES were above the values used in prior 4–6 studies to cull samples, i.e. 5 mg per filter for 25 mm filters,

7 mg per filter for the GSP sampler filters and 10 mg per filter

for the CFC sampler filters. Since the cull from the CFC

samplers would have been excessive in this study, 7 mg per filter

was used for the CFC samples, otherwise filters below the

values given above were excluded from the analysis. These

values are under study as the limits of quantitation for the

XRF technique, and this is discussed below.

Iron makes up the bulk of the metal recycled at this facility

and so is present in the largest quantities on the filters. For the

CFC samples, this represents iron concentration ranges of

47 to 1670 m -3 g m (mean: 380 mg m-3

, geometric mean:

300 mg m-3

). Zinc, being a very volatile metal, is also found

at high concentrations, ranging from 52 to 4500 mg m-3 (mean:

350 m 3 g m- , geometric mean: 190 mg m-3

). Several of the

metals present at the next highest concentrations were not

digested properly by the 50% nitric acid used, and so the ICP

OES results are suspect. Chromium, nickel and manganese are

in this group. Approximate arithmetic mean concentrations

after adjustment for low ICP OES recovery are 230 mg m-3

,

120 mg m-3

, and 110 mg m-3, respectively. Magnesium is found

at similar levels. Lead (9.9 to 450 mg m-3

, mean: 54 mg m-3,

geometric mean: 35 mg m-3

) and cadmium (16 to 420 mg m-3,

mean: 64 mg m-3

, geometric mean: 50 mg m-3) are present in

smaller quantities at approximately equal amounts. Concen

trations of lead were relatively consistent from area to area

within the facility, although, as might be expected, the casting

area had generally the lowest results.

Table 1 Summary statistics for portable XRF analysis for lead in comparison to ICP OES for different samplers used in the nickel chromium

recycler (filter catch only). (N) is the total number of samples collected, N is the number of samples used in the analysis after removing low sample

results as per the text

Sampler (N) N Loading range (mg per filter) Algorithm

Accuracy

20% 25% 30%

CFC without pad (24) 21 o7 160 NIOSH

OSHA

42.9

81.0

57.1

90.5

61.9

90.5

GSP (44) 44 o7 250 Average

Middle

72.7

61.4

86.4

65.9

93.2

70.5

IOM (46) 39 o5 110 3.46

2.84

87.2

74.4

94.9

87.2

94.9

94.9

Button

25 mm

(43) 40

(49) 46

o5 170

o5 150

92.5

89.1

92.5

93.5

92.5

95.7

Nearly all of 18 field blanks (37 mm and 25 mm filters) were

either non detectable for lead on ICP OES analysis, or slightly

negative after subtraction of media blanks, with one single

high value of 5.1 mg per filter. All wipe sample field blanks

were below the ICP OES limit of detection. These results are

in line with our other studies. Table 1 includes the numbers of

samples taken, and the numbers of samples used in the

analysis after subtraction of low values, the loading range
 

 

 

 

 

 

 

 

 

 

 

for lead and the percentage numbers of samples whose XRF

values fell within ± 20%, 25% or 30% of the corresponding

ICP OES values. No sampler met the criterion of 95% of

samples with XRF values within ± 25% of the ICP OES

values, although there did not appear to be a specific bias in

the regression analysis. For the CFC analyses, Table 2 shows

that the confidence interval of the bias with support pad using

either the NIOSH or OSHA algorithm did not include zero.

Table 2 Comparison of fractional bias estimates between CFC samples from the nickel chromium recycler with and without the support pad

Algorithm

No support

Mean (95%

pad

CI) (N 24)

Support pad

Mean (95% CI) (N 29) p valuea

NIOSH

OSHA

0.129 (

0.023 (

0.01, 0.267)

0.142, 0.097)

0.534 (0.412,

0.329 (0.223,

0.656)

0.436)

o.0001

o.0001

a p value of the difference between bias estimates is from t statistic for two independent samples.

The confidence intervals for the bias without support pad do

include zero, and thus it can be concluded that the data

without support pad are closer to the ICP OES reference.

This is in agreement with the similar experiment at the

lead acid battery recycler, although in that case, only the

confidence interval for the OSHA algorithm without support

pad included zero.

