Development and validation of a novel UV-TOF MS method for real-time exhaled propofol analysis in Beagles

Propofol, a fast-acting anesthetic, requires precise titration to minimize adverse effects. While plasma-based monitoring is slow, exhaled propofol offers a real-time, non-invasive alternative, though its clinical application remains limited. This study evaluates ultraviolet time-of-flight mass spectrometry (UV-TOF MS) for real-time monitoring, presenting its calibration and validation in Beagle dogs. Calibration showed excellent linearity (R2 = 0.9939) over 3.23-46.13 ppbv. The intra-day imprecision at propofol concentrations of 4.61 and 23.06 ppbv was below 5.83% and 7.75%, respectively, while the inter-day imprecision was 9.69% and 9.75%, respectively. Carry-over effects were minimal, with signal recovery within 40-60 s, measuring 8.7%, 9.1%, and 4.7% at 4.61, 9.30, and 23.06 ppbv, respectively. In Beagle dogs, Cexhaled exhibited a moderately strong linear correlation with Cplasma (R2 = 0.7950) and a moderate correlation with sedative effects, as indicated by the bispectral index (R2 = 0.5501) after a single bolus injection. Pharmacokinetic (PK) analysis revealed a delay in peak concentration (Tmax) for Cexhaled (2.00 ± 0.21 min) compared to Cplasma (1.00 ± 0.00 min). While AUC values were not directly comparable, both exhibited R_AUC > 80%, indicating reliable drug kinetic reflection. Mean residence time (MRT) and elimination rate constants (λz) showed no significant differences. These results suggest that exhaled breath analysis provides pharmacokinetic insights comparable to plasma, with a slight delay in peak concentration. UV-TOF MS proved to be an efficient method for detecting exhaled propofol, offering potential for real-time anesthesia monitoring in clinical settings.

Recent developments and applications of selected ion flow tube mass spectrometry (SIFT-MS)

Selected ion flow tube mass spectrometry (SIFT-MS) is now recognized as the most versatile analytical technique for the identification and quantification of trace gases down to the parts-per-trillion by volume, pptv, range. This statement is supported by the wide reach of its applications, from real-time analysis, obviating sample collection of very humid exhaled breath, to its adoption in industrial scenarios for air quality monitoring. This review touches on the recent extensions to the underpinning ion chemistry kinetics library and the alternative challenge of using nitrogen carrier gas instead of helium. The addition of reagent anions in the Voice200 series of SIFT-MS instruments has enhanced the analytical capability, thus allowing analyses of volatile trace compounds in humid air that cannot be analyzed using reagent cations alone, as clarified by outlining the anion chemistry involved. Case studies are reviewed of breath analysis and bacterial culture volatile organic compound (VOC), emissions, environmental applications such as air, water, and soil analysis, workplace safety such as transport container fumigants, airborne contamination in semiconductor fabrication, food flavor and spoilage, drugs contamination and VOC emissions from packaging to demonstrate the stated qualities and uniqueness of the new generation SIFT-MS instrumentation. Finally, some advancements that can be made to improve the analytical capability and reach of SIFT-MS are mentioned.

Developing a PK-PD model for propofol in exhaled air and the BIS following fospropofol disodium in beagles

BACKGROUND

Fospropofol, a water-soluble prodrug of propofol, is metabolized into propofol by alkaline phosphatase after administration. This study aimed to develop a pharmacokinetic-pharmacodynamic (PK-PD) model that correlates the propofol concentration in exhaled air (Ce-pro-f) with its anesthetic effects, as measured by the bispectral index (BIS) in beagles.

METHODS

Beagles receiving a single intravenous infusion of fospropofol at varying doses were divided into three groups (n = 6): the DBL-fospro group (15 mg/kg), the DBM-fospro group (30 mg/kg), and the DBH-fospro group (60 mg/kg). Propofol levels were monitored using VUV-TOF MS from pre-administration to recovery. Correlations between Ce-pro-f and blood concentration (Cblood-pro), as well as between Ce-pro-f and the BIS were investigated. PK, PD, and PK-PD models describing the relationship between Ce and BIS were also analyzed.

RESULTS

Propofol concentration in exhaled air can be quantified using VUV-TOF MS at a mass-to-charge ratio of 177.6. After fospropofol injection, the peak Ce-pro-f was delayed compared to Cblood-pro. The PK model of Ce-pro-f can be described using a noncompartmental approach, corresponding to the linear PK characteristics. Additionally, Ce-pro-f showed a moderate to strong negative correlation with BIS values. In the PK-PD model, the PK component was well characterized by a two-compartment model incorporating a first-order delay to account for the time lag of Ce-pro-f relative to Cblood-pro. The PD component was well fitted by the inhibitory sigmoid Emax model, with an indirect connection model selected to explain the observed lag between BIS signals and Ce-pro-f peaks.

CONCLUSIONS

This study is the first to develop a PK-PD model for exhaled propofol in beagles after fospropofol disodium administration. The PK profile was described by a two-compartment model with a first-order delay, and the PD profile was modeled using an inhibitory sigmoid Emax model with an indirect connection model to capture the lag between BIS and exhaled propofol peaks.

Exhaled propofol monitoring for plasma drug prediction in rats

While propofol can be detected in exhaled breath in rats, robust evidence supporting its correlation with plasma concentrations or its use in predicting plasma levels remains lacking. In this study, eighteen mechanically ventilated rats were divided into three groups and injected with low (Group BL, n = 6), medium (Group BM, n = 6), or high (Group BH, n = 6) doses of propofol. The propofol concentration in exhaled breath (Ce-pro) was determined online using vacuum ultraviolet time-of-flight mass spectrometry (VUV-TOF MS), while the propofol concentration in plasma (Cp-pro) were measured using high-performance liquid chromatograph. The results indicated that after propofol injection, the peak Ce-pro was 5.87 ± 1.67 ppbv, 16.54 ± 7.22 ppbv, and 25.40 ± 3.68 ppbv, respectively. Across the different dose groups, Cmax of Ce-pro and Cp-pro were linearly correlated (P BL = 0.032, P BM = 0.031, P BH = 0.049). Tmax of Ce-pro was 1.22 ± 0.17 min, 1.28 ± 0.13 min, and 1.33 ± 0.01 min, respectively (P = 0.341), similar to the Tmax of Cp-pro (1.00 ± 0.00 min). After natural logarithm transformation, the correlation between LN(Ce-pro) and LN(Cp-pro) was well fitted by a linear model, withR B L 2 = 0.94,R B M 2 = 0.95,R B H 2 = 0.98, andR A L L 2 = 0.96. Using the obtained regression equation LN(Cp-pro) = 1.42*LN(Ce-pro)-1.70, the predicted Cp-pro values showed excellent agreement with the actual values within groups (ICCBL = 0.92; ICCBM = 0.97, ICCBH = 0.99, all P < 0.001). This study demonstrates a strong correlation between exhaled and plasma propofol concentrations in rats, indicating that exhaled concentrations can be effectively used to estimate plasma levels.

