Improvement of Sensitivity in Mass Spectrometry Using Silanized Quartz Probe Tips J. G. A. M. Raaymakers and D. J. C. Engel Gist-Brocades N. V. Research & Development, Haarlem. P.O. Box 523, The Netherlands
The electron impact spectra of many compounds under investigation in our laboratories show uncomfortably small molecular ion peaks. Since it is sometimes essential to study the whole spectrum-inclusive of the molecular ion peak-of such compounds, we have investigated whether or not a mass spectrometric technique could be evolved producing intenser spectra. The silanization procedure used in the Biemann separator and applied successfully ( I ) to various GC-MS techniques, was considered to offer good possibilities. We therefore silanized a number of quartz sample probes employed in the ion source of the AEI MS 902 mass spectrometer.
Table I. Benzhydryl E t h e r s Tested
EXPERIMENTAL
Compound
Each sample probe was treated in a 5% solution of dimethyldichlorosilane in toluene at room temperature for 24 hours, rinsed with ethanol and dried, first with paper tissue and then in a n oven a t 200 "C for 3 hours. A considerable number of benzhydrylethers, a group of compounds that has been under investigation in our laboratories for a long period of time, has been tested. As examples, three structures of this type are presented in Table I. Moreover some other compounds were tested in the same way, such as cholesterol, 1,4-bis(2-hydroxypropyl)-2-methylpiperazine, methyl-a-phenyl cyclopentylglycolate.
a b
H H
C
CHI
RESULTS Use of these silanized probes resulted in a considerably improved sensitivity. This appeared from an increased total ion current with a factor varying between 2-30 and much more intense mass spectra at exactly the same source temperature and settings of the instrument. The peaks of the mass spectra increased with a factor of 5-10 in most cases, using the silanized probe tip. In several cases the molecular ion was favored. These results war( 1 ) J. Throck Watson in "Ancillary Techniques of Gas Chromatography." L. S Ettre and W. H. McFadden, E d . , Wiley Interscience, New Y o r k . N . Y , 1969, p 267.
RI
R2
W.H.O. name
H CH, CH,
tofenacine orphenadrine
rant the assumption that adsorption on the untreated quartz surface is responsible for the diminished evaporation. Silanization improves the evaporation of small samples considerably. Viscous liquids which form films on the untreated quartz surface, seem to show this effect more pronouncedly than crystalline samples. In addition, silanization may decrease pyrolysis of compounds, which decompose rather quickly at higher temperatures. More systematic investigations will be necessary to evaluate all aspects of these phenomena.
ACKNOWLEDGMENT The authors thank J . Bongaardt and G. A . de Vogel for experimental assistance. Received for review November 12,1973. Accepted March 14, 1974.
Increasing Gas Chromatograph/Mass Spectrometer Usage by Interfacing a Second Gas Chromatograph
J. P. Mieure, G. W. Mappes, and M. W. Dietrich Monsanto Company, 800
N. Lindbergh Blvd., St. Louis, Mo. 63166
Many analyses by gas chromatography/mass spectrometry (GC/MS) require considerable GC method development prior to MS analysis. The MS frequently remains idle during these periods. Initially, we combined the activities of method development and analysis on the GC/ MS. This does not provide effective utilization of available MS time. We next tried developing the method on a GC not connected to the MS, then transferring the column and conditions to the GC/MS. This reduced idle
time of the MS, but analyst time and convenience were sacrificed when transferring the method. This inefficient use of the costly M S can be minimized by interfacing a second GC to the MS. The effluent from either GC can be directed into the MS by opening a miniature valve. With this system, GC method development can be carried out on either GC, completely isolated from the MS. Simultaneously, the M S can be used for other analyses. Once the method development is completed, the
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
1357
5 PL InlER SEPARATOR
Figure 1. Block
diagram 0
effluent is directed into the M S for analysis. The operation of the other GC is unaffected by these manipulations.
