of 252Cfin that region of the wire. Readings were taken at 0.1-mm intervals and averaged over 5 to 10 readings. The values are stored in the upper 4K words of core memory and, on completion of scanning the image, are averaged, and the percent differences are calculated. The average absorbance and its standard deviation are calculated and printed. The percent difference values are plotted by the teletype giving a visual profile of the relative californium per unit length along the wire. An example of this plot is given in Figure 1B for the autoradiographic image in Figure 1A. The quality of the image depends on the distance between the photoplate and the wire, the accuracy of positioning the plate parallel to the wire, and the radiation received during positioning of the plate. For sources containing greater than 50 pg per 25 mm, blurring of the image was excessive due to radiation received by the photoplate during positioning of the plate. Insertion of radiation absorbers increased the exposure times and minimized the blurring effect. The accuracy of the technique, estimated to be f 5 % , depends upon the reproducibility of positioning the wire relative to the photoplage (f3%) and variations in the emulsion re-
sponse at low exposure levels (62%).The electronic instability of the photomultiplier tube and the analog-to-digital converter have only a minor effect (f0.1%). The technique of computerized densitometry can be applied to interpreting any photographic image. A computer program can be written to generate a two-dimensional array of points in any desired configuration. Calculations and curve smoothing and fitting on a dedicated minicomputer provide a rapid, flexible technique that can be applied to interpreting a variety of photographic images.
LITERATURE CITED (1) Californium-252 Progress, No. 1 through 20 (1969-1976); issued serniannually by Savannah River Operations Office, ERDA. Aiken, S.C.
RECEIVEDfor review August 27,1976. Accepted November 15, 1976. The information contained in this article was developed during the course of work under Contract No. AT(072)-1 with the U.S. Energy Research and Development Administration.
Gas Chromatography/Mass Spectrometry Interface Simplified and Quantified James P. Lehman Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
The value of the combined gas chromatography/mass spectrometry technique (GC/MS) needs no elaboration here. A perusal of the table of contents of this journal demonstrates the great utility of the technique and the wide use that is being made of it. There are, however, three problems associated with the technique as it is generally used which are apparent to workers in this field. These problems are: 1)the determination of the sample enrichment in that portion of the carrier gas stream which enters the MS ion source from the separator, 2) optimization of the GC/MS system for sensitivity while retaining MS resolution, and 3) the transfer of GC methodology from the GC laboratory to the G U M S system. A technique is described which readily and reliably determines the sample enrichment seen in that portion of the carrier gas stream which enters the MS ion source from the separator. This capability permits one to evaluate separator performance and to compare separators with each other under the various conditions in which they are used. Another technique is described which evaluates MS performance as a function of carrier gas flow into the MS ion source. One can then select the optimum operating conditions for best GC/MS sensitivity. In the GC/MS interface system shown in Figure 1,the full functionality of a two-column GC is maintained. This greatly facilitates the transfer of GC methodology to the GC/MS system and reduces considerably the amount of time that the expensive and sophisticated MS is used simply as a GC detector.
EXPERIMENTAL Gas Flow Determinations. The variable effluent splitters were adjusted to the desired split ratio by measuring the He flows a t the flame ionization detectors (FID'S) and the vent a t 3 in Figure 1. The conductance of the vent a t 2 was then adjusted to match that a t 3 by adding glass wool through the tube fitting a t the Valco High Temperature 4-port switching valve (Valco Instruments, Houston, Texas 518
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
77055), 1in Figure 1, until the gas flows a t 2 and 3 were equal when the valve was turned to switch the flow from 2 to 3. The He flow (HesEp) into the separator was controlled by R s ~ p (Figure 1)which consisted of '/I6 inch 0.d. X 0.010 inch i.d. stainless steel (SS) capillary or, more conveniently, the length of 0.016-inch SS wire inserted into a 30-cm length of %6 inch 0.d. X 0.020 inch i.d. SS capillary. HesEp was determined by shutting off the make-up He supply with the valve a t 5 in Figure 1,adjusting the He flow from the column (HecoL) that was connected to the separator (column B in Figure 1)such that a positive flow was measured and recorded a t 3 (HecoL > HesEp), and then switching the column flow to 2 and measuring HecoL. HesEp is the difference of the two measurements. The He flow from the separator into the MS ion source (HeMs) was controlled by RMS(Figure 1) which was inch 0.d. X 0.020 inch or 0.030-inch i.d. SS capillary. The pressure measured a t any point in the MS analyzer tube is a measure of the mass flow through the MS. The pressure registered a t the ionization vacuum gauge (IG), 8 in Figure 1,was used in these experiments. The difference between the pressure registered with the GC/MS shut off valve, 7 in Figure 1, closed (BACKGROUND P I G )and that registered with the valve open (GC/MS PIG) was directly a measure of HeMs. Molecular Leak Calibration. The volume of the inlet system was determined by expanding a known volume of Ar into the inlet system, the initial and final pressures being determined with a McLeod gauge at the gas inlet. The conductance of the molecular leak for He was determined by introducing He into the inlet system and measuring the decreasing gas pressure as a function of time with the McLeod gauge. The IG at 8 in Figure 1 was calibrated in terms of He flow by recording the BACKGROUND PIG,introducing He to the inlet system and recording both P H and ~ the total PIGas the P H was ~ incremented. GC/MS Operating Procedure. T o prevent contamination of the interface and MS with air, a positive flow of He is maintained a t all times a t 3 in Figure 1 by adjustment of the metering valve a t 4. Differences between HecoL and HesEp are accommodated by the vent and metering valve a t 3 and 4. When HecoL > HesEp, the excess column effluent is vented a t 3 and if HesEp > HecoL, the deficit gas flow is provided by the make-up He supply controlled a t 4. The GC/MS shut off valve is opened fully and the sample is injected, the analytical column effluent being vented a t 2. When it is seen that the solvent peak has passed a t the FID, the switching valve is rotated and the column effluent is now being sent to the separator and MS.
