Photographic Quantitation in Spark Source Mass Spectrography Using an On-Line Densitometer and Ion Intensity Areas R . A. Burdo,’ J. R. Roth, and G. H. Morrison2 Department of Chemistry, Cornell University, Ithaca, N. Y . 74850
A microdensitometer is interfaced with a PDP-11/20 computer. The open-loop system acquires and stores digital transmittance profiles. Intensity profiles are obtained by the point-by-point transformation of transmittance profiles according to the Hull equation. An emulsion calibration method is developed which is applicable to peakheight, line-width, or integrated intensities. Elemental concentrations are automatically and simultaneously calculated by two methods using any of three intensity types. The effectiveness of the system is tested in the determination of isotope ratios by various intensity methods. The use of peak-height and integrated intensities is compared in terms of precision and accuracy for external standards, and precision for internal standards.
Spark source mass spectrometry (SSMS) is an invaluable technique for the analysis of trace elements in a wide variety of synthetic and natural materials. However, precision and accuracy in photographic detection methods can be degraded by failure to account for variations in spectral line width and shape. Although methods for linewidth corrections have been suggested in the literature (1-51, such corrections must necessarily assume a constant line shape. Because line shape can vary throughout a given spectrum (6) and because statistical emulsion variations can introduce significant error in the measurement of line width, a number of investigators have developed computer-oriented systems (7-11) for the derivation and integration of ion intensity profiles. Ion intensity profiles are obtained from experimentally-recorded transmittance profiles according to a calibration curve or photographic response function. It is the area under the ion intensity profile, not t h a t under the transmittance profile, which is proportional to the number of ions striking that portion of the photoplate. The computer system described here has several advantages over some of the systems already reported (7-9). 1 Present address, Department of Chemistry, University of Rhode Island, Kingston, R.I. 02881. Author to whom reprint requests should be sent.
F. DeGreve and D Champetier de Ribes Int. J. Mass Spectrom. /on Physics, 4, 1 2 5 (1970) E. B. Owens and N.A . Giardino, A n a / . Chem., 35, 1172 (1963). M . Menetrier. Advan. Mass Spectrom., 4, 465 (1968) A Cornu. Thirteenth A n n . Conf. Mass Spectrom.. St Louis. Mo., 1965, p 496. G . G . Cookson. W . Fletcher. and R . Tushingham, Thirteenth A n n . Conf. Mass Spectrom.. St. Louis, Mo., 1965, p 49. C. W . H u l l , Tenth A n n . Conf Mass Spectrorn.. New Orleans, La.. 1962, p 404. P. J . Paulsen and P. E Branch, Fourteenth A n n . C o d Mass Spec-
trom., Dallas, Texas. 1966, p 290. D . M Desiderio. Jr . and T . E. Mead. Anal. Chem., 40, 2090 (1968). R . W . Bonharn and J 0. Humphries, Sixteenth A n n Conf. Mass Spectrom , Pittsburgh. Pa.. 1968, p 281. J. R. Woolston, W . L Harrington, and R . E. Honig. Sixteenth A n n . Conf. MassSpectrom.. Pittsburgh, Pa., 1968, p 274. P. R . Kennicott. Fourteenth A n n . Conf. Mass Spectrom., Dallas, Texas, 1966, p 278.
First, knowledge of the calibration curve or photographic response constant is not required a t the time of profile acquisition. Second, permanent and intermediate data storage is provided so that manual determinations of calibration curves and concentrations can be eliminated. Third, there are no “preset” threshold levels which can lead to inaccurate determination of the background and to improper profile integration limits. Background and threshold levels are independently determined for each profile according to background measurements which are taken in the immediate regions adjacent to the profile and which are acquired on the same scan as the profile measurements. Since data are stored on a disk or magnetic tape, the slow punching rate of a paper-tape system (10) is avoided. The system described by Kennicott (11) possesses the several advantages mentioned above and is stated to provide analytical accuracies of about f 15% and precisions in the order of 10-20%. The distinction between Kennicott’s system and the system described herein is based on differences in methods of data acquisition, approaches and functions used for calibration, and in other capabilities described later. In addition, a more complete quantitative study and a broader scope of application is presented here. The authors’ system has been routinely used in this laboratory for the past year and requires less operator time than that previously spent for peak-height analysis only.