Wipe samples were taken from the interior of CFC and

25 mm open faced cassettes, but almost every value for lead

was below the ICP OES reporting limit. As this may be a

result of the generally low lead loadings, zinc data were

examined as a surrogate. Detectable zinc was found on the

interior surfaces of approximately 60% of the 25 mm open

faced cassettes, but in less than 25% of the CFCs. In no case

did the percentage of sample found in the cassette exceed 28%

of the total sample (filter plus cassette). Iron was also

examined and has a similar distribution for CFC samples:

40% with deposits, maximum 29%, but nearly all 25 mm

open faced samplers had detectable iron in the wipes with a

maximum 62% of total sample. These differences may indicate

an association of these metals with particles of different

sizes.

Comparison of XRF analyzers with each other and with

ICP-OES and NAA

As noted, while lead data did not appear to show any specific

bias between XRF and ICP OES analysis, a rather extreme

bias was noted for chromium, and possibly also for nickel and

manganese. For example, XRF analyses of the 25 mm

open face cassette filters were higher than ICP OES results

by approximately 850% for Cr, 20% for Ni, and 60% for Mn.

Thus the two XRF instruments, the THERMO NITON and

Innov X, were compared for these metals and summary

data are presented in Table 3.

Table 3 Correlation between two XRF instruments for measurement of Cr, Ni, and Mn on air filters. Sealed source instrument (THERMO

NITON) plotted on the x axis and tube type (Innov X) instrument on the y axis. CFC filter data not shown as the algorithm for the sealed source

instrument was not appropriate for the different analyzer geometry of the tube type instrument. GSP data not shown as intercepts for Cr and Mn

were outside the normal range of 1.5 to +1.5 mg, also likely a result of different analyzer geometry, combined with the inhomogeneity of the filter

deposit

Chromium Nickel Manganese

Sampler Slope 2

r Slope 2

r Slope 2r

Button

IOM

25 mm open faced

1.07

1.05

1.02

0.96

1.00

0.99

1.08

1.08

1.07

0.96

0.99

0.99

1.30

1.28

1.25

0.96

0.99

0.99

For chromium, the tube type

XRF results were very close to those from the sealed source

XRF, suggesting the ICP OES analysis was biased low

due to the extraction technique. For manganese and nickel,

the differences between XRF and ICP OES are less

pronounced, so there is less confidence that the extraction

technique has resulted in a negative bias, but it is certainly

suggestive. The NAA analyses in Table 4 showed plenty of

scatter in comparison to the XRF data, but did not

confirm the ICP OES analyses for chromium. NAA in this

instance had some value in explaining the anomalies in the

ICP/XRF results, but there is no published analytical method

for air filters by NAA so that these results can not be

considered as reference values. The cause of the ICP OES

discrepancy is probably the strength of the acid digestion.

Spinels are oxides of metals that are notoriously difficult to

digest in acid, and these can be formed in the fume from metal

combustion. For example, they are often found in welding

fume.11 A typical formula for a spinel from the environment of

the nickel chromium recycler is (Mg,Mn)(Cr(III),Al2)O4, and

it can be seen that the analysis of chromium would be severely

compromised if this species is only sparingly soluble in 50%

nitric acid.

Results from the copper smelter

In the previous study at a bronze foundry, lead masses by

XRF appeared to be lower than by ICP OES. Three other

elements (Cu, Fe, Zn) were present at this site besides lead, at

similar levels. It was postulated that copper might have

affected the lead result according to the following sequence.

If copper was being underestimated by the analyzer, then,

since copper can absorb the emitted Pb Lb radiation, an

underestimation of copper would lead to an underestimation

of its effect on lead and, therefore, an underestimation of the

lead. To study this possible effect further, samples were taken

from the copper smelter, where concentration values were an

order of magnitude or more above the lead. At this facility,
 

copper and iron concentrations (by ICP OES analysis of CFC

filters) could be found in excess of 4000 mg m-3

, but were more

commonly in the range 100 500 mg m-3

, while lead typically

was less than 100 mg m-3

. Analysis of variance (ANOVA) and

a non parametric test was carried out on the CFC and IOM

samples from both the bronze foundry and the copper smelter

to test whether there was a significant difference between (a)

the copper results by XRF and ICP OES, and (b) the lead

results by XRF and ICP OES, and finally to determine if the

magnitude of the copper result had any effect on either of these

differences. No significant effects were found, even after

removal of outliers, and we conclude that there is no signifi

cant interference at either site.