Validation of a method for estimating pulmonary dead space in ventilated beagles to correct exhaled propofol concentration in mixed air

BACKGROUND

Mixed exhaled air has been widely used to determine exhaled propofol concentrations with online analyzers, but changes in dead space proportions may lead to inaccurate assessments of critical drug concentration data. This study proposes a method to correct propofol concentration in mixed air by estimating pulmonary dead space through reconstructing volumetric capnography (Vcap) from time-CO2 and time-volume curves, validated with vacuum ultraviolet time-of-flight mass spectrometry (VUV-TOF MS).

METHODS

Existing monitoring parameters, including time-volume and time-CO2 curves, were used to determine Vcap. The ratio of physiological dead space to tidal volume (VD/VT) was calculated using Bohr's formula. Additionally, an animal experiment on beagles was conducted with continuous propofol administration until a pseudo-steady state in exhaled propofol concentration was achieved. The propofol concentration in mixed air (CONCmix), and in alveolar air combined with N2 (CONCAN) were measured using VUV-TOF MS to calculate VD/VT. The agreements between VD/VT values from the two methods, along with the predicted CONCAN values based on Vcap and the actual measured CONCAN values were evaluated using the intraclass correlation coefficient (ICC) and Pearson correlation analysis.

RESULTS

After 30 min of continuous propofol administration, a stable respiratory cycle was selected for analysis in each beagle. The calculated VD/VT-Bohr values were 0.535 for beagle A, 0.544 for beagle B, and 0.552 for beagle C. Additionally, based on CONCmix and CONCAN, the calculated VD/VT-VUV-TOF MS values were 0.494, 0.504, and 0.513, respectively. Strong agreement between the two methods was demonstrated by an ICC of 0.994 (P = 0.003) and Pearson's r of 0.995 (P = 0.045). Additionally, the predicted CONCAN values from mixed exhaled air (5.11 parts per billion by volume (ppbv) for beagle A, 5.93 ppbv for beagle B, and 2.56 ppbv for beagle C) showed strong agreement with the actual CONCAN values, with an ICC of 0.996 (P = 0.002) and Pearson's r of 0.994 (P = 0.046).

CONCLUSION

The physiological dead space to tidal volume ratio from mixed air in beagles can be accurately measured using the existing time-volume and time-CO2 curves from the anesthesia machine, enabling corrections of exhaled propofol concentrations in mixed air samples.

Online monitoring of propofol concentrations in exhaled breath

Propofol, a widely used intravenous anesthetic agent, requires accurate monitoring to ensure therapeutic efficacy and prevent oversedation. Recent developments in modern analytical instrumentation have led to significant breakthroughs in on-line analysis of exhaled breath. This review discusses several sophisticated analytical methods that have been explored for noninvasive, real-time monitoring of propofol concentrations, including proton transfer reaction mass spectrometry, selected ion flow tube mass spectrometry, ion mobility spectrometry, and gas chromatography coupled to surface acoustic wave sensors. These techniques have demonstrated good correlations between plasma and exhaled propofol concentrations and between exhaled propofol concentrations and its cerebral effects. Despite these advances, the use of these technologies in clinical settings is hampered by challenges such as equipment noise, bulkiness, and high cost, as well as limitations related to endotracheal intubation, strong adsorption of propofol to components of the respiratory circuit, variability in respiratory patterns, susceptibility to changes in pulmonary ventilation and blood flow, inconsistencies in calibration methods, and the influence of other drugs and temperature fluctuations on measurement accuracy. Overcoming these technical and procedural challenges is critical to advancing the clinical application of breath analysis for propofol monitoring. This article reviews published studies and summarizes the progress and ongoing challenges in the field.

SARS CoV-2 infection screening via the exhaled breath fingerprint obtained by FTIR spectroscopic gas-phase analysis. A proof of concept

The COVID-19 pandemic remains a global challenge now with the long-COVID arising. Mitigation measures focused on case counting, assessment and determination of variants and their likely targets of infection and transmission, the pursuit of drug treatments, use and enhancement of masks, social distancing, vaccination, post-infection rehabilitation, and mass screening. The latter is of utmost importance given the current scenario of infections, reinfections, and long-term health effects. Research on screening platforms has been developed to provide more sensitive, specific, and reliable tests that are accessible to the entire population and can be used to assess the prognosis of the disease as well as the subsequent health follow-up of patients with sequelae of COVID-19. Therefore, the aim of the present study was the simulation of exhaled breath of COVID-19 patients by evaluation of three identified COVID-19 indicator breath biomarkers (acetone (ACE), acetaldehyde (ACH) and nitric oxide (NO)) by gas-phase infrared spectroscopy as a proof-of-concept principle for the detection of infected patients' exhaled breath fingerprint and subsequent follow-up. The specific fingerprints of each of the compounds and the overall fingerprint were obtained. The synthetic exhaled breath evaluation concept revealed a linearity of r = 0.99 for all compounds, and LODs of 6.42, 13.81, 9.22 ppm, and LOQs of 42.26, 52.57, 69.23 ppm for NO, ACE, and ACH, respectively. This study proves the fundamental feasibility of gas-phase infrared spectroscopy for fingerprinting lung damage biomarkers in exhaled breath of patients with COVID-19. This analysis would allow faster and cheaper screening and follow-up of infected individuals, which could improve mass screening in POC settings.

A preclinical study on online monitoring of exhaled ciprofol concentration by the ultraviolet time-of-flight spectrometer and prediction of anesthesia depth in beagles

BACKGROUND

Exhaled air has been demonstrated as a reliable medium for monitoring propofol concentration. However, online monitoring of exhaled ciprofol have not been reported.

METHODS

Thirty-six beagles undergoing mechanical ventilation were divided into 6 groups, including bolus injection of low (Group BL, n = 6), medium (Group BM, n = 6), and high dose of ciprofol (Group BH, n = 6) groups; as well as 1 h continuous infusion of low (Group IL, n = 6), medium (Group IM, n = 6), and high dose of ciprofol (Group IH, n = 6) groups. The ciprofol concentration in exhaled air (CE) was determined by the ultraviolet time-of-flight mass spectrometer (UV-TOFMS). The correlations of CE and plasma concentration (Cp), CE and the bispectral index (BIS) were explored. Additionally, the pharmacokinetics (PK) models of CE and Cp, the pharmacodynamics (PD) models of CE and BIS were also established.

