Hyphenated laser ablation–mass spectrometry instruments have been recognized
as useful analytical tools for the detection and chemical characterization
of aerosol particles. Here we describe the performances of a laser ablation
aerosol particle time-of-flight mass spectrometer (LAAP-ToF-MS) which was
designed for aerodynamic particle sizing using two 405 nm scattering lasers
and characterization of the chemical composition of single aerosol particle
via ablation/ionization by a 193 nm excimer laser and detection in a bipolar
time-of-flight mass spectrometer with a mass resolving power of
We describe a laboratory based optimization strategy for the development of an analytical methodology for characterization of atmospheric particles using the LAAP-ToF-MS instrument in combination with a particle generator, a differential mobility analyzer and an optical particle counter. We investigated the influence of particle number concentration, particle size and particle composition on the detection efficiency. The detection efficiency is a product of the scattering efficiency of the laser diodes and the ionization efficiency or hit rate of the excimer laser. The scattering efficiency was found to vary between 0.6 and 1.9 % with an average of 1.1 %; the relative standard deviation (RSD) was 17.0 %. The hit rate exhibited good repeatability with an average value of 63 % and an RSD of 18 %. In addition to laboratory tests, the LAAP-ToF-MS was used to sample ambient air during a period of 6 days at the campus of Aix-Marseille University, situated in the city center of Marseille, France. The optimized LAAP-ToF-MS methodology enables high temporal resolution measurements of the chemical composition of ambient particles, provides new insights into environmental science, and a new investigative tool for atmospheric chemistry and physics, aerosol science and health impact studies.
Atmospheric aerosols, defined as an assembly of solid or liquid particles
suspended in a gas (Finlayson-Pitts and Pitts, 2000), have a
large impact on human health (Dockery and Pope, 2006) and global
climate (Poeschl, 2005). Ambient aerosols typically span a
size range from 3 nm to 10
A detailed understanding of the particle sizes and the chemical composition of atmospheric particles is of paramount importance to understand their impact on health and climate. Hence, there is a need for the development of appropriate analytical methods for on-line, time-resolved measurements of atmospheric particles. In the last decade several hyphenated laser ablation – mass spectrometry instruments have been developed (see for instance Gaie-Levrel et al. (2012) with the aim of chemically characterizing aerosol particles. Murphy (2007) has reviewed the development and implementation of single particle laser mass spectrometers. These instruments appear promising for aerodynamic sizing of particles and characterization of their chemical composition. The advantage of using laser ionization compared to methods based on thermal desorption, such as that applied in the aerosol mass spectrometer (AMS), is the ability to analyze both non-refractory (e.g., organics, ammonium nitrate) and refractory (e.g., mineral dust, soot) components of individual atmospheric aerosol particles (Pratt and Prather, 2011). However, a deeper investigation is required in order to promote the laser ionization technique as a suitable experimental device for the elemental quantification of individual aerosol particles. The recently launched Laser Ablation Aerosol Particle Time-of-Flight Mass Spectrometer (LAAP-ToF-MS), based on laser desorption and ionization, provides information on the aerodynamic diameter and chemical composition of individual aerosol particles. LAAP-ToF-MS is intended for on-line and continuous measurement of atmospheric particles with an analysis time in the order of milliseconds per particle.
Here we present a laboratory-based study of the LAAP-ToF-MS instrument performance and a novel approach to developing an analytical methodology for continuous monitoring of particle size distribution and their composition using this instrument. It will allow both qualitative information on single particles and quantitative information about ambient particle ensembles to be obtained simultaneously.
The LAAP-ToF-MS instrument (AeroMegt, GmbH) features an aerodynamic particle lens inlet, a particle-sizing region using two scattering lasers, a bipolar time-of-flight mass spectrometer and an excimer laser as ablation/ionization laser. The particle inlet is comprised of an aerodynamic lens with a transmission for particles with an aerodynamic diameter between 80 and 600 nm. The working principle of the LAAP-ToF-MS is shown in Fig. 1a.
The aerosol particles leave the differential pumping stages (inlet) and
enter into the detection region where they pass through the region
irradiated with light (
After ablation, the charged ions are extracted into a bi-polar
time-of-flight mass spectrometer (Tofwerk, BTOF) with a resolving power of
Two types of particles were used for laboratory experiments, spherical
particles of Polystyrene Latex beads (PSL, Duke Scientific Corp) with a
factor shape equal to 1 and a density of 1.05 g mL
The experimental configurations were designed to investigate the
instrument's performance in the first mode of operation, with particle
sizing. The first outline (Fig. 1b–c) was employed to study the
repeatability, the size calibration and the effect of the particle size and
the particle number concentration on the hit rate of the excimer laser (HR)
and the scattering efficiency of the scattering lasers (
The second configuration (Fig. 1d–c) was used for the measurement of atmospheric particles. This second configuration was designed to assess the potential effect of chemical composition on the hit rate and the scattering efficiency of real particles and to assess the effect of the number concentration. The chemical composition, particle size and the number evolution of the ambient particles were measured continuously by the LAAP-ToF-MS and an optical particle counter (OPC 1.109, Grimm, Germany).
