that CS2oxidizes to SO2before reaching the ion forming region so that under these conditions it behaves like an equivalent quantity of SOp. A similar effect was observed by Dressler and Jansik whose experiments were made with a very high air flow rate (15). Preoxidation of the CS1is an important factor in determining its response (171, and the SO, produced is able to oppose the formation of a positive peak in the presence of CH4. The negative responses for SOZ,H S , and SCO with the FID have apparently not been reported, probably because they are only observed with a very pure column, and with hydrocarbon in the supply gases, whereas normal operation of the
FID usually reverses these specifications; most operators avoid high backgrounds and use hydrocarbon contaminated columns. ACKNOWLEDGMENT
The assistance of D. M. Douglas and M. Kecskemeti is gratefully acknowledged. RECEIVED for review October 31, 1969. Accepted December 29,1969.
Digitization of Time-of-Flight Mass Spectra M. A. Grayson and R. J. Conrads’ McDonnell Research Laboratories, McDonnell Douglas Corp., S t . Louis, Mo. 63166 A means of digitizing the mass spectra from a time-offlight mass spectrometer has been devised. The output of the mass spectrometer is recorded on FM magnetic tape along with a time base taken from the analog scanner. The magnetic tape is played back into an analog-to-digital converter operating at a sampling rate of 2000 samples per second. The digitized data are searched by means of a computer program which locates mass peaks, determines their intensity, and measures the value of the time base at which the mass peak occurred. With the aid of the time base, mass numbers are calculated and rounded off to integral values. The output of the program lists mass numbers in ascending order with their corresponding peak heights. An option provides the peak heights in raw or normalized form. The digitized spectrum is also available on digital magnetic tape as input to other data-handling programs. The signal-conditioning circuitry for data acquisition is described.
MASSSPECTRA are most easily interpreted and identified when they are in the digital form of mass number us. peak height. Digitized mass spectra enable the spectroscopist to subtract background accurately and quickly, to compare mass spectra against known standards rapidly, and to facilitate the use of the computer as a tool for the various tasks involved in interpretation. The digitized form is also a convenient and permanent means of storing mass spectra. The most widely used means of recording mass spectra-the oscillographic record-lacks all of these important datahandling features. As a result, many spectra are digitized by hand from oscillographic records. The task of assigning mass ncmbers to all of the peaks in a mass spectrum, reading the peak heights, and recording these data manually is tedious and prone to errors due to fatigue on the part of the person performing the work. Digitized mass spectra are a necessity when the mass spectrometer is used in conjunction with a gas chromatograph. A chromatogram may contain as many as 200 individual peaks. Since it is the usual practice to take two or more mass spectra per G C peak, several hundred mass spectra may be taken during one GC run. Techniques for digitizing low resolution mass spectra from a magnetic mass spectrometer have been described by Hites and 1 Present address, Georgia Institute of Technology, P. 0. Box 31872, Atlanta, Ga. 30332
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Biemann. Analog to digital conversion is performed in the laboratory and the data are written onto digital magnetic tape for reduction at a later time (1) or fed directly into a digital computer for reduction in real time (2). One of the major problems in mass spectrum digitizing routines is determining the mass number correctly. Hites and Biemann solve this problem by matching peaks in the unknown spectrum against known peaks in a standard spectrum. The mass of peaks which have no match in the standard spectrum can be assigned by interpolation or extrapolation. This approach requires a standard spectrum for each scan speed the operator desires to use and assumes that the scan is fairly reproducible. Commercial mass spectrum digitizers are available and have the advantage of producing the digitized mass spectra in real time. However, at high scan speeds-1 to 2 seconds per mass decade-the digitized mass spectrum may be incomplete because of the slow response of the printer. A technique for digitizing mass spectra from a time-offlight mass spectrometer is given below. The mass spectra and a time base taken from the analog scanner are recorded on F M magnetic tape (3). The recorded data are later digitized and reduced at an analog hybrid computer center. The method by which mass numbers are determined differentiates this from previous work. Mass numbers are calculated directly from the equations governing the time-of-flight of ions in the mass spectrometer rather than by comparison with a known spectrum. The equation used for calculating the mass of an ion is ( 4 ) :
m=[d,+-] t p - tl k
(1)
Since the mass numbers are being determined for a recorded spectrum, physical time is not used for tl and f2. Rather, the value of a variable which is linearly related to physical timeLe., a time base-is used. (1) R. A. Hites and K. Biemann, ANAL.CHEM., 39,965 (1967). (2) Zbid., 40, 1217 (1968). (3) P. Issenberg, M. L. Bazinet, and C. Merritt, Jr., Zbid., 37, 1074 (1965). (4) R. W. Kiser, “Introduction to Mass Spectrometry and Its Applications,” Prentice Hall, Englewood Cliffs, N. J., 1965.