RESULTS

Online monitoring of exhaled ciprofol can be achieved with the UV-TOFMS instrument. The CE of ciprofol in beagles was found at parts per billion by volume (ppbv) level. The linear correlation of CE and Cp was weak in bolus injection groups (R2 = 0.01) nonetheless moderate in continuous infusion groups (R2 = 0.53). The i.v. bolus PK model of CE and Cp can be fitted with the non-compartment models. Additionally, the the PD models of CE and BIS can be well fitted with the inhibitory sigmoid Emax model with the estimate values of IC50 = 0.05 ± 0.01 ppbv, γ = 4.74 ± 1.51, E0 = 81.40 ± 3.75, Imax = 16.35 ± 4.27 in bolus injection groups; and IC50 = 0.05 ± 0.01 ppbv, γ = 6.92 ± 1.30, E0 = 83.08 ± 1.62, Imax = 12.58 ± 1.65 in continuous infusion groups.

CONCLUSIONS

Online monitoring of exhaled ciprofol concentration in beagles can be achieved with the UV-TOFMS instrument. Good correlations can be observed between exhaled ciprofol concentration and its cerebral effects reflected by the BIS value, demonstrating the potential of exhaled ciprofol monitoring for titrating depth of anesthesia in future clinical setting.

Calibration and validation of ultraviolet time-of-flight mass spectrometry for online measurement of exhaled ciprofol

Ciprofol (HSK 3486, C14H20O), a novel 2,6-disubstituted phenol derivative similar to propofol, is a new type of intravenous general anaesthetic. We found that the exhaled ciprofol concentration could be measured online by ultraviolet time-of-flight mass spectrometry (UV-TOFMS), which could be used to predict the plasma concentration and anaesthetic effects of ciprofol. In this study, we present the calibration method and validation results of UV-TOFMS for the quantification of ciprofol gas. Using a self-developed gas generator to prepare different concentrations of ciprofol calibration gas, we found a linear correlation between the concentration and intensity of ciprofol from 0 parts per trillion by level (pptv) to 485.85 pptv (R2 = 0.9987). The limit of quantification was 48.59 pptv and the limit of detection was 7.83 pptv. The imprecision was 12.44% at 97.17 pptv and was 8.96% at 485.85 pptv. The carry-over duration was 120 seconds. In addition, we performed a continuous infusion of ciprofol in beagles, measured the exhaled concentration of ciprofol by UV-TOFMS, determined the plasma concentration by high-performance liquid chromatography, and monitored the anaesthetic effects as reflected by the bispectral index value. The results showed that the exhaled and plasma concentrations of ciprofol were linearly correlated. The exhaled ciprofol concentration correlated well with the anaesthetic effect. The study showed that we could use UV-TOFMS to provide a continuous measurement of gaseous ciprofol concentration at 20 second intervals.

Influence of ventilatory parameters on the concentration of exhaled volatile organic compounds in mechanically ventilated patients

Analysis of volatile organic compounds (VOC) within exhaled breath is subject to numerous sources of methodological and physiological variability. Whilst breathing pattern is expected to influence the concentrations of selected exhaled VOCs, it remains challenging to investigate respiratory rate and depth accurately in awake subjects. Online breath sampling was performed in 20 mechanically ventilated patients using proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). The effect of variation in respiratory rate (RR) and tidal volume (TV) on the VOC release profiles was examined. A panel of nineteen VOCs were selected, including isoprene, acetone, propofol, volatile aldehydes, acids and phenols. Variation in RR had the greatest influence on exhaled isoprene levels, with maximum and average concentrations being inversely correlated with RR. Variations in RR had a statistically significant impact on acetone, C3-C7 linear aldehydes and acetic acid. In comparison, phenols (including propofol), C8-C10 aldehydes and C3-C6 carboxylic acids were not influenced by RR. Isoprene was the only compound to be influenced by variation in TV. These findings, obtained under controlled conditions, provide useful guidelines for the optimisation of breath sampling protocols to be applied on awake patients.

Detecting Hexafluoroisopropanol Using Soft Chemical Ionization Mass Spectrometry and Analytical Applications to Exhaled Breath

Here we explore the potential use of proton transfer reaction/selective reagent ion-time-of-flight-mass spectrometry (PTR/SRI-ToF-MS) to monitor hexafluoroisopropanol (HFIP) in breath. Investigations of the reagent ions H₃O⁺, NO⁺, and O₂^+• are reported using dry (relative humidity (rH) ≈ 0%) and humid (rH ≈ 100%)) nitrogen gas containing traces of HFIP, i.e., divorced from the complex chemical environment of exhaled breath. HFIP shows no observable reaction with H₃O⁺ and NO⁺, but it does react efficiently with O₂^+• via dissociative charge transfer resulting in CHF₂⁺, CF₃⁺, C₂HF₂O⁺, and C₂H₂F₃O⁺. A minor competing hydride abstraction channel results in C₃HF₆O⁺ + HO₂^• and, following an elimination of HF, C₃F₅O⁺. There are two issues associated with the use of the three dominant product ions of HFIP, CHF₂⁺, CF₃⁺, and C₂H₂F₃O⁺, to monitor it in breath. One is that CHF₂⁺ and CF₃⁺ also result from the reaction of O₂^+• with the more abundant sevoflurane. The second is the facile reaction of these product ions with water, which reduces analytical sensitivity to detect HFIP in humid breath. To overcome the first issue, C₂H₂F₃O⁺ is the ion marker for HFIP. The second issue is surmounted by using a Nafion tube to reduce the breath sample's humidity prior to its introduction into drift tube. The success of this approach is illustrated by comparing the product ion signals either in dry or humid nitrogen gas flows and with or without the use of the Nafion tube, and practically from the analysis of a postoperative exhaled breath sample from a patient volunteer.

Humidity and measurement of volatile propofol using MCC-IMS (EDMON)

The bedside Exhaled Drug MONitor - EDMON measures exhaled propofol in ppbv every minute based on multi-capillary column - ion mobility spectrometry (MCC-IMS). The MCC pre-separates gas samples, thereby reducing the influence of the high humidity in human breath. However, preliminary analyses identified substantial measurement deviations between dry and humid calibration standards. We therefore performed an analytical validation of the EDMON to evaluate the influence of humidity on measurement performance. A calibration gas generator was used to generate gaseous propofol standards measured by an EDMON device to assess linearity, precision, carry-over, resolution, and the influence of different levels of humidity at 100% and 1.7% (without additional) relative humidity (reference temperature: 37°C). EDMON measurements were roughly half the actual concentration without additional humidity and roughly halved again at 100% relative humidity. Standard concentrations and EDMON values correlated linearly at 100% relative humidity (R²=0.97). The measured values were stable over 100min with a variance ≤ 10% in over 96% of the measurements. Carry-over effects were low with 5% at 100% relative humidity after 5min of equilibration. EDMON measurement resolution at 100% relative humidity was 0.4 and 0.6 ppbv for standard concentrations of 3 ppbv and 41 ppbv. The influence of humidity on measurement performance was best described by a second-order polynomial function (R²≥0.99) with influence reaching a maximum at about 70% relative humidity. We conclude that EDMON measurements are strongly influenced by humidity and should therefore be corrected for sample humidity to obtain accurate estimates of exhaled propofol concentrations.

