Br.J. Anaesth. (1979), 51, 987
A MODIFICATION OF THE MGA 200 MASS SPECTROMETER TO
ENABLE MEASUREMENT OF ANAESTHETIC GAS MIXTURES
W. O. M. DAVIS AND A. A. SPENCE
SUMMARY
The measurement of anaesthetic gas mixtures with a mass spectrometer is complicated by the
overlap of fragmentation products of the anaesthetics with the normal respiratory gas. For the MGA
200 quadrupole instrument the problem has been overcome by incorporating simple circuitry which,
for an appropriate mass number, sums the concentrations of the unwanted fragments and subtracts
them from the total output for the particular channel; this provides an accurate and stable system
(within ± 1% f.s.d.) over at least 24 h.
Mass spectrometers for gas analysis in clinical
practice are capable of measuring the partial pressures
of several gases simultaneously with a response time
of less than 100 ms and total channel scan time of
20 ms. In recent years we have had available for
respiratory investigation the MGA 200 (Centronic
Limited, Croydon), capable of accurate measurement
of nitrogen, oxygen, carbon dioxide and argon. It has
an automatic stability control mode (ASC) which
expresses the partial pressures of each gas present
(excluding water vapour) as a proportion of the total
mixture (100%). Used in this manner, it has proved
to be reliable having a stability of + 1% (f.s.d.) over
24 h. In anaesthesia, however, where the gas mixture
may include nitrous oxide and halothane, it is not
possible to use the MGA 200 in the ASC mode
because of interference caused by overlapping
spectra produced by the fragmentation of the gas
components. It is possible to set the MGA 200 to read
gas mixtures of oxygen, carbon dioxide, nitrous oxide,
argon and halothane and operate without ASC, but at
best there is a drift of approximately + 4% per hour.
We have developed a system (a spectrum overlap
eraser, SOE) to remove the interference caused by
overlapping spectra and which allows the ASC mode
to be used with anaesthetic gas mixtures with, in
addition, a notable improvement in stability. The SOE
is a four-channel, three-input per channel, system
(fig. 1) utilizing simple analog operational amplifier
computing techniques which enable it continuously
to add, subtract, multiply and divide the MGA 200
outputs, removing the unwanted spectra overlap. The
interference-free outputs are fed back to the ASC
W. O. M. DAVIS, T.ENG.(CEI), M.I.T.E.; A. A. SPENCE, M.D.,
F.F.A.R.C.S.; University Department of Anaesthesia, Western
Infirmary, Glasgow G i l 6NT.
0007-0912/79/100987-02 S01.00
I/Ps
N 2 O(30)
SOE
|O/PTRUEN 2 (28)
MGA
O/Ps
CHART RECORDER
RETURNED to
MGA ASC CCT
FIG. 1. Spectrum overlap eraser (SOE) circuitry. ASC =
automatic stability control; CCT = circuit; I/P = input;
O/P = output.
circuit, allowing it to perform to its ASC mode
specification.
METHODS
The spectrum of each gas or vapour of interest was
obtained by analysing nominal 100% gases (BOC
Medical Gas specification) in the MGA 200. It was
evident that N 2 on mass 28, O 2 on mass 32, CO 2 on
mass 44 and N 2 O on mass 30 would be affected by a
mixture containing N 2 O, CO 2 and halothane. For
example, a mixture of 10% N 2 , 36% 0%, 9% CO-j,
32% N 2 O, 13% Ar, read: 14.5% N ^ 38% O^ 30%
CO^ 32% N 2 O, 13% Ar. The N 2 output was increased by the contribution of mass 28 from CO 2 and
N 2 O; the O 2 output by mass 32 from N 2 O; CO 2
output by mass 44 from N 2 O, and so on. From the
above example it can be seen that the "sum" of the
gases measured exceeded 100% (approximately
127%). Had the MGA 200 been operated in the ASC
mode, the output would be compressed to 11.4%
N,, 29.8% Ov 23.5% CO,, 25.1% N 2 O, 10.2% Ar.
© Macmillan Journals Ltd 1979
988
As the ratio between fragmentation peaks is
constant for any one gas or vapour, for more than 24 h,
circuitry was developed into which this ratio could be
set; as more than one gas was being analysed, circuitry
to add the undesired fragmentation peaks and
subtract them from the inflated reading from the
MGA 200 enabled the erroneous reading from the
mass spectrometer to be corrected to read the true
value of the gas analysed. Therefore, in the case of N 2
measured on mass 28 in the presence of N 2 O on mass
30 (fig. 1) the N 2 O output, if fed into the potential
divider circuit set to the ratio of peak 30 to peak 28,
will give a continuous signal for mass 28 which is
interfering with the N 2 reading. If the potentiometer
output is then fed to a subtraction circuit and is
subtracted from the mass 28 output attributable to
N 2 plus the interference from N 2 O, the output signal
from the subtractor will signify the true amount of N 2
in the mixture. When N 2 is to be measured in the
presence of N 2 O mass 30, CO 2 mass 44 and halothane
mass 117, the ratio between their measured peaks and
peak 28 is set in the divider circuits, their outputs are
added in the adder circuit and subtracted from the
inflated 28 peak to read the true N 2 percentage in the
mixture.
