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Rapid detection of volatile organic compounds in a drop urine by ultrasonic nebulization extraction proton transfer reaction mass spectrometry Xue Zou, Yan Lu, Lei Xia, Yating Zhang, Aiyue Li, Hongmei Wang, Chaoqun Huang, Chengyin Shen, and Yannan Chu Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04563 • Publication Date (Web): 27 Dec 2017 Downloaded from http://pubs.acs.org on December 28, 2017

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Analytical Chemistry

Rapid detection of volatile organic compounds in a drop urine by ultrasonic nebulization extraction proton transfer reaction mass spectrometry Xue Zou†, Yan Lu†, Lei Xia†, Yating Zhang†, Aiyue Li†, Hongmei Wang‡, Chaoqun Huang†, Chengyin Shen*,†, Yannan Chu† †

Anhui Province Key Laboratory of Medical Physics and Technology，Center of Medical Physics and Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, China

‡

Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, Anhui 230031, China ABSTRACT: Detection of volatile organic compounds (VOCs) in human urine has potential application value in screening for disease and toxic exposure. However, the current technologies are too slow to get the concentration of VOCs in fresh urine. In this study, we developed a novel ultrasonic nebulization extraction proton transfer reaction mass spectrometry (UNE-PTR-MS) technology. The urinary VOCs can be rapidly extracted to gaseous VOCs using the UNE system and then delivered using a carrier gas to the PTR-MS instrument for rapid detection. The carrier gas flow and sample size were optimized to 100 mL/min and 100 µL, respectively. The limits of detection (LODs) and response time of the UNE-PTR-MS were evaluated by detecting 3 VOCs that are common in human urine: methanol, acetaldehyde, and acetone. The LODs determined for methanol (4.47 µg/L), acetaldehyde (1.98 µg/L), and acetone (3.47 µg/L) are 2~3 orders of magnitude lower than the mean concentrations of that in healthy human urine. The response time of the UNE-PTR-MS is 34 seconds and only 0.66 mL of urine is required for a full scan. The repeatability of this UNE-PTR-MS was evaluated and the relative standard deviations of 5 independent determinations were between 4.62% and 5.21%. Lastly, the UNE-PTR-MS was applied for detection of methanol, acetaldehyde, and acetone in real human urine to test matrix effects, yielding relative recoveries of between 88.39% and 94.54%. These results indicate the UNE-PTR-MS can be used for the rapid detection of VOCs in a drop urine and has practical potential for diagnosing disease or toxic exposure.

There are many kinds of volatile organic compounds (VOCs) in human urine.1 Anomalies in these VOCs are closely related to human metabolism disorders2-5 and give evidence of some human diseases such as bladder cancer,6 urinary tract infection,7 lung cancer,8 and esophageal cancer.9 In addition, urinary VOCs have been found to be secreted in higher levels after exposure to toxic substances.10 Therefore, detection of urinary VOCs has an important application value in screening for disease or toxic exposure. The urinary VOCs changes during storage,11 which makes rapid detection speed a key to accurate determination of VOCs in fresh urine. Headspace solid phase micro-extraction gas chromatography mass spectrometry (HS-SPME-GC-MS) is the most widely used technology for the detection of urinary VOCs with high sensitivity,11-13 but the preconcentration process took