Volatile organic compound profiling to explore primary graft dysfunction after lung transplantation

Primary graft dysfunction (PGD) is a major determinant of morbidity and mortality following lung transplantation. Delineating basic mechanisms and molecular signatures of PGD remain a fundamental challenge. This pilot study examines if the pulmonary volatile organic compound (VOC) spectrum relate to PGD and postoperative outcomes. The VOC profiles of 58 bronchoalveolar lavage fluid (BALF) and blind bronchial aspirate samples from 35 transplant patients were extracted using solid-phase-microextraction and analyzed with comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry. The support vector machine algorithm was used to identify VOCs that could differentiate patients with severe from lower grade PGD. Using 20 statistically significant VOCs from the sample headspace collected immediately after transplantation (< 6 h), severe PGD was differentiable from low PGD with an AUROC of 0.90 and an accuracy of 0.83 on test set samples. The model was somewhat effective for later time points with an AUROC of 0.80. Three major chemical classes in the model were dominated by alkylated hydrocarbons, linear hydrocarbons, and aldehydes in severe PGD samples. These VOCs may have important clinical and mechanistic implications, therefore large-scale study and potential translation to breath analysis is recommended.

Combination of Exhaled Breath Analysis with Parallel Lung Function and FeNO Measurements in Infants

Breath analysis by secondary electrospray ionization-high resolution mass spectrometry (SESI-HRMS) offers the possibility to measure comprehensive metabolic profiles. The technology is currently being deployed in several clinical settings in Switzerland and China. However, patients are required to exhale directly into the device located in a dedicated room. Consequently, clinical implementation in patients incapable of performing necessary exhalation maneuvers (e.g., infants) or immobile (e.g., too weak, elderly, or in intensive care) remains a challenge. The aim of this study was to develop a method to extend such breath analysis capabilities to this subpopulation of patients by collecting breath samples remotely (offline) and promptly (within 10 min) transfer them to SESI-HRMS for chemical analysis. We initially assessed the method in adults by comparing breath mass spectra collected offline with Nalophan bags against spectra of breath samples collected in real time. In total, 13 adults provided 176 pairs of real-time and offline measurements. Lin's concordance correlation coefficient (CCC) was used to estimate the agreement between offline and real-time analyses. Here, 1249 mass spectral features (55% of total detected) exhibited Lin's CCC > 0.6. Subsequently, the method was successfully deployed to analyze breath samples from infants (n = 16), obtaining as a result SESI-HRMS breath profiles. To demonstrate the clinical feasibility of the method, we measured in parallel other clinical variables: (i) lung function, which characterizes the breathing patterns, and (ii) nitric oxide, which is a surrogate marker of airway inflammation. As a showcase, we focused our analysis on the exhaled oxidative stress marker 4-hydroxynonenal and its association with nitric oxide and minute ventilation.

Hyphenated Mass Spectrometry versus Real-Time Mass Spectrometry Techniques for the Detection of Volatile Compounds from the Human Body

Mass spectrometry (MS) is an analytical technique that can be used for various applications in a number of scientific areas including environmental, security, forensic science, space exploration, agri-food, and numerous others. MS is also continuing to offer new insights into the proteomic and metabolomic fields. MS techniques are frequently used for the analysis of volatile compounds (VCs). The detection of VCs from human samples has the potential to aid in the diagnosis of diseases, in monitoring drug metabolites, and in providing insight into metabolic processes. The broad usage of MS has resulted in numerous variations of the technique being developed over the years, which can be divided into hyphenated and real-time MS techniques. Hyphenated chromatographic techniques coupled with MS offer unparalleled qualitative analysis and high accuracy and sensitivity, even when analysing complex matrices (breath, urine, stool, etc.). However, these benefits are traded for a significantly longer analysis time and a greater need for sample preparation and method development. On the other hand, real-time MS techniques offer highly sensitive quantitative data. Additionally, real-time techniques can provide results in a matter of minutes or even seconds, without altering the sample in any way. However, real-time MS can only offer tentative qualitative data and suffers from molecular weight overlap in complex matrices. This review compares hyphenated and real-time MS methods and provides examples of applications for each technique for the detection of VCs from humans.

Selected ion flow tube mass spectrometry for targeted analysis of volatile organic compounds in human breath

The analysis of volatile organic compounds (VOCs) within breath for noninvasive disease detection and monitoring is an emergent research field that has the potential to reshape current clinical practice. However, adoption of breath testing has been limited by a lack of standardization. This protocol provides a comprehensive workflow for online and offline breath analysis using selected ion flow tube mass spectrometry (SIFT-MS). Following the suggested protocol, 50 human breath samples can be analyzed and interpreted in <3 h. Key advantages of SIFT-MS are exploited, including the acquisition of real-time results and direct compound quantification without need for calibration curves. The protocol includes details of methods developed for targeted analysis of disease-specific VOCs, specifically short-chain fatty acids, aldehydes, phenols, alcohols and alkanes. A procedure to make custom breath collection bags is also described. This standardized protocol for VOC analysis using SIFT-MS is intended to provide a basis for wider application and the use of breath analysis in clinical studies.

Real-time continuous measurement of intraoperative trace exhaled propofol by planar differential mobility spectrometry

In order to study anesthetic pharmacokinetics and adequately adjust the anaesthesia depth of patients, real-time measurement of the intraoperative exhaled propofol concentration is of significant importance for anaesthetists. Although a series of analytical techniques and methods have been developed for the detection of exhaled propofol, differential mobility spectrometry (DMS) with the advantages of a much smaller instrument, faster response time and cheaper cost shows great potential for the point of care in the operating room. In this paper, a planar DMS was constructed for real-time continuous measurement of trace propofol in exhaled air. The effects of DMS parameters, such as the radio frequency voltage, the drift gas flow rate and the sampling flow rate of exhaled air on the propofol measurement under high humidity conditions were carefully investigated and discussed. Under the optimum experimental conditions, the limit of detection (LOD) for propofol was achieved in ppbv with a linear range of 0.5 to 25 ppbv, both of which meet clinical requirements. Finally, the planar DMS was performed on a patient undergoing thyroidectomy surgery to real-time monitor the intraoperative exhaled propofol, which demonstrated the capability of DMS for sensitive and breath-by-breath continuous measurement of intraoperative trace exhaled propofol.