EXPERIMENTAL Apparatus. Photoplates are exposed to ions using a Nuclide Graf 2.1 mass spectrograph. Photoplate d a t a are then acquired and analyzed by the open-loop, densitometer-computer system illustrated in Figure 1. The output current of the densitometer’s analytical photomultiplier is converted to a n amplified voltage by a Keithley 301 electrometer amplifier which feeds a Philbrick/ Nexus QFT-5 operational amplifier. A battery-operated potentiometer circuit supplies an offset voltage to the second input terminal of the QFT-5 thereby allowing the output voltage of the QFT-5 to be adjusted t o zero when no light is transmitted to the photomultiplier. The photomultiplier sensitivity is adjusted so that the QFT-5 output is 10 volts when the transmittance of the photoplate is defined a s 10070,t h a t is, a t the most transparent region of the photoplate. The combination of the voltage-to-frequency converter and the electronic counter (Figure 1) operates as a n A-to-D converter. Digital voltage measurements are made available to a Digital Equipment Corporation PDP-11/20 computer a t the counter conversion rate of 10 per second, each measurement corresponding to a n accumulation of frequency pulses a t the counter for a period of 100 milliseconds. After a n hour of warmup for the entire system, the maximum error in a digital measurement caused by both instantaneous fluctuation and drift in the system corresponds to an error of *0.2% in transmittance a t any transmittance from 0 to 100%. Transmittances measured by the digital system agreed within 1% transmittance with those measured by normal (analog) densitometer operation. In actual operation, the densitometer scan rate is adjusted in the range of 1.0-2.5 mm/min so that the narrowest profile to be scanned produces a t least 40 measurements (points) along its ANALYTICAL CHEMISTRY, VOL. 46, NO. 6 , MAY 1974
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701
DIGITAL POP-I1 COMPUTER
- ----
Find Mass, Abundonce Code
Figure 1. On-line d e n s i t o m e t e r s y s t e m
I
base plus at least 50 background points on each immediate side of the profile. The actual plate travel per measurement lies in the range 1.67-4.16 micrometers depending on the scan rate which is held constant for all profiles on a given photoplate. Velocity fluctuations in the scan rate produced a relative imprecision of about 2% in the area under any given transmittance profile. A list of additional instrumental operating conditions is given in Table I. Reagents. A solution-doped graphite matrix containing the nominal concentrations of 32, 166, 224, and 162 ppmw of Cu, Cd, Nd, and Re, respectively, was prepared according to a special freeze-drying technique. A standard solution containing the appropriate ratios of the four elements was added to a high-purity graphite matrix contained in a Teflon beaker which was halfimmersed in a low-power ultrasonic bath. Sufficient wetting agent (ethanol) and standard solution was added so as to produce a consistent slurry which was immediately frozen by insertion of the beaker into liquid nitrogen. The frozen slurry was freeze-dried in the same beaker, transferred to a plastic vial, and blended for 1 hour with a Spex Industries Mixer/Mill. Eight sets of disk-type electrodes were pressed from the resulting graphite mix. Half of these sets were designated as standard and the remaining half as sample electrodes. A second graphite matrix containing 182 ppmw of indium was prepared as above and mixed with an equal weight of previously prepared (12) USGS W-1 Standard Rock. Three electrode pairs were pressed. Procedure. The analysis of mass spectral line profiles on photoplates is divided into three main sections: the acquisition of digital transmittance profiles, calibration of the emulsion, and calculation of elemental concentrations.
Table I. Experimental Operating Conditions Spectrometer conditions
Magnetic analyzer vacuum Electrostatic analyzer vacuum Source v a c u u m G a p voltage Electrostatic potential Accelerating p o t e n t i a l Pulse frequency Pulse duration Proportionality Offset P r i m a r y slit w i d t h Mass ranges covered Detector
~ 1 0 - 8T o r r -10 -8 T o r r -10 -7 T o r r -60 k V 1800 V -17 kV 100 Hz 100 Wsec 10.00 5.30 0.001 inch 6-190 or 10-250 Ilford Q2 P h o t o p l a t e
Darkroom conditions
Developer S t o p bath Fixer
Wash Dry
Modified Ilford ID-13 p r e p a r e d fresh for e a c h use. D e v e l o p for 3 m i n u t e s at 20 "C. K o d a k F o r m u l a SB-la Acetic Acid S t o p Bath for 10 seconds at 20 "C Eastman R a p i d F i x e r w i t h h a r d e n e r for 1 m i n u t e at 20 "C. F i l t e r e d t a p w a t e r for 5 m i n u t e s at 20 "C. Warm a i r for 3 m i n u t e s .