Once a satisfactory time base has been chosen, the value of the constant k can be calculated from the relationship
Naturally, the system must be calibrated with two known masses in terms of the time base being used. Equation 1 can then be used to calculate an unknown mass from a known mass, their respective time base values, and the predetermined constant, k . The gate driving voltage (GDV) from the analog scanner is an excellent time base which will provide accurate determination of mass numbers up to the resolving power of our instrument. There is a one-to-one correspondence between each gate driving voltage value and each point on the mass scale. There are several advantages to using the GDV as a time base for the computation of mass numbers. Nonlinearities in the mass scan are no longer a problem in the accurate determination of mass numbers. Any desired scan speed can be chosen, irrespective of the scan speed with which the system is calibrated. Small irregularities from scan to scan which would normally cause miscalculation of mass above mje 150 are no longer a problem. Less computer memory is required for the mass calculation portion of the program. For the digitization of spectra from a time-of-flight mass spectrometer this technique is more easily implemented and better suited to handling the problem of mass number determination than techniques described for magnetic instruments. EXPERIMENTAL
Data Acquisition. A Bendix Model 12-107 time-of-flight mass spectrometer equipped with a 3012 retrofit analog scanner is used to take the mass spectra. The 3012 scanner has a low noise level in the output circuit, and the scan can be triggered by a single switch. Hence, it is preferred over the older 321 scanners. The GDV, which is recorded as a time base, is tapped off of the plate of the 6U8A tube in the scan generator circuit of the analog scanner. The plate voltage varies from 0 to 300 volts direct current (Vdc). The GDV is recorded within the limited range of the tape recorder- -10 to f10Vdc-with the aid of a signal conditioner and ramp expander circuit discussed in detail in the appendix. The use of the ramp expander circuit provides a means of obtaining a high degree of voltage resolution in recording the GDV, so that mass numbers up to 300 amu can be determined accurately. The multiple operations involved in scanning and recording the mass spectrum are controlled by an electronic circuit. The block diagram in Figure 1 shows the details of the recording process. When the master switch is turned on, the tape recorder is started in the record mode. After a 2-second time delay which allows the tape to reach a constant speed, the initial level of the GDV is recorded for 0.2 second and the scan started. At the end of the scan, the tape is stopped. A convenient feature of this single-switch operation is that the mass spectrometer may be automated when doing GC/MS work by using the signal from the total output integrator to trigger the scan. Four channels of data are recorded simultaneously in the operation initiated by the master switch. Channel 1 is a recording of the low sensitivity mass spectral data in analog form as they appear at the output of the scanner. Channel 2 is a recording of the high sensitivity mass spectral data at a gain of ten times channel 1. The use of two channels at different gains provides a dynamic range of about 1000 for the peak height. Channel 3 is a recording of the GDV after
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I
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Figure 1. Block diagram re- Figure 2. Playback at computer center cording system
it has been conditioned by the ramp expander circuit. Channel 4 is a recording of the digital control gate which is negative whenever a spectrum is being recorded and zero at all other times. A negative voltage is used for the control gate, since the tape stop-start sequence gives rise to positive transients during playback. The digital control gate controls the A to D converter during playback, so that digitization occurs only when channel 4 is negative. The 0.2-second delay between the time the control line goes negative and the scanning of the mass spectrum (Figure 1 , A ) allows for digitizing the dc level of the GDV at the beginning of the spectrum. Prior to data acquisition, the analog scanner is adjusted so that the scan begins at m/e 14, and a calibration spectrum is recorded. Unknown spectra are then recorded by triggering the master switch. Thus all of the spectra start from a known mass, mje 14. Data Reduction. At the analog hybrid computer center, the tape is played back into an analog computer. The block diagram in Figure 2 shows the details of this operation. The analog computer is used to scale and bias the recorded data to the specifications of the analog to digital converter. The analog to digital converter digitizes each data channel at the rate of 2000 samples per second. The converted data are written immediately on digital magnetic tape. This tape contains raw data in packed binary form and must be unpacked and written onto another tape which serves as input to the digital program. The program has three data points for each sample interval taken in the A to D conversion of the spectrum: high sensitivity mass spectral data, called "data hi"; low sensitivity mass spectral data, called "data lo"; and GDV data, called "data tb." The first operation performed by the program is to construct a time base from the expanded GDV that has been recorded on channel 4. The program detects the biases impressed by the ramp expander circuit during recording, measures them, and subtracts them cumulatively from the recorded GDV. The time base thus constructed has a linear relationship with the GDV of the analog scanner. ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
457
Figure 3. Peak finding program logic VL1, VL2. (M-1)st and Mth spectral data bits P1. Two-value index; 0 if spectral data bit is below threshold, 1 if spectral data bit is above threshold PL. Two-value index; 0 if spectral data bit is above 10 % peak height, 1 if spectral data bit is below 10%peak height. PHT. Peak height
The flow chart in Figure 3 shows the details of the peakfinding logic. The program tests data hi points to determine if they are above an arbitrary threshold level of 1 volt. This test provides a means of filtering out the low level noise present on the base line. A data hi point above 1 volt starts the peak-finding routine and the center of mass (centroid) calculation. The program then tests data hi to decide which of the two mass spectral data channels to use.
