An exploratory comparative study of volatile compounds in exhaled breath and emitted by skin using selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) has been used to carry out a pilot parallel study on five volunteers to determine changes occurring in several trace compounds present in exhaled breath and emitted from skin into a collection bag surrounding part of the arm, before and after ingesting 75 g of glucose in the fasting state. SIFT-MS enabled real-time quantification of ammonia, methanol, ethanol, propanol, formaldehyde, acetaldehyde, isoprene and acetone. Following glucose ingestion, blood glucose and trace compound levels were measured every 30 min for 2 h. All the above compounds, except formaldehyde, were detected at the expected levels in exhaled breath of all volunteers; all the above compounds, except isoprene, were detected in the collection bag. Ammonia, methanol and ethanol were present at lower levels in the bag than in the breath. The aldehydes were present at higher levels in the bag than in breath. The blood glucose increased to a peak about 1 h post-ingestion, but this change was not obviously correlated with temporal changes in any of the compounds in breath or emitted by skin, except for acetone. The decrease in breath acetone was closely mirrored by skin-emitted acetone in three volunteers. Breath and skin acetone also clearly change with blood glucose and further work may ultimately enable inferences to be drawn of the blood glucose concentration from skin or breath measurements in type 1 diabetes.

A selected ion flow tube mass spectrometry study of ammonia in mouth- and nose-exhaled breath and in the oral cavity

A study has been carried out, involving three healthy volunteers, of the ammonia levels in breath exhaled via the mouth and via the nose and in the static oral cavity using on-line, selected ion flow tube mass spectrometry (SIFT-MS), obviating the problems associated with sample collection of ammonia. The unequivocal conclusion drawn is that the ammonia appearing in the mouth-exhaled breath of the three volunteers is largely generated in the oral cavity and that the ammonia originating at the alveolar interface in the lungs is typically at levels less than about 100 parts-per-billion, which is a small fraction of the total breath ammonia. This leads to the recommendation that exhaled breath analyses should focus on nose-exhaled breath if the objective is to use breath analysis to investigate systemic, metabolic disease.

The challenge of breath analysis for clinical diagnosis and therapeutic monitoring

The potential of breath analysis for clinical diagnosis and the strengths and weaknesses of the analytical methods used are discussed. Special attention is given to selected ion flow tube mass spectrometry, SIFT-MS, using which on-line real-time analyses of single breath exhalations can be carried out. Illustrative data on the concentration distributions of several breath metabolites amongst the healthy population are presented and their relations to disease when elevated above the normal are alluded to.

A longitudinal study of methanol in the exhaled breath of 30 healthy volunteers using selected ion flow tube mass spectrometry, SIFT-MS

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to monitor the volatile compounds in the exhaled breath of 30 volunteers (19 male, 11 female) over a six-month period. Volunteers provided breath samples each week between 8:45 am and 1 pm (before lunch), and the concentrations of several trace compounds were obtained. In this paper the focus is on methanol in breath. The median methanol level determined using the H(3)O(+) precursor ions for all samples was 461 parts-per-billion (ppb), the concentrations for all the samples ranging from 32 to 1684 ppb. The distribution of breath methanol concentration is seen to be log-normal for this healthy population; the geometric mean was 450 ppb, close to the median value, and the multiplicative (geometric) standard deviation was 1.62. Breath methanol is not correlated with age, breath ethanol or ethanol consumed in the previous 24 h, but there was an inverse correlation with body mass index (BMI) for the cohort of volunteers recruited for this study. Observed breath methanol levels are well compatible with the previously published blood methanol levels. Some tentative suggestions are made concerning the origin of endogenous methanol.