Densitometer conditions
Slit w i d t h Slit h e i g h t
3 micrometers 0 . 5 millimeters
(12) G . H. Morrison and A . M. Rolhenberg, Anal. Chem., 44, 515(19721
702
Acquire
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1974
t Profile 1
t
I
Check Doto
t
Write Identity EL Check Results
t t t
[ w r i t e Parameters
1
Check Plot Control
Autoplot]
I
-
t
1 Redetermine
Profile with new controls or determine the next pro-
1
Figure 2. F l o w c h a r t of a c q u i s i t i o n p r o g r a m The procedure by which digital transmittance profiles are acquired is diagrammed in the flow chart of the computer acquisition program (Figure 2). A profile scan is initiated from the densitometer-Teletype console by the user. The scan is completed when the computer has acquired a predetermined number of digital transmittance measurements (points or channels). Upon computer acknowledgment of scan completion and printing of certain characteristic profile parameters via the Teletype, a number of user options can be exercised. These include the initiation of a new scan, disk storage of the acquired profile, plotting of the profile, etc. The user not only selects the profiles to be scanned but also determines those to be stored. Thus, spurious profiles due to emulsion imperfections or dust spots are never written on the storage file. Profiles of the same ion are separately scanned in order of their exposure index (spectrum number) in a graded series of exposures. The user identifies only the first profile in a given ion exposure series. Subsequent profiles are automatically identified by the computer. If a digital transmittance profile is stored, its identifying and characteristic parameters are included in the stored data. Thus, the storage file is well-structured so that any given profile can be identified and retrieved at a later time. The manner in which characteristic profile parameters--e.g., background and threshold levels-are determined is important and is explained with reference to Figure 3. B,M , and E are the channels or point index numbers ( i = 1+V) corresponding to the start of the profile, the peak-height channel, and the end of the profile, respectively. Channel B is initially defined as 1 and first derivatives of successive transmittance values CT, - T,+1)are taken until a positive derivative is found. If (i - B)is less than a minimum expected half-base width ( W ) defined at the start of the program, then channel B is redefined as channel i and the derivative search continues. Otherwise. M = ( i - l ) and E becomes the first of two channels yielding a negative transmittance derivative where E must be greater than ( M + W). Thus, channels 4-7, 13-17, 23-27, 32-35, and 42-47 all become defined as B before M is found. The results of the first derivative approach ( B = 47, E = 104) are only approximate because B and E are likely to occur for transmittances somewhat above the true background (Figure 3) or somewhat below it in the case of a n anomalous shoulder on the profile (not shown). Therefore. a n approximate background level is determined by averaging the transmittances
from channels 1 to B and E to N . The initial threshold level is obtained by subtracting the standard deviation of the average background from the average background. Channels B and E are adjusted so that their transmittances fall just below or a t the initial threshold level. The adjusted values of B and E can then he used to recompute the final background and threshold levels from which the final values of B and E are obtained as before. The above procedure correctly determines about 95% of the profiles scanned. Since profile parameters are immediately printed following profile acquisition. anomalous behavior is easily detected by the user who can manually override the computer determinations (see Options in Figure 2 ) . Replicate scans of the same profile have shown that the base width ( E - B ) varies by 1-3 channels. the integrated transmittance area by about 270 relative. and the average background transmittance by less than 0.570 in per cent transmittance. At least part of this imprecision can he attributed to fluctuations in densitometer scan speed. Following data acquisition. the stored and characterized transmittance profiles are treated for calibration and concentration determinations according to the Hull equation ( 6 ) which can he written in the form ( I , 13, 14)
where for a given isotope j of element x a t the peak height p of the line, K,, is a constant directly proportional to the concentration of the element in the entire ion beam striking the photoplate and to the plate sensitivity. A is the isotopic abundance. m is an exponent representing the dependence of plate sensitivity on mass M, E is the beam monitor exposure in nanocoulombs. T,] is the peak-height transmittance in per cent, T. is the saturation transmittance in per cent, R is the maximum slope of the plot of log ((100 - T,,/(T, - T , ) ) L'S. log (A,,,E,M,,,m) and Ii, designates the peak-height (ion) intensity which is the expression between equals signs. K,, differs from the quoted references ( I , 13, 14) by the factor M m which should be included if KPi is to be valid for a n element independent of the isotope used. If T , in Equation 1 is the transmittance T , a t any distance x along the base of the profile. then the integrated intensity (intensity area) relationship can he written as
