ARTICLE

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Memory Effects on Adsorption Tubes for Mercury Vapor Measurement

in Ambient Air: Elucidation, Quantification, and Strategies for
Mitigation of Analytical Bias
Richard J. C. Brown,*,† Yarshini Kumar,† Andrew S. Brown,† and Ki-Hyun Kim‡
†
Analytical Science Division, National Physical Laboratory, Teddington, TW11 0LW, United Kingdom
‡
Atmospheric Environment Laboratory, Department of Environment and Energy, Sejong University, 98 Goon Ja Dong, Gwang Jin Goo,
Seoul 143-747, Republic of Korea

bS Supporting Information
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ABSTRACT: The short- and long-term memory effects associated with measurements of
mercury vapor in air using gold-coated silica adsorption tubes have been described. Data
are presented to quantify these effects and to determine their dependence on certain
relevant measurement parameters, such as number of heating cycles used for each analysis,
age of adsorption tube, mass of mercury on adsorption tube, and the length of time between
analyses. The results suggest that the long-term memory effect is due to absorption of
mercury within the bulk gold in the adsorption tube, which may only be fully liberated by
allowing enough time for this mercury to diffuse to the gold surface. The implications of
these effects for air quality networks making these measurements routinely has been
discussed, and recommendations have been made to ensure any measurement bias is
minimized.

’ INTRODUCTION To achieve this goal, many analytical instruments and stan-

Air quality continues to be a concern because of its potentially dardized measurement methodologies have been developed. The
adverse effects on human health and environmental sustainabil- majority of measurements of total gaseous mercury in air rely on
ity. Of the pollutants present in air, mercury is of particular “trap and desorb” technology.5 Ambient air is pulled through a
interest because of its combined qualities of toxicity and potential device capable of trapping any mercury contained; usually, the
for bioaccumulation in aquatic and terrestrial biosystems. More- mechanism of trapping is by surface amalgamation on a high surface
over, because of the long atmospheric half-life and long-range area gold structure contained within the “trap” or “adsorption
transport of gaseous mercury species, emissions produced in one tube”. The adsorption tube can then be heated up to high
country or continent can have a significant impact on the ambient temperatures at which point the mercury desorbs and may be
concentrations in a different country or continent. Therefore, detected using atomic fluorescence spectroscopy or atomic absorp-
pollution of the atmosphere and wider environment by mercury tion spectroscopy. The use of an adsorption tube has two major
is now acknowledged to be a global problem.1,2 benefits: (1) it allows preconcentration of the sample to improve
One of the major routes of human exposure to mercury is overall method detection limits in terms of ambient mass
through inhalation. In order to assess the exposure of the general concentrations, so ambient background concentrations (on the
population to mercury in air, to ensure compliance with national order of 1 ng m
3) are easily measurable, and (2) it allows for the
and international legislation to limit mercury in air concentra- remote sampling of air at multiple locations followed by mea-
tions, and to further the global research effort, many nations now surement at on central location. The latter ensures that operating
have air quality monitoring programs in place.3 In most cases, a multisite network is cost-effective as only one analytical instrument
total gaseous mercury (mostly gaseous elemental mercury with
varying but very small amounts of reactive gaseous mercury) is Received: April 30, 2011
measured. It is clearly essential that the concentration data Accepted: August 15, 2011
produced should be accurate and moreover traceable to the SI Revised: July 13, 2011
system of units to ensure comparability over time and location.4 Published: August 15, 2011

r 2011 American Chemical Society 7812 dx.doi.org/10.1021/es201454u | Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

