Radiant energy – Ionic separation or analysis – Methods
Reexamination Certificate
2003-09-04
2004-12-28
Wells, Nikita (Department: 2881)
Radiant energy
Ionic separation or analysis
Methods
C250S281000, C250S287000, C250S288000, C250S292000
Reexamination Certificate
active
06835928
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
2. Discussion of the Prior Art
Quadrupole rod sets are known which comprise two pairs of parallel rods. Each pair of diametrically opposed rods are electrically connected to each other and to the same phase of an RF voltage supply. The RF voltage supply is arranged such that the RF voltage applied to one pair of diametrically opposed rods has a 180° phase difference with respect to the other pair of rods.
The quadrupole rod set can be operated as a mass filter to transmit ions having specific mass to charge ratios and to attenuate other ions by maintaining a DC potential difference between adjacent pairs of rods. When a DC potential difference is maintained between the pairs of rods certain ions will remain stable in the quadrupole rod set and will be transmitted from one end of the quadrupole rod set to the other. However, other ions will become unstable and hence will not be transmitted by the quadrupole rod set. The DC potential difference maintained between the rods may be arranged, for example, such that ions with mass to charge ratios outside of a narrow range are destabilised and are not transmitted. The DC potential difference can also be increased or scanned so that eventually only ions having a specific mass to charge ratio will be stable in the quadrupole rod set whilst other ions have been filtered out. A further increase in the DC voltage may result in all of the ions being destabilised such that no ions are transmitted. Accordingly, appropriate selection of the RF and DC voltages applied to the quadrupole rod set allows ions of only selected mass to charge ratios to be transmitted whilst all other ions are discarded.
The quadrupole rod set mass filter efficiently transmits ions having a specific mass to charge ratio. However, when ions having a range of mass to charge ratios are required to be recorded the RF and DC voltages applied to the quadrupole rod set must be scanned so as to successively transmit ions of one mass to charge ratio at a time. This results in the duty cycle for transmitting ions of any specific mass to charge ratio decreasing as the range of mass to charge ratios to be recorded increases. For example, if the mass range to be scanned is 500 mass units and the mass peak width at base is one mass unit, then the time spent transmitting ions having the same mass to charge ratio to within one mass to charge ratio unit is 1/1000 of the total scan time and hence the duty cycle drops to 0.1%. This is to be compared with a duty cycle of 100% when the quadrupole rod set mass filter is used to transmit ions having a single mass to charge ratio.
A further limitation of using a quadrupole rod set mass filter/mass analyser to record ions having a range of mass to charge ratios is the time taken to acquire a complete mass spectrum. Ions transmitted through a quadrupole mass filter typically have a relatively low energy, e.g. only a few eV. Therefore, the ions tend to take a relatively long period of time to travel the length of the quadrupole rod set. The length of time is dependent upon the length of the quadrupole rod set and the energy of the ions. The quadrupole rod set mass filter cannot therefore be scanned at a rate faster than the time taken for ions to travel the length of the quadrupole rod set otherwise the ions will not be allowed adequate time to be transmitted. For examples the ions may require between 0.1 ms and 1 ms to travel the length of the quadrupole rod set. Therefore, the quadrupole rod set mass filter cannot be scanned much faster than 1 ms per mass unit otherwise ions will no longer have adequate time to be transmitted. Accordingly, the minimum time required to scan 500 mass units is typically between 0.1 and 0.5 seconds.
It is apparent from the above considerations that the quadrupole rod set mass filter is suited to applications in which it is only required to record and quantify ions having a single or limited range of mass to charge ratios. A quadrupole rod set mass filter is not particularly suited to applications where it is required to record ions having a relatively wide range of mass to charge ratios with high sensitivity and at relatively high speed.
A Time of Flight mass analyser is another known mass analyser. A Time of Flight mass analyser comprises a drift or flight region and a fast ion detector. Ions entering the drift or flight region are arranged to have a constant energy and therefore separate as they travel through the drift or flight region according to their mass to charge ratio. A fast Analogue to Digital Converter (“ADC”) or a Time to Digital Converter (“TDC”) may be used to record the arrival times of the ions at the ion detector. The arrival times enable the mass to charge ratios of the ions to be calculated since the mass to charge of an ion is proportional to the square of the flight time of the ion from the entrance of the drift region to the ion detector.
A Time of Flight mass spectrometer may record a full mass spectrum for each pulse of ions leaving the ion source. If the ion source is a pulsed ion source, such as a Laser Ablation or a Matrix Assisted Laser Desorption and Ionisation (“MALDI”) ion source, then the duty cycle for recording the full mass spectrum can be 100%. If the ion source is continuous, such as an Electrospray or Electron Impact ion source, then the duty cycle is determined by the means by which the continuous beam of ions is sampled and packets of ions are injected into the drift or flight region of the Time of Flight mass analyser.
Orthogonal acceleration Time of Flight mass spectrometers typically achieve a sampling duty cycle in the range of 5-25%. The combination of a non-mass selective ion trap used in conjunction with an orthogonal acceleration Time of Flight mass spectrometer may increase the duty cycle to around 100% for ions having a specific narrow range of mass to charge ratios, whilst the duty cycle for ions outside of that range of mass to charge ratios will fall to 0%.
A Time of Flight mass spectrometer is not ideal for recording ions having a narrow range of mass to charge ratios e.g. ions having a range of only one or two mass to charge ratio units. The duty cycle and transmission of a Time of Flight mass spectrometer required to record ions having a narrow spread of only one or two mass to charge ratio units does not match that of a quadrupole rod set mass filter in a comparable situation. Furthermore, the linear dynamic range of the ion detection systems typically used in a conventional Time of Flight mass spectrometer is inferior to that used in a mass spectrometer incorporating a quadrupole rod set mass analyser. This is due to the fact that ions are recorded in very short bursts in a Time of Flight mass spectrometer whereas ions are recorded continuously in a mass spectrometer incorporating a quadrupole mass analyser.
Although Time of Flight mass spectrometers are suited to applications where it is required to acquire a full mass spectrum quickly and with high sensitivity, Time of Flight mass spectrometers are not particularly suited to applications where it is required to record and quantify ions having mass to charge ratios which differ by a few mass to charge ratio units.
SUMMARY OF THE INVENTION
It is desired to provide an improved mass spectrometer.
According to an aspect of the present invention there is provided a mass spectrometer comprising:
a multi-mode quadrupole rod set; and
an ion detector;
wherein in a first mode of operation the quadrupole rod set acts as a mass filter and wherein in a second mode of operation the quadrupole rod set forms a time of flight region of a Time of Flight mass analyser.
In the first mode of operation ions having mass to charge ratios within a first range are preferably transmitted by the quadrupole rod set and ions having mass to charge ratios outside of the first range are preferably substantially attenuated by the quadrupole rod set. AC or RF voltages are applied to the rods of t
Diederiks & Whitelaw PLC
Micromass UK Limited
Smith II Johnnie L
Wells Nikita
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