Analytical and Bioanalytical Chemistry

, Volume 403, Issue 7, pp 1907–1921

Use of the HS-PTR-MS for online measurements of pyrethroids during indoor insecticide treatments

  • Aude Vesin
  • Guy Bouchoux
  • Etienne Quivet
  • Brice Temime-Roussel
  • Henri Wortham
Original Paper

DOI: 10.1007/s00216-012-6003-x

Cite this article as:
Vesin, A., Bouchoux, G., Quivet, E. et al. Anal Bioanal Chem (2012) 403: 1907. doi:10.1007/s00216-012-6003-x

Abstract

A high-sensitivity proton transfer reaction mass spectrometer (HS-PTR-MS) has been used to study the temporal evolution of pesticide concentrations in indoor environments. Because of the high time variability of the indoor air concentrations during household pesticide applications, the use of this online high time resolution instrument is found relevant. Four pyrethroid pesticides of the latest generation that are commonly found in electric vaporizer refills, namely, transfluthrin, empenthrin, tetramethrin, and prallethrin, were considered. A controlled pesticide generation system was settled and coupled to a HS-PTR-MS analyzer, and a calibration procedure based on the fragmentation patterns of the protonated molecules was performed. To illustrate the functionality of the method, measurements of the concentration–time profiles of transfluthrin contained in an electric vaporizer were carried out in a full-scale environmental room under air exchange rate-controlled conditions. This study demonstrates that the HS-PTR-MS technique can provide online and high time-resolved measurements of semi-volatile organic compounds such as pyrethroid insecticides.

Keywords

HS-PTR-MS Pesticides Indoor air quality Gas phase Proton affinity 

Introduction

According to the National Research Council [1] and to several international studies [2,3], people spend on average more than 80 % of their time in indoor environments. As a result, the evaluation of the inhalation exposure to environmentally significant and health-relevant compounds in indoor environments has become a growing issue of concern. Up to now, due to the constraints of the conventional off-line analytical techniques, the concentrations are averaged over several hours, mainly because of the time-consuming sampling step. As a result, the studies carried out on pesticides in indoor atmospheres mainly provide data on background levels (Non-Occupational Pesticide Exposure Study [4]; Minnesota Children’s Pesticide Exposure Study [5]; Children’s Total Exposure to Persistent Pesticides and other Persistent organic Pollutants [6]; Evaluation of the Population Exposure to Organophosphate Pesticides in the Environment [7]). Thanks to the possibility of accumulating contaminants over a long period during the sampling step, these off-line analysis techniques are actually especially suitable for the measurement of background air level at the sub-micrograms per cubic meter level, which is more or less constant over time. On the other hand, these techniques provide very low time-resolved measurements.

Due to the high temporal variability of the indoor air concentrations during and after household pesticide applications, such as electric vaporizer plug-in insecticides, the concentration–time profiles of these atmospheric contaminants in the gaseous phase are currently unknown. For obtaining relevant data sets on the temporal profiles, the measurement frequency turns out to be the key parameter of the analytical procedure. As a result, the high-sensitivity proton transfer reaction mass spectrometer (HS-PTR-MS), originally designed to provide online and high time-resolved measurements of volatile organic compounds (VOCs) [8, 9, 10], was tested on semi-volatile pesticides in indoor atmospheres.

Up to now, only a few studies involving HS-PTR-MS, generally dealing with VOC measurements, have been carried out in the field of indoor air quality [11,12] or under specific confined conditions such as aircraft cabins [13, 14, 15]. The present study thus intends to evaluate the ability of this online analyzer to measure semi-volatile organic compounds in the gaseous phase. Both laboratory calibration and full-scale measurements were carried out. The targeted active substances are chosen among the pyrethroid pesticide family, which is the most recent generation of household insecticides very commonly applied nowadays [16]. Due to their representativeness of the pyrethroid family in terms of chemical structure and to their prevalence in the current commercial formulations, the study specifically focuses on transfluthrin (1), empenthrin (2), prallethrin (3) and tetramethrin (4). These compounds, for which the chemical structures are presented in Fig. 1, are classified as semi-volatile organic compounds due to their vapor pressures [17] of the order of 10−3 Pa or less (Table 1).
Fig. 1

Chemical structure of the studied pyrethroids

Table 1

Main features of the studied pesticides

Chemical

Transfluthrin (1)

Empenthrin (2)

Prallethrin (3)

Tetramethrin (4)

CAS no.