is required. In addition, measurement results only rely on the attributable to the adsorption tube, Atube, is given by:
calibration of one instrument, so they are likely to show better
comparability and less systematic bias. The role of the absorption Atube ¼ A1 þ A2
2A3 ð1Þ
tube is clearly critical to the accuracy of the measurement results
obtained. It has previously been observed that the efficiency of where Ai is the analytical intensity associated with the ith sequential
collection and desorption on these tubes contributes to the heating cycle. Because of the analytical procedure used, it is assumed
overall measurement uncertainty.6 This paper demonstrates that that the adsorption tube is clean following analysis and ready to be
there may be drawbacks to the use of remote sampling meth- used again for air sampling.
odologies rather than installing automatic measuring instruments As a result of remote sampling and centralized analysis con-
at each monitoring site as a result of a memory effect which these ducted by the air quality network and ongoing research activities
adsorption tubes can display under some conditions. into mercury vapor measurement, NPL maintains an ensemble of
While there has been some recent measurement science re- approximately 100 adsorption tubes, replacing broken and degraded
search into determinations of mercury vapor in air,7
11 very little tubes periodically. In this way, the ensemble contains tubes of
of this has focused on the performance of the adsorption tube many different ages. At any one time, about 25 adsorption tubes
and none on possible memory effects. A couple of studies have are out in the field sampling air, 25 sampled adsorption tubes are
identified the presence of a possible memory effect12,13 and awaiting analysis, 25 are awaiting dispatch to monitoring sites,
proposed that short and long-term effects exist; there has been no and 25 are engaged in research investigations. Given that adsorption
work to quantify this effect, its dependence on input variables, or tubes are additionally periodically rotated between research and
its potential to bias measurement results. This paper now addresses
air quality network duties, the period between analysis for any
that deficiency.
given tube may be several months. If an adsorption tube has been
inactive for a prolonged period or its status is unknown, it is good
’ EXPERIMENTAL SECTION practice to reclean it prior to its next use by running an analysis
The National Physical Laboratory (NPL) currently measures on the tube. During this process, it has been noticed on occasions
total gaseous mercury at 13 monitoring stations around the UK that the analytical response from some adsorption tubes is
as part of NPL’s operation of the UK Heavy Metals Monitoring substantially higher than expected. This indicated that there might
Network on behalf of the UK Government.3 Sampling is per- be a long-term memory effect associated with these adsorption
formed by pulling ambient air though a mercury adsorption tube tubes.
containing gold-coated silica (Amasil, PS Analytical, UK) at an In order to investigate this effect and its short-tem analogue, a
approximate rate of 100 mL/min using a pump (NMP 05 S, KNF full study was made on a variety of adsorption tubes to elucidate
Neuberger, UK) for sampling periods of one week to one month the dependence of the memory effects on relevant parameters
(depending on the expected ambient concentrations). The pump such as: time between analyses, age of the adsorption tube, and
is calibrated, traceable to national standards, and its flow is mass of mercury originally collected on the desorption tube. To
measured at the beginning and end of each sample. An in-line this end, synthetic dosing of tubes took place in the laboratory
particulate filter (0.4 μm GN-4 Metricel, Pall, UK) at the sample using known quantities of mercury removed from the bell-jar
line inlet protects the adsorption tube from exposure to particu- calibration apparatus and injected into a stream of mercury-free
late matter. PFA tubing is used throughout, and the distance nitrogen flowing through the adsorption tube. Each analysis
between the sample line inlet and the adsorption tube is kept to a comprised five consecutive heating cycles, six in the case of
minimum (approximately 5 cm). Typical masses collected on the investigation of the short-term memory effect. To investigate the
adsorption tubes range from 0.1 ng to 50 ng. Sampled adsorption short-term memory effect, adsorption tubes were dosed with
tubes are retuned to NPL where analysis takes place using a between 5 and 50 ng of mercury and then analyzed using
10.525 Sir Galahad analyzer with an atomic fluorescence detector 6 repetitive heating cycles. To investigate the effect of adsorption
(PS Analytical, UK). The instrument is calibrated by use of a tube age on the long-term memory effect, adsorption tubes of
gastight syringe, making multiple injections of known amounts of varying age were dosed with 5 ng of mercury, analyzed imme-
mercury vapor, from a “bell-jar” calibration source8 onto the diately, then analyzed for a second time after 1 week. To investigate
permanent trap of the analyzer. Prior to a series of measurements, the effect of the time between analysis on the long-term memory
the linearity of the instrument is confirmed using a series of five effect, adsorption tubes were dosed with 5 ng of mercury,
injections across the measurement range and the recovery of analyzed immediately, and then analyzed for a second time after
mercury from an adsorption tube dosed with a known mass of x days and for a third time after 2x days. To investigate the effect
mercury is measured. Once linearity is established and the recovery of originally dosed mass of mercury on the long-term memory
is within allowable method parameters, the mercury content of effect, adsorption tubes were dosed between 5 and 50 ng of
the adsorption tubes is determined relative to an additional one- mercury, analyzed immediately, and then analyzed for a second
point calibration. This calibration injection is made prior to the time after 1 week.
measurement of each adsorption tube and tailored to a level close Measurement points presented are an average of the results
to the mass expected to be present on the tube. Sampled adsorption from five adsorption tubes unless otherwise stated. The results of
tubes are placed in the remote port of the instrument and heated these analyses are presented either as analytical intensities or as
to 500 °C, desorbing the mercury onto a permanent trap. Sub- masses of mercury liberated during analysis calculated using the
sequent heating of this trap to a similar temperature then desorbs instrument sensitivity determined by calibration of the instru-
the mercury onto the detector. The adsorption tubes go through ment using the bell-jar apparatus prior to each heating cycle.
this heating cycle three times. It is assumed that the result of Ordinary least-squares fitting, weighting according to the un-
the third analysis of the adsorption tube represents the blank certainty in the y-component of the data, has been performed
associated with the adsorption tube, such that the analytical intensity throughout using NPL’s XLGENLINE software.14
7813 dx.doi.org/10.1021/es201454u |Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

