A PASSIVE TUBE-TYPE SAMPLER FOR THE DETERMINATION OF FORMALDEHYDE
VAPOURS IN MUSEUM ENCLOSURES
L.T. Gibson and A.W. Brokerhof
Summary—A tube-type passive sampling method has been developed and assessed for the
quantification of formaldehyde (methanal) vapours in indoor air. The sampler was designed for use
in museums where test sites often include small enclosures with low air movement. The procedure
involves collection of formaldehyde vapours in a Palmes diffusion tube containing a paper support
impregnated with an acidified solution of 2,4-dinitrophenylhydrazine (2,4-DNPH). After sampling,
quantification of the trapped F-DNPH is achieved by high performance liquid chromatography
(HPLC) analysis with UV detection at 350nm. To validate the procedure, permeation devices were
used to generate formaldehyde-containing atmospheres, 81-2975ppb, in a 20dm3 chamber so that
experimentally derived sampling rates could be calculated and compared with the theoretical value.
Three 2,4-DNPH solutions were investigated to obtain an efficient and stable trapping solution.
Best results were achieved with a 27mg.ml-1 solution of 2,4-DNPH which contained 4-5%v/v
orthophosphoric acid. At 55% RH, and with low airflow in the chamber, the experimentally derived
sampling rate of 1-34 ± 0-17ml.min-1was in good agreement with the theoretically derived sampling
rate of l-36ml.min-1. The passive sampling method was repeatable and reproducible with RSD
(relative standard deviation) values below 7% for long-term exposures at low air velocities.
Introduction
Indoor air pollution in museums originates from a number of sources including the off-gassing of
unsuitable structural or decorative materials. As a result, deleterious microenvironments have been
measured inside cabinets, drawers or cases currently used to store or display valuable museum
artifacts. One of the principal groups of pollutants of concern are carbonyl pollutants which include,
among others, formaldehyde (methanal) vapours. Synthetic wood boards such as plywood,
particleboard, chipboard and fibreboard can emit significant quantities of formaldehyde vapour, as
they commonly contain urea-formaldehyde (UF) or phenol-formaldehyde (PF) resin binders [1-8].
Both resins contain a free source of formaldehyde which will, during the early lifetime of the board,
diffuse from the surface of the board into the surrounding air. After the free formaldehyde has been
released, reactive methylol end-groups and dimethylene ether bridges in the resin can undergo
hydrolytic degradation resulting in further, lower but continuous, emission of formaldehyde vapour
[9]. This extractable formaldehyde does decrease with time, but to non-zero values. Boards bound
with PF resin, which have greater hydrolytic stabilities, emit approximately 10 times less
formaldehyde vapour during the chronic release stage [10]. If emissions occur in well-sealed enclosures with low air-exchange rates, an atmosphere containing steady-state concentrations of
formaldehyde will exist, placing susceptible objects at risk. The deleterious effects of formaldehyde
vapour on a wide range of objects have been reported previously [11-13]. In the case of metals,
formaldehyde is first thought to oxidize to formic (methanolic) acid, via the Cannizzaro reaction,
before corrosion of the metal surface is initiated [14, 15]. This reaction is promoted on the surface
of metals, with zinc being cited as a particularly suitable catalytic surface [16]. and in the presence
of oxidants [17]. High RH values accelerate the reaction, but even at 20% RH metal corrosion has
been observed [16]. Lead, zinc, brass, copper, silver, sterling silver and bronze have all corroded in
the presence of formaldehyde (1200ppb) at ambient humidities for 100 days [18]. It has also been
reported that formaldehyde alters the characteristics of cellulosic materials, such as paper and
textiles [11]. Druzik et al. refer to a study which states that increased brittleness, denaturing and
loss of elasticity in protein materials such as gelatine, animal glue and casein occur due to crosslinking with formaldehyde [19].
It could in fact be argued that some of the damage previously attributed to the presence of
formaldehyde vapours might in fact be due to formic acid which was not routinely measured and
hence remained largely undetected in the 1980s and the early 1990s. Moreover, a recent study into
the effects of formaldehyde, formic acid and acetic (ethanoic) acid on lead, copper and silver
concluded that these gases pose a negligible threat to silver and that only the organic acids pose a
threat to copper alloys [20]. In order to understand the threat
formaldehyde vapours pose to different materials, a device that can provide an accurate
measurement of the formaldehyde concentration in museum enclosures is essential. The device can
then be used (i) to determine the correlation between formaldehyde concentration and corrosion of
susceptible materials and (ii) to assess the risk associated with long-term storage or display of
susceptible objects in contaminated enclosures.