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several hours and the detection process took tens of minutes.12 Multicapillary column ion mobility spectrometry (MCC-IMS) technology has also been used for the detection of urinary VOCs.14 Although the detection process was rapid, the sampling process took more than 4 hours.14 In recent years, research efforts have been made to develop technologies for improving the detection speed. Electronic nose (EN) technology has greatly shortened the detection time to 10 minutes.15 This technology, however, cannot detect unknown VOCs in urine, which limits its application in searching for biomarkers of disease or toxic exposure. Selected ion flow tube mass spectrometry (SIFT-MS) is another fast detection technology but the sample pretreatment process was time-consuming.9 In a conclusion, the current technologies, especially the sample pretreatment technologies, are too slow to get the real concentration of VOCs in fresh urine. In this study, we developed an ultrasonic nebulization extraction (UNE) system for rapidly extracting the urinary VOCs to gaseous VOCs. And proton transfer reaction mass spectrometry (PTR-MS) instrument can detect the gaseous VOCs with fast speed and lower limit of detection (LOD), which has been applied in environmental monitoring, medical research, public security, and food inspection.16-21 By combining the UNE and PTR-MS instrument, we developed a novel UNE-PTR-MS technology for detection of VOCs in liquid sample like urine. Because the methanol, acetaldehyde, and acetone are common in human urine and they are also the potential biomarkers of gastro-esophageal cancer, 9 we choose these 3 VOCs as the examples to examine the performance of UNE-PTR-MS. The sample size and carrier gas flow were optimized in the experiment using standard solution. And then the LODs, response times, and repeatability of the UNE-PTR-MS were evaluated. Lastly, the UNE-PTR-MS was applied for the detection of VOCs in real human urine to test the matrix effects. And the concentration changes of the urinary VOCs after urination was also studied in this work. EXPERIMENTAL SECTION Ultrasonic Nebulization Extraction System. The ultrasonic nebulization extraction system is designed for extracting VOCs from the urine. As shown in Figure 1, it mainly consists of a finnpipette (20-200 µL, Thermo Fisher Scientific, USA), a bottle of high purity nitrogen (99.999%, Nanjing Speciality Gas Co., Ltd., Nanjing, China), a hot water circulation system, an ultrasonic nebulizer (HW-16-112E-EO, Huajingda Electronics Co., Ltd., Shengzhen, China), and a nebulizing chamber (i.d. 47 mm, o.d. 51 mm, height 151 mm) with two glass tubes inside and outside.

Figure 1. The UNE-PTR-MS system. Figure 1 (a) shows the schematic diagram of the UNE-PTR-MS. Figure 1 (b) shows the UNE device. The 1, 2, 3, 4, 5, 6, 7, 8, and 9 in Figure 1 (b) are nebulizing chamber, hot water inlet, hot water outlet, carrier gas inlet, gaseous VOCs outlet,


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power switch, peristaltic pump, thermoses of hot water, and ultrasonic nebulizer, respectively. The urine sample is transferred to the ultrasonic nebulizer using the finnpipette, and then nebulized to small droplets in the nebulizing chamber. Because the contact area between these droplets and the carrier gas is much larger than that between the original urine sample and the headspace air, the VOCs in the droplets evaporate much rapidly to the carrier gas, and reach a gas-liquid equilibrium. The gaseous VOCs are then transferred to the PTR-MS instrument by the carrier gas controlled by a mass flow controller (MFC, D07-15/ZM, Beijing Sevenstar Electronics Co., Ltd., Beijing, China). The carrier gas is high purity nitrogen. The hot water is pumped into the chamber between two glass tubes by a peristaltic pump (BT360, Yingxiang Science and Technology Ltd., Hebei, China) and kept at 95-100 oC to heat the nebulizing chamber, reducing the condensation of the VOCs on the inside chamber wall. PTR-MS Instrument. The extracted gaseous VOCs are detected on our home-made PTR-MS instrument (Ion Sniffer 2020Q). Detailed information about this instrument can be found in our previous works.21-25 Briefly, this instrument mainly consists of a glow discharge ion source, a drift tube, and a quadrupole mass spectrometer (QMS). The reagent ions H3O+ are produced in the ion source by glow discharge with water vapor and then driven to the drift tube under the action of an electric field. The extracted VOCs are introduced to the drift tube by the carrier gas. The VOCs can undergo proton transfer reaction with H3O+ (Eq. 1) if their proton affinities (PAs) are larger than 691 kJ/mol (PA of H2O). H3O++VOCs→H2O+VOCsH+ (1) After passing through a differentially pumping intermediate chamber, the product ions VOCsH+ and reaction ions H3O+ at the end of the drift tube are detected by the QMS. Ion intensity is given in counts per second (cps). The temperature in the laboratory was kept at 20 (±1) oC. The pressure in the drift tube was 1.46 Torr. Because the intensities of ions at m/z 19 and m/z 37 were too high, which may cause irreversible damage to the ion detector, the mass range of full scan was set as m/z 20-36 and m/z 38-150. Reagents and Standards. Methanol and acetone were analytical reagent grade. Acetaldehyde was a 40% aqueous solution. The above 3 chemical reagents were all obtained from Sinopharm Chemical Reagent Co., Ltd., in China. Stock solutions (88 mg/L) were prepared daily by dissolving calculated amounts of these reagents in pure water. Fresh working solutions were prepared by diluting the stock solution in the pure water. The pure water was produced by two-stage lab water purification systems, KNTR-I-10 and Micropure UF. During the solution preparation, the laboratory temperature was also kept at 20 (±1) oC. The real urine sample was collected from 4 healthy volunteers before breakfast. RESULTS AND DISCUSSION Optimization of carrier gas flow. The VOCs in the liquid sample evaporated into the carrier gas and then were delivered to the PTR-MS instrument for detection. The flow of carrier gas has important effects on the VOCs extraction efficiency, so it was optimized in this study. The UNE-PTR-MS was used for detection of 100 µL standard solution spiked with 112.64 µg/L methanol, acetaldehyde, and acetone. The carrier gas flow was set at 100, 200, 300, 400, and 500 mL/min. The response time is defined as the time between starting sampling and signals reaching their maximum values as shown in Figure 2(a). The response times and intensities of ions at m/z 33



