How can we collect samples for GC-MS-analysis?
Samples on site are collected in Nalophan sampling bags. These sampling bags are identical to the ones used for olfactometric analyses. Fur certain purposes, especially for high concentrated airflows, samples can be collected in glass sample containers (gas mouse) which are easily transported in a special protective case.
In the lab the sample bag is connected to an adsorbent (Tenax) and a pre-concentration step takes places. Transferring samples to Tenax TA out of the Nalophan sampling bag is performed within 30 hours after sampling, according the similar guide lines for analysing olfactometric samples (cf. EN131725). In certain circumstances (e.g. high temperatures, high humidity content with risk on condensation), dynamic predilution with odour free nitrogen might be necessary during sampling.
How do we get sampling materials, and how to ship them? Is special packaging required?
Sampling on site can be performed by an OLFASCAN sampling team or sampling media/material can be provided by an OLFASCAN company (e.g. by express mail).
Samples collected in Nalophan sampling bags are shipped in cardboard boxes (e.g. 4 samples of each 10 l, can be shipped in a box of 0,4 x 0,4 x 0,35 m). Glass ‘gas mouse’ sample gas containers of 500 ml are shipped in specially padded boxes.
Samples must reach to the OLFASCAN lab within approximately 24 hours after sampling to allow adsorption on Tenax within 30 hours (similar to guidelines for analysing olfactometric samples according to EN131725).
Can I send my samples adsorbed on my own tubes?
The tubes used are specific to our GC-MS system. In general it is not possible to connect other brands and types of adsorption tubes. If the time frame of 30 hours is to small, possible arrangements can be made to transfer the samples onto Tenax on site. Please consult with the lab if you want to send adsorption tube samples (e-mail: firstname.lastname@example.org).
Quality control and quality assurance of a GC-MS-analysis
Determination of new response factors (explained in more detail in FAQ: ‘how is quantification performed’) is performed monthly and after any maintenance activities. This approach, using approx 80 compounds, ensures a good correlation between the actual response of the instrument and the response factors used.
Blank samples are analysed on a regular basis to ensure the system is not in any way contaminated.
What compounds can be detected with a GC-MS-analysis?
A broad range of Volatile Organic Compounds (VOC) can be detected with a GC-MS-analysis, e.g.: hydrocarbons (aromatic, aliphatic, cyclic), alcohols, ketones, aldehydes, esters, ethers, furans, organic acids, organic sulphur compounds (dimethylsulphide, dimethyldisulphide, …, thiols), chlorinated compounds, nitrogen containing compounds (amines, nitriles, pyridine), etc.…
The detectable compounds generally have a molecular mass in the range between 30 and 200/300. Detection also depends on volatility (vapour pressure/boiling point) of the compound. Compounds which are extremely volatile (e.g. methane, ethylene, …) will not be detected, since these are not retained on the adsorption tubes. Other factors influencing the detectability are among others the stability of the compound during sampling and analysis and the weight of the molecular fragments (detection starts at a mass of m/z=30, hence lighter fragments are not detected).
Additional information on the ability of detection of specific compounds can be requested at email@example.com
Are all odorants detected by GC-MS-analysis?
Many odorants, especially those with an odour threshold above a few ppb, are detected. However, there are quite a few odorous compounds with olfactory detection thresholds well below the GC-MS detection limit, in the range between 0,1 ppt and 1 ppb. These compounds may be sensory relevant yet not instrumentally detectable. The presence of such odorants can be confirmed using a sniffing port combined with the GC separation column, in parallel with the MS. This method, called GC-Sniffing is provided in our laboratory. For more information on GC-Sniffing: firstname.lastname@example.org.
What compounds can not be detected?
Referring to the previous question, following compounds cannot be detected:
What is the detection limit of a GC-MS-analysis?
Generally speaking, 200 ml of air is transferred onto Tenax TA tubes which results in a detection limit in the order of magnitude of a few µg/m³ (ppb-level). When transferring higher volumes of air lower limits of detection could be obtained, however highly volatile compounds will breakthrough the tube and will lead to in incorrect results.
The detection limit also depends on the compound of interest. Different molecules act in different ways within the analytical system resulting in better or worse response ( ‘response factor’ is explained in FAQ: ‘how is quantification performed’). for the same amount of compound.
When very high total VOC-concentrations (e.g. up to several g/m³) are expected, detection limits will become higher (towards mg/m³) since only low volumes can be analysed (typically 1 ml or less).
On a regular basis, a calibration is performed. Each tube is conditioned and checked after each analysis. Each run of analyses is accompanied by a blank and an external standard.
How is quantification performed with a GC-MS-analysis?
The area of the peaks in the chromatogram of the GC-MS-analysis is calculated and multiplied by a ‘response factor’. This ‘response factor’ is determined by injecting a known amount of a compound, and is expressed as a number of ng per surface unit. The ratio of the injected amount of the compound and the corresponding peak area in the chromatogram is called the ‘response factor’. Such response factors are determined regularly in our laboratory for approximately 80 different compounds, all spread over the different groups of possible detectable compounds. Multiplying the peak area of a detected compound by a corresponding response factor and taking into account the volume analysed, results in the concentration of the compound in the sample.
How long does it take to get my results?
In general, the period between arrival of the samples at the OLFASCAN lab and delivery of the results is within ten working days. Delivery time of the results also will depend on the amount of samples to be analysed, lab availability, …. If special (short) delivery times are required/requested, OLFASCAN will make every possible effort to meet these requests.
How are detected compounds identified?
The identification of each present peak in the chromatogram is first performed automatically by comparing the scanned mass spectra to the ones in the NIST Library which contains over 180 000 mass spectra. Confirmation is performed manually by an experienced interpreter. This implies that the mass spectrum of each compound is interpreted manually and compared with the spectra of the compounds in our database.
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