JPH03100463A - Method and instrument for measuring cetane value or cetane index - Google Patents

Abstract

PURPOSE:To accurately measure the cetane value or cetane index in a sample from an extremely small volume of sample oil in a short period of time by separating and eluting the respective components contained in the sample to be measured by using gas chromatograph. CONSTITUTION:The components in the sample oil are first separated and eluted by the gas chromatograph and the mass spectra of the respective components are measured. The signal intensities at the respective mass numbers of all the mass spectra obtd. with the light oil fractions are totaled and the total value A is calculated. The signal intensities of plural numbers of the characteristic mass number are totaled and the total value B is calculated. The total value B of the resulting signal intensities is divided by the total value A of the previously obtd. signal intensities, by which plural pieces of variables are determined. The cetane value or cetane index is determined by substituting the variables with the regression formula in which parameters are previously determined.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method and apparatus for quickly, easily, and highly accurately measuring the cetane number or cetane index of light oil, A heavy oil, and hydrocarbon oil base materials used in producing them. [Prior art] Light oil and A heavy oil are hydrocarbon oils with a boiling point range of about 160 to 380°C, and various types such as straight-run kerosene, straight-run light oil, cracked light oil, desulfurized kerosene, desulfurized light oil, vacuum light oil, heavy oil, etc. Obtained by mixing a hydrocarbon oil base material. The final products, light oil and A heavy oil, are mainly used as diesel fuel, and the cetane number is an extremely important item in understanding the properties of these products and the hydrocarbon oil base material. The cetane number is one of the values that expresses the self-ignitability of diesel fuel. Cetane, which has good ignitability, is set at 100, heptamethylnonane, which has poor ignitability, has a cetane number of 15, and a standard mixture of cetane and heptamethylnonane is used. The ignition properties of the fuel and the sample were compared, and the ignition properties were calculated using the following formula from the respective volumes t (%) of cetane and heptamethylnonane in a standard fuel showing the same ignition properties as the sample, and expressed as an integer. Cetane number - capacity of cetane (%) + capacity of heptamethylnonane (%)
Re-5search) engine is used. However, as is clear from JIS-2280, the method of determining the cetane number using the CFR engine does not specify repeatability and has a drawback in measurement accuracy. Also, the measurement requires approximately 1000m I! . There are problems in that this method requires a large amount of sample, involves complicated operations, and requires approximately 1 hour of measurement time. Therefore, the cetane index is often used as a value representing the self-ignition property of diesel fuel instead of the above-mentioned cetane number. The cetane index is calculated by a formula from the API degree and the 50% distillation temperature at 760 msHg, as specified in JIS-2204. The cetane index can be determined by a simpler method than the cetane number, but it is necessary to determine the 50% distillation temperature using specific gravity (API degree) and distillation operation, which is approximately 500 mj! There is a problem in that it requires a certain amount of sample, and cannot be determined if only a small amount of sample oil is available. For example, research on various processes and catalysts has recently been progressing in various fields as crude oil becomes heavier, and in the development of catalysts for fluid catalytic cracking, light circulating oil (L CO (Li
ght Cycle Oil)) It is essential to measure the cetane index of the fraction. However, in this case, since the catalyst is evaluated using a small tester, the produced oil distilled from the tester contains both light and heavy fractions, and the amount of distilled oil itself is small. General properties cannot be measured, and cetane index cannot be calculated. [Problems to be Solved by the Invention] As mentioned above, conventionally, when measuring the cetane number using the CFR engine specified in JIS-2280, a large amount of sample is required and complicated operations are required. Accordingly, there are also difficulties in measurement accuracy. In addition, as mentioned above, the cetane index can be determined by a simpler method than the cetane number, but the specific gravity of the sample oil (