Semi-quantitative formaldehyde measurements have been obtained using colorimetric techniques
such as dosimeter tubes [21, 22]. However, the colorimetric detector tubes are prone to interference,
have low sensitivities, often incorporate large precision errors (demonstrated in one study as
ranging from 15 to 60% [23]), and their detection limits are often above those values of concern in
museums [24]. Since 1979 [25], active sampling methods using chemically modified solid traps
have been used successfully for the accurate and sensitive determination of formaldehyde vapour in
indoor air. Solid phase extraction cartridges impregnated with 2,4-dinitrophenylhydrazine, one of
the most common solid traps, have been used since the early 1980s [26]. However, the use of noisy,
obtrusive, expensive sampling pumps which have to be calibrated on site make these systems less
attractive for museum applications. Moreover, when small museum cabinets or drawers (<1dm3) are
actively sampled, the volume of air extracted is too small to permit detection of formaldehyde in the
low ppb range. When too much air is extracted (approximately >10% of the air volume to be
sampled), dilution effects are observed [27].
For implementation in a museum environment, passive sampling methods which are sufficiently
unobtrusive, simple to use, economical, and do not require on-site calibration are recommended.
Further advantages over active sampling methods include the possibility of shipping samplers to
and from the museum for deployment and analysis, respectively, and the ability to measure in areas
without using electricity. Although a number of badge-type passive samplers are commercially
available, this design of sampler is not suited to areas with stagnant air or where there are low
airflow rates [28]. Laminar boundary layers commonly form over the surface of the badge when the
surrounding air velocity falls below 10cm.s-1 [29-32]. If this increase in diffusion length is not
considered, measured concentrations will underestimate the true value, leading to a negative
sampling bias in certain situations.
A new passive sampling method for the detection and accurate quantification of formaldehyde
vapour in museum air is, therefore, necessary. This paper describes the development and assessment
of a tube-type passive sampler, based on the Palmes diffusion tube [33], for the quantification of
formaldehyde vapours in museum enclosures. Similar devices have been successfully validated to
collect acetic acid and formic acid vapours in museum air [34, 35]. As defined in the European
Committee for Standardisation's (CEN) protocol on diffusive sampler testing [36], the effects of
sampling time, pollution concentration and storage were studied under laboratory conditions. The
sampler was tested over a range of formaldehyde vapour concentrations (88-2941ppb) and exposure
times (5-168 hours) at 55% RH and room temperature (approximately 23°C).
Diffusion theory
The Palmes diffusion tube operates according to Pick's first law of diffusion. The equation which is
used to determine the atmospheric concentration (μg.m-3) of formaldehyde in air is given by:
where M is the mass (μg) of pollutant collected, L is the length of the diffusion tube (m), D,, is the
diffusion coefficient of formaldehyde in air (m2s-1), A is the cross-sectional area of the tube (m2)
and t is the exposure time (s). More details regarding the derivation of this equation from Fick's law
can be found in a previous paper [34]. Frequently, the concentration of vapours in air is given in
parts per billion (ppb). To convert the concentration units from μg.m-3 to ppb, the concentration
must be multiplied by 1/d, where d is the density of the gas at the temperature of interest. This is
equivalent to multiplication by 24-46/MW, where 24-46 is the molar volume of one mole of gas at
standard temperature and pressure (STP) and MW is the molecular weight of the gas. In this study
the conversion factor used equates 1mg.m-3 of formaldehyde to 0-814ppb, which assumes a
temperature of 25°C. In a field application, the conversion factor used should consider the
temperature of the measured environment.
Using a tube 7-1 X 10-2m long, with a cross-sectional area of 9-5 X 10-5m2, the theoretical sampling
rate, SR, of formaldehyde in the sampler (with D1,2 = 1-7 X 10-5m2s-1) is 2-27 X 10-8m2s-1 , which is
equivalent to l-36ml.min-1.
SR = D1,2A/L
(2)
It is possible to determine the experimental sampling rate, SRe, of the sampler exposed to known
formaldehyde concentrations when the mass of
formaldehyde collected over time is known, by:
SRe = M/Ct
(3)
In order to determine any significance between differences in theoretical sampling rates and those
derived experimentally, statistical t-tests and F-tests were employed [37].