Figure 1. Analytical intensity obtained for each heating cycle, normal- Figure 2. Analytical intensity from an adsorption tube dosed with
ized to the fifth and sixth heating cycle, during the initial analysis of approximately 5 ng of mercury, which was analyzed immediately (the
adsorption tubes dosed with different masses of mercury (in ng, as first analysis, Day 1), and then subsequently after 1 week (the second
indicated in the key). The inset shows the intensity ratio, in percent, of analysis, Day 8) and then 2 weeks (the third analysis, Day 15), normal-
the second to first heating cycles as a function of the mass of mercury ized to the minimum response from each analysis. Each analysis com-
initially dosed onto the adsorption tube, with a best-fit line included as a prised 5 heating cycles, as shown. Note that the x-scale is logarithmic.
guide to the eye.
tube. Hence, a more rigorous but time-consuming analysis
A field-emission scanning electron microscope (FE-SEM), procedure to obtain the analytical intensity attributable to the
Carl-Zeiss-Supra 40, was also used to examine the gold-coated adsorption tube for high accuracy work would be:
silica material in the adsorption tubes. The SEM operating param-
Atube ¼ A1 þ A2 þ A3 þ A4
4A5 ð2Þ
eters were as follows: an accelerating voltage of 10 kV, a working
distance of 3.7 mm, and a final lens aperture of size 30 mm. Furthermore, it is observed that Ai for i g 5 is slightly greater
Secondary electron images were acquired using an In-Lens that the intensity observed for an injection of blank gas onto the
detector. permanent trap of the instrument. This could be as a result of
very small quantities of mercury liberated during analysis in the
’ RESULTS AND DISCUSSION pipework between the remote trap (where the adsorption tubes
are placed) and the permanent trap (where the calibrations
Short-Term Memory Effect. We indentify the presence of a injections are made, and where mercury liberated from adsorp-
short-term memory effect as a result of the incomplete desorp- tion tubes in the remote trap are reabsorbed). However, this
tion of mercury after the first heating cycle of the adsorption tube. slightly elevated response is still present even after new pipework
This effect and its dependence on the mass of mercury loaded is installed, suggesting that the origin of the signal is from the
onto the adsorption tubes is shown in Figure 1. It is clear that the adsorption tube. These baseline levels are discussed later.
vast majority of mercury is liberated from the adsorption tube Long-Term Memory Effect. An extreme example of the long-
during the first heating cycle. For tubes with relatively low masses term memory effect is displayed in Figure 2. The nature of the
of Hg (for instance 10 ng or less), as might be experienced during long-term memory effect is clear from these results. It seems that
ambient air sampling, the intensity is generally only a few percent by the fifth heating cycle on day 1 that a relatively stable baseline
of the intensity obtained during the first heating cycle. Above has been reached. However, when the adsorption tube is reanalysed
10 ng, the percentage of the analytical intensity observed on the after one week, there appears to be a significant quantity of mercury
second heating cycle to that on the first rises to (by interpolation liberated, which then decreases to a baseline value again after five
of the experimental data) about 5% at 25 ng and 10% at 50 ng. heating cycles. The third analysis after two weeks results in only
The profile of the repeat analysis from the second heating cycle small quantities of mercury being liberated over and above the
onward is very similar, regardless of mass loading, showing a baseline response. Hence, we may quantify the long-term memory
further significant decrease on the third heating cycle, and then effect as the quantity of mercury liberated from the adsorption
more minor decreases in the fourth and fifth heating cycles, with tube, over and above the baseline level established during the first
almost no change observed between the fifth and sixth heating set of heating cycles, as a result of additional sets of heating cycles
cycles. Regardless of original mass loading, a relatively similar (the second, third, and subsequent analyses). We will call this
intensity is recorded from the adsorption tubes after the fifth and quantity the excess mercury. In general, the second analysis seems
sixth heating cycles. to liberate an additional quantity of mercury from the adsorption
For most practical purposes, it is clear that using eq 1 to tube, whereas it is much rarer to see significant mercury liberated
calculate the total intensity from an adsorption tube is fit for on the third and subsequent analyses.
purpose, achieving better than 99.5% recovery of the mercury Three parameters have been investigated with respect to the
adsorbed on the tube for a 5 ng loading. Using eq 1 for much long-term memory effect of adsorption tubes: (1) age of the
higher loading may result in recoveries only in excess of 97%, but adsorption tube (which is taken to be an approximate surrogate
this is still fit for purpose given the target combined standard for the number of previous heating cycles experienced); (2) the
uncertainty for measurements made under the European air mass of mercury originally dosed onto the tube; and (3) the
quality directive is 25% (relative). It is clear therefore that a short- length of time between the first set of heating cycles and con-
term memory effect exists such that it requires several heating secutive sets of heating cycles. The results of these investigations
cycles to reach a steady state response for the mercury adsorption are given in Figures 3, 4, and 5.
7814 dx.doi.org/10.1021/es201454u |Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