There is a problem that it is necessary to measure the 50% distillation temperature (API degree) and the 50% distillation temperature, which cannot be determined if only a small amount of sample oil is available. The present invention was proposed in order to solve the above-mentioned problems, and makes it possible to determine the cetane number or cetane index even for a small amount of sample oil. and a method capable of quickly and accurately measuring the cetane number or cetane index of a gas oil fraction even if the sample oil contains light and heavy fractions other than the gas oil fraction. The purpose is to provide equipment. [Means for Solving the Problem] The present inventors have determined the types of hydrocarbons contained in the gas oil fraction, such as paraffinic hydrocarbons, naphthenic hydrocarbons, monoaromatic hydrocarbons, and polyaromatic hydrocarbons. We focused on the fact that each of these has a weak correlation with the cetane number. As a result of further research, the signal intensity of the mass spectrum becomes large at the characteristic mass number of each type of hydrocarbon, but the relationship between the signal intensity at these multiple mass numbers and the cetane number was calculated numerically. Ten types of sample oils were analyzed using multiple regression analysis, and the cetane numbers for the various sample oils were determined from the regression equation, and it was found that they matched extremely well with the cetane numbers obtained by the JIS method. The present invention has been made based on the above findings in order to achieve the above object, and the first invention includes a step of separating and eluting each component contained in a sample to be measured using a gas chromatograph; The step of introducing each separated and eluted component into a mass spectrometer and measuring a mass spectrum, and calculating the signal strength and the measured cetane number or measured cetane from the signal intensity at each mass number of the mass spectrum of the gas oil fraction among the mass spectra. The second invention is characterized by the step of performing multiple regression analysis with the index to determine the cetane number or cetane index based on a regression formula determined in advance. a step of introducing the mass spectrometer into a mass spectrometer and measuring a mass spectrum; and determining the signal intensity at each mass number of the mass spectrum;
The third invention is characterized by comprising the step of performing multiple regression analysis between the signal strength and the measured cetane number or cetane index to obtain the cetane number or cetane index based on a regression formula determined in advance. , a gas chromatograft that separates and elutes each component contained in the sample to be measured, a mass spectrometer that measures the mass spectrum of each of the separated and eluted components, and each mass of the mass spectrum of the light oil fraction among the mass spectra. a calculation means and stage for calculating a plurality of variables from the total value of signal strength in a number and the total value of signal strength in a part of mass numbers; or calculation means for calculating the cetane number or the cetane index based on a regression equation determined in advance by performing multiple analysis analysis with the actually measured cetane index, the fourth invention is characterized in that the sample to be measured is a mass spectrometer that measures a mass spectrum of a component contained in a sample by vaporizing it; and a calculation means that calculates a plurality of variables from the signal intensity at each mass number and the signal intensity at some mass numbers in the mass spectrum. and, performing multiple regression analysis between the variables, each of the variables of the plurality of standard samples, and each measured cetane number or measured cetane index to obtain the cetane number or cetane index based on a regression formula determined in advance. It is characterized by having a calculation means. [Operation] First, the measurement principle of the present invention will be explained using an example in which the cetane number of light oil is determined. The cetane number of the hydrocarbon oil base material used in producing heavy oil, light oil, and heavy oil A,
The same applies when determining the cetane index. A mass spectrometer is a mass spectrometer that uses a method to electrically detect ions, and is used for measuring the abundance of isotopes and chemical analysis. The mass spectrum obtained from a mass spectrometer is the mass spectra obtained from a mass spectrometer.
/e value (m is the mass of the ion, e is the charge of the ion) and arranged in order of size.
/e value (mass number) and signal intensity are shown. When measuring the mass spectrum of a gas oil fraction, it is found that the content of paraffinic, naphthenic, monoaromatic, polyaromatic, etc. hydrocarbons contained in the gas oil fraction
Mass spectra with different signal intensities at the characteristic mass numbers of each hydrocarbon type are obtained. In this case, the gas oil fraction is vaporized and introduced into the mass spectrometer in a uniform state, so only one spectrum is obtained. In the obtained mass spectra, there are several characteristic mass numbers for the hydrocarbon type, for example 7 for paraffinic hydrocarbons.