(protonated methanol),26-28 m/z 45 (protonated acetaldehyde),26,28 and m/z 59 (protonated acetone)26-28 were evaluated at each carrier gas flow as shown in Figure 2 (b). When the carrier gas flow increased from 100 to 500 mL/min, the intensities of these 3 kinds of ions decreased by 60%-70%, meanwhile, the response time did not decrease obviously. Accordingly, we set the carrier gas flow to 100 mL/min to guarantee the sensitivity and the detection speed of the UNE-PTR-MS at the same time. The response time was 34 s when the carrier gas was set to 100 mL/min, which was much shorter than the detection times in previous studies.9, 11-15

Figure 2. The optimization results for the carrier gas. Figure 2(a) shows the monitoring of ions at m/z 33, m/z 45, and m/z 59 over time, with response time factors labeled. Figure 2(b) shows the change trends of response time and intensities of protonated methanol (m/z 33), acetaldehyde (m/z 45), and acetone (m/z 59) when the carrier gas flow was set to 100, 200, 300, 400, and 500 mL/min. Optimization of sample size. During this experiment, we used the finnpipette to transfer a drop liquid sample to the ultrasonic nebulization extraction system. The sample size was optimized to evaluate its effect on the detection result. Working solutions were prepared by spiking with 112.64 µg/L methanol, acetaldehyde, and acetone. The intensities of ions at m/z 33, m/z 45, and m/z 59 were recorded when 20, 40, 60, 80, 100, and 120 µL of the standard solution were used for detection.

Figure 3 Dependence of intensities of target ions on the sample size. The sample size was 20, 40, 60, 80, 100, 120 µL.


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As shown in Figure 3, the ion intensities increased when the sample size varied from 20 to 100 µL and remained on a plateau afterward. According to Henry’s law, when a solute is introduced into an air-water binary system, it distributes itself between the two phases so as to re-establish equilibrium.29 When the sample size was below 100 µL, all the gaseous VOCs were transferred to the PTR-MS instrument before the equilibrium was re-established between the droplets and surrounding carrier gas. Therefore, the sample size was set to 100 µL to ensure the detection sensitivity. Method validation. To calculate the LODs of the UNE-PTR-MS, these 3 common urinary VOCs were calibrated using standard solution spiked with 7.04, 14.08, 28.16, 56.32, 112.64, 225.28, 450.56, 901.12, and 1802.24 µg/L methanol, acetaldehyde, and acetone. Calibration curves were obtained on the basis of 3-fold analyses as shown in Figure 4. Detailed information about the calibration curves is shown in Table 1. The calibration curves were linear in the range studied for each VOC with correlation coefficients R2 between 0.987 and 0.999. Then a directly proportional relationship between the intensities of protonated VOCs and the initial concentration in the liquid sample was established.

Figure 4. Linear calibration curves for varying concentrations of methanol, acetaldehyde, and acetone. Fitted equations for methanol, acetaldehyde, and acetone are y=-31.80+9.69x, y=-14.55+13.16x, and y=-37.80+14.21x, respectively. The LODs were calculated at signal-to-noise (S/N) ratio = 3. The noise level was calculated according to a previous reference.30 In this study, the signal intensities of carrier gas were defined as the background signal. The dwell time of mass spectrometry was set to 10 s. Using the fitted equations, the LODs of methanol, acetaldehyde, and acetone were calculated as 4.47 µg/L, 1.98 µg/L, and 3.47 µg/L (Table 1), which are 2~3 orders of magnitude lower than the mean concentration of that in healthy human urine.31-33 Table 1 Response times, limits of detection (LODs), linear regression values, and relative standard deviation (RSD) of the UNE-PTR-MS for detection of standard solution samples containing methanol, acetaldehyde, and acetone.