The first step in the analysis of the processed raw data is to evaluate the
detection efficiency and to test the repeatability of the performed
analysis. To this end we need to introduce three different concepts of
instrumental efficiency. The detection efficiency (
The repeatability of the LAAP-ToF-MS instrument was tested by continuous
analysis of polystyrene latex (PSL) particles with a diameter of 450 nm and
a number concentration of 39
Repeatability of the scattering efficiency (
Every point in this figure corresponds to an average of detected particles during a period of 3 min, which is a minimum time interval necessary to attain sufficient number of detected particles. The scattering efficiency varies between 0.6 and 1.9 % with an average of 1.1 %. The relative standard deviation (RSD) is 17 % over the entire period of 53 h of analysis. The hit rate exhibits good repeatability with an average value of 63 % and the RSD is 18 %. The scattering efficiency may decrease due to larger particles passing through the critical orifice leading to a lower flow rate in the inlet. The argon fluoride gas lifetime is another important parameter which influences the hit rate. To test this parameter we generated PSL particles with a diameter of 450 nm seven times for few minutes each and then measured the hit rate. The first measurement was made immediately after refilling the excimer laser and the time difference between the first measurement and the last one was 12 days. Figure 3 shows the variation of the hit rate with time. During the first week the hit rate is considered constant, and from the eighth day it begins to decrease. Four weeks after refilling the excimer laser the hit rate has dropped down to zero upon daily use of the laser. According to the Laser Gam Ex5 specifications, laser energy drops to 50 % after a shelf life of 12 days or after 12 million pulses of ArF excimer laser. It seems that the shelf life is the limiting factor when using the laser in association with single particle mass spectrometer, at least in the diode trigger modes. Therefore, the data shown in Fig. 3 correspond only to the first 12 days.
The influence of the ArF gas life time on the evolution of the laser hit rate (HR) over the time.
The total particle number concentration detected by LAAP-ToF-MS
and OPC as a function of time; indicated are peaks corresponding to smoking
events (
The alignments of the scattering laser, aerodynamics lenses and the ionization laser are done manually. Therefore, the average of scattering efficiency and the hit rate are not the same as above for the experiments discussed in the rest of this article. However, the values of repeatability are expressed as relative standard deviation, which is not based on the alignment. Therefore, for a good repeatability of the scattering efficiency during a field campaign it is important to filter out large particles to maintain a constant flow in the inlet for as long as possible, while for a good repeatability of the hit rate it is strongly recommended that the excimer laser is refilled once a week.
Ambient aerosol measurements were performed on the campus of Aix-Marseille
University, situated in the city center of Marseille, France. The ambient
air was simultaneously sampled by LAAP-ToF-MS and OPC for a period of 6 days. A total of 62 813 bipolar mass spectra of single particles with
different sizes were recorded, among which 36433 spectra were useful.This
corresponded to a hit rate of 58 %. The number of particles detected
every 5 min by OPC, in the range between 265 nm and 3
As shown in Fig. 4, there are three peak events detected during this
monitoring campaign. Two of these particle number concentration spikes (a
and b), with maxima of 510.9 and 607.5 particles cm
The LAAP-ToF-MS measurements permit the identification and the monitoring of several types of ions. Figure 10 shows the standard deviation of all superimposed positive and negative ions mass spectra.
The standard deviation of all positive and negative ion mass spectra.
The negative ion mass spectra contain peaks associated with elemental carbon
(
The detection efficiency of the particles can be influenced by the particle
number concentration in the sample flow, the size of the particles and the
chemical composition which can vary during the analysis. For this purpose,
five different number concentrations of ferric sulfate particles ranging
between 50 and 1200 particles cm
To test the influence of particle size on the efficiency of the scattering
lasers and the hit rate of the excimer laser, five different sizes of PSL
particles (350, 450, 500, 600, 700 nm) were analyzed at constant particle
concentration of 20 particles cm
The scattering efficiency and the hit rate as a function of the
size of various PSL particles (350, 450, 500, 600, and 700 nm) at a particle
number concentration of 20 cm
Figure 6 shows that the hit rate decreases with the particle diameter, from
93 to 83 % when the diameter decreases from 600 to 350 nm. This
behavior can be explained by the fact that smaller particles drift with
higher velocity. Thus, the ions generated by the ionization laser have a
higher kinetic energy resulting in aberrations (Murphy, 2007).