where K,, is an areal ( a ) constant proportional to the concentration of' ,y and to the plate sensitivity, I x is the intensity at any point x along the profile base having width I, and where the profile base is divided into (n)discrete intervals (i) of equal width ( u , ) so that the integrated intensity becomes the sum of individual intensity contributions ( w I I ) when (n)is large. I , is computed from the average transmittance ( T , )of the interval. By subtracting the average background intensity ( I * ) from each profile intensity value. the background-compensated forms of Equations 1 and 2 are obtained:
K , , A , M "'E = ( I , - I , )
(3)
and
where I* = ((100 - Tj,)/(Tb - T , ) ) I and Tb is the average background transmittance in per cent determined in regions adjacent to the line. If the shape of the ion intensity profile is known. then the integrated intensity can he determined from the width of the intensity profile at half-height ( W d ) and from the peak-height intensity. For a Gaussian profile. it can be shown (1.5)that
where Wt is the width of the transmittance profile a t half-height. I , = ((100 - T , ) / ( T , - T \ ) ) l H > , and Tu is the per cent trans(13) J Rogers Woolston. Thirteenth Ann Conf. Mass S p e c t r o n , St Louis, Mo , 1965. p 79 (14) J. Rogers Woolston, RCA Rev. 539 (Dec. 1965) (151 R . A . Burdo. Ph D. Thesis. Cornell University, lthaca N Y , 1973
'. Half-
6ol
,
.7-' width ----bo8f-telght ~ e r e l
50
75
0
I50
C h a n n e l Number
Figure 3. Idealized digital transmittance profile
+
mittance a t half-height ( T , Tb)/2 on the transmittance profile. Equations 5 and 6 are background-compensated. Since the value of R is initially unknown, it is determined by a n iterative procedure in which an initial value of R is assumed ( R = 1.00) and a least-squares fitted plot of log intensity i's. log ( A M m E )is made for a graded series of exposures of the same ion. or for several isotopes of the same element on the same exposure. The intensity can he peak-height. integrated. or line-width derived according to the right-hand sides of Equations :3. ?. and 6. respectively, which are all background-compensated If the correct value of R is chosen, then the log-log plot should ideally have a slope of 1.00. If the least-squares plot deviates from a slope of 1.00, then the sign and magnitude of the deviation directly indicates the change in the chosen value of R which is required to approach the ideal plot. In this way. R is adjusted and interim loglog plots are produced until the consecutive adjustments of R generate a plot slope in the range 1.00 f 0.05. or the numher of interim plots exceeds three. or the interim-adjusted value of R falls outside the acceptable range of 0.8 to 1.5, The determined value of R is considered to be that adjusted value leading to any one of the three conditions mentioned. The calibration program employed here is capable of automatically determining R using any one of the intensity expressions mentioned (Equations 3. 4. 6) for least-squares plots involving either a graded series of exposures for a given ion or isotopes of the same element on the same exposure. The values of R obtained for a number of elements are averaged to produce a single value which is used for all later concentration calculations. The calibration program requires the user to supply only the beam-monitor values E and the value of T , for a given mass position. The computer calculates all other T. values assuming that T.varies directly and approximately with the fifth root of the maqs for singly-charged ions. The computed T , value for a given mass p o sition is divided by the charge of the ion. The dependence of T , on ion mass and charge is nor generally known. hut for the specific ion energies, emulsion type. and developing conditions used in this study, the dependence was empirically determined to tollow the approximate relationships stated above. The approximat ion of T , causes no significant intensity error if T,, is higher than T. by about 10'7%in per cent transmittance. Once the average value of R is computed. conrentrations ,are automatically calculated according to the equation
c, = C,(K,iK.)(i;M~~.I;,/Mlt'~J(S
17)
where ? refers to the analyte element, x refers to the htandard or reference element, C is the concentration in piimw. MU' is i.he molecular weight. and S is the sensitivity coefficient. K , and K . are found from Equations 3. 4. or 6 depending on the intensity type of interest ( K , = K,, or K,, for analyte. K . = K , or K,,, for standard). When analyte and standard lines appear simultaneousl>- on each of several exposures. the "intensity ratio" K , , ' K . is computed for each exposure and the average of these ratios. K , j ' K . , is substituted for K , , / K , in Equation 7 to produce the average concentration, C,, which is independent of E ( E , = E . ) , Whether or not the analyte and standard lines appear on the same exposure. the analyte concentration can still be calculated by averaging I he analyte constants from several exposurer (I(,)and taking i-he Substiratio to a similar average for the standard element tuting K , , / K , in Equation 7 yields the concentration C, which is affected by errors in the beam monitor exposure values. The com-
(K,).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
NO. 6.