VOCs

Response time (s)

LOD (µg/L)

Correlation coefficient (R2)


Linear range (µg/L)

RSD (%, n=5)


Methanol Acetaldehyde Acetone

34 34 34

4.47 1.98 3.47

0.995 0.987 0.999

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7.04-1802.24 7.04-1802.24 7.04-1802.24

4.78 5.21 4.62

The repeatability of this UNE-PTR-MS method, expressed as relative standard deviation (RSD), was evaluated with 5 replicate determinations (112.64 µg/L methanol, acetaldehyde, and acetone solution). As shown in Table 1, the calculated results demonstrate good repeatability of the UNE-PTR-MS with RSDs of 4.62%-5.21%. Real human urine analysis. The PTR-MS instrument has 2 detection mode, full scan (FS) mode and multiple ions monitoring (MIM) mode. For the FS mode, the m/z range was set from 20 to 150 (excluding 37) in this study, the dwell time was set to 1 s, and the settle time was set to 0.1 s, so the scan time can be calculated as 143 s. Our aim is to reduce the sample volume needed for a FS to less than 1 mL. Therefore, to guarantee the reliability of the detection result, the VOCs concentration must be kept constant for at least 143 s when only 1 mL urine is sampled. We used the MIM mode (ion at m/z 33) to check the stability of the VOCs concentration when 1 mL methanol solution (112.64 µg/L) was analyzed. As shown in Figure 5(a), the intensity can remain stable for 216 s, longer than the 143 s. So 1 mL sample suffices for a full scan. The minimum volume can be calculated to be 0.66 mL.

Figure 5. The signal intensity of ions at m/z 33 when 1 mL methanol solution was analyzed using MIM mode, and the PTR-MS mass spectrum of one real urine sample using FS mode. Figure 5(a) shows the change trend of ion at m/z 33 when 1 mL standard solution containing 112.64 µg/L methanol was analyzed using the MIM mode. Figure 5(b) showed the mass spectrum of one real human urine sample (volunteer 1) analyzed using the FS mode. And then the UNE-PTR-MS was applied for detection of VOCs in real urine using the FS mode. One mL of morning urine sample was collected from 4 volunteers and analyzed immediately. With the background subtracted, the PTR-MS spectrum contained a number of obvious peaks, including peaks for protonated methanol (m/z 33), acetaldehyde (m/z 45), and acetone (m/z 59). Using the fitted equations shown in Figure 4, the concentrations of methanol, acetaldehyde, and acetone in the urine can be calculated as 1139.40~1472.12 µg/L, 54.75~217.75 µg/L, and 228.14~1034.96 µg/L, respectively. The detailed VOCs concentrations are shown in Table S-2 in the Supporting Information for Publication. The concentrations of urinary methanol, acetaldehyde,


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and acetone reported in previous studies were estimated to be 770.00~1030.00 µg/L31, 660.00~1100.00 µg/L32, and 133.40~6206.00 µg/L33 from their tables or figures, respectively. Obviously, the concentration range of every compound is large in this work and previous studies. And the difference of acetaldehyde concentration between this work and previous study32 may be attributed to the difference of human species, food, drink, metabolic ability of human body, and sampling time. This needs to be studied in detail in next work with our UNE-PTR-MS. The mass spectrum of one volunteer （volunteer 1）is shown in Figure 5(b). In addition to the above 3 peaks, there are many other peaks. Tentative identification is done by depending on the fragmentation pattern of ions or using isotopic ratios34 or comparing the m/z-1 of these peaks and the molecular weight searched in the NIST35 as shown in Table 2. The peaks at m/z 36 and 38 should be the interference from the primary ion with high intensity at m/z 37, so they are not listed in Table 2. The accurate qualitative analysis of these ions will be performed in our next study on the metabolism of urinary VOCs. Table 2 Tentative identification of the main peaks in the mass spectrum of the urine sample based on molecular weight of compound in NIST, the fragmentation pattern of ions, or using isotopic ratios.

m/z

31 33, 51 39 42 43, 59 47 55 61, 62 68, 136 73 75 85

87 89 93 95 a

Possible protonated -monomer (M) -fragment (F) -cluster (C) -isotope (I) Formaldehyde (M). Methanol (M, C). Water cluster(C). Acetonitrile (M). Acetone (M, F). Ethanol (M). Water cluster(C). Acetic acid (M, I). Adenine. Succinic acid (F); Hexanoic acid (F). Methional (F). 3-Hydroxy butyric acid (F); Succinic acid (F). 3-Hydroxy butyric acid (F); Hexanoic acid (F). Butyric acid (M). Toluene (M). Dimethyl disulfide (M).