Experimental procedures
Preparation of the sampler
The sampler consists of an acrylic (polymethyl-methacrylate) tube, 7-1cm long and Hem in diameter, with two acrylic end-caps. Formaldehyde vapours, if present in the surrounding air, will diffuse
into the open tube towards a filterpaper disc impregnated with 60u,l of an acidified solution of 2.4dinitrophenylhydrazine (hereafter referred to as 2,4-DNPH) where it is trapped as its hydrazone
derivative (hereafter referred to as F-DNPH). The trapping solution prepared was studied to ensure
a high collection efficiency and stability. The reaction between formaldehyde and 2,4-DNPH
involves nucleophilic attack of the carbonyl (thought to be the rate determining step) followed by a
1,2-elimination of water. Thus, the presence of an acid is necessary to protonate the carbonyl
moiety prior to nucleophilic attack in order to increase the nucleophilicity of the 2,4-DNPH. Three
trapping reagents with different 2,4-DNPH concentrations and acid content were prepared by
dissolution of orthophos-phoric acid and doubly recrystallized crystals of 2.4-DNPH in acetonitrile
(Table 1). Ethylene glycol dimethyl ether was added to two solutions as it is hygroscopic and aids
impregnation of the paper support. The three solutions were prepared in order to demonstrate:
• the necessity to control the amount of acid added to the solution (solution A vs solution B)
Table 1
Preparation of acidified 2,4-DNPH impregnating solutions
•
the effect of using a wetting agent (solution B vs solution C)
•
that the concentration of the 2,4-DNPH in the trapping solution does not have to be controlled
when the amount of 2,4-DNPH used in the sampler is in excess (solution B vs solution C)
High performance liquid chromatography (HPLC) analysis
After a specified sampling time the F-DNPH trapped in the sampler is washed from the filter by
sonicating the filter for one minute in 2ml of acetonitrile. To measure the concentration of F-DNPH
in the acetonitrile extract, HPLC analysis was performed. A Waters 600S pump was used to obtain
an isocratic flow of a methanol-water (67:33v/v) eluent through a Waters Nova-Pak CIS column at
1ml.min-1. The eluent was filtered before use. using Millipore O45mm HAWP filters (47mm
diameter). Samples were injected using a Waters 717 autosampler, and detection of the F-DNPH
was achieved with a Waters 996 photo diode array with UV detection at 350nm. Data were
collected using a Waters Millennium data system. The F-DNPH peak was detected at approximately
seven minutes run time; however, when samples were injected, the run time was increased to ensure
that there was no carry-over of later eluting analytes (e.g., from degradation products, see later
section on stability of trapping solutions).
Daily calibration of the HPLC required F-DNPH solutions of known concentration. Due to the high
expense of commercially available F-DNPH salt (approximately £50 for 20mg), the salt was prepared in the laboratory by reacting doubly recrystallized 2,4-DNPH with formalin solution. The
purity of the resulting F-DNPH salt was validated by dissolving the appropriate weight of the salt in
acetonitrile to produce a solution concentration of approximately 25mg.ml-1. The solution was
injected into the HPLC and the resulting chromatographic peak was compared with a calibration
graph obtained from F-DNPH solutions using commercially available salt (>99% pure). Once the
concentration of the calibration solution was verified as 26mg.ml"1, increasing volumes (l-20ml) of
the solution were injected into the HPLC to provide daily calibrants corresponding to F-DNPH
weights of 26-520ng. The mass of F-DNPH trapped by the sampler was determined by injecting
10ml of the extracted solution into the HPLC and by comparing the peak area of the F-DNPH from
the extracted solution with the daily calibration graph. A linear least squares regression programme
was used to determine exact solution concentrations and
to periodically check the concentration of the daily calibration solution.
Preparation of standard atmospheres and sampler exposure
A 20dm3 exposure chamber was used to generate environments for sampler investigation. Accurate
vapour phase concentrations of formaldehyde vapour in air were prepared by passing dry air over a
formaldehyde permeation device which continually flowed through the exposure chamber, then to
waste (i.e., a dynamic system was employed). The permeation device was held in an oven to control
the temperature and to provide a constant permeation rate of formaldehyde vapour. The concentration C of formaldehyde in the flowing stream was calculated using
where Km is the molar volume of the gas divided by its molecular weight (used to convert the
concentration in ng.ml-1 to ppm), P is the permeation rate in ng.min-1 and f is the flow rate in
rnl.min-1. Prior to all experiments, the permeation device was allowed to equilibrate to the oven
temperature for two weeks, after which time the permeation rate was measured gravimetrically for
each experiment. To increase the permeation rate, and thus formaldehyde concentration in the
chamber, the temperature of the oven was increased and the new permeation rate was determined
gravimetrically, again after a two-week equilibration period. Before entering the exposure chamber,
the formaldehyde-containing flowing stream was first diluted with a humidified air stream to
provide concentrations in the desired range at approximately 55% RH. Formaldehyde
concentrations were generated at four levels of concentration: approximately 100. 400. 1200 and
3000ppb. The vapour phase concentrations generated inside the chamber were calculated for each
experiment using equation (4); the measured permeation rates were determined gravimetrically and
the airflow rates over the permeation device and the humidification system were measured using
mass flow controllers. The concentrations generated inside the chamber are referred to as "chamber
concentration' in the following result sections. In all experiments, quadruplicate samplers were
prepared with 60u.l of either solution A, B or C (see Table 1), and all 12 samplers were exposed
simultaneously in the formaldehyde-containing environments. Only solution A was assessed at the
lowest concentration. At approximately 400ppb two experiments were performed over 23 hours to
assess the repeatability of the procedure. Triplicate blank samplers were
also prepared with each solution, but remained capped at room temperature during each period of
exposure to obtain the mean F-DNPH background concentration. After blank correction, the mean
masses of formaldehyde collected by each set of four samplers were used to determine the mean
experimentally derived vapour phase concentration (referred to as "measured concentration" in the
following result sections) and experimental sampling rate. The associated errors were propagated
throughout the calculations.