Bronchoalveolar lavage examined by solid phase microextraction, gas chromatography–mass spectrometry and selected ion flow tube mass spectrometry

Samples (210 in total) of broncholaveolar lavages (BALs), obtained from patients hospitalized with pneumonia in various departments of two hospitals, were analysed using the method of solid phase microextraction-gas chromatography (SPME-GC) with FID detection. Up to 20% (9% unequivocally, 11% probably) of these samples was found to contain volatile fatty acids (VFAs) in the series from acetic acid to heptanoic acid. Importantly, the presence of these acids indicates the presence of fermenting anaerobic bacteria, which were not detected by the conventional microbiological examination. Other compounds, namely the heptanol and cyclohexanone, were also detected by this method in some samples. Cyclohexanone occurred almost exclusively in samples from patients receiving intensive care with mechanical ventilation, and is suspected to originate from plastic parts of ventilators. Selected representative samples were also analysed using further methods, namely gas chromatography-mass spectrometry (GC-MS) of native and silylated samples, and selected ion flow tube mass spectrometry (SIFT-MS). These methods confirmed the identities of above mentioned compounds, and detected numerous other compounds tentatively identified as various alcohols, aldehydes, ketones, esters and hydrogen cyanide, HCN. Most of these compounds occurred in small amounts and their origin and diagnostic significance remains uncertain, except, that is, for the HCN, which indicates the presence of Pseudomonas aeruginosa.

The increase of breath ammonia induced by niacin ingestion quantified by selected ion flow tube mass spectrometry

The ingestion of relatively large doses of the vitamin niacin by healthy volunteers results in a reddening of the skin, a skin 'flush'. Thus, we have carried out a study of the breath metabolites of two healthy volunteers following (i) the ingestion of 200 mg of immediate-release niacin, (ii) as (i) but preceded by the ingestion of 325 mg of aspirin that diminishes the skin 'flush', (iii) ingestion of 500 mg of slow-release niacin. On-line breath analysis was carried out using selected ion flow tube mass spectrometry, SIFT-MS. The interesting new observation is that the breath ammonia levels of both volunteers clearly increased following (i) and (ii), and an obvious skin flush did occur following (i) but not following (ii). The slow-release niacin (iii) did not result in a flush and the breath ammonia levels increased more slowly and did not reach the higher levels produced by (i) and (ii). The results of these experiments demonstrate that breath ammonia levels are dependent on the blood/plasma levels of niacin, but are not directly related to the flushing phenomenon, and that the observed increases in blood/breath ammonia levels are consistent with current knowledge of the metabolic pathways of niacin. The parallel measurements of breath isoprene are presented, which demonstrate the quality of breath analyses that can be achieved using SIFT-MS.

A longitudinal study of ammonia, acetone and propanol in the exhaled breath of 30 subjects using selected ion flow tube mass spectrometry, SIFT-MS

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to monitor the volatile compounds in the exhaled breath of 30 volunteers (19 males, 11 females) over a 6 month period. Volunteers provided breath samples each week between 8:45 am and 1 pm (before lunch), and the concentrations of several trace compounds were obtained. In this paper the focus is on ammonia, acetone and propanol. It was found that the concentration distributions of these compounds in breath were close to log-normal. The median ammonia level estimated as a geometric mean for all samples was 833 parts per billion (ppb) with a multiplicative standard deviation of 1.62, the values ranging from 248 to 2935 ppb. Breath ammonia clearly increased with increasing age in this volunteer cohort. The geometric mean acetone level for all samples was 477 parts per billion (ppb) with a multiplicative standard deviation of 1.58, the values ranging from 148 to 2744 ppb. The median propanol level for all samples was 18 ppb, the values ranging from 0 to 135 ppb. A weak but significant correlation between breath propanol and acetone levels is apparent in the data. The findings indicate the potential value of SIFT-MS as a non-invasive breath analysis technique for investigating volatile compounds in human health and in the diseased state.