118712-89-3

54406-48-3

23031-36-9

1166-46-7

Formula

C15H12Cl2F4O2

C18H26O2

C19H24O3

C19H25NO4

Nominal mass (Da)

370

274

300

331

Vapor pressurea (Pa)

4.12 × 10−4 (25 °C)

7.89 × 10−3 (25 °C)

3.71 × 10−4 (25 °C)

2.8 × 10−6 (25 °C)

Saturated gas phase concentration (ppbv)

4.07

77.9

3.66

0.028

Proton affinity (kJ mol−1) ± 5 kJ mol−1

880

900

900

900

Polarizabilityb3)

30.20

32.26

33.41

35.18

Dipole momentb (D)

3.334

2.102

1.779

2.353

Proton transfer reaction rate coefficientc, kSC (cm3 s−1)

4.09 × 10−9

3.67 × 10−9

3.60 × 10−9

3.87 × 10−9

Henry’s law constantd (atm m3 mol−1)

2.57 × 10−5 (20 °C)

3.42 × 10−4 (24 °C)

1.02 × 10−7 (25 °C)

5.57 × 10−9 (25 °C)

LogKowa

5.46

5.42

4.16

4.68

LogKoae

8.43

7.27

9.54

11.32

Generation temperature (°C) ± 0.5 °C

65

65

75

100

Characteristic ions for calibration

163, 191/193/195, 371/373/375

107, 123, 151, 169, 275

123, 149, 151, 169

NA

aSource: Scifinder, estimate from ACD/Labs software V11.02

bSource: Hyperchem 8.0

cCalculated from [23]

dSource: SRC PhysProp Database, estimate from VP/WSol or HENRYWIN

eCalculated from [15]

Materials and methods

HS-PTR-MS principle

The analytical procedure of the HS-PTR-MS technique consists of a chemical ionization occurring through H3O+ transfer reactions, combined with subsequent mass spectrometric ion detection. HS-PTR-MS provides real-time online measurement of the compounds present in the gaseous phase. A brief description of the instrument is given below, and further details can be found in Lindinger et al. [8]. The HS-PTR-MS device consists of:
  • An ion source in which H3O+ is produced from gaseous distilled water with a hollow cathode

  • A drift tube where the transfer reactions between H3O+ and the molecules under study occur

  • A quadrupole mass spectrometer which differentiates the ions based on their mass-to-charge ratio

In the drift tube, compounds having a proton affinity (PA) higher than that of water (PA(H2O) = 691.0 kJ mol−1) [18] undergo efficient proton transfer reactions with the H3O+ ions to produce protonated molecules. Proton transfer does not occur for the most abundant species in the air since their proton affinities are lower than that of water (PA(N2) = 493.8 kJ mol−1, PA(CO2) = 540.2 kJ mol−1, PA(O2) = 421.0 kJ mol−1, PA(O3) = 625.5 kJ mol−1) [16]. These species are consequently pumped off, undetected. The concentration of the detected compounds can be estimated using a known reaction rate constant of the proton transfer reaction or can be accurately determined using independent calibration with gaseous standards [19].

A commercial quadrupole HS-PTR-MS analyzer (Ionicon Analytik, Austria) was used throughout this study.

Pesticides gas phase generation

No gas standard is commercially available for the pesticides under study. As a result, it is necessary to set up a specific device to generate a controlled and constant flow of pesticide in order to calibrate the HS-PTR-MS. For the generation of the gaseous standard flux of pesticides, solid or liquid pure compounds are used: transfluthrin (1) (99.0 %, Clouzeau), empenthrin (2) (95.5 %, Sigma Aldrich), prallethrin (3) (96.2 %, Sigma Aldrich), and tetramethrin (4) (98.6 %, Sigma Aldrich). These four pesticides under study, whose main characteristics are presented in Table 1, can be classified into two categories representative of pyrethroids according to their chemical structure. Empenthrin (2), prallethrin (3), and tetramethrin (4) contain a common fragment, viz., the chrysanthemic acid moiety. In transfluthrin (1), two methyl groups pertaining to the terminalvinyl part are replaced by two chlorine atoms (Fig. 1).

A schematic description of the system developed for the generation of the gas flow of transfluthrin (1), empenthrin (2), prallethrin (3), and tetramethrin (4) is shown in Fig. 2. This generation system was adapted from Temime et al. [20] and Pflieger et al. [21] who originally designed the setup for PAHs and pesticides, respectively.
Fig. 2

Experimental setup for pesticide flow generation

The experimental setup is constituted of a heated permeation glass chamber in which a permeation cell (1.8 mL amber vial) containing the pesticide under study in the liquid or solid phase is located (Fig. 2). The vial is capped with a hydrophobic PTFE membrane filter (Mitex filter, 10.0 μm) and sealed with an open-top closure. Because of temperature, the pesticide volatilizes up to reach the saturation vapor pressure in the permeation cell. On the other hand, due to the continuous nitrogen flux (80 mL min−1, monitored by a mass flow controller, ±0.9 % full scale) that flows throughout the permeation chamber, the partial pressure of the pesticide under study in the chamber is considered to be zero. Because of the difference of pesticide vapor pressure between the permeation cell and the permeation chamber, the molecules leave the sealed vial by diffusion through the membrane. Since the diffusion rate is low compared to volatilization, the difference in vapor pressure between the permeation cell and the permeation chamber is maintained constant over time, which guarantees a regular generation of the pesticide.

