title: Freeze-drying: an alternative method for the analysis of volatile organic compounds in the headspace of urine samples using solid phase micro-extraction coupled to gas chromatography - mass spectrometry.
abstract: BACKGROUND
  Volatile organic compounds (VOCs) can be intermediates of metabolic pathways and their levels in biological samples may provide a better understanding about diseases in addition to potential methods for diagnosis. Headspace analysis of VOCs in urine samples using solid phase micro extraction (SPME) coupled to gas chromatography - mass spectrometry (GC-MS) is one of the most used techniques. However, it generally produces a limited profile of VOCs if applied to fresh urine. Sample preparation methods, such as addition of salt, base or acid, have been developed to improve the headspace-SPME-GC-MS analysis of VOCs in urine samples. These methods result in a richer profile of VOCs, however, they may also add potential contaminants to the urine samples, result in increased variability introduced by manually processing the samples and promote degradation of metabolites due to extreme pH levels. Here, we evaluated if freeze-drying can be considered an alternative sample preparation method for headspace-SPME-GC-MS analysis of urine samples.
  
  
  RESULTS
  We collected urine from three volunteers and compared the performances of freeze-drying, addition of acid (HCl), addition of base (NaOH), addition of salt (NaCl), fresh urine and frozen urine when identifying and quantifying metabolites in 4 ml samples. Freeze-drying and addition of acid produced a significantly higher number of VOCs identified than any other method, with freeze-drying covering a slightly higher number of chemical classes, showing an improved repeatability and reducing siloxane impurities.
  
  
  CONCLUSIONS
  In this work we compared the performance of sample preparation methods for the SPME-GC-MS analysis of urine samples. To the best of our knowledge, this is the first study evaluating the potential of freeze-dry as an alternative sample preparation method. Our results indicate that freeze-drying has potential to be used as an alternative method for the SPME-GC-MS analysis of urine samples. Additional studies using internal standard, synthetic urine and calibration curves will allow a more precise quantification of metabolites and additional comparisons between methods.Graphical abstractEnhancing VOC profiling from urine samples.
authors.affiliation: Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Crown Street, L693BX Liverpool, UK.
authors.forename: Raphael B M
authors.identifier: 0000-0001-6345-0859
authors.initials: RB
authors.lastname: Aggio
authors.affiliation: Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Crown Street, L693BX Liverpool, UK.
authors.forename: Arno
authors.initials: A
authors.lastname: Mayor
authors.affiliation: Marie Curie Palliative Care Institute Liverpool, University of Liverpool, London Road, L39TA Liverpool, UK.
authors.forename: Séamus
authors.initials: S
authors.lastname: Coyle
authors.affiliation: Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Crown Street, L693BX Liverpool, UK.
authors.forename: Sophie
authors.initials: S
authors.lastname: Reade
authors.affiliation: Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Crown Street, L693BX Liverpool, UK ; Department of Surgery and Cancer, South Kensington Campus, Imperial College London, SW72AZ London, UK.
authors.forename: Tanzeela
authors.initials: T
authors.lastname: Khalid
authors.affiliation: Faculty of Health and Applied Sciences, Frenchay Campus, University of the West of England, Coldharbour Lane, BS161QY Bristol, UK.
authors.forename: Norman M
authors.initials: NM
authors.lastname: Ratcliffe
authors.affiliation: Department of Cellular and Molecular Physiology, Institute of Translational Medicine, University of Liverpool, Crown Street, L693BX Liverpool, UK.
authors.forename: Chris S J
authors.initials: CS
authors.lastname: Probert
keywords: Freeze-dry; GC-MS; Metabolomics; SPME; Urine; VOC; Volatile organic compounds