Here is an example of a syringe pump:. Chemyx Fusion X Syringe Pump. Pneumatic intensifier , which operates under constant pressure, i. Reciprocating pumps , which are an economical solution that provides a constant flow and high pressure, but can cause pulsing.
The sample injector should work within very small volumes and withstand the high pressure of the solvent. Most devices use sample injection valves instead of direct injection because the former have superior characteristics.
Other types of detectors can be of use, e. Mass spectrometry MS ionizes atoms or molecules to facilitate their separation and detection in accordance with their molecular masses and charges mass to charge ratio.
MS is used in various applications, e. Sometimes, especially with thermally labile compounds, it is possible to introduce samples directly to the spectrometer in the liquid phase.
This method is called direct infusion. In this case, ionization takes place in the condensed phase, and a syringe pump is necessary to continuously deliver the sample into the spectrometer ion source.
Syringe pumps are the most common and reliable method for direct infusions. Syringe pumps are also commonly used for delivery calibration solution and matrix addition in MS. MS is a very accurate and highly sensitive technique for both separation and detection.
LC-MS is an analytical technique that involves physical separation of target compounds or analytes followed by their mass-based detection. Although relatively new , its sensitivity, selectivity and accuracy have made it a technique of choice for detecting microgram or even nanogram quantities of a variety of analytes ranging from drug metabolites, pesticides and food adulterants, to natural product extracts.
LC brings about a physical separation of the analytes in a liquid sample or a solution of a solid sample. A few microliters of sample solution are injected into a flowing stream of a solvent, called the mobile phase. While the optimal injection volume is dependent on the experimental conditions, it is possible to inject as little as 0. When the sample solution-mobile phase mix reaches the column, its components will differentially interact with the stationary phase which remains in the column depending upon their chemical composition or physical properties.
Based on the mechanism of interaction between the analyte and the stationary phase, LC separations have been classified into different modes, such as: - Partition chromatography — based on the differing solubility and hydrophobicity of the analytes in the stationary phase as compared to the mobile phase.
Some analytes will interact more strongly with the stationary phase than others, resulting in their separation as they pass through the column. The analytes that have the least interaction with the stationary phase emerge from the column first.
As the mobile phase continues to flow through the column, the remaining analytes are flushed out sequentially, those with the strongest interactions emerging last. The time a specific analyte spends in the column is characteristic of that analyte and is called its retention time RT. The identity of a compound in a sample can be confirmed by comparing its RT with the RT of a known compound.
While this is not an accurate method of compound identification, it helps when some information about the sample is known a priori. Although a wide variety of detectors of differing technologies and sensitivities have been coupled with LC for analyzing different sample types, the mass spectrometer has emerged as a selective, sensitive and universal detector. Unlike other detectors, the LC eluent carrying the separated analytes is not allowed to flow into the mass spectrometer.
While the LC system is operated at ambient pressures, the mass spectrometer is operated under vacuum and the two are coupled through an interface. As the column eluent flows into the interface, the solvent is evaporated by applying heat and the analyte molecules are vaporized and ionized. This is a crucial step as the mass spectrometer is only capable of detecting and measuring the gas phase ions.
As the analyte ions are generated at atmospheric pressure in the interface, the process is called atmospheric pressure ionization API and the interface is known as the API source. Post-separation, the ions can be collected and detected by a variety of mass detectors , 2 of which the most common one is the electron-multiplier. When the separated ions strike the surface of the electron-multiplier a dynode , secondary electrons are released. These secondary electrons are multiplied by cascading them through a series of dynodes.
The amplified current generated by the flow of the secondary electrons is measured and correlated to the ion concentrations in the mass spectrometer at any given instant in time Figure 1. This plot displays the peak intensities of the analyte ions versus their RT. Further, each point in the chromatogram is associated with a mass spectrum.
The area of the analyte peak is used for its quantification. The mass spectrometer can be operated in two modes, a scan and b selected ion monitoring SIM. This mode is used when analyzing unknown samples or when there is no available information about the ions present in a sample.
This is the preferred mode of operation for accurate quantification of known compounds in a sample. Further improvements in sample identification and accurate quantification can be achieved by coupling two mass analyzers that are operated in series.
These configurations offer several possibilities for sample analysis. The resulting isolated product ions are then quantified with an electron multiplier. This transition of ions from the precursor to product ion also referred to as MS 2 is highly specific to the structure of the compound of interest and therefore provides a high degree of selectivity.
EAG has unparalleled experience with the operation and application of this powerful technique to quantify a wide variety of compounds in a wide variety of matrices.
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