Unfortunately it was not possible to study the effect of a range of relative humidities or wind
velocities with the apparatus used in this study. However, an atmosphere with low humidity (<5%)
was achieved by removing the diluent flow from the humidifying system, and a different air
velocity was created by placing a small rotary fan on the floor of the chamber. Samplers were
therefore assessed under these conditions over the same range of formaldehyde concentrations.
Temperature control of the chamber was also not possible with the equipment used: however,
kinetic theory of gases dictates that the total mass of analyte collected by a diffusion-controlled
sampler is only slightly dependent on temperature. It was therefore assumed that temperature effects
on the diffusive sampler would be negligible when used in a controlled museum environment.
Stability of samplers
Samplers will usually be prepared and analysed at a laboratory which is remote to the museum test
site: therefore, it is important to determine the stability of the samplers to ensure that degradation
does not occur during shipping or storage. A number of replicate samplers were prepared using
solutions A. B or C to compare the effect of different acid concentrations and the presence of the
wetting agent on stored samplers. The samplers remained capped and were held at room
temperature (approximately 23°C) or in a refrigerator at 4°C. Over a period of one month, every
two or three days, two samplers were removed from the set of stored blanks and analysed. In
addition, the trapping solutions used to impregnate the filters were also stored at room temperature
or in the refrigerator, and samples were extracted every two or three days and analysed.
Results and discussion
Performance of samplers at 55% RH
Various sets of samplers prepared using trapping solution
solution,
A
(22-2mg.mgl-1
2,4-DNPH
Table 2
A
Experimentally derived sampling rate, SReA , of samplers prepared with trapping solution
Errors were calculated as ± 1 standard deviation of the mean result obtained by quadruplicate tubes.
22%v/v acid) were exposed to formaldehyde-containing environments in the range 81-2975ppb.
The relative standard deviation (RSD) values for masses (ng) of formaldehyde collected by
quadruplicate samplers in each experiment ranged from 1 to 7%, thus demonstrating acceptable
repeatability of measurement (Table 2). At each level of concentration (approximately 100, 400,
1200 or 3000ppb), sampler sets were exposed for different times. As expected, the masses of
formaldehyde collected increased linearly with sampling times regardless of chamber concentration
(Figure 1). From the mean masses obtained, the corresponding mean vapour phase concentrations
were calculated as described in equation (4) and converted to ppb concentration units (Table 2).
When measured concentrations were compared with the chamber concentrations a positive linear
correlation (n = 14) was obtained with a slope of 0-87 ± 0-02 and intercept of 9 ± 42. Regression
analysis was performed on the data and a Pearson correlation coefficient of 0-995 was obtained. In
addition, analysis of the residuals using one-way ANOVA and examination of the residual plot
confirmed that a linear correlation was obtained. The mean experimentally derived sampling rate,
SReA , over the full range of concentrations studied was 1-29 ± 0-16ml.min-1 (n = 56, 14 experiments
each with quadruplicate tube exposure), which is not statistically different from the
theoretical sampling rate of l-36ml.min -1 at the 95% confidence level (α = 0-05).
Lowering the acid concentration of the trapping solution used in the sampler and removing the wetting agent (solution B, 4-5% acid) had little effect on the experimental sampling rate of the device
(Table 3). The masses of formaldehyde collected
Figure I Increase in mass of HCHO collected over time for samplers containing Solution A.
Concentrations in chamber ranged from 100(◊) 400 (□), 1200 (∆) and 3000 (X) ppb
Table 3
B
Experimentally derived sampling rate, SReB , of samplers prepared with trapping solution
Errors calculated as described in Table 2.
over time still increased linearly at approximately 400 and 3000ppb (Figure 2) and a linear
correlation (n = 10) was obtained between the measured and chamber concentrations over the
concentration range studied. Regression analysis was performed on the results giving a slope of 0823 ± 0-002 and intercept of 62 ± 34. The Pearson correlation coefficient was determined as 0-998,
and again the residual plot indicated a good fit of the linear model as the residuals were randomly
scattered around zero. Differences between the overall mean experimentally derived sampling rate
SReB , 1-34 ± 0-17ml.min-1 (n = 40), and the theoretical value
Figure 2 Increase in mass of HCHO collected over time for samplers containing Solution B.
Concentrations in chamber were 400 (□) and 3000 (X) ppb.
were not statistically significant (a = 0-05) and could be attributed solely to random variation. In
individual experiments, the masses collected by quadruplicate samplers demonstrated an acceptable
level of repeatability as RSD values of 1-5% were obtained.