M A Y 1974
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~
T a b l e 11. C o m p a r i s o n of Precision and Accuracy of Normalized Isotope Ratios for Different I n t e n s i t y Expressions
% % % %
RSD Error Cum. error Cum. error
No. of ratios
Integrated intensity
Gaussian intensity
Peak-height intensity
670 670 670 74
7 7 6.1 0.0 1.0
12.5 9.6 0.9 3.1
8.3 6.5 0.4 1.6
puter program used here calculates both c, and c, in order that a concentration value be obtained even if analyte and standard lines do not appear on the same exposure. One additional capability of the concentration program is that all designated isotopes of the standard element are considered when concentrations are computed. If only one standard isotope were considered, then some K y / K s ratios for E , = E,7 could not be obtained because the one standard isotope chosen might not produce a measurable line in all exposures. To avoid this problem, a standard element is chosen such that in each spectrum over a wide exposure range, a measurable line will be produced by at least one isotope of that element.
RESULTS AND DISCUSSION In order to determine “normalized” isotope ratios, the four standard electrode pairs containing Cu, Cd, Nd, and Re (see Reagents) were sparked such that each pair produced a series of 11 graded exposures on the upper half of each of two photoplates. thus producing eight half-exposed photoplates. Four of these were run a t low mass dispersion and the other four a t higher mass dispersion. The four sample electrodes were run in the same manner except that their exposures appeared on the bottom half of each photoplate for the additional purpose of analysis by external standards. If two isotopes of the same element appearing on the same exposure are designated y and s, then the normalized isotope ratio (NIR) is obtained by rearrangement of Equation 7 yielding:
‘VIR
= C,/C,
=
K,/K,
(8:
where MW cancels and the sensitivity coefficients are assumed to be equal for isotopes of the same element. The NIR should be unity-that is, the concentration of a given element should be the same regardless of the isotope used for calculation. NIR’s are a good measure of the precision and accuracy of intensity calculations because: 1) they are not affected by sample heterogeneity, 2) fluctuations in sparking conditions tend not to discriminate between isotopes of the same element, 3) instrumental ion transmission should not vary greatly over the narrow mass range covered by isotopes of the same element, and 4) errors in the beam-monitor value ( E ) cancel when E is the same for both lines. NIR’s were computed for nine different isotope pairs appearing on the eight photoplates described above. These pairs are 65/63 Cuc, 111-,113- and 114/112 Cd-, 142 and 143/146 Nd+,187/185 Re+, and 142 and 145/143 N d Z + . Table I1 lists the results for peak-height, integrated, and line-width intensities. The same value of R and the same input data were used to compute each intensity type. Each isotope pair produced about 74 ratios on the average for the eight photoplates. The nine isotope pairs produced a total of 670 ratios. The % RSD is the per cent relative standard deviation for the number of ratios indicated, the 70Error is the deviation of a single ratio from unity on the average, and the ’7c Cumulative Error is the deviation of the mean of the number of ratios indicated from unity. The results of Table I1 are decidedly unfavorable for the 704
~
_
~
_
_
~
~
Table 111. Precision and Accuracy of External Standard C o n c e n t r a t i o n Ratios by I n t e g r a t e d and Peak-Height Intensities
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 6, M A Y 1974
Element
Mean ratio (1nteg.j (Height) 70 RSD (Integ.) (Height) % Error i1nteg.j (Height)
Cu
Cd
Nd
1.14 0.93 19.3 22.6 14. 7.
1.02 0.82 16.6 37.8 2. 18.
0.88 0.73 13.6 26.0 12. 27.
Re
Aver age ofrows
0.92 0.99 0 . 7 6 0.81 17.4 16.7 32.9 29.8 8. 9. 24. 19.