A longitudinal study of ethanol and acetaldehyde in the exhaled breath of healthy volunteers using selected-ion flow-tube mass spectrometry

Selected-ion flow-tube mass spectrometry (SIFT-MS) has been used to monitor the volatile compounds in the exhaled breath of 30 volunteers (19 male, 11 female) over a 6-month period. Volunteers provided breath samples each week between 8:45 and 13:00 (before lunch), and the concentrations of several trace compounds were obtained. In this paper the focus is on ethanol and acetaldehyde, which were simultaneously quantified by SIFT-MS using H3O+ precursor ions. The mean ethanol level for all samples was 196 parts-per-billion (ppb) with a standard deviation of 244 ppb, and the range of values for breath samples analysed is 0 to 1663 ppb. The mean acetaldehyde level for all samples was 24 ppb with a standard deviation of 17 ppb, and the range of values for breath samples analysed is 0 to 104 ppb. Background (ambient air) levels of ethanol were around 50 ppb, whereas any background acetaldehyde was usually undetectable. Increased ethanol levels were observed if sweet drink/food had been consumed within the 2 h prior to providing the breath samples, but no increase was apparent when alcohol had been consumed the previous evening. The measured endogenous breath ethanol and acetaldehyde levels were not correlated. These data relating to healthy individuals are a prelude to using breath analysis for clinical diagnosis, for example, the recognition of bacterial overload in the gut (ethanol) or the possibly of detecting tumours in the body (acetaldehyde).

Combined use of gas chromatography and selected ion flow tube mass spectrometry for absolute trace gas quantification

The value of the gas chromatography (GC) and selected ion flow tube mass spectrometry (SIFT-MS) combination for the analysis of trace gases is demonstrated by the quantification of acetone in air samples using the three precursor ions available to SIFT-MS, viz. H3O+, NO+ and O2+, and by the separation of the isomers 1-propanol and 2-propanol, and their analysis using H3O+ precursor ions. It is shown that the GC/SIFT-MS combination allows for accurate trace gas quantification obviating the regular, time-consuming calibrations that are usually required for the more commonly used detectors of GC systems, and the positive identification of isomers in mixtures that is often challenging using SIFT-MS alone. Thus, the GC/SIFT-MS combination paves the way to more confident analyses of complex mixtures such as exhaled breath.

Increase of acetone emitted by urine in relation to ovulation

BACKGROUND

Selected ion flow tube mass spectrometry allows trace gas quantification in exhaled breath and in the air/vapor above liquids (headspace) down to the 10 parts-per-billion level. During selected ion flow tube mass spectrometry investigation of the volatile compounds emitted by urine, high acetone levels were incidentally identified in the headspace of urine from healthy female volunteers around their mid-cycle. Hence, this study was designed to measure urine headspace acetone levels throughout the menstrual cycle.

METHODS

Using selected ion flow tube mass spectrometry we measured daily urine headspace acetone concentrations of seven ovulating (group 1) and three postmenopausal volunteers (group 2).

RESULTS

A several-fold increase in urine headspace acetone level was detected 2-3 days after the predicted day of ovulation in 5 of the 7 volunteers in group 1. No such rise was detected in group 2.

CONCLUSION

This study provides the basis for future research to understand the reason for and the potential utility of this phenomenon.

A longitudinal study of breath isoprene in healthy volunteers using selected ion flow tube mass spectrometry (SIFT-MS)

Thirty volunteers (19 males, 11 females) were recruited for a 6-month study of the volatile compounds in their exhaled breath using the selected ion flow tube mass spectrometry (SIFT-MS) analytical technique. Volunteers provided weekly breath samples between 8:45 am and 1 pm (before lunch), and the concentrations of several trace compounds were obtained. In this paper, we focus on the isoprene in alveolar breath, which was monitored by SIFT-MS using NO(+) precursor ions. The mean isoprene level for all samples was 118 parts per billion (ppb) with a standard deviation of 68 ppb and the range of values for breath samples given is 0-474 ppb. Variability in isoprene levels was similar in most volunteers. Isoprene levels increased immediately after moderate exercise, but returned to normal within 2-3 min for those few volunteers that were investigated. Cholesterol levels analysed for only three of the subjects were not obviously correlated with isoprene concentration in breath. Differences in isoprene levels were not directly correlated to gender, age or body mass index.