Similar results were obtained when the concentration of 2,4-DNPH used in the sampler was
lowered (Table 4). The masses of formaldehyde increased linearly over time (Figure 3) despite the
lower concentration of 2.4-DNPH available for reaction with formaldehyde, and measured
concentrations were in good agreement with those generated in the chamber as demonstrated by the
regression analysis. The linear model obtained (n = 10) had a slope of 0-87 ± 0-03 and intercept of
37 ± 60 with a Pearson correlation coefficient of 0-994. Examination of the residual plot confirms
that the residuals are randomly scattered around zero confirming that the linear model adequately
describes the correlation in the data. The overall mean experimental sampling rate SReC, 1-29 ± 017ml.min-1 (n = 40), was again not statistically different from the theoretical sampling rate over the
range of concentrations studied (a = 0-05). Acceptable repeatability was demonstrated as individual
RSD values ranged from 1-8% for individual experiments.
Effect of increased air movement
Higher air movement was created inside the chamber using a small fan, and the three sets of
quadruplicate samplers were exposed to the same range of formaldehyde concentrations and
exposure times described above. The overall mean sampling rates
Table 4
C
Experimentally derived sampling rate, SReC , of samplers prepared with trapping solution
Errors calculated as described in Table 2.
of sampler sets deployed in turbulent conditions, labelled SReA(t) , SReB(t) and SReC(t) in Table 5, were
not statistically different from those derived in the previous experiments (α = 0-05). Comparison of
the mean measured concentrations were still linearly correlated with the chamber concentrations (n
= 15, five measurements for each solution) despite increased air movement across the face of the
sampler and the potential for eddy formation at the mouth of the tube. The linear regression model
between the measured concentrations and the chamber concentrations gave a slope of 0-88 ± 0-03,
an intercept of 57 ± 44 and a Pearson corre-
Figure 3 Increase in mass of HCHO collected over time for samplers containing Solution C.
Concentrations in chamber were 400 (□) and 3000 (X) ppb.
lation coefficient of 0-994 (n = 60). Examination of the residual plot confirmed_the random
distribution of residuals around zero, hence indicating no lack of fit of the linear model to the
experimental data points. The increased air movement did have a slight effect on the repeatability of
measurement which decreased slightly, giving RSD values for masses collected by quadruplicate
samplers of 1-9. 5-11 or 1-14% for solutions A. B or C, respectively.
Sampling in dry atmospheres (<5% RH)
The diluent airflow was removed from the humidifier and the humidity inside the chamber was
reduced to <5% RH. The results (Table 6) show that the relative humidity does have an effect on
samplers prepared with solutions containing a high acid concentration (solution A, 22%v/v acid).
The sampling rate decreased significantly from 1-29 ± 0-16 to 0-74 ± 0-14ml.min-1 (Table 6) and
measured concentrations, although still linearly correlated with chamber concentrations, were
consistently lower (Figure 4), despite the use of a wetting agent. In contrast, samplers prepared with
solutions containing approximately 5% acid concentration were affected to a much lesser extent and
even at this humidity level, sampling rates were only reduced by 12 and 6%, to 1-18 ± 0-08 and 121 ± 0-06ml.min-1, respectively.
The low yield in samplers using solution A at 5% RH indicates that the formaldehyde will only be
completely absorbed in the tube when the ratio of water to acid in the impregnated support is in the
correct range. Other workers have shown that an
Table 5 Experimentally derived sampling rates of samplers deployed in the chamber with higher
air movement: three different trapping solutions (A-C) were used
Solutions A. B and C contained 22%, 4-5% or 5% acid concentration, respectively. Errors
calculated as described in Table 2.
incomplete reaction occurs at low RH because the pH of the collection medium is lower in dry air
than in humid air. In water, the formation of the F-DNPH derivative is optimised at pH 1-5, and
recoveries decrease significantly if the pH of the solution rises above 2-5 or falls below 1 [38].
However, in contrast, other workers have shown that the influence of water on aldehyde derivatives
in acidic and non-acidic solutions of acetonitrile was negligible [39]. This suggests that the lowrecoveries for the 22% acid solution might be due to protonation of the amine group, thus
decreasing its nucleophilicity. Clearly, to avoid this situation occurring, solutions must be prepared
with lower acid concentration.