’
Gaussian line-width approximation. A single ratio by this method deviates from the known unit ratio by 9.6% on the average and even after 670 ratios are averaged, the mean still deviate.s from unity by 0.9?&relative. Because the intensity profiles were not usually Gaussian nor any other simple shape and because line shape did vary significantly over the photoplates, the use of line-width approximations was abandoned for further quantitation. On the other hand, the results for peak-height and integrated intensities are much better with integrated intensities providing slightly but not significantly better precision and accuracy over peak-height intensities. The fact that integrated intensity does not prove inferior t o peakheight intensity is important because it is entirely possible that scanning errors, errors in the determination of B or E, errors in the background determination, and failure to acquire a sufficient number of channels could discriminate decidedly against integrated intensities. The integrated intensity accuracy reported here is considerably better than the 1.26 probable error factor reported by Desiderio and Mead (8) and the greater than 10% cumulative error (based on 10 measurements) reported by Bonham and Humphries (9). The eight photoplates used to obtain NIR‘s are also suitable for external standard analyses (16-18) as a series of 11 standard exposures appear on the top half of each photoplate and a series of 11 sample exposures appear on the bottom half. Approximately six of the 11 exposures for the standard and sample electrodes produced readable lines for the elements Cu. Cd, Nd? and Re on each photoplate. For each plate, the average constants R, and R , were computed using integrated intensities and then peak-height intensities for given isotopes of each of the four elements. According to Equation 7 the concentration ratio (C,./C,) is simply equal to K,./K. (or K,/Kg when several lines are available) for external standards. The other factors in the equation cancel when the standard and sample isotopes and matrices are the same. Table I11 lists the mean concentration ratio for eight analyses of the element specified where the mean ratio is based on integrated intensity or peak-height intensity as indicated. Since the sample and standard electrodes have the same nominal concentrations of the four elements, the concentration ratios should all be unity (1.00). The %Error is then the per cent deviation of the mean ratio from 1.00. The overall results in Table I11 (see Average of Rows) show that the plate-to-plate precision is, on the average, much worse for peak-height intensities (29.8% RSD) than for integrated intensities 116.7% RSD) and that integrated intensities have produced twice the accuracy (9% Error) compared to peak-height intensity (19% Error). Also, the average concentration ratio for all 32 analyses by integrat(16) J. Franzen a n d K . D. Schuy. Fresenius’ 2. Anal. Chem.. 225, 295 (1967). (17) W H . Wadlin a n d W W Harrison.Ana/. Chem.. 42, 1399 (1970) ( 1 8 ) W W . Harrison a n d G G Clernena. Ana/. Chem , 44, 940 (1972)
Table IV. Precisions for Internal Standard Concentrations by Integrated and Peak-Height Intensities
Isotope
“CU
Mean concn (Integ.) !Height) % RSD iInteg.) I between) (Height) % RSD (Integ.) (within) (Height)
35.4 40.7 16.8 18.8 10.1 12.4
”JCd
lbjRe
88.2 90.8 8.2 10.2 11.0 12.5
145. 131. 11.8 15.2 8.0 9.6
Average of rows
. . . . . .
12.3 14.7 9.7 11.5
ed intensities (0.99) deviates only by 1% from the expected value of 1.00 whereas the deviation is 19% from the expected value for peak-height intensities. Such disparity in precision and accuracy for the two intensity types is explained by the large differences in line width and line shape for the standard and analyte lines which occurred on five of the eight photoplates. A simple visual inspection indicated that shading of the analyte lines caused broadening and shape distortion leading to low results for the analytes. Broadening of this nature is not likely to affect isotope ratios by peak-height intensity because isotope lines on the same exposure are usually broadened and distorted to similar extents. On the other hand, line-width intensities are affected by distortion in a more serious manner when isotope ratios are taken. Four of the plates which were run at the same mass dispersion for the external standard study were chosen for internal standard analysis of Cu, Cd, and Re with Nd as the standard element. T h e upper and lower halves of each plate correspond to different electrode pairs. However, since all electrodes were pressed from the same batch of doped graphite, the upper and lower halves of each plate are valid as separate internal analyses of the same material so that eight separate analyses are recorded on the four photoplates. As many as five neodymium isotopes (142, 143, 146. 148, and 150) were used to calculate analyte conaccording to Equation 7 and assuming centrations that S,, = SV.Table IV lists the mean value of C,’s based on eight analyses for each of three elements using the analyte isotopes indicated. Results are given for both integrated and peak-height intensities. Contrary to the case for external standards, integrated intensities do not effect such large improvements for internal standards. The precision between analyses (from one electrode pair to another) is improved by only 2.4% (relative standard deviation) with the use of integrated intensity and the precision within a n analysis (%RSD of K,,/K, for a single run) is improved by only 1.8’7~. No conclusion can be drawn from the fact that concentrations computed by peak-height and integrated intensity are different. When concentrations are computed by peakheights, the line width and shape factors are included in the sensitivity coefficients so that equally accurate analyses can be performed with peak intensities for the same number of experiments if relative line widths and shapes do not vary. The comparable precisions indicate for both intensity types in this experiment t h a t the relationship of line width and shape for analyte and standard lines is remaining relatively constant from one exposure to the next. although absolute variations in line width and shape may occur between exposures and between runs. Thus, integrated intensities improve precision for external standards because variations in line width and shape are compensated and do not greatly improve precision for internal standards because relative variations are small.