Breath-by-breath measurement of intraoperative propofol by unidirectional anisole-assisted photoionization ion mobility spectrometry via real-time correction of humidity

Humidity as a major issue affects the quantitative performance of ion mobility spectrometry (IMS) in field applications. According to the kinetic equations of ion-molecular reaction, the intensity ratio of the product ion peak (PIP) over the reactant ion peak (RIP) is proposed as a quantitative factor to correct real-time humidity variation. By coupling this method with a unidirectional anisole-assisted photoionization IMS, direct breath-by-breath measurement of intraoperative propofol was achieved for the first time, which provided more clinical information for studying the anesthetics pharmacokinetics. Although the signal intensities of the RIP and the propofol PIP both declined along with the increase of humidity, the intensity ratio of Propofol/(RIP + Propofol) kept almost constant in a wide relative humidity range of 0%-98%, enabling direct quantitation of exhaled propofol with varying humidity. Furthermore, interfering ion peaks resulted from the high concentration humidity and anesthetics in single exhalation were eliminated during the balanced anesthesia as the exhaled sample was diluted by the unidirectional gas flow scheme. As a demonstration, breath-by-breath variation profiles of propofol were obtained via monitoring end-tidal propofol concentration of intraoperative anesthetized patients (n = 7). The analyses were quantitative, corrected for humidity in real-time, without measuring the humidity content of each breath sample during operation, which show potential for the quantitative analysis of other high humidity samples.

CO2 Laser Photoacoustic Spectrometer for Measuring Acetone in the Breath of Lung Cancer Patients

A CO₂ laser has the advantages of being high in power and having many laser lines in the 9–11 µm infrared region. Thus, a CO₂ laser photoacoustic spectrometer (PAS) can have a multi-component measurement capability for many gas compounds that have non-zero absorption coefficients at the laser lines, and therefore can be applied for measuring several volatile organic compounds (VOCs) in the human breath. We have developed a CO₂ laser PAS system for detecting acetone in the human breath. Although acetone has small absorption coefficients at the CO₂ laser lines, our PAS system was able to obtain strong photoacoustic (PA) signals at several CO₂ laser lines, with the strongest one being at the 10P20 line. Since at the 10P20 line, ethylene and ammonia also have significant absorption coefficients, these two gases have to be included in a multi-component measurement with acetone. We obtained the lowest detection limit of our system for the ethylene, acetone, and ammonia are 6 ppbv, 11 ppbv, and 31 ppbv, respectively. We applied our PAS system to measure these three VOCs in the breath of three groups of subjects, i.e., patients with lung cancer disease, patients with other lung diseases, and healthy volunteers.

Breath Volatile Organic Compound Profiling of Colorectal Cancer Using Selected Ion Flow-tube Mass Spectrometry

OBJECTIVE

BACKGROUND:: Breath VOCs have the potential to noninvasively diagnose cancer.

METHODS

Exhaled breath samples were collected using 2-L double-layered Nalophan bags, and were analyzed using selected-ion-flow-tube mass-spectrometry. Gold-standard test for comparison was endoscopy for luminal inspection and computed tomography (CT) to confirm cancer recurrence. Three studies were conducted: RESULTS:: CONCLUSION:: This study suggests the association of a single breath biomarker with the primary presence and recurrence of CRCa. Further multicenter validation studies are required to validate these findings.

Assessment of a Noninvasive Exhaled Breath Test for the Diagnosis of Oesophagogastric Cancer

Question

What is the diagnostic accuracy of a breath test for esophagogastric cancer?

Findings

In a multicenter diagnostic study of 335 patients, including 172 patients with esophagogastric cancer, the breath test demonstrated good diagnostic accuracy.

Meaning

This study suggests the potential of breath analysis as a noninvasive tool in the diagnosis of esophagogastric cancer.

Importance

Early esophagogastric cancer (OGC) stage presents with nonspecific symptoms.

Objective

The aim of this study was to determine the accuracy of a breath test for the diagnosis of OGC in a multicenter validation study.

Design, Setting, and Participants

Patient recruitment for this diagnostic validation study was conducted at 3 London hospital sites, with breath samples returned to a central laboratory for selected ion flow tube mass spectrometry (SIFT-MS) analysis. Based on a 1:1 cancer:control ratio, and maintaining a sensitivity and specificity of 80%, the sample size required was 325 patients. All patients with cancer were on a curative treatment pathway, and patients were recruited consecutively. Among the 335 patients included; 172 were in the control group and 163 had OGC.

Interventions

Breath samples were collected using secure 500-mL steel breath bags and analyzed by SIFT-MS. Quality assurance measures included sampling room air, training all researchers in breath sampling, regular instrument calibration, and unambiguous volatile organic compounds (VOCs) identification by gas chromatography mass spectrometry.

Main Outcomes and Measures

The risk of cancer was identified based on a previously generated 5-VOCs model and compared with histopathology-proven diagnosis.

Results

Patients in the OGC group were older (median [IQR] age 68 [60-75] vs 55 [41-69] years) and had a greater proportion of men (134 [82.2%]) vs women (81 [47.4%]) compared with the control group. Of the 163 patients with OGC, 123 (69%) had tumor stage T3/4, and 106 (65%) had nodal metastasis on clinical staging. The predictive probabilities generated by this 5-VOCs diagnostic model were used to generate a receiver operator characteristic curve, with good diagnostic accuracy, area under the curve of 0.85. This translated to a sensitivity of 80% and specificity of 81% for the diagnosis of OGC.

Conclusions and Relevance

This study shows the potential of breath analysis in noninvasive diagnosis of OGC in the clinical setting. The next step is to establish the diagnostic accuracy of the test among the intended population in primary care where the test will be applied.