A constant flow of gaseous pesticide diluted in pure nitrogen is thus obtained at the outlet of the permeation chamber. To ensure a constant permeation and avoid any pesticide condensation, the whole setup is housed in a thermostated oven at specific generation temperatures displayed in Table 1.

In order to both determine the pesticide gaseous concentration in the nitrogen stream and to guarantee the stability of generation, the loss of mass in the permeation cell is regularly weighted using a highly sensitive mass balance (maximum weight of 60 g and accuracy of ±0.01 mg; Denver Instrument). This gravimetric calibration is a tried and tested technique that has been widely used in many studies to determine the generation rate [20, 21, 22, 23]. At least 3 days of evaporation is necessary to get a quantifiable loss of mass. Under these conditions, the uncertainty on the mass of evaporated pesticide (4 weigh-in replicated) is <3 % for a mass loss of 1 mg. The generation turned out to be stable over time for all the investigated compounds.

To obtain realistic atmospheric concentrations, a dilution system is added downstream from the generation setup. It consists of an additional nitrogen flow controlled by a mass flow controller (0–2 L min−1 ± 0.9 %, full scale) and a mixing glass chamber in which the pesticide flow and the dilution flow are homogenized. A stop valve is placed on the dilution tube to prevent any pesticide backflow in the system when the dilution flow is set to zero (Fig. 2).

Quantum chemistry calculation of proton affinities and proton transfer reaction rate coefficients

Both proton affinity and reaction rate coefficient have an influence on proton transfer reaction efficiency. First and foremost, the proton affinity of the compound under study is required to be higher than that of water to make the proton transfer reaction possible. Besides, the higher the proton affinity and the reaction rate, the better the sensitivity.

Since no data concerning the PAs of pyrethroid compounds could be found in the literature, quantum chemistry calculations were performed to determine the proton affinity values. It has been chosen to use the composite G4MP2 method which is expected to provide thermochemical quantities within <4 kJ mol−1 [24]. These calculations have been conducted using the GAUSSIAN09 suites of programs [25].

The expected most basic sites of four molecules are (i) the chrysanthemic acid part and (ii) the carbonylated cycles present in prallethrin (3) and tetramethrin (4). Accordingly, fluoroaromatic rings, alcynes, and alkenes have proton affinities around 750 kJ mol−1, while protonation at the oxygen of a carbonyl group corresponds to PA close to 900 kJ mol−1 [26]. Since calculation at the G4MP2 level of the entire molecules (1)–(4) would be prohibitive in memory size and computational time, simplified model compounds (5)–(8) were considered (Fig. 3).
Fig. 3

Computed G4MP2 proton affinities (in kilojoules per mole) of model compounds (5)–(8) (values in bold correspond to the most basic site)

G4MP2 computation demonstrates clearly that protonation at the carbonyl oxygen of chrysanthemic ester is always favored. A second observation is that replacing the two methyl groups of the vinyl moiety of molecule (5) by two chlorine atoms in molecule (6) results in a decrease in PA of 20 kJ mol−1. Combining these data, it can be concluded that transfluthrin (1) possesses a proton affinity PA(1) close to 880 kJ mol−1, while the three other compounds, (2)–(4), are characterized by a PA close to 900 kJ mol−1. Consequently, the four molecules, (1)–(4) present proton affinities higher than that of water (PA(H2O) = 691 kJ mol−1) by around 200 kJ mol−1, thus allowing their efficient protonation in the HS-PTR-MS drift tube.

Kinetics may also have a strong influence on the feasibility of the proton transfer reaction. Proton transfer reaction rate coefficients (kSC, in cubic centimeters per second) are therefore calculated from the dipole moment (μD, in debyes) and the polarizability (α, in cubic Angstrom) of the molecules using the parameterized reaction rate theory by Su and Chesnavich [27]. HyperChem software (version 8.0, Hypercube Inc.) was used for quantum chemical calculations of the dipole moment and the polarizability of the compounds under study. The estimated dipole moment and polarizability as well as the calculated transfer reaction rates are presented in Table 1. The transfer reaction rate values for the molecules under study range from 3.60×10−9 to 4.09×10−9 cm3 s−1, which is relatively high compared to the values calculated by Zhao and Zhang for VOCs [28]. Such values therefore indicate that the kinetic characteristics of the molecules under study are not a limiting factor for the detection of the compounds.

HS-PTR-MS calibration

Calibration consists of converting the raw HS-PTR-MS signal, measured in counts per second, to concentration in parts per billion in volume. According to Warneke et al. [29], the slope of the linear fit of the calibration curve defines the sensitivity (i.e., the calibration factor, in counts per seconds per part per billion in volume).