(c,)
Table V. Concentrationsa of Some Elements in W-1 Rock Computed for Integrated and Peak-Height Intensities Integrated intensity
Peak-height
cc
Isotope
Concn
12 9 322 6 69 5 71 57 4 33 4 124 46 2 76 7 125 215 370
RSD
11 3 14 20 13 13 8 14 12 12 7 7 11 12
9 5
9 2 2 8 5 1 4 9 9 1 7% 3%
10 9 %
5< Concn
19 2 468 10 3 8 89 72 5 47 8 155 51 1 84 7 140 216 319
RSD
24 12 19 19 17 16 15 12 14 12 3 8 14 17
3 4 0 5 4 2 9 7 7 2 4 1 7% 8%
Accepted concnb
12
63 11 6 230 200 190 120 50 78 110 16 22
10 2 %
Concentrations in ppmw uncorrected for sensitivity factor. Indium used as internal standard, c;’, RSD is interplate precision of average concentration based on plates P1342 Lo PI348 inclusive # n oPlate P134: . ‘Values compiled by Cornell Group (5 47/71’, ‘For integrated measurements, only the leading “half-intensity” is employed for all lines of this isotope to avoid contributions of secondary lines in a multiplet. Mixture of “half-intensity” and normal full-intensity measurements, due to variable resolution on some plates.
The general conclusion that relative variations in line width and shape are small for internal standard analyses cannot be justified by the preceding experiment because all of the lines employed occurred at moderate to high mass positions where such variations may be small as compared to low mass positions for elements such as Li, Be, B, and F. For this reason, a second internal standard experiment was performed to determine whether or not integrated intensities would more significantly improve precision for internal standards when the standard element occurs at relatively high mass and the analyte elements occur at low mass. USGS W-1 standard rock (see Reagents) provides several suitable low mass elements. The three electrode pairs containing W - l and doped indium produced six photoplates for the internal standard analysis of a number of elements with indium as the standard. The results are given in Table V for both integrated and peak-height intensities. As a matter of convenience, the five analyzed elements from Co to Rb are considered to be in a high mass region and the seven elements from Li to Cr to be in a low mass region. If the overall precision between analyses (plate-to-plate) for integrated intensities is compared t o that for peakheight intensities for all 12 elements. then the comparison is similar to that of the previous experiment for internal standards. However, if only the low mass elements are considered, it is evident that large improvements in precision are realized for integrated intensities over peakheight intensities (12.3% RSD us. l’i.8% RSD on the average). In fact, the overall improvement of integrated intensity for the 12 elements together (11.770RSD us. 14.7% RSD for peak-height) consists entirely of improvements in the low mass region. The results for fluorine are notably poor by either intensity type, a condition which is attributed t o volatility effects which also explain the extremely low concentrations computed for fluorine and chlorine when sensitivity coefficients are unknown. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1974
705
Finally, the validity of employing “half-intensity” integrations is tested. When full resolution of a profile is lacking but the central portion of the profile is not affected by interferences which occur on only one side of the profile, then the profile is scanned from its free side and the intensity integration is performed only for the free half of the profile, that is, u p to the peak-height channel. A user option programs profiles for half-intensity integration a t the time of acquisition. The results in Table V indicate excellent precision for 9Be, a n isotope for which all lines were computed by half -intensity integration. Also, the effect of mixing half-intensity and full-intensity calculations
has not harmed the precision for integrated intensity us. peak-height intensity for those isotopes so indicated in Table V .
ACKNOWLEDGMENT The authors are indebted t o M. D. Lawless for designing and building parts of the acquisition system. Received for review June 12, 1973. Accepted December 13, 1973. Financial support was provided by the National Science Foundation under Grant No. GP-30940X and through the Cornel1 Materials Science Center.