Stability of the trapping solutions and passive samplers
2,4-DNPH solutions prepared with 22%v/v acid (solution A) or approximately 5%v/v acid
(solutions B and C) showed no evidence of solution degradation after 35 days regardless of storage
temperature (i.e., at 4 or 23°C). 2,4-DNPH solutions extracted from stored samplers prepared with
solution B or C were also stable regardless of temperature (Figure 5). Samplers prepared with
solution A (22% acid), however, were significantly altered when stored at room temperature. The
chromatogram of the extracted solution after 35 days (Figure 6b) had a small 2.4-DNPH peak at 4-5
minutes, thus only a small amount of unreacted 2,4-DNPH solution remained on the filter support
after this length of time. In fact the concentration of unreacted 2,4-DNPH solution (as indicated by
the peak area in the chromatogram) decreased even after a few days when stored at room
temperature. After 28 days, less than 10% of the 2.4-DNPH solution was available for reaction with
formaldehyde (Figure 7). There was also significant evidence of by-products as major peaks
appeared at 5, 30 and 35 minutes, with more minor components being recorded between 7 and 28
minutes (see Figure 6b). Identification of the unknowns was attempted using gas chromatographymass spectrometry: however, the high acid concentration in the extracted solutions interfered with
the analysis and the unknowns were not identified. It was thought that acid hydro-
Table 6 Experimentally derived sampling rates of samplers in the chamber with 5% RH: three
trapping solutions (A-C) were used
Solutions A, B and C contained 22%, 4-5% or 5% acid concentration, respectively. Errors
calculated as described in Table 2.
lysis of the cellulose filter support produced a range of higher molecular weight aldehydes and
ketones which, after reaction with the 2,4-DNPH. Formed
Figure 4 Chamber at 5% RH. Comparison of measured formaldehyde concentrations with expected
chamber concentrations. Samplers contained trapping solutions A (X), B (□) or C (∆).
the unidentified hydrazones observed in the chromatogram.
Comparison of tube-type sampler with a commercially available badge sampler
Quadruplicate tubes, prepared with impregnating solution B (see Table 1), were placed inside the
Figure 5 Chromatograms of extracted solutions from samplers prepared with trapping solution B
14-5% acid) after storage for 35 days (a) in refrigerator at 4°C, (b) at room temperature (23°C).
Figure 6 Chromatograms of extracted solutions from samplers prepared with trapping solution A
(22% acid), after storage for 35 days (a) in refrigerator at 4°C, (b) at room temperature (23°Cj.
chamber with duplicate badge-type passive samplers bought from a commercial supplier and
exposed to a range of formaldehyde concentrations. The initial idea was to use the commercially
supplied badges to validate the concentration inside the exposure chamber. The badges were
analysed in-house and formaldehyde concentrations were calculated as instructed by the
manufacturer. Both sampler types w'ere deployed for the same exposure time. Regression analysis
confirmed a linear correlation for the measured concentrations obtained for the badge and tube
(Table 7). The regression line had a slope of 2-01 ± 0-07. intercept" of 36-3 ± 43-2 and Pearson
correlation coefficient of 0-996. Examination of the residual plot confirmed that the residuals were
randomly scattered around zero, thus indicating a good fit of the linear model to the data. However,
the concentrations measured by the badge samplers were consistently 50-60% lower than those
measured by the tubes. The mean tube
Figure 7 Variation of peak area of unreacted hydrazine in stored samplers with time, prepared with
trapping solution A (22%v/v acid).
concentrations (n = 36) were not found to be statistically different from the chamber concentrations
(α = 0-05). whereas the overall mean (n = 18) experimental sampling rate of the badge. 11-71 ± l08ml.min-1. was significantly lower than the expected sampling rate (supplied by the manufacturer)
of 25-2ml.min-1. Sampler saturation was not expected under the experimental conditions chosen,
and this was supported by the presence of a large 2.4-DNPH peak in the chromatograms of the
badge sampler after exposure. A fan was placed inside the chamber to increase the airflow over the
face of the badge, but the sampling rates of the sampler were still lower than expected (Table 8).
The mean experimentally derived sampling rate did increase to 14-6 ± 4-3ml.min-1 but this value
was still significantly lower than the theoretical sampling
Table 7
Results of commercially available badge-type sampler at 55% RH
Table 8 Results of commercially available badge-type sampler at 55% RH and higher air
movement inside the exposure chamber
Errors calculated as described in Table 2.
rate. It is thought that the addition of the fan did not increase the velocity inside the chamber significantly, thus confirming the potential of badge-type samplers to underestimate concentrations when
used in areas with a low air velocity. The formation of laminar boundary layers and the subsequent
increase in diffusion length are the most probable sources of error in the experimental chamber used
in this study, since numerous studies published in the literature support the use of badge-type samplers in many other applications. It is thought that the tests raised some doubts on the validity of the
badge device when used under these test chamber conditions only. Further field studies confirmed
reasonable agreement between the badges and the tubes, when air velocities were greater than
lOcm.s-1 and the recommended length of badge exposure time was not exceeded.