Figure 3. Mass of excess mercury obtained from the second analysis of

adsorption tubes originally dosed with 5 ng of mercury as a function of Figure 5. Mass of excess mercury obtained from the second and third
adsorption tube age. Each measurement point is the average of five analyses (as indicated on the chart) after one and two weeks, respec-
repeats. The error bars represent the standard deviation of each point, tively, as a function of the mass of mercury originally dosed onto the
and the dotted line represents a weighted least-squares best fit linear adsorption tube. Each measurement point is the average of five repeats.
relationship. The error bars represent the standard deviation of each point. The
dotted line represents a weighted least-squares best fit linear relation-
ship, for the data associated with the second analysis. The dashed line
represents a weighted least-squares best-fit linear relationship, for the
data associated with the third analysis.

the original mass for mercury dosed onto the tube: a gradient of
+0.0016 ( 0.0004 ng/ng. However, the third analysis shows an
insignificant gradient of +0.0002 ( 0.0008 ng/ng. The uncer-
tainty in the relationship determined for the second analysis is
sufficiently large so that it is not possible to propose with any
confidence whether the relationship shows a plateau, similar to
Figure 3, for high mercury masses or whether it continues to
increase linearly.
Several observations may be made about the long-term memory
effect from the results presented. It is worth noting that the data
Figure 4. Mass of excess mercury obtained from the second and third shows a relatively high spread. This is thought to arise mainly
analyses (as indicated on the chart) of adsorption tubes originally dosed from variability in the adsorption tubes themselves, and there-
with 5 ng of mercury as a function of the time between analyses. Each fore, to some extent, the manifestation of the long-term memory
measurement point is the average of five repeats. The error bars represent effect is related to the individual history and characteristics of
the standard deviation of each point. The dotted line represents a each adsorption tube. This imposed variability decreases the
weighted least-squares best fit quadratic relationship (up to 20 d) and certainty with which conclusions may be drawn. This notwith-
weighted linear relationship (after 20 d), for the data associated with the
standing, it is possible to determine some general characteristics
second analysis. The dashed line represents a weighted least-squares
best-fit linear relationship, for the data associated with the third analysis. of the long-term memory effect. First, the excess mercury mea-
sured shows no dependence on tube age and history. This implies
that there is no dependence on the structure of the gold surface in
The gradient on the relationship determined in Figure 3 by the adsorption tube, which is thought to change rapidly upon first
weighted least-squares regression was +0.002 ( 0.003 ng/yr. Not usage of the tube, owing to the first Hg adsorption and heating
only is this gradient small compared to the absolute values observed cycles, but only to change very slowly after that.15 Second, it
but also, because of the large standard deviations associated with seems that the second analysis removes the majority of the excess
each point, the uncertainty in this gradient is larger than the mercury present on the tubes. The third analysis in most cases
gradient itself. It is therefore reasonable to state that there is no shows very little excess mercury above the baseline level. Third, it
significant relationship between the excess mercury recovered as is apparent that the longer an adsorption tube is left after the first
a result of the long-term memory effect and the adsorption analysis the greater the quantity of excess mercury is liberated
tube age. during the second analysis. However, there seems to be a max-
The data in Figure 4 shows a clear increase in the mass of imum to the quantity of excess mercury liberated during the
excess mercury recovered from the second analysis as the time second analysis, presumably equal to the sum total of mercury
between analyses increases. The increase slows as the time remaining on the adsorption tube after the first analysis. Finally,
between analyses increases, resulting in a plateau for times in the excess mercury recovered shows some proportionality to the
excess of approximately 20 days, where no increase in excess mass of mercury initially dosed onto the adsorption tube.
mercury is observed. For the third analysis, no significant gradient is It is also instructive to consider the microstructure of the
observed (
0.0001 ( 0.0010 ng/d) although, similarly to Figure 3, adsorption media within the adsorption tube. This has been done
a small quantity of between 10 and 20 pg continues to be by scanning electron microscopy (an exemplar image is shown
liberated consistently from the adsorption tube. in Figure S1, Supporting Information). This has shown the
Figure 5 indicates a significant positive relationship between porous nature of the surface silica structure and the relatively
the excess mercury measured following the second analysis and low surface coverage of gold on this silica surface. Generally, the
7815 dx.doi.org/10.1021/es201454u |Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