The interior surfaces of both CFC and IOM samples were

wiped and the wipes analyzed separately. Lead was not present

in sufficient quantities to be quantifiable. An average of 25%

(maximum 55%, n 17) of the total copper sample was found

on the walls of CFC samplers, and 19% (max 38%, n 18) of

the total copper sample was found on the walls of IOM

samplers.

Table 4 Comparison of sealed source XRF measurement of Cr, Ni,

Fe and Mn on air filters (n 4) with neutron activation analysis of the

same filters

Low bias% High bias% Mean bias%

Chromium

Nickel

Manganese

Iron

27

40

23

28

44

6.1

32

50

9.5

17

13

12

Accuracy

The accuracy estimates in Table 5 were obtained using data

from the three studies where lead was the dominant metal

present in the analysis (lead ore processing mill, solder manu

facturer and lead acid battery recycler).

Table 5 Accuracy after bias correction of XRF Pb method for n data

points between 10 mg and 300 mg in comparison to ICP OES method

with assumed RSDref 5%. The ISO GUM approach to estimated

uncertainty, reduced by observed outliers, is presented together with

comparable bootstrap estimates. For complete explanation see ref. 10

ISO GUM Bootstrap

Sampler N Bias Outliers Aˆ A95% Aˆ A95%

GSP (ave.) 83 0.077 0 23.1% 27.4% 20.5% 25.1%

CFC (OSHA) 70 0.006 0 24.6% 29.8% 29.8% 31.8%

25 mm 57 0.062 1 26.0% 33.5% 33.1% 37.6%

Button 68 0.089 2 21.0% 30.1% 24.0% 26.5%

IOM 58 0.00 1 34.2% 44.0% 39.7% 43.3%

The three studies

where lead was not the dominant metal (bronze foundry,

nickel chromium recycler, copper smelter) were not included

in the analysis. Two methods for determining the overall

accuracy of the XRF analysis of filters were used. Accuracy

was determined after correction for bias, and any method

based on these results should include this bias correction to

assure accuracy within the ranges given. The CFC correction

however is insignificant and could be ignored. A full discussion

of the statistical methods used to obtain these results has been

published.11

Limit of detection and quantitation

The lowest sample ICP OES value detected by the sealed

source XRF analyzer on a CFC filter was 3.8 mg. This ICP

OES value was given for two filters, and they were recorded by

the THERMO NITON XRF analyzer as 2.5 mg and 3.3 mg.

Values from 1 to 3 mg were not detected. On 25 mm filters

(IOM or 25 mm open face cassette), values of 2 mg by

ICP OES were detected about half of the time. Thus, a

conservative limit of detection (LOD) is probably 4 mg, and

thus blank filters were generally recorded as ‘‘non detect’’ by

the analyzer. Twenty eight data points falling between 5 and

15 mg per CFC filter by ICP OES were used to develop a limit

of quantitation (LOQ), in accordance with Appendix 1 in the

ESI.w At the 90% confidence level, the LOQ is 4.6 mg per filter.

At the 95% confidence level the LOQ is 12.3 mg per filter,

which is less than one half the OSHA action level. However,

even with triplicate filter measurements, the tube type instru

ment could not achieve these limits as it provided negative

values for lead at low loadings. An LOD and LOQ were not

calculated for the tube type instrument.

