Gas chromatography-mass spectrometry

Gas chromatography-mass spectrometry (GC-MS) is one of the so-called hyphenated analytical techniques. Gas chromatography separates the components of a mixture and mass spectroscopy characterizes each of the components individually. GC-MS was mainly used to identify degradation products and quantify some major products in this work.

3.5.1 Apparatus description

The GC-MS instrument was an Agilent 7890A gas chromatograph coupling with a 5975C mass spectrometer. An Agilent 7683B automatic liquid sampler was used for sample injection. GC/MSD ChemStation software was used to control the entire system and acquire and process the data.

Two different chromatographic capillary columns were tried in the GC for analyte separation, one polar column, and one non-polar column. The polar column was expected to perform better for separation of polar degradation products in samples, while a non-polar

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column was expected to perform better for non-polar products. The polar column was an Agilent Technologies J&W CAM (60 m × 0.32 mm × 1.02column, having a non-bonded base deactivated polyethylene glycol stationary phase. The non-polar capillary column was an Agilent Technologies J&W DB-5MS Ultra Inert (60 m × 0.32 mm × 1.0 2 column having a Poly(dimethylsiloxy)poly (1,4-bis(dimethylsiloxy)phenylen)siloxane stationary phase. Both of the two columns provided analyte separation, but the non-polar column provided the better separation. Thus the DB-5MS UI column was used as the analytical column for GC separation in the subsequent experiments.

3.5.2 Analysis procedure

The samples were diluted 5:1 to 10:1 with Milli-Q water in terms of the initial amine concentration and degradation time and conditions. The diluted sample was transferred to a 2 mL glass sample vial and placed into the autosampler that holds 8 samples. Click the icon

‘GCMS_online’ on the desktop of the computer to start the GC/MSD ChemStation. In this ChemStation, a sequence can be made to direct the GC-MS and analyze the samples of interested. A sequence contained all the information needed for the GC-MS to determine which method to use, which sample to inject and an identifying name of each sample.

A method determines all the operating parameters of the GC-MS system. The method used was titled ‘GCMS Amine.M’ and main parameters are given in Table 3.5. The data were automatically saved in ‘GCMS_off Data Analysis’ when a method or a sequence was run. A typical chromatogram for partially degraded AMP sample is shown in Figure 3.14. A number of degradation products can be seen except for AMP and water.

Most compounds will fragment in a unique mass pattern with a certain ionization method. This unique mass pattern can be compared to the fragment pattern of standards available in an electronic library. Using a computer matching technique, the mass fragmentation patterns of the GC-separated analytes were compared with the standards available in the National Institute of Standards and Technology (NIST) library. The library match feature provides a list of probable compounds that match the mass of the parent analyte, based on the fragmentation pattern of the parent compound. Each compound on the list of matches is assigned a probability for an exact match. In some cases, the match with high match probability does not mean that this probable compound must be the parent

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compound, since the NIST library returns many different compounds with the same mass.

Therefore, in this work, computer fitting of the mass spectrum to the mass spectra database (MS Search 2.0) was used for initial product identification. Verification of the species was subsequently performed by comparing both the mass spectra and the GC retention time of commercially available pure standards with those of the initially identified compounds.

Table 3.5Operating parameters of GC-MS.

1. Inlet

type split/splitless, in split mode

temperature 250 °C

pressure 12.8 PSI

split ratio 50:1

2. GC Oven

initial temperature 110 °C initial temperature hold time 1 min

ramp rate 5 °C/min

final temperature 220°C

final temperature hold time 10 min

Total run time 33min

3. Carrier gas

type UHP-grade helium

flow rate 1.5 mL/min

4. MSD parameters

interface temperature 250 °C

EM voltage 1200 V

quad temperature 150 °C

source temperature 230 °C

5. Injector

water washes before injection 3 times

sample washes 3 times

water washes after injection 3 times

injection volume 0.2 ȝL

syringe size 10ȝL

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For estimating relative concentration, some of identified degradation compounds were quantified in SIM mode, which means that the three main ions for each product were selectively chosen with the MS detector to enhance the sensitivity and confidence of the quantification. The calibration curves were obtained from commercial standards at different concentrations and performed before analysis of the degraded sample. Due to commercial unavailability of 4, 4-dimethyl-2-oxazolidinone, the estimation of its concentration was based on a standard with a similar chemical structure, 2-oxazolidinone.

Figure 3.14 A typical GC-MS chromatogram for partially degraded AMP sample.

3.5.3 Analysis error

As mentioned in Section 3.3.4, the GC technique has been developed and utilised for analysis of partially degraded aqueous ethanolamine samples by a number of investigators.

However, water is a difficult solvent for use with the GC method and analysis of high polarity analytes is a demanding application. The quantitative analysis of GC-MS was only employed to estimate some uncharged degradation products of AMP.

The error of GC-MS analysis was mainly due to peak area deviation and unsteady baseline. The uncertainty of quantification with GC-MS was ±10%. The error of GC-MS analysis was estimated on standard deviation of peak areas across all concentrations.

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