After choosing the appropriate mass spectral data bit channel, the program proceeds to test the slope between data bits. The first negative slope found causes the last highest data bit to be stored. The program is designed to finish defining a peak if the voltage level returns below threshold, or if an inflection from negative to positive slope occurs at a voltage level less than 10% of the highest data bit. If a peak is defined by the above tests, the centroid of the mass peak is calculated. If an inflection from negative to positive slope occurs at a voltage level above 10% of the highest data bit, the program continues to find a higher data bit for the peak or a lower inflection point to complete the peak-finding routine. The inflection point test serves as a filter for low level noise that may occur on a peak. As peaks are found by the program, their mass is computed using Equation 1. The reference mass, ml,is some earlier peak in the spectrum; the constant k has been determined earlier by the program and the GDV values are used for tl and tz. If a mass number falls within the range m -0.3 to rn f0.3 mass unit, it is rounded off and stored as an integer; otherwise, the peak is disregarded. The program proceeds in this fashion to reduce all of the data in the spectrum. The results are stored on magnetic disk for formating, and the program continues onto the next spectrum. After all spectra are reduced, the results are printed and also recorded on digital magnetic tape.
Table I. Detailed Listing of Variables Used by Data Reduction Program to Calculate Mass Numbers Peak m2 intensity, volts t z , GDV, volts Calcd mass Rounded-off mass m ~ ref , mass tl, ref. GDV, volts 58.7 2.9 61.7 45.5 90.7 10.0 9.1 31.3 7.5 123.5 42.4 179.8 12.9 12.5 7.5 10.9 110.9 20.9 58.5 68.1 6.4 35.3 9.3 21.7 8.9 743.2 78.5 13.7 15.9 251.8 25.4 11.8 10.8 21.1 9.2 23.9 5.8 49.2 19.4 6.8 44.8
458
-11.54 36.15 40.07 43.76 47.39 54.37 57.66 80.73 83.70 87.20 90.19 93.29 95.98 98.91 120.83 123.38 126.07 128.66 131.26 136.21 155.64 160.27 162.49 164.69 169.11 171.30 173.41 190.57 192.51 198.77 200.92 218.78 220.70 226.14 252.10 276.74 278 52 296.41 299.65 300.98 344.44 I
14.9 26.0 27.1 28.0 29.0 30.9 31.9 39.0 40.1 41.1 42.1 43.0 43.9 44.9 52.9 53.9 55.0 56.0 57.0 58.9 66.9 68.9 69.9 70.9 72.9 73.9 74.9 82.8 83.8 86.9 88.0 96.9 97.9 100.8 114.8 129.1 130.1 140.8 142.8 143.8 172.1
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
15 26 27 28 29 31 32 39 40 41 42 43 44 45 53 54 55 56 57 59 67 69 70 71 73 74 75 83 84 87 88 97 98 101 115 129 130 141 143 144 172
14 14 15 26 27 28 29 31 32 39 40 41 42 43 44 45 53 54 55 56 57 59 67 69 70 71 73 74 75 83 84 87 88 97 98 101 115 129 130 141 143
-16.45 -16.45 -11.54 36.15 40.07 43.76 47.39 54.37 57.66 80.73 83.70 87.20 90.19 93.29 95.98 98.91 120.83 123.38 126.07 128.66 131.26 136.21 155.64 160.27 162.49 164.69 169.11 171.30 173.41 190.57 192.51 198.77 200.92 218.78 220.70 226.14 252.10 276.74 278.52 296.41 299.65
700
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3ZU
360
Gate Driving Voltage
Figure 4. Mass spectrum of methyl nonanoate I I
RESULTS AND DISCUSSION
The primary problem in any digitization routine, whether for high or low resolution mass spectra, is the accurate determination of mass number. In handling low resolution mass spectra it is desirable to obtain mass number calculations accurate enough to be rounded off to integral mass numbers. The integral mass number has obvious advantages in storing and handling the mass spectra with a computer. The manner in which our technique performs the task is detailed below. The first spectrum encountered by the data reduction program is that of the calibration material (in our case, benzene). The value of the GDV at the beginning of the scan and at the most intense mass peak is measured by the program. It then calculates the constant k from Equation 2 with the measured GDV