The choice of the concentration range should address minimum and maximum values. Since HS-PTR-MS is intended to measure compounds in the gaseous phase, the concentration range should not exceed the maximum gas phase concentration governed by vapor pressure. The saturated gas phase concentrations of the molecules under study, expressed in parts per billion in volume, under 1 atm of total air pressure are presented in Table 1. The chosen concentration range used for calibration does, therefore, not exceed these concentrations. It can readily be seen from this calculation that tetramethrin (4) presents a very low saturated concentration and is therefore very unlikely to be present in the gaseous phase under typical room conditions at a sufficient concentration to be detected by the HS-PTR-MS device.

The HS-PTR-MS is connected to the standard pesticide generator using the following conditions: drift tube pressure, 2.2 mbar; E/N = 134 Td, where E/N, expressed in Townsend (1 Td = 10−17 V cm2), is the ratio of the electric field strength (E, in volts per centimeter) to the gas number density (N, in cubic centimeters). Lindinger et al. [8] established that the best experimental conditions correspond to an E/N ratio ranging from 120 to 140 Td, thus avoiding water cluster production (low E/N) and too much fragmentation (high E/N). An increase of the E/N ratio actually leads to an increase of the collision energy between ions and the respective neutral collision partners (H2O and M, the analyte gas), therefore leading to more fragmentation, but less cluster formation [8, 9, 10,30]. Tests at different E/N ratios (134, 112, 90, and 70 Td) have been carried out on transfluthrin (1) to observe the effect on the fragmentation patterns and to determine whether fragmentation could be reduced. The fragment ion abundances actually depend on two factors, namely, the activation energies of the fragmentations and the internal energy distribution of fragmenting ions. The activation energies of the fragmentations of the protonated molecules under study are around 100 kJ mol−1 based on the G4MP2 calculations realized on the model compounds (5) and (6) (enthalpy difference between the most stable ions MH+ with a protonation of the most basic site C=O and the products of acylium ions + CH3OH). Besides, the internal energy distribution of fragmenting ions depends on the experimental conditions. The transfer reaction initially forms MH+ ions, having a surplus of energy equal to the difference between the PA of water and the PA of the molecule under study, which is around 200 kJ mol−1. Under these conditions, since this internal energy is higher than the energy of activation, some of the MH+ ions are likely to spontaneously fragment. On the contrary, if a thermal equilibrium can be reached in the drift tube, the internal energy of the MH+ will be lower than the difference in proton affinity (calculation realized at 300 K for prallethrin (3) gives an average energy of around 60 kJ mol−1). In that case, the MH+ ions would produce few or no fragments, which is expected at low E/N values.

HS-PTR-MS is operated in scan mode with a dwell time of 1 s. The m/z scan ranges are set between m/z 21, corresponding to the precursor ion (H318O+), which is used as the reference ion necessary for quantification and a maximum m/z depending on the pseudo-molecular mass of the compound under study (MH+, i.e., M + 1).

The calibration of each pesticide is performed from the most diluted to the most concentrated pesticide flow to reduce the eventual memory effects of the generation system. The mass ranges are also scanned, with the generation setup being empty in order to consider the contribution of eventual impurities present in the generation system.

Results and discussion

According to their proton affinities estimated in “Quantum chemistry calculation of proton affinities and proton transfer reaction rate coefficients,” the molecules under study should be detectable by the HS-PTR-MS device. However, no significant signal was observed for tetramethrin (4). This is probably due to the low vapor pressure of this molecule, which induces a low gaseous phase concentration at atmospheric temperature. The calibration was therefore only performed for transfluthrin (1), empenthrin (2), and prallethrin (3).

Fragmentation patterns

The proton transfer ionization technique is considered as a soft ionization mode enabling no or little fragmentation of the protonated molecules [8,10,19]. However, the investigated compounds (1)–(3) showed several fragmentation reactions of the protonated molecule. Moreover, the MH+ signal is not predominant in the mass spectra, as widely observed with the HS-PTR-MS [8] (Fig. 4).
Fig. 4

Transfluthrin (a), prallethrin (b), and empenthrin (c) HS-PTR-MS spectra

The mass spectra of empenthrin (2) and prallethrin (3) show several common peaks characteristic of the chrysanthemic acid fragment. These fragments potentially correspond to protonated chrysanthemic acid (m/z 169), chrysanthemyl acylium ion (m/z 151), and chrysanthemyl cation (m/z 123) [31] (Fig. 5). In addition to these three common fragment ions, characteristic signals, specific to each pyrethroid molecule, are also detected at m/z 149 and 301 for prallethrin (2) and m/z 107 and 275 for empenthrin (3), which makes a reliable identification of each pesticide possible. Transfluthrin (1) is easily identifiable since the chlorinated cyclopropane yields to the formation of an acylium fragment at m/z 191, with isotope distribution characteristic of the presence of two chlorine atoms (100 %/65 %/11 %). Also, a specific fragment corresponding to the aromatic part of transfluthrin (1) is detected at m/z 163.
Fig. 5