Exhaled acetone and isoprene in perioperative lung cancer patients under intensive oral care: possible indicators of inflammatory responses and metabolic changes

One of the most severe complications of lung resection is postoperative pneumonia, and its prevention and prediction are critical. Exhaled acetone and isoprene are thought to be related to metabolism; however, little is known on their relationship with bacteria living in the oral cavity or their meaning in the acute phase in perioperative lung cancer patients. We measured acetone and isoprene in exhaled breath of 13 Japanese patients with lung cancer (3 women and 10 men, age range 62-82 years, mean 72.4 years) before breakfast during hospitalization, and compared with two acute-phase proteins, C-reactive protein (CRP) and albumin in blood serum, as well as the total number of bacteria in saliva and their activity to produce acetone and isoprene. Before operation, intensive oral care was carried out for each patient to prevent postoperative pneumonia, and swallowing and cough reflexes were measured for 12 of 13 patients to assess risk of postoperative pneumonia. Breath and saliva were sampled before intensive oral care (T1), after oral care but before operation (T2), and after operation (T3) during hospitalization. The total number of oral bacteria in saliva decreased significantly from T1 to T2 among 13 patients. No acetone or isoprene was detected from saliva after in vitro incubation under anaerobic or aerobic conditions, but both acetone and isoprene were detected in breath. After operation, breath acetone correlated significantly with CRP (Spearman's ρ = 0.559, P = 0.03), but not with albumin. Breath isoprene correlated significantly with albumin (Spearman's ρ = 0.659, P = 0.008), but not with CRP after operation. Although the number of subjects was small, our results support the hypothesis that breath acetone and isoprene may be related with these acute-phase proteins, which reflect inflammatory reactions and subsequent changes in metabolism in the early postoperative phase of lung resection.

Dopant-assisted photoionization positive ion mobility spectrometry coupled with time-resolved purge introduction for online quantitative monitoring of intraoperative end-tidal propofol

Online monitoring of end-tidal propofol provides important information of anesthesia deepness for anesthetists as propofol concentrations in plasma and breath are well correlated. In this work, a dopant-assisted photoionization positive ion mobility spectrometry (DAPI-PIMS) coupled with time-resolved purge introduction was developed for online quantitative monitoring end-tidal propofol. With optimized dopant, toluene, the selectivity and sensitivity of propofol was improved as interference from sevoflurane was eliminated. Using the time-resolved purge introduction, the response of propofol and moisture was separated due to their absorption differences on the inwall of the fluorinated ethylene propylene (FEP) sample loop, ensuring sensitive measurement of end-tidal propofol with a short response time of 4 s. The quantitative equation derived from the second order reaction kinetics model extended the quantitative range of propofol from 0.2 ppbv to 45 ppbv. Finally, the method was used to monitor the intraoperative end-tidal propofol of six patients, and the results nicely demonstrated its feasibility in practical clinical environment.

Validation of liquid and gaseous calibration techniques for quantification of propofol in breath with sorbent tube Thermal Desorption System GC-MS

Plasma concentrations of intravenous drugs cannot currently be evaluated in real time to guide clinical dosing. However, a system for estimating plasma concentration of the anesthetic propofol from exhaled breath may soon be available. Developing reliable calibration and analytical validation techniques is thus necessary. We therefore compared the established sorbent tube liquid injection technique with a gas injection procedure using a reference gas generator. We then quantified propofol with Tenax sorbent tubes in combination with gas-chromatography coupled mass spectrometry in the breath of 15 patients (101 measurements). Over the clinically relevant concentration range from 10 to 50 ppb_v, coefficient of determination was 0.995 for gas calibration; and over the range from 10 to 100ng, coefficient of determination was 0.996 for liquid calibration. A regression comparing gas to liquid calibration had a coefficient of determination of 0.89; slope 1.05±0.01 (standard deviation). The limit of detection was 0.74ng and the lower limit of quantification was 1.12ng for liquid; the limit of detection was 0.90 ppb_v and the lower limit of quantification was 1.36 ppb_v for gas. Loaded sorbent tubes were stable for at least 14days without significant propofol loss as determined with either method. Measurements from liquid or gas samples were comparably suitable for evaluation of patient breath samples.

Adherence of volatile propofol to various types of plastic tubing

Propofol is an intravenous anesthetic. Currently, it is not possible to routinely measure blood concentration of the drug in real time. However, multi-capillary column ion-mobility spectrometry of exhaled gas can estimate blood propofol concentration. Unfortunately, adhesion of volatile propofol on plastic materials complicates measurements. Therefore, it is necessary to consider the extent to which volatile propofol adheres to various plastics used in sampling tubing. Perfluoralkoxy (PFA), polytetrafluorethylene (PTFE), polyurethane (PUR), silicone, and Tygon tubing were investigated in an experimental setting using a calibration gas generator (HovaCAL). Propofol gas was measured for one hour at 26 °C, 50 °C, and 90 °C tubing temperature. Test tubing segments were then flushed with N₂ to quantify desorption. PUR and Tygon sample tubing absorbed all volatile propofol. The silicone tubing reached the maximum propofol concentration after 119 min which was 29 min after propofol gas exposure stopped. The use of PFA or PTFE tubing produced comparable and reasonably accurate propofol measurements. The desaturation time for the PFA was 10 min shorter at 26 °C than for PTFE. PFA tubing thus seems most suitable for measurement of volatile propofol, with PTFE as an alternative.

Status of selected ion flow tube MS: accomplishments and challenges in breath analysis and other areas

This article reflects our observations of recent accomplishments made using selected ion flow tube MS (SIFT-MS). Only brief descriptions are given of SIFT-MS as an analytical method and of the recent extensions to the underpinning analytical ion chemistry required to realize more robust analyses. The challenge of breath analysis is given special attention because, when achieved, it renders analysis of other air media relatively straightforward. Brief overviews are given of recent SIFT-MS breath analyses by leading research groups, noting the desirability of detection and quantification of single volatile biomarkers rather than reliance on statistical analyses, if breath analysis is to be accepted into clinical practice. A 'strengths, weaknesses, opportunities and threats' analysis of SIFT-MS is made, which should help to increase its utility for trace gas analysis.

Techniques and issues in breath and clinical sample headspace analysis for disease diagnosis

Analysis of volatile organic compounds (VOCs) from breath or clinical samples for disease diagnosis is an attractive proposition because it is noninvasive and rapid. There are numerous studies showing its potential, yet there are barriers to its development. Sampling and sample handling is difficult, and when coupled with a variety of analytical instrumentation, the same samples can give different results. Background air and the environment a person has been exposed to can greatly affect the VOCs emitted by the body; however, this is not an easy problem to solve. This review investigates the use of VOCs in disease diagnosis, the analytical techniques employed and the problems associated with sample handling and standardization. It then suggests the barriers to future development.

Microextraction techniques in breath biomarker analysis

Breath volatile organic compound analysis may open a non-invasive window onto (patho)physiological and metabolic processes in the body. Breath tests require controlled sampling with respect to different breath phases and on-site and point-of-care applicability. Microextraction techniques such as solid phase microextraction (SPME) or needle-trap microextraction (NTME) meet these requirements. Small sample volumes and fast and controlled sample preparation combine on-site sampling and pre-concentration in one step. Detection limits in the low ppbV range and fast and simple processing facilitate the application of distribution-based SPME for screening and targeted analysis. Exhaustive NTME has shown further advantages such as fast and automated sampling, improved stability and reproducibility with improved detection limits. Combinations of different sorbents and thermal expansion desorption have shown most promising properties when applied to water saturated breath samples. This article addresses major challenges and advantages of microextraction techniques in breath analysis. Important progress, current applications and future trends are discussed.

