Real-time monitoring of the work environment using ion-mobility spectrometry

Abstract

Objectives: Ion-mobility spectrometry (IMS) is a promising system for on-site real-time monitoring of volatile organic compounds (VOCs). Calibration curves derived from shifts in nominal arrival-time spectra of chemical substances relative to those of water clusters enable quantitative analysis at high concentrations. Methods: This study investigated the adaptability of IMS to real-time monitoring of VOCs in the work environment, using toluene as a test case. Toluene concentrations were measured by IMS at one-minute intervals during a ten-minute simulated cleaning operation. Results: The arrival-time shift was lower at high concentrations because ion production saturates as the toluene concentration approaches the limit of ionizability, with a resulting decrease in slope of the calibration curve. The lower limit of quantification for toluene was assumed to be 13.3 ppm because no arrival-time shift was observed at lower concentrations. The time-averaged toluene concentration measured by IMS for 10 minutes of operation was 45.8 ppm, which is comparable to that measured by gas chromatography–mass spectrometry (GC–MS; 44.3 ppm) within ~3%. Conclusions: Our results indicate that the measurement of toluene concentrations is possible at one-minute intervals by IMS, making it possible to track rapid changes in workplace conditions. Therefore, IMS can measure exposure to VOCs in real-time with an accuracy similar to that of GC–MS.

Introduction

The accurate monitoring of airborne concentrations of chemicals is crucial in assessing exposure in the work environment, where a time-weighted average (TWA) concentration for an 8-h working day during a 40-h working week is commonly used as an indicator of health risk¹⁾. Many occupational exposure limits have been established worldwide^2,3,4). However, short-term exposure to high concentrations can occur during a specific task, even when TWA concentrations are within acceptable limits. Some hazardous chemicals have ceiling values⁴⁾ that must not be exceeded even momentarily, with corresponding short-term (15 min) exposure limits¹⁾.

Gas chromatography–mass spectroscopy (GC–MS) is commonly used to determine chemical concentrations in the atmosphere^5,6,7,8,9) and is an accepted method for measurements in the workplace, where it is used in post-process monitoring with adsorption of airborne substances in a collection tube. However, these measurements are time-consuming, and short-term and real-time exposure data are lacking.

Photo-ionization detectors (PIDs) have been a popular alternative for real-time monitoring of the workplace^10,11), estimating concentrations through ionization by ultraviolet (UV) light. However, all substances in the air with an ionization energy lower than the UV energy are ionized, making individual quantifications difficult when multiple substances are used.

Airborne concentrations of chemicals may change continuously in the work environment owing to factors such as changes in air temperature and air flow caused by air conditioning and ventilation, sometimes increasing by multiple times in short timeframes¹²⁾. Instantaneous exposure to high concentrations may occur in specific tasks¹³⁾.

We have developed an ion-mobility spectrometry (IMS) system to monitor airborne chemicals in real time in the workplace¹⁴⁾. IMS performs qualitative and quantitative analyses at atmospheric pressure and so has no vacuum requirement. It is also useful in real-time monitoring because it can perform short-term analyses. In previous studies, we found that calibration curves derived from shifts in nominal arrival-time spectra of chemical substances overlapping those of clusters enable quantitative analyses at high concentrations^14,15).

This study examined the applicability of IMS to real-time monitoring of fluctuating concentrations during a manual operation. Performance was evaluated by monitoring airborne toluene concentrations during a simulated cleaning operation, with the concentration varying more than in the breathing zone. Solvent-poisoning accidents have been reported during cleaning operations that utilize toluene, particularly in connection to cleaning inside drums and degreasing¹⁶⁾.