Main fragmentation reactions of transfluthrin (1), empenthrin (2), and prallethrin (3) under HS-PTR-MS conditions

The fragmentation pattern of protonated molecules (1)–(3) may be rationalized by considering the two structures resulting from the protonation of either the two oxygen atoms of the chrysanthemic ester moiety (Fig. 5). The protonation of the most basic site (C=O) leads to a structure able to expel the chrysanthemic acid part, giving rise to m/z 163 from transfluthrin (1) and to m/z 169 from empenthrin (2) and prallethrin (3) after the passage through ion-neutral complex intermediates INC-A. The proton transfer inside such electrostatic complex explains the formation of protonated chrysantemic acid (m/z 169).

Similarly, protonation on the ether-like oxygen leads to a structure characterized by an elongation of the C=O…+OHR bond and, thus, the precursor of acylium ions m/z 191 from transfluthrin (1) or m/z 151 from empenthrin (2) and prallethrin (3). The formation of an intermediate ion-neutral complex INC-B (Fig. 5) allows internal hydride ions exchange and, thus, the formation of ions m/z 123 and 149 from empenthrin (2) and prallethrin (3), respectively.

The weak nature of the ester bond present in the O ether protonated pyrethroid structures actually results in a facilitated formation of INC-B and its subsequent fragmentations. Quantum chemical computations show that approx. 120 kJ mol−1 is required to break the CO…OHR+ bond in the O ether protonated species and that the enthalpy difference between this structure and the most stable form (C=O protonated) is equal to 65 kJ mol−1. Thus, 185 kJ mol−1 is needed to form ions m/z 151. This value is below the exothermicity of the ionization (~200 kJ mol−1, see above), in agreement with the observation of extensive fragmentations involving the fission of the ester bond.

For some compounds, it appears that the amount of fragmentation can be controlled by reducing the E/N value. This is for instance the case for ethylbenzene, which fragments at 140 Td, but not at 106 Td [34]. However, the tests realized on transfluthrin making the E/N ratio vary between 70 and 134 Td did not show a significant change of the fragmentation patterns. These tests thus revealed that fragmentation always occurs even at a low E/N ratio, probably because of the high exothermicity of the protonation process with regard to the endothermicity of the fragmentations, thus leading to spontaneous fragmentation. Besides, even if a slight upward trend can be observed when E/N is lowered, the proportion of MH+ relative to the fragments was not significantly increased (Fig. 6).
Fig. 6

Proportion of protonated molecule MH+ relative to the sum of MH+ and fragments

Consequently, it does not seem to be worthwhile decreasing the E/N ratio to reduce fragmentation of the pyrethroid molecules during the HS-PTR-MS analysis. Fragmentation therefore seems unavoidable for pyrethroid analysis via the HS-PTR-MS technique in the presently explored range of experimental conditions.

Sensitivity

Calibration is performed considering the sum of the signals of the protonated molecule MH+ and those of the main fragments. The characteristic ions of the molecules under study summed for calibration are listed in Table 1.

Note that since experimental applications involve commercial insecticides possibly containing additives and solvents, complex mass spectra are likely to arise, especially for m/z below 100. Several compounds may therefore overlap at a single m/z. To reduce the uncertainty associated to mass spectra deconvolution, only fragment ions with m/z larger than 100 are considered for calibration. Schwarz et al. [32] suggest an optimized approach for the quantification of compounds with overlapping fragments based on the study of the fragmentation patterns, but this requires knowing in advance all the compounds present in the gas mixture, which would not be the case if commercial formulations are investigated.

Besides, since most pyrethroid molecules contain common fragments, several m/z may overlap if different pyrethroids are simultaneously detected. This difficulty can be bypassed through mass spectra deconvolution, thanks to the specific ion of each pyrethroid.

The influence of the concentration variations of the precursor ion H3O+ on the sensitivity of the HS-PTR-MS is taken into account through the normalization of the measured ion count rates to the H318O+ count rate (m/z 21). Besides, the molecules under study can also react with water clusters H3O+,(H2O)n (m/z 37, 55, and 73 for respectively n = 1, 2, or 3). The efficiency of proton transfer reactions may be different depending on the value of n (0, 1, 2, or 3). As a result, water vapor level in the drift tube, either due to the humidity of the sample itself or due to the water level in the ion source, may influence the HS-PTR-MS signal [29,32,33]. To correct the influence of the atmospheric humidity, normalization to the signal of the water clusters (generally only to the signal of H3O+,(H2O) (m/z 37) because of the very low proportion of m/z 55 and 73) can also be performed [10].

