Analytica Chimica Ada, 104 (1979) 167-171 8 EIsevier Scientific Publishing Company, Amsterdam - Printed in The NetherIands

Short Communication

AN ION COUNTIN~MULTICHANNEL ANALYSER SYSTEM FOR NEGATIVE-ION QUADRUPOLE MASS SPECTROMETRY

TOSHMiRO FUJII

Division of Chemistry and Physics, Nationoi Institute for ~~~~ron~e~~u~ Studies. Yatabe, T’suhuba. fbaruhi 300-21 [Japan)

(Received 2nd May 1978)

Detection is an important problem in negative-ion quadrupole mass spectrometry. The negative-ion signals obtainrd from an electron multiplier have been processed with a commercially available negative-ion electrometer [ 1). but the electron multiplier and subsequent detection system must float at a high voltage; most mass spectrometers do not have this capacity. In addition the electrometer methodology does not meet sensitivity requirements sufficiently, particularly for negative-ion work in electron impact ionization mass spectrometry (m.s.).

The ioncounting technique enables very small ion currents [ 2.31 to be measured, and a digital output of the ioncounting device is easily acquired with a multichannel analyser, which gives an accumulating facility similar to photoplate recording of sector-type m-s. Combination of the ion-counting device with the multichannel analyser f 4,5] therefore facilitates the processing of relatively low ion signals in negative-ion m.s.

The present communication reports the development of the ion counting- multichannel analyser system to meet sensitivity requirements in quadrupole m.s.; the combination is applied to negative-ion work at low signal levels and me~urement of the negative-ion mass spectrum of acetonitrile given by con- ventional electron impact ionization quadrupole m-s. with the new method.

Experimen tai Negative-ion counting techniques. Figure 1 shows a block diagram of the

proposed system. The ion detector was a continuous dynode electron multi- plier (Type I-l 751, Galileo ElectroOptics Corp., Sturbridge, Mass.). The first dynode was maintained at 1000 V. The pulse output of the multiplier was capacitively coupled to the ioncounting device (Princeton Applied Research, Princeton, NJ). The O.OOl-PF condenser coupling of the output permitted the multiplier anode to be operated at the masimum potential (5 kV) required for negative-ion detection,

The ioncounting device consisted of a p&amplifier and an amplifier/dis- criminator [ 61 which discriminated against signals from background noise, e.g. photons, x-rays, or neutral molecules arriving at the detector.

Ampltfier ond dircrmun0lOr

- Pulse wqnol

Pulse stretcher .

Channel

Scanning ~ odronce Mulllchonnel

OMS stqnol * LOlJlC 0utre onolyser

- control ’ qcnerotor sto.l/s?op

L

INTERFACE UNIT

Fig. 1. Ulock diagram for ioncounting device-multichannel analyscr combination and ion detector for negative-ion quadrupole m.s.

The system was desipcd for use with a 4100-C multichannel analyser (Canberra, Meriden, Conn.) without modification. To acquire the output of the ion-counting device at the analyser, an interface unit was manufactured in this laboratory. It consisted of a pulse stretcher to adjust the pulse signal of the ionxounting device to the acceptable input of the analyser and a logic pulse generator (triggering electronics). The quadrupole m.s. scan signal (ramp voltage, O-10 V) was converted to supply logic pulses to the analyscr for channel advance and start/stop of channel advance.

The analyser was operated in the multiscaling mode; the intensity is recorded in the channel memory as a function of m/e_ Data in the channel memory were displayed in analog form on the oscilloscope (c.r.t.) of the analyser. Vertical displacement was proportional to ion count level and hori- zontal displacement was proportional to mass number. An alphanumeric indication of parameters, such as integrated counts over the channel memory and channel number, could be provided on the c.r.t. When the analyser was operated in the recurrent multiscaling mode, the periodic signals of the m.s. peaks were stored and accumulated in the memory of the analyser.

Figure 2 shows the timing diagram of the triggering logic pulses. The channel advance was automatically started by the pulse obtained from the starting signal of the periodical scan of the m.s. The stopping pulse was given by the returning signal of the scan. Both pulses were repeated throughout the duration of measurement. The pulse for channel advance was generated whcrxver a change of ca. 10 mV occurred in the ramp voltage of the m.s. scan signal _

169

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v . 1~. 2. Timing of the logic pulses for the snalyscr in the recurrent multiscaling mode

Mass spectrometer. Negative-ion mass spectra were obtained with a Finnigan 3300F quadrupole m-s., opctrated in the electron impact ionization mode at UI ion chamber potential of --2.5 V and a focusing lens potentia! of 22 V. I’he electron trap electrode was connected electrically to the ion chamber. rhe ion source power supply was modified to provide the optimum electrode roltage for negative-ion operations.

Operations. Measurement was performed as follows. The repetitive scanning If the m-s. started in the preset interval when the sample was introduced ;hrough the batch inlet. The resulting negative-ion currents, processed by the oncounting device, were stored and accumulated with the analyscr in the .ecurrent multiscaling mode until peaks of sufficient intensity were obtained. fhe accumulated mass spectral data were read out on a chart recorder.

Pesults and discussion Figure 3 shows the negative-ion mass spectrum of acetonitrile in the single

canning mode; the intensity of the peaks is indicated by ion count numbers. fhe scan time was 5 s and the scanning mass range was from 1 to 50 amu. Icetonitrile (reagent grade, Wako Chemicals, Osaka, Japan) was introduced Yom a conventional batch inlet system with a needle valve as a molecular leak .o the m-s. ion source. Sample pressure was 5 X 10m6 Torr. An electron energy >f 8 eV gave the most abundant peak at m/e 40 (CH,CN-). The emission cur- ,ent was 12 PA.

The negative-ion mass spectrum of acetonitrile obtained in the 100 times ecurrent multiscaling mode is shown in Fig. 4. The repetitive scan interval vas 5 s. Other conditions were as in the single scanning mode.

Two additional peaks, CNH- and C2H-, appeared in the accumulated mass ‘pectrum compared with the mass spectrum in the single scanning mode. The accumulating property of the analyser enabled these negative fragment ions o be observed at very low signal levels.

170

I CN- 168 counts

!I

:I

Fig. 3. Negative-ion IIILLSS spectrum of CH,CN given by quadrupole ms. in the single scanning mode.

-f ;lY-

100 times aCCunlulation /ii.

/ CM-

i f H-i

i f

ii

I

i

I

i if

-!$rl-I

Fig. 4. Negative-ion mass spectrum of CH,CN obtained in the 100 times recurrent multi- scaling mode, showing fragment ion peaks of CNH-and C,If-.

This report may stimulate further applications of this method, e.g. negativc- ion mass spectra corresponding to unknown peaks in gas chromatographic traces can be obtained after modification of the triggering electronics.

171

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