CAS

Comparison with previous studies

VOC identificationa

50-00-0 67-56-1 75-05-8 67-64-1 64-17-5 64-19-7 73-24-5 110-15-6 142-62-1 3268-49-3

Spanel et al.36 Kawai et al.37 Zou et al.24 Abbott et al.38 Diskin et al.39 Bergstorm et al.40 Zou et al.24 Smith et al.41 Goyal et al.42 Deja et al.43 Huang et al.9 Troccaz et al.44

NIST NIST; CCA CCA NIST NIST; FCA NIST CCA NIST; ICA NIST; FCA NIST; FCA NIST; FCA NIST; FCA

300-85-6

Deja et al.43

NIST; FCA

110-15-6

Deja et al.43

NIST; FCA

300-85-6

Deja et al.43

NIST; FCA

142-62-1 107-92-6 108-88-3

Huang et al.9 Mochalski et al.45 Fastinoni et al.46

NIST; FCA NIST NIST

624-92-0

Troccaz et al.44

NIST

NIST, identification by comparing the m/z-1 of these peaks and the molecular weight searched in



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the NIST;35 CCA, cluster correlation analysis, using clusters formed due to the reaction with water; FCA, fragment correlation analysis, correlation between potential fragment and monomer ions; ICA, identification by isotopic correlation analysis. To test the matrix effect, the UNE-PTR-MS was applied to detect VOCs in a real urine sample spiked with 112.64 µg/L methanol, acetaldehyde, and acetone. The relative recovery rates for these 3 VOCs were between 88.39% and 94.54% as shown in Table 3. A minor matrix effect in the UNE-PTR-MS technique is probably due to the volatilization of these VOCs during the mixing process in spike recovery experiment. Because the VOCs concentration of fresh urine will change after 8 minutes without any special preservation (see Figure S-1), so it is essential to guarantee the freshness of the urine sample to get a good repeatability. In UNE-PTR-MS method, the sampling time and detection time in MIM or FS mode is no more than 3 minutes. Therefore, this method can be used for accurate quantification of volatile compounds in fresh urine samples in time. Table 3 The concentration (µg/L) of methanol, acetaldehyde, and acetone in fresh urine sample and the accuracy of the established method. Concentration (µg/L)

Urine sample

After spiking with 112.64 µg/L methanol, acetaldehyde, and acetone

Methanol

1352.82

1452.38

88.39

Acetaldehyde

78.92

185.41

94.54

Acetone

1034.96

1135.28

89.06

VOC s

Relative recoverya (%)

a

Relative recovery (%) = (the amount found in the spiked urine sample − the amount in the urine sample)/the amount added ×100 Comparison of UNE-PTR-MS with HS-SPME-GC-MS. HS-SPME-GC-MS is the most widely used technology for the detection of urinary VOCs11-13, so we compared the newly developed UNE-PTR-MS with the HS-SPME-GC-MS in terms of reproducibility, sensitivity, recovery rate, and response time. The same standard solution contained acetone was detected using these two technologies. The details of the instrument condition and detection process of HS-SPME-GC-MS are shown in the Supporting Information for Publication. As shown in Table 4, the RSD and relative recovery are equivalent. However, the LOD and response time of UNE-PTR-MS are much smaller than that of the HS-SPME-GC-MS. The high LOD of HS-SPME-GC-MS is result from the high concentration of acetone in the headspace air, which means the signal of acetone sample with too low concentration will be in the intensive background signal noise. Obviously, the UNE-PTR-MS is more rapid and sensitive than the traditional HS-SPME-GC-MS and can’t be interfered by the ambient air. Table 4 Comparison of the UNE-PTR-MS with HS-SPME-GC-MS for the determination of VOCs in urine.