values: rl = -16.45 Vdc and t2 = 179.50 Vdc and the preassigned mass numbers ml = 14 and mz = 78. The constant resulting from this calibration is 38.49 volts The manner in which the data reduction program computes mass numbers can best be understood by inspecting Table I, The data are from the spectrum of methyl nonanoate (Figure 4). Each row shows the data used by the program in the computation of a mass number. Column one is the raw peak height which has been amplified to match the dynamic range of the analog to digital converter (0 to +128 Vdc). Peaks greater than 128 volts have been taken from the low sensitivity channel and multiplied by a factor of 10. Thus the dynamic range of the intensity is 0 to +1280 Vdc. Column two shows values of the GDV determined by the center of mass calculation for each of the mass peaks found. The data-reduction program has already reconstructed the GDV from the expanded ramp that was recorded on tape. The figures shown are amplified about a factor of 5 greater than the GDV from the analog scanner. The digital data in columns one and two are the input to the data-reduction program. Equation 1 is used to calculate mass numbers using the values of ml, tI, and tz shown in columns five, six, and two, respectively. The results of this calculation are shown in column three. In this spectrum, all of the mass peaks calculated fall within the round off limits of m h0.3 mass unit. If a mass number falls outside of this range, it will be disregarded and will not appear in the final printout. Column four shows the integral mass numbers which result from rounding off the masses in column three.
L
q
Figure 5. Ramp expander circuit Fairchild U5B770939X operational amplifiers 2N706 *1 resistors. All operational switches A3Q2 through A1&2 identical to AzQl except as shown AI-&.
QI-QIZ.
The program is designed to shift the reference point (ml, tl) so that calculation over large mass gaps is avoided. There are two reasons why this is advantageous: The electron beam collimating magnets and the compensating magnets cause small variations in the physical time-of-flight of the lighter (m/e < 50) ions; consequently, Equation 1 does not strictly hold over the complete mass range. Small errors (=tO.1 Vdc) in reconstructing the ramp occur at each ramp discontinuity (Figure 5). These errors will accumulate and become large enough to result in inaccurate mass calculations if the gap between reference mass and unknown mass is too large. By continually changing the reference point from the starting mass to previously determined integral masses in the spectrum, the combined effects of these two problems are minimized. The approach of shifting the reference point is much more easily implemented than trying to account for these perturbations by adding corrective terms in the mass calculation equation. Despite these problems, the mass gap over which a mass number can be correctly calculated is nearly 150 mass units, This was determined by manually calculating the molecular ion of a CI9 hydrocarbon from different reference points in the spectrum of that compound (Table 11). The correct mass for the molecular ion is calculated within roundoff limits over a gap of 140 mass units. Beyond that (reference point masses 85 and 57) the calculated mass of the molecular ion starts to slip below the correct value. The choice of calibration material for determination of the constant k is not restricted to benzene. Any compound in which the most intense mass peak is unequivocably known will suffice as a calibrant for the system. Our work with materials ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
159
300 V d c
Table 11. Accuracy of Mass Calculation from Different Reference Points in Mass Spectrum (rnz = 268) rnl tl r n 2 (calcd) 253 225 183 113 85 57
451.09 416.54 359.27 248.15 194.17 130.90
267.9 267.7 268.2 267.8 267.5 266.7
k = 38.49, tz = 468.96.