Comparison of samplers with each other

One extremely surprising result of this study was the compar

ison of lead concentration results (by ICP OES analysis of the

filters) between pairs of different samplers taken from the same

location or person at the same time (some studies involved

area sampling only, some personal sampling only, and some a

mixture of both). A comparison of mean sampler ratios for all

pairings is shown in Table 6. All 95% confidence intervals

include a ratio of unity, so it is not possible to detect a

difference between these samplers with the numbers of pairs

listed. However, it is possible to consider the 95% confidence

limits as upper limits on the possible differences between

samplers, especially when the standard deviation is small (note

the Button sampler is the source of the largest standard

deviations). For example, the result expected from the pub

lished studies of sampler performance with coarse particles in

the laboratory, or from most previously published field studies

where coarse particles are observed, is an IOM/CFC ratio of

1.5 or higher. Clearly, this is not the case with these results. In

particular, the similarity between the filter concentrations

derived from the 51 pairs of IOM and CFC samplers is

striking, and we believe it important to reproduce the figure

published elsewhere12 again here, as Fig. 1. It is tempting to

suggest that it is the result of sampling aerosols with a size

distribution with most mass contribution from particles smal

ler than about 30 mm, where most samplers are expected to

have similar results. However, the range of environments

sampled within these six industries was large, and included

processes such as crushing lead ore and waste batteries, and

the handling of the crushed materials, where the presence of

large particles is very likely. The observation of similar filter

concentrations prompted us to examine the possibility of other

deposits within the sampler, revealing that for some samplers,

half of the aspiration could be on the filter and the other half

on the interior sampler walls. This was recognized as being

important of 13 during the development the IOM sampler and,

therefore, protocols for the use of the IOM mandate inclusion
 

 

 

 

 

of the wall deposits as part of the sample. For the CFC, this

observation had also been made previously by the US 14 OSHA, whose methods include the contribution of wall

deposits in both gravimetric analyses, through the use of an

internal capsule in the CFC, and analyses for metals by 12 including a wipe of the CFC interior. Unfortunately, this

was not considered in almost all of the studies that were

carried out comparing the CFC and IOM samplers, except 15 for those carried out in France. As a part of the program

summarized here, the CFC and IOM sampler wall deposits

were compared at the lead ore processing plant (for lead) and

at the copper smelter (for copper). A different relative dis

tribution between filter and wall deposits was observed when

compared to results from the French studies, but this is not

surprising as the two sites in our study probably represent a

different size distribution of the aerosol (although it may also

due to a slight difference in the geometry of the IOM 12 capsule). The conclusion of these studies is that the aspira

tion efficiency of the two samplers (CFC and IOM) is not as

different as previously supposed. However, the problem for

the analysis of air samples by on filter XRF is that wall

deposits cannot be accounted for, and so the result can only

be a minimum estimate of the concentration, perhaps as little

as 50% of the total sampler aspiration. Given that the

percentage of wall deposit varies considerably between sam

plers from even the same environment, it is impossible to

account for the wall deposits by a correction factor.

Table 6 Comparison of concentration values between samplers located side by side (ICP OES analysis of lead in filters only). SD standard

deviation of the mean sampler ratio, CI confidence interval

Sampler pairing n Mean sampler ratio SD Lower 95% CI Upper 95% CI

Button/GSP 35 1.26 0.792 0.996 1.52

37 mm/Button 43 1.11 1.06 0.793 1.43

37 mm/25 mm 53 1.11 0.713 0.916 1.30

IOM/GSP 41 0.987 0.471 0.843 1.13

37 mm/GSP 45 1.09 0.608 0.915 1.27

IOM/Button 41 0.991 0.418 0.864 1.12

Button/25 mm 29 1.24 1.02 0.863 1.61

25 mm /GSP 37 1.04 0.435 0.896 1.18

IOM/25 mm 34 0.875 0.388 0.745 1.01

37 mm/IOM 51 0.984 0.378 0.880 1.09

Fig. 1 Comparison of filter only analyses for lead (Pb) from side by

side IOM and CFC samplers (n 51; y 1.02x, r

2 0.93). x 

lead acid battery recycler, B bronze foundry, m lead ore processing

mill, + copper ore smelter, & nickel chromium recycler, K solder

manufacturer. Also published in ref. 12.

Study conclusions

Air samples have been collected in six workplaces where lead is

encountered, selected to represent a range of different envir

onments. In each case, the sample filters were analyzed using a

non destructive, portable XRF analyzer, and then by acid

digestion and ICP OES. Results from two sites are provided

here, while results from the other four have been published

previously. XRF results from the three facilities where lead

was the major element present in the airborne dust have been

combined and tested for accuracy against the corresponding

ICP OES results. The CFC, GSP, Button and 25 mm open

face samplers all gave results within 35% of the corresponding

ICP OES values once corrected for bias in the XRF analysis.