Conclusions
Samplers prepared with a high acid concentration (solution A) demonstrate the need to control the
amount of acid added to the trapping reagent used in this study. Samplers prepared with approximately 5% acid trapping solution (solutions B and C) performed well over the formaldehyde
concentration range used in this study. The amount of 2,4-DNPH used in the trapping solution does
not have to be rigorously controlled as long as the number of moles of reagent used is far in excess
of the number of moles of formaldehyde it will react with (see comparison of solutions B and C). In
addition, the presence of a wetting agent did not appear to affect the performance of the sampler;
however, blank tubes prepared with the solution without the wetting agent did give slightly lower
blank values with better reproducibility. Mean experimentally derived sampling rates at 55% RH
and with low airflow in the chamber differed from the theoretical sampling rate by 1 and 5% for
trapping solutions B and C, respectively. When the humidity was decreased to 5%, the mean
experimentally derived sampling rates of the sampler differed from the expected value by 9 and 5%
for solutions B and C, respectively. Since this humidity was extremely low and the sampling rate
was little affected, it is thought that small fluctuations in humidity experienced in the museum
environment would not affect the sampling rate of the tube sampler. Repeatability of the sampler
has shown that, within individual experiments, typical RSD values were below 7%. Samplers were
stable, even at room temperature, for up to 35 days.
It is advisable to deploy the device in the museum environment for a minimum exposure time of
seven days to permit a reasonable volume of air to be sampled by the device. This time frame provides a detection limit of 100ppb, determined as three times the standard deviation of F-DNPH
measured in blank tubes held at room temperature for seven days. During this time, a saturation
level of 4000ppb is determined, using the conservative estimate that only 30% of the trapping
reagent is available for reaction with formaldehyde. The passive sampler can provide museum staff
with an alternative method of determining accurate concentrations of formaldehyde vapours in
artifact enclosures so that possible risks to collections can be assessed.
The samplers can also be used in laboratory situations where formaldehyde-containing
environments are generated and the correlation between concentration and material damage can be
investigated. Although some colorimetric sampling devices have
a detection limit similar to the samplers discussed in this study, it is thought advantageous to
provide an accurate value for the formaldehyde concentration in air. This will aid laboratory studies
that aim to correlate formaldehyde concentration with material damage and assist when comparing
formaldehyde concentrations at different locations. Further, the samplers can be used to test
materials, either by determination of an equilibrium vapour phase concentration (when the test
material is sealed in a closed environment), or by fixing the tube directly onto the surface of the
material to determine the emission rate. Several laboratory tests (including the material emission
tests) and field applications have been successfully performed with the tube-type sampler, and the
results will be reported in a future publication.
Acknowledgements
The authors wish to acknowledge Dr Brian Cooksey and Prof. David Littlejohn. University of
Strathclyde. for supervision of initial experimental work, and Henk van Keulen. Instituut Collectie
Nederland, for construction of the exposure chamber. In addition we would like to acknowledge the
work undertaken by Cecily Grzywacz at the Getty Conservation Institute which involved
assessment of the samplers both in the laboratory and in the field after the initial validation work.
The results of this study will be reported in a future publication.
Materials
Filter paper discs: Antibiotic Assay Disc. 0-2mm pore size. Hem diameter. Whatman. UK.
2,4-dinitrophenylhydrazine (70%). orthophosphoric acid (ACS reagent), acetonitrile (HPLC grade),
methanol (HPLC grade), ethylene glycol dimethyl ether (HPLC grade), formalin solution (37%.
ACS Reagent): Sigma. Dorset. UK.
Formaldehyde 2.4-dinitrophenylhydrazone: >99%, Aldrich, WI 53201, USA.
Formaldehyde permeation device: Dynacal*. VICI Metronics Inc.
570 Series Formaldehyde Dosimeter Badge: GMD Systems Inc.. 625 Alpha Drive. Pittsburah. PA
15238, USA.
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Authors
LORRAINE GIBSON. BSc
(Hons) in chemistry with computing applications at Glasgow University.
1991, and PhD in analytical chemistry at the University of Strathclyde, 1995. Postdoctoral research
directed towards the implementation of analytical science in preventive conservation was
undertaken at the University of Strathclyde and the Netherlands Institute for Cultural Heritage in
Amsterdam. Recently appointed lecturer in analytical chemistry at the University of Strathclyde.
Address: Department of Pure and Applied Chemistrv, University of Strathclyde, 295 Cathedral
Street. Glasgow Gl 1XL, UK.
AGNES BROKERHOF. MSc
in chemistry at Leiden University, 1987. and BA in art history. 1988.
Postgraduate research at University of Canberra and CSIRO. Australia, into safe pest control
methods for museum objects. Conservation scientist at the Central Research Laboratory, now the
Netherlands Institute for Cultural Heritage in Amsterdam since 1992. Address: Netherlands
Institute for Cultural Heritage, Gabriel Metsustraat 8. 1071 EA Amsterdam, The Netherlands.