A maximum efficiency of 2.5 % for the laser scattering diodes was
observed for particles with a diameter of 450 nm and a lower efficiency for
smaller particles. When the size of the individual particles becomes
equivalent to or greater than the wavelength of the laser (
The scattering efficiency decreases again for particles with a
A comparison between the scattering efficiencies of LAAP-ToF-MS, the single
particle laser ablation mass spectrometer (SPLAM) (Gaie-Levrel et al., 2012) and the single particle
laser ablation time-of-flight mass spectrometer (SPLAT)
(Zelenyuk and Imre, 2005) has been undertaken (Fig. 6). The
scattering efficiency of SPLAT decreases slightly for particles higher than
300 nm compared to SPLAM or LAAP-ToF-MS. The scattering efficiency shows the
same behavior for LAAP-ToF-MS and SPLAM which can be ascribed to the same
operating wavelengths of the scattering lasers (
Finally, a comparison was carried out with another similar instrument named
Aerosol Time of Flight Mass Spectrometer (ATOFMS)
(Gard et al., 1997). This instrument operates at
266 nm unlike the LAAP-TOF-MS (
In any case, it should be noted that size has an impact on the detection efficiency as we mentioned above.
We assessed the size effect of ambient aerosols on the hit rate and on the
scattering efficiency. For each size in the range between 10 nm and 2.5
The optimum particle size for detection is in the range between 400 and
600 nm (aerodynamic diameter), in the same range as the wavelength of
ionization (
In addition, Fig. 7a shows that the hit rate for ambient aerosol as function of the size range is different from the laboratory results. This difference can be ascribed to the effect of chemical composition which is detailed in Sect. 3.4.3.
Figure 7c shows the evolution of the number of spectra in each size range
every 5 min during the measurements. Since the scattering efficiency and the
hit rate are affected by the particle size, so is the detection efficiency
(Fig. 7c). Most of the usable spectra are in the range between 400 and
500 nm. The effect of particle size is overcome by clustering the spectra
obtained for each size range and multiplying the number of ionized particle
by the detection efficiency (
We investigated the transmission efficiency between the first and the second
scattering laser, considering that the two laser diodes have the same
characteristics. However, the first scattering laser exhibits a much higher
efficiency (
The ratio
Smaller particles with a diameter of 350 nm exhibit higher velocities and diverge much more than bigger particles with a size of 600 nm. This curve also explains the lower scattering efficiency of particles with a diameter of 350 nm displayed in Fig. 8.
In this study there are no information about the values of detection limit in number concentration for each particle size, because this limit is different for each type of particle.
Liu et al. (1995) have demonstrated that the morphology of the particles is a very important parameter that influences the divergence of particles during their drift between the two scattering lasers. In fact, the divergence of the particles increases for non-spherical particles implying a reduction of the scattering efficiency of the laser diodes.
The ionization efficiency of the excimer laser depends on the chemical
composition of the particles (Pratt and Prather, 2011).
Experiments were carried out with two types of particles containing ammonium
nitrate and ammonium sulfate in order to assess the effect of chemical
composition on LAAP-ToF-MS performance. Although, both particles have the
same density (1.74
The complete set of spectra can be clustered using the software MATLAB version 2013b into different chemical classes of particles.
Figure 9a illustrates four of these clusters and their repartition every 5 min
in different size range. These clusters were chosen as example to show
different kind of inorganics particles, and one cluster with major
carbonaceous ions. The inorganic particles are those containing sulfate and
nitrate that are considered as secondary particles and particles containing
TiO
Every cluster has its own repartition, which is defined as a number of particles detected every 5 min in different size range. Thus, the chemical composition of the particles detected during the measurements is not constant. To show the effect of chemical composition on the hit rate we calculated the hit rate of particles with different size range every 5 min during the entire time of the measurements. Then we calculated the RSD of the hit rate for each size range. The RSD varies between 51 % for the aerodynamic size range between 400 and 500 nm to 96 % for aerodynamic size range between 800 and 1000 nm (Fig. 9b). Comparing the RSD of ambient particles to the RSD calculated of spherical PSL particles during the laboratory tests (Sect. 3.2, repeatability 18 %), it can be concluded that chemical composition of particles affects the hit rate.