V A R I A N I740 GC
4
7.0 E
E
6.0
I
cH
SEPARATOR G A S INLET
5.0-
M
'MS
E
ION S O U R C E
A
i
4.0-
210'
N
BATCH INLET SYSTEM
Gu 3.0-
j
2.0 UITACHI P E R K I N - E L M E R
RMU-7
1
MS
1.0
Flgure 1. The GC/MS system. Numbered components are referred to
I
.
SPLITTERS
in the text 10
30
20
50
40
HE~~~,CC/MIN
GC/iqS
,'PEAK\ / AREA
G AMiS I F\ LRE3TV - ~ - 3ASrLlUE
_
I-
"A" designates a separator supplied by R. H. Allen Co. (Boulder, Colo.); "B" a separator supplied by Perkin-Elmer (Norwalk, Conn.). "I" indicates the probable error of the mean, the 50% confidence interval. Temperatures ('(2) are those of the separator at the time N was determined
\
J, ! I " Ri'EREruCE ~ ~ _
_
_
AREA
--
~
Figure 3. Values of N for two molecular effusion separators
'
i4'CkCM
1
MIN.
Figure 2. Method of data collection for the determination of
D 40
RESULTS AND DISCUSSION The method of determining the G C N S sample yield at the MS ion source using benzoate esters described earlier ( I ) was found to be unreliable in this and other laboratories (personal communication). Yields of 100%and higher have been determined by this method. Thus a more reliable but experimentally equally simple method was developed to determine the sample enrichment, N , seen in HeMs. Naphthalene (NAPH) was chosen as a reference compound because its vapor pressure at ambient temperature is high enough for it to be introduced to the MS from the gas inlet of the MS and it is chromatographically simple and inert. The intensity of the M+ of NAPH and hence the area (REFERENCE AREA) if the M+ intensity is integrated over time when introduced to the MS from the gas inlet is a linear function of the square root of the temperature of the molecular ) the vapor pressure of the NAPH (PNAPH), leak ( T L E A Kand the latter being a function of the temperature of the NAPH (TNAPH). From the foregoing we may define the quantity REDUCED REFERENCE AREA
-
REFERENCE AREA
(1)
PNAPH ~TLEA (oc) K+ 273 I where PNAPH is from the table "Vapor Pressure, Variation with Temperature" ( 2 ) .When NAPH is introduced to the MS via the GC/MS inlet and the M+ intensity of the NAPH is integrated (PEAK AREA) by selected ion recording, we may similarly define the quantity PEAK AREA X HesEp REDUCED PEAK AREA = GramsNApH Injected x PIG (2)
30
PI G X
iM' INTENSITY ?O
IO
7
.
1.0
8
I
3.0
I
I
I
5.0
PI
I
7.0
I
I
9.0
x105
Figure 4. Plot of ion intensity and pressure data used to determine the
optimum HeMs
where
PIG= GC/MS PIG- BACKGROUND PIG
(3)
If HecoL > HesEp, the quantity GramsNApH Injected is corrected by the factor HesEp/HecoL. The quantity REDUCED PEAK AREA is a linear function of N as is the ratio REDUCED PEAK AREA REDUCED REFERENCE AREA since the denominator is an instrumental constant. For these experiments N can be defined as
D=
(4)
NAPHMs/HeMs (5) NAPHsEdHesEP from which it follows that N = 1for an effluent splitter. The instrumental constant DSPLITis determined by substituting a simple effluent splitter, constructed from a Swagelok Tee N=
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
519
and appropriate capillary tubing, for the separator and determining D. Since the value of D calculated from the data obtained for a separator (DSEP)is a linear function of N , it follows that
N = - DSEP (6) DSPLIT D is determined experimentally by simultaneously measuring PEAK AREA and REFERENCE AREA as shown in Figure 2. The combined NAPH M+ intensity arising from both the molecular leak and the GC/MS inlet is integrated on a strip chart recorder or other recording device. The NAPH solution concentration and injection volume are chosen to give a PEAK AREA approximately equal to the REFERENCE AREA integrated over a minute's time so as to reduce experimental error. By collecting data in this manner errors due to instrumental drift and electron multiplier anomalies are reduced or eliminated. DSPLITis an instrumental constant dependent only on the conductance of the molecular leak so it need be determined only once unless the molecular leak is changed a t p readily determined for any set of sepaa later date. D s ~ is rator operating conditions and reliably yields the previously elusive value N . The enrichment data for two molecular effusion separators are given in Figure 3. No attempt was made to explain the apparently anomalous behavior of the two separators as a function of separator temperature. Due to this behavior, no attempt was made to plot the data according to Fock ( 3 ) . Fock in his recent elegant theoretical treatment of the molecular effusion separator ( 3 )has shown that N is a function of the ratio (3, (7) and concluded that should be kept small for maximum enrichment. The relationship between N and P is described by the inequality
No -