Selected ion flow tube mass spectrometry (SIFT-MS) for on-line trace gas analysis

Selected ion flow tube mass spectrometry (SIFT-MS) is a new analytical technique for the real-time quantification of several trace gases simultaneously in air and breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas using H(3)O(+), NO(+), and O(2) (+.) precursor ions. Reactions between the precursor ions and trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer, thus effecting quantification. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to ppb levels, obviating sample collection and calibration. Illustrative examples of SIFT-MS studies include (i) analysis of gases from combustion engines, animals and their waste, and food; (ii) breath and urinary headspace studies of metabolites, ethanol metabolism, elevated acetone during ovulation, and exogenous compounds; and (iii) urinary infection and the presence of tumors, the influence of dialysis on breath ammonia, acetone, and isoprene, and acetaldehyde released by cancer cells in vitro. Flowing afterglow mass spectrometry (FA-MS) is briefly described, which allows on-line quantification of deuterium in breath water vapor.

Detection of volatile compounds emitted by Pseudomonas aeruginosa using selected ion flow tube mass spectrometry

Pseudomonas aeruginosa (PA) is associated with a distinctive smell produced by a combination of volatile compounds (VCs). Selected ion flow tube mass spectrometry (SIFT-MS) provides a novel and rapid methodology for rapid, accurate detection of trace quantities (parts per billion; ppb) of VCs in air. We studied the VCs produced by different isolates of PA cultures in vitro from patients with cystic fibrosis. Twenty-one patients with cystic fibrosis provided sputum and cough swab samples for culture. These were used to inoculate blood agar (BA) and Pseudomonas-selective media (PSM). These plates were incubated for 48 hr at 37 degrees C inside sealed plastic bags. The air surrounding the samples after 48 hr (headspace) was analyzed using SIFT-MS. PA growth was commonly associated with the production of significant quantities of VCs, notably hydrogen cyanide gas (HCN). This was detectable in the headspace of 15/22 of PA-positive samples. In contrast, it was only seen in the headspace of 1/13 control samples (6 sterile plates and 7 plates with only mixed upper respiratory tract flora). The concentration of HCN was significantly higher above PA-positive samples than above other bacterial growth (P < 0.01), and in our study, levels of HCN greater than 100 ppb were a sensitive (68%) and highly specific (100%) biomarker of PA. SIFT-MS can detect a range of VCs from PA in vitro. HCN may be a specific indicator of PA infection in vivo, and offers promise as a biomarker for noninvasive detection of PA infection by breath analysis.

Coordinated FA-MS and SIFT-MS analyses of breath following ingestion of D2O and ethanol: total body water, dispersal kinetics and ethanol metabolism

A coordinated study of the dispersal of water between the various body compartments (stomach and gut, blood stream and tissue) and the similar dispersal kinetics of ethanol and its metabolism has been carried out involving two healthy volunteers using flowing afterglow mass spectrometry, FA-MS, and selected ion flow tube mass spectrometry, SIFT-MS. Thus, using these techniques, the variations of HDO and ethanol in breath, measured in successive single exhalations, were followed in real time after the ingestion of measured quantities of D2O and ethanol in proportion to the body weights of the subjects at the dose rates D2O approximately 0.283 g kg-1, ethanol approximately 0.067 g kg-1. During the FA-MS experimental periods (about 2 h), the dispersion of HDO into the body water and finally its equilibration in the total body water is observed from which total body water for each subject was determined. In the SIFT-MS measurements, the dispersion of ethanol into the body water and its loss via metabolism was observed until the physiological (pre-dose) breath level of ethanol for each individual was restored. A simple linear transformation is used to derive the time variations of the blood levels of HDO and ethanol. This has allowed a comparison of the fractions of the ingested ethanol that are metabolized during first-pass metabolism for the two subjects. Thus, in one subject 30% and in the other subject 40% of the ingested alcohol is metabolized in the first 20 min following ingestion. The good time resolution allowed by non-invasive breath analysis ensures that the rates of processes such as ethanol metabolism can be accurately measured. Simultaneous measurements of breath acetaldehyde (largely formed via the ethanol metabolism) and acetone were also performed during the SIFT-MS single breath exhalations.