For most compounds, the transfer reaction of analyte gases with bare H3O+ is as efficient as the reaction with H3O+,(H2O). However, for certain compounds such as toluene or benzene, the transfer reaction rates for the clusters are close to zero [29]. In such specific cases, humidity is likely to strongly affect the sensitivity, especially if a low E/N ratio is used, which promotes the proportion of clusters relative to bare H3O+. If the HS-PTR-MS is operated in the typical default conditions (E/N ranging from 120 to 140 Td), the proportion of water clusters formed with H3O+ is on the contrary expected to be very low [10,34] due to the energetic collisions breaking the clusters in the drift tube, even if the relative humidity of the sampled air is important.

The effect of humidity on prallethrin (3) was investigated by adding a humidification system to the dilution channel in the generation setup. While a constant concentration flux of prallethrin (3) was generated over time, different relative humidities (RH) were tested (0, 20, 40, and 60 % at 27 °C) at E/N of 111 and 134 Td.

At an E/N of 134 Td, it is experimentally confirmed that few water clusters are present in the drift tube since the relative abundance of m/z 37 (corresponding to the cluster H3O+,(H2O)) never exceeds 6 % of the potential precursor ions present (H3O+ and H3O+,(H2O)). Bare H3O+ is therefore always largely predominant, which considerably limits the eventual effect of humidity. On the contrary, at an E/N of 111 Td, the proportion of clusters is much higher and noticeably increases when the relative humidity is increasing (25 % of cluster H3O+,(H2O) at 60 % RH). In the case of a compound being sensitive to humidity, this high proportion of clusters is likely to limit its sensitivity. The HS-PTR-MS signal of prallethrin (3), studied under varying RH conditions, is either normalized to m/z 21 alone or to m/z 21 + 37 at E/N of 111 and 134 Td (Fig. 7).
Fig. 7

Influence of relative humidity on the HS-PTR-MS signal of prallethrin (3) normalized to m/z21 and to m/z21+37 at E/N=134 and 111 Td

At an E/N of 134 Td, when the raw signal is normalized to m/z 21 alone, only small and insignificant variations of the signal can be noted, indicating that the increase of humidity does not affect the sensitivity (no fall of the response is observed). When the signal is normalized to m/z 21 + 37, the variations are not particularly smoothed, which confirms that the influence of the cluster is very limited because its proportion is very low compared to m/z 21. Actually, if the cluster m/z 37 had a significant importance as a precursor ion, the normalization through its value would improve the stability of the signal.

At an E/N of 111 Td, the signal increases with relative humidity when the raw data are normalized to m/z 21 alone (Fig. 7) because the cluster ion proportion also increases, but is not accounted for as a precursor ion (no normalization through its abundance). Such a rising trend also confirms that the transfer reaction efficiency is as good with H3O+,(H2O) as with bare H3O+. Moreover, the variations of the signal are smoothed by the normalization to m/z 21 + 37, which confirms the role of the cluster ion H3O+,(H2O) as a significant precursor ion when the humidity goes up.

Humidity has therefore no significant influence on the overall sensitivity of prallethrin (3). Under default HS-PTR-MS conditions (E/N ratios ranging from 120 to 140 Td, as chosen for the present study since fragmentation was not significantly reduced at a lower E/N ratio), since the proportion of water clusters is very limited, normalization to H3O+ ion alone is sufficient, even under high relative humidity conditions. If a lower E/N ratio is applied, it is necessary to consider all the precursor ions present in the drift tube for normalization., i.e., bare H3O+ and H3O+,(H2O), to account for humidity. These tests anyhow confirm that humidity does not limit the sensitivity of the compound.

The calibration curve shows the ion count rates normalized to m/z 21 (in normalized counts per second) versus the concentrations of the pesticides (in parts per billion in volume; Fig. 8), whose slope defines the sensitivity [29], expressed in normalized counts per second per part per billion in volume (Table 2). All data show the linear behavior of the response over the investigated concentration range.
Fig. 8

Calibration curves of transfluthrin (a), prallethrin (b), and empenthrin (c) in dry nitrogen (0 % RH)

Table 2

Experimentally determined sensitivities of transfluthrin, prallethrin, and empenthrin

 

Transfluthrin (1)

Empenthrin (2)

Prallethrin (3)

Sensitivity (ncps ppbv−1) ± uncertainty

(0.8 ± 0.2) × 10−3

(7.8 ± 0.4) × 10−3

(4.6 ± 0.6) × 10−3

The uncertainty associated with the sensitivity values is found to range from 15 to 25 %, which includes the uncertainties of both HS-PTR-MS measurements and pesticide generation. Such an instrumental accuracy was already obtained in previous works for the analysis of VOCs involving HS-PTR-MS [19,32,34].