2
TIME (MINI
1358
I
I
EXPERIMENTAL The M S used in our laboratory is a n Atlas MAT CH4B with a fritted glass molecular separator. The GC's connected to the M S are Hewlett-Packard 7620 and 5750 (Hewlett-Packard Co., Avondale, Pa.). However, we believe this technique can be used with most GC/separator/MS combinations. The experimental arrangement is shown in block diagram form in Figure 1. The two GC's are connected to the MS through a Tpattern micro-capillary valve (V2) (Precision Sampling Corp., Baton Rouge, La.) with Ih6-h. X 0.020-in. inlet and outlet lines. This valve is constructed so that only the line to GC2 can be closed. The line from GCI t o the M S is always open. VI isolates GC1 from the remainder of the system. With VI and Vz closed, no effluent enters the MS. Opening either VI or V2 admits effluent from GC1 or GC2, respectively. V3 is used to adjust the flow into the molecular separator and MS. This is important if wide differences in GC flow rates are used, as when using capillary and %-in. o.d. columns. V3 is normally preset and needs adjustment only when making a large change in flow rate. V1 and V3 are L-pattern micro-capillary valves from Precision Sampling Corp., with Ihe-in. X 0.020-in. inlet and outlet lines. Each GC is equipped with an effluent splitter (Hewlett-Packard No. 19034A) connected to both the M S system and the regular GC detector. When either GC is isolated from the M S system, all the column effluent of the isolated GC is directed into the detector as in normal GC operation. When either V1 or Vp is open, column effluent from the corresponding GC is split between the detector and the MS. The split ratio is determined by the flow properties downstream from the splitter. The desired flow range is obtained by utilizing proper lengths of connecting tubing. Precise control of the flow needed for a given application is provided by Vs. We have found that 8-in. X 0.020-in. i.d. stainless steel tubing provides a satisfactory resistance for connecting the splitter and detector. The M S side of the splitter has about 18 inches of the same tubing connecting the splitter, valves, and MS molecular separator operating at reduced pressure. With this arrangement, V3 can be adjusted to give a split ratio of about 5 to 1 ( M S to Torr range. GC) a t an ion source pressure in the All vacuum connections were made with lhe-in. Cajon VCR vacuum couplings (Cajon Co., Solon, Ohio). Connecting lines and valves were wrapped with heating tape and insulated. The system is heated a t temperatures up to 300 "C with variable transformers. Figure 2 shows a flame ionization detector (FID) chromatogram and total ion monitor (TIM) plots of a mixture to demonstrate
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,' ; J .i I.
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Trichloropropene chromatograms demonstrating sys-
tem performance the performance of the system. One microliter of a 10% solution of trichloropropenes was chromatographed isothermally at 140 "C using a %-in. 0.d. X 6-ft stainless steel column packed with 5% OV-11 on 80-100 mesh H.P. Chromosorb W. The TIM plots (chromatograms reconstructed by a Varian Spectro System 100) are virtually identical, regardless of which GC was used as the inlet.
DISCUSSION This system has permitted us to more effectively use available MS time. For example, an extensive study of polymer systems was recently completed using pyrolysis/ GC/MS without interrupting our normal sample load. The latter were analyzed on one GC between pyrolysis/ GC/MS experiments carried out on the other GC. The addition of the micro-capillary valves has not decreased resolution or sensitivity to a detectable extent. Having two GC's coupled to the MS has also permitted the inclusion of more capability (flame ionization, flame photometric, and thermal conductivity detectors, subambient programming) than might be practical with a single GC. An additional advantage is the fact that the GC/MS system is still usable when one of the GC's requires maintenance. The valve configuration is also very convenient for venting solvent or other unwanted components. Since flow into the MS is preset using V3, VI and Va function merely as on/off valves. Hence, effluent can be rapidly switched into or out of the MS; the valve manipulation requires only a few tenths of a second. Opening both VI and Vz permits effluent from both GC's to enter the MS simultaneously, but to date we have found no practical application for this capability. Received for review November 12, 1973. Accepted March 14, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 9 , AUGUST 1974
