Biomedical Engineering Reference
In-Depth Information
Fig. 30
Spectrum of Ne, Ar, and air 2007 [
19
]
at the base, respectively, in good agreement with theory. Due to the maximum avail-
able signal frequency of 35 MHz the lower mass detection limit at the chosen ion
energy of 50 eV the theoretical detection limit
m/z
was about 6, as confirmed by the
carbon peak at
m/z
= 12.
Based on the simulations discussed in Sect.
3
for the succeeding chip generation
the total length of the mass separator was extended from 2.3 to 3.9 mm. This exten-
sion, for identical finger width
d
1
and the gap
d
2
, should increase the base resolution
of the mass separator R according to the theory from Sect.
3.5
to 9.5. Additionally
the maximum operating frequency of the mass separator and the ion energy were
doubled (70 MHz and 100 eV) allowing for a detection limit of
m/z
= 0.5.
According to Sect.
3.5
, this increases the detection limit by a factor of 4. In this
measurement methane was fed into the system. The resolution at the base is approx-
imately
m
,
m = 8.5, which is smaller than what was theoretically expected (9.5).
Resolution measured at FWHM is 32. Hence, the improvement in comparison to the
previous chips generation is evident. In order to push the detection limit down to
m/z
= 0.5, the finger electrodes were wired such that the spacing d
1
was arti fi cially
doubled.
Figure
31a, b
, show a measurement with the extended mass separator. The
methane spectrum in Fig.
31a
is shown with a hydrogen peak with
m
/
z
= 1.
The spectrum in Fig.
31b
was measured with a Xenon plasma, this allows to
detect the 400 ppm argon in air. The current performance of the PIMMS is
summarized in Table
4
.