As shown in Table 2, significant differences are observed between sensitivities of the molecules under study. In particular, transfluthrin (1) exhibits a sensitivity one order of magnitude below that of empenthrin (2) and prallethrin (3). These deviations may be due to both the differences in the transmission efficiency of the characteristic ions and to the thermodynamic and kinetic characteristics of the molecules.

On the one hand, it has been shown that the transmission efficiency noticeably decreases for m/z higher than 120 in a PTR-MS analyzing device [35]. As a result, the transfluthrin (1) calibration that relies on characteristic ions ranging from m/z 163 to 375 suffers from poor transmission efficiency and therefore presents a lower raw signal compared to prallethrin (3) or empenthrin (2), whose characteristic ions do not exceed m/z 169.

On the other hand, both proton affinity and proton transfer reaction rates may have an influence on the overall sensitivity. As regards kinetics, since no significant difference is observed between the transfer reaction rate values for the molecules under study, it is likely that this factor does not have an important influence on the overall sensitivity. Concerning thermodynamics, the higher the PA of the molecule, the more efficient the protonation. As a result, for equivalent amounts of the parent molecules, a larger quantity of protonated molecules is obtained, thus increasing the sensitivity of detection. On the contrary, it has been shown that when the PA difference between the molecule and water is small, as for formaldehyde for instance (PA(H2CO) = 711.0 kJ mol−1) [36], involving a difference of 20 kJ mol−1, the back-reaction of the protonated molecule with water occurs and reduces the sensitivity [37]. As indicated in “Quantum chemistry calculation of proton affinities and proton transfer reaction rate coefficients”, PA(1) is close to 880 kJ mol−1, while PA(2) and PA(3) are close to 900 kJ mol−1. These values are significantly higher than PA(H2O), and moreover, the small difference between PA(1) and PA(2) or PA(3) has probably only a very limited influence on the overall sensitivity. Probably, transmission efficiency is the most influent factor that explains the lower sensitivity observed for transfluthrin (1).

Field measurements

The following section illustrates the possible experimental applications of the analytical development presented above. The present experiment focuses on the time profile of transfluthrin (1) emitted in an indoor atmosphere. An electric vaporizer containing 13.4 % (w/w) transfluthrin (1), adsorbed on a solid pad refill is applied in a full-scale environmental room (32.3 m3) under controlled conditions (air exchange rate, temperature, and relative humidity).

The vaporizer is plugged in for 8 h according to typical night duration. The air is monitored online with the HS-PTR-MS during and after application until the concentration levels return to the background level measured before application (reference situation). The HS-PTR-MS device is connected to the sampling line and operated at the conditions used for calibration (drift tube pressure, 2.2 mbar; E/N = 134 Td; multiple ion detection mode, 1-s integration time).

As soon as the electric vaporizer is plugged in, the concentration emitted from the vaporizer rapidly increases (Fig. 9), then decreases after unplugging to come down to the reference situation in more than 17 h. The obtained concentration profile provides two key parameters for an eventual future exposure evaluation: the peak concentration and the apparition and decay rates.
Fig. 9

Concentration versus time profile of transfluthrin (1) during and after vaporizer application (the dashed gray line represents the time of unplugging)

Advantages and limitations of the analytical device

HS-PTR-MS appears to be an effective analytical device for the analysis of semi-volatile organic compounds in the atmosphere. Actually, it makes it possible to detect three of the four pesticides under study. Only tetramethrin (4) is not detected by the HS-PTR-MS despite its proton affinity being higher than that of water. This phenomenon is due to the low volatility of this compound which is mainly present in the particulate phase. For this class of compounds, other analytical techniques such as aerosol mass spectrometry should therefore be considered.

The absence of a chromatographic system upstream from the HS-PTR-MS gives the opportunity to obtain real-time high time-resolved measurements, potentially for extended periods of time. All the atmospheric compounds having a proton affinity higher than that of water are actually simultaneously analyzed, thanks to this analytical procedure. Moreover, due to the soft ionization mode, the individual compounds are identified according to the m/z of the few ions corresponding to each compound. Unfortunately, for analysis of complex gas mixtures, several compounds may overlap at a single m/z, which is likely to lead to an overestimation of the compounds under study. The use of a PTR time-of-flight MS (described in [38] and [39]) to increase the m/z resolution could bypass this difficulty. However, it is inefficient when the overlaps are caused by isobaric ions. With regard to the context of the study, this case can occur if several pyrethroids containing a common structure (viz., chrysanthemic acid or chlorinated chrysanthemic acid) are simultaneously analyzed. For instance, empenthrin (2) and prallethrin (3) would produce several common ions (i.e., chrysanthemic acid, m/z 169; chrysanthemyl cation, m/z 123; and chrysanthemyl acylium ion, m/z 151) if they are concurrently detected. To solve this difficulty, it is necessary to study the fragmentation patterns of the pure compounds and to use the mass spectrum deconvolution technique to assign the contribution of each molecule on the ions under study, thanks to the specific ions.