Subpicogram Detection System for Gas Phase Analysis Based upon Atmospheric Pressure Ionization (API) Mass Spectrometry D. I . Carroll, I . Dzidic, R. N. Stillwell, M. G. Horning, and E. C. Horning institute for i / p / d Research. B a y i o r Coilege of Medicine Houston. Texas 66025
A new atmospheric pressure ionization source of simplified and improved design, for use with a modified quadrupole mass spectrometer, is described. Ionization reactions. initiated by electrons from a nickel-63 source, are carried out in a flowing gas stream at atmospheric pressure. The sample is injected directly into the source in common organic solvents. Both positive and negative sample ions result from a complex series of ion molecule reactions. The positive ions are generally M - and M H - ; negative ions are usually (M-H)-. These ions are sampled continuously through a 0.025-cm aperture into the mass analyzer high vacuum region and analyzed with a mass analyzer-detector-computer assembly. The source has an active volume of 0.025 cm3 which is compatible with gas chromatography capillary column requirements. For some biologic samples, such as those from blood or urine, sample preparation consists of solvent extraction. Derivative formation or concentration is often not necessary. I t was found that 150 ferntograms of sample could be easily detected by single ion monitoring.
A preliminary investigation ( I ) of atmospheric pressure ionization (API) mass spectrometry was carried out with a quadrupole mass spectrometer with an ion source of Plasma Chromatography (Franklin GNO Corp.) design ( 2 ) . The instrument in current use, described in this paper, has a source of different design, but has the same form of introduction of ions into the mass analyzer region. Positive or negative ions are generated from sample components through a complex series of ion molecule reactions occurring in the source; primary ionization is due to electrons from a 63Ni foil. In operation, ions and neutral molecules from the reaction chamber enter the mass analyzer through a small aperture (23-micron diameter). In effect, the source is a reaction chamber which is sampled continuously. Horning. M . G Horning. D . I , Carroll I Dzidic. and R N Stillwell. Anal. Chem . 4 5 . 936 (19731 D I . Carroll. R F. Wernlund. and M J. Cohen, U S . Patent
( 1 ) E. C (21
3.639.757. Feb. 1 . 1972
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A N A L Y T I C A L C H E M I S T R Y , V O L . 46, N O . 6, M A Y 1974
The primary purpose of the work described here was to find a way of studying ion concentrations within the source in a dynamic fashion, to establish that the response of the analytical system is in the femtogram range, and to examine a n ion profile of organic bases in human urine in order to determine if samples of biologic origin could be studied without a separation step prior to ionization.
EXPERIMENTAL Mass Spectrometer. The mass spectrometer-detector assemand high vacuum system was originally designed (Franklin 0 Corp. j (2) to be used with a Plasma Chromatograph for the purpose of identifying ions present in the drift tube. The basic mass spectrometer is a Finnigan Model 1015, with radio frequency shielding, which has been modified to accept ions from a grounded source and to operate in either the positive or negative ion mode using pulse counting techniques. The vacuum chamber housing is provided with a heating jacket (Briskheat Corp.) for bakeout a t 250 “C. In practice, since ionization occurs external to the vacuum system. bakeout is very rarely required (no cleaning or maintenance within the vacuum system was required during these studies). The vacuum housing is usually maintained a t the same temperature as the source, although it has also been operated for extended periods a t room temperature without degradation of overall system performance. A schematic diagram of the API mass spectrometer is shown in Figure 1. A single aperture ion lens was added to a conventional axial beam electron impact (EI) ionizer to collect the ions conducted from the external source into the high vacuum region through a 25-micron sampling orifice. The E1 ionizer can be used conventionally for calibration of the mass spectrometer. Provision for a quadrupole rod bias voltage, which is required for a grounded ion source, is made by interposing an adjustable voltage supply in both dc ramp lines to the radio frequency generator. The dual voltage supplies impose a selectable dc voltage upon the normal ramp voltages which are then added to the RF voltage ramp. The control voltage for these ramps is supplied by a computer system. The polarity of all lens and rod bias voltages can be reversed to permit negative ion operation without changing the calibration of the mass Spectrometer. The usual 14-stage dynode multiplier has been replaced by a Ekndix Model 4039 Spiratron multiplier. This multiplier is well suited for pulse counting techniques. A special voltage divider is provided for negative ion operation. In this mode, a cathode accelerating potential of +2 kV is provided t o give the negative ions the required initial energy to release secondary electrons. In this case the signal anode is operated a t about +5 kV off ground and