Influence of Convection on the Diffusive Transport and Sieving of Water and Small Solutes across the Peritoneal Membrane

The three-pore model of peritoneal membrane physiology predicts sieving of small solutes as a result of the presence of a water-exclusive pathway. The purpose of this study was to measure the diffusive and convective components of small solute transport, including water, under differing convection. Triplicate studies were performed in eight stable individuals using 2-L exchanges of bicarbonate buffered 1.36 or 3.86% glucose and icodextrin. Diffusion of water was estimated by establishing an artificial gradient of deuterated water (HDO) between blood/body water and the dialysate. (125)RISA (radio-iodinated serum albumin) was used as an intraperitoneal volume marker to determine the net ultrafiltration and reabsorption of fluid. The mass transfer area coefficient (MTAC) for HDO and solutes was estimated using the Garred and Waniewski equations. The MTAC of HDO calculated for 1.36% glucose and icodextrin were similar (36.8 versus 39.7 ml/min; P = 0.3), whereas for other solutes, values obtained using icodextrin were consistently higher (P < 0.05). A significant increase in the MTAC of HDO was demonstrated with an increase in the convective flow of water when using 3.86% glucose (mean value, 49.5 ml/min; P < 0.05). MTAC for urea was also increased with 3.86% glucose. The identical MTAC for water using 1.36% glucose and icodextrin indicates that diffusion is predominantly through small pores, whereas the difference in MTAC for the remaining solutes is a reflection of their sieving. The increase in the MTAC of water and urea associated with an increase in convection is most likely due to increased mixing within the interstitium.

Measuring transport of water across the peritoneal membrane

INTRODUCTION

Mechanisms of water flow across the peritoneal membrane include diffusion, convection, and reabsorption.

OBJECTIVES

To understand these processes more clearly we have developed a method to measure transport of water across the peritoneal membrane.

METHODS

An artificial gradient of deuterated water (HDO) between blood and dialysate compartments was created in five subjects who took 0.3g per kg of body weight of D2O, which was allowed to equilibrate with total body water. During a test dwell (2 L, bicarbonate:lactate buffer, 1.36% glucose to minimize convection), frequent dialysate samples were drawn to determine the abundance of deuterium and other solutes and to calculate their time constants. Dialysate deuterium abundance was measured using flowing afterglow mass spectrometry (FA-MS). The method was combined with 125iodine-labeled albumin (RISA) to enable simultaneous estimates of intraperitoneal volume and thus calculation of the mass transfer area coefficient (MTAC) for small solutes using the Garred equation.

RESULTS

The appearance of HDO in dialysate in four subjects is described by a single exponential fit with residuals of <1%, similar to method precision. In a fifth subject, the resolution of this method demonstrated that the best fit was a double exponential. When compared to other solutes, the time constant for water was as predicted by its molecular weight, with a MTAC of 38.7 +/- 4.4 mL/min. Total body water could also be estimated from the equilibrated dialysate deuterium abundance, with repeat estimates within 0.5%.

CONCLUSION

Transport of water across the peritoneum can be measured with remarkable accuracy and when combined with an intraperitoneal volume estimation can be used to determine mass transfer. In conditions of low convection, the relative rate of deuterium appearance and mass transfer compared to other solutes suggests that water diffuses predominantly through the intercellular small pores.

