Kenneth S. Webb

Intercomparison study on accurate mass measurement of small molecules in mass spectrometry

The results from an intercomparison of accurate mass measurement of a small molecule (molecular weight 475 Da) across a broad range of mass spectrometers are reported. The intercomparison was designed to evaluate the relative capabilities and the optimum methodology of the diverse range of mass spectrometers currently used to record accurate mass measurements. The data will be used as a basis for developing guidance on accurate mass measurement. The need for guidance has resulted from the continued growth in the use of accurate mass measurements for assignment of elemental formula in the chemical and biochemical industries. This has been fuelled by a number of factors and includes the rapid pace of instrument development, which has enabled accurate mass measurements to be made in a less costly, yet robust fashion. The data from the intercomparison will allow us to compare those protocols that produced excellent accuracy and precision with those that produced poorer accuracy and/or precision for each type of mass spectrometer. The key points for best practice will then be established from this comparison for each type of mass spectrometer and accurate mass measurement technique. A compound was sent to the participating laboratories (in the UK, Europe, and USA), the identity of which was not revealed. Each laboratory was asked to record a minimum of five repeat mass measurements of the molecular species using their local protocols and their preferred ionization technique or techniques. To the best of our knowledge there were no interfering (unresolved) ions that originated from the sample. A questionnaire was also completed with the experimental work. The information from the questionnaires was used to evaluate the protocols used to record the measurements. Forty-five laboratories have reported their results. To summarize the performance of mass spectrometers in the intercomparison, magnetic sector field mass spectrometers used in peak matching mode and FTMS reported the highest mean mass measurement accuracy (88 and 83%, respectively, achieved ≤1 ppm). Magnetic sector field mass spectrometers used in voltage scanning produced 60% of the mean mass measurements with accuracy ≤1 ppm. Magnetic sector field mass spectrometers used in magnet scanning modes, quadrupole-TOF and TOF instruments generally achieved mean mass measurement accuracy between 5 and 10 ppm. The two low resolution triple quadrupoles used in the inter-comparison produced mean mass measurement accuracy of <2 ppm. The precision of the data from each instrument and experiment type is an important consideration when evaluating their relative capabilities. Using both the precision and accuracy, it will be possible to define the uncertainty associated with the elemental formulae derived from accurate mass measurements. Therefore, a thorough statistical evaluation of the data is underway and will be presented in a subsequent publication.

Poor reproducibility of in-source collisional atmospheric pressure ionization mass spectra of toxicologically relevant drugs.

The purpose of the study was to examine the intra- and interlaboratory reproducibility of mass spectra obtained with liquid chromatography-atmospheric pressure ionization mass spectrometry (LC--API-MS) both in electrospray (ESI) and atmospheric pressure chemical ionization (APCI) modes. Toxicologically relevant drugs of different polarity were selected as test substances: morphine-6-glucuronide, 6-monoacetylmorphine, codeine, lysergic acid diethylamide, methylenedioxymethamphetamine. The study was performed in two laboratories using identical instruments and in one using a slightly different instrument. Basic instrument settings and mobile phase were identical in all laboratories. Mass spectra of drugs were taken at four collision energy voltages and using mobile phase of different composition (four concentration levels of acetonitrile and of ammonium formate buffer). The experiments demonstrated that mass spectra of given drugs, obtained in identical conditions with identical instruments, may show very different degrees of fragmentation. Mass spectra obtained with different instruments differed profoundly not only in the degree of fragmentation, but also different fragments and adducts were observed. Short-term intralaboratory reproducibility of mass spectra was satisfactory. On the other hand, the long-term experiments showed different degrees of fragmentation of APCI-generated mass spectra at nominally identical fragmentation energy. The changes in the composition of the mobile phase (concentration of organic modifier or buffer molarity) did not affect the reproducibility of fragmentation to any relevant degree. The study showed that the interlaboratory exchange and use of mass spectrum library, generated by single-quadrupole (LC--API-MS instruments, is hardly feasible at the moment, even under very carefully standardized conditions.

A method for the detection of traces of nitrosamines using combined gas chromatography and mass spectrometry

Abstract A gas chromatograph and high-resolution mass spectrometer, coupled via a membrane separator, are used for the analysis of samples for traces of nitrosamines. The nitrosamines are detected by parent ion monitoring with a detection limit of 1 mg/l on injected material. The gas chromatograph incorporates a pressure-programming and peak-cutting device which is described in detail. Overall analysis time is substantially shorter than for isothermal or temperature-programmed runs.

Speciation of organo-tin compounds using liquid chromatography–atmospheric pressure ionisation mass spectrometry and liquid chromatography–inductively coupled plasma mass spectrometry as complementary techniques

Abstract A liquid chromatographic method for the determination of dibutyltin (DBT), tributyltin (TBT), diphenyltin (DPhT) and triphenyltin (TPhT) in sediments has been developed, which is compatible with both atmospheric pressure ionisation (API) mass spectrometry and inductively coupled plasma (ICP) mass spectrometry. As a result of this development both techniques may be used for the complementary speciation of organo-tin compounds. The chromatographic system comprises of a Kromasil-100 5 μm C18 (150×2.1 mm) column and a mobile phase of 0.05% triethylamine in acetonitrile–acetic acid–water (65:10:25), at a flow-rate of 0.2 ml min−1. The optimisation of the LC–API-MS conditions is discussed, together with the analysis of a real sediment sample for DBT and TBT using selected ion monitoring (SIM).