1. The variable Xn is determined by dividing the signal intensity at 91 for monoaromatic hydrocarbons, 97 for naphthenic hydrocarbons, etc. by the total value of the signal intensities for each mass number (for example, 1 to 500) of the mass spectrum. This variable Xn is an explanatory variable in multiple regression analysis, and Xn exists as many as the number (N) of the plurality of characteristic mass numbers. Note that N is optional, but usually about 5 to 15 is used. Now, for example, if the mass number of the mass spectrum is 1 (i-1, 2,
..., 500) is Pi, the nth mass number among the plurality of characteristic mass numbers is xn, and the signal strength at this mass number x7 is Pxn, then the variable Xn
is denoted by . The reason for performing division here is that the amount of sample introduced into the mass spectrometer is not always constant, and if the amount of sample differs, the absolute value of the signal intensity will differ, so this is to remove these influences. In the present invention, the variable Xn is determined from the mass spectrum of each sample oil of various types (for example, several dozen types) whose cetane numbers have been actually measured, and multiple regression analysis is performed. For example, if the number N of variables Xn is 10, the multiple regression equation is as follows. Here, k: 1, 2. ..., KYl: Actual measured value of cetane number (explained variable) Xk, .: Variable obtained from the fit spectrum (explanatory variable) a7: Regression coefficient b: Constant. The above multiple regression analysis is performed on the standard samples, the parameters a n (a In 82+"'+ 810) and b are calculated, and the regression equation, Y=Σa, Xn+b...(2) is specified. Here, Y: Actual value of cetane number The cetane number Y can be obtained by substituting the above into the previously determined regression equation (2).Next, the effect of the present invention will be explained using the case of determining the cetane number of light oil as an example.Heavy oil A, Or, the same applies when determining the cetane number or cetane index of the hydrocarbon oil base material used when producing light oil or A heavy oil.In the first and third inventions, the components are separated using a gas chromatograph. The reason is that the sample oil may contain light and heavy components other than the light oil fraction, so even in such cases, the cetane number can be determined using the mass spectrum of only the portion corresponding to the light oil fraction. This is to calculate.When separating components using a gas chromatograph,
It is only necessary to separate the n-paraffins, confirm the boiling point range of the separated and eluted components from the chromatogram, and obtain components corresponding to the gas oil fraction, and it is not necessary to completely separate each component. Components separated by a gas chromatograph are continuously introduced into a mass spectrometer and their mass spectra are measured.This measurement is performed by repeatedly scanning the mass number range, for example, from 1 to 500, so the mass The spectrum measurement results will be discrete. Therefore, although mass spectra for all separated components cannot be obtained, a plurality of mass spectra can be obtained. Among the above multiple mass spectra, the optimal determination of the mass spectrum corresponding to the gas oil fraction is based on n-paraffins separated by gas chromatography, such as n-decane (C +
. If the component between n-eicosane (C, Hz) and n-eicosane (C, .H4□) is a gas oil fraction, it is determined based on the tensigon time of n-decane and n-eicosane. In other words, the components separated by the gas chromatograph are continuously introduced into the mass spectrometer, so from the time the components are introduced into the mass spectrometer, the difference between the retention time of n-decane and the retention time of n-eicosane The mass spectrum can be a spectrum corresponding to a gas oil fraction. Next, the signal intensities at each mass number for all of the multiple mass spectra obtained for the gas oil fraction are summed, and the signal intensities at multiple characteristic mass numbers in the obtained mass spectra are calculated. The above-mentioned variable Xn is obtained by summing the total. In the second and fourth inventions, even when the sample oil is directly vaporized and introduced into the mass spectrometer without using a gas chromatograph,