Table 111. Mass Spectra of Methyl Nonanoate at Two Different Scan Speeds 4-secimass decade 2-sec/mass decade Pk ht mje (calcd) Pk ht rn/e (calcd) 15
35.8
27 28 29 39 41 42 43 45 53 54 55 56 57 59
37.6 34.8 64.1 17.9 77.7 26.6 115.0 6.9 5.7 5.3 63.6 15.0 40.8 40.7
69 71 74 75
22.7 12.9 396.2 38.4
83 84 87 88 101 129 141 143 172
8.3 13.6 137.3 15.4 14.8 17.6 29.6 12.8 28.1
15 26 27 28 29 31 32 39 40 41 42 43 44 45 53 54 55 56 57 59 67 69 70 71 73 74 75 83 84 87 88 97 98 101 115 129 130 141 143 144 172
58.7 3.0 61.7 45.6 90.8 10.1 9.1 31.3 7.5 123.6 42.5 179.8 12.9 12.6 7.6 11.0 110.9 20.9 58.6 68.1 6.4 35.4 9.3 21.7 9.0 743.3 78.6 13.7 16.0 251.9 25.5 11.8 10.9 21.1 9.3 24.0 5.9 49.2 19.4 6.9 44.8
& = 38.49 (recorded at 10 secimass decade).
up to m/e 268 shows that the use of a low molecular weight compound for calibration is satisfactory. Another example of the flexibility of this technique for handling T-0-F mass spectra is shown in Table 111. Columns one and two show the digitized mass spectrum of methyl nonanoate scanned at 2 seconds per mass decade. Columns three and four show the spectrum of the same material scanned at 4 seconds per mass decade. The calibration scan was made at 10 second per mass decade. The only difference in the spectra is the loss of some of the smaller peaks due to lower 460
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
R o m p /,om R o m p E x p o n d e r
sensitivity of the scanner at higher scan speeds. The mass numbers in the spectra are correctly determined for each scan speed. Thus, any variety of scan speeds can be chosen during the recording of the mass spectra and correctly digitized mass spectra will result. Prior to use of the GDV as a time base for the calculation of mass numbers, the time of occurrence of the mass peak (clock time) was employed as a time base. Generally, clock time is an accurate time base up to m/e 100. Above this mass range the scan is not linear, nor reproducible, enough for accurate mass number determination using clock time as a time base. The GDV can also be used where A to D conversion is performed in the laboratory. If the A to D is capable of handling a 0- to 300-Vdc signal, the GDV can be digitized directly. Otherwise the ramp expander circuit can be used to accommodate the dynamic range of the A to D converter without losing accuracy in measuring the GDV. The technique of using the gate driving voltage as a time base for determination of mass numbers with T-0-F mass spectra has several advantages. Mass numbers can be determined accurately over the usable mass range of the time-of-flight mass spectrometer. The operator has the option of scanning spectra at any desired speed, irrespective of the scan speed used during calibration. Such a technique circumvents problems arising from electronic instabilities in the circuitry used to scan the spectrum and lends greater flexibility to the use of the mass spectrometer in conjunction with the data reduction facilities. APPENDIX. DISCUSSION OF RAMP EXPANDER CIRCUIT
Since the scan function of the analog scanner is not linear with time, it is important that the ramp function (GDV) which generates the scan be recorded as the time base for calculation of mass numbers. A means of expanding the ramp function while keeping it within the dynamic range of the recording equipment was devised. The 0- to 300-volt ramp function is obtained at the plate of the 6U8A tube in the Model 3012 analog scanner. It is passed directly to a high impedance divider network and subsequently to the current to voltage amplifier, A I , Figure 5. A1 buffers the ramp and conditions it for chopping and amplification. The 0- to 5-volt ramp output from A I is passed to 12 operational switches AP through A H and to the output amplifier, Ala. At each switch the conditioned signal is summed with a weighted reference voltage, the sum serving as input to a transistor bias switch. Each operational switch is aligned by means of the variable input resistor to switch from -15 to - 15 Vdc at 12 different points in the 0- to 5-volt range of the ramp. These signals trigger the transistor bias switches, each of which supplies output amplifier A14with a 0.415-volt bias when it is on. The ramp from A1 is summed with these switched biases in the output amplifier, The ramp, therefore, supplies its own means of triggering a stepped bias to the output stage. Since all of the summing A149
points of the operational switches remain at a virtual ground potential, each trigger point may be adjusted independently. The gain and bias of A I 4are adjusted so that the output ranges from - 10 to +10 volts, as shown in Figure 6. This chopped ramp is then recorded on magnetic tape along with the mass spectral data and the digital control gate. f i e digital cornputer reconstructs the ramp and uses the instantaneous ramp voltage as a time base to assign mass numbers in the mass spectrum. The fall times of the expanded ramp are less than 200 microseconds, which is faster than the 500-microsecond sampling of the analog to digital converter, thus ensuring that the
reconstructed ramp will not have discontinuities at the ramp resets. ACKNOWLEDGMENT
The authors are grateful to Eldon Beran, McDonnell Automation cos, for his assistance in the analog and digital programming. RECEIVED for review June 30, 1969. Accepted January 14, 1970. Research conducted under the McDonnell Douglas Independent Research and Development Program,
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Inert Carrier-Gas Fusion Determination of Total Nitrogen in Rocks and Meteorites Everett K. Gibson, Jr.,l and Carleton B. Moore Center for Meteorite Studies and Department of Chemistry, Arizona State University, Tempe, Ariz. 85281
An inert carrier-gas extraction-gas chromatography technique has been adapted for the determination of total nitrogen in rocks and meteorites. Samples are fused in a graphite crucible at 2400 O C and the nitrogen and other evolved gases are separated and analyzed using a gas chromatograph. The excess carbon monoxide produced from the silicates is converted to carbon dioxide and removed from the system. The limit of detection was found to be 2 pg of N and the error of the technique was less than 5% of the normal working range between 10 and 200 pg of N. The method is rapid, accurate, and simpler than other methods now in use for the determination of trace quantities of nitrogen in a wide variety of materials.