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Method

RSD (%, n=5)

LOD (µg/L)

Relative recovery (%)

Response time

UNE-PTR-MS HS-SPME-GC-MS

4.62 4.32

3.47 178.80

89.06 92.03

34 s 5h

CONCLUSION In this study, a novel UNE-PTR-MS technology was developed for rapid detection of VOCs in a drop urine. The time required by the detection process is much shorter than what is required by previous technologies. The LODs of the UNE-PTR-MS for methanol, acetaldehyde, and acetone are 2~3 orders of magnitude lower than the mean concentrations of that in healthy human urine. UNE-PTR-MS was also applied for detection of VOCs in real urine samples in this experiment, revealing that the urine sample had only a minor negative effect on the detection result. With the help of the UNE-PTR-MS, we also found that VOCs concentration of fresh urine would change after 8 minutes without any special preservation. In conclusion, the UNE-PTR-MS technology introduced in this paper has good application value in the rapid detection of urinary VOCs, and is a potential method for diagnosing disease or exposure to toxic substances. ASSOCIATED CONTENT Supporting Information Available The concentration change of urinary acetone after collection, experimental section of HS-SPME-GC-MS for analysis of urinary acetone, and detailed concentrations of methanol, acetaldehyde, and acetone in the urine of 4 volunteers. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected]. Fax: +86-551-65595179. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was supported by grants from the National Key R&D Program of China (No. 2016YFC0200200), the National Natural Science Foundation of China (Nos. 21577145, 21705152 and 21477132), the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology, China (No. 2014FXCX007), the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No. 2015BAI01B04), and the Science and Technology Service Network Initiative, Chinese Academy of Sciences (No. KFJ-SW-STS-161). REFERENCES (1) de Lacy Costello, B.; Amann, A.; Al-Kateb, H.; Flynn, C.; Filipiak, W.; Khalid, T.; Osborne, D.; Ratcliffe, N. M. J. Breath Res. 2014, 8, 014001. (2) Rhodes, G.; Holland, M. L.; Wiesler, D.; Novotny, M.; Moore, S. A.; Peterson, R. G.; Felten, D. L. J. Chromatogr. 1982, 228, 33-42. (3) Liebich, H. M.; Albabbili, O.; Zlatkis, A.; Kim, K. Clin. Chem. 1975. 21, 1294-1296. (4) Burke, D. G.; Halpern, B.; Malegan, D.; McCairns, E.; Danks, D.; Schlesinger, P.; Wilken, B. Clin. Chem. 1983, 29, 1834-1838.



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Figure 1. The UNE-PTR-MS system. Figure 1 (a) shows the schematic diagram of the UNE-PTR-MS. Figure 1 (b) shows the UNE device. The 1, 2, 3, 4, 5, 6, 7, 8, and 9 in Figure 1 (b) are nebulizing chamber, hot water inlet, hot water outlet, carrier gas inlet, gaseous VOCs outlet, power switch, peristaltic pump, thermoses of hot water, and ultrasonic nebulizer, respectively. 200x105mm (200 x 200 DPI)


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Figure 2. The optimization results for the carrier gas. Figure 2(a) shows the monitoring of ions at m/z 33, m/z 45, and m/z 59 over time, with response time factors labeled. Figure 2(b) shows the change trends of response time and intensities of protonated methanol (m/z 33), acetaldehyde (m/z 45), and acetone (m/z 59) when the carrier gas flow was set to 100, 200, 300, 400, and 500 mL/min. 441x207mm (300 x 300 DPI)



Figure 3 Dependence of intensities of target ions on the sample size. The sample size was 20, 40, 60, 80, 100, 120 µL. 237x181mm (300 x 300 DPI)


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Figure 4. Linear calibration curves for varying concentrations of methanol, acetaldehyde, and acetone. Fitted equations for methanol, acetaldehyde, and acetone are y=-31.80+9.69x, y=-14.55+13.16x, and y=37.80+14.21x, respectively. 239x171mm (300 x 300 DPI)



Figure 5. The signal intensity of ions at m/z 33 when 1 mL methanol solution was analyzed using MIM mode, and the PTR-MS mass spectrum of one real urine sample using FS mode. Figure 5(a) shows the change trend of ion at m/z 33 when 1 mL standard solution containing 112.64 µg/L methanol was analyzed using the MIM mode. Figure 5(b) showed the mass spectrum of one real human urine sample (volunteer 1) analyzed using the FS mode. 230x180mm (300 x 300 DPI)


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For TOC only 186x146mm (200 x 200 DPI)


Rapid detection of volatile organic compounds in a ... - ACS Publications

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