Longitudinal measurements of total body water and body composition in healthy volunteers by online breath deuterium measurement and other near-subject methods

Rapid quantification of breath deuterium abundance by flowing afterglow mass spectrometry (FA-MS) enables accurate measurement of total body water (TBW), which combined with other techniques such as bioelectrical impedance analysis (BIA) and anthropometrics enables near-subject assessment of body composition. This study assessed the comparative reproducibility and inter-relationship of these methods in healthy subjects over 12 months. Detailed bedside composition was performed in 22 subjects, (10 male) aged 28-79 with body mass index (BMI) ranging from 21-38 at baseline and again at one year. Techniques included FA-MS deuterium dilution, BIA, skin-fold thickness (SFT) and soft tissue ultrasound measurement of fat and muscle depth. Short-term reproducibility for each method was established. Within and between technique comparisons of measurement were made from Pearson's linear regression, coefficient of variation (CV) and Bland-Altman analysis. Weight and TBW estimated by FA-MS, BIA and SFT at baseline and one year later were highly correlated (R² = 0.96-0.98), slope 1.02-1.03, CV = 4.5-11.6%. Systematic errors between the different methods in determining TBW were effectively identical at baseline and after one year. There was a tendency for subjects to gain weight during the study period, due to an increase, predominantly in younger women, of body water (FA-MS and SFT) and loss of upper body fat (ultrasound). BIA was relatively insensitive to these changes. It is concluded that over a 12-month period, TBW determined by FA-MS deuterium breath analysis has reproducibility similar to conventional weighing. The stability of between method errors would suggest that these techniques might be used in conjunction with each other in the longitudinal determination of body composition and so detect relatively subtle changes. The value of including an absolute determinant of TBW by FA-MS that is independent of the need to employ population derived equations, appears to be of value in the near-subject determination of body composition as required in clinical practice.

Time variation of ammonia, acetone, isoprene and ethanol in breath: a quantitative SIFT-MS study over 30 days

A study of the concentrations of the common breath metabolites ammonia, acetone, isoprene, ethanol and acetaldehyde in the breath of five subjects over a period of 30 days has been carried out. Breath samples were taken and analysed in the early morning on arrival at the laboratory. The real time analyses of three consecutive breath exhalations were carried out using selected ion flow tube mass spectrometry (SIFT-MS) on line to the instrument. Sufficient data were obtained to allow meaningful concentration distributions to be obtained for ammonia, acetone, isoprene and ethanol. These showed that the ammonia, acetone and isoprene concentrations exhibited sensibly normal distributions, with coefficients of variation of typically 0.3. Obvious and statistically significant (p < 0.01) differences are apparent in the mean concentrations of these metabolites between the five individuals. The acetaldehyde concentrations were relatively low and close to the instrument detection limit, and the differences between the mean concentrations of the five subjects were not statistically significant (p = 0.4), so distributions were not obtained. The mean concentrations, in parts per billion (ppb), of each metabolite range amongst the five subjects are as follows: ammonia, 422-2389: acetone, 293-870; isoprene, 55-121; ethanol, 27-153; acetaldehyde, 2-5. There are no obvious patterns in the distributions of these particular metabolites for these individuals, except that the ammonia levels were greatest in the breath of the two oldest subjects.

Increase of acetone and ammonia in urine headspace and breath during ovulation quantified using selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry (SIFT-MS) has been used for a detailed study of the daily variations in the acetone and ammonia content of the headspace above urine from a healthy female subject over the course of three separate menstrual cycles. Midstream urine samples were taken every morning prior to any food intake and the headspace subsequently analysed for a number of metabolites. Concurrent with the time of ovulation, a 3-to- 12-fold increase in the level of acetone in the urine headspace was observed. The successive peaks in acetone level and the subsequent return to baseline values were mirrored by similar increases in the ammonia levels, but these were a day out of phase. Interestingly, parallel breath analyses at ovulation showed no great increase in either acetone or ammonia above their normal morning levels, suggesting that these metabolites had been removed from the body during the night by the usual metabolic and physiological processes. The results of this study reveal what may be an important phenomenon at the time of ovulation and illustrate the potential and power of online SIFT-MS analysis in this area of research.