As in the case of using the gas chromatograph described above, the cetane number can be obtained by determining a regression equation in advance and calculating the variable Xn for the sample oil to be measured. In addition, 1. Third invention, second invention. In the fourth invention, when measuring the cetane index, the cetane index can be determined by using the cetane index as the explained variable and using a regression equation for the cetane index using the explanatory variable Xn. The above series of steps of the first and second inventions are performed using the gas chromatograph, mass spectrometer, mass spectrum signal intensity integration means, variable calculation calculation means, and cetane number (cetane index) calculation means of the third and fourth inventions. can be executed. As explained above, in the first to fourth inventions, the cetane number (cetane index) of the gas oil fraction is determined by using a mass spectrometer, and the sample oil contains light and heavy substances other than the gas oil fraction. When components are included, the cetane number (cetane index) of the light oil fraction can be determined by separating each component using a gas chromatograph and then introducing each component into a mass spectrometer. The gas chromatograph used in the first invention and the third invention is
In order to efficiently separate the components of the sample oil in a short time, it is preferable to use a device equipped with a temperature raising mechanism. Various types of columns can be used for separation and elution, such as backed columns and capillary columns. However, in the present invention, it is not necessary to completely separate each component in the sample oil, and the n-paraffin component is separated, the boiling point range of the separated component is confirmed from the chromatogram, and the component corresponding to the gas oil fraction is determined. A backed column is preferably used as long as it can be obtained. The packing material for the column is not particularly limited as long as it can achieve the above separation, but a packing material in which a non-polar or slightly polar substance such as a silicon-based polymer is supported on a carrier such as diatomaceous earth is preferably used. As the carrier gas, commonly used hydrogen, helium, etc. can be used, but inert and stable helium is preferable, and the carrier gas velocity is generally about 10 to 10.
40mj! A range of /min is preferred. The oven temperature conditions for the column are related to the separation performance of the column, the carrier gas velocity, etc., and various aspects can be considered. The final temperature is about 250-300℃, and the sample injection amount is about 0.2-1μ! It is. The components separated and eluted by the above gas chromatograph are sent to the separator together with the above carrier gas. The separator is in a vacuum state, the carrier gas is removed here, and each component is introduced into the mass spectrometer. If you want to vaporize the sample oil to be measured and directly introduce it into the mass spectrometer without using a gas chromatograph, vaporize the sample oil using an indirect introduction device (vaporization device).
It is continuously introduced into the mass spectrometer at zero speed for about 17 seconds. The indirect introduction device used has a capacity of about 1000 ml, and is in a vacuum state of about 10-'Torr. Further, the temperature is usually maintained at 100 to 350°C, although it depends on the sample oil to be measured. The components introduced into the mass spectrometer are E I (Elect
Fragment ions are generated by electron impact ionization using a ronImpact ion source. That is, about 2
Set an ionization voltage of about 0 to 80 eV on the ion source,
Bombard sample molecules with electrons. Then, the sample molecule is cleaved from the weakest bond within the molecule, resulting in small fragments. These fragment ions pass through the magnetic field in order of decreasing mass number by scanning the magnetic field from a low magnetic field to a high magnetic field, and a mass spectrum is measured by an ion multiplier. If the sample oil is introduced using an indirect introduction device, only one mass spectrum can be obtained, but if the sample oil is separated using a gas chromatograph, multiple spectra can be obtained. That is, for example, assume that it takes about 13 minutes for each component to be separated and eluted after introducing a sample oil into a gas chromatograph. Furthermore, if the mass spectrum measurement is to be performed over a range of mass numbers from 1 to 500, approximately 6 seconds are required. Therefore, in this case, the mass spectrum is analyzed 130 times (13x6
