Chromatography Forum

dear all
I am trying to understand how the quadropole works in the mass spectrometer. From what i understand there is an Rf current and an axial DC current passed through the quadropole. Could someone please explain clearly and with as few jargon as possible how the direct current filters out ions which are not of interest ??

You can go to

http://www.waters.com/WatersDivision/Co ... MNYR#quads

Regards

.The Quadrupole Mass Analyser
This, as the name suggests, consists of four rods arranged.
The opposite pairs are connected electrically and a voltage, consisting of both
radiofrequency (RF) and direct-current (DC) components, is applied, with the RF
components on the two pairs of rods being 180◦ out-of-phase. At a specific value
of these voltages, ions of a particular m/z follow a stable trajectory through the
rods and reach the detector. A mass spectrum is therefore produced by varying
the RF and DC voltages in a systematic way to bring ions of increasing or
decreasing m/z ratios to the detector.
The quadrupole analyser is an ideal detector for chromatography as it is capable
of fast scanning and uses low voltages which make it tolerant of relatively high
operating pressures, such as those encountered in LC–MS.
The quadrupole is classified as a low-resolution device, i.e. it is capable of
measuring the m/z ratio of an ion to the nearest integer value, and thus is unable
to provide the elemental composition of an ion.

hope this hepl!

Without jargon?

It's easier to visualize in only two dimensions. Think of a beam of ions moving down the analyzer. The RF "slaps" the ions from side to side. If the slap is too hard, the ion will get knocked out of the system and escape. If the slap is too soft, the ion will wander out of the beam and escape. If the slap is "just right", the ion will be knocked back into the right path.

How hard is "just right" depends on the size of the ion (heavier ions need to be slapped harder). The DC controls how hard you slap.

i know about the rf current. I was specifically looking for information about the DIRECT CURRENT (DC) !!! but thanks anyway

i know about the rf current. I was specifically looking for information about the DIRECT CURRENT (DC) !!! Mad but thanks anyway

Sorry I didn't make it clear. If you think of the constant voltage ("DC") as controlling how hard the slap is, for any given voltage, light ions get swatted out of the beam, and heavy ions aren't forced back into the beam. Intermediate ions are hit "just right".

As you change the voltage, the "just right" size range changes.

It's easier to visualize with diagrams. I've uploaded a few pages from our Bioanalytical LC-MS/MS course that covers it.

Click on the LC Resources Inc. link in the sponsors area. Once you're on our web site, click on "Resources", then on "Exchange" from the links on the left-side of the page. The file you want is "quadrupole.pdf".