gold is dispersed across the surface as 20
50 nm islands, with known to be significantly in favor of surface adsorbed
occasional larger agglomerations on the surface. At a rough ap- mercury.16,17 Thus, with the previously surface adsorbed mercury
proximation, on the basis of the observed surface coverage of removed by the first analysis, it is proposed that this equilibrium is
gold on each silica sphere and the number of silica spheres in the re-established as a result of mercury absorbed a few nanometers
adsorption tube, the theoretical capacity of each tube is on the into the bulk gold diffusing toward the surface and becoming
order of 10 μg of mercury. surface adsorbed. Given the relative small size of the gold deposits
From these observations, some theories may be proposed to on the silica structure, the diffusion distances can only be a
explain the long- and short-term memory effects. It may be maximum of 10 nm and probably much less; however, as Figure 4
assumed that when mercury is dosed onto the adsorption tube suggests, this process takes several days to complete. Therefore,
the vast majority is adsorbed onto the gold surface where it forms after a period of time when the second analysis occurs, all of the
an amalgam; the affinity of silica for mercury is significantly less mercury, which has migrated to the surface of the gold, is now
than that of gold. We now propose that a small proportion of this desorbed during heating and measured, resulting in observation of
mercury in the surface amalgam diffuses into the bulk gold. the long-term memory effect. The same partitioning would be
Previously, it has been found that only reactive gaseous mercury expected to happen during the second analysis as well, but by the
(RGM) species (i.e., oxidized mercury) diffused into bulk time of the third analysis, the remaining memory effect will be very
gold16,17 and that elemental mercury at room temperature would small compared to the baseline response and the variability of
be expected to remain as an amalgam on the gold surface, in the tubes and is so not detectable.
which case the measurement of excess mercury measured as a It is interesting to note that the proposed diffusion rate for
result of the long-term memory effect would be a possible way of mercury in bulk gold at room temperature, as observed in
measuring RGM using a standard TGM sampling tube. How- Figure 4, is on the order of many days. However, the long-term
ever, as discussed later, the mercury diffusion into the bulk may memory effect, which is proposed here to occur as a result of
not occur during sampling but instead during analysis when mercury becoming absorbed into the bulk, is seen in adsorption
temperatures are elevated. It is not unreasonable to suggest that tubes that are first analyzed only a few hours after being dosed by
the quantity of mercury diffusing into the bulk under these cir- only gaseous elemental mercury, when it has previously been
cumstances might be a relatively constant percentage of the total observed that usually only oxidized mercury species diffuse into
mercury dosed onto the tube (hence, the results of Figure 5), bulk gold.18 Hence, these observations provide additional evi-
with variability introduced by subtle changes in individual tube dence that the heating of the tubes during analysis may initially
characteristics. act to increase the rate at which mercury is absorbed within the
During the first analysis, the majority of the mercury is bulk gold, possibly by lowering the activation energy barrier from
liberated the first time the adsorption tube is heated. However, the surface mercury to bulk mercury partitioning equilibrium.
some mercury will not be completely removed by this first heating This observation has significant implications for monitoring in