Online breath analysis of propofol during anesthesia: clinical application of membrane inlet-ion mobility spectrometry

BACKGROUND

Breath analysis of propofol is a potential noninvasive method for approximating the plasma propofol concentration. There have been various reported techniques for measuring the exhaled propofol concentration at steady state; however, the propofol concentration undergoes marked changes during clinical anesthesia. Therefore, this study investigated the use of membrane inlet-ion mobility spectrometry (MI-IMS) to monitor exhaled propofol discontinuously and continuously during propofol anesthesia.

METHODS

The study included 19 patients of American Society of Anesthesiologists physical status I or II. In experiment I (discontinuous study), breath and blood samples were collected discontinuously, with stable target propofol concentrations of 2.8 μg/ml, 3.2 μg/ml, 3.5 μg/ml, and 3.8 μg/ml. In experiment II (continuous study), propofol concentration was maintained at 3.5 μg/ml after induction, and exhaled breath was collected continuously every 3 min during propofol infusion. Relationships of the exhaled propofol concentration with the plasma propofol concentration, measured by high-performance liquid chromatography and the continuously measured bispectral (BIS) index were investigated.

RESULTS

Comparison of the exhaled and plasma propofol concentrations revealed a bias ± precision of 2.1% ± 14.6% (95% limits of agreement: - 26.5-30.7%) in experiment I and - 10.4% ± 13.2 (- 36.3-15.4%) in experiment II. In both experiments, exhaled propofol concentrations measured by MI-IMS were consistent with, the propofol effect represented by the BIS index.

CONCLUSIONS

MI-IMS may be a suitable method to predict plasma propofol concentration online during propofol anesthesia. Monitoring exhaled propofol may improve the safety of propofol anesthesia.

Improved analytical performance of negative 63Ni ion mobility spectrometry for on-line measurement of propofol using dichloromethane as dopant

On-line monitoring of propofol in exhaled air is a potential way to evaluate the anaesthesia depth for patients during surgery. In this study, a negative (63)Ni ionization high resolution ion mobility spectrometer with Bradbury-Nielsen-Gate-Grid structure was built to measure propofol with reactant ions Cl(-)(H2O) n using dichloromethane as dopant. Instead of forming three propofol ions (M - H)(-), M · O2(-), and (M2 - H)(-) with reactant ions O2(-)(H2O)n, only product ion M · Cl(-) was produced when introducing dichloromethane gas. The peak-to-peak resolution (R p-p) between reactant ions Cl(-)(H2O)n and product ion M · Cl(-) was 17.4, which was 1.6 times larger than that between O2(-)(H2O)n and product ion. Furthermore, the linear response range using reactant ions Cl(-)(H2O)n was 3.5 times wider than that obtained with reactant ions O2(-)(H2O)n.

SIFT-MS and FA-MS methods for ambient gas phase analysis: developments and applications in the UK

The origins of SIFT created to study interstellar chemistry and SIFT-MS developed for ambient gas and exhaled breath analysis and the UK centres in which these techniques are being exploited.

Real-time monitoring of exhaled drugs by mass spectrometry

Future individualized patient treatment will need tools to monitor the dose and effects of administrated drugs. Mass spectrometry may become the method of choice to monitor drugs in real time by analyzing exhaled breath. This review describes the monitoring of exhaled drugs in real time by mass spectrometry. The biological background as well as the relevant physical properties of exhaled drugs are delineated. The feasibility of detecting and monitoring exhaled drugs is discussed in several examples. The mass spectrometric tools that are currently available to analyze breath in real time are reviewed. The technical needs and state of the art for on-site measurements by mass spectrometry are also discussed in detail. Off-line methods, which give support and are an important source of information for real-time measurements, are also discussed. Finally, some examples of drugs that have already been successfully detected in exhaled breath, including propofol, fentanyl, methadone, nicotine, and valproic acid are presented. Real-time monitoring of exhaled drugs by mass spectrometry is a relatively new field, which is still in the early stages of development. New technologies promise substantial benefit for future patient monitoring and treatment.

Hydrogen sulphide in human nasal air quantified using thermal desorption and selected ion flow tube mass spectrometry

The discovery that hydrogen sulphide (H2S) acts as a gasotransmitter when present at very low concentrations (sub-parts per billion (ppbv)) has resulted in the need to quickly quantify trace amounts of the gas in complex biological samples. Selected ion flow tube mass spectrometry (SIFT-MS) is capable of real-time quantification of H2S but many SIFT-MS instruments lack sufficient sensitivity for this application. In this study we investigate the utility of combining thermal desorption with SIFT-MS for quantifying H2S in the 0.1-1 ppbv concentration range. Human orally or nasally derived breath, and background ambient air, were collected in sampling bags and dried by passing through CaCl2 and H2S pre-concentrated using a sorbent trap optimised for the capture of this gas. The absorbed H2S was then thermally desorbed and quantified by SIFT-MS. H2S concentrations in ambient air, nasal breath and oral breath collected from 10 healthy volunteers were 0.12  ±  0.02 (mean ± SD), 0.40  ±  0.11 and 3.1  ±  2.5 ppbv respectively, and in the oral cavity H2S, quantified by SIFT-MS without pre-concentration, was present at 13.5  ±  8.6 ppbv. The oral cavity H2S correlates well with oral breath H2S but not with nasal breath H2S, suggesting that oral breath H2S derives mainly from the oral cavity but nasal breath is likely pulmonary in origin. The successful quantification of such low concentrations of H2S in nasal air using a rapid analytical procedure paves the way for the straightforward analysis of H2S in breath and may assist in elucidating the role that H2S plays in biological systems.

Breath analysis of ammonia, volatile organic compounds and deuterated water vapor in chronic kidney disease and during dialysis

The volatile metabolites present in trace amounts in exhaled breath of healthy individuals and patients, for example those with advanced chronic kidney disease (CKD), can now be detected and quantified by sensitive analytical techniques. In this review, special attention is given to the major retention metabolites resulting from dialysis-dependent CKD stage 5 and especially ammonia, as a potential estimator of the severity of uremia. However, other biomarkers are important, including the hydrocarbons isoprene, ethane and pentane, in that they are likely to indicate tissue injury associated with the dialysis treatment itself. Evaluation of over-hydration, a serious complication of CKD stage5 can be improved by analysis of deuterium in exhaled water vapor after ingestion of a known amount of deuterated water, so providing total body water measurements at the bedside to support clinical management of volume status.