Resume—Une methode d'echantillonnage passif en tube a ete mise au point et son utilisation pour
quantifier les vapeurs de formaldehyde dans I'air a ete evaluee. L'e chant illonneur a ete confu pour
etre utilise dans les musses, oil les sites devant etre inspectes consistent souvent en de petits
endroits fermes ou il y a tres pen de mouvement d'air. Le precede consiste a recueillir les vapeurs
de formaldehyde dans un tube de diffusion de type Palmes contenant un support papier impregne
d'une solution acidifiee de 2,4-dinitrophenylhydrazine (2,4-DNPH). Apres echantillonnage, on
effectue le dosage de la F-DNHP piegee par chromatographie liquide a haute performance (HPLC)
avec detection UV a 350nm. Pour valider la methode, on a utilise des dispositifs de permeation
pour generer des atmospheres contenant de 81 a 2975 ppb de formaldehyde dans une chambre de
20dm3, afin que les taux d'echantillonnage experimentaux puissent etre calcules et compares aux
valeurs theoriques. Trois solutions de 2,4-DNPH ont ete etudiees pour mettre au point une solution
piege stable et efficace. Les meilleurs resultats ont ete obtenus avec une solution de 2,4-DNHP a
27mg.ml~' contenant 4,5% (v/v) d'acide orthophosphorique. A 55% d'HR et avec un ecoulement
d'air faible dans la chambre, le taux d'echantillonnage experimental calcule de 1,34 ± 0,17ml.min~'
etait en bon accord avec le taux d'echantillon-nage theorique de 1,36ml.min~'. La methode
d'echantillonnage passif est reproductible avec des deviations standard relatives inferieures a 7%
pour des exposition d long terme d de basses vitesses de deplacement de I'air.
Zusammenfassung—Ein passiv arbeitendes Probenrohrchen :ur quantitativen Bestimmung von
gasformigen Formaldehyd ( Methanal) in Innenrdumen wurde entwickelt und getestet. Das
Probenrohrchen wurde spe:iell fur die Verwendung im Museum konzipiert, wo es Fldchen mit sehr
geringer Luftbewegung gibt. Bei der Methode werden die Formaldehydddmpfe in einem PalmesDiffusionsrohrchen gesammelt, in das ein mit einer angesduerten Losung von 2,4Dinitrophenylhydrazin (2,4-DNPH) impragniertes Papier gelegt ist. Die Menge des gesammelten
F-DNPH wird durch Hochdruckflussigkeitschromatographie (HPLC) bei einer Wellenldnge von
350nm bestimmt. Zur Uberpriifung der Methode wurden Formaldehyd-Konzentrationen von 812975 ppb in einer 200cm3 groflen Kammer erzeugt, so daft die theoretisch berechneten
Aufnahmeraten mit experi-mentellen Daten verglichen werden konnten. Drei verschiedene 2,4DNPH-Losungen wurden zur Optimierung der Stabilitdt und Effektivitdt der Apparatur untersucht.
Die besten Ergebnisse wurden mit einer Losung der Konzentration 27mg.ml~' erreicht, die
zusdtzlich 4,5% (v/v) Orthophosphorsaure enthielt. Bei 55% RH und einer geringen Luftbewegung
stimmten der experimentell ermittelte Aufnahmewert von 1,34 ± 0,17ml.min~' gut mit dem
theoretisch berechneten von l,36ml.min~' uberein. Die passive Bestimmungsmethode wurde fiber
lange Zeitrdume und bei geringen Luftbewegungen reproduzierbar mit RSD- Werten von 7%
angewandt.
Resumen—Se ha desarrollado un sistema pasivo de toma de muestras 'tipo-tubo' y ha sido
evaluada la cuan-tificacion de los vapores de formaldehido tmetanal) en interiores. El aparato se
ha diseiiado para su uso en
museos donde los lugares a ser comprobados a menudo induyen pequenos receptdculos con poco
movimiento de aire. El procedimiento se basa en la recoleccion de vapores de formaldehido en un
tubo de difusion Palmes conteniendo un papel de soporte impregnado con una disolucion
acidificada de 2,4-dinitrifenilhidraiina (2,4-DNPH). Despues de la toma de muestras la
cuantificacion del F-DNPH atrapado se consigue por cro-matografia liquida de alta resolution
(HPLC) y por andlisis con una detection UV a 350nm. Para validar el procedimiento se usaron
sistemas de permeabilizacion para generar atmosferas con un contenido en formaldehido de 812975 ppb, en una cdmara de 20dm3, de tal forma que los niveles de muestreo derivados experimentalmente puedan ser calculados y comparados con los valores teoricos. Se investigaron tres
disoluciones de 2,4-DNPH para obtener un solution eficiente y estable que atrape el compuesto en
cuestion. Los me/ores resultados se obtuvieron con una disolucion de 27mg.ml~' de 2,4-DNPH
conteniendo un 4,5% v/v de dcido ortofosforico. Al 55% de RH, y con un flujo de aire bajo dentro
de la cdmara, el nivel de muestreo derivado experimentalmente de 1,34 ± 0,17ml.min~' se
considero un buen acuerdo con el nivel de muestreo tedrico derivado de l,36ml.min~'. Fue posible
repetir y reproducir el metodo de muestreo pasivo con valores de RSD por debajo del 7% para
exposiciones a largo plazo y a bajas velocidades de aire.