Methods

In the IMS system, sample ions produced by a corona discharge move in a drift tube filled with a buffer gas (air in this study), which acts as the target particle under a uniform electric field. Chemicals are identified on the basis of ion velocity in the drift tube^17,18,19). Sample ions moving in the weak uniform electric field are decelerated by collision with buffer gas molecules, accelerated by the electric field, and they finally move in a direction perpendicular to the equipotential surface of the field. If the sample ions are of complex shape or large size, their drift velocity decreases owing to their higher frequency of collision with buffer gas molecules. Information on the shape and size of molecules can thus be obtained from the ion velocity, allowing the substance to be identified. The drift tube method is less affected by non-environmental factors than other methods. The drawback of this method is low resolution in measuring chemical substances at high concentrations.

The IMS system used in this study is illustrated in Figure 1. The device comprises 11 guard ring electrodes, D1–D11, with D1 and D2 forming an ionization chamber; a gate electrode, D3, and a drift tube of 11 cm length with electrodes D4–Dll. A high voltage (HV) set by a resistor chain is applied to each guard ring to produce an electric field gradient (upper left, Figure 1). Adjacent guard rings are insulated with 2-mm-thick Teflon plates. Sample vapor with a concentration adjusted by a calibration-gas generation system PD-1B (Gastec Corporation, Ayase, Japan) is continuously injected into the IMS. The sample vapor is ionized by a corona discharge induced by applying a high voltage (HV3 = 4.7 and HV4 = 2.6 kV; 1 μA) to the two discharge needles in the ionization chamber D2 (Figure 1).

Fig. 1. Schematic of the experimental apparatus. Sample vapor is ionized by a high voltage corona discharge (HV3 = 4.7 kV, HV4 = 2.6 kV), and ions are pulsed using an applied high voltage (HV1 = 2.2 kV, HV2 = 2.0 kV).

The ionization mechanism is described by the following processes¹⁷⁾:

  

$${\mathrm{e}}^{-}+{\mathrm{N}}_2\to {{\mathrm{N}}_2}^{+}+2{\mathrm{e}}^{-}$$(1)

  

$${{\mathrm{N}}_2}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{N}}_2+{\mathrm{H}}_2{\mathrm{O}}^{+}$$(2)

  

$${\mathrm{H}}_2{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+\mathrm{O}\mathrm{H}$$(3)

  

$${\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{M}\to {\left({\mathrm{H}}_2\mathrm{O}\right)}_2{\mathrm{H}}^{+}+\mathrm{M}$$(4)

  

$${\left({\mathrm{H}}_2\mathrm{O}\right)}_{n-1}{\mathrm{H}}^{+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{M}\to {\left({\mathrm{H}}_2\mathrm{O}\right)}_n{\mathrm{H}}^{+}+\mathrm{M}$$(5)

  

$${\left({\mathrm{H}}_2\mathrm{O}\right)}_n{\mathrm{H}}^{+}+\mathrm{M}\to {\left({\mathrm{H}}_2\mathrm{O}\right)}_n+\mathrm{M}{\mathrm{H}}^{+}$$(6)

where M is sample molecule. N₂ molecules in the air are ionized by a corona discharge (1) followed by electron transfer (2); the sample ion, MH⁺, with the addition of a proton, is generated through a series of reactions including proton transfer (3), three-body reactions (4, 5), and further proton transfer (6). The water cluster ion (H₂O)_nH⁺ in (6), the “reactant ion”, plays an important role in ionization.

The continuously incident sample ion, MH⁺, ionized in the ionization chamber, is pulsed with a function generator DG1022U (RIGOL Inc., Suzhou, China) by a blocking voltage applied to two meshes installed in D3, typically of duration 3 ms and frequency 10 Hz. The ions cannot overcome the potential barrier in the gate (D3). Ion flight time is measured using this electrical gate pulse as the start signal and that from the detector as the stop signal. The ion signal monitored using the detector is amplified via a current amplifier DLPCS-200 (FEMTO Messtechnik GmbH, Berlin, Germany), and signal intensity is measured using the oscilloscope. A drift gas (N₂, 50 mL min⁻¹) flows constantly from the end of the drift tube to keep the interior clean.