0x 1/6 = 130) will be measured, and 1
Thirty mass spectra are obtained. Among these 130 mass spectra, a plurality of mass spectra corresponding to the gas oil fraction are determined based on the retention time of the gas chromatogram. When integrating signal intensities or determining variables based on mass spectra, it is preferable to perform isotopic correction on the obtained signal intensities since approximately 0.1% of carbon atoms with a mass number of 13 are present. [Example] Hereinafter, examples of the method and apparatus of the first to fourth inventions will be described with reference to the drawings, taking as an example the case where the cetane number of light oil is determined. The same applies when determining the cetane number or cetane index of a hydrocarbon oil base material used in producing A heavy oil, light oil, or heavy oil. FIG. 1 is a flowchart showing an embodiment of the first invention, and FIG. 3 is a block diagram showing an embodiment of the third invention. Note that FIG. 3 is a diagram showing functions. For example, each calculation unit shown in the figure is configured by the same central processing unit (CPU) in the actual circuit, and each memory in the figure is configured by the same central processing unit (CPU) in the actual circuit. However, it does not correspond to specific connections of constituent elements, such as being configured by a common memory. Furthermore, this is just an example, and it goes without saying that other block configurations can be adopted. In FIG. 3, double arrows indicate the flow of the sample, and double arrows indicate the flow of the signal. In the figure, first, each component in a sample oil is separated and eluted using a gas chromatograph 1. Thereafter, the carrier gas is removed by the separator 2, and each component is introduced into the mass spectrometer 4. In the mass spectrometer 4, sample molecules are converted into fragment ions by an ion source 4A, passed through a magnetic field 4B, and a mass spectrum is measured by an ion multiplier 4C. In the third invention, each component is continuously introduced from the gas chromatograph 1 into the mass spectrometer 4;
Since scanning is performed at regular intervals (for example, every 6 seconds), a plurality of mass spectra are obtained. Next, the detection signals S of the plurality of mass spectra (hereinafter referred to as "mass spectrum signals J") are transmitted to the arithmetic device 5. This calculation device 5 includes a mass spectrum signal intensity integrating means 6
, a variable calculation calculation means 7, a cetane number (cetane index) calculation means 8, and other interfaces (not shown). The mass spectrum signal S is stored in a memory (not shown) or in a memo I76B in the mass spectrum signal intensity integrating means 6. Here, since the mass spectrum signal S also includes signals regarding components other than light oil fractions such as light fractions and heavy fractions, the mass spectrum signal intensity integrating means 6 collects signals from the mass spectrum signal S. , selects a plurality of mass spectrum signals SR corresponding to light oil fractions, and performs isotope correction as necessary. The mass spectrum and signal SIl are selected based on specific components (for example, Judgment signals T + , T based on the retention times of n-decane and n-eicosane)
This is done by z. For example, each matrix element of the mass spectrum signal S is Si,
It can also be expressed as a matrix <S> of j. Here, St,j
is the j-th (j=1.2゜”, +JM
) mass number i (for example, i=1°21.・
., 500) represents the intensity of the mass spectrum. Note that j9 above is a number indicating the final scan of the gas oil fraction. Furthermore, the calculation section 6 in the mass spectrum signal intensity integration means 6
A sums up the signal intensities at each mass number i of a plurality of (jx) spectra corresponding to these gas oil fractions, and calculates the total value. Here, Pi can also be expressed as an i-th column vector <p>, <P>! =(P+Pz... Pi...
pj, )...(3)'. Note that the subscript T means a transposed matrix. Then, the calculation unit 6A calculates the total value of all the above-mentioned total values Pi, that is, <P> ” <E> ... (
4) Calculate '. Here, <E> is a unit matrix. Next, the calculation unit 6A calculates a plurality of characteristic N mass numbers X, , , based on the mass numbers stored in advance in the memory 6B.