The effect of chemical composition on the hit rate was assessed for particles ranging between 400 and 500 nm (aerodynamic diameter). Figure 10a shows the evolution of the scattering efficiency and the hit rate for the detected particles between 400 and 500 nm (aerodynamic diameter).
It can be seen that the hit rate and the scattering efficiency are not
constant all the time. As was already seen for a single type of particles
the instrument exhibits good repeatability. Therefore the variation in HR (%) and
Therefore, a simple separation by size range and a correction of the
detection efficiency according to the size can no longer lead to the real
concentration number because of the variation of the chemical composition.
Thus, the average of the detection efficiency calculated for each size range
is no longer adequate for a time interval of few minutes. Therefore, it is
necessary to have a particle counter (like an OPC) to calculate the
detection efficiency (
Ambient measurements showed that a significant amount of particles could be
related to particles with a diameter less than 350 nm, which is not the case
for experiments with the spherical PSL particles during the calibration of
the instrument. This can be explained by the fact that particles in ambient
air have different optical characteristics, enabling them to scatter the
light more efficiently at the scattering wavelength used in this instrument
(
When a particle drifts through the particle-time-of-flight (P-ToF) chamber, it crosses the beam of two light scattering lasers. Upon passing the first laser beam, the scattered light from the particle is detected by the first photomultiplier tube (PMT). As explained above in the description of LAAP-ToF-MS, the flight time of an individual particle between the first and second scattering lasers is used to determine its velocity and associated vacuum-aerodynamic diameter. For the given beam separation distance of 11.5 cm between the two scatterings lasers the particle velocity was determined and plotted against the aerodynamic particle diameter (Fig. 11).
Figure 11 shows the calibration curve for aerodynamic particle sizing measurements carried out for five certified sizes of PSL particles (a) and five different sizes of ammonium nitrate particles (b).
Plot of aerodynamic particle size versus particle velocity for
The experimental data were fitted with a first order exponential decay curve. The smallest PSL particles that can be precisely size-calibrated have a diameter of 350 nm. However, the fitting equation depicted in Fig. 11 can serve to roughly estimate the size of atmospheric particles with an aerodynamic diameter smaller than 350 nm.
Prior to study the effect of number concentration, an upper limit of the
particle number concentration (
Variation of
For a particle size of 350 nm, which is the smallest particle size that has
been tested,
Concerning the scattering efficiency
The hit rate and the scattering efficiency of 450 nm ferric sulfate particles as a function of the particle number concentration and the percentage of the particles having a size lower than 200 nm for different concentrations of generated particles.
The detected particles in the range between 250 and 350 nm (aerodynamic
diameter) could be the result of two phenomena. The first one is the
presence of a total concentration number higher than the
Considering that the scattering laser is blind with respect to the particles
with
A recently developed LAAP-ToF-MS instrument has been calibrated and characterized.
In this work the performance of LAAP-TOF-MS has been characterized on standard spherical particles under controlled laboratory conditions and on ambient particles.
Prolonged on-line measurements revealed that the detection efficiency of LAAP-ToF-MS and the hit rate exhibits good repeatability with RSD of 17 and 18 %, respectively.
A comparison between the detection efficiency of LAAP-ToF-MS and the scattering efficiency of single particle laser ablation mass spectrometer (SPLAM) showed that the detection efficiency as a function of particle size is very similar.
A maximum detection efficiency of 2.5 % was observed for particles with a diameter of 450 nm with a decreasing efficiency towards smaller sized particles. Therefore, to further increase the accuracy of the data it is essential to improve the detection efficiency for smaller particle sizes.
Many parameters such as particle number concentration in the sample flow,
the size of the particles, and the chemical composition, could change during
a field campaign and affect the detection efficiency of the LAAP-ToF-MS. For
this reason, the changing in the performances of this instrument caused by
the parameters cited above was studied using laboratory and atmospheric
particles. The temporal evolution of the particles was validated during the
ambient aerosol measurements performed at the campus of Aix-Marseille
University, situated in the city center of Marseille, France. The obtained
results are in good agreement with the data obtained by optical particle
counter and the PM
Therefore, LAAP-ToF-MS is a suitable instrument for on-line monitoring of atmospheric particles that can provide information on size distribution, number concentration and chemical composition of the detected particles.
This work is a contribution to the LABEX SERENADE (no. ANR-11-LABX-0064) funded by the “Investissements d'Avenir”, French Government program of the French National Research Agency (ANR) through the A*Midex project (No. ANR-11-IDEX-0001-02).
The authors gratefully acknowledge the support of this work by French National Agency of Research within the ANR-10-EQPX-39-01. Edited by: P. Herckes