Quantification of acetaldehyde released by lung cancer cells in vitro using selected ion flow tube mass spectrometry

The production of volatile compounds from cancer cell lines in vitro has been investigated using selected ion flow tube mass spectrometry (SIFT-MS). This technique enables on-line quantitative analyses of the headspace above cell/medium cultures. This paper reports the discovery that acetaldehyde is released by the lung cancer cell lines SK-MES and CALU-1. The concentration of acetaldehyde in the headspace of the medium/cell culture was measured after 16 h incubation at 37 degrees C and found to be proportional to the number of cancer cells in the medium (typically 10(8)). From these data, the acetaldehyde production rates of the SK-MES cells and the CALU-1 cells in vitro are determined to be 1 x 10(6) and 1.5-3 x 10(6) molecules/cell/min, respectively. The potential value of this new technique in cell biology and in industrial cell biotechnology is discussed.

Comparative measurements of total body water in healthy volunteers by online breath deuterium measurement and other near-subject methods

BACKGROUND

We developed a new near-subject approach, using flowing afterglow-mass spectrometry (FA-MS) and deuterium dilution, which enables the immediate measurement of total body water (TBW) from single exhalations.

OBJECTIVES

The objectives were to show the efficacy of the new FA-MS method in measuring TBW in healthy subjects and to compare these measurements with values derived from multifrequency bioelectrical impedance analysis, skinfold-thickness (SFT) measurements, and both recent and historical published regression equations.

DESIGN

After baseline measurement of breath deuterium abundance, 24 healthy subjects ingested 0.3 g D(2)O/kg body wt. A second breath sample was taken after 3 h to measure the increase in deuterium, from which TBW was calculated. Bioelectrical impedance analysis was carried out with a multifrequency analyzer, and SFT was measured by a single trained observer. Methods were compared with the use of Pearson's correlation coefficient and Bland-Altman analyses.

RESULTS

TBW measures obtained by all methods were highly correlated (r = 0.95-0.98, P < 0.001), especially those between FA-MS, SFT measurement, and recent regression equations. The mean values obtained were within 2% of those published for age-matched control subjects and varied by 1-6% when all methods were compared. Systematic bias was greatest when FA-MS was compared with bioelectrical impedance analysis, which tended to underestimate TBW in smaller, female subjects. No bias related to subject size was observed in a comparison of FA-MS with SFT measurement or with more recent regression equations.

CONCLUSIONS

FA-MS is a simple and effective new approach to TBW measurement in healthy subjects. The difficulty of using population-derived equations to estimate TBW in individual subjects is emphasized.

Quantification of volatile compounds in the headspace of aqueous liquids using selected ion flow tube mass spectrometry

We describe a method by which the concentrations of volatile compounds in the headspace of their dilute aqueous solutions in sealed containers can be determined using on-line selected ion flow tube mass spectrometry (SIFT-MS). Thus, the changing number density of the molecules of the volatile compound in the carrier gas of the SIFT-MS instrument is described in terms of its changing flow rate as the pressure in the sealed container decreases during the sampling procedure. It is shown that the best analytical procedure is to determine the mean concentration of the trace gas in the liquid headspace over a given sampling time and relate this to the required concentration, which is the initial equilibrium concentration established before the pressure in the sealed container reduces significantly. To test the validity of this analytical approach, the headspace concentrations of acetaldehyde, ethanol and acetone above aqueous solutions of known concentrations have been determined. Hence, the Henry's Law constants for these compounds have been determined and found to agree with the published values. The confirmation of the quality of this sampling methodology combined with SIFT-MS for the analysis of volatile compounds in liquid headspace paves the way for the rapid analyses of biological liquids such as urine and serum for clinical diagnosis and physiological monitoring.