(X , 1""' X L+ X 2+ "'+ X
N), the total signal strength of X2+...+XN is each parameter a,,b
is the mass number used when determining (see formula (3)),
Specifically, it is a characteristic mass number of various hydrocarbons such as paraffinic, monoaromatic, and naphthenic hydrocarbons. Next, the above Ptot and Po are output to the variable calculation calculation means 7, and the calculation unit 7A calculates the variable Xn as follows: (5). Note that Px can also be expressed as an N-th column vector (Px), (Px) ”= (Px+ Pxz ・・
・Px, ---PXN)...(5)'. In addition, here, XI+X2+...+XN is any number from 1 to 500 in the case of the upper side, and P in formula (5)
Since x7 is one of Pi in equation (3), when calculating equation (3), Px, , may be stored in the memory 6B and this data may be substituted for the calculation of equation (5). Also, this pre-stored mass number X n−X l
+ or expressed as a matrix, the following is performed, where <X> is a matrix in which Xn is expressed as an Nth-order column vector. Then, the variable Xn or <X> of the above formula (6) or (6)' is sent to the cetane number (cetane index) calculation means 8, and the parameters a, b, which are stored in advance in the memory 8B.
Or, if you read it out and express it in a matrix, Y= <a>'<X> + b
,,,Based on (7)', the cetane number (
cetane index) Y is calculated. Here, <a> is an N-th column vector with a7 as a column element, and <a>7=[aI a2... all... aN]
It is. FIG. 1 is a flowchart showing an example of the cetane number or cetane index measuring method of the first invention when carried out using the apparatus shown in FIG. Note that the first invention is not limited to the flowchart in FIG. 1; for example, the following steps 3 and 4
It goes without saying that the first invention also includes those in which the execution order is exchanged. In FIG. 1, components in the sample oil are first separated and eluted using a gas chromatograph 1 (step 1), and a mass spectrum of each component is measured (step 2). Next, the signal intensities at each mass number i for all of the mass spectra obtained for the gas oil fraction are summed to calculate a total value ΣPi (step 3). Then, a plurality of characteristic mass numbers (X I+ X l+
'－・. XN), and calculate the total value PXFI (step 4). Next, the total value Px of the signal strength obtained in step 4,
A plurality of variables Xn are obtained by dividing + by the total signal strength value ΣPi obtained in step 3 (step 5). Then, the cetane number (cetane index) is determined by substituting the above variables into regression equation (7) or (7)' in which parameters a, b are determined in advance (step 6). FIG. 2 is a flowchart showing an embodiment of the second invention, and FIG. 4 is a block diagram showing an embodiment of the fourth invention. 4 differs from the third invention shown in FIG. 3 in that the sample oil is introduced into the mass spectrometer 4 using an indirect introduction device 3 without using a gas chromatograph. Note that the same reference numerals in FIG. 4 as in FIG. 3 indicate the same elements as in FIG. 3. In FIG. 2, a sample is vaporized and introduced into the mass spectrometer 4, a mass spectrum is measured (step 101), and the signal intensity at each mass number of the obtained mass spectrum is summed (step 1o2). Next, a plurality of variables Xn are obtained by dividing the signal intensity at a plurality of characteristic mass numbers by the signal intensity obtained in step 102 (step 1o3), and the above variables are added to the regression equation obtained in advance. The cetane number (cetane index) is determined by substituting (step 1o4). [Experimental Example] Specific experiments were conducted based on the first and third inventions described above. The results are shown below. In this experiment, the characteristic mass number, ? 1.91,97,1
29°131.141.167, 179.191 (in formulas (5), (6), (7), N = 9
) and 60 types with known cetane number and cetane index (
In equation (2), mass spectra are measured for the sample K = 60), and the variable Xn (n = 1.2+...
, 9) were obtained, multiple regression analysis was performed, and parameters a, l, and b of regression equation (7) were obtained. The measurement conditions of the gas chromatograph and mass spectrometer are as follows.

[Gas chromatograph measurement conditions]

・Column: Packing material; Diatomaceous earth liquid phase methyl silicone Length: 0.3 m Inner diameter: 3 ■ Material: stainless steel ・Column oven temperature conditions: (*heating rate) ・Inlet temperature: 270℃ ・Carrier gas: He20m1! ,/min・Sample injection amount:
0.5μ!