At the three facilities where lead was not the major element

present, results were not quite as good, but this might be a

result of increased variance in the reference method. For the

sealed source XRF instrument and the CFC sampler, the limit

of detection is 4 mg per filter and the LOQ (95% confidence

limit) is 12.3 mg per filter. Insufficient low lead samples were

available to calculate these figures of merit for the other

samplers, but they are expected to be similar. All samplers

had some bias, but in the case of the CFC it is included in the

algorithm for calculating the lead mass, so that no further

correction is necessary.

Samples had been collected side by side, and when the filter

analyses were compared, there were no significant differences

between them. This was a surprising result in view of published

accounts of differences, particularly between the IOM and

CFC samplers. However, protocols call for analysis of wall

deposits for the IOM capsule, but not always for the CFC.

When wall deposits were compared from samples at the lead

mine and the copper smelter, they were found to be not

significantly different, and of a magnitude that would account

for at least some of the differences found in other studies. This

confirms previously published observations. The presence of

wall deposits in many samplers begs the question as to their

inclusion as part of the sample. Further studies are taking

place to determine the particle size of these deposits as an aid

to understanding their significance. Obviously, if wall deposits

must be included, XRF analysis, which can only be performed
on the filter, can only give a minimum estimate of the total

lead in the sample.

The conclusions regarding portable XRF analysis may be

summarized by sampler:

CFC—without support pad and using three readings combined according to the OSHA algorithm, results of the filter

analysis meet accuracy requirements, and the limit of quantitation is below one-half of the OSHA action level. However,

wall deposits are significant and are not included in the XRF

analysis.

IOM—sample deposit is not sufficiently homogenous for

this kind of on-filter analysis. Wall deposits, while less than

with the CFC for similar environments of sampling, are still

often significant and are not included in the XRF analysis.

GSP—results of the filter analysis using the average of three

readings across the filter meet accuracy requirements after a

small bias correction. Wall deposits have not traditionally

been included with analysis of this sampler, although they

were found to exist in the one study where they were examined.

The fact that the GSP filter-only analyses were not significantly different from the filter-only analyses of the other

samplers suggests that it might be necessary to account for

wall deposits in this sampler too.

Button—results of the filter analysis with a single XRF

reading met accuracy requirements once corrected for a large

negative bias, which is probably a result of heavy filter

loading. The filter loading causes a practical problem in that

sampling times must be kept short (around 90 minutes maximum) in dusty areas to avoid pump shut-off. This sampler has

no walls for sample to collect on. The fact that the filter-only

analysis is not significantly different from other samplers with

which it was paired suggests that the aspiration efficiency of

this sampler is lower than the others. Under-sampling of very

large particles compared to other samplers has been noted in

some other studies, but this cannot be confirmed in this

program.16

25 mm open-face cassette—results of the filter analysis with

a single XRF reading met accuracy requirements after a small

bias correction. However, wall deposits are very significant

and, as noted, cannot easily be included in the XRF analysis.

The main conclusion of the study bears repeating. While

most samplers (except the IOM) provided filter deposits that

could be analyzed for lead acceptably by a portable XRF

analyzer when lead was the dominant metal present, all

samplers had a good probability of having wall deposits that

cannot be analyzed by either portable or bench-top XRF

analyzers. These wall deposits are often a significant part of

the sample and there is no likelihood of correcting for their

absence in the analysis. If these sampler wall deposits are

considered part of the sample then direct on-filter analysis is

not possible. The one sampler that is unlikely to have wall

deposits, the Button sampler, had comparable filter deposits to

the others, suggesting that it might be undersampling the

atmosphere. The GSP sampler has been considered to meet

the inhalable sampling convention without considering wall

deposits. However, the results of this study suggest that this

might not be the case.

Disclaimer

The findings and conclusions in this report are those of the

author(s) and do not necessarily represent the views of the

National Institute for Occupational Safety and Health.

The mention of any company names or products does not

imply an endorsement by NIOSH or the Centers for Disease

Control, and nor does it imply that alternative products are

unavailable, or unable to be substituted after appropriate

evaluation.

Acknowledgements

Thanks to Yinghua Sun (NIOSH/HELD/BEB) for assistance

with parts of the statistical analysis.

References

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