In previous studies we proposed a quantitative analysis technique in which a calibration curve is obtained from the shift in arrival-time spectra of chemical substances relative to that of water clusters^14,15). The peak of a chemical substance is overlapped with the water-cluster peak, with the observed peak being a “nominal” peak. The arrival time of a substance is calculated from the position of the nominal peak, based on Gaussian fitting. If the concentration of a substance increases, the concentration of water cluster ions decreases correspondingly, with the nominal peak position shifting from the reactant-ion peak (RIP) position. The conventional IMS technique cannot be applied to quantitative analyses of chemical substances with a high proton affinity at high concentrations because multimeric complexes are generated. Therefore, we suggested a quantitative analysis technique in which a calibration curve was obtained from the shift of the arrival-time spectrum of chemical substances overlapped with that of water clusters by increasing the pulse width.

In this study, toluene concentrations were measured in real time, and quantitative analysis was performed using the calibration curve technique. We measured the concentration at 1-min intervals because based on our previous study the IMS system requires ~50 s for one set of toluene intakes and exhausts¹⁵⁾. Toluene concentration was measured over a range below the ceiling value of 300 ppm defined by the United States Occupational Safety and Health Administration (OSHA)²⁰⁾.

The experimental setup for the simulated sheet metal cleaning with toluene is shown in Figure 2. The process comprised (a) opening the toluene container and soaking a cloth in toluene using a dropper; (b) closing the container; (c) cleaning the metal with the toluene-soaked cloth; then repeating (a) through (c). In a previous study, quantitative analysis at around the legislated ceiling value of toluene was successfully performed¹⁵⁾. To ensure safety in this study, we measured toluene concentrations of ~100 ppm, whereas in practice, the toluene container would not be sealed between uses. Here, the container was opened and closed to increase variation in airborne toluene concentration.

Fig. 2. Sampling setup for real-time measurement during simulated working.

Results and Discussion

Arrival-time spectra of toluene at concentrations of 13.3, 22.6, 86.0, and 162.1 ppm, measured under identical conditions of ambient temperature, humidity, and device setup, are shown in Figure 3, where the vertical axis indicates relative signal intensity. The RIP is the background spectrum obtained from water cluster ions present in air. Table 1 summarizes the relationship between toluene concentration and arrival time calculated using Gaussian fitting. Peaks observed at around 0 ms are the start signal for ions when a voltage pulse is applied to the gate electrode. The arrival-time spectra shown in Figure 3 represent nominal peaks obtained by overlapping the spectrum of toluene with that of water clusters. The nominal-peak positions shift to longer arrival times as the toluene concentration increases because the toluene signal intensity increases relative to that of the water cluster. The arrival-time spectra of toluene obtained in this study differ from those of our previous study¹⁵⁾, suggesting that the spectra are affected by humidity (current humidity 57%, previously 20%; temperature was similar in both cases here [19.9°C] and previously [20.0°C]¹⁵⁾). Arrival times of nominal peaks thus vary with water cluster size because of this effect.

Fig. 3. Arrival time spectra of toluene (13.3, 22.6, 30.9, 86.0, and 162.1 ppm), relative to the RIP. The reactant-ion peak (RIP) is the background spectrum obtained from water cluster ions. These spectra were obtained with an electric field, E, of 186 V cm⁻¹.

Table 1. Relationship between toluene concentration and arrival time

Concentration (ppm)	Arrival time (ms)
13.3	44.07
22.6	45.74
30.9	47.70
86.0	49.66
162.1	52.34