cycle. There may be a number of reasons for this. First, just like the field where non-negligible RGM components in ambient air
any multiphase system, there will be a partitioning of mercury in are to be expected, furthering the possibility of additional mer-
the gas and solid phase, and while the high temperatures which cury diffusion into the bulk gold.
the tube is exposed to will mean that this equilibrium is shifted Adsorption Tube Baseline. It is notable that the baseline
heavily in favor of mercury in the gas phase, some will still remain levels from these adsorption tubes correspond to approximately
amalgamated to the gold. Second, heating is only conducted for a 10 pg above a measurement of mercury-free gas. The origins of
limited time in order to protect the instrument and adsorption these baselines, which show some variation between tubes, are
tube from damage, and while most mercury will be released very not entirely clear. The baseline shows very little variation with
shortly after heating, the mass of mercury liberated will show an repeated analysis, suggesting perhaps that it is less to do with the
exponential decay with time; this function may not fully decay by continual and consistent release of very low levels of mercury and
the time the heating is stopped. Third, the porous structure of the more to do with the measurement of an interfering substance,
silica in the adsorption tube and the tight packing of the silica most probably submicrometer sized particulate matter. The
spheres may mean that some mercury liberated from the surface origin of this particulate matter is either from the carrier gas
of the gold is not carried away efficiently by the gas stream used for analysis (which although undergoing prefiltration will
flowing through the tube and reabsorbs elsewhere as the tube still contain some particles above 100 nm and many particles
cools. These proposals explain the nature of the short-term below 100 nm in diameter) or more probably from the degrada-
memory effect, whereby the adsorption tube needs to be heated tion of material in the adsorption tube during heating. The
several times in succession before a baseline response is pro- particulate matter acts to cause a false positive signal from the
duced. At this point, the adsorption tube produces no excess atomic fluorescence instrument by scattering the incident radia-
mercury over and above its baseline response despite further tion in the direction of the detector. This “mercury-free” baseline
heating. Furthermore, we now propose that the high tempera- is significantly higher for new adsorption tubes; indeed, the
tures experienced during analysis will also allow greater mobility manufacturers recommend repeated heating of new adsorption
of the mercury to move within the gold and therefore may en- tubes prior to use to reduce this baseline to normal values. There
courage some surface adsorbed mercury to move deeper into the is also experimental evidence gained during this study that sug-
bulk of the gold, possibly only a few nanometers into the bulk. gests that this baseline might increase again for older tubes,
Any mercury absorbed into the bulk gold in this manner will most possibly as they start to degrade more rapidly after a very large
probably not be liberated at any significant rate by continued number of heating cycles.
heating, unlike any remaining surface adsorbed mercury. Implications for Ambient Air Monitoring of Memory
Just as we propose a partitioning between mercury in the gas Effects. Providing the adsorption tube is heated at least three
phase and surface adsorbed mercury, so there is a partitioning times during a first analysis, it has been shown that the short-term
between surface adsorbed and bulk absorbed mercury which is memory effect may be largely eliminated. For normal ambient
7816 dx.doi.org/10.1021/es201454u |Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