The main limitation of the HS-PTR-MS for the measurement of pyrethroid compounds in indoor air is probably its poor sensitivity compared to other analytical techniques. While the limits of detection with classical methods are of the order of <0.1 pptv for transfluthrin [40,41], the experimental detection limit of the HS-PTR-MS, observed during the field experiment for transfluthrin (1), is of the order of 50 pptv. HS-PTR-MS is therefore not suited for the analysis of the background air levels in indoor air. However, the time resolution provided by this instrument is not required in the context of background air concentration measurements since no or only little variation of the concentration is expected. On the contrary, in the perspective of monitoring the temporal evolution of pesticide concentration during the application of a commercial insecticide, the measurement frequency is probably more crucial, but a poorer sensitivity is meanwhile achieved.

With regard to the uncertainty associated with the HS-PTR-MS measurements, it should be kept in mind that the measurement integrates the whole analytical chain. To get comparable results, the uncertainties corresponding to these steps in off-line methods should therefore be considered. Several authors observe between 10 and 25 % uncertainty including all steps of the analytical procedure for off-line techniques [42, 43, 44, 45], which is comparable to the uncertainty found with the HS-PTR-MS (15–25 %). Different studies [29,34,46] moreover find very good agreement of the results between the HS-PTR-MS technique and alternative and more established analytical methods on different compounds, accounting for the measurement uncertainties of both instruments.

The main strength of the HS-PTR-MS as a new analytical device to study semi volatile compound behavior in indoor environments (i.e., pesticide temporal evolution after application) is the possibility of monitoring the evolution of the contaminant concentrations versus time with a high time resolution (more than one measurement per minute). Compared to the analytical methods used so far, which typically involve time-consuming sampling and off-line analytical steps [40,47, 48, 49], the time resolution between two measurements has been considerably enhanced. Due to the analytical constraints and to the low background levels of pesticides found indoor that require extended time of accumulation, the sampling time typically ranged from more than 1 h up to several days [50, 51, 52]. Leva et al. [47], Berger-Preiss et al. [48], and Ramesh and Vijayalakshmi [53] managed to reduce it to <1 h, but for a relatively limited number of measures. In contrast, the measurement frequency drops to one measurement per minute or less with the HS-PTR-MS, potentially for extended periods of measurement time (24 h or more). Moreover, no pretreatment of the sample is required, which involves potential risk of analyte loss, e.g., during transport, storage, extraction, and cleanup [46,54] or through photodecomposition [40,55]. Finally, the implementation and the use of the analytical device for experimental study are relatively easy once the initial parameters are adjusted. Both the fast time response and ease of use therefore give this analytical technique an innovative perspective for online measurements in indoor environments.

Conclusion

In addition to the wide range of volatile organic pollutants the HS-PTR-MS can monitor, this online analytical method with fast time response has shown the potential to measure semi-volatile pyrethroid compounds in indoor environments. Thanks to its good time resolution, the use of this technique makes it possible to follow concentrations having a high temporal variability, for instance during and just after insecticide treatment.

With regard to the performance of this analytical method, the measurement uncertainty is comparable to those found for state-of-the-art off-line methods. However, its limit of detection is undoubtedly poorer than with other off-line techniques that accumulate compounds for several hours. Depending on the context, it is clear that other methods are more suited, especially for background air level measurements. A compromise has therefore to be found between time resolution and sensitivity. However, in order to concurrently get high time-resolved and highly sensitive measurements, thus solving the main weaknesses of the device, off-line sampling on different adsorbents followed by a GC-MS analysis and online HS-PTR-MS monitoring could simultaneously be performed since these techniques provide complementary results.

The potential applications of HS-PTR-MS are thus considerably extended, now involving indoor air pollutions with semi-volatile organic compounds. Further experimental applications of different commercial insecticides in indoor environments with HS-PTR-MS monitoring are foreseen.

Acknowledgments

This project has been supported by the French Agency for Food, Environmental and Occupational Health Safety (ANSES, project EST-2008/1/37). Aude Vesin received a doctoral grant from the French Environment and Energy Management Agency (ADEME) and the French National Centre for Scientific Research (CNRS).

Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • Aude Vesin
    • 1
    • 2
  • Guy Bouchoux
    • 3
  • Etienne Quivet
    • 1
    • 2
  • Brice Temime-Roussel
    • 1
    • 2
  • Henri Wortham
    • 1
    • 2
  1. 1.Aix-Marseille Univ, LCE-IRAMarseilleFrance
  2. 2.CNRS, FRE 3416MarseilleFrance
  3. 3.Département de Chimie, CNRS UMR 7651Ecole PolytechniquePalaiseau CedexFrance

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