Lecture 5 - coupling considerations Flashcards
What does each detector detect and is it destructive?
Flame ionization detector - Carbon compounds and yes
Flame photometric detector - Sulphur, P and halogenated compounds and yes
Mass spectrometry - ionized molecular fragments and yes
Nitrogen-phosphorus detector - N, P and halogenated compounds and yes
Atomic-emission detector - yes
Thermal conductivity detector - thermal conductivity and no
Electron capture device - electronegative groups and no
How does the FID work?
- Heats analyte in H flame and component in analyte loses electron. Unselective and widely used.
How does the FPD work?
- Involves burning compound in a flame. A photomultiplier tube is then used to detect lines as specific elements emit light. More specific than FID so less analytes can be used.
How does TCD work?
- Helium or H used as a standard, has a response to all compounds as they all have a different thermal conductivity from helium and H.
- There are 2 parallel tubes containing the reference gas and sample (in heating coils).
- It compares the heat loss rate from sample into reference gas, compares the changes in TC of the sample to reference flow of carrier gas.
How does MS work?
- In GC-MS packed columns are used but with flow rates of 20-30 mL min-1. To reduce large flow rates a jet separator is used.
- A vacuum is used to allow sample to be put into an inlet system, while keeping the rest of the S under vacuum.
- Not ideal for detecting tiny molecules.
- GC and MS can be combined as during GC the sample is already in a capillary tube which is needed for MS.
- Systems now contain HP pumping systems.
- Flow rates now <5 mL or less, a direct interface is possible.
- Bench top systems now can handle these easily, are more sensitive and give better preservation of GC results.
What are the detectors coupled with LC?
- Mass spec which is destructive and detects ions.
- Optical detection (can be UV/vis or fluorescent compounds) which is non-destructive.
- Refractive index which is non-destructive and detects all compounds.
How does the spectroscopic detection work?
- By emission (fluorescence) or absorbance (UV) analyte.
- For UV/vis abs is plotted against elution time (min)
- For fluorescence % flu is plotted against elution time (min)
How does refractive index work?
- A beam irradiates the cell.
- When only a MP is present the beam will be straight and wont reflect, when a sample is added in the beam will bend.
- A reflex index difference is then found.
- Has a lower sensitivity than UV and used for samples which don’t abord UV, like alcohols, sugars or inorganic ions.
How is the conductivity detector used?
- Measure the ability to pass an electric current by recording electronic resistance. Also, electrochemical measurements such as amperometry, voltammetry, coulometry
How does MS-LC work?
Samples A, B and C injected into HPLC (A is sample of interest). HPLC will separate sample A, then MS ion source will split A into its corresponding ion fragments. Then the mass analyser will sort these corresponding to mass/z ratio.
What are the problems with LC-MS?
- Large volume of MP
- Buffers and additives cause problems for MS ionisation (these must be volatile to reduce these effects)
- Large flow rates and low sample concs
- Liquid samples need to be vaporised first
How does electrospray work? and how does ACPI differ?
Analyte flows through capillary, the solution is then drawn out in a cone shape which become droplets. The droplets progressively get smaller and smaller. The droplets vaporise rapidly and only gas-phase ions remain which then go to detector
ACPI uses a UV light to separate the analyte ions from the solvent
Why are UHPLC systems now being used?
- Flow rates are 0.01 - 3 mL min-1 and up to 15000 psi.
- They have sub 2 um particles which permit faster separations, higher resolution and unmatched efficiency.
- Needs low volumes (25 mL)
- These separations can be done in 5 mins.