Having abolished the spectra overlap, it is possible
with simple modification to feed the SOE outputs to
the MGA 200 ASC summing circuit, enabling it to
operate in the ASC mode.
BRITISH JOURNAL OF ANAESTHESIA
Careful calibration can produce measurement of
the components of a N 2 , O 2 , CO 2 , N 2 O, Ar and
halothane mixture to within 1% of the value for each
gas and will maintain this accuracy for more than 24 h.
The present system with its four-channel, threeinput per channel capability could, with ease, be
extended to a six-channel, six-input per channel
system capable of coping with trichloroethylene and
enflurane.
MODIFICATION DU SPECTROMETRE DE
MASSE MGA 200 PERMETTANT LA MESURE
DBS MELANGES DE GAZ ANESTHESIANTS
RESUME
La mesure des melanges de gaz anesthesiants, a l'aide d'un
spectrometre de masse, est rendue plus difficile par le
chevauchement des produits de fragmentation des anesth6siques et des gaz respiratoires normaux. On a pu surmonter
les difficulties rencontrees avec l'instrument quadripolaire
MGA 200, en y incorporant un ensemble de circuits simples
qui permet, pour un nombre de masse approprie3 d'additionner les concentrations des fragments indesirables et de
les soustraire du debit total pour la bande en question. On
obtient ainsi un systeme stable et precis (dans les limites
de ± 1 % de deviation maximale) sur une periode d'au
moins 24 h.
EINE MODIFIZIERUNG DES
MASSENSPEKTROMETERS MGA 200,
UM NARKOSEGASMISCHUNGEN MESSEN
ZU KONNEN
ZUSAMMENFASSUNG
Die Messungen von Narkosegasmischungen mittels Massenspektrometer sind kompliziert aufgrund der Uberlagerung
CALIBRATION
von Partikeln des Narkosemittels mit dem normalen
Bei dem vierpoligen Instrument MGA 200
Calibration can be achieved by two methods. The Atmungsgas.
wurde dieses Problem durch Aufhahme einer einfachen
ratio between wanted and unwanted peaks can be Schaltung uberwunden, die fur eine entsprechende Massenmeasured and dialled into the digital potentiometer zahl die Konzentrationen der unerwunschten Partikel
divider circuits. The MGA is calibrated from one or summiert und sie dann von der Gesamtleistung des
two accurate gas mixtures (BOC Gold Star). How- betreffenden Kanals subtrahiert. Dies gewahrleistet ein
genaues und stabiles System (Genauigkeit ± 1 %) tiber
ever, this method relies on the shape of the peak wenigstens 24 Stunden.
remaining constant.
In routine practice, calibration can be obtained by
using medical grade gases; for example, 100% N 2 O
will produce a wanted peak at mass 30 and unwanted
peaks at mass 28, 32 and 44. With the mass 30 peak
adjusted to read 100%, the unwanted peaks can be
reduced to zero by increasing the potentiometer
setting associated with that peak, hence abolishing the
overlap caused by N 2 O on the Ng, O 2 and CO 2
channels. Similarly, spectra overlap attributable to
O^ CO 2 and halothane is overcome. The MGA is then
switched to ASC mode and the calibration checked
with a suitably certificated gas mixture (BOC Gold
Star).
UNA MODIFICACION DEL ESPECTOMETRO
DE MASA MGA 200 PARA PERMITIR
MEDICIONES DE LAS MEZCLAS DE GASES
ANESTETICOS
SUMARIO
Se complica la medicion de mezclas de gases anesteticos con
un espect6metro de masa a raiz de la superposition de los
productos de fragmentaci6n de los anesteticos con los gases
respiratorios normales. En el instrumento cuadripolar
MGA 200, se solucion6 el problema al incorporarle un
simple sistema de circuitos que, para un numero de masa
apropiado, suma las concentraciones de los fragmentos
indeseados y los sustrae de la producci6n total del canal en
cuesti6n; esto provee un sistema exacto y estable (dentro
de± 1% f.s.d.) durante 24 h por lo menos.