conditions where approximately 2 ng of mercury may be col- The effect of the issues discussed above on automatic analyses
lected during a normal sampling period, heating the adsorption of mercury vapor are less simple to discern. In general, only one
tube three times should recover in excess of 99.5% of the mercury heating cycle is performed to desorb each sample, and samples
that may be liberated during the first analysis cycle. comprise very low mercury masses (often <10 pg) and are usually
For a similar sample of 2 ng of mercury collected on an equally spaced and highly time-resolved (samples are often
adsorption tube, it has been shown that maximum long-term analyzed as regularly as every 15 min). In such a situation, both
memory effects corresponding to between 5% of this mass (0.1 ng of the short and long-term memory effects are likely to be an issue.
excess mercury) have been observed in the second analysis. This However, given that concentration levels are unlikely to change
potential bias is significant when compared to the overall un- dramatically over short time periods, these effects are likely to
certainty of the measurement method (on the order of 15% at the cancel out between subsequent analyses, as per the steady state
95% confidence level). However, when the likely operation of an case for the manual sampling case. Only when concentration
air quality network employing remote sampling and central levels change dramatically from one sample to the next is the
measurement is considered, a large proportion of this possible memory effect likely to have some influence on the measured
measurement bias would be expected to be eliminated. The values. In this case, a slight smoothing out of peaks and troughs of
reason for this is that, providing the adsorption tubes are exposed concentrations would be observed (for similar reasons as given
to a similar quantity of mercury during each sampling period and for the manual case), but longer term averages, such as daily
experience a similar length of time between analyses, the excess values, would be unlikely to be significantly affected. Moreover,
mercury, resulting from the long-term memory effect from there is relatively little one may do to alter parameters associated
sampling period 1 and not recovered during the single analysis with automatic analyses, apart from sampling period length so, in
of this adsorption tube, would instead be liberated during the terms of the recommendations given above for manual sampling,
analysis of the sample collected during sampling period 2 and only point (4) is really valid for automatic measurements.
would be equal to the excess mercury not recovered as a result of A universally applicable suggestion to counter the effects of the
the long-term memory effect attributable to sampling period 2. In memory effects presented (even when they appear to cancel out)
this respect, the long-term memory effect is probably only relevant if is to consider increasing the uncertainty of the measurement
sampling or analysis conditions change significantly for indivi- results to take into account any possible bias resulting from these
dual tubes from one analysis to the next. (This effect is explained memory effects. We would also suggest that the good practice
and displayed diagrammatically in Figure S2, Supporting Infor- suggestions made in this paper are implemented by all operators
mation). of air quality networks using a manual sampling regime for
In general, then, the long-term memory effect would be ex- mercury vapor measurement. While the results presented here
pected to have relatively little impact on air quality networks that are quantitatively specific to the adsorption tube type used in this
study, it is proposed that the qualitative conclusions are transfer-
rely on remote sampling followed by analysis at a central laboratory.
able to all similar trap and desorb measurement regimes using
However, it is conceivable that exposure of the same adsorption
adsorption tubes.
tube at an industrial site with high ambient concentrations, fol-
lowed by exposure at a rural background site, might result in
biases of up to 15%, comparable with the expanded uncertainty of ’ ASSOCIATED CONTENT
the entire measurement. Therefore, there are certain recommen-
dations that should be followed to ensure that the long-term bS Supporting Information. Scanning electron micrograph
memory effect imposes as little measurement bias as possible on of the surface of the sorbent material within the adsorption tube;
air quality measurements: (1) Individual adsorption tubes should illustration of the effect of the long-term memory effect on
be deployed to locations where they are likely to sample similar consecutive sampling periods of equal length using the same
masses of mercury during successive sampling periods. This may adsorption tube and subsequent analytical measurement. This
mean ensuring a set of tubes is used only at one particular site or material is available free of charge via the Internet at http://pubs.
at several sites where concentrations are very similar. Given that acs.org.
TGM concentrations in air show relatively little variation, plan-
ning such a strategy should be fairly simple. (2) New adsorption ’ AUTHOR INFORMATION
tubes should be conditioned with repeated injections of mercury
Corresponding Author
(of expected ambient sample masses) and subsequent analysis
*Tel.: +44 20 8943 6409; fax: +44 20 8614 0423; e-mail: richard.
prior to dispatch to monitoring locations (this should be part of
brown@npl.co.uk.
the validation procedure for new adsorption tubes in any case).
(3) As far as is practicable, the period of time between successive
analyses of the same adsorption tube should be kept constant. If
large differences in the time between analyses occur, it may be ’ ACKNOWLEDGMENT
worth redosing and reanalyzing the adsorption tube in question This work forms part of the Chemical and Biological Metrol-
in the laboratory to minimize the possible bias resulting from the ogy Programme of the National Measurement System of the UK
long-term memory effect. (4) Account should be taken of the Department of Business, Innovation and Skills (BIS). Support
monitoring history of an adsorption tube when ratifying results from the UK Heavy Metals Air Quality Monitoring Network by
from air quality measurements, and the uncertainty of the mea- the UK Department for Environment, Food and Rural Affairs is
surement should be increased if necessary to take account of any also gratefully acknowledged. The fourth author also acknowl-
likely bias as a result of the long-term memory effect. (Attempting edges the support made by the National Research Foundation of
correction for the long-term memory effect is not recommended Korea (NRF) Grant funded by the Ministry of Education,
because of the variability observed in the effect.) Science and Technology (MEST) (No. 2010-0007876).
7817 dx.doi.org/10.1021/es201454u |Environ. Sci. Technol. 2011, 45, 7812–7818
Environmental Science & Technology ARTICLE

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