How does a gas analyser work

Chromatography can test blood alcohol, drug purity, food purity, and essential oil quality. GC may be used on either organic or inorganic analytes, but the sample must be volatile. Ideally, the components of a sample should have different boiling points. First, a liquid sample is prepared. The sample is mixed with a solvent and is injected into the gas chromatograph. Typically the sample size is small -- in the microliters range. Although the sample starts out as a liquid, it is vaporized into the gas phase.

An inert carrier gas is also flowing through the chromatograph. This gas shouldn't react with any components of the mixture. Common carrier gases include argon, helium, and sometimes hydrogen. The sample and carrier gas are heated and enter a long tube, which is typically coiled to keep the size of the chromatograph manageable. The tube may be open called tubular or capillary or filled with a divided inert support material a packed column. The tube is long to allow for a better separation of components.

At the end of the tube is the detector, which records the amount of sample hitting it. In some cases, the sample may be recovered at the end of the column, too. The signals from the detector are used to produce a graph, the chromatogram, which shows the amount of sample reaching the detector on the y-axis and generally how quickly it reached the detector on the x-axis depending on what exactly the detector detects. The chromatogram shows a series of peaks.

The size of the peaks is directly proportional to the amount of each component, although it can't be used to quantify the number of molecules in a sample. Usually, the first peak is from the inert carrier gas and the next peak is the solvent used to make the sample.

Subsequent peaks represent compounds in a mixture. In order to identify the peaks on a gas chromatogram, the graph needs to be compared to a chromatogram from a standard known mixture, to see where the peaks occur. At this point, you may be wondering why the components of the mixture separate while they are pushed along the tube.

The inside of the tube is coated with a thin layer of liquid the stationary phase. Gas or vapor in the interior of the tube the vapor phase moves along more quickly than molecules that interact with the liquid phase. Compounds that interact better with the gas phase tend to have lower boiling points are volatile and low molecular weights, while compounds that prefer the stationary phase tend to have higher boiling points or are heavier.

Other factors that affect the rate at which a compound progresses down the column called the elution time include polarity and the temperature of the column. Total cost of ownership is normally much lower for a DOAS gas analyser, at the same time as it usually produces more reliable data with less downtime for maintenance.

Legal Notice Privacy Policy. About us. News Events Career. Opsis blog. How Does a Gas Analyser Work? About the Author. All vivas related to this topic. Judging by the college model answer, the bare minimum of expected knowledge resembles the contents of the following three paragraphs: pH measurement pH is measured with a glass electrode suspended in the blood sample.

The blood sample acts a a conducting electrolyte. The potential difference across the electrode is proportional to the pH difference, and this can be measured.

The electrode is bathed in a solution which contains some sodium bicarbonate, and generates a known potential difference. The CO 2 from the blood sample diffuses across a semipermeable membrane into the bicarbonate solution, The reaction changes the pH in the electrode, which corresponds to a change in potential difference, and this is measured.

The CO 2 is then inferred from the change in pH. PaO 2 measurement PaO 2 is measured with a Clark electrode The Clark electrode measures the change in current flowing through a reaction chamber where O 2 is reduced to OH - ions by a change in voltage.

O 2 from the blood sample diffuses through a semipermeable membrane into an aqueous buffer. In the aqueous buffer it is reduced to OH - ions with the application of a potential difference mV ; this causes a current to flow between two submerged electrodes. Increasing the voltage across this system also increases the current - up to a plateau. The plateau level depends upon, and is proportional to, the concentration of oxygen.