Volatile nitrosamines in salt-preserved fish before and after cooking.

Abstract Uncooked, steamed and fried samples of salted fish were analysed for volatile nitrosamines by gas chromatography with detection by chemiluminescence and some were also analysed by combined gas chromatography and high-resolution mass spectrometry. N-Nitrosodimethylamine (NDMA) was detected in all of the samples that were analysed, whether cooked or uncooked. It was also detected in the aqueous phases derived from the steamed samples, and in the oil from frying in two out of three batches. In one of these the NDMA level was the highest encountered in the present study. N-Nitrosodiethylamine (NDEA), on the other hand, was detected in more batches of the steamed fish than of the uncooked or fried fish. It was not detected in the oil used for frying. In half of the steamed batches NDEA was detected only in the aqueous phases and in one batch it was detected only in the steamed fish. N-Nitrosodi-n-propylamine (NDPA) was detected in both the steamed and fried samples and N-nitrosodi-n-butylamine (NDBA) in only the fried samples. Neither was detected in the uncooked samples. N-Nitrosomorpholine (NMOR) occurred in the uncooked samples. Salted fish heads are sometimes used by southern Chinese to prepare soup. NDMA and NDEA occurred in both uncooked fish heads and in the soups prepared from them. The results confirm that certain nitrosamines are formed from precursors during cooking. A possible relationship between salted fish and certain cancers that are particularly prevalent among southern Chinese populations is discussed.

The Analysis of Lysergide (LSD): The Development of Novel Enzyme Immunoassay and Immunoaffinity Extraction Procedures Together with an HPLC-MS Confirmation Procedure

A forensic procedure for the screening and confirmation of the presence of lysergide (lysergic acid diethylamide, LSD) in urine is described together with the evaluation of a novel enzyme immunoassay (EIA) and immunoaffinity extraction procedure. Following initial screening using either an established radioimmunoassay (RIA) or a novel EIA procedure, a quantitative estimate is established using a conventional high performance liquid chromatography-fluorescence (HPLC) technique following solid phase extraction. Final confirmation and quantitation, without derivatization, is established using HPLC in combination with electrospray ionization (ESI) mass spectrometry using methysergide as an internal standard. The detection limit of LSD in urine is 0.5 ng/mL. A blind trial confirmed the validity of the results. The choice of internal standard is discussed. Consideration is given to the photosensitivity of LSD solutions. A study of potential interferants in the HPLC-MS confirmation of LSD is presented and shows that for the wide range of compounds studied, there are none that would interfere with this confirmation technique. A comparison is shown between solid phase and immunoaffinity extraction/clean up procedures, and between RIA and EIA screening procedures.

Volatile nitrosamines from ion-exchange resins.

Abstract Deionized water was analysed for volatile nitrosamines, and was found to contain minute amounts of these compounds. Ion-exchange resins, including those used in water deionization plants, were examined and the anion exchangers were identified as the source of the nitrosamines. Gas chromatography combined with chemiluminescence and mass spectrometry was used to identify and quantify the nitrosamines. The relevance of these observations to food analysis is discussed.

THE DETERMINATION OF LYSERGIDE (LSD) IN URINE BY HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY-ISOTOPE DILUTION MASS SPECTROMETRY (IDMS)

The use of isotope dilution mass spectrometry (IDMS) has been investigated for the forensic confirmation of lysergic acid diethylamide (LSD) in urine by LC-MS. The advantages of using a deuterated analog of LSD as an internal standard over methysergide are discussed. This study includes a comparison of the electrospray mass spectra of LSD, LSD-d3 and methysergide, and discusses the choice of suitable ions for use in selected ion monitoring (SIM) mode. An IDMS method is presented for the LC-MS confirmation of LSD in urine, with a limit of quantification (LOQ) of 0.5 ng/mL, reflecting the forensic requirement at this laboratory. Under some circumstances the LOQ can be improved to 0.1 ng/mL. This method is linear in the range tested (up to 10 ng/mL LSD in urine) and has been validated in terms of accuracy and precision.

Determination of lysergide in urine by high-performance liquid chromatography combined with electrospray ionisation mass spectrometry.

A quantitative method which avoids derivatisation is described for the determination of lysergide (LSD) levels in urine. Sample preparation included addition of methysergide as an internal standard followed by solid-phase extraction. LSD was analysed on a system consisting of a C18 stationary phase and a mobile phase of 0.1 M acetate buffer pH 8.0-acetonitrile-triethylamine (75:25:0.25, v/v). LSD was detected by electrospray ionisation mass spectrometry with selected ion monitoring. The quantification limit was 0.5 ng/ml and the method was linear up to 10 ng/ml of LSD in urine.

Simple chemiluminescent detector for the screening of foodstuffs for the presence of volatile nitrosamines

The construction and subsequent evaluation of an apparatus for the detection of trace amounts of nitrosamines is described. The apparatus consists of a gas chromatograph, a catalytic chamber to generate nitric oxide from eluted nitrosamines, and a chemiluminescent detector to measure the infra-red emission resulting from interaction of this gas with ozone. Examples of the use of the system for determining the nitrosamine concentration in food extracts and other materials are given.