Ultrafast response humidity sensor using supramolecular nanofibre and its application in monitoring breath humidity and flow

Measuring humidity in dynamic situations calls for highly sensitive fast response sensors. Here we report, a humidity sensor fabricated using solution processed supramolecular nanofibres as active resistive sensing material. The nanofibres are built via self- assembly of donor and acceptor molecules (coronene tetracarboxylate and dodecyl methyl viologen respectively) involved in charge transfer interactions. The conductivity of the nanofibre varied sensitively over a wide range of relative humidity (RH) with unprecedented fast response and recovery times. Based on UV-vis, XRD and AFM measurements, it is found that the stacking distance in the nanofibre decreases slightly while the charge transfer band intensity increases, all observations implying enhanced charge transfer interaction and hence the conductivity. It is demonstrated to be as a novel breath sensor which can monitor the respiration rate. Using two humidity sensors, a breath flow sensor was made which could simultaneously measure RH and flow rate of exhaled nasal breath. The integrated device was used for monitoring RH in the exhaled breath from volunteers undergoing exercise and alcohol induced dehydration.

Analysis of exhaled breath for disease detection

Breath analysis is a young field of research with great clinical potential. As a result of this interest, researchers have developed new analytical techniques that permit real-time analysis of exhaled breath with breath-to-breath resolution in addition to the conventional central laboratory methods using gas chromatography-mass spectrometry. Breath tests are based on endogenously produced volatiles, metabolites of ingested precursors, metabolites produced by bacteria in the gut or the airways, or volatiles appearing after environmental exposure. The composition of exhaled breath may contain valuable information for patients presenting with asthma, renal and liver diseases, lung cancer, chronic obstructive pulmonary disease, inflammatory lung disease, or metabolic disorders. In addition, oxidative stress status may be monitored via volatile products of lipid peroxidation. Measurement of enzyme activity provides phenotypic information important in personalized medicine, whereas breath measurements provide insight into perturbations of the human exposome and can be interpreted as preclinical signals of adverse outcome pathways.

Detection of volatile organic compounds as biomarkers in breath analysis by different analytical techniques

Breath is a rich mixture containing numerous volatile organic compounds at trace amounts (ppbv–pptv level) such as: hydrocarbons, alcohols, ketones, aldehydes, esters or heterocycles. The presence of some of them depends on health status. Therefore, breath analysis might be useful for clinical diagnostics, therapy monitoring and control of metabolic or biochemical cell cycle products. This Review presents an update on the latest developments in breath analysis applied to diagnosing different diseases with the help of high-quality equipment. Efforts were made to fully and accurately describe traditional and modern techniques used to determine the components of breath. The techniques were compared in terms of design, function and also detection limit of different volatile organic compounds. GC with different detectors, MS, optical sensor and laser spectroscopic detection techniques are also discussed.

Selected ion flow tube mass spectrometry analysis of exhaled breath for volatile organic compound profiling of esophago-gastric cancer

Exhaled breath analysis of volatile organic compounds (VOCs) has great potential in terms of disease diagnosis and measuring physiological response to treatment. In this study, selected ion flow tube mass spectrometry (SIFT-MS) has been applied for the quantification of VOCs in the exhaled breath from 3 groups of patients, viz., those with esophago-gastric cancer, noncancer diseases of the upper gastro-intestinal tract, and a healthy upper gastrointestinal tract cohort. A total of 17 VOCs have been investigated in this study. The concentrations of 4 VOCs, hexanoic acid, phenol, methyl phenol, and ethyl phenol, were found to be significantly different between cancer and positive control groups using the Mann-Whitney U test. Receiver operating characteristics (ROC) analysis was applied for a combination of 4 VOCs (hexanoic acid, phenol, methyl phenol, and ethyl phenol) to discriminate the esophago-gastric cancer cohort from positive controls. The integrated area under the ROC curve (AUC) is 0.91. The results highlight the potential of VOC profiling as a noninvasive test to identify those with esophago-gastric cancer.

Determination of breath isoprene allows the identification of the expiratory fraction of the propofol breath signal during real-time propofol breath monitoring

Real-time measurement of propofol in the breath may be used for routine clinical monitoring. However, this requires unequivocal identification of the expiratory phase of the respiratory propofol signal as only expiratory propofol reflects propofol blood concentrations. Determination of CO2 breath concentrations is the current gold standard for the identification of expiratory gas but usually requires additional equipment. Human breath also contains isoprene, a volatile organic compound with low inspiratory breath concentration and an expiratory concentration plateau. We investigated whether breath isoprene could be used similarly to CO2 to identify the expiratory fraction of the propofol breath signal. We investigated real-time breath data obtained from 40 study subjects during routine anesthesia. Propofol, isoprene, and CO2 breath concentrations were determined by a combined ion molecule reaction/electron impact mass spectrometry system. The expiratory propofol signal was identified according to breath CO2 and isoprene concentrations and presented as median of intervals of 30 s duration. Bland-Altman analysis was applied to detect differences (bias) in the expiratory propofol signal extracted by the two identification methods. We investigated propofol signals in a total of 3,590 observation intervals of 30 s duration in the 40 study subjects. In 51.4 % of the intervals (1,844/3,590) both methods extracted the same results for expiratory propofol signal. Overall bias between the two data extraction methods was -0.12 ppb. The lower and the upper limits of the 95 % CI were -0.69 and 0.45 ppb. Determination of isoprene breath concentrations allows the identification of the expiratory propofol signal during real-time breath monitoring.

On the features, successes and challenges of selected ion flow tube mass spectrometry

The major features of the selected ion flow tube mass spectrometry (SIFT-MS) analytical method that was conceived and designed for the analysis, in real time, of air obviating sample collections into bags or extraction by pre-concentration of trace compounds onto surfaces are reviewed. The unique analytical capabilities of SIFT-MS for ambient analysis are stressed that allow quantification of volatile organic and inorganic compounds directly from the measurement of physical parameters without the need for regular instrumental calibration using internal or external standards. Then, emphasis is placed on the challenging real-time accurate analysis of single exhalations of humid breath, which is now achieved and readily facilitates wider applications of SIFT-MS in other fields where trace gas analysis has value. The quality of the data obtained by SIFT-MS is illustrated by the quantification of some exhaled breath metabolites that are of immediate relevance to physiology and medicine, including that of hydrogen cyanide in the breath of patients with cystic fibrosis. The current status of SIFT-MS is revealed by a form of a strengths, weakness, opportunities and threats (SWOT) analysis intended to present an objective view of this analytical technique and the likely way forward towards its further development and application.