or collision cells are required. The limitation is sensitivity, which decreases with each
MS experiment, although the claimed record in an ion trap is currently MS^14.9.3.4 Nanospray and on-line tandem mass spectrometry
The sensitivity with ESI can be greatly improved with a reduction in flow rate.
Nanosprayis therefore the technique of choice for ultimate sensitivity when sample
amounts are limited. There are two ways of achieving this. Bothstaticanddynamic
nanospray techniques are widely used. Flow rates in both nanospray techniques are in
the order of tens of nm^3 min^1 , which leads to low sample consumption and low signal-
to-noise ratio.
Firstly, in static nanospray, glass needles are used with a very finely drawn out
capillary tip (coated with gold to allow the needle to be held at the correct kV
potential; see Fig. 9.4). The needles are filled with 1–2 mm^3 of sample and accurately
positioned at the entrance to the source. Closed-circuit television (CCTV) is used to
determine accurately the position of the capillary. The solution is drawn into the
source by electrostatic pressure, although a low pressure may be applied with an air-
filled syringe behind the other (open) end of the needle if necessary.
In dynamic nanospray experiments, small-diameter microbore HPLC or capillary
columns are also used to achieve separation at low flow rates. This can be combined
with astreamsplitterdevicethat can furtherreduce flow rate(Section 11.9.3). Thestream
splitter can be used to divert a percentage of the solvent flow from the pump, say 99% to
99.9% towasteand allow the remaindertopass through the column.This allows for much
more accurate flow rates since it is extremely difficult to directly and accurately pump
at 0.5 mm^3 or even 50 nm^3 min^1 with a high-pressure pump. Therefore one can use
a pump that functions more efficiently at flow rates of 50 to 500 mm^3 min^1 to pass
0.5 nm^3 min^1 or less into the micro column.
Nanospray sources are used in triple quadrupole, ion trap and hybrid MALDI
instruments.
Computer programs can be set up to perform tandem MS during the chromatographic
separation on each component as it elutes from the column, if it gives a signal above a
threshold that is set by the operator.9.3.5 Magnetic sector analyser
A magnetic sector analyser is shown diagrammatically in Fig. 9.9. The ions are
accelerated by an electric field. The electricsector acts as a kinetic energy filter and
allows only ions of a particular kinetic energy to pass, irrespective of them/z.This
greatly increases the resolution since the ions emerge from theelectrostatic ana-
lyser(ESA) with the whole range of masses but the same velocity. A given ion with
the appropriate velocity then enters the magnetic sector analyser. It will travel in a
curved trajectory in the magnetic field with a radius depending on them/zand the
velocity of the ion (the latter has already been selected). Thus only ions of a
particularm/zwill be detected at a particular magnetic field strength. The trajectory364 Mass spectrometric techniques