Quadrupole mass spectrometer

a type of an analyzer used in mass spectrometer. separates the ions based on mass to charge ratios. useful for the detection of ions present in the sample

1. QUADRUPOLE MASS ANALYZER

2. QUADRUPOLE MASS ANALYZER (QMA) The QMA is one type of mass analyser which is used in mass spectroscopy. It is also called as transmission QMA, quadrupole mass filter. It consists of four cylindrical rods which are parallel to each other. Here electric field is applied to the rods.

3. PRINCIPLE It consists of four metal rods which are parallel to each other. The opposing rod pair is connected together electrically. A R.F voltage and a D.C voltage is applied. Ions will travel through the quadrupole. Only ions of certain m/z ratio will reach the detector

4. WORKING Ions from the ion source will make their way to the quadrupole mass filter and finally reach the detector. The main functions of QMA is to separate the particles based on m/z ratio Now, a mixture of ions comes from the ion source. This mixture will have positive, negative, small and large particles. For instance let us consider the pair of positive electrodes first Now let us consider the positive charged particles are approaching the positive electrode.

5. WORKING Let us now consider the large size particle first. A positive D.C current is applied to the pair of electrodes. When they approach the positive electrode, they get repelled They just make the way out of the filter.This is not how a filter works An A.C current is applied to the electrode. This A.C will have both positive and negative phase. When positive phase exists, it will make the electrode positive Thus ions get repelled.

6. WORKING But when negative phase exists, it will make the electrode negative. Thus the large particles will get attracted. But when positive phase again exists, they repel and make their way out of the filter. But the small particles, since they have small mass,they will be attracted quickly than large particles. but when positive phase exists, they get repelled. by the time they get repelled, the negative phase exists and they get attracted and they finally crash on the electrode. hence large particles reach the detector first. This is called as high pass filter.

7. WORKING Now let us consider the negative electrode. A D.C current of negative is applied to the electrode. Taking again, the positive charged particles into consideration,we see that they get attracted towards the electrode and they crash. This is not a filter. An A.C current is applied to the electrode. The large particles will get attracted towards negative electrode. By the time it gets attracted, the electrode will become positive.

8. WORKING since the large particles have greater mass, they will have to give greater force to get repelled away from the electrode. By the time it is getting repelled, the electrode will turn into positive one and the large ions are attracted and they crash. But the small size particles will never get attracted towrads the electrode and they just get out of the filter. Thus the small ions alone will make their way out of the filter. This is called low pass filter. Since the quadrupole has both charge electrodes, they act as low and high pass filter.i.e double mass filter.

9. ADVANTAGES compact and reliable. low cost analytical tool Determine the molecular weight in many compounds. To identify the structure of many compounds.

10. DISADVANTAGES It is a low resolution mass analyzer. Only one m/z ratio value can be obtained at a time. Broader m/z ratios values can be obtained only after long scanning periods. Not suited for pulsed ionisation method.

11. APPLICATIONS Compositional analysis of gases and volatile liquids. Mainly used in residual gas analysis. Used in liquid chromatography- mass spectrometry and gas chromatography- mass spectrometry Used in both qualitative and quantitative analyses.