A calibration curve obtained by calculating the nominal arrival time of toluene relative to the RIP is shown in Figure 4, where the vertical axis indicates the shift in nominal arrival time from that of the RIP, and the horizontal axis shows concentration. The measurement error in the flow rate of the calibration–gas generation system was estimated to be ≤3%. The slope of the calibration curve shows shifts in arrival time for increments in sample concentration²¹⁾, changing by ~30 ppm per increment. In the high-concentration range (≥30 ppm), the arrival-time shift is lower because ion production saturates as the concentration of toluene approaches the limit of ionizability. No arrival-time shift was observed at concentrations of <13.3 ppm, which is considered the lower limit of quantification for toluene under these conditions. Although the relationship between peak shift and concentration obtained from our calibration technique is well correlated and is stable¹⁴⁾, the cross point of these calibration curves is affected by temperature and humidity. We have to obtain a calibration curve before a measurement because the calibration curve must be recalibrated if temperature and humidity change. Therefore, this device is not suitable for practical use currently. In the future, we will install a heater and desiccant system to maintain constant temperature and humidity in the device.

Fig. 4. Relationship between concentration and arrival time shift at toluene. The measurement error of the flow rate at the calibration-gas generation system was estimated to be about 3%.

Toluene concentrations measured at 1-min intervals for 10 minutes during a cleaning operation are plotted in Figure 5, where the solid line is the concentration obtained using the IMS, the dashed line is the 10-min average concentration, and the thick red line is the average concentration measured using GC–MS. Toluene concentrations during the simulated work ranged from 20.3 to 83.6 ppm. The concentration generated during the cleaning process was low (20–30 ppm) because the operation used only small amounts of toluene (~5 mL per wipe). Therefore, the toluene concentration generated by opening the toluene container ([a]: 60–80 ppm) was the highest recorded. The toluene concentration dropped when the container was closed (b). In this study, the sampling inlet of IMS is located near the operator’s hand. Thus, IMS measures non-uniform fluctuating concentrations due to air flow caused by manual operation. Resultantly, the toluene concentration during the cleaning operation (c) fluctuate greatly. The 10-min average toluene concentration measured using GC–MS was 44.3 ppm, whereas that measured using IMS was 45.8 ppm; the two results were consistent within ~3%. These results indicate that IMS is able to measure exposure in real-time with an accuracy similar to that of GC–MS. In our previous study, the other chemical substance had little influence on target chemical substance if the proton affinity of the target chemical substance was higher¹⁵⁾. One measurement using IMS is performed for 1 min, which is much shorter than the measurement time of GC-MS (about 30 to 60 min).

Fig. 5. Relationship between process time and toluene concentration during the simulated cleaning. The process comprised opening of the toluene container and soaking a cloth with toluene using a dropper (a); closing the toluene container (b); and cleaning by wiping with the cloth (c).

Conclusion

The capability of IMS in real-time monitoring of the workplace atmosphere was demonstrated in this study of a simulated cleaning operation using toluene. The measured airborne concentration of toluene was lower than the OSHA ceiling value of 300 ppm. A wide range of toluene concentrations was measured using a calibration curve based on the arrival-time shift in the nominal toluene spectrum. The slope of the calibration curve changed by ~30 ppm, with a high-concentration range of >30 ppm. The arrival-time shift decreases with ion production saturation as the toluene concentration approaches the limit of the ionizability. No arrival-time shift was observed at concentrations of <13.3 ppm, suggesting this is the lower limit of quantification under the conditions applied. The GC–MS method recorded a 10-min average toluene concentration of 44.3 ppm, compared with the IMS result of 45.8 ppm, indicating a possible IMS error of ~3%. Our results indicate that the measurement of toluene concentrations is possible at 1-min intervals using IMS, making it possible to track rapid changes in workplace conditions.

Author contributions

K.T. designed and conducted the study, collected and analyzed data, and wrote the manuscript; N.S. gave conceptual advice and wrote the manuscript; M.H. and M.T. gave conceptual advice; S.M. gave technical support. All authors read and approved the final manuscript.

Conflict of interest

The authors declare that they have no conflict of interest.

Data availability statement

All data collected in the study are available upon reasonable request to the authors.

Disclosure

Approval of the research protocol: N/A. Informed Consent: N/A Registry and the Registration No. of the study/trial: N/A Animal studies: N/A

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