COMBUSTION-THERMAL CONDUCTIVITY ANALYZERS for determining total nitrogen in steels and refractory metals have been described by Bryan and Bonfiglio ( I ) , Dallmann and Fassel (2), and others. The sample is heated to 2400 "C in helium in a graphite crucible and the molecular nitrogen and other gases that are formed are swept into a molecular sieve trap and collected. After a fixed collection period, the gases are swept into the molecular sieve column of a gas chromatograph and, upon elution from the column, the molecular nitrogen and the other evolved gases are detected with a thermal conductivity detector. In the determination of nitrogen in silicates and various metal oxides with this technique, problems arise because of the excessively large amounts of carbon monoxide formed upon combustion of the sample in the graphite crucible. The excess carbon monoxide prevents resolution of the nitrogen peak on the chromatogram obtained. In this investigation the technique was modified to remove the carbon monoxide and permit nitrogen determinations in silicate materials. After combustion of the silicate samples, the evolved gases are swept through a gas train in which they pass over a copper oxide-rare earth oxide catalyst held at 400 "C,which converts Present address, TN7, Geochemistry Branch, NASA Manned Spacecraft Center, Houston, Texas 77058 (1) F. R. Bryan and S. Bonfiglio, J. Gas Chromatogr., 2,97 (1964). (2) W. E. Dallrnann and V. A. Fassel, ANAL.CHEM.,39, 133R
(1967).
all of the carbon monoxide to carbon dioxide. After the conversion, the gases are passed through an Ascarite-Anhydrone trap to remove the carbon dioxide and water. The remaining gases are transferred into the molecular sieve column of the chromatograph, where the nitrogen peak is easily resolved. By removing the carbon dioxide and water, the life of the molecular sieve column is increased. Advantages of this method are that high sensitivity is obtained by concentrating the nitrogen in the cold trap from the helium carrier gas, and the chromatographic separation allows the separation of nitrogen from the rare gases which interfere in the v a c u h fusion determination of nitrogen. The samples are fused directly in the graphite crucible and no dissolution of the sample is required. The loss of nitrogen, which often occurs during Kjeldahl dissolution procedures, is eliminated. Although the total nitrogen content of the sample is determined, the technique does not allow determining the form in which the nitrogen occurs. Our previously reported work (3, 4 ) gives the results of the determination of total nitrogen content of chondritic meteorites. The work reported here discusses a technique for the determination of the total nitrogen content of silicate samples of the U S . Geological Survey standard rocks, synthetic silicate standards, National Bureau of Standards standard steels, and meteorites. EXPERIMENTAL
The equipment consists of a Leco Nitroxd Analyzer (Model 534-800). Its operation for nitrogen and oxygen determinations has been described (1). The schematic of the apparatus with the modifications made for this work is given in Figure 1. Combustion Apparatus. A Leco induction furnace (Type 537-100) was used with Leco combustion tube No. 589-625. Samples were melted in graphite crucibles (Leco No. 534-352). Theoperatingtemperature for the silicate samples was 2400 OC. A Leeds & Northrup optical pyrometer was used to calibrate the temperatures of the induction furnace. (3) C. B. Moore and E. K. Gibson, Scierzce, 163, 174 (1969). (4) C. B. Moore, E. K. Gibson, and K. Keil, Earth Planet, Sci. Letters, 6, 457 (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
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