[Measurement conditions for IT quantity analyzer]

・Ion source: Er (electron impact ionization) ・Ionization voltage near 0 eV ・Separator temperature: 270°C ・Scan range: Mass number 1 to 500 Table 1 shows the actual measured values of various sample oils according to the JIS method and the present invention This shows the results obtained in an experiment. In the above experiment, the time required to determine the lycetane number or cetane index per sample was about 30 minutes. Table 2 shows the JIS method cetane number: 53.6. These are the results of repeated measurements five times on a commercially available gas oil sample with a JIS method cetane index of 54.5, and show the repeatability of the final method. [Effects of the Invention] As described above, the present invention employs the signal strength of the mass number of a component showing a characteristic mass number as an explanatory variable, and performs multiple regression analysis using the measured cetane number or the measured cetane index as the explained variable. We decided to use the mass analysis results of the component showing the characteristic mass number of the sample to calculate the cetane number or cetane index of the sample based on the regression equation. , the following effects can be achieved. (1) The cetane number or cetane index of the sample can be determined from the regression equation by simply measuring the signal intensity at each mass number of the sample without actually measuring the cetane number or cetane index of the sample. (2) Therefore, in the conventional technology, it took a long time to measure the ceta number and cetane index per sample, and required complicated operations and a large amount of samples, but the present invention improves these difficulties. I can do it. That is, in the present invention, the cetane number or cetane index of an extremely small amount of sample oil can be measured easily and accurately in a short time. (3) As a result, the method and apparatus of the present invention can be effectively used in many fields including process and quality control in oil refineries or oil depots that produce hydrocarbon oils such as light oil and heavy oil. The industrial value is extremely high.

[Brief explanation of drawings]

1 and 2 are flowcharts showing one embodiment of each of the first and second inventions, and FIGS. 3 and 4 are functional block diagrams showing one embodiment of each of the third and fourth inventions. be. 1... Gas chromatograph 2... Separator 3.
...Indirect introduction device 4...Mass spectrometer 5...
Calculating means 6...Mass spectrum signal intensity integrating means 7...Variable calculation calculating means 8...Cetane number (index) calculating means 1 Fig. Fig. Fig. Fig.

Claims (4)

[Claims] (1) separating and eluting each component contained in the sample to be measured using a gas chromatograph; introducing each of the separated and eluted components into a mass spectrometer to measure a mass spectrum; From the signal intensity at each mass number of the mass spectrum of the gas oil fraction, the cetane number or cetane number is determined based on a regression formula determined in advance by performing multiple regression analysis between the signal intensity and the measured cetane number or cetane index. A method for measuring a cetane number or cetane index, comprising the steps of determining the index. (2) Vaporizing the sample to be measured and introducing it into a mass spectrometer to measure a mass spectrum; and determining the difference between the signal intensity and the measured cetane number or cetane index from the signal intensity at each mass number in the mass spectrum; A method for measuring a cetane number or cetane index, comprising the step of performing multiple regression analysis to obtain a cetane number or cetane index based on a regression equation determined in advance. (3) A gas chromatograph that separates and elutes each component contained in the sample to be measured, a mass spectrometer that measures the mass spectrum of each of the separated and eluted components, and a mass spectrum of the light oil fraction among the mass spectra. calculation means for calculating a plurality of variables from the total value of signal intensity at each mass number and the total value of signal strength at some mass numbers; or a calculation means for calculating the cetane number or cetane index based on a regression equation determined in advance by performing multiple regression analysis with the actually measured cetane index. (4) A mass spectrometer that vaporizes the sample to be measured and measures the mass spectrum of the components contained in the sample; calculation means for calculating the variables; and based on a regression equation determined in advance by performing multiple regression analysis between the variables, each of the variables of the plurality of standard samples, and each measured cetane number or measured cetane index. 1. An apparatus for measuring a cetane number or cetane index, comprising a calculation means for calculating a cetane number or cetane index.